Patent Application
20230402602
One embodiment of the present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a power storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a manufacturing method thereof.
Note that electronic devices in this specification mean 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.
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 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).
Lithium-ion secondary batteries are known to enter thermal runaway after passing through several states when the temperature rises (Non-Patent Document 1).
Lithium-ion secondary batteries can be used in next-generation clean energy vehicles typified by hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). The secondary batteries can be mounted not only on next-generation clean energy vehicles but also on transport vehicles such as agricultural machines, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, electric carts, boats and ships, submarines, aircraft typified by fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.
Lithium-ion secondary batteries used for a variety of applications as described above have been subjected to safety verification assuming a variety of situations. For example, nail penetration tests and the like have been conducted assuming internal short circuits in secondary batteries. Internal short circuits in secondary batteries might cause heat generation and battery ignition.
Lithium-ion secondary batteries have been required to have further improved thermal safety to stably supply power.
To improve thermal safety, fluorine is contained in a surface portion of a positive electrode active material or adsorbed on a surface of the positive electrode active material so that overheating or ignition of a lithium-ion secondary battery is inhibited.
The positive electrode active material contains a transition metal M and oxygen. The transition metal M contains nickel, manganese, and cobalt. Such a positive electrode active material is referred to as NCM. NCM has a layered rock-salt crystal structure. NCM can have an improved energy density when the nickel content is increased.
Fluorine may be dissolved in the positive electrode active material. For example, part of oxygen in the crystal structure of NCM may be substituted by fluorine. It is known that fluorine has high electronegativity and is likely to form stable compounds with many kinds of element.
One embodiment of the invention disclosed in this specification is a positive electrode active material in which fluorine is adsorbed on a surface. Specifically, a fluoro group is adsorbed on the surface of the positive electrode active material; thus, the adsorbed fluoro group can react with an electrolyte solution or the like near the fluoro group, leading to inhibition of thermal decomposition or the like of the electrolyte solution. When fluorine can be detected by X-ray photoelectron spectroscopy (XPS), for example, using a measurement instrument for measuring the element concentration in the positive electrode active material while the fluorine adsorbed on the surface is regarded to be contained in the positive electrode active material, the positive electrode active material can be regarded to contain fluorine. Note that fluorine on a particle surface or inside the particle can be detected by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) instead of XPS. Analysis results of energy dispersive X-ray spectroscopy (EDX), gas chromatography mass spectroscopy (GC/MS), pyrolysis gas chromatography mass spectroscopy (Py-GC/MS), liquid chromatography mass spectroscopy (LC/MS), or the like can also be used for judging whether fluorine is contained or not.
One embodiment of the invention disclosed in this specification is a positive electrode active material containing a transition metal M, oxygen, and fluorine. The transition metal M contains nickel, manganese, and cobalt. The positive electrode active material has a surface portion and an inner portion. A surface of the surface portion contains fluorine. The surface portion has a higher magnesium concentration than the inner portion.
A secondary battery including the positive electrode active material is also one embodiment of the present invention. Specifically, one embodiment of the present invention is a secondary battery including a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an ionic liquid or an organic solvent between the positive electrode and the negative electrode. The positive electrode contains a positive electrode active material and a conductive material. The positive electrode active material contains a transition metal M, oxygen, and fluorine. The transition metal M contains nickel, manganese, and cobalt. The positive electrode active material has a surface portion and an inner portion. The surface portion has a higher magnesium concentration than the inner portion. A surface of the surface portion contains fluorine.
In the above structure, fluorine may be adsorbed on a surface of the conductive material.
In the above structure, the conductive material may be any carbon material, specifically, a carbon nanotube or acetylene black.
Another embodiment of the present invention is a formation method of the positive electrode active material. In the formation method, a positive electrode active material containing nickel, manganese, and cobalt is formed and then a mixture of the positive electrode active material and a fluoride is put in a container, and heating is performed while the container is covered with a lid, so that fluorine is adsorbed on a surface of the positive electrode active material. The heating is preferably performed under pressure.
Putting the mixture of the positive electrode active material and a fluoride in a container followed by heating while the container is covered with a lid may be performed a plurality of times in order that fluorine can be adsorbed on the surface of the positive electrode active material. Repeated addition of fluorine can increase the concentration of fluorine on the surface of the positive electrode active material. An increase in the concentration of fluorine on the surface of the positive electrode active material can inhibit thermal decomposition or the like of an electrolyte solution more effectively.
In the above structure, the surface portion of the positive electrode active material contains magnesium and the surface portion has a higher magnesium concentration than the inner portion. An appropriate concentration of magnesium in lithium sites of the surface portion facilitates maintenance of a layered rock-salt crystal structure of the inner portion. This is probably because magnesium in the lithium sites serves as a column supporting MO2 layers. Thus, the surface portion containing magnesium can improve the structure stability of the positive electrode active material.
An appropriate magnesium concentration can bring the above advantage without an adverse effect on insertion and extraction of lithium in charging and discharging. Meanwhile, excess magnesium might adversely affect insertion and extraction of lithium and might reduce the effect of stabilizing the crystal structure. Moreover, an undesired magnesium compound (e.g., an oxide or 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 the secondary battery. As the magnesium concentration 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.
The surface portion containing fluorine and magnesium can inhibit overheating or ignition of the lithium-ion secondary battery and can further improve thermal safety.
Note that it is not preferable that the surface portion be occupied by only a compound of an additive element (magnesium or aluminum) and oxygen because it becomes difficult to insert and extract lithium. For example, it is not preferable that the surface portion be occupied by only magnesium oxide and a structure in which magnesium oxide and an oxide of a divalent transition metal M form a solid solution. Thus, the surface portion needs to contain at least the transition metal M, and also contain cobalt or lithium in a discharged state to have a path through which lithium is inserted and extracted.
A region containing magnesium or fluorine may have an island shape. The positive electrode active material can function as a positive electrode active material even when part of the inner portion is exposed due to insufficient covering by the surface portion. The island-shaped surface portion can have higher energy density than the inner portion.
To ensure a sufficient path through which lithium is inserted and extracted, the total number of atoms of the transition metal M is preferably larger than the total number of atoms of the additive element, in other words, the concentration of the transition metal M is preferably high, in the surface portion.
An additive element may be added to the positive electrode active material. The additive element is one or more selected from aluminum, calcium, titanium, chromium, and zirconium.
An oxide of titanium, which is an example of the additive element, is known to have superhydrophilicity. Accordingly, the positive electrode active material containing an oxide of titanium at the surface portion presumably has good wettability with respect to a high-polarity solvent. Such a positive electrode active material and a high-polarity electrolyte solution can have favorable contact at the interface therebetween and presumably inhibit an internal resistance increase when a secondary battery is formed using such a positive electrode active material.
A barrier film may be provided to cover a positive electrode active material and the barrier film may function as part of the positive electrode active material.
The positive electrode active material contains a transition metal M and oxygen. The transition metal M contain nickel, manganese, and cobalt. Such a positive electrode active material is referred to as NCM. NCM has a layered rock-salt crystal structure. NCM can have an improved energy density when the nickel content is increased.
The barrier film contains at least magnesium and cobalt and has a layered rock-salt crystal structure. The barrier film functions as part of the positive electrode active material and thus can be referred to as a surface portion.
When fluorine is adsorbed on a surface of the barrier film, overheating or ignition of the lithium-ion secondary battery is inhibited and thermal safety is improved.
Fluorine may be dissolved in the barrier film, which is part of the positive electrode active material. For example, part of oxygen in the crystal structure of NCM may be substituted by fluorine. It is known that fluorine has high electronegativity and is likely to form stable compounds with many kinds of element.
Another embodiment of the present invention is a formation method of the positive electrode active material. In the formation method, a positive electrode active material containing nickel, manganese, and cobalt is formed and then the positive electrode active material and a mixture of a cobalt compound and a fluoride (magnesium fluoride) are put in a container, and heating is performed while the container is covered with a lid, so that fluorine is adsorbed on a surface of the positive electrode active material. The heating is preferably performed under pressure.
According to one embodiment of the present invention, a positive electrode active material that is less likely to deteriorate can be provided. A novel positive electrode active material can be provided. A highly safe or highly reliable secondary battery can be provided. A lithium-ion secondary battery with a long lifetime can be provided.
According to one embodiment of the present invention, overheating or ignition of a lithium-ion secondary battery can be inhibited and thermal safety can be improved.
Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not need to have all the effects. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways. In addition, the present invention should not be construed as being limited to the description in the following embodiments.
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.
Uniformity refers to a state 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 nature 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 detected amount of the certain element (e.g., the count number obtained by a scanning transmission electron microscope-energy dispersive X-ray analysis, STEM-EDX) between the specific regions needs to be 10% or less. Examples of the specific regions include a surface portion, a surface, a projection, a depression, 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 complex.
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.
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, short-circuit current is preferably inhibited even at a high charge voltage. With the positive electrode active material of one embodiment of the present invention, short-circuit current is inhibited even at a high charge voltage. Thus, a secondary battery with both high discharge capacity and high safety can be obtained.
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 (also referred to as burn-in treatment) during the fabrication process of a secondary battery is not regarded as deterioration. For example, a state 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 Japanese Industrial Standards (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 not only JIS described above but also JIS, standards defined by the International Electrotechnical Commission (IEC), and the like for electric vehicle propulsion, industrial use, and the like.
In some cases, materials included in a secondary battery that have not been degraded are referred to as initial products or materials in an initial state, and materials that have been degraded (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.
In this embodiment, a positive electrode active material 100 of one embodiment of the present invention will be described with reference to
The positive electrode active material 100 contains lithium, a transition metal M, oxygen, and fluorine. The transition metal M is one or two or more selected from nickel, manganese, and cobalt. The positive electrode active material 100 preferably contains an additive element in addition to the transition metal M. The positive electrode active material 100 can contain lithium nickel-manganese-cobalt oxide to which an additive element is added.
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. The positive electrode active material 100 of one embodiment of the present invention contains nickel, manganese, and cobalt as the transition metal M taking part in an oxidation-reduction reaction.
Nickel preferably accounts for a large percentage of the transition metal M contained in the positive electrode active material 100 because it is easier to increase the charge and discharge capacity than in the case where cobalt accounts for more than half, even when charge voltage is low. Thus, nickel preferably accounts for 50% or more of the transition metal M contained in the positive electrode active material 100, further preferably 60% or more, still further preferably 75% or more, for example. The positive electrode active material 100 contains fluorine, and a Ni—Co—Mn-based positive electrode active material (also referred to as NCM) whose composition excluding fluorine is represented by LiNixCoyMnzO2 (x>0, y>0, and z>0) is used as the positive electrode active material 100. Note that x, y, and z preferably satisfy x:y:z=5:2:3 or the neighborhood thereof, or x:y:z=6:2:2 or the neighborhood thereof, for example. Alternatively, for example, x, y, and z preferably satisfy x:y:z=8:1:1 or the neighborhood thereof. Alternatively, for example, x, y, and z preferably satisfy x:y:z=9:0.5:0.5 or the neighborhood thereof.
An additive element may be added to the positive electrode active material 100. For example, one or more selected from magnesium, aluminum, calcium, titanium, zirconium, fluorine, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium may be used as the additive element. The ratio of the number of atoms of the additive element to the total number of atoms of the transition metal M is preferably lower than 25 atomic %, further preferably lower than 10 atomic %, still further preferably lower than 5 atomic %. In the case where a transition metal (e.g., titanium) is added as the additive element, the additive element is not included in the transition metal M.
Such an additive element further stabilizes the crystal structure of the positive electrode active material 100 as described later. In this specification and the like, an additive element can be rephrased as part of a raw material or a mixture.
A surface portion 100b preferably has a higher fluorine concentration than an inner portion 100c. The inner portion 100c preferably has a higher nickel concentration than the surface portion. The concentration of fluorine preferably has a gradient that increases from the inner portion toward the surface. An additive element, e.g., magnesium, may be added to the surface portion 100b so that the surface portion 100b can function as a barrier layer. In the case of using magnesium, magnesium fluoride and NCM are mixed and heated so that the concentrations of magnesium and fluorine in the surface portion 100b can be higher than those in the inner portion 100c.
The surface portion 100b containing fluorine and magnesium can inhibit overheating or ignition of the lithium-ion secondary battery and can further improve thermal safety.
The positive electrode active material 100 of one embodiment of the present invention preferably has a region whose resistance can be increased. Such a region is referred to as a first region, in some cases, to be distinguished from other regions. The first region preferably has, in a cross-sectional view, a narrow width of greater than or equal to 2 nm and less than or equal to 20 nm, further preferably greater than or equal to 2 nm and less than or equal to 10 nm, still further preferably greater than or equal to 2 nm and less than or equal to 5 nm. Such a region having a narrow width is referred to as a “shell” in this specification and the like, in some cases.
