Positive electrode and secondary battery

Abstract

To provide a stable positive electrode and a highly safe secondary battery under high potential or high temperature conditions. [Solution] The positive electrode comprises a first material and a second material, wherein at least a portion of the surface of the first material has a region covered with the second material, the first material having lithium cobalt oxide having magnesium, fluorine, aluminum, and nickel, and the second material being a composite oxide having an olivine-type crystal structure (having one or more selected from Fe, Ni, Co, and Mn).

Description

[Technical Field]

[0001] One aspect of the present invention relates to a method for producing a positive electrode active material, or a method for producing a positive electrode, or a method for producing a secondary battery, or a portable information terminal, energy storage system, vehicle, etc., 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. One aspect of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a method for manufacturing the same. One aspect of the present invention particularly relates to a method for producing a positive electrode active material, or to a positive electrode active material. Alternatively, one aspect of the present invention particularly relates to a method for producing a positive electrode, or to a positive electrode. Alternatively, one aspect of the present invention particularly relates to a method for producing a secondary battery, or to a secondary battery.

[0003] In this specification, the term "semiconductor device" refers to all devices that can function by utilizing semiconductor properties, and electro-optical devices, semiconductor circuits, and electronic devices are all considered semiconductor devices.

[0004] In this specification, "electronic equipment" refers to all devices having a positive electrode active material, a secondary battery, or an energy storage device, and electro-optical devices having a positive electrode active material, a positive electrode, a secondary battery, or an energy storage device, as well as information terminal devices having an energy storage device, are all considered electronic equipment.

[0005] In this specification, the term "energy storage device" refers to all elements and devices that have an energy storage function. For example, this includes energy storage devices such as lithium-ion secondary batteries (also called secondary batteries), lithium-ion capacitors, and electric double-layer capacitors. [Background technology]

[0006] 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, with their high output and high energy density, are seeing rapidly expanding demand in conjunction with the development of the semiconductor industry. They are used in mobile information terminals such as mobile phones, smartphones, and notebook computers, portable music players, digital cameras, medical equipment, home energy storage systems, industrial energy storage systems, and next-generation clean energy vehicles such as hybrid vehicles (HV), electric vehicles (EV), and plug-in hybrid vehicles (PHV). As a rechargeable energy source, they have become indispensable to today's information society.

[0007] Among these, composite oxides such as lithium cobaltate and lithium nickel-cobalt-manganate, which have a layered rock salt structure, are widely used. These materials possess useful properties as active materials for energy storage devices, such as high capacity and high discharge voltage. However, in order to achieve high capacity, the positive electrode is exposed to a high lithium potential during charging. In such a high potential state, a large amount of lithium is desorbed, which can reduce the stability of the crystal structure and lead to greater degradation during charge-discharge cycles. Against this backdrop, efforts are being made to improve the positive electrode active material of secondary batteries in order to create high-capacity and highly stable secondary batteries (for example, Patent Documents 1 to 3). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2018-088400 [Patent Document 2] International Publication 2018 / 203168 Brochure [Patent Document 3] Japanese Patent Publication No. 2020-140954 [Non-patent literature]

[0009] [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, pp.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) ,2009, 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] WE Counts et al, Journal of the American Ceramic Society, 1953, 36[1] pp.12-17. Fig.01471 [Non-Patent Document 5] 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 pp.364-369. [Overview of the project] [Problems that the invention aims to solve]

[0010] Although improvements to positive electrode active materials have been actively pursued as described in the above-mentioned Patent Documents 1 to 3, there is still room for improvement in various aspects of lithium-ion secondary batteries and the positive electrode active materials used therein, such as charge / discharge capacity, cycle characteristics, reliability, safety, and cost.

[0011] Therefore, one aspect of the present invention aims to provide a method for producing a positive electrode active material that is stable in high potential and / or high temperature states. Alternatively, it aims to provide a method for producing a positive electrode active material whose crystal structure is less likely to collapse even after repeated charging and discharging. Alternatively, it aims to provide a method for producing a positive electrode active material with excellent charge-discharge cycle characteristics. Alternatively, it aims to provide a method for producing a positive electrode active material with a large charge-discharge capacity. Alternatively, it aims to provide a secondary battery with high reliability or safety.

[0012] Furthermore, one aspect of the present invention aims to provide a method for manufacturing a positive electrode that is stable in high-potential and / or high-temperature states. Alternatively, it aims to provide a method for manufacturing a positive electrode with excellent charge-discharge cycle characteristics. Alternatively, it aims to provide a method for manufacturing a positive electrode with a large charge-discharge capacity. Alternatively, it aims to provide a secondary battery with high reliability or safety.

[0013] Furthermore, one aspect of the present invention aims to provide a novel substance, active material particles, electrode, secondary battery, energy storage device, or method for producing the same. Another aspect of the present invention aims to provide a method for producing a secondary battery having one or more characteristics selected from high purity, high performance, and high reliability, or to provide a secondary battery.

[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] A desirable form of the composite having a positive electrode active material is a structure in which at least a portion of the particle surface of a particulate first material that functions as a positive electrode active material is covered with a second material, and more preferably, a structure in which substantially the entire particle surface of the particulate first material is covered with the second material. Here, substantially the entire surface is covered in a state in which the particulate first material and the electrolyte do not come into direct contact.

[0016] In a state where at least a portion, preferably substantially the entire, of the particle surface of the first material is covered with the second material, the area in direct contact with the electrolyte of the first material is reduced, thereby suppressing the detachment of transition metal elements and / or oxygen from the first material under high-voltage charging conditions. As a result, capacity degradation due to repeated charging and discharging can be suppressed. Furthermore, when a material with a stable crystal structure even under high-voltage charging conditions is used as the second material, a secondary battery using a composite according to one embodiment of the present invention can obtain effects such as improved stability at high temperatures and improved fire resistance.

[0017] Furthermore, by using a material with excellent stability under high-voltage charging conditions as the first material, the durability and stability of the composite under high-voltage charging can be further improved. In addition, the heat resistance and / or fire resistance of the secondary battery using the composite can be further improved.

[0018] As the first material, it is preferable to use, for example, lithium cobaltate excellent in stability in a high-voltage charged state and / or a metal oxide-coated composite oxide excellent in stability in a high-voltage charged state. As lithium cobaltate excellent in stability in a high-voltage charged state, for example, lithium cobaltate added with magnesium and fluorine, lithium cobaltate added with magnesium, fluorine, aluminum and nickel, etc. can be used. Further, as the metal oxide-coated composite oxide excellent in stability in a high-voltage charged state, it is preferable to use a metal oxide-coated composite oxide in which secondary particles of lithium nickel-cobalt-manganese oxide are coated with aluminum oxide, etc. As lithium nickel-cobalt-manganese oxide, the atomic number ratio can be, for example, nickel:cobalt:manganese = 8:1:1, nickel:cobalt:manganese = 9:0.5:0.5, etc.

[0019] Note that lithium cobaltate added with magnesium, fluorine, aluminum and nickel is a particularly preferable material as the first material because when the initial heating described later is performed, the charge-discharge repetition characteristics at high voltage are significantly excellent.

[0020] As the second material that covers at least a part of the particle surface of the first material, preferably substantially the whole, one or both of an oxide and LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, Mn) can be used. Examples of the oxide include aluminum oxide, zirconium oxide, hafnium oxide, and niobium oxide, etc. Further, examples of LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, Mn) include LiFePO4, LiNiPO4, LiCoPO4, LiMnPO4, LiFe a Ni b PO4, LiFe a Co b PO4, LiFe a Mn b PO4, LiNi a Co b PO4, LiNi a Mn b PO4 (a + b is 1 or less, 0 < a < 1, 0 < b < 1), LiFe c Nid Co e PO4, LiFe c Ni d Mn e PO4, LiNi c Co d Mn e PO4 (where c + d + e is less than or equal to 1, 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe f Ni g Co h Mn i PO4 (where f + g + h + i is less than or equal to 1, 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), etc.

[0021] Note that as the positive electrode of the present invention, it may have a structure in which at least a part of the surface of the composite is covered with a graphene compound. Preferably, a structure in which 80% or more of the particle surface of the composite and / or the aggregate having the composite is covered with a graphene compound is preferred.

[0022] One aspect of the present invention has a first material and a second material that covers at least a part of the surface of the first material. The first material has a first composite oxide represented by LiM1O2 (M1 is one or more selected from Fe, Ni, Co, Mn, Al), and the second material has a second composite oxide represented by LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, Mn), and is a positive electrode.

[0023] Another aspect of the present invention has a first material and a second material that covers at least a part of the surface of the first material. The first material has lithium cobalt oxide having magnesium, fluorine, aluminum, and nickel, and the second material has a second composite oxide represented by LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, Mn), and is a positive electrode.

[0024] Furthermore, one aspect of the present invention is a positive electrode comprising a first material and a second material covering at least a portion of the surface of the first material, wherein the first material has lithium cobalt oxide having magnesium, fluorine, aluminum, and nickel, and the lithium cobalt oxide has a region on its surface where the concentration of magnesium, fluorine, or aluminum is maximum, and the second material has a second composite oxide represented as LiM2PO4 (where M2 is one or more selected from Fe, Ni, Co, and Mn).

[0025] Furthermore, one aspect of the present invention provides a positive electrode comprising a first material and a second material covering at least a portion of the surface of the first material, wherein the first material has a first composite oxide represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al), and the second material has aluminum oxide.

[0026] Furthermore, one aspect of the present invention provides a positive electrode comprising a first material and a second material covering at least a portion of the surface of the first material, wherein the first material has lithium cobalt oxide having magnesium, fluorine, aluminum, and nickel, and the second material has aluminum oxide.

[0027] Furthermore, one aspect of the present invention is a positive electrode comprising a first material and a second material covering at least a portion of the surface of the first material, wherein the first material has lithium cobalt oxide having magnesium, fluorine, aluminum, and nickel, and the lithium cobalt oxide has a region on its surface where the concentration of magnesium, fluorine, or aluminum is maximum, and the second material has aluminum oxide.

[0028] Furthermore, one aspect of the present invention is a positive electrode comprising a first material and a second material, wherein the first material has a first composite oxide represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al), and the second material has a second composite oxide represented by LiM2PO4 (where M2 is one or more selected from Fe, Ni, Co, and Mn).

[0029] One aspect of the present invention is a secondary battery having the positive electrode described in any one of the above.

[0030] One aspect of the present invention is a vehicle having the above-mentioned secondary battery.

[0031] One aspect of the present invention is an energy storage system having the above-described secondary battery.

[0032] One aspect of the present invention is an electronic device having the above-mentioned secondary battery.

[0033] Furthermore, one aspect of the present invention is a method for producing a positive electrode active material comprising a first material and a second material, the method comprising: a first step of covering at least a portion of the surface of the first material with the second material to form a composite; and a second step of heating the composite, wherein the first material comprises lithium cobaltate having magnesium, fluorine, aluminum, and nickel, and the second material comprises a second composite oxide represented by LiM2PO4 (where M2 is one or more selected from Fe, Ni, Co, and Mn), and the heating is carried out in an oxygen-containing atmosphere.

[0034] Furthermore, one aspect of the present invention is a method for producing a positive electrode active material comprising a first material and a second material, the method comprising: a first step of covering at least a portion of the surface of the first material with the second material to form a composite; and a second step of heating the composite, wherein the first material comprises lithium cobalt oxide having magnesium, fluorine, aluminum, and nickel, the second material comprises aluminum oxide, and the heating is carried out in an oxygen atmosphere.

[0035] In one aspect of the present invention, in any one of the above, it is preferable that the heating is carried out at a temperature of 450°C or higher and 800°C or lower. [Effects of the Invention]

[0036] One aspect of the present invention can provide a method for producing a positive electrode active material that is stable in high potential and / or high temperature conditions. Alternatively, it can provide a method for producing a positive electrode active material whose crystal structure is resistant to collapse even after repeated charging and discharging. Alternatively, it can provide a method for producing a positive electrode active material with excellent charge-discharge cycle characteristics. Alternatively, it can provide a method for producing a positive electrode active material with a large charge-discharge capacity. Alternatively, it can provide a secondary battery with high reliability or safety.

[0037] Furthermore, according to one aspect of the present invention, it is possible to provide novel materials, active material particles, secondary batteries, energy storage devices, or methods for producing the same. Also, according to one aspect of the present invention, it is possible to provide a method for producing a secondary battery having one or more characteristics selected from high purity, high performance, and high reliability, or a secondary battery.

[0038] Furthermore, the description of these effects does not preclude the existence of other effects. Moreover, one aspect of the present invention does not necessarily have 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]

[0039] [Figure 1] Figures 1A to 1C illustrate an example of a method for producing a positive electrode active material according to one embodiment of the present invention. [Figure 2] Figures 2A and 2B are diagrams relating to calculations of an example of a positive electrode active material according to one embodiment of the present invention. [Figure 3] Figures 3A to 3C are diagrams relating to the calculation of an example of a positive electrode active material according to one embodiment of the present invention. [Figure 4] Figure 4 is a diagram illustrating the calculation of an example of a positive electrode active material according to one embodiment of the present invention. [Figure 5] Figures 5A and 5B are diagrams relating to the calculation of an example of a positive electrode active material according to one embodiment of the present invention. [Figure 6]Figures 6A and 6B illustrate an example of a method for manufacturing a positive electrode according to one aspect of the present invention. [Figure 7] Figures 7A and 7B illustrate an example of a method for manufacturing a positive electrode according to one embodiment of the present invention. [Figure 8] Figure 8 illustrates an example of a method for manufacturing a positive electrode according to one aspect of the present invention. [Figure 9] Figure 9 illustrates an example of a method for manufacturing a positive electrode according to one aspect of the present invention. [Figure 10] Figures 10A and 10B illustrate an example of a method for producing a positive electrode active material according to one embodiment of the present invention. [Figure 11] Figures 11A to 11C illustrate an example of a method for producing a positive electrode active material according to one embodiment of the present invention. [Figure 12] Figure 12 illustrates an example of a method for producing a positive electrode active material according to one embodiment of the present invention. [Figure 13] Figures 13A to 13C illustrate an example of a method for producing a positive electrode active material according to one embodiment of the present invention. [Figure 14] Figures 14A to 14C illustrate the method for preparing the positive electrode active material. [Figure 15] Figure 15 is a diagram illustrating the method for preparing the positive electrode active material. [Figure 16] Figures 16A to 16C illustrate the method for preparing the positive electrode active material. [Figure 17] Figure 17A is a top view of a positive electrode active material according to one embodiment of the present invention, and Figure 17B is a cross-sectional view of a positive electrode active material according to one embodiment of the present invention. [Figure 18] Figure 18 illustrates the Li occupancy rate and crystal structure of a positive electrode active material according to one embodiment of the present invention. [Figure 19] Figure 19 shows the XRD pattern calculated from the crystal structure. [Figure 20] Figure 20 illustrates the Li occupancy rate and crystal structure of the positive electrode active material in the comparative example. [Figure 21] Figure 21 shows the XRD pattern calculated from the crystal structure. [Figure 22]Figures 22A to 22C show the lattice constants calculated from XRD. [Figure 23] Figures 23A to 23C show the lattice constants calculated from XRD. [Figure 24] Figure 24 is a graph of charging capacity and voltage. [Figure 25] Figure 25A is a graph of the dQ / dV of a secondary battery according to one embodiment of the present invention. Figure 25B is a graph of the dQ / dV of a secondary battery according to one embodiment of the present invention. Figure 25C is a graph of the dQ / dV of a comparative secondary battery. [Figure 26] Figure 26 is a schematic cross-sectional view of the positive electrode active material. [Figure 27] Figures 27A and 27B are SEM images of the positive electrode. [Figure 28] Figure 28A is a front view showing three-dimensional information, Figure 28B is an enlarged view of a part thereof, Figure 28C is a cross-sectional view thereof, Figure 28D is a side view showing three-dimensional information, Figure 28E is an enlarged view of a part thereof, and Figure 28F is a cross-sectional view thereof. [Figure 29] Figures 29A to 29C are SEM images of the positive electrode. [Figure 30] Figures 30A to 30C are SEM images of the positive electrode. [Figure 31] Figures 31A and 31B are STEM images of the positive electrode. [Figure 32] Figures 32A to 32C show the EDX analysis results of the positive electrode. [Figure 33] Figures 33A and 33B are cross-sectional TEM images of the positive electrode active material layer. [Figure 34] Figures 34A to 34C show micro-electron diffraction patterns of the positive electrode active material layer. [Figure 35] Figures 35A to 35C show examples of crystal structures. [Figure 36] Figure 36A is a cross-sectional STEM image of the particles after pressing, and Figures 36B and 36C are schematic cross-sectional diagrams. [Figure 37] Figure 37 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 38]Figure 38 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 39] Figures 39A to 39E illustrate an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 40] Figure 40 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 41] Figure 41 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 42] Figure 42 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 43] Figure 43 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 44] Figure 44 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 45] Figure 45 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 46] Figure 46 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 47] Figure 47 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 48] Figures 48A and 48B are cross-sectional views of the positive electrode active material. [Figure 49] Figures 49A to 49C illustrate the concentration distribution within the positive electrode active material. [Figure 50] Figure 50 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 51] Figure 51 illustrates an example of a method for producing a positive electrode active material according to one aspect of the present invention. [Figure 52] Figure 52 is a diagram illustrating an example of a positive electrode according to one aspect of the present invention. [Figure 53] Figure 53A is an exploded perspective view of a coin-type rechargeable battery, Figure 53B is a perspective view of a coin-type rechargeable battery, and Figure 53C is a cross-sectional perspective view thereof. [Figure 54]Figure 54A shows an example of a cylindrical secondary battery. Figure 54B shows an example of a cylindrical secondary battery. Figure 54C shows an example of multiple cylindrical secondary batteries. Figure 54D shows an example of an energy storage system with multiple cylindrical secondary batteries. [Figure 55] Figures 55A and 55B illustrate examples of secondary batteries, while Figure 55C shows the inside of a secondary battery. [Figure 56] Figures 56A to 56C illustrate examples of secondary batteries. [Figure 57] Figures 57A and 57B show the external appearance of a secondary battery. [Figure 58] Figures 58A to 58C illustrate the method for manufacturing a secondary battery. [Figure 59] Figures 59A to 59C show examples of battery pack configurations. [Figure 60] Figures 60A and 60B illustrate examples of secondary batteries. [Figure 61] Figures 61A to 61C illustrate an example of a secondary battery. [Figure 62] Figures 62A and 62B illustrate an example of a secondary battery. [Figure 63] Figure 63A is a perspective view of a battery pack showing one embodiment of the present invention, Figure 63B is a block diagram of the battery pack, and Figure 63C is a block diagram of a vehicle having a motor. [Figure 64] Figures 64A to 64D illustrate an example of a transport vehicle. [Figure 65] Figures 65A and 65B illustrate an energy storage device according to one embodiment of the present invention. [Figure 66] Figure 66A shows an electric bicycle, Figure 66B shows the secondary battery of an electric bicycle, and Figure 66C is a diagram illustrating an electric motorcycle. [Figure 67] Figures 67A to 67D illustrate an example of an electronic device. [Figure 68]Figure 68A shows an example of a wearable device, Figure 68B shows a perspective view of a wristwatch-type device, and Figure 68C is a diagram illustrating the side view of a wristwatch-type device. Figure 68D is a diagram illustrating an example of wireless earphones. [Figure 69] Figures 69A to 69C are SEM images of the surface of the positive electrode active material. [Figure 70] Figures 70A and 70B are graphs showing the cycle characteristics. [Modes for carrying out the invention]

[0040] The 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 that 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.

[0041] A secondary battery has, for example, a positive electrode and a negative electrode. The positive electrode is composed of a positive electrode active material. The positive electrode active material is, for example, a substance that performs a reaction that contributes to the charge and discharge capacity. However, the positive electrode active material may also contain a portion of a substance that does not contribute to the charge and discharge capacity.

[0042] In this specification, the positive electrode active material of one aspect of the present invention may be expressed as a positive electrode material, a positive electrode material for secondary batteries, a composite oxide, etc. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention comprises a compound. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention comprises a composition. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention comprises a composite having a positive electrode active material.

[0043] Furthermore, in this specification, the term "particle" is not limited to spherical shapes (circular cross-sections), but may include elliptical, rectangular, trapezoidal, conical, rounded-cornered quadrilateral, asymmetrical shapes, and individual particles may have irregular shapes.

[0044] Particle size can be measured, for example, using laser diffraction particle size distribution measurement, and can be compared using the D50 value. Here, D50 is the particle size at which the integrated particle amount accounts for 50% of the integrated particle amount curve of the particle size distribution measurement results, i.e., the median. 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 major axis of the particle cross-section may be measured by analysis such as SEM (Scanning Electron Microscope) or TEM (Transmission Electron Microscope).

[0045] Furthermore, Miller indices are used to indicate crystal planes and directions in this specification. Individual planes are indicated by parentheses ( ). In crystallography, crystal planes, directions, and space groups are indicated by superscripts above the numbers, but in this specification, due to limitations in patent application notation, a minus sign (-) may be placed before the number instead of a superscript above it.

[0046] Furthermore, in this specification, the layered rock salt-type crystal structure of a composite oxide containing lithium and a transition metal refers to a crystal structure having a rock salt-type ionic arrangement in which cations and anions are arranged alternately, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, thereby enabling two-dimensional diffusion of lithium. Defects such as vacancies in cations or anions may be present. Also, strictly speaking, a layered rock salt-type crystal structure may have a distorted lattice structure of the rock salt-type crystal.

[0047] Furthermore, in this specification, a rock salt-type crystal structure refers to a structure in which cations and anions are arranged alternately. Note that there may be vacancies of cations or anions in part of the crystal structure.

[0048] Furthermore, in this specification, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all of the insertable and detachable lithium present in the positive electrode active material has been detached. For example, the theoretical capacity of LiFePO4 is 170 mAh / g, the theoretical capacity of LiCoO2 is 274 mAh / g, the theoretical capacity of LiNiO2 is 274 mAh / g, and the theoretical capacity of LiMn2O4 is 148 mAh / g.

[0049] In this specification, the extent to which insertable and detachable lithium remains in the positive electrode active material is expressed by x in the composition formula, for example, Li x x in CoO2, or Li x Indicated by x in MO2. Li in this specification. x CoO2 contains Li as appropriate. x This can be interpreted as MO2. In the case of the positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, when a secondary battery using LiCoO2 as the positive electrode active material is charged at 219.2 mAh / g, Li 0.2 It can be expressed as CoO2 or x=0.2. x A small x in CoO2 means, for example, 0.1 <x≦0.24をいう。

[0050] When properly synthesized lithium cobalt oxide, before being used as the positive electrode, approximately satisfies the stoichiometric ratio, it is LiCoO2, and the Li occupancy rate of the lithium sites is x=1. Similarly, a secondary battery after discharge is also LiCoO2, and it can be said that x=1. Here, "discharge completed" means, for example, with a discharge current of 100mA / g and a voltage of 2.5V (vs Li / Li + This refers to a state that falls below the following conditions.

[0051] Furthermore, while this specification and other documents may show an example in which lithium metal is used as the negative electrode in a secondary battery using a positive electrode and positive electrode active material according to one aspect of the present invention, the secondary battery according to one aspect of the present invention is not limited to this. Other materials, such as graphite or lithium titanate, may be used for the negative electrode. The properties of the positive electrode and positive electrode active material according to one aspect of the present invention, such as resistance to crystal structure collapse even after repeated charging and discharging and obtaining good cycle characteristics, are not affected by the material of the negative electrode. Also, while examples of charging and discharging at a relatively high voltage, such as 4.6V, with a lithium counter electrode are sometimes shown for a secondary battery according to one aspect of the present invention, charging and discharging may be performed at a lower voltage. When charging and discharging at a lower voltage, it is expected that the cycle characteristics will be even better than those shown in this specification and other documents.

[0052] Furthermore, in this specification, "kiln" refers to a device for heating a material to be processed. For example, instead of "kiln," you may use terms such as furnace, oven, or heating device.

[0053] (Embodiment 1) This embodiment describes a composite having a positive electrode active material according to one aspect of the present invention, a method for producing the composite, and a method for producing the positive electrode.

[0054] Figures 1A to 1C show the method for fabricating a composite material containing a positive electrode active material. Figures 2A to 5B show the calculations for the composite material containing a positive electrode active material. Figures 6A to 9 show the method for fabricating the positive electrode.

