Batteries, electronic devices, energy storage systems and mobile devices

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

[Technical Field]

[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 aspect of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One aspect 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. [Background technology]

[0005] In recent years, there has been active development of various types of power storage devices, 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 (PDAs) such as mobile phones, smartphones, and notebook computers, portable music players, digital cameras, medical devices, and next-generation clean energy vehicles (CEVs), including hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Demand for lithium-ion secondary batteries has expanded rapidly alongside 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 up of lithium cobalt oxide (LiCoO2), lithium nickel-cobalt-manganese oxide (LiNi 1-x-y Co x Mn y It consists of a positive electrode containing a positive electrode active material such as lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4), a negative electrode containing a negative electrode active material such as graphite or other carbon materials that can absorb and release 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. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] WO2020 / 128699 Brochure [Non-patent literature]

[0010] [Non-Patent Document 1] Toyoki Okumura et al, “Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3-and O2-lithium cobalt oxides from first-principle calculation”, Journal of Materials Chemistry, 2012, 22, p. 17340-17348 [Non-patent document 2] Motohashi, T. et al, “Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”, Physical Review B, 80(16);165114 [Non-patent document 3] Zhaohui Chen et al, “Staging Phase Transitions in Li▲x▼CoO▲2▼”, Journal of The Electrochemical Society, 2002, 149(12)A1604-A1609 [Non-patent document 4] Belsky, A. et al., “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Cryst., (2002) B58 364-369. [Non-patent document 5] A. van de Walle, “Multicomponent multisublattice alloys, nonconfigurational entropy and other additions to the Alloy Theoretic Automated Toolkit”, Calphad Journal 33, 266, (2009). [Non-patent document 6] Rasband, WS, ImageJ, US National Institutes of Health, Bethesda, Maryland, USA, http: / / rsb.info.nih.gov / ij / , 1997-2012. [Non-Patent Document 7] Schneider, CA, Rasband, WS, Eliceiri, KW “NIH Image to ImageJ:25 years of image analysis”.Nature Methods 9,671-675,2012. [Non-patent document 8] Abramoff, MD, Magelhaes, PJ, Ram, SJ “Image Processing with ImageJ”.Biophotonics International,volume 11,issue 7,pp.36-42,2004. Summary of the Invention [Problem to be solved by the invention]

[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 onto 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 electrodes with fewer voids, a secondary battery of the same volume can achieve a larger battery capacity and improve the capacity density per volume. An electrode with an active material layer with fewer voids is sometimes called 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. [Means for solving the problem]

[0015] One embodiment of the present invention is a battery that includes 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.

[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 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 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 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.

[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 to an interior portion of 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, 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, 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 portion and an interior portion. The surface portion is a region extending from a surface of the second active material to an interior portion of 10 nm or less. The surface portion and the interior portion are 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 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 to an interior portion of the second active material, the surface portion and the interior portion 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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

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

[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, 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. in the description. Furthermore, the ordinal numbers used to identify one embodiment of the present invention may not match the ordinal numbers used in this specification.

[0047] In this specification, particles are not limited to those having a spherical shape (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 results indicates 50% of the cumulative amount, i.e., the median diameter. The measurement of particle size is not limited to laser diffraction particle size distribution measurement; when the particle size is below the lower limit of measurement for laser diffraction particle size distribution measurement, the cross-sectional diameter of the particle cross section may be measured by analysis using an SEM (scanning electron microscope) or TEM (transmission electron microscope). When the cross-sectional shape of a particle is not circular, for example, the particle size can be measured by measuring the area of ​​the particle cross section using image processing, 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, they may be represented by a minus sign (-) before the number instead of a bar above it. Individual orientations representing directions within a crystal are represented using [ ], collective orientations representing all equivalent directions are represented using < >, individual planes representing 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 lithium contained in the positive electrode active material is deintercalated. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, that of lithium nickel oxide (LiNiO2) is 275 mAh / g, and that of lithium manganese oxide (LiMn2O4) is 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 the x in the composition formula, for example, Lix x in CoO2, or Li x is represented by x in MO2 (M is a transition metal). It can be said that x is the occupancy rate of Li in the lithium site. In the case of a positive electrode active material in a secondary battery, x = (theoretical capacity - charged capacity) / theoretical capacity can be used. For example, when a secondary battery using LiCoO2 as the positive electrode active material is charged to 219.2 mAh / g, Li 0.2 CoO2 or x = 0.2 can be said. Li x When x in LiCoO2 is small, for example, it means 0.1 < x ≤ 0.24. The transition metal M can be selected from the elements described in Groups 4 to 13 shown in the periodic table. For example, at least one of manganese, cobalt, and nickel is used.

[0052] When lithium cobaltate approximately satisfies the stoichiometric ratio, it is LiCoO2 and the occupancy rate x of Li in the lithium site is 1. Also, a secondary battery after discharge is completed is also LiCoO2, and it can be said that x = 1. Here, the completion of discharge means, for example, a state where the voltage becomes 2.5 V or less (vs. counter electrode Li) at a current of 100 mA / g. In a lithium-ion secondary battery, when the occupancy rate of lithium in the lithium site becomes x = 1 and no more lithium can enter, the voltage drops rapidly. At this time, it can be said that the discharge is completed. Generally, in a lithium-ion secondary battery using LiCoO2, since the discharge voltage drops rapidly until the discharge voltage reaches 2.5 V, it is assumed that the discharge is completed under the above conditions.

[0053] Li x The charged capacity and / or discharge capacity used for calculating x in LiCoO2 are preferably measured under conditions where there is no influence or little influence of short circuit and / or electrolyte decomposition. For example, it is preferable not to use the data of a secondary battery in which a rapid change in capacity regarded as a short circuit occurs for the calculation of 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 with a mutually offset structure, such as ABCABC, then this is referred to as a cubic close-packed structure. Therefore, the anions do not need to be arranged in a strict cubic lattice. At the same time, because real crystals always have defects, analytical results do not necessarily have to be theoretical. For example, in electron diffraction patterns or FFT (fast Fourier transform) patterns such as TEM images, spots may appear at positions slightly different from the theoretical positions. For example, if the orientation from the theoretical positions is less than 5 degrees or less than 2.5 degrees, then the structure can be said to have a cubic close-packed structure.

[0056] Homogeneity refers to the phenomenon in which a certain element (e.g., A) is distributed with similar characteristics in a specific region of a solid composed of multiple elements (e.g., A, B, C). The concentration of the element in each specific region should be substantially the same. For example, the difference in element concentration between specific regions should be within 10%. Examples of specific regions include the surface layer, surface, convex portions, concave portions, and 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 the current collector is called a single-sided coated electrode, and an electrode in which active material layers are provided on both sides of the 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] [Layered electrode 1] An electrode of one embodiment of the present invention will be described with reference to FIGS. 1A to 6D. The 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 example of an electrode with a two-layer structure according to 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 two-layer 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 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 to reduce 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 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. This means that 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.

[0070] 1A and the like, the cross-sectional shape of the first active material 411a and the like in the first layer 414a is schematically shown 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 laminated structure positive electrode] As a particularly preferred example of a positive electrode active material included in a positive electrode having 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 the surface layer portion. Enlarged views of the vicinity of AB in FIG. 1B are shown in FIGS. 2A and 2B. Enlarged views of the vicinity of CD in FIG. 1B are shown in FIGS. 2C and 2D. An example of a positive electrode active material 100 having a topotactic region in the surface layer portion will now be described.

[0072] In FIG. 1B, dotted lines indicate crystal planes parallel to the cation arrangement. Arrows indicate the direction of lithium insertion and desorption during charge and discharge. The cation arrangement 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 cation arrangement 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 other substances chemically adsorbed after preparation. It also does not contain electrolytes, binders, conductive materials, or compounds derived from these substances that adhere 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 bond 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 also be determined in conjunction with the results of higher spatial resolution analyses, such as electron energy loss spectroscopy (EELS).

[0076] The term "grain boundary" refers to, for example, a portion where particles of the positive electrode active material 100 adhere to each other, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repeated bright and dark lines in a STEM image become discontinuous, a portion containing many crystal defects, or a portion where the crystal structure is disrupted. The term "crystal defect" refers to defects that can be observed in a cross-sectional TEM (transmission electron microscope) or cross-sectional STEM image, i.e., a structure in which other elements have entered between lattices, a cavity, etc. The grain boundary can be considered a type of planar defect. The term "vicinity of a grain boundary" refers to a region within 10 nm of the 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 desorption of oxygen 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 and the like, "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 roughly 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. Strictly speaking, the layered rock-salt crystal structure may have a distorted rock-salt crystal lattice.

[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 deficiencies are also acceptable.

[0083] Furthermore, 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, etc.

[0084] While the rock-salt type has no distinction between cation sites, the layered rock-salt type has two types of cation sites in its crystal structure: one is mostly occupied by lithium and the other by a transition metal (M). Both the rock-salt type and the layered rock-salt type share a layered structure, with alternating two-dimensional planes of cations and two-dimensional planes of anions. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form these two-dimensional planes, if the central spot (transmitted spot) is taken as the origin (000), the bright spot closest to the central spot would be, for example, the (111) plane in an ideal rock-salt type, and, for example, the (003) plane in a layered rock-salt type. For example, when comparing the electron diffraction patterns of rock-salt MgO and layered rock-salt LiCoO2, the bright spot on the (003) plane of LiCoO2 is observed at a distance roughly half the distance of the bright spot on the (111) plane of MgO. Therefore, if the analyzed region contains two phases, for example, rock-salt MgO and layered rock-salt LiCoO2, the electron diffraction image will show plane orientations in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock-salt and layered rock-salt types will have strong brightness, while bright spots occurring only in the layered rock-salt type will have weak brightness.

[0085] Furthermore, when a layered rock-salt crystal structure is observed perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternating. This characteristic is not seen 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 alternating in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in part of the low-brightness layers, i.e., the lithium layers.

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

[0087] Alternatively, it can be explained as follows: Anions on the {111} plane of the cubic crystal structure have a triangular lattice. Layered rocksalt has a space group of 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 rocksalt 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 rocksalt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structure.

