Battery, electronic device, power storage system, and mobile body

Abstract

Provided are an electrode and a secondary battery which have a high capacity density and are excellent in terms of rapid charge and rapid discharge. A battery has a positive electrode and a negative electrode, wherein: the positive electrode has a current collector, a first layer overlapping the current collector, and a second layer overlapping the first layer; the first layer has a first active material having a first particle size; the second layer has a second active material having a second particle size; and the first particle size is smaller than the second particle size. The second active material has a surface layer part and an internal part, the surface layer part is a region extending no more than 10 nm inward from the surface of the second active material, and the surface layer part and the internal part preferably exhibit topotaxy.

Description

Batteries, electronic devices, energy storage systems and mobile devices

[0001] BACKGROUND OF THE INVENTION 1. Field of the Invention One embodiment of the present invention relates to a secondary battery and a manufacturing method thereof, a method for manufacturing an electrode, and an apparatus for manufacturing the electrode. Alternatively, the present invention relates to an electronic device, a power storage system, a mobile object, or the like that includes a secondary battery.

[0002] 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.

[0003] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like.

[0004] In this specification, the term "power storage device" refers to all elements and devices having a power storage function, including, for example, power storage devices such as lithium ion secondary batteries (also referred to as batteries, secondary batteries, etc.), lithium ion capacitors, and electric double layer capacitors.

[0005] In recent years, various types of power storage devices have been actively developed, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. In particular, lithium-ion secondary batteries, which have high output and high energy density, are applied to mobile devices such as mobile information terminals, such as mobile phones, smartphones, and notebook computers, portable music players, digital cameras, medical devices, and next-generation clean energy vehicles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Demand for lithium-ion secondary batteries has expanded rapidly in tandem with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0006] Lithium-ion secondary batteries are made of lithium cobalt oxide (LiCoO ２ ), lithium nickel-cobalt-manganese oxide (LiNi １−ｘ−ｙ Co ｘ Mn ｙ O２ ) or lithium iron phosphate (LiFePO ４ The battery is composed of a positive electrode containing a positive electrode active material such as ethylene carbonate (EC) or diethyl carbonate (DEC), a negative electrode containing a negative electrode active material such as graphite or other carbon material capable of absorbing and releasing lithium, and an electrolyte containing an organic solvent such as ethylene carbonate (EC) or diethyl carbonate (DEC).

[0007] Furthermore, lithium ion secondary batteries are required to have high capacity density, high performance, and safety in various operating environments.

[0008] Patent Document 1 discloses a method for producing an electrode that can increase the capacity density of a secondary battery.

[0009] WO2020 / 128699 Brochure

[0010] Toyoki Okumura et al,“Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348Motohashi,T.et al,“Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et al, “Staging Phase Transitions in Li▲x▼CoO▲2▼”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609Belsky, A. et al. , “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Crystal. , (2002) B58 364-369. A. van de Walle, “Multicomponent multisublattice alloys, nonconfigural entropy and other additions to the Alloy Theoretic Automated Toolkit”, Calphad Journal 33, 266, (2009). Rasband, W. S. , ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http: / / rsb. info. nih. gov / ij / , 1997-2012. Schneider, C. A. , Rasband, W. S. , Eliceiri, K. W. “NIH Image to ImageJ: 25 years of image analysis”. Nature Methods 9, 671-675, 2012. Abramoff, M. D. , Magelhaes, P. J. , Ram, S. J. “Image Processing with ImageJ”. Biophotonics International, volume 11, issue 7, pp. 36-42, 2004. ;

[0011] The present invention aims to provide a method for manufacturing a secondary battery that can achieve a high capacity density, a method for manufacturing a secondary battery that can be rapidly charged and rapidly discharged, and a method for manufacturing a safe and reliable secondary battery.

[0012] Electrodes (positive and negative electrodes) for lithium-ion secondary batteries are fabricated by applying a slurry containing particulate active material to a metal foil called a current collector and drying the slurry. Electrodes fabricated in this manner have an active material layer on the current collector. The active material layer contains active material and voids, and in order to increase the capacity density of secondary batteries, it is necessary to minimize the voids. By using an electrode with fewer voids, a secondary battery with the same volume can achieve a larger battery capacity and improve the capacity density per volume. Note that an electrode with a layer of active material with fewer voids is sometimes referred to as a high-density electrode, a densified electrode, or an electrode with high film density.

[0013] Electrodes for lithium ion secondary batteries desirably have good electron conduction paths at the interface between the current collector and the active material layer and in the active material layer. It is also desirable to have good lithium ion conduction paths in the active material layer in a region adjacent to the separator or solid electrolyte layer. While electrodes with good electron conduction paths and good lithium ion conduction paths are suitable for rapid charging and rapid discharging, it cannot be said that electrode structures with good electron conduction paths and good lithium ion conduction paths and methods for fabricating such electrodes have yet to be fully achieved. Another challenge is to provide a structure of a high-capacity density electrode with good electron conduction paths and good lithium ion conduction paths and a method for fabricating such an electrode.

[0014] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims.

[0015] One embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer overlapping with the current collector, and a second layer overlapping with the first layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size. The first particle size is smaller than the second particle size.

[0016] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer overlapping with the current collector, and a second layer overlapping with the first layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size. The first particle size is smaller than the second particle size. The second active material has a sphericity of 0.8 to 1.0.

[0017] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer overlapping with the current collector, and a second layer overlapping with the first layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size. The first particle size is smaller than the second particle size. The second active material has a surface portion and an interior portion. The surface portion is a region extending from a surface of the second active material toward the interior by 10 nm or less. The surface portion and the interior portion are each topotaxis.

[0018] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer overlapping the current collector, and a second layer overlapping the first layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size. The first particle size is smaller than the second particle size. The second active material has a surface portion and an interior portion. The surface portion is a region extending from a surface of the second active material toward the interior by 10 nm or less. The surface portion and the interior portion are topotaxis. The second active material has a sphericity of 0.8 or more and 1.0 or less.

[0019] In any one of the above-described batteries having a first layer and a second layer, it is preferable that the first layer is on the current collector, and the second layer is on the first layer.

[0020] When a first layer is provided on a current collector and a second layer is provided on the first layer, it is preferable that the first layer and the second layer contain a conductive material, and that the mass of the conductive material in the second layer is greater than the mass of the conductive material in the first layer.

[0021] When a first layer is provided on a current collector and a second layer is provided on the first layer, it is preferable that the first layer and the second layer each contain a solid electrolyte, and that the mass of the solid electrolyte in the first layer is greater than the mass of the solid electrolyte in the second layer.

[0022] In any one of the above-described batteries, it is preferable that the battery has a second layer on the current collector, and the first layer on the second layer.

[0023] When the current collector has a second layer on it and a first layer on it, it is preferable that the first layer and the second layer contain a conductive material, and that the mass of the conductive material in the first layer is greater than the mass of the conductive material in the second layer.

[0024] When the current collector has a second layer on it and a first layer on it, it is preferable that the first layer and the second layer each have a solid electrolyte, and that the mass of the solid electrolyte in the second layer is greater than the mass of the solid electrolyte in the first layer.

[0025] Alternatively, one embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size, the second layer includes a second active material having a second particle size, and the third layer includes a third active material having a third particle size. The first particle size is smaller than the second particle size, and the third particle size is smaller than the second particle size.

[0026] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size. The third layer includes a third active material having a third particle size. The first particle size is smaller than the second particle size, the third particle size is smaller than the second particle size, and the second active material has a sphericity of 0.8 to 1.0.

[0027] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size and a fourth active material having a fourth particle size. The third layer includes a third active material having a third particle size. The first particle size is smaller than the second particle size, the third particle size is smaller than the second particle size, and the fourth particle size is smaller than the second particle size.

[0028] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size and a fourth active material having a fourth particle size. The third layer includes a third active material having a third particle size. The first particle size is smaller than the second particle size, the third particle size is smaller than the second particle size, and the fourth particle size is smaller than the second particle size. The second active material has a sphericity of 0.8 to 1.0.

[0029] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size, the second layer includes a second active material having a second particle size, and the third layer includes a third active material having a third particle size. The first particle size and the third particle size are smaller than the second particle size. The second active material has a surface portion and an interior, and the surface portion is a region extending from a surface of the second active material toward the interior by 10 nm or less. The surface portion and the interior are each topotaxis.

[0030] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size, the second layer includes a second active material having a second particle size, and the third layer includes a third active material having a third particle size. The first particle size and the third particle size are smaller than the second particle size. The second active material has a surface layer portion and an interior, and the surface layer portion is a region extending from a surface of the second active material toward the interior by 10 nm or less. The surface layer portion and the interior are each topotaxis. The second active material has a sphericity of 0.8 to 1.0.

[0031] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size and a fourth active material having a fourth particle size. The third layer includes a third active material having a third particle size. The first particle size, the third particle size, and the fourth particle size are all smaller than the second particle size. The second active material has a surface portion and an interior. The surface portion is a region extending from a surface of the second active material toward the interior by 10 nm or less. The surface portion and the interior are each topotaxis.

[0032] Another embodiment of the present invention is a battery including a positive electrode and a negative electrode. The positive electrode includes a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer. The first layer includes a first active material having a first particle size. The second layer includes a second active material having a second particle size and a fourth active material having a fourth particle size. The third layer includes a third active material having a third particle size. The first particle size, the third particle size, and the fourth particle size are all smaller than the second particle size. The second active material has a surface layer portion and an interior, and the surface layer portion is a region extending from a surface of the second active material toward the interior by 10 nm or less. The surface layer portion and the interior are each topotaxis. The second active material has a sphericity of 0.8 to 1.0.

[0033] In any one of the above-described batteries having a first layer, a second layer, and a third layer, it is preferable that the first layer, the second layer, and the third layer contain conductive materials, and that the mass of the conductive material contained in the third layer is greater than the mass of the conductive material contained in the second layer, and that the mass of the conductive material contained in the second layer is greater than the mass of the conductive material contained in the first layer.

[0034] In any one of the above-described batteries having a first layer, a second layer, and a third layer, the first layer, the second layer, and the third layer preferably have a solid electrolyte, the mass of the solid electrolyte in the first layer being greater than the mass of the solid electrolyte in the second layer, and the mass of the solid electrolyte in the second layer being greater than the mass of the solid electrolyte in the third layer.

[0035] In any one of the above-described batteries having a solid electrolyte, it is preferable that the second active material has a surface layer portion and an interior portion, the surface layer portion being a region of 10 nm or less from the surface of the second active material toward the interior portion, and when the surface layer portion and the interior portion are each topotaxis, the second active material has an edge surface having a region where the surface layer portion and the solid electrolyte are in contact with each other.

[0036] One aspect of the present invention is a mobile object including any one of the batteries described above.

[0037] One embodiment of the present invention is a power storage system including any one of the above batteries.

[0038] One embodiment of the present invention is an electronic device including any one of the above batteries.

[0039] It is possible to provide a secondary battery that can achieve a high capacity density, a secondary battery that can be rapidly charged and rapidly discharged, and a secondary battery that is safe and highly reliable.

[0040] According to one embodiment of the present invention, a positive electrode active material or composite oxide in which a decrease in charge / discharge capacity during charge / discharge cycles is suppressed can be provided. Alternatively, a positive electrode active material or composite oxide in which a crystal structure is not easily deformed even after repeated charge / discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide in which a charge / discharge capacity is large can be provided. Alternatively, a secondary battery with high safety or reliability can be provided.

[0041] It is possible to realize a manufacturing method that enables a secondary battery to have a high capacity density. It is also possible to realize a manufacturing method that enables a secondary battery that can be rapidly charged and rapidly discharged. It is also possible to provide a manufacturing method for a safe and reliable secondary battery. Or, it is possible to realize a manufacturing method that can reduce defects that occur in the active material in a sufficiently densified electrode. A high-density electrode with few defects in the active material makes it possible to realize an excellent secondary battery that satisfies high capacity density, high performance, and safety in various operating environments.

[0042] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0043] FIG. 1A illustrates an example of an electrode according to one embodiment of the present invention. FIG. 1B illustrates a cross-sectional view of a positive electrode active material. FIGS. 2A to 2D illustrate portions of cross-sectional views of positive electrode active materials. FIG. 3 illustrates an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 4A illustrates an example of a STEM image in which the crystal orientations are roughly consistent. FIG. 4B illustrates an FFT pattern of a region of the rock-salt-type crystal RS. FIG. 4C illustrates an FFT pattern of a region of the layered rock-salt-type crystal LRS. FIGS. 5A and 5B illustrate an example of an electrode according to one embodiment of the present invention. FIGS. 6A to 6D illustrate an example of an electrode according to one embodiment of the present invention. FIG. 7 illustrates an example of a method for manufacturing an electrode according to one embodiment of the present invention. FIGS. 8A and 8B illustrate an example of a method for manufacturing an electrode according to one embodiment of the present invention. FIGS. 9A and 9B illustrate an example of a method for manufacturing an electrode according to one embodiment of the present invention. FIG. 10 illustrates an example of a method for manufacturing an electrode according to one embodiment of the present invention. FIG. 11 illustrates an example of calculations for an electrode according to one embodiment of the present invention. FIGS. 12A to 12F illustrate an example of calculations for an electrode according to one embodiment of the present invention. FIGS. 13A to 13C show an example of calculations for an electrode of one embodiment of the present invention. FIGS. 14A to 14C show an example of calculations for an electrode of one embodiment of the present invention. FIG. 15 shows an example of calculations for an electrode of one embodiment of the present invention. FIGS. 16A and 16B show an example of an electrode of one embodiment of the present invention. FIGS. 17A and 17B show an example of an electrode of one embodiment of the present invention. FIGS. 18A to 18D show an example of an electrode of one embodiment of the present invention. FIGS. 19A and 19B show an example of an electrode of one embodiment of the present invention. FIGS. 20A and 20B show an example of an electrode of one embodiment of the present invention. FIGS. 21A and 21B show an example of an electrode of one embodiment of the present invention. FIGS. 22A and 22B show an example of an electrode of one embodiment of the present invention. FIGS. 23A and 23B show an example of a battery including an electrode of one embodiment of the present invention. FIG. 24 shows an example of a battery including an electrode of one embodiment of the present invention. FIGS. 25A and 25B show an example of a battery including an electrode of one embodiment of the present invention. 26A1 to 26B3 are diagrams illustrating the crystal structure and calculation results. FIGS. 27A1 to 27A3 are diagrams illustrating the crystal structure. FIGS. 28A and 28B are diagrams illustrating the crystal structure and calculation results. FIGS. 29A and 29B are diagrams illustrating the crystal structure. FIG. 30 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 31 is a diagram illustrating the crystal structure of a conventional positive electrode active material.FIGS. 32A and 32B are cross-sectional views of a positive electrode active material, and FIGS. 32C1 and 32C2 are partial cross-sectional views of the positive electrode active material. FIG. 33 shows an XRD pattern calculated from the crystal structure. FIG. 34 shows an XRD pattern calculated from the crystal structure. FIG. 35 is a cross-sectional view of a positive electrode active material. FIGS. 36A to 36C are diagrams illustrating a manufacturing method of a positive electrode active material. FIG. 37 shows an example of a manufacturing flow of a positive electrode active material, illustrating one embodiment of the present invention. FIG. 38 is a cross-sectional view of a reaction vessel used in one embodiment of the present invention. FIG. 39A is an exploded perspective view of a coin-type secondary battery, FIG. 39B is a perspective view of the coin-type secondary battery, and FIG. 39C is a cross-sectional perspective view thereof. FIG. 40A shows an example of a cylindrical secondary battery. FIG. 40B shows an example of a cylindrical secondary battery. FIG. 40C shows an example of a plurality of cylindrical secondary batteries. FIG. 40D shows an example of a power storage system including a plurality of cylindrical secondary batteries. FIGS. 41A and 41B are diagrams illustrating an example of a secondary battery, and FIG. 41C is a diagram illustrating the inside of the secondary battery. FIGS. 42A to 42C are diagrams illustrating an example of a secondary battery. FIGS. 43A and 43B are diagrams illustrating the appearance of a secondary battery. FIGS. 44A to 44C are diagrams illustrating a manufacturing method of a secondary battery. FIGS. 45A to 45C are diagrams illustrating a configuration example of a battery pack. FIGS. 46A to 46C are diagrams illustrating an example of a secondary battery. FIGS. 47A and 47B are diagrams illustrating an example of a secondary battery. FIG. 48A is a perspective view of a battery pack illustrating one embodiment of the present invention, FIG. 48B is a block diagram of the battery pack, and FIG. 48C is a block diagram of a vehicle having a motor. FIGS. 49A to 49D are diagrams illustrating an example of a transportation vehicle. FIG. 49E is a diagram illustrating an example of an artificial satellite. FIGS. 50A and 50B are diagrams illustrating a power storage device of one embodiment of the present invention. Fig. 51A is a diagram showing an electric bicycle, Fig. 51B is a diagram showing a secondary battery for the electric bicycle, and Fig. 51C is a diagram explaining an electric motorcycle. Figs. 52A to 52D are diagrams explaining an example of an electronic device. Fig. 53A shows an example of a wearable device, Fig. 53B is a perspective view of a wristwatch-type device, and Fig. 53C is a diagram explaining a side view of the wristwatch-type device. Fig. 53D is a diagram explaining an example of a wireless earphone. Figs. 54A and 54B are cross-sectional SEM images of electrodes of the example.55A and 55B are cross-sectional SEM images of an electrode according to an embodiment.

[0044] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0045] In addition, in the drawings, the size, layer thickness, or area may be exaggerated for clarity, and therefore are not necessarily limited to the scale.

[0046] In addition, in this specification and the like, ordinal numbers such as first, second, etc. are used for convenience and do not indicate the order of processes or stacking. Therefore, for example, "first" can be appropriately replaced with "second" or "third," etc., for explanation. Furthermore, the ordinal numbers used to identify one embodiment of the present invention may not match the ordinal numbers used in this specification and the like.

[0047] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), and examples of the cross-sectional shape of individual particles include ellipsoids, rectangles, trapezoids, triangles, squares with rounded corners, and asymmetric shapes, and further, individual particles may have an irregular shape.

[0048] The particle size can be measured, for example, by laser diffraction particle size distribution measurement, and can be expressed as D50. D50 is the particle size when the cumulative particle amount curve of the particle size distribution measurement result accounts for 50%, i.e., the median diameter. The measurement of particle size is not limited to laser diffraction particle size distribution measurement, and when the particle size is below the lower limit of measurement of laser diffraction particle size distribution measurement, the cross-sectional diameter of the particle cross section may be measured by analysis such as SEM (scanning electron microscope) or TEM (transmission electron microscope). As a method for measuring particle size when the cross-sectional shape of the particle is not circular, for example, the area of ​​the particle cross section can be measured by image processing, etc., and the particle size can be calculated as the diameter of a circle having that area.

[0049] In this specification, space groups are represented using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are represented using Miller indices. Individual planes representing crystal planes are represented using ( ). While space groups, crystal planes, and crystal directions are represented by a bar above the number in crystallography, in this specification, due to formatting constraints, a minus sign (-) may be placed before the number instead of a bar above it. Individual orientations indicating directions within a crystal are represented using [ ], collective orientations indicating all equivalent directions are represented using < >, individual planes indicating crystal planes are represented using ( ), and collective planes with equivalent symmetry are represented using {}. Trigonal crystals represented by the space group R-3m are generally represented as a hexagonal composite hexagonal lattice for ease of understanding the structure, and Miller indices such as (hkl) and (hkil) are sometimes used. Here, i is -(h+k).

[0050] The theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. For example, LiCoO ２ The theoretical capacity of lithium nickel oxide (LiNiO) is 274 mAh / g. ２ ) has a theoretical capacity of 275 mAh / g, and lithium manganese oxide (LiMn ２ O ４ ) has a theoretical capacity of 148 mAh / g.

[0051] The amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ CoO ２ x in, or Li ｘ MO ２ (M is a transition metal). x can also be said to be the occupancy rate of Li at the lithium site. In the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２The small value of x in the formula (1) means, for example, that 0.1<x≦0.24. The transition metal M can be selected from elements in Groups 4 to 13 of the periodic table, and for example, at least one of manganese, cobalt, and nickel is used.

[0052] When the lithium cobalt oxide satisfies the stoichiometric ratio, LiCoO ２ The occupancy rate of Li on the lithium site is x = 1. The secondary battery after discharge also has LiCoO ２ In this case, x=1. The end of discharge here refers to the state where the voltage is 2.5V (vs. Li counter electrode) or less at a current of 100mA / g. In a lithium ion secondary battery, when the occupancy rate of lithium in the lithium site reaches x=1 and no more lithium can enter, the voltage drops sharply. At this point, discharge can be said to be complete. Generally, LiCoO ２ In the case of a lithium ion secondary battery using the above method, the discharge voltage drops rapidly until it reaches 2.5 V, so it is assumed that the discharge is completed under the above conditions.

