Secondary batteries

Abstract

To provide electrodes and secondary batteries with high capacity density and excellent rapid charging and discharging capabilities. [Solution] A battery having 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 having a first active material having a first particle size, and the second layer having a second active material having a second particle size, the first particle size being smaller than the second particle size. The second active material has a surface layer and an interior, the surface layer is a region of 10 nm or less from the surface to the interior of the second active material, and it is preferable that the surface layer and the interior are topotaxis.

Description

[Technical Field]

[0001] One aspect of the present invention relates to a secondary battery and a method for manufacturing the same; to a method for manufacturing electrodes and an apparatus for manufacturing electrodes; or to electronic devices, energy storage systems, and mobile devices having a secondary battery.

[0002] One aspect of the present invention relates to a product, a method, or a method of manufacture. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. Another 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 method of manufacturing the same.

[0003] In this specification, "electronic equipment" refers to all devices that have an energy storage device, and all electro-optical devices with an energy storage device, information terminal devices with an energy storage device, etc., are considered electronic equipment.

[0004] In this specification, the term "energy storage device" refers to all elements and devices that have an energy storage function. This includes, for example, energy storage devices such as lithium-ion secondary batteries (also called batteries, secondary batteries, etc.), lithium-ion capacitors, and electric double-layer capacitors. [Background technology]

[0005] In recent years, there has been a great deal of development on various energy storage devices, including lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. In particular, lithium-ion secondary batteries, which offer high output and high energy density, are being applied to mobile devices such as mobile phones, smartphones, and notebook computers, portable music players, digital cameras, medical equipment, and next-generation clean energy vehicles such as hybrid vehicles (HV), electric vehicles (EV), and plug-in hybrid vehicles (PHV). Along with the development of the semiconductor industry, demand for these batteries has been rapidly expanding, and they have become indispensable to today's information society as a rechargeable energy source.

[0006] Lithium-ion rechargeable batteries use lithium cobalt oxide (LiCoO2) and lithium nickel-cobalt-manganate (LiNi 1-x-y Co x Mn y It consists of a positive electrode containing a positive electrode active material such as O2 or lithium iron phosphate (LiFePO4), a negative electrode containing a negative electrode active material such as a carbon material like graphite that can intercalate and deintercalate 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 manufacturing electrodes that can increase the capacity density of secondary batteries. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] WO2020 / 128699 pamphlet [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 LixCoO2(0.0≦x≦1.0)”, Physical Review B, 80(16);165114 [Non-Patent Document 3] Zhaohui Chen et al, “Staging Phase Transitions in LixCoO2”, 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. [Overview of the project] [Problems that the invention aims to solve]

[0011] The objectives are to realize a manufacturing method that enables high capacity density of secondary batteries, to realize a manufacturing method that enables rapid charging and rapid discharging of secondary batteries, and to provide a manufacturing method for secondary batteries that is safe and reliable.

[0012] Electrodes (positive and negative electrodes) for lithium-ion secondary batteries are manufactured by coating a slurry containing particulate active material onto a metal foil called a current collector and drying it. Electrodes manufactured in this way have an active material layer on the current collector. The active material layer contains active material and voids, and minimizing the voids is necessary to increase the capacity density of secondary batteries. By using electrodes with fewer voids, a larger battery capacity can be obtained even with the same volume of secondary battery, improving the capacity density per unit volume. Electrodes with an active material layer with fewer voids are sometimes called high-density electrodes, densified electrodes, or electrodes with high film density.

[0013] Electrodes for lithium-ion secondary batteries are desirable to have good electron conduction paths at the interface between the current collector and the active material layer, and within the active material layer. Furthermore, it is desirable for the active material layer to have good lithium-ion conduction paths in regions adjacent to the separator or solid electrolyte layer. Electrodes with good electron conduction paths and good lithium-ion conduction paths are suitable for rapid charging and rapid discharging; however, electrode structures and methods for fabricating such electrodes are not yet sufficiently established. Another challenge is to provide structures and methods for fabricating high-capacity-density electrodes that possess good electron conduction paths and good lithium-ion conduction paths.

[0014] Furthermore, the description of these problems does not preclude the existence of other problems. Moreover, one aspect of the present invention does not need to solve all of these problems. It is possible to extract other problems from the description, drawings, and claims. [Means for solving the problem]

[0015] One aspect of the present invention is a battery having 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 having a first active material having a first particle size, and the second layer having a second active material having a second particle size, the first particle size being smaller than the second particle size.

[0016] Furthermore, one aspect of the present invention is a battery having 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 having a first active material having a first particle size, and the second layer having a second active material having a second particle size, the first particle size being smaller than the second particle size, and the sphericity of the second active material being 0.8 or more and 1.0 or less.

[0017] Furthermore, one aspect of the present invention is a battery having 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, and the second layer has a second active material having a second particle size, the first particle size being smaller than the second particle size, the second active material having a surface layer and an interior, the surface layer being a region of 10 nm or less from the surface to the interior of the second active material, and the surface layer and the interior being topotaxy.

[0018] Furthermore, one aspect of the present invention is a battery having 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, the first particle size is smaller than the second particle size, the second active material has a surface layer and an interior, the surface layer is a region of 10 nm or less from the surface to the interior of the second active material, the surface layer and the interior are both topotaxy, and the sphericity of the second active material is 0.8 or more and 1.0 or less.

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

[0020] In the case where a current collector has a first layer and a second layer has a second layer 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] In the case where a current collector has a first layer and a second layer has a second layer on the first layer, it is preferable that the first layer and the second layer have 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 the battery described in any one of the above, it is preferable that a second layer is provided on the current collector and a first layer is provided on the second layer.

[0023] In the case where a second layer is placed on the current collector and a first layer is placed on the second 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 first layer is greater than the mass of the conductive material in the second layer.

[0024] In the case where a second layer is placed on the current collector and a first layer is placed on the second layer, it is preferable that the first layer and the second layer contain 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 aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein the positive electrode has a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer, the first layer having a first active material having a first particle size, the second layer having a second active material having a second particle size, and the third layer having a third active material having a third particle size, the first particle size being smaller than the second particle size, and the third particle size being smaller than the second particle size.

[0026] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein the positive electrode comprises a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer, the first layer having a first active material having a first particle size, the second layer having a second active material having a second particle size, and the third layer having a third active material having a third particle size, the first particle size being smaller than the second particle size, the third particle size being smaller than the second particle size, and the sphericity of the second active material being 0.8 or more and 1.0 or less.

[0027] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein the positive electrode has a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer, the first layer having a first active material having a first particle size, the second layer having a second active material having a second particle size and a fourth active material having a fourth particle size, and the third layer having a third active material having a third particle size, the first particle size being smaller than the second particle size, the third particle size being smaller than the second particle size, and the fourth particle size being smaller than the second particle size.

[0028] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein 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 and a fourth active material having a fourth particle size, and 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 fourth particle size is smaller than the second particle size, and the sphericity of the second active material is 0.8 or more and 1.0 or less.

[0029] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein 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, and the third layer has a third active material having a third particle size, the first and third particle sizes are smaller than the second particle size, the second active material has a surface layer and an interior, the surface layer is a region of 10 nm or less from the surface of the second active material toward the interior, and the surface layer and the interior are topotaxy.

[0030] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein the positive electrode comprises a current collector, a first layer on the current collector, a second layer on the first layer, and a third layer on the second layer, the first layer having a first active material having a first particle size, the second layer having a second active material having a second particle size, and the third layer having a third active material having a third particle size, the first and third particle sizes being smaller than the second particle size, the second active material having a surface layer and an interior, the surface layer being a region of 10 nm or less extending from the surface to the interior of the second active material, the surface layer and the interior being topotaxy, and the sphericity of the second active material being 0.8 or more and 1.0 or less.

[0031] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein 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 and a fourth active material having a fourth particle size, and the third layer has 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 layer and an interior, the surface layer is a region of 10 nm or less from the surface of the second active material toward the interior, and the surface layer and the interior are topotaxy.

[0032] Furthermore, one aspect of the present invention is a battery having a positive electrode and a negative electrode, wherein 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 and a fourth active material having a fourth particle size, and the third layer has 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 layer and an interior, the surface layer is a region of 10 nm or less from the surface to the interior of the second active material, the surface layer and the interior are both topotaxy, and the sphericity of the second active material is 0.8 or more and 1.0 or less.

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

[0034] In a battery having a first layer, a second layer, and a third layer as described in any one of the above, it is preferable that the first layer, the second layer, and the third layer each have 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, and the mass of the solid electrolyte in the second layer is greater than the mass of the solid electrolyte in the third layer.

[0035] In a battery having a solid electrolyte as described in any one of the above, the second active material has a surface layer and an interior, the surface layer is a region of 10 nm or less extending from the surface of the second active material inward, and if the surface layer and the interior are topotaxy, it is preferable that the edge surface of the second active material has a region in contact with the surface layer and the solid electrolyte.

[0036] One aspect of the present invention is a mobile body having the battery described in any one of the above.

[0037] One aspect of the present invention is an energy storage system having the battery described in any one of the above.

[0038] One aspect of the present invention is an electronic device having the battery described in any one of the above. [Effects of the Invention]

[0039] We can provide secondary batteries that enable high capacity density. Furthermore, we can provide secondary batteries that enable rapid charging and rapid discharging. We can also provide secondary batteries that are safe and reliable.

[0040] According to one aspect of the present invention, it is possible to provide a positive electrode active material or composite oxide in which the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Alternatively, it is possible to provide a positive electrode active material or composite oxide in which the crystal structure is less likely to collapse even after repeated charge / discharge cycles. Alternatively, it is possible to provide a positive electrode active material or composite oxide with a large charge / discharge capacity. Alternatively, it is possible to provide a secondary battery with high safety or reliability.

[0041] This method enables the fabrication of secondary batteries with high capacity density. It also enables the fabrication of secondary batteries capable of rapid charging and rapid discharging. Furthermore, it provides a method for fabricating safe and reliable secondary batteries. Alternatively, it enables a fabrication method that reduces defects in the active material in sufficiently high-density electrodes. High-density electrodes with fewer active material defects enable the realization of superior secondary batteries that meet high capacity density, high performance, and safety requirements in various operating environments.

[0042] Furthermore, the description of these effects does not preclude the existence of other effects. Moreover, one embodiment of the present invention does not need to possess all of these effects. Other effects will naturally become apparent from the description in the specification, drawings, and claims, and it is possible to extract other effects from the description in the specification, drawings, and claims. [Brief explanation of the drawing]

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

[0044] Embodiments of the present invention will be described in detail below with reference to the drawings. However, it will be readily apparent to those skilled in the art that the present invention is not limited to the following description, and its form and details can be modified in various ways. Furthermore, the present invention is not to be interpreted as being limited to the embodiments described below.

[0045] Furthermore, in drawings, size, layer thickness, or area may be exaggerated for clarity. Therefore, the scale is not necessarily limited.

[0046] Furthermore, the ordinal numbers used in this specification, such as "first," "second," etc., are for convenience only and do not indicate the order of processes or stacking. Therefore, for example, "first" can be replaced with "second" or "third," etc., as appropriate in the explanation. Also, the ordinal numbers described in this specification may not be the same as the ordinal numbers used to specify an aspect of the present invention.

[0047] In this specification, the term "particle" is not limited to spherical shapes (circular cross-sections), but may also refer to individual particles with elliptical, rectangular, trapezoidal, triangular, rounded quadrilateral, or asymmetrical cross-sections, and individual particles may also have irregular shapes.

[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, or median diameter, at which the integrated amount accounts for 50% of the integrated particle amount curve of the particle size distribution measurement result. The measurement of particle size is not limited to laser diffraction particle size distribution measurement; if it 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 such as SEM (scanning electron microscope) or TEM (transmission electron microscope). As a method for measuring the particle size when the cross-sectional shape of the particle is not circular, for example, the area of ​​the particle cross-section can be measured by image processing, and the particle size can be calculated as the diameter of a circle having that area.

[0049] In this specification and the like, the space group is represented using the Short notation of the international notation (or Hermann-Mauguin symbol). Also, crystal planes and crystal directions are represented using Miller indices. Individual planes indicating crystal planes are represented using ( ). In crystallography, bars are attached above numbers when representing the space group, crystal planes, and crystal directions, but in this specification and the like, due to formatting constraints, instead of attaching a bar above the number, a -(minus sign) may be attached before the number for representation. Also, individual orientations indicating directions within a crystal are represented using [ ], collective orientations indicating all equivalent directions are represented using < >, individual planes indicating crystal planes are represented using ( ), and collective planes having equivalent symmetries are represented using {}, respectively. Also, a trigonal crystal represented by the space group R-3m is generally represented by a composite hexagonal lattice of a hexagonal crystal for easier understanding of the structure, and (hkil) may be used in addition to (hkl) as Miller indices. Here, i is -(h + k).

[0050] The theoretical capacity of the positive electrode active material refers to the amount of electricity when all the insertable and removable lithium in the positive electrode active material has been removed. For example, the theoretical capacity of LiCoO₂ is 274 mAh / g, the theoretical capacity of lithium nickelate (LiNiO₂) is 275 mAh / g, and the theoretical capacity of lithium manganate (LiMn₂O₄) is 148 mAh / g.

[0051] Also, the degree to which insertable and removable lithium remains in the positive electrode active material is represented by x in the composition formula, for example, x in Li x CoO₂, or x in Li x 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 the 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, it can be said that Li 0.2 CoO₂ or x = 0.2. A small x in Li x CoO₂ means, for example, 0.1 < x ≤ 0.24. Note that the transition metal M can be selected from the elements described in Groups 4 to 13 shown in the periodic table, and for example, at least one of manganese, cobalt, and nickel is used.

[0052] When lithium cobalt oxide approximately satisfies the stoichiometric ratio, it is LiCoO2 and the lithium occupancy rate x=1 at the lithium sites. Similarly, a secondary battery that has finished discharging is also LiCoO2, and it can be said that x=1. Discharge completion here refers to the state where, for example, with a current of 100mA / g, the voltage falls below 2.5V (vs. counter electrode Li). In lithium-ion secondary batteries, when the lithium occupancy rate at the lithium sites becomes x=1 and no more lithium can be added, the voltage drops sharply. At this point, discharge can be said to have finished. Generally, in lithium-ion secondary batteries using LiCoO2, the discharge voltage drops sharply before reaching 2.5V, so discharge is considered to have finished under the above conditions.

[0053] Li x The charging and / or discharging capacities used to calculate x in CoO2 should preferably be measured under conditions where there is little or no influence from short circuits and / or electrolyte decomposition. For example, it is preferable not to use data from secondary batteries that have experienced a sudden change in capacity that appears to be due to a short circuit in calculating x.

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

[0055] Furthermore, in layered rock salt crystals and rock salt crystals, if the A, B, and C layers containing anions are stacked offset from each other in a structure like ABCABC, it will be called a cubic close-packed structure. Therefore, the anions do not need to be strictly in a cubic lattice. At the same time, since real crystals always have defects, the analysis results do not necessarily conform to theory. For example, in FFT (Fast Fourier Transform) patterns such as electron diffraction patterns or TEM images, spots may appear at positions slightly different from the theoretical positions. For example, if the orientation from the theoretical position is 5 degrees or less, or 2.5 degrees or less, it can be said that it has a cubic close-packed structure.

[0056] Homogeneity, in a solid composed of multiple elements (e.g., A, B, C), refers to the phenomenon where a particular element (e.g., A) is distributed in specific regions with similar characteristics. It is sufficient that the elemental concentrations in these specific regions are substantially the same. For example, the difference in elemental concentrations between specific regions should be within 10%. Examples of specific regions include the surface, surface layer, convex areas, concave areas, and interior.

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

[0058] Furthermore, positive electrode active materials to which additive elements have been added may be expressed as composite oxides, positive electrode materials, positive electrode materials, positive electrode materials for secondary batteries, etc. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention has a compound. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention has a composition. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention has a composite.

[0059] As the charging voltage of a secondary battery increases, the voltage at the positive electrode generally rises. The positive electrode active material according to one aspect of the present invention has a stable crystal structure even at high voltages. The stability of the crystal structure of the positive electrode active material in the charged state suppresses the decrease in charge / discharge capacity that occurs with repeated charging and discharging.

[0060] (Embodiment 1) This embodiment describes an electrode according to one aspect of the present invention and a method for manufacturing the same.

[0061] [Laminated electrode 1] An electrode according to one embodiment of the present invention will be described with reference to Figures 1A to 6D. The electrode according to one embodiment of the present invention can be used as either a positive electrode or a negative electrode, or both. In the case of a positive electrode, a positive electrode active material is used as the active material, and in the case of a negative electrode, a negative electrode active material is used as the active material. As the active material used in the electrode according to one embodiment of the present invention, the active materials shown in Embodiments 1 to 4 can be used.

[0062] Schematic diagrams of an electrode having a laminated structure according to one aspect of the present invention, viewed from the side, are shown in Figures 1A, 5A, and 5B. Furthermore, particularly preferred examples of the positive electrode active material of the positive electrode having a laminated structure according to one aspect of the present invention are described in Figures 1B, 2A to 2D, 3, and 4A to 4C.

[0063] As an example of a two-layer electrode according to one aspect of the present invention, electrode 400A is shown in Figure 1A. Electrode 400A has an active material layer 414 on a current collector 413, and the active material layer 414 is a two-layer electrode having a first layer 414a and a second layer 414b. The first layer 414a has a first active material 411a, and the second layer 414b has a second active material 411b.

[0064] Preferably, the particle size Ra of the first active material 411a in the first layer 414a is 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. Here, the particle size of the active material refers to the median diameter of the active material, which can be measured using any one of the measurement methods described above.

[0065] Here, the size ratio of the first active material 411a to the second active material 411b, namely 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. When the first active material 411a and the second active material 411b satisfy the relationship shown above, 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 interfacial resistance between the current collector 413 and the active material layer 414.

[0067] Although not shown in Figure 1A, the first layer 414a and the second layer 414b may contain a conductive material and a binder, which will be described later. Alternatively, the first layer 414a and the second layer 414b may contain a conductive material, a binder, and a solid electrolyte, which will be described later.

[0068] Here, the thickness of the first layer 414a is preferably 1 μm to 20 μm, more preferably 1 μm to 10 μm. This is because, although it is important for the first layer 414a to reduce interfacial resistance, the first active material 411a in the first layer 414a has a small particle size in the active material layer 414 and contributes little to the efficient storage of lithium ions, so it is preferable to make the thickness of the 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 therefore contributes significantly to the efficient storage of lithium ions. Furthermore, as will be described later in Figure 6, using large-particle active material in combination with small-particle and medium-particle active material enables more efficient storage of lithium ions. In other words, the volumetric capacity density of the electrode can be increased. For this reason, the thickness of the second layer 414b in the active material layer 414 is preferably 10 μm to 200 μm, and more preferably 20 μm to 150 μm. The higher the proportion of the second layer 414b in the active material layer 414, the higher the volumetric capacity density of the electrode 400A can be.

[0070] In Figure 1A and other figures, the cross-sectional shape of the first active material 411a in the first layer 414a is schematically represented as a circle or a perfect circle for ease of understanding. In reality, the cross-sectional shape of the active material may be other than a circle or a perfect circle (such as a shape with irregularities or an ellipse), but these shapes are also included in one aspect of the present invention.

[0071] [Positive electrode active material in a layered positive electrode structure] As a particularly preferred example of a positive electrode active material in a laminated structure positive electrode according to one aspect of the present invention, Figure 1B shows a positive electrode active material 100 having a topotaxy region in its surface layer. Figures 2A and 2B show enlarged views of the area around AB in Figure 1B. Figures 2C and 2D show enlarged views of the area around CD in Figure 1B. Now, an example of a positive electrode active material 100 having a topotaxy region in its surface layer will be described.

