POSITIVE ELECTRODE ACTIVE MATERIAL

Abstract

A positive electrode active material that is unlikely to generate defects even when charging and discharging at a high voltage and/or at a high temperature is provided. A positive electrode active material in which crystal structures are unlikely to collapse even when charging and discharging are repeated is also provided. The positive electrode active material contains lithium, cobalt, oxygen, and an additive element. The positive electrode active material includes a surface portion, an inner portion. The positive electrode active material contains the additive element in the surface portion. The surface portion is a region extending from a surface of the positive electrode active material to a depth of 10 nm or less toward the inner portion, and the surface portion and the inner portion are topotaxy. A degree of diffusion of the additive element vary between crystal planes of the surface portion, and the additive element is at least one or two or more selected from nickel, aluminum, and magnesium.

Description

TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof.

Note that an electronic device in this specification means all devices including power storage devices, and electro-optical devices including power storage devices, information terminal devices including power storage devices, and the like are all electronic devices.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and a high capacity has rapidly grown with the development of the semiconductor industry. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today's information society.

In particular, secondary batteries for mobile electronic devices, for example, are highly demanded to have high discharge capacity per weight and excellent cycle performance. In order to meet such demands, positive electrode active materials in positive electrodes of secondary batteries have been actively improved (e.g., Patent Document 1 to Patent Document 3). In addition, crystal structures of positive electrode active materials have been studied (Non-Patent Document 1 to Non-Patent Document 3).

In addition, X-ray diffraction (XRD) is one of methods used for analysis of the crystal structure of a positive electrode active material. With the use of ICSD (Inorganic Crystal Structure Database) described in Non-Patent Document 4, XRD data can be analyzed. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 5) can be used, for example.

REFERENCES

Patent Documents

[Patent Document 1] Japanese Published Patent Application No. 2019-179758

[Patent Document 2] International Publication WO 2020/026078 Pamphlet

[Patent Document 3] Japanese Published Patent Application No. 2020-140954

Non-Patent Documents

• [Non-Patent Document 1] Toyoki Okumura et al., “Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3- and O2-lithium cobalt oxides from first-principle calculation”, Journal of Materials Chemistry, 2012, 22, p. 17340-17348
• [Non-Patent Document 2] Motohashi, T. et al., “Electronic phase diagram of the layered cobalt oxide system Li_xCoO₂ (0.0≤x≤1.0)”, Physical Review B, 80 (16): 165114
• [Non-Patent Document 3] Zhaohui Chen et al., “Staging Phase Transitions in Li_xCoO₂”, Journal of The Electrochemical Society, 2002, 149 (12), A1604-A1609
• [Non-Patent Document 4] Belsky, A. et al., “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Cryst., (2002) B58, 364-369.
• [Non-Patent Document 5] F. Izumi and K. Momma, Solid State Phenom., (2007) 130, 15-20 (2007).
• [Non-Patent Document 6] A. van de Walle, “Multicomponent multisublattice alloys, nonconfigurational entropy and other additions to the Alloy Theoretic Automated Toolkit”, Calphad Journal 33, 266, (2009).
• [Non-Patent Document 7] Rasband, W. S., ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http://rsb.info.nih.gov/ij/, 1997-2012.
• [Non-Patent Document 8] Schneider, C. A., Rasband, W. S., Eliceiri, K. W., “NIH Image to ImageJ: 25 years of image analysis”, Nature Methods, 9, 671-675, 2012,
• [Non-Patent Document 9] Abramoff, M. D., Magelhaes, P. J., Ram, S. J., “Image Processing with ImageJ”, Biophotonics International, volume 11, issue 7, pp. 36-42, 2004.

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

Development of lithium-ion secondary batteries and positive electrode active materials used therein has room for improvement in terms of charge and discharge capacity, cycle performance, reliability, safety, cost, and the like.

An object of one embodiment of the present invention is to provide a positive electrode active material or a composite oxide which inhibits a decrease in charge and discharge capacity during charge and discharge cycles when used in a lithium-ion secondary battery. Another object is to provide a positive electrode active material or a composite oxide whose crystal structure is not easily broken even when charging and discharging are repeated. Another object is to provide a positive electrode active material or a composite oxide with high charge and discharge capacity. Another object is to provide a highly safe or highly reliable secondary battery.

Another object of one embodiment of the present invention is to provide a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof.

Note that the description of these objects does not preclude the existence of other objects. Note that one embodiment of the present invention does not have to achieve all the objects. Other objects can be derived from the description of the specification, the drawings, and the claims.

Means for Solving the Problems

In a conventional positive electrode active material, it is known that by charging and discharging at a high voltage, e.g., 4.5 V or higher, or at a high temperature, e.g., 45° C. or higher, a progressive defect that progresses deeply from the surface toward the inner portion is generated. Progress of a defect in a positive electrode active material to form a hole can be referred to as pitting corrosion, and the hole generated by this phenomenon is also referred to as a pit in this specification. The pit is considered to originate from distortion in a crystal structure and/or shift of an atomic arrangement generated in a surface portion of the positive electrode active material.

Thus, in one embodiment of the present invention, a positive electrode active material where an inner portion and a surface portion containing an additive element are topotaxy is to be formed. Alternatively, a positive electrode active material in which a crystal orientation of the inner portion and that of the surface portion containing the additive element are substantially aligned with each other is formed. By the surface portion and the inner portion being topotaxy, distortion in a crystal structure generated due to insertion and extraction of Li during charging and discharging and/or shift of an atomic arrangement can be reduced. This can inhibit the cause of the pit. Moreover, by containing the additive element in the surface portion, shift of a layer structure formed of octahedrons of a transition metal M and oxygen can be inhibited and/or release of oxygen from the positive electrode active material can be inhibited. Thus, a positive electrode active material with little deterioration even when charging at a high voltage and charging and discharging at a high temperature environment are performed can be provided.

One embodiment of the present invention is a positive electrode active material containing lithium, cobalt, oxygen, and an additive element. The positive electrode active material includes a surface portion and an inner portion. The positive electrode active material contains an additive element in the surface portion. The surface portion is a region extending from a surface of the positive electrode active material to a depth of 10 nm or less toward the inner portion. The surface portion and the inner portion are topotaxy. A degree of diffusion of the additive element vary between crystal planes of the surface portion. The additive element is at least one or two or more selected from nickel, aluminum, and magnesium.

In the above embodiment, the positive electrode active material has a crystal structure identified as a space group R-3m, and in the surface portion, the additive element exists deeper in a region not parallel to the arrangement of cations than in a region parallel to the arrangement of cations.

In the above embodiment, the number of atoms of nickel contained in the positive electrode active material is preferably greater than or equal to 0.1% and less than or equal to 2% of the number of atoms of cobalt, and the number of atoms of aluminum contained in the positive electrode active material is preferably greater than or equal to 0.1% and less than or equal to 2% of the number of atoms of cobalt.

In the above embodiment, fluorine is preferably further contained as the additive element.

Effect of the Invention

One embodiment of the present invention can provide a positive electrode active material or a composite oxide which inhibits a decrease in charge and discharge capacity during charge and discharge cycles when used in a lithium-ion secondary battery. Another embodiment can provide a positive electrode active material or a composite oxide whose crystal structure is not easily broken even when charging and discharging are repeated can be provided. A positive electrode active material or a composite oxide with high charge and discharge capacity can be provided. A highly safe or highly reliable secondary battery can be provided.

Another embodiment of the present invention can provide a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof.

Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not need to have all the effects. Other effects will be apparent from the description of the specification, the drawings, the claims, and the like, and other effects can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a positive electrode active material, and FIG. 1B1 to FIG. 1C2 are cross-sectional views of part of the positive electrode active material.

FIG. 2 is an example of a TEM image showing crystal orientations substantially aligned with each other.

FIG. 3A is an example of a STEM image showing crystal orientations substantially aligned with each other. FIG. 3B shows an FFT pattern of a region of a rock-salt crystal RS. FIG. 3C shows an FFT pattern of a region of a layered rock-salt crystal LRS.

FIG. 4A1 to 4B3 are diagrams showing crystal structures and calculation results.

FIG. 5A1 to 5A3 are diagrams showing crystal structures.

FIGS. 6A and 6B are diagrams showing crystal structures and calculation results.

FIGS. 7A and 7B are diagrams showing crystal structures.

FIG. 8 is a diagram showing crystal structures of a positive electrode active material.

FIG. 9 is a diagram showing crystal structures of a conventional positive electrode active material.

FIG. 10A and FIG. 10B are cross-sectional views of a positive electrode active material, and FIG. 10C1 and FIG. 10C2 are parts of cross-sectional views of the positive electrode active material.

FIG. 11 is a diagram showing XRD patterns calculated from crystal structures.

FIG. 12 is a diagram showing XRD patterns calculated from crystal structures.

FIG. 13 is a cross-sectional view of a positive electrode active material.

FIG. 14A to FIG. 14C are diagrams showing a manufacturing method of a positive electrode active material.

FIG. 15A and FIG. 15B are cross-sectional views of an active material layer in the case where graphene or a graphene compound is used as a conductive material.

FIG. 16A and FIG. 16B are diagrams showing a coin-type secondary battery. FIG. 16C is a diagram showing charging and discharging of a secondary battery.

FIG. 17A to FIG. 17D are diagrams showing a cylindrical secondary battery.

FIG. 18A and FIG. 18B are diagrams showing an example of a secondary battery.

FIG. 19A to FIG. 19D are diagrams showing examples of a secondary battery.

FIG. 20A to FIG. 20H are diagrams showing examples of electronic devices.

FIG. 21A to FIG. 21C are diagrams showing an example of electronic devices.

FIG. 22 is a diagram showing examples of electronic devices.

FIG. 23A to FIG. 23D are diagrams showing examples electronic devices.

FIG. 24A to FIG. 24C are diagrams showing examples of electronic devices.

FIG. 25A to FIG. 25C are diagrams showing examples of vehicles.

MODE FOR CARRYING OUT THE INVENTION

Examples of embodiments of the present invention will be described in detail below with reference to the drawings and the like. Note that the present invention should not be interpreted as being limited to the examples of embodiments given below. Embodiments of the invention can be changed unless it deviates from the spirit of the present invention.

In this specification and the like, a space group is represented using the short notation of the international notation (or the Hermann-Mauguin notation). In addition, the Miller index is used for the expression of crystal planes and crystal orientations. An individual plane that shows a crystal plane is denoted by “( )”. In the crystallography, a bar is placed over a number in the expression of space groups, crystal planes, and crystal orientations: in this specification and the like, because of format limitations, crystal planes, crystal orientations, and space groups are sometimes expressed by placing a minus sign (−) in front of a number instead of placing a bar over the number. Furthermore, an individual direction which shows an orientation in a crystal is denoted with “[ ]”, a set direction which shows all of the equivalent orientations is denoted with “< >”, an individual plane which shows a crystal plane is denoted with “( )”, and a set plane having equivalent symmetry is denoted with “{ }”. A trigonal system represented by the space group R-3m is generally represented by a composite hexagonal lattice for easy understanding of the structure and, in some cases, not only (hkl) but also (hkil) is used as the Miller index. Here, i is −(h+k).

In this specification and the like, particles are not necessarily spherical (with a circular cross section). Other examples of the cross-sectional shapes of particles include an ellipse, a rectangle, a trapezoid, a triangle, a quadrilateral with rounded corners, and an asymmetrical shape, and a particle may have an indefinite shape.

A theoretical capacity of a positive electrode active material refers to the amount of electricity obtained when all lithium that can be inserted and extracted and is contained in the positive electrode active material is extracted. For example, the theoretical capacity of LiCoO₂ is 274 mAh/g, the theoretical capacity of LiNiO₂ is 274 mAh/g, and the theoretical capacity of LiMn₂O₄ is 148 mAh/g.

The remaining amount of lithium in a positive electrode active material, which is compared to the theoretical capacity, is represented by x in a compositional formula, e.g., Li_xCoO₂ or Li_xMO₂. In this specification, Li_xCoO₂ can be replaced with Li_xMO₂ as appropriate. In the case of a positive electrode active material in a secondary battery, x can be charge capacity/theoretical capacity. For example, in the case where a secondary battery using LiCoO₂ as a positive electrode active material is charged to 219.2 mAh/g, it can be said that the positive electrode active material is represented by Li_0.2CoO₂ or x=0.2. Note that “x in Li_xCoO₂ is small” means, for example, 0.1<x≤0.24.

Lithium cobalt oxide to be used for a positive electrode, which has been appropriately synthesized and almost satisfies the stoichiometric proportion, is LiCoO₂, and the occupancy rate x of Li in the lithium sites is 1. In a secondary battery after its discharging ends, it can be said that contained lithium cobalt oxide is also LiCoO₂ and x=1. Here, “discharging ends” means that a voltage becomes 3.0 V or 2.5 V or lower at a current of 100 mAh or lower, for example.

Charge capacity and/or discharge capacity used for calculation of x in Li_xCoO₂ is preferably measured under the condition of no influence or small influence of a short circuit and/or decomposition of an electrolyte solution. For example, data of a secondary battery, suffering from a sudden change of capacity that seems to result from a short circuit, should not be used for calculation of x.

The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, or the like. Thus, in this specification and the like, belonging to a space group, being attributed to a space group, or being a space group can be rephrased as being identified as a space group.

Note that in this specification and the like, a structure is referred to as a cubic close-packed structure when three layers of anions are shifted and stacked like “ABCABC” in the structure. Accordingly, anions do not necessarily form a cubic lattice structure. At the same time, actual crystals always have a defect and thus, analysis results are not necessarily consistent with the theory. For example, in an electron diffraction pattern or an FFT (fast Fourier transform) pattern of a TEM image or the like, a spot may appear in a position slightly different from a theoretical position. For example, anions may be regarded as forming a cubic close-packed structure when a difference in orientation from a theoretical position is 5° or less or 2.5° or less.

Note that in this specification and the like, uniformity refers to a phenomenon in which, in a solid made of a plurality of elements (e.g., A, B, and C), a certain element (e.g., A) is distributed with similar features in specific regions. Note that it is acceptable for the specific regions to have substantially the same concentration of the element. For example, a difference in the concentration of the element between the specific regions can be 10% or less. Examples of the specific regions include a surface portion, a surface, a projected portion, a depressed portion, and an inner portion.

A positive electrode active material to which an additive element is added is sometimes referred to as a composite oxide, a positive electrode member, a positive electrode material, a secondary battery positive electrode member, or the like. In this specification and the like, the positive electrode active material of one embodiment of the present invention preferably contains a compound. In this specification and the like, the positive electrode active material of one embodiment of the present invention preferably contains a composition. In this specification and the like, the positive electrode active material of one embodiment of the present invention preferably contains a composite.

In the case where the features of individual particles of a positive electrode active material are described in the following embodiment and the like, not all the particles necessarily have the features. When 50% or more, preferably 70% or more, further preferably 90% or more of three or more randomly selected particles of a positive electrode active material have the features, for example, it can be said that an effect of improving the characteristics of the positive electrode active material and a secondary battery including the positive electrode active material is sufficiently obtained.

The voltage of a positive electrode generally increases with increasing charge voltage of a secondary battery. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at a high voltage. The stable crystal structure of the positive electrode active material in a charged state can inhibit a charge and discharge capacity decrease due to repeated charging and discharging.

A short circuit of a secondary battery might cause not only a malfunction in charging operation and/or discharging operation of the secondary battery but also heat generation and ignition. In order to obtain a safe secondary battery, a short-circuit current is preferably inhibited even at a high charge voltage. In the positive electrode active material of one embodiment of the present invention, a short-circuit current is inhibited even at a high charge voltage. Thus, a secondary battery having a high discharge capacity and a high level of safety can be obtained.

Note that the description is made on the assumption that materials (such as a positive electrode active material, a negative electrode active material, an electrolyte, and a separator) of a secondary battery have not deteriorated unless otherwise specified. A decrease in discharge capacity due to aging treatment and burn-in treatment during the manufacturing process of a secondary battery is not regarded as deterioration. For example, the case where discharge capacity is higher than or equal to 97% of the rated capacity of a lithium-ion secondary battery cell and an assembled lithium-ion secondary battery (hereinafter, referred to as a lithium-ion secondary battery) can be regarded as a non-deteriorated state. The rated capacity conforms to JIS C 8711:2019 in the case of a lithium-ion secondary battery for a portable device. The rated capacities of other lithium-ion secondary batteries conform to JIS described above, JIS for electric vehicle propulsion, industrial use, and the like, standards defined by IEC, and the like.

Note that in this specification and the like, in some cases, materials included in a secondary battery that have not deteriorated are referred to as initial products or materials in an initial state, and materials that have deteriorated (have discharge capacity lower than 97% of the rated capacity of the secondary battery) are referred to as products in use, materials in a used state, products that are already used, or materials in an already-used state.

Embodiment 1

In this embodiment, a positive electrode active material that can be used for a secondary battery of one embodiment of the present invention and a manufacturing method thereof are described with reference to FIG. 1A to FIG. 14.

[Positive Electrode Active Material]

FIG. 1A is a cross-sectional view of a positive electrode active material 100 that can be used for a secondary battery of one embodiment of the present invention. FIG. 1B1 and FIG. 1B2 show enlarged views of a portion near A-B in FIG. 1A. FIG. 10C1 and FIG. 10C2 show enlarged views of a portion near the line C-D in FIG. 1A.

FIG. 1A shows a crystal plane parallel to the arrangement of cations with dotted lines. Arrows represent directions in which lithium is inserted and extracted in a charging and discharging state. Here, the arrangement of cations refers to the arrangement of cations other than lithium typified by a transition metal M, which is likely to be easily observed by STEM or the like. The crystal plane parallel to the arrangement of cations refers to a crystal plane parallel to a direction in which lithium ions can diffuse. As shown in FIG. 1B1 to FIG. 1C2, the positive electrode active material 100 includes a surface portion 100a and an inner portion 100b. In each drawing, a dashed line denotes a boundary between the surface portion 100a and the inner portion 100b. Although not shown, the positive electrode active material 100 may have a grain boundary.

In this specification and the like, the surface portion 100a of the positive electrode active material 100 refers to a region that is within 50 nm, preferably within 35 nm, further preferably within 20 nm in depth from the surface toward the inner portion, and most preferably a region positioned within 10 nm in depth from the surface toward the inner portion. A plane generated by a fissure and/or a crack (also referred to as a crevice) can be considered as a surface. The surface portion 100a can be rephrased as the vicinity of a surface, a region in the vicinity of a surface, or a shell.

The inner portion 100b refers to a region deeper than the surface portion 100a of the positive electrode active material. The inner portion 100b can be rephrased as an inner region or a core.

A surface of the positive electrode active material 100 refers to a surface of a composite oxide including the surface portion 100a, the inner portion 100b, a convex portion, and the like. Thus, the positive electrode active material 100 does not include a carbonate, a hydroxy group, or the like which is chemically adsorbed after formation of the positive electrode active material 100. Furthermore, an electrolyte, a binder, a conductive material, and a compound originating from any of these that are attached to the positive electrode active material 100 are not included either. The surface of the positive electrode active material 100 in, for example, a cross-sectional STEM (scanning transmission electron microscope) image is a boundary between a region where a bonding image of an electron is observed and a region where the image is not observed, and is determined as the outermost surface of a region where a bright spot derived from an atomic nucleus of a metal element that has a larger atomic number than lithium is observed. The surface in a cross-sectional STEM image or the like may be determined also on the basis of higher spatial-resolution analysis results, e.g., electron energy loss spectroscopy (EELS).

The crystal grain boundary refers to, for example, a portion where particles of the positive electrode active material 100 adhere to each other or a portion where a crystal orientation changes inside the positive electrode active material 100, i.e., a portion where repetition of bright lines and dark lines is discontinuous in a STEM image or the like, a portion including a large number of crystal defects, a portion with a disordered crystal structure, or the like. A crystal defect refers to a defect that can be observed in a cross-sectional image observed with a cross-sectional transmission electron microscope (TEM), a cross-sectional STEM image, or the like, i.e., a structure containing another atom between lattices, a hollow, or the like. The crystal grain boundary is one of plane defects. The vicinity of the crystal grain boundary refers to a region positioned within 10 nm from the crystal grain boundary.

<Topotaxy>

It is preferable that the crystal structure of the positive electrode active material 100 change continuously from the inner portion 100b toward the surface. Alternatively, it is preferable that the surface portion 100a and the inner portion 100b have the same or substantially the same crystal orientation. Note that in the following description, “the structure in which the crystal orientations are aligned or substantially aligned with each other” is simply referred to as “the crystal orientations are aligned with each other”. Alternatively, it is preferable that the surface portion 100a and the inner portion 100b be topotaxy.

Topotaxy refers to having similarity in a three-dimensional structure such that crystal orientations are substantially aligned with each other, or to having the same orientations crystallographically. Epitaxy refers to having similarity in structures of two-dimensional interfaces.

By the surface portion 100a and the inner portion 100b being topotaxy, distortion in a crystal structure and/or shift in atomic arrangement of crystal structure can be reduced. This can prevent the cause of a pit. By the surface portion 100a containing an additive element, shift of a layer structure formed of octahedrons of a transition metal M and oxygen can be inhibited and/or release of oxygen from the positive electrode active material 100 can be inhibited. Thus, a positive electrode active material with little deterioration even when charging at a high voltage and charging and discharging at a high temperature environment are performed can be provided.

For example, a crystal structure preferably changes continuously from the layered rock-salt inner portion 100b toward the surface and the surface portion 100a that have a rock-salt structure or have features of both a rock-salt structure and a layered rock-salt structure. Alternatively, the crystal orientations of the surface portion 100a are preferably substantially aligned with that of the layered rock-salt inner portion 100b.

In this specification and the like, a layered rock-salt crystal structure, which belongs to the space group R-3m, of a composite oxide containing lithium and the transition metal M such as cobalt refers to a crystal structure in which a rock-salt ion arrangement where cations and anions are alternately arranged is included and lithium and the transition metal M are regularly arranged to form a two-dimensional plane, so that lithium can be diffused two-dimensionally. Note that a defect such as a cation or anion vacancy may exist. Moreover, in the layered rock-salt crystal structure, strictly, a lattice of a rock-salt crystal is distorted in some cases.

A rock-salt crystal structure refers to a structure in which a cubic crystal structure with the space group Fm-3m or the like is included and cations and anions are alternately arranged. Note that a cation or anion vacancy may be included.

Having features of both a layered rock-salt crystal structure and a rock-salt crystal structure can be determined by electron diffraction pattern, a TEM image, a cross-sectional STEM image, and the like.

There is no distinction among cation sites in a rock-salt structure. Meanwhile, a layered rock-salt crystal structure has two types of cation sites: one type is mostly occupied by lithium, and the other is occupied by the transition metal M. A stacked-layer structure where two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same in a rock-salt structure and a layered rock-salt structure. Given that the center spot (transmission spot) among bright spots in an electron diffraction pattern corresponding to crystal planes that form the two-dimensional planes is at the origin point 000, the bright spot nearest to the center spot is on the (111) plane in an ideal rock-salt crystal structure, for instance, and on the (003) plane in a layered rock-salt crystal structure, for instance. For example, when electron diffraction patterns of rock-salt MgO and layered rock-salt LiCoO₂ are compared to each other, the distance between the bright spots on the (003) plane of LiCoO₂ is observed at a distance approximately half the distance between the bright spots on the (111) plane of MgO. Thus, for instance, when two phases of rock-salt MgO and layered rock-salt LiCoO₂ are included in a region to be analyzed, a plane orientation in which bright spots with high luminance and bright spots with low luminance are alternately arranged is seen in an electron diffraction pattern. A bright spot common between the rock-salt structure and the layered rock-salt structure has high luminance, whereas a bright spot caused only in the layered rock-salt structure has low luminance.

When a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in a cross-sectional STEM image and the like, layers observed with high luminance and layers observed with low luminance are alternately observed. Such a feature is not observed in a rock-salt crystal structure because there is no distinction among cation sites therein. When a crystal structure having the features of both a rock-salt crystal structure and a layered rock-salt crystal structure is observed from a given crystal orientation, layers observed with high luminance and layers observed with low luminance are alternately observed in a cross-sectional STEM image and the like, and a metal that has a lager atomic number than lithium is present in part of the layers with low luminance, i.e., the lithium layers.

Anions of a layered rock-salt crystal and anions of a rock-salt crystal form a cubic close-packed structure (face-centered cubic lattice structure). Anions of an O3′ crystal and a monoclinic O1(15) crystal described later are presumed to form a cubic close-packed structure. Thus, when a layered rock-salt crystal and a rock-salt crystal are in contact with each other, there is a crystal plane at which orientations of cubic closest packed structures composed of anions are aligned with each other.

The description can also be made as follows. Anions on the {111} plane of a cubic crystal structure have a triangle lattice. A layered rock-salt structure, which belongs to the space group R-3m and is a rhombohedral structure, is generally represented by a composite hexagonal lattice for easy understanding of the structure, and the (0001) plane of the layered rock-salt structure has a hexagonal lattice. The triangle lattice on the {111} plane of the cubic crystal has atomic arrangement similar to that of the hexagonal lattice on the (0001) plane of the layered rock-salt structure. These lattices being consistent with each other can be expressed as “orientations of the cubic close-packed structures are aligned with each other”.