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

[0089] The general agreement of the crystal orientations of the two regions 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 diffraction patterns, and FFT patterns of TEM and STEM images. XRD (X-ray Diffraction), electron diffraction, neutron diffraction, etc. can also be used as materials for determination.

[0090] Figure 3 shows an example of a TEM image in which the orientation of the layered rock salt crystal LRS and the rock salt crystal RS roughly coincides. Images that reflect the crystal structure can be obtained in TEM, STEM, HAADF-STEM, ABF-STEM, etc.

[0091] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicular 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 LRSWhen the angle between the dark lines is between 0 and 5 degrees, or between 0 and 2.5 degrees, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly 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 roughly 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 approximate alignment.

[0092] Furthermore, HAADF-STEM images exhibit contrast proportional to the atomic number, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobaltate, which belongs 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, resulting in the arrangement of the cobalt atoms being observed as bright lines or an array 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 bright lines or an array of highly bright dots, while the arrangements of the 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, if repeated 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 the same, i.e., that the crystal orientations are roughly the same. Similarly, if 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 the same.

[0094] Elements with smaller atomic numbers appear brighter in ABF-STEM, but like HAADF-STEM, contrast is obtained according to atomic number, making it possible to determine crystal orientation in the same way as with HAADF-STEM images.

[0095] Figure 4A shows an example of a STEM image in which the orientations of the layered rock-salt-type crystal LRS and the rock-salt-type crystal RS are roughly aligned. Figure 4B shows an FFT of the region of the rock-salt-type crystal RS, and Figure 4C shows an FFT of the region of the layered rock-salt-type 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 these. The right side shows the measured values. The spot marked with an O is the zeroth-order diffraction, and an X is marked at the center of the spot.

[0096] The spot labeled A in Figure 4B is due to the 11-1 reflection of the cubic crystal. The spot labeled A in Figure 4C is due to 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 diffraction, when the orientations of the layered rock salt crystal and the rock salt crystal roughly coincide, the <0003> orientation of the layered rock salt crystal and the <11-1> orientation of the rock salt crystal 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. Spot-like reciprocal lattice points that are not continuous with other reciprocal lattice points indicate high crystallinity.

[0098] Furthermore, even if 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 roughly the same as described above, 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 Figure 4C originates 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 Figure 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 identical. 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 different reciprocal lattice space from the spot where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in Figure 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° to 56° (i.e., ∠AOB is 54° to 56°) from the orientation of the reflection originating from the 11-1 reflection of the cubic crystal (A in Figure 4B). Note that these Miller indices are merely an example and do not necessarily have to be identical. For example, equivalent reciprocal lattice points in each may also be used.

[0100] It is known that layered rock-salt cathode active materials, such as lithium cobalt oxide, tend to exhibit 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 cathode active material using an SEM or similar, it is possible to thin-section the observation sample using an FIB or similar technique so that the electron beam is [12-10] incident in a TEM or similar technique, making it easier to observe the (0003) plane. When determining whether the crystal orientation is consistent, it is preferable to thin-section the layered rock-salt cathode active material so that the (0003) plane can be easily observed.

[0101] 1A preferably has a cathode active material 100 having a topotaxis region in its surface layer portion. 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 its surface layer portion.

[0102] As described above, a positive electrode having a stacked structure according to one embodiment of the present invention preferably includes a 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 a 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 additive elements obtained by the manufacturing methods shown 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 composite 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 composite oxide prepared using the preparation method described in Embodiment 2 preferably has an additive element in the surface layer. The additive element in the surface layer of the composite 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 the above hexagonal layered structure has one or more selected from the first transition metal, the second transition metal, and the third transition metal. Specifically, the first transition metal is nickel, the second transition metal is cobalt, the third transition metal is manganese, and LiNi x Co y Mn z O2 (where x > 0, y > 0, 0.8 < x + y + z < 1.2), a NiCoMn system (also referred to as NCM) can be used. Specifically, for example, it is preferable to satisfy 0.1x < y < 8x and 0.1x < z < 8x. As an example, x, y, and z preferably satisfy x:y:z = 1:1:1 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 5:2:3 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 8:1:1 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 9:0.5:0.5 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 6:2:2 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 1:4:1 or values in the vicinity thereof.

[0108] In addition, the composite oxide having the 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 required. From the viewpoint of increasing the capacity retention rate after charge-discharge cycles of the 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. A 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. First layer 414a has first active material 411a, second layer 414b has second active material 411b, and third layer 414c has third active material 411c. Like electrode 400A shown in FIG. 1A, electrode 400B shown in FIG. 5A preferably has positive electrode active material 100 having a topotaxis region in its surface layer portion. That is, it is preferable that positive electrode active material 100 having a topotaxis region in its surface layer portion be one or more of first active material 411a, second active material 411b, and 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 to 5 μm, more preferably 1 μm to 5 μm. The particle size Rb of the second active material 411b in the second layer 414b is preferably 1 μm to 35 μm, more preferably 5 μm to 25 μm. The particle size 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 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.

[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. Moreover, 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 414a, the second layer 414b, and the third layer 414c may contain a conductive material and a binder, which will be described later, or the first layer 414a, the second layer 414b, and the third layer 414c 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 become 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 to reduce 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 in the first layer 414a is schematically shown 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 these 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, and therefore has high electronic resistance and a relatively lower potential, which tends to slow down the battery reaction. For example, unlike the structure of an electrode of one embodiment of the present invention, when the third active material 411c in the third layer 414c and the second active material 411b in 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), causing 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 in the third layer 414c has a smaller particle size than the second active material 411b 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] Second active material 411b in second layer 414b has the largest particle size in active material layer 414 and is likely to contribute significantly to efficient lithium ion storage. Furthermore, as will be described later with reference to FIG. 6, a large particle size active material can be used in combination with a small particle size active material and a medium particle size active material to more efficiently store lithium ions. This means that the volumetric capacity density of the electrode can be increased. Therefore, in active material layer 414, the thickness of 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 second layer 414b in active material layer 414, the higher the volumetric capacity density of electrode 400B.

[0120] The presence of 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 or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less.

[0121] As described above, FIG. 5A illustrates electrode 400B having a three-layer stack structure including first layer 414a, second layer 414b, and third layer 414c. The structure of the electrode including third layer 414c according to one embodiment of the present invention is not limited to the three-layer structure. For example, as shown in FIG. 5B , electrode 400C may have a two-layer stack structure including second layer 414b and third layer 414c. Rapid charging and rapid discharging are also possible in electrode 400C due to the relationship between second layer 414b and third layer 414c described above for 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, but in reality, it 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, it is easy to achieve a structure in which the fourth active material 411d is present between the particles of the second active material 411b, resulting in a high density of the second layer 414b. Furthermore, when the battery has a large-particle-size second active material 411b, a small-particle-size fourth active material 411d, and a medium-particle-size fifth active material 411e as shown in FIG. 6D, 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 indicates the sphericity of the active material particles, i.e., how close the shape of the active material particles is to a perfect sphere. For example, the sphericity can be determined by processing particles having a particle diameter of D50±50% of the median diameter for cross-sectional observation, then measuring the perimeter L and area S of the particle cross-section, and then calculating the sphericity (SP) using the following formula:

[0126]

number

[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 FIGS. 6C and 6D , the density of the second layer 414b can be further increased by pressing the second active material 411b, which includes the second active material 411b having a large particle size, the fourth active material 411d having a small particle size, and / or the fifth active material 411e having a medium particle size. In this case, too, as described above, it is preferable to use active materials with high sphericity. Therefore, the sphericity of the fourth active material 411d having a small particle size and the fifth active material 411e having a medium particle size 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 to 35 μm, more preferably 5 μm to 25 μm. The particle size of the fourth active material 411d contained in the second layer 414b is preferably 500 nm to 5 μm, more preferably 1 μm to 5 μm. Furthermore, the particle size of the fifth active material 411e contained in the second layer 414b is preferably 1 μm to 20 μm, more preferably 5 μm to 15 μm.

[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), if 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 with a two-layer structure and an electrode with a three-layer structure, an electrode with four or more layers may also be used. For example, the two-layer electrode shown in FIG. 1A may have a layer of medium-sized active material between a layer of small-sized active material (first layer 414a) and a layer of large-sized active material (second layer 414b). Furthermore, the three-layer electrode shown in FIG. 5A may have a five-layer structure, with a layer of medium-sized active material between a layer of small-sized active material (first layer 414a) and a layer of large-sized active material (second layer 414b), and a layer of medium-sized active material between a layer of large-sized active material (second layer 414b) and a layer of 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 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 may each contain the same type (and combination) of additive element or different types. Furthermore, 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 may each contain the same concentration of additive element or different types.

[0133] For example, a structure 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 can be used.

[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 producing laminated electrode 1] 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, and in step S12, a mixture 501 is prepared.

[0139] Next, in step S13 of FIG. 7, the mixture 501 is applied to a current collector. Highly conductive materials, such as metals like stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof, can be used for 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 a continuous coater with hot air drying and infrared drying. 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. The pressing method can be any of plate pressing, hydrostatic pressing, and roll pressing. When roll pressing is used, it is preferable that the temperature of the roll is adjusted so that the active material layer is at 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 coated electrode 511, and then in step S24, the mixture 502 applied to the coated 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 coated 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 explanation 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 application method in step S33 can be the method described in the description of step S13. Furthermore, the drying method in step S34 can be the method described in the description of step S14. 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, but, 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 steps S15, S25, and one or two of steps S35 do not have to be performed.

[0147] Electrode 400B having first layer 414a, second layer 414b, and third layer 414c can be fabricated by the fabrication steps described above. In the fabrication method described with reference to FIG. 7, electrode 400A having first layer 414a and second layer 414b, and electrode 400C having second layer 414b and third layer 414c can be fabricated by completing the fabrication of the electrode in step S31.

[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 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 is in a state in which the binder 110 is well dispersed in the dispersion medium 120.