[0053] Li ｘ CoO ２ The charge capacity and / or discharge capacity used to calculate x in the above table should preferably be measured under conditions where there is no or little influence of short circuit and / or decomposition of the electrolyte. For example, data on a secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0054] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0055] Furthermore, in layered rock salt crystals and rock salt crystals, if the anion-containing layers A, B, and C are stacked in a mutually shifted pattern, such as ABCABC, then this is referred to as a cubic close-packed structure. Therefore, the anions do not necessarily have to be in a cubic lattice. At the same time, because real crystals always have defects, the analysis results do not necessarily conform to theory. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM image, spots may appear at positions slightly different from the theoretical positions. For example, if the orientation relative to the theoretical positions is 5 degrees or less, or 2.5 degrees or less, then the structure can be said to have a cubic close-packed structure.

[0056] Homogeneity refers to the phenomenon in which, in a solid consisting of multiple elements (e.g., A, B, C), a certain element (e.g., A) is distributed with similar characteristics in specific regions. It is sufficient that the concentrations of elements in the specific regions are substantially the same. For example, it is sufficient that the difference in element concentration between the specific regions is within 10%. Examples of specific regions include the surface layer, the surface, the convex portions, the concave portions, and the interior.

[0057] The electrodes (positive and negative electrodes) have an active material layer and a current collector. An electrode in which an active material layer is provided on one side of a current collector is called a single-sided coated electrode, and an electrode in which active material layers are provided on both sides of a current collector is called a double-sided coated electrode. The electrode and its manufacturing method according to one embodiment of the present invention are applicable to both single-sided coated electrodes and double-sided coated electrodes.

[0058] A cathode active material to which an additive element is added may be referred to as a composite oxide, a cathode material, a cathode ingredient, a cathode material for a secondary battery, or the like. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a compound. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composition. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composite.

[0059] As the charging voltage of a secondary battery increases, the voltage of the positive electrode generally increases. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge.

[0060] Embodiment 1 In this embodiment, an electrode of one embodiment of the present invention and a manufacturing method thereof will be described.

[0061] 1A to 6D . An electrode of one embodiment of the present invention can be used for one or both of a positive electrode and a negative electrode. In the case of a positive electrode, a positive electrode active material is used as an active material, and in the case of a negative electrode, a negative electrode active material is used as an active material. As the active material used in the electrode of one embodiment of the present invention, the active materials described in Embodiments 1 to 4 can be used.

[0062] 1A, 5A, and 5B are schematic side views of an electrode having a stacked structure according to one embodiment of the present invention. Particularly preferred examples of a positive electrode active material included in a positive electrode having a stacked structure according to one embodiment of the present invention are described with reference to FIG. 1B, 2A to 2D, 3, and 4A to 4C.

[0063] 1A shows an electrode 400A as an example of a bilayer electrode of one embodiment of the present invention. The electrode 400A has an active material layer 414 over a current collector 413. The active material layer 414 has a bilayer structure including a first layer 414a and a second layer 414b. The first layer 414a is over the current collector 413, and the second layer 414b is over the first layer 414a. The first layer 414a includes a first active material 411a, and the second layer 414b includes a second active material 411b.

[0064] The particle size Ra of the first active material 411a in the first layer 414a is preferably smaller than the particle size Rb of the second active material 411b in the second layer 414b. For example, the particle size Ra of the first active material 411a in the first layer 414a is preferably 500 nm or more and 5 μm or less, more preferably 1 μm or more and 5 μm or less. The particle size Rb of the second active material 411b in the second layer 414b is preferably 1 μm or more and 35 μm or less, more preferably 5 μm or more and 25 μm or less. The particle size of the active material here refers to the median diameter of the active material, which can be measured using any one of the above-mentioned measurement methods.

[0065] Here, the ratio of particle size Rb to particle size Ra between the first active material 411a and the second active material 411b is preferably 2 or more and 15 or less, more preferably 3 or more and 10 or less, and even more preferably 4 or more and 8 or less. When the first active material 411a and the second active material 411b satisfy the above-described relationship, rapid charging and rapid discharging become possible.

[0066] This is because the first active material 411a in the first layer 414a has a smaller particle size than the second active material 411b in the second layer 414b, which increases the number of contact points between the current collector 413 and the active material layer 414, thereby reducing the interface resistance between the current collector 413 and the active material layer 414.

[0067] 1A, the first layer 414a and the second layer 414b may contain a conductive material and a binder, which will be described later, or may contain a conductive material, a binder, and a solid electrolyte, which will be described later.

[0068] Here, the thickness of first layer 414a is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less. This is because, although it is important for first layer 414a to function as reducing the interface resistance, first active material 411a contained in first layer 414a has a small particle size of the active material in active material layer 414 and therefore contributes little to efficient storage of lithium ions, and therefore it is preferable to make first layer 414a thin.

[0069] The second active material 411b contained in the second layer 414b has the largest particle size in the active material layer 414 and is likely to contribute greatly to the efficient storage of lithium ions. Furthermore, as will be described later with reference to FIG. 6 , the use of a large particle size active material in combination with a small particle size active material and a medium particle size active material enables more efficient storage of lithium ions. In other words, the volumetric capacity density of the electrode can be increased. Therefore, in the active material layer 414, the thickness of the second layer 414b is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 150 μm or less. The higher the proportion of the second layer 414b in the active material layer 414, the higher the volumetric capacity density of the electrode 400A can be.

[0070] 1A and the like, the cross-sectional shape of the first active material 411a and the like included in the first layer 414a is schematically represented as a circle or a perfect circle for ease of understanding. The cross-sectional shape of the actual active material may be a shape other than a circle or a perfect circle (e.g., a shape with irregularities or an ellipse), and such shapes are also included in one embodiment of the present invention.

[0071] [Positive Electrode Active Material in a Positive Electrode with a Laminated Structure] As a particularly preferred example of a positive electrode active material in a positive electrode with a laminated structure according to one embodiment of the present invention, FIG. 1B shows a positive electrode active material 100 having a topotactic region in a surface layer portion. Enlarged views of the vicinity of A-B in FIG. 1B are shown in FIGS. 2A and 2B. Enlarged views of the vicinity of C-D in FIG. 1B are shown in FIGS. 2C and 2D. Here, an example of the positive electrode active material 100 having a topotactic region in a surface layer portion will be described.

[0072] In FIG. 1B, dotted lines indicate crystal planes parallel to the arrangement of cations. Arrows indicate the direction of lithium insertion and desorption during charge and discharge. The arrangement of cations here refers to the arrangement of cations other than lithium, such as transition metal M, which is easily observed in STEM images. The crystal planes parallel to the arrangement of cations refer to crystal planes parallel to the direction in which lithium ions can diffuse. As shown in FIGS. 2A to 2D, the positive electrode active material 100 has a surface layer 100a and an interior 100b. In these figures, dashed lines indicate the boundary between the surface layer 100a and the interior 100b. Although not shown, the positive electrode active material 100 may have grain boundaries.

[0073] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region within 50 nm from the surface toward the inside, more preferably within 35 nm from the surface toward the inside, even more preferably within 20 nm from the surface toward the inside, and most preferably within 10 nm from the surface toward the inside. Surfaces caused by cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0074] The region of the positive electrode active material deeper than the surface layer 100a is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0075] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer 100a, the interior 100b, and the protrusions. Therefore, the positive electrode active material 100 does not contain carbonates, hydroxyl groups, or the like that are chemically adsorbed after preparation. It also does not contain electrolytes, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 100. The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image, etc., is the boundary between the region where an electron beam combined image is observed and the region where it is not observed, and is the outermost region where bright spots originating from the atomic nuclei of metal elements with atomic numbers greater than that of lithium are observed. The surface in a cross-sectional STEM image, etc., may be determined in conjunction with the results of higher spatial resolution analyses, such as electron energy loss spectroscopy (EELS).

[0076] The term "crystal grain boundary" refers to, for example, a portion where the positive electrode active material 100 adheres to itself, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. Crystal defects refer to defects that can be observed in a cross-sectional TEM (transmission electron microscope), cross-sectional STEM image, etc., that is, a structure in which other elements have entered between the lattices, cavities, etc. The crystal grain boundary can be said to be one type of planar defect. The vicinity of the crystal grain boundary refers to a region within 10 nm from the crystal grain boundary.

[0077] <Epitaxy, Topotaxis> It is preferable that the crystal structure of the positive electrode active material 100 continuously changes from the interior 100b toward the surface. Alternatively, it is preferable that the crystal orientations of the surface layer 100a and the interior 100b are the same or approximately the same. Note that, hereinafter, a configuration in which the crystal orientations are the same or approximately the same may be simply referred to as "the crystal orientations are approximately the same." Alternatively, it is preferable that the surface layer 100a and the interior 100b are topotaxy.

[0078] Topotaxis refers to the three-dimensional structural similarity of the crystals, or the crystallographically identical orientation, while epitaxy refers to the structural similarity of the two-dimensional interface.

[0079] The topotaxis between the surface layer portion 100a and the interior portion 100b can reduce distortion of the crystal structure and / or misalignment of the atomic arrangement. This can suppress the occurrence of pits. Furthermore, the presence of an additive element in the surface layer portion 100a can suppress misalignment of the layered structure consisting of an octahedron of a transition metal M and oxygen, as described below, and / or suppress oxygen desorption from the positive electrode active material 100. This can result in a positive electrode active material that is less susceptible to deterioration even when charged at high voltages and charged and discharged in high-temperature environments. In other words, the positive electrode active material 100 having topotaxis in the surface layer portion can also be said to be a positive electrode active material that is less susceptible to deterioration even when charged at high voltages and charged and discharged in high-temperature environments. In this specification, etc., "pits" refers to holes formed by the progression of defects in the positive electrode active material.

[0080] For example, it is preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type.Alternatively, it is preferable that the orientation of the surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are approximately the same.

[0081] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal M such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a structure in which the lattice of the rock-salt crystal is distorted.

[0082] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0083] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0084] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal M. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction image corresponding to the crystal planes that form these two-dimensional planes, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the rock salt type in an ideal state, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２The bright spots on the (003) plane of MgO are observed at a distance about half the distance of the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0085] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0086] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3'-type crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0087] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0088] However, since the space group of the layered rock salt crystal and the O3'-type crystal is R-3m, which is different from the space group Fm-3m (the space group of a general rock salt crystal) of the rock salt crystal, the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt crystal and the O3'-type crystal and the rock salt crystal. In this specification, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt crystal, the O3'-type crystal, and the rock salt crystal, it may be said that the crystal orientations are approximately aligned, or that they are topotaxis, or that they are epitaxy. Note that the crystal orientations that are approximately aligned are not limited to the above combination of the layered rock salt type and the rock salt type. Regarding combinations with other crystal structures such as spinel type and perovskite type, it can also be said that the crystal orientations are approximately aligned when the orientations of the cubic close-packed structures formed by anions are aligned.

[0089] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, and FFT patterns such as TEM and STEM images. XRD (X-ray diffraction), electron diffraction, neutron diffraction, etc. can also be used as materials for the determination.

[0090] 3 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and the rock salt crystals RS are roughly the same. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0091] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in Figure 3) ＲＳ and L ＬＲＳ When the angle between the dark lines is 0 degrees or more and 5 degrees or less, or 0 degrees or more and 2.5 degrees or less, it can be determined that the crystal planes are approximately aligned, i.e., the crystal orientations are approximately aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are approximately aligned. In general, it is difficult to clearly distinguish between "aligned" and "approximately aligned." For this reason, in this specification, "aligned" includes both complete alignment (for example, when the angle between the bright lines is 0 degrees) and approximately aligned.

[0092] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers being observed brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the cobalt atoms, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate with a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0093] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0094] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0095] Figure 4A shows an example of an STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS are roughly the same. Figure 4B shows the FFT of the region of the rock salt crystal RS, and Figure 4C shows the FFT of the region of the layered rock salt crystal LRS. The left side of Figures 4B and 4C shows the composition, the JCPDS (Joint Committee on Powder Diffraction Standard) card number, and the d value and angle calculated from this. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction, and an X is marked at the center position of the spot.

[0096] The spot marked A in Figure 4B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 4C is derived from the 0003 reflection of the layered rock salt type. From Figures 4B and 4C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 4B is roughly parallel to the line passing through AO in Figure 4C. Here, "roughly coincident" and "roughly parallel" mean that the angle is between 0 degrees and 5 degrees, or between 0 degrees and 2.5 degrees.

[0097] In this way, in FFT and electron beam diffraction, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0098] Furthermore, as described above, when the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt crystal may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock salt crystal. For example, the spot marked B in FIG. 4C is originating from the 1014 reflection of the layered rock salt crystal. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point originating from the 0003 reflection of the layered rock salt crystal (A in FIG. 4C ) and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be the same. For example, equivalent reciprocal lattice points in each may be used.

[0099] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the spot where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 4B is originating from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 4B) originating from the 11-1 reflection of the cubic crystal. Note that these Miller indices are merely an example and do not necessarily need to be identical. For example, equivalent reciprocal lattice points in each may be used.

[0100] It is known that layered rock-salt type positive electrode active materials, including lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material using an SEM or the like, it is possible to thin-section the observation sample using an FIB or the like so that the electron beam is [12-10] incident in a TEM or the like, making the (0003) plane easier to observe. When determining whether the crystal orientation is consistent, it is preferable to thin-section the layered rock-salt type so that the (0003) plane can be easily observed.

[0101] 1A preferably has a cathode active material 100 having a topotaxis region in the surface layer portion thereof. That is, it is preferable that either or both of the first active material 411a and the second active material 411b have a cathode active material 100 having a topotaxis region in the surface layer portion thereof.

[0102] As described above, the positive electrode having a stacked structure according to one embodiment of the present invention preferably includes the positive electrode active material 100 having a topotaxis region in a surface layer portion. For example, a composite oxide having an additive element can be used as the positive electrode active material 100 having a topotaxis region in a surface layer portion. Details of the composite oxide having an additive element will be described in Embodiment 2. Below, an overview of the composite oxide having an additive element and its use in a positive electrode having a stacked structure will be described.

[0103] The composite oxides containing an additive element obtained by the manufacturing methods described in Embodiments 2 and 3 have crystals with a hexagonal layer structure. The crystals are not limited to single crystals (also called crystallites). In the case of polycrystals, several crystallites gather to form primary particles. A primary particle refers to a particle that is recognized as a single grain when observed with an SEM. A secondary particle refers to a mass formed by aggregation of primary particles. The aggregation of primary particles does not depend on the bonding force acting between multiple primary particles. The bonding force may be a covalent bond, an ionic bond, a hydrophobic interaction, a van der Waals force, or any other intermolecular interaction, or multiple bonding forces may be acting.

[0104] When a complex oxide is prepared using the preparation method described in Embodiment 2, primary particles that do not aggregate are often formed, but secondary particles having a small number of primary particles (e.g., less than 10) may be formed. As shown in FIGS. 2A to 2D , the complex oxide prepared using the preparation method described in Embodiment 2 preferably has an additive element in the surface layer portion. The additive element in the surface layer portion of the complex oxide can be 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. The additive element preferably includes one or more selected from bromine and beryllium. However, since bromine and beryllium are toxic to living organisms, it is preferable to use the additive elements described above.

[0105] As the composite oxide having an additional element, for example, the composite oxides described in Embodiment 2 can be used, such as lithium cobalt oxide having magnesium, lithium cobalt oxide having magnesium and fluorine, lithium cobalt oxide having magnesium and aluminum, lithium cobalt oxide having magnesium, aluminum, and fluorine, lithium cobalt oxide having magnesium, aluminum, and nickel, lithium cobalt oxide having magnesium, aluminum, nickel, and fluorine, lithium cobalt oxide having magnesium, aluminum, nickel, and barium, and lithium cobalt oxide having magnesium, aluminum, nickel, barium, and fluorine.

[0106] When a composite oxide is produced using the coprecipitation method shown in the third embodiment, secondary particles having a large number (for example, 10 or more) of primary particles may be formed.

[0107] The crystal having a hexagonal layer structure has one or more selected from the group consisting of a first transition metal, a second transition metal, and a third transition metal. Specifically, the first transition metal is nickel, the second transition metal is cobalt, the third transition metal is manganese, and LiNiｘ Co ｙ Mn ｚ O ２ A NiCoMn-based material (also referred to as NCM) represented by (x > 0, y > 0, 0.8 < x + y + z < 1.2) can be used. Specifically, for example, it is preferable to satisfy 0.1x < y < 8x and 0.1x < z < 8x. For example, it is preferable that x, y, and z satisfy x:y:z = 1:1:1 or a value thereabout. For example, it is preferable that x, y, and z satisfy x:y:z = 5:2:3 or a value thereabout. For example, it is preferable that x, y, and z satisfy x:y:z = 8:1:1 or a value thereabout. For example, it is preferable that x, y, and z satisfy x:y:z = 9:0.5:0.5 or a value thereabout. For example, it is preferable that x, y, and z satisfy x:y:z = 6:2:2 or a value thereabout. For example, it is preferable that x, y, and z satisfy x:y:z = 1:4:1 or a value thereabout.

[0108] Furthermore, the composite oxide having an additive element obtained by the above method may contain, in addition to the first transition metal, the second transition metal, and the third transition metal, one or more selected from the group consisting of magnesium, aluminum, calcium, zirconium, vanadium, chromium, iron, copper, zinc, gallium, germanium, strontium, yttrium, niobium, molybdenum, tin, barium, and lanthanum, as necessary. In order to increase the capacity retention rate after charge-discharge cycling of a secondary battery using the above positive electrode active material, it is preferable to contain one or more selected from magnesium, calcium, aluminum, and zirconium.

[0109] Next, another example of an electrode having a stacked structure according to one embodiment of the present invention will be described. FIG. 1A shows a two-layer electrode as an example of an electrode having a stacked structure according to one embodiment of the present invention. Here, FIG. 5A shows a schematic side view of a three-layer electrode as another example of an electrode having a stacked structure according to one embodiment of the present invention. The three-layer electrode 400B according to one embodiment of the present invention includes an active material layer 414 over a current collector 413. The active material layer 414 includes a first layer 414a, a second layer 414b, and a third layer 414c. The active material layer 414 includes the first layer 414a, the second layer 414b over the first layer 414a, and the third layer 414c over the second layer 414b. The first layer 414a includes a first active material 411a, the second layer 414b includes a second active material 411b, and the third layer 414c includes a third active material 411c. Similar to the electrode 400A shown in FIG. 1A, the electrode 400B shown in FIG. 5A preferably includes a positive electrode active material 100 having a topotaxis region in its surface layer portion. That is, it is preferable that the positive electrode active material 100 has a topotaxis region in its surface layer portion as one or more of the first active material 411a, the second active material 411b, and the third active material 411c.

[0110] The particle size Ra of the first active material 411a in the first layer 414a is preferably smaller than the particle size Rb of the second active material 411b in the second layer 414b. The particle size Rc of the third active material 411c in the third layer 414c is preferably smaller than the particle size Rb of the second active material 411b in the second layer 414b. For example, the particle size Ra of the first active material 411a in the first layer 414a is preferably 500 nm or more and 5 μm or less, more preferably 1 μm or more and 5 μm or less. The particle size Rb of the second active material 411b in the second layer 414b is preferably 1 μm or more and 35 μm or less, more preferably 5 μm or more and 25 μm or less. The particle diameter Rc of the third active material 411c in the third layer 414c is preferably 500 nm to 5 μm, more preferably 1 μm to 5 μm. Note that the particle diameter of the active material here refers to the median diameter of the active material, which can be measured using any one of the above-mentioned measurement methods.

[0111] Here, the ratio of the sizes of the first active material 411a and the second active material 411b, that is, particle size Rb / particle size Ra, is preferably 2 or more and 15 or less, more preferably 3 or more and 10 or less, and even more preferably 4 or more and 8 or less. Furthermore, the ratio of the sizes of the third active material 411c and the second active material 411b, that is, particle size Rb / particle size Rc, is preferably 2 or more and 10 or less, and more preferably 3 or more and 5 or less.

[0112] 5A , the first layer 414 a, the second layer 414 b, and the third layer 414 c may contain a conductive material and a binder, which will be described later, or may contain a conductive material, a binder, and a solid electrolyte, which will be described later.

[0113] When the first active material 411a in the first layer 414a, the second active material 411b in the second layer 414b, and the third active material 411c in the third layer 414c satisfy the above-described relationship, rapid charging and rapid discharging are possible.

[0114] This is because the first active material 411a in the first layer 414a has a smaller particle size than the second active material 411b in the second layer 414b, which increases the number of contact points between the current collector 413 and the active material layer 414, thereby reducing the interface resistance between the current collector 413 and the active material layer 414.

[0115] Here, the thickness of first layer 414a is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less. This is because, although it is important for first layer 414a to function as reducing the interface resistance, first active material 411a contained in first layer 414a has a small particle size of the active material in active material layer 414 and therefore contributes little to efficient storage of lithium ions, and therefore it is preferable to make first layer 414a thin.

[0116] 5A and the like, the cross-sectional shape of the first active material 411a and the like included in the first layer 414a is schematically represented as a circle or a perfect circle for ease of understanding. The cross-sectional shape of the actual active material may be a shape other than a circle or a perfect circle (e.g., a shape with irregularities or an ellipse), and such shapes are also included in one embodiment of the present invention.