The shell 100d preferably contains cobalt, in which case the shell 100d allows insertion and extraction of lithium ions (Li+) and a reduction in speed of current that flows because of an internal short-circuit. The positive electrode active material 100 preferably has the first region and a second region which is at a position deeper than the first region; magnesium is preferably contained at least in the first region, and the second region does not necessarily contain magnesium. Cobalt contained in the first region and the second region probably allows insertion and extraction of lithium ions (Li+).
The shell 100d or the adsorbed fluorine makes release of oxygen in the positive electrode active material 100 difficult and can inhibit a thermal decomposition reaction. The use of the positive electrode active material 100 including the shell 100d for a positive electrode can inhibit overheating or ignition of the lithium-ion secondary battery and can improve thermal safety.
Note that adsorption may be chemical adsorption or physical adsorption. Chemical adsorption refers to formation of a chemical bond due to a chemical reaction between at least one of additive elements and the surface 100a of the positive electrode active material, whereas physical adsorption refers to adsorption due to intermolecular force (van der Waals force) exerted between at least one of additive elements and the surface of the positive electrode active material 100. As is described later, fluorine may be substituted for part of oxygen in the positive electrode active material 100. Sufficient fluorine contained in the positive electrode active material 100 means that there are fluorine adsorbed on the surface and fluorine substituted for part of oxygen.
Examples of a fluoride contained in the lithium-ion secondary battery include, as described later, LiPF6 and LiBF4 as lithium salts and polyvinylidene fluoride (PVDF) as a binder. Fluorine originating from such a fluoride may be adsorbed on the surface 100a of the positive electrode active material.
The surface portion 100b of the positive electrode active material 100 in
The shell 100d preferably has a narrow width (short range). Furthermore, the width of the shell 100d is preferably larger on a plane through which lithium can be inserted and extracted, that is, a plane other than the (001) plane, than on the (001) plane. In addition, the shell 100d preferably has a structure in which a region containing magnesium and a region containing nickel overlap with each other, connected to each other, or combined with each other on the plane through which lithium can be inserted and extracted, that is, the plane other than the (001) plane. This structure makes it possible to inhibit release of oxygen from the positive electrode active material or a structural change of the positive electrode active material. In other words, when the shell 100d is provided on the plane other than the (001) plane, release of oxygen from the plane other than the (001) plane can be inhibit in some cases. The (001) plane, the (003) plane, and the like are sometimes collectively referred to as the (00l) plane. The (00l) plane is sometimes referred to as a C-plane, a basal plane, or the like. Furthermore, NCM has a two-dimensional diffusion path of lithium. In other words, the diffusion path of lithium extends along a plane. In this specification and the like, a plane where the diffusion path of lithium is exposed or a plane through which lithium is inserted and extracted, i.e., a plane other than the (001) plane, is sometimes referred to as an edge plane.
The surface portion 100b has a crystal structure of LiMeO2 belonging to the space group R-3m.
The surface portion 100b having the LiMeO2 crystal structure has a two-dimensional diffusion path of lithium. In other words, the diffusion path of lithium extends along a plane. In this specification and the like, a plane where the diffusion path of lithium is exposed or a plane through which lithium is inserted and extracted, i.e., a plane other than the (001) plane, is sometimes referred to as an edge plane.
The positive electrode active material 100 contains at least nickel, manganese, and cobalt. Since nickel is contained, the positive electrode active material 100 preferably has a structure in which a region containing magnesium and a region containing nickel overlap with each other, connected to each other, or combined with each other on the plane through which lithium can be inserted and extracted, that is, the surface having an orientation other than the (001) orientation. This structure makes it possible to inhibit release of oxygen from the positive electrode active material or a structural change of the positive electrode active material. As illustrated in
The inner portion 100c refers to a region deeper than the surface portion 100b of the positive electrode active material. The inner portion 100c can be rephrased as an inner region or a core.
The surface 100a of the positive electrode active material refers to a surface of a composite oxide that includes the surface portion 100b and the inner portion 100c. Thus, the positive electrode active material 100 does not contain a material to which a metal oxide that does not contain a lithium site contributing to charging and discharging, such as aluminum oxide (Al2O3), is attached, or a carbonate, a hydroxy group, or the like which is chemically adsorbed after formation of the positive electrode active material. The attached metal oxide refers to, for example, a metal oxide having a crystal structure different from the crystal structures of the inner portion 100c and the surface portion 100b.
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 exist and a region where oxygen and the transition metal M do not exist is considered as the surface 100a of the positive electrode active material. When the positive electrode active material is analyzed, a protective 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.
Therefore, the position of the surface of the positive electrode active material in, for example, STEM-EDX line analysis refers to a point where the detected amount of the characteristic X-ray of the transition metal M is equal to 50% of the sum of the average value MAVE of the detected amounts of the characteristic X-ray of the transition metal M in the inner portion and the average value MBG of the detected amounts of the characteristic X-ray of the transition metal M of the background or a point where the detected amount of the characteristic X-ray of oxygen is equal to 50% of the sum of the average value OAVE of the detected amounts of the characteristic X-ray of oxygen in the inner portion and the average value OBG of the detected amounts of the characteristic X-ray of oxygen of the background. Note that when the position of the point where the detected amount of the characteristic X-ray of the transition metal M is equal to 50% of the sum of the average value of the detected amounts of the characteristic X-ray of the transition metal Min the inner portion and the average value of the detected amounts of the characteristic X-ray of the transition metal M of the background is different from the position of the point where the detected amount of the characteristic X-ray of oxygen is equal to 50% of the sum of the average value of the detected amounts of the characteristic X-ray of oxygen in the inner portion and the average value of the detected amounts of the characteristic X-ray of oxygen of the background, 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, in such a case, the point where the detected amount of the characteristic X-ray of the transition metal M is equal to 50% of the sum of the average value MAVE of the detected amounts of the characteristic X-ray of the transition metal M in the inner portion and the average value MBG of the detected amounts of the characteristic X-ray of the transition metal M of the background can be employed as the position of the surface of the positive electrode active material. In the case of a positive electrode active material containing a plurality of the transition metals M, the position of its surface can be determined using MAVE and MBG of the element whose detected amount of the characteristic X-ray in the inner portion is larger than that of any other element.
The average value MBG of the detected amounts of the characteristic X-ray of the transition metal M of the background can be calculated by averaging the detected amounts 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 detected amount of the characteristic X-ray of the transition metal M begins to increase, for example. The average value MAVE of the detected amounts of the characteristic X-ray of the transition metal Min the inner portion can be calculated by averaging the detected amounts in the range greater than or equal to 2 nm, preferably greater than or equal to 3 nm in a portion that is at a depth larger than, by greater than or equal to 30 nm, preferably greater than 50 nm, the depth at which the detected amounts of the characteristic X-ray of the transition metal M and oxygen are saturated and stabilized, e.g., the depth at which the detected amount of the characteristic X-ray of the transition metal M begins to increase. The average value OBG of the detected amounts of the characteristic X-ray of oxygen of the background and the average value OAVE of the detected amounts of the characteristic X-ray of oxygen in the inner portion can be calculated in a similar manner.
The surface 100a of the positive electrode active material in, for example, a cross-sectional STEM 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. The surface of the positive electrode active material is also determined as the outermost surface of a region where an atomic column derived from an atomic nucleus of, among metal elements which constitute the positive electrode active material, a metal element that has a larger atomic number than lithium is observed in the cross-sectional STEM image. 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 employing also 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 exists outside the surface determined in the above-described manner, it can be said that a difference between the maximum value and the surface is within the margin of error when the difference is less than 1 nm.
A peak in STEM-EDX linear 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 linear 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 performing scanning six times can be used as the profile of each element. The times of scanning is not limited to six and an average of measured values obtained by performing scanning seven or more times can be used as the profile of each element.
STEM-EDX linear analysis can be performed as follows. First, a protective film is deposited over a surface of a positive electrode active material. For example, carbon can be deposited with an ion sputtering 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-EDX linear 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 can be, for example, 10 kV.
The STEM-EDX linear analysis can be performed using HD-2700 produced by Hitachi High-Tech Corporation as a STEM apparatus and two Octane T Ultra W produced by EDAX Inc as EDX detectors. In the EDX linear analysis, the emission current of the STEM apparatus is set to be in the range of 6 μA to 10 μA, 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 linear 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 6 or more.
The concentration of at least one of cobalt and manganese in the transition metal M is preferably higher in the surface portion 100b of the positive electrode active material than in the inner portion 100c. Furthermore, the concentration of nickel is preferably higher in the inner portion 100c than in the surface portion. The concentration of at least one of cobalt and manganese preferably has a gradient that increases toward the surface of the positive electrode active material 100. Furthermore, the concentration of nickel preferably has a gradient that increases toward the inner portion of the positive electrode active material 100.
It is further preferable that additive elements contained in the positive electrode active material 100 be differently distributed. For example, it is preferable that the additive elements exhibit concentration peaks at different depths from a surface. The concentration peak here refers to the local maximum value of the concentration in the surface portion 100b or the concentration in 200 nm or less in depth from the surface 100a.
It is preferable that the crystal structure continuously change from the inner portion toward the surface owing to the above-described concentration gradient of the additive element. Alternatively, it is preferable that the orientations of crystal structure of the surface portion and crystal structure of the inner portion 100c be substantially aligned with each other.
For example, the crystal structure preferably changes continuously from the inner portion 100c that has a layered rock-salt crystal structure toward the surface and the surface portion that have a feature of a rock-salt crystal structure or features of both a rock-salt crystal structure and a layered rock-salt crystal structure. Alternatively, the orientations of the surface portion that has the feature of a rock-salt crystal structure or the features of both a rock-salt crystal structure and a layered rock-salt crystal structure and the layered rock-salt inner portion 100c are preferably substantially aligned with each other.
Note that 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 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 are regularly arranged to form a two-dimensional plane, so that lithium can diffuse two-dimensionally. Note that a defect such as a cation or anion vacancy may exist. 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 such as a crystal structure belonging to the space group Fm-3m is included and cations and anions are alternately arranged. Note that a cation or anion vacancy may exist.
Having features of both a layered rock-salt crystal structure and a rock-salt crystal structure can be judged by electron diffraction, a transmission electron microscope (TEM) image, a cross-sectional STEM image, and the like.
There is no distinction among cation sites in a rock-salt crystal 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. 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 crystal structure and a layered rock-salt crystal 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 LiMO2 are compared to each other, the distance between the bright spots on the (003) plane of LiMO2 is observed at a distance approximately half the distance between the bright spots on the (111) plane of MgO. Thus, when two phases of rock-salt MgO and layered rock-salt LiMO2 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 exists in an electron diffraction pattern. A bright spot common between the rock-salt and layered rock-salt crystal structures has high luminance, whereas a bright spot caused only in the layered rock-salt crystal 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 exists 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). 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 close-packed structures formed of anions are aligned with each other.
The description can also be made as follows. An anion on the {111} plane of a cubic crystal structure has a triangle lattice. A layered rock-salt crystal 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 crystal 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 crystal 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 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 is different from that in the rock-salt crystal. In this specification, in the layered rock-salt crystal and the rock-salt crystal, a state where the orientations of the cubic close-packed structures formed of anions are aligned with each other may be referred to as a state where crystal orientations are substantially aligned with each other. In addition, a state where three-dimensional structures have similarity, e.g., crystal orientations are substantially aligned with each other, or orientations are crystallographically the same is referred to as “topotaxy”.
The orientations of crystals in two regions being substantially aligned with each other can be judged, for example, from a TEM image, a STEM image, a high-angle annular dark field STEM (HAADF-STEM) image, an annular bright-field STEM (ABF-STEM) image, an electron diffraction pattern, and an FFT pattern of a TEM image, a STEM image, or the like. X-ray diffraction (XRD), electron diffraction, neutron diffraction, and the like can also be used for judging.
The positive electrode active material 100 of one embodiment of the present invention 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 LiMO2 (M is at least one of Ni, Co, and Mn) is 1. A composite oxide having a layered rock-salt crystal structure is favorably used as a positive electrode active material of a secondary battery because it has high discharge capacity and a two-dimensional diffusion path of lithium ions and is thus suitable for insertion and extraction reactions of lithium ions. For this reason, it is particularly preferable that the inner portion 100c, which accounts for the majority of the volume of the positive electrode active material 100, have a layered rock-salt crystal structure.
Meanwhile, the surface portion 100b of the positive electrode active material 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 100c so that the layered structure does not break even when a large amount of lithium is extracted from the positive electrode active material 100 by charging. Alternatively, the surface portion 100b preferably functions as a barrier layer of the positive electrode active material 100. Alternatively, the surface portion, 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 100b and the inner portion 100c of the positive electrode active material such as release of oxygen and/or inhibition of oxidative decomposition of an electrolyte on the surface 100a of the positive electrode active material. That is, “the surface portion 100b functions as a barrier layer” means that, for example, the surface portion inhibits a change in the structure of the positive electrode active material 100 and inhibits oxidative decomposition of an electrolyte.