[0072] Figure 1B shows dotted lines indicating crystal planes parallel to the arrangement of cations. The arrows indicate the direction of lithium insertion and removal during charging and discharging. Here, the arrangement of cations refers to the arrangement of cations other than lithium, represented by the transition metal M, which is easily observable in STEM images, etc. The crystal planes parallel to the arrangement of cations refer to crystal planes parallel to the direction in which lithium ions can diffuse. As shown in Figures 2A to 2D, the positive electrode active material 100 has a surface layer 100a and an interior layer 100b. In these figures, the boundary between the surface layer 100a and the interior layer 100b is indicated by a dashed line. Although not shown, the positive electrode active material 100 may also have crystal 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 interior, more preferably within 35 nm from the surface toward the interior, even more preferably within 20 nm from the surface toward the interior, and most preferably within 10 nm from the surface toward the interior. Surfaces formed by cracks and / or fissures may also be considered the surface. The surface layer 100a is synonymous with the vicinity of the surface, the vicinity of the surface region, or the shell.

[0074] Furthermore, the region deeper than the surface layer 100a of the positive electrode active material is called the interior 100b. Interior 100b is synonymous with the interior region or core.

[0075] Furthermore, 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, etc. Therefore, the positive electrode active material 100 does not contain carbonates, hydroxyl groups, etc. that have been chemically adsorbed after fabrication. It also does not contain electrolytes, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 100. Furthermore, the surface of the positive electrode active material 100 in cross-sectional STEM (scanning transmission electron microscope) images, etc., refers to the boundary between the region where the electron beam coupling 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 lithium are confirmed. The surface in cross-sectional STEM images, etc., may also be determined in conjunction with the results of analyses with higher spatial resolution, such as electron energy loss spectroscopy (EELS).

[0076] Furthermore, grain boundaries refer to areas where positive electrode active materials 100 are fixed together, areas where the crystal orientation changes within the positive electrode active material 100, i.e., areas where the repetition of bright and dark lines in STEM images becomes discontinuous, areas containing many crystal defects, and areas where the crystal structure is disordered. Crystal defects refer to defects observable by cross-sectional TEM (transmission electron microscope), cross-sectional STEM images, i.e., structures where other elements are inserted between lattice spaces, cavities, etc. A grain boundary can be considered a type of surface defect. Furthermore, the vicinity of a grain boundary refers to the region within 10 nm of the grain boundary.

[0077] <epitaxy, topotaxy> Preferably, the positive electrode active material 100 has a crystal structure that changes continuously 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. In the following, a configuration in which the crystal orientations are the same or approximately the same may simply be described as "the crystal orientations are approximately the same." Alternatively, it is preferable that the surface layer 100a and the interior 100b are topotaxy.

[0078] Topotaxy refers to a three-dimensional structural similarity where the orientation of the crystals is roughly the same, or to having the same crystallographic orientation. Epitaxy, on the other hand, refers to a structural similarity at a two-dimensional interface.

[0079] The topotaxy nature of the surface layer 100a and the interior 100b reduces distortion of the crystal structure and / or displacement of atomic arrangement. This suppresses the formation of pits. Furthermore, the presence of additive elements in the surface layer 100a suppresses displacement of the layered structure consisting of octahedra of the transition metal M and oxygen, as described later, and / or suppresses the desorption of oxygen from the positive electrode active material 100. Therefore, a positive electrode active material that does not degrade even when charged at high voltage and charged and discharged in high-temperature environments can be obtained. In other words, a positive electrode active material 100 having topotaxy in its surface layer can be said to be a positive electrode active material that does not degrade even when charged at high voltage and charged and discharged in high-temperature environments. In this specification, a pit refers to a hole formed in the positive electrode active material due to the progression of a defect.

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

[0081] In this specification, the layered rock salt crystal structure belonging to space group R-3m, which is found in composite oxides containing lithium and transition metals M including cobalt, refers to a crystal structure that has a rock salt-type ionic arrangement in which cations and anions are arranged alternately, and in which the transition metal M and lithium are arranged regularly to form a two-dimensional plane, thereby enabling two-dimensional diffusion of lithium. Defects such as vacancies in cations or anions may be present. Furthermore, strictly speaking, the layered rock salt crystal structure may have a distorted lattice structure of the rock salt crystal.

[0082] Furthermore, a rock salt-type crystal structure refers to a cubic crystal structure, including the space group Fm-3m, in which cations and anions are arranged alternately. It is also acceptable for there to be vacancies in either the cation or anion.

[0083] Furthermore, the presence of characteristics of both layered rock salt and rock salt crystal structures can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc.

[0084] In rock salt-type MgO, there is no distinction in the sites of cations, but in layered rock salt-type MgO, there are two types of cation sites in the crystal structure: one is mostly occupied by lithium, and the other by transition metal M. The layered structure, in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately, is the same for both rock salt-type and layered rock salt-type MgO. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmission spot) is taken as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in ideal rock salt-type MgO and the (003) plane in layered rock salt-type MgO. For example, when comparing the electron diffraction patterns of rock salt-type MgO and layered rock salt-type LiCoO2, the bright spot on the (003) plane of LiCoO2 is observed at approximately half the distance of the bright spot on the (111) plane of MgO. Therefore, when the analysis region contains two phases, for example, rock salt type MgO and layered rock salt type LiCoO2, the electron diffraction pattern will show plane orientations in which bright spots of high and low brightness are arranged alternately. Bright spots common to both rock salt type and layered rock salt type will have high brightness, while bright spots occurring only in the layered rock salt type will have low brightness.

[0085] Furthermore, in cross-sectional STEM images, when a layered rock salt crystal structure is observed from a direction perpendicular to the c-axis, layers with high brightness and layers with low brightness are observed alternately. This feature is not seen in rock salt crystals because there is no distinction in the sites of cations. In the case of a crystal structure that possesses characteristics of both rock salt and layered rock salt crystals, when observed from a specific crystal orientation, layers with high brightness and layers with low brightness are observed alternately in cross-sectional STEM images, and furthermore, a metal with an atomic number greater than lithium is present in a part of the low-brightness layer, i.e., the lithium layer.

[0086] Layered rock salt crystals and the anions in rock salt crystals adopt a cubic close-packed structure (face-centered cubic lattice structure). It is also presumed that the anions in the O3' type crystals, which will be discussed later, adopt a cubic close-packed structure. Therefore, when layered rock salt crystals and rock salt crystals are in contact, there exists a crystal plane in which the orientation of the cubic close-packed structure composed of anions is aligned.

[0087] Alternatively, it can be explained as follows: The anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt type has a space group R-3m and a rhombohedral structure, but to facilitate understanding of the structure, it is generally represented as a composite hexagonal lattice, and the (0001) plane of the layered rock salt type has a hexagonal lattice. The triangular lattice of the {111} plane of the cubic crystal has a similar atomic arrangement to the hexagonal lattice of the (0001) plane of the layered rock salt type. The consistency between the two lattices can be described as the orientation of the cubic close-packed structure being aligned.

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

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

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

[0091] For example, in high-resolution images of TEM, etc., contrast derived from crystal planes can be obtained. Due to the diffraction and interference of the electron beam, for example, when the electron beam is incident perpendicular to the c-axis of the composite hexagonal lattice of the layered rock salt type, the contrast derived from the (0003) plane is obtained as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, the repetition of bright and dark lines is observed in the TEM image, and between bright lines (for example, L shown in Fig. 3 RS and L LRSIf the angle between the bright and 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 coincidental, that is, the crystal orientations are roughly coincidental. Similarly, if 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 coincidental. In general, it is difficult to clearly distinguish between "coincidence" and "approximate coincidence." Therefore, in this specification, "coincidence" includes both cases where there is a perfect coincidence (for example, when the angle between the bright lines is 0 degrees) and cases where there is an approximate coincidence.

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

[0093] Therefore, in HAADF-STEM images, if the repetition of bright and dark lines is 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 arrangement is roughly consistent, i.e., the crystal orientation is roughly consistent. 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 orientation is roughly consistent.

[0094] In ABF-STEM, elements with smaller atomic numbers appear brighter, but since contrast corresponding to atomic number is obtained, similar to HAADF-STEM, the orientation of the crystal can be determined in the same way as with HAADF-STEM images.

[0095] Figure 4A shows an example of a STEM image where the orientations of layered rock salt crystals (LRS) and rock salt crystals (RS) are roughly consistent. Figure 4B shows the FFT of the region of rock salt crystals (RS), and Figure 4C shows the FFT of the region of layered rock salt crystals (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 it. The right side shows the measured values. Spots marked with O represent zero-order diffraction, and the center position of the spot is marked with X.

[0096] The spot labeled A in Figure 4B originates from the 11-1 reflection of the cubic crystal. The spot labeled A in Figure 4C originates from the 0003 reflection of the layered rock salt type. From Figures 4B and 4C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type are roughly coincide. That is, the line passing through AO in Figure 4B and the line passing through AO in Figure 4C are roughly parallel. Here, roughly coincidental and roughly parallel means that the angle is between 0 degrees and 5 degrees, or between 0 degrees and 2.5 degrees.

[0097] Thus, in FFT and electron diffraction, if the orientations of the layered rock salt crystal and the rock salt crystal are roughly the same, 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. The fact that the reciprocal lattice points are spot-like and not continuous with other reciprocal lattice points means that the crystallization is high.

[0098] Furthermore, as mentioned above, if 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, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt type may be observed in a reciprocal space different from the orientation of the 0003 reflection of the layered rock salt type. For example, the spot labeled B in Figure 4C originates from the 1014 reflection of the layered rock salt type. This spot may be observed at an angle of 52° to 56° from the orientation of the reciprocal point (A in Figure 4C) originating from the 0003 reflection of the layered rock salt type (i.e., ∠AOB is between 52° and 56°), and where d is between 0.19 nm and 0.21 nm. Note that this index is just an example and does not necessarily have to match. For example, equivalent reciprocal points in each case would also be acceptable.

[0099] Similarly, spots not originating from the cubic 11-1 reflection may be observed in a different reciprocal space from the spot where the cubic 11-1 reflection was observed. For example, the spot labeled B in Figure 4B originates from the cubic 200 reflection. This is because diffraction spots may be observed at an angle between 54° and 56° from the orientation of the cubic 11-1 reflection (A in Figure 4B) (i.e., ∠AOB is between 54° and 56°). Note that these Miller indices are just examples and do not necessarily have to match. For example, equivalent reciprocal points in each case would also be acceptable.

[0100] It is known that layered rock salt type cathode active materials, including lithium cobalt oxide, tend to exhibit (0003) planes and equivalent planes, as well as (10-14) planes and equivalent planes, as crystal planes. Therefore, by carefully observing the shape of the cathode active material with an SEM or similar device, it is possible to thin-section the observation sample using a FIB or similar device so that the electron beam is incident at [12-10] in a TEM or similar device, making the (0003) plane easier to observe. When it is necessary to determine the consistency of the crystal orientation, it is preferable to thin-section the layered rock salt type so that the (0003) plane is easily observable.

[0101] The electrode 400A shown in Figure 1A preferably has a positive electrode active material 100 having a topotaxy region in its surface layer. In other words, it is preferable that either one or both of the first active material 411a and the second active material 411b have a positive electrode active material 100 having a topotaxy region in its surface layer.

[0102] As described above, a positive electrode having a laminated structure according to one embodiment of the present invention preferably has a positive electrode active material 100 having a topotaxy region in its surface layer. For example, a composite oxide having an additive element can be used as the positive electrode active material 100 having a topotaxy region in its surface layer. Details of the composite oxide having an additive element will be described in Embodiment 2. Below, an overview of the composite oxide having an additive element and its use in the positive electrode of a laminated 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 layered structure. The crystals are not limited to single crystals (also called crystallites); if they are polycrystalline, several crystallites gather to form primary particles. A primary particle refers to a particle that is recognized as a single particle during SEM observation. Secondary particles refer to aggregates of primary particles. The bonding forces acting between multiple primary particles are not limited to covalent bonds, ionic bonds, hydrophobic interactions, van der Waals forces, or other intermolecular interactions, and multiple bonding forces may be at work.

[0104] When producing a composite oxide using the manufacturing method shown in Embodiment 2, non-aggregating primary particles are often formed, but secondary particles having a small number (e.g., fewer than 10) primary particles may be formed. As shown in Figures 2A to 2D, it is preferable that the composite oxide produced using the manufacturing method shown in Embodiment 2 has additive elements in its surface layer. As additive elements in the surface layer of the composite oxide, one or more can be 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. Furthermore, it is preferable to have one or more additive elements selected from bromine and beryllium. However, since bromine and beryllium are toxic to living organisms, it is preferable to use the additive elements mentioned above.

[0105] As composite oxides having additive elements, for example, 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 and nickel and fluorine, lithium cobalt oxide having magnesium, aluminum, nickel and barium, and lithium cobalt oxide having magnesium, aluminum, nickel, barium and fluorine, as shown in Embodiment 2, can be used.

[0106] When producing a composite oxide using the coprecipitation method shown in Embodiment 3, secondary particles having a large number (e.g., 10 or more) primary particles may be formed.

[0107] The crystal having the above-described hexagonal layered structure has one or more selected from a first transition metal, a second transition metal, and a third transition metal. Specifically, the first transition metal is nickel, the second transition metal is cobalt, the third transition metal is manganese, and LiNi x Co y Mn z O2 (x > 0, y > 0, 0.8 < x + y + z < 1.2), which is also referred to as NiCoMn (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 necessary. 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 laminated structure according to one aspect of the present invention will be described. Figure 1A shows a two-layer electrode as an example of an electrode having a laminated structure according to one aspect of the present invention. Here, Figure 5A shows a schematic diagram of a three-layer electrode viewed from the side as another example of an electrode having a laminated structure according to one aspect of the present invention. The three-layer electrode 400B according to one aspect of the present invention has an active material layer 414 on a current collector 413, and the active material layer 414 has a first layer 414a, a second layer 414b, and a third layer 414c. The active material layer 414 has a first layer 414a, a second layer 414b on the first layer 414a, and a third layer 414c on the second layer 414b. The first layer 414a has a first active material 411a, the second layer 414b has a second active material 411b, and the third layer 414c has a third active material 411c. Similar to the electrode 400A shown in Figure 1A, the electrode 400B shown in Figure 5A preferably has a positive electrode active material 100 having a topotaxy region in its surface layer. In other words, it is preferable that the positive electrode active material 100 has a topotaxy region in its surface layer, which is one or more of the first active material 411a, the second active material 411b, and the third active material 411c.

[0110] Preferably, the particle size Ra of the first active material 411a in the first layer 414a is smaller than the particle size Rb of the second active material 411b in the second layer 414b. Also, preferably, the particle size Rc of the third active material 411c in the third layer 414c is 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 Rc of the third active material 411c in the third layer 414c is preferably 500 nm or more and 5 μm or less, more preferably 1 μm or more and 5 μm or less. Here, the particle size of the active material refers to the median diameter of the active material, which can be measured using any one of the measurement methods described above.

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

[0112] Although not shown in Figure 5A, the first layer 414a, the second layer 414b, and the third layer 414c may contain a conductive material and a binder, as described later. Alternatively, the first layer 414a, the second layer 414b, and the third layer 414c may contain a conductive material, a binder, and a solid electrolyte, as described later.

[0113] If 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 relationship described above, then 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 interfacial resistance between the current collector 413 and the active material layer 414.

[0115] Here, the thickness of the 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 the first layer 414a to reduce interfacial resistance, the first active material 411a in the first layer 414a has a small particle size in the active material layer 414 and contributes little to the efficient storage of lithium ions, so it is preferable to make the thickness of the first layer 414a thinner.

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

[0117] Next, the relationship between the second layer 414b and the third layer 414c will be explained. Because the third layer 414c is farther from the current collector 413 than the first layer 414a and the second layer 414b, it has higher electronic resistance and a relatively lower potential, which tends to slow down the battery reaction rate. For example, unlike the electrode structure of one embodiment of the present invention, if the third active material 411c in the third layer 414c and the second active material 411b in the second layer 414b have similar particle sizes, the battery reaction will be slower in the region farther from the current collector 413 (the position corresponding to the third layer 414c), resulting in uneven reaction in the active material layer 414. As a result, when rapid charging and rapid discharging are performed, the chargeable and dischargeable capacity will decrease.

[0118] In the electrode structure of one embodiment of the present invention described above, the particle size of the third active material 411c in the third layer 414c is smaller than that of the second active material 411b in the second layer 414b. As a result, the rate of the battery reaction in the third layer 414c is higher than the rate of the battery reaction in the second layer 414b. Therefore, it is possible to reduce reaction unevenness in the active material layer 414 during rapid charging and rapid discharging, and as a result, it is possible to suppress a decrease in the chargeable and dischargeable capacity even when rapid charging and rapid discharging are performed.

[0119] The second active material 411b in the second layer 414b has the largest particle size in the active material layer 414, and therefore contributes significantly to the efficient storage of lithium ions. Furthermore, as will be described later in Figure 6, using large-particle active material in combination with small-particle and medium-particle active material enables more efficient storage of lithium ions. In other words, the volumetric capacity density of the electrode can be increased. For this reason, the thickness of the second layer 414b in the active material layer 414 is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 150 μm or less. The higher the proportion of the second layer 414b in the active material layer 414, the higher the volumetric capacity density of the electrode 400B can be.

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

[0121] As described above, Figure 5A shows an electrode 400B having a three-layer laminated structure consisting of a first layer 414a, a second layer 414b, and a third layer 414c. The structure of an electrode having a third layer 414c in one aspect of the present invention is not limited to the three-layer structure described above. For example, as shown in Figure 5B, an electrode 400C may have a two-layer laminated structure consisting of a second layer 414b and a third layer 414c. In electrode 400C as well, rapid charging and rapid discharging are possible due to the relationship between the second layer 414b and the third layer 414c, as described above for electrode 400B.

[0122] Next, the second active material 411b of the second layer 414b will be explained using Figures 6A to 6D.

[0123] The second active material 411b in the second layer 414b has the largest particle size in the active material layer 414 and contributes significantly to the efficient storage of lithium ions. Ideally, the second layer 414b should have a dense structure as shown in Figure 6A, but in reality, it often has a structure with gaps as shown in Figure 6B. Therefore, as shown in Figure 6C, when a fourth active material 411d with small particle size is present in addition to the second active material 411b with large particle size, it is easier to adopt a structure in which the fourth active material 411d is placed between the particles of the second active material 411b, resulting in a higher density of the second layer 414b. Furthermore, as shown in Figure 6D, when there are small-particle fourth active material 411d and medium-particle fifth active material 411e in addition to the large-particle second active material 411b, it is easier to form a structure in which the fourth active material 411d and fifth active material 411e are placed between the particles of the second active material 411b, and as a result the density of the second layer 414b increases.

[0124] Here, by pressing the second layer 414b-2 shown in Figure 6B, it is possible to obtain a second layer 414b with a higher density than that shown in Figure 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 will crack. Here, if the sphericity of the second active material 411b is high, the second active material 411b is less likely to crack even when pressed at higher pressures. In other words, it is easier to obtain a high-density second layer 414b. Therefore, it is preferable that the sphericity of the second active material 411b is high.

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

[0126]

number

[0127] The sphericity of the first active material 411a, the second active material 411b, and the 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, in the second active material 411b, as shown in Figures 6C and 6D, which uses a second active material 411b with a large particle size, along with a fourth active material 411d with a small particle size and / or a fifth active material 411e with a medium particle size, it is possible to further increase the density of the second layer 414b by pressing. In this case as well, as described above, it is preferable to have an active material with high sphericity. Therefore, the sphericity of the fourth active material 411d with a small particle size and the fifth active material 411e with 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, it is preferable that the particle size of the second active material 411b is larger than that of the fifth active material 411e, and the particle size of the fifth active material 411e is larger than that of the fourth active material 411d. As for the particle size of the active material in the second layer 414b, for example, the particle size of the second active material 411b is preferably 1 μm or more and 35 μm or less, more preferably 5 μm or more and 25 μm or less, as described above. The particle size of the fourth active material 411d in the second layer 414b is preferably 500 nm or more and 5 μm or less, more preferably 1 μm or more and 5 μm or less. Furthermore, the particle size of the fifth active material 411e in the second layer 414b is preferably 1 μm or more and 20 μm or less, more preferably 5 μm or more and 15 μm or less.

[0130] When the second layer 414b has a second active material 411b (large particle size) and a fourth active material 411d (small particle size), if 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, then Ma is preferably 0.05 or more and 0.5 or less, more preferably 0.1 or more and 0.4 or less. Furthermore, if 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), it is preferable that 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, where Mb is between 0.1 and 0.5, and Mc is between 0.1 and 0.5.

