Method for producing positive electrode active material

Abstract

The present invention provides a novel method for producing a positive electrode active material. This method for producing a positive electrode active material includes: a first step for mixing lithium cobaltate that comprises magnesium, aluminum, and nickel and a carbon quantum dot dispersion liquid with each other so as to prepare a first mixture; a second step for drying the first mixture at a temperature of 90-200°C inclusive for a time period of 0.5-3 hours; and a third step for heating the dried first mixture at a temperature of 200-500°C inclusive for a time period of 1-10 hours.

Description

Method for producing positive electrode active material

[0001] One embodiment of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One embodiment of the present invention relates to a power storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a manufacturing method thereof.

[0002] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like.

[0003] In recent years, there has been active development of various types of energy storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. Demand for high-power, high-capacity lithium-ion secondary batteries has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0004] In particular, there is a high demand for secondary batteries with a large discharge capacity per weight and excellent cycle characteristics for mobile electronic devices, etc. To meet this demand, improvements to the positive electrode active materials of the positive electrodes of secondary batteries have been actively pursued (e.g., Patent Documents 1 and 2). Research on the crystalline structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 3).

[0005] X-ray diffraction (XRD) is one of the techniques used to analyze the crystal structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) introduced in Non-Patent Document 4. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 5 can be referenced from the ICSD. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 6) can be used. VESTA (Non-Patent Document 7) can be used as software for drawing crystal structures.

[0006] Also, image processing software such as ImageJ (Non-Patent Documents 8 to 10) is known. By using this software, for example, the shape of the positive electrode active material can be analyzed.

[0007] Microelectron beam diffraction is also effective for identifying the crystalline structure of the positive electrode active material, particularly the crystalline structure of the surface layer. For example, the analysis program ReciPro (Non-Patent Document 11) can be used to analyze the electron beam diffraction pattern.

[0008] In order to improve the conductivity of the positive electrode active material particles, they may be coated with carbon. For example, LiCoO ２ There is a known study on carbon coating of the surface of a substrate (Non-Patent Document 12).

[0009] JP 2018-206747 A JP 2022-070247 A

[0010] Toyoki Okumura et al.,“Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348T.Motohashi,et al.,“Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et al.,“Staging Phase Transitions in Li▲x▼CoO▲2▼”,Journal of The Electro chemical Society,2002,149(12)A1604−A1609A.Belsky,et al.,“New developments in the Inorganic Crystal Structure Database(ICSD):accessibility in support of materials research and design”,Acta Cryst.,(2002)B58 364−369.J.Akimoto,Y.Gotoh,Y.Oosawa,“Synthesis and structure refinement of LiCoO▲2▼ single crystals”Journal of Solid State Chemistry(1998)141,p.298−302.F.Izumi and K.Momma,“Three−Dimensional Visualization in Powder Diffraction”Solid State Phenom.130,15−20(2007)K.Momma and F.Izumi,”VESTA 3 for three−dimensional visualization of crystal,volumetric and morphology data”J.Appl.Cryst.(2011).44,1272−1276Rasband,W.S.,ImageJ,U.S.National Institutes of Health,Bethesda,Maryland,USA,http: / / rsb.info.nih.gov / ij / ,1997−2012.Schneider,C.A.,Rasband,W.S.,Eliceiri,K.W.”NIH Image to ImageJ:25 years of image analysis”.Nature Methods 9,671−675,2012.Abramoff,M.D.,Magelhaes,P.J.,Ram,S.J.”Image Processing with ImageJ”.Biophotonics International,volume 11,issue 7,pp.36−42,2004.Seto,Y.&Ohtsuka,M.”ReciPro:free and open−source multipurpose crystallographic software integrating a crystal model database and viewer,diffraction and microscopy simulators,and diffraction data analysis tools“(2022).J.Appl.Cryst.55.Sun,Yiping,et al.“Graphene quantum dots coated LiCoO▲2▼ fwor improved cycling stability and thermal safety at high voltage.”Journal of electroanalytical chemistry 866(2020):114109.,

[0011] Lithium-ion secondary batteries have room for improvement in various aspects, such as output characteristics, discharge capacity, cycle characteristics, reliability, safety, and cost. For example, in order to suppress changes in the crystalline structure of the surface of the positive electrode active material, the surface of the positive electrode active material may be coated with a low-activity oxide. However, this coating may inhibit lithium intercalation and deintercalation or electronic conduction. If the intercalation and deintercalation of lithium ions or electronic conduction is inhibited, there is concern about deterioration in secondary battery characteristics, such as a decrease in discharge capacity during high-rate discharge (also known as a decrease in output characteristics or a decrease in rate characteristics) and a decrease in charge / discharge capacity in low-temperature environments.

[0012] In view of the above, an object of one embodiment of the present invention is to provide a cathode active material that can be used in a lithium-ion secondary battery and that promotes lithium ion insertion and desorption even when a coating is provided. Another object is to provide a cathode active material that promotes electronic conduction. Another object is to provide a cathode active material or a composite oxide in which a decrease in discharge capacity during high-rate discharge is suppressed. Another object is to provide a cathode active material or a composite oxide in which a decrease in discharge capacity in a low-temperature environment is suppressed. Another object is to provide a cathode active material or a composite oxide in which a decrease in discharge capacity during charge-discharge cycles is suppressed. Another object is to provide a cathode active material or a composite oxide whose crystalline structure is not easily destroyed even with repeated charge-discharge cycles. Another object is to provide a cathode active material or a composite oxide that has a large discharge capacity. Another object is to provide a secondary battery or a vehicle that is safe or highly reliable.

[0013] Another object of one embodiment of the present invention is to provide a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof.

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

[0015] One aspect of the present invention is a method for producing a positive electrode active material, the method including: a first step of mixing lithium cobalt oxide containing magnesium, aluminum, and nickel with a carbon quantum dot dispersion to produce a first mixture; a second step of drying the first mixture at a temperature of 90° C. or higher and 200° C. or lower for a time period of 0.5 hours or higher and 3 hours or lower; and a third step of heating the first mixture that has been subjected to the second step at a temperature of 200° C. or higher and 500° C. or lower for a time period of 1 hour or higher and 10 hours or lower.

[0016] Furthermore, in the above, it is preferable to use a carbon quantum dot dispersion liquid prepared by a method including: step 1a of mixing 1,3,6-trinitropyrene, water, and sodium sulfite to prepare mixture 1a; step 2a of placing mixture 1a in a sealed container and heating it at a temperature of 180°C or higher and 220°C or lower for 10 hours or higher and 24 hours or lower; step 3a of filtering mixture 1a that has undergone step 2a; and step 4a of collecting the filtrate obtained through step 3a and mixing it with water.

[0017] Furthermore, in the above, it is preferable to use lithium cobalt oxide containing magnesium, aluminum, and nickel produced by a method including: a step 1b of heating lithium cobalt oxide at a temperature of 850°C or higher and 950°C or lower for a time period of 2 hours or higher and 10 hours or lower; a step 2b of mixing a magnesium source with the lithium cobalt oxide that has undergone the step 1b to produce a mixture 1b; a step 3b of heating the mixture 1b at a temperature of 850°C or higher and 950°C or lower for a time period of 2 hours or higher and 60 hours or lower; a step 4b of mixing a nickel source and an aluminum source with the mixture 1b that has undergone the step 3b to produce a mixture 2b; and a step 5b of heating the mixture 2b at a temperature of 800°C or higher and 900°C or lower for a time period of 2 hours or higher and 20 hours or lower.

[0018] According to one embodiment of the present invention, a cathode active material that can be used in a lithium-ion secondary battery and that promotes lithium ion insertion and desorption can be provided. Alternatively, a cathode active material that promotes electron conduction can be provided. Alternatively, a cathode active material or composite oxide that suppresses a decrease in discharge capacity during high-rate discharge can be provided. Alternatively, a cathode active material or composite oxide that suppresses a decrease in discharge capacity in a low-temperature environment can be provided. Alternatively, a cathode active material or composite oxide that suppresses a decrease in discharge capacity during charge-discharge cycles can be provided. Alternatively, a cathode active material or composite oxide that does not easily lose its crystal structure even after repeated charge-discharge cycles can be provided. Alternatively, a cathode active material or composite oxide that has a large discharge capacity can be provided. Alternatively, a secondary battery or vehicle that is highly safe or highly reliable can be provided.

