Patent Application
20240030429
One embodiment of the present invention relates to an object, a method, or a manufacturing method. Another embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof. In particular, the present invention relates to a positive electrode active material for a lithium-ion secondary battery, a fabrication method thereof, and a lithium-ion secondary battery including the positive electrode active material.
In recent years, demand for lithium-ion secondary batteries with high output and high capacity has rapidly grown with the development of the semiconductor industry, and the lithium-ion secondary batteries are essential for modern society as energy sources that can be repeatedly used.
In particular, as lithium-ion secondary batteries for mobile electronic devices, secondary batteries with a large discharge capacity per weight and excellent charge and discharge characteristics have been required. In order to meet such demands, positive electrode active materials included in positive electrodes of secondary batteries have been actively improved (see Patent Document 1, for example).
Patent Document 1 discloses, focusing on a crack in a positive electrode active material, a technique for repairing the crack by performing sol-gel treatment a plurality of times. The crack is generated before the positive electrode active material is completed.
However, the deterioration state of the positive electrode active material after a cycle test is not taken into account in Patent Document 1. In view of this, an object of the present invention is to provide a positive electrode active material with a suppressed decrease in discharge capacity retention rate in a cycle test, as a result of earnest consideration on the deterioration state of the positive electrode active material after a cycle test.
Another object of one embodiment of the present invention is to provide a lithium-ion secondary battery including the positive electrode active material and an electronic device including the lithium-ion secondary battery, or to provide fabrication methods thereof.
Note that the description of these objects does not preclude the existence of other objects. One embodiment of the present invention does not have to achieve all of these objects. Other objects can be derived from the description of the specification, the drawings, and the claims.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, the positive electrode active material includes a defect, and contains at least an element identical to the additive element in a region in the vicinity of the defect.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, the positive electrode active material includes a defect, and contains at least an element identical to the additive element in a region in the vicinity of a side surface of the defect.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, the positive electrode active material includes a defect, and contains at least an element identical to the additive element in a region in the vicinity of a tip of the defect.
In one embodiment of the present invention, the defect preferably has a constant width.
In one embodiment of the present invention, an upper limit voltage of the cycle test can be 4.65 V or 4.7 V.
In one embodiment of the present invention, the additive element contained in the lithium cobalt oxide is preferably positioned in a surface portion of the lithium cobalt oxide.
In one embodiment of the present invention, the additive element contained in the lithium cobalt oxide preferably contains at least magnesium or aluminum.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed at an upper limit voltage of 4.7 V and at 25° C. on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, a surface portion of the positive electrode active material includes a region where a rock-salt structure exists.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed at an upper limit voltage of 4.7 V and at 25° C. on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, a surface portion of the positive electrode active material includes a region where a rock-salt structure exists, and the additive element contained in the lithium cobalt oxide is positioned in a surface portion of the lithium cobalt oxide.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed at an upper limit voltage of 4.7 V and at 45° C. on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, a surface portion of the positive electrode active material includes a region where a rock-salt structure exists and a region where a spinel structure exists.
One embodiment of the present invention is a positive electrode active material used for a secondary battery. The positive electrode active material includes lithium cobalt oxide containing an additive element. After a cycle test is performed at an upper limit voltage of 4.7 V and at 45° C. on a cell using the positive electrode active material for a positive electrode and a lithium electrode as a counter electrode, a surface portion of the positive electrode active material includes a region where a rock-salt structure exists and a region where a spinel structure exists, and the additive element is not detected in the surface portion of the positive electrode active material in EDX line analysis.
In one embodiment of the present invention, the rock-salt structure preferably exists in a region from a surface of the lithium cobalt oxide to a depth of greater than or equal to 0.8 nm and less than or equal to 0.9 nm.
In one embodiment of the present invention, the spinel structure preferably exists in a region from a surface of the lithium cobalt oxide to a depth of greater than or equal to 1.5 nm and less than or equal to 4.5 nm.
In one embodiment of the present invention, the lithium cobalt oxide preferably includes a defect having a constant width in a cross section.
In one embodiment of the present invention, the additive element preferably contains at least magnesium and aluminum.
One embodiment of the present invention is a lithium-ion secondary battery including the above positive electrode active material in a positive electrode, and graphite in a negative electrode.
One embodiment of the present invention is a vehicle including the above lithium-ion secondary battery.
A decrease in discharge capacity retention rate, i.e., deterioration in a cycle test is suppressed in the positive electrode active material of the present invention. A lithium-ion secondary battery including the positive electrode active material has a long lifetime and an improved safety. In addition, an electronic device including the lithium-ion secondary battery can be used for a long period and has an improved safety.
Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not have to have all of these effects. Note that other effects will be apparent from the description of the specification, the drawings, the claims, and the like, and other effects can be derived from the description of the specification, the drawings, the claims, and the like.
FIG. 52A1 to
FIG. 53A1 to
FIG. 56A1 to
FIG. 57A1 to
Examples of embodiments of the present invention are described below with reference to the drawings and the like. Note that the present invention should not be construed as being limited to the description of the examples of the embodiments below. Embodiments of the invention can be changed unless it deviates from the spirit of the present invention.
In this specification and the like, the Miller index is used for the expression of crystal planes and crystal orientations. An individual plane that shows a crystal plane is denoted by “( )”. In the crystallography, a bar is placed over a number in the expression of crystal planes, crystal orientations, and space groups; in this specification and the like, because of format limitations, crystal planes, crystal orientations, and space groups are sometimes expressed by placing − (minus sign) in front of the number instead of placing a bar over the number.
In this specification and the like, a charge depth is used as an indicator; the charge depth obtained when all the lithium that can be inserted and extracted is inserted is 0, and the charge depth obtained when all the lithium that can be inserted and extracted and is contained in a positive electrode active material is extracted is 1.
In this specification and the like, a theoretical capacity of a positive electrode active material refers to the amount of electricity obtained when all lithium that can be inserted and extracted and is contained in the positive electrode active material is extracted. For example, the theoretical capacity of LiCoO2 is 274 mAh/g, the theoretical capacity of LiNiO2 is 274 mAh/g, and the theoretical capacity of LiMn2O4 is 148 mAh/g.
The remaining amount of lithium that can be inserted into and extracted from a positive electrode active material can be represented by x in a compositional formula, e.g., x in LixCoO2 or x in Lix/MO2. In the case of a positive electrode active material in a secondary battery, x can be represented by (theoretical capacity−charge capacity)/theoretical capacity. In the case where a secondary battery using LiCoO2 as a positive electrode active material is charged to 219.2 mAh/g, it can be said that the positive electrode active material is represented by Li0.2CoO2, or x=0.2. Small x in LixCoO2 means, for example, 0.1<x≤0.24.
In the case where lithium cobalt oxide substantially satisfies the stoichiometric composition, the lithium cobalt oxide is LiCoO2 and the occupancy rate x of Li in lithium sites is 1. In a secondary battery after its discharging ends, it can be said that the lithium cobalt oxide is LiCoO2 and x=1. Here, “discharging ends” means that a voltage becomes 2.5 V or lower (in the case of a lithium counter electrode) at a current of 100 mAh/g, for example. In a lithium-ion secondary battery, a voltage decreases steeply when the occupancy rate x of lithium in lithium sites becomes 1 and no more lithium is allowed to be inserted. It can be said that discharging ends at this time. In a general lithium-ion secondary battery using LiCoO2, the discharge voltage decreases steeply before the discharge voltage reaches 2.5 V; thus, discharging is regarded as ending when the above condition is satisfied.
Examples of a cycle test include a half-cell test using a lithium metal for a counter electrode and a full-cell test using graphite or the like for a counter electrode. A positive electrode active material after a cycle test deteriorates. The deterioration caused by a cycle test is sometimes referred to as deterioration over time. The deterioration over time generates a defect in some cases. The defect is not generated uniformly in a positive electrode active material and generated locally in some cases. Furthermore, the defect sometimes progresses due to an influence of a cycle test or the like.
It is considered that the deterioration over time tends to be apparent after a cycle test under a severe condition such as a high temperature or a high upper limit voltage.
In order to improve the characteristics of a lithium-ion secondary battery, inhibiting deterioration of a positive electrode active material should be taken into account. The deterioration over time has not been fully elucidated yet. In view of this, as a result of earnest consideration on the deterioration over time, the present inventors and the like have considered that the deterioration over time reflects a decrease in discharge capacity retention rate.
In this embodiment, a structure example 1 of a positive electrode active material of one embodiment of the present invention is described.
The positive electrode active material 100 illustrated in
In lithium composite oxide other than LCO, one or more of Fe, Mn, Ni, and Co may be contained. Examples of the lithium composite oxide containing Ni, Mn, and Co include a NiCoMn-based material (NCM, also referred to as lithium nickel-cobalt-manganese oxide) represented by LiNixCoyMnzO2 (x>0, y>0, and 0.8<x+y+z<1.2). Specifically, 0.1x<y<8x and 0.1x<z<8x are preferably satisfied in the above. For example, x, y, and z preferably satisfy x:y:z=1:1:1 or the neighborhood thereof. Alternatively, for example, x, y, and z preferably satisfy x:y:z=5:2:3 or the neighborhood thereof. 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=6:2:2 or the neighborhood thereof. Alternatively, for example, x, y, and z preferably satisfy x:y:z=1:4:1 or the neighborhood thereof. In the case of NCM, primary particles are aggregated to form a secondary particle in some cases.
The positive electrode active material 100 contains an additive element as an element other than a main component, for example, an element other than cobalt, oxygen, and lithium, in the case of lithium cobalt oxide. The additive element contains at least magnesium or aluminum, and Embodiment 5 can be referred to for additive elements other than magnesium and aluminum. When an additive element is mixed and then heating is performed, the additive element is positioned at least in a surface portion of the positive electrode active material 100 in some cases. Regions in each of which the additive element is positioned sometimes exhibit a function of inhibiting deterioration of the inner portion 50, and are referred to as barrier layers 53a to 53c here. The barrier layers 53a to 53c may each contain a main component element, and may contain cobalt, for example. Containing the main component, the barrier layers 53a to 53c can transmit at least a carrier ion (e.g., lithium ion) and can be regarded as parts of the positive electrode active material 100.
Being positioned in the surface portion, the barrier layers 53a to 53c are in contact with a liquid electrolyte (sometimes also referred to as electrolyte solution). In this state, the additive element might be eluted to the electrolyte solution from any of the barrier layers 53a to 53c by a chemical reaction or an electrochemical reaction in a cycle test. Needless to say, the main component of the positive electrode active material, such as cobalt or oxygen contained in the barrier layers 53a to 53c, might also be eluted to the electrolyte solution. Because of various reasons including the possibility of elution, the states of the barrier layers 53a to 53c change after deterioration over time. For example, a barrier layer existing as a continuous film is divided in some cases.
In the case where an additive element exists in the surface portion of the positive electrode active material 100, the concentration is higher than that in the inner portion 50 in some cases. This state can be expressed as a state where the additive element is unevenly distributed in the surface portion of the positive electrode active material 100. The unevenly distributed element may be referred to as an additive element.
In the case where the additive element is unevenly distributed in the surface portion, the additive element is not detected in the inner portion 50 in some cases. “The additive element is not detected” means that the concentration of the additive element is lower than or equal to the lower detection limit of a measurement device. Preferably, an additive element that does not contribute to an improvement in capacitance of the positive electrode active material 100 is not detected in the inner portion 50.
The positive electrode active material 100 includes a defect that is a factor of deterioration.
The crack 57 is a crevice generated by application of physical stress. It is considered that the positive electrode active material 100 expands and contracts repeatedly during a cycle test. Volume change due to the repeated expansion and contraction probably applies physical stress on the positive electrode active material 100. The crack 57 generated by application of stress sometimes horizontally crosses the crystal plane 52, and has a tapered shape in a cross-sectional view in some cases.
The pits 58a and 58b are holes formed by extraction of some layers of a main component such as cobalt or oxygen in a cycle test, and include holes generated by pitting corrosion. For example, cobalt is considered to be eluted to the electrolyte solution in some cases, and elution of one layer of cobalt might result in formation of a hole that is referred to as a pit. The pit might progress in a cycle test to be a deep hole. In addition, the pits 58a and 58b are sometimes triggered by entering and leaving of lithium ions in a cycle test. In addition, the pits 58a and 58b might progress in accordance with volume change such as expansion and contraction of the positive electrode active material 100 in a cycle test; furthermore, the pits 58a and 58b might be triggered by the crack 57. The pits 58a and 58b rarely cross the crystal plane 52 horizontally in a cross-sectional view, and extend in a direction along the crystal plane 52. In most cases, the pit progresses with a constant width and has a tapered tip.
Although both the crack 57 and the pits 58a and 58b are defects, there seems to be various differences between them as described above.
The shape of an opening portion corresponding to entrance of the pits 58a and 58b is not limited to a circular shape and may be a rectangular shape. The width of the pits 58a and 58b or simply the width of a defect in a cross section is greater than or equal to 3 nm and less than or equal to 50 nm, preferably greater than or equal to 5 nm and less than or equal to 40 nm. The depth of the pits 58a and 58b or simply the depth of a defect is greater than or equal to 10 nm and less than or equal to 2 μm, preferably greater than or equal to 50 nm and less than or equal to 1.8 μm. The width and depth of the pits 58a and 58b are probably determined depending on the cycle test condition such as the number of cycles or the ambient temperature.
Since the electrolyte solution can enter the pits 58a and 58b with the above size, the positive electrode active material 100 in the vicinity of the pits 58a and 58b is also impregnated with the electrolyte solution. The positive electrode active material 100 impregnated with the electrolyte solution allows entering and leaving of lithium ions, but cannot maintain the layered rock-salt crystal structure because of the contact with the electrolyte solution, and thus is decreased in crystallinity in some cases. That is, the positive electrode active material 100 in the vicinity of the pit sometimes has lower crystallinity than the inner portion and includes an amorphous region, for example.
Such a positive electrode active material in the vicinity of a pit preferably includes a region containing the same element as the additive element. In
The pit is often generated after a cycle test; at the time when the positive electrode active material 100 is completed, no pit is generated and of course no additive element exists in the vicinity of a pit. When a pit is generated due to a cycle test, the positive electrode active material 100 in the vicinity of the pit preferably includes a region containing the additive element. The present inventors and the like have found the correlation between the region and suppression of a decrease in discharge capacity retention rate. For example, the present inventors have assumed that the region suppresses the pit progress and accordingly inhibits a decrease in discharge capacity retention rate. In order to suppress the pit progress, the additive element preferably exists at least in the vicinity of the tip of the pit rather than in the vicinity of a side surface of the pit. The region becomes an amorphous region in some cases.
It can be considered that the additive element in the barrier layer enters the positive electrode active material through the pit. Alternatively, it can be considered that the additive element in the inner portion of the positive electrode active material 100 diffuses outwardly. In any cases, the additive element preferably exists in the positive electrode active material 100 in the vicinity of the pit.
When a model where the additive element comes from the barrier layer is considered, the additive element is probably eluted from the barrier layer to the electrolyte solution in a cycle test. In the subsequent cycle test, the additive element probably enters the positive electrode active material in the vicinity of the pit through the electrolyte solution, like lithium. In the case where the positive electrode active material is lithium cobalt oxide and the additive element is magnesium, the magnesium is sometimes positioned in lithium sites in the lithium cobalt oxide in the vicinity of the pit, and in the case where the additive element is aluminum, the aluminum is sometimes positioned in cobalt sites in the lithium cobalt oxide in the vicinity of the pit. The additive element positioned in each site preferably suppresses the pit progress.
The state where the additive element is positioned in each site in the positive electrode active material is regarded as a state where the additive element forms a solid solution in the positive electrode active material 100. That is, an additive element that forms a solid solution in the positive electrode active material is preferably used. Therefore, among the additive elements in the barrier layers, an element that does not form a solid solution is not positioned in the vicinity of the pit, in some cases. In this case, the kind of the additive element detected in the barrier layer is sometimes different from the kind of the additive element detected in the positive electrode active material in the vicinity of the pit.
Note that a cycle test can be regarded as corresponding to a usage mode of a lithium-ion secondary battery. Analysis of the positive electrode active material after a cycle test allows understanding of the state of the positive electrode active material of a lithium-ion secondary battery.
Note that distortion that causes the pits 58a and 58b is generated by difference in a lattice constant between a portion where a large amount of Li is extracted and a portion where Li is not extracted so much. The difference in a lattice constant can probably be reduced by a pit. A pit generated to reduce the difference in a lattice constant is formed deeply, and the depth of an adjacent pit is small. That is, adjacent pits are different from each other at least in the depth (see the pit 58a and the like in
In consideration of such an effect of the pit, a positive electrode active material including a pit whose progress is suppressed seems to be preferable as compared with a positive electrode active material including no pit.
Since a pit is likely to be generated in a positive electrode active material with a layered rock-salt crystal structure, the pit progress is desired to be suppressed.
The positive electrode active material 100 illustrated in
When a cycle test is performed while the positive electrode active material is in contact with the electrolyte solution or the like, oxidation decomposition of an organic solvent of an electrolyte occurs and the decomposed product might form a coating film on the positive electrode active material. The coating film includes the organic solvent, and thus has a composition different from that of the barrier layer containing the above additive element.
This embodiment can be used in appropriate combination with any of the other embodiments.
In this embodiment, a structure example 2 of the positive electrode active material of one embodiment of the present invention is described.
The positive electrode active material 100 illustrated in
The positive electrode active material 100 illustrated in
Note that the state of the barrier layers 53a to 53c might change due to a cycle test. For example, the barrier layers 53a to 53c before a cycle test preferably have a larger area for covering the positive electrode active material 100 than those after the cycle test, and further preferably form a barrier layer that is like a continuous film. Needless to say, the barrier layers may be separated from each other before a cycle test. The barrier layers 53a to 53c preferably exist with a uniform thickness in a cross section of the positive electrode active material, and for the existence with a uniform thickness, the inner portion 50 preferably has a smooth surface.
In the case where the additive element exists in the surface portion of the positive electrode active material 100, the concentration of the additive element is higher than that in the inner portion 50 in some cases. This state can be expressed as a state where the additive element is unevenly distributed in the surface portion of the positive electrode active material 100. The unevenly distributed element may be referred to as an additive element.
In the case where the additive element is unevenly distributed in the surface portion, the additive element is not detected in the inner portion 50 in some cases. “The additive element is not detected” means that the concentration of the additive element is lower than or equal to the lower detection limit of a measurement device. An additive element that does not contribute to an improvement in battery characteristics such as capacitance of the positive electrode active material 100, for example, preferably has a concentration that does not allow the additive element to be detected in the inner portion 50.
The barrier layers 53a to 53c may contain a main component element, and may contain cobalt when the positive electrode active material is LCO. Containing the main component, the barrier layers 53a to 53c can transmit at least a carrier ion (e.g., lithium ion) and can be regarded as parts of the positive electrode active material 100.
The positive electrode active material 100 illustrated in
The inner portion 50 of the positive electrode active material 100 preferably has a layered rock-salt crystal structure. A composite oxide containing cobalt can be used as the positive electrode active material having a layered rock-salt crystal structure; examples of the composite oxide include lithium cobalt oxide (LiCoO2, sometimes simply referred to as LCO) and lithium nickel-cobalt-manganese oxide (sometimes simply referred to as NCM). In the case of LCO, the positive electrode active material 100 is often a primary particle. In the case of NCM, primary particles are often aggregated to form a secondary particle.