[0152] In step S111 of FIG. 8B, binder mixture 1001 is prepared, and in step S112, conductive material 1002 is prepared. In order to knead the mixture in a later step, the amount of binder mixture 1001 prepared in step S111 can be an amount less than the total amount necessary 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, etc. 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. They preferably have a curved shape. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds preferably have functional groups. Graphene compounds may also be rolled up into a shape like a carbon nanofiber. 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 the mixture 1020 in step S132. For example, a propeller mixer, a planetary rotation mixer, or a thin film swirl mixer can be used as the mixing method. In the mixing in step S131, it is preferable to perform kneading at high viscosity (sometimes called thick kneading). By kneading at 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, in step S111, an amount of binder mixture 1001 prepared is less than the total amount necessary to form the active material layer, 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 the subsequent 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 a simplified example of the electrode fabrication method, in which 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, can be performed using the methods described in the description 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, that is, active material 10c is added to active material 10a and active material 10b. 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] [Calculation for electrode 1 of the laminated structure] Calculations for an example of an electrode according to one embodiment of the present invention will be described with reference to FIGS.

[0164] Figure 11 shows a schematic diagram of the structural model used in the calculations. While current collectors are shown in Figure 11 for clarity, the calculations were performed without including the current collectors, using structures ranging from d = 0 μm to d = 120 μm. Note that d represents the distance from the interface between the positive electrode current collector (current collector 1) and the positive electrode active material layer toward the negative electrode current collector (current collector 2). The interface between the positive electrode current collector and the positive electrode active material layer is d = 0 μm, the interface between the positive electrode active material layer and the separator is d = 50 μm, the interface between the separator and the negative electrode active material layer is d = 70 μm, and the interface 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, which keeps the particle size, void ratio, and volume ratio of the active material 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, Model A had a constant particle size of 20 μm. Model B had a three-layer structure. In Model B, the first, second, and third layers were arranged in this order, from d = 0 μm to d = 120 μm. The particle size of the active material in the first layer was 5 μm, that in the second layer was 20 μm, and that in the third layer was 5 μm. The thickness of the separator was 20 μm. The negative electrode active material layer was the same for Model A and Model B (film thickness 50 μm, active material particle size 1 μm). The calculation conditions for the charge / discharge current were 0.1 C, 1 C, 2 C, 3 C, 4 C, and 5 C.

[0166] The voltage-capacity curves for charge and discharge were calculated using the calculation structure model shown in Figure 11 and Figures 12A to 12F. The charge and 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 Marquis2019 included with PyBaMM.

[0167] As a result of the charge / discharge simulation, the calculated 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 these 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 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] [Layered 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 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 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 layer portion. That is, any 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 preferably includes the positive electrode active material 100 having a topotaxis region in its surface layer portion.

[0171] Here, a preferred structure when a cathode active material 100 having a topotaxis region in the surface layer portion is used for an electrode 400E will be described with reference to 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 the surface of a particle of the cathode active material 100 where the end of the cation arrangement is exposed can be referred to as the edge surface.

[0172] In an electrode having a solid electrolyte 421 like electrode 400E, as shown in the schematic diagram of FIG. 17A, it is preferable to have the solid electrolyte 421 in the direction of lithium insertion / extraction in the positive electrode active material 100. In other words, it is preferable to have a region on the edge surface of the positive electrode active material 100 where the surface portion of the positive electrode active material 100 and the solid electrolyte 421 are in contact. Here, it is particularly preferable if the surface portion of the positive electrode active material 100 in contact with the solid electrolyte 421 is topotactic with the interior of the positive electrode active material 100, because this allows for good migration of lithium ions in the contact region between the positive electrode active material 100 and the 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 particularly preferred structure is shown in FIG. 17B. FIG. 17B illustrates 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 portion, and the second layer 414b has a cathode active material 411Tb having a topotaxis region in its surface portion. Thus, 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 positive electrode active materials 411Ta included 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 positive electrode active material 411Ta. Similarly, the plurality of positive electrode active materials 411Tb included 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 positive electrode active material 411Tb. In other words, the direction of lithium intercalation and deintercalation in the positive electrode active material 411Ta and the direction of lithium intercalation and deintercalation in the positive electrode 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 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 of high electron transfer resistance (also referred to as a region of low electron mobility) in the active material layer 414. Similarly, the second layer 414b has a 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 shows the 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 second layer 414b is greater than the proportion of conductive material in first layer 414a, and the proportion of conductive material in third layer 414c is greater than the proportion of conductive material in second layer 414b, it is possible to reduce the difference in electron transfer resistance among first layer 414a, second layer 414b, and third layer 414c. In other words, it is preferable that the mass of the conductive material in third layer 414c is greater than the mass of the conductive material in second layer 414b, and that the mass of the conductive material in second layer 414b is greater than the mass of the conductive material in first layer 414a. This makes it possible to reduce unevenness in the battery reaction in active material layer 414 during rapid charging and rapid discharging.

[0179] Here, we 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 FIGS. 18C and 18D. FIG. 18D shows the profile of the solid electrolyte ratio between B1 and B2 in FIG. 18C. 18D, the second layer 414b has a larger proportion of solid electrolyte than the third layer 414c, and the first layer 414a has a larger proportion of solid electrolyte than the second layer 414b. This structure (electrode 400G) reduces the difference in ion migration resistance among the first layer 414a, the second layer 414b, and the third layer 414c. In other words, the mass of the solid electrolyte in the first layer 414a is preferably greater than the mass of the solid electrolyte in the second layer 414b, and the mass of the solid electrolyte in the second layer 414b is preferably greater than the mass of the solid electrolyte in the third layer 414c. This reduces the unevenness of 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 FIGS. 18A and 18B and the solid electrolyte proportion profiles shown in FIGS. 18C and 18D are superimposed. FIG. 19B shows 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] [Positive electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and may include a conductive material 415 and a binder, which will be described later. The positive electrode active material layer preferably has the above-described stacked structure.

[0184] [Negative electrode] 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 highly conductive material that 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 collectors can be in any suitable shape, such as sheet, mesh, punched metal, or expanded metal. The current collectors should preferably have a thickness of 10 μm to 30 μm.

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

[0190] As a current collector, a titanium compound may be provided by laminating it on the above-described metal. As the titanium compound, for example, titanium nitride, titanium oxide, titanium nitride in which a part of nitrogen is substituted with oxygen, titanium oxynitride (TiO x N y where 0 < x < 2 and 0 < y < 1), and titanium oxide in which a part of oxygen is substituted with nitrogen, one selected therefrom, or two or more of them may be mixed or laminated and used. Among them, titanium nitride is particularly preferable because it has high conductivity and a high function of suppressing oxygen diffusion. By providing the titanium compound on the surface of the current collector, for example, the reaction between the material of the active material layer formed on the current collector and the metal is suppressed. When the active material layer contains a compound having oxygen, the 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, there may be a concern about the oxidation reaction between the oxygen of graphene oxide and aluminum. In such a case, by providing a titanium compound on aluminum, the oxidation reaction between the current collector and graphene oxide can be suppressed.

[0191] [Conductive material] The conductive material, also called a conductivity-imparting agent or a conductive auxiliary agent, is a carbon material. By attaching the conductive material between a plurality of active materials, the plurality of active materials are electrically connected to each other, enhancing the conductivity. Note that "attachment" does not only refer to the case where the active material and the conductive material are physically in close contact, but also includes cases where a covalent bond occurs, cases where they are bonded by van der Waals forces, cases where the conductive material covers a part of the surface of the active material, cases where the conductive material fits into the surface irregularities of the active material, and cases where they are electrically connected even if they are not in contact with each other.

[0192] Active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably have 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 materials 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, and nitrocellulose.

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

[0202] Fluorine-containing polymer materials, specifically polyvinylidene fluoride (PVDF), can be used. PVDF is a resin with a melting point between 134°C and 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 aforementioned rubber material.

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

[0206] [Graphene compounds] 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, etc. 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 called 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. Although a single sheet of reduced graphene oxide can function, multiple sheets may also be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is between 2 atomic % and 15 atomic %. By achieving these carbon and oxygen concentrations, it can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D between the G band and 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 almost 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 plurality of graphene compounds are bonded together to form a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net). When an active material is covered with a graphene net, the graphene net can also function as a binder that binds 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 (SiO2, 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 placed between the positive electrode and the negative electrode. The separator can be made of, for example, cellulose-containing fibers such as paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane. The separator is preferably processed into a bag shape and placed so as to encase either the positive electrode or the negative electrode.

[0217] The separator is a porous material with pores of at least 2 nm in diameter. Preferably, the separator has pores of 6.5 nm or larger, and more preferably pores 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 material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials that can be used include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials that can be used include PVDF and polytetrafluoroethylene. Examples of polyamide materials that can be used 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 of 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] Sulfide-based solid electrolytes include thiolithium-based (Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glasses (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), sulfide crystallized glasses (Li7P3S 11 , Li 3.25 P 0.95 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] Oxide-based solid electrolytes include materials with a perovskite crystal structure (La 2 / 3-x Li 3x TiO3, etc.), materials with NASICON-type crystal structure (Li 1-Y Al Y Ti 2-Y (PO4)3, etc.), materials with garnet-type crystal structure (Li7La3Zr2O 12 etc.), materials with LISICON-type crystal structure (Li14 ZnGe4O 16 etc.), LLZO (Li7La3Zr2O 12 ), oxide glasses (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3, etc.), oxide crystallized glasses (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, etc.) are included. Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0225] Halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, LiI, etc. Also, composite materials in which these halide-based solid electrolytes are filled in the pores of porous aluminum oxide or porous silica can be used as solid electrolytes.

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

[0227] Among them, Li 1-x Al x Ti 2-x (PO4)3 (0 < x < 1) (hereinafter, LATP) contains aluminum and titanium, which are elements that the positive electrode active material used in the secondary battery of one aspect of the present invention may have. Therefore, a synergistic effect can be expected for improving the cycle characteristics, which is preferable. Also, an improvement in productivity due to reduction of processes can be expected. In this specification, etc., the NASICON-type crystal structure refers to a structure in which MO6 octahedra and XO4 tetrahedra share vertices and are three-dimensionally arranged in a compound represented by M2(XO4)3 (M: transition metal, X: S, P, As, Mo, W, etc.).