[0055] The positive electrode comprises a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a composite comprising a first material that functions as a positive electrode active material and a second material that covers at least a portion of the first material, and may further contain a conductive agent and a binder. The composite comprising the positive electrode active material is sometimes simply referred to as the positive electrode active material.

[0056] A composite having a positive electrode active material is obtained by a composite treatment described below, using at least a first material and a second material. The composite treatment can include, for example, one or more composite treatments using mechanical energy such as mechanochemical methods, mechanofusion methods, and ball milling methods; composite treatments using liquid-phase reactions such as coprecipitation methods, hydrothermal methods, and sol-gel methods; and composite treatments using gas-phase reactions such as barrel sputtering, ALD (Atomic Layer Deposition), vapor deposition, and CVD (Chemical Vapor Deposition). Furthermore, it is preferable to perform a heat treatment after the composite treatment. In this specification, the composite treatment is also referred to as a surface coating treatment or coating treatment.

[0057] When heat treatment is performed after composite processing, the second material covering at least a portion of the first material is expected to sinter or melt and spread, thereby reducing the area of ​​direct contact between the first material and the electrolyte. On the other hand, if the temperature of the heat treatment after composite processing is too high, elements of the second material may diffuse into the interior of the first material more than necessary, potentially reducing the charge / discharge capacity of the first material and diminishing the effectiveness of the second material as a coating layer. Therefore, when heat treatment is performed after composite processing, the heating temperature, heating time, and heating atmosphere must be appropriately set.

[0058] Method 1 for fabricating the composite material describes a method for fabricating a composite material by mechanical energy treatment using a first material 100x and a second material 100y. However, the present invention is not limited to these descriptions.

[0059] [Method for preparing the composite 1] An example of a method for producing a composite having a positive electrode active material, which is one aspect of the present invention, will be explained with reference to Figures 1A to 1C.

[0060] In step S101 of Figure 1A, the first material 100x is prepared, and in step S102, the second material 100y is prepared.

[0061] As the first material 100x, a composite oxide represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al) prepared by the manufacturing method shown in Embodiment 3 described below, to which an additive element X is added, can be used. For example, lithium cobalt oxide with magnesium and fluorine added, or lithium cobalt oxide with magnesium, fluorine, aluminum, and nickel added can be used. In particular, the lithium cobalt oxide with magnesium, fluorine, aluminum, and nickel added is preferably one that has undergone the initial heating shown in Embodiment 3. As another example of the first material 100x, lithium nickel-cobalt-manganate can be used. Here, a high nickel ratio is preferred as the transition metal ratio of the lithium nickel-cobalt-manganate, for example, materials with atomic ratios of nickel:cobalt:manganese = 8:1:1 and nickel:cobalt:manganese = 9:0.5:0.5 are preferred. Furthermore, a metal oxide coated composite oxide can be used in which secondary particles of lithium nickel-cobalt-manganate are coated with aluminum oxide. Here, the coating layer (aluminum oxide) of the metal oxide-coated composite oxide is preferably thin, for example, 1 nm to 200 nm, more preferably 1 nm to 100 nm. Furthermore, it is preferable that the nickel-cobalt-manganate lithium described above has calcium added to it.

[0062] As the second material 100y, LiM2PO4 (where M2 is one or more selected from Fe, Ni, Co, and Mn) can be used. Alternatively, an oxide can be used as the second material 100y. Examples of oxides include aluminum oxide, zirconium oxide, hafnium oxide, and niobium oxide. As LiM2PO4, the aforementioned materials can be used, for example, LiFePO4, LiMnPO4, LiFe a Mn b PO4(a+b is less than or equal to 1, 0 <a<1、0<b<1)、LiFe a Ni bPO4 (where a + b ≤ 1, 0 < a < 1, 0 < b < 1) can be used. Further, the particle surface of the second material 100y may have a carbon coating layer.

[0063] When using a material that functions as a positive electrode active material as the second material 100y, as a combination of the first material 100x and the second material 100y, according to the characteristics required for the secondary battery, it is possible to select a combination that is unlikely to cause a step in the charge-discharge curve, or to select a combination that causes a step in the charge-discharge curve at a desired charging rate.

[0064] Next, as step S103, a composite treatment of the above-mentioned first material 100x and second material 100y is performed. When performing the composite treatment by mechanical energy, it can be performed by a mechanochemical method. Further, it may be treated using a mechano-fusion method.

[0065] Also, as step S103, when using a ball mill, it is preferable to use zirconia balls as media, for example. For the ball mill treatment, when the purpose is mixing, a dry treatment is desirable. When performing the ball mill treatment wet, acetone can be used. When performing a wet ball mill treatment, dehydrated acetone with a moisture content of 100 ppm or less, preferably 10 ppm or less, may be used.

[0066] By the composite treatment in step S103, it is possible to create a state in which at least a part, preferably substantially the whole, of the particle surface of the particulate first material 100x is covered with the second material 100y.

[0067] Through the above steps, a composite 100z having the positive electrode active material of one aspect of the present invention shown in Fig. 1A can be produced (step S104). Here, the composite 100z having the positive electrode active material obtained here may sometimes be simply referred to as a positive electrode active material.

[0068] In the manufacturing method shown in Figure 1B, the procedure is the same as in the manufacturing method shown in Figure 1A up to step S103, and after step S103, a heat treatment is performed as step S104. The heating in step S104 should be carried out in an oxygen-containing atmosphere at a temperature of 400°C to 950°C, preferably 450°C to 800°C, for 1 hour to 60 hours, preferably 2 hours to 20 hours.

[0069] By following the above steps, a composite 100z having a positive electrode active material according to one embodiment of the present invention, as shown in Figure 1B, can be produced (step S105). The composite 100z obtained here is sometimes simply referred to as the positive electrode active material.

[0070] In addition, in order to obtain a good coating state during the compounding process, the ratio of the particle size of the second material 100y to the particle size of the first material 100x (particle size of the second material 100y / first material 100x) is preferably 1 / 100 or more and 1 / 50 or less, and more preferably 1 / 200 or more and 1 / 100 or less. As adjustment of the particle size of the second material 100y, a micronization treatment may be performed using the method shown in Figure 1C.

[0071] [Calculations related to the complex 1] As an example of a composite having a positive electrode active material, a structure in which LiCoO2 and LiFePO4 are bonded, and LiCoO2 and LiFe 0.5 Mn 0.5 PO4 or LiFe 0.5 Ni 0.5 The structure in which PO4 is coupled was optimized and evaluated using Density Functional Theory (DFT). The main calculation conditions are shown in Table 1, and the initial state of the model used in the calculation is shown in Figures 2A and 2B.

[0072] [Table 1]

[0073] As an initial state of the model used for the calculation, the structure in which LiCoO2 and LiFePO4 are combined is shown in FIG. 2A. Also, FIG. 2B shows the structure in which LiCoO2 and LiFe 0.5 Mn 0.5 PO4 or LiFe 0.5 Ni 0.5 PO4 are combined. In the models of these structures, the potential difference before and after the extraction of Li (corresponding to the potential difference during charging) was calculated. The calculation results are shown as graphs of theoretical capacity - charging voltage in FIGS. 3A, 3B, and 3C.

[0074] As the calculation results shown in FIGS. 3A, 3B, and 3C, it was confirmed that the charging voltage increases in the order of LiFePO4 < LiMnPO4 < LiNiPO4. Also, it was confirmed that the charging voltage is higher when a part of Fe in LiFePO4 is replaced with Mn than in LiFePO4, and the charging voltage is even higher when a part of Fe in LiFePO4 is replaced with Ni.

[0075] [Calculation 2 Regarding the Composite] In the composite in which the particle surface of the first material 100x is covered with the second material 100y, the lattice strain at the interface between the first material 100x and the second material 100y was verified by first - principle calculation.

[0076] Here, for the structure in which the first material 100x is LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 O2 (Ni:Co:Mn = 8:1:1) and the second material 100y is LiFePO4 (hereinafter referred to as the NCM - LFP junction), calculations were performed. The (001) plane of LiFePO4 was joined to the (104) plane of LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 O2. In the particle, the molar ratio was made approximately LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 O2:LiFePO4 = 9:1.

[0077] Also, LiNi 8 / 10 Co 1 / 10Mn 1 / 10 We also conducted tests using only O2 particles (hereinafter referred to as NCM only) and only LiFePO4 particles (hereinafter referred to as LFP only).

[0078] Also, Li Sta 8 / 10 Co 1 / 10 Mn 1 / 10 We also investigated the case where O2 particles and LiFePO4 particles were mixed without bonding (hereinafter referred to as NCM-LFP mixing). In the mixed particles, the molar ratio was approximately LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 The ratio of O2 to LiFePO4 was set to 9:1.

[0079] First, optimization calculations were performed using density functional theory (DFT). The main calculation conditions are shown in Table 2. The number of atoms used in the calculations was as follows: for NCM-LFP junctions, 116 Li, 82 Ni, 11 Co, 11 Mn, 12 Fe, 12 P, and 256 O. For NCM only, 60 Li, 48 Ni, 6 Co, 6 Mn, and 120 O. For LFP only, 32 Li, 32 Fe, 32 P, and 128 O.

[0080] [Table 2]

[0081] Figure 4 shows the state of the bonding interface after the optimization calculation. As shown in region 991 in Figure 4, the bonding with LiFePO4 near the bonding interface resulted in LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 The structure of the O2 molecule was observed to be distorted.

[0082] Next, for the optimized structure, the potential difference before and after the extraction of lithium atoms (corresponding to the potential difference during charging) was calculated. For the NCM-LFP mixture, the potential difference of NCM alone was multiplied by the potential difference of LFP alone.

[0083] Figure 5A shows the relationship between theoretical capacity and charging voltage, as calculated. Figure 5B shows an enlarged view of a portion of the graph in Figure 5A.

[0084] LiRing 8 / 10 Co 1 / 10 Mn 1 / 10 In a structure where LiFePO4 is bonded to the surface of O2 (Ni:Co:Mn=8:1:1), the change in charging voltage with respect to capacitance is linear. This is because, as described in Figure 4, LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 This is thought to be because the structure of O2 was distorted, weakening the interaction between the nickel and cobalt atoms.

[0085] [Cathode active material] As the first material 100x, a composite oxide represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al) having a layered rock salt-type crystalline structure can be used. Alternatively, as the first material 100x, a composite oxide represented by LiM1O2 with added element X can be used. Preferably, the added element X in the first material 100x is one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic. These elements may further stabilize the crystalline structure of the first material 100x. In other words, the first material 100x can include lithium cobalt oxide with added magnesium and fluorine, lithium cobalt oxide with added magnesium, fluorine, aluminum and nickel, lithium cobalt oxide with added magnesium, fluorine and titanium, lithium nickel-cobalt oxide with added magnesium and fluorine, lithium cobalt-aluminate with added magnesium and fluorine, lithium nickel-cobalt-aluminate with added magnesium and fluorine, lithium nickel-cobalt-aluminate with added magnesium and fluorine, lithium nickel-cobalt-manganate with added magnesium and fluorine, and the like. A high nickel ratio is preferred as the transition metal ratio of the lithium nickel-cobalt-manganate; for example, materials with atomic ratios of nickel:cobalt:manganese = 8:1:1 and nickel:cobalt:manganese = 9:0.5:0.5 are preferred. Furthermore, it is preferable to have lithium nickel-cobalt-manganate with added calcium as the above-mentioned lithium nickel-cobalt-manganate.

[0086] Furthermore, as the first material 100x, secondary particles of a composite oxide represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al) coated with a metal oxide may be used. As the metal oxide, oxides of one or more metals selected from Al, Ti, Nb, Zr, La, and Li can be used. For example, a metal oxide-coated composite oxide, in which secondary particles of a composite oxide represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al) are coated with aluminum oxide, can be used as the first material 100x. For example, a metal oxide-coated composite oxide can be used in which secondary particles of lithium nickel-cobalt-manganate with an atomic ratio of nickel:cobalt:manganese = 8:1:1 or nickel:cobalt:manganese = 9:0.5:0.5 are coated with aluminum oxide. Here, the coating layer is preferably thin, for example, 1 nm to 200 nm, more preferably 1 nm to 100 nm. Furthermore, it is preferable that the above-mentioned lithium nickel-cobalt-manganate has calcium added to it.

[0087] As a method for producing the first material 100x, the production methods described in Embodiments 3 and 4 below can be used.

[0088] As the second material 100y, one or more oxides and LiM2PO4 having an olivine-type crystal structure (where M2 is one or more selected from Fe, Ni, Co, and Mn) can be used (also called a composite oxide having an olivine-type crystal structure (containing one or more selected from Fe, Ni, Co, and Mn)). Examples of oxides include aluminum oxide, zirconium oxide, hafnium oxide, and niobium oxide. Examples of LiM2PO4 include LiFePO4, LiNiPO4, LiCoPO4, LiMnPO4, and LiFe a Ni b PO4, LiFe a Co b PO4, LiFe a Mn b PO4, LiNi a Co bPO4, LiNi a Mn b PO4 (a + b is less than or equal to 1, 0 < a < 1, 0 < b < 1), LiFe c Ni d Co e PO4, LiFe c Ni d Mn e PO4, LiNi c Co d Mn e PO4 (c + d + e is less than or equal to 1, 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe f Ni g Co h Mn i PO4 (f + g + h + i is less than or equal to 1, 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), etc. Also, the particle surface of the second material 100y may have a carbon coating layer. As a method for producing LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, and Mn) having an olivine crystal structure, the production method described in Embodiment 5 below can be used.

[0089] [Composite] In this embodiment, an example of a method for producing a composite that covers at least a part of the particle surface of the particulate first material 100x that functions as a positive electrode active material with the second material 100y is shown in the above-described composite production method 1. A desirable form of the composite having a positive electrode active material is a structure in which at least a part of the particle surface of the particulate first material 100x is covered with the second material 100y, and more preferably, a structure in which substantially the entire particle surface of the particulate first material 100x is covered with the second material 100y. Here, the state of covering substantially the entire surface means a state in which the particulate first material 100x and the electrolyte do not directly touch.

[0090] In a state where at least a portion, preferably substantially the entire, of the particle surface of the particulate first material 100x, which functions as a positive electrode active material, is covered with the second material 100y, the area in direct contact between the first material 100x and the electrolyte is reduced. Therefore, the desorption of transition metal elements and / or oxygen from the first material 100x can be suppressed in a high-voltage charging state, thereby suppressing capacity degradation due to repeated charging and discharging. Furthermore, by being covered with the second material 100y, which has a stable crystal structure even in a high-voltage charging state, a secondary battery using a composite having a positive electrode active material according to one embodiment of the present invention can obtain effects such as improved stability at high temperatures and improved fire resistance.

[0091] In particular, by using a material with excellent stability under high-voltage charging conditions as the first material 100x, the durability and stability of the composite under high-voltage charging can be further improved. Furthermore, the heat resistance and / or fire resistance of the secondary battery using the composite can be further improved.

[0092] As a material with excellent stability under high-voltage charging conditions, it is preferable to use lithium cobalt oxide with magnesium and fluorine added, or lithium cobalt oxide with magnesium, fluorine, aluminum, and nickel added. Furthermore, as a material with excellent stability under high-voltage charging conditions, it is preferable to use a metal oxide-coated composite oxide, such as one in which secondary particles of nickel-cobalt-manganate lithium are coated with aluminum oxide. The atomic ratio of nickel-cobalt-manganate lithium can be nickel:cobalt:manganese = 8:1:1, nickel:cobalt:manganese = 9:0.5:0.5, etc.

[0093] Lithium cobalt oxide with added magnesium, fluorine, aluminum, and nickel exhibits remarkably superior charge-discharge cycle characteristics at high voltages when subjected to the initial heating described later, making it a particularly preferred material as the first material 100x.

[0094] As the second material that covers at least a part, preferably substantially the whole, of the particle surface of the particulate first material 100x that functions as a positive electrode active material, one or more of oxides and LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, Mn) can be used. Examples of the oxides include aluminum oxide, zirconium oxide, hafnium oxide, niobium oxide, and the like. Examples of LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, Mn) include LiFePO4, LiNiPO4, LiCoPO4, LiMnPO4, LiFe a Ni b PO4, LiFe a Co b PO4, LiFe a Mn b PO4, LiNi a Co b PO4, LiNi a Mn b PO4 (a + b is 1 or less, 0 < a < 1, 0 < b < 1), LiFe c Ni d Co e PO4, LiFe c Ni d Mn e PO4, LiNi c Co d Mn e PO4 (c + d + e is 1 or less, 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe f Ni g Co h Mn i PO4 (f + g + h + i is 1 or less, 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), and the like.

[0095] Also, in a state where substantially the whole of the particle surface of the particulate first material 100x that functions as a positive electrode active material is covered with the second material 100y, charge-discharge characteristics different from those obtained when the first material 100x and the second material 100y are simply mixed may be obtained.

[0096] Furthermore, the positive electrode of the present invention may have a structure in which at least a portion of the surface of the composite having the positive electrode active material is covered with a graphene compound. Preferably, a structure is preferred in which 80% or more of the particle surface of the composite having the positive electrode active material and / or the aggregate having the composite is covered with a graphene compound. Graphene compounds will be described later.

[0097] [Method for fabricating the positive electrode 1] An example of a method for manufacturing a positive electrode, which is one aspect of the present invention, will be explained with reference to Figures 6A and 6B.

[0098] In step S101 of Figure 6A, the binder 110 is prepared, and in step S102, the dispersion medium 120 is prepared.

[0099] For example, one or more of the following materials can be used as binder 110: 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.

[0100] Polyimides possess excellent thermal, mechanical, and chemical stability. Furthermore, when polyimides are used as a binder, dehydration and cyclization (imidization) reactions are performed. These reactions can be carried out, for example, by heat treatment. In one embodiment of the present invention, when graphene having an oxygen-containing functional group is used as the graphene compound and polyimides as the binder, the reduction of the graphene compound can also be performed by the heat treatment, simplifying the process. Due to its excellent heat resistance, the heat treatment can be performed at a heating temperature of, for example, 200°C or higher. By performing the heat treatment at a heating temperature of 200°C or higher, the reduction reaction of the graphene compound can be sufficiently carried out, further improving the conductivity of the electrode.

[0101] 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.

[0102] Furthermore, rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer can be used as binders. Fluororubber can also be used as a binder.

[0103] Furthermore, it is preferable to use a water-soluble polymer as the binder. Examples of water-soluble polymers include polysaccharides. Examples of polysaccharides include cellulose derivatives such as carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, diacetylcellulose, and regenerated cellulose, or starch. It is even more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0104] You may use a combination of several of the binders mentioned above.

[0105] As the dispersion medium 120, for example, one or more of the following can be used: water, methanol, ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

[0106] As a suitable combination of the binder 110 and the dispersion medium 120, it is preferable to use a combination of polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP).

[0107] 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.

[0108] In step S111 of Figure 6B, the binder mixture 1001 is prepared, and in step S112, the conductive agent 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.

[0109] As the conductive agent 1002, 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.

[0110] In this specification, graphene compounds refer to graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multigraphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multigraphene oxide, graphene quantum dots, etc. A graphene compound is a substance that contains carbon, has a plate-like or sheet-like shape, and has a two-dimensional structure formed by a six-membered carbon ring. The two-dimensional structure formed by the six-membered carbon ring may also be called a carbon sheet. Graphene compounds may have functional groups containing oxygen. Furthermore, graphene compounds preferably have a bent shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0111] In this specification, graphene oxide refers to, for example, a material having carbon and oxygen, having a sheet-like structure, and having functional groups, particularly epoxy groups, carboxyl groups, or hydroxyl groups.

[0112] 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 stacked. 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%. With such carbon and oxygen concentrations, it can function as a highly conductive material even in small amounts. It is also preferable that reduced graphene oxide has a G / D intensity ratio of 1 or more in the Raman spectrum. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0113] In some cases, pores can be created in graphene compounds by reducing graphene oxide.

[0114] Alternatively, a material in which the ends of graphene are terminated with fluorine may be used.

[0115] 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. Multiple graphene compounds are formed to partially cover multiple granular active materials or to adhere to the surfaces of multiple granular active materials, and thus are in surface contact with one another.

[0116] 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 and electrode weight. In other words, the charge and discharge capacity of the secondary battery can be increased.

[0117] 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.

[0118] Furthermore, by using a spray-drying device beforehand, a graphene compound, which is a conductive agent, 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.

[0119] 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.

[0120] Graphene compounds preferably have pores in a portion of the carbon sheet. By providing pores in a portion of the carbon sheet of a graphene compound that allow carrier ions such as lithium ions to pass through, the insertion and removal of carrier ions becomes easier on the surface of the active material covered with the graphene compound, thereby improving the rate characteristics of the secondary battery. The pores provided in a portion of the carbon sheet may be called voids, defects, or cavities.

[0121] Next, in step S113, the binder mixture 1001 and the conductive agent 1002 are mixed to obtain the mixture 1010 from step S121. For mixing, for example, a propeller-type mixing device, a planetary rotating mixing device, or a thin-film swirling mixing device can be used.

[0122] Next, in step S122 of Figure 6B, a composite 100z having a positive electrode active material is prepared.

[0123] Next, in step S123, the mixture 1010 and the composite 100z having the positive electrode active material are mixed to obtain the mixture 1020 from step S131. 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 S123, if the viscosity is appropriately adjusted, the aggregation of powders such as the positive electrode active material can be undone by kneading.

[0124] Next, the binder mixture 1001 is prepared in step S132, and the dispersion medium 1003 is prepared in step S133. If, in step S111, less than the total amount of binder mixture 1001 required to form the positive electrode 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 positive electrode active material layer was prepared in step S111, it is not necessary to prepare the binder mixture 1001 in step S132. As the dispersion medium 1003, the same dispersion medium as in step S102 in Figure 6A 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.

[0125] Next, in step S134, the mixture 1020 and dispersion medium 1003 from step S131 are mixed with the binder mixture 1001 prepared in step S132 to obtain the mixture 1030 from step S135. Mixture 1030 is sometimes called positive electrode slurry.

[0126] Next, in step S136, the mixture 1030 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. For the material used for the current collector, the materials described in Embodiment 6 may be used. As the application method in step S136, a slot die method, gravure, blade method, or a method combining these can be used. A continuous coating machine may also be used for application. Following step S136, in step S137, the mixture 1030 applied to the current collector is dried. As the drying method, for example, batch type methods such as hot plates, drying ovens, ventilated drying ovens, and vacuum drying ovens, as well as continuous type methods combining hot air drying and infrared drying with a continuous coating machine, can be used.

[0127] By following the above steps, a positive electrode 2000 according to one aspect of the present invention can be manufactured (step S140).

[0128] This embodiment can be implemented in appropriate combination with other embodiments.

[0129] (Embodiment 2) This embodiment describes a method for manufacturing a positive electrode according to one aspect of the present invention.

[0130] The positive electrode comprises a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer comprises a first material 100x and a second material 100y that function as positive electrode active material, and may further comprise a conductive agent and a binder.

[0131] As the first material 100x, the materials described above and the materials described in Embodiments 3 and 4 can be used. Furthermore, as the second material 100y, the materials described above and the materials described in Embodiment 5 can be used.

[0132] In this embodiment, as examples of methods for manufacturing a positive electrode having a first material 100x and a second material 100y, positive electrode manufacturing method 2, positive electrode manufacturing method 3, and positive electrode manufacturing method 4 are shown. As a desirable form of the positive electrode, it is desirable that the first material 100x and the second material 100y are well dispersed in the positive electrode active material layer and that a good conductive network is present. For example, in the first material 100x and the second material 100y, it is desirable to have a structure in which the amount of conductive agent in contact with one positive electrode active material with lower electronic conductivity is greater than the amount of conductive agent in contact with the other positive electrode active material. Here, the amount of conductive agent in contact with the positive electrode active material can be considered as the coverage rate of the conductive agent on the particle surface of the positive electrode active material, and can be measured, for example, by surface SEM observation, cross-sectional SEM observation, or cross-sectional TEM observation.