[0019] According to one embodiment of the present invention, a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0020] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0021] FIG. 1 is a diagram illustrating a method for preparing a carbon-coated cathode active material. FIG. 2 is a diagram illustrating a method for preparing a carbon quantum dot colloidal solution. FIGS. 3A to 3D are diagrams illustrating a method for preparing a cathode active material. FIG. 4 is a diagram illustrating a method for preparing a cathode active material. FIGS. 5A to 5C are diagrams illustrating a method for preparing a cathode active material. FIG. 6A is a cross-sectional view illustrating the internal structure of a secondary battery, and FIG. 6B is a cross-sectional view illustrating the cathode and electrolyte of the secondary battery. FIGS. 7A and 7B are cross-sectional views illustrating a cathode active material. FIGS. 8A to 8F are cross-sectional views illustrating a cathode active material. FIG. 9 is a diagram illustrating the crystalline structure of a cathode active material. FIG. 10 is a diagram illustrating the crystalline structure of a conventional cathode active material. FIG. 11 is a diagram illustrating an XRD pattern calculated from the crystalline structure. FIG. 12 is a diagram illustrating an XRD pattern calculated from the crystalline structure. FIGS. 13A to 13G are diagrams illustrating the positional relationship of distributions in EDX-ray analysis. FIG. 14A is an exploded perspective view of a coin-type secondary battery, FIG. 14B is a perspective view of the coin-type secondary battery, and FIG. 14C is a cross-sectional perspective view thereof. FIG. 15A shows an example of a cylindrical secondary battery. FIG. 15B shows an example of a cylindrical secondary battery. FIG. 15C shows an example of a plurality of cylindrical secondary batteries. FIG. 15D shows an example of a power storage system including a plurality of cylindrical secondary batteries. FIGS. 16A and 16B are diagrams illustrating an example of a secondary battery, and FIG. 16C is a diagram illustrating the internal state of the secondary battery. FIGS. 17A to 17C are diagrams illustrating an example of a secondary battery. FIGS. 18A and 18B are diagrams illustrating the appearance of a secondary battery. FIGS. 19A to 19C are diagrams illustrating a manufacturing method of a secondary battery. FIGS. 20A to 20C show a configuration example of a battery pack. FIG. 21A is a perspective view of a battery pack illustrating one embodiment of the present invention, FIG. 21B is a block diagram of the battery pack, and FIG. 21C is a block diagram of a vehicle including the battery pack. 22A to 22D are diagrams illustrating an example of a transportation vehicle. FIG. 22E is a diagram illustrating an example of an artificial satellite. FIG. 23A and FIG. 23B are diagrams illustrating a power storage device according to one embodiment of the present invention. FIG. 24A is a diagram illustrating an electric bicycle, FIG. 24B is a diagram illustrating a secondary battery of the electric bicycle, and FIG. 24C is a diagram illustrating a scooter.Figures 25A to 25D are diagrams illustrating an example of an electronic device. Figure 26A shows an example of a wearable device, Figure 26B shows a perspective view of a wristwatch-type device, and Figure 26C is a diagram illustrating a side view of a wristwatch-type device. Figure 27 is a graph illustrating the results of a discharge rate test. Figures 28A and 28B are graphs illustrating the results of a charge / discharge cycle test. Figure 29 is a graph illustrating the results of a charge / discharge cycle test. Figures 30A and 30B are graphs illustrating the results of a DSC test.

[0022] Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings, etc. However, the present invention should not be construed as being limited to the following examples. The embodiments of the present invention can be modified within the scope of the spirit of the present invention.

[0023] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. For ease of understanding the structure, trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice. Unless otherwise specified, the space group R-3m will also be expressed as a composite hexagonal lattice in this specification. Miller indices may also be expressed as (hkil) rather than (hkl). Here, i is −(h+k).

[0024] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), and examples of the cross-sectional shape of individual particles include ellipsoids, rectangles, trapezoids, triangles, squares with rounded corners, and asymmetric shapes, and further, individual particles may have an irregular shape.

[0025] The theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. For example, LiCoO ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 275mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0026] The amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ MO ２ In the above formula, M represents a transition metal, and unless otherwise specified in this specification, M is cobalt and / or nickel. In the case of a positive electrode active material in a lithium ion secondary battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, LiMO ２ When a lithium ion secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ MO ２ Or we can say x = 0.2. ｘ MO ２ The small value of x in the formula means, for example, 0.1<x≦0.24.

[0027] When properly synthesized lithium cobalt oxide is used in the positive electrode, the stoichiometric ratio is approximately satisfied. ２ and x = 1. The lithium cobalt oxide contained in the lithium ion secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The state where discharge is completed (discharged state) here refers to a state where the voltage is 3.0 V or 2.5 V or less at a current of, for example, 100 mA / g or less.

[0028] Li ｘ MO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data from a lithium-ion secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0029] The space group of the positive electrode active material and the like is identified by XRD, electron beam diffraction, neutron beam diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0030] Furthermore, if the anions have a structure in which three layers are stacked with a mutual shift, such as ABCABC, it is called a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. At the same time, since real crystals always have defects, the analysis results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, if the deviation from the theoretical position and orientation is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0031] The distribution of a certain element refers to a region in which the element is continuously detected within a noise-free range using a certain continuous analytical method. A region in which the element is continuously detected within a noise-free range can also be defined as a region in which the element is always detected when the analysis is performed multiple times.

[0032] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode substance, a positive electrode material for secondary batteries, a positive electrode material for lithium ion secondary batteries, or the like.

[0033] Furthermore, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, etc., it is not necessary for all particles to have that characteristic. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of the positive electrode active material have that characteristic, it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material and a secondary battery containing it.

[0034] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in the secondary battery are described in their pre-degradation state. A decrease in discharge capacity due to aging and burn-in treatments during secondary battery manufacturing is not considered to be degradation. For example, a secondary battery consisting of a single cell or a battery pack can be said to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. For secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0035] In this specification and the like, the state of the materials of a secondary battery before deterioration is sometimes referred to as an initial product or initial state, and the state after deterioration (the state when the secondary battery has a discharge capacity of less than 97% of the rated capacity) is sometimes referred to as a product in use or a state in use, or a used product or a used state.

[0036] In this specification, the (001) plane, the (003) plane, and the like may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C plane, the basal plane, and the like. In addition, in lithium cobalt oxide, lithium has a two-dimensional diffusion path. In other words, it can be said that the lithium diffusion path exists along the plane. In this specification, a plane on which the lithium diffusion path is exposed, that is, a plane other than the plane where lithium is inserted and extracted (specifically, the (00l) plane), may be referred to as an edge plane.

[0037] In this specification, etc., secondary particles refer to particles formed by aggregation of primary particles. Also in this specification, etc., primary particles refer to particles that do not have grain boundaries on their appearance. Also in this specification, etc., single particles refer to particles that do not have grain boundaries on their appearance. Also in this specification, etc., single crystals refer to crystals in which there are no grain boundaries inside the particles, and polycrystals refer to crystals in which there are grain boundaries inside the particles. Polycrystals can be said to be an aggregate of multiple crystallites, and grain boundaries can be said to be the interface that exists between two or more crystallites. In polycrystals, it is preferable that the crystallites are aligned in the same direction.

[0038] In this specification etc., the term "A and / or B" may be used, but this is an example of a description that includes only A, only B, or both A and B.

[0039] In this specification, the words "first" and "second" may be used for the convenience of understanding the technical content or to identify each component. Therefore, the words "first" and "second" do not limit the number of each component. Furthermore, the words "first" and "second" do not limit the order of each component. Furthermore, the words "first" and "second" or identifying symbols used in this specification may not match the words or identifying symbols in the claims.

[0040] In this specification, terms such as "first step" and "second step" indicating the order of steps, "step 1a" and "step 2a" indicating the order of steps, and "step 1b" and "step 2b" indicating the order of steps may be used for the convenience of understanding the technical content or to identify each component.

[0041] Embodiment 1 In this embodiment, a method for manufacturing a carbon-coated cathode active material 120, which is a cathode active material of a battery according to one embodiment of the present invention, is described. The carbon-coated cathode active material 120, which is a cathode active material according to one embodiment of the present invention, has a configuration in which a carbon coating layer 110 is provided on the surface of a cathode active material 100, and the carbon coating layer 110 is manufactured using carbon quantum dots. Furthermore, as the cathode active material 100, a cathode active material such as lithium cobalt oxide containing magnesium, aluminum, and nickel, the manufacturing method of which is described in this embodiment, can be used.

[0042] [Method for Producing Carbon-Coated Positive Electrode Active Material] Fig. 1 is a diagram illustrating a method for producing a carbon-coated positive electrode active material 120. Fig. 2 is a diagram illustrating a method for producing a carbon quantum dot (CQDs: Carbon Quantum Dots) colloidal solution used in the description of Fig. 1. In this specification, carbon quantum dots refer to a carbon material having a diameter of 10 nm to 500 nm and a thickness of 1 nm to 10 nm, preferably a diameter of 10 nm to 100 nm and a thickness of 1 nm to 5 nm.

[0043] In FIG. 1 , a carbon quantum dot colloid solution (CQDs colloid solution) is prepared in step S101. Examples of carbon quantum dots that can be used include sulfonated carbon quantum dots (SCQDs: Sulfonated Carbon Quantum Dots) as described in FIG. 2 , as well as hydroxylated carbon quantum dots (HCQDs: Hydroxylated Carbon Quantum Dots) and aminated carbon quantum dots (ACQDs: Aminated Carbon Quantum Dots). Furthermore, water is prepared in step S102. It is preferable to use pure water as the water used in step S102.