[0228] When a liquid electrolyte 576 is used in a 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 a short circuit or overcharging. Ionic liquids are 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, such as LiPF, LiClO, LiAsF, LiBF, LiAlCl, LiSCN, LiBr, LiI, LiSO, and LiB. 10 Cl 10 , Li2B 12 Cl 12 , LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)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] Fluorinated cyclic carbonates include fluorinated ethylene carbonates, 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 electrodes during charging and discharging. Using fluorinated cyclic carbonates not as a small additive but as a catalyst for lithium ion transport during charging and discharging enables low-temperature operation. Lithium ions move in clusters of several to several tens of ions within a secondary battery.

[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 or graphene compound 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, and their coordination bond with lithium is weaker than FEC, which has one fluorine atom bonded. 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. 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 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, it is possible to realize 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.

[0242] The electrolyte may also contain additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive may be, for example, 0.1% by volume or more and less than 5% by volume of the entire 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. The term "semi-solid" here does not mean that the ratio of solid material is 50%. Semi-solid means that the battery has the properties of a solid, such as small volume change, while also possessing 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, the battery may be made by infiltrating a porous solid material 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 conducting 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. For this reason, when using PEO as a lithium-ion conductive polymer, it is preferable to charge and discharge at temperatures above 60°C.

[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-coordinate -1 nm, 0.92 × 10 for 8-coordinated -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-coordinate -1 nm, 1.40 × 10 for 6-coordinated -1 nm, 1.42 × 10 for 8-coordinated -1 nm. 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 mentioned above, segmental motion occurs, so it is not necessary to maintain a constant distance at all times. It is sufficient that the distance 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, LiPF6, LiN(FSO2)2 (lithium bis(fluorosulfonyl)imide, LiFSI), LiN(SO2CF3)2 (lithium bis(trifluoromethanesulfonyl)amide, LiTFSA), LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, Li2B 10 Cl 10 , Li2B 12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, and lithium bis(oxalato)borate (LiBOB) can be used alone or in any combination and ratio of two or more of these.

[0257] The use of LiFSI is particularly favorable due to its excellent low-temperature properties. Furthermore, LiFSI and LiTFSA are less reactive with water than LiPF6 and other compounds. This makes it easier to control the dew point when fabricating electrodes and electrolyte layers using LiFSI. For example, LiFSI can be used in an inert atmosphere such as argon, which minimizes moisture, or in a dry room with a controlled dew point, as well as in normal air. This improves productivity and is therefore favorable. Furthermore, the use of highly dissociable and plasticizing Li salts such as LiFSI and LiTFSA is particularly favorable when using lithium conduction utilizing the segmental motion of ether chains, as they can be used over a wide temperature range.

[0258] The absence or very little amount of organic solvents makes it possible to make a secondary battery less likely to catch fire or ignite, which is preferable and improves safety. Furthermore, if the electrolyte 576 is an electrolyte layer containing no or very little organic solvent, it has sufficient strength even without a separator and can electrically insulate the positive and negative electrodes. Since a separator is not required, a secondary battery with high productivity can be made. Using an electrolyte layer containing the electrolyte 576 and an inorganic filler further increases strength, resulting in a secondary battery with higher safety.

[0259] [Exterior body] The exterior of a secondary battery can be made of a metal material such as aluminum or a resin material. A film-like exterior can also be used. Examples of films include a three-layer structure in which a flexible metal thin film made of aluminum, stainless steel, copper, nickel, or the like is provided on a membrane made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film made of polyamide resin or polyester resin is further provided on the metal thin film as the exterior surface of the exterior. A fluororesin film is also preferred. Fluororesin films are highly stable against acids, alkalis, organic solvents, and the like, and can suppress side reactions, corrosion, and other problems associated with secondary battery reactions, thereby achieving an excellent secondary battery. Examples of fluororesin films include PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane: a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether), FEP (perfluoroethylenepropene copolymer: a copolymer of tetrafluoroethylene and hexafluoropropylene), and ETFE (ethylenetetrafluoroethylene copolymer: a copolymer of tetrafluoroethylene and ethylene).

[0260] [Internal structure of a battery] A battery including an electrode of 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 has the electrode structure shown in FIG. 18A , and can use, as a positive electrode active material, one or more of the positive electrode active materials described in Embodiments 2 and 3. The secondary battery shown in FIG. 23A preferably includes a liquid electrolyte 576, and the liquid electrolyte 576 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 has the electrode structure shown in FIG. 19A, and can use one or more of the positive electrode active materials described in Embodiments 2 and 3 as the positive electrode active material. An active material layer 414 included in the positive electrode may include the conductive material and the binder.

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

[0264] As shown in FIG. 24, a negative electrode 430 has a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 has a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also have 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, a negative electrode 430 without a solid electrolyte 421 can be obtained. Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery.

[0265] As the solid electrolyte 421 of 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.

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

[0269] [Cathode active material] 1B is a cross-sectional view of a positive electrode active material 100A that can be used in a secondary battery according to one embodiment of the present invention. Enlarged views of the vicinity of AB in FIG. 1B are shown in FIGS. 2A and 2B. Enlarged views of the vicinity of CD in FIG. 1B are shown in FIGS. 2C and 2D.

[0270] 1B and 2A to 2D, a positive electrode active material 100A has a surface layer portion 100a and an interior portion 100b. In these figures, the boundary between the surface layer portion 100a and the interior portion 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 (LiMO2) containing lithium and a transition metal M to which the additive element A has been added. However, the composition of the composite oxide is not strictly limited to Li:M:O=1:1:2. A positive electrode active material to which the additive element A has been added may also be referred to as 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 extracted. 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 for the positive electrode active material 100A to contain 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt among the transition metals M, as this has many advantages, such as relatively easy synthesis and 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, the transition metal M is more easily converted into cobalt than in a composite oxide such as lithium nickel oxide (LiNiO2) in which nickel accounts for the majority of the transition metal M. xCoO2 exhibits superior stability when x is small. This is thought to be because the influence of distortion due to the Jahn-Teller effect is smaller with cobalt than with nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the transition metal's d orbital. Layered rock-salt complex oxides, such as lithium nickelate, in which octahedral low-spin nickel(III) dominates, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This raises concerns about the collapse of the crystal structure during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions, closer in size to lithium ions. Therefore, layered rock-salt complex oxides, such as lithium nickelate, in which nickel dominates, are prone to cation mixing between nickel and lithium.

[0274] On the other hand, if 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 be increased, 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 is synonymous with a mixture or a part of a raw material.

[0278] The additional element A does not necessarily have to include 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, as described above, 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 calculating the crystal structure of the surface layer portion when an additive element is present and when it is not present will be described with reference to FIGS.

[0281] Cobalt oxide may be present in the surface layer of lithium cobalt oxide without any added elements. Cobalt oxide may also contain metal defects. 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. However, 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 of the {1-11} plane perpendicular to the {110} plane of CoO.

[0282] Figure 26B1 is a schematic diagram of lithium cobalt oxide (LCO) with cobalt oxide (CoO) in the surface layer. Figure 26B2 shows an enlarged view of the surface layer. Figure 26B3 shows the results of classical molecular dynamics calculations of 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 misalignments in the atomic arrangement occur, as shown by the dotted circle in Figure 26B3. Such unstable locations are thought to be prone to cobalt and / or oxygen desorption, which could potentially serve as the initiation 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 an additive element. As shown in Figures 27A1 to 27A3, the oxygen arrangements of the {110} planes of LCO and the {110} planes of CoO and MgO are the same, resulting in topotaxis. Furthermore, the 6x 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 the {1-11} plane of CoO. Therefore, it is thought 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 the ATAT (Alloy Theoretic Automated Toolkit) software described in Non-Patent Document 5. ATAT is software that combines first-principles calculations and cluster expansion methods to efficiently advance structural exploration. 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 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 indicates an Mg-O octahedron (MgO6) with Mg at the center, and the black parallelogram indicates a Co-O octahedron (CoO6) with Co at the center.

[0285] [Table 1]

[0286] As shown in Figure 28A, Co (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, making it difficult to determine in which crystal orientation it can form topotaxis with LCO. Therefore, the volume of the structural model at each ratio was divided by the number of metal atoms in the structural model, and the volume per metal atom (10 -3 nm 3 ) and the results of calculating the tendency of change in interplanar spacing 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 thought that the deviation from the {001} lattice spacing of LCO becomes smaller 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 thought that O can be formed. (1-x) Mg x The lattice mismatch between LCO and O is smaller than that between LCO and CoO. (1-x) Mg x The surface layer 100a having O is more likely to undergo topotaxis with the LCO in the interior 100b, and the stress is reduced as indicated by the length of the white arrows in Figures 29A and 29B.

[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 less susceptible to pit formation.

[0290] <Crystal structure> <Li x When x in CoO2 is 1≫ The positive electrode active material 100A according to one embodiment of the present invention is in a discharged state, i.e., Li x When x = 1 in CoO2, it is preferable for the layered rock-salt complex oxide to have a layered rock-salt crystal structure belonging to the space group R-3m. Layered rock-salt complex oxides have high discharge capacity, two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent cathode active materials for secondary batteries. Therefore, it is particularly preferable for the inner portion 100b, which occupies the majority of the volume of the cathode active material 100A, to have a layered rock-salt crystal structure. Figure 30 shows the layered rock-salt crystal structure, labeled R-3m O3.

[0291] On the other hand, the surface layer 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 octahedrons of the transition metal M and oxygen, so that the layered structure formed by the octahedrons of the transition metal M and oxygen is not destroyed even when lithium is released from the cathode active material 100A upon charging. Alternatively, the surface layer 100a preferably functions as a barrier film for the cathode active material 100A. Alternatively, the surface layer 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 100a and inner portion 100b of the cathode active material 100A, such as oxygen release, 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 from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, the atoms on the surface of the positive electrode active material 100A in the surface layer 100a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and can be said to be 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 Even when x in CoO2 is small, for example, 0.24 or less, the layered structure consisting of the transition metal M and oxygen octahedra in the inner 100b can be made less likely to break. Furthermore, it is possible to suppress the displacement of the layer consisting of the transition metal M and oxygen octahedra in the inner 100b.