[0133] In one embodiment of the present invention, when mixing a first material 100x and a second material 100y as the positive electrode, and using LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al) as the first material 100x and LiM2PO4 (where M2 is one or more selected from Fe, Ni, Co, and Mn) as the second material 100y, the higher the mixing ratio of the second material 100y, which has high stability at high temperatures and a stable crystal structure even in a high-voltage charging state, the higher the fire resistance and heat resistance of the secondary battery using the positive electrode of one embodiment of the present invention. Furthermore, even when the mixing ratio of the first material 100x to the second material 100y is such that the ratio of the second material 100y decreases to 7:3, 8:2, or 9:1, the secondary battery using the positive electrode of one embodiment of the present invention may still possess fire resistance.

[0134] Method 2 for manufacturing a positive electrode provides an example of a method for manufacturing a positive electrode, comprising a manufacturing step of mixing a first material 100x, a second material 100y, and a mixture of a conductive agent and a binder. Method 3 for manufacturing a positive electrode provides an example of a method for manufacturing a positive electrode, comprising a first manufacturing step of mixing the first material 100x and the second material 100y, and a second manufacturing step of mixing the mixture obtained in the first step with a mixture of a conductive agent and a binder. Method 4 for manufacturing a positive electrode provides an example of a method for manufacturing a positive electrode, comprising a first manufacturing step of mixing the second material 100y, a mixture of a conductive agent and a binder, and a second manufacturing step of mixing the mixture obtained in the first step with the first material 100x. However, the present invention is not to be construed as being limited to these descriptions.

[0135] [Method for fabricating the positive electrode 2] An example of a method for manufacturing a positive electrode, which is one aspect of the present invention, will be explained with reference to Figures 7A and 7B.

[0136] In step S101 of Figure 7A, the binder 110 is prepared, and in step S102, the dispersion medium 120 is prepared. The materials shown in Embodiment 1 can be used as the binder 110 and the dispersion medium 120.

[0137] 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.

[0138] In step S111 of Figure 7B, the binder mixture 1001 is prepared, and in step S112, the conductive agent 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.

[0139] As the conductive agent 1002, 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.

[0140] Next, in step S113, the binder mixture 1001 and the conductive agent 1002 are mixed to obtain the mixture 1010 from step S121. For mixing, for example, a propeller-type mixing device, a planetary rotating mixing device, or a thin-film swirling mixing device can be used.

[0141] In step S122 of Figure 7B, the first material 100x is prepared, and in step S123, the second material 100y is prepared.

[0142] The first material 100x and the second material 100y can be the materials shown in Embodiment 1, respectively.

[0143] Furthermore, when selecting a combination of the first material 100x and the second material 100y, it is possible to choose a combination that minimizes the occurrence of steps in the charge-discharge curve, or a combination that causes steps in the charge-discharge curve at a desired charge level, depending on the characteristics required for the secondary battery.

[0144] Next, in step S124, the mixture 1010, the first material 100x, and the second material 100y are mixed to obtain the mixture 1020 from step S131. For mixing, for example, a propeller-type mixer, a planetary-type mixer, or a thin-film swirling mixer can be used. In the mixing in step S124, if the viscosity is appropriately adjusted, the aggregation of powders such as the positive electrode active material can be undone by solid kneading.

[0145] Next, the binder mixture 1001 is prepared in step S132, and the dispersion medium 1003 is prepared in step S133. If, in step S111, less than the total amount of binder mixture 1001 required to form the positive electrode 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 positive electrode active material layer was prepared in step S111, then it is not necessary to prepare the binder mixture 1001 in step S132. As the dispersion medium 1003, the same dispersion medium as in step S102 in Figure 7A 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.

[0146] Next, in step S134, the mixture 1020 from step S131, the binder mixture 1001 prepared in step S132, and the dispersion medium 1003 prepared in step S133 are mixed to obtain the mixture 1030 from step S135. The mixture 1030 is sometimes called the positive electrode slurry.

[0147] Next, in step S136, the mixture 1030 is applied to the current collector. The material shown in Embodiment 1 can be used as the current collector. Furthermore, the application in step S136 and the drying in step S137 can be carried out in the same manner as steps S136 and S137 shown in Figure 6.

[0148] By following the above steps, a positive electrode 2000 according to one aspect of the present invention can be manufactured (step S140).

[0149] [Method for fabricating the positive electrode 3] Another example of a method for manufacturing a positive electrode, which is one aspect of the present invention, will be explained with reference to Figure 8.

[0150] In step S111 of Figure 8, prepare the binder mixture 1001, and in step S112, prepare the conductive agent 1002. The binder mixture 1001 shown in Figure 7A can be used as the binder mixture 1001. In order to knead the mixture in a later step, the amount of binder mixture 1001 prepared in step S111 should be less than the total amount required to form the positive electrode active material layer, resulting in a mixture suitable for kneading. In this case, any shortage of binder mixture 1001 can be added in a step after kneading. Note that kneading refers to mixing with high viscosity.

[0151] As the conductive agent 1002, 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.

[0152] Next, in step S113, the binder mixture 1001 and the conductive agent 1002 are mixed to obtain the mixture 1010 from step S121. For mixing, for example, a propeller-type mixing device, a planetary rotating mixing device, or a thin-film swirling mixing device can be used.

[0153] As step S131 in FIG. 8, prepare the first material 100x, and in step S132, prepare the second material 100y.

[0154] For the first material 100x and the second material 100y, the materials shown in Embodiment 1 can be used respectively.

[0155] Note that as the combination of the first material 100x and the second material 100y, according to the characteristics required for the secondary battery, it is possible to select a combination in which steps are unlikely to occur in the charge-discharge curve, or to select a combination in which steps occur in the charge-discharge curve at a desired charge rate.

[0156] Next, in step S133, mix the first material 100x and the second material 100y to obtain the mixture 1100 in step S141. As a mixing method, for example, a ball mill, a propeller-type mixing device, a planetary rotation-type mixing device, or a thin-film swirling-type mixing device can be used.

[0157] Next, in step S142, mix the mixture 1010 in step S121 and the mixture 1100 in step S141 to obtain the mixture 1020 in step S151. As a mixing method, for example, a propeller-type mixing device, a planetary rotation-type mixing device, or a thin-film swirling-type mixing device can be used. In the mixing in step S142, when the viscosity is appropriately adjusted, the aggregation of powders such as the positive electrode active material can be loosened by kneading.

[0158] Next, the binder mixture 1001 is prepared in step S152, and the dispersion medium 1003 is prepared in step S153. In step S111, if an amount of the binder mixture 1001 less than the total amount required to form the positive electrode active material layer is prepared, the shortage of the binder mixture 1001 can be added in step S152. If the total amount of the binder mixture 1001 required to form the positive electrode active material layer is prepared in step S111, the binder mixture 1001 does not have to be prepared in step S152. As the dispersion medium 1003, the same dispersion medium as in step S102 of FIG. 7A can be used. It is desirable to adjust the amount of the dispersion medium 1003 to be prepared so that the viscosity is appropriate for coating in later steps.

[0159] Next, in step S154, the mixture 1020 in step S151, the binder mixture 1001 prepared in step S152, and the dispersion medium 1003 prepared in step S153 are mixed to obtain the mixture 1030 in step S155. The mixture 1030 may be referred to as a positive electrode slurry.

[0160] Next, in step S156, the mixture 1030 is applied to the current collector. As the current collector, the material shown in Embodiment 1 can be used. Also, the application in step S156 and the drying in step S157 can be performed in the same manner as steps S136 and S137 shown in FIG. 6.

[0161] Through the above steps, the positive electrode 2000 of one aspect of the present invention can be manufactured (step S160).

[0162] [Manufacturing Method of Positive Electrode 4] Another example of the manufacturing method of the positive electrode, which is one aspect of the present invention, will be described using FIG. 9.

[0163] In step S111 of Figure 9, prepare the binder mixture 1001, and in step S112, prepare the conductive agent 1002. The binder mixture 1001 shown in Figure 7A can be used as the binder mixture 1001. In order to knead the mixture into a solid mass in a later step, the amount of binder mixture 1001 prepared in step S111 should be less than the total amount required to form the positive electrode active material layer, resulting in a mixture 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.

[0164] As the conductive agent 1002, 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.

[0165] Next, in step S113, the binder mixture 1001 and the conductive agent 1002 are mixed to obtain the mixture 1010 from step S121. For mixing, for example, a propeller-type mixing device, a planetary rotating mixing device, or a thin-film swirling mixing device can be used.

[0166] Next, in step S122 of Figure 9, the second material 100y is prepared.

[0167] As the second material 100y, LiM2PO4 (where M2 is one or more selected from Fe, Ni, Co, and Mn) having an olivine-type crystal structure, prepared by the manufacturing method shown in Embodiment 5 described later, can be used. As LiM2PO4, the above-mentioned materials can be used, for example, LiFePO4, LiMnPO4, LiFe a Mn b PO4(a+b is less than or equal to 1, 0 <a<1、0<b<1)、LiFe a Ni bPO4 (where a + b ≤ 1, 0 < a < 1, 0 < b < 1) can be used. Also, the surface of the particles of the second material 100y may have a carbon coating layer.

[0168] Next, in step S123, the mixture 1010 and the second material 100y are mixed to obtain the mixture 1021 in step S131. As a mixing method, for example, a propeller-type mixing device, a planetary rotation-type mixing device, a thin-film swirling-type mixing device, etc. can be used. In the mixing in step S123, when the viscosity is appropriately adjusted, agglomeration of powders such as the positive electrode active material can be loosened by kneading.

[0169] In the process of manufacturing a positive electrode having the first material 100x and the second material 100y, when the second material 100y has lower electronic conductivity than the first material 100x, it is desirable to mix the second material 100y and a conductive agent before the step of mixing the first material 100x. By doing this, it becomes possible to obtain a structure in which the amount of the conductive agent in contact with the second material 100y is larger than the amount of the conductive agent in contact with the first material 100x.

[0170] Next, in step S132, the first material 100x is prepared.

[0171] The first material 100x and the second material 100y can each use the materials shown in Embodiment 1.

[0172] Note that as the combination of the first material 100x and the second material 100y, according to the characteristics required for the secondary battery, it is possible to select a combination in which it is difficult for a step to occur in the charge-discharge curve, or to select a combination in which a step occurs in the charge-discharge curve at a desired charge rate.

[0173] Next, in step S142, the mixture 1021 and the first material 100x are mixed to obtain the mixture 1022 of step S151. For mixing, for example, a propeller-type mixer, a planetary-type mixer, or a thin-film swirling mixer can be used. In the mixing of step S142, if the viscosity is appropriately adjusted, the aggregation of powders such as the positive electrode active material can be undone by solid kneading.

[0174] Next, the binder mixture 1001 is prepared in step S152, and the dispersion medium 1003 is prepared in step S153. If, in step S111, less than the total amount of binder mixture 1001 required to form the positive electrode active material layer was prepared, the deficit can be added in step S152. If the entire amount of binder mixture 1001 required to form the positive electrode active material layer was prepared in step S111, it is not necessary to prepare the binder mixture 1001 in step S152. As the dispersion medium 1003, the same dispersion medium as in step S102 in Figure 7A 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.

[0175] Next, in step S154, the mixture 1022 from step S151, the binder mixture 1001 prepared in step S152, and the dispersion medium 1003 prepared in step S153 are mixed to obtain the mixture 1030 from step S155. The mixture 1030 is sometimes called the positive electrode slurry.

[0176] Next, in step S156, the mixture 1030 is applied to the current collector. The material shown in Embodiment 1 can be used as the current collector. Furthermore, the application in step S156 and the drying in step S157 can be carried out in the same manner as steps S136 and S137 shown in Figure 6.

[0177] By following the above steps, a positive electrode 2000 according to one aspect of the present invention can be manufactured (step S160).

[0178] This embodiment can be implemented in appropriate combination with other embodiments.

[0179] (Embodiment 3) In this embodiment, an example of a method for producing a positive electrode active material according to one aspect of the present invention will be described using FIGS. 10A to 16C. Further, a positive electrode active material according to one aspect of the present invention will be described using FIGS. 17A to 25C.

[0180] [Production Method 1 of Positive Electrode Active Material] An example of a method for producing a positive electrode active material according to one aspect of the present invention will be described using FIGS. 10A and 10B.

[0181] [End]] <Step S11> In step S11 of FIG. 10A, a lithium source and a transition metal source are prepared as materials for lithium and the transition metal. In the drawing, the transition metal source is illustrated as the M1 source.

[0182] As the lithium source, for example, lithium carbonate, lithium fluoride, or the like can be used.

[0183] As the transition metal source, for example, at least one of manganese, cobalt, and nickel can be used. For example, when using only cobalt as the transition metal source, when using only nickel, when using two types of cobalt and manganese, when using two types of cobalt and nickel, or when using three types of cobalt, manganese, and nickel.

[0184] Note that it is preferable to use a high-purity material as the transition metal source used in the synthesis. Specifically, the purity of the material is 3N (99.9%) or more, preferably 4N (99.99%) or more, more preferably 4N5 (99.995%) or more, and still more preferably 5N (99.999%) or more. By using a high-purity material, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be enhanced.

[0185] In addition, it is preferable that the transition metal source has high crystallinity. For example, it is preferable that the transition metal source has single crystal grains. The crystallinity of the transition metal source can be evaluated from, for example, 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, etc. Furthermore, X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. can also be used as criteria for evaluating the crystallinity of the transition metal source. Note that the above evaluation of crystallinity can be applied not only to the transition metal source but also to the evaluation of the crystallinity of primary or secondary particles.

[0186] Furthermore, when using metals capable of forming layered rock salt-type composite oxides, it is preferable to use a mixing ratio of cobalt, manganese, and nickel within a range that allows for a layered rock salt-type crystal structure. Additionally, an additive element X may be added to these transition metals within a range that allows for a layered rock salt-type crystal structure. An example of the process of adding the additive element X is shown in Figure 10B. In step S11, a lithium source, a transition metal source, and an additive element X source are prepared, and then step S12 is carried out.

[0187] As the additive element X source, one or more elements selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic can be used. In addition to the above elements, bromine and beryllium may also be used as the additive element X source. However, since bromine and beryllium are toxic to living organisms, it is preferable to use the additive element X source described above.

[0188] Furthermore, as a transition metal source, oxides, hydroxides, etc., of the above-mentioned metals exemplified as transition metals can be used. As a cobalt source, for example, cobalt oxide, cobalt hydroxide, etc., can be used.

[0189] Furthermore, manganese oxide, manganese hydroxide, etc., can be used as manganese sources. Nickel oxide, nickel hydroxide, etc., can be used as nickel sources. Aluminum oxide, aluminum hydroxide, etc., can be used as aluminum sources.

[0190] <Step S12> Next, in step S12, the lithium source and transition metal source are crushed and mixed. The crushing and mixing can be done dry or wet. It is particularly preferable to crush using dehydrated acetone with a purity of 99.5% or higher, with a moisture content of 10 ppm or less. In this specification, the term "crushing" may be read as "grinding". For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use zirconia balls as the media. When using a ball mill or a bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media or material. In step S12, the peripheral speed will be 838 mm / s (rotation speed 400 rpm, ball mill container diameter 40 mm). Furthermore, by using the above-mentioned dehydrated acetone in the crushing and mixing, it is possible to reduce impurities that may be mixed into the material.

[0191] <Step S13> Next, in step S13, the mixed materials are heated. The heating temperature in this step is preferably 800°C or higher and less than 1100°C, more preferably 900°C or higher and 1000°C or lower, 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 source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to reasons such as lithium evaporating from the lithium source and / or the metal used as the transition metal source being excessively reduced. For example, if cobalt is used as the transition metal, a defect in which the cobalt becomes divalent may occur.

[0192] The heating time can be, for example, 1 hour or more and 100 hours or less, and preferably 2 hours or more and 20 hours or less. Heating is preferably carried out in an atmosphere with little water, such as dry air (for example, with a dew point of -50°C or lower, more preferably with a dew point of -80°C or lower). In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. Furthermore, heating is preferable in an atmosphere where the impurity concentrations of CH4, CO, CO2, and H2 are each 5 ppb (parts per billion) or less, as this suppresses the incorporation of impurities into the material.

[0193] Furthermore, for example, when heating at 1000°C for 10 hours, it is preferable to raise the temperature at 200°C / h and the flow rate of dry air at 10 L / min. After that, the heated material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to 10 hours or more and 50 hours or less. However, cooling to room temperature in step S13 is not essential.

[0194] Furthermore, the crucible or sheath used during heating in step S13 is preferably made of a highly heat-resistant material such as alumina (aluminum oxide), mullite cordierite, magnesia, or zirconia. An alumina crucible is preferable because it is made of a material that does not contain impurities. In this embodiment, it is preferable to use an alumina crucible with a purity of 99.9%. It is preferable to place a lid on the crucible or sheath before heating. This prevents the material from volatilizing.

[0195] Furthermore, when collecting the material after heating in step S13, it is preferable to transfer it from the crucible to a mortar before collection, as this prevents impurities from contaminating the material. It is also preferable that the mortar itself be made of a material that does not allow impurities to enter. Specifically, it is preferable to use a mortar made of alumina with a purity of 90% or higher, preferably 99% or higher. Note that the same conditions as in step S13 can be applied to the heating processes described later, other than step S13.

[0196] <Step S14> Through the above steps, a positive electrode active material 100A according to one embodiment of the present invention can be produced in Figure 10A, and a positive electrode active material 100B can be produced in Figure 10B (step S14). The positive electrode active material 100A and the positive electrode active material 100B can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0197] [Method for preparing positive electrode active material 2] Next, another example of a method for producing a positive electrode active material according to one aspect of the present invention will be explained using Figures 11A, 11B, and 11C.

[0198] In Figure 11A, steps S11 to S14 are performed in the same manner as in Figure 10A to prepare a composite oxide (LiM1O2) having lithium, a transition metal, and oxygen.

[0199] In addition, a pre-synthesized composite oxide may be used as step S14. In this case, steps S11 to S13 can be omitted. When preparing a pre-synthesized composite oxide, it is preferable to use a high-purity material. The purity of the material should be 99.5% or higher, preferably 99.9% or higher, and more preferably 99.99% or higher.

[0200] <Step S20> As step S20 in Figure 11A, a source of additive element X is prepared. The materials described above can be used as the source of additive element X. In addition, multiple elements may be used as additive element X. The case in which multiple elements are used as additive element X will be explained using Figures 11B and 11C.

[0201] <Step S21> In step S21 of Figure 11B, a magnesium source (Mg source) and a fluorine source (F source) are prepared. Alternatively, a lithium source may be prepared in conjunction with the magnesium source and the fluorine source.

[0202] For example, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc., can be used as magnesium sources.

[0203] Examples of fluorine sources that can be used include lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF2), lanthanum fluoride (LaF3), and sodium aluminum hexafluoride (Na3AlF6). Furthermore, the fluorine source is not limited to a solid; for example, fluorine (F2), carbon fluoride, sulfur fluoride, oxygen fluoride (OF2, O2F2, O3F2, O4F2, O2F), etc., may be used and mixed in the atmosphere during the heating process described later. Multiple fluorine sources may also be used in combination. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted during the heating process described later.

[0204] For example, lithium fluoride and lithium carbonate can be used as lithium sources. In other words, lithium fluoride can be used as both a lithium source and a fluorine source. Similarly, magnesium fluoride can be used as both a fluorine source and a magnesium source.

[0205] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as both the fluorine source and the magnesium source. Mixing lithium fluoride (LiF) and magnesium fluoride (MgF2) in a molar ratio of approximately LiF:MgF2 = 65:35 yields the highest effect in lowering the melting point (Non-Patent Literature 4). 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 (LiF) to magnesium fluoride (MgF2) 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 = around 0.33). In this specification, "around" means a value greater than 0.9 times and less than 1.1 times the value.

[0206] Furthermore, if the following mixing and crushing steps are carried out wet, a solvent should be 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.

[0207] <Step S22> Next, in step S22 of Figure 11B, the above materials are mixed and crushed. Mixing can be done dry or wet, but wet mixing is preferred because it can crush the materials into smaller pieces. For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use zirconia balls as the media. The conditions for the ball mill or bead mill can be the same as those in step S12.

[0208] <Step S23> Next, in step S23, the materials crushed and mixed above are recovered to obtain the additive element X source. Note that the additive element X source shown in step S23 is formed from multiple materials and may therefore be referred to as a mixture.

[0209] The above mixture preferably has a median diameter (D50) of 600 nm to 20 μm, and more preferably 1 μm to 10 μm. When the mixture is finely powdered in this way, it is easier to uniformly adhere the mixture to the surface of the composite oxide particles when it is mixed with lithium, a transition metal, and oxygen in a later process. When the mixture is uniformly adhered to the surface of the composite oxide particles, it is preferable because it is easier to distribute halogen and magnesium evenly near the surface of the composite oxide particles after heating. If there are regions near the surface that do not contain halogen and magnesium, it may be difficult to form the O3' type crystal structure described later in the charged state.

[0210] While step S21 in Figure 11B illustrates a method for mixing two types of materials, the method is not limited to this. For example, as shown in Figure 11C, a source of four types of materials (magnesium source (Mg source), fluorine source (F source), nickel source (Ni source), and aluminum source (Al source)) may be mixed to prepare the additive element X source. Alternatively, a single material, i.e., one type of material, may be used to prepare the additive element X source. Nickel oxide, nickel hydroxide, etc., can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc., can be used as the aluminum source.

[0211] <Step S31> Next, in step S31 of Figure 11A, the LiM1O2 obtained in step S14 is mixed with the additive element X source. The ratio of the number of transition metal atoms M in the composite oxide having lithium, a transition metal, and oxygen to the number of magnesium atoms Mg in the additive element X is preferably M:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0212] The mixing in step S31 is preferably carried out under milder conditions than the mixing in step S12 in order to avoid destroying the particles of the composite oxide. For example, it is preferable to use conditions with a lower rotation speed or a 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 zirconia balls as the media.

[0213] Here, the mixing will be carried out in a ball mill using 1 mm diameter zirconia balls at 150 rpm for 1 hour in a dry state. The mixing will be performed in a dry room with a dew point between -100°C and -10°C.

[0214] <Step S32> Next, in step S32 of Figure 11A, the materials mixed above are collected to obtain mixture 903.

[0215] In this embodiment, a method of adding a mixture of lithium fluoride and magnesium fluoride to lithium cobalt oxide with few impurities is described, but the present invention is not limited to this. Instead of the mixture 903 in step S32, a starting material of lithium cobalt oxide to which a magnesium source and a fluorine source, etc., have been added and heated may be used. In this case, it is not necessary to separate the processes in steps S11 to S14 and steps S21 to S23, making it simpler and more productive.

[0216] Alternatively, lithium cobalt oxide with magnesium and fluorine added beforehand may be used. Using lithium cobalt oxide with magnesium and fluorine added makes the process up to step S32 easier to complete.

[0217] Alternatively, magnesium and fluorine sources may be added to lithium cobalt oxide that has already been treated with magnesium and fluorine.

[0218] <Step S33> Next, in step S33, the mixture 903 is heated in an oxygen-containing atmosphere. It is preferable to heat the mixture 903 in a way that prevents the particles from sticking together.

[0219] If the particles of mixture 903 adhere to each other during heating, the distribution of additive elements, which are preferably distributed near the surface as described later, may deteriorate. Furthermore, even the surface of particles that are preferably smooth and have few irregularities may become more irregular and develop more defects such as cracks and / or fissures if the particles adhere to each other. This is thought to be due to the adhesion of mixture 903 particles, which reduces the contact area with oxygen in the atmosphere and obstructs the diffusion pathways of the additive elements.

[0220] Furthermore, heating in step S33 may be performed using a rotary kiln. Heating with a rotary kiln can be performed while stirring, whether in a continuous or batch system. Alternatively, heating in step S33 may be performed using a roller hearth kiln.

[0221] The heating temperature in step S33 must be above the temperature at which the reaction between LiM1O2 and the additive element X source proceeds. The temperature at which the reaction proceeds here refers to any temperature at which elemental interdiffusion occurs between LiM1O2 and the additive element X source. Therefore, it may be possible to set the temperature lower than the melting temperature of these materials. For example, in oxides, the melting temperature T m 0.757 times (Tammann temperature T) d Solid-phase diffusion occurs from this point. Therefore, the heating temperature in step S33 should be, for example, 500°C or higher.

[0222] However, it is preferable that the heating temperature in step S33 be 742°C or higher, as this facilitates the reaction. For example, if LiF and MgF2 are used as the source of additive element X, the eutectic point of LiF and MgF2 is around 742°C, so it is preferable to set the heating temperature in step S33 to 742°C or higher.