The particle of the positive electrode active material 100 is preferably a single crystal or polycrystalline. A single crystal particle included in the positive electrode active material 100 is referred to as a single particle in some cases. The crystallite size of the positive electrode active material 100 is preferably large.
When primary particles are large, formation of secondary particles by aggregation and sintering of the primary particles is inhibited. A large primary particle leads to a large crystallite size calculated using the half width of the XRD pattern, as a foregone conclusion. Accordingly, the positive electrode active material 100 which is a single particle or which has a large crystallite size calculated using the XRD pattern has no crack or fewer cracks that might be generated between primary particles as compared to a positive electrode active material formed by sintering of a large number of primary particles. Thus, generation of cracks can be inhibited even when the volume of the positive electrode active material 100 is changed by charging and discharging.
The crystallite size calculated using the half width of the XRD pattern is preferably greater than or equal to 150 nm, further preferably greater than or equal to 180 nm, still further preferably greater than or equal to 200 nm, for example.
Note that heating for a long time, heating at high temperatures, a plurality of heating processes, or heating after addition of excess lithium is necessary to obtain a large single crystal or a large crystallite size in some cases. However, heating for a long time reduces the productivity. Heating at high temperatures might cause cation mixing between nickel ions and lithium ions. Furthermore, excess lithium might cause gelling of a binder. The single crystal size and the crystallite size are preferably set as appropriate while avoiding the above problems. For example, the crystallite size calculated from the XRD pattern is preferably less than or equal to 1000 nm, further preferably less than or equal to 800 nm.
A positive electrode active material whose crystallite size calculated from the XRD pattern is within the above range can be regarded as a positive electrode active material having a sufficiently large crystallite size and having features close to those of a single particle.
To obtain an XRD pattern for calculation of a crystallite size, a positive electrode that includes a positive electrode active material, a current collector, a binder, a conductive material, and the like may be subjected to XRD, although it is preferable that only the positive electrode active material be subjected to XRD. Note that the positive electrode active material particles present in the positive electrode might be oriented such that the crystal planes of the positive electrode active material particles are oriented in the same direction owing to, for example, pressure application in a formation process. When many of the positive electrode active material particles are oriented in the above manner, the crystallite size might fail to be calculated accurately; thus, it is preferable that to obtain an XRD pattern, a positive electrode active material layer be removed from the positive electrode, the binder and the like in the positive electrode active material layer be eliminated to some extent using a solvent or the like, and a sample holder be filled with the resultant positive electrode active material, for example. Alternatively, a powder sample of the positive electrode active material or the like may be attached onto a reflection-free silicon plate to which grease is applied, for example.
The crystallite size can be calculated using ICSD coll. code. 172909 as literature data of lithium cobalt oxide and a diffraction pattern that is obtained with Bruker D8 ADVANCE, for example, under the following conditions: CuKα is used as an X-ray source, the 2θ range is from 15° to 90°, an increment is 0.005, and a detector is LYNXEYE XE-T. Analysis can be conducted using DIFFRAC.TOPAS ver. 6 as crystal structure analysis software, and the following settings can be used, for example.
A value of LVol-IB, which is a crystallite size calculated by the above method, is preferably employed as a crystallite size. Note that in a sample whose preferred orientation is calculated to be less than 0.8, too many particles are oriented in the same direction; thus, this sample is not suitable for calculation of a crystallite size in some cases.
The concentration gradient of each of fluorine and the transition metal M contained in the positive electrode active material 100 can be evaluated by, for example, exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, electron probe microanalysis (EPMA), or the like.
EDX measurement for two-dimensional evaluation of an area by area scan is referred to as EDX area analysis. EDX measurement for evaluation of the atomic concentration distribution in a positive electrode active material by line scan is referred to as EDX 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 and the transition metal M in the surface portion and the inner portion 100c of the positive electrode active material 100 can be quantitatively analyzed. By EDX line analysis, the concentration distribution and the highest concentration of the additive element 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 area 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 fluorine in the surface portion 100b is higher than that in the inner portion 100c. An increase in the concentration of fluorine on the surface of the positive electrode active material can inhibit thermal decomposition or the like of an electrolyte solution effectively.
The positive electrode active material 100 does not need to have an island-shaped surface portion as long as fluorine is adsorbed on the surface 100a, and a structure illustrated in
Although fluorine adsorbed so as to cover the surface 100a of the positive electrode active material entirely is indicated by a dotted line also in
This embodiment can be implemented in combination with any of the other embodiments.
In this embodiment, an example of a formation method of the positive electrode active material 100 described in Embodiment 1 will be described with reference to
First, in Step S11, a lithium source and a transition metal M191 source for the inner portion 100c of the positive electrode active material 100 are prepared. The transition metal M191 source is NCM containing nickel, manganese, and cobalt.
Next, in Step S12, synthesis is performed using the lithium source and the transition metal M191 source for the inner portion 100c of the positive electrode active material 100. The synthesis can be performed by, for example, a method in which the lithium source and the transition metal source for the inner portion 100c of the positive electrode active material 100 are mixed by a solid phase method and then heating is performed. Owing to the heating performed after the lithium source is added, secondary particles of NCM each become single particles.
In this manner, a composite oxide C191 contained in the inner portion 100c of the positive electrode active material 100 is formed (Step S13). Although an example in which the composite oxide C191 is synthesized is described in this embodiment, a commercially available product equivalent to the composite oxide C191 may be used. The composite oxide C191 is referred to as a single particle of NCM.
Next, in Step S21, an X192 source for the surface portion 100b of the positive electrode active material 100 and a fluorine source for the surface portion 100b of the positive electrode active material 100 are prepared. As the fluorine source, a fluorine compound typified by lithium fluoride (LiF) can be used. Note that LiF is preferable because it is easily melted in an annealing process described later owing to its relatively low melting point of 848° C. As the X192 source, magnesium fluoride (MgF2) is used. Since magnesium fluoride is also a fluorine source, either the fluorine source or the X192 source may be used. The use of magnesium fluoride allows magnesium to be placed in the vicinity of the surface of the positive electrode active material at high concentration, leading to formation of the surface portion 100b.
The use of lithium fluoride and magnesium fluoride is advantageous in forming the positive electrode active material 100 because the eutectic point of lithium fluoride and magnesium fluoride is around 742° C. A low eutectic point is preferable because it facilitates a later reaction in Step S31, in which case the annealing time can be shortened and the productivity can be increased.
Then, in Step S31, synthesis is performed using the composite oxide C191 contained in the inner portion 100c of the positive electrode active material 100, the X192 source for the surface portion 100b of the positive electrode active material 100, and a halogen source for the surface portion 100b of the positive electrode active material 100. The synthesis can be performed by a method in which the composite oxide C191, the X192 source, and the halogen source are mixed by a solid phase method and then heating is performed. A sol-gel method may also be used for the synthesis.
Here, an example of the annealing method in Step S31 is described with reference to
A heating furnace 220 illustrated in
Thus, before heating in the space 202 in the heating furnace is performed, a step of providing an atmosphere containing oxygen in the space 202 in the heating furnace and a step of placing the container 216 in which the mixture 903 is placed in the space 202 in the heating furnace are performed. The steps performed in this order enable the mixture 903 to be annealed in an atmosphere containing oxygen and a fluoride. For example, flowing of a gas is performed during the annealing. During the annealing, the space 202 in the heating furnace can be sealed to prevent the gas from being discharged to the outside.
Although there is no particular limitation on a method for providing an atmosphere containing oxygen in the space 202 in the heating furnace, a method in which air is exhausted from the space 202 in the heating furnace and then an oxygen gas or a gas containing oxygen such as dry air is introduced, or a method in which an oxygen gas or a gas containing oxygen such as dry air is introduced into the space 202 in the heating furnace for a certain period of time can be used, for example. In particular, a method in which air is exhausted from the space 202 in the heating furnace and then an oxygen gas is introduced (oxygen purge) is preferably employed. Note that the air in the space 202 in the heating furnace may be regarded as an atmosphere containing oxygen.
The fluoride or the like attached to the inner walls of the container 216 and the lid 218 can be fluttered again by the heating to be attached to the mixture 903.
There is no particular limitation on the step of heating the heating furnace 220. The heating may be performed using a heating mechanism included in the heating furnace 220.
Although there is no particular limitation on the way of placing the mixture 903 in the container 216, as illustrated in
The annealing in Step S31 is preferably performed while the pressure in the heating furnace is controlled using the pressure gauge 221. The atmosphere in the heating furnace is preferably in an atmospheric pressure state or a pressurized state. When heating is performed under pressure, the surface of the positive electrode active material 100 becomes smooth. Such a positive electrode active material may be less likely to be physically broken by pressure application or the like than a positive electrode active material whose surface is not smooth. For example, the positive electrode active material 100 is unlikely to be broken in a test involving pressure application such as a nail penetration test, meaning that the positive electrode active material 100 has high safety.
The annealing in Step S31 is preferably performed at an appropriate temperature for an appropriate time. The time for lowering the temperature after the annealing is preferably longer than or equal to 10 hours and shorter than or equal to 50 hours, for example. Then, the material subjected to the annealing is collected, whereby the positive electrode active material 100 is obtained.
In this manner, the positive electrode active material 100 illustrated in
In the case of adding fluorine at high concentration, the surface 100a may be in contact with a solution containing fluorine a plurality of times.
A positive electrode active material 101 illustrated in
First, in Step S11, a lithium source and a transition metal M191 source for the inner portion 100c are prepared.
Next, in Step S12, synthesis is performed using the lithium source and the transition metal M191 source for the inner portion 100c of the positive electrode active material 101. The synthesis can be performed by, for example, a method in which the lithium source and the transition metal source for the inner portion 100c of the positive electrode active material 101 are mixed by a solid phase method and then heating is performed.
In this manner, the composite oxide C191 contained in the inner portion 100c of the positive electrode active material 101 is formed (Step S13). Although an example in which the composite oxide C191 is synthesized is described in this embodiment, a commercially available product equivalent to the composite oxide C191 may be purchased and used.
Next, in Step S41, a lithium source and a transition metal M193 source for the surface portion 100b are prepared. The transition metal M193 source contains cobalt.
Then, in Step S51, synthesis is performed using the composite oxide C191 contained in the inner portion 100c of the positive electrode active material 101, the lithium source, and the transition metal M193 source. The synthesis can be performed by a method in which the composite oxide C191, the lithium source, and the transition metal M193 source are mixed by a solid phase method and then heating is performed.
In this manner, a composite oxide C191+193 including the inner portion 100c covered with the surface portion 100b is formed (Step S52). The composite oxide C191+193 has a region of LCO on an outer side than NCM.
Next, in Step S61, an X194 source for the shell 100d and a fluorine source used for making fluorine adsorbed on a surface of the composite oxide C191+193 are prepared. Lithium fluoride is used as the fluorine source and magnesium fluoride (MgF2) is used as the X194 source.
After that, in Step S71, synthesis is performed using the fluorine source (fluorine compound), the composite oxide C191+193, and the X194 source for the shell 100d. The synthesis can be performed by a method in which the fluorine source, the composite oxide C191+193, and the X194 source are mixed by a solid phase method and then heating is performed. A sol-gel method may also be used for the synthesis.
In this manner, the positive electrode active material 101 illustrated in
This embodiment can be implemented in combination with any of the other embodiments.
Although an example in which the inner portion 100c and the surface portion 100b of the positive electrode active material 100 are formed using a solid phase method is described in Embodiment 2, the inner portion 100c of the positive electrode active material 100 can be formed using a coprecipitation method.
In this embodiment, an example of a formation method of the positive electrode active material 100 of one embodiment of the present invention will be described with reference to
In Step S111 in
It is particularly preferable that the transition metal M contained in the positive electrode active material 100 contain a large amount of nickel, in which case the cost of the raw material may be lower than that in the case of containing a large amount of cobalt and charge and discharge capacity per weight may be increased. For example, the proportion of nickel in the transition metal M is preferably higher than 25 atomic %, further preferably higher than or equal to 60 atomic %, and still further preferably higher than or equal to 80 atomic % of the total of nickel, manganese, and cobalt. However, when the proportion of nickel is too high, the chemical stability and heat resistance might decrease. Thus, the proportion of nickel in the transition metal M is preferably lower than or equal to 95 atomic % of the total of nickel, manganese, and cobalt.
The transition metal M preferably contains cobalt, in which case the average discharging voltage is high and a secondary battery can be highly reliable because cobalt contributes to stabilization of the layered rock-salt crystal structure.
The transition metal M preferably contains manganese, in which case the heat resistance and chemical stability are improved. However, a too high proportion of manganese tends to decrease a discharging voltage and a discharge capacity. For this reason, the proportion of manganese in the transition metal M is preferably higher than or equal to 2.5 atomic % and lower than or equal to 34 atomic % of the total of nickel, manganese, and cobalt.