[0294] To give the surface layer 100a a stable composition and crystal structure, the surface layer 100a preferably contains an additive element A, and more preferably contains a plurality of additive elements A. The surface layer 100a preferably has a higher concentration of one or more selected from the additive elements A than the interior 100b. The positive electrode active material 100A preferably has a concentration gradient of one or more selected from the additive elements A. The positive electrode active material 100A more preferably has a different distribution 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. The concentration peak here refers to the maximum concentration value in the surface layer 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 Figure 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 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 presumably because the magnesium present at the lithium site functions as a pillar supporting the CoO2 layers. In addition, the presence of magnesium makes it easier to maintain the Li x When x in CoO2 is, for example, 0.24 or less, the desorption of oxygen from around the 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, it may have a reduced effect on stabilizing the crystal structure. This is thought to be because magnesium occupies both the lithium site and the transition metal M 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 in 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 in 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% to 5%, more preferably 0.5% to 2%, and even more preferably about 1%. The amount of magnesium contained in the entire cathode active material 100A referred to here may be a value obtained by performing elemental analysis of the entire cathode active material 100A using, for example, GD-MS or ICP-MS, or may be based on the value of the composition of raw materials used in the production of the cathode active material 100A.

[0300] Nickel, one of the additive elements X, can exist in either the transition metal M site or the lithium site. When nickel exists in the transition metal M site, it has a lower redox 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, it can suppress the shift in the layered structure consisting of the transition metal M and oxygen octahedra. It also suppresses the volume change during charging and discharging. It also increases the elastic modulus, meaning the battery becomes harder. This is presumably because the nickel present at the lithium site also functions as a pillar supporting the CoO2 layers. Therefore, it is expected that the crystal structure will be more stable, especially at high temperatures, such as 45°C or higher, during charging.

[0302] On the other hand, excessive nickel may increase the influence of strain due to the Jahn-Teller effect, and may also 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 number of cobalt atoms, more preferably 0.05% to 4%, more 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. Aluminum also suppresses the elution of the surrounding transition metal M, 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, it is possible to create a positive electrode active material 100A whose crystal structure is less likely to collapse even after 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 positive electrode active material 100A be an appropriate amount. For example, the number of aluminum atoms contained in the entire positive electrode 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 positive electrode active material 100A referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100A using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials used in the production of the positive electrode active material 100A.

[0307] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When some of the oxygen in the surface layer 100a is replaced by 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 cobalt ion's oxidation-reduction potential differs between the presence and absence of fluorine, from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine. Therefore, when some of the oxygen in the surface layer 100a of the positive electrode active material 100A is replaced by fluorine, the desorption and insertion of lithium ions near the fluorine can occur more smoothly. This can improve the charge / discharge characteristics and current characteristics when used in secondary batteries. Furthermore, the presence of fluorine in the surface layer 100a, which is the surface that comes into contact with 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 superhydrophilic properties. 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 good 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 the 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 When x in CoO2 is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that CoO2 exists 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] When the electrolyte contains LiPF6, hydrogen fluoride may be generated by hydrolysis. Hydrogen fluoride may also be generated by the reaction of polyvinylidene fluoride (PVDF), a component of the positive electrode, with alkali. Reducing the hydrogen fluoride concentration in the electrolyte may prevent corrosion and / or peeling of the current collector. It may also prevent a decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0312] When the positive electrode active material 100A contains phosphorus together with magnesium, Li x This is preferable because stability is extremely high when x in CoO2 is small. When 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 preferred. Alternatively, 1% to 8% is preferred. Alternatively, 2% to 20% is preferred. Alternatively, 2% to 8% is preferred. Alternatively, 3% to 20% is preferred. Alternatively, 3% to 10% is preferred. 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 preferred. Alternatively, 0.1% to 4% is preferred. Alternatively, 0.5% to 10% is preferred. Alternatively, 0.5% to 4% is preferred. Alternatively, 0.7% to 10% is preferred. 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 ​​obtained by mixing 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 the divalent magnesium can exist more stably near the divalent nickel. x Even when x in CoO2 is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0315] Furthermore, the presence of additive elements A with different distributions, such as additive element X and additive element Y, is preferable because it stabilizes the crystal structure over a wider region. For example, when positive electrode active material 100A contains both magnesium and nickel, which are part of additive element X, and aluminum, which is part of additive element Y, it can stabilize the crystal structure over a wider region than when it contains only one of additive element X and additive element Y. In this way, when 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 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 5 nm to 50 nm deep from the surface, because this stabilizes the crystal structure over 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 greater. 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 hinder the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 100a. For example, it is preferable that the number of nickel atoms is 1 / 6 or less of the number of magnesium atoms.

[0320] Furthermore, while 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 can 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 CoO2 with small x≫ The positive electrode active material 100A of one embodiment of the present invention has the above-described distribution of the additive element A and / or the crystal structure in a discharged state, and therefore, x The crystal structure when x in CoO2 is small is different from that of conventional positive electrode active materials. <x≦0.24をいうこととする。

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

[0323] The change in the crystal structure of a conventional positive electrode active material is shown in Figure 31. The conventional positive electrode active material shown in Figure 31 is lithium cobalt oxide (LiCoO) that does not contain any additional element A. The change in the crystal structure of lithium cobalt oxide that does not contain any additional element A is described in Non-Patent Documents 1 to 3, etc.

[0324] Figure 31 shows R-3m O3 and Li x This shows the crystal structure of lithium cobalt oxide with x=1 in CoO2. In this crystal structure, lithium occupies octahedral sites, and there are three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is coordinated with six oxygen atoms, and these structures are connected in a plane with edge sharing. This is sometimes called a layer consisting of octahedra of cobalt and oxygen.

[0325] Conventional lithium cobalt oxide is known to have a crystal structure that belongs to the monoclinic space group P2 / m when the symmetry of lithium increases when x is around 0.5. This structure has one CoO2 layer in the unit cell. For this reason, it is sometimes called the O1 type or monoclinic O1 type.

[0326] When x = 0, the positive electrode active material has a trigonal space group P-3m1 crystal structure, with one CoO2 layer in each unit cell. This crystal structure is sometimes called the O1 type or trigonal O1 type. The trigonal structure may also be converted to a composite hexagonal lattice, which is sometimes called the hexagonal O1 type.

[0327] Furthermore, conventional lithium cobalt oxide (LiCOO) with x = 0.12 or so has a crystal structure of the space group R-3m. This structure can be described as a structure in which a trigonal O1-type CoO2 structure and an R-3m O3-type LiCoO2 structure are alternately stacked. Therefore, this crystal structure is sometimes referred to as the H1-3-type crystal structure. Because actual lithium insertion and desorption can be uneven, the H1-3-type crystal structure is experimentally observed from x = 0.25 or so. Furthermore, the H1-3-type crystal structure actually has twice the number of cobalt atoms per unit cell compared to other structures. However, in Figure 31 and other parts of this specification, the c-axis of the H1-3-type crystal structure is shown as half the unit cell to facilitate comparison with other crystal structures.

[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 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, the unit cell with the smallest GOF (goodness of fit) value should be used.

[0329] Li x When conventional lithium cobalt oxide is repeatedly charged and discharged so that x in CoO2 becomes 0.24 or less, the crystal structure changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0330] However, these two crystal structures have a large misalignment of the CoO2 layers. As shown by the dotted lines and arrows in Figure 31, in the H1-3 crystal structure, the CoO2 layers are significantly misaligned from those in the R-3m O3 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: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3m O3 crystal structure exceeds 3.5%, typically 3.9% or more.

[0332] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers like the trigonal O1 type, 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 The change in the crystal structure between the discharge state where x in CoO2 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 deviation of the CoO2 layer between the state where x is 1 and the state where x is 0.24 or less can be reduced. Furthermore, the change in volume compared per cobalt atom can be reduced. Therefore, the positive electrode active material 100A of one embodiment of the present invention is less likely to collapse in 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 has a low crystalline structure even when Li is charged and discharged. x When x in CoO2 is 0.24 or less, the positive electrode active material 100A according to one embodiment of the present invention can have a more stable crystal structure than conventional positive electrode active materials. x When x in CoO2 is kept at 0.24 or less, short circuits are less likely to occur, which is preferable as it further improves the safety of the secondary battery.

[0335] Li xThe crystal structure of the interior 100b of the positive electrode active material 100A when x in CoO2 is approximately 1 and 0.2 is shown in Fig. 30. The interior 100b occupies the majority of the volume of the positive electrode active material 100A and is the part that contributes greatly to charge and discharge, so it can be said that the displacement of the CoO2 layer and changes in volume are the most problematic part.

[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] A positive electrode active material 100A according to one embodiment of the present invention, when x is approximately 0.2, has a crystal structure belonging to the trigonal space group R-3m. The symmetry of the CoO2 layer is the same as that of O3. Therefore, this crystal structure is referred to as an O3'-type crystal structure. This crystal structure is shown in Figure 30, labeled R-3m O3'.

[0339] The O3' type crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), with the range of 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' crystal structure, ions of cobalt, nickel, magnesium, etc. occupy six oxygen coordination positions, while light elements such as lithium may occupy four oxygen coordination positions.

[0341] As shown by the dotted line in Figure 30, there is almost no deviation in the CoO2 layer between the R-3m O3 in the discharged state and the O3'-type crystal structure.

[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] In this way, in the positive electrode active material 100A of one embodiment of the present invention, Li x When x in CoO2 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 per the same number of cobalt atoms is also suppressed. Therefore, the crystal structure of the positive electrode active material 100A is resistant 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 it can stably utilize more lithium than conventional positive electrode active materials, the positive electrode active material 100A has a large discharge capacity per weight and per volume. Therefore, the use of the positive electrode active material 100A enables the fabrication of secondary batteries with high discharge capacity per weight and per volume.