[0044] 1, the CQDs colloidal solution and water are mixed and stirred. The amount of water to be mixed should be an amount that allows the cathode active material 100 to be sufficiently dispersed in the next step S121. For example, stirring in step S103 can be performed using a stirrer. In this manner, a carbon quantum dot dispersion (CQDs dispersion) can be prepared in step S111.

[0045] Next, the cathode active material 100 prepared in step S110 is mixed with the carbon quantum dot dispersion liquid prepared in step S111. The mixture is mixed at a weight ratio of, for example, cathode active material 100:carbon quantum dot colloid solution=99:1, and then stirred (step S121). For example, a stirrer may be used for stirring in step S121.

[0046] Next, in step S122, the material after stirring is dried. Drying can be performed by leaving the material on a hot plate at a temperature of 90° C. to 200° C. The drying time is preferably 0.5 hours to 3 hours.

[0047] Next, in step S123, the dried material is heated. The heating in step S123 is preferably performed at a temperature of 200° C. to 500° C., more preferably 250° C. to 400° C., and even more preferably about 300° C. If the temperature is too high, there is a concern that the carbon coating layer will be carbonized, and if the temperature is too low, there is a concern that the functional groups (e.g., sulfonic groups) possessed by the CQDs will not be released, so heating at the above temperatures is preferred.

[0048] After heating, the material is crushed in step S124. Sieving may be performed after step S124. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. Furthermore, it is preferable to use a mortar made of zirconium oxide. Zirconium oxide mortars are made of a material that does not easily release impurities. Specifically, it is recommended to use a mortar made of zirconium oxide with a purity of 90% or more, preferably 99% or more.

[0049] As a result of the above, the carbon-coated positive electrode active material 120 can be produced in step S130.

[0050] <Preparation of Carbon Quantum Dot Colloidal Solution> Next, an example of a method for preparing a carbon quantum dot colloidal solution will be described with reference to FIG.

[0051] In step S140 of FIG. 2, 1,3,6-trinitropyrene is prepared, in step S141, water is prepared, and in step S142, sodium sulfite (Na ２ SO ３) are prepared. Next, in step S151, these are mixed and stirred. The stirring in step S151 may be performed using, for example, a stirrer. In this manner, a mixed solution is prepared. The mixing ratio of 1,3,6-trinitropyrene and sodium sulfite may be, for example, 1,3,6-trinitropyrene:sodium sulfite = 1:21 by weight. The mixing ratio of 1,3,6-trinitropyrene and sodium sulfite to water may be determined so as to achieve a viscosity that allows sufficient stirring in the next step S152. For example, when 1,3,6-trinitropyrene is taken as 1, the weight of water may be mixed such that the viscosity is 10,000 to 500,000, preferably 100,000 to 300,000.

[0052] Next, in step S152, the mixed solution is placed in a heat-resistant and pressure-resistant container (also referred to as a sealed container) such as an autoclave, and then heated at a temperature of 150° C. to 250° C., more preferably 180° C. to 220° C., under a pressure of 0.5 MPa to 3.0 MPa, more preferably 1.0 MPa to 2.0 MPa, for 2 hours to 24 hours, more preferably 10 hours to 24 hours. Note that the inner tube of the heat-resistant and pressure-resistant container can be made of glass, fluororesin, or the like.

[0053] After heating in step S152, in step S153, the mixture is cooled to room temperature, and then the heated mixture is taken out and filtered.

[0054] 2, the filtrate obtained by the filtration in step S153 is collected. The filtrate collected in step S260 is called a carbon quantum dot colloid solution (CQDs colloid solution). In this manner, a carbon quantum dot colloid solution can be prepared.

[0055] Next, an example of a method for manufacturing the positive electrode active material 100 will be described. The drawings are diagrams illustrating the method for manufacturing the positive electrode active material 100.

[0056] The method of adding the additive element is important for producing the positive electrode active material 100. At the same time, it is also important that the crystallinity inside the positive electrode active material is good.

[0057] Therefore, in the process of producing the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, and then mix in the additive element source and perform a heat treatment.

[0058] In the method of synthesizing lithium cobalt oxide containing an additive element by mixing an additive element source simultaneously with a cobalt source and a lithium source, it is difficult to increase the concentration of the additive element in the surface layer portion 100a. Furthermore, if the additive element source is simply mixed without heating after synthesizing lithium cobalt oxide, the additive element will simply adhere to the lithium cobalt oxide without dissolving in the lithium cobalt oxide. Without sufficient heating, it is difficult to achieve a good distribution of the additive element. Therefore, it is preferable to mix the additive element source after synthesizing lithium cobalt oxide and then perform a heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

[0059] However, if the temperature of the heat treatment is too high, cation mixing occurs, increasing the possibility that the added element, for example, magnesium, enters the cobalt site. ｘ CoO ２ When x in the matrix is ​​small, the layered rock salt crystal structure of R-3m is not maintained. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent and the evaporation of lithium.

[0060] Therefore, it is preferable to mix a material that functions as a flux with the additive element source. If the melting point is lower than that of lithium cobalt oxide, the material can function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting point of the additive element source and lithium cobalt oxide. Lowering the melting point makes it easier to distribute the additive element well at a temperature where cation mixing is unlikely to occur.

[0061] [Initial Heating] It is more preferable to heat the lithium cobalt oxide after synthesis and before mixing with the additive element. This heating is sometimes called initial heating.

[0062] The initial heating causes lithium compounds and the like that remain unintentionally on the surface of the lithium cobalt oxide to be desorbed, resulting in a better distribution of the added elements.

[0063] More specifically, it is believed that initial heating facilitates different distributions depending on the additive elements through the following mechanism. First, lithium compounds unintentionally remaining on the surface are desorbed by initial heating. Next, this lithium-deficient surface layer 100a is mixed with additive element sources such as nickel, aluminum, and magnesium sources, and the mixture is heated. Of the additive elements, magnesium is a divalent typical element, and nickel is a transition metal, but is prone to becoming a divalent ion. Therefore, Mg is present in part of the surface layer 100a. ２＋ and Ni ２＋ and Co reduced by lithium deficiency. ２＋ However, since this phase is formed only in a part of the surface layer portion 100a, it may not be clearly observed in an electron microscope image such as an STEM or an electron beam diffraction pattern.

[0064] Among the additive elements, nickel is likely to dissolve and diffuse into the interior 100b when the surface layer 100a is a layered rock-salt lithium cobaltate, but when part of the surface layer 100a is a rock-salt lithium cobaltate, nickel tends to remain in the surface layer 100a. Therefore, initial heating can make it easier for divalent additive elements such as nickel to remain in the surface layer 100a. The effect of this initial heating is particularly significant on the surface other than the (001)-oriented surface of the positive electrode active material 100 and on the surface layer 100a.

[0065] Among the additive elements, aluminum is thought to be more stable in the non-lithium sites of the layered rock-salt type than of the rock-salt type, and therefore aluminum is more likely to be distributed in the deeper region having the layered rock-salt type and / or the interior 100b than in the region close to the surface having the rock-salt type phase in the surface layer portion 100a.

[0066] Furthermore, the initial heating is expected to have the effect of increasing the crystallinity of the layered rock salt type crystal structure in the inner portion 100b.

[0067] Therefore, especially Li ｘCoO ２ When x is, for example, 0.15 or more and 0.17 or less, it is preferable to perform this initial heating in order to produce a positive electrode active material 100 having a monoclinic O1(15) type crystal structure.

[0068] However, initial heating is not necessarily required. In other heating steps, the atmosphere, temperature, time, etc. can be controlled to obtain Li ｘ CoO ２ When x in the formula (1) is small, it may be possible to produce a cathode active material 100 having an O3' type and / or a monoclinic O1(15) type. The cathode active material 100 having an O3' type and a monoclinic O1(15) type will be described in detail in the second embodiment.

[0069] <<Method 1 for Producing Positive Electrode Active Material 100>> Method 1 for producing the positive electrode active material 100, which undergoes heat treatment and initial heating, will be described with reference to FIGS. 3A to 3D. FIG.

[0070] <Step S11> In step S11 shown in FIG. 3B, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0071] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0072] As the cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide.

[0073] The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity and / or reliability of the secondary battery can be increased.

[0074] In addition, the cobalt source preferably has high crystallinity, for example, single crystal grains. The crystallinity of the cobalt source can be evaluated using a TEM image, a STEM (Scanning Transmission Electron Microscope) image, a HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) image, an ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) image, or the like, or evaluations such as X-ray diffraction, electron diffraction, and neutron diffraction. The above-described method for evaluating the crystallinity can be applied not only to the evaluation of the cobalt source but also to the evaluation of the crystallinity of other materials.