[0223] Furthermore, when mixture 903 is prepared 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 heating temperature of 830°C or higher is more preferable.

[0224] A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and increases productivity.

[0225] However, the heating temperature must be below the decomposition temperature of LiM1O2 (1130°C in the case of LiCoO2). Furthermore, at temperatures near the decomposition temperature, there is a concern that a small amount of LiM1O2 may decompose. For this reason, the heating temperature in step S33 is preferably 1130°C or lower, more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 900°C or lower.

[0226] Therefore, the heating temperature in step S33 is preferably 500°C to 1130°C, more preferably 500°C to 1000°C, even more preferably 500°C to 950°C, and even more preferably 500°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 830°C to 1130°C, more preferably 830°C to 1000°C, even more preferably 830°C to 950°C, and even more preferably 830°C to 900°C.

[0227] Furthermore, when heating mixture 903, it is preferable to control the partial pressure of fluorine or fluoride in the atmosphere to an appropriate range.

[0228] 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 LiM1O2, for example, between 742°C and 950°C, enabling the distribution of additive elements, including magnesium, near the surface and the production of a positive electrode active material with good properties.

[0229] However, since LiF is less dense than oxygen in its gaseous state, gaseous LiF easily escapes from the top of the heating container. Therefore, when LiF volatilizes due to heating, the amount of LiF in mixture 903 decreases. This weakens its function as a flux. Thus, it is necessary to heat while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, Li and F on the surface of LiM1O2 may react 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 necessary.

[0230] 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.

[0231] Furthermore, when heating with a rotary kiln, it is preferable to heat the mixture 903 by controlling the flow rate of the oxygen-containing atmosphere inside the kiln. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to purge the atmosphere first and then not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln.

[0232] 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.

[0233] Heating should preferably be carried out for an appropriate amount of time. The heating time varies depending on conditions such as the heating temperature, the size of the LiM1O2 particles in step S14, and the composition. When the particles are small, a lower temperature or shorter time may be preferable than when the particles are large.

[0234] For example, if the average particle size (D50) of the composite oxide in step S14 of Figure 11A is about 12 μm, the heating temperature is preferably, for example, 600°C to 950°C. The heating time is preferably 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more.

[0235] On the other hand, if the average particle size (D50) of the composite oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 600°C to 950°C. The heating time is preferably, for example, 1 hour to 10 hours, and more preferably about 2 hours. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0236] <Step S34> Next, the heated material is recovered to produce the positive electrode active material 100C. At this time, it is preferable to further sift the recovered particles. By the above steps, a positive electrode active material 100C according to one embodiment of the present invention can be produced (step S34). The positive electrode active material 100C can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0237] [Method for preparing positive electrode active material 3] Next, another example of a method for producing a positive electrode active material according to one aspect of the present invention will be explained using Figure 12, as well as Figures 13A, 13B, and 13C.

[0238] In Figure 12, steps S11 to S14 are performed in the same manner as in Figure 10A to prepare a composite oxide (LiM1O2) having lithium, a transition metal, and oxygen.

[0239] In addition, a pre-synthesized composite oxide containing lithium, a transition metal, and oxygen may be used as step S14. In this case, steps S11 to S13 can be omitted.

[0240] <Step S20a> As step S20a in Figure 12, a source of additive element X1 is prepared. The additive element X1 source can be selected from the additive element X described above. For example, one or more of magnesium, fluorine, and calcium can be suitably used as additive element X1. In this embodiment, a configuration using magnesium and fluorine as additive element X1 is illustrated in Figure 13A. Steps S21 and S22 included in step S20a shown in Figure 13A can be prepared in the same process as steps S21 and S22 shown in Figure 11B.

[0241] Step S23, shown in Figure 13A, is a process in which the material crushed and mixed in step S22, also shown in Figure 13A, is recovered to serve as the source of additive element X1.

[0242] Furthermore, steps S31 to S33 shown in Figure 12 can be manufactured using the same process as steps S31 to S33 shown in Figure 11.

[0243] <Step S34a> Next, the material heated in step S33 is recovered to produce a composite oxide.

[0244] <Step S40> As step S40 in Figure 12, a source of additive element X2 is prepared. The additive element X2 source can be selected from the additive element X described above. For example, as additive element X2, one or more selected from nickel, titanium, boron, zirconium, and aluminum can be suitably used. In this embodiment, a configuration using nickel and aluminum as additive element X2 is illustrated in Figure 13B. Steps S41 and S42 included in step S40 shown in Figure 13B can be manufactured in the same process as steps S21 and S22 shown in Figure 11B.

[0245] Step S43, shown in Figure 13B, is a process in which the material crushed and mixed in step S42, also shown in Figure 13B, is recovered to serve as the source of additive element X2.

[0246] Furthermore, step S40 shown in Figure 13C is a modified example of step S40 shown in Figure 13B. In Figure 13C, a nickel source and an aluminum source are prepared (step S41), and each is crushed independently (step S42a) to prepare multiple additive element X2 sources (step S43).

[0247] <Steps S51 to S53> Next, step S51 in Figure 12 is a process of mixing the composite oxide prepared in step S34a with the additive element X2 source prepared in step S40. Step S51 in Figure 12 can be processed in the same way as step S31 shown in Figure 11A. Step S52 in Figure 12 can be processed in the same way as step S32 shown in Figure 11A. The material prepared in step S52 in Figure 12 is mixture 904. Mixture 904 is a material that contains the additive element X2 source added in step S40 in addition to the material of mixture 903. Step S53 in Figure 12 can be processed in the same way as step S33 shown in Figure 11A.

[0248] <Step S54> Next, the heated material is recovered to produce the positive electrode active material 100D. At this time, it is preferable to further sift the recovered particles. By the above steps, a positive electrode active material 100D according to one embodiment of the present invention can be produced (step S54). The positive electrode active material 100D can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0249] As shown in Figures 12 and 13A to 13C, by separating the processes for introducing the transition metal, additive element X1, and additive element X2, it is sometimes possible to change the depth profile of each element. For example, the concentration of the additive element can be increased near the surface compared to the interior of the particle. Also, using the number of atoms of the transition metal as a reference, the ratio of the number of atoms of the additive element to this reference can be made higher near the surface than in the interior.

[0250] [Method for preparing positive electrode active material 4] <Step S11> In step S11 shown in Figure 14A, lithium sources (Li sources) and transition metal sources (M sources) are prepared as the starting materials, lithium and transition metals, respectively.

[0251] 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.

[0252] The transition metal can be selected from elements listed in groups 4 through 13 of the periodic table, for example, at least one of manganese, cobalt, and nickel can be used. The transition metal 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).

[0253] As a transition metal source, it is preferable to use a compound having the above-mentioned transition metal. For example, oxides of the metals exemplified above as transition metals, or hydroxides of the exemplified metals, etc., can be used. As a cobalt source, cobalt oxide, cobalt hydroxide, etc., can be used. As a manganese source, manganese oxide, manganese hydroxide, etc., can be used. As a nickel source, nickel oxide, nickel hydroxide, etc., can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, etc., can be used.

[0254] The transition metal 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.

[0255] In addition, it is preferable that the transition metal source has high crystallinity, for example, having single crystal grains.

[0256] Furthermore, when using two or more transition metal sources, it is preferable to prepare them in a proportion (mixing ratio) such that the two or more transition metal sources can adopt a layered rock salt-type crystalline structure.

[0257] <Step S12> Next, as shown in step S12 in Figure 14A, the lithium source and the transition metal 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 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.

[0258] A ball mill or bead mill can be used for mixing and other processes. When using a ball mill, alumina balls or zirconia balls are preferable as the grinding media. Zirconia 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 container diameter 40 mm).

[0259] <Step S13> Next, in step S13 shown in Figure 14A, 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 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 source. Such defects include, for example, when cobalt is used as the transition metal, excessive reduction can cause the cobalt to change from trivalent to divalent, inducing oxygen defects.

[0260] The heating time should ideally be between 1 hour and 100 hours, and preferably between 2 hours and 20 hours.

[0261] 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.

[0262] 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. Furthermore, 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.

[0263] 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.

[0264] 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 and then filled with oxygen up to 50 hPa.

[0265] 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.

[0266] Heating in this process may be carried out using a rotary kiln or a roller hearth kiln. Heating using a rotary kiln can be performed while stirring, whether in a continuous or batch system.

[0267] 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. An alumina crucible is preferable if it is made of a material that does not contain impurities. In this embodiment, it is preferable to use an alumina crucible with a purity of 99.9%. It is preferable to place a lid on the crucible or sheath before heating. This prevents the material from volatilizing.

[0268] 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 an alumina mortar. An alumina mortar is made of a material that does not contain impurities. Specifically, an alumina mortar with a purity of 90% or higher, preferably 99% or higher, should be used. In addition, the same heating conditions as in step S13 can be applied to the heating processes described later, other than step S13.

[0269] <Step S14> Through the above process, a composite oxide containing a transition metal (LiM1O2) can be obtained in step S14 shown in Figure 14A. The composite oxide only needs to have the crystal structure of a lithium composite oxide represented as LiM1O2, and its composition is not strictly limited to Li:M1:O=1:1:2. When cobalt is used as the transition metal, 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.

[0270] 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.

[0271] <Step S15> Next, as step S15 shown in Figure 14A, 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.

[0272] Initial heating may cause some lithium to detach from the lithium composite oxide in step S14. Furthermore, it is expected to improve the crystallinity of the lithium composite oxide. Additionally, since the lithium source and / or transition metal M1 prepared in step S11, etc., contain impurities, initial heating can reduce these impurities from the lithium composite oxide in step S14.

[0273] After initial heating, the surface of the composite oxide becomes smooth. A smooth surface is one with few irregularities, an overall rounded shape, and rounded corners. Furthermore, a smooth surface is defined as one with 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. A smooth active material can have a surface roughness of at least 10 nm or less, preferably less than 3 nm, when the surface irregularity information is quantified from the measurement data in a cross-section observed with a scanning transmission electron microscope (STEM).

[0274] Initial heating involves heating the composite oxide after it has been completed, and its purpose is to smooth the surface, thereby reducing degradation after charging and discharging. A lithium source is not required for initial heating to smooth the surface.

[0275] Alternatively, in the initial heating phase to smooth the surface, it is not necessary to prepare an additive element source.

[0276] Alternatively, a flux agent does not need to be prepared during the initial heating to smooth the surface.

[0277] The lithium source and transition metal source prepared in step S11, etc., may contain impurities. It is possible to reduce impurities from the composite oxide completed in step S14 by initial heating.

[0278] The heating conditions for this step should be such that the surface of the composite oxide becomes smooth. For example, the heating conditions can be selected from those described in step S13. To add to the heating conditions, the heating temperature in this step 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 step 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.

[0279] 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.

[0280] Furthermore, differences in shrinkage can cause microscopic displacements in the composite oxide, such as crystal misalignment. This process is recommended to reduce such misalignment. This process makes it possible to homogenize the misalignment of the composite oxide. When the misalignment is homogenized, the surface of the composite oxide may become smoother. This can also be described as crystal grain alignment. In other words, it is believed that step S15 mitigates the crystal misalignment in the composite oxide, resulting in a smoother surface.

[0281] 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.

[0282] A smooth surface of a composite oxide can be described as having a surface roughness of at least 10 nm, preferably less than 3 nm, when the surface irregularities information is quantified from measurement data in a cross-section of the composite oxide. A cross-section is, for example, a cross-section obtained when observing with a scanning transmission electron microscope (STEM).

[0283] Furthermore, a pre-synthesized composite oxide containing lithium, a transition metal, and oxygen may be used as step S14. 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.

[0284] It is possible that the lithium in the composite oxide decreases due to initial heating. The reduced lithium, due to the decrease in the added elements (explained in the next step, S20, etc.), may make it easier for the lithium to enter the composite oxide.

[0285] <Step S20> Additive element X 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 X to a composite oxide with a smooth surface allows for uniform addition of the additive element X. Therefore, it is preferable to add the additive element X after initial heating. The step of adding additive element X will be explained using Figures 14B and 14C.

[0286] <Step S21> In step S21 shown in Figure 14B, a Mg source and an F source are prepared as additive element sources (X sources) to be added to the composite oxide. A lithium source may also be prepared along with the additive element sources.

[0287] As additive element X, 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 as additive element X. However, since bromine and beryllium are toxic to living organisms, it is preferable to use the additive elements mentioned above.

[0288] When magnesium is selected as the additive element X, the additive element source 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.

[0289] When fluorine is selected as the additive element X, the additive element source can be called a fluorine source. Examples of such fluorine sources include lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF2), lanthanum fluoride (LaF3), or sodium aluminum hexafluoride (Na3AlF6). 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.

[0290] 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.

[0291] The fluorine source may also be a gas, such as fluorine (F2), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF2, O2F2, O3F2, O4F2, O2F), which may be mixed into the atmosphere during the heating process described later. Multiple fluorine sources may also be used.

[0292] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as both the fluorine source and the magnesium source. Mixing lithium fluoride and magnesium fluoride in a molar ratio of approximately LiF:MgF2 = 65:35 maximizes the effect of lowering the melting point (see Non-Patent Literature 4). 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 (around x = 0.33). Note that "around" means a value greater than 0.9 times and less than 1.1 times that value.

[0293] <Step S22> Next, in step S22 shown in Figure 14B, 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.

[0294] 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.

[0295] <Step S23> Next, in step S23 shown in Figure 14B, the material that has been crushed and mixed above is recovered to obtain an additive element source (X source). The additive element source shown in step S23 has multiple starting materials and can be called a mixture.

[0296] 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 added elements, 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.

[0297] Such finely powdered mixtures (including those containing only one additive element) are more likely to uniformly adhere to the surface of the composite oxide particles when mixed with the composite oxide 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 the additive element to the surface layer of the composite oxide after heating. The region where the additive element is distributed can also be called the surface layer. If there are regions in the surface layer that do not contain the additive element, it may be difficult to achieve the O3' type crystal structure described later in the charged state. Although fluorine was used in this explanation, fluorine can also be replaced with chlorine, and these can be considered as halogens.

[0298] <Step S21> A process different from that shown in Figure 14B will be explained using Figure 14C. In step S21 shown in Figure 14C, four types of additive element sources are prepared to be added to the composite oxide. In other words, the types of additive element sources in Figure 14C are different from those in Figure 14B. A lithium source may also be prepared along with the additive element sources.

[0299] Four types of additive element 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 14B. Nickel sources such as nickel oxide and nickel hydroxide can be used. Aluminum sources such as aluminum oxide and aluminum hydroxide can be used.

[0300] <Steps S22 and S23> Next, steps S22 and S23 shown in Figure 14C are the same as the steps described in Figure 14B.

[0301] <Step S31> Next, in step S31 shown in Figure 14A, the composite oxide and the additive element source (X source) are mixed. The ratio of the number of transition metal atoms M in the composite oxide having lithium, a transition metal, and oxygen to the number of magnesium atoms Mg in the additive element X is preferably M:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0302] 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 zirconia balls as the media.

[0303] In this embodiment, the mixing is performed dry using a ball mill with zirconia 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.

[0304] <Step S32> Next, in step S32 of Figure 14A, the materials mixed above are recovered to obtain mixture 903. During recovery, if necessary, the materials may be crushed and then sieved.

[0305] 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 transition metal source. Then, in step S13, heating is performed to obtain LiM1O2 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.

[0306] Alternatively, lithium cobalt oxide with magnesium and fluorine added beforehand may be used. Using lithium cobalt oxide with added magnesium and fluorine allows for the omission of steps S11 to S32 and step S20. This method is simple and highly productive.

[0307] Alternatively, magnesium and fluorine sources may be added to lithium cobalt oxide that has already been treated with magnesium and fluorine, according to step S20 in Figure 14B, or magnesium, fluorine, nickel, and aluminum sources may be added according to step S20 in Figure 14C.

[0308] <Step S33> Next, in step S33 shown in Figure 14A, the mixture 903 is heated. The heating can be performed by selecting from the heating conditions described in step S13. A heating time of 2 hours or more is preferable.

[0309] Let's add some information about the heating temperature. The lower limit of the heating temperature in step S33 must be above the temperature at which the reaction between the composite oxide (LiM1O2) and the additive element source proceeds. The temperature at which the reaction proceeds is simply the temperature at which interdiffusion of the elements in LiM1O2 and the additive element source occurs, and it may be lower than the melting temperature of these materials. Let's explain using an oxide as an example, but the melting temperature T m0.757 times (Tammann temperature T) d It is known that solid-phase diffusion occurs from ). Therefore, the heating temperature in step S33 should be 500°C or higher.

[0310] 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, when LiF and MgF2 are used as the additive element sources, the eutectic point of LiF and MgF2 is around 742°C. Therefore, it is preferable to set the lower limit of the heating temperature in step S33 to 742°C or higher.

[0311] 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.

[0312] A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and increases productivity.

[0313] The upper limit of the heating temperature should be below the decomposition temperature of LiM1O2 (the decomposition temperature of LiCoO2 is 1130°C). At temperatures near the decomposition temperature, there is a concern that a small amount of LiM1O2 may decompose. 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.

[0314] 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 S13.

[0315] 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.

[0316] 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 (LiM1O2), for example, between 742°C and 950°C, enabling the distribution of additive elements, including magnesium, to the surface layer and producing a positive electrode active material with good properties.

[0317] 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 LiM1O2 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.

[0318] 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.

[0319] In this process, it is preferable to heat the mixture 903 in a way that prevents the particles from sticking together. If the particles of the mixture 903 stick together during heating, the contact area with oxygen in the atmosphere decreases, and the diffusion pathway of the added elements (e.g., fluorine) is obstructed, which may worsen the distribution of added elements (e.g., magnesium and fluorine) to the surface layer.

[0320] Furthermore, it is believed that if the additive elements (e.g., fluorine) are 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 particles do not adhere to each other.

[0321] 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. Allowing oxygen to flow may cause the fluorine source to evaporate, which is undesirable for maintaining surface smoothness.

[0322] 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.

[0323] A note regarding heating time: The heating time varies depending on conditions such as the heating temperature, the size of the LiM1O2 particles in step S14, and the composition. When the LiM1O2 particles are small, a lower temperature or shorter heating time may be preferable than when the LiM1O2 particles are large.

[0324] When the median diameter (D50) of the composite oxide (LiM1O2) in step S14 of Figure 14A is approximately 12 μm, the heating temperature is preferably, for example, 600°C to 950°C. The heating time is preferably, for example, 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0325] On the other hand, if the median diameter (D50) of the composite oxide (LiM1O2) in step S14 is about 5 μm, the heating temperature is preferably, for example, 600°C to 950°C. The heating time is preferably, for example, 1 hour to 10 hours, and more preferably about 2 hours. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0326] <Step S34> Next, in step S34 shown in Figure 14A, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100E. At this time, it is preferable to further sift the recovered particles. Through the above steps, one embodiment of the positive electrode active material 100E of the present invention can be produced. The positive electrode active material of one embodiment of the present invention has a smooth surface. The positive electrode active material 100E can be used as the first material 100x shown in Embodiments 1 and 2.

[0327] [Method for preparing positive electrode active material 5] Next, we will describe one embodiment of the present invention that differs from method 2 for producing the positive electrode active material.

[0328] In Figure 15, steps S11 to S15 are performed in the same manner as in Figure 14A to prepare a composite oxide (LiM1O2) with a smooth surface.

[0329] <Step S20a> As mentioned above, it is possible to add element X to the composite oxide to the extent that it can take on a layered rock salt type crystal structure. However, in this preparation method 2, the step of adding the element in two or more separate steps will be explained with reference to Figure 16A.

[0330] <Step S21> Figure 16A shows the details of step S20a. In step S21, a Mg source and an F source are prepared as the first additive element source (X1 source). The X1 source can be selected from the additive element X described in step S21 shown in Figure 14B. For example, one or more of magnesium, fluorine, and calcium can be suitably used as the additive element X1. Figure 16A illustrates the case where a magnesium source (Mg source) and a fluorine source (F source) are used as the first additive element source (X1 source).

[0331] Steps S21 to S23 shown in Figure 16A can be prepared under the same conditions as steps S21 to S23 shown in Figure 14B. As a result, the first additive element source (X1 source) can be obtained in step S23. The first additive element source (X1 source) is the X1 source in step S20a shown in Figure 15.

[0332] Furthermore, steps S31 to S33 shown in Figure 15 can be manufactured using the same process as steps S31 to S33 shown in Figure 14A.

[0333] <Step S34a> Next, the material heated in step S33 shown in Figure 15 is recovered to produce a composite oxide containing the additive element X1. This is also called the second composite oxide to distinguish it from the composite oxide of step S14.

[0334] <Step S40> In step S40, shown in Figure 15, a second source of added elements (X2 source) is added. Step S40 will be explained in detail with reference to Figures 16B and 16C.

[0335] <Step S41> In step S41 shown in Figure 16B, a Ni source and an Al source are prepared as the second additive element source (X2 source). The X2 source can be selected from the additive element X described in step S21 shown in Figure 14B. For example, one or more of nickel, titanium, boron, zirconium, and aluminum can be suitably used as the additive element X2. Figure 16B illustrates the case where a nickel source and an aluminum source are used as the second additive element source (X2 source).

[0336] Steps S41 to S43 shown in Figure 16B can be prepared under the same conditions as steps S21 to S23 shown in Figure 14B. As a result, a second additive element source (X2 source) can be obtained in step S43.

[0337] Furthermore, Figure 16C shows a modified version of the steps described using Figure 16B. In step S41 shown in Figure 16C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are each pulverized independently. As a result, in step S43, multiple second additive element sources (X2 sources) are prepared. The steps in Figure 16C differ from those in Figure 16B in that the additive elements are pulverized independently in step S42a.

[0338] <Steps S51 to S53> Next, steps S51 to S53 shown in Figure 15 can be carried out under the same conditions as steps S31 to S33 shown in Figure 14A. The conditions for step S53 regarding the heating process may be at a lower temperature and for a shorter time than in step S33. Through the above steps, a positive electrode active material 100F of one embodiment of the present invention can be produced in step S53. The positive electrode active material of one embodiment of the present invention has a smooth surface. The positive electrode active material 100F can be used as the first material 100x shown in Embodiments 1 and 2.

[0339] As shown in Figures 15 and 16, in manufacturing method 2, the additive elements to the composite oxide are introduced separately as a first additive element X1 and a second additive element X2. By introducing them separately, the depth profile of each additive element can be changed. For example, it is possible to profile the first additive element so that its concentration is higher in the surface layer than in the interior, and the second additive element so that its concentration is higher in the interior than in the surface layer.

[0340] As demonstrated in this embodiment, a positive electrode active material with a smooth surface can be obtained through the initial heating process.

[0341] The initial heating described in this embodiment is performed on a composite oxide. Therefore, it is preferable that the initial heating is performed under conditions that are lower than the heating temperature required to obtain the composite oxide and shorter than the heating time required to obtain the composite oxide. When adding additive elements to the composite oxide, it is preferable to perform the addition step after the initial heating. This addition step can be divided into two or more steps. Following this order of steps is preferable because it maintains the surface smoothness obtained in the initial heating. When the composite oxide contains cobalt as a transition metal, it can be interpreted as a composite oxide containing cobalt.

[0342] The positive electrode active material 100F may be expressed as a composite oxide (LiM1O2) having lithium, a transition metal, and oxygen. However, the positive electrode active material in one embodiment of the present invention only needs to have the crystal structure of a lithium composite oxide represented as LiM1O2, and its composition is not strictly limited to Li:M1:O=1:1:2.

[0343] As described above, in one embodiment of the present invention, a high-purity material is used as the transition metal source during synthesis, and the positive electrode active material is produced in a process that minimizes the inclusion of impurities during synthesis. The positive electrode active material obtained by such a method is a material with a low impurity concentration, in other words, a highly purified material. Furthermore, the positive electrode active material obtained by such a method is a material with high crystallinity. Moreover, the positive electrode active material obtained by the method of producing a positive electrode active material according to one embodiment of the present invention can increase the capacity of a secondary battery and / or improve the reliability of a secondary battery.

[0344] [Structure of the positive electrode active material] A positive electrode active material according to one aspect of the present invention will be described with reference to Figures 17 to 25.

[0345] Figure 17A is a schematic top view of a positive electrode active material 100, which is one embodiment of the present invention. A schematic cross-sectional view of AB in Figure 17A is shown in Figure 17B.