[0344] The positive electrode active material 100A is Li x It has been confirmed that when x in CoO2 is between 0.15 and 0.24, it may have an O3' type crystal structure, and it is also presumed that when x is between 0.24 and 0.27, it also has an O3' type crystal structure. However, the crystal structure is Li x Since it is affected not only by x in CoO2 but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it is not necessarily limited to the above range of x.

[0345] Therefore, the positive electrode active material 100A is Li x When x in CoO2 is more than 0.1 and not more 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 To make the x in CoO2 small, it is generally necessary to charge at a high charging voltage. x A state in which x in CoO2 is small can be rephrased as a state in which the battery is charged at a high charging voltage. For example, when conventional positive electrode active materials are charged at a voltage of 4.6 V or higher relative to the potential of lithium metal in a 25°C environment under CC / CV charging, an H1-3 crystal structure appears. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be considered a high charging voltage. Unless otherwise specified, charging voltages in this specification and elsewhere 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 O3 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 O3′-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, the H1-3 type crystal may be finally observed when the charge voltage is further increased. 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 relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the battery has a similar crystal structure 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, for example, in the monoclinic O1 (Li 0.5 The lithium distribution can be analyzed, for example, by neutron diffraction.

[0351] The O3' type crystal structure can also be said to be similar to the CdCl2 type crystal structure, although it has random lithium between the layers. This CdCl2 type-like crystal structure is similar to the CdCl2 type crystal structure, but the O3' type crystal structure has random lithium between the layers. 0.06 Although the crystal structure is similar to that when charged to NiO2, it is known that pure lithium cobaltate or layered rock salt type positive electrode active materials containing a large amount of cobalt do not usually adopt a CdCl2 type 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 resulting 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 100a of the positive electrode active material 100A. An example of the distribution of the additional element X near the CD in FIG. 1B is shown in FIG. 2C, and an example of the distribution of the additional element Y near the CD in FIG. 2D.

[0354] Here, the surface near CD 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 concentration peak distribution of one or more 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 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 selected from the additional element X and the additional element Y below the lower detection limit.

[0355] In the layered rock-salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This can be said to be a structure in which CoO2 layers and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane.

[0356] The CoO2 layer is relatively stable, so the surface on which the CoO2 layer is present is relatively stable, and the main diffusion path of lithium ions during charging and discharging is not exposed on this surface.

[0357] On the other hand, the diffusion paths of lithium ions are exposed on surfaces that are not parallel to the cation arrangement, i.e., surfaces that are not parallel to the CoO layer. Therefore, the surface and surface layer 100a that are not parallel to the cation arrangement are important regions for maintaining the diffusion paths of lithium ions, but at the same time, they are prone to instability because they are the regions from which lithium ions first desorb. Therefore, reinforcing the surface and surface layer 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 according to another embodiment of the present invention, it is important that the distribution of additive element A in the surface not parallel to the cation arrangement and in its surface layer 100a is not limited to the outermost layer as shown in Figures 2A and 2B, but exists at a desired depth. On the other hand, the concentration of additive element A in the surface parallel to the cation arrangement and in its surface layer 100a may be low or absent, as described above.

[0359] In a manufacturing method described in a later embodiment, in which high-purity LiCoO is manufactured, and then the additive element A is mixed and heated, the additive element A spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additive element A in the surface not parallel to the arrangement of cations and in the surface layer portion 100a thereof can be easily controlled to a preferred range.

[0360] Furthermore, while it is preferable that the surface of the cathode active material 100A be smooth and minimally uneven, this need not necessarily be the case for the entire cathode active material 100A. Composite oxides with an R-3m layered rock-salt crystal structure are prone to slippage in planes parallel to the arrangement of cations, such as the 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 arrows in FIG. 32B, resulting in deformation, after a process such as pressing.

[0361] In this case, the additional element A may be absent or below the detection limit on the surface and its surface layer 100a newly formed as a result of the slip. EF in FIG. 32B is an example of the surface and its surface layer 100a newly formed as a result of the slip. Enlarged views of the vicinity of EF are shown in FIGS. 32C1 and 32C2. Unlike FIGS. 2A to 2D, additional element X and 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, in a composite oxide with a composition of LiCoO2 and a layered rock-salt structure of R-3m, cobalt and lithium are arranged parallel to the (001) plane. Furthermore, in HAADF-STEM images, cobalt, which has the highest atomic number in LiCoO2, exhibits the highest brightness. Therefore, the arrangement of highly bright atoms in HAADF-STEM images can be considered to be the arrangement of cobalt. The repetition of this highly bright arrangement is synonymous with crystal fringes or lattice fringes.

[0364] <Grain boundary> In addition to the distribution described above, the additional element A contained in the positive electrode active material 100A of one embodiment of the present invention is more preferably at least partially distributed unevenly at 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 areas with high concentration and areas with low concentration.

[0366] For example, the magnesium concentration at and near the grain boundaries of the positive electrode active material 100A is preferably higher than that in other regions of the interior 100b. The fluorine concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 100b. The nickel concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 100b. The aluminum concentration at and near the grain boundaries is also preferably 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 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 and fluorine concentrations are high at and near the grain boundaries, even if cracks occur along the grain boundaries of cathode active material 100A of one embodiment of the present invention, the magnesium and fluorine concentrations 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 to 100 μm, more preferably 2 μm to 40 μm, and even more preferably 5 μm to 30 μm. Alternatively, 1 μm to 40 μm is preferred. Alternatively, 1 μm to 30 μm is preferred. Alternatively, 2 μm to 100 μm is preferred. Alternatively, 2 μm to 30 μm is preferred. Alternatively, 5 μm to 100 μm is preferred. Alternatively, 5 μm to 40 μm is preferred.

[0370] <Analysis method> A certain positive electrode active material is Li x When x in CoO2 is small, whether the positive electrode active material 100A of one embodiment of the present invention has an O3'-type crystal structure or not can be determined by Li x This can be determined by analyzing a positive electrode having a positive electrode active material with a small x in CoO2 using XRD, electron beam diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[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 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 Li x A characteristic of CoO2 is that there is little change in the crystal structure when x in CoO2 is 1 and when it is 0.24 or less. When charged at high voltage, materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more are not desirable 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 with magnesium and fluorine or lithium cobalt oxide with magnesium and aluminum has something in common, depending on the concentration and distribution of the additional element A, the structure may be different from Li x When x in CoO2 is 0.24 or less, the O3' type crystal structure accounts for 60% or more, and when the H1-3 type crystal structure accounts for 50% or more.

[0374] Furthermore, even in the case of cathode active material 100A of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be formed if 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 the cathode active material 100A of one embodiment of the present invention is a cathode active material, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

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

[0376] Furthermore, whether the distribution of the additive element A in a certain positive electrode active material is in the state described above can be determined by analyzing it using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

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

[0378] ≪Charging method≫ 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) is fabricated using the composite oxide as a positive electrode and a lithium counter electrode as a negative electrode, and high-voltage charging is 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 used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution can be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC).

[0382] The separator can 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). Such a low current is desirable for observing the phase change of the positive electrode active material. The temperature is 25°C or 45°C. After charging in this manner, the coin cell is disassembled in an argon-filled glove box and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. For subsequent analyses, the cell is preferably sealed in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in an airtight container in an argon atmosphere. After charging is complete, the positive electrode is preferably removed and analyzed promptly. Specifically, within 1 hour, and preferably within 30 minutes, after charging is complete.

[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 can 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 XRD measurement apparatus and conditions are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD equipment: Bruker AXS, D8 ADVANCE X-ray source:CuKα1 ray Output: 40KV, 40mA 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 xThe ideal XRD patterns calculated from the crystal structures of LiCoO2O3 with x = 1 in CoO2, H1-3 type, and trigonal O1 with x = 0 are also shown. The LiCoO2(O3) and CoO2(O1) patterns were created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 4). The 2θ range was 15° to 75°, the step size was 0.01, and the wavelength λ1 was 1.540562 × 10 -10 m and λ2 were not set, and the monochromator was set to 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, fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating an XRD pattern in the same way 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 Figure 34, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. x 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 in CoO2 is small 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 with x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures with x = 1 and x ≦ 0.24 that appear at 2θ angles of 42° to 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0393] The positive electrode active material 100A according to one embodiment of the present invention is Li x When x in CoO2 is small, the material has an O3'-type crystal structure, but it does not have to be entirely O3'-type. It may contain other crystal structures, or it may be partially 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 preferably accounts for 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0395] Furthermore, the sharpness of diffraction peaks in an XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., has a narrow half-width. The half-width varies depending on the XRD measurement conditions and the 2θ value, even for peaks arising from the same crystalline phase. Under the measurement conditions described above, for peaks observed between 2θ = 43° and 46°, 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 that crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after charging.

[0396] In addition, the crystallite size of the O3'-type crystal structure of the positive electrode active material 100A is reduced to only about 1 / 20 of that of LiCoO2(O3) in the discharged state. x When x in CoO2 is small, a clear peak of the O3'-type crystal structure can be confirmed. On the other hand, with conventional LiCoO2, even if a portion of the material has a structure similar to the O3'-type crystal structure, the crystallite size becomes small and the peak becomes broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0397] XPS In the case of inorganic oxides, X-ray photoelectron spectroscopy (XPS) can analyze a region from the surface to a depth of approximately 2 to 8 nm (typically 5 nm or less) by using monochromated aluminum Kα rays as the X-ray source. This allows 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 portion 100a than in the interior portion 100b. This is equivalent to saying that the concentration of one or more selected additive elements A in the surface layer portion 100a is preferably higher than the average concentration of 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 portion 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 portion 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 portion 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] The surface and surface layer 100a of the cathode active material 100A according to one embodiment of the present invention are assumed to be free of carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the cathode active material 100A. The surface of the cathode active material 100A is also assumed to be free of electrolyte, binder, conductive material, and compounds derived therefrom that adhere 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 subjecting the sample to various analyses, the sample of 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 from these adhering to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element A is unlikely to dissolve, and therefore the atomic ratio of the added 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 magnesium to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 to 1.5. Meanwhile, the ratio (Mg / Co) determined by ICP-MS analysis is preferably 0.001 to 0.06.