[0075] <Step S12> Next, in step S12 shown in FIG. 3B , the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method allows for finer particle pulverization and mixing. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the cobalt source with dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0076] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as pulverizing media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed: 400 rpm, ball mill diameter: 40 mm).

[0077] <Step S13> Next, in step S13 shown in FIG. 3B , the mixed material is heated. Heating is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably at approximately 950°C. If the temperature is too low, the decomposition and melting of the lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to lithium evaporation from the lithium source and / or excessive reduction of cobalt. For example, cobalt may change from trivalent to divalent, which may induce oxygen defects.

[0078] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0079] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0080] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0081] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and allowing oxygen to flow through the reaction chamber is called flow.

[0082] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen (which may also be called purging) may be used to prevent the oxygen from entering or leaving the reaction chamber. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0083] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0084] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0085] The container for containing the object to be heated during heating is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, a setter made of aluminum oxide with a purity of 99.9% is used. It is preferable to cover the crucible or setter before heating, as this prevents the material from volatilizing. Mullite-cordierite may also be used as the material for the crucible and setter.

[0086] Furthermore, it is preferable to use a used crucible rather than a new one. In this specification, a new crucible refers to one that has undergone two or fewer heating processes with materials containing lithium, transition metal M, and / or additive elements. A used crucible refers to one that has undergone three or more heating processes with materials containing lithium, transition metal M, and / or additive elements. This is because using a new crucible may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the sheath during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, may not fall within the desired range. On the other hand, this risk is less likely with a used crucible.

[0087] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use a zirconium oxide mortar. Zirconium oxide mortars are made of a material that does not easily release impurities. Specifically, a zirconium oxide mortar with a purity of 90% or more, preferably 99% or more, is used. Heating conditions equivalent to those of step S13 can also be applied to heating steps other than step S13, which will be described later.

[0088] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ The lithium cobalt oxide (LiCoO) thus prepared can be used to synthesize the starting material LiCoO in step S10 of FIG. ２ It can be used as:

[0089] Although an example of producing a composite oxide by a solid phase method as in steps S11 to S14 has been shown, a composite oxide may also be produced by a coprecipitation method. Alternatively, a composite oxide may also be produced by a hydrothermal method. The lithium cobalt oxide produced in this way is added to the starting material LiCoO prepared in step S10 of FIG. ２ It can be used as:

[0090] <Step S15> Next, in step S15 shown in Fig. 3A, the lithium cobalt oxide is heated. Because this is the first heating of the lithium cobalt oxide, the heating in step S15 may be referred to as initial heating. Alternatively, because this heating is performed before step S33 described below, it may be referred to as preheating or pretreatment.

[0091] As described above, the initial heating causes lithium to be desorbed from a portion of the surface layer 100a of the lithium cobalt oxide. It is also expected to have the effect of improving the crystallinity of the interior 100b. Furthermore, impurities may be mixed into the lithium source and / or cobalt source prepared in step S11, etc. The initial heating can reduce the amount of impurities in the lithium cobalt oxide completed in step S14.

[0092] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. A smooth surface of lithium cobalt oxide means that there are few irregularities, the lithium cobalt oxide is rounded overall, and the corners are rounded. Furthermore, a state in which there is little foreign matter adhering to the surface is called smooth. Foreign matter is thought to be a cause of irregularities, so it is preferable that it does not adhere to the surface.

[0093] For this initial heating, it is not necessary to provide a lithium source, an additive element source, or a material that functions as a flux.

[0094] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. In addition to the heating conditions, the heating temperature in this step should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in this step should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0095] The effect of increasing the crystallinity of the inner portion 100b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the difference in shrinkage of the lithium cobalt oxide produced in step S13.

[0096] The heating in step S13 may cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference may induce a shrinkage difference. It is also believed that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is believed that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. Homogenizing the strain energy relieves the strain in the lithium cobalt oxide. This may result in a smoother surface for the lithium cobalt oxide. This is also referred to as an improved surface. In other words, it is believed that the shrinkage difference in the lithium cobalt oxide is relieved after step S15, resulting in a smoother surface for the composite oxide.

[0097] Furthermore, the difference in shrinkage may cause microscopic misalignment, such as crystal misalignment, in the lithium cobalt oxide. This step is preferably carried out to reduce this misalignment. This step makes it possible to equalize the misalignment in the composite oxide. When the misalignment is equalized, the surface of the composite oxide may become smooth. This is also referred to as the alignment of crystal grains. In other words, it is believed that step S15 reduces the misalignment of crystals and other particles in the composite oxide, resulting in a smoother surface.

[0098] When lithium cobalt oxide with a smooth surface is used as the positive electrode active material, deterioration during charge and discharge in a secondary battery is reduced and cracking of the positive electrode active material can be prevented.

[0099] Note that pre-synthesized lithium cobalt oxide may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on pre-synthesized lithium cobalt oxide, lithium cobalt oxide with a smooth surface can be obtained.

[0100] <Step S20> Next, as shown in step S20, it is preferable to add the additional element A to the lithium cobalt oxide that has undergone the initial heating. When the additional element A is added to the lithium cobalt oxide that has undergone the initial heating, the additional element A can be added evenly. Therefore, it is preferable to add the additional element A after the initial heating. The step of adding the additional element A will be described with reference to FIGS. 3A and 3C.

[0101] 3A and 3C, the steps of preparing a source of additional element A (A source) will be described. A lithium source may be prepared together with the source of additional element A.

[0102] The additive element A can be one or more selected from the additive elements described in the previous embodiments, such as magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. Also, one or two selected from bromine and beryllium can be used.

[0103] <Step S21> Step S21 shown in Fig. 3C will be described. When magnesium is selected as the additive element, the source of the additive element can be called a magnesium source (M source). As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0104] When fluorine is selected as the additive element, the source of the additive element can be called a fluorine source (F source). Examples of the fluorine source include lithium fluoride (LiF), magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0105] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0106] The fluorine source may also be gaseous, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F) or the like may be used and mixed into the atmosphere in the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0107] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and magnesium source. ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or its vicinity) is more preferable. In this specification, "or its vicinity" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0108] 3C, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0109] 3C, the pulverized and mixed materials are collected to obtain a source of the additional element A. The source of the additional element A shown in step S23 includes a plurality of starting materials and can be called a mixture.

[0110] The particle size of the mixture is preferably D50 (median diameter) of 600 nm to 10 μm, more preferably 1 μm to 5 μm. Even when a single material is used as the additive element source, the D50 (median diameter) is preferably 600 nm to 10 μm, more preferably 1 μm to 5 μm.

[0111] Such a finely pulverized mixture (including the case where only one type of additive element is included) is preferable because when the mixture is mixed with lithium cobalt oxide in a later step, the mixture can be easily adhered uniformly to the surfaces of the lithium cobalt oxide particles, since this facilitates the uniform distribution or diffusion of the additive element in the surface layer portion 100 a of the positive electrode active material 100 after heating.

[0112] <Step S21> A step different from that shown in FIG. 3C will be described with reference to FIG. 3D. In step S21 shown in FIG. 3D, four types of additive element sources to be added to lithium cobalt oxide are prepared. That is, the types of additive element sources in FIG. 3D are different from those in FIG. 3C. A lithium source may be prepared together with the additive element sources.

[0113] As sources of four additive elements, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described with reference to FIG. 3C. Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0114] <Steps S22 and S23> Steps S22 and S23 shown in FIG. 3D are similar to the steps described with reference to FIG. 3C.

[0115] <Step S31> Next, in step S31 shown in FIG. 3A, the lithium cobalt oxide that has been initially heated is mixed with an additional element A source (A source).

[0116] In the present embodiment, the number of magnesium atoms contained in the additive element A source is preferably 0.50% or more and 3.0% or less, more preferably 0.75% or more and 2.0% or less, and still more preferably 0.75% or more and 1.0% or less, relative to the number of cobalt atoms contained in the lithium cobalt oxide.

[0117] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter mixing time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium.

[0118] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0119] <Step S32> Next, in step S32 of Fig. 3A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, sieving may be performed as necessary.

[0120] 3A to 3D illustrate a fabrication method in which the additive element is added only after initial heating, but the present invention is not limited to this method. The additive element may be added at a different timing or multiple times. The timing may vary depending on the element.

[0121] For example, an additive element may be added to the lithium source and the cobalt source in step S11, i.e., in the stage of the starting material for the composite oxide. Then, in step S13, lithium cobalt oxide containing the additive element can be obtained. In this case, it is not necessary to separate steps S11 to S14 from steps S21 to S23. This method can be said to be simple and highly productive.

[0122] Alternatively, lithium cobalt oxide containing some of the additive elements may be used. For example, if lithium cobalt oxide containing magnesium and fluorine is used, steps S11 to S14 and some of step S20 can be omitted. This method is simple and has high productivity.

[0123] Alternatively, lithium cobalt oxide to which magnesium and fluorine have been added in advance may be heated in step S15, and then a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added as in steps S20 to S31.