As the transition metal M source, an aqueous solution containing the transition metal M is prepared. As the nickel source, an aqueous solution of nickel salt can be used. As the nickel salt, nickel sulfate, nickel chloride, nickel nitrate, or a hydrate thereof can be used, for example. Furthermore, an organic acid salt of nickel typified by nickel acetate or a hydrate thereof can also be used. As the nickel source, an aqueous solution of nickel alkoxide or an organic nickel complex can also be used. In this specification and the like, the term “organic acid salt” denotes a compound of a metal and an organic acid such as an acetic acid, a citric acid, an oxalic acid, a formic acid, or a butyric acid.
Similarly, an aqueous solution of cobalt salt can be used as the cobalt source. As the cobalt salt, cobalt sulfate, cobalt chloride, cobalt nitrate, or a hydrate thereof can be used, for example. Furthermore, an organic acid salt of cobalt typified by cobalt acetate or a hydrate thereof can also be used. As the cobalt source, an aqueous solution of cobalt alkoxide or an organic cobalt complex can be used.
Similarly, an aqueous solution of manganese salt can be used as the manganese source. As the manganese salt, manganese sulfate, manganese chloride, manganese nitrate, or a hydrate thereof can be used. Furthermore, an organic acid salt of manganese typified by manganese acetate or a hydrate thereof can also be used as the manganese salt. As the manganese source, an aqueous solution of manganese alkoxide or an organic manganese complex can be used.
In this embodiment, an aqueous solution in which nickel sulfate, cobalt sulfate, and manganese sulfate are dissolved in pure water is prepared as the transition metal M source. In this case, the atomic ratio of nickel, cobalt, and manganese is expressed by Ni:Co:Mn=8:1:1 or in the neighborhood thereof. The aqueous solution is acidic.
As shown in Step S113 in
Next, in Step S114 in
Next, in Step S121 in
The pure water that is preferably used for the transition metal M source and the alkaline solution is water with a resistivity of 1 MΩ·cm or higher, preferably 10 MΩ·cm or higher, further preferably 15 MΩ·cm or higher. Water with the above-described resistivity has high purity and an extremely small amount of impurities.
As shown in Step S122 in
Next, in Step S131 in
During the coprecipitation reaction of Step S131, the pH of the reaction system is preferably higher than or equal to 9.0 and lower than or equal to 11.5.
For example, when an alkaline solution is put in a reaction vessel and an acid solution is dropped into the reaction vessel, the pH of the aqueous solution in the reaction vessel is preferably kept in the above range. Similarly, the same applies to a case where the acid solution is put in the reaction vessel and the alkaline solution is dropped thereinto. The dropping rate of the acid solution or the alkaline solution is preferably higher than or equal to 0.01 mL/min when 200 mL to 350 mL of the solution is in the reaction vessel, in which case the pH conditions can be easily controlled. The reaction vessel contains a reaction container or the like.
The aqueous solution in the reaction vessel is preferably stirred with a stirring means. The stirring means includes a stirrer, an impeller, or the like. The impeller can have two to six blades, for example, when an impeller with four blades is employed, the four blades may be arranged to make a cross shape seen from above. The rotation number of the stirring means may be from 800 rpm to 1200 rpm, inclusive. A baffle plate may be provided in the reaction vessel to change the stirring direction and the rate of flow. The use of a baffle plate improves mixing efficiency and allows synthesis of more uniform composite hydroxide particles.
The temperature of the reaction vessel is preferably controlled to be higher than or equal to 50° C. and lower than or equal to 90° C. After the temperature of the reaction vessel falls within the above temperature range, dropping of the alkaline solution or the acid solution is preferably started.
The reaction vessel preferably has an inert atmosphere. Nitrogen or argon can be used as the inert atmosphere. In the case of the nitrogen atmosphere, a nitrogen gas may be introduced at a flow rate of 0.5 L/min to 2 L/min, inclusive.
In the reaction vessel, a reflux condenser is preferably placed. The nitrogen gas can be released from the reaction vessel and water vapor can be returned to the reaction vessel with use of the reflux condenser.
Through above-described coprecipitation reaction, the composite hydroxide 98 containing the transition metal M is precipitated.
Filtration is preferably performed to collect the composite hydroxide 98 as in Step S132 in
As shown in Step S133 in
In this manner, the composite hydroxide 98 containing the transition metal M can be obtained. In this specification and the like, the composite hydroxide 98 denotes a hydroxide of a plurality of metals. The composite hydroxide 98 can be referred to as a precursor of the inner portion 100c of the positive electrode active material 100.
Next, in Step S141 in
As the lithium source, for example, lithium hydroxide, lithium carbonate, or lithium nitrate can be used. In particular, a material having a low melting point among lithium compounds, such as lithium hydroxide with the melting point of 462° C., is preferably used. Since a positive electrode active material containing nickel at a high proportion easily causes cation mixing as compared with lithium cobalt oxide or the like, heating in Step S143 and the like needs to be performed at low temperatures. Therefore, it is preferable to use a material having a low melting point.
The particle diameter of the lithium source is preferably small because it facilitates a favorable reaction. A lithium source microparticulated by fluidized bed jet milling can be used, for example. The particle diameter here is a particle diameter at an accumulated value of 50% in particle diameter distribution obtained using a laser diffraction and scattering method (also referred to as an average particle diameter). The average particle diameter refers to D50 of the case where the particle diameter distribution is symmetrical. Note that D50 refers to a 50% cumulative particle diameter calculated using a particle size distribution analyzer (SALD-2200 manufactured by Shimadzu Corporation) using a laser diffraction and scattering method. The particle size may be measured by measuring the major diameter of the cross section of the particle obtained by analysis with a SEM, a transmission electron microscope (TEM), or the like, instead of using laser diffraction particle size distribution measurement. Note that an example of a method for measuring D50 with a SEM, TEM, or the like includes a method for measuring 20 or more particles to make a particle size distribution curve, and setting a particle diameter when the accumulation of particles accounts for 50% as D50.
<Step S142>
Next, in Step S142 in
Then, heating is performed on the mixture of the composite hydroxide 98 and the lithium source (Step S143). The heating in Step S143, heating in Step S153, and heating in Step S155 in
An electric furnace or a rotary kiln furnace can be used as a firing device for the heating. A crucible, a sagger, a setter, or a container used in the heating is preferably made of a material that hardly releases impurities. For example, a crucible made of aluminum oxide with a purity of 99.9% can be used. In the case of mass production, a sagger made of mullite cordierite (Al2O3·SiO2·MgO) can be used, for example. Such a container is preferably heated with the lid on.
The heating in Step S143 is preferably performed at a temperature higher than or equal to 400° C. and lower than or equal to 750° C., further preferably higher than or equal to 650° C. and lower than or equal to 750° C. The time for the heating in Step S143 is preferably longer than or equal to 1 hour and shorter than or equal to 30 hours, further preferably longer than or equal to 2 hours and shorter than or equal to 20 hours.
The heating atmosphere is preferably an oxygen atmosphere or an oxygen-containing atmosphere that is dry air with few moisture (e.g., with a dew point lower than or equal to −50° C., preferably lower than or equal to −80° C.).
Furthermore, crushing is preferably performed in Step S144 after the heating. The crushing can be performed in a mortar, for example. Furthermore, classification may be performed using a sieve. Through the above steps, a composite oxide 99 is obtained in Step S145. The composite oxide 99 is a material that can be called NCM.
Next, in Step S151, a fluorine source (fluorine compound) is prepared. Examples of the fluorine compound include hydrogen fluoride, halogen fluoride (e.g., ClF3 or IF5), a gaseous fluoride (e.g., BF3, NF3, PF5, SiF4, or SF6), and a metal fluoride (e.g., LiF, NiF2, AlF3, or MgF2). In this embodiment, magnesium fluoride or lithium fluoride is used as the fluorine source. In the case of forming the positive electrode active material 100 as a single particle, lithium fluoride is preferably used and heated so that crystal growth occurs. In the case of using lithium fluoride, the amount of lithium in Step S151 is adjusted such that the total amount of lithium prepared in Step S151 and Step S141 becomes the final required amount of lithium. The final required amount of lithium is, when the total number of atoms of nickel, cobalt, and manganese is 1, for example, preferably greater than or equal to 0.95 and less than or equal to 1.25, further preferably greater than or equal to 1.00 and less than or equal to 1.05 (atomic ratio). Although a method in which lithium is added twice in Step S141 and Step S151 and heating is performed after each of the step is described here, one embodiment of the present invention is not limited thereto. Lithium may be added three or more times and heating is performed every time after lithium is added.
Then, the composite oxide 99 obtained in Step S145 and the fluorine source (fluorine compound) are mixed.
Subsequently, heating is performed on the mixture of the composite oxide 99 and the fluorine source. The heating in Step S153 is preferably performed at sufficiently high temperatures to increase the crystallite size of the positive electrode active material 100. The temperature range may depend on the composition of the transition metal M.
In the case where the proportion of nickel in the transition metal M is high, e.g., higher than or equal to 70%, the heating temperature is preferably higher than or equal to 750° C., further preferably higher than or equal to 800° C., still further preferably higher than or equal to 850° C., for example. However, too high temperatures might cause reduction of the transition metal M including nickel to the divalent state, for example. Accordingly, the heating temperature is preferably lower than or equal to 950° C., further preferably lower than or equal to 920° C., still further preferably lower than or equal to 900° C., for example.
In the case where the proportion of nickel in the transition metal M is higher than or equal to 40% and lower than or equal to 60%, the heating temperature is preferably higher than or equal to 900° C., further preferably higher than or equal to 950° C., still further preferably around 970° C., for example. However, too high temperatures might cause the above disadvantage; accordingly, the heating temperature is preferably lower than or equal to 1020° C., further preferably lower than or equal to 990° C. For the other conditions of the heating, the description of Step S143 can be referred to.
Furthermore, crushing is preferably performed in Step S154 after the heating. The description of Step S144 can be referred to for the crushing.
In addition, the heating in Step S155 is preferably performed. The heating can reduce the residue of the lithium source or the like. The heating in Step S155 is preferably performed at a temperature higher than or equal to 400° C. and lower than or equal to 900° C., further preferably higher than or equal to 750° C. and lower than or equal to 850° C. The time for the heating in Step S155 is preferably longer than or equal to 1 hour and shorter than or equal to 30 hours, further preferably longer than or equal to 2 hours and shorter than or equal to 20 hours. Note that the heating in Step S155 is not necessarily performed.
Furthermore, crushing is preferably performed in Step S156 after the heating. The description of Step S144 can be referred to for the crushing.
Although a method in which heating is performed twice in Step S153 and Step S155 after the fluorine source is mixed in Step S152 is described with reference to
Through the above steps, the positive electrode active material 100 can be formed (Step S175). The average particle diameter of particles of the positive electrode active material 100, which is measured using a laser diffraction and scattering method, is preferably greater than or equal to 2 μm and less than or equal to 20 μm.
This embodiment can be freely combined with any of the other embodiments.
An example of a positive electrode formed for fabricating a secondary battery using the positive electrode active material described in any one of Embodiments 1 to 3 is described below. The secondary battery at least includes an exterior body, a current collector, an active material (a positive electrode active material or a negative electrode active material), a conductive material, and a binder. The secondary battery also includes an electrolyte solution in which a lithium salt or the like is dissolved. In the secondary battery including an electrolyte solution, a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode are provided. The electrolyte solution preferably contains fluorine, in which case fluorine can be adsorbed on the surface of the positive electrode active material. When the electrolyte solution contains a fluoro group, fluorine adsorbed on the surface of the positive electrode active material can be stably maintained.
First, the positive electrode is described.
A current collector 400 is metal foil, and the positive electrode is formed by applying slurry onto the metal foil and drying the slurry. Pressing may be performed after drying. The positive electrode is a component obtained by forming an active material layer over the current collector 400.
Slurry refers to a material solution that is used to form an active material layer over the current collector 400 and includes at least an active material, a binder, and a solvent, preferably also a conductive material mixed therewith. Slurry may also be referred to as slurry for an electrode or active material slurry; in some cases, slurry for forming a positive electrode active material layer is referred to as slurry for a positive electrode, and slurry for forming a negative electrode active material layer is referred to as slurry for a negative electrode.
A conductive material is also referred to as a conductivity-imparting agent and a conductive additive, and a carbon material is used as the conductive material. A conductive material is attached between a plurality of active materials, whereby the plurality of active materials are electrically connected to each other, and the conductivity increases. Note that the term “attach” refers not only to a state where an active material and a conductive material are physically in close contact with each other, and includes, for example, the following concepts: the case where covalent bonding occurs, the case where bonding with the Van der Waals force occurs, the case where a conductive material covers part of the surface of an active material, the case where a conductive material is embedded in surface roughness of an active material, and the case where an active material and a conductive material are electrically connected to each other without being in contact with each other.
Typical examples of the carbon material used as the conductive material include carbon black (e.g., furnace black, particulate carbon such as acetylene black, and graphite).