Note that a space group of the layered rock-salt crystal and the O3′ crystal is R-3m, which is different from the space group Fm-3m of a rock-salt crystal (the space group of a general rock-salt crystal); thus, the Miller index of the crystal plane satisfying the above conditions in the layered rock-salt crystal and the O3′ crystal is different from that in the rock-salt crystal. In this specification, a state where the orientations of the cubic close-packed structures composed of anions in the layered rock-salt crystal, the rock-salt crystal, and O3′ type are aligned with each other is sometimes referred to as a state where crystal orientations are substantially aligned with each other, topotaxy, or epitaxy. Note that the crystal orientations being aligned with each other is not limited to the combination of the layered rock-salt type and the rock-salt type described above. Even for combinations having other crystal structures such as a spinel type crystal structure and a perovskite crystal structure, it can be said that when orientations of a cubic-close packed structure constituted by anions are aligned with each other, the crystal orientations are substantially aligned with each other.

The crystal orientations in two regions being substantially aligned with each other can be determined, for example, from a TEM (Transmission Electron Microscope) image, a STEM (Scanning Transmission Electron Microscope) image, a HAADF-STEM (High-angle Annular Dark Field Scanning TEM) image, an ABF-STEM (Annular Bright-Field Scanning Transmission Electron microscope) image, an electron diffraction pattern, and an FFT pattern of a TEM image, 20) a STEM image, and the like. XRD (X-ray Diffraction), electron diffraction, neutron diffraction, and the like can also be used for judging.

FIG. 2 shows an example of a TEM image in which orientations of a layered rock-salt crystal LRS and a rock-salt crystal RS are substantially aligned with each other. In a TEM image, a STEM image, a HAADF-STEM image, an ABF-STEM image, and the like, an image reflecting a crystal structure is obtained.

For example, in a high-resolution TEM image, a contrast derived from a crystal plane is obtained. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, a contrast derived from the (0003) plane is obtained as repetition of bright bands (bright strips) and dark bands (dark strips) because of diffraction and interference of the electron beam. Thus, when repetition of bright lines and dark lines is observed and the angle between the bright lines (e.g., L_RS and L_LRS in FIG. 2) is greater than or equal to 0° or less than or equal to 5° or greater than or equal to 0° or less than or equal to 2.5° in the TEM image, it can be determined that the crystal planes are substantially aligned with each other, that is, crystal orientations are substantially aligned with each other. Similarly, when the angle between the dark lines is greater than or equal to 0° or less than or equal to 5° or greater than or equal to 0° or less than or equal to 2.5°, it can be judged that crystal orientations are substantially aligned with each other. In general, it is difficult to clearly differentiate “aligned” from “substantially aligned”. In this specification, the expression “aligned” includes both “perfectly aligned” (where the angle between bright lines is 0°, for example) and “substantially aligned”.

In a HAADF-STEM image, a contrast proportional to the atomic number is obtained, and an element having a larger atomic number is observed to be brighter. For example, in the case of lithium cobalt oxide that has a layered rock-salt structure belonging to the space group R-3m, cobalt (atomic number: 27) has the largest atomic number: hence, an electron beam is strongly scattered at the position of a cobalt atom, and arrangement of the cobalt atoms is observed as bright lines or arrangement of high-luminance dots. Thus, when the lithium cobalt oxide having a layered rock-salt crystal structure is observed in the direction perpendicular to the c-axis, arrangement of the cobalt atoms is observed as bright lines or arrangement of high-luminance dots, and arrangement of lithium atoms and oxygen atoms is observed as dark lines or a low-luminance region in the direction perpendicular to the c-axis. The same applies to the case where fluorine (atomic number: 9) and magnesium (atomic number: 12) are included as the additive elements of the lithium cobalt oxide.

Consequently, in the case where repetition of bright lines and dark lines is observed in two regions having different crystal structures and the angle between the bright lines is 5° or less or 2.5° or less in a HAADF-STEM image, it can be judged that arrangements of the atoms are substantially aligned with each other, that is, orientations of the crystals are substantially aligned with each other. Similarly, when the angle between the dark lines is 5° or less or 2.5° or less, it can be judged that orientations of the crystals are substantially aligned with each other.

With an ABF-STEM, an element having a smaller atomic number is observed to be brighter, but a contrast corresponding to the atomic number is obtained as with a HAADF-STEM: hence, in an ABF-STEM image, crystal orientations can be judged as in a HAADF-STEM image.

FIG. 3A shows an example of a STEM image in which orientations of the layered rock-salt crystal LRS and the rock-salt crystal RS are substantially aligned with each other. FIG. 3B shows an FFT pattern of a region of the rock-salt crystal RS, and FIG. 3C shows an FFT pattern of a region of the layered rock-salt crystal LRS. In FIG. 3B and FIG. 3C, the composition, the JCPDS card number, and d values and angles to be calculated are shown on the left. The measured values are shown on the right. A spot denoted by O is zero-order diffraction.

A spot denoted by A in FIG. 3B is derived from 11-1 reflection of a cubic structure. A spot denoted by A in FIG. 3C is derived from 0003 reflection of a layered rock-salt structure. It is found from FIG. 3B and FIG. 3C that the direction of the 11-1 reflection of the cubic structure and the direction of the 0003 reflection of the layered rock-salt structure are substantially aligned with each other. That is, a straight line that passes through AO in FIG. 3B is substantially parallel to a straight line that passes through AO in FIG. 3C. Here, the terms “substantially aligned” and “substantially parallel” mean that the angle between the two is greater than or equal to 0° and less than or equal to 5° or greater than or equal to 0° and less than or equal to 2.5°.

When the orientations of the layered rock-salt crystal and the rock-salt crystal are substantially aligned with each other in the above manner in FFT pattern and electron diffraction pattern, the <0003> orientation of the layered rock-salt crystal and the <11-1> orientation of the rock-salt crystal may be substantially aligned with each other. In that case, it is preferable that these reciprocal lattice points be spot-shaped, that is, they be not connected to other reciprocal lattice points. The state where reciprocal lattice points are spot-shaped and not connected to other reciprocal lattice points means high crystallinity.

When the direction of the 11-1 reflection of the cubic structure and the direction of the 0003 reflection of the layered rock-salt structure are substantially aligned with each other as described above, a spot that is not derived from the 0003 reflection of the layered rock-salt structure may be observed, depending on the incident direction of the electron beam, on a reciprocal lattice space different from the direction of the 0003 reflection of the layered rock-salt structure. For example, a spot denoted by B in FIG. 3C is derived from 1014 reflection of the layered rock-salt structure. This is sometimes observed at a position where the difference in orientation from the reciprocal lattice point derived from the 0003 reflection of the layered rock-salt crystal structure (A in FIG. 3C) is greater than or equal to 52° and less than or equal to 56° (i.e., ∠AOB is 52° to) 56° and d is greater than or equal to 0.19 nm and less than or equal to 0.21 nm. Note that these indices are just an example, and the spot does not necessarily correspond with them. For example, the spot may be a reciprocal lattice point equivalent to the indices.

Similarly, a spot that is not derived from the 11-1 reflection of the cubic structure may be observed on a reciprocal lattice space different from the direction where the 11-1 reflection of the cubic structure is observed. For example, a spot denoted by B in FIG. 3B is derived from 200 reflection of the cubic structure. This diffraction spot is sometimes observed at a position where the difference in orientation of reflection derived from the 11-1 reflection of the cubic structure (A in FIG. 3B) is greater than or equal to 54° and less than or equal to 56° (i.e., φAOB is greater than or equal to 54° and less than or equal to) 56°. Note that these indices are just an example, and the spot does not necessarily correspond with them. For example, the spot may be a reciprocal lattice point equivalent to the indices.

It is known that in a layered rock-salt positive electrode active material, such as lithium cobalt oxide, the (0003) plane and a plane equivalent thereto and the (10-14) plane and a plane equivalent thereto are likely to be crystal planes. Thus, a sample to be observed can be processed to be thin by FIB or the like such that an electron beam of a TEM, for example, enters in [12-10], in order to easily observe the (0003) plane in careful observation of the shape of the positive electrode active material with a SEM or the like. To judge whether crystal orientations are aligned, a sample is preferably processed to be thin so that the (0003) plane of the layered rock-salt structure is easily observed.

<Elements Contained in Positive Electrode Active Material>

The positive electrode active material 100 contains lithium, the transition metal M, oxygen, and an additive element A. Alternatively, the positive electrode active material 100 can contain a composite oxide (LiMO₂) containing lithium and the transition metal M to which the additive element A is added. Note that the composition of the composite oxide is not limited to Li:M:O=1:1:2. In some cases, a positive electrode active material to which an additive element A is added is referred to as a composite oxide.

A positive electrode active material of a lithium-ion secondary battery needs to contain a transition metal which can take part in an oxidation-reduction reaction in order to maintain a neutrally charged state even when lithium ions are inserted and extracted. It is preferable that the positive electrode active material 100 of one embodiment of the present invention mainly contain cobalt as a transition metal M taking part in an oxidation-reduction reaction. In addition to cobalt, at least one or two or more of nickel and manganese may be contained. Using cobalt at greater than or equal to 75 at %, preferably higher than or equal to 90 at %, further preferably higher than or equal to 95 at % as the transition metal M contained in the positive electrode active material 100 brings many advantages such as relatively easy synthesis, easy handling, and excellent cycling performance, which is preferable.

When cobalt is used as the transition metal M contained in the positive electrode active material 100 at greater than or equal to 75 at %, preferably greater than or equal to 90 at %, further preferably greater than or equal to 95 at %, LiCoO₂ with small x is more stable than a composite oxide in which nickel accounts for the majority of the transition metal M, such as lithium nickel oxide (LiNiO₂). This is probably because the influence of distortion by the Jahn-Teller effect is smaller in the case of using cobalt than in the case of using nickel. The Jahn-Teller effect in a transition metal compound varies in degree according to the number of electrons in the d orbital of the transition metal. The influence of the Jahn-Teller effect is large in a composite oxide having a layered rock-salt crystal structure, such as lithium nickel oxide, in which octahedral coordinated low-spin nickel (III) accounts for the majority of the transition metal, and a layer having an octahedral structure formed of nickel and oxygen is likely to be distorted. Thus, there is a concern that the crystal structure might break in charge and discharge cycles. The size of a nickel ion is larger than the size of a cobalt ion and close to that of a lithium ion. Thus, there is a problem in that cation mixing between nickel and lithium is likely to occur in a composite oxide having a layered rock-salt crystal structure in which nickel accounts for the majority of the transition metal, such as lithium nickel oxide.

Using nickel at greater than or equal to 33 at %, preferably greater than or equal to 60 at %, further preferably greater than or equal to 80 at % as the transition metal M contained in the positive electrode active material 100 is preferable because in that case, the cost of the raw materials might be lower than that in the case of using a large amount of cobalt and charge and discharge capacity per weight might be increased.

As the additive element A contained in the positive electrode active material 100, one or two or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, cobalt, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium is preferably used. The total percentage of the transition metal among the additive elements A is preferably less than 25 at %, further preferably less than 10 at %, still further preferably less than 5 at % among the entire transition metal. That is, (the additive element A of the transition metal)/(transition element M+additive element A of the transition metal) is preferably less than 5 at %, further preferably less than 10 at %, still further preferably less than 5 at %.

That is, the positive electrode active material 100 can contain lithium cobalt oxide to which magnesium and fluorine are added, lithium cobalt oxide to which magnesium, fluorine, and titanium are added, lithium cobalt oxide to which magnesium, fluorine, and aluminum are added, lithium cobalt oxide to which magnesium, fluorine, and nickel are added, lithium cobalt oxide to which magnesium, fluorine, nickel, and aluminum are added, or the like.

These additive elements A further stabilize the crystal structure of the positive electrode active material 100 in some cases as described later. In this specification and the like, the additive element A can be rephrased as part of a raw material or a mixture.

Note that as the additive element A, magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium is not necessarily contained.

For example, when the positive electrode active material 100 is substantially free from manganese, the above advantages such as relatively easy synthesis, easy handling, and excellent cycling performance are sometimes enhanced. The weight of manganese contained in the positive electrode active material 100 is preferably less than or equal to 600 ppm, further preferably less than or equal to 100 ppm, for example. The weight of manganese can be analyzed by GD-MS, for example.

Next, results of calculation of crystal structures in the surface portion containing an additive element and not containing an additive element will be described with reference to FIG. 4 to FIG. 7.

In some cases, cobalt oxide may exist in a surface portion of lithium cobalt oxide in which no particular additive elements are contained. Cobalt oxide may contain metal vacancies. FIG. 4A1 shows the crystal structure of lithium cobalt oxide (LCO), and FIG. 4A2 shows the crystal structure of cobalt oxide (CoO). As shown in FIG. 4A1 and FIG. 4A2, the crystal orientations of {110} of LCO and {110} of CoO are substantially aligned with each other, but there is 5.1% deviation between an interplanar spacing of {001}, which is a plane perpendicular to {110} of LCO, and a value six times the interplanar spacing of {1-11}, which is a plane perpendicular to {110} of CoO.

FIG. 4B1 is a schematic diagram of lithium cobalt oxide containing cobalt oxide in its surface portion. An enlarged view of the surface portion is shown in FIG. 4B2. FIG. 4B3 shows the results of the classical molecular dynamics calculation on part of the surface portion containing LCO and CoO. Since there is deviation of more than 5% between an interplanar spacing of {001}, which is a plane perpendicular to {110} of LCO, and a value six times the interplanar spacing of {1-11}, which is a plane perpendicular to {110} of CoO, a deviation of the atomic arrangement occurs at a plurality portion circled with dashed lines as shown in FIG. 4B3. It is thought that cobalt and/or oxygen are likely to be extracted at such an unstable portion. Thus, the unstable portion may become the starting point of a pit.

Furthermore, even when lithium cobalt oxide containing an additive element is used, cobalt oxide may exist in the surface portion (in some cases). FIG. 5A1 shows the crystal structure of lithium cobalt oxide (LCO), FIG. 5A2 shows the crystal structure of cobalt oxide (CoO), and FIG. 5A3 shows the crystal structure of magnesium oxide (MgO) where magnesium is used as an additive element. As shown in FIG. 5A1 to FIG. 5A3, oxygen arrangements of {110} of LCO and {110} of CoO and MgO are aligned with each other, and are topotaxy. Alternatively, a value six times the interplanar spacing of {1-11}, which is a plane perpendicular to {110} of MgO, is longer than the interplanar spacing of {1-11} of LCO and shorter than that of CoO. Thus, lattice mismatch and distortion are smaller in the case where the LCO and MgO are in contact with each other than in the case where LCO and CoO are in contact with each other.

Next, to determine whether CoO and MgO form a solid solution, the formation energy of a solid solution Co_(1-x)Mg_xO is analyzed using ATAT (Alloy Theoretic Automated Toolkit) software described in Non-Patent Document 6. ATAT is software that combines first-principles calculation and cluster expansion method is used for searching structures efficiently. In the software of the first principles calculation, VASP (Vienna Ab initio Simulation Package) was used. The details of calculation conditions are listed in Table 1. FIG. 6A shows the results of searching the placement using ATAT when x in Co_(1-x)Mg_xO is 0.125, 0.143, 0.250, 0500, and 0.833. The gray parallelogram in FIG. 6A is a schematic diagram of an Mg—O octahedron (MgO₆) having Mg at the center, and the black parallelogram is a schematic diagram of a Co—O octahedron (CoO₆) with Co at the center.

TABLE 1
Software           VASP
Functional         GGA + U (DFT-D2)
Pseudopotential    PAW
Cut-off energy     600
(eV)
U potential        Co 4.91
Calculation target Optimization of lattices and
                   atomic positions

As shown in FIG. 6A, the graph of the formation energy of Co_(1-x)Mg_xO is convex downward and CoO and MgO are more stable when in a solid solution. This suggests that CoO and MgO can be in a state of solid solution. In addition, it is suggested Co and Mg in a solid solution are distributed in a dispersed manner.

Co_(1-x)Mg_xO in a state of a solid solution has anisotropic properties in its interplanar spacing: it is difficult to determine which crystal orientations can be topotaxy with LCO. Thus, a tendency of change in interplanar spacing on the basis of volumes are calculated, and the calculation results are shown in FIG. 6B. FIG. 6B indicates that the volume of Co_(1-x)Mg_xO decreases and gets closer to that of MgO as the ratio of Mg in a solid solution of becomes higher. Accordingly, it is considered that at the surface where LCO and Co_(1-x)Mg_xO are in contact with each other, the deviation from the {001} interplanar spacing of LCO becomes smaller.

Thus, CoO and MgO are likely to form a solid solution, and by heating after adding the additive element, a solid solution Co_(1-x)Mg_xO can be formed in the surface portion 100a of the positive electrode active material 100 as shown in FIG. 7A and FIG. 7B. Co_(1-x)Mg_xO has smaller lattice mismatch with LCO than with CoO. Thus, the surface portion 100a having Co_(1-x)Mg_xO becomes more likely to be topotaxy with LCO in the inner portion 100b. As indicated by the length of white arrows in FIG. 7A and FIG. 7B, compressive stress becomes small.

In this manner, even in a case where cobalt oxide is present in the surface portion of lithium cobalt oxide, by adding the additive element and by heating, the surface portion 100a can be a solid solution of cobalt oxide and an oxide containing the additive element. Thus, the surface portion 100a and the inner portion 100b of the positive electrode active material 100 easily become topotaxy. Thus, it is possible that in the positive electrode active material 100, pits are less likely to be formed.

<Crystal Structure>

<<x in Li_xCoO₂ being 1>>

The positive electrode active material 100 of one embodiment of the present invention having the above characteristics preferably has a layered rock-salt crystal structure belonging to the space group R-3m in a discharged state, i.e., a state where x in Li_xCoO₂ is 1. A composite oxide having a layered rock-salt structure excels as a positive electrode active material of a secondary battery because it has high discharge capacity and a two-dimensional diffusion path for lithium ions and is thus suitable for an insertion/extraction reaction of lithium ions. For this reason, it is particularly preferable that the inner portion 100b, which accounts for the majority of the volume of the positive electrode active material 100, have a layered rock-salt crystal structure. In FIG. 8, the layered rock-salt crystal structure denoted by R-3m O3 is shown.

Meanwhile, the surface portion 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has a function of reinforcing the layered structure, which is formed of octahedrons of the transition metal M and oxygen, of the inner portion 100b so that the layered structure does not break even when lithium is extracted from the positive electrode active material 100 by charging. Alternatively, the surface portion 100a preferably functions as a barrier film of the positive electrode active material 100. Alternatively, the surface portion 100a, which is the outer portion of the positive electrode active material 100, preferably reinforces the positive electrode active material 100. Here, the term “reinforce” means inhibition of a change in the structures of the surface portion 100a and the inner portion 100b of the positive electrode active material 100 such as extraction of oxygen and/or inhibition of oxidative decomposition of an electrolyte on the surface of the positive electrode active material 100.

Accordingly, the surface portion 100a preferably has a crystal structure different from that of the inner portion 100b. The surface portion 100a preferably has a more stable composition and a more stable crystal structure than those of the inner portion 100b at room temperature (25° C.). For example, at least part of the surface portion 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has the rock-salt crystal structure. Alternatively, the surface portion 100a preferably has both a layered rock-salt crystal structure and a rock-salt crystal structure. Alternatively, the surface portion 100a preferably has features of both a layered rock-salt crystal structure and a rock-salt crystal structure.

The surface portion 100a is a region from which lithium ions are extracted initially in charging, and is a region that tends to have a lower concentration of lithium than the inner portion 100b. It can be said that bonds between atoms are partly cut on the surface of the particle of the positive electrode active material 100 included in the surface portion 100a. Thus, the surface portion 100a is regarded as a region that tends to be unstable and easily starts deterioration of the crystal structure. Meanwhile, when the surface portion 100a can be made sufficiently stable, the layered structure, which is formed of octahedrons of the transition metal M and oxygen, of the inner portion 100b is unlikely to be broken even with small x in Li_xCoO₂, e.g., with x of less than or equal to 0.24. Furthermore, a shift in layers, which are formed of octahedrons of the transition metal M and oxygen, of the inner portion 100b can be inhibited.

In order that the surface portion 100a can have a stable composition and a stable crystal structure, the surface portion 100a preferably contains an additive element A, further preferably contains a plurality of additive elements A. The surface portion 100a preferably has a higher concentration of one or more selected from the additive elements A than the inner portion 100b. The one or more selected from the additive elements A contained in the positive electrode active material 100 preferably have a concentration gradient. In addition, it is further preferable that the additive elements A in the positive electrode active material 100 be differently distributed. For example, it is further preferable that the additive elements A exhibit concentration peaks at different depths from the surface. The concentration peak here refers to the local maximum value of the concentration in the surface portion 100a or the concentration in a region from the surface to a depth of 50 nm.

For example, as shown by hatching density of FIG. 1B1, some of the additive elements A such as magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, and calcium preferably have a concentration gradient in which the concentration increases from the inner portion 100b toward the surface. An element having such a concentration gradient is referred to as an additive element X.

Another additive element A such as aluminum or manganese preferably has a concentration gradient as shown in FIG. 1B2 by hatching density and exhibits a concentration peak in a deeper region than the additive element in FIG. 1B1. The concentration peak may be located in the surface portion 100a or located deeper than the surface portion 100a. For example, the concentration peak is preferably located in a region extending, toward the inner portion, from a depth from the surface of 5 nm to a depth from the surface of 30 nm. An element having such a concentration gradient is referred to as an additive element Y.

For example, magnesium, which is one of the additive elements X, is divalent, and a magnesium ion is more stable in lithium sites than in transition metal M sites in the layered rock-salt crystal structure and thus is likely to enter the lithium sites. An appropriate concentration of magnesium in the lithium sites of the surface portion 100a can facilitate maintenance of the layered rock-salt crystal structure. This is probably because magnesium in the lithium sites serves as a column supporting the CoO₂ layers. Moreover, magnesium can inhibit extraction of oxygen therearound in a state where x in Li_xCoO₂ is, for example, 0.24 or less. Magnesium is also expected to increase the density of the positive electrode active material 100. In addition, a high concentration of magnesium in the surface portion 100a probably increases the corrosion resistance to hydrofluoric acid generated by the decomposition of the electrolyte solution.

An appropriate concentration of magnesium can bring the above-described advantages without an adverse effect on insertion and extraction of lithium in charging and discharging. However, excess magnesium might adversely affect insertion and extraction of lithium. Furthermore, the effect of stabilizing the crystal structure might be reduced. This is probably because magnesium enters the transition metal M sites in addition to the lithium sites. Moreover, an undesired magnesium compound (e.g., an oxide and a fluoride) which is substituted for neither the lithium site nor the transition metal M site might segregate at the surface of the positive electrode active material or the like to serve as a resistance component of a secondary battery. As the concentration of magnesium in the positive electrode active material increases, the discharge capacity of the positive electrode active material decreases in some cases. This is probably because excess magnesium enters the lithium sites and the amount of lithium contributing to charging and discharging decreases.

Thus, the entire positive electrode active material 100 preferably contains an appropriate amount of magnesium. For example, the proportion of magnesium to the sum of the transition metal M (Mg/Co) in the positive electrode active material 100 of one embodiment of the present invention is preferably higher than or equal to 0.25% and lower than or equal to 5%, further preferably higher than or equal to 0.5% and lower than or equal to 2%, still further preferably approximately 1%. The amount of magnesium contained in the entire positive electrode active material 100 here may be a value obtained by element analysis on the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be a value based on the ratio of the raw materials mixed in the formation process of the positive electrode active material 100, for example.

Nickel, which is an example of the additive elements X, can be present in both the transition metal M sites and the lithium sites. Nickel is preferably present in the transition metal M sites because a lower redox potential can be obtained as compared with the case where only cobalt is present in the transition metal M sites, leading to an increase in discharge capacity.

In addition, when nickel is present in lithium sites, a shift in the layered structure formed of octahedrons of the transition metal M and oxygen can be inhibited. Moreover, a change in volume in charging and discharging is inhibited. Furthermore, an elastic modulus becomes large, i.e., hardness increases. This is probably because nickel in the lithium sites serves as a column supporting the CoO₂ layers. Thus, in particular, the crystal structure is expected to be more stable in a charged state at high temperatures, e.g., 45° C. or higher, which is preferable.

Meanwhile, excess nickel might increase the influence of distortion due to the Jahn-Teller effect. Moreover, excess nickel might adversely affect insertion and extraction of lithium.

Thus, the entire positive electrode active material 100 preferably contains an appropriate amount of nickel. For example, in the positive electrode active material 100, the number of nickel atoms is preferably greater than 0% and less than or equal to 7.5%, further preferably greater than or equal to 0.05% and less than or equal to 4%, still further preferably greater than or equal to 0.1% and less than or equal to 2%, yet still further preferably greater than or equal to 0.2% and less than or equal to 1% of the number of cobalt atoms. Alternatively, it is preferably greater than 0% and less than or equal to 4%. Alternatively, it is preferably greater than 0% and less than or equal to 2%. Alternatively, it is preferably greater than or equal to 0.05% and less than or equal to 7.5%. Alternatively, it is preferably greater than or equal to 0.05% and less than or equal to 2%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 7.5%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 4%. The amount of nickel described here may be a value obtained by element analysis on the entire positive electrode active material by GD-MS, ICP-MS, or the like, or may be a value based on the ratio of the raw materials mixed in the process of forming the positive electrode active material, for example.

Aluminum, which is one of additive elements Y, can exist in a transition metal M site in a layered rock-salt crystal structure. Since aluminum is a trivalent representative element and its valence does not change, lithium around aluminum is unlikely to move even in charging and discharging. Thus, aluminum and lithium therearound serve as columns to inhibit a change in the crystal structure. Furthermore, aluminum has effects of inhibiting elution of the transition metal M around aluminum and improving continuous charge tolerance. Moreover, an Al—O bond is stronger than a Co—O bond and thus extraction of oxygen around aluminum can be inhibited. These effects improve thermal stability. Thus, a secondary battery that includes the positive electrode active material 100 containing aluminum as the additive element Y can have higher level of safety. Furthermore, the positive electrode active material 100 can have a crystal structure that is unlikely to be broken by repeated charging and discharging.