[0117] Next, the relationship between the second layer 414b and the third layer 414c will be described. The third layer 414c is farther from the current collector 413 than the first layer 414a and the second layer 414b. Therefore, the third layer 414c has high electronic resistance and a relatively lower potential, which tends to decrease the rate of the battery reaction. For example, unlike the structure of an electrode of one embodiment of the present invention, when the third active material 411c of the third layer 414c and the second active material 411b of the second layer 414b have approximately the same particle size, the battery reaction slows down in a region farther from the current collector 413 (a position corresponding to the third layer 414c). This causes uneven reaction in the active material layer 414. As a result, the chargeable and dischargeable capacity decreases during rapid charging and rapid discharging.

[0118] In the above-described electrode structure of one embodiment of the present invention, the third active material 411c included in the third layer 414c has a smaller particle size than the second active material 411b included in the second layer 414b, and therefore the battery reaction rate in the third layer 414c is higher than the battery reaction rate in the second layer 414b. Therefore, unevenness in the reaction in the active material layer 414 can be reduced during rapid charging and rapid discharging. As a result, even during rapid charging and rapid discharging, a decrease in the chargeable and dischargeable capacity can be suppressed.

[0119] The second active material 411b contained in the second layer 414b has the largest particle size in the active material layer 414 and is likely to contribute greatly to the efficient storage of lithium ions. Furthermore, as will be described later with reference to FIG. 6 , the use of a large particle size active material in combination with a small particle size active material and a medium particle size active material enables more efficient storage of lithium ions. In other words, the volumetric capacity density of the electrode can be increased. Therefore, in the active material layer 414, the thickness of the second layer 414b is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 150 μm or less. The higher the proportion of the second layer 414b in the active material layer 414, the higher the volumetric capacity density of the electrode 400B can be.

[0120] The third layer 414c enables rapid charging and rapid discharging by reducing unevenness in the battery reaction in the active material layer 414. In this case, the thickness of the third layer 414c is preferably 1 μm to 20 μm, more preferably 1 μm to 10 μm.

[0121] As described above, FIG. 5A illustrates the electrode 400B having a three-layer stacked structure including the first layer 414a, the second layer 414b, and the third layer 414c. The structure of the electrode including the third layer 414c of one embodiment of the present invention is not limited to the three-layer structure. For example, as shown in FIG. 5B , the electrode 400C may have a two-layer stacked structure including the second layer 414b and the third layer 414c. The electrode 400C also enables rapid charging and rapid discharging due to the relationship between the second layer 414b and the third layer 414c described above for the electrode 400B.

[0122] Next, the second active material 411b included in the second layer 414b will be described with reference to FIGS. 6A to 6D.

[0123] The second active material 411b contained in the second layer 414b has the largest particle size in the active material layer 414 and contributes greatly to the efficient storage of lithium ions. Ideally, the second layer 414b would have a dense structure as shown in FIG. 6A . However, in reality, the second layer 414b often has a structure with gaps as shown in FIG. 6B . Therefore, as shown in FIG. 6C , when the second layer 414b contains a fourth active material 411d having a small particle size in addition to the second active material 411b having a large particle size, the fourth active material 411d is likely to be present between the particles of the second active material 411b, resulting in a high density of the second layer 414b. Furthermore, as shown in FIG. 6D , when a large-particle-size second active material 411b is combined with a small-particle-size fourth active material 411d and a medium-particle-size fifth active material 411e, a structure in which the fourth active material 411d and the fifth active material 411e are present between the particles of the second active material 411b is easily formed, resulting in an increased density of the second layer 414b.

[0124] Here, by pressing the second layer 414b-2 shown in FIG. 6B, it is possible to obtain a second layer 414b with a higher density than that shown in FIG. 6B. However, if a pressing pressure exceeding the strength of the second active material 411b is applied, there is a problem that the second active material 411b may crack. Here, if the second active material 411b has high sphericity, the second active material 411b is less likely to crack even when pressed with a higher pressure. In other words, a high-density second layer 414b is easily obtained. Therefore, it is preferable that the second active material 411b has high sphericity.

[0125] <Sphericity> The sphericity of an active material is a numerical value that represents the sphericity of the active material particles, that is, how close the shape of the active material particles is to a perfect sphere. To determine the sphericity, for example, particles having a particle size of D50±50% of the median diameter are processed for cross-sectional observation, and then the cross-sectional observation is performed to measure the perimeter L and the area S of the particle cross-section, whereby the sphericity (SP) can be calculated using the following formula:

[0126]

[0127] The sphericity of first active material 411a, second active material 411b, and third active material 411c is preferably 0.6 or more and 1.0 or less, more preferably 0.8 or more and 1.0 or less, and most preferably 0.9 or more and 1.0 or less.

[0128] Furthermore, as shown in Figures 6C and 6D, even in the case of a second active material 411b using a large-particle-size second active material 411b together with a small-particle-size fourth active material 411d and / or a medium-particle-size fifth active material 411e, the density of the second layer 414b can be further increased by pressing. In this case, as described above, active materials with high sphericity are preferred. Therefore, the sphericity of the small-particle-size fourth active material 411d and the medium-particle-size fifth active material 411e is preferably 0.6 or more and 1.0 or less, more preferably 0.8 or more and 1.0 or less, and most preferably 0.9 or more and 1.0 or less.

[0129] <Particle Size> In the second layer 414b, the particle size of the second active material 411b is preferably larger than that of the fifth active material 411e, and the particle size of the fifth active material 411e is preferably larger than that of the fourth active material 411d. As described above, the particle size of the active material contained in the second layer 414b is, for example, preferably 1 μm or more and 35 μm or less, more preferably 5 μm or more and 25 μm or less. The particle size of the fourth active material 411d contained in the second layer 414b is preferably 500 nm or more and 5 μm or less, more preferably 1 μm or more and 5 μm or less. Furthermore, the particle size of the fifth active material 411e contained in the second layer 414b is preferably 1 μm or more and 20 μm or less, more preferably 5 μm or more and 15 μm or less.

[0130] When the second layer 414b has a second active material 411b (large particle size) and a fourth active material 411d (small particle size), the mass ratio of the second active material 411b to the fourth active material 411d is expressed as second active material 411b:fourth active material 411d=1:Ma, where Ma is preferably 0.05 or more and 0.5 or less, more preferably 0.1 or more and 0.4 or less. Furthermore, when the second layer 414b has a second active material 411b (large particle size), a fourth active material 411d (small particle size), and a fifth active material 411e (medium particle size), when the mass ratio of the second active material 411b, the fourth active material 411d, and the fifth active material 411e is expressed as second active material 411b:fourth active material 411d:fifth active material 411e = 1:Mb:Mc, it is preferable that Mb is 0.1 or more and 0.5 or less, and Mc is 0.1 or more and 0.5 or less.

[0131] Although the above examples show examples of electrodes having a two-layer structure and an electrode having a three-layer structure, an electrode having four or more layers may also be used. For example, the two-layer electrode shown in FIG. 1A may have a layer having a medium-sized active material between a layer having a small-sized active material (first layer 414a) and a layer having a large-sized active material (second layer 414b). Furthermore, the three-layer electrode shown in FIG. 5A may have a five-layer structure, in which a layer having a medium-sized active material is between a layer having a small-sized active material (first layer 414a) and a layer having a large-sized active material (second layer 414b), and a layer having a medium-sized active material is between a layer having a large-sized active material (second layer 414b) and a layer having a small-sized active material (third layer 414c).

[0132] When the above composite oxide containing an additive element is used for the positive electrode having a stacked structure of one embodiment of the present invention, the first active material 411 a in the first layer 414 a, the second active material 411 b in the second layer 414 b, and the third active material 411 c in the third layer 414 c may each contain the same type (and combination) of additive elements or different types of additive elements. Furthermore, the first active material 411 a in the first layer 414 a, the second active material 411 b in the second layer 414 b, and the third active material 411 c in the third layer 414 c may each contain the same concentration of additive elements or different types of additive elements.

[0133] For example, a structure can be used in which lithium cobalt oxide containing magnesium, aluminum, and nickel is used as the second active material 411b of the second layer 414b, and lithium cobalt oxide containing magnesium is used as the third active material 411c of the third layer 414c.

[0134] Furthermore, for example, the concentration of the additive element contained in the first active material 411a in the first layer 414a and the third active material 411c in the third layer 414c can be set to be higher than the concentration of the additive element contained in the second active material 411b in the second layer 414b.

[0135] In the above example, the active material in the active material layer 414 has been described, but the active material layer 414 may include one or more of a conductive material, a binder, a solid electrolyte, and the like, which will be described later. As the active material included in the active material layer 414, any of the positive electrode active materials and the negative electrode active materials described in any of Embodiments 2 to 4 can be used.

[0136] [Method for Manufacturing Electrode 1 Having Stacked Structure] An example of a method for manufacturing an electrode of one embodiment of the present invention will be described with reference to FIGS.

[0137] 7 shows an example of a method for manufacturing an electrode in which a first layer 414a is manufactured in steps S11 to S21, a second layer 414b is manufactured in steps S21 to S31, and a third layer 414c is manufactured in steps S31 to S41. In FIG. 7, the mixture 501 prepared in step S12, the mixture 502 prepared in step S22, and the mixture 503 prepared in step S32 can be manufactured by any of the manufacturing methods shown in FIGS. 8B, 9A, 9B, and 10, or by combining a plurality of manufacturing methods.

[0138] 7, a current collector is prepared in step S11, and a mixture 501 is prepared in step S12.

[0139] Next, in step S13 of FIG. 7 , the mixture 501 is applied to a current collector. Highly conductive materials, such as metals such as stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof, can be used as the current collector. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. The application method in step S13 can be a slot die method, a gravure method, a blade method, or a combination thereof. A continuous coater or the like can also be used for application. Following step S13, in step S14, the mixture 501 applied to the current collector is dried. Examples of drying methods that can be used include batch methods using a hot plate, a drying oven, a forced air drying oven, and a vacuum drying oven, as well as continuous methods combining hot air drying and infrared drying with a continuous coater. After drying, a coated electrode 511 is obtained in step S21.

[0140] 7, after drying in step S14, pressing can be performed in step S15. As a pressing method, any of plate pressing, hydrostatic pressing, and roll pressing can be used. When roll pressing is used, it is preferable that the temperature of the roll is adjusted so that the active material layer has a temperature of 10°C or higher and 200°C or lower, preferably 80°C or higher and 150°C or lower.

[0141] Next, in step S22 of FIG. 7, a mixture 502 is prepared.

[0142] 7, the mixture 502 is applied to the coating electrode 511, and then in step S24, the mixture 502 applied to the coating electrode 511 is dried. Here, the application method in step S23 can be the method described in the description of step S13. Furthermore, the drying method in step S24 can be the method described in the description of step S14. After drying, the coating electrode 512 is obtained in step S31.

[0143] Here, after drying in step S24, pressing can be performed in step S25 as shown in Fig. 7. As a pressing method, the method described in the description of step S15 can be used.

[0144] Next, in step S32 of FIG. 7, a mixture 503 is prepared.

[0145] 7, the mixture 503 is applied to the coated electrode 512, and then in step S34, the mixture 503 applied to the coated electrode 512 is dried. Here, the method described in the description of step S13 can be used as the application method in step S33. Furthermore, the method described in the description of step S14 can be used as the drying method in step S34. After drying, the coated electrode 513 is obtained in step S41.

[0146] 7, after drying in step S34, pressing can be performed in step S35. The pressing method can be the same as that described in the description of step S15. It is preferable to perform all of steps S15, S25, and S35. However, for example, if the density of active material layer 414 can be increased without performing one or two of steps S15, S25, and S35, then one or two of steps S15, S25, and S35 may not be performed.

[0147] The electrode 400B having the first layer 414a, the second layer 414b, and the third layer 414c can be manufactured by the manufacturing process described above. Note that in the manufacturing method described with reference to FIG. 7, by completing the manufacturing of the electrode in step S31, the electrode 400A having the first layer 414a and the second layer 414b and the electrode 400C having the second layer 414b and the third layer 414c can be manufactured.

[0148] 8 to 10 show methods for producing mixtures that can be used as the mixtures 501, 502, and 503 shown in FIG.

[0149] In step S101 of FIG. 8A, the binder 110 is prepared, and in step S102, the dispersion medium 120 is prepared.

[0150] Examples of materials that can be used as the binder 110 include one or more of polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl 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, and nitrocellulose. Examples of the dispersion medium 120 that can be used include one or more of water, methanol, ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO). A combination of polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) is preferably used as a suitable combination of the binder 110 and the dispersion medium 120. Details of the binder will be described later.

[0151] Next, in step S103, the binder 110 and the dispersion medium 120 are mixed to obtain a binder mixture 1001 in step S104. For example, a propeller mixer, a planetary rotation mixer, or a thin film swirl mixer can be used as the mixing method. It is desirable that the binder mixture 1001 be in a state in which the binder 110 is well dispersed in the dispersion medium 120.

[0152] 8B, a binder mixture 1001 is prepared in step S111, and a conductive material 1002 is prepared in step S112. In order to knead the mixture in a later step, the amount of binder mixture 1001 prepared in step S111 can be an amount smaller than the total amount required to form the positive electrode active material layer, and can be a mixing amount suitable for kneading. In this case, the shortage of binder mixture 1001 can be added in a step after kneading. Note that kneading refers to kneading with high viscosity.

[0153] For example, one or more of the following can be used as the conductive material 1002: carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fiber such as carbon nanofiber and carbon nanotube, and graphene compound. Details of the conductive material will be described later.

[0154] In this specification and the like, graphene compounds include multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, and the like. Graphene compounds contain carbon, have a shape such as a flat plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. They preferably have a curved shape. The two-dimensional structure formed by six-membered carbon rings may also be referred to as a carbon sheet. Graphene compounds preferably have functional groups. Graphene compounds may also be rolled into a shape similar to carbon nanofibers. Details of graphene compounds will be described later.

[0155] Next, in step S121, the binder mixture 1001 and the conductive material 1002 are mixed to obtain a mixture 1010 in step S122. As a mixing method, for example, a propeller type mixer, a planetary rotation type mixer, or a thin film swirl type mixer can be used.

[0156] Next, in step S123 of FIG. 8B , active material 10 is prepared. For example, when mixture 1030 prepared in FIG. 8B is used to prepare first layer 414a, first active material 411a is used as active material 10. When mixture 1030 prepared in FIG. 8B is used to prepare second layer 414b, second active material 411b is used as active material 10. When mixture 1030 prepared in FIG. 8B is used to prepare third layer 414c, third active material 411c is used as active material 10.

[0157] Next, in step S131, the mixture 1010 and the active material 10 are mixed to obtain a mixture 1020 in step S132. For example, a propeller mixer, a planetary rotation mixer, or a thin film swirl mixer can be used as a mixing method. In the mixing step S131, it is preferable to perform kneading at a high viscosity (sometimes called "hard kneading"). By kneading at a high viscosity, it is possible to break up the aggregation of powders such as the active material.

[0158] Next, in step S133, binder mixture 1001 is prepared, and in step S134, dispersion medium 1003 is prepared. If an amount of binder mixture 1001 less than the total amount necessary to form the active material layer is prepared in step S111, the shortage of binder mixture 1001 can be added in step S132. If the entire amount of binder mixture 1001 necessary to form the active material layer is prepared in step S111, it is not necessary to prepare binder mixture 1001 in step S133. The same dispersion medium as in step S102 of FIG. 8A can be used as dispersion medium 1003. It is desirable to adjust the amount of dispersion medium 1003 prepared so that the viscosity is appropriate for application in a later step.

[0159] Next, in step S141, the mixture 1020 from step S132 and the dispersion medium 1003 from step S134 are mixed with the binder mixture 1001 prepared in step S133 to obtain a mixture 1030 from step S142. When a positive electrode active material is used as the active material, the mixture 1030 may be called a positive electrode slurry. When a negative electrode active material is used as the active material, the mixture 1030 may be called a negative electrode slurry.

[0160] 9A shows an example in which the method for producing an electrode is simplified and binder mixture 1001, conductive material 1002, and active material 10 are mixed at once in step S121. The mixing method in step S121 and step S131 in FIG. 9A, and the adjustment of the viscosity of mixture 1031 by the amount of dispersion medium 1003, etc. can be performed using the methods described in the explanation of FIG. 8B.

[0161] 9B shows an example of a method for producing a mixture 1032 having two types of active materials 10. The mixture 1032 can be produced in the same manner as the production method shown in FIG. 8B , except that active material 10a is prepared in step S123, active material 10b is prepared in step S124, and mixture 1010, active material 10a, and active material 10b are mixed in step S131. While FIG. 9B shows an example in which two types of active materials 10 are used, three types may be used, including active material 10a, active material 10b, and active material 10c. Furthermore, four or more types of active materials may be used.

[0162] FIG. 9B shows an example in which mixture 1010, active material 10a, and active material 10b are mixed at once in step S131, but as shown in FIG. 10, active material 10a and active material 10b may be mixed in advance.

[0163] [Calculations for Electrode 1 with Stacked Structure] Calculations for an example of an electrode according to one embodiment of the present invention will be described with reference to FIGS.

[0164] A schematic diagram of the structural model used in the calculation is shown in Fig. 11. For ease of understanding, a current collector is shown in Fig. 11, but the calculation does not include the current collector and is performed using a structure ranging from d = 0 μm to d = 120 μm. Note that d indicates the distance from the interface between the positive electrode current collector (current collector 1) and the positive electrode active material layer (positive electrode active material layer) toward the negative electrode current collector (current collector 2), and the interface position between the positive electrode current collector and the positive electrode active material layer is d = 0 μm, the interface position between the positive electrode active material layer and the separator is d = 50 μm, the interface position between the separator and the negative electrode active material layer is d = 70 μm, and the interface position between the negative electrode active material layer and the negative electrode current collector is d = 120 μm. 12A to 12C show the profiles of the calculation settings for Model A, where the particle size, void ratio, and volume ratio of the active material are constant, from d = 0 μm to d = 50 μm. Also, 12D to 12F show the profiles of the calculation settings for Model B, which has a three-layer structure, from d = 0 μm to d = 50 μm.

[0165] In the positive electrode active material layer of Model A, the particle size of the active material was constant at 20 μm. Model B had a three-layer structure. In Model B, the first layer, second layer, and third layer were arranged in this order from d = 0 μm to d = 120 μm, with the particle size of the active material in the first layer being 5 μm, the particle size in the second layer being 20 μm, and the particle size in the third layer being 5 μm. The thickness of the separator was 20 μm. The negative electrode active material layer was used under the same conditions (film thickness 50 μm, active material particle size 1 μm) in Model A and Model B. Calculations were performed under the conditions of 0.1 C, 1 C, 2 C, 3 C, 4 C, and 5 C for the charge / discharge current.

[0166] Calculations were performed on the voltage-capacity curves during charging and discharging using the calculation structure models shown in Figure 11 and Figures 12A to 12F. The charge / discharge simulation was performed using PyBaMM (version 0.4.0). The calculation model used was the Doyle-Fuller-Newman (DFN) model included with PyBaMM. The parameter set used was a partially modified version of Marquis 2019 included with PyBaMM.

[0167] As a result of the charge / discharge simulation, calculation results of discharge curves are shown in FIGS. 13A to 14C. FIG. 13A shows the discharge curve at 0.1C, FIG. 13B shows the discharge curve at 1C, FIG. 13C shows the discharge curve at 2C, FIG. 14A shows the discharge curve at 3C, FIG. 14B shows the discharge curve at 4C, and FIG. 14C shows the discharge curve at 5C. In the calculations under the present conditions, as shown in FIG. 13A, under low discharge rate conditions (low discharge current conditions), the capacity of Model A was higher than that of Model B with a three-layer structure. However, under high discharge rate conditions (high discharge current conditions) as shown in FIG. 14C, the relationship was reversed, and the capacity of Model B with a three-layer structure was calculated to be higher than that of Model A. These results are summarized in FIG. 15, which shows the relationship between discharge energy and C rate. As shown by the above calculations, the stacked electrode of one embodiment of the present invention is expected to be suitable for rapid charging and rapid discharging.

[0168] [Stacked Electrode 2] An electrode according to another embodiment of the present invention will be described with reference to FIGS.

[0169] Fig. 16A is a schematic diagram showing an electrode 400D obtained by evenly distributing conductive material 415 on electrode 400B having the three-layer structure shown in Fig. 5A. Fig. 16B is a schematic diagram showing an electrode 400E obtained by evenly distributing conductive material 415 and solid electrolyte 421 on electrode 400B having the three-layer structure shown in Fig. 5A. Electrode 400D has an electrode structure suitable for batteries using a liquid electrolyte. Electrode 400E has an electrode structure suitable for all-solid-state batteries and semi-solid-state batteries using solid electrolyte 421.

[0170] The positive electrode of one embodiment of the present invention preferably includes the positive electrode active material 100 having a topotaxis region in its surface portion. That is, it is preferable that one or more of the first active material 411a, the second active material 411b, the third active material 411c, the fourth active material 411d, and the fifth active material 411e include the positive electrode active material 100 having a topotaxis region in its surface portion.