[0346] <Elemental composition and distribution> The positive electrode active material 100 comprises lithium, a transition metal, oxygen, and an additive element. The additive element should preferably be different from the transition metal present in the positive electrode active material 100. In other words, the positive electrode active material 100 can be described as a composite oxide represented by LiM1O2 with elements other than M1 added.

[0347] As the transition metal in the positive electrode active material 100, it is preferable to use a metal that can form a layered rock salt type composite oxide belonging to space group R-3m together with lithium. For example, at least one of manganese, cobalt, and nickel can be used. In other words, the transition metal in the positive electrode active material 100 may be cobalt only, nickel only, two types of cobalt and manganese, two types of cobalt and nickel, or three types of cobalt, manganese, and nickel. In other words, the positive electrode active material 100 can have composite oxides containing lithium and a transition metal, such as lithium cobalt oxide, lithium nickel oxide, lithium cobalt oxide in which part of the cobalt is substituted with manganese, lithium cobalt oxide in which part of the cobalt is substituted with nickel, and nickel-manganese-lithium cobalt oxide. It is preferable to have nickel in addition to cobalt as a transition metal because the crystal structure becomes more stable in the charged state at high voltage.

[0348] The additive element X in the positive electrode active material 100 is preferably one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic. These additive elements may further stabilize the crystal structure of the positive electrode active material 100. In other words, the positive electrode active material 100 can include lithium cobalt oxide with magnesium and fluorine, lithium cobalt oxide with magnesium, fluorine and titanium, lithium nickel-cobalt oxide with magnesium and fluorine, lithium cobalt-aluminate with magnesium and fluorine, lithium nickel-cobalt-aluminate with magnesium and fluorine, lithium nickel-manganese-cobalt oxide with magnesium and fluorine, etc. In this specification, the additive element X may be referred to as a mixture, part of a raw material, etc.

[0349] As shown in Figure 17B, the positive electrode active material 100 has a surface layer 100a and an interior layer 100b. It is preferable that the surface layer 100a has a higher concentration of the added element than the interior layer 100b. Also, as shown by the gradient in Figure 17B, it is preferable that the added element has a concentration gradient that increases from the interior to the surface. In this specification, the surface layer 100a refers to the region of the positive electrode active material 100 from the surface down to about 10 nm. Surfaces created by cracks and / or fissures may also be considered the surface. The region of the positive electrode active material 100 that is deeper than the surface layer 100a is referred to as the interior layer 100b.

[0350] In one embodiment of the present invention, the positive electrode active material 100 is reinforced by a surface layer 100a, i.e., the outer periphery of the particles, which has a high concentration of additive elements, so that even if lithium is removed from the positive electrode active material 100 due to charging, the layered structure consisting of octahedrons of cobalt and oxygen does not break down.

[0351] Furthermore, it is preferable that the concentration gradient of the additive elements is present throughout the entire surface layer 100a of the positive electrode active material 100, and moreover, that it be present homogeneously. This is because 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, which is undesirable. If stress concentrates in a part of the particles, defects such as cracks may occur from there, which may lead to cracking of the positive electrode active material and a decrease in charge / discharge capacity.

[0352] Magnesium is divalent and is more stable in lithium sites than transition metal sites in layered rock salt crystal structures, thus readily occupying lithium sites. The presence of magnesium at an appropriate concentration in the lithium sites of the surface layer 100a facilitates the maintenance of the layered rock salt crystal structure. Furthermore, magnesium has a strong binding affinity for oxygen, which can suppress the release of oxygen from its surroundings. At appropriate concentrations, magnesium is preferable as it does not adversely affect lithium insertion and removal during charging and discharging. However, excessive magnesium may adversely affect lithium insertion and removal.

[0353] Aluminum is trivalent and can exist at transition metal sites in layered rock salt crystal structures. Aluminum can suppress the leaching of surrounding cobalt. Furthermore, because aluminum has a strong bonding force with oxygen, it can suppress the departure of oxygen from around the aluminum. Therefore, by including aluminum as an additive element, a positive electrode active material 100 can be made that is less prone to crystal structure collapse even after repeated charging and discharging.

[0354] Fluorine is a monovalent anion, and if some of the oxygen in the surface layer 100a is replaced by fluorine, the lithium release energy decreases. This is because the change in the valence of cobalt ions accompanying lithium release is different: from trivalent to tetravalent when fluorine is absent, and from divalent to trivalent when fluorine is present, resulting in different oxidation-reduction potentials. Therefore, if some of the oxygen in the surface layer 100a of the positive electrode active material 100 is replaced by fluorine, the release and insertion of lithium ions near the fluorine can occur more smoothly. This is preferable because it improves charge-discharge characteristics, rate characteristics, etc., when used in a secondary battery.

[0355] Titanium oxide is known to be superhydrophilic. Therefore, by using a positive electrode active material 100 having titanium oxide in its surface layer 100a, it is possible to improve wettability with highly polar solvents. When used in a secondary battery, good contact at the interface between the positive electrode active material 100 and the highly polar electrolyte may be achieved, potentially suppressing an increase in resistance. In this specification, the term "electrolyte" refers to a liquid electrolyte.

[0356] 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 capacity associated with repeated charging and discharging.

[0357] Furthermore, a short circuit in a secondary battery can not only cause malfunctions in the charging and / or discharging operations of the secondary battery, but also lead to overheating and ignition. To realize a safe secondary battery, it is preferable that the short-circuit current is suppressed even at high charging voltages. The positive electrode active material 100 in one aspect of the present invention suppresses the short-circuit current even at high charging voltages. Therefore, it is possible to create a secondary battery that achieves both high capacity and safety.

[0358] A secondary battery using the positive electrode active material 100 according to one embodiment of the present invention preferably satisfies high capacity, excellent charge-discharge cycle characteristics, and safety simultaneously.

[0359] The concentration gradient of additive elements can be evaluated, for example, using energy dispersive X-ray spectroscopy (EDX). Among EDX measurements, the method of scanning within a region to evaluate it in two dimensions is sometimes called EDX surface analysis. Furthermore, the method of extracting data from a linear region from EDX surface analysis and evaluating the distribution of atomic concentrations within the positive electrode active material is sometimes called line analysis.

[0360] EDX surface analysis (e.g., elemental mapping) allows for the quantitative analysis of the concentrations of additive elements in the surface layer 100a, interior 100b, and near grain boundaries of the positive electrode active material 100. Furthermore, EDX radiation analysis allows for the analysis of peak concentrations of additive elements.

[0361] When EDX radiation analysis is performed on the positive electrode active material 100, the magnesium 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 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm.

[0362] Furthermore, it is preferable that the distribution of fluorine in the positive electrode active material 100 overlaps with the distribution of magnesium. Therefore, when EDX radiation analysis is performed, the peak of fluorine concentration 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 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm.

[0363] It should be noted that not all additive elements have the same concentration distribution. For example, if the positive electrode active material 100 contains aluminum as an additive element, it is preferable that its distribution is slightly different from that of magnesium and fluorine. For example, when EDX radiation analysis is performed, it is preferable that the peak of magnesium concentration is closer to the surface than the peak of aluminum concentration in the surface layer 100a. For example, it is preferable that the peak of aluminum concentration is located at a depth of 0.5 nm to 20 nm from the surface toward the center of the positive electrode active material 100, and more preferably at a depth of 1 nm to 5 nm.

[0364] Furthermore, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the additive element I to the transition metal (I / M) 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. Even more preferably 0.030 or more and 0.20 or less. For example, when the additive element is magnesium and the transition metal is cobalt, the ratio of the number of atoms of magnesium to cobalt (Mg / Co) is preferably 0.020 or more and 0.50 or less. Even more preferably 0.025 or more and 0.30 or less. Even more preferably 0.030 or more and 0.20 or less.

[0365] As mentioned above, if the additive elements in the positive electrode active material 100 are in excess, they may adversely affect the insertion and removal of lithium. Furthermore, when used in a secondary battery, this may lead to increased resistance and decreased capacity. On the other hand, if there are insufficient elements, they may not be distributed throughout the entire surface layer 100a, resulting in insufficient maintenance of the crystal structure. Thus, the additive elements in the positive electrode active material 100 need to be at an appropriate concentration, but adjusting this concentration is not easy.

[0366] Therefore, for example, the positive electrode active material 100 may have regions where excess additive elements are unevenly distributed. The presence of such regions removes excess additive elements from other regions, allowing for an appropriate additive element concentration in most of the interior and near the surface of the positive electrode active material 100. By achieving an appropriate additive element concentration in most of the interior and near the surface of the positive electrode active material 100, it is possible to suppress increases in resistance and decreases in capacity when used as a secondary battery. The ability to suppress increases in the resistance of a secondary battery is an extremely desirable characteristic, especially during high-rate charging and discharging.

[0367] Furthermore, in positive electrode active material 100 having regions where excess additive elements are unevenly distributed, it is permissible to mix the additive elements in excess to some extent during the manufacturing process. This is preferable as it widens the margin in production.

[0368] In this specification, "non-uniformity" refers to a difference in the concentration of an element between a region A and a region B. Other terms that may be used include segregation, precipitation, heterogeneity, bias, high concentration, or low concentration.

[0369] <Crystal structure> Materials with a layered rock salt crystal structure, such as lithium cobalt oxide (LiCoO2), are known to have high discharge capacity and are excellent as positive electrode active materials for secondary batteries. Examples of materials with a layered rock salt crystal structure include composite oxides represented by LiM1O2.

[0370] It is known that the Jahn-Teller effect in transition metal compounds differs in strength depending on the number of electrons in the d orbitals of the transition metal.

[0371] In nickel-containing composite oxides, distortion can easily occur due to the Jahn-Teller effect. Therefore, when LiNiO2 is charged and discharged at high voltages, there is a concern that the crystal structure may collapse due to distortion. In LiCoO2, the effect of the Jahn-Teller effect is suggested to be smaller, and it may have better resistance to high-voltage charging and discharging, making it preferable.

[0372] The positive electrode active material will be described with reference to FIGS. 18 to 21. FIGS. 18 to 21 describe the case where cobalt is used as the transition metal included in the positive electrode active material.

[0373] ≪Li x When x in CoO2 is 1≫ The positive electrode active material 100 according to one embodiment of the present invention is in a discharged state, that is, Li x When x = 1 in CoO2, it preferably has a layered rock salt-type crystal structure belonging to the space group R-3m. The layered rock salt-type composite oxide has a high discharge capacity, has a two-dimensional lithium ion diffusion path, is suitable for the insertion / desorption reaction of lithium ions, and is excellent as a positive electrode active material for a secondary battery. Therefore, in particular, it is preferable that the interior 100b, which occupies most of the volume of the positive electrode active material 100, has a layered rock salt-type crystal structure. [[ID=1,2]]

[0374] In FIG. 18, in addition to the layered rock salt-type crystal structure belonging to the space group R-3m, O3 is added. The name O3 is based on the fact that in this crystal structure, lithium occupies an octahedral site and there are three CoO2 layers in the unit cell. In some cases, this crystal structure is also referred to as an O3-type crystal structure. The CoO2 layer refers to a structure in which an octahedral structure in which oxygen is six-coordinated to cobalt is continuous in the plane direction in a state of sharing edges. In some cases, the CoO2 layer is also referred to as a layer composed of octahedra of cobalt and oxygen.

[0375] ≪Li x When x in CoO2 is in a small state≫ The positive electrode active material 100 according to one embodiment of the present invention has a crystal structure different from that of a conventional positive electrode active material when x in Li x CoO2 is in a small state. Here, when x is small, it means 0.1 <x ≦ 0.24. FIG. 18 shows the crystal structure when x = 0.2.

[0376] Li x The change in the crystal structure accompanying the change in x in CoO2 is compared between a conventional positive electrode active material and the positive electrode active material 100 according to one embodiment of the present invention.

[0377] <Conventional positive electrode active material> Figure 20 shows the change in the crystal structure of a conventional positive electrode active material. The conventional positive electrode active material shown in Figure 20 is lithium cobalt oxide (LiCoO2, LCO) without the addition of halogens and magnesium or other additive elements. As described in Non-Patent Documents 1 to 3, etc., the crystal structure of lithium cobalt oxide shown in Figure 20 changes.

[0378] Figure 20 shows Li x The crystal structure of lithium cobalt oxide at x=1 in CoO2 is shown with R-3m O3. x=1 corresponds to the discharge state of a secondary battery. Next, the crystal structure of lithium cobalt oxide at x=0.5 is shown with P2 / m monoclinic O1. Conventional lithium cobalt oxide has a crystal structure that belongs to the monoclinic space group P2 / m when x=0.5, as the symmetry of lithium increases. In this structure, one CoO2 layer exists in the unit cell. For this reason, this crystal structure is sometimes referred to as O1 type or monoclinic O1 type.

[0379] Also Li x The crystal structure of lithium cobalt oxide in CoO2 at x=0 is shown, with the P-3m1 trigonal O1 structure added. Conventional lithium cobalt oxide has a crystal structure that belongs to the P-3m1 space group of the trigonal system at x=0. In this structure, there is one CoO2 layer in the unit cell. For this reason, this crystal structure is sometimes called the O1 type or trigonal O1 type. In addition, the trigonal structure is sometimes converted to a composite hexagonal lattice, and this crystal structure is sometimes called the hexagonal O1 type.

[0380] Also Li xThe crystal structure of lithium cobalt oxide in CoO2 at x=0.12 is shown with R-3m H1-3. Conventional lithium cobalt oxide has a crystal structure belonging to the space group R-3m at x=0.12. This structure can be described as a structure in which the structure of CoO2, such as the trigonal O1 type, and the structure of LiCoO2, such as the R-3m O3, are alternately stacked. For this reason, this crystal structure is sometimes referred to as the H1-3 type crystal structure. In reality, irregularities can occur in the insertion and removal of lithium, so experimentally, the H1-3 type crystal structure is observed from x=0.25. In addition, the H1-3 type crystal structure actually has twice the number of cobalt atoms per unit cell compared to other structures. However, in this specification, including Figure 20, the c-axis of the H1-3 type crystal structure is shown as half the unit cell to facilitate comparison with other crystal structures.

[0381] As an example, in the H1-3 type crystal structure, as described in Non-Patent Literature 2, 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 be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis using XRD. In this case, the unit cell that yields the smallest GOF (goodness of fit) value should be adopted.

[0382] 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.

[0383] However, these two crystal structures exhibit a significant displacement of the CoO2 layer. As shown by the dotted line and arrows in Figure 20, in the H1-3 type crystal structure, the CoO2 layer is significantly shifted from R-3m O3. Such dynamic structural changes can negatively affect the stability of the crystal structure.

[0384] 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.

[0385] In addition, the H1-3 crystal structure, unlike the trigonal O1 type, lacks lithium between the CoO2 layers, and the continuous CoO2 layers are likely to be unstable.

[0386] 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.

[0387] <Positive electrode active material according to one aspect of the present invention> In the positive electrode active material 100 of one embodiment of the present invention shown in Figure 18, Li x The change in crystal structure between the discharge state where x is 1 in CoO2 and the state where x is 0.24 or less, for example x=0.2, 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 0.2 (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 100 according to 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.

[0388] Furthermore, the positive electrode active material 100 in one aspect of the present invention is Li xWhen 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 100 in one aspect of the present invention is Li x Maintaining a state where x in CoO2 is 0.24 or less is preferable because it reduces the likelihood of short circuits and further improves the safety of the secondary battery.

[0389] Li x Figure 18 shows the crystal structure of lithium cobalt oxide when x in CoO2 is approximately 1 and 0.2. It is a composite oxide containing lithium cobalt oxide, cobalt as a transition metal, and oxygen. In addition to the above, it is preferable to have magnesium as an additive element. Furthermore, it is preferable to have halogens such as fluorine and chlorine as additive elements.

[0390] In one embodiment of the present invention, lithium cobalt oxide has the same R-3m O3 crystal structure as conventional lithium cobalt oxide when x=1. However, in one embodiment of the present invention, lithium cobalt oxide has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2, which is the value at which conventional lithium cobalt oxide has an H1-3 type crystal structure.

[0391] When x = approximately 0.2, lithium cobalt oxide according to one embodiment of the present invention has a crystal structure that belongs 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 called the O3' type crystal structure. In Figure 18, the R-3m O3' is shown alongside the crystal structure when x = approximately 0.2.

[0392] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co(0,0,0.5), O(0,0,x), and within the range of 0.20≦x≦0.25.

[0393] As shown by the dotted line in Figure 18, there is almost no displacement of the CoO2 layer between the R-3m O3 in the discharged state and the O3' type crystal structure.

[0394] 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%, indicating a small volume difference.

[0395] Thus, in the positive electrode active material 100, Li x When x in CoO2 is small, that is, when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume per unit of the same number of cobalt atoms is also suppressed. Therefore, the crystal structure of positive electrode active material 100 is less likely to collapse even when repeatedly charging and discharging in a manner 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 100. Also, because it can stably utilize more lithium than conventional positive electrode active materials, positive electrode active material 100 has a high discharge capacity per unit weight and per unit volume. Therefore, by using positive electrode active material 100, it is possible to manufacture secondary batteries with high discharge capacity per unit weight and per unit volume.

[0396] The positive electrode active material 100 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 it is affected not only by x in CoO2 but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it may have an O3' type crystal structure regardless of the above range of x.

[0397] Furthermore, the positive electrode active material 100 is Li x When x in CoO2 is greater than 0.1 and less than or equal to 0.24, the entire interior of the positive electrode active material 100 does not have to be of the O3' type crystal structure. It may contain other crystal structures, or a part of it may be amorphous.

[0398] Also Li x To make x in CoO2 small, it is generally necessary to charge with a high charging voltage. xA 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 charging 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 high charging voltage relative to the potential of lithium metal can be said to be a charging voltage of 4.6V or higher. Furthermore, unless otherwise specified in this specification, the charging voltage is expressed relative to the potential of lithium metal.

[0399] When the positive electrode active material 100 is charged with a high charging voltage, it is preferable because it can maintain a crystal structure with the symmetry of R-3m O3. Examples of high charging voltages include a voltage of 4.6V or higher at 25°C. Examples of even higher charging voltages include a voltage of 4.65V to 4.7V at 25°C.

[0400] Even with the positive electrode active material 100, if the charging voltage is further increased, H1-3 type crystals may gradually be observed. 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 more and less than 4.6V at 25°C, the positive electrode active material 100 in one embodiment of the present invention may take on an O3' type crystal structure.

[0401] 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.

[0402] Furthermore, while Figure 18 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 20, 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.

[0403] 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 CoO2 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.

[0404] Additive elements, such as magnesium, that are randomly and dilutely present between the CoO2 layers, i.e., at the lithium sites, have the effect of suppressing the displacement of the CoO2 layers when charged at high voltage. Therefore, when magnesium is present between the CoO2 layers, an O3' type crystal structure is more likely to be formed. For this reason, it is preferable that magnesium is distributed in at least the surface layer of the positive electrode active material 100 according to one embodiment of the present invention, and moreover, distributed throughout the positive electrode active material 100. Furthermore, in order to distribute magnesium throughout the positive electrode active material 100, it is preferable to perform a heat treatment in the manufacturing process of the positive electrode active material 100 according to one embodiment of the present invention.

[0405] However, if the heat treatment temperature is too high, cation mixing will occur, increasing the likelihood that additive elements, such as magnesium, will enter the cobalt site. Magnesium present in the cobalt site does not help maintain the R-3m structure during high-voltage charging. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to its divalent state and the evaporation of lithium.

[0406] Therefore, it is preferable to add a halogen compound such as a fluorine compound to lithium cobalt oxide before the heat treatment to distribute magnesium to the positive electrode active material 100. Adding a halogen compound causes a decrease in the melting point of lithium cobalt oxide. By lowering the melting point, it becomes easier to distribute magnesium to the positive electrode active material 100 at a temperature at which cation mixing is less likely to occur. Furthermore, if a fluorine compound is present, it can be expected that the corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte will be improved.

[0407] Furthermore, if the magnesium concentration is increased beyond the desired value, the effect on stabilizing the crystal structure may decrease. This is thought to be because magnesium will enter not only the lithium sites but also the cobalt sites. The number of magnesium atoms in the positive electrode active material according to one embodiment of the present invention is preferably 0.001 times or more and 0.1 times or less the number of transition metal atoms such as cobalt, more preferably greater than 0.01 times and less than 0.04 times, and even more preferably about 0.02 times. The magnesium concentration shown here may be, for example, a value obtained from elemental analysis of the entire particle of the positive electrode active material 100 using ICP-MS, or it may be based on the value of the raw material blend during the process of manufacturing the positive electrode active material.

[0408] Lithium cobalt oxide may be mixed with one or more metals other than cobalt (hereinafter referred to as metal Z), such as nickel, aluminum, manganese, titanium, vanadium, and chromium, and it is particularly preferable to add nickel and one or more aluminum. Manganese, titanium, vanadium, and chromium may be more likely to stably form the tetravalent state and may contribute significantly to structural stability. Adding metal Z may make the crystal structure more stable in a high-voltage charged state. Here, in one embodiment of the present invention, it is preferable that metal Z is added at a concentration that does not significantly change the crystallinity of lithium cobalt oxide. For example, it is preferable that the amount is such that the aforementioned Jahn-Teller effect does not occur.

[0409] Nickel, manganese, and other transition metals, as well as aluminum, are preferably present at the cobalt site, but some may be present at the lithium site. Magnesium is also preferably present at the lithium site. Oxygen may be partially substituted with fluorine.

[0410] In one embodiment of the present invention, the capacity of the positive electrode active material may decrease as the magnesium concentration increases. One possible reason for this is that the amount of lithium contributing to charging and discharging may decrease due to the presence of magnesium at the lithium sites. In addition, excess magnesium may generate magnesium compounds that do not contribute to charging and discharging. In one embodiment of the present invention, the capacity per unit weight and per unit volume may be increased by including nickel as metal Z in addition to magnesium. In another embodiment of the present invention, the capacity per unit weight and per unit volume may be increased by including aluminum as metal Z in addition to magnesium. In yet another embodiment of the present invention, the capacity per unit weight and per unit volume may be increased by including nickel and aluminum in addition to magnesium.

[0411] The concentrations of elements such as magnesium and metal Z in the positive electrode active material according to one embodiment of the present invention are examined below.

[0412] In one embodiment of the present invention, when the positive electrode active material contains magnesium in addition to element X, it exhibits extremely high stability in a high-voltage charged state. When element X is 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%. 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%. The concentrations of phosphorus and magnesium shown herein may be values ​​obtained, for example, from elemental analysis of the entire particle of the positive electrode active material using ICP-MS, or they may be based on the values ​​of the raw material composition during the manufacturing process of the positive electrode active material.

[0413] The number of nickel atoms in the positive electrode active material according to one embodiment of the present invention is preferably 10% or less of the number of cobalt atoms, more preferably 7.5% or less, even more preferably 0.05% to 4%, and particularly preferably 0.1% to 2%. The nickel concentration shown herein may be, for example, a value obtained from elemental analysis of the entire particle of the positive electrode active material using ICP-MS, or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material.

[0414] If a high-voltage charge is maintained for an extended period, transition metals may dissolve from the positive electrode active material into the electrolyte, potentially disrupting its crystal structure. However, by including nickel in the above proportion, it may be possible to suppress the dissolution of transition metals from the positive electrode active material 100.

[0415] The number of aluminum atoms in the positive electrode active material according to one embodiment of the present invention is preferably 0.05% to 4% of the number of cobalt atoms, and more preferably 0.1% to 2%. The aluminum concentration shown here may be, for example, a value obtained from elemental analysis of the entire particle of the positive electrode active material using ICP-MS, or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material.

[0416] The positive electrode active material according to one aspect of the present invention preferably contains element X, and it is preferable that phosphorus is used as element X. Furthermore, the positive electrode active material according to one aspect of the present invention more preferably contains a composite oxide containing phosphorus and oxygen.

[0417] In one embodiment of the present invention, the positive electrode active material has a composite oxide containing element X, which may make it less likely for a short circuit to occur when a high-voltage charge state is maintained.

[0418] In one embodiment of the present invention, if the positive electrode active material contains phosphorus as element X, the hydrogen fluoride generated by the decomposition of the electrolyte may react with the phosphorus, potentially reducing the concentration of hydrogen fluoride in the electrolyte.