[0131] The above examples show electrodes with a two-layer structure and electrodes with a three-layer structure, but electrodes with four or more layers are also possible. For example, in the two-layer electrode shown in Figure 1A, there may be a layer with medium-sized active material between the layer with small-sized active material (first layer 414a) and the layer with large-sized active material (second layer 414b). Also, in the three-layer electrode shown in Figure 5A, there may be a layer with medium-sized active material between the layer with small-sized active material (first layer 414a) and the layer with large-sized active material (second layer 414b), and a layer with medium-sized active material between the layer with large-sized active material (second layer 414b) and the layer with small-sized active material (third layer 414c), resulting in a five-layer electrode.

[0132] When a composite oxide having the above-mentioned additive elements is used as the positive electrode of a laminated structure according to one aspect 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 have the same type (and combination) of additive elements, or they may be different. 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 have the same concentration of additive elements, or they may be different.

[0133] For example, the structure may be such that the second active material 411b in the second layer 414b is lithium cobalt oxide containing magnesium, aluminum, and nickel, and the third active material 411c in the third layer 414c is lithium cobalt oxide containing magnesium.

[0134] Furthermore, for example, the concentrations of the additive elements 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 a higher concentration than the concentrations of the additive elements 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 was described, but the active material layer 414 may have one or more of the conductive material, binder, and solid electrolyte described later. Furthermore, the positive electrode active material and negative electrode active material described in Embodiments 2 to 4 can be used as the active material in the active material layer 414.

[0136] [Method for fabricating a layered electrode 1] An example of a method for manufacturing electrodes according to one aspect of the present invention will be explained with reference to Figures 7 to 10.

[0137] Figure 7 shows an example of an electrode fabrication method in which a first layer 414a is fabricated in steps S11 to S21, a second layer 414b is fabricated in steps S21 to S31, and a third layer 414c is fabricated in steps S31 to S41. In Figure 7, any of the fabrication methods shown in Figures 8B, 9A, 9B, and 10 can be used to fabricate the mixture 501 prepared in step S12, the mixture 502 prepared in step S22, and the mixture 503 prepared in step S32, and multiple fabrication methods can also be used in combination.

[0138] In step S11 of Figure 7, the current collector is prepared. Also, in step S12, the mixture 501 is prepared.

[0139] Next, in step S13 of Figure 7, the mixture 501 is applied to the current collector. As the current collector, materials with high conductivity such as stainless steel, gold, platinum, aluminum, titanium, and alloys thereof can be used. Furthermore, it is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. As the application method in step S13, a slot die method, gravure method, blade method, or a method combining these can be used. A continuous coating machine may also be used for application. Following step S13, in step S14, the mixture 501 applied to the current collector is dried. As the drying method, for example, batch methods such as hot plates, drying ovens, forced-air drying ovens, and vacuum drying ovens, as well as continuous methods combining hot air drying and infrared drying with a continuous coating machine, can be used. After drying, the coated electrode 511 of step S21 is obtained.

[0140] Here, as shown in Figure 7, after drying in step S14, pressing can be performed as step S15. Any of the following methods can be used for pressing: a flat plate press, a hydrostatic press, or a roll press. When using a roll press, it is preferable that the roll temperature is adjusted so that the active material layer reaches a temperature of 10°C to 200°C, preferably 80°C to 150°C.

[0141] Next, in step S22 of Figure 7, the mixture 502 is prepared.

[0142] Next, in step S23 of Figure 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 explanation of step S13. Also, the drying method in step S24 can be the method described in the explanation of step S14. After drying, the coated electrode 512 of step S31 is obtained.

[0143] Here, as shown in Figure 7, after drying in step S24, pressing can be performed as step S25. The method of pressing can be the method described in the explanation of step S15.

[0144] Next, in step S32 of Figure 7, the mixture 503 is prepared.

[0145] Next, in step S33 of Figure 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 explanation of step S13. Also, the drying method in step S34 can be the method described in the explanation of step S14. After drying, the coated electrode 513 of step S41 is obtained.

[0146] Here, as shown in Figure 7, after drying in step S34, pressing can be performed as step S35. The method of pressing can be the method 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 the active material layer 414 can be increased without performing one or two of steps S15, S25, and S35, then one or two of steps S15, S25, and S35 may be omitted.

[0147] By the manufacturing process described above, an electrode 400B having a first layer 414a, a second layer 414b, and a third layer 414c can be manufactured. Furthermore, in the manufacturing method described in Figure 7, by completing the electrode manufacturing in step S31, an electrode 400A having a first layer 414a and a second layer 414b, and an electrode 400C having a second layer 414b and a third layer 414c can be manufactured.

[0148] Figures 8 to 10 show methods for preparing the mixtures that can be used as mixture 501, mixture 502, and mixture 503 shown in Figure 7.

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

[0150] As the binder 110, one or more of the following materials can be used: polystyrene, methyl polyacrylate, 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, nitrocellulose, etc. As the dispersion medium 120, one or more of the following can be used: water, methanol, ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO), etc. A preferred combination of binder 110 and dispersion medium 120 is polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP). 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 the binder mixture 1001 of step S104. For example, a propeller-type mixer, a planetary-type mixer, or a thin-film swirling mixer can be used as a mixing method. It is desirable that the binder mixture 1001 is in a state where the binder 110 is well dispersed in the dispersion medium 120.

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

[0153] As the conductive material 1002, one or more of the following can be used: acetylene black and carbon black such as furnace black; graphite such as artificial graphite and natural graphite; carbon fibers such as carbon nanofibers and carbon nanotubes; and graphene compounds. Details of the conductive material will be described later.

[0154] In this specification, the term "graphene compound" includes multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multigraphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multigraphene oxide, etc. A graphene compound is defined as a material having carbon atoms, having a plate-like or sheet-like shape, and possessing a two-dimensional structure formed by six-membered carbon rings. It is also preferable that it has a bent shape. The two-dimensional structure formed by the six-membered carbon rings may also be called a carbon sheet. It is preferable that the graphene compound has functional groups. Furthermore, the graphene compound may be rolled up to resemble carbon nanofibers. Details of graphene compounds will be described later.

[0155] Next, in step S121, the binder mixture 1001 and the conductive material 1002 are mixed to obtain the mixture 1010 in step S122. For example, a propeller-type mixing device, a planetary-type mixing device, or a thin-film swirling-type mixing device can be used as a mixing method.

[0156] Next, in step S123 of Figure 8B, the active material 10 is prepared. For example, when the mixture 1030 prepared in Figure 8B is used to prepare the first layer 414a, the first active material 411a is used as the active material 10. Also, when the mixture 1030 prepared in Figure 8B is used to prepare the second layer 414b, the second active material 411b is used as the active material 10. Also, when the mixture 1030 prepared in Figure 8B is used to prepare the third layer 414c, the third active material 411c is used as the 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 mixing, for example, a propeller-type mixing device, a planetary rotating mixing device, or a thin-film swirling mixing device can be used. In the mixing in step S131, it is preferable to perform kneading at high viscosity (sometimes called solid kneading). By kneading at high viscosity, the aggregation of powders such as the active material can be undone.

[0158] Next, the binder mixture 1001 is prepared in step S133, and the dispersion medium 1003 is prepared in step S134. If, in step S111, less than the total amount of binder mixture 1001 required to form the active material layer was prepared, the deficit can be added in step S132. If the entire amount of binder mixture 1001 required to form the active material layer was prepared in step S111, it is not necessary to prepare the binder mixture 1001 in step S133. As the dispersion medium 1003, the same dispersion medium as in step S102 in Figure 8A can be used. It is desirable to adjust the amount of dispersion medium 1003 prepared so that it has a viscosity suitable for coating in later steps.

[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 the mixture 1030 from step S142. When a positive electrode active material is used as the active material, the mixture 1030 is sometimes called a positive electrode slurry. Similarly, when a negative electrode active material is used as the active material, the mixture 1030 is sometimes called a negative electrode slurry.

[0160] Figure 9A shows a simplified electrode fabrication method in which the binder mixture 1001, conductive material 1002, and active material 10 are mixed at once in step S121. For the mixing method in step S121, step S131, and the viscosity adjustment of the mixture 1031 by adjusting the amount of dispersion medium 1003, the methods described in the explanation of Figure 8B can be used.

[0161] Figure 9B shows an example of a method for preparing a mixture 1032 having two types of active materials 10. The mixture 1032 can be prepared in the same manner as shown in Figure 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 Figure 9B shows an example using two types of active materials 10, three types may be used, including active material 10a, active material 10b, and active material 10c. Furthermore, four or more types of active materials may be used.

[0162] In Figure 9B, an example is shown in which mixture 1010, active material 10a, and active material 10b are mixed all at once in step S131. However, as shown in Figure 10, active material 10a and active material 10b may be mixed beforehand.

[0163] [Calculations regarding electrode 1 in the layered structure] A calculation relating to an example of an electrode according to one aspect of the present invention will be explained with reference to Figures 11 to 15.

[0164] Figure 11 shows a schematic diagram of the structural model used in the calculations. For clarity, a current collector is shown in Figure 11, but the calculations were performed using structures in the range of d=0μm to d=120μm without including the current collector. 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 towards the negative electrode current collector (current collector 2). The interface between the positive electrode current collector and the positive electrode active material layer is at d=0μm, the interface between the positive electrode active material layer and the separator is at d=50μm, the interface between the separator and the negative electrode active material layer is at d=70μm, and the interface between the negative electrode active material layer and the negative electrode current collector is at d=120μm. Here, the calculation settings profile for Model A, in which the particle size, void ratio, and volume ratio of the active material are kept constant from d=0μm to d=50μm, is shown in Figures 12A to 12C. Furthermore, the calculation settings profile for Model B, which has a three-layer structure, in which the particle size, void ratio, and volume ratio of the active material are kept constant from d=0μm to d=50μm, is shown in Figures 12D to 12F.

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

[0166] In the calculation structure models shown in Figures 11 and 12A to 12F, calculations were performed for the voltage-capacity curves during charging and discharging. The charge-discharge simulations were performed using PyBaMM (version 0.4.0). The calculation model used was the DFN (Doyle-Fuller-Newman) model included with PyBaMM. The parameter set used was a modified version of Marquis2019 included with PyBaMM.

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

[0168] [Laminated electrode structure 2] Another embodiment of the electrode according to the present invention will be described with reference to Figures 16 to 21.

[0169] Figure 16A is a schematic diagram showing electrode 400D when the conductive material 415 is evenly distributed on the three-layer electrode 400B shown in Figure 5A. Figure 16B is a schematic diagram showing electrode 400E when the conductive material 415 and solid electrolyte 421 are evenly distributed on the three-layer electrode 400B shown in Figure 5A. Electrode 400D is a suitable electrode structure for batteries using a liquid electrolyte. Electrode 400E is a suitable electrode structure for all-solid-state batteries and semi-solid-state batteries using a solid electrolyte 421.

[0170] In one aspect of the present invention, it is preferable to have a positive electrode active material 100 having a topotaxy region in its surface layer as the positive electrode. That is, it is preferable that one or more of the first active material 411a, the second active material 411b, the third active material 411c, the fourth active material 411d, and the fifth active material 411e have a positive electrode active material 100 having a topotaxy region in its surface layer.

[0171] Here, a preferred structure for the electrode 400E using a positive electrode active material 100 having a topotaxis region on its surface will be explained using Figures 17A and 17B. Similar to Figure 1B, in Figures 17A and 17B, dotted lines indicate crystal planes parallel to the arrangement of cations. The arrows indicate the direction of lithium (Li) insertion and removal during charging and discharging. In other words, lithium can be inserted and removed from the positive electrode active material 100 at the ends of the arrangement of cations. The surface of the particle surface of the positive electrode active material 100 where the ends of the arrangement of cations are exposed can be called the edge surface.

[0172] In electrodes having a solid electrolyte 421, such as electrode 400E, it is preferable to have the solid electrolyte 421 in the direction of lithium insertion and deinsertion in the positive electrode active material 100, as shown in the schematic diagram of Figure 17A. In other words, it is preferable to have a region on the edge surface of the positive electrode active material 100 where the surface layer of the positive electrode active material 100 and the solid electrolyte 421 are in contact. Here, it is particularly preferable that the surface layer of the positive electrode active material 100 in contact with the solid electrolyte 421 is topotaxis with the interior of the positive electrode active material 100, because lithium ion movement in the contact region between the positive electrode active material 100 and the solid electrolyte 421 is performed well.

[0173] Furthermore, when two positive electrode active materials 100 are in contact via a solid electrolyte 421, it is preferable that each of the two positive electrode active materials 100 has the solid electrolyte 421 in the direction of lithium insertion and deinsertion in the positive electrode active material 100, as shown in Figure 17A. In other words, it is preferable that each of the two positive electrode active materials 100 has a region in contact with the edge surface and the solid electrolyte 421. Although the example in Figure 17A shows an example in which two positive electrode active materials 100 and one solid electrolyte 421 are in contact, the example is not limited to this, and it is also possible for three positive electrode active materials 100 and one solid electrolyte 421 to be in contact, or for two positive electrode active materials 100 and two solid electrolytes 421 to be in contact, and there are no particular restrictions on the number of positive electrode active materials 100 and the number of solid electrolytes 421.

[0174] Figure 17B shows a schematic diagram of a particularly preferred structure in which the solid electrolyte 421 is in the direction of lithium insertion and deinsertion in the positive electrode active material 100. Figure 17B shows an electrode having a first layer 414a on a current collector 413, a second layer 414b on the first layer 414a, and a solid electrolyte 421. The first layer 414a has a positive electrode active material 411Ta with a topotaxy region on its surface, and the second layer 414b has a positive electrode active material 411Tb with a topotaxy region on its surface. Thus, it is preferable that the positive electrode active material 411Ta and the positive electrode active material 411Tb have a region in contact with each other via the solid electrolyte 421. Here, it is preferable that the plurality of positive electrode active materials 411Ta in the first layer 414a are arranged such that the second layer 414b is located ahead of the direction of lithium insertion and deinsertion in the positive electrode active material 411Ta, as shown in Figure 17B. Similarly, it is preferable that the plurality of positive electrode active materials 411Tb in the second layer 414b are arranged such that the first layer 414a is located ahead of the direction of lithium insertion and deinsertion in the positive electrode active material 411Tb. In other words, it is preferable that the direction of lithium insertion and deinsertion in the positive electrode active material 411Ta and the direction of lithium insertion and deinsertion in the positive electrode active material 411Tb are approximately parallel.

[0175] In this case, the speed of lithium movement from the first layer 414a to the second layer 414b can be improved. In other words, the speed of lithium movement from the first layer 414a toward the negative electrode can be improved, making it a structure advantageous for rapid charging and charging in low-temperature environments. Furthermore, similar to charging, this structure is also advantageous for rapid discharge and discharge in low-temperature environments during discharge (movement of lithium from the negative electrode side toward the first layer 414a).

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

[0177] As explained 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 each at different distances from the current collector 413. Alternatively, as shown in Figure 23A, they are at different distances from the separator 440. Or, as shown in Figure 23B, they are at different distances from the solid electrolyte layer 420.

[0178] Here, we consider the third layer 414c, which is relatively far from the current collector 413. In the third layer 414c, due to its distance from the current collector 413, it becomes a region with high electron transfer resistance (also called a region with low electron mobility) within the active material layer 414. Similarly, the second layer 414b has higher electron transfer resistance compared to the first layer 414a. Structures to reduce this difference in electron transfer resistance are shown in Figures 18A and 18B. Figure 18B is a diagram showing the conductive material ratio profile between A1 and A2 in Figure 18A. As shown in Figure 18B, by creating a structure (electrode 400F) in which the proportion of conductive material in the second layer 414b is greater than that in the first layer 414a, and the proportion of conductive material in the third layer 414c is greater than that in the second layer 414b, it is possible to reduce the difference in electron transfer resistance between the first layer 414a, the second layer 414b, and the third layer 414c. In other words, it is preferable that the mass of conductive material in the third layer 414c is greater than the mass of conductive material in the second layer 414b, and the mass of conductive material in the second layer 414b is greater than the mass of conductive material in the first layer 414a. This makes it possible to reduce the unevenness of the battery reaction in the 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. In the first layer 414a, due to its distance from the solid electrolyte layer 420, it becomes a region with high ion transfer resistance (also called a region with low ionic conductivity) within the active material layer 414. Similarly, the second layer 414b has higher ion transfer resistance compared to the third layer 414c. Structures to reduce this difference in ion transfer resistance are shown in Figures 18C and 18D. Figure 18D shows the profile of the solid electrolyte ratio between B1 and B2 in Figure 18C. As shown in Figure 18D, by creating a structure (electrode 400G) in which the proportion of solid electrolyte in the second layer 414b is greater than that of the third layer 414c, and the proportion of solid electrolyte in the first layer 414a is greater than that of the second layer 414b, it is possible to reduce the difference in ion transfer resistance between the first layer 414a, the second layer 414b, and the third layer 414c. In other words, it is preferable that the mass of the solid electrolyte in the first layer 414a is greater than the mass of the solid electrolyte in the second layer 414b, and the mass of the solid electrolyte in the second layer 414b is greater than the mass of the solid electrolyte in the third layer 414c. This makes it possible to reduce the unevenness of the battery reaction in the active material layer 414 during rapid charging and rapid discharging.

[0180] Figures 19A and 19B show the electrode structure (electrode 400H) when the conductive material ratio profile shown in Figures 18A and 18B and the solid electrolyte ratio profile shown in Figures 18C and 18D are superimposed. Figure 19B shows the conductive material ratio profile and the solid electrolyte ratio profile between C1 and C2 in Figure 19A. When an all-solid-state battery has electrode 400H, an all-solid-state battery more suitable for rapid charging and rapid discharging can be realized by reducing the difference in electron transfer resistance in the active material layer 414 and by reducing the difference in ion transfer resistance in the active material layer 414.

[0181] Examples of applying the profiles of the conductive material ratio and the solid electrolyte ratio shown in FIGS. 18 and 19 to the two-layer electrodes (electrode 400I and electrode 400J) are shown in FIGS. 20 and 21. Also in the two-layer electrodes, as described above, by reducing the difference in electron transfer resistance in the active material layer 414 and reducing the difference in ion transfer resistance in the active material layer 414, an all-solid-state battery suitable for faster charging and discharging becomes possible.

[0182] Next, additional explanations will be made regarding the positive electrode, negative electrode, current collector, conductive material, binder, graphene compound, separator, electrolyte, and exterior body shown in the explanations so far in the descriptions of each item below.

[0183] [Positive Electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a positive electrode active material and may have a conductive material 415 and a binder described later. As the structure of the positive electrode active material layer, it is preferable to have the above-described laminated structure.

[0184] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material and may have a conductive material 415 and a binder described later. As the structure of the negative electrode active material layer, it is preferable to have the above-described laminated structure.

[0185] Also, as another form of the negative electrode, a negative electrode that does not have a negative electrode active material at the end of battery fabrication may be used. As a negative electrode without a negative electrode active material, for example, a negative electrode that has only a negative electrode current collector at the end of battery fabrication, and lithium ions that are desorbed from the positive electrode active material by charging of 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 may be called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

[0186] When using a negative electrode without a negative electrode active material, a film may be provided on the negative electrode current collector to homogenize the deposition of lithium. As a film to homogenize the deposition of lithium, for example, a solid electrolyte having lithium ion conductivity can be used. As a solid electrolyte, sulfide particle-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes can be used. Among these, polymer-based solid electrolytes are suitable as a film to homogenize the deposition of lithium because it is relatively easy to form a uniform film on the negative electrode current collector. Alternatively, as a film to homogenize the deposition of lithium, for example, a metal film that forms an alloy with lithium can be used. As a metal film that forms an alloy with lithium, for example, a magnesium metal film can be used. Since lithium and magnesium form a solid solution over a wide composition range, it is suitable as a film to homogenize the deposition of lithium.