[0171] Here, a preferred structure when using a cathode active material 100 having a topotaxis region in the surface layer portion of the electrode 400E will be described using FIGS. 17A and 17B. As in FIG. 1B, in FIGS. 17A and 17B, dotted lines indicate crystal planes parallel to the cation arrangement. Arrows indicate the direction of lithium (Li) insertion and desorption during charge and discharge. In other words, the cathode active material 100 is capable of lithium insertion and desorption at the end of the cation arrangement. Note that, on the particle surface of the cathode active material 100, the surface where the end of the cation arrangement is exposed can be called the edge surface.

[0172] 17A , an electrode having a solid electrolyte 421, such as electrode 400E, preferably has solid electrolyte 421 in the direction of lithium insertion / extraction in cathode active material 100. In other words, it is preferable that the edge surface of cathode active material 100 has a region where the surface portion of cathode active material 100 and solid electrolyte 421 are in contact with each other. Here, it is particularly preferable that the surface portion of cathode active material 100 in contact with solid electrolyte 421 is topotactic with the interior of cathode active material 100, because this allows for good migration of lithium ions in the contact region between cathode active material 100 and solid electrolyte 421.

[0173] Furthermore, when two cathode active materials 100 are in contact with each other via a solid electrolyte 421, as shown in FIG. 17A , the two cathode active materials 100 are preferably in contact with each other such that the solid electrolyte 421 is in the direction of lithium insertion / extraction in the cathode active materials 100. In other words, it is preferable that each of the two cathode active materials 100 has a region where the edge surface and the solid electrolyte 421 are in contact with each other. Note that while the example in FIG. 17A shows an example in which two cathode active materials 100 are in contact with one solid electrolyte 421, this is not limiting, and three cathode active materials 100 may be in contact with one solid electrolyte 421, or two cathode active materials 100 may be in contact with two solid electrolytes 421. There are no particular limitations on the number of cathode active materials 100 and the number of solid electrolytes 421.

[0174] As an example of a cathode active material 100 having a solid electrolyte 421 in the direction of lithium insertion / extraction, a schematic diagram of a particularly preferred structure is shown in Figure 17B. Figure 17B shows an electrode having a first layer 414a on a current collector 413, a second layer 414b on the first layer 414a, and a solid electrolyte 421. The first layer 414a has a cathode active material 411Ta having a topotaxis region in its surface layer portion, and the second layer 414b has a cathode active material 411Tb having a topotaxis region in its surface layer portion. In this way, the cathode active material 411Ta and the cathode active material 411Tb preferably have a region in contact with each other via the solid electrolyte 421. 17B , the plurality of cathode active materials 411Ta contained in the first layer 414a are preferably provided so that the second layer 414b is located ahead of the direction of lithium intercalation and deintercalation in the cathode active material 411Ta. Similarly, the plurality of cathode active materials 411Tb contained in the second layer 414b are preferably provided so that the first layer 414a is located ahead of the direction of lithium intercalation and deintercalation in the cathode active material 411Tb. In other words, the direction of lithium intercalation and deintercalation in the cathode active material 411Ta and the direction of lithium intercalation and deintercalation in the cathode active material 411Tb are preferably approximately parallel to each other.

[0175] In this case, the rate at which lithium ions move from the first layer 414a to the second layer 414b can be improved. That is, the rate at which lithium ions move from the first layer 414a toward the negative electrode can be improved, so this structure can be said to be advantageous for rapid charging and charging in low-temperature environments. Furthermore, this structure can be said to be advantageous for rapid discharging and discharging in low-temperature environments, both in terms of charging and discharging (lithium movement from the negative electrode side toward the first layer 414a).

[0176] In this embodiment, an example of an electrode structure that is a further development of the electrode structure shown in FIGS. 16A and 16B, which has a high capacity density and is suitable for rapid charging and rapid discharging, will be described.

[0177] As described above, the first active material 411a in the first layer 414a, the second active material 411b in the second layer 414b, and the third active material 411c in the third layer 414c are all located at different distances from the current collector 413. This can also be said to be due to the different distances from the separator 440 as shown in Fig. 23A . Alternatively, this can also be said to be due to the different distances from the solid electrolyte layer 420 as shown in Fig. 23B .

[0178] Here, the third layer 414c, which is relatively far from the current collector 413, will be considered. Because the third layer 414c is far from the current collector 413, it becomes a region with high electron transfer resistance (also referred to as a region with low electron mobility) in the active material layer 414. Similarly, the second layer 414b has higher electron transfer resistance than the first layer 414a. Structures for reducing this difference in electron transfer resistance are shown in FIGS. 18A and 18B. FIG. 18B is a diagram showing a profile of the conductive material ratio between A1 and A2 in FIG. 18A. 18B , by using a structure (electrode 400F) in which the proportion of conductive material in the second layer 414b is greater than the proportion of conductive material in the first layer 414a, and the proportion of conductive material in the third layer 414c is greater than the proportion of conductive material in the second layer 414b, it is possible to reduce the difference in electron transfer resistance among the first layer 414a, the second layer 414b, and the third layer 414c. In other words, it is preferable that the mass of the conductive material in the third layer 414c is greater than the mass of the conductive material in the second layer 414b, and that the mass of the conductive material in the second layer 414b is greater than the mass of the conductive material in the first layer 414a. This makes it possible to reduce unevenness in the battery reaction in the active material layer 414 during rapid charging and rapid discharging.

[0179] Here, consider the first layer 414a, which is relatively far from the solid electrolyte layer 420. Because the first layer 414a is far from the solid electrolyte layer 420, it becomes a region of high ion migration resistance (also referred to as a region of low ion conductivity) within the active material layer 414. Similarly, the second layer 414b has a higher ion migration resistance than the third layer 414c. Structures for reducing this difference in ion migration resistance are shown in Figures 18C and 18D. Figure 18D is a diagram showing the profile of the solid electrolyte ratio between B1 and B2 in Figure 18C. 18D , by using a structure (electrode 400G) in which the proportion of solid electrolyte in the second layer 414b is greater than the proportion of solid electrolyte in the third layer 414c, and the proportion of solid electrolyte in the first layer 414a is greater than the proportion of solid electrolyte in the second layer 414b, it is possible to reduce the difference in ion migration resistance among the first layer 414a, the second layer 414b, and the third layer 414c. In other words, it is preferable that the mass of the solid electrolyte in the first layer 414a is greater than the mass of the solid electrolyte in the second layer 414b, and that the mass of the solid electrolyte in the second layer 414b is greater than the mass of the solid electrolyte in the third layer 414c. This makes it possible to reduce unevenness in the battery reaction in the active material layer 414 during rapid charging and rapid discharging.

[0180] 19A and 19B show an electrode structure (electrode 400H) in which the conductive material proportion profiles shown in Fig. 18A and 18B are superimposed on the solid electrolyte proportion profiles shown in Fig. 18C and 18D. Fig. 19B is a diagram showing the conductive material proportion profile and solid electrolyte proportion profile between C1 and C2 in Fig. 19A. When an all-solid-state battery has electrode 400H, reducing the difference in electron transfer resistance in active material layer 414 and reducing the difference in ion transfer resistance in active material layer 414 makes it possible to realize an all-solid-state battery that is more suitable for rapid charging and rapid discharging.

[0181] 20 and 21 show examples in which the conductive material proportion profiles and solid electrolyte proportion profiles shown in Figures 18 and 19 are applied to electrodes with a two-layer structure (electrodes 400I and 400J). As explained above, even in electrodes with a two-layer structure, reducing the difference in electron transfer resistance in active material layer 414 and reducing the difference in ion transfer resistance in active material layer 414 enables an all-solid-state battery that is more suitable for rapid charging and rapid discharging.

[0182] Next, the positive electrode, negative electrode, current collector, conductive material, binder, graphene compound, separator, electrolyte, and exterior body described above will be further described in the following sections.

[0183] 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 415 and a binder, which will be described later. The positive electrode active material layer preferably has the above-described stacked structure.

[0184] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and may include a conductive material 415 and a binder, which will be described later. The negative electrode active material layer preferably has the above-described stacked structure.

[0185] Another example of a negative electrode is one that does not have a negative electrode active material at the end of the battery fabrication. A negative electrode that does not have a negative electrode active material can be, for example, a negative electrode that has only a negative electrode current collector at the end of the battery fabrication, in which lithium ions released from the positive electrode active material upon charging the battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

[0186] When a negative electrode without a negative electrode active material is used, a film for uniforming lithium deposition may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniforming lithium deposition. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as films for uniforming lithium deposition because they are relatively easy to form uniformly on the negative electrode current collector. Furthermore, for example, a metal film that forms an alloy with lithium can be used as the film for uniforming lithium deposition. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Lithium and magnesium form a solid solution over a wide composition range, making it suitable as a film for uniforming lithium deposition.

[0187] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0188] [Current Collector] The positive electrode current collector and the negative electrode current collector can be made of a material that has high conductivity and does not alloy with carrier ions such as lithium, such as metals such as stainless steel, gold, platinum, zinc, iron, copper, aluminum, and titanium, and alloys thereof. The current collector can be appropriately shaped, such as a sheet, mesh, punched metal, or expanded metal. The current collector should preferably have a thickness of 10 μm or more and 30 μm or less.

[0189] The negative electrode current collector is preferably made of a material that does not alloy with carrier ions such as lithium.

[0190] A titanium compound may be provided as a current collector by laminating it on the metals listed above. Examples of titanium compounds include titanium nitride, titanium oxide, titanium nitride in which part of the nitrogen is replaced with oxygen, and titanium oxynitride (TiO ｘ N ｙ, 0<x<2, 0<y<1), and titanium oxide in which part of the oxygen has been substituted with nitrogen, or a mixture or stack of two or more of these can be used. Titanium nitride is particularly preferred among these because of its high conductivity and its excellent ability to suppress oxygen diffusion. By providing a titanium compound on the surface of the current collector, for example, reaction between the material of the active material layer formed on the current collector and metal can be suppressed. When the active material layer contains a compound containing oxygen, oxidation reaction between the metal element and oxygen can be suppressed. For example, when aluminum is used as the current collector and the active material layer is formed using graphene oxide (described later), oxidation reaction between oxygen in the graphene oxide and aluminum may be a concern. In such a case, providing a titanium compound on aluminum can suppress oxidation reaction between the current collector and graphene oxide.

[0191] [Conductive Material] The conductive material, also called a conductivity imparting agent or a conductivity aid, is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0192] The active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably contain a conductive material.

[0193] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fibers such as carbon nanofibers and carbon nanotubes, and graphene compounds can be used.

[0194] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0195] The active material layer may also contain, as a conductive material, metal powder or metal fiber such as copper, nickel, aluminum, silver, or gold, or a conductive ceramic material.

[0196] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0197] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the secondary battery.

[0198] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the spaces between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. A secondary battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and stability, making it effective as an in-vehicle secondary battery.

[0199] [Binder] The active material layer preferably contains a binder. The binder binds or fixes, for example, the electrolyte and the active material. The binder can also bind or fix the electrolyte and a carbon-based material, the active material and another carbon-based material, multiple active materials together, multiple carbon-based materials, etc.

[0200] As the binder, it is preferable to use a material such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl 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.

[0201] Polyimide has excellent thermal, mechanical and chemical stability.

[0202] A fluorine-containing polymer material, such as a fluoropolymer, can be used. PVDF is a resin with a melting point in the range of 134°C to 169°C, and is a material with excellent thermal stability.

[0203] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. As the binder, fluororubber can also be used.

[0204] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0205] The binder may be used in combination with two or more of the above.

[0206] [Graphene Compound] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, 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. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be referred to as a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0207] In this specification and the like, graphene oxide refers to, for example, a material that contains carbon and oxygen, has a sheet-like shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0208] In this specification, reduced graphene oxide refers to, for example, a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. It may also be called a carbon sheet. A single reduced graphene oxide can function, but multiple sheets may be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % or more and 15 atomic % or less. By achieving these carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0209] By reducing graphene oxide, it may be possible to provide holes in the graphene compound.

[0210] Alternatively, a material in which the ends of graphene are terminated with fluorine may be used.

[0211] In the longitudinal cross section of the active material layer, the sheet-like graphene compound is dispersed substantially uniformly in the inner region of the active material layer. As shown in Fig. 22A and Fig. 22B , the plurality of graphene compounds are formed so as to partially cover the plurality of granular active material particles or to be attached to the surfaces of the plurality of granular active material particles, and are in surface contact with each other.

[0212] Here, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphene compounds together. When an active material is covered with a graphene net, the graphene net can also function as a binder that bonds the active materials together. Therefore, the amount of binder can be reduced or no binder can be used, thereby improving the ratio of the active material to the electrode volume or weight. In other words, the charge / discharge capacity of a secondary battery can be increased.

[0213] Here, it is preferable to use graphene oxide as the graphene compound, mix it with an active material to form a layer that will become an active material layer, and then reduce it. That is, the completed active material layer preferably contains reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in a polar solvent, to form the graphene compound, it is possible to substantially uniformly disperse the graphene compound in the internal region of the active material layer. Since the solvent is volatilized and removed from the dispersion medium containing the uniformly dispersed graphene oxide and the graphene oxide is reduced, the graphene compound remaining in the active material layer partially overlaps and is dispersed to such an extent that it is in surface contact with each other, thereby forming a three-dimensional conductive path. Note that the reduction of graphene oxide may be performed, for example, by heat treatment or using a reducing agent.

[0214] Furthermore, by using a spray dryer in advance, a graphene compound, which is a conductive material, can be formed as a coating that covers the entire surface of the active material, and further, the graphene compound can be used to electrically connect the active materials together, thereby forming a conductive path.

[0215] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO ２ , SiO ｘ (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a D50 of 1 μm or less, more preferably 100 nm or less.

[0216] [Separator] A separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0217] The separator is a porous material having pores with a diameter of at least 2 nm. Preferably, the separator has pores with a diameter of 6.5 nm or more, and more preferably, pores with a diameter of about 20 nm. In the case of the semi-solid secondary battery described above, the separator can be omitted.

[0218] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0219] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0220] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0221] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0222] [Electrolyte] As the solid electrolyte 421 included in the solid electrolyte layer 420, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, or the like can be used.

[0223] The sulfide-based solid electrolytes include thiolithium-based (Li １０ GeP ２ S １２ , Li ３．２５ Ge ０．２５ P ０．７５ S ４ etc.), sulfide glass (70Li ２ S・30P ２ S ５ , 30Li ２ S・26B ２ S ３ ・44LiI, 63Li ２ S・36SiS ２ ・1Li ３ P.O. ４ , 57Li ２ S・38SiS ２ ・5Li ４ SiO ４ , 50Li ２ S・50GeS ２ etc.), sulfide crystallized glass (Li７ P ３ S １１ , Li ３．２５ P ０．９５ S ４ Sulfide-based solid electrolytes have the advantages of being highly conductive, being able to be synthesized at low temperatures, and being relatively soft, which makes it easy to maintain conductive paths even after charging and discharging.

[0224] The oxide-based solid electrolyte includes a material having a perovskite crystal structure (La ２／３−ｘ Li ３ｘ TiO ３ etc.), materials having a NASICON type crystal structure (Li １−Ｙ Al Ｙ Ti ２−Ｙ (P.O. ４ ) ３ etc.), materials having a garnet-type crystal structure (Li ７ La ３ Zr ２ O １２ etc.), materials having a LISICON type crystal structure (Li １４ ZnGe ４ O １６ etc.), LLZO (Li ７ La ３ Zr ２ O １２ ), oxide glass (Li ３ P.O. ４ -Li ４ SiO ４ , 50Li ４ SiO ４ ・50Li ３ BO ３ etc.), oxide crystallized glass (Li １．０７ Al ０．６９ Ti １．４６ (P.O. ４ ) ３ , Li １．５ Al ０．５ Ge １．５ (P.O. ４ ) ３ Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0225] Halide-based solid electrolytes include LiAlCl ４ , Li ３InBr ６ , LiF, LiCl, LiBr, LiI, etc. Composite materials in which these halide-based solid electrolytes are filled into the pores of porous aluminum oxide or porous silica can also be used as the solid electrolyte.

[0226] Also, different solid electrolytes may be mixed and used.

[0227] Among them, Li having a NASICON type crystal structure １−ｘ Al ｘ Ti ２−ｘ (P.O. ４ ) ３ (0<x<1) (hereinafter, referred to as LATP) contains aluminum and titanium, which are elements that may be contained in the positive electrode active material used in the secondary battery of one embodiment of the present invention, and is therefore expected to have a synergistic effect in improving cycle characteristics, which is preferable. In addition, productivity can be improved by reducing the number of steps. Note that in this specification and the like, the NASICON-type crystal structure refers to a structure having a structure such as M ２ (XO ４ ) ３ In the compound represented by the formula (M: transition metal, X: S, P, As, Mo, W, etc.), MO ６ Octahedron and XO ４ It refers to a structure in which tetrahedrons are arranged three-dimensionally with their vertices shared.

[0228] When a liquid electrolyte 576 is used in the secondary battery, for example, the electrolyte 576 can be 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, sultone, and the like, or two or more of these can be used in any combination and ratio.

[0229] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte 576, it is possible to prevent the secondary battery from exploding or catching fire even if the temperature of the internal region of the secondary battery rises due to short-circuiting or overcharging. The ionic liquid is composed of cations and anions, including organic cations and anions. Examples of organic cations include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, as well as aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0230] In particular, in the case where silicon is used as an active material in the negative electrode of the secondary battery of one embodiment of the present invention, the liquid electrolyte 576 containing an ionic liquid is preferably used.

[0231] The secondary battery of one embodiment of the present invention includes, as carrier ions, alkali metal ions such as lithium ions, sodium ions, and potassium ions, and alkaline earth metal ions such as calcium ions, strontium ions, barium ions, beryllium ions, and magnesium ions.

[0232] When lithium ions are used as carrier ions, the electrolyte contains a lithium salt, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２, LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ etc. can be used.

[0233] The electrolyte preferably contains fluorine. For example, an electrolyte containing one or more fluorinated cyclic carbonates and lithium ions can be used as the fluorine-containing electrolyte. The fluorinated cyclic carbonate improves non-flammability and can enhance the safety of the lithium ion secondary battery.

[0234] Examples of fluorinated cyclic carbonates that can be used include fluorinated ethylene carbonate, such as monofluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. For low-temperature operation, it is important to use one or more fluorinated cyclic carbonates as an electrolyte to solvate lithium ions and transport them within the electrolyte contained in the electrode during charging and discharging. Using a fluorinated cyclic carbonate not as a small amount of additive but contributing to the transport of lithium ions during charging and discharging enables low-temperature operation. Within a secondary battery, lithium ions move in clusters of several to several tens of ions.

[0235] The use of a fluorinated cyclic carbonate in the electrolyte reduces the desolvation energy required for lithium ions solvated in the electrolyte contained in the electrode to enter active material particles. Reducing this desolvation energy facilitates insertion and desorption of lithium ions into active material particles, even at low temperatures. While lithium ions may migrate in a solvated state, a hopping phenomenon, in which the coordinated solvent molecules switch positions, may also occur. When lithium ions are more easily desolvated, they may be more susceptible to migration via the hopping phenomenon, which may facilitate lithium ion migration. There is a concern that electrolyte decomposition products cling to the surface of the active material during charging and discharging of secondary batteries, causing deterioration of the secondary battery. However, when the electrolyte contains fluorine, the electrolyte is smooth, making it difficult for electrolyte decomposition products to adhere to the surface of the active material. This reduces secondary battery degradation.

[0236] A plurality of solvated lithium ions may form clusters in the electrolyte and move within the negative electrode, between the positive electrode and the negative electrode, within the positive electrode, etc.

[0237] In this specification, the term "electrolyte" is a general term that includes solid electrolytes, liquid electrolytes, semi-solid electrolytes, and the like.

[0238] Deterioration is likely to occur at interfaces present in secondary batteries, such as the interface between the active material and the electrolyte. In a secondary battery according to one embodiment of the present invention, the presence of a fluorine-containing electrolyte can prevent deterioration, typically electrolyte alteration or increased viscosity, that can occur at the interface between the active material and the electrolyte. Furthermore, a binder, a graphene compound, or the like may be attached to or retained by the fluorine-containing electrolyte. This configuration can maintain a reduced viscosity of the electrolyte, in other words, a smooth electrolyte, thereby improving the reliability of the secondary battery. DFEC, which has two fluorine atoms bonded, and F4EC, which has four fluorine atoms bonded, have lower viscosity and are smoother than FEC, which has one fluorine atom bonded, and thus have weaker coordination bonds with lithium. Therefore, adhesion of viscous decomposition products to active material particles can be reduced. Adhesion or adhesion of viscous decomposition products to active material particles can hinder the movement of lithium ions at the interfaces of the active material particles. Fluorine-containing electrolytes mitigate the formation of decomposition products on the surface of the active material (positive electrode active material or negative electrode active material) by solvating the electrolyte. Furthermore, the use of fluorine-containing electrolytes can prevent the formation and growth of dendrites by preventing the adhesion of decomposition products.

[0239] Another feature is that a fluorine-containing electrolyte is used as the main component, and the fluorine-containing electrolyte is 5% by volume or more, 10% by volume or more, preferably 30% by volume or more and 100% by volume or less.