[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 are higher than those of each of the additional elements A in the surface layer portion 100a. 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 positive electrode 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 ratio of the number of magnesium atoms to the number of cobalt atoms was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times. The ratio of the number of nickel atoms to the number of cobalt atoms was preferably 0.15 to 0.15 times, more preferably 0.03 to 0.13 times. The ratio of the number of aluminum atoms to the number of cobalt atoms was preferably 0.12 to 0.09 times. The ratio of the number of fluorine atoms 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°. For example, measurement can be performed using the following equipment and conditions. Measurement equipment: PHI Quantera II X-ray source: Monochromatic Al Kα (1486.6eV) 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 showing the bond energy between fluorine and other elements is preferably 682 eV or more 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 analyzed by XPS, the peak showing the bond energy between magnesium and other elements 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. More preferably, the positive electrode active material 100A has a concentration peak at a different depth from the surface 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 in two dimensions. Linear analysis is performed by scanning a line to evaluate the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also sometimes used to extract data from a linear area of ​​EDX area analysis. Point analysis is performed by measuring 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 line 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 containing magnesium as the additive element X, the magnesium concentration in the surface layer 100a is preferably higher than the magnesium concentration in the interior 100b. Furthermore, when EDX line 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 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. Furthermore, the magnesium concentration preferably decays 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 decays 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 additive element X, the fluorine distribution preferably overlaps with the magnesium distribution. For example, the difference in depth 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 analysis is performed, the fluorine concentration peak of the surface layer 100a is preferably present 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, it is preferable for the fluorine concentration peak to be slightly closer to the surface than the magnesium concentration peak, as this increases resistance to hydrofluoric acid. For example, it is more preferable for the fluorine concentration peak to be at least 0.5 nm closer to the surface than the magnesium concentration peak, and even more preferable for it to be at least 1.5 nm closer to the surface.

[0415] In addition, in a cathode active material 100A containing nickel as the additive element X, the nickel concentration peak in the surface layer 100a preferably exists within a depth of 3 nm from the surface toward the center of the cathode active material 100A, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. In addition, in a 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 positive electrode active material 100A contains aluminum as added element Y, EDX analysis preferably shows that the magnesium, nickel, or fluorine concentration peak is closer to the surface than the aluminum concentration peak in surface layer 100a. For example, the aluminum concentration peak is preferably present at a depth of 0.5 nm to 50 nm, more preferably 5 nm to 50 nm, from the surface to the center of positive electrode active material 100A.

[0417] Furthermore, when EDX line 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 to 0.6, more preferably 0.1 to 0.4. 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 to 0.6, more preferably 0.1 to 0.45. 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 to 0.2, more preferably 0.01 to 0.1. 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 to 1.6, more preferably 0.1 to 1.4.

[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 chemisorption or background, is found in the area that can be clearly determined to be outside the surface. background If O is detected, the measurement background The average oxygen concentration is calculated by subtracting aveThis 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 positive electrode active material 100A is subjected to linear or area analysis, the ratio (A / Co) of 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. 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.

[0422] For example, when the additive element X is magnesium, when a line analysis or area analysis is performed on the positive electrode active material 100A, 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 positive electrode active material 100A according to 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 for 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] <Charging curve and dQ / dV vs V curve> Positive electrode active material 100A according to one embodiment of the present invention may exhibit a characteristic voltage change during charging. The voltage change can be read from a dQ / dV vs. V curve obtained by differentiating capacity (Q) with voltage (V) from a charging curve (dQ / dV). For example, it is believed that a non-equilibrium phase change occurs around the peak in the dQ / dV vs. V curve, resulting in a significant change in the crystal structure. In this specification and elsewhere, 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 during the phase change from the O3 type to the O3' type. Therefore, a broad peak indicates a smaller change in the energy required for lithium extraction, i.e., a smaller change in the crystal structure, compared to a sharper peak. The smaller these changes are, the less the effects of CoO2 layer displacement and volume change are, which is preferable.

[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-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 charge capacity is the product of the current value and time.

[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 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 can be set to, for example, 500.

[0433] Specifically, the average value of dQ from the nth to the (n+500th) data is calculated, and the average value of dV from the nth to the (n+500th) data is calculated in the same way. 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 of a dQ / dV vs. V graph. Note that when using a moving average such as the one described above, 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 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 reaches 10 mA / g, and discharging at a constant current of 100 mA / g at 2.5 V.

[0435] At around 4.55V, the phase changes from O3 type to O3' type, and the O3 type at this time is Li x The value of x in CoO2 is approximately 0.3. This has the same symmetry as the O3 type with x = 1 described in Figure 31, but the distance between the CoO2 layers is slightly different. In this specification, when distinguishing between O3 types with different values ​​of x, the O3 type with x = 1 will be referred to as O3 (2θ = 18.85°), and the O3 type with x = 0.3 or so will be referred to as O3 (2θ = 18.57°). This is because the position of the peak that appears in XRD measurements at 2θ around 19° corresponds to the CoO2 interlayer distance.

[0436] <Discharge curve and dQ / dV vs V curve> Furthermore, when 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 range of dQ / dV vs. V calculated from the discharge curve, which is lower than the peak that appears around 3.9 V and falls within the range of 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 + Mg 2+ It is preferred that Li be substituted. + Mg 2+ With the replacement of Ni 3+ is reduced to Ni2+ In addition, some Li + Mg 2+ is replaced by Mg 2+ Nearby Co 3+ is reduced to Co 2+ In addition, some Co 3+ 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 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 following 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. If the magnesium concentration is too high, the Ni 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) or the like.

[0440] <Surface roughness and specific surface area> The positive electrode active material 100A according to one embodiment of the present invention preferably has a smooth surface with minimal irregularities. A smooth surface with minimal irregularities indicates that the effect of the flux, which will be described later, is fully exerted, melting the surface of the additive element A source and the composite oxide. This is one factor indicating that the additive element A is well distributed in the surface layer portion 100a. A good distribution means, for example, that the concentration of the additive element A is uniformly distributed in the surface layer portion 100a.

[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 FIB or the like to expose its 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. The interface is then extracted using image processing software. The interface line between the protective film or the like and the cathode active material 100A is selected using an automatic selection tool, and the data is extracted to a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed using a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction. 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 smoothness.

[0447] Ideal specific surface area S i is calculated assuming that all 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 should be the same.

[0452] Next, using image processing software (such as "ImageJ"), the above SEM image is converted to, for example, an 8-bit image (called a grayscale image). Grayscale images contain luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 levels. Dark areas have lower levels of gradation, and bright areas have higher levels of gradation. Changes in luminance can be quantified by relating them to the number of levels of gradation. This number is called the grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material numerically.

[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 pause method≫ The distribution of the additive element A, such as magnesium, present in the surface layer 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 the 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(0.1s) 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 higher, even for a positive electrode active material that does not contain an additive element. Therefore, n is preferably, for example, between 2 and 10. 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] <Pit> Positive electrode active materials may develop progressive defects that progress from the surface to the interior when charged at 4.5 V or higher, or when charged and discharged in a high-temperature environment, such as 45°C or higher. The phenomenon in which defects progress to form holes in a positive electrode active material is also called pitting corrosion, and the holes generated by this phenomenon are also referred to as pits in this specification. The opening shapes 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 of the positive electrode active material 51 where the additive element A is present is indicated by 53 and 56. The surface layer where the pits have formed has 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 near the pits, 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 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 charge and discharge. In this specification, cracks and pits are different. Cracks may be present immediately after the preparation of the positive electrode active material, but pits do not. Pits can be considered holes formed by the loss of several layers of transition metal M and oxygen due to charge and discharge 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 eluted. Cracks refer to, for example, new surfaces formed by the application of physical pressure or fractures caused by grain boundaries. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charge and discharge. Pits may also occur from cracks and / or cavities within the positive electrode active material.

[0462] [Method for producing positive electrode active material] To prepare a positive electrode active material 100A having the distribution, composition, and / or crystal structure of the additive element A as described in the previous embodiment, it is important to add the additive element A. At the same time, it is also important that the crystallinity of the 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 the 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 simultaneously mixing a transition metal M source and an additive element A source, 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 a 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, will enter the transition metal M site. Magnesium present in the transition metal M site is Li x When x in CoO2 is small, it is not effective in maintaining the R-3m layered rock salt crystal structure. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as cobalt being reduced to divalent and lithium evaporating.

[0466] Therefore, it is preferable to mix a material that functions as a flux 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 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 additive 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 portion 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, through the following mechanism. First, lithium is released from 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 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 structures, the bond distance between metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt structure.

[0472] For example, rock salt 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 Even if a spinel-type phase is formed in a part of the surface layer 100a, the Me-O distance of the spinel-type NiAl2O4 is 2.0125 × 10 nm.-1 nm, and the Me-O distance in spinel-type MgAl2O4 is 2.02 × 10 -1 In both cases, the Me-O distance is 2×10 -1 More than nm.

[0473] On the other hand, in layered rocksalt structures, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, the Al-O distance in layered rocksalt LiAlO2 is 1.905 × 10 -1 nm (Li-O distance is 2.11 × 10 -1 nm), and the Co-O distance in layered rocksalt LiCoO2 is 1.9224 × 10 -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 within the surface layer 100a.

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

[0477] However, initial heating is not necessarily required. In other heating processes, such as annealing, the atmosphere, temperature, time, etc. can be controlled to achieve Li x When x in CoO2 is small, a positive electrode active material 100A having an O3' type may be produced.

[0478] An example of a manufacturing flow of positive electrode active material 100A through 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 for example, it is recommended to use a material with a purity of 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, only cobalt may be used, only nickel may be used, two elements (cobalt and manganese), two elements (cobalt and nickel), or three elements (cobalt, manganese, and nickel) may be used. When only cobalt is used, the resulting positive electrode active material has lithium cobalt oxide (LCO), and when three elements (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 materials.