[0187] Furthermore, when using a negative electrode without negative electrode active material, a negative electrode current collector with irregularities can be used. When using a negative electrode current collector with irregularities, the recesses in the negative electrode current collector become cavities where lithium can easily be deposited, thus suppressing the formation of dendrite-like shapes when lithium is deposited.

[0188] [Current collector] As the positive electrode current collector and negative electrode current collector, materials with high conductivity that do not alloy with carrier ions such as lithium can be used, such as metals like stainless steel, gold, platinum, zinc, iron, copper, aluminum, and titanium, and their alloys. The current collector can be in the shape of a sheet, mesh, perforated metal, expanded metal, etc., as appropriate. The current collector should preferably have a thickness of 10 μm to 30 μm.

[0189] Furthermore, it is preferable to use a material for the negative electrode current collector that does not alloy with carrier ions such as lithium.

[0190] A titanium compound may be provided by laminating it on the above-described metal as a current collector. As the titanium compound, for example, titanium nitride, titanium oxide, titanium nitride in which part of nitrogen is substituted by oxygen, titanium oxynitride (TiO x N y where 0 < x < 2 and 0 < y < 1), and titanium oxide in which part of oxygen is substituted by nitrogen, may be selected, or one 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 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 part of the surface of the active material is covered by the conductive 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 conductive materials, one or more of the following can be used: 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.

[0194] As carbon fibers, for example, mesophase pitch carbon fibers and isotropic pitch carbon fibers can be used. Alternatively, carbon nanofibers or carbon nanotubes can be used. Carbon nanotubes can be fabricated, for example, by vapor deposition.

[0195] The active material layer may also contain metal powders or metal fibers such as copper, nickel, aluminum, silver, or gold, or conductive ceramic materials as conductive materials.

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

[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. Therefore, a smaller amount of graphene compound can improve the electrical conductivity between the granular active material and the graphene compound compared to conventional conductive materials. Consequently, the ratio of the active material in the active material layer can be increased. This, in turn, can increase 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, readily penetrate minute spaces. These minute spaces refer, for example, to regions between multiple active materials. By combining a carbon-containing compound that readily penetrates minute spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the electrode density can be increased, and excellent conductive paths can be formed. A secondary battery obtained by the manufacturing method according to one embodiment of the present invention can have high capacity density and stability, making it effective as a secondary battery for automotive use.

[0199] [Binder] The active material layer preferably has 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 a carbon-based material, multiple active materials to each other, multiple carbon-based materials to each other, etc.

[0200] It is preferable to use materials such as polystyrene, methyl polyacrylate, 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 as binders.

[0201] Polyimides possess 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 in the range of 134°C to 169°C, and is a material with excellent thermal stability.

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

[0204] Further, as the binder, for example, it is preferable to use a water-soluble polymer. As the water-soluble polymer, for example, polysaccharides and the like can be used. As the polysaccharides, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, or starch and the like can be used. Further, it is more preferable to use these water-soluble polymers in combination with the above-mentioned rubber materials.

[0205] The binder may be used in combination of a plurality of the above.

[0206] [Graphene compound] In this specification and the like, the graphene compound includes graphene, multilayer graphene, multi-graphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. The graphene compound refers to a substance having carbon, having a flat plate shape, sheet shape, etc., and having a two-dimensional structure formed by carbon six-membered rings. The two-dimensional structure formed by the carbon six-membered rings may be referred to as a carbon sheet. The graphene compound may have a functional group. Further, the graphene compound preferably has a bent shape. Further, the graphene compound may be rounded to be like a carbon nanofiber.

[0207] In this specification and the like, graphene oxide refers to, for example, a substance having carbon and oxygen, having a sheet shape, and having a functional group, particularly an epoxy group, carboxy group or hydroxy group.

[0208] In this specification, reduced graphene oxide refers to, for example, a material having carbon and oxygen, having a sheet-like shape, and possessing a two-dimensional structure formed by a six-membered carbon ring. It may also be called a carbon sheet. Reduced graphene oxide can function as a single sheet, but multiple sheets may be laminated together. It is preferable that reduced graphene oxide has a portion where the carbon concentration is greater than 80 atomic%, and the oxygen concentration is between 2 atomic% and 15 atomic%. By having such carbon and oxygen concentrations, it can function as a highly conductive material even in small quantities. Furthermore, it is preferable that the intensity ratio of the G band to the D band in the Raman spectrum of reduced graphene oxide is 1 or greater. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small quantities.

[0209] In some cases, pores can be created in graphene compounds by reducing graphene oxide.

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

[0211] In the longitudinal section of the active material layer, sheet-like graphene compounds are dispersed approximately uniformly within the internal region of the active material layer. As shown in Figures 22A and 22B, the multiple graphene compounds are formed to partially cover or adhere to the surfaces of the multiple granular active materials, and thus are in surface contact with one another.

[0212] Here, multiple graphene compounds can bond together to form a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net). When the active material is coated with the 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 eliminated, thereby improving the ratio of active material to electrode volume or electrode weight. In other words, the charge and discharge capacity of the secondary battery can be increased.

[0213] Here, it is preferable to use graphene oxide as the graphene compound, mix it with the active material to form a layer that will become the active material layer, and then reduce it. In other words, it is preferable that the completed active material layer has reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in polar solvents, for the formation of the graphene compound, the graphene compound can be dispersed approximately uniformly within the internal region of the active material layer. By volatilizing and removing the solvent from the dispersion medium containing the uniformly dispersed graphene oxide and reducing the graphene oxide, the graphene compounds remaining in the active material layer partially overlap and are dispersed to the extent that they are in surface contact with each other, thereby forming three-dimensional conductive paths. The reduction of graphene oxide may be carried out, for example, by heat treatment or by using a reducing agent.

[0214] Furthermore, by using a spray-drying device beforehand, a graphene compound, which is a conductive material, can be formed as a coating to cover the entire surface of the active material, and then the active material particles can be electrically connected with the graphene compound to form conductive paths.

[0215] Furthermore, the active material layer may be made by mixing the graphene compound with the material used to form the graphene compound. For example, particles used as a catalyst when forming the graphene compound may be mixed together with the graphene compound. Examples of catalysts used when forming the graphene compound include silicon dioxide (SiO2, SiO2). x Examples of particles include those having (x<2), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. Preferably, the D50 of the particles is 1 μm or less, and more preferably 100 nm or less.

[0216] [Separator] A separator is placed between the positive and negative electrodes. The separator can be made from materials such as cellulose fibers including paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane. It is preferable that the separator be processed into a bag shape and positioned to enclose either the positive or negative electrode.

[0217] The separator is a porous material having pores with a diameter of at least 2 nm. Preferably, the separator has pores of 6.5 nm or larger, and more preferably, pores of about 20 nm in size. 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 material, a polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

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

[0220] For example, a polypropylene film may be coated on both sides with a mixture of aluminum oxide and aramid. Alternatively, the side of the polypropylene film in contact with the positive electrode may be coated with a mixture of aluminum oxide and aramid, and the side in contact with the negative electrode may be coated with a fluorine-based material.

[0221] By using a multi-layered separator, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, thus increasing the capacity per unit volume of the secondary battery.

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

[0223] Sulfide-based solid electrolytes include thiolysicone-based (Li 10 GeP2S 12 Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glass (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), sulfide crystallized glass (Li7P3S 11 Li 3.25 P 0.95 It contains S4, etc. Sulfide-based solid electrolytes have advantages such as the availability of materials with high conductivity, the ability to be synthesized at low temperatures, and their relatively soft nature which helps maintain conductive paths even after charging and discharging.

[0224] Oxide-based solid electrolytes include materials having a perovskite-type crystal structure (La 2 / 3-x Li 3x Materials having a NASICON-type crystal structure (Li 1-Y Al Y Ti 2-Y (PO4)3 etc.) Materials having a garnet-type crystal structure (Li7La3Zr2O 12 Materials having a 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 using liquid electrolyte 576 in a secondary battery, for example, one of the following can be used as electrolyte 576: 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, etc., 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 electrolyte 576, it is possible to prevent the secondary battery from rupturing or igniting even if the internal temperature rises due to a short circuit or overcharging within the internal region of the secondary battery. Ionic liquids consist of cations and anions, and include 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, fluorosulfonic acid anions, perfluoroalkyl sulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, or perfluoroalkyl phosphate anions.

[0230] In particular, in a secondary battery according to one aspect of the present invention, when silicon is used as the active material of the negative electrode, it is preferable to use a liquid electrolyte 576 having an ionic liquid.

[0231] A secondary battery according to one aspect of the present invention has, for example, 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 as carrier ions.

[0232] When lithium ions are used as carrier ions, the electrolyte, for example, contains a lithium salt. Examples of lithium salts include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B. 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] Furthermore, the electrolyte preferably contains fluorine. As a fluorine-containing electrolyte, for example, an electrolyte having one or more types of fluorinated cyclic carbonates and lithium ions can be used. Fluorinated cyclic carbonates can improve flammability and enhance the safety of lithium-ion secondary batteries.

[0234] As fluorinated cyclic carbonates, fluorinated ethylene carbonates such as monofluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC) can be used. Note that DFEC has isomers such as cis-4,5 and trans-4,5. As an electrolyte, it is important to solvate lithium ions using one or more types of fluorinated cyclic carbonates and transport them within the electrolyte contained in the electrodes during charging and discharging in order to operate at low temperatures. By contributing to the transport of lithium ions during charging and discharging, rather than using fluorinated cyclic carbonates as small additives, low-temperature operation becomes possible. In secondary batteries, lithium ions move in clusters of several to several dozen ions.

[0235] By using fluorinated cyclic carbonates as the electrolyte, the desolvation energy required for lithium ions solvated within the electrolyte to enter the active material particles is reduced. Reducing this desolvation energy makes it easier for lithium ions to insert into or detach from the active material particles, even at low temperatures. While lithium ions may move while remaining solvated, a hopping phenomenon can occur where the coordinating solvent molecules are replaced. Easier desolvation of lithium ions can facilitate hopping, thus improving lithium ion movement. There is a concern that the decomposition products of the electrolyte during charging and discharging of secondary batteries can adhere to the surface of the active material, leading to battery degradation. However, when the electrolyte contains fluorine, the electrolyte is fluid, making it difficult for the decomposition products to adhere to the surface of the active material. Therefore, battery degradation can be suppressed.

[0236] Solvated lithium ions may form clusters in the electrolyte and move within the negative electrode, between the positive and negative electrodes, within the positive electrode, etc.

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

[0238] Degradation is prone to occur at interfaces within secondary batteries, such as the interface between the active material and the electrolyte. In one embodiment of the present invention, the presence of a fluorine-containing electrolyte prevents degradation, typically alteration of the electrolyte or increased viscosity of the electrolyte, that can occur at the interface between the active material and the electrolyte. Alternatively, a binder or graphene compound may be attached to or retained around the fluorine-containing electrolyte. This configuration makes it possible to maintain a state where the viscosity of the electrolyte is reduced, in other words, a free-flowing state of the 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 freer than FEC, which has one fluorine atom bonded, resulting in weaker coordination bonds with lithium. Therefore, the adhesion of highly viscous decomposition products to the active material particles can be reduced. When highly viscous decomposition products adhere to or cling to the active material particles, lithium ions become less able to move at the interface 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) through solvation. Furthermore, by using fluorine-containing electrolytes, the formation and growth of dendrites can be prevented by preventing the adhesion of decomposition products.

[0239] Another characteristic is the use of an electrolyte containing fluorine as the main component, with the fluorine-containing electrolyte being 5% or more by volume, 10% or more by volume, preferably 30% to 100% by volume.

[0240] In this specification, the main component of the electrolyte refers to a component that accounts for 5% or more by volume of the total electrolyte of the secondary battery. Furthermore, "5% or more by volume of the total electrolyte of the secondary battery" here refers to the proportion of the total electrolyte measured during the manufacturing of the secondary battery. In addition, when a secondary battery is disassembled after its manufacture, it is difficult to quantify the proportion of each of the multiple types of electrolytes, but it is possible to determine whether a particular organic compound accounts for 5% or more by volume of the total electrolyte.

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

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

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

[0244] Furthermore, the presence of a polymer material that gels the electrolyte enhances safety against leakage and other issues. Typical examples of polymer materials that gel include silicone gels, acrylic gels, acrylonitrile gels, polyethylene oxide gels, polypropylene oxide gels, and fluorine-based polymer gels.

[0245] As polymer materials, for example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, and polyacrylonitrile, as well as copolymers containing these, can be used. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. Furthermore, the polymer formed may have a porous structure.

[0246] Furthermore, the above configuration is an example of a secondary battery using a liquid electrolyte, but is not particularly limited. For example, semi-solid-state batteries and all-solid-state batteries can also be fabricated.

[0247] In this specification, whether it is a secondary battery using a liquid electrolyte or a semi-solid battery, the layer placed between the positive and negative electrodes will be referred to as the electrolyte layer. The electrolyte layer of a semi-solid battery is a layer formed by film deposition and can be distinguished from the liquid electrolyte layer.

[0248] Furthermore, in this specification, a semi-solid battery refers to a battery having a semi-solid material in at least one of its components: the electrolyte layer, the positive electrode, and the negative electrode. Here, "semi-solid" does not mean that the solid material makes up 50% of the battery. "Semi-solid" means possessing solid properties, such as small volume change, while also having some liquid-like properties, such as flexibility. As long as these properties are met, the battery may consist of a single material or multiple materials. For example, a liquid material may be impregnated into a porous solid material.

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

[0250] Electrolyte 576 comprises a lithium-ion conductive polymer and a lithium salt.

[0251] In this specification, a lithium-ion conductive polymer is a polymer that has the conductivity of a cation such as lithium. More specifically, it is a polymer compound having a polar group to which a cation can coordinate. Preferably, the polar group is an ether group, ester group, nitrile group, carbonyl group, siloxane, etc.

[0252] Examples of lithium-ion conductive polymers that can be used include polyethylene oxide (PEO), derivatives having polyethylene oxide as the main chain, polypropylene oxide, polyacrylic acid esters, polymethacrylate esters, polysiloxanes, and polyphosphazenes.

[0253] The lithium-ion conductive polymer may be branched, crosslinked, or copolymerized. Its molecular weight is preferably, for example, 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 through partial motion (also called segmental motion) of the polymer chains. For example, in PEO, lithium ions move while changing the oxygen groups they interact with through segmental motion of the ether chains. When the temperature is close to or higher than the melting or softening point of the lithium-ion conductive polymer, the crystalline region dissolves and the amorphous region increases, and the motion of the ether chains becomes more active, resulting in higher ionic conductivity. Therefore, when using PEO as a lithium-ion conductive polymer, it is preferable to perform charging and discharging at 60°C or higher.

[0255] According to Shannon et al., Acta A 32(1976) 751, the radius of a monovalent lithium ion is 0.590 × 10⁻⁶ in the 4-coordinate state. -1 nm, 0.76 × 10 for 6-coordinate. -1 nm, 0.92 × 10 for 8-coordinate. -1 It is in nm. The radius of a divalent oxygen ion is 1.35 × 10⁻⁶ when it is two-coordinate. -1 nm, 1.36 × 10 for 3-coordinate.-1 nm, 1.38 × 10 for 4-coordinate. -1 nm, 1.40 × 10 for 6-coordinate. -1 nm, 1.42 × 10 for 8-coordinate. -1 The distance is in nm. The distance between polar groups in adjacent lithium-ion conductive polymer chains is preferably greater than or equal to the distance at which lithium ions and anions from the polar groups can stably exist while maintaining the ionic radius described above. Furthermore, it is preferable that the distance is sufficient for sufficient interaction between lithium ions and 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 if the distance is appropriate when lithium ions pass through.

[0256] Furthermore, as lithium salts, compounds can be used that contain lithium along with at least one of the following: phosphorus, fluorine, nitrogen, sulfur, oxygen, chlorine, arsenic, boron, aluminum, bromine, and iodine. 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, lithium bis(oxalate) borate (LiBOB), etc., can be used individually or in any combination and ratio of two or more of these.

[0257] In particular, using LiFSI is preferable because it exhibits good low-temperature characteristics. Furthermore, LiFSI and LiTFSA are less reactive with water compared to LiPF6, etc. Therefore, it is easier to control the dew point when fabricating electrodes and electrolyte layers using LiFSI. For example, they can be handled not only in an inert atmosphere such as argon with moisture removed as much as possible, and in a dry room with controlled dew point, but also in a normal atmospheric atmosphere. This improves productivity, which is preferable. Moreover, using Li salts with high dissociability and plasticizing effects, such as LiFSI and LiTFSA, is particularly preferable when using lithium conduction utilizing the segmental motion of the ether chain, because it can be used over a wide temperature range.

[0258] The absence or very low amount of organic solvents makes it possible to create a secondary battery that is less likely to ignite or burn, thus improving safety, which is preferable. Furthermore, if the electrolyte 576 is an electrolyte layer that has no organic solvents or very little, it has sufficient strength to electrically insulate the positive and negative electrodes even without a separator. Since a separator is not required, a secondary battery with high productivity can be created. If the electrolyte layer contains both electrolyte 576 and an inorganic filler, the strength is further increased, resulting in a secondary battery with even higher safety.

[0259] [Exterior] The outer casing of a secondary battery can be made of metal materials such as aluminum or resin materials. A film-like outer casing can also be used. As a film, for example, a three-layer film can be used, in which a highly flexible metal thin film such as aluminum, stainless steel, copper, or nickel is provided on a film made of materials such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as polyamide resin or polyester resin is provided on the metal thin film as the outer surface of the outer casing. Furthermore, it is preferable to use a fluororesin film as the film. Fluororesin films have high stability against acids, alkalis, organic solvents, etc., and suppress side reactions, corrosion, etc. associated with the reactions of the secondary battery, thereby realizing a superior secondary battery. Examples of fluororesin films include PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane: copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether), FEP (perfluoroethylenepropene copolymer: copolymer of tetrafluoroethylene and hexafluoropropylene), and ETFE (ethylenetetrafluoroethylene copolymer: copolymer of tetrafluoroethylene and ethylene).

[0260] [Internal structure of the battery] A battery having electrodes according to one embodiment of the present invention will be described with reference to Figures 23 and 24.

[0261] As shown in Figure 23A, a secondary battery according to one embodiment of the present invention has a positive electrode 410, a separator 440, and a negative electrode 430. The positive electrode 410 in Figure 23A is a positive electrode using the electrode structure shown in Figure 18A, and one or more of the positive electrode active materials described in Embodiment 2 and Embodiment 3 can be used as the positive electrode active material. The secondary battery shown in Figure 23A has a liquid electrolyte 576, and it is preferable that the liquid electrolyte 576 fills the spaces between the particles of the layered structure of the positive electrode 410.

[0262] As shown in Figures 23B and 24, a secondary battery according to one embodiment of the present invention has a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430. The positive electrode 410 shown in Figures 23B and 24 is a positive electrode using the electrode structure shown in Figure 19A, and one or more of the positive electrode active materials described in Embodiments 2 and 3 can be used as the positive electrode active material. The active material layer 414 of the positive electrode may also have the conductive material and binder described above.

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

[0264] As shown in Figure 24, the 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 it into particles, so the negative electrode 430 can be made without the solid electrolyte 421, as shown in Figure 23B. Using metallic lithium in the negative electrode 430 is preferable because it can improve the energy density of the secondary battery.

[0265] As the solid electrolyte 421 in the solid electrolyte layer 420, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halogen-based solid electrolyte, etc., can be used.

[0266] Furthermore, modified examples of the battery having the solid electrolyte 421 shown in Figures 23B and 24 are shown in Figures 25A and 25B. The batteries shown in Figures 25A and 25B have not only the solid electrolyte 421 but also a liquid electrolyte 576. Because these batteries have both a solid electrolyte and a liquid electrolyte, they are sometimes called semi-solid batteries. Semi-solid batteries have the advantages of both the flame retardancy of the solid electrolyte 421 and the increased contact interface between the active material and the electrolyte of the liquid electrolyte 576. In this case, using an electrolyte having an ionic liquid as the liquid electrolyte 576 is particularly preferable because it results in a flame-retardant battery.

[0267] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0268] (Embodiment 2) In this embodiment, a positive electrode active material 100A and a method for producing the positive electrode active material 100A, which can be used in a secondary battery according to one aspect of the present invention, will be described with reference to Figures 26A to 36.