Meanwhile, excess aluminum might adversely affect insertion and extraction of lithium.

Thus, the entire positive electrode active material 100 preferably contains an appropriate amount of aluminum. For example, in the entire positive electrode active material 100, the number of aluminum atoms is preferably greater than or equal to 0.05% and less than or equal to 4%, further preferably greater than or equal to 0.1% and less than or equal to 2%, still further preferably greater than or equal to 0.3% and less than or equal to 1.5% of the number of cobalt atoms. Alternatively, it is preferably greater than or equal to 0.05% and less than or equal to 2%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 4%. Here, the amount of aluminum contained in the entire positive electrode active material 100 may be a value obtained by element analysis on the entire positive electrode active material 100 with GD-MS, ICP-MS, or the like or may be a value based on the ratio of the raw materials mixed in the process of forming the positive electrode active material 100, for example.

When fluorine, which is an example of the additive element X, is substituted for part of oxygen in the surface portion 100a, the lithium extraction energy is lowered. This is because the change in valence of cobalt ions associated with lithium extraction is from trivalent to tetravalent in the case of not containing fluorine and is from divalent to trivalent in the case of containing fluorine, and the oxidation-reduction potentials in these cases differ from each other. It can thus be said that when fluorine is substituted for part of oxygen in the surface portion 100a of the positive electrode active material 100, lithium ions near fluorine are likely to be extracted and inserted smoothly. Thus, a secondary battery including the positive electrode active material 100 can have improved charge and discharge characteristics, improved current characteristics, or the like. When fluorine is present in the surface portion 100a, which has a surface in contact with the electrolyte solution, the corrosion resistance to hydrofluoric acid can be effectively increased. As will be described in the following embodiment, a fluoride such as lithium fluoride that has a lower melting point than another additive element A source can serve as a fusing agent (also referred to as a flux) for lowering the melting point of another additive element A source.

An oxide of titanium, which is an example of the additive element X, is known to have superhydrophilicity. Accordingly, the positive electrode active material 100 that contains titanium oxide in the surface portion 100a presumably has good wettability with respect to a high-polarity solvent. In a secondary battery formed using this positive electrode active material 100, the positive electrode active material 100 and a high-polarity electrolyte solution can have favorable contact at the interface therebetween, which may inhibit an internal resistance increase.

When the surface portion 100a contains phosphorus, which is an example of the additive element X, a short circuit can be inhibited while a state with small x in Li_xCoO₂ is maintained, in some cases, which is preferable. For example, a compound containing phosphorus and oxygen is preferably contained in the surface portion 100a.

When the positive electrode active material 100 contains phosphorus, phosphorus may react with hydrogen fluoride generated by the decomposition of the electrolyte solution or the electrolyte, which can decrease the concentration of hydrogen fluoride in the electrolyte and is thus preferable.

In the case where the electrolyte contains LiPF₆, hydrogen fluoride might be generated by hydrolysis. In some cases, hydrogen fluoride might be generated by the reaction of polyvinylidene fluoride (PVDF) used as a component of the positive electrode and alkali. The decrease in the concentration of hydrogen fluoride in the electrolyte can inhibit corrosion of a current collector and/or separation of the coating film in some cases. Furthermore, the decrease in the concentration of hydrogen fluoride in the electrolyte can inhibit a reduction in adhesion properties due to gelling and/or insolubilization of PVDF in some cases.

In the case where the positive electrode active material 100 has a crack, crack development can be inhibited by phosphorus, more specifically, a compound containing phosphorus and oxygen, for example, being in the inner portion of the positive electrode active material having the crack on its surface, e.g., a filling portion.

When the surface portion 100a contains both magnesium and nickel, divalent nickel can exist more stably in the vicinity of divalent magnesium. Thus, elution of magnesium can be inhibited even in the state where x in Li_xCoO₂ is small, which can contribute to stabilization of the surface portion 100a.

Additive elements A that are differently distributed, such as the additive element X and the additive element Y, are preferably contained at a time, in which case the crystal structure of a wider region can be stabilized. For example, the crystal structure of a wider region can be stabilized in the case where the positive electrode active material 100 contains all of magnesium and nickel, which are examples of the additive element X, and aluminum, which is an example of the additive element Y, as compared with the case where either only one of the additive element X and the additive element Y is contained. In the case where the positive electrode active material 100 contains both the additive element X and the additive element Y as described above, the surface can be sufficiently stabilized by the additive element X such as magnesium and nickel: thus, the additive element Y such as aluminum is not necessary for the surface. On the contrary, aluminum is preferably widely distributed in a deep region, e.g., in a region that is 5 nm to 50 nm inclusive in depth from the surface, in which case the crystal structure in a wider region can be stabilized.

When a plurality of the additive elements A are contained as described above, the effects of the additive elements A contribute synergistically to further stabilization of the surface portion 100a. In particular, magnesium, nickel, and aluminum are preferably contained, in which case a high effect of stabilizing the composition and the crystal structure can be obtained.

Note that the surface portion 100a occupied by only a compound of an additive element A and oxygen is not preferred because this surface portion 100a would make insertion and extraction of lithium difficult. For example, it is not preferable that the surface portion 100a be occupied by only MgO, a structure in which MgO and NiO (II) form a solid solution, and/or a structure in which MgO and CoO (II) form a solid solution. Thus, the surface portion 100a should contain at least cobalt, also contain lithium in a discharged state, and have the path through which lithium is inserted and extracted.

To ensure the sufficient path through which lithium is inserted and extracted, the concentration of cobalt is preferably higher than that of magnesium in the surface portion 100a. For example, the ratio of the number of magnesium (Mg) atoms to the number of cobalt (Co) atoms (Mg/Co) is preferably less than or equal to 0.62. Alternatively, the concentration of cobalt is preferably higher than that of nickel in the surface portion 100a. Alternatively, the concentration of cobalt is preferably higher than that of aluminum in the surface portion 100a. Alternatively, the concentration of cobalt is higher than that of fluorine in the surface portion 100a.

Moreover, excess nickel might hinder diffusion of lithium; thus, the concentration of magnesium is preferably higher than that of nickel in the surface portion 100a. For example, the number of nickel atoms is preferably less than or equal to one sixth that of magnesium atoms.

It is preferable that some additive elements A, in particular, magnesium, nickel, and aluminum have higher concentrations in the surface portion 100a than in the inner portion 100b and exist randomly also in the inner portion 100b to have low concentrations. When magnesium and aluminum exist in the lithium sites of the inner portion 100b at appropriate concentrations, an effect of facilitating maintenance of the layered rock-salt crystal structure can be obtained in a manner similar to the above. When nickel exists in the inner portion 100b at an appropriate concentration, a shift in the layered structure formed of octahedrons of the transition metal M and oxygen can be inhibited in a manner similar to the above. Also in the case where both magnesium and nickel are contained, a synergistic effect of inhibiting elution of magnesium can be expected since divalent magnesium can be present more stably in the vicinity of divalent nickel.

<<The State where x in Li_xCoO₂ is Small>>

The crystal structure in a state where x in Li_xCoO₂ is small of the positive electrode active material 100 of one embodiment of the present invention is different from that of a conventional positive electrode active material because the positive electrode active material 100 has the above-described additive element A distribution and/or crystal structure in a discharged state. Here, “x is small” means 0.1<x≤0.24.

A conventional positive electrode active material and the positive electrode active material 100 of one embodiment of the present invention are compared and changes in crystal structures owing to a change in x in Li_xCoO₂ will be described with reference to FIG. 8 to FIG. 12.

A change in the crystal structure of the conventional positive electrode active material is shown in FIG. 9. The conventional positive electrode active material shown in FIG. 9 is lithium cobalt oxide (LiCoO₂) without an additive element A in particular. A change in the crystal structure of lithium cobalt oxide containing no additive element A is described in Non-Patent Document 1 to Non-Patent Document 3 and the like.

In FIG. 9, the crystal structure of lithium cobalt oxide with x in Li_xCoO₂ of 1 is denoted by R-3m O3. In this crystal structure, lithium occupies octahedral sites and a unit cell includes three CoO₂ layers. Thus, this crystal structure is referred to as an O3 type crystal structure in some cases. Note that the CoO₂ layer has a structure in which an octahedral structure with cobalt coordinated to six oxygen atoms continues on a plane in an edge-shared state. Such a layer is sometimes referred to as a layer formed of octahedrons of cobalt and oxygen.

Conventional lithium cobalt oxide with x being approximately 0.5 is known to have an improved symmetry of lithium and have a monoclinic crystal structure belonging to the space group P2/m. This structure includes one CoO₂ layer in a unit cell. Thus, this crystal structure is referred to as an O1 type structure or a monoclinic O1 type structure in some cases.

A positive electrode active material with x of 0 has the trigonal crystal structure belonging to the space group P-3 ml and includes one CoO₂ layer in a unit cell. Thus, this crystal structure is referred to as an O1 type structure or a trigonal O1 type structure in some cases. Moreover, in some cases, this crystal structure is referred to as a hexagonal O1 type structure when the trigonal crystal is converted into a composite hexagonal lattice.

When x is approximately 0.24, conventional lithium cobalt oxide has the crystal structure belonging to the space group R-3m. This structure can also be regarded as a structure in which CoO₂ structures such as trigonal O1 type structures and LiCoO₂ structures such as R-3m O3 are alternately stacked. Thus, this crystal structure is referred to as an H1-3 type crystal structure in some cases. The number of cobalt atoms per unit cell in the actual H1-3 type crystal structure is twice that in other structures. However, in this specification including FIG. 9, the c-axis of the H1-3 type crystal structure is half that of the unit cell for easy comparison with the other crystal structures.

For the H1-3 type crystal structure, as disclosed in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell can be expressed as follows, for example: Co (0, 0, 0.42150±0.00016), O1 (0, 0, 0.27671±0.00045), and O2 (0, 0, 0.11535±0.00045). O₁ and O₂ are each an oxygen atom. A unit cell that should be used for representing a crystal structure in a positive electrode active material can be judged by the Rietveld analysis of XRD patterns, for example. In this case, a unit cell is selected such that the value of GOF (goodness of fit) is small.

When charging that makes x in Li_xCoO₂ be 0.24 or less and discharging are repeated, the crystal structure of conventional lithium cobalt oxide repeatedly changes between the R-3m O3 type crystal structure in a discharged state and the H1-3 type crystal structure (i.e., an unbalanced phase change).

However, there is a large shift in the CoO₂ layers between these two crystal structures. As denoted by the dotted lines and the arrows in FIG. 9, the CoO₂ layer in the H1-3 type crystal structure largely shifts from that in the structure belonging to R-3m O3 in a discharged state. Such a dynamic structural change can adversely affect the stability of the crystal structure.

A difference in volume between these two crystal structures is also large. When the H1-3 type crystal structure and the R-3m O3 type crystal structure in a discharged state contain the same number of cobalt atoms, these structures have a difference in volume of greater than 3.5%, typically greater than or equal to 3.9%.

In addition, a structure in which CoO₂ layers are arranged continuously, such as the trigonal O1 type structure, included in the H1-3 type crystal structure is highly likely to be unstable.

Accordingly, when charging that makes x be 0.24 or less and discharging are repeated, the crystal structure of conventional lithium cobalt oxide is gradually broken. The broken crystal structure triggers deterioration of the cycle performance. This is because the broken crystal structure has a smaller number of sites where lithium can exist stably and makes it difficult to insert and extract lithium.

On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. 8, a change in the crystal structure between a discharged state with x in Li_xCoO₂ being 1 and a state with x being 0.24 or less is smaller than that in a conventional positive electrode active material. Specifically, a shift in the CoO₂ layers between the state with x being 1 and the state with x being 0.24 or less can be small. Furthermore, a change in the volume can be small in the case where the positive electrode active materials have the same number of cobalt atoms. Thus, the positive electrode active material 100 of one embodiment of the present invention can have a crystal structure that is difficult to break even when charging that makes x be 0.24 or less and discharging are repeated, and enables excellent cycle performance. In addition, the positive electrode active material 100 of one embodiment of the present invention with x in Li_xCoO₂ being 0.24 or less can have a more stable crystal structure than a conventional positive electrode active material. Thus, the positive electrode active material 100 of one embodiment of the present invention with x in Li_xCoO₂ being kept at 0.24 or less inhibits a short circuit. This is preferable because the safety of the secondary battery is improved.

FIG. 8 shows crystal structures of the inner portion 100b of the positive electrode active material 100 in a state where x in Li_xCoO₂ is approximately 1, in a state where x in Li_xCoO₂ is approximately 0.2, and in a state where x in Li_xCoO₂ is approximately 0.15. The inner portion 100b, accounting for the majority of the volume of the positive electrode active material 100, largely contributes to charging and discharging and is accordingly a portion where a shift in CoO₂ layers and a volume change matter most.

The positive electrode active material 100 with x being 1 has the R-3m O3 type crystal structure, which is the same as that of conventional lithium cobalt oxide.

However, the positive electrode active material 100 has a crystal structure different from the H1-3 type crystal structure when x is 0.24 or less, e.g., approximately 0.2 or approximately 0.15, with which conventional lithium cobalt oxide has the H1-3 type crystal structure. The positive electrode active material 100 of one embodiment of the present invention with x being approximately 0.2 has a trigonal crystal structure belonging to the space group R-3m. The symmetry of the CoO₂ layers of this structure is the same as that of O3. Thus, this crystal structure is called an O3′ type crystal structure. In FIG. 8, this crystal structure is denoted by R-3m O3′.

In the unit cell of the O3′ type crystal structure, the coordinates of cobalt and oxygen can be represented as follows: Co (0, 0, 0.5) and O (0, 0, x) within the range of 0.20≤ x≤0.25. In the unit cell, the lattice constant of the a-axis is preferably 2.797≤a≤2.837 (Å), further preferably 2.807≤a≤2.827 (Å), typically a=2.817 (Å). The lattice constant of the c-axis is preferably 13.681≤c≤13.881 (Å), further preferably 13.751≤c≤13.811 (Å), typically c=13.781 (Å).

In both of the O3′ type crystal structure and the monoclinic O1 (15) type crystal structure, an ion of cobalt, nickel, magnesium, or the like occupies a site coordinated to six oxygen atoms. Note that light elements such as lithium and magnesium sometimes occupy a site coordinated to four oxygen atoms.

As denoted by the dotted lines in FIG. 8, the CoO₂ layers hardly shift between the R-3m O3 in the discharged state, the O3′ type crystal structure, and the monoclinic O1 (15) type crystal structure.

The R-3m O3 type crystal structure in a discharged state and the O3′ type crystal structure which contain the same number of cobalt atoms have a difference in volume of 2.5% or less, specifically 2.2% or less, typically 1.8%.

As described above, in the positive electrode active material 100 of one embodiment of the present invention, a change in the crystal structure caused when x in Li_xCoO₂ is small, i.e., when a large amount of lithium is extracted, is smaller than that in a conventional positive electrode active material. In addition, a change in the volume between the compared structures having the same number of cobalt atoms is inhibited. Thus, the crystal structure of the positive electrode active material 100 is less likely to break even when charging that makes x be 0.24 or less and discharging are repeated. Thus, a decrease in charge and discharge capacity of the positive electrode active material 100 in charge and discharge cycles is inhibited. Furthermore, the positive electrode active material 100 can stably use a larger amount of lithium than a conventional positive electrode active material and thus has high discharge capacity per weight and per volume. Thus, with the use of the positive electrode active material 100, a secondary battery with high discharge capacity per weight and per volume can be manufactured.

Note that the positive electrode active material 100 is confirmed to have the O3′ type crystal structure in some cases when x in Li_xCoO₂ is greater than or equal to 0.15 and less than or equal to 0.24, and is assumed to have the O3′ type crystal structure even when x is greater than 0.24 and less than or equal to 0.27. In addition, the positive electrode active material 100 is confirmed to have the monoclinic O1 (15) type crystal structure in some cases when x in Li_xCoO₂ is greater than 0.1 and less than or equal to 0.2, typically greater than or equal to 0.15 and less than or equal to 0.17. However, the crystal structure is influenced by not only x in Li_xCoO₂ but also the number of charge and discharge cycles, a charge current and a discharge current, temperature, an electrolyte, and the like, so that the range of x is not limited to the above.

Thus, when x in Li_xCoO₂ is greater than 0.1 and less than or equal to 0.24, the positive electrode active material 100 may have the O3′ type crystal structure only or both the O3′ type crystal structure and the monoclinic O1 (15) type crystal structure. Not all of the inner portion 100b of the positive electrode active material 100 necessarily has the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure. The positive electrode active material 100 may have another crystal structure or may be partly amorphous.

In order to make x in Li_xCoO₂ small, charging with a high charge voltage is necessary in general. Thus, the state where x in Li_xCoO₂ is small can be rephrased as a state where charging with a high charge voltage has been performed. For example, when CC/CV charging is performed at 25° C. and 4.6 V or higher using the potential of a lithium metal as a reference, the H1-3 type crystal structure appears in a conventional positive electrode active material. Thus, a charge voltage of 4.6 V or higher can be regarded as a high charge voltage with reference to the potential of a lithium metal. In this specification and the like, unless otherwise specified, a charge voltage is shown with reference to the potential of a lithium metal.

Thus, the positive electrode active material 100 of one embodiment of the present invention is preferable because the crystal structure with the symmetry of R-3m O3 can be maintained even when charging with a high charge voltage of 4.6 V or higher is performed at 25° C., for example. Moreover, the positive electrode active material 100 of one embodiment of the present invention is preferable because the O3′ type crystal structure can be obtained when charging with a higher charge voltage, e.g., a voltage higher than or equal to 4.65 V and lower than or equal to 4.7 V is performed at 25° C. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is preferable because the monoclinic O1 (15) type crystal structure can be obtained when charging at a much higher charge voltage, e.g., a voltage higher than 4.7 V and lower than or equal to 4.8 V is performed at 25° C.

In the positive electrode active material 100, when the charge voltage is increased, the H1-3 type crystal structure is eventually observed in some cases. As described above, the crystal structure is influenced by the number of charge and discharge cycles, a charge current and a discharge current, temperature, an electrolyte, and the like, so that the positive electrode active material 100 of one embodiment of the present invention sometimes has the O3′ type crystal structure even at a lower charge voltage, e.g., a charge voltage of higher than or equal to 4.5 V and lower than 4.6 V at 25° C. Similarly, the positive electrode active material 100 may sometimes have the monoclinic O1 (15) type crystal structure at a charge voltage of higher than or equal to 4.65 V and lower than or equal to 4.7 V at 25° C.

Note that in the case where graphite is used as a negative electrode active material in a secondary battery, for example, the voltage of the secondary battery is lower than the above-mentioned voltage by the potential of graphite. The potential of graphite is approximately 0.05 V to 0.2 V with reference to the potential of a lithium metal. Thus, for a secondary battery using graphite as a negative electrode active material, a similar crystal structure is obtained at a voltage corresponding to a difference between the above-described voltage and the potential of the graphite.

Although a chance of the existence of lithium is the same in all lithium sites in O3′ and monoclinic O1 (15) in FIG. 8, one embodiment of the present invention is not limited thereto. Lithium may exist unevenly in only some of the lithium sites: for example, lithium may symmetrically exist as in the monoclinic O1 (Li_0.5CoO₂) shown in FIG. 9. Distribution of lithium can be analyzed by neutron diffraction, for example.

The O3′ type crystal structure and the monoclinic O1 (15) structure can be regarded as a crystal structure that contains lithium between layers randomly but is similar to a CdCl₂ type crystal structure. The crystal structure similar to the CdCl₂ type crystal structure is close to a crystal structure of lithium nickel oxide when charged up to a charge depth of Li_0.06NiO₂: however, pure lithium cobalt oxide or a layered rock-salt positive electrode active material containing a large amount of cobalt is known not to have the CdCl₂ type crystal structure in general.

The additive-element A concentration gradient is preferably similar in a plurality of portions of the surface portion 100a of the positive electrode active material 100. In other words, it is preferable that a barrier film derived from the additive element A be uniformly formed in the surface portion 100a. When the surface portion 100a partly has a barrier film, stress might be concentrated on parts that do not have a barrier film. The concentration of stress on part of the positive electrode active material 100 might cause defects such as cracks from that part, leading to breakage of the positive electrode active material and a decrease in discharge capacity.

Note that the additive elements A do not necessarily have similar concentration gradients throughout the surface portion 100a of the positive electrode active material 100. For example, FIG. 1C1 shows an example of distribution of the additive element X in the portion in the vicinity of C-D in FIG. 1A and FIG. 1C2 shows an example of distribution of the additive element Y in the portion in the vicinity of the line C-D.

Here, the surface in the vicinity of C-D is assumed to be parallel to the arrangement of cations. The surface parallel to the arrangement of cations may have different distribution of the additive element A from other surfaces. For example, in the surface parallel to arrangement of cations and the surface portion 100a thereof, concentration peak distribution of one or more selected from the additive element X and the additive element Y may be limited to a portion at a shallower depth from the surface than a surface having other orientations. Alternatively, the surface parallel to the arrangement of cations and its surface portion 100a may have smaller concentration of one or more selected from the additive element X and the additive element Y than a surface having other orientations. Further alternatively, at the surface parallel to the arrangement of cations and at the surface portion 100a, the concentration of one or more selected from the additive element X and the additive element Y may be below the lower detection limit. In a layered rock-salt crystal structure belonging to R-3m, cations are arranged parallel to the (001) plane. In other words, a CoO₂ layer and a lithium layer are alternately stacked parallel to the (001) plane. Accordingly, a diffusion path of lithium ions also exists parallel to a (001) plane.

Since the CoO₂ layer is relatively stable, a plane having the CoO₂ layer in the surface is relatively stable. A main diffusion path of lithium ions in charging and discharging is not exposed at this plane.

On the other hand, the diffusion path of lithium ions is exposed at a plane not parallel to the arrangement of cations, i.e., a plane not parallel to the CoO₂ layer. Thus, the surface not parallel to the arrangement of cations and the surface portion 100a thereof easily lose stability because they are regions where extraction of lithium ions starts as well as important regions for maintaining a diffusion path of lithium ions. It is thus extremely important to reinforce the surface not parallel to the arrangement of cations and the surface portion 100a thereof so that the crystal structure of the whole positive electrode active material 100 is maintained.

Thus, in the positive electrode active material 100A of another embodiment of the present invention, it is important that the additive elements A are not distributed only in the outermost layer in the plane not parallel to the arrangement of cations and the surface portion 100a thereof, but exist within a preferred depth, as illustrated in FIG. 1BA and FIG. 1B2. By contrast, at the plane not parallel to the arrangement of cations and the surface portion 100a thereof, the concentration of the additive element may be low as described above or the additive element may be absent.

In the formation method as described in the following embodiment, in which high-purity LiCoO₂ is formed, the additive element A is mixed afterwards, and heating is performed, the additive element A spreads mainly through a diffusion path of lithium ions. Thus, the range of distribution of the element A in the plane not parallel to the arrangement of cations and in the surface portion 100a thereof is easily made preferable.

The positive electrode active material 100 preferably has a smooth surface with little unevenness: however, it is not necessary that the whole surface of the positive electrode active material 100 be in such a state. In a composite oxide with a layered rock-salt crystal structure belonging to R-3m, slipping easily occurs at a plane parallel to the arrangement of cations, e.g., a plane where lithium atoms are arranged. In a case where there is a plane in which lithium atoms are arranged like that in FIG. 10A, by going through the process such as pressing, slipping might occur in parallel to the plane where lithium atoms are arranged, as indicated by the arrows in FIG. 10B, resulting in deformation.

In this case, at a surface newly formed as a result of slipping and the surface portion 100a thereof, the additive element A is not present or the concentration of the additive element A is below the lower detection limit in some cases. The line E-F in FIG. 10B denotes examples of the surface newly formed as a result of slipping and its surface portion 100a. FIG. 10C1 and FIG. 10C2 show enlarged views of the vicinity of the line E-F. In FIG. 10C1 and FIG. 10C2, neither the additive element X nor the additive element Y is distributed, unlike in FIG. 10B1 to FIG. 10C2.

However, since slipping is likely to occur in parallel to the arrangement of cations, it is likely that the newly generated surface and its surface portion 100a becomes parallel to a diffusion path of lithium. In this case, since a diffusion path of lithium ions is not exposed and is relatively stable, substantially no problem is caused even when the additive element A is not present or concentration of the additive element is below the lower detection limit.

Note that as described above, in a composite oxide whose composition is LiCoO₂ and which has a layered rock-salt crystal structure belonging to R-3m, cobalt atoms and lithium atoms are arranged parallel to a (001) plane. In a HAADF-STEM image, the luminance of cobalt, which has the largest atom number in LiCoO₂, is the highest. Thus, in a HAADF-STEM image, arrangement of atoms with a high luminance may be regarded as arrangement of atoms of cobalt. Repetition of such arrangement with a high luminance can be rephrased as crystal fringes or lattice fringes.

<<Crystal Grain Boundary>>

It is further preferable that the additive element A contained in the positive electrode active material 100 of one embodiment of the present invention have the above-described distribution and be at least partly unevenly distributed at the crystal grain boundary and the vicinity thereof.