The generation mechanism of the pits 58a and 58b illustrated in
The pits 58a and 58b are holes formed by extraction of some layers of cobalt or oxygen that is a main component of the positive electrode active material 100 due to a cycle test, and include holes generated by pitting corrosion. The pits 58a and 58b begin to be generated from the surface portion of the positive electrode active material 100, which is because the surface portion is under the condition where cobalt or oxygen is released. The surface of the positive electrode active material 100 is in contact with the electrolyte solution and the positive electrode active material is impregnated with the electrolyte solution. That is, the surface portion of the positive electrode active material 100 is under the condition of being in contact with the electrolyte solution. When a material that easily reacts with oxygen is used for the electrolyte solution, oxygen contained in the positive electrode active material 100 reacts with the electrolyte solution; accordingly, a Co—O bond (cobalt-oxygen bond) at least in the surface portion is cut. The cobalt cut from the bond is considered to diffuse into the inner portion 50 as well as the electrolyte solution. This is because cobalt is considered to move mainly to lithium sites. Lithium ions do not exist in the lithium sites at the time of charging, which allows cobalt to easily move to the lithium sites. When the Co—O bond in the surface portion is cut, oxygen is desorbed, and cobalt diffuses as described above, the crystal structure of the surface portion probably changes.
The change in the crystal structure is described.
Note that CoO has a rock-salt structure in which no Li layer is observed. In addition, LiCo2O4 has a spinel structure in which lithium can be observed but a lithium layer is different from that in a layered rock-salt crystal structure, and Co3O4 has a spinel structure in which no Li layer can be observed. Li is less likely to enter and leave in the spinel structure than in the LCO structure. Furthermore, CoO is positioned closer to the surface of the positive electrode active material 100 than LiCo2O4 or Co3O4. CoO exists at a position where the contact with the electrolyte solution is the largest, but has a crystal structure that does not allow entering and leaving of Li.
The surface region 105b is in contact with the electrolyte solution or impregnated with the electrolyte solution, which probably causes a change in the crystal structure before and after a cycle test. The present inventors and the like have considered that it is important to understand at least the crystal structure, particularly the crystal structure of the surface portion after the cycle test to grasp the generation mechanism of a pit.
Note that the surface portion is a region that can be impregnated with the electrolyte solution, and include the surface of the positive electrode active material 100. Specifically, the surface portion includes a region from the surface of the positive electrode active material 100 to a depth of 20 nm.
The pits 58a and 58b illustrated in
Examples of the defect other than the pits 58a and 58b include a crack and a slip.
As described above, the surface portion of the positive electrode active material 100 easily deteriorates due to the contact with the electrolyte solution, and the reaction between the electrolyte solution and the positive electrode active material, for example, is also deemed to be a factor of generation trigger and progress of the pits 58a and 58b that are defects.
The above suggests that the positive electrode active material 100 where at least a pit is observed after a cycle test deteriorates. This is because lithium ions are less likely to enter and leave in the spinel structure than in the LCO structure.
In view of this, the present inventors have found a structure where the barrier layers 53a to 53c are provided in the surface portion of the positive electrode active material 100, as illustrated in
The barrier layers 53a to 53c contain an additive element as an element other than a component element, for example, an element other than cobalt, oxygen, and lithium, in the case of lithium cobalt oxide. The additive element contains at least magnesium, fluorine, nickel, aluminum, or zirconium. Refer to Embodiment 3 for the other additive elements. When lithium cobalt oxide or the like where an additive element is mixed is heated, the additive element positioned at least in the surface portion of the positive electrode active material 100 can be observed.
Even when having the same crystal structure as the inner portion 50, the barrier layers 53a to 53c are different from the inner portion 50 in containing the additive element. When the additive element is contained, lithium is less likely to be extracted even in a charged state and thus the crystal structure is less likely to be broken. When lithium is not extracted, no lithium site is formed and thus cobalt diffusion into the lithium site does not occur. Therefore, the barrier layers 53a to 53c are considered to be able to maintain the same crystal structure as the inner portion 50 when being positioned in the surface portion. Thus, a pit is less likely to be generated in the barrier layers 53a to 53c, which indicates a high function of preventing contact between the electrolyte solution and the inner portion 50.
In the case where an additive element exists in the surface portion of the positive electrode active material 100, the concentration is higher than that in the inner portion 50 in some cases. This state can be expressed as a state where the additive element is unevenly distributed in the surface portion of the positive electrode active material 100. The unevenly distributed element may be referred to as an additive element. The concentration of the additive element in the barrier layers 53a to 53c positioned in the surface portion is also higher than that in the inner portion 50 in many cases.
In the case where the additive element is unevenly distributed in the surface portion, the additive element is not detected in the inner portion 50 in some cases. “The additive element is not detected” means that the concentration of the additive element is lower than or equal to the lower detection limit of a measurement device. It is preferable that an additive element that does not contribute to an improvement in capacitance of the positive electrode active material 100 be not detected in the inner portion 50.
When a cycle test is performed while the positive electrode active material is in contact with the electrolyte solution or the like, oxidation decomposition of an organic solvent of an electrolyte occurs and the decomposed product might form a coating film on the positive electrode active material. The coating film includes the organic solvent, and thus has a composition different from that of the barrier layer containing the above additive element.
Note that a cycle test is regarded as corresponding to a usage mode of a lithium-ion secondary battery. Analysis of the positive electrode active material after a cycle test allows understanding of the state of the positive electrode active material of a lithium-ion secondary battery.
Note that distortion that causes the pits 58a and 58b is generated by difference in a lattice constant between a portion where a large amount of Li is extracted and a portion where Li is not extracted so much. The difference in a lattice constant can probably be reduced by a pit. A pit generated to reduce the difference in the lattice constant is formed deeply, and the depth of an adjacent pit is small. That is, adjacent pits are different from each other at least in the depth (see the pit 58a and the like in
This embodiment can be used in appropriate combination with any of the other embodiments.
In this embodiment, a structure example 3 of a positive electrode active material of one embodiment of the present invention is described.
The positive electrode active material 100 illustrated in
The positive electrode active material 100 illustrated in
The positive electrode active material 100 includes a defect after a cycle test; in
The split 61 exists in the inner portion 50, and thus is considered to be generated by factors including a factor different from that of a pit. For example, the splits 61 are formed a lot in the positive electrode active material 100 exposed to high temperature (higher than or equal to ° C.) in a cycle test. In the positive electrode active material 100 exposed to room temperature (25° C.) in a cycle test, no split is detected or only undetectable split is formed.
The positive electrode active material 100 illustrated in
The change in the crystal structure is described.
The positive electrode active material 100 in which a split is observed after a cycle test is found to have a spinel structure, which indicates that lithium ions are less likely to enter and leave in the spinel structure. It is probable that the split is also a factor of deterioration after a cycle test. Thus, an additive element, for example, is preferably added so as to be distributed uniformly in the inner portion 50 to inhibit generation of a split.
In addition to the pit formation and crystal structure change in the surface portion which are described in Embodiment 1, the split or the like described in this embodiment is also considered to be a factor of deterioration.
This embodiment can be used in appropriate combination with any of the other embodiments.
In this embodiment, a structure example 4 of a positive electrode active material of one embodiment of the present invention is described.
The positive electrode active material 190 illustrated in
The positive electrode active material 190 illustrated in
The positive electrode active material 190 illustrated in
The barrier layer 192 and the shell layer 193 are positioned in the surface portion, not in the inner portion 191. The inner portion 191 is sometimes referred to as a core with respect to a cell. The barrier layer 192 contains an additive element different from the main component of the inner portion 191. Therefore, the barrier layer 192 is referred to as an impurity layer in some cases.
The thickness of the shell layer 193 is preferably larger than the thickness of the barrier layer 192. This enables effective protection of the barrier layer 192. For example, the thickness of the shell layer 193 in a cross section is preferably greater than or equal to 1.2 times and less than or equal to 3 times the thickness of the barrier layer 192.
The shell layer 193 can be obtained through a composite-making process using a first material and a second material. The first material corresponds to the positive electrode active material including the barrier layer. The second material corresponds to the shell layer. As the second material, an active material that can occlude and release lithium may be used. For example, as the second material, one or more of an oxide and LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, and Mn) that has an olivine crystal structure can be used. Examples of the oxide include aluminum oxide, zirconium oxide, hafnium oxide, and niobium oxide. Examples of the LiMPO4 include LiFePO4, LiNiPO4, LiCoPO4, LiMnPO4, LiFeaNibPO4, LiFeaCobPO4, LiFeaMnbPO4, LiNiaCobPO4, LiNiaMnbPO4 (a+b≤1, 0<a<1, and 0<b<1), LiFecNidCoePO4, LiFecNidMnePO4, LiNicCodMnePO4 (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), and LiFefNigCohMniPO4 (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and 0<i<1).
As the composite-making process, any one or more of the following composite-making processes can be used: a composite-making process utilizing mechanical energy such as a mechanochemical process, a mechanofusion process, and a ball mill process; a composite-making process utilizing a liquid phase reaction such as a coprecipitation process, a hydrothermal process, and a sol-gel process; and a composite-making process utilizing a gas phase reaction such as a barrel sputtering process, an ALD (Atomic Layer Deposition) process, an evaporation process, and a CVD (Chemical Vapor Deposition) process. After the composite-making process, heat treatment is preferably performed. Note that the composite-making process is sometimes referred to as a surface coating process or a coating process.
The positive electrode active material 190 illustrated in
This embodiment can be used in appropriate combination with any of the other embodiments.
In this embodiment, a method for fabricating a positive electrode active material of one embodiment of the present invention is described.
In Step S11 shown in
A compound containing lithium is preferably used as the lithium source, and for example, lithium carbonate, lithium hydroxide, lithium nitrate, lithium fluoride, or the like can be used. The lithium source preferably has a high purity and is preferably a material having a purity of higher than or equal to 99.99%, for example.
The transition metal can be selected from the elements belonging to Group 4 to Group 13 of the periodic table and for example, at least one of manganese, cobalt, and nickel is used. As the transition metal, for example, cobalt alone; nickel alone; two metals of cobalt and manganese; two metals of cobalt and nickel; or three metals of cobalt, manganese, and nickel may be used. When the transition metal is cobalt alone, the positive electrode active material to be obtained contains lithium cobalt oxide (LCO); when the transition metal is three metals of cobalt, manganese, and nickel, the positive electrode active material to be obtained contains lithium nickel cobalt manganese oxide (NCM).
As the transition metal source, a compound containing the above transition metal is preferably used and for example, an oxide, a hydroxide, or the like of any of the metals given as examples of the transition metal can be used. As a cobalt source, for example, cobalt oxide, cobalt hydroxide, or the like can be used. As a manganese source, manganese oxide, manganese hydroxide, or the like can be used. As a nickel source, nickel oxide, nickel hydroxide, or the like can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, or the like can be used.
The transition metal source preferably has high purity, and for example, it is preferable to use a material having purity higher than or equal to 3N (99.9%), further preferably higher than or equal to 4N (99.99%), still further preferably higher than or equal to 4N5 (99.995%), yet still further preferably higher than or equal to 5N (99.999%). Impurities of the positive electrode active material can be controlled by using such a high-purity material. As a result, the capacity of a secondary battery is increased and/or the reliability of the secondary battery is improved.
Furthermore, the transition metal source preferably has high crystallinity and for example, preferably includes single crystal particles. To evaluate the crystallinity of the transition metal source, for example, the crystallinity can be judged with a TEM (transmission electron microscope) image, a STEM (scanning transmission electron microscope) image, an HAADF-STEM (high-angle annular dark field scanning transmission electron microscope) image, an ABF-STEM (annular bright-field scan transmission electron microscope) image, or the like, or can be judged by X-ray diffraction (XRD), electron diffraction, neutron diffraction, or the like. Note that the above methods for evaluating crystallinity can also be employed to evaluate the crystallinity of other materials in addition to the transition metal source.
In the case of using two or more transition metal sources, the two or more transition metal sources are preferably prepared to have proportions (mixing ratio) such that a layered rock-salt crystal structure would be obtained.
Next, in Step S12 shown in
As a means for the mixing or the like, a ball mill, a bead mill, or the like can be used. In the case where a ball mill is used, alumina balls or zirconia balls are preferably used as a grinding medium. Zirconia balls are preferable because they release fewer impurities. In the case where a ball mill, a bead mill, or the like is used, the peripheral speed is preferably greater than or equal to 100 mm/s and less than or equal to 2000 mm/s in order to inhibit contamination from the media or the material. In this embodiment, mixing is performed at a peripheral speed of 838 mm/s (the rotation speed: 400 rpm, the ball mill diameter: 40 mm).
Next, in Step S13 shown in
The heating time is preferably longer than or equal to 1 hour and shorter than or equal to 100 hours, further preferably longer than or equal to 2 hours and shorter than or equal to 20 hours.
A temperature raising rate is preferably higher than or equal to 80° C./h and lower than or equal to 250° C./h, although depending on the end-point temperature of the heating. For example, in the case of heating at 1000° C. for 10 hours, the temperature raising is preferably at 200° C./h.
The heating is preferably performed in an atmosphere with little water such as a dry-air atmosphere and for example, the dew point of the atmosphere is preferably lower than or equal to −50° C., further preferably lower than or equal to −80° C. In this embodiment, the heating is performed in an atmosphere with a dew point of −93° C. To reduce impurities that might enter the material, the concentrations of impurities such as CH4, CO, CO2, and H2 in the heating atmosphere are each preferably lower than or equal to 5 ppb (parts per billion).
The heating atmosphere is preferably an oxygen-containing atmosphere. In a method, a dry air is continuously introduced into a reaction chamber. The flow rate of a dry air in this case is preferably 10 L/min. Continuously introducing oxygen into a reaction chamber to make oxygen flow therein is referred to as “flowing”.
In the case where the heating atmosphere is an oxygen-containing atmosphere, flowing of oxygen is not necessarily performed. For example, a method may be employed in which the pressure in the reaction chamber is reduced and then the reaction chamber is filled with oxygen so that the oxygen does not enter or exit in/from the reaction chamber; this method is referred to as purging. For example, the pressure in the reaction chamber may be reduced to −970 hPa, and then the reaction chamber may be filled with oxygen until the pressure becomes 50 hPa.
Cooling after the heating can be performed by natural cooling, and the time it takes for the temperature to decrease to room temperature from a predetermined temperature is preferably longer than or equal to 10 hours and shorter than or equal to 50 hours. Note that the temperature does not necessarily need to decrease to room temperature as long as it decreases to a temperature acceptable to the next step.
The heating in this step may be performed with a rotary kiln or a roller hearth kiln. The heating with a rotary kiln can be performed while stirring is performed in either case of a sequential rotary kiln or a batch-type rotary kiln. In a rotary kiln or a roller hearth kiln, oxygen is preferably flowed.
A crucible or a sagger used in heating is preferably made of alumina, in which case impurities are less likely to enter during mixing with the crucible or the sagger. In this embodiment, a crucible made of alumina with a purity of 99.9% is used. Heating is preferably performed with a rid put on the crucible. This can prevent evaporation or sublimation of a material.
After the heating is finished, the heated material is ground as needed and may be made to pass through a sieve. Before collection of the heated material, the material may be moved from the crucible to a mortar. An alumina mortar is preferably used as the mortar, in which case impurities are less likely to enter during mixing with the mortar. Specifically, it is preferable to use a mortar made of alumina with a purity of higher than or equal to 90%, preferably higher than or equal to 99%. Note that heating conditions equivalent to those in Step S13 can be employed in a later-described heating step other than Step S13.
Through the above steps, a composite oxide containing the transition metal (LiMO2) can be obtained in Step S14 shown in
Although the example is described in which the composite oxide is fabricated by a solid phase method as in Step S11 to Step S14, the composite oxide may be fabricated by a coprecipitation method. Alternatively, the composite oxide may be fabricated by a hydrothermal method.
An additive element X may be added to the composite oxide as long as a layered rock-salt crystal structure is obtained. A step of adding the additive element is described.
In Step S20 shown in
As the additive element, one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic can be used. As the additive element, one or both of bromine and beryllium can be used. Note that the above-described additive elements are more suitably used because bromine and beryllium are elements having toxicity to living things.
For the addition of the additive element X, a solid phase method, a liquid phase method such as a sol-gel method, a sputtering method, an evaporation method, a CVD (chemical vapor deposition) method, a PLD (pulsed laser deposition) method, or the like can be used.
When magnesium is selected as the additive element, the additive element source can be referred to as a magnesium source. As the magnesium source, for example, magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate can be used. Two or more of these magnesium sources may be used.
When fluorine is selected as the additive element, the additive element source can be referred to as a fluorine source. As the fluorine source, for example, lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2 or CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF2), lanthanum fluoride (LaF3), sodium aluminum hexafluoride (Na3AlF6), or the like can be used. Among them, lithium fluoride, which has a relatively low melting point of 848° C., is preferable because it is easily melted in a heating process described later.
Magnesium fluoride can be used as both the fluorine source and the magnesium source. Lithium fluoride can also be used as both the fluorine source and the lithium source. Another example of the lithium source that can be used in Step S20 is lithium carbonate.
The fluorine source may be a gas, and for example, fluorine (F2), carbon fluoride, sulfur fluoride, oxygen fluoride (OF2, O2F2, O3F2, O4F2, or O2F), or the like may be used and mixed in the atmosphere in a heating step described later. Two or more of these fluorine sources may be used.
In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as the fluorine source and the magnesium source. When lithium fluoride and magnesium fluoride are mixed at approximately LiF:MgF2=65:35 (molar ratio), the effect of lowering the melting point becomes the highest. On the other hand, when the amount of lithium fluoride increases, cycle performance might deteriorate because of too large an amount of lithium. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably LiF:MgF2=x:1 (0≤x≤1.9), further preferably LiF:MgF2=x:1 (0.1≤x≤0.5), still further preferably LiF:MgF2=x:1 (x=0.33 and the neighborhood thereof). Note that the neighborhood means a value greater than 0.9 times and less than 1.1 times 0.33.
Next, as the additive element source (X source), a magnesium source and a fluorine source are ground and mixed. This step can be performed under any of the conditions for the grinding and mixing selected from those described for Step S12.
Next, a heating step may be performed as needed. The heating step can be performed under any of the heating conditions selected from those described for Step S13. The heating time is preferably longer than or equal to 2 hours and the heating temperature is preferably higher than or equal to 800° C. and lower than or equal to 1100° C. The materials ground and mixed in the above step are collected, so that the additive element source (X source) can be obtained. Note that the obtained additive element source is formed of a plurality of starting materials and can be referred to as a mixture. Note that the obtained additive element source is referred to as a mixture even when being formed of one kind of starting material.
As for the particle diameter of the mixture, the median diameter (D50) is preferably greater than or equal to 600 nm and less than or equal to 20 μm, further preferably greater than or equal to 1 μm and less than or equal to 10 μm. Also when one kind of material is used as the additive element source, the median diameter (D50) is preferably greater than or equal to 600 nm and less than or equal to 20 μm, and further preferably greater than or equal to 1 μm and less than or equal to 10 μm.
When mixed with a composite oxide in a later step, the mixture thus pulverized is easily attached to surfaces of composite oxide particles uniformly. The mixture is preferably attached to the surfaces of the composite oxides uniformly, in which case at least magnesium is easily distributed in or diffused to the surface portions of the composite oxides uniformly after heating. A region where magnesium is distributed can be referred to as a surface portion. When the surface portion includes a region not containing magnesium, the positive electrode active material might be less likely to have an O3′ type crystal structure, which is described later, in a charged state.
Although the example where two kinds of additive element sources, which are the magnesium source and the fluorine source, are prepared is described above, three or more kinds of additive element sources may be added to the composite oxide.
For example, four kinds of additive element sources, which are a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source), can be prepared. Note that the magnesium source and the fluorine source can be selected from the compounds and the like described above. As the nickel source, nickel oxide, nickel hydroxide, or the like can be used. As the aluminum source, aluminum oxide, aluminum hydroxide, or the like can be used.