[0240] In this specification, the term "main component of the electrolyte" refers to 5% by volume or more of the total electrolyte of the secondary battery. Furthermore, "5% by volume or more of the total electrolyte of the secondary battery" refers to the percentage of the total electrolyte measured during the manufacture of the secondary battery. Furthermore, when disassembling a secondary battery after fabrication, it is difficult to quantify the percentage of each of multiple types of electrolytes, but it is possible to determine whether a certain type of organic compound accounts for 5% by volume or more of the total electrolyte.

[0241] By using an electrolyte containing fluorine, a secondary battery that can operate over a wide temperature range, specifically, from -40°C to 150°C, preferably from -40°C to 85°C, can be realized.

[0242] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte. The concentration of the additive may be, for example, 0.1% by volume or more and less than 5% by volume with respect to the total volume of the electrolyte.

[0243] In addition to the above, the electrolyte may contain one or more aprotic organic solvents such as γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran.

[0244] Furthermore, the use of a gelling polymer material in the electrolyte increases safety against leakage, etc. Typical examples of gelling polymer materials include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0245] Examples of polymeric materials that can be used include polymers having a polyalkylene oxide structure, such as polyethylene oxide (PEO), PVDF, and polyacrylonitrile, as well as copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymers formed may also have a porous shape.

[0246] Although the above configuration shows an example of a secondary battery using a liquid electrolyte, it is not particularly limited. For example, semi-solid batteries and all-solid batteries can also be produced.

[0247] In this specification, the layer disposed between the positive electrode and the negative electrode is referred to as the electrolyte layer in both the case of a secondary battery using a liquid electrolyte and the case of a semi-solid battery. The electrolyte layer of a semi-solid battery can be said to be a layer formed by film formation, and can be distinguished from a liquid electrolyte layer.

[0248] In addition, in this specification, a semi-solid battery refers to a battery having a semi-solid material in at least one of the electrolyte layer, positive electrode, and negative electrode. Here, semi-solid does not mean that the ratio of solid material is 50%. Semi-solid means that while having solid properties such as small volume change, it also has some liquid-like properties such as flexibility. As long as these properties are met, the battery may be made of a single material or multiple materials. For example, a porous solid material may be impregnated with a liquid material.

[0249] In this specification, a polymer electrolyte secondary battery refers to a secondary battery having a polymer in the electrolyte layer between a positive electrode and a negative electrode. Polymer electrolyte secondary batteries include dry (or intrinsic) polymer electrolyte batteries and polymer gel electrolyte batteries.

[0250] The electrolyte 576 comprises a lithium ion conductive polymer and a lithium salt.

[0251] In this specification, the lithium ion conductive polymer is a polymer that has conductivity for cations such as lithium. More specifically, it is a polymer compound having a polar group to which a cation can be coordinated. The polar group preferably has an ether group, an ester group, a nitrile group, a carbonyl group, a siloxane group, or the like.

[0252] Examples of the lithium ion conductive polymer that can be used include polyethylene oxide (PEO), derivatives having polyethylene oxide as the main chain, polypropylene oxide, polyacrylic acid ester, polymethacrylic acid ester, polysiloxane, and polyphosphazene.

[0253] The lithium ion conductive polymer may be branched or crosslinked, or may be a copolymer. The molecular weight is preferably 10,000 or more, and more preferably 100,000 or more.

[0254] In lithium ion conductive polymers, lithium ions move while changing the polar groups they interact with due to the partial motion (also called segmental motion) of the polymer chain. For example, in the case of PEO, lithium ions move while changing the oxygen they interact with due to the segmental motion of the ether chain. When the temperature is close to or higher than the melting point or softening point of the lithium ion conductive polymer, the crystalline regions dissolve, the amorphous regions increase, and the motion of the ether chains becomes more active, resulting in higher ionic conductivity. Therefore, when using PEO as a lithium ion conductive polymer, it is preferable to charge and discharge at 60°C or higher.

[0255] According to Shannon's ionic radius (Shannon et al., Acta A 32 (1976) 751.), the radius of a monovalent lithium ion is 0.590 × 10 when it is four-coordinated. −１ nm, 0.76 × 10 for 6 coordinations −１ nm, 0.92 × 10 for 8 coordinations −１ The radius of a divalent oxygen ion is 1.35 × 10 −１ nm, 1.36 × 10 for three-coordinate −１ nm, 1.38 × 10 for 4-coordination −１ nm, 1.40 × 10 for 6 coordinations −１ nm, 1.42 × 10 for 8 coordinations −１ The distance between the polar groups of adjacent lithium ion conductive polymer chains is preferably at least the distance at which the lithium ions and the anions of the polar groups can stably exist while maintaining the ionic radius as described above. It is also preferable that the distance be such that sufficient interaction occurs between the lithium ions and the polar groups. However, as described above, segmental motion occurs, so it is not necessary to maintain a constant distance at all times. It is sufficient to maintain a distance that is appropriate for the lithium ions to pass through.

[0256] As the lithium salt, for example, a compound containing lithium and at least one of phosphorus, fluorine, nitrogen, sulfur, oxygen, chlorine, arsenic, boron, aluminum, bromine, and iodine can be used. For example, LiPF ６ , LiN(FSO ２ ) ２(lithium bis(fluorosulfonyl)imide, LiFSI), LiN(SO ２ CF ３ ) ２ (lithium bis(trifluoromethanesulfonyl)amide, LiTFSA), LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (LiBOB), or the like, can be used alone or in any combination and ratio of two or more of these.

[0257] In particular, the use of LiFSI is preferable because it provides good low-temperature properties. ６It is less reactive with water than other materials. This makes it easier to control the dew point when fabricating electrodes and electrolyte layers using LiFSI. For example, it can be handled not only in an inert atmosphere such as argon, which minimizes moisture, or in a dry room with a controlled dew point, but also in normal air. This improves productivity and is therefore preferable. Furthermore, the use of Li salts with high dissociation properties and plasticizing effects, such as LiFSI and LiTFSA, is particularly preferable when using lithium conduction utilizing the segmental motion of ether chains, because they can be used over a wide temperature range.

[0258] The absence or very small amount of organic solvents makes it possible to produce a secondary battery that is less likely to catch fire or ignite, which is preferable and improves safety. Furthermore, if the electrolyte 576 is an electrolyte layer that contains no or very little organic solvent, it is possible to achieve sufficient strength and electrically insulate the positive and negative electrodes without a separator. Since a separator is not required, a secondary battery with high productivity can be produced. If the electrolyte layer contains the electrolyte 576 and an inorganic filler, the strength is further increased and a secondary battery with higher safety can be produced.

[0259] [Exterior Body] Exterior bodies of secondary batteries can be made of metal materials such as aluminum and resin materials. Film-like exterior bodies can also be used. Examples of films that can be used include a three-layer structure in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body. It is also preferable to use a fluororesin film as the film. Fluororesin films have high stability against acids, alkalis, organic solvents, etc., and can suppress side reactions, corrosion, etc. associated with secondary battery reactions, thereby achieving excellent secondary batteries. Examples of fluororesin films include PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane: a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether), FEP (perfluoroethylene propene copolymer: a copolymer of tetrafluoroethylene and hexafluoropropylene), and ETFE (ethylene tetrafluoroethylene copolymer: a copolymer of tetrafluoroethylene and ethylene).

[0260] [Internal Structure of Battery] A battery including an electrode according to one embodiment of the present invention will be described with reference to FIGS. 23 and 24. FIG.

[0261] 23A , the secondary battery of one embodiment of the present invention includes a positive electrode 410, a separator 440, and a negative electrode 430. The positive electrode 410 in FIG. 23A is a positive electrode using the electrode structure shown in FIG. 18A , and any one or more of the positive electrode active materials described in Embodiments 2 and 3 can be used as the positive electrode active material. The secondary battery shown in FIG. 23A includes a liquid electrolyte 576, and the liquid electrolyte 576 preferably fills spaces between particles in the stacked structure of the positive electrode 410.

[0262] 23B and 24 , the secondary battery of one embodiment of the present invention includes a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430. The positive electrode 410 shown in FIGS. 23B and 24 is a positive electrode using the electrode structure shown in FIG. 19A , and any one or more of the positive electrode active materials described in Embodiments 2 and 3 can be used as the positive electrode active material. The active material layer 414 included in the positive electrode may include the conductive material and the binder.

[0263] The solid electrolyte layer 420 has a solid electrolyte 421. The solid electrolyte layer 420 is located between the positive electrode 410 and the negative electrode 430, and is a region that does not have either the positive electrode 410 or the negative electrode 430.

[0264] As shown in Fig. 24, 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 a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also include a conductive material and a binder. When metallic lithium is used as the negative electrode active material 431, it is not necessary to form the material into particles, and therefore, as shown in Fig. 23B, the negative electrode 430 can be one that does not include the solid electrolyte 421. Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery.

[0265] The solid electrolyte 421 of the solid electrolyte layer 420 may be, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, or a halide-based solid electrolyte.

[0266] 25A and 25B show modified versions of the battery having the solid electrolyte 421 shown in FIGS. 23B and 24 . The batteries shown in FIGS. 25A and 25B include not only the solid electrolyte 421 but also a liquid electrolyte 576. These batteries are sometimes referred to as semi-solid batteries because they contain both a solid electrolyte and a liquid electrolyte. Semi-solid batteries have both the flame retardancy of the solid electrolyte 421 and the increased contact interface between the active material and the electrolyte of the liquid electrolyte 576. In this case, using an electrolyte containing an ionic liquid as the liquid electrolyte 576 is particularly preferable because it results in a flame retardant battery.

[0267] The content of this embodiment mode can be freely combined with the content of other embodiment modes.

[0268] Embodiment 2 In this embodiment, a positive electrode active material 100A as an example of the positive electrode active material 100 that can be used for the secondary battery of one embodiment of the present invention and a manufacturing method thereof will be described with reference to FIGS. 26A to 36 .

[0269] [Positive Electrode Active Material] Fig. 1B is a cross-sectional view of a positive electrode active material 100A that can be used in a secondary battery of one embodiment of the present invention. Figs. 2A and 2B are enlarged views of the vicinity of A-B in Fig. 1B. Figs. 2C and 2D are enlarged views of the vicinity of C-D in Fig. 1B.

[0270] 1B and 2A to 2D, a positive electrode active material 100A has a surface layer 100a and an interior 100b. In these figures, the boundary between the surface layer 100a and the interior 100b is indicated by a dashed line.

[0271] <Elements contained in the positive electrode active material> The positive electrode active material 100A contains lithium, a transition metal M, oxygen, and an additive element A. Alternatively, the positive electrode active material 100A may contain a composite oxide (LiMO) containing lithium and a transition metal M. ２ ) to which the additive element A is added. However, the composition of the composite oxide is not strictly limited to Li:M:O=1:1:2. In addition, a positive electrode active material to which the additive element A is added may also be called a composite oxide.

[0272] The positive electrode active material of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. In one embodiment of the present invention, the positive electrode active material 100A preferably uses cobalt as the transition metal M responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or two selected from nickel and manganese may also be used. It is preferable that the positive electrode active material 100A contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals M, because of its many advantages, such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0273] Furthermore, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metal M in the positive electrode active material 100A, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal M, ｘ CoO ２ The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) dominates, are significantly affected by the Jahn-Teller effect, making the nickel-oxygen octahedral layers more susceptible to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel dominates, have the problem of nickel and lithium cation mixing being more likely to occur.

[0274] On the other hand, when nickel is used as the transition metal M in the positive electrode active material 100A at 33 atomic % or more, preferably at 60 atomic % or more, and more preferably at 80 atomic % or more, the raw material may be cheaper than when cobalt is used in large amounts, and the charge / discharge capacity per weight may increase, which is preferable.

[0275] The additive element A contained in the positive electrode active material 100A is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. The sum of the transition metals among the additive elements A is preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0276] That is, the positive electrode active material 100A can include lithium cobalt oxide to which magnesium and fluorine are added, lithium cobalt oxide to which magnesium, fluorine and titanium are added, lithium cobalt oxide to which magnesium, fluorine and aluminum are added, lithium cobalt oxide to which magnesium, fluorine and nickel are added, lithium cobalt oxide to which magnesium, fluorine, nickel and aluminum are added, and the like.

[0277] As will be described later, these additional elements A further stabilize the crystal structure of the positive electrode active material 100A. In this specification and the like, the additional element A has the same meaning as a mixture or a part of a raw material.

[0278] The additional element A does not necessarily have to contain magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0279] For example, if the cathode active material 100A is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, are further enhanced. The weight of manganese contained in the cathode active material 100A is preferably, for example, 600 ppm or less, more preferably 100 ppm or less. The weight of manganese can be analyzed, for example, using GD-MS.

[0280] Next, the results of calculations of the crystal structure of the surface layer portion when an additive element is present and when an additive element is not present will be described with reference to FIGS.

[0281] In particular, cobalt oxide may be present in the surface layer of lithium cobalt oxide that does not contain any additive elements. Cobalt oxide may also contain metal vacancies. Figure 26A1 shows the crystal structure of lithium cobalt oxide (LCO), and Figure 26A2 shows the crystal structure of cobalt oxide (CoO). As shown in Figures 26A1 and 26A2, the {110} crystal orientations of LCO and CoO roughly coincide, but there is a 5.1% difference between the 1.405 nm spacing of the {001} plane perpendicular to the {110} plane of LCO and the 1.477 nm spacing, which is six times the 1-11} plane perpendicular to the {110} plane of CoO.

[0282] Figure 26B1 is a schematic diagram of lithium cobalt oxide (LCO) having cobalt oxide (CoO) in the surface layer. Figure 26B2 shows an enlarged view of the surface layer. Figure 26B3 shows the results of calculations using classical molecular dynamics on a portion of the surface layer containing LCO and CoO. Because there is a difference of more than 5% between the interplanar spacing of the {001} plane perpendicular to the {110} plane of LCO and the interplanar spacing of the {1-11} plane perpendicular to the {110} plane of CoO, which is six times the interplanar spacing, multiple deviations in atomic arrangement occur, as shown in the dotted circle in Figure 26B3. It is believed that such unstable locations are prone to cobalt and / or oxygen desorption. Therefore, they can become the starting points for pits.

[0283] Even in the case of lithium cobalt oxide containing an additive element, cobalt oxide may be present in the surface layer. Figure 27A1 shows the crystal structure of lithium cobalt oxide (LCO), Figure 27A2 shows the crystal structure of cobalt oxide (CoO), and Figure 27A3 shows the crystal structure of magnesium oxide (MgO) when magnesium is used as the additive element. As shown in Figures 27A1 to 27A3, the arrangement of oxygen in the {110} plane of LCO is the same as that in the {110} planes of CoO and MgO, resulting in topotaxis. Furthermore, the six-fold spacing of the {1-11} plane, which is perpendicular to the {110} plane of MgO, is 1.461 nm, which is longer than the 1.405 nm spacing of the {001} plane of LCO and shorter than the 1.477 nm spacing of six-fold spacing of the {1-11} plane of CoO. Therefore, it is believed that the lattice mismatch and strain are smaller when LCO and MgO are in contact than when LCO and CoO are in contact.

[0284] Next, we investigated whether CoO and MgO form a solid solution. （１−ｘ） Mg ｘ The formation energy of O is analyzed using ATAT (Alloy Theoretical Automated Toolkit) software described in Non-Patent Document 5. ATAT is software for efficiently advancing structure search by combining first-principles calculations and cluster expansion methods. The software used for the first-principles calculations was VASP (Vienna Ab initio Simulation Package), and the calculation conditions were as shown in Table 1. Using ATAT, the formation energy of Co （１−ｘ） Mg ｘ The results of searching for the configurations when x in O is 0.125, 0.143, 0.250, 0.500, and 0.833 are shown in Figure 28A. The gray parallelogram in Figure 28A represents an Mg-O octahedron with Mg at the center (MgO ６ ), and the black parallelogram represents a Co—O octahedron (CoO ６ )

[0285]

[0286] As shown in FIG. （１−ｘ） Mg ｘ The graph of the formation energy of O is convex downward, and the solid solution is more stable, suggesting that CoO and MgO can form a solid solution. It is also suggested that Co and Mg are dispersed and distributed in the solid solution state.

[0287] Co in solid solution （１−ｘ） Mg ｘ O has anisotropy in the interplanar spacing, and it is difficult to determine in which crystal orientation it can form topotaxis with LCO. Therefore, the volume of the structural model of each ratio is divided by the number of metal atoms in the structural model, and the volume per metal atom (10 −３ nm ３ The results of calculating the tendency of change in interplanar spacing based on the Mg solid solution ratio are shown in Figure 28B. （１−ｘ） Mg ｘ This suggests that the volume of O decreases and tends to approach that of MgO.（１−ｘ） Mg ｘ It is believed that the deviation from the {001} lattice spacing of LCO becomes small on the plane in contact with O.

[0288] Therefore, it is considered that CoO and MgO easily form a solid solution. As the solid solution of CoO and MgO progresses due to heating, a solid solution of CoO and MgO appears in the surface layer 100a of the positive electrode active material 100A as shown in FIGS. 29A and 29B. （１−ｘ） Mg ｘ It is believed that O can be formed. （１−ｘ） Mg ｘ The lattice mismatch between LCO and O is smaller than that between CoO and Co. （１−ｘ） Mg ｘ The surface layer 100a containing O is more likely to undergo topotaxis with the LCO in the interior 100b. Also, as shown by the length of the white arrows in Figures 29A and 29B, the stress is reduced.

[0289] Thus, even when cobalt oxide is present in the surface layer of lithium cobalt oxide, adding an additive element and heating the material can convert the surface layer 100a into a solid solution of cobalt oxide and an oxide containing the additive element. This facilitates topotaxis between the surface layer 100a and the interior 100b of the positive electrode active material 100A. This allows the positive electrode active material 100A to be resistant to pit formation.

[0290] <Crystal structure> <Li ｘ CoO ２ When x is 1 in the positive electrode active material 100A of one embodiment of the present invention is in a discharged state, that is, Li ｘ CoO ２ When x = 1 in the formula, it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies the majority of the volume of the positive electrode active material 100A, has a layered rock-salt type crystal structure. Figure 30 shows the layered rock-salt type crystal structure, labeled R-3m O3.

[0291] On the other hand, the surface layer portion 100a of the cathode active material 100A according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b, which is made up of an octahedron of the transition metal M and oxygen, so that the layered structure formed by the octahedron of the transition metal M and oxygen is not destroyed even when lithium is removed from the cathode active material 100A upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100A. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100A, preferably reinforces the cathode active material 100A. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and the inner portion 100b of the cathode active material 100A, such as oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100A.

[0292] Therefore, the surface layer portion 100a preferably has a different crystal structure from the interior portion 100b. Furthermore, the surface layer portion 100a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100A of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0293] The surface layer 100a is the region where lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 100b. In addition, it can be said that the atoms on the surface of the positive electrode active material 100A in the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 100a can be made sufficiently stable, Li ｘ CoO ２ Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of the transition metal M and oxygen octahedra can be made less likely to break. Furthermore, displacement of the layer made of the transition metal M and oxygen octahedra in the inner portion 100b can be suppressed.

[0294] In order to give the surface layer portion 100a a stable composition and crystal structure, the surface layer portion 100a preferably contains an additive element A, and more preferably contains a plurality of additive elements A. Furthermore, the surface layer portion 100a preferably has a higher concentration of one or more selected from the additive elements A than the interior portion 100b. Furthermore, the one or more selected from the additive elements A contained in the positive electrode active material 100A preferably have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive element A in the positive electrode active material 100A differs depending on the additive element A. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additive element A. Here, the concentration peak refers to the maximum value of the concentration in the surface layer portion 100a or within 50 nm from the surface.

[0295] For example, some of the additive elements A, such as magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, and calcium, preferably have a concentration gradient that increases from the interior 100b toward the surface, as shown by the density of the hatching in Fig. 2A. Elements having such a concentration gradient will be referred to as additive elements X.

[0296] Another additive element A, such as aluminum or manganese, preferably has a concentration gradient and a concentration peak in a region deeper than that shown in FIG. 2A, as shown by the hatching density in FIG. 2B. The concentration peak may be present in the surface layer 100a, or may be deeper than the surface layer 100a. For example, it is preferable that the peak be present in a region of 5 nm to 30 nm from the surface toward the interior. An element having such a concentration gradient will be referred to as an additive element Y.

[0297] For example, magnesium, which is one of the additive elements X, is divalent, and magnesium ions are more stable at the lithium site than at the transition metal M site in the layered rock-salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock-salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２When x in the positive electrode active material 100A is, for example, 0.24 or less, oxygen desorption from the surroundings of magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100A. Furthermore, a high magnesium concentration in the surface layer 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0298] At an appropriate concentration, magnesium does not adversely affect the intercalation and deintercalation of lithium during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect the intercalation and deintercalation of lithium. Furthermore, the effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying the transition metal M site in addition to the lithium site. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) that do not substitute for either the lithium site or the transition metal M site may segregate on the surface of the positive electrode active material and become resistance components in the secondary battery. Furthermore, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium site, reducing the amount of lithium contributing to charging and discharging.