Note that in this specification and the like, uneven distribution means that the concentration of an element in a certain region differs from that in another region. This may be rephrased as segregation, precipitation, unevenness, deviation, a mixture of a high-concentration portion and a low-concentration portion, or the like.

For example, the concentration of magnesium at the crystal grain boundary and the vicinity thereof in the positive electrode active material 100 is preferably higher than that in the other regions in the inner portion 100b. In addition, the concentration of fluorine at the crystal grain boundary and the vicinity thereof is preferably higher than that in the other regions in the inner portion 100b. In addition, the concentration of nickel at the crystal grain boundary and the vicinity thereof is preferably higher than that in the other regions in the inner portion 100b. In addition, the concentration of aluminum at the crystal grain boundary and the vicinity thereof is preferably higher than that in the other regions in the inner portion 100b.

The crystal grain boundary is a type of plane defect. Thus, the crystal grain boundary tends to be unstable and the crystal structure easily starts to change like the surface of the particle. Thus, the higher the concentration of the additive element A in the crystal grain boundary and its vicinity is, the more effectively the change in the crystal structure can be inhibited.

When the concentration of magnesium and the concentration of fluorine are high at the crystal grain boundary and the vicinity thereof, the concentration of magnesium and the concentration of fluorine in the vicinity of a surface generated by a crack are also high even when the crack is generated along the crystal grain boundary of the positive electrode active material 100 of one embodiment of the present invention. Thus, the positive electrode active material including a crack can also have an increased corrosion resistance to hydrofluoric acid.

<Particle Diameter>

When the particle diameter of the positive electrode active material 100 of one embodiment of the present invention is too large, there are problems such as difficulty in lithium diffusion and large surface roughness of an active material layer at the time when the material is applied to a current collector. By contrast, too small a particle diameter causes problems such as difficulty in loading of the active material layer at the time when the material is applied to the current collector and overreaction with the electrolyte solution. Thus, the median diameter (D50) is preferably greater than or equal to 1 μm and less than or equal to 100 μm, further preferably greater than or equal to 2 μm and less than or equal to 40 μm, still further preferably greater than or equal to 5 μm and less than or equal to 30 μm. Alternatively, it is preferably greater than or equal to 1 μm and less than or equal to 40 μm. Alternatively, it is preferably greater than or equal to 1 μm and less than or equal to 30 μm. Alternatively, it is preferably greater than or equal to 2 μm and less than or equal to 100 μm. Alternatively, it is preferably greater than or equal to 2 μm and less than or equal to 30 μm. Alternatively, it is preferably greater than or equal to 5 μm and less than or equal to 100 μm. Alternatively, it is preferably greater than or equal to 5 μm and less than or equal to 40 μm.

<Analysis Method>

Whether or not a given positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention, which has the O3′ type crystal structure and/or monoclinic O1 (15) type crystal structure when x in Li_xCoO₂ is small, can be judged by analyzing a positive electrode including the positive electrode active material with small x in Li_xCoO₂ by XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

XRD is particularly preferable because the symmetry of the transition metal M such as cobalt contained in the positive electrode active material can be analyzed with high resolution, the degrees of crystallinity and the crystal orientations can be compared, the distortion of lattice periodicity and the crystallite size can be analyzed, and a positive electrode obtained by disassembling a secondary battery can be measured without any change with sufficient accuracy, for example. A diffraction peak reflecting the crystal structure of the inner portion 100b of the positive electrode active material 100, which accounts for the majority of the volume of the positive electrode active material 100, is obtained through XRD, in particular, powder XRD.

As described above, the positive electrode active material 100 of one embodiment of the present invention has a feature of a small change in the crystal structure between when x in Li_xCoO₂ is 1 and when x is less than or equal to 0.24. A material where 50% or more of the crystal structure largely changes in high-voltage charging is not preferable because the material cannot withstand high-voltage charging and discharging.

It should be noted that the O3′ type crystal structure or the monoclinic O1 (15) type crystal structure is not obtained in some cases only by addition of the additive element A. For example, when x in Li_xCoO₂ is less than or equal to 0.24, lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure at 60% or more in some cases, and has the H1-3 type crystal structure at 50% or more in other cases, depending on the concentration and distribution of the additive element A.

In addition, in the case where x is too small, e.g., 0.1 or less, or under the condition where charge voltage is higher than 4.9 V, the positive electrode active material 100 of one embodiment of the present invention sometimes has the H1-3 type crystal structure or the trigonal O1 type crystal structure. Thus, determining whether or not a positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention requires analysis of the crystal structure by XRD and other methods and data such as charge capacity or charge voltage.

Note that a positive electrode active material with small x sometimes causes a change in the crystal structure when exposed to the air. For example, the O3′ type crystal structure and the monoclinic O1 (15) type crystal structure change into the H1-3 type crystal structure in some cases. For that reason, all samples subjected to analysis of crystal structures are preferably handled in an inert atmosphere such as an argon atmosphere.

Whether the additive element A contained in a given positive electrode active material in the above-described state can be judged by, for example, analysis using XPS, energy dispersive X-ray spectroscopy (EDX), EPMA (electron probe microanalysis), or the like.

The crystal structure of the surface portion 100a, the crystal grain boundary, or the like can be analyzed by electron diffraction of a cross section of the positive electrode active material 100, for example.

<<Charging Method>>

Whether or not a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be determined by high-voltage charging. For example, the high-voltage charging is performed on a coin cell (CR2032 type with a diameter of 20 mm and a height of 3.2 mm) formed using the composite oxide for a positive electrode and a lithium counter electrode for a negative electrode.

More specifically, a positive electrode can be formed by application of a slurry in which the positive electrode active material, a conductive material, and a binder are mixed to a positive electrode current collector made of aluminum foil.

A lithium metal can be used for a counter electrode. Note that when the counter electrode is formed using a material other than the lithium metal, the potential of a secondary battery differs from the potential of the positive electrode. Unless otherwise specified, the voltage and the potential in this specification and the like refer to the potential of a positive electrode.

As an electrolyte contained in the electrolyte solution, 1 mol/L lithium hexafluorophosphate (LiPF₆) can be used. As the electrolyte solution, an electrolyte solution in which ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2 wt % are mixed can be used.

As a separator, a 25-μm-thick polypropylene porous film can be used.

Stainless steel (SUS) can be used for a positive electrode can and a negative electrode can.

The coin cell fabricated with the above conditions is subjected to constant current charging to a freely selected voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V) and then constant voltage charging to a sufficiently small current value. The sufficiently small current value can be 20 mA/g or 10 mA/g, for example. To observe a phase change of the positive electrode active material, charging with such a small current value is preferably performed. The temperature is set to 25° C. or 45° C. After charging is performed in this manner, the coin cell is disassembled in a glove box with an argon atmosphere to take out the positive electrode, whereby the positive electrode active material with predetermined charge capacity can be obtained. In order to inhibit a reaction with components in the external environment, the positive electrode is preferably enclosed in an argon atmosphere in performing various analyses later. For example, XRD can be performed on the positive electrode active material enclosed in an airtight container with an argon atmosphere. After charging is completed, the positive electrode is preferably taken out immediately and subjected to the analysis. Specifically, the positive electrode is preferably subjected to the analysis within an hour after the completion of charging, further preferably within 30 minutes after the completion of charging.

In the case where the crystal structure in a charged state after multiple charge-discharge cycles is analyzed, the conditions of the multiple charge-discharge cycles may be different from the above-described charge conditions. For example, charging can be performed by constant current charging at 100 mA/g up to a freely selected voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), and then constant voltage charging to a current value of 10 mA/g, and as discharging, constant current discharging can be performed at 100 mA/g to 2.5 V.

Also in the case where the crystal structure in a discharged state after the multiple charge-discharge cycles is analyzed, constant current discharging can be performed at a current value of 100 mA/g to 2.5 V, for example.

<<XRD>>

The apparatus and conditions for the XRD measurement are not particularly limited. The measurement can be performed with the apparatus and conditions as described below, for example.

XRD apparatus: D8 ADVANCE produced by Bruker AXSX-ray source: CuKα₁ radiation

Output: 40 kV, 40 mA

Slit width: Div. Slit, 0.5°

Detector: LynxEye

Scanning method: 2θ/θ continuous scanningMeasurement range (2θ): from 15° to 90° Step width (2θ): 0.01°Counting time: 1 second/stepRotation of sample stage: 15 rpm

In the case where the measurement sample is a powder, the sample can be set by, for example, being put in a glass sample holder or being sprinkled on a reflection-free silicon plate to which grease is applied. In the case where the measurement sample is a positive electrode, the sample can be set in such a manner that the positive electrode is attached to a substrate with a double-sided adhesive tape so that the position of the positive electrode active material layer can be adjusted to the measurement plane required by the apparatus.

FIG. 11 and FIG. 12 show ideal powder XRD patterns with CuKα1 radiation that are calculated from models of the O3′ type crystal structure, the monoclinic O1 (15) type crystal structure, and the H1-3 type crystal structure. For comparison, ideal XRD patterns calculated from the crystal structure of LiCoO₂ O3 with x in Li_xCoO₂ of 1, the crystal structure of the H1-3 type, and the crystal structure of the trigonal O1 with x of 0 are also shown. Note that the patterns of LiCoO₂ (O3) and CoO₂ (O1) are made from crystal structure data obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 4) with Reflex Powder Diffraction, which is a module of Materials Studio (BIOVIA). The 20 range is from 15° to 75°, the step size is 0.01, the wavelength λ1 is 1.540562×10⁻¹⁰ m, the wavelength λ2 is not set, and a single monochromator is used. The pattern of the H1-3 type crystal structure is similarly made from the crystal structure data disclosed in Non-Patent Document 3. The O3′ type crystal structure and the monoclinic O1 (15) type crystal structure are estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, the crystal structure is fitted with TOPAS Ver. 3 (crystal structure analysis software produced by Bruker Corporation), and the XRD patterns of the O3′ type crystal structure and the monoclinic O1 (15) type crystal structure are made in a manner similar to that for other structures.

As shown in FIG. 11, the O3′ type crystal structure exhibits diffraction peaks at 2θ of 19.25±0.12° (greater than or equal to 19.13° and less than) 19.37° and 2θ of 45.47±0.10° (greater than or equal to 45.37° and less than 45.57°).

However, as shown in FIG. 12, the H1-3 type crystal structure and trigonal O1 do not exhibit peaks at these positions. Thus, the diffraction peaks at 2θ of 19.25±0.12° (greater than or equal to 19.13° and less than) 19.37° and 2θ of 45.47±0.10° (greater than or equal to 45.37° and less than) 45.57° in a state where x in Li_xCoO₂ is small can be the features of the positive electrode active material 100 of one embodiment of the present invention.

It can be said that the positions of the XRD diffraction peaks exhibited by the crystal structure with x being 1 and the crystal structure with x being 0.24 or less are close to each other. More specifically, it can be said that in the 20 range of 42° to 46°, a difference in 2θ between the main diffraction peak exhibited by the crystal structure with x of 1 and the main diffraction peak exhibited by the crystal structure with x of 0.24 or less is 0.7° or less, preferably 0.5° or less.

Although the positive electrode active material 100 of one embodiment of the present invention has the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure when x in Li_xCoO₂ is small, not all of the positive electrode active material 100 necessarily has the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure. The positive electrode active material 100 may have another crystal structure or may be partly amorphous. Note that when the XRD patterns are subjected to the Rietveld analysis, the O3′ type crystal structure preferably accounts for greater than or equal to 50%, further preferably greater than or equal to 60%, still further preferably greater than or equal to 66%. The positive electrode active material in which the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure account for greater than or equal to 50%, preferably greater than or equal to 60%, further preferably greater than or equal to 66% can have sufficiently good cycle performance.

Furthermore, even after 100 or more cycles of charging and discharging after the measurement starts, the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure preferably account for more than or equal to 35%, further preferably more than or equal to 40%, still further preferably more than or equal to 43% when the Rietveld analysis is performed.

Sharpness of a diffraction peak in an XRD pattern indicates the degree of crystallinity. It is thus preferable that the diffraction peaks after charging be sharp, in other words, have a small half width. Even peaks that are derived from the same crystal phase have different half widths depending on the XRD measurement conditions and the 2θ value. In the case of the above-described measurement conditions, the peak observed at 2θ of greater than or equal to 43° and less than or equal to 46° preferably has a small half width of less than or equal to 0.2°, further preferably less than or equal to 0.15°, still further preferably less than or equal to 0.12°. Note that not all peaks need to fulfill the requirement. A crystal phase can be regarded as having high crystallinity when one or more peaks derived from the crystal phase fulfill the requirement. Such high crystallinity contributes to stability of the crystal structure after sufficient charging.

The crystallite sizes of the O3′ type crystal structure and the monoclinic O1 (15) type crystal structure included in the positive electrode active material 100 do not decrease to less than approximately one-twentieth that of LiCoO₂ (O3) in a discharged state. Thus, a clear peak of the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure can be observed when x in Li CoO₂ is small, even under the same XRD measurement conditions as those of a positive electrode before charging and discharging. In contrast, conventional LiCoO₂ has a small crystallite size and a broad and small peak even when it can have a structure part of which is similar to the O3′ type crystal structure and/or the monoclinic O1 (15) type crystal structure. The crystallite size can be calculated from the half width of the XRD peak.

<<XPS>>

In an inorganic oxide, a region that is approximately 2 nm to 8 nm (typically, equal to or less than 5 nm) in depth from a surface can be analyzed by X-ray photoelectron spectroscopy (XPS) using monochromated aluminum Kα radiation as an X-ray source: thus, the concentrations of elements in approximately half the depth of the surface portion 100a can be quantitatively analyzed. The bonding states of the elements can be analyzed by narrow scanning. Note that the quantitative accuracy of XPS is approximately ±1 at % in many cases. The lower detection limit is approximately 1 at % but depends on the element.

In the positive electrode active material 100 of one embodiment of the present invention, the concentration of one or more selected from the additive elements A is preferably higher in the surface portion 100a than in the inner portion 100b. This means that the concentration of one or more selected from the additive elements A in the surface portion 100a is preferably higher than the average concentration of the additive elements A in the entire positive electrode active material 100. For this reason, for example, it is preferable that the concentration of one or more additive elements A selected from the surface portion 100a, which is measured by XPS or the like, be higher than the average concentration of the additive elements A in the entire positive electrode active material 100, which is measured by ICP-MS (an inductively coupled plasma-mass spectrometry), GD-MS (a glow discharge mass spectrometry), or the like. For example, the concentration of magnesium in at least part of the surface portion 100a, which is measured by XPS or the like, is preferably higher than the concentration of magnesium in the entire positive electrode active material 100. The concentration of nickel in at least part of the surface portion 100a is preferably higher than the concentration of nickel in the entire positive electrode active material 100. The concentration of aluminum in at least part of the surface portion 100a is preferably higher than the concentration of aluminum in the entire positive electrode active material 100. The concentration of fluorine in at least part of the surface portion 100a is preferably higher than the fluorine concentration of the entire positive electrode active material 100.

Note that the surface and the surface portion 100a of the positive electrode active material 100 of one embodiment of the present invention do not contain a carbonate, a hydroxy group, or the like which is chemically adsorbed after formation of the positive electrode active material 100. Furthermore, an electrolyte solution, a binder, a conductive material, and a compound originating from any of these that are attached to the surface of the positive electrode active material 100 are not contained either. Thus, in quantitative analysis of the elements contained in the positive electrode active material, correction may be performed to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that might be detected in surface analysis such as XPS. For example, in XPS, the kinds of bonds can be identified by analysis, and a C—F bond originating from a binder may be excluded by correction.

Furthermore, before any of various kinds of analyses is performed, a sample of a positive electrode active material or a positive electrode active material layer or the like may be washed, for example, to eliminate an electrolyte solution, a binder, a conductive material, and a compound originating from any of these that are attached to the surface of the positive electrode active material. Although lithium might be dissolved into a solvent or the like used in the washing at this time, the additive element A is not easily dissolved even in that case: thus, the atomic ratio of the additive element A is not affected.

The concentration of the additive element A may be compared using the ratio of the additive element A to cobalt. The use of the ratio of the additive element to cobalt preferably enables comparison while reducing the influence of a carbonate or the like which is chemically adsorbed after formation of the positive electrode active material. For example, in the XPS analysis, the atomic ratio of magnesium to cobalt (Mg/Co) is preferably greater than or equal to 0.4 and less than or equal to 1.5. In the ICP-MS analysis, Mg/Co is preferably greater than or equal to 0.001 and less than or equal to 0.06.

Similarly, to ensure the sufficient path through which lithium is inserted and extracted, the concentrations of lithium and cobalt are preferably higher than those of the additive elements A in the surface portion 100a of the positive electrode active material 100. This means that the concentrations of lithium and cobalt in the surface portion 100a are preferably higher than that of one or more selected from the additive elements A contained in the surface portion 100a, which is measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. In addition, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. The concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. The concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

It is further preferable that the additive element Y such as aluminum be preferably widely distributed in a region that is greater than or equal to 5 nm and less than or equal to 50 nm in depth from the surface, for example. Thus, the additive element Y such as aluminum is detected by analysis on the entire positive electrode active material 100 by ICP-MS, GD-MS, or the like, but the concentration of the additive element Y such as aluminum is preferably lower than or equal to the lower detection limit in XPS or the like.

Furthermore, when XPS analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, the number of magnesium atoms is preferably greater than or equal to 0.4 times and less than or equal to 1.2 times, further preferably 0.65 times or more and 1.0 times or less the number of cobalt atoms. The number of nickel atoms is preferably 0.15 times or less, further preferably 0.03 times or more and 0.13 times or less the number of cobalt atoms. The number of aluminum atoms is preferably 0.12 times or less, further preferably 0.09 times or less the number of cobalt atoms. The number of fluorine atoms is preferably 0.3 times or more and 0.9 times or less, further preferably 0.1 times or more and 1.1 times or less the number of cobalt atoms.

In the XPS analysis, monochromatic aluminum Kα radiation can be used as an X-ray, for example. An extraction angle is, for example, 45°. For example, the measurement can be performed using the following device and conditions.

Measurement device: Quantera II produced by PHI, Inc.X-ray source: monochromatic Al Kα (1486.6 eV)

• • Detection area: 100 μmφDetection depth: approximately 4 to 5 nm (extraction angle) 45°
  • Measurement spectrum: wide scanning, narrow scanning of each detected element

In addition, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, a peak indicating the bonding energy of fluorine with another element is preferably at greater than or equal to 682 eV and less than 685 eV, further preferably approximately 684.3 eV. The above value is different from 685 eV, which is the bonding energy of lithium fluoride, and 686 eV, which is the bonding energy of magnesium fluoride. That is, the positive electrode active material 100 of one embodiment of the present invention containing fluorine is preferably in the bonding state other than lithium fluoride and magnesium fluoride.

Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, a peak indicating the bonding energy of magnesium with another element is preferably at greater than or equal to 1302 eV and less than 1304 eV, further preferably at approximately 1303 eV. This bonding energy is different from that of magnesium fluoride (1305 eV) and is close to that of magnesium oxide. That is, the positive electrode active material 100 of one embodiment of the present invention containing magnesium is preferably in the bonding state other than magnesium fluoride.

<<EDX>>

The one or more selected from the additive elements A contained in the positive electrode active material 100 preferably have a concentration gradient. It is further preferable that the additive elements A contained in the positive electrode active material 100 exhibit concentration peaks at different depths from the surface. The concentration gradient of the additive element A can be evaluated by exposing a cross section of the positive electrode active material 100 using FIB or the like and analyzing the cross section using EDX, EPMA (electron probe microanalysis), or the like.

In the EDX measurement for two-dimensional evaluation of an area by area scan is referred to as EDX area analysis. The measurement for evaluation of the atomic concentration distribution in a positive electrode active material by line scan is referred to as line analysis. Furthermore, extracting data of a linear region from EDX area analysis is referred to as line analysis in some cases. The measurement of a region without scanning is referred to as point analysis.

By EDX area analysis (e.g., element mapping), the concentrations of the additive element A in the surface portion 100a, the inner portion 100b, the vicinity of the crystal grain boundary, and the like of the positive electrode active material 100 can be semi-quantitatively analyzed. By EDX line analysis, the concentration distribution and the highest concentration of the additive element A can be analyzed. An analysis method in which a thinned sample is used, such as STEM-EDX, is preferred because the method makes it possible to analyze the concentration distribution in the depth direction from the surface toward the center in a specific region of the positive electrode active material regardless of the distribution in the front-back direction.

EDX surface analysis or EDX point analysis of the positive electrode active material 100 of one embodiment of the present invention preferably reveals that the concentration of each additive element A, in particular, the additive element X in the surface portion 100a is higher than that in the inner portion 100b.

For example, EDX area analysis or EDX point analysis of the positive electrode active material 100 containing magnesium as the additive element X reveals that the concentration of magnesium in the surface portion 100a is preferably higher than that in the inner portion 100b. Thus, in the EDX line analysis, a peak of the concentration of magnesium in the surface portion 100a is preferably observed in a region extending, toward the center of the positive electrode active material 100, from the surface thereof to a depth of 3 nm, further preferably 1 nm, still further preferably 0.5 nm. In addition, the concentration of magnesium preferably attenuates, at a depth of 1 nm from the point where the concentration reaches the peak, to less than or equal to 60% of the peak concentration. In addition, the concentration of magnesium preferably attenuates, at a depth of 2 nm from the point where the concentration reaches the peak, to less than or equal to 30% of the peak concentration. Here, a “peak of concentration” refers to the local maximum value of concentration.

When the positive electrode active material 100 contains magnesium and fluorine as the additive elements X, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, a difference in the depth direction between a peak of the concentration of fluorine and a peak of the concentration of magnesium is preferably within 10 nm, further preferably within 3 nm, still further preferably within 1 nm.

Thus, in the EDX line analysis, a peak of the concentration of fluorine in the surface portion 100a is preferably observed in a region extending, toward the center of the positive electrode active material 100, from the surface thereof to a depth of 3 nm, further preferably 1 nm, still further preferably 0.5 nm. It is further preferable that a peak of the concentration of fluorine be exhibited slightly closer to the surface side than a peak of the concentration of magnesium is, which increases resistance to hydrofluoric acid. For example, it is preferable that a peak of the concentration of fluorine be exhibited slightly closer to the surface side than a peak of the concentration of magnesium is by 0.5 nm or more, further preferably 1.5 nm or more.

When the positive electrode active material 100 containing nickel as the additive element X, a peak of the concentration of nickel in the surface portion 100a is preferably observed in a region extending, toward the center of the positive electrode active material 100, from the surface thereof to a depth of 3 nm, further preferably 1 nm, still further preferably 0.5 nm. When the positive electrode active material 100 contains magnesium and nickel as the additive elements, the distribution of nickel preferably overlaps with the distribution of magnesium. For example, a difference in the depth direction between a peak of the concentration of nickel and a peak of the concentration of magnesium is preferably within 10 nm, further preferably within 3 nm, still further preferably within 1 nm.

In the case where the positive electrode active material 100 contains aluminum as the additive element Y, the peak of the concentration of magnesium, the concentration of nickel, or the concentration of fluorine is preferably closer to the surface than the peak of the concentration of aluminum is in the surface portion 100a in the EDX line analysis. For example, the peak of the concentration of aluminum is preferably exhibited by a region that is greater than or equal to 0.5 nm and less than or equal to 50 nm in depth, further preferably greater than or equal to 5 nm and less than or equal to 50 nm in depth from the surface toward the center of the positive electrode active material 100.

EDX line, area, or point analysis of the positive electrode active material 100 preferably reveals that the atomic ratio of magnesium to cobalt (Mg/Co) at a peak of the concentration of magnesium is preferably greater than or equal to 0.05 and less than or equal to 0.6, further preferably greater than or equal to 0.1 and less than or equal to 0.4. The atomic ratio of aluminum to cobalt (Al/Co) at a peak of the concentration of aluminum is preferably greater than or equal to 0.05 and less than or equal to 0.6, further preferably greater than or equal to 0.1 and less than or equal to 0.45. The atomic ratio of nickel to cobalt (Ni/Co) at a peak of the concentration of nickel is preferably greater than or equal to 0) and less than or equal to 0.2, further preferably greater than or equal to 0.01 and less than or equal to 0.1. The atomic ratio of fluorine to cobalt (F/Co) at a peak of the concentration of fluorine is preferably greater than or equal to 0 and less than or equal to 1.6, further preferably greater than or equal to 0.1 and less than or equal to 1.4.

Since the positive electrode active material 100 is a compound containing oxygen and a transition metal into and from which lithium can be inserted and extracted, an interface between a region where oxygen and the transition metal M (Co, Ni, Mn, Fe, or the like) that is oxidized or reduced due to insertion and extraction of lithium are present and a region where oxygen and the transition metal M are absent is considered as the surface of the positive electrode active material. A plane generated by slipping and/or a crack also can be considered as the surface of the positive electrode active material. When the positive electrode active material is analyzed, a protective 20) film is attached on its surface in some cases: however, the protective film is not included in the positive electrode active material. As the protective film, a single-layer film or a multilayer film of carbon, a metal, an oxide, a resin, or the like is sometimes used.