Next, in Step S31 shown in
The conditions of the mixing in Step S31 are preferably milder than those of the mixing in Step S12 in order not to damage the composite oxide particles. For example, conditions with a lower rotation frequency or shorter time than the mixing in Step S12 are preferable. In addition, it can be said that the dry process has a milder condition than the wet process. As a means for the mixing, a ball mill, a bead mill, or the like can be used. In the case where a ball mill is used, zirconia balls are preferably used as media, for example.
In this embodiment, the mixing is performed with a ball mill using zirconia balls having a diameter of 1 mm by a dry process at 150 rpm for 1 hour. The mixing step is performed in a dry room with a dew point higher than or equal to −100° C. and lower than or equal to −10° C.
Next, in Step S32 in
Note that this embodiment describes a method for adding lithium fluoride as the fluorine source and magnesium fluoride as the magnesium source to the composite oxide in a later step. However, the present invention is not limited to the above method. The magnesium source, the fluorine source, and the like can be added to the lithium source and the transition metal source at the stage of Step S11, i.e., at the stage of the starting materials of the composite oxide. Then, the heating in Step S13 is performed, so that LiMO2 to which magnesium and fluorine are added can be obtained. In this case, there is no need to separate steps of Step S11 to Step S14 and steps of Step S31 to Step S32. This method can be regarded as being simple and highly productive.
Alternatively, lithium cobalt oxide to which magnesium and fluorine are added in advance may be used. When lithium cobalt oxide to which magnesium and fluorine are added is used, the steps of Step S11 to Step S32 and Step S20 can be omitted. This method can be regarded as being simple and highly productive.
Alternatively, in accordance with Step S20, a magnesium source and a fluorine source may be further added to the lithium cobalt oxide to which magnesium and fluorine are added in advance. Alternatively, a magnesium source, a fluorine source, a nickel source, and an aluminum source may be further added to the lithium cobalt oxide to which magnesium and fluorine are added in advance.
Next, in Step S33 shown in
Here, a supplementary explanation of the heating temperature is provided. The lower limit of the heating temperature in Step S33 needs to be higher than or equal to the temperature at which a reaction between the composite oxide (LiMO2) and the additive element source proceeds. The temperature at which the reaction proceeds is the temperature at which interdiffusion of the elements contained in LiMO2 and the additive element source occurs, and may be lower than the melting temperatures of these materials. When description is made using an oxide as an example, it is known that solid phase diffusion occurs at a temperature 0.757 times the melting temperature Tm (what is called the Tamman temperature Td). Accordingly, it is only required that the heating temperature in Step S33 be higher than or equal to 500° C., for example.
Of course, the reaction proceeds more easily at a temperature higher than or equal to the temperature at which at least part of the mixture 903 is melted. For example, in the case where LiF and MgF2 are contained as the additive element source, the eutectic point of LiF and MgF2 is around 742° C., and the lower limit of the heating temperature in Step S33 is preferably higher than or equal to 742° C.
The mixture 903 obtained by mixing such that LiCoO2:LiF:MgF2=100:0.33:1 (molar ratio) exhibits an endothermic peak at around 830° C. in differential scanning calorimetry measurement (DSC measurement). Therefore, the lower limit of the heating temperature is further preferably higher than or equal to 830° C.
A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and enables high productivity.
The upper limit of the heating temperature is lower than the decomposition temperature of LiMO2 (the decomposition temperature of LiCoO2 is 1130° C.). At around the decomposition temperature, a slight amount of LiMO2 might be decomposed. Thus, the heating temperature is preferably lower than or equal to 1000° C., further preferably lower than or equal to 950° C., still further preferably lower than or equal to 910° C.
In view of the above, the heating temperature in Step S33 is preferably higher than or equal to 500° C. and lower than or equal to 1130° C., further preferably higher than or equal to 500° C. and lower than or equal to 1000° C., still further preferably higher than or equal to 500° C. and lower than or equal to 950° C., and yet still further preferably higher than or equal to 500° C. and lower than or equal to 910° C. Furthermore, the heating temperature is preferably higher than or equal to 742° C. and lower than or equal to 1130° C., further preferably higher than or equal to 742° C. and lower than or equal to 1000° C., still further preferably higher than or equal to 742° C. and lower than or equal to 950° C., and yet still further preferably higher than or equal to 742° C. and lower than or equal to 910° C. Furthermore, the heating temperature is higher than or equal to 800° C. and lower than or equal to 1100° C., preferably higher than or equal to 830° C. and lower than or equal to 1130° C., further preferably higher than or equal to 830° C. and lower than or equal to 1000° C., still further preferably higher than or equal to 830° C. and lower than or equal to 950° C., and yet still further preferably higher than or equal to 830° C. and lower than or equal to 910° C. Note that the heating temperature in Step S33 is preferably lower than that in Step 13.
In addition, at the time of heating the mixture 903, the partial pressure of fluorine or a fluoride originating from the fluorine source or the like is preferably controlled to be within an appropriate range.
In the fabrication method described in this embodiment, some of the materials, e.g., LiF as the fluorine source, function as a flux in some cases. Owing to this function, the heating temperature can be lower than the decomposition temperature of the composite oxide (LiMO2), e.g., a temperature higher than or equal to 742° C. and lower than or equal to 950° C., which allows distribution of the additive element such as magnesium in the surface portion and fabrication of the positive electrode active material having favorable performance.
However, since LiF in a gas phase has a specific gravity less than that of oxygen, heating volatilizes or sublimates LiF in some cases. When LiF is volatilized or sublimated, the amount of LiF in the mixture 903 is reduced. As a result, the function of LiF as a flux deteriorates. Thus, heating is preferably performed while volatilization or sublimation of LiF is inhibited. Note that even when LiF is not used as the fluorine source or the like, Li at the surface of LiMO2 and F of the fluorine source might react to produce LiF, which might be volatilized or sublimated. Therefore, inhibition of volatilization or sublimation is needed even when a fluoride having a higher melting point than LiF is used.
In view of this, the mixture 903 is preferably heated in an atmosphere containing LiF, i.e., the mixture 903 is preferably heated in a state where the partial pressure of LiF in a heating furnace is high. Such heating can inhibit volatilization or sublimation of LiF in the mixture 903.
The heating in this step is preferably performed such that particles of the mixture 903 are not adhered to one another. Adhesion of the particles of the mixture 903 during the heating might decrease the area of contact with oxygen in the atmosphere and block a path of diffusion of the additive element (e.g., fluorine), thereby hindering distribution of the additive element (e.g., magnesium and fluorine) in the surface portion.
It is considered that uniform distribution of the additive element (e.g., fluorine) in the surface portion leads to a smooth positive electrode active material with little unevenness. In view of this, the particles are preferably not adhered to each other.
In the case of using a rotary kiln for the heating, the flow rate of an oxygen-containing atmosphere in the kiln is preferably controlled during the heating. For example, the flow rate of an oxygen-containing atmosphere is preferably set low, or no flowing of an atmosphere is preferably performed after an atmosphere is purged first and an oxygen atmosphere is introduced into the kiln. Flowing of oxygen is not preferable because it might cause evaporation of the fluorine source, which prevents maintaining the smoothness of the surface.
In the case of using a roller hearth kiln for the heating, the mixture 903 can be heated in an atmosphere containing LiF with the container containing the mixture 903 covered with a lid, for example.
A supplementary explanation of the heating time is provided. The heating time is changed depending on conditions, such as the heating temperature, and the particle size and composition of LiMO2 in Step S14. In the case where the particle size is small, the heating is preferably performed at a lower temperature or for a shorter time than the case where the particle size is large, in some cases.
In the case where the median diameter (D50) of the composite oxide (LiMO2) in Step S14 in
Meanwhile, in the case where the median diameter (D50) of the composite oxide (LiMO2) in Step S14 is approximately 5 μm, the heating temperature is preferably higher than or equal to 600° C. and lower than or equal to 950° C., for example. The heating time is preferably longer than or equal to 1 hour and shorter than or equal to 10 hours, and further preferably approximately 2 hours, for example. Note that the temperature decreasing time after the heating is, for example, preferably longer than or equal to 10 hours and shorter than or equal to 50 hours.
Next, in Step S34 shown in
Through the above steps, the positive electrode active material 100 of one embodiment of the present invention can be fabricated.
As shown in
Steps S11 to S14 shown in
The initial heating refers to heating after the composite oxide is completed. When the initial heating is performed aiming at smoothing a surface, an additive element can be added uniformly and a continuous barrier layer can be formed.
For the initial heating, a lithium compound source does not need to be prepared.
For the initial heating, an additive element source does not need to be prepared.
For the initial heating, a flux (also referred to as flux agent) does not need to be prepared.
The initial heating is heating performed before addition of an additive element, and is referred to as preheating or pretreatment in some cases.
The initial heating can reduce impurities in the composite oxide completed in Step 14.
The heating conditions in this step can be freely set as long as the heating makes the surface of the above composite oxide smooth. For example, any of the heating conditions described for Step S13 can be selected. Additionally, the heating temperature in this step is preferably lower than the temperature in Step S13 so that the crystal structure of the composite oxide is maintained. The heating time in this step may be shorter than the time in Step S13 so that the crystal structure of the composite oxide is maintained. For example, the heating is preferably performed at a temperature higher than or equal to 700° C. and lower than or equal to 1000° C. for approximately 2 hours.
In the composite oxide, a temperature difference between the surface and the inner portion of the composite oxide might be caused by the heating in Step S13. The temperature difference sometimes induces differential shrinkage. It can also be deemed that the temperature difference leads to a fluidity difference between the surface and the inner portion, thereby causing differential shrinkage. The energy involved in differential shrinkage causes a difference in internal stress in the composite oxide. The difference in internal stress is also called distortion, and the above energy is sometimes referred to as distortion energy. The internal stress is eliminated by the initial heating in Step S15 and in other words, the distortion energy is probably equalized by the initial heating in Step S15. When the distortion energy is equalized, the distortion in the composite oxide is relieved. Thus, it is deemed that Step S15 makes the surface of the composite oxide smooth. In other words, it is deemed that Step S15 reduces the differential shrinkage caused in the composite oxide to make the surface of the composite oxide smooth. This is also rephrased as modification of the surface.
Such differential shrinkage might cause a micro shift in the composite oxide such as a shift in a crystal. To reduce the shift, this initial heating is preferably performed. Performing the initial heating can distribute a shift uniformly in the composite oxide. When the shift is distributed uniformly, the surface of the composite oxide might become smooth. “Shift is distributed uniformly” is also rephrased as “crystal grains are aligned”. In other words, it is deemed that Step S15 reduces the shift due to a crystal or the like which is caused in the composite oxide to make the surface of the composite oxide smooth.
The use of a composite oxide with a smooth surface as a positive electrode active material can prevent cracking of the positive electrode active material, thereby reducing deterioration after a cycle test.
It can be said that when surface unevenness in a cross section of a composite oxide is measured and quantified, a smooth surface of the composite oxide has a surface roughness of at least less than or equal to 10 nm. The one cross section is, for example, a cross section obtained in observation using a scanning transmission electron microscope.
Note that when Step S15 is performed on the pre-synthesized composite oxide containing lithium, a transition metal, and oxygen, a composite oxide with a smooth surface can be obtained.
The initial heating might reduce the amount of lithium in the composite oxide. The reduction in the amount of lithium might promote entry of an additive element into the composite oxide when the additive element source is added after Step S20.
Through the above steps, the positive electrode active material 100 of one embodiment of the present invention can be fabricated. The positive electrode active material of one embodiment of the present invention has a smooth surface.
Next, as one embodiment of the present invention, a method different from the fabrication methods 1 and 2 of the positive electrode active material is described.
Steps S11 to S14 in
As described above, the additive element X may be added to the composite oxide as long as a layered rock-salt crystal structure can be obtained. A fabrication method 3 here describes the addition of the additive element divided into two or more steps.
First, in Step S20a shown in
For the addition of the first additive element X1, a solid phase method, a liquid phase method such as a sol-gel method, a sputtering method, an evaporation method, a CVD (chemical vapor deposition) method, a PLD (pulsed laser deposition) method, or the like can be used.
Here, as the first additive element source (X1 source), a magnesium source (Mg source) and a fluorine source (F source) are prepared. Next, with reference to
Steps S31 to S33 shown in
Next, the material heated in Step S33 is collected to fabricate a composite oxide containing the first additive element X1. This composite oxide is called a second composite oxide to be distinguished from the composite oxide in Step S14.
In Step S40 shown in
For the addition of the second additive element X2, a solid phase method, a liquid phase method such as a sol-gel method, a sputtering method, an evaporation method, a CVD (chemical vapor deposition) method, a PLD (pulsed laser deposition) method, or the like can be used.
Here, in the case where a sol-gel method is used for the addition of the second additive element X2, a solvent used for the sol-gel method is prepared in addition to the second additive element source (X2 source). In the sol-gel method, a metal alkoxide can be used as a metal source and alcohol can be used as the solvent, for example. In the case of adding aluminum, for example, aluminum isopropoxide can be used as the metal source and isopropanol(2-propanol) can be used as the solvent. For example, in the case of adding zirconium, zirconium(IV) tetraisopropoxide can be used as the metal source and isopropanol can be used as the solvent.
In Step S40 shown in
In the case where a plurality of element sources are contained as the second additive element source, the plurality of element sources may be prepared by independently performing the steps up to and including the step of grinding. As a result, a plurality of second additive element sources (X2 sources) are independently prepared in Step S40.
For example, the second additive element source using a solid phase method and the second additive element source using a sol-gel method may be independently prepared. An example is described where a nickel source and an aluminum source are prepared by a wet process and a sol-gel method, respectively.
First, nickel hydroxide is prepared and ground to prepare a nickel source. Heating may be performed after grinding to remove a solvent.
Next, aluminum isopropoxide, zirconium(IV) tetrapropoxide, and isopropanol are prepared separately from the nickel source, and then stirring is performed. After that, filtration is performed for collection and drying under reduced pressure is performed at 70° C. for one hour to prepare an aluminum source.
Next, Step S51 to Step S53 shown in
As shown in
Next, as one embodiment of the present invention, a method different from the fabrication methods 1 to 3 of the positive electrode active material is described.
As described above, the additive element X may be added to the composite oxide as long as a layered rock-salt crystal structure can be obtained. In
In Step S40a shown in
For the addition of X2a, a solid phase method, a liquid phase method such as a sol-gel method, a sputtering method, an evaporation method, a CVD (chemical vapor deposition) method, a PLD (pulsed laser deposition) method, or the like can be used.
In Step S40a shown in
In the case where a plurality of additive element sources are prepared, grinding may be performed independently.
In Step S40b shown in
In the case where a sol-gel method is used, a solvent used for the sol-gel method is prepared in addition to X2b. In the sol-gel method, a metal alkoxide can be used as a metal source and alcohol can be used as the solvent, for example. Aluminum isopropoxide can be used as aluminum alkoxide in the case of preparing an aluminum source, zirconium isopropoxide can be used as zirconium alkoxide in the case of preparing a zirconium source, and isopropanol can be used as the solvent.
Next, the aluminum alkoxide, the zirconium alkoxide, and the isopropanol are mixed (stirred). The sol-gel reaction may be progressed here, or the sol-gel reaction may be progressed in the next step. In the case where the sol-gel reaction is progressed, heating may be performed at the time of mixing. In this manner, a mixture (also referred to as a mixed solution) containing the aluminum source and the zirconium source is prepared as the X2b source.
Next, Step S51 to Step S53 shown in
This embodiment can be used in combination with any of the other embodiments.
In this embodiment, structures of a positive electrode active material of one embodiment of the present invention are described.
A material having the layered rock-salt crystal structure, such as lithium cobalt oxide (LiCoO2), is known to have a high discharge capacity and excel as a positive electrode active material of a secondary battery. As an example of the material having the layered rock-salt crystal structure, a composite oxide represented by LiMO2 (M is a transition metal) is given.
It is known that the Jahn-Teller effect in a transition metal compound varies in degree according to the number of electrons in the d orbital of the transition metal. For example, in the case where the transition metal M is nickel, a composite oxide containing an excess amount of nickel might be greatly affected by distortion due to the Jahn-Teller effect. Accordingly, when charging and discharging at a high voltage are performed on LiNiO2, which is a composite oxide containing Ni, the crystal structure might be broken because of the distortion. Meanwhile, the influence of the Jahn-Teller effect is suggested to be small in a composite oxide containing cobalt as the transition metal M (LiCoO2); hence, LiCoO2 has higher tolerance to high-voltage charging in some cases.
Described here is a crystal structure or the like of the case where a composite oxide containing cobalt as the transition metal M (LiCoO2) is used as the positive electrode active material.
As shown in
Conventional lithium cobalt oxide with x in LixCoO2 of approximately 0.5 is known to have an improved symmetry of lithium and have a monoclinic crystal structure belonging to the space group P2/m. This structure includes one CoO2 layer in a unit cell. Thus, this crystal structure is referred to as an O1 type crystal structure or a monoclinic O1 type crystal structure in some cases.
Furthermore, when the charge depth is 1, that is, when x in LixCoO2 is 0, the conventional lithium cobalt oxide has a trigonal crystal structure belonging to the space group P-3m1, and a unit cell includes one CoO2 layer. Thus, this crystal structure is referred to as an O1 type crystal structure or a trigonal O1 type crystal structure in some cases. Moreover, in some cases, this crystal structure is referred to as a hexagonal O1 type crystal structure when a trigonal crystal is converted into a composite hexagonal lattice.
Conventional lithium cobalt oxide with, for example, x in LixCoO2 of approximately 0.12 has a crystal structure belonging to the space group R-3m. This crystal structure can also be regarded as a structure in which the CoO2 structure belonging to, for example, P-3m1 O1, and the LiCoO2 structure belonging to, for example, R-3m O3, are alternately stacked. Thus, this crystal structure is referred to as an H1-3 type crystal structure in some cases. Note that since insertion and extraction of lithium do not necessarily uniformly occur in reality, the H1-3 type crystal structure starts to be observed when x is approximately 0.25 in practice. The number of cobalt atoms per unit cell in the actual H1-3 type crystal structure is twice that in other structures. However, in this specification including
For the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as follows, for example: Co (0, 0, 0.42150±0.00016), O1 (0, 0, 0.27671±0.00045), and O2 (0, 0, 0.11535±0.00045). Note that O1 and O2 are each an oxygen atom. In this manner, the H1-3 type crystal structure is represented by a unit cell including one cobalt atom and two oxygen atoms. Meanwhile, the O3′ type crystal structure of one embodiment of the present invention is preferably represented by a unit cell containing one cobalt atom and one oxygen atom, as described later. This means that the symmetry of cobalt and oxygen differs between the O3′ type crystal structure and the H1-3 type structure, and the amount of change from the O3 type crystal structure is smaller in the O3′ type crystal structure than in the H1-3 type structure.
A preferred unit cell for representing a crystal structure of the lithium cobalt oxide is selected such that the value of goodness of fit (GOF) is smaller in Rietveld analysis of XRD, for example.
When charging at a high charge voltage of 4.6 V or more with reference to the redox potential of a lithium metal or charging that makes x in LixCoO2 be 0.25 or less and discharging are repeated, the crystal structure of conventional lithium cobalt oxide changes (i.e., an unbalanced phase change occurs) repeatedly between the H1-3 type crystal structure and the O3 type crystal structure in a discharged state.
As indicated by a dotted line and an arrow in
A difference in volume is also large. The H1-3 type crystal structure and the O3 type crystal structure in a discharged state that contain the same number of cobalt atoms have a difference in volume of more than or equal to 3.0%.
In addition, a structure in which CoO2 layers are continuous, such as the trigonal O1 type crystal structure, included in the H1-3 type crystal structure is highly likely to be unstable.