[0124] 3A, the mixture 903 is heated. The heating conditions can be selected from those described in step S13. The heating time is preferably 2 hours or more.

[0125] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 650° C. or higher.

[0126] Of course, the reaction is more likely to proceed if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF and MgF are used as the additive element source. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0127] Also, LiCoO ２ :LiF:MgF ２ Mixture 903 obtained by mixing so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830°C in differential scanning calorimetry (DSC test). Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

[0128] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0129] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 900°C or lower.

[0130] In consideration of these, the heating temperature in step S33 is preferably 650°C or higher and 1130°C or lower, more preferably 650°C or higher and 1000°C or lower, even more preferably 650°C or higher and 950°C or lower, and even more preferably 650°C or higher and 900°C or lower. Also, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Also, it is preferably 800°C or higher and 1100°C or lower, or 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, and even more preferably 830°C or higher and 900°C or lower.

[0131] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0132] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered below the decomposition temperature of lithium cobalt oxide, for example, to 742° C. or higher and 950° C. or lower, and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0133] However, since LiF has a smaller specific gravity in a gaseous state than oxygen, there is a possibility that LiF may volatilize or sublimate when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, etc., LiCoO ２ There is a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0134] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 903.

[0135] The heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additive elements (e.g., fluorine) diffuse is blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0136] Furthermore, it is believed that uniform distribution of an additive element (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the particles of mixture 903 do not adhere to each other.

[0137] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0138] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 903 .

[0139] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of the lithium cobalt oxide in step S14, etc. When the lithium cobalt oxide is small, a lower heating temperature or a shorter heating time may be preferable than when the lithium cobalt oxide is large.

[0140] 3B , when the median diameter (D50) of the lithium cobalt oxide is about 12 μm, the heating temperature is preferably, for example, 650° C. or more and 950° C. or less. The heating time is, for example, preferably, 3 hours or more and 60 hours or less, more preferably, 10 hours or more and 30 hours or less, and even more preferably, about 20 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or more and 50 hours or less.

[0141] On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 5 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0142] <Step S34> Next, in step S34 shown in FIG. 3A , the heated material is recovered to obtain the positive electrode active material 100. At this time, the recovered particles can be crushed by sieving as necessary. Through the above steps, the positive electrode active material 100 of one embodiment of the present invention can be produced. The positive electrode active material of one embodiment of the present invention has a smooth surface.

[0143] <<Method 2 for Producing Positive Electrode Active Material 100>> Next, a method 2 for producing the positive electrode active material 100, which is one embodiment of the present invention and differs from method 1 for producing the positive electrode active material 100, will be described with reference to FIGS. 4 to 5C . Method 2 for producing the positive electrode active material differs from method 1 mainly in the number of times the additive elements are added and the mixing method. For other descriptions, the description of method 1 can be referred to.

[0144] In FIG. 4, steps S11 to S15 are carried out in the same manner as in FIGS. 3A and 3B to prepare lithium cobalt oxide that has undergone initial heating.

[0145] <Step S20a> Next, as shown in step S20a, it is preferable to add an additional element A1 to the lithium cobalt oxide that has undergone the initial heating.

[0146] <Step S21> In steps S21 to S23 shown in Fig. 5A, a first additive element source (A1 source) is prepared. The first additive element source can be selected from the additive elements A described in step S21 shown in Fig. 3C and used. For example, one or more selected from magnesium, fluorine, and calcium can be suitably used as the additive element A1. Fig. 5A illustrates an example in which a magnesium source (Mg source) and a fluorine source (F source) are used as the first additive element source.

[0147] Steps S21 to S23 shown in Fig. 5A can be performed under the same conditions as steps S21 to S23 shown in Fig. 3C. As a result, an additional element source (Al source) can be obtained in step S23.

[0148] Furthermore, steps S31 to S33 shown in FIG. 4 can be performed in the same manner as steps S31 to S33 shown in FIG. 3A.

[0149] <Step S34a> Next, the material heated in step S33 is recovered to produce lithium cobalt oxide containing the additional element A1. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0150] <Step S40> In step S40 shown in Fig. 4, a second additive element source (A2 source) is prepared. This will be described with reference to Figs. 5B and 5C.

[0151] <Step S41> In steps S41 to S43 shown in FIG. 5B, a second additive element source (A2 source) is prepared. The additive element A2 contained in the second additive element source can be selected from the additive elements A described in step S21 shown in FIG. 3C. For example, the additive element A2 can be one or more selected from nickel, titanium, boron, zirconium, and aluminum. FIG. 5B illustrates an example in which a nickel source (Ni source) and an aluminum source (Al source) are used as the second additive element source. Examples of nickel sources that can be used include nickel hydroxide and nickel fluoride. Examples of aluminum sources that can be used include aluminum hydroxide and aluminum fluoride.

[0152] The number of nickel atoms contained in the second additive element source (A2 source) is preferably 0.05% or more and 4.0% or less, more preferably 0.20% or more and 2.0% or less, and even more preferably 0.20% or more and 1.0% or less, relative to the number of cobalt atoms in the lithium cobalt oxide. For example, when nickel hydroxide is used as the nickel source, when the number of moles of lithium cobalt oxide in step S10 is taken as 100, the number of moles of nickel hydroxide contained in the second additive element source is preferably 0.05 or more and 4.0 or less (0.05 mol% or more and 4.0 mol% or less), more preferably 0.20 or more and 2.0 or less (0.20 mol% or more and 2.0 mol% or less), and even more preferably 0.20 or more and 1.0 or less (0.20 mol% or more and 1.0 mol% or less).

[0153] The number of aluminum atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0% of the number of cobalt atoms in the lithium cobalt oxide, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%. For example, when aluminum hydroxide is used as the aluminum source, when the number of moles of lithium cobalt oxide in step S10 is taken as 100, the number of moles of aluminum hydroxide contained in the second additive element source is preferably 0.05 to 4.0 (0.05 mol% to 4.0 mol%), more preferably 0.20 to 2.0 (0.20 mol% to 2.0 mol%), and even more preferably 0.20 to 1.0 (0.20 mol% to 1.0 mol%).

[0154] Steps S41 to S43 shown in Fig. 5B can be performed under the same conditions as steps S21 to S23 shown in Fig. 3C. As a result, an additional element source (A2 source) can be obtained in step S43.

[0155] 5C shows a modified example of the steps described with reference to FIG. 5B. In step S41 shown in FIG. 5C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are independently pulverized. That is, in step S43 shown in FIG. 5C, a plurality of second additive element sources (A2 sources) are prepared. The steps in FIG. 5C differ from those in FIG. 5B in that the additive elements are independently pulverized in step S42a.

[0156] <Steps S51 to S53> Next, steps S51 to S53 shown in FIG. 4 can be performed under the same conditions as steps S31 to S34 shown in FIG. 3A. The heating in step S53 is preferably performed at 800° C. or higher and 1000° C. or lower, more preferably 800° C. or higher and 950° C. or lower, and even more preferably 800° C. or higher and 900° C. or lower. The heating time is preferably 1 hour or higher and 60 hours or lower, more preferably 2 hours or higher and 20 hours or lower, and even more preferably 2 hours or higher and 10 hours or lower. The heating in step S53 is preferably performed at a lower heating temperature and for a shorter heating time than in step S33. Through the above steps, the cathode active material 100 of one embodiment of the present invention can be produced in step S54. The cathode active material of one embodiment of the present invention has a smooth surface.

[0157] 4 and 5 , in production method 2, the addition of an additive element to lithium cobalt oxide is carried out separately as additive element A1 and additive element A2. By separately adding additive element A1 and additive element A2, the distribution of each additive element in the depth direction can be changed. For example, it is possible to distribute additive element A1 so that its concentration is higher in the surface layer portion of the positive electrode active material than in the interior thereof, and to distribute additive element A2 so that its concentration is higher in the interior than in the surface layer portion.

[0158] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0159] Embodiment 2 In this embodiment, a battery and a positive electrode active material of one embodiment of the present invention will be described.

[0160] [Battery] A lithium ion battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. When the electrolyte contains an electrolytic solution, the battery also includes a separator between the positive electrode and the negative electrode. The battery may further include an exterior body that covers at least a portion of the periphery of the positive electrode, the negative electrode, and the electrolyte.

[0161] In this embodiment, a description will be given mainly of a positive electrode and a positive electrode active material of a battery according to one embodiment of the present invention. Note that the positive electrode active material refers to a carbon-coated positive electrode active material 120 having a carbon coating layer 110 on particles of the positive electrode active material 100, an example of which is described in Embodiment 1. Details of the remaining components of the lithium-ion battery according to one embodiment of the present invention will be described in Embodiment 3.