[0419] When the electrolyte contains LiPF6 as a lithium salt, hydrolysis may generate hydrogen fluoride. Hydrogen fluoride may also be generated by the reaction between PVDF, used as a component of the positive electrode, and alkali. Reducing the hydrogen fluoride concentration in the electrolyte may suppress corrosion of the current collector and / or peeling of the coating. Furthermore, it may suppress the decrease in adhesion due to gelation and / or insolubilization of the PVDF.

[0420] If the positive electrode active material has cracks, the presence of phosphorus, or more specifically, a complex oxide containing phosphorus and oxygen, within the cracks may suppress the progression of the cracks.

[0421] As is evident from the oxygen atoms indicated by arrows in Figure 18, the symmetry of the oxygen atoms differs slightly between the O3-type and O3'-type crystal structures. Specifically, in the O3-type crystal structure, the oxygen atoms are aligned along the dotted line, whereas in the O3'-type crystal structure, the oxygen atoms are not strictly aligned. This is because, in the O3'-type crystal structure, the amount of tetravalent cobalt increases with decreasing lithium, leading to a larger Jahn-Teller strain and distortion of the octahedral structure of CoO6. The increased repulsion between oxygen atoms in the CoO2 layer with decreasing lithium also plays a role.

[0422] ≪Surface layer 100a≫ It is preferable that magnesium is distributed throughout the particles of the positive electrode active material 100 according to one embodiment of the present invention, but in addition, it is preferable that the magnesium concentration in the surface layer 100a is higher than the average of the entire particle. For example, it is preferable that the magnesium concentration in the surface layer 100a measured by XPS or the like is higher than the average magnesium concentration of the entire particle measured by ICP-MS or the like.

[0423] Furthermore, in one embodiment of the present invention, when the positive electrode active material 100 contains one or more metals other than cobalt, such as nickel, aluminum, manganese, iron, and chromium, it is preferable that the concentration of the metal near the particle surface is higher than the average concentration of the entire particle. For example, it is preferable that the concentration of elements other than cobalt in the surface layer 100a, as measured by XPS or the like, is higher than the average concentration of the elements in the entire particle, as measured by ICP-MS or the like.

[0424] The surface layer of the positive electrode active material is, in essence, entirely composed of crystal defects, and during charging, lithium escapes from the surface, making it a region where the lithium concentration tends to be lower than in the interior. Therefore, it is prone to instability and structural collapse. A higher magnesium concentration in the surface layer 100a can more effectively suppress changes in the crystal structure. Furthermore, a higher magnesium concentration in the surface layer 100a can also be expected to improve corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte.

[0425] Furthermore, it is preferable that the concentration of halogens such as fluorine in the surface layer 100a of the positive electrode active material 100 in one embodiment of the present invention is higher than the average concentration of the positive electrode active material 100 as a whole. The presence of halogens in the surface layer 100a, which is the region in contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid.

[0426] Thus, it is preferable that the surface layer 100a of the positive electrode active material 100 in one aspect of the present invention has a different composition from the interior 100b, with higher concentrations of additive elements, such as magnesium and fluorine. It is also preferable that the composition adopts a crystalline structure that is stable at room temperature. For this reason, the surface layer 100a may have a different crystalline structure from the interior 100b. For example, at least a part of the surface layer 100a of the positive electrode active material 100 in one aspect of the present invention may have a rock salt type crystalline structure. Furthermore, if the surface layer 100a and the interior 100b have different crystalline structures, it is preferable that the crystal orientations of the surface layer 100a and the interior 100b are roughly the same.

[0427] The anions in layered rock salt crystals and rock salt crystals adopt a cubic close-packed structure (face-centered cubic lattice structure). It is also presumed that the anions in O3'-type crystals adopt a cubic close-packed structure. When these are in contact, there exists a crystal plane in which the orientation of the cubic close-packed structure composed of anions is aligned. However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group of rock salt crystals Fm-3m (the space group of a typical rock salt crystal) and Fd-3m (the space group of a rock salt crystal with the simplest symmetry). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different for layered rock salt crystals and O3'-type crystals and for rock salt crystals. In this specification, when the orientation of the cubic close-packed structure composed of anions is aligned in layered rock salt crystals, O3'-type crystals, and rock salt crystals, it is sometimes said that the crystal orientation is approximately the same.

[0428] The general agreement of crystal orientation between two regions can be determined from 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, etc. X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc., can also be used as a basis for determination. When the crystal orientations are general agreement, it can be observed in TEM images, etc., that the difference in direction of the rows in which cations and anions are arranged alternately in a linear fashion is 5 degrees or less, more preferably 2.5 degrees or less. Note that light elements such as oxygen and fluorine may not be clearly visible in TEM images, etc., but in such cases, the agreement of orientation can be determined from the arrangement of metallic elements.

[0429] However, if the surface layer 100a consists only of MgO, or only of a solid solution of MgO and CoO(II), lithium insertion and removal becomes difficult. 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. Furthermore, it is preferable that the concentration of cobalt is higher than that of magnesium.

[0430] Furthermore, it is preferable that element X is located in the surface layer 100a of the positive electrode active material 100 according to one aspect of the present invention. For example, the positive electrode active material 100 according to one aspect of the present invention may be covered with a coating (barrier layer) having element X.

[0431] ≪Grain boundary≫ The additive element X in the positive electrode active material 100 according to one aspect of the present invention may be present randomly and dilutely within the material, but it is more preferable that a portion of it is segregated at the grain boundaries.

[0432] In other words, in one embodiment of the present invention, it is preferable that the concentration of the additive element X at the grain boundaries and in the vicinity of the positive electrode active material 100 is higher than that of other regions inside.

[0433] Similar to particle surfaces, grain boundaries are also surface defects. Therefore, they are prone to instability and easily initiate changes in crystal structure. For this reason, increasing the concentration of additive element X at and near the grain boundaries can more effectively suppress changes in crystal structure.

[0434] Furthermore, if the concentration of additive element X is high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material 100 according to one embodiment of the present invention, the concentration of additive element X will be high near the surface created by the cracks. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after cracks have occurred.

[0435] In this specification, the term "near the grain boundary" refers to the region extending approximately 10 nm from the grain boundary.

[0436] ≪Particle size≫ If the particle size of the positive electrode active material 100 in one aspect of the present invention is too large, problems arise such as difficulty in lithium diffusion or the surface of the active material layer becoming too rough when coated onto a current collector. On the other hand, if the particle size is too small, problems arise such as difficulty in supporting the active material layer when coating onto a current collector or excessive reaction with the electrolyte. For this reason, the average particle size (D50: also called the median diameter) 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.

[0437] <Analysis method> Whether a certain positive electrode active material is a positive electrode active material 100 according to one embodiment of the present invention that exhibits an O3' type crystal structure when charged with a high voltage can be determined by analyzing the positive electrode charged with a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. XRD is particularly preferable 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 distortion of the lattice and crystallite size, and obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery.

[0438] As described above, the positive electrode active material 100 according to one aspect of the present invention has the characteristic of showing little change in crystal structure between a high-voltage charged state and a discharged state. Materials in which the crystal structure that changes significantly from the discharged state accounts for 50 wt% or more when charged at high voltage are undesirable because they cannot withstand high-voltage charging and discharging. It is important to note that simply adding additive elements may not result in the desired crystal structure. For example, even if they are both magnesium and lithium cobaltate containing fluorine, there are cases where the O3' type crystal structure accounts for 60 wt% or more when charged at high voltage, and cases where the H1-3 type crystal structure accounts for 50 wt% or more. Furthermore, at a certain voltage, the O3' type crystal structure may account for almost 100 wt%, and if the voltage is further increased, the H1-3 type crystal structure may be generated. Therefore, in order to determine whether or not a material is the positive electrode active material 100 according to one aspect of the present invention, analysis of the crystal structure, including XRD, is necessary.

[0439] However, positive electrode active materials in a high-voltage charged or discharged state 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 in an inert atmosphere such as an argon atmosphere.

[0440] ≪Charging method≫ High-voltage charging to determine whether a certain composite oxide is the positive electrode active material 100 according to one aspect of the present invention can be performed, for example, by fabricating a coin cell (CR2032 type, 20 mm in diameter and 3.2 mm in height) with lithium as the counter electrode.

[0441] More specifically, the positive electrode can be made by coating an aluminum foil positive electrode current collector with a slurry containing a mixture of positive electrode active material and a conductive agent.

[0442] 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.

[0443] 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%.

[0444] Polypropylene with a thickness of 25 μm can be used for the separator.

[0445] The positive electrode and negative electrode cans can be made of stainless steel (SUS).

[0446] The coin cell prepared under the above conditions is charged with a constant current of 4.6V and 0.5C, and then charged with a constant voltage until the current value becomes 0.01C. Here, 1C is defined as 200mA / g. The temperature is set to 25℃. 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 charged at high voltage. When performing various analyses thereafter, it is preferable to seal it under an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing it in a sealed container under an argon atmosphere.

[0447] ≪XRD≫ Figures 19 and 21 show the ideal powder XRD patterns calculated from the O3' type crystal structure and the H1-3 type crystal structure using the CuKα1 line. For comparison, Li x The ideal XRD patterns calculated from the crystal structures of LiCoO2O3 at x=1 in CoO2, H1-3 type, and trigonal O1 at x=0 are also shown. The patterns for LiCoO2O3 and CoO2O1 were created using Reflex Powder Diffraction, a module in Materials Studio (BIOVIA), from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). 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.

[0448] As shown in Figure 19, in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.30 ± 0.20° (19.10° to 19.50°) and 2θ = 45.55 ± 0.10° (45.45° to 45.65°). More specifically, sharp diffraction peaks appear at 2θ = 19.30 ± 0.10° (19.20° to 19.40°) and 2θ = 45.55 ± 0.05° (45.50° to 45.60°). However, as shown in Figure 21, these peaks do not appear in the H1-3 type crystal structure and CoO2O1. Therefore, the appearance of peaks at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10° when charged at high voltage can be said to be a characteristic of the positive electrode active material 100 in one embodiment of the present invention.

[0449] This also means that the positions where the XRD diffraction peaks appear are close at x=1 and x≦0.24. More specifically, for two or more, more preferably three or more, of the main diffraction peaks in both cases, the difference in the positions where the peaks appear is 2θ=0.7 or less, and more preferably 2θ=0.5 or less.

[0450] In one embodiment of the present invention, the positive electrode active material 100 has an O3' type crystal structure when x in LixCoO2 is small, but not all of the positive electrode active material 100 has to have an O3' type crystal structure. It may contain other crystal structures, or 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 is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more. If the O3' type crystal structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, a positive electrode active material with sufficiently excellent cycle characteristics can be obtained.

[0451] Furthermore, even after more than 100 charge-discharge cycles from the start of cycle measurement, it is preferable that the O3' type crystal structure is 35 wt% or more, more preferably 40 wt% or more, and even more preferably 43 wt% or more when Rietveld analysis is performed.

[0452] Furthermore, the crystallite size of the O3'-type crystal structure in the positive electrode active material decreases to only about 1 / 10th of that of LiCoO2O3 in the discharged state. Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, a clear peak of the O3'-type crystal structure can be observed when x in LixCoO2 is small. On the other hand, in simple 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.

[0453] In one embodiment of the present invention, the positive electrode active material preferably has a small effect of the Jahn-Teller effect, as described above. The positive electrode active material in one embodiment of the present invention preferably has a layered rock salt type crystal structure and mainly contains cobalt as a transition metal. In addition, in one embodiment of the present invention, the positive electrode active material may contain metal Z as described above, in addition to cobalt, as long as the effect of the Jahn-Teller effect is small.

[0454] In the cathode active material, we will use XRD analysis to investigate the range of lattice constants in which the Jahn-Teller effect is presumed to be small.

[0455] Figure 22 shows the results of estimating the lattice constants of the a-axis and c-axis using XRD when the positive electrode active material of one embodiment of the present invention has a layered rock salt type crystal structure and contains cobalt and nickel. The positive electrode active material is prepared using steps S11 to S34 described later, with at least a nickel source used in step S21. Figure 22A shows the results for the a-axis, and Figure 22B shows the results for the c-axis. Note that Figures 22A and 22B are results for the powder of the positive electrode active material obtained according to steps S11 to S34. That is, the results are for the material before it is incorporated into the positive electrode. The nickel concentration (%) on the horizontal axis represents the concentration ratio (percentage) of nickel when the sum of the number of atoms of cobalt and nickel is taken as 100%. The concentration ratio (percentage) of nickel can be determined using a cobalt source and a nickel source.

[0456] Figure 23 shows the results of estimating the lattice constants of the a-axis and c-axis using an XRD pattern when the positive electrode active material according to one embodiment of the present invention has a layered rock salt type crystal structure and contains cobalt and manganese. The positive electrode active material is prepared using steps S11 to S34 described later, with at least a manganese source used in step S21. Figure 23A shows the results for the a-axis, and Figure 23B shows the results for the c-axis. Note that Figures 23A and 23B are results for the powder of the positive electrode active material obtained according to steps S11 to S34. That is, the results are for the material before it is incorporated into the positive electrode. The manganese concentration (%) on the horizontal axis represents the manganese concentration ratio (percentage) when the sum of the number of atoms of cobalt and manganese is taken as 100%. The manganese concentration ratio (percentage) can be determined using a cobalt source and a manganese source.

[0457] Figure 22C shows the a-axis lattice constant divided by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active material whose lattice constant results are shown in Figures 22A and 22B. Figure 23C shows the a-axis lattice constant divided by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active material whose lattice constant results are shown in Figures 23A and 23B.

[0458] Figure 22C shows a significant change in the a / c axis at nickel concentrations of 5% and 7.5%, suggesting that the a-axis distortion is increasing. This distortion may be Jahn-Teller distortion. This suggests that a superior cathode active material with low Jahn-Teller distortion can be obtained at nickel concentrations below 7.5%.

[0459] Next, Figure 23A suggests that when the manganese concentration is 5% or higher, the behavior of the lattice constant changes is different and does not follow Vegard's law. Therefore, it is suggested that the crystal structure is different when the manganese concentration is 5% or higher. Thus, a manganese concentration of, for example, 4% or less is preferable.

[0460] It should be noted that the above-mentioned ranges for nickel and manganese concentrations do not necessarily apply to the surface layer 100a of the particles. In other words, the concentrations in the surface layer 100a of the particles may be higher than those mentioned above.

[0461] Based on the above considerations regarding the preferred range of lattice constants, in a positive electrode active material according to one embodiment of the present invention, the layered rock salt type crystal structure of the particles of the positive electrode active material in a non-charged or discharged state, as estimated from the XRD pattern, has a lattice constant of 2.814 × 10⁻¹⁴ in the a-axis. -10 Larger than m, 2.817 × 10 -10 It is smaller than m, and the lattice constant of the c axis is 14.05 × 10⁻¹⁰. -10 Larger than m, 14.07 × 10 -10 It was found that a value smaller than m is preferable. The state without charging and discharging may, for example, be the powder state before the positive electrode of the secondary battery is manufactured.

[0462] Alternatively, in the layered rock salt-type crystal structure of the positive electrode active material particles in a non-charging or discharged state, it is preferable that the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is greater than 0.20000 and less than 0.20049.

[0463] Alternatively, when XRD analysis is performed on the layered rock salt-type crystal structure of the positive electrode active material particles in a non-charging or discharged state, a first peak may be observed when 2θ is between 18.50° and 19.30°, and a second peak may be observed when 2θ is between 38.00° and 38.80°.

[0464] The peaks appearing in the powder XRD pattern reflect the crystal structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100. The crystal structure of the surface layer 100a and other parts can be analyzed by electron diffraction of the cross-section of the positive electrode active material 100.

[0465] ≪XPS≫ X-ray photoelectron spectroscopy (XPS) allows for analysis of regions from the surface to a depth of 2 nm to 8 nm (usually around 5 nm), enabling quantitative analysis of the concentration of each element in approximately half of the surface layer 100a. Furthermore, narrow-scan analysis allows for the analysis of elemental bonding states. The quantitative accuracy of XPS is typically around ±1 atomic percent, and the detection limit is approximately 1 atomic percent, although this varies depending on the element.

[0466] When XPS analysis is performed on the positive electrode active material 100 according to one embodiment of the present invention, the number of atoms of the additive element is preferably 1.6 to 6.0 times the number of atoms of the transition metal, and more preferably 1.8 to less than 4.0 times. When the additive element is magnesium and the transition metal is cobalt, the number of magnesium atoms is preferably 1.6 to 6.0 times the number of cobalt atoms, and more preferably 1.8 to less than 4.0 times. Furthermore, the number of halogen atoms such as fluorine is preferably 0.2 to 6.0 times the number of atoms of the transition metal, and more preferably 1.2 to 4.0 times.

[0467] When performing XPS analysis, for example, monochromatic aluminum can be used as the X-ray source. The extraction angle can also be set to, for example, 45°.

[0468] Furthermore, when the positive electrode active material 100 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 100 according to one embodiment of the present invention contains fluorine, it is preferable that the bond is with something other than lithium fluoride and magnesium fluoride.

[0469] Furthermore, when the positive electrode active material 100 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 100 according to one embodiment of the present invention contains magnesium, it is preferable that the bond is with an element other than magnesium fluoride.

[0470] For additive elements that are preferably present in large quantities in the surface layer 100a, such as magnesium or aluminum, it is preferable that the concentration measured by XPS or the like is higher than the concentration measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry).

[0471] When magnesium or aluminum is processed to expose its cross-section and analyzed using TEM-EDX, it is preferable that the concentration in the surface layer 100a is higher than the concentration in the interior 100b. Processing can be performed, for example, by FIB (Focused Ion Beam).

[0472] In XPS (X-ray photoelectron spectroscopy) analysis, the number of magnesium atoms is preferably 0.4 to 1.5 times the number of cobalt atoms. On the other hand, the ratio of magnesium atoms (Mg / Co) determined by ICP-MS analysis is preferably 0.001 to 0.06.

[0473] On the other hand, it is preferable that the nickel contained in the transition metal is not concentrated in the surface layer 100a but is distributed throughout the entire positive electrode active material 100. However, this does not apply if there are regions where the aforementioned excess additive elements are concentrated.

[0474] ≪Charging curve and dQ / dV curve≫ It is believed that a non-equilibrium phase change occurs around the peak in the dQ / dV curve, which is obtained by differentiating the capacitance (Q) with respect to voltage (V) from the charging curve (dQ / dV), and that the crystal structure changes significantly. In this specification, a non-equilibrium phase change refers to a phenomenon that causes a nonlinear change in a physical quantity.

[0475] Figure 24 shows the charging curves of a secondary battery using a positive electrode active material according to one embodiment of the present invention and a secondary battery using a comparative example positive electrode active material.

[0476] The positive electrode active material 1 of the present invention shown in Figure 24 was prepared by the method shown in Figures 14A and 14B. More specifically, lithium cobalt oxide (C-10N manufactured by Nippon Chemical Industrial Co., Ltd.) was used as LiM1O2 in step S14, and LiF and MgF2 were mixed and heated. A half cell was prepared and charged using this positive electrode active material in the same manner as in the XRD measurement.

[0477] The positive electrode active material 2 of the present invention shown in Figure 24 was prepared by the method shown in Figures 14A and 14C. More specifically, lithium cobalt oxide (C-10N manufactured by Nippon Chemical Industrial Co., Ltd.) was used as LiM1O2 in step S14, and LiF, MgF2, Ni(OH)2, and Al(OH)3 were mixed and heated. A half cell was prepared and charged using this positive electrode active material in the same manner as in the XRD measurement.

[0478] The positive electrode active material in the comparative example shown in Figure 24 was prepared by forming an aluminum-containing layer on the surface of lithium cobalt oxide (C-5H, manufactured by Nippon Chemical Industrial Co., Ltd.) using the sol-gel method, and then heating it at 500°C for 2 hours. Using this positive electrode active material, a half-cell was prepared and charged in the same manner as in the XRD measurement.

[0479] Figure 24 shows the charging curves when these half-cells were charged to 4.9V at 25°C with 10mA / g. For positive electrode active material 1, the comparison example is n=2, while for positive electrode active material 2, n=1.

[0480] Figures 25A to 25C show the dQ / dV curves representing the change in voltage with respect to charging capacity, obtained from the data in Figure 24. Figure 25A shows the dQ / dV curve for a half-cell using positive electrode active material 1 according to one embodiment of the present invention, Figure 25B shows the dQ / dV curve for a half-cell using positive electrode active material 2 according to one embodiment of the present invention, and Figure 25C shows the dQ / dV curve for a half-cell using a comparative positive electrode active material.

[0481] As is clear from Figures 25A to 25C, in both one embodiment of the present invention and the comparative example, peaks were observed at voltages of approximately 4.06V and 4.18V, and the change in capacitance with respect to voltage was nonlinear. Between these two peaks, Li x This is thought to be the crystal structure (space group P2 / m) in CoO2 when x is 0.5. x In CoO2, the space group P2 / m with x = 0.5 shows that lithium atoms are aligned, as shown in Figure 20. It is thought that energy is used for this lithium alignment, resulting in a nonlinear change in capacitance with respect to voltage.

[0482] Furthermore, in the comparative example shown in Figure 25C, large peaks were observed at approximately 4.54V and 4.61V. The area between these two peaks is thought to represent an H1-3 phase crystal structure.

[0483] On the other hand, in the secondary battery of one embodiment of the present invention shown in Figures 25A and 25B, which exhibits extremely good cycle characteristics, a small peak was observed at around 4.55V, but it was not clear. Furthermore, in positive electrode active material 2, no further peaks were observed even above 4.7V, indicating that the O3' structure was maintained. Thus, in the dQ / dV curve of a secondary battery using the positive electrode active material of one embodiment of the present invention, some peaks may be extremely broad or small at 25°C. In such cases, it is possible that two crystal structures are coexisting. For example, it is possible that two phases of O3 and O3' are coexisting, or two phases of O3' and H1-3 are coexisting.

[0484] ≪Discharge curve and dQ / dV curve≫ Furthermore, in one embodiment of the present invention, when the positive electrode active material is charged at a high voltage and then discharged at a low rate of, for example, 0.2C or less, a characteristic voltage change may appear near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the dQ / dV curve obtained from the discharge curve, at a voltage lower than the peak that appears around 3.9V and in the range up to 3.5V.

[0485] ≪Surface roughness and specific surface area≫ In one embodiment of the present invention, the positive electrode active material 100 preferably has a smooth surface with few irregularities. A smooth surface with few irregularities is one factor indicating that the distribution of additive elements in the surface layer 100a is good.

[0486] 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 100, or from the specific surface area of ​​the positive electrode active material 100.

[0487] For example, the surface smoothness of the positive electrode active material 100 can be quantified from a cross-sectional SEM image, as shown below.

[0488] First, the positive electrode active material 100 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 100 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 100 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. Interface extraction is then performed using image processing software. Furthermore, the interface line between the protective film, etc. and the positive electrode active material 100 is selected using a tool such as Magic Hand, and the data is extracted into a spreadsheet program. Using the functions of the spreadsheet program, correction is performed from a regression curve (quadratic regression), and 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. This surface roughness is the surface roughness of the positive electrode active material at least at 400 nm from the outer circumference of the particle.

[0489] In this embodiment, the particle surface of the positive electrode active material 100 preferably has a roughness index, namely, root mean square surface roughness (RMS), which is less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

[0490] The image processing software used for noise reduction, interface extraction, etc., is not particularly limited, but for example, "ImageJ" can be used. Similarly, the spreadsheet software is not particularly limited, but for example, Microsoft Office Excel can be used.

[0491] For example, the actual specific surface area A measured by the gas adsorption method using the constant-volume method. R And the ideal specific surface area A i The surface smoothness of the positive electrode active material 100 can also be quantified from this ratio.

[0492] Ideal specific surface area A i This is calculated by assuming that all particles have the same diameter as D50, the same weight, and are ideal spheres.

[0493] 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.

[0494] In one embodiment of the present invention, the positive electrode active material 100 has an ideal specific surface area A determined from the median diameter D50. i And the actual specific surface area A R Ratio A R / A i It is preferable that this value is 2 or less.

[0495] [Defects in positive electrode active material] Examples of defects that may occur in the positive electrode active material are shown in Figures 26 to 36. According to one embodiment of the present invention, the positive electrode active material is expected to have an effect in suppressing the occurrence of such defects.