Thus, the surface of the positive electrode active material in, for example, STEM-EDX linear analysis refers to a point where the value of the amount of the detected transition metal M is equal to 50% of the sum of the average value MAVE of the amount of the detected transition metal M in the inner portion and the average value MBG of the amount of the background transition metal M and a point where a value of the amount of the detected oxygen is equal to 50% of the sum of the average value OAVE of the amount of detected oxygen in the inner portion and the average value OBG of the amount of background oxygen. Note that in the case where the positions of the points differ between the transition metal M and oxygen, the difference is probably due to the influence of a carbonate, a metal oxide containing oxygen, or the like, which is attached to the surface. Thus, the point that is equal to 50% of the sum of the average value MAVE of the amount of the detected transition metal M in the inner portion and the average value MBG of the amount of the background transition metal M can be used. In the case of a positive electrode active material containing a plurality of transition metals M, its surface can be determined using MAVE and MBG of an element whose count number is the largest in the inner portion 100b.

The average value MBG of the amount of the background transition metal M can be calculated by averaging the amount in the range greater than or equal to 2 nm, preferably greater than or equal to 3 nm, which is outside a portion in the vicinity of the portion at which the amount of the detected transition metal M begins to increase, for example. The average value MAVE of the amount of the detected transition metal M in the inner portion can be calculated by averaging the amount in the range greater than or equal to 2 nm, preferably greater than or equal to 3 nm in a region where the count numbers of the transition metals M and oxygen atoms are saturated and stabilized, e.g., a portion that is greater than or equal to 30 nm, preferably greater than 50 nm in depth from the portion where the amount of the detected transition metal M begins to increase, for example. The average value OBG of the amount of background oxygen and the average value OAVE of the amount of detected oxygen in the inner portion can be calculated in a similar manner.

The surface of the positive electrode active material 100 in, for example, a cross-sectional STEM (scanning transmission electron microscope) image is a boundary between a region where an image derived from the crystal structure of the positive electrode active material is observed and a region where the image is not observed, and is determined as the outermost surface of a region where an atomic column derived from an atomic nucleus of a metal element that has a greater atomic number than lithium among the metal elements constituting the positive electrode active material is confirmed. Alternatively, the surface refers to an intersection of a tangent drawn at a luminance profile from the surface toward the bulk and an axis in the depth direction in a STEM image. The surface in a STEM image or the like may be judged in combination with analysis with higher spatial resolution.

The spatial resolution of STEM-EDX is approximately 1 nm. Thus, the maximum value of an additive element profile may be shifted by approximately 1 nm. For example, even when the maximum value of the profile of an additive element such as magnesium is outside the surface determined in the above-described manner, it can be said that a difference between the maximum value and the surface can be referred to as within the margin of error as long as the difference is less than 1 nm.

A peak in STEM-EDX line analysis refers to the detection intensity in each element profile or the maximum value of the characteristic X-ray of each element. As a noise in STEM-EDX line analysis, a measured value having a half width smaller than or equal to spatial resolution (R), for example, smaller than or equal to R/2 can be given.

The adverse effect of a noise can be reduced by scanning the same portion a plurality of times under the same conditions. For example, an integrated value obtained by measurement by scanning six times can be used as the profile of each element. The number of scanning is not limited to six and an average obtained by performing scanning seven or more times can be used as the profile of each element.

STEM-EDX line analysis can be performed as follows, for example. First, a protective film is deposited over a surface of a positive electrode active material. For example, carbon can be deposited with an ion sputter apparatus (MC1000, produced by Hitachi High-Tech Corporation).

Next, the positive electrode active material is thinned to fabricate a cross-section sample to be subjected to STEM analysis. For example, the positive electrode active material can be thinned with an FIB-SEM apparatus (XVision 200TBS, produced by Hitachi High-Tech Corporation). Here, picking up can be performed by a micro probing system (MPS), and an accelerating voltage at final processing condition can be, for example, 10 kV.

The STEM-EDX line analysis can be performed using (HD-2700 produced by Hitachi High-Tech Corporation) as a STEM apparatus and Octane T Ultra W (with two detectors) produced by EDAX Inc as EDX detectors. In the EDX line analysis, the emission current of the STEM apparatus is set to be in the range of 6 μA to 10 μA, both inclusive, and a portion of the thinned sample, which is not positioned at a deep level and has little unevenness, is measured. The magnification is 150,000 times, for example. The EDX line analysis can be performed under conditions where drift correction is performed, the line width is 42 nm, the pitch is 0.2 nm, and the number of frames is six or more.

<<Surface Roughness and Specific Surface Area>>

The positive electrode active material 100 of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness indicates that an effect of a fusing agent described later adequately functions and the surfaces of the additive element A source and the composite oxide melt. Thus, a smooth surface with little unevenness indicates favorable distribution of the additive element A in the surface portion 100a. Having a favorable distribution indicates, for example, that the concentration distribution of the additive element A in the surface portion 100a is uniform.

A smooth surface with little unevenness can be determined from, for example, a cross-sectional SEM image or a cross-sectional TEM image of the positive electrode active material 100 or the specific surface area of the positive electrode active material 100.

The level of the surface smoothness of the positive electrode active material 100 can be quantified from its cross-sectional SEM image, as described below, for example.

First, the positive electrode active material 100 is processed with an FIB or the like such that its cross section is exposed. At this time, the positive electrode active material 100 is preferably covered with the protective film, a protective agent, or the like. Next, a SEM image of the interface between the positive electrode active material 100 and the protective film or the like is taken. The SEM image is subjected to noise processing using image processing software. For example, the Gaussian Blur (σ=2) is performed, followed by binarization. In addition, interface extraction is performed using image processing software. Moreover, an interface line between the positive electrode active material 100 and the protective film or the like is selected with an automatic selection tool or the like, and data is extracted to spreadsheet software or the like. With the use of the function of the spreadsheet software or the like, correction is performed using regression curves (quadratic regression), parameters for calculating roughness are obtained from data subjected to slope correction, and root-mean-square surface roughness (RMS) is obtained by calculating standard deviation. This surface roughness refers to the surface roughness in at least 400 nm of the particle periphery of the positive electrode active material.

On the surface of the particle of the positive electrode active material 100 of this embodiment, root-mean-square (RMS) surface roughness, which is an index of roughness, is preferably less than 3 nm, further preferably less than 1 nm, still further preferably less than 0.5 nm.

Note that the image processing software used for the noise processing, the interface extraction, or the like is not particularly limited, and for example, “ImageJ” described in Non-Patent Document 7 to Patent Document 9 can be used.

For example, the level of surface smoothness of the positive electrode active material 100 can also be quantified from the ratio of an actual specific surface area SR measured by a constant-volume gas adsorption method to an ideal specific surface area S_i.

The ideal specific surface area S_i is calculated on the assumption that all the particles of the positive electrode active material have the same diameter as D50, have the same weight, and have ideal spherical shapes.

The median diameter D50 can be measured with a particle size distribution analyzer or the like using a laser diffraction and scattering method. The specific surface area can be measured with a specific surface area analyzer or the like by a constant-volume gas adsorption method, for example.

In the positive electrode active material 100 of one embodiment of the present invention, the ratio of the actual specific surface area SR to the ideal specific surface area S_i obtained from the median diameter D50,S_R/S_i, is preferably lower than or equal to 2.1.

The level of the surface smoothness of the positive electrode active material 100 can be quantified from its cross-sectional SEM image by the following method, for example.

First, a surface SEM image of the positive electrode active material 100 is obtained. At this time, conductive coating may be performed as pretreatment for observation. The surface to be observed is preferably vertical to an electron beam. In the case of comparing a plurality of samples, the same measurement conditions and the same observation area are adopted.

Then, the above SEM image is converted into an 8-bit image (which is referred to as a grayscale image) with the use of image processing software (e.g., ImageJ). The grayscale image includes luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be represented by 28=256 gradation levels. A dark portion has a low gradation level and a bright portion has a high gradation level. A variation in luminance can be quantified in relation to the number of gradation levels. The quantified value is referred to as a grayscale value. By obtaining such a grayscale value, the unevenness of the positive electrode active material can be evaluated quantitatively.

In addition, a variation in luminance in a target region can also be represented with a histogram. A histogram three-dimensionally shows distribution of gradation levels in a target region and is also referred to as a luminance histogram. A luminance histogram enables visually easy-to-understand evaluation of unevenness of the positive electrode active material.

In the positive electrode active material 100 of one embodiment of the present invention, the difference between the maximum grayscale value and the minimum grayscale value is preferably less than or equal to 120, further preferably less than or equal to 115, still further preferably greater than or equal to 70 and less than or equal to 115. The standard deviation of the grayscale value is preferably less than or equal to 11, further preferably less than or equal to 8, still further preferably greater than or equal to 4 and less than or equal to 8.

<Pit>

When a positive electrode active material undergoes charging and discharging under conditions, including charging at 4.5 V or more, or at a high temperature, e.g., 45° C. or higher, a progressive defect that progresses deeply from the surface toward the inner portion might be generated. Progress of a defect in a positive electrode active material to form a hole can be referred to as pitting corrosion, and the hole generated by this phenomenon is also referred to as a pit in this specification.

FIG. 13 shows a schematic cross-sectional view of a positive electrode active material 51 including the pit. A crystal plane 55 parallel to the arrangement of cations is also shown. Although a pit 54 and a pit 58 are shown as holes since FIG. 13 is a cross-sectional view, their opening shape is not circular but a wide groove-like shape. Unlike a depression 52, the pit 54 and the pit 58 are likely to be generated parallel to the arrangement of lithium ions as shown in the drawing.

In the positive electrode active material 51, surface portions where the additive elements A exist are denoted by reference numerals 53 and 56. A surface portion where the pit is generated contains a smaller amount of the additive element A than the surface portions 53 and 56 or contains the additive element A whose concentration is below the lower detection limit, and thus probably has a poor function of a barrier film. Presumably, the crystal structure of the composite oxide in the vicinity of a portion where a pit is formed is broken and differs from a layered rock-salt crystal structure. The breakage of the crystal structure inhibits diffusion and release of lithium ions that are carrier ions: thus, a pit is probably a cause of deterioration of cycle performance.

A source of a pit can be a point defect. It is considered that a pit is generated when a point defect included in a positive electrode active material changes due to repetitive charging and discharging, and the positive electrode active material undergoes chemical or electrochemical erosion or degradation due to the electrolyte or the like surrounding the positive electrode active material. This degradation does not occur uniformly in the surface of the positive electrode active material but occurs locally in a concentrated manner.

In addition, as shown in FIG. 13 as a crack 57, a defect such as a crack (also referred to as a crevice) might be generated by expansion and contraction of the positive electrode active material due to charging and discharging. In this specification, a crack and a pit are different from each other. Immediately after formation of a positive electrode active material, a crack can exist but a pit does not exist. A pit can also be regarded as a hole formed by extraction of some layers of the transitional metal M and oxygen due to charging and discharging under conditions such as a high-voltage conditions at 4.5 V or more, or at a high temperature (45° C. or higher), i.e., a portion from which the transition metal M has been eluted. A crack refers to, for example, a surface newly generated by application of physical pressure or a crevice generated owing to a crystal grain boundary. A crack might be caused by expansion and contraction of a positive electrode active material due to charging and discharging. A pit might be generated from a void inside a positive electrode active material and/or a crack.

[Formation Method of Positive Electrode Active Material]

A way of adding the additive element A is important in forming the positive electrode active material 100 having the distribution of the additive element A, the composition, and/or the crystal structure described in the above embodiment. Favorable crystallinity of the inner portion 100b is important as well.

Thus, in the formation process of the positive electrode active material 100, preferably, the composite oxide containing a transitional metal is synthesized first, then the additive element source A is mixed, and heat treatment is performed.

In a method of synthesizing a composite oxide containing the additive element A, lithium, and the transition metal M by mixing the additive element A source concurrently with a transition metal M source and a lithium source, it is difficult to increase the concentration of the additive element A in the surface portion 100a. In addition, after a composite oxide containing lithium and the transition metal M is synthesized, only mixing the additive element source A without performing heating causes the additive element A to be just attached to, not dissolved in, the composite oxide. It is difficult to distribute the additive element A favorably without sufficient heating. Thus, it is preferable that the composite oxide be synthesized, and then the additive element A source be mixed and heat treatment be performed. The heat treatment after mixing of the additive element A source may be referred to as annealing.

However, annealing at an excessively high temperature may cause cation mixing, which increases the possibility of entry of the additive element A such as magnesium into the transition metal M. Magnesium that exists at the transition metal M sites does not have an effect of maintaining a layered rock-salt crystal structure belonging to R-3m when x in Li_xCoO₂ is small. Furthermore, heat treatment at an excessively high temperature might have an adverse effect: for example, cobalt might be reduced to have a valence of two or lithium might be evaporated.

Here, a material functioning as a fusing agent is preferably mixed together with the additive element source A. The material can be regarded as functioning as a fusing agent when having a melting point lower than that of the composite oxide containing lithium and the transition metal M. For example, a fluorine compound such as lithium fluoride is preferably used. Adding the fusing agent decreases the melting points of the additive element A source and the composite oxide containing lithium and the transition metal M. The decrease in the melting points makes it easier to favorably distribute the additive element A at a temperature at which the cation mixing is unlikely to occur.

It is further preferable that heat treatment be performed between the synthesis of the composite oxide containing lithium and the transition metal M and the mixing of the additive element A. This heating is referred to as initial heating in some cases.

Owing to influence of lithium extraction from part of the surface portion 100a of the composite oxide containing lithium and the transition metal M by the initial heating, the distribution of the additive element A becomes more favorable.

Specifically, the distributions of the additive elements A can be easily made different from each other by the initial heating in the following mechanism. First, by the initial heating, a lithium compound (e.g., lithium carbonate) or the like unintentionally remaining on a surface of lithium cobalt oxide is extracted. Then, a composite oxide containing lithium and the transition metal M from which the unintentional lithium compound is eliminated and the additive elements A source such as a nickel source, an aluminum source, and the magnesium source, are mixed and heated. Among the additive elements A, magnesium is a divalent representative element, and nickel is a transition metal but is likely to be a divalent ion. Thus, in part of the surface portion 100a, a rock-salt phase containing Mg²⁺ and Ni²⁺ along with Co²⁺ is formed.

Among the additive elements A, nickel is likely to be dissolved and is diffused to the inner portion 100b in the case where the surface portion 100a is the composite oxide that contains lithium and the transition metal M and has a layered rock-salt crystal structure, but nickel is likely to remain in the surface portion 100a in the case where part of the surface portion 100a has a rock-salt crystal structure.

Furthermore, in such a rock-salt crystal structure, the bond distance between a metal Me and oxygen (Me—O distance) tends to be longer than that in a layered rock-salt crystal structure.

For example, Me—O distance is 2.09 Å and 2.11 Å in Ni_0.5Mg_0.5O having a rock-salt crystal structure and MgO having a rock-salt crystal structure, respectively. Even when a spinel phase is formed in part of the surface portion 100a, Me—O distance is 2.0125 Å and 2.02 Å in NiAl₂O₄ having a spinel structure and MgAl₂O₄ having a spinel structure, respectively. In each case, Me—O distance is longer than 2 Å.

Meanwhile, in a layered rock-salt crystal structure, the bond distance between oxygen and a metal other than lithium is shorter than the above-described distance. For example, Al—O distance is 1.905 Å (Li—O distance is 2.11 Å) in LiAlO₂ having a layered rock-salt crystal structure. In addition, Co—O distance is 1.9224 Å (Li—O distance is 2.0916 Å) in LiCoO₂ having a layered rock-salt crystal structure.

According to the ionic radius of Shannon (Shannon et al., Acta A 32 (1976) 751.), the ion radius of hexacoordinated aluminum and the ion radius of hexacoordinated oxygen are 0.535 Å and 1.4 Å, respectively, and the sum of those values is 1.935 Å.

From the above, aluminum is considered to exist at sites other than lithium sites more stably in a layered rock-salt crystal structure than in a rock-salt crystal structure. Thus, in the surface portion 100a, aluminum is more likely to be distributed in a region having a layered rock-salt phase at a larger depth and/or the inner portion 100b than in a region having a rock-salt phase that is close to the surface.

Moreover, the initial heating is expected to increase the crystallinity of the layered rock-salt crystal structure of the inner portion 100b.

However, the initial heating is not necessarily performed. In some cases, by controlling atmosphere, temperature, time, or the like in another heating step, e.g., annealing, the positive electrode active material 100 that has the O3′ type structure and/or the monoclinic O1 (15) type structure when x in Li_xCoO₂ is small can be formed.

An example of a formation flow of the positive electrode active material 100, in which annealing and the initial heating are performed, is described with reference to FIG. 14A to FIG. 14C.

<Step S11>

In Step S11 shown in FIG. 14A, a lithium source (Li source) and a transition metal M source (M source) are prepared as materials for lithium and a transition metal M which are starting materials.

As the lithium source, a lithium-containing compound is preferably used and for example, lithium carbonate, lithium hydroxide, lithium nitrate, lithium fluoride, or the like can be used. The lithium source preferably has a high purity and is preferably a material having a purity higher than or equal to 99.99%, for example.

The transition metal M can be selected from the elements belonging to Groups 4 to 13 of the periodic table and for example, at least one of manganese, cobalt, and nickel is used. That is, as the transition metal M, for example, cobalt alone, nickel alone, two elements of cobalt and manganese, two metals of cobalt and nickel, or three metals of cobalt, manganese, and nickel may be used. When cobalt alone is used, the positive electrode active material to be obtained contains lithium cobalt oxide (LCO); when three metals of cobalt, manganese, and nickel are used, the positive electrode active material to be obtained contains lithium nickel cobalt manganese oxide (NCM).

As the transition metal M source, a compound containing the above transition metal M is preferably used and for example, an oxide, a hydroxide, or the like of any of the metals given as examples of the transition metal M can be used. As a cobalt source, tricobalt tetraoxide, cobalt hydroxide, or the like can be used. As a manganese source, manganese oxide, manganese hydroxide, or the like can be used. As a nickel source, nickel oxide, nickel hydroxide, or the like can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, or the like can be used.

The transition metal M source preferably has a high purity and is preferably a material having a purity higher than or equal to 3N (99.9%), further preferably higher than or equal to 4N (99.99%), still further preferably higher than or equal to 4N5 (99.995%), yet still further preferably higher than or equal to 5N (99.999%), for example. Impurities of the positive electrode active material can be controlled by using such a high-purity material. As a result, a secondary battery with an increased capacity and/or increased reliability can be obtained.

Furthermore, the transition metal M source preferably has high crystallinity, and preferably includes single crystal particles, for example. To evaluate the crystallinity of the transition metal M source, the crystallinity can be determined by a TEM (transmission electron microscope) image, a STEM (scanning transmission electron microscope) image, a HAADF-STEM (high-angle annular dark field scanning transmission electron microscope) image, an ABF-STEM (annular bright-field scanning transmission electron microscope) image, or the like, or can be judged by X-ray diffraction (XRD), electron diffraction, neutron diffraction, or the like. Note that the above methods for evaluating crystallinity can also be employed to evaluate the crystallinity of other materials in addition to the transition metal M source.

In the case of using two or more transition metal M sources, the two or more transition metal M sources are preferably prepared to have proportions (mixing ratio) such that a layered rock-salt crystal structure would be obtained.

<Step S12>

Next, in Step S12 shown in FIG. 14A, the lithium source and the transition metal M source are ground and mixed to form a mixed material. The grinding and mixing can be performed by a dry method or a wet method. A wet method is preferred because it can grind particles into a smaller size. When the grinding and mixing are performed by a wet method, a solvent is prepared. As the solvent, ketone such as acetone, alcohol such as ethanol or isopropanol, ether, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), or the like can be used. An aprotic solvent, which is unlikely to react with lithium, is preferably used. In this embodiment, dehydrated acetone with a purity higher than or equal to 99.5% is used. It is preferable that the lithium source and the transition metal M source be mixed into dehydrated acetone whose moisture content is less than or equal to 10 ppm and which has a purity higher than or equal to 99.5% in the grinding and mixing. With the use of dehydrated acetone with the above-described purity, impurities that might be mixed can be reduced.

A ball mill, a bead mill, or the like can be used for the mixing and the like. When a ball mill is used, aluminum oxide balls or zirconium oxide balls are preferably used as a grinding medium. Zirconium oxide balls are preferable because they release fewer impurities. When a ball mill, a bead mill, or the like is used, the peripheral speed is preferably higher than or equal to 100 mm/s and lower than or equal to 2000 mm/s in order to inhibit contamination from the medium. In this embodiment, the peripheral speed is set to 838 mm/s (the rotational frequency is 400 rpm, and the diameter of the ball mill is 40 mm).

<Step S13>

Next, in Step S13 shown in FIG. 14A, the above mixed material is heated. The heating is preferably performed at higher than or equal to 800° C. and lower than or equal to 1100° C., further preferably at higher than or equal to 900° C. and lower than or equal to 1000° C., still further preferably at approximately 950° C. An excessively low temperature might lead to insufficient decomposition and melting of the lithium source and the transition metal M source. An excessively high temperature might lead to a defect due to evaporation of lithium from the lithium source and/or excessive reduction of the metal used as the transition metal M source, for example. The defect is, for example, an oxygen defect which could be induced by a change of trivalent cobalt into divalent cobalt due to excessive reduction, in the case where cobalt is used as the transition metal M.

When the heating time is too short, LiMO₂ is not synthesized, but when the heating time is too long, the productivity is lowered. For example, the heating time is preferably longer than or equal to 1 hour and shorter than or equal to 100 hours, further preferably longer than or equal to 2 hours and shorter than or equal to 20 hours.

A temperature rising rate is preferably higher than or equal to 80° C./h and lower than or equal to 250° C./h, although depending on the end-point temperature of the heating. For example, in the case of heating at 1000° C. for 10 hours, the temperature raising rate is preferably 200° C./h.

The heating is preferably performed in an atmosphere with little water such as a dry-air atmosphere and for example, the dew point of the atmosphere is preferably lower than or equal to −50° C., further preferably lower than or equal to −80° C. In this embodiment, the heating is performed in an atmosphere with a dew point of −93° C. To reduce impurities that might enter the material, the concentrations of impurities such as CH₄, CO, CO₂, and H₂ in the heating atmosphere are each preferably lower than or equal to 5 ppb (parts per billion).

The heating atmosphere is preferably an oxygen-containing atmosphere. For example, one method, a dry air is continuously introduced into a reaction chamber. The flow rate of a dry air in this case is preferably 10 L/min. Continuously introducing oxygen into a reaction chamber to make oxygen flow therein is referred to as flowing.

In the case where the heating atmosphere is an oxygen-containing atmosphere, flowing is not necessarily performed. For example, the following method may be employed: the pressure in the reaction chamber is reduced, then the reaction chamber is filled with oxygen, and the oxygen is prevented from entering or exiting from the reaction chamber. Such a method is referred to as purging. For example, the pressure in the reaction chamber may be reduced to −970 hPa and then, the reaction chamber may be filled with oxygen until the pressure becomes 50 hPa. Cooling after the heating can be performed by letting the mixed material stand to cool, and the time it takes for the temperature to decrease to room temperature from a predetermined temperature is preferably longer than or equal to 10 hours and shorter than or equal to 50 hours. Note that the temperature does not necessarily need to decrease to room temperature as long as it decreases to a temperature acceptable to the next step.

The heating in this step may be performed with a rotary kiln or a roller hearth kiln. Heating with stirring can be performed in either case of a sequential rotary kiln or a batch-type rotary kiln.

A crucible or a saggar used at the time of the heating is preferably made of alumina (aluminum oxide), mullite cordierite, magnesia, or zirconia, i.e., preferably includes a highly heat-resistant material. Furthermore, since aluminum oxide is a material into which impurities are less likely to enter, the purity of a crucible made of alumina or a saggar is 99% or higher, preferably 99.5% or more. In this embodiment, a crucible made of aluminum oxide with a purity of 99.9% is used. The heating is preferably performed with the crucible or the saggar covered with a lid. Volatilization of the materials can be prevented.

The heated material is ground as needed and may be made to pass through a sieve. Before collection of the heated material, the material may be moved from the crucible to a mortar. As the mortar, a zirconium oxide mortar can be suitably used. A zirconium oxide mortar is made of a material that hardly releases impurities. Specifically, a mortar made of zirconium oxide with a purity higher than or equal to 90%, preferably higher than or equal to 99% is used. Note that heating conditions equivalent to those in Step S13 can be employed in a later-described heating step other than Step S13.

<Step S14>

Through the above steps, a composite oxide including the transition metal M (LiMO₂) can be obtained in Step S14 shown in FIG. 14A. The composite oxide needs to have a crystal structure of a lithium composite oxide represented by LiMO₂, but the composition is not strictly limited to Li:M:O=1:1:2. In the case where cobalt is used as the transition metal M, the composite oxide is referred to as a composite oxide containing cobalt and is represented by LiCoO₂. Note that the composition is not strictly limited to Li:Co:O=1:1:2.

Although the example is described in which the composite oxide is formed by a solid phase method as in Step S11 to Step S14, the composite oxide may be formed by a coprecipitation method. Alternatively, the composite oxide may be formed by a hydrothermal method.

<Step S15>

Next, in Step S15 shown in FIG. 14A, the above composite oxide is heated. The heating in Step S15 is the first heating performed on the composite oxide and thus, this heating is sometimes referred to as the initial heating. The heating is performed before Step S20 described below and thus is sometimes referred to as preheating or pretreatment.

Through the initial heating, as described above, lithium compound remaining unintentionally on the surface of the composite oxide are extracted. In addition, an effect of increasing the crystallinity of the inner portion 100b can be expected. The lithium source and/or transition metal M source prepared in Step S11 and the like might contain impurities. The initial heating can reduce impurities in the composite oxide completed in Step S14.