[0162] 6A is a cross-sectional schematic diagram illustrating the internal structure of a lithium-ion battery 10. The lithium-ion battery 10 has a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11 has a positive electrode current collector 21 and a positive electrode active material layer 22 on the positive electrode current collector 21, and the negative electrode 12 has a negative electrode current collector 31 and a negative electrode active material layer 32. As shown in the figure, the positive electrode active material layer 22 and the negative electrode active material layer 32 face each other with the separator 13 interposed therebetween. Although not shown in FIG. 6A , electrolytes are present in the voids of the positive electrode active material layer 22, the voids of the separator 13, and the voids of the negative electrode active material layer 32.

[0163] 6A illustrates one positive electrode 11, one negative electrode 12, and one separator 13, the lithium ion battery of one embodiment of the present invention is not limited to this structure. The lithium ion battery may have a structure including two positive electrodes 11, two negative electrodes 12, and two separators 13, or may have more than two electrodes stacked. Furthermore, the lithium ion battery may have a wound structure instead of the stacked structure illustrated in FIG. 6A.

[0164] FIG. 6B is an enlarged view of a portion A enclosed by a dashed line in FIG. 6A.

[0165] The positive electrode active material layer 22 includes a positive electrode active material 100 and a conductive material 41. As shown in Fig. 6B, the carbon-coated positive electrode active material 120 includes a carbon coating layer 110 on particles of the positive electrode active material 100. Although not shown, the positive electrode active material layer 22 may include a binder in addition to the positive electrode active material 100 and the conductive material 41.

[0166] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with an electrolyte 51 as shown in the figure. For example, it is preferable that 60% or more of the voids in the positive electrode active material layer 22 be filled with the electrolyte 51, more preferably 70% or more of the voids, even more preferably 70% or more of the voids, even more preferably 80% or more of the voids, even more preferably 90% or more of the voids, even more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the positive electrode active material layer 22 refer to regions in the positive electrode active material layer 22 other than the solid components (positive electrode active material, conductive material, etc.).

[0167] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 includes a carbon-coated positive electrode active material 120, which is a particle group consisting of a plurality of particles. When the particle group includes a plurality of first particles and a plurality of second particles, the plurality of first particles and the plurality of second particles preferably have different particle diameters. For example, the particle size distribution of the plurality of first particles may be such that the median diameter D50 is 9 μm or more and 25 μm or less, and the particle size distribution of the plurality of second particles may be such that the median diameter D50 is 0.1 μm or more and less than 9 μm. The carbon-coated positive electrode active material 120 includes a carbon coating layer 110 and a positive electrode active material 100.

[0168] <Positive Electrode Active Material> The positive electrode active material 100 has the function of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material used in one embodiment of the present invention can be a material that exhibits minimal deterioration during charge and discharge (hereinafter also referred to as "charge and discharge") even at high charge voltages. Specifically, a positive electrode active material (composite oxide) having a particle diameter (median diameter (D50)) of 10 μm to 50 μm, preferably 9 μm to 25 μm, obtained by the method for producing the positive electrode active material described in embodiment 1 can be used. The positive electrode active material 100 contains one or more of an additive element X, an additive element Y, and an additive element Z. The additive element X, the additive element Y, and the additive element Z will be described in detail in the section <Containing Elements>. The additive element X, the additive element Y, and the additive element Z may be collectively referred to as additive element A.

[0169] The positive electrode active material 100 is the main constituent material of the positive electrode active material layer 22, and the weight of the positive electrode active material 100 is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more of the weight of the solid components of the positive electrode active material layer 22. If the particle diameter of the positive electrode active material 100 is too small, the surface area becomes too large, which may result in excessive reaction between the positive electrode active material surface and the electrolyte. For this reason, the particle diameter (median diameter (D50)) of the positive electrode active material is preferably 10 μm or more. Furthermore, if the particle diameter of the positive electrode active material is larger than the thickness of the active material layer described below, the particle density of the active material layer cannot be increased, so the particle diameter of the largest particle is preferably 50 μm or less.

[0170] The particle size can be measured using a particle size distribution meter (laser diffraction particle size distribution measuring device) using a laser diffraction / scattering method. D50 is the particle size when the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. The measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may be measured by analysis such as SEM (Scanning Electron Microscope) or TEM. Note that, as a method for measuring D50 by analysis such as SEM or TEM, for example, 20 or more particles are measured, a cumulative curve is created, and the particle size when the cumulative amount accounts for 50% can be taken as D50.

[0171] Unless otherwise specified in this specification, the "charging voltage" is expressed based on the potential of lithium metal. Furthermore, in this specification, the "high charging voltage" refers to a charging voltage of, for example, 4.5 V or higher, preferably 4.55 V or higher, more preferably 4.6 V or higher, 4.65 V or higher, or 4.7 V or higher.

[0172] As mentioned above, in this specification, a "high charging voltage" is defined as 4.6 V or higher based on the potential when the negative electrode is made of lithium metal, but when the potential when the negative electrode is made of a carbon material (e.g., graphite) is used as the reference, a "high charging voltage" is defined as 4.5 V or higher. In short, in the case of a half cell using lithium metal as the negative electrode, a charging voltage of 4.6 V or higher is defined as a high charging voltage, and in the case of a full cell using a carbon material (e.g., graphite) as the negative electrode, a charging voltage of 4.5 V or higher is defined as a high charging voltage.

[0173] A carbon-coated positive electrode active material 120 that is less susceptible to deterioration due to repeated charging and discharging at a high charging voltage will be described with reference to FIGS. 7A to 8F.

[0174] 7A and 7B are cross-sectional views of a carbon-coated cathode active material 120 according to one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 7B are shown in FIGS. 8A to 8C. Enlarged views of the vicinity of C-D in FIG. 7B are shown in FIGS. 8D to 8F.

[0175] As shown in FIG. 7A , the carbon-coated positive electrode active material 120 has a carbon coating layer 110 and a positive electrode active material 100 .

[0176] The carbon coating layer 110 preferably has a thickness of 1 nm or more and 100 nm or less, and more preferably has a thickness of 1 nm or more and 20 nm or less.

[0177] The positive electrode active material 100 has a surface layer 100a and an inner portion 100b. In these drawings, the boundary between the surface layer 100a and the inner portion 100b is indicated by a dashed line.

[0178] The surface layer 100a of the positive electrode active material 100 refers to, for example, a region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface to the interior. Note that "approximately perpendicular" refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0179] The region of the positive electrode active material deeper than the surface layer 100a is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0180] Furthermore, when the positive electrode active material 100 has a layered rock salt crystal structure of space group R-3m, the surface layer portion 100a has an edge region 100a1 and a basal region 100a2, as shown in FIG. 7B . In FIGS. 7A and 7B , the straight line labeled (00l) represents the (00l) plane. Here, the edge region 100a1 has a surface exposed in a direction intersecting the (00l) plane, and the edge region 100a1 refers to a region extending from the surface toward the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or substantially perpendicular to the surface. Here, "intersect" means that the angle formed by the perpendicular to the first plane (the (00l) plane) and the normal to the second plane (the surface of the positive electrode active material 100) is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less.

[0181] The basal region 100a2 has a surface parallel to the (00l) plane, and the region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or substantially perpendicular from the surface to the interior is referred to as the basal region 100a2. Note that "parallel" here means that the angle between the perpendicular to the first surface (the (00l) plane) and the normal to the second surface (the surface of the positive electrode active material 100) is 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0182] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the inner portion 100b. Therefore, the positive electrode active material 100 is made of aluminum oxide (Al ２ O ３ The term "adhered metal oxide" does not include metal oxides attached to the surface of the positive electrode active material, such as the metal oxides attached to the surface of the positive electrode active material, carbonates chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. Note that the "adhered metal oxide" refers to, for example, metal oxides whose crystal orientation does not match that of the interior 100b.

[0183] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM images, ABF-STEM images, electron beam diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and FFT patterns of STEM images, etc. XRD, neutron beam diffraction, etc. can also be used as materials for determination.

[0184] It also does not include the electrolyte, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 100.

[0185] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen capable of lithium insertion and desorption, the interface between the region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion and desorption and the region where oxygen is present and the region where oxygen is not present is defined as the surface of the positive electrode active material. Surfaces created by slips, cracks, and / or cracks may also be considered the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be applied to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multi-layer film of carbon, metal, oxide, resin, etc.

[0186] <Containing Elements> The positive electrode active material 100 contains lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 contains lithium cobalt oxide (LiCoO ２ However, the positive electrode active material 100 of one embodiment of the present invention may have a crystal structure described later. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0187] The positive electrode active material of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 100 of one embodiment of the present invention preferably uses cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or more selected from nickel and manganese may also be used. It is preferable that the positive electrode active material 100 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals contained therein, because of its many advantages, such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0188] Furthermore, when the cobalt content of the transition metals in the positive electrode active material 100 is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal, ｘ CoO ２The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) accounts for the majority of the transition metal, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel accounts for the majority of the transition metal, are prone to cation mixing between nickel and lithium.