In
In the positive electrode of the secondary battery, a binder (a resin) is mixed in order to fix the current collector 400 such as metal foil and the active material. The binder is also referred to as a binding agent. Since the binder is a high molecular material, a large amount of binder lowers the proportion of the active material in the positive electrode, thereby reducing the discharge capacity of the secondary battery. Therefore, the amount of binder mixed is reduced to a minimum. In
In
Although
In the positive electrode in
Graphene, which has electrically, mechanically, or chemically remarkable characteristics, is a carbon material that is expected to be applied to a variety of fields, such as field-effect transistors and solar batteries.
Graphene in this specification and the like includes multilayer graphene and multi graphene. In other words, graphene 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. The carbon material used as the conductive material in the positive electrode is not limited to graphene, and can be a graphene compound. A graphene compound includes 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. In other words, a graphene compound may include a functional group. Graphene or a graphene compound is preferably bent. Graphene or a graphene compound may be rolled, and rolled graphene is referred to as a carbon nanofiber in some cases.
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.
As a graphene compound, fluorine-containing graphene may be used. Fluorine-containing graphene can be formed by making graphene and a fluorine compound contact each other (which is called fluorination treatment). The fluorination treatment is preferably performed using fluorine (F2) or a fluorine compound. The fluorine compound is preferably hydrogen fluoride, halogen fluoride (e.g., ClF3 or IF5), a gaseous fluoride (e.g., BF3, NF3, PF5, SiF4, or SF6), a metal fluoride (e.g., LiF, NiF2, AlF3, or MgF2), or the like. The fluorination treatment is preferably performed using a gaseous fluoride, which may be diluted with an inert gas. The fluorination treatment is preferably performed at room temperature or in a temperature range higher than or equal to 0° C. and lower than or equal to 250° C., which includes room temperature. Performing the fluorination treatment at higher than or equal to 0° C. enables adsorption of fluorine onto a surface of graphene.
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-resistance 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 allows a conductive path to be efficiently formed in an active material layer. Hence, the use of the graphene compound as a conductive material can increase the area where the active material and the conductive material are in contact with each other. The graphene compound preferably covers 80% or more of the area of the active material. Note that a graphene compound preferably clings to at least part of an active material particle. A graphene compound preferably overlays at least part of an active material particle. The shape of a graphene compound preferably conforms to at least part of the shape of an active material particle. The shape of an active material particle means, for example, an uneven surface of a single active material particle or an uneven surface formed by a plurality of active material particles. A graphene compound preferably surrounds at least part of an active material particle. A graphene compound may have a hole.
In
In the step of mixing the graphene 404 and the acetylene black 403 to obtain an electrode slurry, the weight of mixed acetylene black is preferably 1.5 to 20 times, further preferably 2 to 9.5 times the weight of graphene.
When the graphene 404 and the acetylene black 403 are mixed in the above ratio range, the acetylene black 403 can be dispersed uniformly and less likely to be aggregated at the time of preparing the slurry. Furthermore, when the graphene 404 and the acetylene black 403 are mixed in the above ratio range, the positive electrode can have a higher density than that using only the acetylene black 403 as the conductive material. As the electrode density is higher, the capacity per unit volume can be higher. Specifically, the density of the positive electrode active material layer measured by gravimetry can be higher than 3.5 g/cc. In addition, it is preferable that the positive electrode active material described in Embodiments 1 to 3 be used for the positive electrode and the graphene 404 and the acetylene black 403 be mixed in the above ratio range, in which case synergy for higher capacity of the secondary battery can be expected.
A positive electrode containing a first carbon material (graphene) and a second carbon material (acetylene black) which are mixed in the above range enables fast charging, although having lower electrode density than a positive electrode containing only graphene as a conductive material. In addition, it is preferable that the positive electrode active material described in Embodiments 1 to 3 be used for the positive electrode and the graphene 404 and the acetylene black 403 be mixed in the above ratio range, in which case synergy for higher stability and compatibility with faster charging of the secondary battery can be expected.
The above features are advantageous for secondary batteries for vehicles.
When a vehicle becomes heavier with increasing number of secondary batteries, more energy is consumed to move the vehicle, which decreases the mileage. With the use of high-density secondary batteries, the mileage of the vehicle can be maintained with almost no increase in the total weight of a vehicle including a secondary battery having the same weight.
Since power is needed to charge the secondary battery with higher capacity in the vehicle, the charging is desirably finished in a short time. What is called a regenerative charging, in which electric power temporarily generated when the vehicle is braked is used for charging, is performed under high rate charging conditions; thus, a secondary battery for a vehicle is desired to have favorable rate characteristics.
Using the positive electrode active material described in Embodiments 1 to 3 for the positive electrode and mixing acetylene black and graphene within an optimal range enable both higher electrode density and formation of an appropriate space needed for ion conduction, whereby a secondary battery for a vehicle which has high energy density and favorable output characteristics can be obtained.
This structure is also effective in a portable information terminal, and using the positive electrode active material described in Embodiments 1 to 3 for the positive electrode and setting the mixing ratio of acetylene black to graphene in the optimal range enable a small secondary battery with high capacity. Setting the mixing ratio of acetylene black to graphene in the optimal range also enables fast charging of a portable information terminal.
In
Using the positive electrode active material described in Embodiments 1 to 3 for the positive electrode and mixing acetylene black and graphene within an optimal range enable both higher electrode density and formation of an appropriate space needed for ion conduction, whereby a secondary battery which has high energy density and favorable output characteristics can be obtained.
A fluorine-containing carbon nanotube may be used. A fluorine-containing carbon nanotube can be formed by making a carbon nanotube and a fluorine compound contact each other (which is called fluorination treatment). The description of the fluorination treatment using graphene can be referred to for the fluorination treatment using carbon nanotube.
In
Fluorine-containing acetylene black may be used. Fluorine-containing acetylene black can be formed by making acetylene black and a fluorine compound contact each other (which is called fluorination treatment). The description of the fluorination treatment using graphene can be referred to for the fluorination treatment using acetylene black.
In
A secondary battery can be fabricated by using any one of the positive electrodes in
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. Alternatively, fluororubber can be used as the binder.
As the binder, for example, a water-soluble polymer is preferably used. As the water-soluble polymer, 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, poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinyl acetate, or nitrocellulose is preferably used.
At least two of the above materials may be used in combination for the binder.
For example, a material having a significant viscosity modifying effect and another material may be used in combination. For example, a rubber material or the like has high adhesion or high elasticity but may have difficulty in viscosity modification when mixed in a solvent. In such a case, a rubber material or the like is preferably mixed with a material having a significant viscosity modifying effect, for example. As a material having a significant viscosity modifying effect, for instance, a water-soluble polymer is preferably used. An example of a water-soluble polymer having a significant viscosity modifying effect is the above-mentioned polysaccharide; for instance, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or starch can be used.
Note that a cellulose derivative such as carboxymethyl cellulose obtains a higher solubility when converted into a salt such as a sodium salt or an ammonium salt of carboxymethyl cellulose, and thus easily exerts an effect as a viscosity modifier. A high solubility can also increase the dispersibility of an active material or other components in the formation of a slurry for an electrode. In this specification, cellulose and a cellulose derivative used as a binder of an electrode include salts thereof.
A water-soluble polymer stabilizes the viscosity by being dissolved in water and allows stable dispersion of the active material or another material combined as the binder, such as styrene-butadiene rubber, in an aqueous solution. Furthermore, a water-soluble polymer is expected to be easily and stably adsorbed onto an active material surface because it has a functional group. Many cellulose derivatives, such as carboxymethyl cellulose, have a functional group such as a hydroxyl group or a carboxyl group. Because of functional groups, polymers are expected to interact with each other and cover an active material surface in a large area.
In the case where the binder that covers or is in contact with the active material surface forms a film, the film is expected to serve also as a passivation film to suppress the decomposition of an electrolyte. Here, a passivation film refers to a film without electric conductivity or a film with extremely low electric conductivity, and can inhibit the decomposition of the electrolyte at a potential at which a battery reaction occurs when the passivation film is formed on the active material surface, for example. It is preferable that the passivation film can conduct lithium ions while suppressing electrical conduction.
Although the above structure is an example of a secondary battery using an electrolyte solution, one embodiment of the present invention is not limited thereto. For example, a semi-solid-state battery can be fabricated using the positive electrode active material 100 described in Embodiments 1 to 3.
In this specification and the like, a semi-solid-state battery refers to a battery in which at least one of an electrolyte layer, a positive electrode, and a negative electrode includes a semi-solid-state material. The term “semi-solid-state” here does not mean that the proportion of a solid-state material is 50%. The term “semi-solid-state” means having properties of a solid, such as a small volume change, and also having some of properties close to those of a liquid, such as flexibility. A single material or a plurality of materials can be used to satisfy the above properties. For example, a porous solid-state material infiltrated with a liquid material may be used.
In this specification and the like, a polymer electrolyte secondary battery refers to a secondary battery in which an electrolyte layer between a positive electrode and a negative electrode includes a polymer. Polymer electrolyte secondary batteries include a dry (or true) polymer electrolyte battery and a polymer gel electrolyte battery. A polymer electrolyte secondary battery may be referred to as a semi-solid-state battery.
A semi-solid-state battery fabricated using the positive electrode active material 100 described in Embodiments 1 to 3 is a secondary battery having high charge and discharge capacity. The semi-solid-state battery can have high charging and discharging voltages. Alternatively, a highly safe or highly reliable semi-solid-state battery can be achieved.
This embodiment can be freely combined with any of the other embodiments.
This embodiment describes examples of shapes of a secondary battery including a positive electrode formed by the formation method described in the foregoing embodiments.
An example of a coin-type secondary battery is described.
For easy understanding,
In
The positive electrode 304 is a stack in which a positive electrode active material layer 306 is formed over a positive electrode current collector 305.
In a coin-type secondary battery 300, the positive electrode can 301 doubling as a positive electrode terminal and the 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. The positive electrode 304 includes the positive electrode current collector 305 and the positive electrode active material layer 306 provided in contact with the positive electrode current collector 305. The 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. The negative electrode 307 is not limited to having a stacked-layer structure, and lithium metal foil or lithium-aluminum alloy foil may be used.
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. 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, for example. 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 the separator 310 are immersed in the electrolyte solution. Then, as illustrated in
With the above structure, the coin-type secondary battery 300 can have a high level of safety.
An example of a cylindrical secondary battery is described with reference to
Inside the battery can 602 having a hollow cylindrical shape, a wound body in which a strip-like positive electrode 604 and a strip-like negative electrode 606 are wound with a strip-like separator 605 located therebetween is provided. Although not illustrated, the wound body is wound around the central axis. 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, and the like in order to prevent corrosion due to the electrolyte solution. Inside the battery can 602, the wound body 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. The inside of the battery can 602 provided with the wound body is filled with a nonaqueous electrolyte solution (not illustrated). As the nonaqueous electrolyte solution, an electrolyte solution similar to that for the coin-type secondary battery can be used.
Since the positive electrode and the negative electrode of the cylindrical storage battery are wound, active materials are preferably formed on both sides of the current collectors.
The positive electrode active material 100 obtained in Embodiments 1 to 3 is used in the positive electrode 604, whereby the cylindrical secondary battery 616 can have a high level of safety.
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 613 and the bottom of the battery can 602, respectively. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 through a positive temperature coefficient (PTC) element 611. The safety valve mechanism 613 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. The PTC element 611, which is 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 ceramic or the like can be used for the PTC element.
The plurality of secondary batteries 616 may be connected in series after being connected in parallel.
A temperature control device may be provided between the plurality of secondary batteries 616. The secondary batteries 616 can be cooled with the temperature control device when overheated, whereas the secondary batteries 616 can be heated with the temperature control device when cooled too much. Thus, the performance of the power storage system 615 is less likely to be influenced by the outside temperature.
In
Structure examples of secondary batteries are described with reference to
A secondary battery 913 illustrated in
Note that as illustrated in
For the housing 930a, an insulating material such as an organic resin can be used. In particular, when a material such as an organic resin is used for the side on which an antenna is formed, blocking of an electric field by the secondary battery 913 can be inhibited. When an electric field is not significantly blocked by the housing 930a, an antenna may be provided inside the housing 930a. For the housing 930b, a metal material can be used, for example.
As illustrated in
The positive electrode active material 100 obtained in Embodiments 1 to 3 is used in the positive electrode 932, whereby the secondary battery 913 can have a high level of safety.
The separator 933 has a larger width than the negative electrode active material layer 931a and the positive electrode active material layer 932a, and is wound to overlap the negative electrode active material layer 931a and the positive electrode active material layer 932a. In terms of safety, the width of the negative electrode active material layer 931a is preferably larger than that of the positive electrode active material layer 932a. The wound body 950a having such a shape is preferable because of its high degree of safety and high productivity.