[0269] [Cathode active material] Figure 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. Figures 2A and 2B show enlarged views of the area around AB in Figure 1B. Figures 2C and 2D show enlarged views of the area around CD in Figure 1B.

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

[0271] <Elements present in the positive electrode active material> The positive electrode active material 100A comprises lithium, a transition metal M, oxygen, and an additive element A. Alternatively, the positive electrode active material 100A may be a composite oxide (LiMO2) containing lithium and a transition metal M, to which the additive element A is added. However, the composition of the composite oxide is not strictly limited to Li:M:O=1:1:2. Furthermore, a positive electrode active material to which the additive element A is added may also be referred to as a composite oxide.

[0272] The positive electrode active material of a lithium-ion secondary battery needs to contain a redox-capable transition metal in order to maintain charge neutrality even when lithium ions are inserted and removed. In one embodiment of the present invention, the positive electrode active material 100A preferably uses cobalt as the transition metal M responsible for the redox reaction. In addition to cobalt, at least one or two selected from nickel and manganese may be used. It is preferable that the cobalt content of the transition metal M in the positive electrode active material 100A is 75 atomic% or more, preferably 90 atomic% or more, and more preferably 95 atomic% or more, as this offers many advantages, such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics.

[0273] Furthermore, if the cobalt content of the transition metal M in the positive electrode active material 100A is 75 atomic percent or more, preferably 90 atomic percent or more, and more preferably 95 atomic percent or more, then compared to composite oxides such as lithium nickelate (LiNiO2) in which nickel accounts for the majority of the transition metal M, Li xThe stability of CoO2 is better when x is small. This is thought to be because cobalt is less affected by strain due to the Jahn-Teller effect than nickel. In transition metal compounds, the strength of the Jahn-Teller effect varies depending on the number of electrons in the d orbitals of the transition metal. In layered rock salt type composite oxides in which octahedral low-spin nickel(III) ions make up the majority, such as lithium nickelate, the Jahn-Teller effect is significant, and strain is likely to occur in the layers consisting of octahedra of nickel and oxygen. Therefore, there is a growing concern that the crystal structure may collapse during charge-discharge cycles. Also, nickel ions are larger than cobalt ions and are close in size to lithium ions. Therefore, in layered rock salt type composite oxides in which nickel makes up the majority, such as lithium nickelate, there is a problem that cation mixing of nickel and lithium is likely to occur.

[0274] On the other hand, if nickel is used as the transition metal M in the positive electrode active material 100A in an amount of 33 atomic percent or more, preferably 60 atomic percent or more, and more preferably 80 atomic percent or more, the raw materials may be cheaper compared to the case where cobalt is abundant, and the charge / discharge capacity per unit weight may increase, which is preferable.

[0275] The additive element A 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. Furthermore, the sum of transition metals among the additive element A is preferably less than 25 atomic%, more preferably less than 10 atomic%, and even more preferably less than 5 atomic%.

[0276] In other words, the positive electrode active material 100A can include lithium cobalt oxide with magnesium and fluorine added, lithium cobalt oxide with magnesium, fluorine and titanium added, lithium cobalt oxide with magnesium, fluorine and aluminum added, lithium cobalt oxide with magnesium, fluorine and nickel added, lithium cobalt oxide with magnesium, fluorine, nickel and aluminum added, and so on.

[0277] These additive elements A further stabilize the crystal structure of the positive electrode active material 100A, as will be described later. In this specification, additive elements A are synonymous with a mixture or part of the raw materials.

[0278] Furthermore, the additive 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 positive electrode active material 100A is substantially free of manganese, the above advantages such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics become even greater. The weight of manganese contained in the positive electrode active material 100A is preferably 600 ppm or less, more preferably 100 ppm or less. The weight of manganese can be analyzed, for example, using GD-MS.

[0280] Next, using Figures 26 to 29, we will explain the results of calculating the crystal structure of the surface layer with and without added elements.

[0281] In particular, cobalt oxide may be present in the surface layer of lithium cobalt oxide that does not contain any additive elements. Cobalt oxide may also contain metal 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 crystal orientation of {110} in LCO and {110} in CoO is roughly the same, but there is a 5.1% difference between the interplanar spacing of {001}, which is perpendicular to {110} in LCO (1.405 nm), and the interplanar spacing of {1-11}, which is perpendicular to {110} in CoO (1.477 nm), which is six times the interplanar spacing.

[0282] Figure 26B1 is a schematic diagram of lithium cobaltate (LCO) with cobalt oxide (CoO) in its surface layer. A magnified view of the surface layer is shown in Figure 26B2. Figure 26B3 shows the results of calculations using classical molecular dynamics for a portion of the surface layer containing LCO and CoO. Because there is a difference of more than 5% between the interplanar spacing of {001}, which is perpendicular to {110} in LCO, and the interplanar spacing of {1-11}, which is six times perpendicular to {110} in CoO, multiple shifts in atomic arrangement occur, as indicated by the dashed circles in Figure 26B3. It is thought that cobalt and / or oxygen are more likely to desorb in such unstable areas. Therefore, these areas can become the starting points for pits.

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

[0284] Next, we will examine 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 structure exploration. For the first-principles calculations, VASP (Vienna Ab initio Simulation Package) was used, and the calculation conditions were as shown in Table 1. Using ATAT, Co (1-x) Mg x Figure 28A shows the results of exploring the arrangements when x in O is 0.125, 0.143, 0.250, 0.500, and 0.833. In Figure 28A, the gray parallelograms represent Mg-O octahedra (MgO6) with Mg at the center, and the black parallelograms represent Co-O octahedra (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 downwards, and it is more stable when in solid solution, suggesting that CoO and MgO can form a solid solution. Furthermore, it is suggested that Co and Mg are dispersed in the solid solution state.

[0287] Co in a solid solution state (1-x) Mg x O exhibits anisotropy in interplanar spacing, making it difficult to determine which crystal orientations can result in LCO and topotaxis. Therefore, the volume per metal atom is calculated by dividing the volume of each structural model by the number of metal atoms in the structural model (10 -3 nm 3 The results of calculating the trend of changes in interplanar spacing based on ) are shown in Figure 28B. From Figure 28B, as the solid solution ratio of Mg increases, Co (1-x) Mg x This suggests that the volume of O decreases and tends to approach that of MgO. From this, we can see that LCO and Co (1-x) Mg x It is thought that the discrepancy between the plane spacing of {001} of LCO and the surface in contact with O will be small.

[0288] Therefore, it is thought that CoO and MgO readily form a solid solution. As heating progresses, the solid solution of CoO and MgO progresses, and as shown in Figures 29A to 29B, a solid solution of CoO forms on the surface layer 100a of the positive electrode active material 100A. (1-x) Mg x It is thought that O can be formed. (1-x) Mg x O has less lattice mismatch with LCO than CoO. Therefore, Co (1-x) Mg x The surface layer 100a containing O is more likely to become topotaxis with the LCO in the interior 100b. Also, as shown by the length of the white arrows in Figures 29A and 29B, the stress is reduced.

[0289] Thus, even when cobalt oxide is present on the surface of lithium cobalt oxide, adding an additive element and heating it can make the surface layer 100a a solid solution of cobalt oxide and an oxide containing the additive element. As a result, the surface layer 100a and the interior 100b of the positive electrode active material 100A become more topotaxis-prone. Therefore, a positive electrode active material 100A that is less prone to pit formation can be obtained.

[0290] <Crystal structure> ≪Li x When x in CoO2 is 1 >> In one aspect of the present invention, the positive electrode active material 100A is in a discharge state, i.e., Li x In the case of x=1 in CoO2, it is preferable to have a layered rock salt type crystal structure belonging to space group R-3m. Layered rock salt type composite oxides have high discharge capacity, possess a two-dimensional lithium ion diffusion pathway, are suitable for lithium ion insertion / desorption reactions, and are excellent as positive electrode active materials for secondary batteries. For this reason, it is particularly preferable that the interior 100b, which accounts for most of the volume of the positive electrode active material 100A, has a layered rock salt type crystal structure. Figure 30 shows the layered rock salt type crystal structure with R-3m O3 attached.

[0291] On the other hand, in one embodiment of the present invention, it is preferable that the surface layer 100a of the positive electrode active material 100A has a function to reinforce the layered structure of the interior 100b, which consists of an octahedron of transition metal M and oxygen, so that it does not break down even if lithium is removed from the positive electrode active material 100A due to charging. Alternatively, it is preferable that the surface layer 100a functions as a barrier film for the positive electrode active material 100A. Alternatively, it is preferable that the surface layer 100a, which is the outer periphery of the positive electrode active material 100A, reinforces the positive electrode active material 100A. Reinforcement as used here means suppressing structural changes in the surface layer 100a and interior 100b of the positive electrode active material 100A, including oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the positive electrode active material 100A.

[0292] Therefore, it is preferable that the surface layer 100a has a different crystal structure from the interior 100b. It is also preferable that the surface layer 100a has a composition and crystal structure that is more stable at room temperature (25°C) than the interior 100b. For example, it is preferable that at least a part of the surface layer 100a of the positive electrode active material 100A in one aspect of the present invention has a rock salt type crystal structure. Alternatively, it is preferable that the surface layer 100a has both layered rock salt type and rock salt type crystal structures. Alternatively, it is preferable that the surface layer 100a has characteristics of both layered rock salt type and rock salt type crystal structures.

[0293] The surface layer 100a is the region where lithium ions first desorb during charging, and it is a region where the lithium concentration tends to be lower than in the interior 100b. Furthermore, the atoms on the surface of the positive electrode active material 100A in the surface layer 100a can be described as having some of their bonds broken. Therefore, the surface layer 100a is prone to instability, and it is a region where degradation 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, when x is 0.24 or less, the layered structure consisting of the transition metal M and oxygen octahedra in the interior 100b can be made less prone to breaking. Furthermore, the displacement of the layers consisting of the transition metal M and oxygen octahedra in the interior 100b can be suppressed.

[0294] To ensure a stable composition and crystal structure for the surface layer 100a, it is preferable that the surface layer 100a contains additive element A, and more preferably that it contains multiple additive element A. Furthermore, it is preferable that the surface layer 100a has a higher concentration of one or more selected additive element A than the interior layer 100b. It is also preferable that the one or more selected additive element A present in the positive electrode active material 100A have a concentration gradient. Moreover, it is more preferable that the distribution of the positive electrode active material 100A differs depending on the additive element A. For example, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element A. Here, "concentration peak" refers to the maximum concentration value in the surface layer 100a or below 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 towards the surface, as shown by the density of the hatches in Figure 2A. Elements with such a concentration gradient will be called additive elements X.

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

[0297] For example, magnesium, one of the additive elements X, is divalent, and magnesium ions are more stable in lithium sites than in transition metal M sites in the layered rock salt crystal structure, so they readily enter the lithium sites. The presence of magnesium at an appropriate concentration in the lithium sites of the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is presumed to be because the magnesium present in the lithium sites functions as pillars that support the CoO2 layers. Furthermore, the presence of magnesium allows Li x When x in CoO2 is, for example, 0.24 or less, the desorption of oxygen around magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100A. In addition, a high magnesium concentration in the surface layer 100a is expected to improve corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte.

[0298] At appropriate concentrations, magnesium does not adversely affect lithium insertion and removal during charging and discharging, and the above benefits can be enjoyed. However, excessive magnesium may adversely affect lithium insertion and removal. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters not only lithium sites but also transition metal M sites. In addition, unwanted magnesium compounds (oxides and fluorides, etc.) that do not substitute for lithium sites or transition metal M sites may segregate on the surface of the positive electrode active material and become a resistive component of the secondary battery. Moreover, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the lithium sites, reducing the amount of lithium that contributes to charging and discharging.

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

[0300] Furthermore, nickel, one of the additive elements X, can be present at both the transition metal M site and the lithium site. When present at the transition metal M site, its oxidation-reduction potential is lower compared to cobalt, which leads to an increase in discharge capacity and is therefore preferable.

[0301] Furthermore, when nickel is present at the lithium site, the displacement of the layered structure consisting of the transition metal M and oxygen octahedra can be suppressed. Volume changes associated with charging and discharging are also suppressed. Additionally, the elastic modulus increases, meaning it becomes harder. This is presumed to be because the nickel present at the lithium site also functions as a pillar supporting the CoO2 layers. Therefore, it is desirable that the crystal structure becomes more stable, especially in the charged state at high temperatures, such as 45°C or above.

[0302] On the other hand, an excess of nickel may exacerbate the distortion caused by the Jahn-Teller effect. Furthermore, an excess of nickel may negatively affect lithium insertion and removal.

[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% and 7.5% or less of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferable that it is more than 0% and 4% or less. Alternatively, it is preferable that it is more than 0% and 2% or less. Alternatively, it is preferable that it is more than 0% and 7.5% or less. Alternatively, it is preferable that it is more than 0% and 2% or less. Alternatively, it is preferable that it is more than 0% and 7.5% or less. Alternatively, it is preferable that it is more than 0% and 4% or less. The amount of nickel shown here may be the value obtained by elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition in the process of manufacturing the positive electrode active material.

[0304] Furthermore, aluminum, one of the additive elements Y, can exist at the transition metal M sites in the layered rock salt crystal structure. Since aluminum is a trivalent typical element and its valency does not change, lithium around the aluminum does not easily move during charging and discharging. Therefore, aluminum and the surrounding lithium can function as pillars, suppressing changes in the crystal structure. In addition, aluminum suppresses the dissolution of the surrounding transition metal M, improving continuous charging endurance. Moreover, since 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, having aluminum as an 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 negatively affect the insertion and removal of lithium.

[0306] Therefore, it is preferable that the total amount of aluminum in the positive electrode active material 100A is appropriate. For example, the total number of aluminum atoms in the positive electrode active material 100A is preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.3% to 1.5% of the total number of cobalt atoms. Alternatively, 0.05% to 2% is preferred. Alternatively, 0.1% to 4% is preferred. The total amount of aluminum in the positive electrode active material 100A referred to here may be, for example, the value obtained by elemental analysis of the entire positive electrode active material 100A using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material 100A.

[0307] Furthermore, fluorine, one of the additive elements X, is a monovalent anion, and if some of the oxygen in the surface layer 100a is replaced by fluorine, the lithium desorption energy decreases. This is because the change in the valence of cobalt ions associated with lithium desorption differs depending on the presence or absence of fluorine; for example, without fluorine, the valence changes from trivalent to tetravalent, while with fluorine, it changes from divalent to trivalent, resulting in a different oxidation-reduction potential for cobalt ions. Therefore, if 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. As a result, when used in a secondary battery, the charge-discharge characteristics, current characteristics, etc., can be improved. In addition, the presence of fluorine in the surface layer 100a, which is the part that comes into contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid. As will be described in later embodiments, if the melting point of fluorides, including lithium fluoride, is lower than the melting point of other additive element A sources, it can function as a flux (also called a fluxing agent) to lower the melting point of the other additive element A sources.

[0308] Furthermore, titanium oxide, one of the additive elements X, is known to be superhydrophilic. Therefore, by using a positive electrode active material 100A having titanium oxide in its surface layer 100a, it is possible that wettability with highly polar solvents will be improved. When used in a secondary battery, good contact at the interface between the positive electrode active material 100A and the highly polar electrolyte may be achieved, potentially suppressing an increase in internal resistance.

[0309] Furthermore, if phosphorus, one of the additive elements X, is present in the surface layer 100a, Li x It is preferable that the state in which x in CoO2 is kept small may suppress short circuits. For example, it is preferable that it exists in the surface layer 100a as a compound containing phosphorus and oxygen.

[0310] If the positive electrode active material 100A contains phosphorus, it is preferable that the hydrogen fluoride generated by the decomposition of the electrolyte reacts with the phosphorus, potentially lowering the concentration of hydrogen fluoride in the electrolyte.

[0311] If the electrolyte contains LiPF6, hydrolysis may generate hydrogen fluoride. Furthermore, the reaction between polyvinylidene fluoride (PVDF), used as a component of the positive electrode, and alkali may also generate hydrogen fluoride. Reducing the hydrogen fluoride concentration in the electrolyte may suppress corrosion of the current collector and / or peeling of the coating. It may also suppress the decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0312] If the positive electrode active material 100A contains phosphorus along with magnesium, Li x The stability is extremely high and preferable when x in CoO2 is small. When the positive electrode active material 100A contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Or 1% to 10% is preferable. Or 1% to 8% is preferable. Or 2% to 20% is preferable. Or 2% to 8% is preferable. Or 3% to 20% is preferable. Or 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and even more preferably 0.7% to 4%. Or 0.1% to 5% is preferable. Or 0.1% to 4% is preferable. Or 0.5% to 10% is preferable. Or 0.5% to 4% is preferable. Or 0.7% to 10% is preferable. Alternatively, a concentration of 0.7% to 5% is preferred. The concentrations of phosphorus and magnesium shown herein may be, for example, values ​​obtained by elemental analysis of the entire positive electrode active material 100A using GC-MS, ICP-MS, etc., or they may be based on the values ​​of the raw material composition during the manufacturing process of the positive electrode active material 100A.

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

[0314] Furthermore, if the surface layer 100a contains both magnesium and nickel, divalent magnesium may be able to exist more stably near divalent nickel. Therefore, Li x Even when x in CoO2 is small, the elution of magnesium can be suppressed. Therefore, this can contribute to the stabilization of the surface layer 100a.

[0315] Furthermore, it is preferable to have both additive elements A, such as additive element X and additive element Y, which have different distributions, as this allows for stabilization of the crystal structure over a wider area. For example, if the 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 area than when it contains only one of additive elements X or Y. In this way, when the positive electrode active material 100A contains both additive elements X and Y, surface stabilization can be sufficiently achieved by additive element X such as magnesium, so additive element Y such as aluminum is not essential for the surface. Rather, it is preferable for aluminum to be widely distributed in deeper regions, for example, in the region between 5 nm and 50 nm from the surface, as this allows for stabilization of the crystal structure over a wider area.

[0316] As described above, having multiple additive elements A allows the effects of each additive element A to synergistically contribute to further stabilization of the surface layer 100a. In particular, having magnesium, nickel, and aluminum is preferable as it is highly effective in achieving a stable composition and crystal structure.

[0317] However, it is undesirable if the surface layer 100a is occupied only by compounds of additive element A and oxygen, as this makes lithium insertion and removal difficult. For example, it is undesirable for the surface layer 100a to be occupied only by MgO, a structure in which MgO and NiO(II) are in solid solution, and / or a structure in which MgO and CoO(II) are in solid solution. Therefore, the surface layer 100a must contain at least cobalt, and in the discharge state, it must also contain lithium and have pathways for lithium insertion and removal.

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

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

[0320] Furthermore, while it is preferable that some of the added elements A, particularly magnesium, nickel, and aluminum, are present at higher concentrations in the surface layer 100a than in the interior 100b, it is also preferable that they be present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites of the interior 100b, it has the effect of making it easier to maintain a layered rock salt-type crystal structure, similar to the above. Also, when nickel is present at an appropriate concentration in the interior 100b, the displacement of the layered structure consisting of octahedra of transition metal M and oxygen can be suppressed, similar to the above. In addition, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, so a synergistic effect of suppressing magnesium leaching can be expected.

[0321] ≪Li x When x is small during CoO2≫ In one aspect of the present invention, the positive electrode active material 100A has the above-described distribution and / or crystal structure of the additive element A in the discharge state, resulting in Li x The crystal structure of CoO2 when x is small differs from that of conventional cathode active materials. Here, x is small when it is 0.1 <x≦0.24をいうこととする。

[0322] Using FIGS. 30 to 34, the change in the crystal structure of Li x accompanying the change in x in CoO2 will be described while comparing the conventional cathode active material with the cathode active material 100A of one embodiment of the present invention.

[0323] The change in the crystal structure of the conventional cathode active material is shown in FIG. 31. The conventional cathode active material shown in FIG. 31 is lithium cobalt oxide (LiCoO2) that does not particularly have the additive element A. The change in the crystal structure of lithium cobalt oxide that does not particularly have the additive element A is described in Non-Patent Documents 1 to 3 and the like.