[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 using 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. 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, LiMO2 will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time should be between 1 hour and 100 hours, and preferably between 2 hours and 20 hours.

[0490] The temperature rise rate depends on the heating temperature reached, but should be between 80°C / h and 250°C / h. For example, if heating at 1000°C for 10 hours, the temperature should be raised at 200°C / h.

[0491] The heating atmosphere is preferably an atmosphere with little water, such as dry air, with a dew point of -50°C or less, more preferably -80°C or less. In this embodiment, heating is performed in an atmosphere with a dew point of -93°C. In order to suppress impurities that may be mixed into the material, the impurity concentrations of CH4, CO, CO2, and H2 in the heating atmosphere should each be 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 having oxygen 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, the reaction chamber may be depressurized and then filled with oxygen to prevent the oxygen from entering or leaving the reaction chamber, a method 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 can be cooled naturally, but it is preferable that the time required to cool the material from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary, 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 place a lid on the crucible or sheath during heating, 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> Through the above steps, a composite oxide (LiMO2) containing a transition metal M can be obtained in step S14 shown in FIG. 36A. The composite oxide only needs to have the crystal structure of a lithium composite oxide represented by LiMO2, and its composition is not strictly limited to Li:M:O=1:1:2. When cobalt is used as the transition metal M, it is called a composite oxide containing cobalt and is represented by LiCoO2. The composition is not strictly limited to Li:Co:O=1:1:2.

[0499] Although the composite oxide is produced by the solid phase method in steps S11 to S14, 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 is sometimes called initial heating. Alternatively, because it is heating before step S20 described below, it is sometimes called preheating or pretreatment.

[0501] As described above, the initial heating causes lithium to be released 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 and 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 to 1000°C for 2 hours to 20 hours is recommended.

[0505] The effect of increasing the crystallinity of the inner portion 100b is, for example, the effect of mitigating 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 interior of the composite oxide. This temperature difference may induce a contraction difference. It is thought that the temperature difference causes a difference in fluidity between the surface and interior, resulting in a contraction difference. The energy associated with the contraction difference causes an internal stress difference 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 misalignment in the composite oxide, such as misalignment of crystals. This step is preferably carried out in order to reduce such misalignment. This step makes it possible to equalize the misalignment of the composite oxide. When the misalignment is 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 the misalignment of crystals and the like that has occurred in the composite oxide is alleviated after step S15, and the surface of the composite oxide becomes smooth.

[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] A smooth surface of a complex oxide can be defined as a surface roughness of at least 10 nm or less when surface irregularity information is quantified from measurement data on a cross section of the complex 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. The added element A, which will be explained in the next step S20, 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 within the range that allows a layered rock salt type crystal structure to be formed. 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 Figures 36B and 36C.

[0513] <Step S21> 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, etc. may 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 (MgF2) is prepared as both the fluorine source and the magnesium source. The melting point is most effectively lowered when lithium fluoride and magnesium fluoride are mixed in a molar ratio of approximately LiF:MgF2 = 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium content will be excessive, resulting in poor cycle performance. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably LiF:MgF2 = x:1 (0≦x≦1.9), more preferably LiF:MgF2 = x:1 (0.1≦x≦0.5), and even more preferably LiF:MgF2 = x:1 (x = 0.33 or thereabouts). In this specification, "nearby" refers to a value greater than 0.9 times but less than 1.1 times the value.

[0520] At the same time, the amount of magnesium added is preferably more than 0.1 atomic % and less than 3 atomic % based on LiCoO2, more preferably 0.5 atomic % to 2 atomic % and even more preferably 0.5 atomic % to 1 atomic %. If the amount of magnesium added is 0.1 atomic % or less, the initial discharge capacity is high, but the discharge capacity drops rapidly after repeated charge / discharge cycles that increase the depth of charge. If the amount of magnesium added is more than 0.1 atomic % and less than 3 atomic %, both the initial discharge characteristics and the charge / discharge cycle characteristics are good, even after repeated charge / discharge cycles that increase the depth of charge. On the other hand, if the amount of magnesium added exceeds 3 atomic %, both the initial discharge capacity and the charge / discharge cycle characteristics tend to gradually deteriorate.

[0521] <Step S22> Next, in step S22 shown in Fig. 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] <Step S23> Next, in step S23 shown in Fig. 36B, the pulverized and mixed materials are collected to obtain the source of the additional element A (A source). Note that the source of the additional element A shown in step S23 contains multiple 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 easy to uniformly adhere 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 preferable because it makes it easy to distribute or diffuse fluorine and magnesium uniformly 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 or 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 explanation, fluorine may also be chlorine, and chlorine can be read as including these and so can be interpreted as halogen.

[0526] <Step S21> A step different from that shown in FIG. 36B will be described with reference to FIG. 36C. In step S21 shown in FIG. 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 FIG. 36C are different from those in FIG. 36B. A lithium source may be prepared together with the additive element A sources.

[0527] As sources of the four additive elements 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, steps S22 and S23 shown in FIG. 36C are similar to the steps described in FIG. 36B.

[0529] <Step S31> 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, transition metal M, and oxygen to the number of atoms Mg of magnesium contained 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, the mixing conditions are preferably lower in rotation speed or shorter in time than 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 the media.

[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 a composite oxide that has undergone initial heating. However, the present invention is not limited to the above method. In step S11, that is, at 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, heating is performed in step S13 to obtain LiMO2 doped with magnesium and fluorine. In this case, there is no need to separate the steps S11 to S14 from the steps S21 to S23. This method can be said to be simple and highly productive.

[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] <Step S33> 36A, the mixture 903 is heated under heating conditions 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 provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the composite oxide (LiMO2) and the additive element A source proceeds. The temperature at which the reaction proceeds may be any temperature at which mutual diffusion of elements contained in LiMO2 and the additive element A source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, and the melting temperature T m 0.757 times (Tanman 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 at a temperature equal to or higher than the temperature at which at least a portion of the mixture 903 melts. For example, when LiF and MgF2 are used as the source of the additional element A, the eutectic point of LiF and MgF2 is around 742°C, so the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0539] Furthermore, a mixture 903 obtained by mixing so as to achieve a molar ratio of LiCoO2:LiF:MgF2=100:0.33:1 exhibits an endothermic peak at around 830°C in differential scanning calorimetry (DSC measurement). 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 the heating temperature is below the decomposition temperature of LiMO2 (the decomposition temperature of LiCoO2 is 1130°C). At temperatures close to the decomposition temperature, there is a concern that LiMO2 may decompose, albeit in a small amount. Therefore, a temperature of 1000°C or less is more preferable, a temperature of 950°C or less is even more preferable, and a temperature of 900°C or less is even more preferable.

[0542] Considering these, 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 fabrication method described in this embodiment, some materials, such as LiF, a fluorine source, may function as a flux. This function allows the heating temperature to be lowered below the decomposition temperature of the composite oxide (LiMO2), for example, to 742°C or higher and 950°C or lower, and allows the additive element A, including magnesium, to be distributed in the surface layer, thereby producing a positive electrode active material with good characteristics.

[0545] However, because LiF has a lighter specific gravity in its gaseous state than oxygen, it may 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 the mixture while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, the Li on the LiMO2 surface may react with the F fluorine source, producing 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, the volatilization of LiF in the mixture 903 can be suppressed.

[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 poor distribution of the additional element A (for example, magnesium and fluorine) in the surface layer.

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

[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 placing a lid on a container containing the mixture 903, for example.

[0551] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of LiMO2 in step S14, etc. When LiMO2 is small, a lower temperature or a shorter time may be more preferable than when LiMO2 is large.

[0552] When the median diameter (D50) of the composite oxide (LiMO2) in step S14 of Figure 36A is about 12 μm, the heating temperature is preferably, for example, 600°C or higher and 950°C or lower. The heating time is preferably, for example, 3 hours or higher, more preferably 10 hours or higher, and even more preferably 60 hours or higher. The temperature reduction time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0553] On the other hand, when the median diameter (D50) of the composite oxide (LiMO2) in step S14 is about 5 μm, the heating temperature is preferably, for example, 600°C or higher and 950°C or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 2 hours. The temperature reduction time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0554] <Step S34> Next, in step S34 shown in FIG. 36A, the heated material is recovered and crushed as necessary to obtain a positive electrode active material 100A. At this time, it is preferable to further sieve the recovered positive electrode active material 100A. Through the above steps, a positive electrode active material 100A according to one embodiment of the present invention can be produced. The positive electrode active material according to one embodiment of the present invention has a smooth surface.

[0555] Note that the positive electrode active material 100A shown in this embodiment can be used as 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 in Embodiment 1.

[0556] (Embodiment 3) [Method for producing positive electrode active material] In this embodiment, an example of a method for manufacturing a positive electrode active material 100B as an example of the positive electrode active material 100 that can be used in the secondary battery of one embodiment of the present invention is described with reference to Figure 37. A positive electrode active material to which an additive element is added is also called a composite oxide having the additive element.

[0557] In this embodiment, a co...

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 has a first active material having a first particle size; the second layer has a second active material having a second particle size; the third layer has 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 contained in the first layer is greater than the mass of the solid electrolyte contained in the second layer; a mass of the solid electrolyte in the second layer being greater than a mass of the solid electrolyte in the third layer;

2. In claim 1, The battery, wherein the second active material has a sphericity of 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 battery, wherein the second active material has a sphericity of 0.8 or more and 1.0 or less.

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

6. In claim 5, The battery, wherein the second active material has a sphericity of 0.8 or more and 1.0 or less.

7. In claim 3, the second active material has a surface layer portion and an interior portion, the surface layer portion is a region of 10 nm or less extending from the surface of the second active material toward the interior, The 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; the mass of the conductive material in the third layer is greater than the 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 contained in the first layer is greater than the mass of the solid electrolyte contained 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 battery having a region on the edge surface of the second active material where the surface layer portion and the solid electrolyte are in contact.

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.

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