Thus, when charging and discharging at a high voltage, that is, charging and discharging that make x be 0.25 or less are repeated, the crystal structure of conventional lithium cobalt oxide is gradually broken. The broken crystal structure triggers degradation of the battery characteristics after a cycle test. This is because the broken crystal structure has a smaller number of sites where lithium can exist stably and makes it difficult to insert and extract lithium.
The crystal structure of the case with a charge depth of 0 (in a discharged state), that is, with x in LixCoO2 of 1, in
In the unit cell of the O3′ type crystal structure, the coordinates of cobalt and oxygen can be represented by Co (0, 0, 0.5) and O (0, 0, x) within the range of 0.20≤x≤0.25. In the unit cell, the lattice constant of the a-axis is preferably 0.2797≤a≤0.2837 (nm), further preferably 0.2807≤a≤0.2827 (nm), typically a=0.2817 (nm). The lattice constant of the c-axis is preferably 1.3681≤c≤1.3881 (nm), and further preferably 1.3751≤c≤1.3811 (nm), typically, c=1.3781 (nm).
Note that in the O3′ type crystal structure, an ion of cobalt, magnesium, or the like occupies a site coordinated to six oxygen atoms. Note that a light element such as lithium sometimes occupies a site coordinated to four oxygen atoms.
Note that a light element such as lithium sometimes occupies a site coordinated to four oxygen atoms.
As indicated by the dotted lines in
Note that the positive electrode active material 100 of one embodiment of the present invention is confirmed to have the O3′ type crystal structure in some cases when x in LixCoO2 is greater than or equal to 0.15 and less than or equal to 0.24, and is assumed to have the O3′ type crystal structure even when x is greater than 0.24 and less than or equal to 0.27. However, the crystal structure is influenced by not only x in LixCoO2 but also the number of charge and discharge cycles, a charge current and a discharge current, temperature, an electrolyte, and the like, so that the range of x is not limited to the above.
Thus, when x in LixCoO2 is greater than 0.1 and less than or equal to 0.24, the entire internal structure of the positive electrode active material 100 of one embodiment of the present invention is not necessarily the O3′ type crystal structure. The positive electrode active material 100 may have another crystal structure or may be partly amorphous.
In order to make x in LixCoO2 small, charging at a high charge voltage is necessary in general. Therefore, the state where x in LixCoO2 is small can be rephrased as a state where a charge voltage is high. For example, when CC/CV (constant current/constant voltage) charging is performed at 25° C. and 4.6 V or higher with reference to the potential of a lithium metal, the H1-3 type crystal structure appears in a conventional positive electrode active material. Therefore, a charge voltage of 4.6 V or higher with reference to the potential of a lithium metal can be regarded as a high charge voltage. In this specification and the like, unless otherwise specified, charge voltage is shown with reference to the potential of a lithium metal.
Thus, the positive electrode active material 100 of one embodiment of the present invention is preferable because the O3 type crystal structure can be maintained even when charging at a high charge voltage of 4.6 V or higher is performed at 25° C., for example. Moreover, the positive electrode active material 100 of one embodiment of the present invention is preferable because the O3′ type crystal structure can be obtained when charging at a higher charge voltage, e.g., a voltage higher than or equal to 4.65 V and lower than or equal to 4.7 V is performed at 25° C.
In the positive electrode active material 100, when the charge voltage is increased, the H1-3 type crystal is eventually observed in some cases. As described above, the crystal structure is influenced by the number of charge and discharge cycles, a charge current and a discharge current, an electrolyte, and the like, so that the positive electrode active material 100 of one embodiment of the present invention sometimes has the O3′ type crystal structure even at a lower charge voltage, e.g., a charge voltage of higher than or equal to 4.5 V and lower than 4.6 V at 25° C.
Note that in the case where graphite is used as a negative electrode active material in a secondary battery, for example, the voltage of the secondary battery is lower than the above-mentioned voltage by the potential of graphite. The potential of graphite is approximately 0.05 V to 0.2 V with reference to the potential of a lithium metal. Therefore, for a secondary battery using graphite as a negative electrode active material, a similar crystal structure is obtained at a voltage corresponding to a difference between the above-described voltage and the potential of the graphite.
Although a chance of the existence of lithium is the same in all lithium sites in the O3′ type crystal structure in
The O3′ type crystal structure can be regarded as a crystal structure that contains lithium between layers randomly and is similar to a CdCl2 crystal structure. The crystal structure similar to the CdCl2 crystal structure is close to a crystal structure of lithium nickel oxide that is charged to be Li0.06NiO2; however, pure lithium cobalt oxide or a layered rock-salt positive electrode active material containing a large amount of cobalt is known not to have the CdCl2 crystal structure in general.
A slight amount of the additive element, such as magnesium randomly existing between the CoO2 layers, i.e., in lithium sites, can suppress a shift in the CoO2 layers at the time of charging at a high voltage. Thus, magnesium between the CoO2 layers makes it easier to obtain the O3′ type crystal structure. Therefore, magnesium is preferably distributed throughout a particle of the positive electrode active material 100 of one embodiment of the present invention. To distribute magnesium throughout the particle, heat treatment is preferably performed in the fabrication process of the positive electrode active material 100 of one embodiment of the present invention.
However, heat treatment at an excessively high temperature may cause cation mixing, which increases the possibility of entry of the additive element such as magnesium into the cobalt sites. Magnesium in the cobalt sites does not have the effect of maintaining the O3′ crystal structure at the time of charging at a high voltage. Furthermore, heat treatment at an excessively high temperature might have an adverse effect; for example, cobalt might be reduced to have a valence of two or lithium might be evaporated.
In view of the above, a fluorine compound is preferably added to lithium cobalt oxide before the heat treatment for distributing magnesium throughout the particle. The addition of the fluorine compound decreases the melting point of lithium cobalt oxide. The decreased melting point makes it easier to distribute magnesium throughout the particle at a temperature at which the cation mixing is unlikely to occur. Furthermore, the fluorine compound probably increases corrosion resistance to hydrofluoric acid generated by decomposition of an electrolyte solution.
When the magnesium concentration is higher than or equal to a desired value, the effect of stabilizing a crystal structure becomes small in some cases. This is probably because magnesium enters the cobalt sites in addition to the lithium sites. The number of magnesium atoms in the positive electrode active material 100 of one embodiment of the present invention is preferably greater than or equal to 0.001 times and less than or equal to 0.1 times, further preferably greater than 0.01 times and less than 0.04 times, still further preferably approximately 0.02 times the number of atoms of the transition metal M. Alternatively, the number of magnesium atoms preferably greater than or equal to 0.001 times and less than 0.04 times the number of cobalt atoms. Alternatively, it is preferably greater than or equal to 0.01 times and less than or equal to 0.1 times. The magnesium concentration described here may be a value obtained by element analysis on the whole particles of the positive electrode active material using ICP-MS or the like, or may be a value based on the ratio of the raw materials mixed in the fabrication process of the positive electrode active material, for example.
As a metal (hereinafter, a metal Z), one or more metals selected from nickel, aluminum, manganese, titanium, vanadium, and chromium may be added to lithium cobalt oxide, for example, and in particular, one or both of nickel and aluminum are preferably added. In some cases, manganese, titanium, vanadium, and chromium are likely to have a valence of four stably and thus contribute highly to a stable structure. The addition of the metal Z may enable the positive electrode active material 100 of one embodiment of the present invention to have a more stable crystal structure in a high-voltage charged state. Here, in the positive electrode active material 100 of one embodiment of the present invention, the metal Z is preferably added at a concentration that does not greatly change the crystallinity of the lithium cobalt oxide. For example, the metal Z is preferably added at an amount that does not cause an adverse effect of the Jahn-Teller effect.
Aluminum and the transition metal typified by nickel and manganese preferably exist in cobalt sites, but part of them may exist in lithium sites. Magnesium preferably exists in lithium sites. Fluorine may be substituted for part of oxygen.
As the magnesium concentration in the positive electrode active material 100 of one embodiment of the present invention increases, the charge and discharge capacity of the positive electrode active material decreases in some cases. As an example, one reason is that the amount of lithium that contributes to charging and discharging decreases when magnesium enters the lithium sites. Another possible reason is that excess magnesium generates a magnesium compound that does not contribute to charging and discharging. When the positive electrode active material 100 of one embodiment of the present invention contains nickel as the metal Z in addition to magnesium, the charge and discharge capacity per weight and per volume can be increased in some cases. When the positive electrode active material 100 of one embodiment of the present invention contains aluminum as the metal Z in addition to magnesium, the charge and discharge capacity per weight and per volume can be increased in some cases. When the positive electrode active material 100 of one embodiment of the present invention contains nickel and aluminum in addition to magnesium, the charge and discharge capacity per weight and per volume can be increased in some cases.
The concentrations of the elements contained in the positive electrode active material 100 of one embodiment of the present invention, such as magnesium and the metal Z, are described below using the number of atoms.
The number of nickel atoms contained in the positive electrode active material 100 of one embodiment of the present invention is preferably greater than 0% and less than or equal to 7.5%, further preferably greater than or equal to 0.05% and less than or equal to 4%, still further preferably greater than or equal to 0.1% and less than or equal to 2%, yet still further preferably greater than or equal to 0.2% and less than or equal to 1% of the number of cobalt atoms. Alternatively, it is preferably greater than 0% and less than or equal to 4%. Alternatively, it is preferably greater than 0% and less than or equal to 2%. Alternatively, it is preferably greater than or equal to 0.05% and less than or equal to 7.5%. Alternatively, it is preferably greater than or equal to 0.05% and less than or equal to 2%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 7.5%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 4%. The nickel concentration described here may be a value obtained by element analysis on the whole particles of the positive electrode active material using GD-MS, ICP-MS, or the like, or may be a value based on the ratio of the raw materials mixed in the fabrication process of the positive electrode active material, for example.
Nickel contained at any of the above concentrations easily forms a solid solution uniformly throughout the positive electrode active material 100 and thus particularly contributes to stabilization of the crystal structure of the inner portion 50. When divalent nickel exists in the inner portion 50, a slight amount of the additive element having a valence of two and randomly existing in lithium sites, such as magnesium, might be able to exist more stably in the vicinity of the divalent nickel. Thus, even when charging and discharging at a high voltage are performed, dissolution of magnesium might be inhibited. Accordingly, charge and discharge cycle performance might be improved. Such a combination of the effect of nickel in the inner portion 50 and the effect of magnesium, aluminum, titanium, fluorine, or the like in the surface portion extremely effectively stabilizes the crystal structure at the time of charging at a high voltage.
The number of aluminum atoms contained in the positive electrode active material 100 of one embodiment of the present invention is preferably greater than or equal to 0.05% and less than or equal to 4%, further preferably greater than or equal to 0.1% and less than or equal to 2%, still further preferably greater than or equal to 0.3% and less than or equal to 1.5% of the number of cobalt atoms. Alternatively, it is preferably greater than or equal to 0.05% and less than or equal to 2%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 4%. The aluminum concentration described here may be a value obtained by element analysis on the whole particles of the positive electrode active material using GD-MS, ICP-MS, or the like, or may be a value based on the ratio of the raw materials mixed in the fabrication process of the positive electrode active material, for example.
It is preferable that the positive electrode active material 100 of one embodiment of the present invention contain an element W and phosphorus be used as the element W. The positive electrode active material 100 of one embodiment of the present invention further preferably includes a compound containing phosphorus and oxygen.
When the positive electrode active material of one embodiment of the present invention includes a compound containing the element W, a short circuit can be inhibited while a high-voltage charged state is maintained, in some cases.
In the case where the positive electrode active material of one embodiment of the present invention contains phosphorus as the element W, the phosphorus may react with hydrogen fluoride generated by the decomposition of the electrolyte solution, which might decrease the hydrogen fluoride concentration in the electrolyte solution.
In the case where the electrolyte solution contains LiPF6, hydrogen fluoride may be generated by hydrolysis. In some cases, hydrogen fluoride is generated by the reaction of PVDF used as a component of the positive electrode and alkali. The decrease in the hydrogen fluoride concentration in the electrolyte solution can inhibit corrosion and coating film separation of a current collector in some cases. Furthermore, the decrease in the hydrogen fluoride concentration in the electrolyte solution can inhibit a reduction in adhesion properties due to gelling or insolubilization of PVDF in some cases.
When containing magnesium in addition to the element W, the positive electrode active material 100 of one embodiment of the present invention is extremely stable in a high-voltage charged state. When the element W is phosphorus, the number of phosphorus atoms is preferably greater than or equal to 1% and less than or equal to 20%, further preferably greater than or equal to 2% and less than or equal to 10%, still further preferably greater than or equal to 3% and less than or equal to 8% of the number of cobalt atoms. Alternatively, it is preferably greater than or equal to 1% and less than or equal to 10%. Alternatively, it is preferably greater than or equal to 1% and less than or equal to 8%. Alternatively, it is preferably greater than or equal to 2% and less than or equal to 20%. Alternatively, it is preferably greater than or equal to 2% and less than or equal to 8%. Alternatively, it is preferably greater than or equal to 3% and less than or equal to 20%. Alternatively, it is preferably greater than or equal to 3% and less than or equal to 10%. In addition, the number of magnesium atoms is preferably greater than or equal to 0.1% and less than or equal to 10%, further preferably greater than or equal to 0.5% and less than or equal to 5%, still further preferably greater than or equal to 0.7% and less than or equal to 4% of the number of cobalt atoms. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 5%. Alternatively, it is preferably greater than or equal to 0.1% and less than or equal to 4%. Alternatively, it is preferably greater than or equal to 0.5% and less than or equal to 10%. Alternatively, it is preferably greater than or equal to 0.5% and less than or equal to 4%. Alternatively, it is preferably greater than or equal to 0.7% and less than or equal to 10%. Alternatively, it is preferably greater than or equal to 0.7% and less than or equal to 5%. The phosphorus concentration and the magnesium concentration described here may each be a value obtained by element analysis on the whole particles of the positive electrode active material using ICP-MS or the like, or may be a value based on the ratio of the raw materials mixed in the fabrication process of the positive electrode active material, for example.
Anions of a layered rock-salt crystal structure and anions of a rock-salt crystal structure each form a cubic close-packed structure (face-centered cubic lattice structure). Anions of the O3′ type crystal structure are also presumed to form a cubic close-packed structure. When these crystal structures are in contact with each other, there is a crystal plane at which orientations of the cubic close-packed structures composed of anions are aligned. Note that a space group of the layered rock-salt crystal structure and the O3′ type crystal structure is R-3m, which is different from a space group Fm-3m of the rock-salt crystal structure (a space group of a general rock-salt crystal) and a space group Fd-3m of the rock-salt crystal structure (a space group of a rock-salt crystal structure having the simplest symmetry); thus, the Miller index of the crystal plane satisfying the above conditions in the layered rock-salt crystal structure and the O3′ type crystal structure is different from that in the rock-salt crystal structure. In this specification, a state where the orientations of the cubic close-packed structures composed of anions in the layered rock-salt crystal structure, the O3′ type crystal structure, and the rock-salt crystal structure are aligned is sometimes referred to as a state where crystal orientations are substantially aligned.
Substantial alignment of the crystal orientations in two regions can be judged from a TEM (transmission electron microscopy) image, a STEM (scanning transmission electron microscopy) image, a HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) image, an ABF-STEM (annular bright-field scanning transmission electron microscopy) image, or the like. X-ray diffraction (XRD), electron diffraction, neutron diffraction, and the like can also be used for judging. In the TEM image and the like, alignment of cations and anions can be observed as repetition of bright lines and dark lines. When the orientations of cubic close-packed structures in the layered rock-salt crystal structure and the rock-salt crystal structure are aligned, a state where the angle made by the repetition of bright lines and dark lines in the crystals is less than or equal to 5°, further preferably less than or equal to 2.5° can be observed. Note that in the TEM image and the like, a light element such as oxygen or fluorine cannot be clearly observed in some cases; however, in such a case, alignment of orientations can be judged by arrangement of metal elements.
Too large a particle diameter of the positive electrode active material 100 of one embodiment of the present invention causes problems such as difficulty in lithium diffusion and too much surface roughness of an active material layer in application to a current collector. In contrast, too small a particle diameter causes problems such as increased difficulty in application of the active material layer to the current collector and overreaction with an electrolyte solution. Therefore, the median diameter (D50) is preferably greater than or equal to 1 μm and less than or equal to 100 μm, further preferably greater than or equal to 1 μm and less than or equal to 35 μm or greater than or equal to 1 μm and less than or equal to 30 μm, still further preferably greater than or equal to 5 μm and less than or equal to 20 μm or greater than or equal to 5 μm and less than or equal to 25 μm.
Whether or not a given positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention, which has the O3′ type crystal structure when charged at a high voltage, can be judged by analyzing a positive electrode charged at a high voltage by XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like. XRD is particularly preferable because the symmetry of a transition metal such as cobalt contained in the positive electrode active material can be analyzed with high resolution, the degrees of crystallinity and the crystal orientations can be compared, the distortion of lattice periodicity and the crystallite size can be analyzed, and a positive electrode itself obtained by disassembling a secondary battery can be measured with sufficient accuracy, for example.
As described above, the positive electrode active material 100 of one embodiment of the present invention has a feature of a small change in the crystal structure between a high-voltage charged state and a discharged state. A material in which 50 wt % or more of the crystal structure largely changes between a high-voltage charged state and a discharged state is not preferable because the material cannot withstand charging and discharging at a high voltage. It should be noted that the intended crystal structure is not obtained in some cases only by addition of the additive element. For example, in a high-voltage charged state, lithium cobalt oxide containing magnesium and fluorine has the O3′ type crystal structure at 60 wt % or more in some cases, and has the H1-3 type crystal structure at 50 wt % or more in other cases. In some cases, lithium cobalt oxide containing magnesium and fluorine may have the O3′ type crystal structure at almost 100 wt % at a predetermined voltage, and increasing the voltage to be higher than the predetermined voltage may cause the H1-3 type crystal structure. Thus, to determine whether or not a positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention, the crystal structure should be analyzed by XRD or other methods.
However, the crystal structure of a positive electrode active material in a high-voltage charged state or a discharged state may be changed with exposure to the air. For example, the O3′ type crystal structure changes into the H1-3 type crystal structure in some cases. For that reason, all samples are preferably handled in an inert atmosphere such as an argon atmosphere.
As a cycle test for the positive electrode active material 100 of one embodiment of the present invention, a half-cell test using a lithium counter electrode is given. As a cell used in the half-cell test, a coin cell (CR2032 type, with a diameter of 20 mm and a height of 3.2 mm) can be fabricated.
A positive electrode formed by application of a slurry in which the obtained positive electrode active material, a conductive additive, and a binder are mixed to a positive electrode current collector made of aluminum foil is used for the coin cell.
A lithium metal can be used for a counter electrode of the coin cell. Note that when the counter electrode is formed using a material other than the lithium metal, a potential of a secondary battery differs from a potential of the positive electrode. Unless otherwise specified, a voltage and a potential in this specification and the like refer to a potential of a positive electrode.
As a lithium salt contained in an electrolyte solution of the coin cell, 1 mol/L lithium hexafluorophosphate (LiPF6) can be used, and as a solvent contained in the electrolyte solution, a solution in which ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2 wt % are mixed can be used.
As a separator of the coin cell, 25-μm-thick polypropylene can be used.
Stainless steel (SUS) can be used for a positive electrode can and a negative electrode can of the coin cell.
The coin cell fabricated under the above conditions is subjected to constant current charging (CC) at a freely selected voltage (e.g., 4.6 V, 4.65 V, or 4.7 V) and 0.5 C and then constant voltage charging (CV) until the current value reaches 0.01 C. Note that 1 C can be 137 mA/g or 200 mA/g. The temperature is set to 25° C. After the charging is performed in this manner, the coin cell is disassembled in a glove box with an argon atmosphere to take out the positive electrode, whereby the positive electrode active material charged at a high voltage can be obtained. The positive electrode active material is preferably enclosed in an argon atmosphere when various analyses are performed later.