[0324] In FIG. 31, R-3m O3 is attached to show the crystal structure of lithium cobalt oxide with x = 1 in Li x CoO2. In this crystal structure, lithium occupies an octahedral site, and there are three CoO2 layers in the unit cell. Therefore, this crystal structure may be called an O3-type crystal structure. Here, the CoO2 layer means a structure in which an octahedral structure in which cobalt is coordinated with six oxygen atoms is continuous in a plane in a state of sharing edges. In some cases, this may be referred to as a layer composed of octahedrons of cobalt and oxygen.

[0325] It is also known that the conventional lithium cobalt oxide has a crystal structure belonging to the monoclinic space group P2 / m with enhanced symmetry of lithium when x is about 0.5. This structure has one CoO2 layer in the unit cell. Therefore, it may be called O1 type or monoclinic O1 type.

[0326] The cathode active material when x = 0 has a crystal structure of the trigonal space group P-3m1, and also has one CoO2 layer in the unit cell. Therefore, this crystal structure may be called O1 type or trigonal O1 type. In some cases, the trigonal crystal is converted into a composite hexagonal lattice and called hexagonal O1 type.

[0327] Furthermore, conventional lithium cobalt oxide at x=0.12 has a crystal structure with space group R-3m. This structure can be described as a structure in which trigonal O1 type CoO2 structures and R-3m O3 type LiCoO2 structures are alternately stacked. For this reason, this crystal structure is sometimes called the H1-3 type crystal structure. However, since actual lithium insertion and deinsertion can be uneven, the H1-3 type crystal structure is experimentally observed from x=0.25. In reality, the H1-3 type crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in this specification, including Figure 31, 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, in the H1-3 type crystal structure, as described in Non-Patent Literature 3, the coordinates of cobalt and oxygen in the unit cell can be represented 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, respectively. The unit cell that should 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 that yields the smallest GOF (goodness of fit) value should be adopted.

[0329] Li x When charging and discharging are repeated such that x in CoO2 becomes 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in its crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0330] However, these two crystal structures exhibit a large displacement of the CoO2 layer. As shown by the dotted line and arrow in Figure 31, in the H1-3 type crystal structure, the CoO2 layer is significantly displaced from the R-3m O3 in the discharge state. Such dynamic structural changes can negatively affect the stability of the crystal structure.

[0331] Furthermore, these two crystal structures also have a large volume difference. When compared per the same number of cobalt atoms, the volume difference between the H1-3 type crystal structure and the discharged R-3m O3 type crystal structure exceeds 3.5%, and is typically 3.9% or more.

[0332] In addition, the H1-3 type crystal structure, which has a continuous CoO2 layer like the trigonal O1 type, is likely to be unstable.

[0333] Therefore, repeated charging and discharging cycles that result in x being 0.24 or less cause the conventional lithium cobalt oxide crystal structure to break down. This breakdown of the crystal structure leads to a deterioration of the cycle characteristics. This is because the breakdown of the crystal structure reduces the number of sites where lithium can exist stably, and also makes it more difficult for lithium to be inserted and removed.

[0334] On the other hand, in the positive electrode active material 100A of one embodiment of the present invention shown in Figure 30, Li x The change in crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less in CoO2 is less than that of conventional positive electrode active materials. More specifically, the displacement 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 when compared per cobalt atom can be reduced. Therefore, the positive electrode active material 100A in one aspect of the present invention is less prone to crystal structure collapse even when repeated charging and discharging where x is 0.24 or less, and can achieve excellent cycle characteristics. In addition, the positive electrode active material 100A in one aspect of the present invention is Li x When x in CoO2 is 0.24 or less, it can adopt a more stable crystal structure than conventional positive electrode active materials. Therefore, the positive electrode active material 100A in one aspect of the present invention is Li x When the value of x in CoO2 remains below 0.24, short circuits are less likely to occur. In such cases, the safety of the secondary battery is further improved, which is preferable.

[0335] Li xFig. 30 shows the crystal structure of the interior 100b of the positive electrode active material 100A when x in CoO2 is about 1 and 0.2. Since the interior 100b occupies most of the volume of the positive electrode active material 100A and is a part that greatly contributes to charge and discharge, it can be said that the deviation and volume change of the CoO2 layer are the most problematic parts.

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

[0337] However, when x is 0.24 or less, for example, about 0.2 and about 0.15, at which the conventional lithium cobaltate has an H1-3 type crystal structure, the positive electrode active material 100A has crystals with a different structure.

[0338] The positive electrode active material 100A of one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. This is because the symmetry of the CoO2 layer is the same as that of O3. Therefore, this crystal structure will be referred to as the O3’ type crystal structure. Fig. 30 shows this crystal structure with R-3m O3’.

[0339] The O3’ type crystal structure can show the coordinates of cobalt and oxygen in the unit cell within the range of Co(0,0,0.5), O(0,0,x), 0.20 ≦ x ≦ 0.25. Also, the lattice constants of the unit cell are preferably 2.797 ≦ a ≦ 2.837 (×10 -1 nm), more preferably 2.807 ≦ a ≦ 2.827 (×10 -1 nm), and typically a = 2.817 (×10 -1 nm). The c-axis is preferably 13.681 ≦ c ≦ 13.881 (×10 -1 nm), more preferably 13.751 ≦ c ≦ 13.811, and typically c = 13.781 (×10 -1 nm).

[0340] In the O3’ type crystal structure, ions such as cobalt, nickel, and magnesium occupy the oxygen six-coordination positions. Light elements such as lithium may occupy the oxygen four-coordination positions.

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

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

[0343] Thus, in the positive electrode active material 100A of one aspect of the present invention, Li x When x in CoO2 is small, that is, when a large amount of lithium is desorbed, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume per unit of the same number of cobalt atoms is also suppressed. Therefore, the crystal structure of positive electrode active material 100A is less likely to collapse even when repeated charging and discharging cycles occur where x is 0.24 or less. As a result, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed in positive electrode active material 100A. Also, because it can stably utilize more lithium than conventional positive electrode active materials, positive electrode active material 100A has a high discharge capacity per unit weight and per unit volume. Therefore, by using positive electrode active material 100A, it is possible to manufacture secondary batteries with high discharge capacity per unit weight and per unit 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 estimated that it also has an O3' type crystal structure when x is greater than 0.24 and less than or equal to 0.27. However, the crystal structure is Li x Because x in CoO2 is affected not only by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it is not necessarily limited to the range of x mentioned above.

[0345] Therefore, the positive electrode active material 100A is Li x When x in CoO2 is greater than 0.1 and less than or equal to 0.24, the entire interior 100b of the positive electrode active material 100A does not necessarily have to have an O3' type crystal structure. It may contain other crystal structures, or a portion may be amorphous.

[0346] Also Li x To make x in CoO2 small, it is generally necessary to charge with a high charging voltage. x A state where x in CoO2 is small can be rephrased as a state where it has been charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6V or higher relative to the potential of lithium metal in an environment of 25°C, the H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6V or higher relative to the potential of lithium metal can be said to be a high charging voltage. Furthermore, unless otherwise specified in this specification, the charging voltage is expressed relative to the potential of lithium metal.

[0347] Therefore, the positive electrode active material 100A according to one aspect 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 4.6V or higher at 25°C. It is also preferable because it can adopt an O3' type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65V or higher and 4.7V or lower at 25°C.

[0348] Even with the positive electrode active material 100A, an H1-3 type crystal may only be observed when the charging voltage is further increased. Furthermore, as mentioned above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, electrolyte, etc., so even at lower charging voltages, for example, when the charging voltage is 4.5V or higher but less than 4.6V at 25°C, the positive electrode active material 100A according to one embodiment of the present invention may take on an O3' type crystal structure.

[0349] Furthermore, in the case of a secondary battery, if graphite is used as the negative electrode active material, for example, the voltage of the secondary battery will decrease by the amount of the graphite's potential compared to the above. The potential of graphite is approximately 0.05V to 0.2V relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is observed when the voltage obtained by subtracting the graphite's potential from the above voltage is obtained.

[0350] Furthermore, while Figure 30 shows O3' with lithium present at all lithium sites with equal probability, this is not the only way. It may be concentrated at some lithium sites, or, for example, as shown in Figure 31, monoclinic O1(Li 0.5 It may have symmetry similar to that of CoO2. The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0351] Furthermore, the O3' type crystal structure can be described as a crystal structure similar to the CdCl2 type crystal structure, although it has lithium randomly placed between the layers. This crystal structure similar to the CdCl2 type is formed when lithium nickelate is used. 0.06 Although the crystal structure is similar to that of NiO2 when charged to this level, it is known that pure lithium cobalt oxide, or layered rock salt-type cathode 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 additive element A is similar at multiple locations on the surface layer 100a of the positive electrode active material 100A. In other words, it is preferable that the reinforcement derived from additive element A is uniformly present on the surface layer 100a. Even if there is reinforcement in a part of the surface layer 100a, if there are parts without reinforcement, stress may concentrate in those parts. If stress concentrates in a part of the positive electrode active material 100A, defects such as cracks may occur from that point, which may lead to cracking of the positive electrode active material and a decrease in discharge capacity.

[0353] However, it is not necessarily required that the additive element A has a similar concentration gradient across the entire surface layer 100a of the positive electrode active material 100A. An example of the distribution of additive element X near CD in Figure 1B is shown in Figure 2C, and an example of the distribution of additive element Y near CD is shown in Figure 2D.

[0354] Here, we assume that the surface near CD is parallel to the arrangement of cations. The distribution of additive element A on the surface parallel to the arrangement of cations may differ from that of the other surfaces. For example, on the surface parallel to the arrangement of cations and its surface layer 100a, the distribution of one or more concentration peaks selected from additive elements X and Y may be limited to a shallower portion from the surface compared to the other orientations. Alternatively, on the surface parallel to the arrangement of cations and its surface layer 100a, the concentrations of one or more selected from additive elements X and Y may be lower compared to the other orientations. Alternatively, on the surface parallel to the arrangement of cations and its surface layer 100a, one or more selected from additive elements X and Y may be below the detection limit.

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

[0356] Since the CoO2 layer is relatively stable, the surface on which the CoO2 layer exists is also relatively stable. The main diffusion pathways of lithium ions during charging and discharging are not exposed on this surface.

[0357] On the other hand, the lithium ion diffusion pathway is exposed on surfaces that are not parallel to the cation arrangement, i.e., surfaces that are not parallel to the CoO2 layer. Therefore, the surface and surface layer 100a that are not parallel to the cation arrangement are important regions for maintaining the lithium ion diffusion pathway, but at the same time, they are prone to instability because they are the regions where lithium ions first desorb. For this reason, reinforcing the surface and surface layer 100a that are not parallel to the cation arrangement is extremely important for maintaining the overall crystal structure of the positive electrode active material 100A.

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

[0359] In the later embodiment, the manufacturing method involves first producing high-purity LiCoO2 and then mixing in additive element A and heating it. In this method, the additive element A spreads mainly through the diffusion pathway of lithium ions. Therefore, it is easier to control the distribution of additive element A on surfaces that are not parallel to the arrangement of cations and on their surface layer 100a to a desirable range.

[0360] Furthermore, while it is preferable that the surface of the positive electrode active material 100A be smooth and have few irregularities, it is not necessarily required that the entire positive electrode active material 100A be so. In composite oxides having a layered rock salt-type crystalline structure of R-3m, slip is likely to occur on planes parallel to the arrangement of cations, for example, on planes where lithium is arranged. For example, if there is a plane where lithium is arranged as shown in Figure 32A, slip may occur parallel to the plane where lithium is arranged during processes such as pressing, as indicated by the arrow in Figure 32B, causing deformation.

[0361] In this case, the newly formed surface and its surface layer 100a resulting from the slip may either not contain additive element A, or may be below the detection limit. EF in Figure 32B is an example of the newly formed surface and its surface layer 100a resulting from the slip. Enlarged views of the area around EF are shown in Figures 32C1 and 32C2. Unlike Figures 2A to 2D, additive elements X and Y are not distributed in Figures 32C1 and 32C2.

[0362] However, since slips tend 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 pathway. In this case, the lithium ion diffusion pathway is not exposed and is relatively stable, so there is little problem even if the additive element A is absent or below the detection limit.

[0363] As mentioned above, in composite oxides with the composition LiCoO2 and a layered rock salt type crystal structure of R-3m, cobalt and lithium are arranged parallel to the (001) plane. Furthermore, in HAADF-STEM images, the brightness of cobalt, which has the largest atomic number among LiCoO2 atoms, is the highest. Therefore, in HAADF-STEM images, the arrangement of atoms with high brightness can be considered to be the arrangement of cobalt. The repetition of this high-brightness arrangement is synonymous with crystal fringes or lattice fringes.

[0364] ≪Grain Boundaries≫ In addition to the distribution described above, it is more preferable that the additive element A in the positive electrode active material 100A of one aspect of the present invention is concentrated at or near the grain boundaries.

[0365] In this specification, "non-uniformity" refers to a situation where the concentration of an element in one region differs from that in other regions. It is synonymous with segregation, precipitation, heterogeneity, bias, or a mixture of areas with high and low concentrations.

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

[0367] Grain boundaries are a type of surface defect. Therefore, like surfaces, they tend to be unstable and prone to initiating changes in crystal structure. For this reason, increasing the concentration of additive element A at and near the grain boundaries can more effectively suppress changes in crystal structure.

[0368] Furthermore, if the magnesium and fluorine concentrations are high at and near the grain boundaries, even if a crack occurs along the grain boundary of the positive electrode active material 100A according to one embodiment of the present invention, the magnesium and fluorine concentrations will be high near the surface created by the crack. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the crack has occurred.

[0369] <Particle size> In one embodiment of the present invention, if the particle size of the positive electrode active material 100A is too large, problems arise such as difficulty in lithium diffusion and excessive roughness of the surface of the active material layer when coated onto the current collector. On the other hand, if it is too small, problems arise such as difficulty in supporting the active material layer when coating onto the current collector and excessive reaction with the electrolyte. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Or preferably 1 μm or more and 40 μm or less. Or preferably 1 μm or more and 30 μm or less. Or preferably 2 μm or more and 100 μm or less. Or preferably 2 μm or more and 30 μm or less. Or preferably 5 μm or more and 100 μm or less. Or preferably 5 μm or more and 40 μm or less.

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

[0371] XRD is particularly favored because it can analyze the symmetry of transition metals such as cobalt in the positive electrode active material with high resolution, compare the crystallinity and crystal orientation, analyze the periodic strain of the lattice and crystallite size, and obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling a secondary battery. Among XRD methods, powder XRD provides diffraction peaks that reflect the crystal structure of the interior 100b of the positive electrode active material 100A, which occupies most of the volume of the positive electrode active material 100A.

[0372] The positive electrode active material 100A in one aspect of the present invention is Li x A characteristic feature 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 changes significantly and where the crystal structure accounts for more than 50% are undesirable because they cannot withstand high-voltage charging and discharging.

[0373] It is also important to note that simply adding element A may not result in the formation of an O3' type crystal structure. For example, even if lithium cobalt oxide has magnesium and fluorine, or lithium cobalt oxide has magnesium and aluminum, the structure may differ depending on the concentration and distribution of element A. x There are two cases in CoO2: one where x is 0.24 or less and the O3' type crystal structure accounts for more than 60%, and another where the H1-3 type crystal structure accounts for more than 50%.

[0374] Furthermore, even with the positive electrode active material 100A according to one aspect of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9V, an H1-3 type or trigonal O1 type crystal structure may occur. Therefore, in order to determine whether or not the positive electrode active material 100A is according to one aspect of the present invention, analysis of the crystal structure, including XRD, and information such as charging capacity or charging voltage are necessary.

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

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

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

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

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

[0380] Lithium metal can be used for the counter electrode. However, if a material other than lithium metal is used for the counter electrode, the potential of the secondary battery and the potential of the positive electrode will differ. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0381] The electrolyte in the electrolyte solution may be 1 mol / L lithium hexafluoride phosphate (LiPF6), and the electrolyte solution may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7, and vinylene carbonate (VC) at 2 wt%.

[0382] A 25 μm thick porous polypropylene film can be used as the separator.

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

[0384] The coin cell prepared under the above conditions is charged with a constant current of 10 mA / g to an arbitrary voltage (e.g., 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V). It is desirable to charge with such a small current to observe the phase change of the positive electrode active material. The temperature should be 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere, and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. When performing various analyses afterward, it is preferable to seal the sample under an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the sample in a sealed container under an argon atmosphere. Furthermore, it is preferable to remove the positive electrode and subject it to analysis as soon as possible after charging is complete. Specifically, it is preferable to do so within 1 hour, and more preferably within 30 minutes.

[0385] Furthermore, when analyzing the crystal structure of the charged state after multiple charge-discharge cycles, the conditions for these multiple charge-discharge cycles may differ from the charging conditions described above. For example, charging can be performed by constant current charging at a current of 100 mA / g up to an arbitrary voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V), then constant voltage charging until the current reaches 10 mA / g, and discharging can be performed by constant current discharge at 2.5V and 100 mA / g.

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

[0387] ≪XRD≫ The equipment and conditions for XRD measurement are not particularly limited. For example, measurements can be performed using the following equipment and conditions. XRD system: Bruker AXS D8 ADVANCE X-ray source: CuKα1 ray Output: 40KV, 40mA Slit width: Div.Slit, 0.5° Detector: LynxEye Scanning 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 sample to be measured is a powder, it can be set up by placing it in a glass sample holder or by sprinkling the sample onto a grease-coated silicone anti-reflective plate. If the sample to be measured is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set up to match the measurement surface required by the device.

[0389] Figures 33 and 34 show the ideal powder XRD patterns calculated using CuKα1 lines, based on the O3' type crystal structure and the H1-3 type crystal structure model. In Figure 34, Li is shown for comparison. xThe ideal XRD patterns calculated from the crystal structures of LiCoO2O3 at x=1, H1-3 type, and trigonal O1 at x=0 in CoO2 are also shown. The patterns for LiCoO2(O3) and CoO2(O1) were created using the Reflex Powder Diffraction module in Materials Studio (BIOVIA) from crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 4). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength λ1 = 1.540562 × 10⁻¹⁰. -10 m and λ2 were not set, and the Monochromator was set to single. The H1-3 type crystal structure pattern was similarly created from the crystal structure information described in Non-Patent Literature 3. For the O3' type crystal structure pattern, the crystal structure was estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, fitted using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker), and the XRD pattern was created in the same manner as the others.

[0390] As shown in Figure 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, peaks do not appear at these positions in the H1-3 type crystal structure and trigonal O1. Therefore, Li 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 is a characteristic feature of the positive electrode active material 100A in one embodiment of the present invention.

[0392] This can also be described as the positions where XRD diffraction peaks appear being close together in the crystal structure at x=1 and x≦0.24. More specifically, for the main diffraction peaks in the crystal structure at x=1 and x≦0.24 where 2θ is between 42° and 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0393] In one embodiment of the present invention, the positive electrode active material 100A is Li x When x in CoO2 is small, it has an O3' type crystal structure, but it does not have to be entirely an O3' type crystal structure. It may contain other crystal structures, and part of it may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3' type crystal structure accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3' type crystal structure accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be made into a cathode active material with sufficiently excellent cycle characteristics.

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

[0395] Furthermore, the sharpness of diffraction peaks in the XRD pattern indicates high crystallinity. Therefore, it is preferable for each diffraction peak after charging to be sharp, i.e., have a narrow full width at half maximum (FWHM). The FWHM varies depending on the XRD measurement conditions and the value of 2θ, even for peaks arising from the same crystalline phase. Under the measurement conditions described above, for peaks observed between 2θ = 43° and 46°, the FWHM is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. It is not necessary for all peaks to satisfy this requirement. If some peaks satisfy this requirement, it can be said that the crystal phase has high crystallinity. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0396] Furthermore, the crystallite size of the O3'-type crystal structure of the positive electrode active material 100A decreases to only about 1 / 20th of that of LiCoO2(O3) in the discharged state. Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, Li x When x is small in CoO2, a clear peak of the O3'-type crystal structure can be observed. On the other hand, in conventional LiCoO2, even if some parts can adopt 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 full width at half maximum of the XRD peak.