[0496] Charging and discharging under high voltage conditions of 4.5V or higher or high temperature (45°C or higher) may cause progressive defects such as pits in the positive electrode active material. Additionally, crack-like defects may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. Figure 26 shows a schematic cross-sectional view of the positive electrode active material 51. In the positive electrode active material 51, pits 54 and 58 are shown as holes, but the opening shape is not circular and has depth. The positive electrode active material 51 may also have cracks 57. The positive electrode active material 51 has crystal planes 55 and may also have recesses 52. The barrier layers 53 and 56 may cover the positive electrode active material 51, but they may be separated. The barrier layer 53 also covers the recesses 52.

[0497] The positive electrode active material of lithium-ion secondary batteries is typically LCO or NCM, which can be described as an alloy containing multiple metallic elements (such as cobalt and nickel). At least one of the positive electrode active materials has a defect, and this defect may change before and after charging and discharging. When used in secondary batteries, the positive electrode active material may be chemically or electrochemically eroded by the surrounding environmental substances (such as the electrolyte), or the material may deteriorate. This deterioration does not occur uniformly on the surface of the positive electrode active material, but rather concentrates locally, and repeated charging and discharging of the secondary battery can cause defects to develop deeply from the surface into the interior.

[0498] The phenomenon in which defects in the positive electrode active material progress and form holes can also be called pitting corrosion.

[0499] In this specification, cracks and pits are different. Immediately after the production of the positive electrode active material, there are cracks but no pits. A pit is a hole where several layers of cobalt or oxygen have escaped under high voltage conditions of 4.5 V or higher or at high temperature (45 °C or higher) during charge and discharge, and can also be said to be a location where cobalt has eluted. Therefore, there are no pits immediately after the production of the positive electrode active material. A crack refers to a new surface caused by the application of physical pressure or a crack resulting from grain boundaries. Cracks may also occur due to the expansion and contraction of particles during charge and discharge. Additionally, pits may occur from cracks or cavities within the particles.

[0500] <Disassembly of the secondary battery> The charge and discharge test was carried out up to 50 cycles. The discharge capacity at the 50th cycle had decreased to less than 40% of that at the 1st cycle. The secondary battery was disassembled, and the positive electrode was taken out. The disassembly was carried out under an argon atmosphere. After disassembly, it was washed with DMC and then the solvent was volatilized. Observations were made on the positive electrode that had undergone the charge and discharge test up to 50 cycles and the positive electrode before being incorporated into the secondary battery, that is, the positive electrode immediately after production.

[0501] <SEM observation> The positive electrode was observed by a scanning electron microscope (SEM). Figure 27A shows the SEM image of the positive electrode of the secondary battery after 50 cycles. Figure 27B shows the SEM image of the positive electrode before being incorporated into the secondary battery. For the SEM observation, a scanning electron microscope device SU8030 manufactured by Hitachi High-Tech Corporation was used. :

[0502] Next, the cross-section of the positive electrode active material was processed by FIB, and the cross-section of the positive electrode active material was observed by SEM. By repeating the cross-section processing by FIB and SEM observation, three-dimensional information of the structure as shown in Figure 28A or Figure 28D can be obtained. For the FIB processing and SEM observation, XVision210B manufactured by Hitachi High-Tech was used.

[0503] Fig. 28B is an enlarged view of a part of the three-dimensional information in Fig. 28A from the front, and the cross-section taken as a cutaway is shown in Fig. 28C. Further, the three-dimensional information of the side surface obtained by rotating the three-dimensional information in Fig. 28A corresponds to Fig. 28D. Fig. 28E is an enlarged view of a part of Fig. 28D, and the cross-section taken as a cutaway is shown in Fig. 28F. As shown in Fig. 28F, the pit is not a hole but a groove having a width, which can also be called a crack-like shape.

[0504] Fig. 29A shows a SEM image of the upper surface of the positive electrode of the secondary battery after 50 cycles. Fig. 29B is a cross-sectional view of the broken line portion in Fig. 29A. Further, Fig. 29C is an enlarged view of the portion surrounded by the square frame in Fig. 29B. Pits 90a, 90b, and 90c are shown in Fig. 29C.

[0505] Fig. 30A shows a SEM image of the upper surface of the positive electrode before being incorporated into the secondary battery. Fig. 30B is a cross-sectional view of the broken line portion in Fig. 30A. Further, Fig. 30C is an enlarged view of the portion surrounded by the square frame in Fig. 30B. A crack 91b is shown in Fig. 30C.

[0506] As described above, when the positive electrode after 50 cycles was observed, pits and cracks were observed.

[0507] <STEM Observation> Next, regarding the positive electrode of the secondary battery after 50 cycles, the cross-section was observed by a scanning transmission electron microscope (STEM). The sample for cross-section observation was processed using FIB.

[0508] <EDX Analysis> Regarding the positive electrode of the secondary battery after 50 cycles, it was evaluated using energy dispersive X-ray spectroscopy (EDX).

[0509] Fig. 31A shows a cross-sectional STEM image of the positive electrode. Fig. 31B is an enlarged view of the portion surrounded by the square frame in Fig. 31A.

[0510] Figures 32A to 32C show the EDX mapping in the region shown in Figure 31B. Figure 32A shows the EDX mapping for magnesium, Figure 32B shows the mapping for aluminum, and Figure 32C shows the mapping for cobalt. A Hitachi High-Tech HD-2700 was used for the EDX analysis. The acceleration voltage was set to 200kV. The EDX mapping suggested that magnesium and aluminum are present in at least a portion of the surface layer of the positive electrode active material particles.

[0511] <Microelectron diffraction> Next, the crystal structure of lithium cobalt oxide at and near the grain boundaries was analyzed using microelectron diffraction.

[0512] Figure 33A is a cross-sectional TEM image of degraded lithium cobalt oxide after 50 cycles. Figure 33B is a magnified image of the area enclosed by the black line in Figure 33A. The areas analyzed by micro-electron diffraction are indicated by the asterisks NBED1, NBED2, and NBED3 in Figure 33B.

[0513] Figure 34A shows the microelectron diffraction pattern of the NBED1 region marked with a star. Transmitted light is denoted as O, and parts of the diffraction spots are denoted as DIFF1-1, DIFF1-2, and DIFF1-3, as shown in the figure. Analysis of the NBED1 region marked with a star revealed that the interplanar spacing of DIFF1-1 was 0.475 nm, DIFF1-2 was 0.199 nm, and DIFF1-3 was 0.238 nm. The interplanar angles were ∠1O2=55°, ∠1O3=80°, and ∠2O3=24°. The electron beam incidence direction was [0-10]. Based on the interplanar spacing and angles, DIFF1-1 was considered to be a layered rock salt type crystal of 10-2, DIFF1-2 was similarly 10-5, and DIFF1-3 was similarly 00-3, suggesting the presence of a LiCoO2 crystal structure.

[0514] Figure 34B shows the microelectron diffraction pattern of the NBED2 region marked with a star. Transmitted light is denoted as O, and parts of the diffraction spots are denoted as DIFF2-1, DIFF2-2, and DIFF2-3, as shown in the figure. Analysis of the NBED2 region marked with a star revealed that the interplanar spacing of DIFF2-1 was 0.468 nm, DIFF2-2 was 0.398 nm, and DIFF2-3 was 0.472 nm. The interplanar angles were ∠1O2=54°, ∠1O3=110°, and ∠2O3=56°. Based on the interplanar spacing and angles, DIFF2-1, DIFF2-2, and DIFF2-3 are spinel-type crystals and are considered to have a crystal structure of either Co3O4 or LiCo2O4.

[0515] Figure 34C shows the microelectron diffraction pattern of the NBED3 region marked with a star. Transmitted light is labeled O, and parts of the diffraction spots are labeled DIFF3-1, DIFF3-2, and DIFF3-3, as shown in the figure. Analysis of the NBED1 region revealed that the interplanar spacing of DIFF3-1 was 0.241 nm, DIFF3-2 was 0.210 nm, and DIFF3-3 was 0.246 nm. The interplanar angles were ∠1O2=55°, ∠1O3=110°, and ∠2O3=55°. Based on the interplanar spacing and angles, DIFF3-1, DIFF3-2, and DIFF3-3 were considered to be rock salt type crystals with a CoO crystal structure.

[0516] Figure 35A shows the crystal structure of LiCoO2, which has a layered rock salt structure. Figure 35B shows the crystal structure of LiCo2O4, which has a spinel structure. Figure 35C shows the crystal structure of CoO, which has a rock salt structure.

[0517] <Slip> Figure 36A is a cross-sectional STEM image of a portion of the positive electrode active material layer after the slurry that forms the positive electrode active material layer has been applied to the current collector and pressed. Due to the pressing, a step is observed on the particle surface perpendicular to the grid pattern (c-axis direction), and evidence of deformation along the grid pattern direction (ab-plane direction) can be seen.

[0518] Figure 36B is a schematic cross-sectional view of the particles before pressing. In the particles before pressing, a relatively uniform barrier layer exists on the particle surface perpendicular to the grid pattern.

[0519] Figure 36C is a schematic cross-sectional view of the particles after pressing. The pressing process causes displacement in the lattice pattern direction (ab-plane direction). The barrier layer also has multiple steps and becomes non-uniform. Regarding the displacement in the ab-plane direction, the same shape of irregularities is observed on the particle surface opposite to the surface where irregularities were observed, indicating that some particles are displaced in the ab-plane direction.

[0520] The multiple steps illustrated in Figure 36C are observed as striped patterns on the particle surface. These striped patterns on the particle surface, observed due to steps caused by shifting during pressing, are called slip (stack faults). Such particle slip can lead to unevenness in the barrier layer, potentially causing degradation. Therefore, it is desirable to minimize or eliminate slip in the positive electrode active material.

[0521] This embodiment can be implemented in appropriate combination with other embodiments.

[0522] (Embodiment 4) In this embodiment, an example of a method for producing a positive electrode active material according to one aspect of the present invention will be described using Figures 37 to 47.

[0523] [Method for preparing positive electrode active material 1] An example of a method for producing a positive electrode active material, which is one aspect of the present invention, will be explained with reference to Figure 37.

[0524] As step S21 in Figure 37, a transition metal M1 source 800 is prepared.

[0525] As the transition metal M1, at least one of manganese, cobalt, and nickel can be used. For example, as the transition metal M1, only cobalt may be used, only nickel may be used, two types of cobalt and manganese may be used, two types of cobalt and nickel may be used, or three types of cobalt, manganese, and nickel may be used. The transition metal M1 source is prepared as an aqueous solution containing the transition metal M1.

[0526] As the transition metal M1 source 800, an aqueous solution containing cobalt can be an aqueous solution of cobalt sulfate or an aqueous solution of cobalt nitrate, an aqueous solution containing nickel can be an aqueous solution of nickel sulfate or an aqueous solution of nickel nitrate, and an aqueous solution containing manganese can be an aqueous solution of manganese sulfate or an aqueous solution of manganese nitrate.

[0527] Furthermore, it is preferable to use a high-purity material as the transition metal M1 source 800 used in the synthesis. Specifically, when using an aqueous solution containing the transition metal M1, the purity of the solute material used to prepare the aqueous solution should be 2N (99%) or higher, preferably 3N (99.9%) or higher, and even more preferably 4N (99.99%) or higher. The water should preferably be pure water with a resistivity of 1 MΩ·cm or higher, more preferably 10 MΩ·cm or higher, and even more preferably 15 MΩ·cm or higher, with few impurities. By using high-purity materials, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0528] Furthermore, when using a metal capable of forming a layered rock salt type composite oxide, it is preferable to use a mixing ratio of cobalt, manganese, and nickel within a range that allows for a layered rock salt type crystalline structure.

[0529] Next, in step S31, the above transition metal M1 source 800 is mixed to obtain the mixture 811 of step S32.

[0530] Next, aqueous solution A812 is prepared as step S33, and aqueous solution B813 as step S34.

[0531] As aqueous solution A812, one or more aqueous solutions containing at least one chelating agent such as glycine, oxine, 1-nitroso-2-naphthol, or 2-mercaptobenzothiazole, or aqueous ammonia can be used.

[0532] As aqueous solution B813, one or more of the following can be used: aqueous solution of sodium hydroxide, aqueous solution of potassium hydroxide, or aqueous solution of lithium hydroxide.

[0533] Next, in step S35, the mixture 811 from step S32, aqueous solution A 812, and aqueous solution B 813 are mixed.

[0534] As a mixing method for step S35, a method can be used in which the mixture 811 from step S32 and the aqueous solution B813 from step S32 are added dropwise to the aqueous solution A812 placed in the reaction vessel. It is desirable to add the mixture 811 from step S32 dropwise at a constant rate, and to add the aqueous solution B813 dropwise as needed to maintain the pH of the mixed solution in the reaction vessel within a predetermined range. In the mixing of step S35, it is desirable to stir the solution in the reaction vessel with a stirring blade or stirrer, and it is desirable to remove dissolved oxygen from the solution in the reaction vessel, the mixture 811 from step S32, the aqueous solution A812, and the aqueous solution B813 by N2 bubbling. In the mixing of step S35, the pH in the reaction vessel should preferably be 9 to 11, more preferably 10.0 to 10.5. In the mixing of step S35, the temperature of the solution in the reaction vessel should preferably be 40°C to 80°C, more preferably 50°C to 70°C.

[0535] Alternatively, as a mixing method in step S35, a method can be used in which aqueous solutions A812 and B813 are added dropwise to the mixture 811 from step S32 placed in the reaction vessel. It is preferable to adjust the dropping rate of aqueous solutions A812 and B813 in order to maintain the solute ion concentration and hydroxyl group concentration of aqueous solution A812 in the reaction vessel within a predetermined range. In the mixing of step S35, it is preferable to stir the solution in the reaction vessel with a stirring blade or stirrer, and it is preferable to remove dissolved oxygen from the solution in the reaction vessel, the mixture 811 from step S32, aqueous solution A812, and aqueous solution B813 by N2 bubbling. In the mixing of step S35, the temperature of the solution in the reaction vessel is preferably 40°C to 80°C, more preferably 50°C to 70°C.

[0536] Alternatively, a method of mixing in step S35 in which aqueous solution A812 is not used will be described. A fixed amount of aqueous solution B813 is added dropwise to the mixture 811 from step S32 placed in the reaction vessel. In the mixing in step S35, it is desirable to stir the solution in the reaction vessel with a stirring blade or stirrer, and it is desirable to remove dissolved oxygen from the solution in the reaction vessel, the mixture 811 from step S32, and aqueous solution B813 by N2 bubbling. In the mixing in step S35, the temperature of the solution in the reaction vessel should preferably be 40°C to 80°C, more preferably 50°C to 70°C.

[0537] Alternatively, as a method of mixing in step S35, a case in which pure water is used in addition to the mixture 811, aqueous solution A812, and aqueous solution B813 from step S32 will be described. The mixture 811 and aqueous solution A812 from step S32 are added dropwise to the pure water in the reaction vessel at a constant rate, and aqueous solution B813 can be added dropwise as appropriate to maintain the pH of the mixed solution in the reaction vessel within a predetermined range. In the mixing in step S35, it is desirable to stir the solution in the reaction vessel with a stirring blade or stirrer, and it is desirable to remove dissolved oxygen from the solution in the reaction vessel, the mixture 811 from step S32, aqueous solution A812, and aqueous solution B813 by N2 bubbling. In the mixing in step S35, the pH in the reaction vessel should preferably be 9 or more and 11 or less, more preferably 10.0 or more and 10.5 or less. In the mixing in step S35, the temperature of the solution in the reaction vessel should preferably be 40°C or more and 80°C or less, more preferably 50°C or more and 70°C or less.

[0538] Next, the solution containing the hydroxide having the transition metal M1, formed by the mixing in step S35, is filtered and then washed with water in step S36. The water used for washing is preferably pure water with a resistivity of 1 MΩ·cm or more, more preferably 10 MΩ·cm or more, and even more preferably 15 MΩ·cm or more, and has few impurities. By using pure water with few impurities for washing, impurities contained in the hydroxide having the transition metal M1 can be removed, and a high-purity hydroxide having the transition metal M1 can be obtained as a reaction precursor.

[0539] Next, in step S36, the hydroxide having the transition metal M1 after washing is dried and recovered to obtain the mixture 821 of step S41.

[0540] Next, in step S42, lithium compound 803 is prepared, and in step S51, the mixture 821 from step S41 and lithium compound 803 are mixed. After mixing, the mixture is recovered in step S52 to obtain the mixture 831 from step S53. Mixing can be done dry or wet. For example, a ball mill or bead mill can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. Also, when using a ball mill or bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media or material. In step S42, the peripheral speed will be 838 mm / s (rotation speed 400 rpm, diameter of ball mill container 40 mm).

[0541] Examples of lithium compound 803 include lithium hydroxide, lithium carbonate, and lithium nitrate.

[0542] Furthermore, it is preferable to use a high-purity material for the lithium compound 803 used in the synthesis. Specifically, the purity of the material should be 4N (99.99%) or higher, preferably 4N5UP (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0543] Next, in step S54, the mixture 831 from step S53 is heated. The heating is preferably carried out at a temperature of 700°C or higher but less than 1100°C, more preferably at a temperature of 800°C or higher but less than 1000°C, and even more preferably at a temperature of 800°C or higher but less than 950°C.

[0544] The heating time can be, for example, 1 hour to 100 hours, and preferably 2 hours to 20 hours. Heating is preferably carried out in an oxygen-containing atmosphere with low water content, such as oxygen or dry air (for example, with a dew point of -50°C or lower, more preferably -80°C or lower). In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. Furthermore, heating is preferable in an atmosphere where the impurity concentrations of CH4, CO, CO2, and H2 are each 5 ppb (parts per billion) or less, as this suppresses the incorporation of impurities into the material.

[0545] Furthermore, for example, when heating at 850°C for 10 hours, it is preferable to raise the temperature at 200°C / h and the flow rate of the dry atmosphere at 10 L / min. After that, the heated material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to 10 hours or more and 50 hours or less. However, cooling to room temperature in step S54 is not essential.

[0546] Furthermore, the crucible or sheath used during heating in step S54 is preferably made of a highly heat-resistant material such as alumina (aluminum oxide), mullite cordierite, magnesia, or zirconia. An alumina crucible is preferable because it is made of a material that does not contain impurities. In this embodiment, it is preferable to use an alumina crucible with a purity of 99.9%. It is preferable to place a lid on the crucible or sheath before heating. This prevents the material from volatilizing.

[0547] Furthermore, when collecting the material after heating in step S54, it is preferable to transfer it from the crucible to a mortar before collection to prevent impurities from contaminating the material. It is also preferable that the mortar itself be made of a material that does not allow impurities to enter. Specifically, it is preferable to use a mortar made of alumina with a purity of 90 wt% or higher, preferably 99 wt% or higher.

[0548] Next, in step S55, the material fired above is recovered to obtain the positive electrode active material 100G in step S56. The positive electrode active material 100G can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0549] [Method for preparing positive electrode active material 2] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be described using Figures 38 and 39A to 39E.

[0550] Steps S21 to S55 in Figure 38 can be carried out in the same manner as shown in Figure 37.

[0551] Next, in step S62, the additive element X source 833 is prepared.

[0552] As the additive element X source, one or more can be selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, bromine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic. The additive element X source 833 in step S62 can be any one or more from an aqueous solution containing additive element X, an alkoxide containing additive element X, or a solid compound containing additive element X. For example, as the additive element X source 833 in step S62 in Figure 38, a solid compound containing one or more additive elements X may be prepared, crushed, and mixed, as shown as S62a or S62b in Figures 39A and 39B, and used as the additive element X source 833 in step S62. When using a solid compound containing one or more additive elements X, it may be mixed after crushing, crushed after mixing, or used as the additive element X source 833 in step S62 without crushing.

[0553] Furthermore, it is preferable to use a high-purity material as the source of additive element X used in the synthesis. Specifically, the purity of the material should be 2N (99%) or higher, preferably 3N (99.9%) or higher, and even more preferably 4N (99.99%) or higher. By using a high-purity material, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0554] When the mixing and crushing steps are carried out wet, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). 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.

[0555] Next, in step S71, the mixture 832 from step S61 and the additive element X source 833 from step S62 are mixed. After mixing, the mixture is recovered in step S72 to obtain the mixture 841 from step S73. Mixing can be carried out dry or wet. For example, a ball mill or a bead mill can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. Also, when using a ball mill or a bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media or material. In step S71, the peripheral speed will be 838 mm / s (rotation speed 400 rpm, diameter of ball mill container 40 mm).

[0556] Next, in step S74, the mixture 841 from step S73 is heated. The heating temperature in step S74 is preferably 500°C to 1100°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.

[0557] Furthermore, heating in step S74 may be performed using a roller hearth kiln. When heat-treating with a roller hearth kiln, the mixture 841 may be processed using a heat-resistant container with a lid.

[0558] The heating time can be, for example, 1 hour to 100 hours, and preferably 2 hours to 20 hours. Heating is preferably carried out in an oxygen-containing atmosphere with low water content, such as oxygen or dry air (for example, with a dew point of -50°C or lower, more preferably -80°C or lower). In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. Furthermore, heating is preferable in an atmosphere where the impurity concentrations of CH4, CO, CO2, and H2 are each 5 ppb (parts per billion) or less, as this suppresses the incorporation of impurities into the material.

[0559] Furthermore, for example, when heating at 850°C for 10 hours, it is preferable to raise the temperature at 200°C / h and the flow rate of the dry atmosphere at 10 L / min. After that, the heated material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to 10 hours or more and 50 hours or less. However, cooling to room temperature in step S74 is not essential.

[0560] Next, in step S75, the material that was calcined above is recovered to obtain the mixture 842 from step S81. The mixture 842 obtained in step S81 can be used as the positive electrode active material 100. Alternatively, the mixture 842 obtained in step S81 can be used in the processes from step S81 onward as shown in Figure 39C.

[0561] Next, the steps from step S81 onwards, as shown in Figure 39C, will be explained. In step S82, the additive element X source 843 is prepared.

[0562] The additive element X added in step S82 can be selected from the additive elements X described above. The additive element X source 843 in step S82 can be any one or more of the following: an aqueous solution containing additive element X, an alkoxide containing additive element X, or a solid compound containing additive element X. For example, as the additive element X source 843 in step S82 in Figure 39C, a solid compound containing one or more additive elements X may be prepared, crushed, and mixed, as shown as S82a or S82b in Figures 39D and 39E, and used as the additive element X source 843 in step S82. When using a solid compound containing one or more additive elements X, it may be mixed after crushing, crushed after mixing, or used as the additive element X source 843 in step S82 without crushing.

[0563] Furthermore, it is preferable to use a high-purity material as the source of additive element X used in the synthesis. Specifically, the purity of the material should be 2N (99%) or higher, preferably 3N (99.9%) or higher, and even more preferably 4N (99.99%) or higher. By using a high-purity material, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0564] Next, in step S91, the mixture 842 from step S81 and the additive element X source 843 from step S82 are mixed. After mixing, the mixture is recovered in step S92 to obtain the mixture 851 from step S93. Mixing can be done dry or wet. For example, a ball mill or a bead mill can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. Also, when using a ball mill or a bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media or material. In step S91, mixing is performed dry in a ball mill using zirconia balls with a diameter of 1 mm at 150 rpm for 1 hour. The mixing is performed in a dry room with a dew point of -100°C to -10°C.

[0565] Next, in step S94, the mixture 851 from step S93 is heated. The heating temperature in step S94 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.

[0566] The heating time can be, for example, 1 hour to 100 hours, and preferably 2 hours to 20 hours. Heating is preferably carried out in an oxygen-containing atmosphere with low water content, such as oxygen or dry air (for example, with a dew point of -50°C or lower, more preferably -80°C or lower). In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. Furthermore, heating is preferable in an atmosphere where the impurity concentrations of CH4, CO, CO2, and H2 are each 5 ppb (parts per billion) or less, as this suppresses the incorporation of impurities into the material.

[0567] Furthermore, for example, when heating at 850°C for 10 hours, it is preferable to raise the temperature at 200°C / h and the flow rate of the dry atmosphere at 10 L / min. After that, the heated material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to 10 hours or more and 50 hours or less. However, cooling to room temperature in step S74 is not essential.

[0568] However, cooling to room temperature in step S94 is not mandatory. If there are no problems with the subsequent processes, cooling may be extended to a temperature higher than room temperature.