For example, XRD can be performed on the positive electrode active material enclosed in an airtight container with an argon atmosphere. The apparatus and conditions for the XRD measurement are not particularly limited. The measurement can be performed with the apparatus and conditions as described below, for example.
In the case where the measurement sample in XRD is a powder, the sample can be set by, for example, being put in a glass sample holder or being sprinkled on a reflection-free silicon plate to which grease is applied. In the case where the measurement sample is a positive electrode, the sample can be set in such a manner that the positive electrode is attached to a substrate with a double-sided adhesive tape so as to fit the measurement plane required by the apparatus.
As shown in
Meanwhile, as shown in
The XRD results in
Although the positive electrode active material 100 of one embodiment of the present invention has the O3′ type crystal structure when charged at a high voltage, not all the positive electrode active materials necessarily have the O3′ type crystal structure. The positive electrode active material 100 may have another crystal structure or may be partly amorphous. Note that when the XRD patterns are subjected to the Rietveld analysis, the O3′ type crystal structure preferably accounts for greater than or equal to 50 wt %, further preferably greater than or equal to 60 wt %, still further preferably greater than or equal to 66 wt %.
Furthermore, after a cycle test of 50 or more cycles, preferably 100 cycles of charging and discharging under the above high voltage condition, the O3′ type crystal structure accounts for preferably greater than or equal to 35 wt %, further preferably greater than or equal to 40 wt %, still further preferably greater than or equal to 43 wt %, in the Rietveld analysis.
The crystallite size of the O3′ type crystal structure is only decreased to approximately one-tenth that of LiCoO2 (O3) in a discharged state. Thus, the diffraction peak of the O3′ type crystal structure can sometimes be clearly observed after high-voltage charging even when XRD measurement is performed on a positive electrode before charging and discharging. By contrast, simple LiCoO2 has a small change in crystallite size and exhibits a broad and small diffraction peak although it can partly have a structure similar to the O3′ type crystal structure after high-voltage charging. Note that the crystallite size can be obtained from the half width of the XRD peak, and thus the crystallite size is considered to correlate with the half width. The crystallite size reduced to 1/10 means the half width reduced to 1/10.
The influence of the Jahn-Teller effect is preferably small in the positive electrode active material 100 of one embodiment of the present invention. It is preferable that the positive electrode active material of one embodiment of the present invention have a layered rock-salt crystal structure and mainly contain cobalt as a transition metal. The positive electrode active material of one embodiment of the present invention may contain the above-described metal Z in addition to cobalt as long as the influence of the Jahn-Teller effect is small, and may contain nickel, manganese, or the like, for example.
The range of the lattice constants where the influence of the Jahn-Teller effect is presumed to be small in the positive electrode active material is examined by XRD analysis.
As shown in
Preferable ranges of the lattice constants of the positive electrode active material 100 of one embodiment of the present invention are examined above. In the layered rock-salt crystal structure, which can be estimated from the XRD patterns, the a-axis lattice constant is preferably greater than 2.814×10−10 m and less than 2.817×10−10 m, and the c-axis lattice constant is preferably greater than 14.05×10−10 m and less than 14.07×10−10 m. Examined above is the state of a powder, which corresponds to a positive electrode active material in a state where charging and discharging are not performed or in a discharged state.
In the layered rock-salt crystal structure of the positive electrode active material in the state where charging and discharging are not performed or in the discharged state, the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis/c-axis) is preferably greater than 0.2000 and less than 0.2005.
When the layered rock-salt crystal structure of the positive electrode active material in the state where charging and discharging are not performed or in the discharged state is subjected to XRD analysis, a first peak is observed at 20 of greater than or equal to 18.50° and less than or equal to 19.30°, and a second peak is observed at 20 of greater than or equal to 38.00° and less than or equal to 38.80°, in some cases.
Note that the peaks appearing in the powder XRD patterns reflect the crystal structure of the inner portion 50 of the positive electrode active material 100, which accounts for the majority of the volume of the positive electrode active material 100. The crystal structure of the surface portion, a crystal grain boundary, or the like can be analyzed by electron diffraction of a cross section of the positive electrode active material 100, for example.
A region from the surface of the positive electrode active material to a depth of greater than or equal to 2 nm and less than or equal to 8 nm (normally, less than or equal to 5 nm) can be analyzed by XPS; thus, the concentration of each element in the surface portion of the positive electrode active material can be quantitatively analyzed. The bonding states of the elements can be analyzed by narrow scanning. Note that the quantitative accuracy of XPS is approximately ±1 atomic % in many cases, and the lower detection limit is approximately 1 atomic % but depends on the element.
When the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, the number of atoms of the additive element is preferably greater than or equal to 1.6 times and less than or equal to 6.0 times, further preferably greater than or equal to 1.8 times and less than 4.0 times the number of atoms of the transition metal M. When the additive element is magnesium and the transition metal M is cobalt, the number of magnesium atoms is preferably greater than or equal to 1.6 times and less than or equal to 6.0 times, further preferably greater than or equal to 1.8 times and less than 4.0 times the number of cobalt atoms. The number of atoms of halogen such as fluorine is preferably greater than or equal to 0.2 times and less than or equal to 6.0 times, further preferably greater than or equal to 1.2 times and less than or equal to 4.0 times the number of atoms of the transition metal M.
In the XPS analysis, monochromatic aluminum can be used as an X-ray source, for example. An extraction angle is, for example, 45°. For example, the measurement can be performed using the following apparatus and conditions.
In addition, when the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, a peak indicating the bonding energy of fluorine with another element is preferably at greater than or equal to 682 eV and less than 685 eV, further preferably approximately 684.3 eV. The above value is different from both the bonding energy of lithium fluoride, which is 685 eV, and the bonding energy of magnesium fluoride, which is 686 eV. That is, in the case where the positive electrode active material 100 of one embodiment of the present invention contains fluorine, the fluorine is preferably in a bonding state other than lithium fluoride and magnesium fluoride.
Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, a peak indicating the bonding energy of magnesium with another element is preferably at greater than or equal to 1302 eV and less than 1304 eV, further preferably approximately 1303 eV. This value is different from the bonding energy of magnesium fluoride, which is 1305 eV, and close to the bonding energy of magnesium oxide. That is, in the case where the positive electrode active material 100 of one embodiment of the present invention contains magnesium, the magnesium is preferably in a bonding state other than magnesium fluoride.
The concentrations of the additive elements that preferably exist in the surface portion in a large amount, such as magnesium and aluminum, measured by XPS or the like are preferably higher than the concentrations measured by ICP-MS (inductively coupled plasma mass spectrometry), GD-MS (glow discharge mass spectrometry), or the like.
When a cross section is exposed by processing and analyzed by TEM-EDX, the concentrations of magnesium and aluminum in the surface portion are preferably higher than those in the inner portion 50. An FIB (Focused Ion Beam) can be used for the processing, for example.
In the XPS analysis, the number of magnesium atoms is preferably greater than or equal to 0.4 times and less than or equal to 1.5 times the number of cobalt atoms. In the ICP-MS analysis, the atomic ratio of magnesium (Mg/Co) is preferably greater than or equal to 0.001 and less than or equal to 0.06.
By contrast, it is preferable that nickel not be unevenly distributed in the surface portion but be distributed throughout the positive electrode active material 100.
As described above, the positive electrode active material of one embodiment of the present invention preferably contains cobalt and nickel as the transition metal and magnesium as the additive element. It is preferable that Ni2+ be substituted for part of Co3+ and Mg2+ be substituted for part of Li+ accordingly. Accompanying the substitution of Mg2+ for Li+, the Ni2+ might be reduced to be Ni3+. Accompanying the substitution of Mg2+ for part of Li+, Co3+ in the vicinity of Mg2+ might be reduced to be Co2+. Accompanying the substitution of Mg2+ for part of Co3+, Co3+ in the vicinity of Mg2+ might be oxidized to be Co4+.
Thus, the positive electrode active material of one embodiment of the present invention preferably contains one or more of Ni2+, Ni3+, Co2+, and Co4+. Moreover, the spin density attributed to one or more of Ni2+, Ni3+, Co2+, and Co4+ per weight of the positive electrode active material is preferably higher than or equal to 2.0×1017 spins/g and less than or equal to 1.0×1021 spins/g. The positive electrode active material preferably has the above spin density, in which case the crystal structure can be stable particularly in a charged state. Note that too high a magnesium concentration might reduce the spin density attributed to one or more of Ni2+, Ni3+, Co2+, and Co4+.
The spin density in the positive electrode active material can be analyzed by ESR, for example.
Elements can be quantified by EPMA. In surface analysis, distribution of each element can be analyzed.
In EPMA, a region from a surface to a depth of approximately 1 μm is analyzed. Thus, the concentration of each element is sometimes different from measurement results obtained by other analysis methods. For example, when surface analysis is performed on the positive electrode active material 100, the concentration of the additive element present in the surface portion might be lower than the concentration obtained in XPS. The concentration of the additive element present in the surface portion might be higher than the concentration obtained in ICP-MS or a value based on the ratio of the raw materials mixed in the fabrication process of the positive electrode active material.
EPMA surface analysis of a cross section of the positive electrode active material 100 of one embodiment of the present invention preferably reveals a concentration gradient in which the concentration of the additive element increases from the inner portion toward the surface portion. The aluminum concentration peak may be located in the surface portion or located deeper than the surface portion. The aluminum concentration peak is preferably located on the inner side of the magnesium concentration peak.
Note that the positive electrode active material of one embodiment of the present invention does not contain a carbonic acid, a hydroxy group, or the like which is chemisorbed after fabrication of the positive electrode active material. Furthermore, the positive electrode active material of one embodiment of the present invention does not contain an electrolyte solution, a binder, a conductive material, and a compound originating from any of these that are attached to the surface of the positive electrode active material. Thus, in quantification of the elements contained in the positive electrode active material, correction may be performed to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that might be detected in surface analysis such as XPS and EPMA.
This embodiment can be used in combination with any of the other embodiments.
In this embodiment, examples of a secondary battery of one embodiment of the present invention are described.
Hereinafter, a secondary battery in which a positive electrode, a negative electrode, and an electrolyte solution are wrapped in an exterior body is described as an example.
The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and may include a conductive material and a binder. As the positive electrode active material, the positive electrode active material described in the above embodiments is used.
The positive electrode active material described in the above embodiments and another positive electrode active material may be mixed to be used.
Other examples of the positive electrode active material include a composite oxide with an olivine crystal structure, a composite oxide with a layered rock-salt crystal structure, and a composite oxide with a spinel crystal structure. For example, a compound such as LiFePO4, LiFeO2, LiNiO2, LiMn2O4, V2O5, Cr2O5, and MnO2 can be given.
As another positive electrode active material, it is preferable to mix lithium nickel oxide (LiNiO2 or LiNi1-xMxO2 (0<x<1) (M=Co, Al, or the like)) with a lithium-containing material that has a spinel crystal structure and contains manganese, such as LiMn2O4. This composition can improve the performance of the secondary battery.
As another positive electrode active material, a lithium-manganese composite oxide that can be represented by a composition formula LiaMnbMcOd can be used. Here, the element M is preferably silicon, phosphorus, or a metal element other than lithium and manganese, and is further preferably nickel. In the case where the whole particles of a lithium-manganese composite oxide are measured, it is preferable to satisfy the following at the time of discharging: 0<a/(b+c)<2; c>0; and 0.26≤(b+c)/d<0.5. Note that the proportions of metals, silicon, phosphorus, and the like in the whole particles of a lithium-manganese composite oxide can be measured with, for example, an ICP-MS (inductively coupled plasma mass spectrometer). The proportion of oxygen in the whole particles of a lithium-manganese composite oxide can be measured by, for example, EDX (energy dispersive X-ray spectroscopy). Alternatively, the proportion of oxygen can be measured by ICPMS combined with fusion gas analysis and valence evaluation of XAFS (X-ray absorption fine structure) analysis. Note that a lithium-manganese composite oxide is an oxide containing at least lithium and manganese, and may contain at least one kind of element selected from a group consisting of chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, phosphorus, and the like.
The active material layer 200 includes particles of the positive electrode active material 100, graphene or a graphene compound 201 serving as the conductive material, and a binder (not illustrated).
The graphene compound 201 in this specification and the like refers to multilayer graphene, multi graphene, graphene oxide, multilayer graphene oxide, multi graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi graphene oxide, graphene quantum dots, and the like. A graphene compound contains carbon, has a plate-like shape, a sheet-like shape, or the like, and has a two-dimensional structure formed of a six-membered ring composed of carbon atoms. The two-dimensional structure formed of the six-membered ring composed of carbon atoms may be referred to as a carbon sheet. A graphene compound may include a functional group. A graphene compound preferably has a bent shape. A graphene compound may be rounded like a carbon nanofiber.
In this specification and the like, graphene oxide contains carbon and oxygen, has a sheet-like shape, and includes a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.
In this specification and the like, reduced graphene oxide contains carbon and oxygen, has a sheet-like shape, and has a two-dimensional structure formed of a six-membered ring composed of carbon atoms. In the case where the reduced graphene includes defects, a 7- or more-membered ring is observed. The reduced graphene oxide may be referred to as a carbon sheet. The reduced graphene oxide functions by itself and may have a stacked-layer structure. The reduced graphene oxide preferably includes a portion where the carbon concentration is higher than 80 atomic % and the oxygen concentration is higher than or equal to 2 atomic % and lower than or equal to 15 atomic %. With such a carbon concentration and such an oxygen concentration, the reduced graphene oxide can function as a conductive material with high conductivity even with a small amount. In addition, the intensity ratio G/D of a G band to a D band of the Raman spectrum of the reduced graphene oxide is preferably 1 or more. The reduced graphene oxide with such an intensity ratio can function as a conductive material with high conductivity even with a small amount.
A graphene compound has excellent electrical characteristics of high conductivity and excellent physical properties of high flexibility and high mechanical strength in some cases. A graphene compound has a sheet-like shape. A graphene compound has a curved surface in some cases, thereby enabling low-resistant surface contact. Furthermore, graphene and a graphene compound have extremely high conductivity even with a small thickness in some cases and thus allow a conductive path to be formed in an active material layer efficiently even with a small amount. Hence, the use of a graphene compound as a conductive material can increase the area where an active material and the conductive material are in contact with each other. A graphene compound preferably covers 80% or more of the area of an active material. Note that a graphene compound preferably clings to at least part of an active material. Alternatively, a graphene compound preferably overlays at least part of an active material. Alternatively, the shape of a graphene compound preferably conforms to at least part of the shape of an active material. The shape of an active material indicates, for example, unevenness of a single active material or unevenness formed by a plurality of active materials. A graphene compound preferably surrounds at least part of an active material. A graphene compound may have a hole. The hole is observed as a poly-membered ring.
In the case where an active material with a small particle size (e.g., 1 μm or less) is used, the specific surface area of the active material is large and thus more conductive paths for the active materials are needed. In such a case, it is preferable to use a graphene compound that can efficiently form a conductive path even with a small amount.
It is particularly effective to use a graphene compound, which has the above-described properties, as a conductive material of a secondary battery that needs to be rapidly charged and discharged. For example, a secondary battery for a two- or four-wheeled vehicle, a secondary battery for a drone, or the like is required to have fast charge and discharge characteristics in some cases. In addition, a mobile electronic device or the like is required to have fast charge characteristics in some cases. Fast charging and discharging may also be referred to as charging and discharging at a high rate, for example, at 1 C, 2 C, or 5 C or more.
Here, the plurality of sheets of graphene or the plurality of graphene compounds can be bonded to each other to form a net-like graphene compound sheet (hereinafter, referred to as a graphene compound net or a graphene net). A graphene net that covers the active material can function as a binder for bonding the active materials. Accordingly, the amount of the binder can be reduced, or the binder does not have to be used, which can increase the proportion of the active material in the electrode volume or the electrode weight. That is, the charge and discharge capacity of the secondary battery can be increased.
Here, it is preferable to perform reduction after a layer to be the active material layer 200 is formed in such a manner that graphene oxide is used as the graphene or the graphene compound 201 and mixed with the positive electrode active material 100. That is, the formed active material layer preferably contains reduced graphene oxide. When graphene oxide with extremely high dispersibility in a polar solvent is used for the formation of the graphene or the graphene compound 201, the graphene or the graphene compound 201 can be substantially uniformly dispersed in the active material layer 200. The solvent is removed by volatilization from a dispersion medium in which graphene oxide is uniformly dispersed, and the graphene oxide is reduced; hence, the sheets of graphene or the graphene compounds 201 remaining in the active material layer 200 partly overlap with each other and are dispersed such that surface contact is made, thereby forming a three-dimensional conduction path. Note that the graphene oxide may be reduced by heat treatment or with the use of a reducing agent, for example.
Unlike a particulate conductive material such as acetylene black, which makes point contact with an active material, the graphene or the graphene compound 201 is capable of making low-resistance surface contact; accordingly, the electrical conduction between the particles of the positive electrode active material 100 and the graphene or the graphene compound 201 can be improved with a small amount of the graphene and the graphene compound 201 as compared with a normal conductive material. This can increase the proportion of the positive electrode active material 100 in the active material layer 200. Hence, the discharge capacity of the secondary battery can be increased.
It is possible to form, with a spray dry apparatus, a graphene compound serving as a conductive material as a coating film to cover the entire surface of the active material in advance and to form a conductive path between the active materials using the graphene compound.
A material used in formation of the graphene compound may be mixed with the graphene compound to be used for the active material layer 200. For example, particles used as a catalyst in formation of the graphene compound may be mixed with the graphene compound. As an example of the catalyst in formation of the graphene compound, particles containing any of silicon oxide (SiO2 or SiOx (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, and the like can be given. The median diameter (D50) of the particles is preferably less than or equal to 1 μm, further preferably less than or equal to 100 nm.
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, water-soluble polymers are preferably used. As the water-soluble polymers, a polysaccharide or the like can be used, for example. As the polysaccharide, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, starch, or the like can be used. It is further preferable that such water-soluble polymers be used in combination with any of the above-described rubber materials.
Alternatively, as the binder, it is preferable to use a material such as polystyrene, poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinyl acetate, or nitrocellulose.
Two or more of the above-described 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 might 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 example, a water-soluble polymer is preferably used. As a water-soluble polymer having a significant viscosity modifying effect, it is possible to use the above-mentioned polysaccharide, for example, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or starch.
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 and the like 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 a 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 includes 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 inhibit the decomposition of the electrolyte solution. 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 an electrolyte solution 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.
The positive electrode current collector can be formed using a material that has high conductivity, such as a metal like stainless steel, gold, platinum, aluminum, and titanium, or an alloy thereof. It is preferable that a material used for the positive electrode current collector not dissolve at the potential of the positive electrode. It is also possible to use an aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added. A metal element that forms silicide by reacting with silicon may be used. Examples of the metal element that forms silicide by reacting with silicon include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can have a foil-like shape, a plate-like shape, a sheet-like shape, a net-like shape, a punching-metal shape, an expanded-metal shape, or the like as appropriate. The current collector having a thickness greater than or equal to 5 μm and less than or equal to 30 μm is preferably used.
Slurry containing the positive electrode active material, a conductive material, a binder, and the like are applied as a positive electrode active material layer on the positive electrode current collector, drying or the like is performed, and then pressing is performed, so that the positive electrode is completed. The pressing is preferably performed in multiple stages. First pressing and second pressing are sequentially performed, and the pressure of the second pressing is preferably set greater than or equal to 5 times and less than or equal to 8 times as high as that of the first pressing.