Furthermore, by going through the initial heating, there is an effect that a surface of the composite oxide becomes smooth. A smooth surface of the composite oxide refers to a state of having little unevenness and being rounded as a whole, and its corner portion is rounded. A smooth surface also refers to a surface to which few foreign matters are attached. Foreign matters are deemed to cause unevenness and are preferably not attached to a surface.

For this initial heating, there is no need to prepare a lithium compound source. For the initial heating, there is no need to prepare an additive element source A. Alternatively, there is no need to prepare a material functioning as a fusing agent.

When the heating time in this step is too short, sufficient effect is not obtained, but when the heating time in this step is too long, the productivity is lowered. For example, any of the heating conditions described for Step S13 can be selected. Additionally, the heating temperature in this step is preferably lower than that in Step S13 so that the crystal structure of the composite oxide is maintained. The heating time in this step is preferably shorter than that in Step S13 so that the crystal structure of the composite oxide is maintained. For example, the heating is preferably performed at a temperature higher than or equal to 700° C. and lower than or equal to 1000° C. for longer than or equal to 2 hours and shorter than or equal to 20 hours.

The effect of increasing the crystallinity of the internal portion 100b is, for example, an effect of reducing distortion, a shift, or the like derived from differential shrinkage or the like of the composite oxide formed in Step S13.

The heating in Step S13 might cause a temperature difference between the surface and an inner portion of the above composite oxide. The temperature difference sometimes induces differential shrinkage. It can also be deemed that the temperature difference leads to a fluidity difference between the surface and the inner portion, thereby causing differential shrinkage. The energy involved in differential shrinkage causes a difference in internal stress in the composite oxide. The difference in internal stress is also called distortion, and the above energy is sometimes referred to as distortion energy. The internal stress is eliminated by the initial heating in Step S15 and in other words, the distortion energy is probably equalized by the initial heating in Step S15. When the distortion energy is equalized, the distortion in the composite oxide is relieved. This is probably why the surface of the composite oxide becomes smooth through Step S15. This is also rephrased as modification of the surface. In other words, it is deemed that Step S15 reduces the differential shrinkage caused in the composite oxide to make the surface of the composite oxide smooth.

Such differential shrinkage might cause a micro shift in the composite oxide such as a shift in a crystal. To reduce the shift, this step is preferably performed. Performing this step can distribute a shift uniformly in the composite oxide. When the shift is distributed uniformly, the surface of the composite oxide might become smooth. This is also referred to as alignment of crystal grains. In other words, it is deemed that Step S15 reduces the shift in a crystal or the like which is caused in the composite oxide to make the surface of the composite oxide smooth.

In a secondary battery including a composite oxide with a smooth surface as a positive electrode active material, degradation by charging and discharging is inhibited and a crack in the positive electrode active material can be prevented.

It can be said that when surface unevenness information in one cross section of a composite oxide is quantified with measurement data, a smooth surface of the composite oxide has a surface roughness at least less than or equal to 10 nm. The one cross section is, for example, a cross section obtained in observation using a scanning transmission electron microscope (STEM).

Note that in Step S14, a composite oxide containing lithium, a transition metal M, and oxygen, synthesized in advance may be used. In that case, Step S11 to Step S13 can be omitted. When Step S15 is performed on the pre-synthesized composite oxide, a composite oxide with a smooth surface can be obtained.

Through the initial heating, the lithium compound remaining in a surface of the composite oxide might be unintentionally extracted. The initial heating might reduce cracks and/or crystal defects in the composite oxide. Due to these effects, the additive element A described for Step S20 or the like below might easily enter the composite oxide.

<Step S20>

The additive element A may be added to the composite oxide having a smooth surface as long as a layered rock-salt crystal structure can be obtained. When the additive element A is added to the composite oxide having a smooth surface, the additive element A can be uniformly added. It is thus preferable that the initial heating precede the addition of the additive element A. The step of adding the additive element A is described with reference to FIG. 14B and FIG. 14C.

<Step S21>

In Step S21 shown in FIG. 14B, an additive element A source (A source) to be added to the composite oxide is prepared. A lithium source may be prepared together with the additive element A sources.

As the additive element A, one or more elements selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic can be used. As the additive element, one or more selected from bromine and beryllium can be used. Note that the additive elements given earlier are more suitable since bromine and beryllium are elements having toxicity to living things.

When magnesium is selected as the additive element A, the additive element A source can be referred to as a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Two or more of these magnesium sources may be used.

When fluorine is selected as the additive element A, the additive element A source can be referred to as a fluorine source. As the fluorine source, for example, lithium fluoride, magnesium fluoride, aluminum fluoride, titanium fluoride, cobalt fluoride, nickel fluoride, zirconium fluoride, vanadium fluoride, manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride, calcium fluoride, sodium fluoride, potassium fluoride, barium fluoride, cerium fluoride, lanthanum fluoride, sodium aluminum hexafluoride, or the like can be used. In particular, lithium fluoride is preferable because it is easily melted in a heating step described later owing to its relatively low melting point of 848° C.

Magnesium fluoride can be used as both the fluorine source and the magnesium source. Lithium fluoride can be used as the lithium source. Another example of the lithium source that can be used in Step S21 is lithium carbonate.

The fluorine source may be a gas, and fluorine, carbon fluoride, sulfur fluoride, oxygen fluoride, or the like may be used and mixed in the atmosphere in a heating step described later. Two or more of these fluorine sources may be used.

In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF₂) is prepared as the fluorine source and the magnesium source. When lithium fluoride and magnesium fluoride are mixed such that LiF:MgF₂ is approximately 65:35 (molar ratio), the effect of lowering the melting point is maximized. Meanwhile, when the proportion of lithium fluoride increases, the cycling performance might be degraded because of an excessive amount of lithium. Thus, the molar ratio of lithium fluoride to magnesium fluoride (LIF:MgF₂) is preferably x:1 (0≤x≤1.9), further preferably x:1 (0.1≤x≤0.5), still further preferably x:1 (x=0.33 or an approximate value thereof). Note that in this specification and the like, “an approximate value of a given value” means a value greater than 0.9 times and less than 1.1 times the given value.

Meanwhile, magnesium is preferably added at greater than 0.1 at % and less than or equal to 3 at %, further preferably greater than or equal to 0.5 at % and less than or equal to 2 at %, still further preferably greater than or equal to 0.5 at % and less than or equal to 1 at %, relative to LiCoO₂. When magnesium is added at less than or equal to 0.1 at %, the initial discharge capacity is high but repeated charging and discharging with a large charge depth rapidly lowers the discharge capacity. In the case where magnesium is added at greater than 0.1 at % and less than or equal to 3 at %, both the initial discharge characteristics and charge and discharge cycle performance are excellent even when charging and discharging with a large charge depth are repeated. By contrast, in the case where magnesium is added at greater than 3 at %, both the initial discharge capacity and the charge and discharge cycle performance tend to deteriorate gradually.

<Step S22>

Next, in Step S22 shown in FIG. 14B, the magnesium source and the fluorine source are ground and mixed. Any of the conditions for the grinding and mixing that are described for Step S12 can be selected to perform this step.

<Step S23>

Next, in Step S23 shown in FIG. 14B, the materials ground and mixed in the above step are collected to give the additive element A source (A source). Note that the additive element A source in Step S23 contains a plurality of starting materials and can be referred to as a mixture.

As for the particle diameter of the mixture, the D50 (median diameter) is preferably greater than or equal to 600 nm and less than or equal to 20 μm, further preferably greater than or equal to 1 μm and less than or equal to 10 μm. Also when one kind of material is used as the additive element A source, the D50 (median diameter) is preferably greater than or equal to 600 nm and less than or equal to 20 μm, further preferably greater than or equal to 1 μm and less than or equal to 10 μm.

Such a pulverized mixture (which may contain only one kind of the additive element A) is easily attached to the surface of a composite oxide particle uniformly in a later step of mixing with the composite oxide. The mixture is preferably attached uniformly to the surface of the composite oxide particle, in which case fluorine and magnesium are easily distributed or dispersed uniformly in a surface portion of the composite oxide after heating. The region where fluorine and magnesium are distributed can be referred to as a surface portion. When there is a region containing neither fluorine nor magnesium in the surface portion, an O3′ type crystal structure, which is described later, might be unlikely to be obtained in a charged state. Note that although fluorine is used in the above description, chlorine may be used instead of fluorine, and a general term “halogen” for these elements can replace “fluorine”.

<Step S21>

A process different from that in FIG. 14B is described with reference to FIG. 14C. In Step S21 shown in FIG. 14C, four kinds of additive element A sources to be added to the composite oxide are prepared. In other words, FIG. 14C is different from FIG. 14B in the kinds of the additive element A sources. A lithium source may be prepared together with the additive element A sources.

As the four kinds of additive element A sources, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. Note that the magnesium source and the fluorine source can be selected from the compounds and the like described with reference to FIG. 14B. As a nickel source, nickel oxide, nickel hydroxide, or the like can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, or the like can be used.

<Step S22 and Step S23>

Next, Step S22 and Step S23 shown in FIG. 14C are similar to the steps described with reference to FIG. 14B.

<Step 31>

Next, in Step S31 shown in FIG. 14A, the composite oxide and the additive element A source (A source) are mixed. The ratio of the number M of the transition metal M atoms in the composite oxide containing lithium, the transition metal M, and oxygen to the number Mg of magnesium atoms contained in the additive element A source is preferably M:Mg=100:y (0.1≤y≤6), further preferably M:Mg=100:y (0.3≤y≤3).

The conditions of the mixing in Step S31 are preferably milder than those of the mixing in Step S12 in order not to damage the composite oxide. For example, conditions with a lower rotation frequency or shorter time than those for the mixing in Step S12 are preferable. In addition, it can be said that a dry method has a milder condition than a wet method. For example, a ball mill or a bead mill can be used for the mixing. When a ball mill is used, zirconium oxide balls are preferably used as a medium, for example.

In this embodiment, the mixing is performed with a ball mill using zirconium oxide balls with a diameter of 1 mm by a dry method at 150 rpm for 1 hour. The mixing is performed in a dry room the dew point of which is higher than or equal to −100° C. and lower than or equal to −10° C.

<Step S32>

Next, in Step S32 in FIG. 14A, the materials mixed in the above step are collected, whereby a mixture 903 is obtained. At the time of the collection, crushing may be performed.

Note that in this embodiment, the method is described in which lithium fluoride as the fluorine source and magnesium fluoride as the magnesium source are added afterward to the composite oxide that has been subjected to the initial heating. However, the present invention is not limited to the above method. The magnesium source, the fluorine source, and the like can be added to the lithium source and the transition metal M source in Step S11, i.e., at the stage of the starting materials of the composite oxide. Then, the heating in Step S13 is performed, so that LiMO₂ to which magnesium and fluorine are added can be obtained. In that case, there is no need to separately perform Step S11 to Step S14 and Step S21 to Step S23. This method can be regarded as being simple and highly productive.

Alternatively, the composite oxide to which magnesium and fluorine are added in advance may be used. When composite oxide to which magnesium and fluorine are added is used, Step S11 to Step S14 and Step S20 can be skipped. This method can be regarded as being simple and highly productive.

Alternatively, to the composite oxide to which magnesium and fluorine are added in advance, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be further added as in Step S20.

<Step S33>

Then, in Step S33 shown in FIG. 14A, the mixture 903 is heated. Any of the heating conditions described for Step S13 can be selected. The heating time is preferably longer than or equal to 2 hours.

Here, a supplementary explanation of the heating temperature is provided. The lower limit of the heating temperature in Step S33 needs to be higher than or equal to the temperature at which a reaction between the composite oxide (LiMO₂) and the additive element A source proceeds. The temperature at which the reaction proceeds is the temperature at which interdiffusion of the elements included in LiMO₂ and the additive element A source occurs, and may be lower than the melting temperatures of these materials. It is known that in the case of an oxide as an example, solid phase diffusion occurs at the Tamman temperature Ta (0.757 times the melting temperature Tm). Accordingly, it is only required that the heating temperature in Step S33 be higher than or equal to 500° C.

Needless to say, the reaction more easily proceeds at a temperature higher than or equal to the temperature at which at least part of the mixture 903 is melted. For example, in the case where LiF and MgF₂ are included in the additive element A source, the lower limit of the heating temperature in Step S33 is preferably higher than or equal to 742° C. because the eutectic point of LiF and MgF₂ is around 742° C.

The mixture 903 obtained by mixing such that LiCoO₂:LiF:MgF₂=100:0.33:1 (molar ratio) exhibits an endothermic peak at around 830° C. in differential scanning calorimetry (DSC). Thus, the lower limit of the heating temperature is further preferably higher than or equal to 830° C.

A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and enables high productivity.

The upper limit of the heating temperature is lower than the decomposition temperature of LiMO₂ (the decomposition temperature of LiCoO₂ is 1130° C.). At around the decomposition temperature, a slight amount of LiMO₂ might be decomposed. Thus, the upper limit of the heating temperature is preferably lower than or equal to 1000° C., further preferably lower than or equal to 950° C., still further preferably lower than or equal to 900° C.

In view of the above, the heating temperature in Step S33 is preferably higher than or equal to 500° C. and lower than or equal to 1130° C., further preferably higher than or equal to 500° C. and lower than or equal to 1000° C., still further preferably higher than or equal to 500° C. and lower than or equal to 950° C., yet still further preferably higher than or equal to 500° C. and lower than or equal to 900° C. Furthermore, the heating temperature is preferably higher than or equal to 742° C. and lower than or equal to 1130° C., further preferably higher than or equal to 742° C. and lower than or equal to 1000° C., still further preferably higher than or equal to 742° C. and lower than or equal to 950° C., yet still further preferably higher than or equal to 742° C. and lower than or equal to 900° C. Furthermore, the heating temperature is preferably higher than or equal to 800° C. and lower than or equal to 1100° C., further preferably higher than or equal to 830° C. and lower than or equal to 1130° C., still further preferably higher than or equal to 830° C. and lower than or equal to 1000° C., yet still further preferably higher than or equal to 830° C. and lower than or equal to 950° C., yet still further preferably higher than or equal to 830° C. and lower than or equal to 900° C. Note that the heating temperature in Step S33 is preferably higher than that in Step S13.

In addition, at the time of heating the mixture 903, the partial pressure of fluorine or a fluoride originating from the fluorine source or the like is preferably controlled to be within an appropriate range.

In the fabrication method described in this embodiment, some of the materials, e.g., LiF as the fluorine source, function as a flux in some cases. Owing to this function, the heating temperature can be lower than the decomposition temperature of the composite oxide (LiMO₂), e.g., a temperature higher than or equal to 742° C. and lower than or equal to 950° C., which allows distribution of the additive element A such as magnesium in the surface portion and fabrication of the positive electrode active material having favorable characteristics.

However, since LiF in a gas phase has a specific gravity less than that of oxygen, heating might volatilize LiF and in that case, LiF in the mixture 903 decreases. As a result, the function of a fusing agent deteriorates. Thus, heating needs to be performed while volatilization of LiF is inhibited. Note that even when LiF is not used as the fluorine source or the like, Li at the surface of LiMO₂ and F of the fluorine source might react to produce LiF, which might be volatilized. Thus, such inhibition of volatilization is needed also when a fluoride having a higher melting point than LiF is used.

In view of this, the mixture 903 is preferably heated in an atmosphere containing LiF, i.e., the mixture 903 is preferably heated in a state where the partial pressure of LiF in the heating furnace is high. Such heating can inhibit volatilization of LiF in the mixture 903.

The heating in this step is preferably performed such that the particles of the mixture 903 are not adhered to each other. Adhesion of the particles of the mixture 903 during the heating might decrease the area of contact with oxygen in the atmosphere and inhibit a path of diffusion of the additive element A (e.g., fluorine), thereby hindering distribution of the additive element A (e.g., magnesium and fluorine) in the surface portion.

It is considered that uniform distribution of the additive element A (e.g., fluorine) in the surface portion leads to a smooth positive electrode active material with little unevenness. Thus, it is preferable that the particles of the mixture 903 not be adhered to each other in order to allow the smooth surface obtained through the heating in Step S15 to be maintained or to be smoother in this step.

In the case of using a rotary kiln for the heating, the heating is preferably performed while the flow rate of an oxygen-containing atmosphere in the kiln is preferably controlled. For example, the flow rate of an oxygen-containing atmosphere is preferably set low, or no flowing of an atmosphere is preferably performed after an atmosphere is purged first and an oxygen atmosphere is introduced into the kiln. Flowing of oxygen is not preferable because it might cause evaporation of the fluorine source, which prevents maintaining the smoothness of the surface.

In the case of using a roller hearth kiln for the heating, the mixture 903 can be heated in an atmosphere containing LiF with the container containing the mixture 903 covered with a lid, for example.

A supplementary explanation of the heating time is provided. The heating time is changed depending on conditions such as the heating temperature and the size and composition of LiMO₂ in Step S14. In the case where the size of LiMO₂ is small, the heating is preferably performed at a lower temperature or for a shorter time than heating in the case where the size is large, in some cases.

In the case where the composite oxide (LiMO₂) in Step S14 in FIG. 14A has a median diameter (D50) of approximately 12 μm, the heating temperature is preferably higher than or equal to 600° C. and lower than or equal to 950° C., for example. The heating time is preferably longer than or equal to 3 hours, further preferably longer than or equal to 10 hours, still further preferably longer than or equal to 60 hours, for example. Note that the temperature decreasing time after the heating is, for example, preferably longer than or equal to 10 hours and shorter than or equal to 50 hours.

In the case where the composite oxide (LiMO₂) in Step S14 has a median diameter (D50) of approximately 5 μm, the heating temperature is preferably higher than or equal to 600° C. and lower than or equal to 950° C., for example. The heating time is preferably longer than or equal to 1 hour and shorter than or equal to 10 hours, further preferably approximately 2 hours, for example. Note that the temperature decreasing time after the heating is, for example, preferably longer than or equal to 10 hours and shorter than or equal to 50 hours.

<Step S34>

Next, the heated material is collected in Step S34 shown in FIG. 14A, in which crushing is performed as needed: thus, the positive electrode active material 100 is obtained. Here, the collected positive electrode active material 100 is preferably made to pass through a sieve. Through the above process, the positive electrode active material 100 of one embodiment of the present invention can be formed. The positive electrode active material of one embodiment of the present invention has a smooth surface.

This embodiment can be used in combination with any of the other embodiments.

Embodiment 2

In this embodiment, examples of a secondary battery of one embodiment of the present invention are described with reference to FIG. 15 to FIG. 18.

Structure Example of Secondary Battery

Hereinafter, a secondary battery in which a positive electrode, a negative electrode, and an electrolyte solution are wrapped in an exterior body is described as an example.

[Positive Electrode]

The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and may include a conductive material and a binder. As the positive electrode active material, the positive electrode active material formed by the formation method described in the above embodiments is used.

The positive electrode active material described in the above embodiments and another positive electrode active material may be mixed to be used.

Other examples of the positive electrode active material include a composite oxide with an olivine crystal structure, a composite oxide with a layered rock-salt crystal structure, and a composite oxide with a spinel crystal structure. For example, a compound such as LiFePO₄, LiFeO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ is given.

As another positive electrode active material, it is preferable to add lithium nickel oxide (LiNiO₂ or LiNi_1-xM_xO₂ (0<x<1) (M=Co, Al, or the like)) to a lithium-containing material with a spinel crystal structure which contains manganese, such as LiMn₂O₄, because the characteristics of the secondary battery including such a material can be improved.

The applicable another example of the positive electrode active material is a lithium-manganese composite oxide that can be represented by a composition formula Li_aMn_bM_cO_d. Here, the element M is preferably silicon, phosphorus, or a metal element other than lithium and manganese, further preferably nickel. In the case where the whole particle of a lithium-manganese composite oxide is measured, it is preferable to satisfy the following at the time of discharging: 0<a/(b+c)<2; c>0; and 0.26≤(b+c)/d<0.5. Note that the proportions of metals, silicon, phosphorus, and the like in the whole particle of a lithium-manganese composite oxide can be measured with, for example, an ICP-MS (inductively coupled plasma mass spectrometer). The proportion of oxygen in the whole particle of a lithium-manganese composite oxide can be measured by, for example, EDX (energy dispersive X-ray spectroscopy). Alternatively, the proportion of oxygen can be measured by ICP-MS analysis combined with fusion gas analysis and valence evaluation of XAFS (X-ray absorption fine structure) analysis. Note that the lithium-manganese composite oxide is an oxide containing at least lithium and manganese, and may contain at least one selected from chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, phosphorus, and the like.

A cross-sectional structure example of an active material layer 200 containing graphene or a graphene compound as a conductive material is described below.

FIG. 15A is a longitudinal cross-sectional view of the active material layer 200. The active material layer 200 includes particles of the positive electrode active material 100, graphene or a graphene compound 201 serving as the conductive material, and a binder (not shown).

The graphene compound 201 in this specification and the like refers to multilayer graphene, multi graphene, graphene oxide, multilayer graphene oxide, multi graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi graphene oxide, graphene quantum dots, and the like. A graphene compound contains carbon, has a plate-like shape, a sheet-like shape, or the like, and has a two-dimensional structure formed of a six-membered ring composed of carbon atoms. The two-dimensional structure formed of the six-membered ring composed of carbon atoms may be referred to as a carbon sheet. A graphene compound may include a functional group. The graphene compound is preferably bent. The graphene compound may be rounded like a carbon nanofiber.

In this specification and the like, graphene oxide contains carbon and oxygen, has a sheet-like shape, and includes a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

In this specification and the like, reduced graphene oxide contains carbon and oxygen, has a sheet-like shape, and has a two-dimensional structure formed of a six-membered ring composed of carbon atoms. The reduced graphene oxide functions by itself and may have a stacked-layer structure. The reduced graphene oxide preferably includes a portion where the carbon concentration is higher than 80 atomic % and the oxygen concentration is higher than or equal to 2 atomic % and lower than or equal to 15 atomic %. With such a carbon concentration and such an oxygen concentration, the reduced graphene oxide can function as a conductive material with high conductivity even with a small amount. In addition, the intensity ratio G/D of a G band to a D band of the Raman spectrum of the reduced graphene oxide is preferably 1 or more. The reduced graphene oxide with such an intensity ratio can function as a conductive material with high conductivity even with a small amount.

A graphene compound sometimes has excellent electrical characteristics of high conductivity and excellent physical properties of high flexibility and high mechanical strength. A graphene compound has a sheet-like shape. A graphene compound has a curved surface in some cases, thereby enabling low-resistant surface contact. Furthermore, a graphene compound sometimes has extremely high conductivity even with a small thickness, and thus a small amount of a graphene compound efficiently allows a conductive path to be formed in an active material layer. Hence, when a graphene compound is used as the conductive material, the area where the active material and the conductive material are in contact with each other can be increased. The graphene compound preferably covers 80% or more of the active material. Note that the graphene compound preferably clings to at least part of an active material particle. The graphene compound preferably overlays at least part of the active material particle. The shape of the graphene compound preferably conforms to at least part of the shape of the active material particle. The shape of an active material particle means, for example, unevenness of a single active material particle or unevenness formed by a plurality of active material particles. The graphene compound preferably surrounds at least part of an active material particle. The graphene compound may have a hole.

In the case where active material particles with a small diameter (e.g., 1 μm or less) are used, the specific surface area of the active material particles is large and thus more conductive paths for the active material particles are needed. In such a case, it is particularly preferable to use a graphene compound that can efficiently form a conductive path even with a small amount.

It is particularly effective to use a graphene compound, which has the above-described properties, as a conductive material of a secondary battery that needs to be rapidly charged and discharged. For example, a secondary battery for a two- or four-wheeled vehicle, a secondary battery for a drone, or the like is required to have rapid charge and rapid discharge characteristics in some cases. In addition, a mobile electronic device or the like is required to have rapid charge characteristics in some cases. Rapid charging and discharging may also be referred to as charging and discharging at a high rate. For example, rapid charging and discharging is referred to as charging and discharging at 1 C, 2 C, or 5 C or more.

The longitudinal cross section of the active material layer 200 in FIG. 15B shows substantially uniform dispersion of the sheet-like graphene or the graphene compound 201 in the active material layer 200. The graphene or the graphene compound 201 is schematically shown by the thick line in FIG. 15B but is actually a thin film having a thickness corresponding to the thickness of a single layer or a multi-layer of carbon molecules. A plurality of sheets of graphene or the plurality of graphene compounds 201 are formed to partly cover or adhere to the surfaces of the plurality of particles of the positive electrode active material 100, so that the plurality of sheets of graphene or the plurality of graphene compounds 201 make surface contact with the particles of the positive electrode active material 100.

Here, the plurality of sheets of graphene or the plurality of graphene compounds can be bonded to each other to form a net-like graphene compound sheet (hereinafter, referred to as a graphene compound net or a graphene net). A graphene net that covers the active material can function as a binder for bonding the active material particles. Accordingly, the amount of the binder can be reduced, or the binder does not have to be used. This can increase the proportion of the active material in the electrode volume and the electrode weight. That is to say, the charge and discharge capacity of the secondary battery can be increased.

Here, it is preferable to perform reduction after a layer to be the active material layer 200 is formed in such a manner that graphene oxide is used as the graphene or the graphene compound 201 and mixed with an active material. That is, the formed active material layer preferably contains reduced graphene oxide. When graphene oxide with extremely high dispersibility in a polar solvent is used for the formation of the graphene or the graphene compound 201, the graphene or the graphene compound 201 can be substantially uniformly dispersed in the active material layer 200. The solvent is removed by volatilization from a dispersion medium in which graphene oxide is uniformly dispersed, and the graphene oxide is reduced: hence, the sheets of graphene or the graphene compounds 201 remaining in the active material layer 200 partly overlap with each other and are dispersed such that surface contact is made, thereby forming a three-dimensional conductive path. Note that graphene oxide can be reduced by heat treatment or with the use of a reducing agent, for example.