[0569] Next, in step S95, the material fired above is recovered to obtain the positive electrode active material 100H in step S101. The positive electrode active material 100H can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0570] [Method for preparing positive electrode active material 3] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be described using Figures 40 and 41.

[0571] In steps S21a, S21b, and S21c of Figure 40, a transition metal M1 source 800 is prepared. In this embodiment, we will describe the case in which three types of transition metal M1 sources 800 are used: nickel source 800a, cobalt source 800b, and manganese source 800c.

[0572] For nickel source 800a, an aqueous solution containing nickel can be a nickel sulfate aqueous solution or a nickel nitrate aqueous solution, etc. For cobalt source 800b, an aqueous solution containing cobalt can be a cobalt sulfate aqueous solution or a cobalt nitrate aqueous solution, etc. For manganese source 800c, an aqueous solution containing manganese can be a manganese sulfate aqueous solution or a manganese nitrate aqueous solution, etc.

[0573] Furthermore, it is preferable to use a high-purity material as the transition metal M1 source 800 used in the synthesis. Specifically, when using an aqueous solution containing the transition metal M1, the purity of the solute material used to prepare the aqueous solution should be 2N (99%) or higher, preferably 3N (99.9%) or higher, and even more preferably 4N (99.99%) or higher. The water should preferably be pure water with a resistivity of 1 MΩ·cm or higher, more preferably 10 MΩ·cm or higher, and even more preferably 15 MΩ·cm or higher, with few impurities. By using high-purity materials, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0574] Furthermore, when using a metal capable of forming a layered rock salt type composite oxide, it is preferable to use a mixing ratio of cobalt, manganese, and nickel within a range that allows for a layered rock salt type crystalline structure.

[0575] Next, in step S31, the nickel source 800a, cobalt source 800b, and manganese source 800c are mixed to obtain the mixture 811 of step S32.

[0576] Next, aqueous solution A812 is prepared as step S33, and aqueous solution B813 as step S34.

[0577] As aqueous solution A812, one or more aqueous solutions containing at least one chelating agent such as glycine, oxine, 1-nitroso-2-naphthol, or 2-mercaptobenzothiazole, or aqueous ammonia can be used.

[0578] As aqueous solution B813, one or more of the following can be used: aqueous solution of sodium hydroxide, aqueous solution of potassium hydroxide, or aqueous solution of lithium hydroxide.

[0579] Next, in step S35, the mixture 811 from step S32, aqueous solution A 812, and aqueous solution B 813 are mixed.

[0580] Steps S35 to S55 in Figure 40 can be carried out in the same manner as shown in Figure 37.

[0581] Next, in steps S63 and S64, a magnesium source 834 and a fluorine source 835 are prepared as the source of additive element X. Subsequently, in step S65, the magnesium source 834 and the fluorine source 835 are crushed and mixed to obtain the mixture 836 in step S66.

[0582] For example, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc., can be used as the magnesium source 834.

[0583] As the fluorine source 835, for example, lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF2), lanthanum fluoride (LaF3), sodium aluminum hexafluoride (Na3AlF6), etc. can be used. Furthermore, the fluorine source is not limited to a solid; for example, fluorine (F2), carbon fluoride, sulfur fluoride, oxygen fluoride (OF2, O2F2, O3F2, O4F2, O2F), etc., may be used and mixed in the atmosphere during the heating process described later. Multiple fluorine sources may also be used in combination. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted during the annealing process described later.

[0584] In this embodiment, lithium fluoride (LiF) 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 (LiF) and magnesium fluoride (MgF2) 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 (LiF) and magnesium fluoride (MgF2) 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 = around 0.33). In this specification, "around" means a value greater than 0.9 times and less than 1.1 times the value.

[0585] Furthermore, if the crushing and mixing process in step S65 is carried out wet, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). 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.

[0586] Furthermore, it is preferable to use high-purity materials as the magnesium and fluorine sources during synthesis. Specifically, the purity of these materials should be 4N (99.99%) or higher, preferably 4N5UP (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using high-purity materials, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0587] Next, in step S71, the mixture 832 from step S61 and the mixture 836 from step S66 are mixed. After mixing, the mixture is recovered in step S72 to obtain the mixture 841 from step S73. Mixing can be done dry or wet. For example, a ball mill or a bead mill can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. Also, when using a ball mill or a bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media or material. In step S71, the peripheral speed will be 838 mm / s (rotation speed 400 rpm, diameter of ball mill container 40 mm).

[0588] Next, in step S74, the mixture 841 from step S73 is heated. The heating temperature in step S74 is preferably 500°C to 1100°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.

[0589] Furthermore, heating in step S74 may be performed using a roller hearth kiln. When heat-treating with a roller hearth kiln, the mixture 841 may be processed using a heat-resistant container with a lid.

[0590] The heating time can be, for example, 1 hour to 100 hours, and preferably 2 hours to 20 hours. Heating is preferably carried out in an oxygen-containing atmosphere with low water content, such as oxygen or dry air (for example, with a dew point of -50°C or lower, more preferably -80°C or lower). In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. Furthermore, heating is preferable in an atmosphere where the impurity concentrations of CH4, CO, CO2, and H2 are each 5 ppb (parts per billion) or less, as this suppresses the incorporation of impurities into the material.

[0591] Furthermore, for example, when heating at 850°C for 10 hours, it is preferable to raise the temperature at 200°C / h and the flow rate of the dry atmosphere at 10 L / min. After that, the heated material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to 10 hours or more and 50 hours or less. However, cooling to room temperature in step S74 is not essential.

[0592] Next, in step S75, the calcined material is recovered to obtain the mixture 842 from step S81. The mixture 842 obtained in step S81 can be used as the positive electrode active material 100. Alternatively, the mixture 842 obtained in step S81 can be used in the processes from step S81 onward as shown in Figure 41.

[0593] Next, the steps from step S81 onwards shown in Figure 41 will be explained. In steps S83 and S84, nickel source 845 and aluminum source 846 are prepared as the additive element X source. In steps S85 and S86, nickel source 845 and aluminum source 846 are crushed respectively, and in step S87 they are mixed to obtain the mixture 847 in step S88.

[0594] Nickel sources such as nickel oxide and nickel hydroxide can be used.

[0595] Aluminum oxide, aluminum hydroxide, and the like can be used as aluminum sources.

[0596] Furthermore, it is preferable to use high-purity materials as the nickel and aluminum sources during synthesis. Specifically, the purity of these materials should be 4N (99.99%) or higher, preferably 4N5UP (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using high-purity materials, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0597] Next, in step S91, the mixture 842 from step S81 and the mixture 847 from step S88 are mixed. After mixing, the mixture is recovered in step S92 to obtain the mixture 851 from step S93. Mixing can be done dry or wet. For example, a ball mill or a bead mill can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. Also, when using a ball mill or a bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media or material. In step S91, the mixture is to be dry-mixed in a ball mill using zirconia balls with a diameter of 1 mm at 150 rpm for 1 hour. The mixing is to be done in a dry room with a dew point of -100°C to -10°C.

[0598] Next, in step S94, the mixture 851 from step S93 is heated. The heating temperature in step S94 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.

[0599] The heating time can be, for example, 1 hour to 100 hours, and preferably 2 hours to 20 hours. Heating is preferably carried out in an oxygen-containing atmosphere with low water content, such as oxygen or dry air (for example, with a dew point of -50°C or lower, more preferably -80°C or lower). In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. Furthermore, heating is preferable in an atmosphere where the impurity concentrations of CH4, CO, CO2, and H2 are each 5 ppb (parts per billion) or less, as this suppresses the incorporation of impurities into the material.

[0600] Furthermore, for example, when heating at 850°C for 10 hours, it is preferable to raise the temperature at 200°C / h and the flow rate of the dry atmosphere at 10 L / min. After that, the heated material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to 10 hours or more and 50 hours or less. However, cooling to room temperature in step S74 is not essential.

[0601] Next, in step S95, the material fired above is recovered to obtain the positive electrode active material 100J in step S101. The positive electrode active material 100J can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0602] [Method for preparing positive electrode active material 4] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be explained using Figure 42.

[0603] As step S21 in Figure 42, a transition metal M1 source 800 is prepared, and as step S22, an additive element X source 801 is prepared.

[0604] As the transition metal M1, at least one of manganese, cobalt, and nickel can be used. For example, as the transition metal M1, only cobalt may be used, only nickel may be used, two types of cobalt and manganese may be used, two types of cobalt and nickel may be used, or three types of cobalt, manganese, and nickel may be used. The transition metal M1 source is prepared as an aqueous solution containing the transition metal M1.

[0605] As the transition metal M1 source 800, an aqueous solution containing cobalt can be an aqueous solution of cobalt sulfate or an aqueous solution of cobalt nitrate, an aqueous solution containing nickel can be an aqueous solution of nickel sulfate or an aqueous solution of nickel nitrate, and an aqueous solution containing manganese can be an aqueous solution of manganese sulfate or an aqueous solution of manganese nitrate.

[0606] Furthermore, it is preferable to use a high-purity material as the transition metal M1 source 800 used in the synthesis. Specifically, when using an aqueous solution containing the transition metal M1, the purity of the solute material used to prepare the aqueous solution should be 2N (99%) or higher, preferably 3N (99.9%) or higher, and even more preferably 4N (99.99%) or higher. The water should preferably be pure water with a resistivity of 1 MΩ·cm or higher, more preferably 10 MΩ·cm or higher, and even more preferably 15 MΩ·cm or higher, with few impurities. By using high-purity materials, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0607] As the additive element X source 801, one or more can be selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, bromine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic. As the additive element X source 801, any one or more can be selected from an aqueous solution containing additive element X, an alkoxide containing additive element X, or a solid compound containing additive element X. It is preferable that the additive element X source 801 in step S22 be prepared as an aqueous solution containing additive element X.

[0608] Furthermore, it is preferable to use a high-purity material as the additive element X source 801 used in the synthesis. Specifically, the purity of the material should be 2N (99%) or higher, preferably 3N (99.9%) or higher, and even more preferably 4N (99.99%) or higher. By using a high-purity material, the capacity of the secondary battery can be increased and / or the reliability of the secondary battery can be improved.

[0609] Next, in step S31, the above-mentioned transition metal M1 source and additive element X source are mixed to obtain the mixture 811 of step S32.

[0610] Next, aqueous solution A812 is prepared as step S33, and aqueous solution B813 as step S34.

[0611] As aqueous solution A812, one or more aqueous solutions containing at least one chelating agent such as glycine, oxine, 1-nitroso-2-naphthol, or 2-mercaptobenzothiazole, or aqueous ammonia can be used.

[0612] As aqueous solution B813, one or more of the following can be used: aqueous solution of sodium hydroxide, aqueous solution of potassium hydroxide, or aqueous solution of lithium hydroxide.

[0613] Next, in step S35, the mixture 811 from step S32, aqueous solution A 812, and aqueous solution B 813 are mixed.

[0614] Steps S35 to S54 in Figure 42 can be carried out in the same manner as shown in Figure 37.

[0615] Next, in step S55, the material fired above is recovered to obtain the positive electrode active material 100K in step S56. The positive electrode active material 100K can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0616] [Method for preparing positive electrode active material 5] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be explained using Figure 43.

[0617] Steps S21 to S41 in Figure 43 can be carried out in the same manner as shown in Figure 37.

[0618] Next, in step S42, lithium compound 803 is prepared, and in step S43, additive element X source 801 is prepared. In step S51, the mixture 821 from step S41, lithium compound 803, and additive element X source 801 are mixed together.

[0619] Steps S51 to S54 in Figure 43 can be carried out in the same manner as shown in Figure 37.

[0620] Next, in step S55, the material fired above is recovered to obtain the positive electrode active material 100L in step S56. The positive electrode active material 100L can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0621] [Method for preparing positive electrode active material 6] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be explained using Figure 44.

[0622] Steps S21 to S74 in Figure 44 can be carried out in the same manner as shown in Figure 38.

[0623] Next, in step S75, the material fired above is recovered to obtain the positive electrode active material 100M in step S76. The positive electrode active material 100M can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0624] [Method for preparing positive electrode active material 7] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be explained using Figure 45.

[0625] Steps S21 to S41 in Figure 45 can be carried out in the same way as shown in Figure 42. Also, steps S42 to S54 in Figure 45 can be carried out in the same way as shown in Figure 43.

[0626] Next, in step S55, the material fired above is recovered to obtain the positive electrode active material 100N in step S56. The positive electrode active material 100N can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0627] [Method for preparing positive electrode active material 8] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be explained using Figure 46.

[0628] Steps S21 to S54 in Figure 46 can be carried out in the same way as shown in Figure 43. Also, steps S55 to S74 in Figure 46 can be carried out in the same way as shown in Figure 38.

[0629] Next, in step S75, the material fired above is recovered to obtain the positive electrode active material 100P in step S76. The positive electrode active material 100P can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0630] [Method for preparing positive electrode active material 9] Another example of a method for producing a positive electrode active material according to one embodiment of the present invention will be explained using Figure 47.

[0631] Steps S21 to S54 in Figure 47 can be carried out in the same way as shown in Figure 45. Also, steps S55 to S74 in Figure 47 can be carried out in the same way as shown in Figure 38.

[0632] Next, in step S75, the material fired above is recovered to obtain the positive electrode active material 100Q in step S76. The positive electrode active material 100Q can be used as the first material 100x shown in Embodiment 1 and Embodiment 2.

[0633] As shown in Figures 38 to 47, by separating the process of introducing the transition metal M1 from the process of introducing the additive element X, it is sometimes possible to change the depth profile of each element. For example, the concentration of the additive element can be increased near the surface compared to the interior of the particle. Also, using the number of atoms of the transition metal M1 as a reference, the ratio of the number of atoms of the additive element to this reference can be made higher near the surface than in the interior.

[0634] The positive electrode active material 100 may be expressed as a composite oxide (LiM1O2) having lithium, a transition metal M1, and oxygen. However, the positive electrode active material in one embodiment of the present invention only needs to have the crystal structure of a lithium composite oxide represented as LiM1O2, and its composition is not strictly limited to Li:M1:O=1:1:2.

[0635] Furthermore, in one embodiment of the present invention, a high-purity material is used as the transition metal M1 source during synthesis, and the positive electrode active material is produced in a process that minimizes the inclusion of impurities during synthesis. In addition, by thoroughly eliminating the inclusion of impurities during synthesis and controlling the introduction of desired additive elements (additive element X, additive element X1, or additive element X2) into the positive electrode active material, it is possible to obtain a positive electrode active material in which regions with low impurity concentrations and regions with introduced additive elements are controlled. Furthermore, the positive electrode active material shown in this embodiment is a material with high crystallinity. Moreover, the positive electrode active material obtained by the method for producing the positive electrode active material according to one embodiment of the present invention can increase the capacity of a secondary battery and / or improve the reliability of a secondary battery.

[0636] [Structure of the positive electrode active material] A positive electrode active material according to one aspect of the present invention will be described with reference to Figures 48A to 49C.

[0637] Figure 48A shows a cross-sectional view of the positive electrode active material 100. The positive electrode active material 100 has a plurality of primary particles 101. At least some of the plurality of primary particles 101 are fixed together to form secondary particles 102. An enlarged view of the secondary particles 102 is shown in Figure 48B. The positive electrode active material 100 may have voids 105. Note that the shapes of the primary particles 101 and secondary particles 102 shown in Figures 48A and 48B are examples and are not limited thereto.

[0638] In this specification, a primary particle is the smallest solid unit that is recognized as having a clear boundary in a microscopic image such as an SEM image, TEM image, or STEM image. A secondary particle is a particle formed by the sintering, bonding, or aggregation of multiple primary particles. The bonding force acting between the multiple primary particles is irrelevant. It may be a covalent bond, an ionic bond, a hydrophobic interaction, a van der Waals force, or any other intermolecular interaction, and multiple bonding forces may be at work. Furthermore, the term "particle" includes both primary and secondary particles.

[0639] <Contained elements> The positive electrode active material 100 comprises lithium, a transition metal M1, oxygen, and an additive element.

[0640] The positive electrode active material 100 can be described as a composite oxide represented by LiM1O2 to which multiple additive elements have been added. However, the positive electrode active material in one embodiment of the present invention only needs to have the crystal structure of a lithium composite oxide represented by LiM1O2, and its composition is not strictly limited to Li:M1:O=1:1:2.

[0641] As the transition metal M1 in the positive electrode active material 100, it is preferable to use a metal that can form a layered rock salt type composite oxide belonging to space group R-3m together with lithium. For example, at least one of manganese, cobalt, and nickel can be used. In other words, as the transition metal in the positive electrode active material 100, only cobalt may be used, only nickel may be used, two types of cobalt and manganese may be used, two types of cobalt and nickel may be used, or three types of cobalt, manganese, and nickel may be used. In other words, the positive electrode active material 100 can have composite oxides containing lithium and the transition metal M1, such as lithium cobalt oxide, lithium nickel oxide, lithium cobalt oxide in which part of the cobalt is substituted with manganese, lithium cobalt oxide in which part of the cobalt is substituted with nickel, and nickel-manganese-lithium cobalt oxide.

[0642] In particular, using cobalt as the transition metal M1 in the positive electrode active material 100 in an amount of 75 atomic% or more, preferably 90 atomic% or more, and more preferably 95 atomic% or more, offers many advantages, such as being relatively easy to synthesize, easy to handle, and having excellent cycle properties.

[0643] On the other hand, if nickel is used as the transition metal M1 in the positive electrode active material 100 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.

[0644] Furthermore, the presence of nickel along with cobalt as the transition metal M1 can suppress the shifting of the layered structure composed of octahedra of cobalt and oxygen. This is preferable, as it can lead to a more stable crystal structure, especially in the charged state at high temperatures. This is because nickel is thought to diffuse easily into the interior of lithium cobalt oxide, and while it may be present at the cobalt sites during discharge, it can be cation-mixed and positioned at the lithium sites during charging. Nickel present at the lithium sites during charging is thought to function as pillars supporting the layered structure composed of octahedra of cobalt and oxygen, contributing to the stabilization of the crystal structure.

[0645] Furthermore, the transition metal M1 does not necessarily have to include manganese, nickel, or cobalt.

[0646] It is preferable to use at least one of the following as additive elements: magnesium, fluorine, aluminum, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic.

[0647] In particular, the positive electrode active material 100 can be made more durable by adding phosphorus, which is preferable as it allows for a safer secondary battery.

[0648] Furthermore, since manganese, titanium, vanadium, and chromium are materials that readily and stably form the tetravalent state, using them as the transition metal M1 in the positive electrode active material 100 can sometimes enhance its contribution to structural stability.

[0649] These additive elements may further stabilize the crystal structure of the positive electrode active material 100, as will be described later. In other words, the positive electrode active material 100 can include lithium cobalt oxide with magnesium and fluorine, lithium nickel-cobalt oxide with magnesium and fluorine, lithium cobalt-aluminate with magnesium and fluorine, lithium nickel-cobalt-aluminate, lithium nickel-cobalt-aluminate with magnesium and fluorine, lithium nickel-manganese-cobalt oxide with magnesium and fluorine, etc. In this specification, the additive elements may be referred to as mixtures, part of raw materials, impurities, etc.

[0650] Furthermore, it is preferable that the additive elements in the positive electrode active material 100 are added at a concentration that does not significantly alter the crystallinity of the composite oxide represented by LiM1O2. For example, it is preferable that the amount is such that it does not exhibit the Jahn-Teller effect.

[0651] Furthermore, the additive elements do not necessarily have to include magnesium, fluorine, aluminum, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, or arsenic.

[0652] <Elemental distribution> It is preferable that at least one of the additive elements in the positive electrode active material 100 has a concentration gradient.

[0653] For example, it is preferable that the primary particle 101 has a surface layer 101a and an interior layer 101b, with the concentration of the added element being higher in the surface layer 101a than in the interior layer 101b. In the cross-sectional views of the positive electrode active material shown in Figures 48A and 48B, the concentration of the added element in the primary particle 101 is shown as a gradient. A darker gradient, i.e., closer to black, means a higher concentration of the added element, while a lighter gradient, i.e., closer to white, means a lower concentration of the added element.

[0654] Furthermore, it is preferable that the concentration of the added element at and near the interface 103 between primary particles is higher than the concentration inside 101b of the primary particle 101. In this specification, the vicinity of interface 103 refers to the region from interface 103 to about 10 nm.

[0655] Figure 49A shows an example of the concentration distribution of added elements between the dashed-dotted line A and B in the positive electrode active material 100 shown in Figure 48B. In Figure 49A, the horizontal axis represents the distance between the dashed-dotted line A and B in Figure 48B, and the vertical axis represents the concentration of added elements.

[0656] Compared to the primary particles 101, the interface 103 and its vicinity have regions with higher concentrations of the added elements. Note that the shape of the concentration distribution of the added elements is not limited to the shape shown in Figure 49A.

[0657] Furthermore, when multiple additive elements are present, it is preferable that the peak concentration positions differ depending on the additive element.

[0658] For example, as shown in Figures 48A, 48B, and 49B, examples of preferred additive elements that have a concentration gradient increasing from the interior 101b toward the surface include magnesium, fluorine, and titanium.

[0659] Furthermore, for some other additive elements, it is preferable that the concentration peak in the positive electrode active material 100 is in a region closer to the interior 101b than the additive elements distributed as shown in Figure 49B, as shown in Figure 49C. Aluminum is an example of an additive element for which such a distribution is preferable. The concentration peak may be located in the surface layer or deeper than the surface layer. For example, it is preferable to have the concentration peak in a region between 5 nm and 30 nm from the surface.

[0660] Furthermore, it is preferable that some of the added elements, such as magnesium, have a concentration gradient that increases from the interior 101b towards the surface, as shown in Figure 49B, but it is also preferable that they are thinly distributed throughout the primary particle 101. For example, it is preferable that the magnesium concentration in the surface layer 101a, as measured by XPS or the like, is higher than the average magnesium concentration of the entire particle, as measured by ICP-MS or the like.

[0661] Furthermore, in one embodiment of the present invention, when the positive electrode active material 100 contains one or more metals other than cobalt, such as nickel, aluminum, manganese, iron, and chromium, it is preferable that the concentration of the metal in the near-surface region of the primary particles 101 is higher than the average concentration of the entire particle. For example, it is preferable that the concentration of elements other than cobalt in the surface layer 101a, as measured by XPS or the like, is higher than the average concentration of the elements in the entire particle, as measured by ICP-MS or the like.

[0662] Unlike the interior of the crystal, the particle surface is in a state where the bonds are broken, and lithium is released from the surface during charging, making it a region where the lithium concentration tends to be lower than in the interior 101b. Therefore, it is a region that is prone to instability and the crystal structure is easily disrupted. If the concentration of additive elements in the surface layer 101a is high, changes in the crystal structure can be suppressed more effectively. In addition, a high concentration of additive elements in the surface layer 101a can also be expected to improve corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte.

[0663] Thus, it is preferable that the surface layer 101a of the positive electrode active material 100 in one aspect of the present invention has a different composition from the interior 101b, with a higher concentration of added elements. Furthermore, it is preferable that the composition adopts a crystalline structure that is stable at room temperature (25°C). For this reason, the surface layer 101a may have a different crystalline structure from the interior 101b. For example, at least a part of the surface layer 101a of the positive electrode active material 100 in one aspect of the present invention may have a rock salt type crystalline structure. Also, if the surface layer 101a and the interior 101b have different crystalline structures, it is preferable that the crystal orientations of the surface layer 101a and the interior 101b are roughly the same.

[0664] However, if the surface layer 101a consists only of additive elements and oxygen, for example, only MgO, or only a solid solution of MgO and CoO(II), then insertion and removal of lithium becomes difficult. Therefore, the surface layer 101a must contain at least a transition metal M1, and in the discharge state, it must also contain lithium and have pathways for lithium insertion and removal. Furthermore, it is preferable that the concentration of the transitio...

Claims

1. It comprises a first material and a second material, At least a portion of the surface of the first material has a region covered with the second material, The first material has lithium cobalt oxide containing magnesium, fluorine, aluminum, and nickel. The lithium cobalt oxide has a region on its surface where the concentration of magnesium, fluorine, or aluminum is maximum. When the first material was subjected to EDX radiation analysis, the ratio of magnesium to cobalt atoms (Mg / Co) near the grain boundaries was 0.020 or greater and 0.

50. The second material is a positive electrode having aluminum oxide.

2. A secondary battery having the positive electrode as described in claim 1.

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