When the positive electrode active material layer is formed on the positive electrode current collector, pressing is performed, and then a cross-sectional STEM image of part of the positive electrode is observed, a step formed on the particle surface in a direction perpendicular to the lattice fringes (the c-axis direction) by the pressing and an evidence of deformation along the lattice fringe direction (the ab plane direction) are observed. The deformation is referred to as a slip in some cases. The slip might trigger a defect such as a crack or a pit. Therefore, the slip is preferably not generated.
The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may contain a conductive material and a binder.
As a negative electrode active material, for example, an alloy-based material or a carbon material can be used.
As the negative electrode active material, an element that enables charge and discharge reactions by an alloying reaction and a dealloying reaction with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, and the like can be used. Such elements have higher charge and discharge capacity than carbon; in particular, silicon has a high theoretical capacity of 4200 mAh/g. For this reason, silicon is preferably used as the negative electrode active material. Alternatively, a compound containing any of the above elements may be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Here, an element that enables charge and discharge reactions by an alloying reaction and a dealloying reaction with lithium, a compound containing the element, and the like may be referred to as an alloy-based material.
In this specification and the like, SiO refers to silicon monoxide, for example. Note that SiO can alternatively be expressed as SiOx. Here, x is preferably 1 or an approximate value of 1. For example, x in SiOx is preferably greater than or equal to 0.2 and less than or equal to 1.5, further preferably greater than or equal to 0.3 and less than or equal to 1.2. Alternatively, x in SiOx is preferably greater than or equal to 0.2 and less than or equal to 1.2. Alternatively, x in SiOx is preferably greater than or equal to 0.3 and less than or equal to 1.5.
As the carbon material, graphite, graphitizing carbon (soft carbon), non-graphitizing carbon (hard carbon), carbon nanotube, graphene, carbon black, or the like can be used.
Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. As artificial graphite, spherical graphite having a spherical shape can be used. For example, MCMB is preferable because it may have a spherical shape. Moreover, MCMB may be preferable because it can relatively easily have a small surface area. Examples of natural graphite include flake graphite and spherical natural graphite.
Graphite has a low potential substantially equal to that of a lithium metal (greater than or equal to 0.05 V and less than or equal to 0.3 V vs. Li/Li+) when lithium ions are inserted into the graphite (while a lithium-graphite intercalation compound is formed). For this reason, a lithium-ion secondary battery can have a high operating voltage. In addition, graphite is preferable because of its advantages such as a relatively high charge and discharge capacity per unit volume, relatively small volume expansion, low cost, and a higher safety than that of a lithium metal.
As the negative electrode active material, an oxide such as titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O12), a lithium-graphite intercalation compound (LixC6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), or molybdenum oxide (MoO2) can be used.
As the negative electrode active material, Li3-xMxN (M=Co, Ni, or Cu) with a Li3N structure, which is a composite nitride of lithium and a transition metal, can be used. For example, Li2.6Co0.4N3 is preferable because of its high charge and discharge capacity (900 mAh/g and 1890 mAh/cm3).
A composite nitride of lithium and a transition metal is preferably used, in which case lithium ions are contained in the negative electrode active material and thus the negative electrode active material can be used in combination with a material for a positive electrode active material that does not contain lithium ions, such as V2O5 or Cr3O8. Note that in the case of using a material containing lithium ions as a positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by extracting the lithium ions contained in the positive electrode active material in advance.
Alternatively, a material that causes a conversion reaction can be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may be used as the negative electrode active material. Other examples of the material that causes a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, sulfides such as CoS0.89, NiS, and CuS, nitrides such as Zn3N2, Cu3N, and Ge3N4, phosphides such as NiP2, FeP2, and CoP3, and fluorides such as FeF3 and BiF3.
For the conductive material and the binder that can be included in the negative electrode active material layer, materials similar to those for the conductive material and the binder that can be included in the positive electrode active material layer can be used.
For the negative electrode current collector, a material similar to that for the positive electrode current collector can be used. Note that a material that is not alloyed with carrier ions of lithium or the like is preferably used for the negative electrode current collector.
The electrolyte solution contains a solvent and an electrolyte. As the solvent of the electrolyte solution, an aprotic organic solvent is preferable; for example, one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used, or two or more of these solvents can be used in an appropriate combination at an appropriate ratio.
The use of one or more ionic liquids (room temperature molten salts) that are unlikely to burn and volatize as the solvent of the electrolyte solution can prevent a secondary battery from exploding or catching fire even when the secondary battery internally shorts out or the internal temperature increases owing to overcharging or the like. An ionic liquid contains a cation and an anion, specifically, an organic cation and an anion. Examples of the organic cation used for the electrolyte solution include an aliphatic onium cation and an aromatic cation. Examples of the aliphatic onium cation include a quaternary ammonium cation, a tertiary sulfonium cation, and a quaternary phosphonium cation. Examples of the aromatic cation include an imidazolium cation and a pyridinium cation. Examples of the anion used for the electrolyte solution include a monovalent amide-based anion, a monovalent methide-based anion, a fluorosulfonate anion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, a perfluoroalkylborate anion, a hexafluorophosphate anion, and a perfluoroalkylphosphate anion.
As the electrolyte dissolved in the solvent, a lithium salt is used. As the lithium salt, for example, one or more selected from LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, Li2B10Cl10, Li2B12Cl12, LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), and LiN(C2F5SO2)2 can be used. In the case where two or more of the above lithium salts are used, the lithium salts can be combined at an appropriate ratio.
As the electrolyte solution used for a secondary battery, it is preferable to use an electrolyte solution that is highly purified and contains a small number of dust particles or elements other than the constituent elements of the electrolyte solution (hereinafter, also simply referred to as “impurities”). Specifically, the weight ratio of impurities to the electrolyte solution is preferably less than or equal to 1%, further preferably less than or equal to 0.1%, still further preferably less than or equal to 0.01%.
Furthermore, an additive agent such as vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or a dinitrile compound such as succinonitrile or adiponitrile may be added to the electrolyte solution. The concentration of the material to be added in the whole solvent is, for example, higher than or equal to 0.1 wt % and lower than or equal to 5 wt %. VC or LiBOB is particularly preferable because it facilitates formation of a favorable coating film.
Alternatively, a polymer gel electrolyte obtained in such a manner that a polymer is swelled with an electrolyte solution may be used.
When a polymer gel electrolyte is used, safety against liquid leakage and the like is improved. Moreover, a secondary battery can be thinner and more lightweight.
As a polymer that undergoes gelation, a silicone gel, an acrylic gel, an acrylonitrile gel, a polyethylene oxide-based gel, a polypropylene oxide-based gel, a fluorine-based polymer gel, or the like can be used.
Examples of the polymer include a polymer having a polyalkylene oxide structure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile; and a copolymer containing any of them. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The formed polymer may be porous.
Instead of the electrolyte solution, a solid electrolyte including an inorganic material such as a sulfide-based inorganic material or an oxide-based inorganic material can be used. Instead of the electrolyte solution, a solid electrolyte solution containing a high-molecular material such as a PEO (polyethylene oxide)-based high molecular material can be used. In the case where the solid electrolyte is used, a separator and a spacer are not necessary. Furthermore, the battery can be entirely solidified; therefore, there is no possibility of liquid leakage and thus the safety of the battery is dramatically improved.
The secondary battery preferably includes a separator. The separator can be formed using, for example, paper, nonwoven fabric, glass fiber, ceramics, or synthetic fiber containing nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane. The separator is preferably formed to have an envelope-like shape to wrap one of the positive electrode and the negative electrode.
The separator may have a multilayer structure. For example, an organic material film of polypropylene, polyethylene, or the like can be coated with a ceramic-based material, a fluorine-based material, a polyamide-based material, a mixture thereof, or the like. As the ceramic-based material, aluminum oxide particles or silicon oxide particles can be used, for example. As the fluorine-based material, PVDF or polytetrafluoroethylene can be used, for example. As the polyamide-based material, nylon or aramid (meta-based aramid or para-based aramid) can be used, for example.
When the separator is coated with the ceramic-based material, the oxidation resistance is improved; hence, deterioration of the separator in charging and discharging at a high voltage can be inhibited and thus the reliability of the secondary battery can be improved. When the separator is coated with the fluorine-based material, the separator is easily brought into close contact with an electrode, resulting in high output characteristics. When the separator is coated with the polyamide-based material, in particular, aramid, the safety of the secondary battery is improved because heat resistance is improved.
For example, both surfaces of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid. Alternatively, a surface of a polypropylene film that is in contact with the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and a surface of the polypropylene film that is in contact with the negative electrode may be coated with the fluorine-based material.
With the use of a separator having a multilayer structure, the charge and discharge capacity per volume of the secondary battery can be increased because the safety of the secondary battery can be maintained even when the total thickness of the separator is small.
For an exterior body included in the secondary battery, a metal material such as aluminum or a resin material can be used, for example. A film-like exterior body can also be used. As the film, for example, it is possible to use a film having a three-layer structure in which a highly flexible metal thin film of aluminum, stainless steel, copper, nickel, or the like is provided over a film formed of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film of a polyamide-based resin, a polyester-based resin, or the like is provided as the outer surface of the exterior body over the metal thin film.
A structure of a secondary battery including a solid electrolyte layer is described below as another structure example of a secondary battery.
As illustrated in
The positive electrode 410 includes a positive electrode current collector 413 and a positive electrode active material layer 414. The positive electrode active material layer 414 includes a positive electrode active material 411 and a solid electrolyte 421. As the positive electrode active material 411, the positive electrode active material fabricated by the fabrication method described in the above embodiments is used. The positive electrode active material layer 414 may also include a conductive material and a binder.
The solid electrolyte layer 420 includes the solid electrolyte 421. The solid electrolyte layer 420 is positioned between the positive electrode 410 and the negative electrode 430 and is a region that includes neither the positive electrode active material 411 nor a negative electrode active material 431.
The negative electrode 430 includes a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 includes the negative electrode active material 431 and the solid electrolyte 421. The negative electrode active material layer 434 may also include a conductive material and a binder. Note that when a lithium metal is used for the negative electrode 430, the negative electrode 430 that does not include the solid electrolyte 421 can be formed, as illustrated in
As the solid electrolyte 421 included in the solid electrolyte layer 420, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, or a halide-based solid electrolyte can be used, for example.
Examples of the sulfide-based solid electrolyte include a thio-LISICON-based material (e.g., Li10GeP2S12 and Li3.25Ge0.25P0.75S4), sulfide glass (e.g., 70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, and 50Li2S·50GeS2), and sulfide-based crystallized glass (e.g., Li7P3S11 and Li3.25P0.95S4). The sulfide-based solid electrolyte has advantages such as high conductivity of some materials, low-temperature synthesis, and ease of maintaining a path for electrical conduction after charging and discharging because of its relative softness.
Examples of the oxide-based solid electrolyte include a material with a perovskite crystal structure (e.g., La2/3-xLi3xTiO3), a material with a NASICON crystal structure (e.g., Li1-XAlXTi2-X(PO4)3), a material with a garnet crystal structure (e.g., Li7La3Zr2O12), a material with a LISICON crystal structure (e.g., Li14ZnGe4O16), LLZO (Li7La3Zr2O12), oxide glass (e.g., Li3PO4—Li4SiO4 and 50Li4SiO4·50Li3BO3), and oxide-based crystallized glass (e.g., Li1.07Al0.69Ti1.46(PO4)3 and Li1.5Al0.5Ge1.5(PO4)3). The oxide-based solid electrolyte has an advantage of stability in the air.
Examples of the halide-based solid electrolyte include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, and LiI. Moreover, a composite material in which pores of porous aluminum oxide or porous silica are filled with such a halide-based solid electrolyte can be used as the solid electrolyte.
Alternatively, different solid electrolytes may be mixed and used.
In particular, Li1+xAlxTi2-x(PO4)3 (0<x<1) having a NASICON crystal structure (hereinafter, LATP) is preferable because it contains aluminum and titanium, each of which is the element the positive electrode active material used in the secondary battery 400 of one embodiment of the present invention is allowed to contain, and thus synergy of improving the cycle performance is expected. Moreover, higher productivity due to the reduction in the number of steps is expected. Note that in this specification and the like, a material with a NASICON crystal structure refers to a compound that is represented by M2(XO4)3 (M: transition metal; X: S, P, As, Mo, W, or the like) and has a structure in which MO6 octahedrons and XO4 tetrahedrons that share common corners are arranged three-dimensionally.
An exterior body of the secondary battery 400 of one embodiment of the present invention can employ a variety of materials and shapes, and preferably has a function of applying pressure to the positive electrode, the solid electrolyte layer, and the negative electrode.
The evaluation material is placed on an electrode plate 751, surrounded by an insulating tube 752, and pressed from above by the electrode plate 753.
A stack of a positive electrode 750a, a solid electrolyte layer 750b, and a negative electrode 750c is illustrated here as an example of the evaluation material, and its cross section is illustrated in
The electrode plate 751 and the lower component 761 that are electrically connected to the positive electrode 750a correspond to a positive electrode terminal. The electrode plate 753 and the upper component 762 that are electrically connected to the negative electrode 750c correspond to a negative electrode terminal. The electric resistance or the like can be measured while pressure is applied to the evaluation material through the electrode plate 751 and the electrode plate 753.
The exterior body of the secondary battery of one embodiment of the present invention is preferably a package having excellent airtightness. For example, a ceramic package or a resin package can be used. The exterior body is sealed preferably in a closed atmosphere where the outside air is blocked, for example, in a glove box.
The external electrode 771 is electrically connected to the positive electrode 750a through the electrode layer 773a and functions as a positive electrode terminal. An external electrode 772 is electrically connected to the negative electrode 750c through the electrode layer 773b and functions as a negative electrode terminal.
This embodiment can be used in appropriate combination with any of the other embodiments.
In this embodiment, examples of a shape of a secondary battery including the positive electrode described in the above embodiment are described. For the materials used for the secondary battery described in this embodiment, the description of the above embodiment can be referred to.
First, an example of a coin-type secondary battery is described.
In a coin-type secondary battery 300, a positive electrode can 301 doubling as a positive electrode terminal and a negative electrode can 302 doubling as a negative electrode terminal are insulated from each other and sealed by a gasket 303 made of polypropylene or the like. A positive electrode 304 includes a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector 305. A negative electrode 307 includes a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector 308.
Note that only one surface of each of the positive electrode 304 and the negative electrode 307 used for the coin-type secondary battery 300 is provided with an active material layer.
For the positive electrode can 301 and the negative electrode can 302, a metal having corrosion resistance to an electrolyte solution, such as nickel, aluminum, or titanium, an alloy of such a metal, or an alloy of such a metal and another metal (e.g., stainless steel) can be used. Alternatively, the positive electrode can 301 and the negative electrode can 302 are preferably covered with nickel, aluminum, or the like in order to prevent corrosion due to the electrolyte solution. The positive electrode can 301 and the negative electrode can 302 are electrically connected to the positive electrode 304 and the negative electrode 307, respectively.
The negative electrode 307, the positive electrode 304, and a separator 310 are immersed in the electrolyte. Then, as illustrated in
When the positive electrode active material described in the above embodiment is used in the positive electrode 304, the coin-type secondary battery 300 with high charge and discharge capacity and excellent cycle performance can be obtained.
Here, a current flow in charging a secondary battery is described with reference to FIG. 22C. When a secondary battery using lithium is regarded as a closed circuit, movement of lithium ions and the current flow are in the same direction. Note that in the secondary battery using lithium, the anode and the cathode interchange in charging and discharging, and the oxidation reaction and the reduction reaction interchange; hence, an electrode with a high reaction potential is called a positive electrode and an electrode with a low reaction potential is called a negative electrode. For this reason, in this specification, the positive electrode is referred to as a “positive electrode” or a “plus electrode” and the negative electrode is referred to as a “negative electrode” or a “minus electrode” in all the cases where charging is performed, discharging is performed, a reverse pulse current is supplied, and a charge current is supplied. The use of the terms “anode” and “cathode” related to an oxidation reaction and a reduction reaction might cause confusion because the anode and the cathode interchange in charging and discharging. Thus, the terms “anode” and “cathode” are not used in this specification. If the term “anode” or “cathode” is used, it should be mentioned that the anode or the cathode is which of the one at the time of charging or the one at the time of discharging and corresponds to which of a positive (plus) electrode or a negative (minus) electrode.
Two terminals illustrated in
Next, an example of a cylindrical secondary battery is described with reference to
Inside the battery can 602 having a hollow cylindrical shape, a battery element in which a strip-like positive electrode 604 and a strip-like negative electrode 606 are wound with a separator 605 interposed therebetween is provided. Although not illustrated, the battery element is wound around a center pin. One end of the battery can 602 is close and the other end thereof is open. For the battery can 602, a metal having corrosion resistance to an electrolyte solution, such as nickel, aluminum, or titanium, an alloy of such a metal, or an alloy of such a metal and another metal (e.g., stainless steel) can be used. Alternatively, the battery can 602 is preferably covered with nickel, aluminum, or the like in order to prevent corrosion due to the electrolyte solution. Inside the battery can 602, the battery element in which the positive electrode, the negative electrode, and the separator are wound is provided between a pair of insulating plates 608 and 609 that face each other. Furthermore, a nonaqueous electrolyte solution (not illustrated) is injected inside the battery can 602 provided with the battery element. As the nonaqueous electrolyte solution, a nonaqueous electrolyte solution that is similar to that of the coin-type secondary battery can be used.
Since a positive electrode and a negative electrode that are used for a cylindrical storage battery are wound, active materials are preferably formed on both surfaces of a current collector. A positive electrode terminal (positive electrode current collecting lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collecting lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be formed using a metal material such as aluminum. The positive electrode terminal 603 and the negative electrode terminal 607 are resistance-welded to a safety valve mechanism 612 and the bottom of the battery can 602, respectively. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 through a PTC element (Positive Temperature Coefficient) 611. The safety valve mechanism 612 cuts off electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold value. The PTC element 611, which serves as a thermally sensitive resistor whose resistance increases as temperature rises, limits the amount of current by increasing the resistance, in order to prevent abnormal heat generation. Barium titanate (BaTiO3)-based semiconductor ceramics or the like can be used for the PTC element.
Furthermore, as illustrated in
When the positive electrode active material described in the above embodiment is used in the positive electrode 604, the cylindrical secondary battery 600 with high charge and discharge capacity and excellent cycle performance can be obtained.
Other structure examples of secondary batteries are described with reference to
The circuit board 900 includes a terminal 911 and a circuit 912. The terminal 911 is connected to the terminal 951, the terminal 952, the antenna 914, and the circuit 912. Note that a plurality of terminals 911 may be provided to serve as a control signal input terminal, a power supply terminal, and the like.
The circuit 912 may be provided on the rear surface of the circuit board 900. Note that the shape of the antenna 914 is not limited to coil shapes, and may be a linear shape or a plate shape, for example. An antenna such as a planar antenna, an aperture antenna, a traveling-wave antenna, an EH antenna, a magnetic-field antenna, or a dielectric antenna may be used. Alternatively, the antenna 914 may be a flat-plate conductor. The flat-plate conductor can serve as one of conductors for electric field coupling. That is, the antenna 914 may serve as one of two conductors of a capacitor. Thus, electric power can be transmitted and received not only by an electromagnetic field or a magnetic field but also by an electric field.
The battery pack includes a layer 916 between the antenna 914 and the secondary battery 913. The layer 916 has a function of blocking an electromagnetic field by the secondary battery 913, for example. As the layer 916, for example, a magnetic body can be used.
Note that the structure of the battery pack is not limited to that in
For example, as illustrated in
As illustrated in
With the above structure, both of the antenna 914 and the antenna 918 can be increased in size. The antenna 918 has a function of communicating data with an external device, for example. An antenna with a shape that can be used for the antenna 914, for example, can be used as the antenna 918. As a system for communication using the antenna 918 between the secondary battery and another device, a response method that can be used between the secondary battery and another device, such as NFC (near field communication), can be employed.