Unlike a conductive material in the form of particles, such as acetylene black, which makes point contact with an active material, the graphene or the graphene compound 201 is capable of making low-resistance surface contact: accordingly, the electrical conduction between the particles of the positive electrode active material 100 and the graphene or the graphene compound 201 can be improved with a small amount of the graphene and the graphene compound 201 compared with a normal conductive material. This can increase the proportion of the positive electrode active material 100 in the active material layer 200. Thus, discharge capacity of the secondary battery can be increased.

It is possible to form, with a spray dry apparatus, a graphene compound serving as a conductive material as a coating film to cover the entire surface of the active material in advance and to form a conductive path between the active materials using the graphene compound.

A material used in formation of the graphene compound may be mixed with the graphene compound to be used for the active material layer 200. For example, particles used as a catalyst in formation of the graphene compound may be mixed with the graphene compound. As an example of the catalyst in formation of the graphene compound, particles containing any of silicon oxide (SiO₂ or SiO_x (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, and the like can be given. The median diameter (D50) of the particles is preferably less than or equal to 1 μm, further preferably less than or equal to 100 nm.

[Binder]

As the binder, a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer is preferably used, for example. Fluororubber can also be used as the binder.

As the binder, for example, water-soluble polymers are preferably used. As the water-soluble polymers, a polysaccharide can be used, for example. As the polysaccharide, starch, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or the like can be used. It is further preferable that such a water-soluble polymer be used in combination with any of the above rubber materials.

Alternatively, as the binder, a material such as polystyrene, poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinyl acetate, or nitrocellulose is preferably used.

Two or more of the above materials may be used in combination for the binder.

[Positive Electrode Current Collector]

The positive electrode current collector can be formed using a material that has high conductivity, such as a metal like stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that a material used for the positive electrode current collector not be eluted at the potential of the positive electrode. Alternatively, it is possible to use an aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added. A metal element that forms silicide by reacting with silicon may be used. Examples of the metal element that forms silicide by reacting with silicon include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The positive electrode current collector can have a foil-like shape, a plate-like shape, a sheet-like shape, a net-like shape, a punching-metal shape, an expanded-metal shape, or the like as appropriate. The positive electrode current collector preferably has a thickness greater than or equal to 5 μm and less than or equal to 30 μm.

[Negative Electrode]

The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may include a conductive material and a binder.

[Negative Electrode Active Material]

As the negative electrode active material, an alloy-based material and/or a carbon-based material can be used, for example.

For the negative electrode active material, an element that enables charge and discharge reactions by an alloying reaction and a dealloying reaction with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, and the like can be used. Such elements have higher charge and discharge capacity than carbon: in particular, silicon has a high theoretical capacity of 4200 mAh/g. For this reason, silicon is preferably used as the negative electrode active material. Alternatively, a compound containing any of the above elements may be used. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂, Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Here, an element that enables charge and discharge reactions by an alloying reaction and a dealloying reaction with lithium, a compound containing the element, and the like may be referred to as an alloy-based material.

In this specification and the like, SiO refers, for example, to silicon monoxide. Alternatively, SiO can be expressed as SiO_x. Here, x is preferably an approximate value of 1. For example, x is preferably greater than or equal to 0.2 and less than or equal to 1.5, further preferably greater than or equal to 0.3 and less than or equal to 1.2. Alternatively, x is preferably greater than or equal to 0.2 and less than or equal to 1.2. Still alternatively, x is preferably greater than or equal to 0.3 and less than or equal to 1.5.

As the carbon-based material, graphite, graphitizing carbon (soft carbon), non-graphitizing carbon (hard carbon), carbon nanotube, graphene, carbon black, or the like is used.

Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. As artificial graphite, spherical graphite having a spherical shape can be used. For example, MCMB is preferably used because it may have a spherical shape. Moreover, MCMB may preferably be used because it can relatively easily have a small surface area. Examples of natural graphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithium metal (higher than or equal to 0.05 V and lower than or equal to 0.3 V vs. Li/Lit) when lithium ions are inserted into graphite (while a lithium-graphite intercalation compound is formed). For this reason, a lithium-ion secondary battery can have a high operating voltage. In addition, graphite is preferred because of its advantages such as a relatively high charge and discharge capacity per unit volume, relatively small volume expansion, low cost, and a higher level of safety than that of a lithium metal.

As the negative electrode active material, an oxide such as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂), a lithium-graphite intercalation compound (Li_xC₆), niobium pentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Alternatively, as the negative electrode active material, Li_3-xM_xN (M=Co, Ni, or Cu) with a Li₃N structure, which is a composite nitride of lithium and a transition metal, can be used. For example, Li_2.6Co_0.4N₃ is preferable because of its high charge and discharge capacity (900 mAh/g and 1890 mAh/cm³).

A composite nitride of lithium and a transition metal is preferably used, in which case lithium ions are contained in the negative electrode active material and thus the negative electrode active material can be used in combination with a material for a positive electrode active material that does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Note that even in the case of using a material containing lithium ions as a positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by extracting the lithium ions contained in the positive electrode active material in advance.

Alternatively, a material that causes a conversion reaction can be used for the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may be used as the negative electrode active material. Other examples of the material that causes a conversion reaction include oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides such as CoS_0.89, NiS, and CuS, nitrides such as Zn₃N₂, CusN, and Ge₃N₄, phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ and BiF₃.

For the conductive material and the binder that can be included in the negative electrode active material layer, materials similar to those for the conductive material and the binder that can be included in the positive electrode active material layer can be used.

[Negative Electrode Current Collector]

For the negative electrode current collector, a material similar to that for the positive electrode current collector can be used. Note that a material that is not alloyed with carrier ions of lithium or the like is preferably used for the negative electrode current collector.

[Electrolyte Solution]

The electrolyte solution contains a solvent and an electrolyte. As the solvent of the electrolyte solution, an aprotic organic solvent is preferably used. For example, one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used, or two or more of these solvents can be used in an appropriate combination in an appropriate ratio.

Alternatively, the use of one or more kinds of ionic liquids (room temperature molten salts) which have features of non-flammability and non-volatility as the solvent of the electrolyte solution can prevent a secondary battery from exploding, catching fire, and the like even when the secondary battery internally shorts out and/or the internal temperature increases owing to overcharging or the like. An ionic liquid contains a cation and an anion, specifically, an organic cation and an anion. Examples of the organic cation used for the electrolyte solution include aliphatic onium cations such as a quaternary ammonium cation, a tertiary sulfonium cation, and a quaternary phosphonium cation, and aromatic cations such as an imidazolium cation and a pyridinium cation. Examples of the anion used for the electrolyte solution include a monovalent amide-based anion, a monovalent methide-based anion, a fluorosulfonate anion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, a perfluoroalkylborate anion, a hexafluorophosphate anion, and a perfluoroalkylphosphate anion.

As the electrolyte dissolved in the above-described solvent, one of lithium salts such as LiPF₆, LiClO₄, LiASF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiCAF₉SO₃, LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LIN (C₄F₉SO₂)(CF₃SO₂), and LiN(C₂F₅SO₂)₂ can be used, or two or more of these lithium salts can be used in an appropriate combination at an appropriate ratio.

The electrolyte solution used for a secondary battery is preferably highly purified and contains a small number of dust particles and elements other than the constituent elements of the electrolyte solution (hereinafter, also simply referred to as “impurities”). Specifically, the weight ratio of impurities to the electrolyte solution is preferably less than or equal to 1%, further preferably less than or equal to 0.1%, still further preferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate) borate (LiBOB), or a dinitrile compound such as succinonitrile or adiponitrile may be added to the electrolyte solution. The concentration of the material to be added in the whole solvent is, for example, higher than or equal to 0.1 wt % and lower than or equal to 5 wt %. VC or LiBOB is particularly preferable because it facilitates formation of a favorable coating film.

A polymer gel electrolyte in which a polymer is swelled with an electrolyte solution may be used.

When a polymer gel electrolyte is used, safety against liquid leakage and the like is improved. Moreover, the secondary battery can be thinner and more lightweight.

As a polymer that undergoes gelation, a silicone gel, an acrylic gel, an acrylonitrile gel, a polyethylene oxide-based gel, a polypropylene oxide-based gel, a fluorine-based polymer gel, or the like can be used.

Examples of the polymer include a polymer having a polyalkylene oxide structure, such as polyethylene oxide (PEO): PVDF: polyacrylonitrile; and a copolymer containing any of them.

For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The formed polymer may be porous.

Instead of the electrolyte solution, a solid electrolyte including an inorganic material such as a sulfide-based inorganic material and an oxide-based inorganic material, and a solid electrolyte including a polymer material such as a PEO (polyethylene oxide)-based polymer material may alternatively be used. When the solid electrolyte is used, provision of a separator and a spacer are not necessary. Furthermore, the battery can be entirely solidified: thus, there is no possibility of liquid leakage and thus the safety of the battery is dramatically improved.

[Separator]

The secondary battery preferably includes a separator. The separator can be formed using, for example, paper, nonwoven fabric, glass fiber, ceramics, or synthetic fiber containing nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane. The separator is preferably formed to have an envelope-like shape to wrap one of the positive electrode and the negative electrode.

The separator may have a multilayer structure. For example, an organic material film of polypropylene, polyethylene, or the like can be coated with a ceramic-based material, a fluorine-based material, a polyamide-based material, a mixture thereof, or the like. Examples of the ceramic-based material include aluminum oxide particles and silicon oxide particles. Examples of the fluorine-based material include PVDF and polytetrafluoroethylene. Examples of the polyamide-based material include nylon and aramid (meta-based aramid and para-based aramid).

When the separator is coated with the ceramic-based material, the oxidation resistance is improved: hence, degradation of the separator during high-voltage charging and discharging can be inhibited and thus the reliability of the secondary battery can be improved. When the separator is coated with the fluorine-based material, the separator is easily brought into close contact with an electrode, resulting in high output characteristics. When the separator is coated with the polyamide-based material, in particular, aramid, the heat resistance is improved: thus, the safety of the secondary battery can be improved.

For example, both surfaces of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid. Alternatively, a surface of a polypropylene film that is to be in contact with the positive electrode may be coated with the mixed material of aluminum oxide and aramid, and a surface of the polypropylene film that is to be in contact with the negative electrode may be coated with the fluorine-based material.

With the use of a separator having a multilayer structure, the charge and discharge capacity per volume of the secondary battery can be increased because the safety of the secondary battery can be maintained even when the total thickness of the separator is small.

[Exterior Body]

For an exterior body included in the secondary battery, a metal material such as aluminum and/or a resin material can be used, for example. A film-like exterior body can also be used. As the film, for example, it is possible to use a film having a three-layer structure in which a highly flexible metal thin film of aluminum, stainless steel, copper, nickel, or the like is provided over a film formed of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film of a polyamide-based resin, a polyester-based resin, or the like is provided over the metal thin film as the outer surface of the exterior body.

Embodiment 3

In this embodiment, examples of the shape of a secondary battery including the positive electrode described in the above embodiment are described. For the materials used for the secondary battery described in this embodiment, the description of the above embodiment can be referred to.

<Coin-Type Secondary Battery>

First, an example of a coin-type secondary battery is described. FIG. 16A is an external view of a coin-type (single-layer flat type) secondary battery, and FIG. 16B is a cross-sectional view thereof.

In a coin-type secondary battery 300, a positive electrode can 301 doubling as a positive electrode terminal and a negative electrode can 302 doubling as a negative electrode terminal are insulated from each other and sealed by a gasket 303 made of polypropylene or the like. A positive electrode 304 includes a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector 305. A negative electrode 307 includes a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector 308.

Note that only one surface of each of the positive electrode 304 and the negative electrode 307 used for the coin-type secondary battery 300 is provided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, a metal having corrosion resistance to an electrolyte solution, such as nickel, aluminum, or titanium, an alloy of such a metal, or an alloy of such a metal and another metal (e.g., stainless steel) can be used. Alternatively, the positive electrode can 301 and the negative electrode can 302 are preferably covered with nickel, aluminum, or the like in order to prevent corrosion due to the electrolyte solution. The positive electrode can 301 and the negative electrode can 302 are electrically connected to the positive electrode 304 and the negative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and a separator 310 are soaked in the electrolyte. Then, as shown in FIG. 16B, the positive electrode 304, the separator 310, the negative electrode 307, and the negative electrode can 302 are stacked in this order with the positive electrode can 301 positioned at the bottom, and the positive electrode can 301 and the negative electrode can 302 are subjected to pressure bonding with the gasket 303 located therebetween. In such a manner, the coin-type secondary battery 300 is manufactured. When the positive electrode active material described in the above embodiment is used in the positive electrode 304, the coin-type secondary battery 300 with high charge and discharge capacity and excellent cycle performance can be obtained.

Here, a current flow in charging a secondary battery is described with reference to FIG. 16C. When a secondary battery using lithium is regarded as one closed circuit, movement of lithium ions and the current flow are in the same direction. Note that in the secondary battery using lithium, the anode and the cathode interchange in charging and discharging, and the oxidation reaction and the reduction reaction interchange: hence, an electrode with a high reaction potential is called a positive electrode and an electrode with a low reaction potential is called a negative electrode. For this reason, in this specification, the positive electrode is referred to as a “positive electrode” or a “plus electrode” and the negative electrode is referred to as a “negative electrode” or a “minus electrode” in all the cases where charging is performed, discharging is performed, a reverse pulse current is supplied, and a charge current is supplied. The use of the terms “anode” and “cathode” related to an oxidation reaction and a reduction reaction might cause confusion because the anode and the cathode interchange in charging and discharging. Thus, the terms “anode” and “cathode” are not used in this specification. If the term “anode” or “cathode” is used, it should be mentioned that the anode or the cathode is which of the one at the time of charging or the one at the time of discharging and corresponds to which of a positive (plus) electrode or a negative (minus) electrode.

Two terminals shown in FIG. 16C are connected to a charger, and the secondary battery 300 is charged. As charging of the secondary battery 300 proceeds, a potential difference between the electrodes increases.

<Cylindrical Secondary Battery>

Next, an example of a cylindrical secondary battery is described with reference to FIG. 17. FIG. 17A shows an external view of a cylindrical secondary battery 600. FIG. 17B is a schematic cross-sectional view of the cylindrical secondary battery 600. The cylindrical secondary battery 600 includes, as shown in FIG. 17B, a positive electrode cap (battery lid) 601 on a top surface and a battery can (outer can) 602 on a side surface and a bottom surface. The positive electrode cap 601 and the battery can (outer can) 602 are insulated from each other by a gasket (insulating gasket) 610.

Inside the battery can 602 having a hollow cylindrical shape, a battery element in which a strip-like positive electrode 604 and a strip-like negative electrode 606 are wound with a separator 605 located therebetween is provided. Although not shown, the battery element is wound around a center pin. One end of the battery can 602 is close and the other end thereof is open. For the battery can 602, a metal having corrosion resistance to an electrolyte solution, such as nickel, aluminum, or titanium, an alloy of such a metal, or an alloy of such a metal and another metal (e.g., stainless steel) can be used. The battery can 602 is preferably covered with nickel, aluminum, or the like in order to prevent corrosion due to the electrolyte solution. Inside the battery can 602, the battery element in which the positive electrode, the negative electrode, and the separator are wound is provided between a pair of insulating plates 608 and 609 that face each other. Furthermore, a nonaqueous electrolyte solution (not shown) is injected inside the battery can 602 provided with the battery element. As the nonaqueous electrolyte solution, a nonaqueous electrolyte solution that is similar to that of the coin-type secondary battery can be used.

Since a positive electrode and a negative electrode that are used for a cylindrical storage battery are wound, active materials are preferably formed on both surfaces of a current collector. A positive electrode terminal (positive electrode current collecting lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collecting lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be formed using a metal material such as aluminum. The positive electrode terminal 603 and the negative electrode terminal 607 are resistance-welded to a safety valve mechanism 612 and the bottom of the battery can 602, respectively. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 through a PTC element (Positive Temperature Coefficient) 611. The safety valve mechanism 612 cuts off electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold value. The PTC element 30) 611, which serves as a thermally sensitive resistor whose resistance increases as temperature rises, limits the amount of current by increasing the resistance, in order to prevent abnormal heat generation. Barium titanate (BaTiO₃)-based semiconductor ceramics or the like can be used for the PTC element.

Furthermore, as shown in FIG. 17C, a plurality of secondary batteries 600 may be provided between a conductive plate 613 and a conductive plate 614 to form a module 615. The plurality of secondary batteries 600 may be connected in parallel, connected in series, or connected in series after being connected in parallel. With the module 615 including the plurality of secondary batteries 600, large electric power can be extracted.

FIG. 17D is a top view of the module 615. The conductive plate 613 is shown by a dotted line for clarity of the diagram. As shown in FIG. 17D, the module 615 may include a wiring 616 electrically connecting the plurality of secondary batteries 600 with each other. The conductive plate can be provided to overlap with the wiring 616. In addition, a temperature control device 617 may be provided between the plurality of secondary batteries 600. The secondary batteries 600 can be cooled with the temperature control device 617 when overheated, whereas the secondary batteries 600 can be heated with the temperature control device 617 when cooled too much. Thus, the performance of the module 615 is unlikely to be affected by the outside temperature. A heating medium included in the temperature control device 617 preferably has an insulating property and incombustibility.

When the positive electrode active material described in the above embodiment is used in the positive electrode 604, the cylindrical secondary battery 600 with high charge and discharge capacity and excellent cycle performance can be obtained.

Structure Examples of Secondary Battery

Other structure examples of secondary batteries are described with reference to FIG. 18 to FIG. 19.

FIG. 18A and FIG. 18B are external views of a battery pack. The battery pack includes a secondary battery 913 and a circuit board 900. The secondary battery 913 is connected to an antenna 914 through the circuit board 900. A label 910 is attached to the secondary battery 913. In addition, as shown in FIG. 18B, the secondary battery 913 is connected to a terminal 951 and a terminal 952. The circuit board 900 is fixed with a seal 915.

The circuit board 900 includes a terminal 911 and a circuit 912. The terminal 911 is connected to the terminal 951, the terminal 952, the antenna 914, and the circuit 912. Note that a plurality of terminals 911 may be provided to serve as a control signal input terminal, a power supply terminal, and the like.

The circuit 912 may be provided on the rear surface of the circuit board 900. Note that the shape of the antenna 914 is not limited to coil shapes, and may be a linear shape or a plate shape, for example. An antenna such as a planar antenna, an aperture antenna, a traveling-wave antenna, an EH antenna, a magnetic-field antenna, or a dielectric antenna may be used. Alternatively, the antenna 914 may be a flat-plate conductor. The flat-plate conductor can serve as one of conductors for electric field coupling. That is, the antenna 914 may serve as one of two conductors of a capacitor. Thus, electric power can be transmitted and received not only by an electromagnetic field or a magnetic field but also by an electric field.

The battery pack includes a layer 916 between the antenna 914 and the secondary battery 913. The layer 916 has a function of blocking an electromagnetic field by the secondary battery 913, for example. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the battery pack is not limited to that in FIG. 18.

For example, as shown in FIG. 19A and FIG. 19B, two opposite surfaces of the secondary battery 913 shown in FIG. 18A and FIG. 18B may be provided with respective antennas. FIG. 19A is an external view seen from one side of the opposite surfaces, and FIG. 19A is an external view seen from the other side of the opposite surfaces. Note that for portions similar to those of the secondary battery shown in FIG. 18A and FIG. 18B, the description of the secondary battery shown in FIG. 18A and FIG. 18B can be appropriately referred to.

As shown in FIG. 19A, the antenna 914 is provided on one of the opposite surfaces of the secondary battery 913 with the layer 916 located therebetween, and as shown in FIG. 19B, an antenna 918 is provided on the other of the opposite surfaces of the secondary battery 913 with a layer 917 located therebetween. The layer 917 has a function of blocking an electromagnetic field by the secondary battery 913, for example. As the layer 917, for example, a magnetic body can be used.

With the above structure, both of the antenna 914 and the antenna 918 can be increased in size. The antenna 918 has a function of communicating data with an external device, for example. An antenna with a shape that can be used for the antenna 914, for example, can be used as the antenna 918. As a system for communication using the antenna 918 between the secondary battery and another device, a response method that can be used between the secondary battery and another device, such as NFC (near field communication), can be employed.

Alternatively, as shown in FIG. 19C, the secondary battery 913 shown in FIG. 18A and FIG. 18B may be provided with a display device 920. The display device 920 is electrically connected to the terminal 911. Note that the label 910 is not necessarily provided in a portion where the display device 920 is provided. Note that for portions similar to those of the secondary battery shown in FIG. 18A and FIG. 18B, the description of the secondary battery shown in FIG. 18A and FIG. 18B can be appropriately referred to.

The display device 920 may display, for example, an image showing whether charging is being carried out, an image showing the amount of stored power, or the like. As the display device 920, electronic paper, a liquid crystal display device, an electroluminescent (EL) display device, or the like can be used. For example, the use of electronic paper can reduce power consumption of the display device 920.

Alternatively, as shown in FIG. 19D, the secondary battery 913 shown in FIG. 18A and FIG. 18B may be provided with a sensor 921. The sensor 921 is electrically connected to the terminal 911 via a terminal 922. Note that for portions similar to those of the secondary battery shown in FIG. 18A and FIG. 18B, the description of the secondary battery shown in FIG. 18A and FIG. 18B can be appropriately referred to.

The sensor 921 has a function of measuring, for example, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays. With the sensor 921, for example, data on an environment (e.g., temperature) where the secondary battery is placed can be detected and stored in a memory inside the circuit 912.

Embodiment 4

In this embodiment, examples of electronic devices each including the secondary battery of one embodiment of the present invention are described.

First, FIG. 20A to FIG. 20G show examples of electronic devices including the bendable secondary battery described in the above embodiment. Examples of electronic devices each including a bendable secondary battery include television sets (also referred to as televisions or television receivers), monitors of computers or the like, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as cellular phones or mobile phone devices), portable game machines, portable information terminals, audio reproducing devices, and large game machines such as pachinko machines.

Furthermore, a flexible secondary battery can be incorporated along a curved inside or outside wall surface of a house or a building: it can also be incorporated along a curved interior or exterior surface of an automobile.

FIG. 20A shows an example of a mobile phone. A mobile phone 7400 is provided with a display portion 7402 incorporated in a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes a secondary battery 7407. When the secondary battery of one embodiment of the present invention is used as the secondary battery 7407, a lightweight mobile phone with a long lifetime can be provided.

FIG. 20B shows the mobile phone 7400 that is curved. When the whole mobile phone 7400 is curved by external force, the secondary battery 7407 provided therein is also curved. FIG. 20C shows the bent secondary battery 7407. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a state of being bent. Note that the secondary battery 7407 includes a lead electrode electrically connected to a current collector. The current collector is, for example, copper foil, and partly alloyed with gallium: thus, adhesion between the current collector and an active material layer in contact with the current collector is improved, and the secondary battery 7407 can have high reliability even in a state of being bent.

FIG. 20D shows an example of a bangle display device. A portable display device 7100 includes a housing 7101, a display portion 7102, operation buttons 7103, and a secondary battery 7104. FIG. 20E shows the bent secondary battery 7104. When the display device is worn on a user's arm while the secondary battery 7104 is bent, the housing changes its shape and the curvature of part or the whole of the secondary battery 7104 is changed. Note that the bending condition of a curve at a given point that is represented by a value of the radius of a corresponding circle is referred to as the radius of curvature, and the reciprocal of the radius of curvature is referred to as curvature. Specifically, part or the whole of the housing or the main surface of the secondary battery 7104 is changed in the range of radius of curvature from 40 mm or more to 150 mm or less. When the radius of curvature at the main surface of the secondary battery 7104 is in the range from 40 mm or more to 150 mm or less, the reliability can be kept high. When the secondary battery of one embodiment of the present invention is used as the secondary battery 7104, a lightweight portable display device with a long lifetime can be provided.

FIG. 20F shows an example of a watch-type portable information terminal. A portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input/output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and a computer game.

The display surface of the display portion 7202 is curved, and images can be displayed on the curved display surface. In addition, the display portion 7202 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, by touching an icon 7207 displayed on the display portion 7202, application can be started.

With the operation button 7205, a variety of functions such as time setting, power on/off, on/off of wireless communication, setting and cancellation of a silent mode, and setting and cancellation of a power saving mode can be performed. For example, the functions of the operation button 7205 can be set freely by setting the operating system incorporated in the portable information terminal 7200.

The portable information terminal 7200 can perform near field communication that is standardized communication. For example, mutual communication between the portable information terminal 7200 and a headset capable of wireless communication enables hands-free calling.

The portable information terminal 7200 includes the input/output terminal 7206, and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging via the input/output terminal 7206 is possible. Note that the charging operation may be performed by wireless power feeding without using the input/output terminal 7206.

The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. When the secondary battery of one embodiment of the present invention is used, a lightweight portable information terminal with a long lifetime can be provided. For example, the secondary battery 7104 shown in FIG. 20E can be provided in the housing 7201 while being curved, or can be provided in the band 7203 such that it can be curved.

The portable information terminal 7200 preferably includes a sensor. As the sensor, for example, a human body sensor such as a fingerprint sensor, a pulse sensor, or a temperature sensor, a touch sensor, a pressure sensitive sensor, or an acceleration sensor is preferably mounted.