As illustrated in
As illustrated in
As illustrated in
Next, examples of the appearance of a laminated secondary battery are illustrated in
An example of a method for fabricating the laminated secondary battery having the appearance illustrated in
First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked.
Then, the negative electrodes 506, the separators 507, and the positive electrodes 503 are placed over the exterior body 509.
Subsequently, the exterior body 509 is folded along a dashed line as illustrated in
Next, the electrolyte solution is introduced into the exterior body 509 from the inlet of the exterior body 509. The electrolyte solution is preferably introduced in a reduced pressure atmosphere or in an inert atmosphere. Lastly, the inlet is sealed by bonding. In this manner, the laminated secondary battery 500 can be fabricated.
The positive electrode active material 100 obtained in Embodiments 1 to 3 is used in the positive electrodes 503, whereby the secondary battery 500 can have a high level of safety.
In this embodiment, examples of vehicles each including the secondary battery of one embodiment of the present invention are described.
The secondary battery can be used in vehicles, typically automobiles. Examples of the automobiles include next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (also referred to as PHEVs or PHVs). The secondary battery can be used as one of the power sources provided in the automobiles. The vehicle is not limited to an automobile. Examples of the vehicles include a train, a monorail train, a ship, a submarine (a deep-submergence vehicle and an unmanned submarine), a flying object (a helicopter, an unmanned aircraft (a drone), an airplane, a rocket, and artificial satellite), an electric bicycle, and an electric motorcycle. The secondary battery of one embodiment of the present invention can be used in these vehicles.
The electric vehicle is provided with first batteries 1301a and 1301b as main secondary batteries for driving and a second battery 1311 that supplies electric power to an inverter 1312 for starting a motor 1304. The second battery 1311 is also referred to as a cranking battery and a starter battery. The second battery 1311 specifically needs high output and does not necessarily have high capacity, and the capacity of the second battery 1311 is lower than that of the first batteries 1301a and 1301b.
The internal structure of the first battery 1301a may be the wound structure illustrated in
Although this embodiment shows an example where the two first batteries 1301a and 1301b are connected in parallel, three or more batteries may be connected in parallel. In the case where the first battery 1301a can store sufficient electric power, the first battery 1301b may be omitted. By constituting a battery pack including a plurality of secondary batteries, large electric power can be extracted. The plurality of secondary batteries may be connected in parallel, connected in series, or connected in series after being connected in parallel. A plurality of secondary batteries can be collectively referred to as an assembled battery.
An in-vehicle secondary battery includes a service plug or a circuit breaker that can cut off high voltage without the use of equipment in order to cut off electric power from a plurality of secondary batteries. The first battery 1301a is provided with such a service plug or a circuit breaker.
Electric power from the first batteries 1301a and 1301b is mainly used to rotate the motor 1304 and is also supplied to in-vehicle parts for 42 V (such as an electric power steering 1307, a heater 1308, and a defogger 1309) through a DC-DC circuit 1306. In the case where there is a rear motor 1317 for the rear wheels, the first battery 1301a is used to rotate the rear motor 1317.
The second battery 1311 supplies electric power to in-vehicle parts for 14 V (such as an audio 1313, power windows 1314, and lamps 1315) through a DC-DC circuit 1310.
Next, the first battery 1301a is described with reference to
Next,
The control circuit portion 1320 includes a switch portion 1324 that includes at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch portion 1324, and a portion for measuring the voltage of the first battery 1301a. The control circuit portion 1320 is set to have the upper limit voltage and the lower limit voltage of the secondary battery used, and imposes the upper limit of current from the outside, the upper limit of output current to the outside, or the like. The range from the lower limit voltage to the upper limit voltage of the secondary battery falls within the recommended voltage range. When a voltage falls outside the range, the switch portion 1324 operates and functions as a protection circuit. The control circuit portion 1320 can also be referred to as a protection circuit because it controls the switch portion 1324 to prevent overdischarging and/or overcharging. For example, when the control circuit 1322 detects a voltage that is likely to cause overcharging, current is interrupted by turning off the switch in the switch portion 1324. Furthermore, a function of interrupting current in accordance with a temperature rise may be set by providing a PTC element in the charge and discharge path. The control circuit portion 1320 includes an external terminal 1325 (+IN) and an external terminal 1326 (−IN).
The switch portion 1324 can be formed by a combination of an n-channel transistor and a p-channel transistor. The switch portion 1324 is not limited to including a switch having a Si transistor using single crystal silicon; the switch portion 1324 may be formed using a power transistor containing germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs), gallium aluminum arsenide (GaAlAs), indium phosphide (InP), silicon carbide (SiC), zinc selenide (ZnSe), gallium nitride (GaN), gallium oxide (GaOx, where x is a real number greater than 0), or the like.
The first batteries 1301a and 1301b mainly supply electric power to in-vehicle parts for 42 V (for a high-voltage system), and the second battery 1311 supplies electric power to in-vehicle parts for 14 V (for a low-voltage system). A lead battery is usually used for the second battery 1311 due to cost advantage.
In this embodiment, an example in which a lithium-ion battery is used as both the first battery 1301a and the second battery 1311 is described. As the second battery 1311, a lead storage battery, an all-solid-state battery, or an electric double layer capacitor may alternatively be used.
Regenerative energy generated by rolling of tires 1316 is transmitted to the motor 1304 through a gear 1305, and is stored in the second battery 1311 through a motor controller 1303, a battery controller 1302, or the control circuit portion 1321. Alternatively, the regenerative energy is stored in the first battery 1301a through the battery controller 1302 and the control circuit portion 1320. Alternatively, the regenerative energy is stored in the first battery 1301b through the battery controller 1302 and the control circuit portion 1320. For efficient charging with regenerative energy, the first batteries 1301a and 1301b are preferably capable of fast charging.
The battery controller 1302 can set the charge voltage, charge current, and the like of the first batteries 1301a and 1301b. The battery controller 1302 can set charge conditions in accordance with charge characteristics of a secondary battery used, so that fast charging can be performed.
Although not illustrated, when the electric vehicle is connected to an external charger, a plug of the charger or a connection cable of the charger is electrically connected to the battery controller 1302. Electric power supplied from the external charger is stored in the first batteries 1301a and 1301b through the battery controller 1302. Some chargers are provided with a control circuit, in which case the function of the battery controller 1302 is not used; to prevent overcharging, the first batteries 1301a and 1301b are preferably charged through the control circuit portion 1320. In addition, a plug of the charger or a connection cable of the charger is sometimes provided with a control circuit. The control circuit portion 1320 is also referred to as an electronic control unit (ECU). The ECU is connected to a controller area network (CAN) provided in the electric motor vehicle. The CAN is a type of a serial communication standard used as an in-vehicle LAN. The ECU includes a microcomputer. Moreover, the ECU uses a CPU or a GPU.
External chargers installed at charge stations and the like have a 100 V outlet, a 200 V outlet, or a three-phase 200V outlet with 50 kW, for example. Furthermore, charging can be performed by electric power supplied from external charge equipment with a contactless power feeding method or the like.
For fast charging, secondary batteries that can withstand charging at high voltage have been desired to perform charging in a short time.
It is possible to achieve a secondary battery in which graphene is used as a conductive material, the electrode layer is formed thick to suppress a reduction in capacity while increasing the loading amount, and the electrical characteristics are significantly improved in synergy with maintenance of high capacity. This secondary battery is particularly effectively used in a vehicle and can achieve a vehicle that has a long range, specifically a driving range per charge of 500 km or greater, without increasing the proportion of the weight of the secondary battery to the weight of the entire vehicle.
Specifically, in the secondary battery in this embodiment, the use of the positive electrode active material 100 described in Embodiments 1 to 3 can increase the operating voltage, and the increase in charge voltage can increase the available capacity. Moreover, using the positive electrode active material 100 described in Embodiments 1 to 3 in the positive electrode can provide an automotive secondary battery having a high level of safety.
Next, examples in which the secondary battery of one embodiment of the present invention is mounted on a vehicle, typically a transport vehicle, will be described.
Mounting the secondary battery illustrated in any one of
The automobile 2001 can be charged when the secondary battery included in the automobile 2001 is supplied with electric power through external charge equipment by a plug-in system, a contactless power feeding system, or the like. In charging, a given method such as CHAdeMO (registered trademark) or Combined Charging System can be employed as a charge method, the standard of a connector, and the like as appropriate. Charge equipment may be a charge station provided in a commerce facility or a power source in a house. For example, with the use of the plug-in technique, the power storage device mounted on the automobile 2001 can be charged by being supplied with electric power from outside. The charging can be performed by converting AC electric power into DC electric power through a converter such as an AC-DC converter.
Although not illustrated, 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 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 two vehicles. Furthermore, a solar cell may be provided in the exterior of the vehicle to charge the secondary battery when the vehicle stops or moves. To supply electric power in such a contactless manner, an electromagnetic induction method or a magnetic resonance method can be used.
The secondary battery module of the aircraft 2004 has eight 4 V secondary batteries connected in series and has a maximum voltage of 32 V, for example. The battery pack 2203 has the same function as that in
This embodiment describes examples in which the lithium-ion battery of one embodiment of the present invention is mounted on a two-wheeled vehicle and a bicycle as examples of mounting a secondary battery in a vehicle.
The electric bicycle 8700 is provided with a power storage device 8702. The power storage device 8702 can supply electric power to a motor that assists a rider. The power storage device 8702 is portable, and
In the motor scooter 8600 illustrated in
In this embodiment, examples of electronic devices each including the secondary battery of one embodiment of the present invention are described. Examples of the electronic device including the secondary battery include a television device (also referred to as a television or a television receiver), a monitor of a computer and the like, a digital camera, a digital video camera, a digital photo frame, a mobile phone (also referred to as a cellular phone or a mobile phone device), a portable game console, a portable information terminal, an audio reproducing device, and a large-sized game machine such as a pachinko machine. Examples of the portable information terminal include a laptop personal computer, a tablet terminal, an e-book reader, and a mobile phone.
The mobile phone 2100 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.
With the operation button 2103, 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 2103 can be set freely by the operating system incorporated in the mobile phone 2100.
The mobile phone 2100 can employ near field communication based on an existing communication standard. For example, mutual communication between the mobile phone 2100 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible.
Moreover, the mobile phone 2100 includes the external connection port 2104, and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging can be performed via the external connection port 2104. Note that the charge operation may be performed by wireless power feeding without using the external connection port 2104.
The mobile phone 2100 preferably includes a sensor. As the sensor, 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, for example.
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 the 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 the 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 further includes, in its inner region, the secondary battery 6409 of one embodiment of the present invention and a semiconductor device or an electronic component. A secondary battery including a positive electrode using the positive electrode active material 100 obtained in Embodiments 1 to 3 has a high energy density and a high degree of safety, and thus can be used safely for a long time over along period of time and is preferable as the secondary battery 6409 included in the robot 6400.
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 that is likely to be caught in the brush 6304 (e.g., a wire) by image analysis, the rotation of the brush 6304 can be stopped. The cleaning robot 6300 further includes, in its inner region, the secondary battery 6306 of one embodiment of the present invention and a semiconductor device or an electronic component. A secondary battery including a positive electrode using the positive electrode active material 100 obtained in Embodiments 1 to 3 has a high energy density and a high degree of safety, and thus can be used safely for a long time over along period of time and is preferable as the secondary battery 6306 included in the cleaning robot 6300.
In this embodiment, thermal runaway, a nail penetration test, and the like of a secondary battery will be described to explain the principle or the like that ignition is less likely to occur when a secondary battery including one or more of the positive electrode active materials 100 and 101, each of which is one embodiment of the present invention, is subjected to a nail penetration test.
In general, when the temperature of the secondary battery reaches 100° C. or the vicinity thereof, (1) collapse of a solid electrolyte interphase (SEI) of a negative electrode and heat generation are caused. When the temperature of the secondary battery exceeds 100° C., (2) reduction of an electrolyte solution by the negative electrode (the negative electrode is C6Li when graphite is used) and heat generation are caused, and (3) oxidation of the electrolyte solution by a positive electrode and heat generation are caused. When the temperature of the secondary battery reaches 180° C. or the vicinity thereof, (4) thermal decomposition of the electrolyte solution is caused and (5) oxygen release from the positive electrode and thermal decomposition of the positive electrode (the thermal decomposition includes a structural change in a positive electrode active material) are caused. After that, when the temperature of the secondary battery exceeds 200° C., (6) decomposition of the negative electrode is caused, and finally, (7) the positive electrode and the negative electrode come into direct contact with each other. The secondary battery enters thermal runaway after passing through the state (5), the state (6), the state (7), or the like. Thus, to prevent thermal runaway, the temperature increase of the secondary battery is preferably inhibited and the negative electrode, the positive electrode, and/or the electrolyte solution is/are preferably kept stable at high temperatures exceeding 100° C.