[0397] ≪XPS≫ In X-ray photoelectron spectroscopy (XPS), when using monochromatic aluminum Kα rays as the X-ray source for inorganic oxides, it is possible to analyze regions from the surface to a depth of approximately 2 nm to 8 nm (typically less than 5 nm). Therefore, the concentration of each element can be quantitatively analyzed in a region that is about half the depth of the surface layer 100a. Furthermore, the bonding state of elements can be analyzed using narrow-scan analysis. The quantitative accuracy of XPS is often around ±1 atomic percent, and the detection limit is also around 1 atomic percent, although this varies depending on the element.

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

[0399] In one embodiment of the present invention, the surface and surface layer 100a of the positive electrode active material 100A do not contain carbonates, hydroxyl groups, etc., that have been chemically adsorbed after the production of the positive electrode active material 100A. Furthermore, electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100A are also excluded. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc., which can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds through analysis, and corrections may be made to exclude CF bonds derived from the binder.

[0400] Furthermore, before subjecting the sample to various analyses, the positive electrode active material and positive electrode active material layer may be washed to remove electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve into the solvent used for washing, but even in that case, the additive element A is unlikely to dissolve, so it will not affect the atomic ratio of additive element A.

[0401] Furthermore, the concentration of additive element A may be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it reduces the influence of carbon dioxide and other substances chemically adsorbed after the production 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 or more and 1.5 or less. On the other hand, the Mg / Co ratio determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0402] Similarly, in order to ensure sufficient pathways for lithium insertion and deinsertion, it is preferable that the concentrations of lithium and cobalt in the surface layer 100a of the positive electrode active material 100A are higher than those of each additive element A. This means that it is preferable that the concentrations of lithium and cobalt in the surface layer 100a are higher than the concentrations of one or more additive elements A selected from the additive elements A present in the surface layer 100a as measured by XPS, etc. For example, it is preferable that the concentration of cobalt in at least a portion of the surface layer 100a as measured by XPS, etc. is higher than the concentration of magnesium in at least a portion of the surface layer 100a as measured by XPS, etc. 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. Similarly, it is 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 that of aluminum. Similarly, it is preferable that the concentration of lithium is higher than that of aluminum. It is also preferable that the concentration of cobalt is higher than that of fluorine. Similarly, it is preferable that the concentration of lithium is higher than that of fluorine.

[0403] Furthermore, it is more preferable that the additive elements Y, including aluminum, are widely distributed in deeper regions, for example, in the region between 5 nm and 50 nm from the surface. Therefore, it is more preferable that, although additive elements Y, including aluminum, are detected in the analysis of the entire cathode active material 100A using ICP-MS, GD-MS, etc., they are below the detection limit in XPS, etc.

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

[0405] For XPS analysis, for example, monochromatic aluminum Kα radiation can be used as the X-ray source. The extraction angle can be set to, for example, 45°. Measurements can be performed using, for example, the following equipment and conditions. Measurement device: PHI QuanteraII X-ray source: Monochromatic Al Kα (1486.6eV) Detection area: 100 μmφ Detection depth: Approximately 4-5 nm (extraction angle 45°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0406] Furthermore, when the positive electrode active material 100A according to one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably around 684.3 eV. This value is different from both the bond energy of lithium fluoride, which is 685 eV, and the bond energy of magnesium fluoride, which is 686 eV. In other words, when the positive electrode active material 100A according to one embodiment of the present invention contains fluorine, it is preferable that the bond is with an element other than lithium fluoride or magnesium fluoride.

[0407] Furthermore, when the positive electrode active material 100A according to one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably around 1303 eV. This value is different from the bond energy of magnesium fluoride, which is 1305 eV, and is close to the bond energy of magnesium oxide. In other words, when the positive electrode active material 100A according to one embodiment of the present invention contains magnesium, it is preferable that the bond is with an element other than magnesium fluoride.

[0408] ≪EDX≫ It is preferable that one or more of the additive elements A present in the positive electrode active material 100A have a concentration gradient. More preferably, the depth of the concentration peaks from the surface differs depending on the additive element A of the positive electrode active material 100A. The concentration gradient of the additive element A can be evaluated, for example, by exposing the cross-section of the positive electrode active material 100A using a FIB (Focused Ion Beam) and analyzing the cross-section using energy dispersive X-ray spectroscopy (EDX), EPMA (Electron Probe Microanalysis), etc.

[0409] Among EDX measurements, EDX surface analysis refers to a method of scanning within a region to evaluate it in two dimensions. Line analysis refers to a method of scanning linearly to evaluate the distribution of atomic concentrations within the positive electrode active material. Furthermore, line analysis is sometimes used to describe the extraction of linear region data from EDX surface analysis. Finally, point analysis refers to a method of measuring a region without scanning.

[0410] EDX surface analysis (e.g., elemental mapping) allows for semi-quantitative analysis of the concentration of additive element A in the surface layer 100a, interior 100b, and near grain boundaries of the positive electrode active material 100A. Furthermore, EDX radiation analysis allows for the analysis of the concentration distribution and maximum value of additive element A. Analysis methods that thin the sample, such as STEM-EDX, are more preferable because they allow for the analysis of 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 surface analysis or EDX point analysis is performed on the positive electrode active material 100A according to one embodiment of the present invention, it is preferable that the concentration of each additive element A, particularly additive element X, in the surface layer 100a is higher than that of the interior 100b.

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

[0413] Furthermore, in positive electrode active material 100A having magnesium and fluorine as additive elements X, it is preferable that the distribution of fluorine overlaps with the distribution of magnesium. For example, it is preferable that the difference in depth between the peak of fluorine concentration and the peak of magnesium concentration is within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0414] Furthermore, when EDX radiation analysis is performed, the fluorine concentration peak in the surface layer 100a is preferably located within a depth of 3 nm from the surface toward the center of the positive electrode active material 100A, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. It is also preferable that the fluorine concentration peak is located slightly closer to the surface than the magnesium concentration peak, as this increases resistance to hydrofluoric acid. For example, it is more preferable that the fluorine concentration peak is located 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferable that it is located 1.5 nm or more closer to the surface.

[0415] Furthermore, in the positive electrode active material 100A having nickel as the additive element X, the nickel concentration peak in the surface layer 100a is preferably located 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, in the positive electrode active material 100A having magnesium and nickel, the distribution of nickel is preferably superimposed on the distribution of magnesium. For example, the difference in depth between the magnesium concentration peak and the magnesium concentration peak is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

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

[0417] Furthermore, when EDX radiation analysis, surface 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 magnesium concentration peak is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum Al to cobalt Co (Al / Co) at the aluminum concentration peak is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel Ni to cobalt Co (Ni / Co) at the nickel concentration peak is preferably 0 or more and 0.2 or less, and more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine F to cobalt Co (F / Co) at the fluorine concentration peak is preferably 0 or more and 1.6 or less, and more preferably 0.1 or more and 1.4 or less.

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

[0419] Since the positive electrode active material 100A is a composite oxide, the surface can be estimated using the amount of oxygen detected. Specifically, first, the average value of the oxygen concentration is obtained from the region where the amount of oxygen detected in the interior 100b is stable. ave We determine the amount of oxygen O in the region that is clearly outside the surface, which is thought to be due to chemiadsorption or background. background If detected, O background Subtracting this gives the average oxygen concentration O aveThis can be done. This average value O ave Half of that value, that is, 1 / 2O ave The measurement point that shows the closest measurement value can be estimated to be the surface of the positive electrode active material.

[0420] The surface can also be estimated in the same way as described above using the amount of cobalt detected. Alternatively, it can be estimated similarly using the sum of the detected amounts of multiple transition metals. The detected amounts of transition metals, including cobalt, are suitable for surface estimation because they are less affected by chemiadsorption.

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

[0422] For example, when the additive element X is magnesium, when line analysis or surface analysis is performed on the positive electrode active material 100A, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) near the grain boundaries is preferably 0.020 or more and 0.50 or less. Furthermore, 0.025 or more and 0.30 or less is preferred. Furthermore, 0.030 or more and 0.20 or less is preferred. Or 0.020 or more and 0.30 or less is preferred. Or 0.020 or more and 0.20 or less is preferred. Or 0.025 or more and 0.50 or less is preferred. Or 0.030 or more and 0.50 or less is preferred. Or 0.030 or more and 0.30 or less is preferred.

[0423] ≪EPMA≫ EPMA (Electron Probe Microanalysis) can also quantify elements. Surface analysis allows for the analysis of the distribution of each element.

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

[0425] However, EPMA analyzes only the area from the surface down to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from those obtained using other analytical methods. For example, when surface analysis of cathode active material 100A is performed using EPMA, the concentration of each additive element A present in the surface layer 100a may be lower than that obtained with XPS.

[0426] ≪Charging curve and dQ / dV vs sV curve≫ In one embodiment of the present invention, the positive electrode active material 100A may exhibit characteristic voltage changes during charging. These voltage changes can be read from the dQ / dVvsV curve, obtained by differentiating the capacitance (Q) with respect to voltage (V) from the charging curve (dQ / dV). For example, around the peak in the dQ / dVvsV curve, a non-equilibrium phase change is thought to occur, resulting in a significant change in the crystal structure. In this specification, a non-equilibrium phase change refers to a phenomenon that causes a nonlinear change in a physical quantity.

[0427] In one embodiment of the present invention, the positive electrode active material 100A may have a broad peak around 4.55V in the dQ / dVvsV curve. The peak around 4.55V reflects the voltage change during the phase change from O3 type to O3' type. Therefore, a broad peak means that the change in energy required to extract lithium is smaller than when the peak is sharp, i.e., the change in crystal structure is smaller. Smaller changes are preferable because they reduce the influence of shift and volume changes in the CoO2 layer.

[0428] More specifically, in the dQ / dV vs V curve of the charging curve, when the maximum value appearing between 4.5V and 4.6V is defined as the first peak, it is considered sufficiently broad and preferable if the full width at half maximum of the first peak is 0.10V or more.

[0429] When acquiring the dQ / dV vs V curve, charging can be performed using a constant current charge of 10mA / g up to, for example, 4.9V. Furthermore, when acquiring the dQ / dV for the initial charge, it is preferable to discharge the battery to 2.5V at 100mA / g before starting the above charging process.

[0430] The data acquisition interval during charging can be set to, for example, every second or to acquire voltage and current when there is a voltage fluctuation of 1 mV. The integrated value of the current and time is used as the charging capacity.

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

[0432] However, since the above data is susceptible to the effects of minute noise, dQ / dV may be calculated from a moving average of a certain number of intervals for the difference between voltage and charging capacity. The number of intervals can be, for example, 500.

[0433] Specifically, the average value of dQ from the nth to the n+500th value is calculated, and similarly, the average value of dV from the nth to the n+500th value is calculated. dQ(average of 500 values) / dV(average of 500 values) can be expressed as dQ / dV. Similarly, the voltage on the horizontal axis of the dQ / dV vs V graph can use the moving average value from the nth to the n+500th value as the interval. However, when using the moving average described above, it is preferable not to use the data from the 501st to the last value onwards in the dQ / dV vs V graph, as the noise effect becomes significant.

[0434] Furthermore, when analyzing the dQ / dV vs. V curve after multiple charge-discharge cycles, the conditions for these multiple cycles may differ from the charging conditions described above. For example, charging can be performed at an arbitrary voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V) with a constant current of 100mA / g, followed by constant voltage charging until the current reaches 10mA / g, and then discharging can be performed at 2.5V with a constant current of 100mA / g.

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

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

[0437] ≪ESR≫ In one embodiment of the present invention, the positive electrode active material 100A contains cobalt, and preferably contains nickel and magnesium as additive elements A. As a result, some Co 3+ Ni 3+ It is replaced by, and also some Li + is Mg 2+ It is preferable that it be replaced with Li. + is Mg 2+ As a result of being replaced by, the Ni 3+ It is reduced to Ni2+ This can happen. Also, some Li + is Mg 2+ It is replaced by Mg 2+ Nearby Co 3+ It is reduced to Co 2+ This can happen. Also, some Co 3+ is Mg 2+ It is replaced by Mg 2+ Nearby Co 3+ It is oxidized to Co 4+ This can happen.

[0438] Therefore, the positive electrode active material 100A is Ni 2+ Ni 3+ Co 2+ and Co 4+ It is preferable to have one or more of the following. Also, the Ni per unit weight of the positive electrode active material 100A 2+ Ni 3+ Co 2+ and Co 4+ The spin density resulting from one or more of the following is 2.0 × 10 17 spins / g or more 1.0×10 21 It is preferable that the spin density is less than or equal to spins / g. Using the positive electrode active material 100A having the aforementioned spin density is preferable because it stabilizes the crystal structure, especially in the charged state. However, if the magnesium concentration is too high, Ni 2+ Ni 3+ Co 2+ and Co 4+ A decrease in spin density may occur due to one or more of the following factors.

[0439] The spin density in the positive electrode active material can be analyzed using methods such as electron spin resonance (ESR).

[0440] ≪Surface roughness and specific surface area≫ In one embodiment of the present invention, the positive electrode active material 100A preferably has a smooth surface with few irregularities. A smooth surface with few irregularities indicates that the effect of the flux described later has been fully exerted, and the surface of the composite oxide and the source of the added element A have melted. Therefore, it is one factor indicating that the distribution of the added element A in the surface layer 100a is good. Good distribution means, for example, that the concentration distribution of the added element A in the surface layer 100a is uniform.

[0441] The smoothness and minimal irregularities of the surface can be determined, for example, from a cross-sectional SEM image or TEM image of the positive electrode active material 100A, or from the specific surface area of ​​the positive electrode active material 100A.

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

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

[0444] In this embodiment, the surface of the positive electrode active material 100A preferably has a root mean square (RMS) surface roughness, which is an indicator 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 used for noise reduction, interface 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 this ratio.

[0447] Ideal specific surface area S i This is calculated by assuming that all positive electrode active materials have the same diameter as D50, the same weight, and are ideally spherical in shape.

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

[0449] In one embodiment of the present invention, the positive electrode active material 100A has an ideal specific surface area A determined from the median diameter D50. i And the actual specific surface area S R Ratio S R / S i It is preferable that the value is between 1.0 and 2.1.

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

[0451] First, a surface SEM image of the positive electrode active material 100A is obtained. A conductive coating may be applied as a pretreatment before observation. It is preferable that the observation surface is 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 (for example, "ImageJ"), the above SEM image is converted to, for example, an 8-bit image (this is called a grayscale image). A grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be represented by 2 to the power of 8 = 256 gradations. Darker areas have a lower number of gradations, and brighter areas have a higher number of gradations. The change in luminance can be quantified in relation to the number of gradations. This numerical value is called the grayscale value. By obtaining the grayscale value, it becomes possible to numerically evaluate the unevenness of the positive electrode active material.

[0453] Furthermore, it becomes possible to represent the brightness changes of the target area using a histogram. A histogram is a three-dimensional representation of the grayscale distribution in a target area, and is also called a brightness histogram. By obtaining a brightness histogram, it becomes possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

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

[0455] ≪Current pause method≫ In one embodiment of the present invention, the magnesium and other additive elements A present in the surface layer of the positive electrode active material 100A may undergo slight changes in their distribution during repeated charge-discharge cycles. For example, the distribution of additive elements A may improve, leading to a decrease in electron conduction resistance. As a result, the electrical resistance, i.e., the fast-responding resistance component R(0.1s) measured by the current pause method, may decrease in the initial stages of the charge-discharge cycle.

[0456] For example, when comparing the nth charge (where n is a natural number greater than 1) with the (n+1)th charge, the fast-responding resistance component R(0.1s), measured by the current pause method, may be lower in the (n+1)th charge than in the nth charge. Consequently, the discharge capacity in the (n+1)th charge may be higher than that in the nth charge. When n is 1, that is, when comparing the first charge with the second charge, the second charge capacity may be larger, which can occur even with positive electrode active materials that do not contain additive elements. Therefore, it is preferable that n is, for example, between 2 and 10. However, this is not limited to the initial stages of the charge-discharge cycle. A charge-discharge capacity that is approximately equal to the rated capacity, for example, 97% or more of the rated capacity, can be considered the initial stage of the charge-discharge cycle.

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

[0458] Figure 35 shows a schematic cross-sectional view of the positive electrode active material 51 having pits. A crystal plane 55 parallel to the arrangement of cations is also shown. Since Figure 35 is a cross-sectional view, pits 54 and 58 are shown as holes, but the shape of these openings is not circular but has depth and a groove-like shape. Also, as shown in pits 54 and 58, unlike recesses 52, they tend to occur parallel to the arrangement of lithium ions.

[0459] Furthermore, the surface layers of the positive electrode active material 51 where the added element A is present are shown as 53 and 56. In the surface layers where pits occur, the amount of added element A is less than in 53 and 56 or below the detection limit, suggesting that the function of the barrier film is reduced. In addition, it is thought that the crystal structure of the composite oxide collapses near where the pits form, resulting in a crystal structure different from that of the layered rock salt type. Since the collapse of the crystal structure inhibits the diffusion and release of lithium ions, which are carrier ions, the pits are considered to be a factor in the deterioration of cycle characteristics.

[0460] The source of the pits may be point defects. It is thought that point defects in the positive electrode active material change with repeated charging and discharging, and are chemically or electrochemically eroded by the surrounding electrolyte, or that the material deteriorates. This deterioration does not occur uniformly on the surface of the positive electrode active material, but rather occurs locally and concentrated.

[0461] Furthermore, as shown in crack 57 in Figure 35, defects such as cracks (also called fissures) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In this specification, cracks and pits are different. Cracks may exist immediately after the positive electrode active material is manufactured, but pits do not. A pit can be described as a hole where several layers of transition metal M and oxygen have been removed due to charging and discharging under high voltage conditions of 4.5V or higher or high temperature (45°C or higher), and can also be described as a location where the transition metal M has dissolved. A crack refers to a new surface that is created by the application of physical pressure, or a fissure that originates from a grain boundary. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In addition, pits may occur from cracks and / or cavities inside the positive electrode active material.

[0462] [Method for preparing positive electrode active material] In order to produce 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, the method of adding the additive element A is important. 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 having lithium and a transition metal, and then mix in the additive element A source and perform a heat treatment.

[0464] In a method of synthesizing a composite oxide containing additive element A, lithium, and transition metal M by mixing a transition metal M source and a lithium source simultaneously with an additive element A source, it is difficult to increase the concentration of additive element A in the surface layer 100a. Furthermore, if the additive element A source is only mixed after synthesizing the composite oxide containing lithium and transition metal M without heating, the additive element will only adhere to the composite oxide without solid dissolution. Without sufficient heating, it is also difficult to properly distribute the additive element A. Therefore, it is preferable to synthesize the composite oxide, then mix in the additive element A source, and then perform a heat treatment. This heat treatment after mixing in the additive element A source is sometimes called annealing.

[0465] However, if the annealing temperature is too high, cation mixing occurs, increasing the likelihood that the added element A, for example, magnesium, will enter the transition metal M site. Magnesium present at the transition metal M site is Li x When x in CoO2 is small, it does not maintain the layered rock salt crystal structure of R-3m. Furthermore, if the heating temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent cobalt and the evaporation of lithium.

[0466] Therefore, it is preferable to mix a material that functions as a flux with the source of additive element A. Any material that has a lower melting point than the composite oxide having lithium and transition metal M can be considered to function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux causes a melting point depression in both the source of additive element A and the composite oxide having lithium and transition metal M. By lowering the melting point, it becomes easier to distribute the additive element A well at a temperature where cation mixing is less likely to occur.

[0467] Furthermore, it is even more preferable to heat the composite oxide containing lithium and transition metal M before mixing in the added element A. This heating is sometimes referred to as initial heating.

[0468] Initial heating causes lithium to desorb from a portion of the surface layer 100a of the composite oxide containing lithium and transition metal M, which further improves the distribution of additive element A.