Alternatively, as illustrated in
The display device 920 is electrically connected to the terminal 911 or the like, and the display device 920 may display, for example, an image showing whether charging is being carried out, an image showing the amount of stored power, or the like. As the display device 920, electronic paper, a liquid crystal display device, an electroluminescent (EL) display device, or the like can be used. For example, the use of electronic paper can reduce power consumption of the display device 920.
Alternatively, as illustrated in
The sensor 921 has a function of measuring, for example, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays. With the sensor 921, for example, data on an environment (e.g., temperature) where the secondary battery is placed can be detected and stored in a memory inside the circuit 912.
Furthermore, structure examples of the secondary battery 913 are described with reference to
The 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 from the secondary battery 913 can be inhibited. When an electric field is not significantly blocked by the housing 930a, an antenna such as the antenna 914 may be provided inside the housing 930a. For the housing 930b, a metal material can be used, for example.
The negative electrode 931 is connected to the terminal 911 via one of the terminal 951 and the terminal 952. The positive electrode 932 is connected to the terminal 911 via the other of the terminal 951 and the terminal 952.
As illustrated in
For the film 981 and the film 982 having a depressed portion, a metal material such as aluminum or a resin material can be used, for example. With the use of a resin material for the film 981 and the film 982 having a depressed portion, the film 981 and the film 982 having a depressed portion can be changed in their forms when external force is applied; thus, a flexible storage battery can be fabricated.
Although the secondary battery 980 is formed using two films in
Next, another example of a laminated secondary battery is described with reference to
A laminated secondary battery 500 illustrated in
In the laminated secondary battery 500 illustrated in
As the exterior body 509 of the laminated secondary battery 500, for example, a laminate film having a three-layer structure can be employed in which a highly flexible metal thin film of aluminum, stainless steel, copper, nickel, or the like is provided over a film formed of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film of a polyamide-based resin, a polyester-based resin, or the like is provided as the outer surface of the exterior body over the metal thin film.
In
Here, an example of a method for fabricating the laminated secondary battery whose external view is illustrated in
First, as illustrated in
After that, the negative electrode 506, the separator 507, and the positive electrode 503 are placed over the exterior body 509.
Subsequently, the exterior body 509 is folded along a portion shown by a dashed line as illustrated in
Next, the electrolyte solution 508 (not illustrated) is introduced into the exterior body 509 from the inlet of the exterior body 509. The electrolyte solution 508 is preferably introduced in a reduced pressure atmosphere or in an inert gas atmosphere. Lastly, the inlet is bonded. In the above manner, the laminated secondary battery 500 can be fabricated.
When the positive electrode active material described in the above embodiment is used in the positive electrode 503, the secondary battery 500 with high charge and discharge capacity and excellent cycle performance can be obtained.
In an all-solid-state battery, the contact state of the inside interfaces can be kept favorable by applying a predetermined pressure in the direction of stacking positive electrodes and negative electrodes. By applying a predetermined pressure in the direction of stacking positive electrodes and negative electrodes, expansion in the stacking direction due to charging and discharging of the all-solid-state battery can be suppressed, and the reliability of the all-solid-state battery can be improved.
This embodiment can be implemented in appropriate combination with any of the other embodiments.
In this embodiment, examples of electronic devices each including the secondary battery of one embodiment of the present invention are described.
First,
Furthermore, a flexible secondary battery can be incorporated along a curved inside/outside wall surface of a house or a building or a curved interior/exterior surface of an automobile.
The portable information terminal 7200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and a computer game.
The display surface of the display portion 7202 is curved, and images can be displayed on the curved display surface. In addition, the display portion 7202 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, by touching an icon 7207 displayed on the display portion 7202, application can be started.
With the operation button 7205, a variety of functions such as time setting, power on/off, on/off of wireless communication, setting and cancellation of a silent mode, and setting and cancellation of a power saving mode can be performed. For example, the functions of the operation button 7205 can be set freely by setting the operating system incorporated in the portable information terminal 7200.
The portable information terminal 7200 can perform near field communication that is standardized communication. For example, mutual communication between the portable information terminal 7200 and a headset capable of wireless communication enables hands-free calling.
The portable information terminal 7200 includes the input/output terminal 7206, and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging via the input/output terminal 7206 is possible. Note that the charging operation may be performed by wireless power feeding without using the input/output terminal 7206.
The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. When the secondary battery of one embodiment of the present invention is used, a lightweight portable information terminal with a long lifetime can be provided. For example, the secondary battery 7104 illustrated in
The portable information terminal 7200 preferably includes a sensor. As the sensor, for example, a human body sensor such as a fingerprint sensor, a pulse sensor, or a temperature sensor, a touch sensor, a pressure sensitive sensor, or an acceleration sensor is preferably mounted.
The display surface of the display portion 7304 is curved, and images can be displayed on the curved display surface. A display state of the display device 7300 can be changed by, for example, near field communication that is standardized communication.
The display device 7300 includes an input/output terminal, and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging via the input/output terminal is possible. Note that the charging operation may be performed by wireless power feeding without using the input/output terminal.
The secondary battery of one embodiment of the present invention can be used as the secondary battery included in the display device 7300.
When the secondary battery of one embodiment of the present invention is used as a secondary battery of a daily electronic device, a lightweight product with a long lifetime can be provided. Examples of the daily electronic device include an electric toothbrush, an electric shaver, and electric beauty equipment. As secondary batteries of these products, small and lightweight stick type secondary batteries with high charge and discharge capacity are desired in consideration of handling ease for users.
Next,
The tablet terminal 9600 includes a power storage unit 9635 inside the housing 9630a and the housing 9630b. The power storage unit 9635 is provided across the housing 9630a and the housing 9630b, passing through the movable portion 9640.
The entire region or part of the region of the display portion 9631 can be a touch panel region, and data can be input by touching text, an input form, an image including an icon, and the like displayed on the region. For example, it is possible that keyboard buttons are displayed on the entire display portion 9631a on the housing 9630a side, and data such as text or an image is displayed on the display portion 9631b on the housing 9630b side.
It is possible that a keyboard is displayed on the display portion 9631b on the housing 9630b side, and data such as text or an image is displayed on the display portion 9631a on the housing 9630a side. Furthermore, it is possible that a switching button for showing/hiding a keyboard on a touch panel is displayed on the display portion 9631 and the button is touched with a finger, a stylus, or the like to display a keyboard on the display portion 9631.
Touch input can be performed concurrently in a touch panel region in the display portion 9631a on the housing 9630a side and a touch panel region in the display portion 9631b on the housing 9630b side.
The switch 9625 to the switch 9627 may function not only as an interface for operating the tablet terminal 9600 but also as an interface that can switch various functions. For example, at least one of the switch 9625 to the switch 9627 may function as a switch for switching power on/off of the tablet terminal 9600. For another example, at least one of the switch 9625 to the switch 9627 may have a function of switching the display orientation between a portrait mode and a landscape mode and a function of switching display between monochrome display and color display. For another example, at least one of the switch 9625 to the switch 9627 may have a function of adjusting the luminance of the display portion 9631. The luminance of the display portion 9631 can be optimized in accordance with the amount of external light in use of the tablet terminal 9600 detected by an optical sensor incorporated in the tablet terminal 9600. Note that another sensing device including a sensor for measuring inclination, such as a gyroscope sensor or an acceleration sensor, may be incorporated in the tablet terminal, in addition to the optical sensor.
Note that as described above, the tablet terminal 9600 can be folded in half, and thus can be folded when not in use such that the housing 9630a and the housing 9630b overlap with each other. By the folding, the display portion 9631 can be protected, which increases the durability of the tablet terminal 9600. With the power storage unit 9635 including the secondary battery of one embodiment of the present invention, which has high charge and discharge capacity and excellent cycle performance, the tablet terminal 9600 that can be used for a long time over a long period can be provided.
The tablet terminal 9600 illustrated in
The solar cell 9633, which is attached on the surface of the tablet terminal 9600, can supply electric power to a touch panel, a display portion, a video signal processing portion, and the like. Note that the solar cell 9633 can be provided on one surface or both surfaces of the housing 9630 and the power storage unit 9635 can be charged efficiently. The use of a lithium-ion battery as the power storage unit 9635 brings an advantage such as a reduction in size.
The structure and operation of the charge and discharge control circuit 9634 illustrated in
First, an operation example in which electric power is generated by the solar cell 9633 using external light is described. The voltage of electric power generated by the solar cell is raised or lowered by the DCDC converter 9636 to a voltage for charging the power storage unit 9635. When the display portion 9631 is operated with the electric power from the solar cell 9633, the switch SW1 is turned on and the voltage is raised or lowered by the converter 9637 to a voltage needed for the display portion 9631. When display on the display portion 9631 is not performed, the switch SW1 is turned off and the switch SW2 is turned on, so that the power storage unit 9635 is charged.
Note that the solar cell 9633 is described as an example of a power generation unit; however, one embodiment of the present invention is not limited to this example. The power storage unit 9635 may be charged using another power generation unit such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the charging may be performed with a non-contact power transmission module that performs charging by transmitting and receiving power wirelessly (without contact), or with a combination of other charge units.
A semiconductor display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), or an FED (Field Emission Display) can be used for the display portion 8002.
Note that the display device includes, in its category, all of information display devices for personal computers, advertisement displays, and the like besides information display devices for TV broadcast reception.
In
Note that although the installation lighting device 8100 provided in the ceiling 8104 is illustrated in
As the light source 8102, an artificial light source that emits light artificially by using electric power can be used. Specifically, an incandescent lamp, a discharge lamp such as a fluorescent lamp, and light-emitting elements such as an LED and an organic EL element are given as examples of the artificial light source.
In
Note that although the split-type air conditioner including the indoor unit and the outdoor unit is illustrated in
In
Note that among the electronic devices described above, a high-frequency heating apparatus such as a microwave oven and an electronic device such as an electric rice cooker require high power in a short time. Therefore, the tripping of a breaker of a commercial power supply in use of the electronic device can be prevented by using the secondary battery of one embodiment of the present invention as an auxiliary power supply for supplying electric power which cannot be supplied enough by a commercial power supply.
In a time period when electronic devices are not used, particularly when the proportion of the amount of electric power which is actually used to the total amount of electric power which can be supplied from a commercial power supply source (such a proportion is referred to as a usage rate of electric power) is low, electric power is stored in the secondary battery, whereby an increase in the usage rate of electric power can be inhibited in a time period other than the above time period. For example, in the case of the electric refrigerator-freezer 8300, electric power is stored in the secondary battery 8304 in night time when the temperature is low and the refrigerator door 8302 and the freezer door 8303 are not opened or closed. Moreover, in daytime when the temperature is high and the refrigerator door 8302 and the freezer door 8303 are opened and closed, the usage rate of electric power in daytime can be kept low by using the secondary battery 8304 as an auxiliary power supply.
According to one embodiment of the present invention, the secondary battery can have excellent cycle performance and improved reliability. Furthermore, according to one embodiment of the present invention, a secondary battery with high charge and discharge capacity can be obtained; thus, the secondary battery itself can be made more compact and lightweight as a result of improved characteristics of the secondary battery. Thus, the secondary battery of one embodiment of the present invention is used in the electronic device described in this embodiment, whereby a more lightweight electronic device with a longer lifetime can be obtained.
This embodiment can be implemented in appropriate combination with the other embodiments.
In this embodiment, examples of electronic devices each including the secondary battery described in the above embodiment are described with reference to
For example, the secondary battery of one embodiment of the present invention can be provided in a glasses-type device 4000 illustrated in
The secondary battery of one embodiment of the present invention can be provided in a headset-type device 4001. The headset-type device 4001 includes at least a microphone portion 4001a, a flexible pipe 4001b, and an earphone portion 4001c. The secondary battery can be provided in the flexible pipe 4001b or the earphone portion 4001c. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.
The secondary battery of one embodiment of the present invention can be provided in a device 4002 that can be attached directly to a body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.
The secondary battery of one embodiment of the present invention can be provided in a device 4003 that can be attached to clothes. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.
The secondary battery of one embodiment of the present invention can be provided in a belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power feeding and receiving portion 4006b, and the secondary battery can be provided inside the belt portion 4006a. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.
The secondary battery of one embodiment of the present invention can be provided in a watch-type device 4005. The watch-type device 4005 includes a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided in the display portion 4005a or the belt portion 4005b. With the use of the secondary battery of one embodiment of the present invention, space saving required with downsizing of a housing can be achieved.
The display portion 4005a can display various kinds of information such as time and reception information of an e-mail or an incoming call.
The watch-type device 4005 is a wearable device that is wound around an arm directly; thus, a sensor that measures the pulse, the blood pressure, or the like of the user may be incorporated therein. Data on the exercise quantity and health of the user can be stored to be used for health maintenance.
The main bodies 4100a and 4100b each include a driver unit 4101, an antenna 4102, and a secondary battery 4103. A display portion 4104 may also be included. Moreover, a substrate where a circuit such as a wireless IC is provided, a terminal for charging, and the like are preferably included. Furthermore, a microphone may be included.
A case 4110 includes a secondary battery 4111. Moreover, a substrate where a circuit such as a wireless IC or a charge control IC is provided, and a terminal for charging are preferably included. Furthermore, a display portion, a button, and the like may be included.
The main bodies 4100a and 4100b can communicate wirelessly with another electronic device such as a smartphone. Thus, sound data and the like transmitted from another electronic device can be played through the main bodies 4100a and 4100b. When the main bodies 4100a and 4100b include a microphone, sound captured by the microphone is transmitted to another electronic device, and sound data obtained by processing with the electronic device can be transmitted to and played through the main bodies 4100a and 4100b. Hence, the wireless earphones can be used as a translator, for example.
The secondary battery 4103 included in the main body 4100a can be charged by the secondary battery 4111 included in the case 4110. As the secondary battery 4111 and the secondary battery 4103, the coin-type secondary battery or the cylindrical secondary battery of the above embodiment, for example, can be used. A secondary battery including a positive electrode using the positive electrode active material 100 obtained in Embodiment 1 has a high energy density; thus, with the use of the secondary battery as the secondary battery 4103 and the secondary battery 4111, a structure that accommodates space saving due to a reduction in size of the wireless earphones can be achieved.
For example, the cleaning robot 6300 can determine whether there is an obstacle such as a wall, furniture, or a step by analyzing images taken by the cameras 6303. In the case where the cleaning robot 6300 detects an object, such as a wire, that is likely to be caught in the brush 6304 by image analysis, the rotation of the brush 6304 can be stopped. The cleaning robot 6300 further includes a secondary battery 6306 of one embodiment of the present invention and a semiconductor device or an electronic component. The cleaning robot 6300 including the secondary battery 6306 of one embodiment of the present invention can be a highly reliable electronic device that can operate for a long time.
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 the secondary battery 6409 of one embodiment of the present invention and a semiconductor device or an electronic component. The robot 6400 including the secondary battery of one embodiment of the present invention can be a highly reliable electronic device that can operate for a long time.
For example, image data taken by the camera 6502 is stored in an electronic component 6504. The electronic component 6504 can analyze the image data to detect whether there is an obstacle in the way of the movement. Moreover, the electronic component 6504 can estimate the remaining battery level from a change in the power storage capacity of the secondary battery 6503. The flying object 6500 further includes the secondary battery 6503 of one embodiment of the present invention. The flying object 6500 including the secondary battery of one embodiment of the present invention can be a highly reliable electronic device that can operate for a long time.
This embodiment can be implemented in appropriate combination with any of the other embodiments.
In this embodiment, examples of vehicles each including the secondary battery of one embodiment of the present invention will be described.
The use of secondary batteries in vehicles enables production of next-generation clean energy vehicles such as hybrid electric vehicles (HVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHVs).
The secondary battery can also supply electric power to a display device included in the automobile 8400, such as a speedometer or a tachometer. Furthermore, the secondary battery can supply electric power to a semiconductor device included in the automobile 8400, such as a navigation system.
An automobile 8500 illustrated in
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 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.
In the motor scooter 8600 illustrated in
According to one embodiment of the present invention, the secondary battery can have improved cycle performance and the charge and discharge capacity of the secondary battery can be increased. Thus, the secondary battery itself can be made more compact and lightweight. The compact and lightweight secondary battery contributes to a reduction in the weight of a vehicle, and thus increases the mileage. Furthermore, the secondary battery included in the vehicle can be used as a power source for supplying electric power to products other than the vehicle. In such a case, the use of a commercial power supply can be avoided at peak time of electric power demand, for example. Avoiding the use of a commercial power supply at peak time of electric power demand can contribute to energy saving and a reduction in carbon dioxide emissions. Moreover, the secondary battery with excellent cycle performance can be used over a long period; thus, the use amount of rare metals typified by cobalt can be reduced.
This embodiment can be implemented in appropriate combination with the other embodiments.
In this example, a positive electrode active material was fabricated with reference to FIG. or the like described in the above embodiment, and coin cells (sometimes also referred to as cells) each using the fabricated positive electrode active material were fabricated and evaluated.
As LiMO2 in Step S14 in
According to Step S20a in
According to Step S31 in
Heating was performed according to Step S33 in
According to Step S40 in
According to Step S51 in
Heating was performed according to Step S53 in
In such a manner, the positive electrode active material 100 of Step S54 in
Next, positive electrodes were fabricated using the positive electrode active material 100 fabricated in the above manner.
The positive electrode active material fabricated in the above manner, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed at the positive electrode active material:AB:PVDF=95:3:2 (weight ratio) using NMP as a solvent, whereby slurry was fabricated. The fabricated slurry was applied to only one surface of the current collector, and then the solvent was vaporized. Aluminum foil having a thickness of 20 μm was used as the current collector.
After that, positive electrodes were fabricated under two conditions: a condition where pressing was performed (denoted as “with pressing”) and a condition where pressing was not performed (denoted as “without pressing”). Under the condition where pressing was performed, a pressure of 210 kN/m was applied at 120° C., and then a pressure of 1467 kN/m was applied at 120° C. The loading amount and the density of the positive electrode active material layer were approximately 6.8 mg/cm 2 and 3.7 g/cc under the condition with pressing, and approximately 7.1 mg/cm 2 and 2.1 g/cc under the condition without pressing.
Through the above steps, the positive electrodes were fabricated.
Next, using the fabricated positive electrodes, CR2032 type coin cells (with a diameter of 20 mm and a height of 3.2 mm) were fabricated.
A lithium metal was used for a counter electrode of the coin cell. This electrode is referred to as a lithium electrode.
As a lithium salt contained in an electrolyte solution of the coin cell, 1 mol/L of lithium hexafluorophosphate (LiPF6) was used. As a solvent contained in the electrolyte solution, a solution in which ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2 wt % are mixed was used.
A polypropylene separator was used for the coin cell.
A cycle test (half-cell test) was performed on the coin cells to evaluate the samples.
First, a discharge rate and a charge rate as the cycle conditions are described. The discharge rate refers to the relative ratio of a current at the time of discharging to battery capacity and is expressed in a unit C. A current corresponding to 1 C in a battery with a rated capacity X (Ah) is X (A). The case where discharging is performed with a current of 2X (A) is rephrased as to perform discharging at 2 C, and the case where discharging is performed with a current of X/5 (A) is rephrased as to perform discharging at 0.2 C. The same applies to the charge rate; the case where charging is performed with a current of 2X (A) is rephrased as to perform charging at 2 C, and the case where charging is performed with a current of X/5 (A) is rephrased as to perform charging at 0.2 C.