[0189] The additive element contained in the positive electrode active material 100 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium. The sum of the transition metals among the additive elements is preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0190] That is, the positive electrode active material 100 can be one or more of lithium cobalt oxide having magnesium, lithium cobalt oxide having magnesium and aluminum, lithium cobalt oxide having magnesium, aluminum, and titanium, lithium cobalt oxide having magnesium and nickel, lithium cobalt oxide having magnesium, aluminum, and nickel, lithium cobalt oxide having magnesium and fluorine, lithium cobalt oxide having magnesium, fluorine, and titanium, lithium cobalt oxide having magnesium, fluorine, and aluminum, lithium cobalt oxide having magnesium, fluorine, titanium, and aluminum, lithium cobalt oxide having magnesium, fluorine, and nickel, lithium cobalt oxide having magnesium, fluorine, nickel, and aluminum, and the like.

[0191] The positive electrode active material 100 may include a positive electrode active material having cobalt, oxygen, and magnesium, a positive electrode active material having cobalt, oxygen, magnesium, and aluminum, a positive electrode active material having cobalt, oxygen, magnesium, aluminum, and titanium, a positive electrode active material having cobalt, oxygen, magnesium, and nickel, a positive electrode active material having cobalt, oxygen, magnesium, aluminum, and nickel, a positive electrode active material having cobalt, oxygen, magnesium, and fluorine, and a positive electrode active material having cobalt, oxygen, and magnesium. It can also be said that any one or more of the following can be used in a lithium ion battery: a positive electrode active material having neodymium, fluorine, and titanium; a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and aluminum; a positive electrode active material having cobalt, oxygen, magnesium, fluorine, titanium, and aluminum; a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and nickel; and a positive electrode active material having cobalt, oxygen, magnesium, fluorine, nickel, and aluminum.

[0192] The additive element is preferably present as a solid solution in the positive electrode active material 100. For example, when performing a line analysis using STEM-EDX, the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the transition metal M detected increases, i.e., closer to the interior of the positive electrode active material 100.

[0193] These added elements further stabilize the crystal structure of the positive electrode active material 100, as will be described later.

[0194] The additive element does not necessarily have to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, or beryllium.

[0195] For example, if the cathode active material 100 is substantially free of titanium, the above-mentioned advantage of having excellent cycle characteristics will be even greater. The weight of titanium contained in the cathode active material 100 is, for example, 600 ppm or less, and more preferably 100 ppm or less. Furthermore, when the cathode active material 100 is analyzed by STEM-EDX, it is preferable that characteristic X-rays attributable to titanium are not detected, that is, the concentration is below the lower detection limit (for example, less than 0.3 atomic %).

[0196] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, etc., are further enhanced. The weight of manganese contained in the cathode active material 100 is, for example, 600 ppm or less, and more preferably 100 ppm or less.

[0197] The surface layer 100a is the region where lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 100b. In addition, the atoms on the surface of the particles of the positive electrode active material 100 in the surface layer 100a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and can be said to be a region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 100a can be made sufficiently stable, Li ｘ CoO ２Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of cobalt and oxygen octahedra can be made less likely to break.Furthermore, displacement of the layers made of cobalt and oxygen octahedra in the inner portion 100b can be suppressed.

[0198] In order to give the surface layer portion 100a a stable composition and crystal structure, it is preferable that the surface layer portion 100a contains an additive element, and more preferably contains multiple additive elements. Furthermore, it is preferable that the surface layer portion 100a has a higher concentration of one or more selected from the additive elements than the interior portion 100b. Furthermore, it is preferable that one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive elements in the positive electrode active material 100 differs depending on the additive element. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The concentration peak here refers to the maximum concentration value in the surface layer portion 100a or within 50 nm from the surface.

[0199] [Distribution] The distribution of the additive element will be described. Figures 8A to 8C are enlarged views of the vicinity of A-B in Figure 7B, and are views illustrating the edge region 100a1 of the positive electrode active material 100. Figures 8D to 8F are enlarged views of the vicinity of C-D in Figure 7B, and are views illustrating the basal region 100a2 of the positive electrode active material 100.

[0200] For example, some of the additive elements, such as magnesium, fluorine, titanium, silicon, phosphorus, boron, and calcium, preferably have a concentration gradient that increases from the interior 100b toward the surface, as shown by the gradation in Figures 8A and 8D. An additive element having such a concentration gradient will be referred to as an additive element X.

[0201] As shown by the density of the hatching in Figures 8B and 8E, other additive elements, such as aluminum and manganese, preferably have a concentration gradient and a concentration peak in a region deeper than additive element X shown in Figures 8A and 8D. The concentration peak may be present in the surface layer portion 100a, or may be deeper than the surface layer portion 100a. For example, it is preferable that the peak be present in a region of 5 nm to 30 nm from the surface toward the interior. An additive element having such a concentration gradient will be referred to as additive element Y.

[0202] As shown by the presence or absence of hatching and the density of the hatching in FIGS. 8C and 8F , other additive elements, such as nickel and barium, may be clearly present in the edge region 100a1 but substantially absent in the basal region 100a2. Here, "clearly present" refers to a case in which a characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 100. Furthermore, "substantially absent" refers to a case in which a characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 100. This also refers to the element being below the lower limit of detection in the STEM-EDX analysis. In this case, this also refers to the element being below the lower limit of detection in the STEM-EDX analysis. An additive element having such a distribution is referred to as additive element Z.

[0203] For example, magnesium, which is one of the additive elements X, is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, so they are more likely to enter the lithium site. When magnesium is present at an appropriate concentration at the lithium site in the surface layer 100a, the layered rock-salt crystal structure can be easily maintained. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. Furthermore, a high magnesium concentration in the surface layer portion 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0204] At an appropriate concentration, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying cobalt sites in addition to lithium sites. Furthermore, excess magnesium compounds (e.g., oxides or fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become a resistance component in the secondary battery. Furthermore, increasing the magnesium concentration of the positive electrode active material can decrease the discharge capacity of the positive electrode active material. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0205] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.001 to 0.1 times the number of cobalt atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0206] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Furthermore, aluminum has the effect of suppressing the elution of surrounding cobalt and improving continuous charging durability. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is less likely to collapse even with repeated charging and discharging.

[0207] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0208] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0209] Nickel, which is one of the additional elements Z, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower oxidation-reduction potential than cobalt, which leads to an increase in discharge capacity, which is preferable.

[0210] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the volume change caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site can also be prevented from shifting to CoO２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0211] On the other hand, an excess of nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and may also adversely affect the insertion and extraction of lithium.

[0212] Therefore, it is preferable that the entire positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 100 is preferably more than 0% but not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, more than 0% but not more than 4% is preferable. Alternatively, more than 0% but not more than 2% is preferable. Alternatively, 0.05% to 7.5% is preferable. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 7.5% is preferable. Alternatively, 0.1% to 4% is preferable. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0213] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When a portion of the oxygen in the surface layer portion 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the valence of cobalt ions changes with lithium desorption (from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine), resulting in different oxidation-reduction potentials. Therefore, when a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 100 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material 100 is used in a secondary battery, charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. Furthermore, as described in the first embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of the other additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source.

[0214] 8A and 8C, when the surface layer 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0215] Furthermore, having additive elements with different distributions, such as additive element X, additive element Y, and additive element Z, in combination is preferable because it can stabilize the crystal structure in a wider region. For example, when the positive electrode active material 100 contains magnesium, which is one of the additive elements X, aluminum, which is one of the additive elements Y, and nickel, which is one of the additive elements Z, it can stabilize the crystal structure in a wider region than when it contains only one or two of the additive elements X, Y, and Z. In this way, when the positive electrode active material 100 contains additive element X, additive element Y, and additive element Z in combination, the surface can be sufficiently stabilized by additive element X such as magnesium and additive element Z such as nickel, so additive element Y such as aluminum is not essential to the surface. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable for aluminum to be continuously detected in a region from the surface to a depth of 1 nm to 25 nm. In this way, a wider distribution of aluminum is preferable because it can stabilize the crystal structure in a wider region.

[0216] 8C and 8F , it is preferable that the additive element Z is contained in a large amount in the edge region 100a1 (also referred to as being contained preferentially or selectively) because this improves the stability of the crystal structure of the edge region 100a1 where lithium ions enter and exit the positive electrode active material 100 during charging and discharging of the lithium ion battery. Furthermore, it is preferable that the additive element Z has the above-described distribution, for example, when the positive electrode active material 100 is lithium cobalt oxide, because this minimizes the effects of adding the additive element Z, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0217] As described above, when multiple additive elements are present, the effects of each additive element are synergistic, which can contribute to further stabilization of the surface layer portion 100a. In particular, the presence of magnesium, nickel, and aluminum is highly effective in achieving a stable composition and crystal structure, making it preferable. In particular, it is preferable that the surface layer portion 100a of the positive electrode active material 100 has a region where aluminum is distributed more inward than magnesium. Furthermore, in addition to the region where magnesium and aluminum are distributed, it is most preferable that the surface layer portion 100a of the positive electrode active material 100 has a region where the nickel distribution and the magnesium distribution overlap in the edge region 100a1.