Each of the positive electrode active materials 100 and 101, which is one embodiment of the present invention described in Embodiments 1 to 3, has a stable crystal structure and inhibits release of oxygen advantageously. Thus, the secondary battery including the positive electrode active material 100 or 101 probably does not come into a state of after at least the state (5) and the temperature increase of the secondary battery is probably inhibited, leading to a significant effect that thermal runaway is less likely to occur.
Next, the nail penetration test is described with reference to
An internal short circuit generally occurs when the nail 1603 penetrates the positive electrode 1503 and the negative electrode 1506 as illustrated in
When an internal short circuit of a secondary battery occurs, its temperature generally changes as shown in the graph of
When a nail penetration test is performed on the secondary battery including one or more of the positive electrode active materials 100 and 101, each of which is one embodiment of the present invention described in Embodiments 1 to 3, the speed of current flowing into the positive electrode in the case of an internal short-circuit is probably reduced because the positive electrode active materials 100 and 101 include the shell described above. In that case, a significant effect that thermal runaway is less likely to occur and thus ignition or the like is less likely to occur can be obtained.
In this embodiment, a positive electrode active material 1201 of one embodiment of the present invention will be described with reference to
The positive electrode active material 1201 contains lithium, a transition metal M, and oxygen. The transition metal M is one or more selected from nickel, manganese, and cobalt. The positive electrode active material 1201 preferably contains an additive element in addition to the elements. The positive electrode active material 1201 can contain lithium nickel-manganese-cobalt oxide to which an additive element is added.
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. The positive electrode active material 1201 of one embodiment of the present invention contains nickel, manganese, and cobalt as the transition metal M taking part in an oxidation-reduction reaction.
Nickel preferably accounts for a large percentage of the transition metal M contained in the positive electrode active material 1201 because it is easier to increase the charge and discharge capacity than in the case where cobalt accounts for more than half, even when charge voltage is low. Thus, nickel preferably accounts for 50% or more of the transition metal M, further preferably 60% or more, still further preferably 75% or more, for example. The positive electrode active material 1201 contains fluorine, and a Ni—Co—Mn-based positive electrode active material (also referred to as NCM) whose composition excluding fluorine is represented by LiNixCoyMnzO2 (x>0, y>0, and z>0) is used as the positive electrode active material. Note that x, y, and z preferably satisfy x:y:z=5:2:3 or the neighborhood thereof, for example. Alternatively, for example, x, y, and z preferably satisfy x:y:z=8:1:1 or the neighborhood thereof. Alternatively, for example, x, y, and z preferably satisfy x:y:z=9:0.5:0.5 or the neighborhood thereof.
An additive element may be added to the positive electrode active material 1201. For example, one or more selected from magnesium, aluminum, calcium, titanium, zirconium, fluorine, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium may be used as the additive element. The ratio of the number of atoms of the additive element to the total number of atoms of the transition metal M (the transition metal M contains nickel, cobalt, and manganese) is preferably lower than 25 atomic %, further preferably lower than 10 atomic %, still further preferably lower than 5 atomic %.
Such an additive element further stabilizes the crystal structure of the positive electrode active material 1201 as described later.
A surface portion 1204 preferably has a higher fluorine concentration than an inner portion 1200c. The inner portion 1200c preferably has a higher nickel concentration than the surface portion. Note that LCO to which an additive element, e.g., magnesium, is added may be used for the surface portion 1204 so that the surface portion 1204 can function as a barrier film. In the case of using magnesium, a cobalt compound, magnesium fluoride, and NCM are mixed and heated so that NCM can be the inner portion 1200c and the concentrations of magnesium and fluorine in the surface portion 1204 can be higher than those in the inner portion 1200c. In the case of using magnesium as the additive element, the cobalt concentration in the surface portion 1204 can be higher than that in the inner portion 1200c because magnesium is compatible with cobalt.
When the surface portion 1204 formed using LCO contains magnesium, thermal safety can be improved. The surface portion 1204 further containing fluorine can inhibit overheating or ignition of the lithium-ion secondary battery and can further improve thermal safety.
The positive electrode active material 1201 of one embodiment of the present invention preferably has a region whose resistance can be increased. Such a region is referred to as a first region, in some cases, to be distinguished from other regions. The first region preferably has, in a cross-sectional view, a narrow width (short range) of greater than or equal to 2 nm and less than or equal to 20 nm, further preferably greater than or equal to 2 nm and less than or equal to 10 nm, still further preferably greater than or equal to 2 nm and less than or equal to 5 nm. Such a region having a narrow width is referred to as a “shell” in this specification and the like, in some cases.
The shell 1200d preferably contains cobalt, in which case the shell 1200d allows insertion and extraction of lithium ions (Li+) and a reduction in speed of current that flows because of an internal short-circuit. The positive electrode active material 1201 preferably has the first region and a second region which is at a position deeper than the first region; magnesium is preferably contained at least in the first region, and the second region does not necessarily contain magnesium. Cobalt contained in the first region and the second region probably allows insertion and extraction of lithium ions (Li+).
When the positive electrode active material 1201 is subjected to EDX linear analysis, a peak of the magnesium concentration in the surface portion 1204 is preferably observed in a region that is 3 nm in depth, further preferably 1 nm in depth, still further preferably 0.5 nm in depth from the boundary between the shell and the surface portion of the positive electrode active material 1201 toward the center of the positive electrode active material 1201. In addition, the magnesium concentration 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. The magnesium concentration 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.
In the EDX linear analysis, the distribution of fluorine preferably overlaps with the distribution of magnesium in the surface portion 1204 of the positive electrode active material 1201. For example, a difference in the depth direction between a peak of the fluorine concentration and a peak of the magnesium concentration is preferably within 10 nm, further preferably within 3 nm, still further preferably within 1 nm.
In the EDX linear analysis, a peak of the nickel concentration in the surface portion 1204 is preferably observed in a region that is 3 nm in depth, further preferably 1 nm in depth, still further preferably 0.5 nm in depth from the boundary between the shell and the surface portion toward the center of the positive electrode active material 1201. When the surface portion 1204 contains magnesium and nickel, the distribution of nickel preferably overlaps with the distribution of magnesium. For example, a difference in the depth direction between a peak of the nickel concentration and a peak of the magnesium concentration is preferably within 10 nm, further preferably within 3 nm, still further preferably within 1 nm.
EDX linear, surface, or point analysis of the positive electrode active material 1201 preferably reveals that the atomic ratio of magnesium to cobalt (Mg/Co) at a peak of the magnesium concentration 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 nickel to cobalt (Ni/Co) at a peak of the nickel concentration 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 fluorine concentration 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.
The shell 1200d or the adsorbed fluorine makes release of oxygen in the positive electrode active material 1201 difficult and can inhibit a thermal decomposition reaction. The use of the positive electrode active material 1201 including the shell 1200d for a positive electrode can inhibit overheating or ignition of the lithium-ion secondary battery and can improve thermal safety.
Note that adsorption may be chemical adsorption or physical adsorption. Chemical adsorption refers to formation of a chemical bond due to a chemical reaction between at least one of additive elements and the surface 1200a of the positive electrode active material, whereas physical adsorption refers to adsorption due to intermolecular force (van der Waals force) exerted between at least one of additive elements and the surface of the positive electrode active material 1201. As is described later, fluorine may be substituted for part of oxygen in the positive electrode active material 1201. Sufficient fluorine contained in the positive electrode active material 1201 means that there are fluorine adsorbed on the surface and fluorine substituted for part of oxygen.
Examples of a fluoride contained in the lithium-ion secondary battery include LiPF6 and LiBF4 as lithium salts and polyvinylidene fluoride (PVDF) as a binder. Fluorine originating from such a fluoride may be adsorbed on the surface 1200a of the positive electrode active material.
The surface portion 1204 of the positive electrode active material 1201 in
The inner portion 1200c refers to a region deeper than the surface portion 1204 of the positive electrode active material. The inner portion 1200c can be rephrased as an inner region or a core. The surface portion 1204 can be referred to as a first shell, and the shell 1200d can be referred to as a second shell. The inner portion 1200c contains NCM, which has a composition different from that of LCO contained in the surface portion 1204. Accordingly, there is a boundary between the inner portion 1200c and the surface portion 1204. Note that a clear boundary is not formed depending on heating conditions, in some cases.
The surface 1200a of the positive electrode active material refers to a surface of the shell 1200d. Thus, the positive electrode active material 1201 does not contain a material to which a metal oxide that does not contain a lithium site contributing to charging and discharging, such as aluminum oxide, is attached on the surface 1200a, or a carbonate, a hydroxy group, or the like which is chemically adsorbed after formation of the positive electrode active material. The attached metal oxide refers to, for example, a metal oxide having a crystal structure different from the crystal structures of the inner portion 1200c and the surface portion 1204.
Since the positive electrode active material 1201 is a compound containing oxygen and a transition metal into and from which lithium ions 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 ions exist and a region where oxygen and the transition metal M do not exist is considered as the surface 1200a of the positive electrode active material. When the positive electrode active material is analyzed, a protective 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.
This embodiment can be implemented in combination with any of the other embodiments.
In this embodiment, an example of a formation method of the positive electrode active material 1201 described in Embodiment 10 will be described with reference to
First, in Step S11, a lithium source and a transition metal M191 source for the inner portion 1200c of the positive electrode active material 1201 are prepared. The transition metal M191 source contains nickel, manganese, and cobalt.
Next, in Step S12, synthesis is performed using the lithium source and the transition metal M191 source for the inner portion 1200c of the positive electrode active material 1201. The synthesis can be performed by, for example, a method in which the lithium source and the transition metal source for the inner portion 1200c of the positive electrode active material 1201 are mixed by a solid phase method and then heating is performed.
In this manner, the composite oxide C191 contained in the inner portion 1200c of the positive electrode active material 1201 is formed (Step S13). Although an example in which the composite oxide C191 is synthesized is described in this embodiment, a commercially available product equivalent to the composite oxide C191 may be used.
Next, in Step S121, a fluorine source, and a cobalt source and a lithium source for the surface portion 1204 of the positive electrode active material 1201 are prepared. As the cobalt source, the lithium source, and the fluorine source, cobalt fluoride, lithium fluoride (LiF), and magnesium fluoride can be used, respectively. Note that LiF is preferable because it is easily melted in an annealing process described later owing to its relatively low melting point of 848° C. The use of magnesium fluoride allows magnesium to be placed in the vicinity of the surface of the positive electrode active material at high concentration.
Then, in Step S131, synthesis is performed using the fluorine source, the composite oxide C191 contained in the inner portion 1200c of the positive electrode active material 1201, and the cobalt source and the lithium source for the surface portion 1204 of the positive electrode active material 1201. As a result, NCM serves as the inner portion 1200c of the positive electrode active material 1201, and the inner portion 1200c is covered with the surface portion 1204. The surface portion 1204 is a barrier film containing at least cobalt, magnesium, and lithium, and has a layered rock-salt crystal structure. The synthesis can be performed by a method in which the fluorine source, the composite oxide C191, the cobalt source, and the lithium source are mixed by a solid phase method and then heating is performed. A sol-gel method may also be used for the synthesis.
Since the annealing method in Step S131 is the same as that in Step S31 in Embodiment 1, the description thereof is omitted here.
The annealing in Step S131 is preferably performed at an appropriate temperature for an appropriate time. When the annealing temperature is too high, the materials are melted and mixed, so that the positive electrode active material 1201 having a barrier layer cannot be obtained. The time for lowering the temperature after the annealing is preferably longer than or equal to 10 hours and shorter than or equal to 50 hours, for example. Then, the material subjected to the annealing is collected, whereby the positive electrode active material 1201 is obtained.
In this manner, the positive electrode active material 1201 illustrated in
In the case of adding fluorine at high concentration, the surface 1200a may be in contact with a solution containing fluorine a plurality of times.
A positive electrode active material 1211 illustrated in
First, in Step S11, a lithium source and a transition metal M191 source for the inner portion 1200c are prepared.
Next, in Step S12, synthesis is performed using the lithium source and the transition metal M191 source for the inner portion 1200c of the positive electrode active material 1211. The synthesis can be performed by, for example, a method in which the lithium source and the transition metal source for the inner portion 1200c of the positive electrode active material 1211 are mixed by a solid phase method and then heating is performed.
In this manner, the composite oxide C191 contained in the inner portion 1200c of the positive electrode active material 1211 is formed (Step S13). Although an example in which the composite oxide C191 is synthesized is described in this embodiment, a commercially available product equivalent to the composite oxide C191 may be used.
Next, in Step S42, a lithium source and a cobalt source for the surface portion 1204 are prepared.
Then, in Step S53, synthesis is performed using the composite oxide C191 contained in the inner portion 1200c of the positive electrode active material 1211, the lithium source, and the cobalt source. The synthesis can be performed by a method in which the composite oxide C191, the lithium source, and the cobalt source are mixed by a solid phase method and then heating is performed.
In this manner, a composite oxide C194 including the inner portion 1200c covered with the surface portion 1204 is formed. At this stage, a particle in which a surface of the inner portion 1200c of the positive electrode active material 1211, i.e., a surface of NCM, is covered with lithium cobalt oxide (LCO) can be obtained (Step S54).