[0299] Therefore, it is preferable that the amount of magnesium contained in the entire cathode active material 100A is appropriate. For example, the ratio of magnesium to the sum of transition metals M (Mg / M) contained in the cathode active material 100A of one embodiment of the present invention is preferably 0.25% or more and 5% or less, more preferably 0.5% or more and 2% or less, and even more preferably about 1%. The amount of magnesium contained in the entire cathode active material 100A here may be a value obtained by performing elemental analysis of the entire cathode active material 100A using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the cathode active material 100A.

[0300] Nickel, which is one of the additive elements X, can exist either at the transition metal M site or at the lithium site. When nickel exists at the transition metal M site, it has a lower oxidation-reduction potential than cobalt, which is preferable as it leads to an increase in discharge capacity.

[0301] Furthermore, when nickel is present at the lithium site, the layer structure consisting of the transition metal M and oxygen octahedra can be prevented from shifting. Also, the volume change caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel present at the lithium site and CoO ２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0302] On the other hand, excessive nickel may increase the strain caused by the Jahn-Teller effect, and excessive nickel may adversely affect lithium insertion and extraction.

[0303] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 100A is appropriate. For example, the number of nickel atoms in the positive electrode active material 100A is preferably more than 0% but not more than 7.5% of the total number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably more than 0% but not more than 4%. Alternatively, it is preferably more than 0% but not more than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0304] Furthermore, aluminum, one of the additive elements Y, can exist at the transition metal M site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Furthermore, aluminum has the effect of suppressing the elution of the surrounding transition metal M and improving continuous charging durability. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as the additive element Y can improve safety when used in secondary batteries. Furthermore, a positive electrode active material 100A can be obtained whose crystal structure is less likely to collapse even with repeated charging and discharging.

[0305] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0306] Therefore, it is preferable that the aluminum content of the entire cathode active material 100A be an appropriate amount. For example, the number of aluminum atoms contained in the entire cathode active material 100A is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire cathode active material 100A referred to here may be, for example, a value obtained by performing elemental analysis of the entire cathode active material 100A using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the cathode active material 100A.

[0307] Furthermore, fluorine, one of the additive elements X, is a monovalent anion, and when a portion of the oxygen in the surface layer portion 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the change in valence of cobalt ions accompanying lithium desorption differs depending on whether or not fluorine is present. For example, the redox potential of cobalt ions differs, from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine. Therefore, when a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 100A is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when used in a secondary battery, charge / discharge characteristics, current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid. Furthermore, as will be described in a later embodiment, when the melting point of a fluoride such as lithium fluoride is lower than the melting point of another additive element A source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element A source.

[0308] Furthermore, titanium oxide, which is one of the additive elements X, is known to have superhydrophilicity. Therefore, by providing the cathode active material 100A with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100A may have improved wettability with highly polar solvents. When used in a secondary battery, this may improve the contact at the interface between the cathode active material 100A and a highly polar electrolyte, thereby suppressing an increase in internal resistance.

[0309] Furthermore, when phosphorus, which is one of the additive elements X, is contained in the surface layer portion 100a, Li ｘ CoO ２ When the value of x in the graph is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that the graphite oxide is present in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0310] When the positive electrode active material 100A contains phosphorus, the hydrogen fluoride generated by decomposition of the electrolyte reacts with the phosphorus, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0311] The electrolyte is LiPF ６In the case of a battery containing a cathode, hydrogen fluoride may be generated by hydrolysis. Hydrogen fluoride may also be generated by the reaction of polyvinylidene fluoride (PVDF), which is used as a component of the cathode, with an alkali. A reduction in the hydrogen fluoride concentration in the electrolyte may prevent corrosion of the current collector and / or peeling of the coating. Furthermore, a decrease in adhesion due to gelation and / or insolubilization of PVDF may be prevented.

[0312] When the positive electrode active material 100A contains phosphorus together with magnesium, Li ｘ CoO ２ This is preferable because stability is extremely high when x is small in the positive electrode active material 100A. When the positive electrode active material 100A contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, 1% to 10% is preferable. Alternatively, 1% to 8% is preferable. Alternatively, 2% to 20% is preferable. Alternatively, 2% to 8% is preferable. Alternatively, 3% to 20% is preferable. Alternatively, 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Alternatively, 0.1% to 5% is preferable. Alternatively, 0.1% to 4% is preferable. Alternatively, 0.5% to 10% is preferable. Alternatively, 0.5% to 4% is preferable. Alternatively, 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 100A using, for example, GC-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 100A.

[0313] Furthermore, when the positive electrode active material 100A has cracks, the progression of the cracks can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on the surface, for example, in the embedded portion.

[0314] Furthermore, when the surface layer 100a contains both magnesium and nickel, there is a possibility that divalent magnesium can exist more stably near divalent nickel. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0315] Furthermore, having additive elements A with different distributions, such as additive element X and additive element Y, in combination is preferable because it can stabilize the crystal structure in a wider region. For example, when the positive electrode active material 100A contains both magnesium and nickel, which are part of the additive element X, and aluminum, which is one of the additive elements Y, it can stabilize the crystal structure in a wider region than when it contains only one of the additive elements X and Y. In this way, when the positive electrode active material 100A contains both additive element X and additive element Y, the surface can be sufficiently stabilized by additive element X, such as magnesium, so that additive element Y, such as aluminum, is not essential to the surface. Rather, it is preferable for aluminum to be widely distributed in a deep region, for example, a region from the surface to a depth of 5 nm to 50 nm, because this can stabilize the crystal structure in a wider region.

[0316] As described above, when a plurality of additional elements A are contained, the effects of the respective additional elements A are synergistic and can contribute to further stabilization of the surface layer portion 100a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0317] However, if the surface layer 100a is occupied only by a compound of the additional element A and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO, a structure in which MgO and NiO(II) are solid-solved, and / or a structure in which MgO and CoO(II) are solid-solved. Therefore, the surface layer 100a must contain at least cobalt, and in the discharged state, it must also contain lithium, and must have a path for the insertion and extraction of lithium.

[0318] In order to ensure sufficient paths for lithium insertion and desorption, it is preferable that the surface layer 100a has a higher cobalt concentration than magnesium. For example, the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is preferably 0.62 or more. It is also preferable that the surface layer 100a has a higher cobalt concentration than nickel. It is also preferable that the surface layer 100a has a higher cobalt concentration than aluminum. It is also preferable that the surface layer 100a has a higher cobalt concentration than fluorine.

[0319] Furthermore, since too much nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium in the surface layer 100a is higher than that of nickel. For example, it is preferable that the number of nickel atoms is 1 / 6 or less of the number of magnesium atoms.

[0320] Furthermore, although it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably present randomly and in a sparse manner in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it is possible to suppress the deviation of the layered structure consisting of the transition metal M and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, which is expected to have a synergistic effect of suppressing magnesium elution.

[0321] <Li ｘ CoO ２ State in which x is small in the positive electrode active material 100A of one embodiment of the present invention has the above-described distribution of the additional element A and / or the crystal structure in the discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0322] Using Figures 30 to 34, Li ｘ CoO２ The change in the crystal structure accompanying the change in x in the positive electrode active material 100A will be described by comparing a conventional positive electrode active material with the positive electrode active material 100A of one embodiment of the present invention.

[0323] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 31. The conventional positive electrode active material shown in FIG. 31 is a lithium cobalt oxide (LiCoO ２ In particular, the change in the crystal structure of lithium cobalt oxide not containing the additive element A is described in Non-Patent Documents 1 to 3, etc.

[0324] Figure 31 shows the R-3m O3 and Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0325] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0326] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0327] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２It can also be said that the structure of and are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 crystal structure. Note that, since actual lithium insertion and desorption can be uneven, the H1-3 crystal structure is experimentally observed from about x = 0.25. In addition, the number of cobalt atoms per unit cell in the H1-3 crystal structure is actually twice that of other structures. However, in Figure 31 and other parts of this specification, to make it easier to compare with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0328] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0329] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0330] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 31, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0331] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0332] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0333] Therefore, when charging and discharging is repeated so that x becomes 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0334] On the other hand, in the positive electrode active material 100A of one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100A of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 100A of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ CoO ２ When x in the positive electrode active material 100A is 0.24 or less, the positive electrode active material 100A can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0335] Li ｘ CoO ２ The crystal structure of the inner portion 100b of the positive electrode active material 100A when x is approximately 1 or 0.2 is shown in FIG. 30. The inner portion 100b occupies the majority of the volume of the positive electrode active material 100A and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0336] When x=1, the positive electrode active material 100A has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0337] However, the positive electrode active material 100A has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.15, which results in an H1-3 type crystal structure.

[0338] The positive electrode active material 100A according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 30 with the notation R-3m O3'.

[0339] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ nm). The c-axis is 13.681≦c≦13.881 (×10 −１ nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0340] In the O3' type crystal structure, ions of cobalt, nickel, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0341] As shown by the dotted line in FIG. 30, the R-3m O3 in the discharged state and the O3′-type crystal structure are different in CoO ２ There is almost no layer misalignment.

[0342] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0343] As described above, in the positive electrode active material 100A of one embodiment of the present invention, Li ｘ CoO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the crystal structure of the positive electrode active material 100A is less likely to collapse even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 100A has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100A, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0344] The positive electrode active material 100A is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is presumed that it also has an O3' type crystal structure when x is more than 0.24 and 0.27 or less. However, the crystal structure is ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0345] Therefore, the positive electrode active material 100A is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the entire interior 100b of the positive electrode active material 100A does not have to have the O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0346] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO ２The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6 V or higher relative to the potential of lithium metal in an environment of 25°C, a H1-3 crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be said to be a high charging voltage. Furthermore, in this specification and the like, unless otherwise specified, charging voltages are expressed relative to the potential of lithium metal.

[0347] Therefore, in other words, positive electrode active material 100A of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it is preferable because it can adopt an O 3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C.

[0348] Even in the case of the positive electrode active material 100A, if the charge voltage is further increased, the H1-3 type crystal may finally be observed. Furthermore, as described above, since the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the electrolyte, and the like, the positive electrode active material 100A of one embodiment of the present invention may be able to adopt the O3′ type crystal structure even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher but lower than 4.6 V at 25°C.

[0349] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0350] In addition, in O3' of FIG. 30, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, in the monoclinic O1 (Li ０．５ CoO ２ The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0351] The O3' type crystal structure has random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0352] Furthermore, it is preferable that the concentration gradient of the additive element A be similar at multiple locations in the surface layer portion 100a of the positive electrode active material 100A. In other words, it is preferable that the reinforcement derived from the additive element A is uniformly present in the surface layer portion 100a. Even if a portion of the surface layer portion 100a is reinforced, if there is a portion without reinforcement, stress may be concentrated in the unreinforced portion. If stress is concentrated in a portion of the positive electrode active material 100A, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0353] However, the additional element A does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 100 a of the positive electrode active material 100 A. An example of the distribution of the additional element X near C-D in FIG. 1B is shown in FIG. 2C , and an example of the distribution of the additional element Y near C-D in FIG. 2D .

[0354] Here, the surface in the vicinity of C-D is assumed to be parallel to the arrangement of cations. The surface parallel to the arrangement of cations may have a different distribution of the additional element A than the other surfaces. For example, the surface parallel to the arrangement of cations and its surface layer 100a may have a distribution of concentration peaks of one or more elements selected from the additional element X and the additional element Y limited to a shallower portion from the surface compared to the other orientations. Alternatively, the surface parallel to the arrangement of cations and its surface layer 100a may have a lower concentration of one or more elements selected from the additional element X and the additional element Y compared to the other orientations. Alternatively, the surface parallel to the arrangement of cations and its surface layer 100a may have one or more elements selected from the additional element X and the additional element Y below the lower detection limit.

[0355] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and therefore the diffusion path of lithium ions is also parallel to the (001) plane.

[0356] CoO ２ The layer is relatively stable, so CoO ２ The surface where the layer is present is relatively stable, as the main diffusion path of lithium ions during charging and discharging is not exposed on this surface.

[0357] On the other hand, the plane that is not parallel to the cation arrangement, i.e., CoO ２ The diffusion path of lithium ions is exposed on the surface that is not parallel to the layer. Therefore, the surface and surface layer portion 100a that are not parallel to the cation arrangement are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and surface layer portion 100a that are not parallel to the cation arrangement is extremely important for maintaining the crystal structure of the entire positive electrode active material 100A.

[0358] Therefore, in positive electrode active material 100A of another embodiment of the present invention, it is important that the distribution of additive element A in the surface not parallel to the arrangement of cations and in surface layer 100a thereof is not limited to the outermost surface as shown in Figures 2A and 2B, but is present at a desired depth. On the other hand, the concentration of additive element A in the surface parallel to the arrangement of cations and in surface layer 100a thereof may be low or absent, as described above.

[0359] High purity LiCoO, which will be described in a later embodiment ２ In the manufacturing method of mixing the additional element A after manufacturing the first layer 100a and heating the mixture, the additional element A spreads mainly through the diffusion path of lithium ions. Therefore, it is easy to make the distribution of the additional element A in the surface not parallel to the arrangement of cations and in the surface layer 100a thereof fall within a preferred range.

[0360] Furthermore, while it is preferable that the surface of the positive electrode active material 100A be smooth and have few irregularities, this need not necessarily be the case for the entire positive electrode active material 100A. A composite oxide having an R-3m layered rock salt crystal structure is prone to slippage in planes parallel to the arrangement of cations, such as planes where lithium is arranged. For example, when a plane where lithium is arranged exists as shown in FIG. 32A , slippage may occur parallel to the plane where lithium is arranged, as indicated by the arrow in FIG. 32B , resulting in deformation, as a result of a process such as pressing.

[0361] In this case, the additional element A may not be present or may be below the detection limit on the surface newly formed as a result of the slip and its surface layer 100a. E-F in FIG. 32B is an example of the surface newly formed as a result of the slip and its surface layer 100a. Enlarged views of the vicinity of E-F are shown in FIGS. 32C1 and 32C2. Unlike FIGS. 2A to 2D, the additional element X and the additional element Y are not distributed in FIGS. 32C1 and 32C2.

[0362] However, since slippage tends to occur parallel to the arrangement of cations, the newly formed surface and its surface layer 100a tend to be parallel to the lithium diffusion path. In this case, the lithium ion diffusion path is not exposed and the structure is relatively stable, so there is almost no problem even if the additive element A is not present or is below the detection limit.

[0363] As mentioned above, the composition is LiCoO ２ In the composite oxide having a layered rock salt type crystal structure of R-3m, cobalt and lithium are arranged parallel to the (001) plane. ２ Among these, cobalt, which has the largest atomic number, has the highest brightness. Therefore, in a HAADF-STEM image, the arrangement of bright atoms can be considered to be the arrangement of cobalt. The repetition of this bright arrangement is synonymous with crystal fringes or lattice fringes.

[0364] <Grain Boundaries> In addition to the distribution described above, at least a portion of the additional element A contained in the positive electrode active material 100A of one embodiment of the present invention is preferably unevenly distributed in and near the grain boundaries.

[0365] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of high-concentration and low-concentration regions.

[0366] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material 100A is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in other regions of the interior 100b.

[0367] Grain boundaries are a type of planar defect. Therefore, like surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of the added element A at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0368] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of positive electrode active material 100A of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the cracked surface, thereby improving the corrosion resistance to hydrofluoric acid even after the cracks occur.

[0369] <Particle size> If the particle size of the positive electrode active material 100A of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector may occur. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte may occur. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0370] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100A of one embodiment of the present invention has an O3′-type crystal structure by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0371] XRD is particularly preferred in that it can analyze the symmetry of the transition metal M, such as cobalt, contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100A, which occupies the majority of the volume of the positive electrode active material 100A.

[0372] As described above, the positive electrode active material 100A according to one embodiment of the present invention is a ｘ CoO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltages accounts for 50% or more of the crystal structure are not preferable because they cannot withstand high-voltage charging and discharging.

[0373] It should also be noted that simply adding an additional element A may not result in an O3'-type crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additional element A, Li ｘ CoO ２ In this case, x is 0.24 or less and the O3' type crystal structure accounts for 60% or more, and in other cases the H1-3 type crystal structure accounts for 50% or more.

[0374] Furthermore, even in the case of positive electrode active material 100A of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not a material is positive electrode active material 100A of one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0375] However, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, it may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0376] Furthermore, whether or not the distribution of the additive element A in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0377] The crystal structure of the surface layer 100a, the grain boundaries, etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100A.

[0378] Whether a certain composite oxide is the positive electrode active material 100A of one embodiment of the present invention can be determined by high-voltage charging. For example, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) may be fabricated using the composite oxide as a positive electrode and a lithium counter electrode as a negative electrode, and high-voltage charging may be performed.

[0379] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0380] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0381] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0382] The separator may be a 25 μm thick porous polypropylene film.

[0383] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0384] The coin cell prepared under the above conditions is charged at a constant current of 10 mA / g to a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). To observe the phase change of the positive electrode active material, charging at such a low current is desirable. The temperature is 25°C or 45°C. After charging in this manner, the coin cell is disassembled in an argon-atmosphere glove box and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. When various analyses are performed, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container in an argon atmosphere. Furthermore, it is preferable to remove the positive electrode promptly after charging is complete and perform the analysis. Specifically, within one hour after charging is complete, and more preferably within 30 minutes.

[0385] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions. For example, charging may be performed by constant current charging at a current value of 100 mA / g up to a desired voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 10 mA / g, and then constant current discharging at 2.5 V and 100 mA / g.

[0386] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at, for example, 2.5 V and a current value of 100 mA / g.

[0387] <<XRD>> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １ Line output: 40 KV, 40 mA Slit width: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0388] If the measurement sample is a powder, it can be set by placing it in a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0389] The ideal powder XRD patterns calculated from the O3' type crystal structure and the H1-3 type crystal structure model using CuKα1 radiation are shown in Figures 33 and 34. In Figure 34, for comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structures of O3, H1-3 type, and trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from the Inorganic Crystal Structure Database (ICSD) (see Non-Patent Document 4). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material of one embodiment of the present invention and fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and an XRD pattern was created in the same manner as the others.

[0390] As shown in FIG. 33, in the O3' type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and less than 19.37°) and 2θ=45.47±0.10° (45.37° or more and less than 45.57°).

[0391] However, as shown in FIG. 34, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°) when x is small in the positive electrode active material 100A of one embodiment of the present invention can be said to be a characteristic of the positive electrode active material 100A of one embodiment of the present invention.

[0392] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0393] Note that the positive electrode active material 100A of one embodiment of the present invention is Li ｘ CoO ２When x in the formula is small, the material has an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0394] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0395] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions and the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ = 43° or more and 46° or less, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after charging.

[0396] The crystallite size of the O3'-type crystal structure of the positive electrode active material 100A is 0.01g / cm2 / cm3. ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the graph is small, a clear peak of the O3' type crystal structure can be confirmed. ２In this case, even if a part of the crystal structure resembles the O3'-type crystal structure, the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0397] <<XPS>> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, using monochromated aluminum Kα rays as the X-ray source allows analysis of a region from the surface to a depth of approximately 2 to 8 nm (typically 5 nm or less), thereby allowing quantitative analysis of the concentration of each element in a region approximately half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often approximately ±1 atomic %, and the lower detection limit is approximately 1 atomic %, depending on the element.

[0398] In one embodiment of the cathode active material 100A of the present invention, the concentration of one or more selected additive elements A is preferably higher in the surface layer 100a than in the interior 100b. This is equivalent to saying that the concentration of one or more selected additive elements A in the surface layer 100a is preferably higher than the average concentration in the entire cathode active material 100A. Therefore, for example, it can be said that the concentration of one or more selected additive elements A in the surface layer 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements A in the entire cathode active material 100A measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer 100a measured by XPS or the like is preferably higher than the magnesium concentration in the entire cathode active material 100A. Furthermore, the nickel concentration in at least a portion of the surface layer 100a is preferably higher than the nickel concentration in the entire cathode active material 100A. It is also preferable that the aluminum concentration in at least a portion of the surface layer portion 100a is higher than the aluminum concentration in the entire positive electrode active material 100A.It is also preferable that the fluorine concentration in at least a portion of the surface layer portion 100a is higher than the fluorine concentration in the entire positive electrode active material 100A.

[0399] Note that the surface and surface layer 100a of the cathode active material 100A according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, or the like that are chemically adsorbed after the preparation of the cathode active material 100A. Also, the surface of the cathode active material 100A does not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the cathode active material 100A. Therefore, when quantifying the elements contained in the cathode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0400] Furthermore, before being subjected to various analyses, samples such as the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. At this time, lithium may dissolve in the solvent used for washing, but even in this case, the additional element A is unlikely to dissolve, and therefore the atomic ratio of the additional element A is not affected.