[0469] More specifically, the following mechanism is thought to make it easier to differentiate the distribution of added element A through initial heating. First, lithium is desorbed from a portion of the surface layer 100a by 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 added element A, including nickel, aluminum, and magnesium, and heated. Of the added element A, magnesium is a divalent typical element, and nickel is a transition metal but is prone to becoming a divalent ion. Therefore, Mg 2+ and Ni 2+ And, due to lithium deficiency, Co 2+ A rock salt-type phase is formed, which has the following characteristics.

[0470] Of the added elements A, nickel readily dissolves in the composite oxide containing layered rock salt-type lithium and transition metal M in the surface layer 100a, and diffuses to the interior 100b. However, if a portion of the surface layer 100a is rock salt-type, nickel tends to remain in the surface layer 100a.

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

[0472] For example, rock salt mold mayonnaise 0.5 Mg 0.5 The Me-O distance in O is 2.09 × 10⁻⁶. -1 The Me-O distance in rock salt-type MgO is 2.11 × 10⁻⁶ nm. -1 The distance is nm. Furthermore, 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 × 10⁻⁶.-1 The Me-O distance of spinel-type MgAl2O4 is 2.02 × 10⁻⁶ nm. -1 The distance is nm. In all cases, the Me-O distance is 2 × 10⁻⁶. -1 It exceeds nm.

[0473] On the other hand, in layered rock salt, the bond distance between metals other than lithium and oxygen is shorter than described above. For example, the Al-O distance in layered rock salt LiAlO2 is 1.905 × 10⁻⁶ -1 nm (Li-O distance is 2.11 × 10⁻¹⁰) -1 The distance is (nm). Furthermore, the Co-O distance in layered rock salt-type LiCoO2 is 1.9224 × 10⁻⁶. -1 nm (Li-O distance is 2.0916 × 10⁻¹⁶) -1 It is (nm).

[0474] According to Shannon et al., Acta A 32(1976) 751, the ionic radius of 6-coordinate aluminum is 0.535 × 10⁻⁶. -1 The ionic radius of oxygen in nm and 6-coordinate state is 1.4 × 10⁻⁶. -1 The units are in nm, and their sum is 1.935 × 10⁻⁶. -1 It is nm.

[0475] From the above, it is considered that aluminum exists more stably at sites other than lithium in the layered rock salt type than in the rock salt type. Therefore, aluminum is more likely to be distributed in deeper regions with the layered rock salt type and / or in the interior 100b of the surface layer 100a than in the region closer to the surface with the rock salt type phase.

[0476] Furthermore, initial heating is expected to enhance the crystallinity of the layered rock salt-type crystalline structure within the internal 100b layer.

[0477] However, initial heating is not always necessary. By controlling the atmosphere, temperature, time, etc., in other heating processes, such as annealing, Li x When x in CoO2 is small, it may be possible to produce a positive electrode active material 100A that has the O3' type.

[0478] An example of the production flow for the positive electrode active material 100A, including annealing and initial heating, will be explained using Figures 36A to 36C.

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

[0480] As a lithium source, it is preferable to use a lithium-containing compound, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. The lithium source should preferably have high purity; for example, a material with a purity of 99.99% or higher is preferable.

[0481] The transition metal M can be selected from elements listed in groups 3 to 11 of the periodic table, for example, at least one of manganese, cobalt, and nickel can be used. In other words, the transition metal M can be cobalt only, nickel only, cobalt and manganese, cobalt and nickel, or cobalt, manganese, and nickel. When cobalt only is used, the resulting positive electrode active material contains lithium cobalt oxide (LCO), and when cobalt, manganese, and nickel are used, the resulting positive electrode active material contains nickel-cobalt-lithium manganese oxide (NCM).

[0482] As the transition metal M source, it is preferable to use a compound having the above-mentioned transition metal M. For example, oxides of the metals exemplified above, or hydroxides of the exemplified metals, etc., can be used as the transition metal M. If it is a cobalt source, cobalt oxide, cobalt hydroxide, etc., can be used. If it is a manganese source, manganese oxide, manganese hydroxide, etc., can be used. If it is a nickel source, nickel oxide, nickel hydroxide, etc., can be used. If it is an aluminum source, aluminum oxide, aluminum hydroxide, etc., can be used.

[0483] The transition metal M source should preferably have high purity; for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N5 (99.995%) or higher, and even more preferably 5N (99.999%) or higher should be used. 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, having single crystal grains. The crystallinity of the transition metal M source can be evaluated by TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle scattering 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, etc., or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. The above methods for evaluating crystallinity can be applied not only to transition metal M sources but also to the evaluation of other materials.

[0485] Furthermore, when using two or more transition metal M sources, it is preferable to prepare them in a proportion (mixing ratio) such that the two or more transition metal M sources can adopt a layered rock salt type crystalline structure.

[0486] <Step S12> Next, as shown in step S12 in Figure 36A, the lithium source and the transition metal M source are crushed and mixed to prepare a mixed material. Crushing and mixing can be done dry or wet. Wet crushing is preferred because it allows for finer crushing. If wet crushing is used, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. It is more preferable to use an aprotic solvent that does not react easily 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 with dehydrated acetone with a purity of 99.5% or higher, with a water content of 10 ppm or less, and then crush and mix them. By using dehydrated acetone of such purity as described above, the amount of impurities that may be introduced can be reduced.

[0487] A ball mill or bead mill can be used for mixing and other processes. When using a ball mill, it is preferable to use aluminum oxide balls or zirconium oxide balls as the grinding media. Zirconium oxide balls are preferable because they produce less impurity. When using a ball mill or bead mill, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less in order 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 Figure 36A, the mixed material is heated. The heating temperature is preferably between 800°C and 1100°C, more preferably between 900°C and 1000°C, and even more preferably around 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 the evaporation of lithium from the lithium source and / or the excessive reduction of the metal used as the transition metal M source. Such defects include, for example, when cobalt is used as the transition metal M, excessive reduction can cause the cobalt to change from trivalent to divalent, inducing 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 heating rate depends on the target temperature, but a rate between 80°C / h and 250°C / h is generally recommended. For example, when heating to 1000°C for 10 hours, a heating rate of 200°C / h is appropriate.

[0491] The heating atmosphere should preferably be a dry air atmosphere with low moisture content, for example, an atmosphere with a dew point of -50°C or lower, more preferably -80°C or lower. In this embodiment, heating will be carried out in an atmosphere with a dew point of -93°C. In order to suppress impurities that may be mixed into the material, the concentrations of impurities such as CH4, CO, CO2, and H2 in the heating atmosphere should be kept below 5 ppb (parts per billion) each.

[0492] An atmosphere containing oxygen is preferred as the heating atmosphere. For example, one method is to continuously introduce dry air 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 oxygen-containing atmosphere, a method that does not involve flowing oxygen is also acceptable. For example, the reaction chamber can be depressurized and then filled with oxygen, preventing the oxygen from entering or leaving the reaction chamber; this method is called purging. For instance, the reaction chamber can be depressurized to -970 hPa using a differential pressure gauge, and then filled with oxygen up to 50 hPa.

[0494] After heating, natural cooling is acceptable, but it is preferable that the cooling time from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary; it is sufficient if it cools to a temperature acceptable for the next step.

[0495] Heating in this process may be carried out using a rotary kiln or a roller hearth kiln. When using a rotary kiln, heating can be performed 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 prone to impurities, the purity of the alumina crucible or sheath should be 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 before heating. This prevents the material from volatilizing.

[0497] After heating is complete, the material may be crushed and sieved as needed. When collecting the heated material, it may be transferred from the crucible to a mortar before collection. It is preferable to use a mortar made of aluminum oxide. Mortars made of aluminum oxide are less likely to release impurities. Specifically, a mortar made of aluminum oxide with a purity of 90% or higher, preferably 99% or higher, should be used. In addition, heating conditions equivalent to those in step S13 can be applied to the heating processes described later, other than step S13.

[0498] <Step S14> Through the above process, a composite oxide (LiMO2) containing a transition metal M can be obtained in step S14 shown in Figure 36A. The composite oxide only needs to have the crystal structure of a lithium composite oxide represented as 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 as LiCoO2. The composition is not strictly limited to Li:Co:O=1:1:2.

[0499] Although examples of producing composite oxides by solid-phase methods have been shown as in steps S11 to S14, composite oxides may also be produced by coprecipitation or by hydrothermal methods.

[0500] <Step S15> Next, as step S15 shown in Figure 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 heated before step S20 shown below, it may be called preheating or pretreatment.

[0501] As described above, initial heating causes lithium to be desorbed from a portion of the surface layer 100a of the composite oxide. It is also expected to improve the crystallinity of the interior 100b. Furthermore, the lithium source and / or transition metal M prepared in step S11, etc., may contain impurities. Initial heating makes it possible to reduce impurities from the composite oxide completed in step 14.

[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 generally rounded, and the corners are also rounded. In addition, a smooth surface is defined as having few foreign substances adhering to it. Foreign substances are thought to be a cause of irregularities, so it is preferable that they do not adhere to the surface.

[0503] For this initial heating, it is not necessary to prepare a lithium compound source, nor is it necessary to prepare a source of additive element A, nor is it necessary to prepare a material that functions as a flux.

[0504] If the heating time in this process is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, it can be performed by selecting from the heating conditions described in step S13. To add to those heating conditions, the heating temperature in this process should be lower than the temperature in step S13 in order to maintain the crystal structure of the composite oxide. Also, the heating time in this process should be shorter than the time in step S13 in order to maintain the crystal structure of the composite oxide. For example, heating at a temperature of 700°C to 1000°C for 2 to 20 hours is recommended.

[0505] Furthermore, the effect of increasing the crystallinity of the internal 100b is, for example, the effect of mitigating strain, displacement, etc., that originate from the difference in shrinkage, etc., of the composite oxide fabricated in step S13.

[0506] The above-mentioned composite oxide may develop a temperature difference between its surface and interior due to heating in step S13. This temperature difference can induce a difference in shrinkage. It is thought that the difference in shrinkage occurs because the fluidity of the surface and interior differs due to the temperature difference. The energy associated with the difference in shrinkage gives the composite oxide a difference in internal stress. This difference in internal stress is also called strain, and the energy associated with it is sometimes called strain energy. The internal stress is removed by the initial heating in step S15, or in other words, the strain energy is considered to be homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain of the composite oxide is relieved. Therefore, after step S15, the surface of the composite oxide may become smoother. This is also referred to as the surface being improved. In other words, it is thought that after step S15, the difference in shrinkage that occurred in the composite oxide is relieved, and the surface of the composite oxide becomes smoother.

[0507] Furthermore, differences in shrinkage can cause microscopic displacements in the composite oxide, such as crystal displacements. This process is also recommended to reduce such displacements. This process makes it possible to homogenize the displacements in the composite oxide. When the displacements are homogenized, the surface of the composite oxide may become smoother. This can also be described as the crystal grains being aligned. In other words, it is believed that step S15 alleviates the crystal displacements and other issues that have occurred in the composite oxide, resulting in a smoother surface.

[0508] Using a composite oxide with a smooth surface as the positive electrode active material reduces degradation during charging and discharging in a secondary battery and prevents cracking of the positive electrode active material.

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

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

[0511] It is possible that the lithium in the composite oxide decreases due to initial heating. The reduced lithium, which will be explained in the next step S20, may make it easier for additive element A to enter the composite oxide.

[0512] <Step S20> Additive element A may be added to a composite oxide with a smooth surface, as long as it can adopt a layered rock salt-type crystalline structure. Adding additive element A to a composite oxide with a smooth surface allows for uniform addition of additive element A. Therefore, it is preferable to add additive element A after initial heating. The step of adding additive element A will be explained using Figures 36B and 36C.

[0513] <Step S21> In step S21 shown in Figure 36B, a source of additive element A (source A) to be added to the composite oxide is prepared. A lithium source may also be prepared together with the additive element source A.

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

[0515] When magnesium is selected as additive element A, the source of additive element A can be called a magnesium source. Examples of such magnesium sources include magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate. Multiple magnesium sources may also be used.

[0516] When fluorine is selected as additive element A, the additive element A source can be called a fluorine source. Examples of suitable fluorine sources 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, or 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 process described later.

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

[0518] The fluorine source may also be a gas, such as fluorine, carbon fluoride, sulfur fluoride, or oxygen fluoride, which may be mixed into the atmosphere during the heating process described later. Multiple 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 effect of lowering the melting point is greatest 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 will be in excess and the cycle characteristics will deteriorate. 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 and its vicinity). In this specification, "nearby" means a value greater than 0.9 times and 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 3 atomic% or less based on LiCoO2, more preferably 0.5 atomic% to 2 atomic% or less, and even more preferably 0.5 atomic% to 1 atomic% or less. If the amount of magnesium added is 0.1 atomic% or less, the initial discharge capacity is high, but the discharge capacity decreases rapidly when repeated charge-discharge cycles with high charge depths are performed. If the amount of magnesium added is more than 0.1 atomic% and 3 atomic% or less, both the initial discharge characteristics and charge-discharge cycle characteristics are good even when repeated charge-discharge cycles with high charge depths are performed. On the other hand, if the amount of magnesium added exceeds 3 atomic%, both the initial discharge capacity and charge-discharge cycle characteristics tend to gradually deteriorate.

[0521] <Step S22> Next, in step S22 shown in Figure 36B, the magnesium source and the fluorine source are crushed and mixed. This step can be performed by selecting from the crushing and mixing conditions described in step S12.

[0522] A heating step may be performed after step S22 if necessary. The heating step can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more, and the heating temperature is preferably 800°C to 1100°C.

[0523] <Step S23> Next, in step S23 shown in Figure 36B, the material that has been crushed and mixed above is recovered to obtain the additive element A source (A source). The additive element A source shown in step S23 has multiple starting materials and can be called a mixture.

[0524] The particle size of the above mixture is preferably such that the D50 (median diameter) is 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. Even when one material is used as the source of additive element A, the D50 (median diameter) is preferably such that it is 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less.

[0525] Such a finely powdered mixture (including cases where only one additive element A is present) makes it easier to uniformly adhere the mixture to the surface of the composite oxide when mixed with it in a later step. Uniform adhesion of the mixture to the surface of the composite oxide is preferable because it facilitates the uniform distribution or diffusion of fluorine and magnesium to the surface layer of the composite oxide after heating. The region where fluorine and magnesium are distributed can also be called the surface layer. If there are regions in the surface layer that do not contain fluorine and magnesium, it may be difficult to form the O3' type crystal structure described later in the charged state. Although fluorine was used in this explanation, fluorine can also be chlorine, and the term "halogen" can be used interchangeably if it includes these.

[0526] <Step S21> A process different from that shown in Figure 36B will be explained using Figure 36C. In step S21 shown in Figure 36C, four types of additive element A sources are prepared to be added to the composite oxide. In other words, the types of additive element A sources in Figure 36C are different from those in Figure 36B. A lithium source may also be prepared along with the additive element A sources.

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

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

[0529] <Step S31> Next, in step S31 shown in Figure 36A, the composite oxide and the additive element A source (A source) are mixed. The ratio of the number of atoms M of the transition metal M in the composite oxide having lithium, transition metal M, and oxygen to the number of atoms Mg of magnesium in the additive 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 in step S31 is preferably carried out under milder conditions than the mixing in step S12 in order to avoid destroying the complex oxide. For example, it is preferable to use conditions with a lower rotation speed or shorter time than the mixing in step S12. Also, dry mixing is generally milder than wet mixing. For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0531] In this embodiment, the mixing is performed dry using a ball mill with zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour. The mixing is carried out 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 Figure 36A, the materials mixed above are recovered to obtain mixture 903. During recovery, if necessary, the materials may be crushed and then sieved.

[0533] In this embodiment, a method is described in which lithium fluoride is added as a fluorine source and magnesium fluoride as a magnesium source to the composite oxide after initial heating. However, the present invention is not limited to the above method. At step S11, that is, at the stage of the starting materials of the composite oxide, the magnesium source and fluorine source can be added to the lithium source and the transition metal M source. Then, in step S13, heating is performed to obtain LiMO2 with added magnesium and fluorine. In this case, it is not necessary to separate the processes of steps S11 to S14 from the processes of steps S21 to S23. This can be said to be a simple and highly productive method.

[0534] Alternatively, a composite oxide with magnesium and fluorine added beforehand may be used. Using a composite oxide with magnesium and fluorine added allows for the omission of steps S11 to S32 and step S20. This can be considered a simple and highly productive method.

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

[0536] <Step S33> Next, in step S33 shown in Figure 36A, the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. A heating time of 2 hours or more is preferable.

[0537] Let me add some information about the heating temperature here. The lower limit of the heating temperature in step S33 must be above 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 is the temperature at which interdiffusion of the elements present in LiMO2 and the additive element A source occurs, and it may be lower than the melting temperature of these materials. Let me explain using an oxide as an example, but the melting temperature T m 0.757 times (Tammann temperature T) dIt is known that solid-phase diffusion occurs from ). Therefore, the heating temperature in step S33 should be 500°C or higher.

[0538] Of course, the reaction proceeds more easily if the temperature is above the melting point of at least a portion of the mixture 903. For example, if LiF and MgF2 are used as the source of additive element A, the eutectic point of LiF and MgF2 is around 742°C, so it is preferable to set the lower limit of the heating temperature in step S33 to 742°C or higher.

[0539] Furthermore, when mixture 903 is obtained by mixing LiCoO2:LiF:MgF2 in a molar ratio of 100:0.33:1, an endothermic peak is observed around 830°C in differential scanning calorimetry (DSC measurement). Therefore, a lower limit of the heating temperature of 830°C or higher is more preferable.

[0540] A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and increases productivity.

[0541] The upper limit of the heating temperature should be below the decomposition temperature of LiMO2 (the decomposition temperature of LiCoO2 is 1130°C). At temperatures near the decomposition temperature, there is a concern that LiMO2 may decompose, albeit in small amounts. Therefore, it is more preferable to be 1000°C or lower, even more preferable to be 950°C or lower, and even more preferable to be 900°C or lower.

[0542] Based on these considerations, 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. Furthermore, 742°C to 1130°C is preferred, 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, 800°C to 1100°C, 830°C to 1130°C are preferred, 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 should be higher than that in step 13.

[0543] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride, which may be caused by the fluorine source, to an appropriate range.

[0544] In the manufacturing 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 to below the decomposition temperature of the composite oxide (LiMO2), for example, between 742°C and 950°C, enabling the distribution of magnesium and other additive elements A to the surface layer and the production of a positive electrode active material with good properties.

[0545] However, since LiF is less dense than oxygen in its gaseous state, it may volatilize when heated, and if it volatilizes, the amount of LiF in mixture 903 will decrease. This weakens 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, Li on the surface of LiMO2 may react with F from the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, the same need to suppress volatilization is required.

[0546] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 while the partial pressure of LiF in the heating furnace is high. Such heating can suppress the volatilization of LiF in the mixture 903.

[0547] In this process, it is preferable to heat the mixture 903 so that it does not stick together. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases, and the diffusion pathway of added element A (e.g., fluorine) is obstructed, which may worsen the distribution of added element A (e.g., magnesium and fluorine) to the surface layer.

[0548] Furthermore, it is believed that if the additive element A (e.g., fluorine) is uniformly distributed on the surface, a smooth positive electrode active material with few irregularities can be obtained. Therefore, in order to maintain or further improve the smooth surface after heating in step S15 of this process, it is preferable that the mixture 903 does not adhere to each other.

[0549] Furthermore, when heating with a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere inside the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to purge the atmosphere initially and then not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln. If oxygen flows, the fluorine source may evaporate, which is undesirable for maintaining surface smoothness.

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

[0551] A...

Claims

1. The electrode comprises a current collector, a first layer on the current collector, a second layer on the first layer, a third layer on the second layer, and a conductive material. 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 proportion of the conductive material in the second layer is greater than the proportion of the conductive material in the first layer. A secondary battery wherein the proportion of the conductive material in the third layer is greater than the proportion of the conductive material in the second layer.

2. In claim 1, A secondary battery in which the sphericity of the second active material is 0.8 or more and 1.0 or less.

3. In claim 1, The first layer, the second layer, and the third layer each have a solid electrolyte, The mass of the solid electrolyte in the first layer is greater than the mass of the solid electrolyte in the second layer. A secondary battery in which the mass of the solid electrolyte in the second layer is greater than the mass of the solid electrolyte in the third layer.

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