FIG. 20G shows an example of an armband display device. A display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can include a touch sensor in the display portion 7304 and can also serve as a portable information terminal.

The display surface of the display portion 7304 is curved, and images can be displayed on the curved display surface. A display state of the display device 7300 can be changed by, for example, near field communication that is standardized communication.

The display device 7300 includes an input/output terminal, and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging via the input/output terminal is possible. Note that the charging operation may be performed by wireless power feeding without using the input/output terminal.

When the secondary battery of one embodiment of the present invention is used as the secondary battery included in the display device 7300, a lightweight display device with a long lifetime can be provided.

Examples of electronic devices each including the secondary battery with excellent cycle performance described in the above embodiment are described with reference to FIG. 20H, FIG. 21, and FIG. 22.

When the secondary battery of one embodiment of the present invention is used as a secondary battery of a daily electronic device, a lightweight product with a long lifetime can be provided. Examples of the daily electronic device include an electric toothbrush, an electric shaver, and electric beauty equipment. As secondary batteries of these products, small and lightweight stick type secondary batteries with high charge and discharge capacity are desired in consideration of handling ease for users.

FIG. 20H is a perspective view of a device called a cigarette smoking device (electronic cigarette). In FIG. 20H, an electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 including a liquid supply bottle, a sensor, and the like. To improve safety, a protection circuit that prevents overcharging and overdischarging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 20H includes an external terminal for connection to a charger. When the electronic cigarette 7500 is held, the secondary battery 7504 is a tip portion: thus, it is preferable that the secondary battery 7504 have a short total length and be lightweight. With the secondary battery of one embodiment of the present invention, which has high charge and discharge capacity and excellent cycle performance, the small and lightweight electronic cigarette 7500 that can be used for a long time over a long period can be provided.

Next, FIG. 21A and FIG. 21B show an example of a tablet terminal that can be folded in half. A tablet terminal 9600 shown in FIG. 21A and FIG. 21B includes a housing 9630a, a housing 9630b, a movable portion 9640 connecting the housing 9630a and the housing 9630b to each other, a display portion 9631 including a display portion 9631a and a display portion 9631b, a switch 9625 to a switch 9627, a fastener 9629, and an operation switch 9628. When a flexible panel is used for the display portion 9631, a tablet terminal with a larger display portion can be provided. FIG. 21A shows the tablet terminal 9600 that is opened, and FIG. 21B shows the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside the housing 9630a and the housing 9630b. The power storage unit 9635 is provided across the housing 9630a and the housing 9630b, passing through the movable portion 9640.

The entire region or part of the region of the display portion 9631 can be a touch panel region, and data can be input by touching text, an input form, an image including an icon, and the like displayed on the region. For example, it is possible that keyboard buttons are displayed on the entire display portion 9631a on the housing 9630a side, and data such as text or an image is displayed on the display portion 9631b on the housing 9630b side.

It is possible that a keyboard is displayed on the display portion 9631b on the housing 9630b side, and data such as text or an image is displayed on the display portion 9631a on the housing 9630a side. Furthermore, it is possible that a switching button for showing/hiding a keyboard on a touch panel is displayed on the display portion 9631 and the button is touched with a finger, a stylus, and the like to display a keyboard on the display portion 9631.

Touch input can be performed concurrently in a touch panel region in the display portion 9631a on the housing 9630a side and a touch panel region in the display portion 9631b on the housing 9630b side.

The switch 9625 to the switch 9627 may function not only as an interface for operating the tablet terminal 9600 but also as an interface that can switch various functions. For example, at least one of the switch 9625 to the switch 9627 may function as a switch for switching power on/off of the tablet terminal 9600. For another example, at least one of the switch 9625 to the switch 9627 may have a function of switching the display orientation between a portrait mode and a landscape mode and a function of switching display between monochrome display and color display. For another example, at least one of the switch 9625 to the switch 9627 may have a function of adjusting the luminance of the display portion 9631. The luminance of the display portion 9631 can be optimized in accordance with the amount of external light in use of the tablet terminal 9600 detected by an optical sensor incorporated in the tablet terminal 9600. Note that another sensing device including a sensor for measuring inclination, such as a gyroscope sensor or an acceleration sensor, may be incorporated in the tablet terminal, in addition to the optical sensor.

FIG. 21A shows an example in which the display portion 9631a on the housing 9630a side and the display portion 9631b on the housing 9630b side have substantially the same display area: however, there is no particular limitation on the display areas of the display portion 9631a and the display portion 9631b, and the display portions may have different sizes or different display quality. For example, one may be a display panel that can display higher-definition images than the other.

The tablet terminal 9600 is folded in half in FIG. 21B. The tablet terminal 9600 includes a housing 9630, a solar cell 9633, and a charge and discharge control circuit 9634 including a DCDC converter 9636. A power storage unit of one embodiment of the present invention is used as the power storage unit 9635.

Note that as described above, the tablet terminal 9600 can be folded in half, and thus can be folded when not in use such that the housing 9630a and the housing 9630b overlap with each other. By the folding, the display portion 9631 can be protected, which increases the durability of the tablet terminal 9600. With the power storage unit 9635 including the secondary battery of one embodiment of the present invention, which has high charge and discharge capacity and excellent cycle performance, the tablet terminal 9600 that can be used for a long time over a long period can be provided.

The tablet terminal 9600 shown in FIG. 21A and FIG. 21B can also have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, or the time on the display portion, a touch-input function of operating or editing data displayed on the display portion by touch input, a function of controlling processing by various kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tablet terminal 9600, can supply electric power to a touch panel, a display portion, a video signal processing portion, and the like. Note that the solar cell 9633 can be provided on one surface or both surfaces of the housing 9630 and the power storage unit 9635 can be charged efficiently. The use of a lithium-ion battery as the power storage unit 9635 brings an advantage such as a reduction in size.

The structure and operation of the charge and discharge control circuit 9634 shown in FIG. 21B are described with reference to a block diagram in FIG. 21C. The solar cell 9633, the power storage unit 9635, the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and the display portion 9631 are shown in FIG. 21C, and the power storage unit 9635, the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 shown in FIG. 21B.

First, an operation example in which electric power is generated by the solar cell 9633 using external light is described. The voltage of electric power generated by the solar cell is raised or lowered by the DCDC converter 9636 to a voltage for charging the power storage unit 9635. When the display portion 9631 is operated with the electric power from the solar cell 9633, the switch SW1 is turned on and the voltage is raised or lowered by the converter 9637 to a voltage needed for the display portion 9631. When display on the display portion 9631 is not performed, the switch SW1 is turned off and the switch SW2 is turned on, so that the power storage unit 9635 is charged.

Note that the solar cell 9633 is described as an example of a power generation unit: however, one embodiment of the present invention is not limited to this example. The power storage unit 9635 may be charged using another power generation unit such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, charging may be performed with a non-contact power transmission module that performs charging by transmitting and receiving power wirelessly (without contact), and with a combination of other charge units.

FIG. 22 shows other examples of electronic devices. In FIG. 22, a display device 8000 is an example of an electronic device including a secondary battery 8004 of one embodiment of the present invention. Specifically, the display device 8000 corresponds to a display device for TV broadcast reception and includes a housing 8001, a display portion 8002, speaker portions 8003, the secondary battery 8004, and the like. The secondary battery 8004 of one embodiment of the present invention is provided in the housing 8001. The display device 8000 can be supplied with electric power from a commercial power supply and can use electric power stored in the secondary battery 8004. Thus, the display device 8000 can be operated with the use of the secondary battery 8004 of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like.

A semiconductor display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), or an FED (Field Emission Display) can be used for the display portion 8002.

Note that the display device includes, in its category, all of information display devices for personal computers, advertisement displays, and the like besides information display devices for TV broadcast reception.

In FIG. 22, an installation lighting device 8100 is an example of an electronic device including a secondary battery 8103 of one embodiment of the present invention. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, the secondary battery 8103, and the like. Although FIG. 22 shows the case where the secondary battery 8103 is provided in a ceiling 8104 on which the housing 8101 and the light source 8102 are installed, the secondary battery 8103 may be provided in the housing 8101. The lighting device 8100 can be supplied with electric power from a commercial power supply and can use electric power stored in the secondary battery 8103. Thus, the lighting device 8100 can be operated with the use of the secondary battery 8103 of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like.

Note that although the installation lighting device 8100 provided in the ceiling 8104 is shown in FIG. 22 as an example, the secondary battery of one embodiment of the present invention can be used in an installation lighting device provided in, for example, a side wall 8105, a floor 8106, or a window 8107 other than the ceiling 8104, and can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source that emits light artificially by using electric power can be used. Specifically, an incandescent lamp, a discharge lamp such as a fluorescent lamp, and light-emitting elements such as an LED and an organic EL element are given as examples of the artificial light source.

In FIG. 22, an air conditioner including an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device including a secondary battery 8203 of one embodiment of the present invention. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, the secondary battery 8203, and the like. Although FIG. 22 shows the case where the secondary battery 8203 is provided in the indoor unit 8200, the secondary battery 8203 may be provided in the outdoor unit 8204. Alternatively, the secondary batteries 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. The air conditioner can be supplied with electric power from a commercial power supply and can use electric power stored in the secondary battery 8203. Particularly in the case where the secondary batteries 8203 are provided in both the indoor unit 8200 and the outdoor unit 8204, the air conditioner can be operated with the use of the secondary battery 8203 of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like.

Note that although the split-type air conditioner including the indoor unit and the outdoor unit is shown in FIG. 22 as an example, the secondary battery of one embodiment of the present invention can be used in an air conditioner in which the function of an indoor unit and the function of an outdoor unit are integrated in one housing.

In FIG. 22, an electric refrigerator-freezer 8300 is an example of an electronic device including a secondary battery 8304 of one embodiment of the present invention. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, the secondary battery 8304, and the like. The secondary battery 8304 is provided in the housing 8301 in FIG. 22. The electric refrigerator-freezer 8300 can be supplied with electric power from a commercial power supply and can use electric power stored in the secondary battery 8304. Thus, the electric refrigerator-freezer 8300 can be operated with the use of the secondary battery 8304 of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like.

Note that among the electronic devices described above, a high-frequency heating apparatus such as a microwave oven and an electronic device such as an electric rice cooker require high electric power in a short time. Thus, the tripping of a breaker of a commercial power supply in use of the electronic device can be prevented by using the secondary battery of one embodiment of the present invention as an auxiliary power supply for supplying electric power which cannot be supplied enough by a commercial power supply.

In a time period when electronic devices are not used, particularly when the proportion of the amount of electric power which is actually used to the total amount of electric power which can be supplied from a commercial power supply source (such a proportion is referred to as a usage rate of electric power) is low, electric power is stored in the secondary battery, whereby an increase in the usage rate of electric power can be inhibited in a time period other than the above time period. For example, in the case of the electric refrigerator-freezer 8300, electric power is stored in the secondary battery 8304 in nighttime when the temperature is low and the refrigerator door 8302 and the freezer door 8303 are not opened or closed. Moreover, in daytime when the temperature is high and the refrigerator door 8302 and the freezer door 8303 are opened and closed, the usage rate of electric power in daytime can be kept low by using the secondary battery 8304 as an auxiliary power supply.

According to one embodiment of the present invention, the secondary battery can have excellent cycle performance and improved reliability. Furthermore, according to one embodiment of the present invention, a secondary battery with high charge and discharge capacity can be obtained: thus, the secondary battery itself can be made more compact and lightweight as a result of improved characteristics of the secondary battery. Thus, the secondary battery of one embodiment of the present invention is used in the electronic device described in this embodiment, whereby a more lightweight electronic device with a longer lifetime can be obtained.

This embodiment can be implemented in appropriate combination with the other embodiments.

Embodiment 5

In this embodiment, examples of electronic devices each including the secondary battery described in the above embodiment are described with reference to FIG. 23A to FIG. 23C.

FIG. 23A shows examples of wearable devices. A secondary battery is used as a power supply of a wearable device. To have improved splash resistance, water resistance, or dust resistance in daily use or outdoor use by a user, a wearable device is desirably capable of being charged with and without a wire whose connector portion for connection is exposed.

For example, the secondary battery of one embodiment of the present invention can be provided in a glasses-type device 4000 shown in FIG. 23A. The glasses-type device 4000 includes a frame 4000a and a display portion 4000b. The secondary battery is provided in a temple portion of the frame 4000a having a curved shape, whereby the glasses-type device 4000 can be lightweight, can have a well-balanced weight, and can be used continuously for a long time. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.

The secondary battery of one embodiment of the present invention can be provided in a headset-type device 4001. The headset-type device 4001 includes at least a microphone portion 4001a, a flexible pipe 4001b, and an earphone portion 4001c. The secondary battery can be provided in the flexible pipe 4001b and/or the earphone portion 4001c. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.

The secondary battery of one embodiment of the present invention can be provided in a device 4002 that can be attached directly to a body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.

The secondary battery of one embodiment of the present invention can be provided in a device 4003 that can be attached to clothes. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.

The secondary battery of one embodiment of the present invention can be provided in a belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power feeding and receiving portion 4006b, and the secondary battery can be provided inside the belt portion 4006a. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.

The secondary battery of one embodiment of the present invention can be provided in a watch-type device 4005. The watch-type device 4005 includes a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided in the display portion 4005a or the belt portion 4005b. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.

The display portion 4005a can display various kinds of information such as time and reception information of an e-mail or an incoming call.

In addition, the watch-type device 4005 is a wearable device that is wound around an arm directly: thus, a sensor that measures the pulse, the blood pressure, or the like of the user may be incorporated therein. Data on the exercise quantity and health of the user can be stored to be used for health maintenance.

FIG. 23B is a perspective view of the watch-type device 4005 that is detached from an arm.

FIG. 23C is a side view. FIG. 23C shows a state where the secondary battery 913 is incorporated in the watch-type device 4005. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913, which is small and lightweight, is provided at a position overlapping with the display portion 4005a.

FIG. 23D shows an example of wireless earphones. The wireless earphones shown here consist of, but are not limited to, a pair of main bodies 4100a and 4100b.

Each of the main bodies 4100a and 4100b includes a driver unit 4101, an antenna 4102, and a secondary battery 4103. Each of the main bodies 4100a and 4100b may also include a display portion 4104. Moreover, each of the main bodies 4100a and 4100b preferably includes a substrate where a circuit such as a wireless IC is provided, a terminal for charging, and the like. Each of the main bodies 4100a and 4100b may also include a microphone.

A case 4110 includes a secondary battery 4111. Moreover, the case 4110 preferably includes a substrate where a circuit such as a wireless IC or a charge control IC is provided, and a terminal for charging. The case 4110 may also include a display portion, a button, and the like.

The main bodies 4100a and 4100b can communicate wirelessly with another electronic device such as a smartphone. Thus, sound data and the like transmitted from another electronic device can be played through the main bodies 4100a and 4100b. When the main bodies 4100a and 4100b include a microphone, sound captured by the microphone is transmitted to another electronic device, and sound data obtained by processing with the electronic device can be transmitted to and played through the main bodies 4100a and 4100b. Hence, the wireless earphones can be used as a translator, for example.

The secondary battery 4103 included in the main body 4100a can be charged by the secondary battery 4111 included in the case 4110. As the secondary battery 4111 and the secondary battery 4103, the coin-type secondary battery or the cylindrical secondary battery of the foregoing embodiment, for example, can be used. A secondary battery whose positive electrode includes the positive electrode active material 100 obtained in Embodiment 1 has a high energy density: thus, with the use of the secondary battery as the secondary battery 4103 and the secondary battery 4111, space saving required with downsizing of the wireless earphones can be achieved.

FIG. 24A shows an example of a cleaning robot. A cleaning robot 6300 includes a display portion 6302 placed on a top surface of a housing 6301, a plurality of cameras 6303 placed on a side surface of the housing 6301, a brush 6304, operation buttons 6305, a secondary battery 6306, a variety of sensors, and the like. Although not shown, the cleaning robot 6300 is provided with a tire, an inlet, and the like. The cleaning robot 6300 is self-propelled, detects dust 6310, and sucks up the dust through the inlet provided on the bottom surface.

For example, the cleaning robot 6300 can determine whether there is an obstacle such as a wall, furniture, or a step by analyzing images taken by the cameras 6303. In the case where the cleaning robot 6300 detects an object, such as a wire, that is likely to be caught in the brush 6304 by image analysis, the rotation of the brush 6304 can be stopped. The cleaning robot 6300 includes a secondary battery 6306 of one embodiment of the present invention and a semiconductor device or an electronic component. The cleaning robot 6300 including the secondary battery 6306 of one embodiment of the present invention can be a highly reliable electronic device that can operate for a long time.

FIG. 24B shows an example of a robot. A robot 6400 shown in FIG. 24B includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display portion 6405, a lower camera 6406, an obstacle sensor 6407, a moving mechanism 6408, an arithmetic device, and the like.

The microphone 6402 has a function of detecting a speaking voice of a user, an environmental sound, and the like. The speaker 6404 has a function of outputting sound. The robot 6400 can communicate with a user using the microphone 6402 and the speaker 6404.

The display portion 6405 has a function of displaying various kinds of information. The robot 6400 can display information desired by a user on the display portion 6405. The display portion 6405 may be provided with a touch panel. Moreover, the display portion 6405 may be a detachable information terminal, in which case charging and data communication can be performed when the display portion 6405 is set at the home position of the robot 6400.

The upper camera 6403 and the lower camera 6406 each have a function of taking an image of the surroundings of the robot 6400. The obstacle sensor 6407 can detect an obstacle in the direction where the robot 6400 advances with the moving mechanism 6408. The robot 6400 can move safely by recognizing the surroundings with the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

The robot 6400 includes the secondary battery 6409 of one embodiment of the present invention and a semiconductor device or an electronic component. The robot 6400 including the secondary battery of one embodiment of the present invention can be a highly reliable electronic device that can operate for a long time.

FIG. 24C shows an example of a flying object. A flying object 6500 shown in FIG. 24C includes propellers 6501, a camera 6502, a secondary battery 6503, and the like and has a function of flying autonomously.

For example, image data taken by the camera 6502 is stored in an electronic component 6504. The electronic component 6504 can analyze the image data to detect whether there is an obstacle in the way of the movement. Moreover, the electronic component 6504 can estimate the remaining battery level from a change in the power storage capacity of the secondary battery 6503.

The flying object 6500 includes the secondary battery 6503 of one embodiment of the present invention. The flying object 6500 including the secondary battery of one embodiment of the present invention can be a highly reliable electronic device that can operate for a long time. This embodiment can be implemented in appropriate combination with the other embodiments.

Embodiment 6

In this embodiment, examples of vehicles each including the secondary battery of one embodiment of the present invention will be described.

The use of secondary batteries in vehicles enables production of next-generation clean energy vehicles such as hybrid electric vehicles (HVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHVs).

FIG. 25 shows examples of a vehicle including the secondary battery of one embodiment of the present invention. An automobile 8400 shown in FIG. 25A is an electric vehicle that runs on the power of an electric motor. Alternatively, the automobile 8400 is a hybrid electric vehicle capable of driving using either an electric motor or an engine as appropriate. The use of one embodiment of the present invention achieves a high-mileage vehicle. The automobile 8400) includes a secondary battery. As the secondary battery, the modules of the secondary batteries shown in FIG. 17C and FIG. 17D may be arranged to be used in a floor portion in the automobile. The secondary battery can be used not only for driving an electric motor 8406, but also for supplying electric power to a light-emitting device such as a headlight 8401 and a room light (not shown).

The secondary battery can also supply electric power to a display device included in the automobile 8400, such as a speedometer or a tachometer. Furthermore, the secondary battery can supply electric power to a semiconductor device included in the automobile 8400, such as a navigation system.

An automobile 8500 shown in FIG. 25B can be charged when the secondary battery included in the automobile 8500 is supplied with electric power through external charge equipment by a plug-in system, a contactless power feeding system, and/or the like. FIG. 25B shows a state where a secondary battery 8024 included in the automobile 8500 is charged with the use of a ground-based charge apparatus 8021 through a cable 8022. Charging can be performed as appropriate by a given method such as CHAdeMO (registered trademark) and Combined Charging System as a charging method, the standard of a connector, or the like. The charge apparatus 8021 may be a charge station provided in a commerce facility or a power supply in a house. For example, with the use of a plug-in technique, the secondary battery 8024 included in the automobile 8500 can be charged by being supplied with electric power from outside. Charging can be performed by converting AC electric power into DC electric power through a converter such as an ACDC converter.

Although not shown, the vehicle may include a power receiving device so that it can be charged by being supplied with electric power from an above-ground power transmitting device in a contactless manner. In the case of the contactless power feeding system, by fitting a power transmitting device in a road and/or an exterior wall, charging can be performed not only when the vehicle is stopped but also when driven. In addition, the contactless power feeding system may be utilized to perform transmission and reception of electric power between vehicles. Furthermore, a solar cell may be provided in the exterior of the vehicle to charge the secondary battery when the vehicle stops and/or moves. To supply electric power in such a contactless manner, an electromagnetic induction method and/or a magnetic resonance method can be used.

FIG. 25C shows an example of a motorcycle including the secondary battery of one embodiment of the present invention. A motor scooter 8600 shown in FIG. 25C includes a secondary battery 8602, side mirrors 8601, and direction indicators 8603. The secondary battery 8602 can supply electric power to the direction indicators 8603.

In the motor scooter 8600 shown in FIG. 25C, the secondary battery 8602 can be held in an under-seat storage 8604. The secondary battery 8602 can be held in the under-seat storage 8604 even when the under-seat storage 8604 is small. The secondary battery 8602 is detachable: thus, the secondary battery 8602 is carried indoors when charged, and is stored before the motor scooter is driven.

According to one embodiment of the present invention, the secondary battery can have excellent cycle performance and the charge and discharge capacity of the secondary battery can be increased. Thus, the secondary battery itself can be made more compact and lightweight. The compact and lightweight secondary battery contributes to a reduction in the weight of a vehicle, and thus increases the mileage. Furthermore, the secondary battery included in the vehicle can be used as a power supply source for supplying electric power to products other than the vehicle. In such a case, the use of a commercial power supply can be avoided at peak time of electric power demand, for example. Avoiding the use of a commercial power supply at peak time of electric power demand can contribute to energy saving and a reduction in carbon dioxide emissions. Moreover, the secondary battery with excellent cycle performance can be used over a long period: thus, the use amount of rare metals typified by cobalt can be reduced.

This embodiment can be implemented in appropriate combination with the other embodiments.

REFERENCE NUMERALS

• • 100 positive electrode active material
  • 100 a: surface portion 100b: inner portion

Claims

1. A positive electrode active material particle comprising lithium, cobalt, oxygen, and an additive element, wherein the positive electrode active material particle comprises a surface portion and an inner portion,wherein the positive electrode active material particle comprises the additive element in the surface portion,wherein the surface portion is a region extending from a surface of the positive electrode active material particle to a depth of 10 nm or less toward the inner portion,wherein the surface portion and the inner portion are topotaxy,wherein a degree of diffusion of the additive element vary between each crystal planes of the surface portion, andwherein the additive element is at least one or two or more selected from nickel, aluminum, and magnesium.

2. The positive electrode active material particle according to claim 1, wherein the positive electrode active material particle comprises a crystal structure identified as a space group R-3m, andwherein, in the surface portion, the additive element exists deeper in a region not parallel to the arrangement of cations than in a region parallel to the arrangement of cations.

3. The positive electrode active material particle according to claim 1, wherein the number of atoms of the nickel is greater than or equal to 0.1% and less than or equal to 2% of the number of atoms of the cobalt, andwherein the number of atoms of the aluminum is greater than or equal to 0.1% and less than or equal to 2% of the number of atoms of the cobalt.

4. The positive electrode active material particle according to claim 3, wherein the positive electrode active material further comprises fluorine as the additive element.

5. A lithium-ion secondary battery comprising a positive electrode and a negative electrode, wherein the positive electrode comprises a positive electrode active material comprising a positive electrode active material particle,wherein the positive electrode active material particle comprises lithium, cobalt, oxygen, magnesium, and aluminum,wherein the positive electrode active material particle comprises a surface portion and an inner portion,wherein the positive electrode active material particle comprises magnesium an aluminum in the surface portion,wherein the surface portion is a region extending from a surface of the positive electrode active material particle to a depth of 10 nm or less toward the inner portion,wherein the surface portion and the inner portion are topotaxy,wherein a degree of diffusion of magnesium in a region not parallel to the arrangement of cations is different from a degree of diffusion of magnesium in a region parallel to the arrangement of cations, andwherein a degree of diffusion of aluminum in a region not parallel to the arrangement of cations is different from a degree of diffusion of aluminum in a region parallel to the arrangement of cations.

6. The lithium-ion secondary battery according to claim 5, wherein the positive electrode active material particle comprises a crystal structure identified as a space group R-3m,wherein, in the surface portion, magnesium exists deeper in the region not parallel to the arrangement of cations than in the region parallel to the arrangement of cations, andwherein, in the surface portion, aluminum exists deeper in the region not parallel to the arrangement of cations than in the region parallel to the arrangement of cations.

7. The lithium-ion secondary battery according to claim 5, wherein the positive electrode active material particle further comprises nickel in the surface portion,wherein the number of atoms of the nickel is greater than or equal to 0.1% and less than or equal to 2% of the number of atoms of the cobalt, andwherein the number of atoms of the aluminum is greater than or equal to 0.1% and less than or equal to 2% of the number of atoms of the cobalt.

8. The lithium-ion secondary battery according to claim 5, wherein the positive electrode active material particle further comprises fluorine.