[0401] The concentration of the additive element A may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances that are chemically adsorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 or more and 1.5 or less. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0402] Similarly, in the positive electrode active material 100A, in order to ensure sufficient paths for lithium insertion and desorption, it is preferable that the concentrations of lithium and cobalt in the surface layer portion 100a be higher than those of each additional element A. This can be said to mean that the concentrations of lithium and cobalt in the surface layer portion 100a are preferably higher than the concentrations of one or more additional elements A selected from the additional elements A contained in the surface layer portion 100a measured by XPS or the like. For example, it is preferable that the concentration of cobalt in at least a portion of the surface layer portion 100a measured by XPS or the like is higher than the concentration of magnesium in at least a portion of the surface layer portion 100a measured by XPS or the like. Similarly, it is preferable that the concentration of lithium is higher than the concentration of magnesium. It is also preferable that the concentration of cobalt is higher than the concentration of nickel. It is also preferable that the concentration of lithium is higher than the concentration of nickel. It is also preferable that the concentration of cobalt is higher than the concentration of aluminum. It is also preferable that the concentration of lithium is higher than the concentration of aluminum. It is also preferable that the concentration of cobalt is higher than the concentration of fluorine. It is also preferable that the concentration of lithium is higher than the concentration of fluorine.

[0403] Furthermore, it is more preferable that the additive element Y, such as aluminum, is widely distributed in a deep region, for example, a region having a depth of 5 nm to 50 nm from the surface. Therefore, although the additive element Y, such as aluminum, is detected in an analysis of the entire cathode active material 100A using ICP-MS, GD-MS, or the like, it is more preferable that this is below the lower limit of detection using XPS, or the like.

[0404] Furthermore, when XPS analysis was performed on the positive electrode active material 100A of one embodiment of the present invention, the number of magnesium atoms relative to the number of cobalt atoms was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times. The number of nickel atoms relative to the number of cobalt atoms was preferably 0.15 times, more preferably 0.03 to 0.13 times. The number of aluminum atoms relative to the number of cobalt atoms was preferably 0.12 times, more preferably 0.09 times. The number of fluorine atoms relative to the number of cobalt atoms was preferably 0.3 to 0.9 times, more preferably 0.1 to 1.1 times.

[0405] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions: Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0406] Furthermore, when the positive electrode active material 100A of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably equal to or greater than 682 eV and less than 685 eV, and more preferably about 684.3 eV. This value is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV). In other words, when the positive electrode active material 100A of one embodiment of the present invention contains fluorine, the bond is preferably other than that of lithium fluoride or magnesium fluoride.

[0407] Furthermore, when the positive electrode active material 100A of one embodiment of the present invention is subjected to XPS analysis, the peak representing the bond energy between magnesium and another element is preferably equal to or greater than 1302 eV and less than 1304 eV, and more preferably about 1303 eV. This value is different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide. In other words, when the positive electrode active material 100A of one embodiment of the present invention contains magnesium, the bond is preferably other than that of magnesium fluoride.

[0408] <EDX> Preferably, one or more selected from the additive elements A contained in the positive electrode active material 100A have a concentration gradient. Furthermore, it is more preferable that the depth from the surface of the concentration peak of the positive electrode active material 100A differs depending on the additive element A. The concentration gradient of the additive element A can be evaluated, for example, by exposing a cross section of the positive electrode active material 100A using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0409] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0410] EDX area analysis (e.g., element mapping) can be used to semi-quantitatively analyze the concentration of the additive element A in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material 100A. Furthermore, EDX ray analysis can be used to analyze the concentration distribution and maximum value of the additive element A. Furthermore, analysis that thins the sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0411] Therefore, when EDX area analysis or EDX point analysis is performed on positive electrode active material 100A of one embodiment of the present invention, the concentration of each additional element A, particularly additional element X, in surface layer portion 100a is preferably higher than that in interior portion 100b.

[0412] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100A having magnesium as the additive element X, it is preferable that the magnesium concentration in the surface layer 100a is higher than that in the interior 100b. Furthermore, when EDX analysis is performed, the peak of the magnesium concentration in the surface layer 100a is preferably present at a depth of 3 nm from the surface of the cathode active material 100A toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, it is preferable that the magnesium concentration decay to 60% or less of the peak at a depth of 1 nm from the peak position. Furthermore, it is preferable that the magnesium concentration decay to 30% or less of the peak at a depth of 2 nm from the peak position.

[0413] In addition, in the positive electrode active material 100A having magnesium and fluorine as the additional element X, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0414] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer portion 100a preferably exists at a depth of 3 nm from the surface toward the center of the positive electrode active material 100A, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, if the fluorine concentration peak exists slightly closer to the surface than the magnesium concentration peak, resistance to hydrofluoric acid is increased, which is more preferable. For example, the fluorine concentration peak is more preferably 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferably 1.5 nm or more closer to the surface.

[0415] In the cathode active material 100A containing nickel as the additive element X, the nickel concentration peak in the surface layer 100a is preferably present at a depth of 3 nm from the surface toward the center of the cathode active material 100A, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. In the cathode active material 100A containing magnesium and nickel, the nickel distribution preferably overlaps with the magnesium distribution. For example, the difference in depth between the magnesium concentration peaks is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0416] Furthermore, when the positive electrode active material 100A contains aluminum as the added element Y, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present at a depth of 0.5 nm to 50 nm, more preferably 5 nm to 50 nm, from the surface toward the center of the positive electrode active material 100A.

[0417] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 100A, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0418] The surface of positive electrode active material 100A in the EDX analysis results can be estimated, for example, as follows: For an element that is uniformly present in interior 100b of positive electrode active material 100A, such as oxygen or cobalt, the point where the detected amount is half that of interior 100b is defined as the surface.

[0419] Since the positive electrode active material 100A is a composite oxide, the surface can be estimated using the detected amount of oxygen. Specifically, first, the average oxygen concentration O ａｖｅ At this time, oxygen O, which is thought to be due to chemical adsorption or background, is found in the area that can be clearly determined to be outside the surface. ｂａｃｋｇｒｏｕｎｄ If detected, O ｂａｃｋｇｒｏｕｎｄ The average oxygen concentration O ａｖｅ This average value O ａｖｅ Half the value of, that is, 1 / 2O ａｖｅ The measurement point showing the measurement value closest to this can be assumed to be the surface of the positive electrode active material.

[0420] The surface can also be estimated in the same way as above using the detected amount of cobalt. Alternatively, the sum of the detected amounts of multiple transition metals can be used to estimate the surface. The detected amounts of transition metals, including cobalt, are less susceptible to the influence of chemical adsorption, making them suitable for estimating the surface.

[0421] Furthermore, when linear or area analysis is performed on the positive electrode active material 100A, the ratio (A / Co) of the added element A to cobalt Co near the grain boundaries is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. Alternatively, it is preferably 0.020 or more and 0.30 or less. Alternatively, it is preferably 0.020 or more and 0.20 or less. Alternatively, it is preferably 0.025 or more and 0.50 or less. Alternatively, it is preferably 0.025 or more and 0.20 or less. Alternatively, it is preferably 0.030 or more and 0.50 or less. Alternatively, it is preferably 0.030 or more and 0.30 or less.

[0422] For example, when the additive element X is magnesium, when the positive electrode active material 100A is subjected to linear analysis or area analysis, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) near the grain boundaries is preferably 0.020 or more and 0.50 or less. It is further preferably 0.025 or more and 0.30 or less. It is further preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0423] <EPMA> EPMA (Electron Probe Microanalysis) can also quantify elements. Area analysis can analyze the distribution of each element.

[0424] When EPMA surface analysis is performed on a cross section of the positive electrode active material 100A of one embodiment of the present invention, it is preferable that one or more selected from the additive elements A have a concentration gradient, similar to the EDX analysis results. It is also more preferable that the depth from the surface of the concentration peak differs depending on the additive element A. The preferred range of the concentration peak of each additive element A is also the same as in the case of EDX.

[0425] However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from the measurement results obtained using other analytical methods. For example, when the surface of the positive electrode active material 100A is analyzed using EPMA, the concentration of each added element A present in the surface layer 100a may be lower than the result obtained using XPS.

[0426] <Charge Curve and dQ / dV vs. V Curve> The positive electrode active material 100A of one embodiment of the present invention may exhibit a characteristic voltage change during charging. The voltage change can be read from the dQ / dV vs. V curve obtained by differentiating (dQ / dV) the capacity (Q) with respect to the voltage (V) from the charge curve. For example, it is believed that a non-equilibrium phase change occurs around the peak in the dQ / dV vs. V curve, causing a significant change in the crystal structure. Note that in this specification, a non-equilibrium phase change refers to a phenomenon that causes a non-linear change in a physical quantity.

[0427] The positive electrode active material 100A according to one embodiment of the present invention may have a broad peak near 4.55 V in the dQ / dV vs. V curve. The peak near 4.55 V reflects the change in voltage that occurs when the phase changes from O3 type to O3' type. Therefore, the broadness of this peak means that the change in energy required for lithium extraction is smaller than when the peak is sharp, i.e., the change in the crystal structure is smaller. The smaller these changes, the more likely CoO ２ This is preferable because it is less affected by layer displacement and volume change.

[0428] More specifically, when the maximum value appearing between 4.5 V and 4.6 V in the dQ / dV vs. V curve of the charging curve is defined as the first peak, it is preferable that the half-value width of the first peak is 0.10 V or more, as this can be said to be sufficiently broad.

[0429] The charging when obtaining the dQ / dV vs. V curve can be, for example, a constant current charge of 10 mA / g up to 4.9 V. When obtaining the dQ / dV of the initial charge, it is preferable to start the charging after discharging to 2.5 V at 100 mA / g before measurement.

[0430] The data acquisition interval during charging can be set to acquire the voltage and current at intervals of 1 second or when there is a voltage fluctuation of 1 mV. The value obtained by integrating the current value and time is taken as the charge capacity.

[0431] The difference between the nth and (n+1)th data of the charge capacity is defined as the nth value of the capacitance change dQ. Similarly, the difference between the nth and (n+1)th data of the voltage is defined as the nth value of the voltage change dV.

[0432] However, since the use of the above data is subject to the large influence of minute noise, dQ / dV may be calculated from the moving average of the voltage and charge capacity differences over a certain number of intervals. The number of intervals may be set to, for example, 500.

[0433] Specifically, the average value of dQ from the nth to the (n+500th) data is calculated, and similarly, the average value of dV from the nth to the (n+500th) data is calculated. dQ (average of 500 data) / dV (average of 500 data) can be used as dQ / dV. Similarly, the moving average value from the nth to the (n+500th) data can be used as the number of sections for the voltage on the horizontal axis in a dQ / dV vs. V graph. Note that when using the above moving average, it is preferable not to use the 501st data from the end to the last data in the dQ / dV vs. V graph because the influence of noise is significant.

[0434] When analyzing the dQ / dV vs. V curve after multiple charge / discharge cycles, the charge / discharge conditions may be different from the above-mentioned charging conditions. For example, charging may be performed at a constant current of 100 mA / g at any voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 10 mA / g, and discharging at a constant current of 100 mA / g at 2.5 V.

[0435] In addition, the phase changes from O3 type to O3' type at around 4.55 V, and the O3 type at this time is Li ｘ CoO ２ The x in the figure is about 0.3. This has the same symmetry as the O3 type with x=1 explained in Figure 31, but ２ The distance between layers is slightly different. In this specification, when distinguishing between O3 types with different values ​​of x, O3 type with x = 1 is referred to as O3 (2θ = 18.85°), and O3 type with x = 0.3 or so is referred to as O3 (2θ = 18.57°). This is because the position of the peak that appears around 2θ = 19° in XRD measurement is due to the CoO ２ This is because it corresponds to the interlayer distance.

[0436] <Discharge Curve and dQ / dV vs. V Curve> Furthermore, when the positive electrode active material 100A of one embodiment of the present invention is charged at a high voltage and then discharged at a low current of, for example, 40 mA / g or less, a characteristic voltage change may appear near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the dQ / dV vs. V curve obtained from the discharge curve, which is lower than the peak that appears around 3.9 V and falls within the range up to 3.5 V.

[0437] <<ESR>> The positive electrode active material 100A of one embodiment of the present invention preferably contains cobalt and nickel and magnesium as the additional element A. As a result, a portion of Co ３＋ Ni ３＋ and some Li ＋ is Mg ２＋ It is preferred that Li be substituted with ＋ is Mg ２＋ With the substitution of ３＋ is reduced to Ni ２＋ In addition, some Li ＋ is Mg ２＋ is replaced by Mg ２＋ Nearby Co ３＋ is reduced to Co ２＋ In addition, some Co ３＋ is Mg ２＋ is replaced by Mg ２＋ Nearby Co ３＋ is oxidized to Co ４＋ This may occur.

[0438] Therefore, the positive electrode active material 100A is Ni ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ It is preferable that the positive electrode active material 100A has at least one of the following: ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ The spin density due to one or more of the above is 2.0 × 10 １７ spins / g or more 1.0×10 ２１ It is preferable that the positive electrode active material 100A has the above-mentioned spin density, because the crystal structure is stable, especially in the charged state. ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ The spin density may be reduced due to one or more of the above.

[0439] The spin density in the positive electrode active material can be analyzed using, for example, electron spin resonance (ESR).

[0440] <Surface Roughness and Specific Surface Area> The positive electrode active material 100A of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness indicates that the effect of the flux described below is fully exerted, and the surface of the additive element A source and the composite oxide are melted. Therefore, this is one factor indicating that the distribution of the additive element A in the surface layer portion 100a is good. Good distribution means, for example, that the concentration distribution of the additive element A in the surface layer portion 100a is uniform.

[0441] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of positive electrode active material 100A, the specific surface area of ​​positive electrode active material 100A, or the like.

[0442] For example, the surface smoothness can be quantified from a cross-sectional SEM image of positive electrode active material 100A as follows.

[0443] First, the cathode active material 100A is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 100A with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material 100A is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 100A is then selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness of at least the outer 400 nm of the cathode active material.

[0444] The surface of positive electrode active material 100A of the present embodiment preferably has a root mean square (RMS) surface roughness, which is an index of roughness, of less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

[0445] The image processing software for noise processing, boundary extraction, etc. is not particularly limited, but for example, "ImageJ" described in Non-Patent Documents 6 to 8 can be used.

[0446] For example, the actual specific surface area S measured by the gas adsorption method using the constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 100A can also be quantified from the ratio of the surface roughness to the surface roughness.

[0447] Ideal specific surface area S ｉ is calculated assuming that all the positive electrode active materials have the same diameter D50, the same weight, and an ideal spherical shape.

[0448] The median diameter D50 can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. The specific surface area can be measured by a specific surface area measuring device using a gas adsorption method based on a constant volume method, for example.

[0449] The positive electrode active material 100A of one embodiment of the present invention has an ideal specific surface area A calculated from the median diameter D50. ｉ and the actual specific surface area S Ｒ The ratio S Ｒ / S ｉ is preferably 1.0 or more and 2.1 or less.

[0450] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100A by the following method.

[0451] First, an SEM image of the surface of the positive electrode active material 100A is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area are the same.

[0452] Next, image processing software (for example, "ImageJ") is used to convert the SEM image into, for example, an 8-bit image (called a grayscale image). The grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 gradations. Dark areas have lower gradations, and bright areas have higher gradations. The luminance change can be quantified in relation to the number of gradations. This numerical value is called the grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material as a numerical value.

[0453] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram is a three-dimensional representation of the gradation distribution in the target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0454] In positive electrode active material 100A of one embodiment of the present invention, the difference between the maximum and minimum grayscale values ​​is preferably 120 or less, more preferably 115 or less, and even more preferably 70 to 115. The standard deviation of the grayscale values ​​is preferably 11 or less, more preferably 8 or less, and even more preferably 4 to 8.

[0455] <Current-Rest Method> The distribution of the additive element A, such as magnesium, contained in the surface layer portion of the positive electrode active material 100A of one embodiment of the present invention may change slightly during repeated charge and discharge. For example, the distribution of the additive element A may become better, resulting in a decrease in electronic conduction resistance. As a result, the electrical resistance at the beginning of the charge and discharge cycle, i.e., the fast-response resistance component R(0.1 s) measured by the current-rest method, may decrease.

[0456] For example, when comparing the nth charge (n is a natural number greater than 1) with the n+1th charge, the fast-response resistance component R(0.1 s) measured by the current-rest method may be lower at the n+1th charge than at the nth charge. Accordingly, the n+1th discharge capacity may be higher than the nth discharge capacity. When n is 1, that is, when comparing the first charge with the second charge, the second charge capacity may be larger, especially for a positive electrode active material that does not contain an additive element. Therefore, n is preferably, for example, 2 or more and 10 or less. However, this is not limited to the initial stage of the charge-discharge cycle. A charge-discharge capacity that is approximately the same as the rated capacity, for example, 97% or more of the rated capacity, can be considered to be at the initial stage of the charge-discharge cycle.

[0457] <Pits> When a positive electrode active material is charged at 4.5 V or higher, or when the material is charged and discharged in a high-temperature environment, for example, at 45° C. or higher, progressive defects may develop that progress from the surface to the interior. The phenomenon in which defects progress to form holes in a positive electrode active material can also be called pitting corrosion, and holes generated by this phenomenon are also referred to as pits in this specification. The opening shape of the holes may be circular, elliptical, rectangular, or may have a deep groove-like shape.

[0458] Figure 35 shows a cross-sectional schematic diagram of a positive electrode active material 51 having pits. Crystal planes 55 parallel to the arrangement of cations are also shown. Because Figure 35 is a cross-sectional view, pits 54 and pits 58 are shown as holes, but their opening shapes are not circular but have depth and are groove-like. Furthermore, as shown by pits 54 and pits 58, unlike recesses 52, they tend to form parallel to the arrangement of lithium ions.

[0459] The surface layer portions of the positive electrode active material 51 where the additive element A is present are indicated by 53 and 56. The surface layer portions where the pits have occurred have less additive element A than 53 and 56 or below the detection limit, and it is expected that the function of the barrier film is reduced. It is also thought that the crystalline structure of the composite oxide breaks down in the vicinity of where the pits are formed, resulting in a crystalline structure different from that of the layered rock salt type. When the crystalline structure breaks down, it inhibits the diffusion and release of lithium ions, which are carrier ions, and therefore the pits are thought to be a factor in the deterioration of cycle characteristics.

[0460] The source of pits may be point defects. Point defects in the positive electrode active material change with repeated charge and discharge, and are thought to be chemically or electrochemically corroded by the surrounding electrolyte, or to be caused by material degradation. This degradation does not occur uniformly on the surface of the positive electrode active material, but rather occurs in localized areas.

[0461] Furthermore, as shown by the crack 57 in Figure 35, defects such as cracks (also called fissures) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In this specification, cracks and pits are different. Even if cracks are present immediately after the preparation of the positive electrode active material, pits are not. Pits can be considered holes formed by the loss of several layers of transition metal M and oxygen due to charging and discharging under high voltage conditions of, for example, 4.5 V or higher or at high temperatures (45°C or higher), and can also be considered locations where transition metal M has dissolved. Cracks refer to, for example, new surfaces formed by the application of physical pressure or fissures caused by grain boundaries. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charging and discharging. Pits may also occur from cracks and / or cavities within the positive electrode active material.

[0462] [Method for Producing Positive Electrode Active Material] In order to produce positive electrode active material 100A having the distribution, composition, and / or crystal structure of additive element A as described in the previous embodiment, the method for adding additive element A is important. At the same time, it is also important that the crystallinity of interior 100b is good.

[0463] Therefore, in the process of producing the positive electrode active material 100A, it is preferable to first synthesize a composite oxide containing lithium and a transition metal, and then mix in a source of the additive element A and perform a heat treatment.

[0464] In a method of synthesizing a composite oxide containing the additive element A, lithium, and the transition metal M by mixing the additive element A source with the transition metal M source and the lithium source at the same time, it is difficult to increase the concentration of the additive element A in the surface layer portion 100a. Furthermore, if the additive element A source is simply mixed without heating after synthesizing the composite oxide containing lithium and the transition metal M, the additive element will simply adhere to the composite oxide without dissolving in the composite oxide. Without sufficient heating, it is also difficult to achieve a good distribution of the additive element A. For this reason, it is preferable to mix the additive element A source after synthesizing the composite oxide and then perform a heat treatment. This heat treatment after mixing the additive element A source is sometimes called annealing.

[0465] However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that the additive element A, for example, magnesium, enters the transition metal M site. Magnesium present in the transition metal M site is Li ｘ CoO ２ When the value of x in the matrix is ​​small, the layered rock salt crystal structure of R-3m cannot be maintained. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent and the evaporation of lithium.

[0466] Therefore, it is preferable to mix a material that functions as a flux together with the source of the additional element A. If the melting point is lower than that of the composite oxide containing lithium and the transition metal M, the material can be said to function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting points of the source of the additional element A and the composite oxide containing lithium and the transition metal M. Lowering the melting point makes it easier to distribute the additional element A well at a temperature where cation mixing is unlikely to occur.

[0467] Furthermore, it is more preferable to carry out heating after synthesizing the composite oxide containing lithium and the transition metal M and before mixing with the additional element A. This heating is sometimes called initial heating.

[0468] The initial heating causes lithium to be released from a part of the surface layer 100a of the composite oxide containing lithium and the transition metal M, and this further improves the distribution of the additive element A.