Next, in Step S62, a magnesium source for the surface portion 1204 and a fluorine source used for making fluorine adsorbed on a surface of the composite oxide C194 are prepared.
After that, in Step S73, synthesis is performed using the composite oxide C194, the magnesium source for the surface portion 1204, and the fluorine source. As a result of this synthesis, the surface portion 1204, which is formed of lithium cobalt oxide (LCO), is doped with magnesium or fluorine. The synthesis can be performed by a method in which the composite oxide C194, the magnesium source, and the fluorine source are mixed by a solid phase method and then heating is performed. A sol-gel method may also be used for the synthesis.
In this manner, the positive electrode active material 1211 illustrated in
This embodiment can be implemented in combination with any of the other embodiments.
Although an example in which the inner portion 1200c and the surface portion 1204 of the positive electrode active material 1201 are formed using a solid phase method is described in Embodiment 11, the inner portion 1200c of the positive electrode active material 1201 can be formed using a coprecipitation method.
In this embodiment, an example of a formation method of a positive electrode active material 1200f of one embodiment of the present invention will be described with reference to
According to Embodiment 3, in Step S111 in
Next, as shown in Step S113 in
Then, in Step S114 in
Subsequently, in Step S121 in
After that, as shown in Step S122 in
Next, in Step S131 in
During the coprecipitation reaction of Step S131, the pH of the reaction system is preferably higher than or equal to 9.0 and lower than or equal to 11.5.
Through above-described coprecipitation reaction, the composite hydroxide 98 containing the transition metal M is precipitated.
Then, filtration is preferably performed to collect the composite hydroxide 98 as in Step S132 in
Next, as shown in Step S133 in
In this manner, the composite hydroxide 98 containing the transition metal M can be obtained.
Next, as in Embodiment 3, a lithium source is prepared in Step S141 in
After that in Step S142 in
Then, heating is performed on the mixture of the composite hydroxide 98 and the lithium source (Step S143). The heating in Step S143, heating in Step S253, and heating in Step S255 in
An electric furnace or a rotary kiln furnace can be used as a firing device for the heating. A crucible, a sagger, a setter, or a container used in the heating is preferably made of a material that hardly releases impurities. For example, a crucible made of aluminum oxide with a purity of 99.9% can be used. In the case of mass production, a sagger made of mullite cordierite (Al2O3·SiO2·MgO) can be used, for example. Such a container is preferably heated with the lid on.
The heating in Step S143 is preferably performed at a temperature higher than or equal to 400° C. and lower than or equal to 750° C., further preferably higher than or equal to 650° C. and lower than or equal to 750° C. The time for the heating in Step S143 is preferably longer than or equal to 1 hour and shorter than or equal to 30 hours, further preferably longer than or equal to 2 hours and shorter than or equal to 20 hours.
The heating atmosphere is preferably an oxygen atmosphere or an oxygen-containing atmosphere that is dry air with few moisture (e.g., with a dew point lower than or equal to −50° C., preferably lower than or equal to −80° C.).
Next, crushing is preferably performed in Step S144 after the heating. Through the above steps, the composite oxide 99 is obtained in Step S145. The composite oxide 99 is a material that can be called NCM.
Next, in Step S181, a lithium source is prepared. As the lithium source, lithium fluoride is used. In the case of using lithium fluoride, the amount of lithium in Step S181 is adjusted such that the total amount of lithium prepared in Step S181 and Step S141 becomes the final required amount of lithium.
Then, the composite oxide 99 obtained in Step S145 and the lithium source (lithium fluoride) are mixed.
Subsequently, heating is performed on the mixture of the composite oxide 99 and the fluorine source. The heating in Step S253 is preferably performed at sufficiently high temperatures to increase the crystallite size of the positive electrode active material 1200f. The temperature range may depend on the composition of the transition metal M.
In the case where the proportion of nickel in the transition metal M is high, e.g., higher than or equal to 70%, the heating temperature is preferably higher than or equal to 750° C., further preferably higher than or equal to 800° C., still further preferably higher than or equal to 850° C., for example. However, too high temperatures might cause reduction of the transition metal M including nickel to the divalent state, for example. Accordingly, the heating temperature is preferably lower than or equal to 950° C., further preferably lower than or equal to 920° C., still further preferably lower than or equal to 900° C., for example.
In the case where the proportion of nickel in the transition metal M is higher than or equal to 40% and lower than or equal to 60%, the heating temperature is preferably higher than or equal to 900° C., further preferably higher than or equal to 950° C., still further preferably around 970° C., for example. However, too high temperatures might cause the above disadvantage; accordingly, the heating temperature is preferably lower than or equal to 1020° C., further preferably lower than or equal to 990° C. For the other conditions of the heating, the description of Step S143 can be referred to.
Furthermore, crushing is preferably performed in Step S254 after the heating. The description of Step S144 can be referred to for the crushing.
In addition, the heating in Step S255 is preferably performed. The heating can reduce the residue of the lithium source or the like. The heating in Step S255 is preferably performed at a temperature higher than or equal to 400° C. and lower than or equal to 900° C., further preferably higher than or equal to 750° C. and lower than or equal to 850° C. The time for the heating in Step S255 is preferably longer than or equal to 1 hour and shorter than or equal to 30 hours, further preferably longer than or equal to 2 hours and shorter than or equal to 20 hours. Note that the heating in Step S255 is not necessarily performed.
Furthermore, crushing is preferably performed in Step S256 after the heating. The description of Step S144 can be referred to for the crushing. After the crushing, the resultant material is collected.
Through the above steps, a composite oxide 199 is obtained in Step S257. The composite oxide 199 is a material that can be called NCM.
Subsequently, a lithium source and a cobalt source are prepared.
Next, in Step S260, synthesis is performed using the composite oxide 199, the lithium source, and the cobalt source. The synthesis in Step S260 can be performed by a method in which the composite oxide 199, the lithium source, and the cobalt source are mixed by a solid phase method and then heating is performed.
In this manner, a composite oxide 299 including an inner portion (the composite oxide 199) covered with a surface portion is formed (Step S262). At this stage, a particle in which the inner portion, i.e., NCM, of the positive electrode active material 1200f is covered with lithium cobalt oxide (LCO) can be obtained.
Next, in Step S263, a magnesium source and a fluorine source used for making fluorine adsorbed on a surface of the composite oxide 299 are prepared.
After that, in Step S271, synthesis is performed using the composite oxide 299, the magnesium source, and the fluorine source. As a result of this synthesis, the surface portion, which is formed of lithium cobalt oxide (LCO), is doped with magnesium or fluorine. The synthesis can be performed by a method in which the composite oxide 299, the magnesium source, and the fluorine source are mixed by a solid phase method and then heating is performed. A sol-gel method may also be used for the synthesis.
Through the above steps, the positive electrode active material 1200f can be formed (Step S275). Note that the steps after Step S257 are the same as the steps after Step S12 described in Embodiment 2 and thus are briefly described here. The steps after Step S121 in
This embodiment can be freely combined with any of the other embodiments.
An example of a positive electrode formed for fabricating a secondary battery using the positive electrode active material 1201, 1211, or 1200f described in any one of Embodiments 10 to 12 is described below. The secondary battery at least includes an exterior body, a current collector, an active material (a positive electrode active material or a negative electrode active material), a conductive material, and a binder. The secondary battery also includes an electrolyte solution in which a lithium salt or the like is dissolved. In the secondary battery including an electrolyte solution, a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode are provided. The electrolyte solution preferably contains a material having a fluoro group, in which case fluorine can be adsorbed on the surface of the positive electrode active material 1201, 1211, or 1200f. When the electrolyte solution contains a material having a fluoro group, fluorine can be adsorbed on the surface of the positive electrode active material 1201, 1211, or 1200f stably.
First, the positive electrode is described.
The current collector 400 is metal foil, and the positive electrode is formed by applying slurry onto the metal foil and drying the slurry. Pressing may be performed after drying. The positive electrode is a component obtained by forming an active material layer over the current collector 400.
In
In the positive electrode of the secondary battery, a binder (a resin) is mixed in order to fix the current collector 400 such as metal foil and the active material. The binder is also referred to as a binding agent. Since the binder is a high molecular material, a large amount of binder lowers the proportion of the active material in the positive electrode, thereby reducing the discharge capacity of the secondary battery. Therefore, the amount of binder mixed is reduced to a minimum. In
In
Although
In the positive electrode in
In
In the step of mixing the graphene 404 and the acetylene black 403 to obtain an electrode slurry, the weight of mixed acetylene black is preferably 1.5 to 20 times, further preferably 2 to 9.5 times the weight of graphene.
When the graphene 404 and the acetylene black 403 are mixed in the above ratio range, the acetylene black 403 can be dispersed uniformly and less likely to be aggregated at the time of preparing the slurry. Furthermore, when the graphene 404 and the acetylene black 403 are mixed in the above ratio range, the positive electrode can have a higher density than that using only the acetylene black 403 as the conductive material. As the electrode density is higher, the capacity per unit volume can be higher. Specifically, the density of the positive electrode active material layer measured by gravimetry can be higher than 3.5 g/cc. In addition, it is preferable that one or more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 be used as the active material 701 of the positive electrode and the graphene 404 and the acetylene black 403 be mixed in the above ratio range, in which case synergy for higher capacity of the secondary battery can be expected.
A positive electrode containing a first carbon material (graphene) and a second carbon material (acetylene black) which are mixed in the above range enables fast charging, although having lower electrode density than a positive electrode containing only graphene as a conductive material. In addition, it is preferable that one or more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 be used as the active material 701 of the positive electrode and the graphene 404 and the acetylene black 403 be mixed in the above ratio range, in which case synergy for higher stability and compatibility with faster charging of the secondary battery can be expected.
The above features are advantageous for secondary batteries for vehicles.
When a vehicle becomes heavier with increasing number of secondary batteries, more energy is consumed to move the vehicle, which decreases the mileage. With the use of high-density secondary batteries, the mileage of the vehicle can be maintained with almost no increase in the total weight of a vehicle including a secondary battery having the same weight.
Since power is needed to charge the secondary battery with higher capacity in the vehicle, the charging is desirably finished in a short time. What is called a regenerative charging, in which electric power temporarily generated when the vehicle is braked is used for charging, is performed under high rate charging conditions; thus, a secondary battery for a vehicle is desired to have favorable rate characteristics.
Using one of more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 as the active material 701 of the positive electrode and mixing acetylene black and graphene within an optimal range enable both higher electrode density and formation of an appropriate space needed for ion conduction, whereby a secondary battery for a vehicle which has high energy density and favorable output characteristics can be obtained.
This structure is also effective in a portable information terminal, and using one of more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 as the active material 701 of the positive electrode and setting the mixing ratio of acetylene black to graphene in the optimal range enable a small secondary battery with high capacity. Setting the mixing ratio of acetylene black to graphene in the optimal range also enables fast charging of a portable information terminal.
In
Using one of more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 as the active material 701 of the positive electrode and mixing acetylene black and graphene within an optimal range enable both higher electrode density and formation of an appropriate space needed for ion conduction, whereby a secondary battery which has high energy density and favorable output characteristics can be obtained.
A fluorine-containing carbon nanotube may be used. A fluorine-containing carbon nanotube can be formed by making a carbon nanotube and a fluorine compound contact each other (which is called fluorination treatment). The description of the fluorination treatment using graphene can be referred to for the fluorination treatment using carbon nanotube.
In
Fluorine-containing acetylene black may be used. Fluorine-containing acetylene black can be formed by making acetylene black and a fluorine compound contact each other (which is called fluorination treatment). The description of the fluorination treatment using graphene can be referred to for the fluorination treatment using acetylene black.
In
A secondary battery can be fabricated by using any one of the positive electrodes in
Although the above structure is an example of a secondary battery using an electrolyte solution, one embodiment of the present invention is not limited thereto.
For example, a semi-solid-state battery can be fabricated using one or more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 as an active material of the positive electrode.
A semi-solid-state battery fabricated using one of more of the positive electrode active materials 1201, 1211, and 1200f described in Embodiments 10 to 12 is a secondary battery having high charge and discharge capacity. The semi-solid-state battery can have high charging and discharging voltages. Alternatively, a highly safe or highly reliable semi-solid-state battery can be achieved.
This embodiment can be freely combined with any of the other embodiments.
This application is based on Japanese Patent Application Serial No. 2022-092766 filed with Japan Patent Office on Jun. 8, 2022, Japanese Patent Application Serial No. 2022-098104 filed with Japan Patent Office on Jun. 17, 2022, and Japanese Patent Application Serial No. 2022-139989 filed with Japan Patent Office on Sep. 2, 2022, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-092766 | Jun 2022 | JP | national |
| 2022-098104 | Jun 2022 | JP | national |
| 2022-139989 | Sep 2022 | JP | national |