Specific conditions are described. The charge conditions are as follows: constant current charging (referred to as CC charging) was performed at 0.5 C until the voltage reached 4.6 V or 4.7 V, and then constant voltage charging (referred to as CV charging) was performed until the current value reached 0.05 C. The voltage of 4.6 V or 4.7 V is referred to as an upper limit voltage. As the discharge condition, constant current discharging was performed at 0.5 C until the voltage reached 2.5 V. The voltage of 2.5 V is referred to as a lower limit voltage. In this cycle test, 1 C was set to 200 mA/g. The coin cells were placed at a temperature of 25° C. or 45° C. A break period longer than or equal to 5 minutes and shorter than or equal to 15 minutes may be provided between charging and discharging, and a break period of 10 minutes was provided in this cycle test. In the above manner, charging and discharging were repeated 50 times.
In a laminated cell (also referred to as full cell), graphite was used instead of lithium for the electrode. The upper limit voltage of the coin cell (also referred to as half cell) is lower than that of the full cell by approximately 0.1 V. The conditions in this cycle test correspond to charging to 4.5 V or 4.6 V in the full cell.
The upper graph of
As the cycle test results, the maximum discharge capacity values are shown in the following table.
As the cycle test results, the discharge capacity retention rates after 50 cycles are shown in the following table.
Table 1 does not show a big difference in the maximum discharge capacity value between the condition without pressing and the condition with pressing.
Next, the coin cells after the cycle test in which charging and discharging were repeated 50 cycles were disassembled, and observation images were obtained by SEM observation. In the case of cross-sectional observation, cross-sections were exposed by processing with FIB for observation. The FIB processing and the SEM observation were performed using XVision manufactured by Hitachi High-Tech Corporation, and an accelerating voltage in the SEM observation was 2.0 kV.
In the SEM image of the top surface of the positive electrode shown in
In the SEM image of the top surface of the positive electrode shown in
It is suggested that the frequency of generation of a pit and a crack is high under the conditions at 4.6 V with pressing. The frequency of generation of a pit and a crack is low under the condition at 4.6 V without pressing, which indicates the possibility that a decrease in discharge capacity retention rate is suppressed.
In this example, a positive electrode active material was fabricated with reference to
As LiMO2 in Step S14 in
According to Step S20a in
According to Step S31 in
Heating was performed according to Step S33 in
According to Step S40a in
According to Step S41 in
According to Step S40b in
According to Step S51, the X2b source and the mixture 991 were mixed at a heating temperature of 95° C. for a heating time of 3 hours to remove the solvent. In such a manner, the mixture 904 of Step S52 was obtained.
Heating was performed according to Step S53 in
In such a manner, the positive electrode active material 100 of Step S54 in
Next, a positive electrode was fabricated using the positive electrode active material 100 fabricated in the above manner.
The positive electrode active material fabricated in the above manner, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed at the positive electrode active material:AB:PVDF=95:3:2 (weight ratio) using NMP as a solvent, whereby slurry was fabricated. The fabricated slurry was applied to only one surface of the current collector, and then the solvent was vaporized. Aluminum foil having a thickness of 20 μm was used as the current collector.
Next, a pressure of 210 kN/m was applied at 120° C., and then a pressure of 1467 kN/m was applied at 120° C. The loading amount and the density of the positive electrode active material layer were approximately 7.3 mg/cm 2 and 3.8 g/cc, respectively.
Through the above steps, the positive electrode was fabricated.
Next, using the fabricated positive electrode, a CR2032 type coin cell (with a diameter of 20 mm and a height of 3.2 mm) was fabricated.
A lithium metal was used for a counter electrode of the coin cell. This is referred to as a lithium electrode.
As a lithium salt contained in an electrolyte solution of the coin cell, 1 mol/L of lithium hexafluorophosphate (LiPF6) was used. As a solvent contained in the electrolyte solution, a solution in which ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2 wt % are mixed was used.
A polypropylene separator was used for the coin cell.
A cycle test (half-cell test) was performed on the coin cell to evaluate the sample.
The charge conditions are as follows: constant current charging was performed at 0.5 C until the voltage reached 4.65 V or 4.7 V, and then constant voltage charging was performed until the current value reached 0.05 C. The voltage of 4.65 V or 4.7 V is referred to as an upper limit voltage. As the discharge condition, constant current discharging was performed at 0.5 C until the voltage reached 2.5 V. The voltage of 4.6 V or 4.7 V is referred to as an upper limit voltage. In this cycle test, 1 C was set to 200 mA/g. The coin cell was placed at a temperature of 25° C. or 45° C. An interval longer than or equal to 5 minutes and shorter than or equal to 15 minutes may be provided between charging and discharging, and an interval of 10 minutes was provided in this cycle test. In the above manner, charging and discharging were repeated 50 times.
In a laminated cell (also referred to as full cell), graphite was used instead of lithium for the electrode. The upper limit voltage of the coin cell (also referred to as half cell) is smaller than that of the full cell by approximately 0.1 V. The conditions in this cycle test correspond to charging to 4.5 V or 4.6 V in the full cell.
As the cycle test results, the maximum discharge capacity values are shown in the following table.
As the cycle test results, the discharge capacity retention rates after 50 cycles are shown in the following table.
Table 3 does not show a big difference in the maximum discharge capacity value.
The coin cell after the cycle test in which charging and discharging were repeated 50 cycles under the conditions of 25° C. and 4.7 V was disassembled, and STEM observation was performed. The cross section was processed by FIB. As the STEM apparatus, JEM-ARM200F manufactured by JEOL Ltd. was used. The accelerating voltage was 200 kV.
EDX analysis was performed on measurement points P1 to P6 shown in
The region 1997 is a STEM image of a region including the tip of the pit; at the measurement points P3 and P4 that are in the surface portion at the tip and the measurement point P5 positioned deeper than the measurement points P3 and P4 by several nanometers, the concentrations of magnesium and aluminum were lower than or equal to 1 atom % and no clear peak was detected. Meanwhile, at the measurement point P6 positioned deeper than the measurement point P5, a magnesium peak was observed although the concentration was lower than or equal to 1 atom %. At the measurement point P1 around the entrance of the pit, both magnesium and aluminum were detected. At the measurement point P2 at a depth of approximately 50 nm from the measurement point P1, aluminum was detected. In addition, a magnesium peak was observed at the measurement point P2 although the concentration was lower than or equal to 1 atom %.
Magnesium and aluminum are the elements that are the same as those added to the lithium cobalt oxide as the additive element sources (the X1 source, the X2 source, or the like).
In the surface portion of the lithium cobalt oxide where a defect such as a pit is formed, an additive element such as magnesium or aluminum is detected. This indicates that the additive element moves due to charging and discharging in a cycle test and is unevenly distributed also in the surface portion that appears only after formation of the defect such as the pit.
In consideration of the pit progress, the region 1997 is formed after the region 1996 and the region 1995. Since magnesium was detected at P6 in the region 1997, magnesium is presumably detected also in P3 to P5 when the cycle test is kept performed. A region at the tip of a pit where magnesium or the like can be distributed unevenly, such as the region 1997, is referred to as a pit tip region or simply referred to as a defect tip region. The thickness of the pit tip region in a cross-sectional image is 15 nm, preferably 10 nm, further preferably 5 nm.
A region in the vicinity of a pit where magnesium or the like is distributed unevenly, such as the region 1995 or the region 1996, is referred to as a region in the vicinity of a pit or simply referred to as a region in the vicinity of a defect. The thickness of the region in the vicinity of a pit in a cross-sectional image is 15 nm, preferably 10 nm, further preferably 5 nm.
This example examines lithium diffusibilities of LiCoO2 (the O3 structure, the layered rock-salt structure, hexagonal: R-3m), LiCo2O4 (the spinel structure, cubic: Fd-3m), Co3O4 (the spinel structure, cubic: Fd-3m), and CoOx (the rock-salt structure in the case of CoO, cubic: Fm-3m), which are structures that lithium cobalt oxide can have.
Among these, lithium exists in the structure of LiCoO2 as illustrated in
As illustrated in
As illustrated in
The y-axis in
In addition, the lithium diffusion coefficient in each structure was calculated and the movement distance of lithium per hour was calculated. The obtained results are shown in the following table.
It is found from Table 6 that LiCoO2 and LiCo2O4 have substantially the same lithium diffusibility even when examined in view of the lithium diffusion coefficient. It is found that lithium hardly diffuses in Co3O4 as compared with that in LiCoO2 and LiCo2O4. When lithium moves the same distance in each of Co3O4, LiCoO2, and LiCo2O4, the time taken in Co3O4 is 104 times longer than that taken in LiCoO2. A difference in the lithium diffusion coefficient or the like between LiCo2O4 and Co3O4, both of which have the spinel structure, was observed. Note that since there is no diffusion path of lithium in CoO, Li diffusion is not considered to occur therein and hyphens are used in Table 6. Thus, it is considered that existence of CoO or Co3O4, an increase in the thickness of a region where CoO or Co3O4 exists, and the like makes lithium diffusion difficult and causes a degradation in the battery characteristics such as a discharge capacity retention rate.
As illustrated in
Next, the electron resistivities in the structures are shown in the following Table.
Table 7 shows that the electron resistivities become higher in the order of the crystal structures illustrated in
As described above, the crystal structure of lithium cobalt oxide changes after a cycle test into a crystal structure with a state where no lithium exists. Accordingly, the electron resistivity of the lithium cobalt oxide increases. These can be regarded as factors of deterioration of the lithium cobalt oxide after a cycle test.
In this example, a positive electrode active material was fabricated according to
As LiMO2 in Step S14 in
After Step S14, heating was performed according to Step S15 shown in
According to Step S20a in
According to Step S31 in
Heating was performed according to Step S33 in
According to Step S40 in
According to Step SM in
Heating was performed according to Step S53 in
In such a manner, the positive electrode active material 100 of Step S54 in
Next, a positive electrode was fabricated using the positive electrode active material 100 fabricated in the above manner.
The positive electrode active material fabricated in the above manner, acetylene black (AB) as a conductive material, and PVDF as a binder were mixed at a weight ratio of 95:3:2 using NMP as a solvent, whereby slurry was fabricated. After the fabricated slurry was applied to only one surface of the current collector, the solvent was vaporized. Aluminum foil having a thickness of 20 μm was used as the current collector.
Next, pressing was performed at 210 kN/m and 120° C., and then pressing was performed at 1467 kN/m and 120° C. The loading amount and the density of the positive electrode active material layer were approximately 7.0 mg/cm 2 and 3.7 g/cc, respectively.
Through the above steps, the positive electrode was fabricated.
Next, using the fabricated positive electrode, CR2032 type coin cells (with a diameter of 20 mm and a height of 3.2 mm) were fabricated.
A lithium metal was used for a counter electrode of the coin cell (referred to as lithium electrode). The electrode is referred to as a lithium electrode.
As a lithium salt contained in an electrolyte solution of the coin cell, 1 mol/L of lithium hexafluorophosphate (LiPF6) was used. As a solvent contained in the electrolyte solution, a solution in which ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2 wt % are mixed was used.
A polypropylene separator was used for the coin cell.
Two coin cells were prepared as Samples A and B.
A cycle test (half-cell test) was performed on Samples A and B to perform evaluation.
Sample A was placed at a temperature of 25° C. As charging, constant current charging was performed at 0.5 C until the voltage reached 4.6 V or 4.7 V, and then constant voltage charging was performed until the current value reached 0.05 C. The voltage of 4.6 V or 4.7 V is referred to as an upper limit voltage. As discharging, constant current discharging was performed at 0.5 C until the voltage reached 2.5 V. The voltage of 2.5 V is referred to as a lower limit voltage. In this cycle test, 1 C was set to 200 mA/g. A break period longer than or equal to 5 minutes and shorter than or equal to 15 minutes may be provided between charging and discharging, and a break period of 10 minutes was provided in this cycle test. In the above manner, charging and discharging were repeated 50 times.
In a laminated cell (also referred to as full cell), graphite was used instead of lithium for the electrode. The upper limit voltage of the coin cell (also referred to as half cell) is smaller than that of the full cell by approximately 0.1 V. The conditions in this cycle test correspond to charging to 4.5 V or 4.6 V in the full cell.
The only difference from the conditions for Sample A is that Sample B was placed at a temperature of 45° C.
As the cycle test results, the maximum discharge capacity values are shown in the following table.
As the cycle test results, the discharge capacity retention rates after 50 cycles are shown in the following table.
Table 8 does not show a big difference in the maximum discharge capacity value.
Samples A and B after the cycle test in which charging and discharging were repeated 50 cycles at a voltage of 4.7 V were disassembled, and observation images were obtained by SEM observation.
The dashed line 1983 shown in
Comparison between the SEM images in
Next, EDX mapping analysis and EDX line analysis were performed on Samples A and B. In each analysis, cobalt as a main component of the surface portion and magnesium and aluminum as additive elements were focused.
First, Samples A and B were observed with a HAADF (High-Angle Annular Dark Field)-STEM (Scanning Transmission Electron Microscope). Hereinafter, an image obtained with a HAADF-STEM is also referred to as a STEM image.
In this example, the STEM images were obtained using an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd. under the following conditions: the acceleration voltage was 200 kV; and irradiation with an electron beam with a diameter of approximately 0.1 nmϕ was performed.
FIG. 52A1 is a cross-sectional STEM image. In FIG. 52A1, the positive electrode active material 1980 and the AB 1981 can be observed. FIG. 52A2 is an enlarged image of a region 1997A denoted by a square in FIG. 52A1. The EDX line analysis was performed in the arrow direction shown in FIG. 52A2. FIG. 52A3 is an enlarged image of a region 1997B denoted by a square in FIG. 52A2. In FIG. 52A3, lattice fringes 1986 can be observed.
The barrier layer containing magnesium and aluminum probably exists in Sample A even after a cycle test. Thus, as shown in
FIG. 53A1 to
It is considered that there is no barrier layer in Sample B after a cycle test. Thus, as shown in
Next, the structures of the surface portions of Samples A and B were examined. The cross-sectional STEM image of Sample A is shown again in
The cross-sectional STEM image of Sample B is shown in
The present inventors considered that the difference in how the lattice fringes in a crystal were seen was due to the difference in the crystal structure. In view of this, the crystal structure of the surface portion after a cycle test was examined.
STEM images of Samples A and B were obtained again. FIG. 56A1 and FIG. 56A2 are the STEM images of Sample A, and FIG. 57A1 and
FIG. 56A1 is a cross-sectional STEM image of Sample A. FIG. 56A2 is an enlarged image of a region 2001A denoted by a square in FIG. 56A1.
FIG. 57A1 is a cross-sectional STEM image of Sample B. FIG. 57A2 is an enlarged image of a region 2002A denoted by a square in FIG. 57A1.
When CoO is compared between Samples A and B, CoO exists in a wider range in Sample B in the depth direction. The depth direction is the depth direction in a cross section of the sample.
It is found from the above that, after a cycle test at a high temperature such as 45° C., the crystal structure of the surface portion changes and at least the spinel structure begins to appear. It is also found that the region where CoO exists becomes wider after the cycle test at high temperature. As described in the above example, the spinel structure and CoO have low lithium diffusibility. Since the crystal structure with low lithium diffusibility is widely formed in the surface portion, the discharge capacity retention rate after a cycle test, especially at high temperature, is considered to become low.
Next, a change in the crystal structure of the surface portion before and after a cycle test was examined. For another sample, a positive electrode active material was fabricated under the same conditions as Example 2 described above.
A coin cell was fabricated using the positive electrode active material under the same conditions as Sample A or the like to obtain Sample C.
Sample C was subjected to a cycle test (half-cell test) and evaluated.
The test conditions for Sample C were the same as those for Sample A. That is, the measurement temperature is 25° C.
HAADF-STEM observation was performed before and after the cycle test.
The region 2004 including the surface portion before the cycle test shown in
The pit 1989 was formed after the cycle test, and CoO, the spinel structure, and the O3 structure were found to exist in both the region 2005 in the vicinity of a pit shown in
The length of each region was measured and quantified. In
It is found from comparison between before and after the cycle test of Sample C that a region where CoO exists is narrower in the region 2004 before the cycle test shown in
In this example, the pit size and the like of Samples A and B after a cycle test in which charging and discharging at a voltage of 4.7 V were repeated 50 cycles were examined.
Furthermore, when the pit width of Sample A is assumed to be a width between arrows shown in
The present inventors have considered that a pit begins to be formed in lithium cobalt oxide from the surface portion of the lithium cobalt oxide and progresses due to repetition of oxygen release (also referred to as oxygen desorption) and cobalt diffusion. The present inventors have considered that oxygen is probably desorbed from the lithium cobalt oxide due to a reaction with an electrolyte solution, for example, and cobalt cut from the Co—O bond might diffuse. The reaction with the electrolyte solution is deemed to progress faster at higher temperature, which probably contributes to making the pit width of Sample B larger than the pit width of Sample A.
The crystal structure of the surface portion when oxygen desorption and cobalt diffusion further progress is considered to change from the O3 structure to CoO.
This example examined what is happened in an atomic level at the initial stage of pit generation.
Sample C before a cycle test, which is examined in the above example, has the O3 structure in its inner portion and includes CoO in its surface portion, and thus includes a region where the O3 structure and the CoO are in contact with each other. CoO corresponds to CoOx where x=1. The crystal orientation in the (110) plane of LCO and that in the (110) plane of CoO are aligned with each other. When a set plane having equivalent symmetry is represented by { }, crystal orientations in {110} of LCO and {110} of CoO can be regarded as being aligned with each other. As shown in
Thus, calculation was performed to examine whether the deviation can cause structure distortion in the O3 structure and CoO. As shown in
It is also considered that a pit is generated in the following manner: a stress difference generated between CoO and the O3 structure tears the surface, the teared portion becomes a starting point of a pit, and then CoO and the O3 structure are eroded from the starting point.
Next, focusing on CoO, a model of CoO whose surface includes a planar portion without distortion, which is illustrated in
The calculation results show that the cobalt atom is more likely to be desorbed when unevenness is included as illustrated in
Furthermore, exposing lithium cobalt oxide to a high temperature such as 45° C. seems to make going over the energy barrier needed for cutting Co—O (a bond between oxygen and cobalt) easy, so that the cobalt atom or the oxygen atom is likely to be desorbed.
In this example, a growth process of a pit, that is, how the pit grows was examined.
Therefore, formation of a Li block layer such as CoO or the spinel structure in the surface of the O3 structure, which inhibits Li diffusion, is considered to be the factor of a decrease in discharge capacity retention rate after a cycle test.
The Li block layer is extremely thin in the case where a barrier layer exists in the lithium cobalt oxide. However, the barrier layer seems to disappear after a cycle test, which probably results in a large thickness of the Li block layer and a decrease in discharge capacity retention rate.
The present inventors have considered that inhibition of Li diffusion into the inner portion due to the formation of the Li block layer in the surface portion of the positive electrode active material is one of factors of battery deterioration, which is different from a conventional main factor of battery deterioration. In addition, the present inventors have considered that the pit itself is not the main factor of deterioration.
In this example, check for splits was conducted on Samples A and B after a cycle test in which charging and discharging at a voltage of 4.7 V were repeated 50 cycles.
The crystal structure of Sample B where the split was observed was checked.
As shown in
As shown in
As described above, the inner portion of the lithium cobalt oxide where a split was generated was found to have the O3 structure. As shown in
50: inner portion, 50a: inner portion, 50b: inner portion, 52: crystal plane, 53a: barrier layer, 53c: barrier layer, 57: crack, 58a: pit, 58b: pit, 59a: vicinity, 59b: vicinity, 60: grain boundary, 61: split, 100: positive electrode active material
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-186742 | Nov 2020 | JP | national |
| 2020-208833 | Dec 2020 | JP | national |
| 2021-144027 | Sep 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/059948 | 10/28/2021 | WO |