[0469] More specifically, it is believed that the initial heating makes it easier to vary the distribution depending on the additive element A, due to the following mechanism. First, lithium is released from a part of the surface layer 100a by the initial heating. Next, a composite oxide containing lithium and a transition metal M, which has this lithium-deficient surface layer 100a, is mixed with a source of the additive element A, such as a nickel source, an aluminum source, or a magnesium source, and heated. Of the additive elements A, magnesium is a divalent typical element, and nickel is a transition metal, but it is prone to becoming a divalent ion. Therefore, Mg is released from a part of the surface layer 100a. ２＋ and Ni ２＋ and Co reduced by lithium deficiency. ２＋ A rock salt type phase having

[0470] Of the added elements A, nickel is easily dissolved and diffuses into the interior 100b when the surface layer 100a is a layered rock-salt type composite oxide containing lithium and a transition metal M, but when part of the surface layer 100a is rock-salt type, nickel tends to remain in the surface layer 100a.

[0471] Furthermore, in these rock salt types, the bond distance between the metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt type.

[0472] For example, rock salt type Ni ０．５ Mg ０．５ The Me-O distance in O is 2.09 × 10 −１ nm, and the Me-O distance in rocksalt MgO is 2.11 × 10 −１ nm. Even if a spinel phase is formed in a part of the surface layer 100a, the spinel phase is ２ O ４ The Me-O distance is 2.0125 × 10 −１ nm, spinel-type MgAl ２ O ４ The Me-O distance is 2.02 × 10 −１ In both cases, the Me-O distance is 2 × 10 −１ Exceeds nm.

[0473] On the other hand, in the layered rock salt type, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, layered rock salt type LiAlO ２ The Al-O distance in−１ nm (Li-O distance is 2.11 × 10 −１ nm). Also, layered rock salt LiCoO ２ The Co-O distance in −１ nm (Li-O distance is 2.0916 × 10 −１ nm).

[0474] According to Shannon's ionic radius (Shannon et al., Acta A 32 (1976) 751.), the ionic radius of hexacoordinated aluminum is 0.535 × 10 −１ nm, and the ionic radius of hexacoordinated oxygen is 1.4 × 10 −１ nm, and the sum of these is 1.935 × 10 −１ nm.

[0475] From the above, it is believed that aluminum exists more stably in non-lithium sites in the layered rock-salt structure than in the rock-salt structure, and therefore aluminum is more likely to be distributed in the deeper region having the layered rock-salt structure and / or the interior 100b than in the region close to the surface having the rock-salt structure in the surface layer portion 100a.

[0476] Furthermore, the initial heating is expected to have the effect of increasing the crystallinity of the layered rock salt type crystal structure in the inner portion 100b.

[0477] However, initial heating is not necessarily required. In other heating steps, such as annealing, the atmosphere, temperature, time, etc. can be controlled to prevent Li ｘ CoO ２ When x in the formula is small, a positive electrode active material 100A having an O3' type can sometimes be produced.

[0478] An example of a manufacturing flow of positive electrode active material 100A that undergoes annealing and initial heating will be described with reference to FIGS. 36A to 36C.

[0479] <Step S11> In step S11 shown in FIG. 36A, a lithium source (Li source) and a transition metal M source (M source) are prepared as starting materials for lithium and transition metal M, respectively.

[0480] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0481] The transition metal M can be selected from elements in Groups 3 to 11 of the periodic table, and for example, at least one of manganese, cobalt, and nickel is used. That is, as the transition metal M, there are cases where only cobalt is used, only nickel is used, two types of transition metals, cobalt and manganese, two types of transition metals, cobalt and nickel, or three types of transition metals, cobalt, manganese, and nickel, are used. When only cobalt is used, the resulting positive electrode active material has lithium cobalt oxide (LCO), and when three types of transition metals, cobalt, manganese, and nickel, are used, the resulting positive electrode active material has lithium nickel-cobalt-manganese oxide (NCM).

[0482] As the transition metal M source, it is preferable to use a compound containing the transition metal M, and for example, an oxide or hydroxide of a metal exemplified as the transition metal M can be used. As a cobalt source, cobalt oxide, cobalt hydroxide, etc. can be used. As a manganese source, manganese oxide, manganese hydroxide, etc. can be used. As a nickel source, nickel oxide, nickel hydroxide, etc. can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0483] The transition metal M source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased and / or the reliability of the secondary battery is improved.

[0484] In addition, it is preferable that the transition metal M source has high crystallinity, for example, single crystal grains. The crystallinity of the transition metal M source can be evaluated using TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, enhanced hollow-cone illumination-TEM (enhanced hollow-cone illumination transmission electron microscope) images, or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. Note that the above-mentioned methods for evaluating crystallinity can be applied not only to transition metal M sources but also to evaluation of the crystallinity of other sources.

[0485] When two or more transition metal M sources are used, the two or more transition metal M sources are preferably prepared in a ratio (mixing ratio) that allows the two or more transition metal M sources to form a layered rock salt type crystal structure.

[0486] <Step S12> Next, in step S12 shown in FIG. 36A , the lithium source and the transition metal M source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller pulverization. When performing the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the transition metal M source in dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0487] A ball mill, a bead mill, or the like can be used as a means for mixing, etc. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as grinding media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0488] <Step S13> Next, in step S13 shown in FIG. 36A , the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably about 950°C. If the temperature is too low, the decomposition and melting of the lithium source and the transition metal M source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to lithium evaporation from the lithium source and / or excessive reduction of the metal used as the transition metal M source. For example, when cobalt is used as the transition metal M, excessive reduction can cause cobalt to change from trivalent to divalent, which can induce oxygen defects.

[0489] If the heating time is too short, LiMO ２ However, if the heating time is too long, productivity decreases. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0490] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0491] The heating atmosphere is preferably an atmosphere with little water, such as dry air, and for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is performed in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４, CO, CO ２ , and H ２ The impurity concentrations of the above should be set to 5 ppb (parts per billion) or less.

[0492] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and allowing oxygen to flow through the reaction chamber is called flow.

[0493] When the heating atmosphere is an atmosphere containing oxygen, a method that does not allow oxygen to flow may be used. For example, a method may be used in which the reaction chamber is depressurized and then filled with oxygen to prevent the oxygen from entering or leaving the reaction chamber, which is called purging. For example, the reaction chamber may be depressurized to -970 hPa on a differential pressure gauge, and then filled with oxygen to 50 hPa.

[0494] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0495] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0496] The crucible or sheath used during heating is preferably made of a highly heat-resistant material such as alumina (aluminum oxide), mullite / cordierite, magnesia, or zirconia. Furthermore, since aluminum oxide is a material that is less susceptible to impurities, the purity of the alumina crucible or sheath is 99% or higher, preferably 99.5% or higher. In this embodiment, a crucible made of aluminum oxide with a purity of 99.9% is used. It is preferable to heat the crucible or sheath with a lid, which prevents the material from volatilizing.

[0497] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use a mortar made of aluminum oxide. A mortar made of aluminum oxide is a material that does not easily release impurities. Specifically, a mortar made of aluminum oxide with a purity of 90% or more, preferably 99% or more, is used. Note that heating conditions equivalent to those of step S13 can be applied to heating steps other than step S13, which will be described later.

[0498] <Step S14> By the above steps, a composite oxide having a transition metal M (LiMO ２ The composite oxide can be obtained by ２ It is sufficient for the lithium composite oxide to have a crystal structure represented by the formula: Li:M:O = 1:1:2, and the composition is not strictly limited to Li:M:O = 1:1:2. When cobalt is used as the transition metal M, the composite oxide is called a cobalt-containing composite oxide and is represented by LiCoO2. The composition is not strictly limited to Li:Co:O = 1:1:2.

[0499] Although the example of producing the composite oxide by the solid phase method in steps S11 to S14 has been shown, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0500] <Step S15> Next, in step S15 shown in Fig. 36A, the composite oxide is heated. Because this is the first heating of the composite oxide, the heating in step S15 may be called initial heating. Alternatively, because this heating is performed before step S20 described below, it may be called preheating or pretreatment.

[0501] As described above, the initial heating causes lithium to be desorbed from a portion of the surface layer 100a of the composite oxide. It is also expected to have the effect of increasing the crystallinity of the interior 100b. Furthermore, the lithium source and / or transition metal M prepared in step S11 or the like may contain impurities. The initial heating can reduce the amount of impurities in the composite oxide completed in step S14.

[0502] Furthermore, initial heating has the effect of smoothing the surface of the composite oxide. A smooth surface means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a smooth surface means that there is little foreign matter adhering to the surface. Foreign matter is thought to be a cause of irregularities, so it is preferable that it does not adhere to the surface.

[0503] For this initial heating, it is not necessary to prepare a lithium compound source, a source of the additional element A, or a material that functions as a flux.

[0504] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. In addition to the heating conditions, the heating temperature in this step should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in this step should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0505] The effect of increasing the crystallinity of the inner portion 100b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the difference in shrinkage of the composite oxide produced in step S13.

[0506] The heating in step S13 may cause a temperature difference between the surface and the interior of the composite oxide. The temperature difference may induce a contraction difference. It is also thought that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a contraction difference. The energy associated with the contraction difference causes a difference in internal stress in the composite oxide. The internal stress difference is also called strain, and this energy is sometimes called strain energy. The internal stress is removed by the initial heating in step S15; in other words, the strain energy is thought to be homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in the composite oxide is alleviated. Therefore, the surface of the composite oxide may become smoother after step S15. This is also called an improved surface. In other words, it is thought that the contraction difference that occurred in the composite oxide is alleviated after step S15, resulting in a smoother surface of the composite oxide.

[0507] Furthermore, the difference in shrinkage may cause microscopic deviations in the composite oxide, such as deviations in crystals. This step is preferably carried out in order to reduce such deviations. This step makes it possible to equalize the deviations in the composite oxide. When the deviations are equalized, the surface of the composite oxide may become smooth. This is also referred to as the alignment of crystal grains. In other words, it is believed that step S15 reduces the deviations of crystals and the like that have occurred in the composite oxide, resulting in a smooth surface of the composite oxide.

[0508] When a composite oxide with a smooth surface is used as a positive electrode active material, deterioration during charge and discharge in a secondary battery is reduced, and cracking of the positive electrode active material can be prevented.

[0509] The smooth surface of a composite oxide can be expressed as a surface roughness of at least 10 nm or less when surface irregularity information is quantified from measurement data at a cross section of the composite oxide. The cross section is, for example, a cross section obtained when observing with a scanning transmission electron microscope (STEM).

[0510] In step S14, a composite oxide containing lithium, a transition metal M, and oxygen that has been synthesized in advance may be used. In this case, steps S11 to S13 can be omitted. By performing step S15 on a composite oxide that has been synthesized in advance, a composite oxide with a smooth surface can be obtained.

[0511] It is possible that the lithium in the composite oxide is reduced by the initial heating, and the added element A, which will be explained in the next step S20, etc., may be more likely to enter the composite oxide due to the reduced lithium.

[0512] <Step S20> The additive element A may be added to a composite oxide having a smooth surface, as long as it can form a layered rock salt crystal structure. Adding the additive element A to a composite oxide having a smooth surface allows the additive element A to be added evenly. Therefore, it is preferable to add the additive element A after the initial heating. The step of adding the additive element A will be described with reference to FIGS. 36B and 36C.

[0513] 36B, a source of an additive element A (A source) to be added to the composite oxide is prepared. A lithium source may be prepared together with the additive element A source.

[0514] The additive element A can be 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. The additive element A can be one or more selected from bromine and beryllium. However, since bromine and beryllium are toxic to living organisms, it is more preferable to use the additive elements described above.

[0515] When magnesium is selected as the additional element A, the source of the additional element A can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0516] When fluorine is selected as the additional element A, the source of the additional element A can be called a fluorine source. Examples of the fluorine source that can be used include lithium fluoride, magnesium fluoride, aluminum fluoride, titanium fluoride, cobalt fluoride, nickel fluoride, zirconium fluoride, vanadium fluoride, manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride, calcium fluoride, sodium fluoride, potassium fluoride, barium fluoride, cerium fluoride, lanthanum fluoride, and sodium aluminum hexafluoride. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating step described below.

[0517] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0518] The fluorine source may be a gas, such as fluorine, carbon fluoride, sulfur fluoride, or oxygen fluoride, which may be mixed into the atmosphere in the heating step described below. A plurality of the above-mentioned fluorine sources may also be used.

[0519] In this embodiment, lithium fluoride is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and the magnesium source. ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２ The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or its vicinity) is more preferable. In this specification, "or its vicinity" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0520] At the same time, the amount of magnesium added is ２ Based on the standard, the content is preferably more than 0.1 atomic % and not more than 3 atomic %, more preferably 0.5 atomic % to 2 atomic %, and even more preferably 0.5 atomic % to 1 atomic %. When the amount of magnesium added is 0.1 atomic % or less, the initial discharge capacity is high, but the discharge capacity drops rapidly when charging and discharging is repeated to increase the depth of charge. When the amount of magnesium added is more than 0.1 atomic % and not more than 3 atomic %, both the initial discharge characteristics and the charge-discharge cycle characteristics are good even when charging and discharging is repeated to increase the depth of charge. On the other hand, when the amount of magnesium added exceeds 3 atomic %, both the initial discharge capacity and the charge-discharge cycle characteristics tend to gradually deteriorate.

[0521] 36B, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0522] If necessary, a heating step may be performed after step S22. The heating step can be performed under heating conditions selected from those described in step S13. The heating time is preferably 2 hours or more, and the heating temperature is preferably 800°C or higher and 1100°C or lower.

[0523] 36B, the pulverized and mixed materials are collected to obtain a source of the additional element A (A source). Note that the source of the additional element A shown in step S23 contains a plurality of starting materials and can be called a mixture.

[0524] The particle size of the mixture is preferably D50 (median diameter) of 600 nm to 20 μm, more preferably 1 μm to 10 μm. Even when a single material is used as the source of the additional element A, the D50 (median diameter) is preferably 600 nm to 20 μm, more preferably 1 μm to 10 μm.

[0525] Such a finely powdered mixture (including the case where only one type of additive element A is used) is likely to be uniformly adhered to the surface of the composite oxide when mixed with the composite oxide in a later step. Having the mixture uniformly adhered to the surface of the composite oxide is preferred because it facilitates uniform distribution or diffusion of fluorine and magnesium in the surface layer of the composite oxide after heating. The region where fluorine and magnesium are distributed can also be referred to as the surface layer. If there is a region in the surface layer that does not contain fluorine and magnesium, it may be difficult to obtain the O3'-type crystal structure described below in the charged state. Although fluorine has been used in the description, fluorine may also be chlorine, and chlorine can be read as including these and therefore halogen.

[0526] <Step S21> A step different from that shown in Figure 36B will be described using Figure 36C. In step S21 shown in Figure 36C, four types of additive element A sources to be added to the composite oxide are prepared. That is, the types of additive element A sources in Figure 36C are different from those in Figure 36B. A lithium source may be prepared in addition to the additive element A sources.

[0527] As sources of four types of additive element A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described in FIG. 36B . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0528] <Step S22> and <Step S23> Next, step S22 and step S23 shown in FIG. 36C are the same as the steps described in FIG. 36B.

[0529] 36A , the composite oxide is mixed with a source of the additional element A. The ratio of the number of atoms M of the transition metal M in the composite oxide containing lithium, the transition metal M, and oxygen to the number of atoms Mg of magnesium in the additional element A is preferably M:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0530] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the composite oxide. For example, it is preferable to use conditions with a lower rotation speed or a shorter time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as media, for example.

[0531] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0532] <Step S32> Next, in step S32 of Fig. 36A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0533] In this embodiment, a method is described in which lithium fluoride as a fluorine source and magnesium fluoride as a magnesium source are added to the composite oxide after the initial heating. However, the present invention is not limited to the above method. In step S11, that is, in the stage of the starting materials for the composite oxide, a magnesium source, a fluorine source, etc. can be added to the lithium source and the transition metal M source. Then, in step S13, heating is performed to obtain LiMO with added magnesium and fluorine. ２ In this case, it is not necessary to separate the steps S11 to S14 from the steps S21 to S23. This method is simple and has high productivity.

[0534] Alternatively, a composite oxide to which magnesium and fluorine have been added in advance may be used. If a composite oxide to which magnesium and fluorine have been added is used, steps S11 to S32 and step S20 can be omitted. This method can be said to be simple and highly productive.

[0535] Alternatively, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source and an aluminum source may be further added to a composite oxide to which magnesium and fluorine have been added in advance in step S20.

[0536] 36A, the mixture 903 is heated. The heating conditions can be selected from those described in step S13. The heating time is preferably 2 hours or more.

[0537] Here, a supplementary note about the heating temperature will be given. The lower limit of the heating temperature in step S33 is ２ The temperature at which the reaction between the LiMO and the additive element A source proceeds must be equal to or higher than the temperature at which the reaction between the LiMO and the additive element A source proceeds. ２ The temperature may be lower than the melting point of these materials as long as it is a temperature at which mutual diffusion of elements contained in the source of the additive element A occurs. ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 500° C. or higher.

[0538] Of course, the reaction proceeds more easily when the temperature is equal to or higher than the temperature at which at least a part of the mixture 903 melts. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0539] Also, LiCoO ２ :LiF:MgF ２ A mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830° C. in differential scanning calorimetry (DSC). Therefore, the lower limit of the heating temperature is more preferably 830° C. or higher.

[0540] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0541] The upper limit of heating temperature is LiMO２ Decomposition temperature of LiCoO ２ The decomposition temperature of LiMO is less than 1130°C. At temperatures close to the decomposition temperature, a small amount of LiMO ２ Therefore, the temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 900°C or lower.

[0542] Taking these factors into consideration, the heating temperature in step S33 is preferably 500°C to 1130°C, more preferably 500°C to 1000°C, even more preferably 500°C to 950°C, and even more preferably 500°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 800°C to 1100°C, or 830°C to 1130°C, more preferably 830°C to 1000°C, even more preferably 830°C to 950°C, and even more preferably 830°C to 900°C. The heating temperature in step S33 is preferably higher than that in step S13.

[0543] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0544] In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be controlled to the temperature of the composite oxide (LiMO). ２ ) can be lowered to a temperature lower than the decomposition temperature, for example, 742°C or higher and 950°C or lower, and additive element A including magnesium can be distributed in the surface layer portion, thereby producing a positive electrode active material with good characteristics.

[0545] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF will volatilize when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, etc., LiMO ２There is a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0546] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 903.

[0547] The heating in this step is preferably performed so as not to stick together the mixture 903. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additional element A (for example, fluorine) diffuses is blocked, which may result in a poor distribution of the additional element A (for example, magnesium and fluorine) in the surface layer portion.

[0548] It is also believed that uniform distribution of the additive element A (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the mixture 903 does not stick to itself.

[0549] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0550] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 903 .

[0551] The heating time is determined by the heating temperature, the LiMO ２ It changes depending on the size and composition of LiMO. ２If is small, a lower temperature or shorter time may be more preferable than if is large.

[0552] The compo...

Claims

1. A positive electrode and a negative electrode are included. the positive electrode has a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer; the first layer having a first active material having a first particle size; the second layer having a second active material having a second particle size; the third layer having a third active material having a third particle size; the first particle size is smaller than the second particle size; The third particle size is smaller than the second particle size. the first layer, the second layer, and the third layer have a solid electrolyte; the mass of the solid electrolyte in the first layer is greater than the mass of the solid electrolyte in the second layer; a mass of the solid electrolyte in the second layer is greater than a mass of the solid electrolyte in the third layer.

2. In claim 1, The sphericity of the second active material is 0.8 or more and 1.0 or less.

3. In claim 1, the second layer has a fourth active material having a fourth particle size; The fourth particle size is smaller than the second particle size.

4. In claim 3, The sphericity of the second active material is 0.8 or more and 1.0 or less.

5. In claim 1, The second active material has a surface portion and an interior portion, the surface layer portion is a region of 10 nm or less extending from a surface of the second active material toward the inside, A battery, wherein the surface portion and the interior portion are topotaxis.

6. In claim 5, The sphericity of the second active material is 0.8 or more and 1.0 or less.

7. In claim 3, The second active material has a surface portion and an interior portion, the surface layer portion is a region of 10 nm or less extending from a surface of the second active material toward the inside, A battery, wherein the surface portion and the interior portion are topotaxis.

8. In claim 7, The battery, wherein the second active material has a sphericity of 0.8 or more and 1.0 or less.

9. In any one of claims 1 to 8, the first layer, the second layer, and the third layer comprise a conductive material; a mass of the conductive material in the third layer is greater than a mass of the conductive material in the second layer; A battery, wherein the mass of the conductive material in the second layer is greater than the mass of the conductive material in the first layer.

10. In claim 5, the first layer, the second layer, and the third layer have a solid electrolyte; the mass of the solid electrolyte in the first layer is greater than the mass of the solid electrolyte in the second layer; the mass of the solid electrolyte in the second layer is greater than the mass of the solid electrolyte in the third layer; the second active material has an edge surface having a region where the surface portion and the solid electrolyte are in contact with each other.

11. A mobile object comprising the battery according to claim 1.

12. An electricity storage system comprising the battery according to claim 1.

13. An electronic device comprising the battery according to claim 1.