[0218] <Crystal structure> <Li ｘ CoO ２ When x is 1 in the positive electrode active material 100 according to one embodiment of the present invention, the positive electrode active material 100 is in a discharged state, that is, Li ｘ CoO ２ In the case where x = 1 in the formula (I), it is preferable that the composite oxide 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 two-dimensional lithium ion diffusion paths, is suitable for lithium ion insertion / extraction reactions, and is excellent as a positive electrode active material for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies most of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure.

[0219] On the other hand, the surface layer portion 100a of the cathode active material 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b, which is made up of an octahedron of the transition metal M and oxygen, so that the layered structure formed by the octahedron of the transition metal M and oxygen is not destroyed even when lithium is released from the cathode active material 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and the inner portion 100b of the cathode active material 100, such as oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0220] Therefore, the surface layer portion 100a preferably has a different crystal structure from the interior portion 100b. Furthermore, the surface layer portion 100a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0221] Furthermore, although it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably present randomly and in a sparse manner in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it is possible to suppress the deviation of the layered structure consisting of the transition metal M and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, which is expected to have a synergistic effect of suppressing magnesium elution.

[0222] It is also preferable that the crystal structure continuously changes from the interior 100b toward the surface due to the concentration gradient of the added element A. Alternatively, it is preferable that the crystal orientations of the surface layer 100a and the interior 100b are roughly the same.

[0223] For example, it is preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has a rock salt type or both a rock salt type and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has the characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are roughly the same.

[0224] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal M such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as vacancies of cations or anions may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a structure in which the lattice of the rock-salt crystal is distorted.

[0225] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately, and may contain cation or anion defects.

[0226] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0227] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal M. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is taken as the origin (000), the bright spot closest to the central spot is, for example, the (111) plane in the rock salt type in an ideal state, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２ The bright spots on the (003) plane of MgO are observed at a distance about half the distance between the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0228] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0229] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3'-type crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0230] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0231] However, the space group of the layered rock salt type crystal and the O3' type crystal is R-3m, which is different from the space group Fm-3m of the rock salt type crystal (the space group of a general rock salt type crystal), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal and the O3' type crystal and the rock salt type crystal. In this specification, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt type crystal, the O3' type and the rock salt type crystal, it may be said that the crystal orientations are approximately the same.

[0232] <Li ｘ CoO ２The positive electrode active material 100 according to one embodiment of the present invention has the above-described distribution of the additive element A and / or the crystal structure in a discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0233] 9 to 12, Li ｘ CoO ２ The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0234] The change in the crystal structure of the conventional positive electrode active material is shown in FIG. 10. The conventional positive electrode active material shown in FIG. 10 is a lithium cobalt oxide (LiCoO ２ )

[0235] In Figure 10, R-3m O3 is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0236] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0237] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0238] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２ It can also be said that the structure of and are stacked alternately. For this reason, this crystal structure is sometimes called an H1-3 crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in this specification, including Figure 10, the c-axis of the H1-3 crystal structure is shown as half the unit cell to make it easier to compare with other crystal structures.

[0239] As an example of an H1-3 type crystal structure, the coordinates of cobalt and oxygen in a unit cell can be expressed as Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0240] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0241] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 10, in the H1-3 type crystal structure, CoO ２The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0242] Furthermore, the difference in volume between these two crystal structures is large: per equivalent number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0243] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0244] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult to insert and extract lithium.

[0245] On the other hand, in the positive electrode active material 100 according to one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ CoO ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0246] Li ｘ CoO ２ 9 shows the crystal structure of the interior 100b of the positive electrode active material 100 when x is approximately 1 or 0.2. The interior 100b occupies the majority of the volume of the positive electrode active material 100 and is the part that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0247] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0248] However, the positive electrode active material 100 has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.12, which results in an H1-3 type crystal structure.

[0249] The positive electrode active material 100 according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 9 with the notation R-3m O3'.

[0250] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ nm). The c-axis is 13.681≦c≦13.881 (×10 −１ nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0251] In the O3' type crystal structure, ions of cobalt, nickel, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0252] As shown by the dotted line in FIG. 9, the difference between R-3m(O3) in the discharged state and the O3′-type crystal structure is ２ There is almost no layer misalignment.

[0253] The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0254] As described above, in the positive electrode active material 100 according to one embodiment of the present invention, Li ｘ CoO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the positive electrode active material 100 suppresses the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because it can stably utilize more lithium than conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0255] The positive electrode active material 100 is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is presumed that even when x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. However, the crystal structure is ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0256] Therefore, the positive electrode active material 100 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the entire interior 100b of the positive electrode active material 100 does not have to have an O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0257] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO ２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6 V or higher relative to the potential of lithium metal in an environment of 25°C, a H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be considered a high charging voltage.

[0258] Therefore, in other words, the positive electrode active material 100 of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it is preferable because it can adopt an O 3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C.

[0259] In some cases, the H1-3 crystal structure is finally observed when the charge voltage is further increased, even in the positive electrode active material 100. Furthermore, as described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher and lower than 4.6 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt the O3′ crystal structure.

[0260] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, a secondary battery using graphite as the negative electrode active material has a similar crystal structure at a voltage obtained by subtracting the potential of graphite from the voltage when lithium metal is used as the negative electrode active material.

[0261] In addition, in the O3' type crystal structure of FIG. 9, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, a monoclinic O1 (Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0262] The O3' type crystal structure has random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0263] Furthermore, it is preferable that the concentration gradient of the additive element A be similar at multiple locations in the surface layer portion 100a of the positive electrode active material 100. In other words, it is preferable that the reinforcement derived from the additive element A is uniformly present in the surface layer portion 100a. Even if a portion of the surface layer portion 100a is reinforced, if there is a portion without reinforcement, stress may be concentrated in the unreinforced portion. If stress is concentrated in a portion of the positive electrode active material 100, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0264] However, the additional element A does not necessarily have the same concentration gradient throughout the entire surface layer portion 100a of the positive electrode active material 100. An example of the distribution of the additional element X near C-D in FIG. 7B is shown in FIG. 8D , and an example of the distribution of the additional element Y near C-D is shown in FIG. 8E .

[0265] Here, the area around C-D has an R-3m layered rock salt type crystal structure, and the surface has a (001) orientation. The (001)-oriented surface may have a different distribution of the additional element A than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a distribution of concentration peaks of one or more elements selected from the additional element X and the additional element Y limited to a shallower portion from the surface compared to surfaces other than the (001) orientation. Alternatively, the (001)-oriented surface and its surface layer 100a may have a lower concentration of one or more elements selected from the additional element X and the additional element Y compared to other orientations. Alternatively, the (001)-oriented surface and its surface layer 100a may have one or more elements selected from the additional element X and the additional element Y below the lower detection limit.

[0266] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and the diffusion path of lithium ions is also parallel to the (001) plane.

[0267] CoO ２ Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0268] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface and the surface layer 100a other than the (001) orientation are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and the surface layer 100a other than the (001) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0269] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, it is important that the distribution of the additional element A in the plane other than the (001) plane and in the surface layer portion 100a1 thereof is as shown in Figures 8A to 8C. On the other hand, the concentration of the additional element A in the (001) plane and in the surface layer portion 100a thereof may be low or absent, as described above.

[0270] As described in the first embodiment, high purity LiCoO ２ In the manufacturing method of mixing the additional element A after manufacturing the silicon nitride film and then heating the silicon nitride film, the additional element A spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additional element A in the planes other than the (001) plane and in the surface layer portion 100 a thereof can be easily controlled to a preferred range.

[0271] <Grain Boundaries> In addition to the distribution described above, the additional element A contained in the positive electrode active material 100 of one embodiment of the present invention is more preferably at least partially distributed unevenly in and near the grain boundaries.

[0272] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of high-concentration and low-concentration regions.

[0273] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material 100 is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in other regions of the interior 100b.

[0274] Grain boundaries are a type of planar defect. Therefore, like surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of the added element A at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0275] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material 100 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid of the positive electrode active material after the cracks occur can be improved.

[0276] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100 of one embodiment of the present invention has an O3′-type crystal structure by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0277] In particular, XRD is preferable in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies the majority of the volume of the positive electrode active material 100.

[0278] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the size.

[0279] As described above, the positive electrode active material 100 according to one embodiment of the present invention is ｘ CoO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0280] It should also be noted that simply adding an additive element may not result in an O3'-type crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li ｘ CoO ２ In this case, x is 0.24 or less and the O3' type crystal structure accounts for 60% or more, and in other cases the H1-3 type crystal structure accounts for 50% or more.

[0281] Furthermore, even in the positive electrode active material 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the positive electrode active material 100 of one embodiment of the present invention is the positive electrode active material 100, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0282] However, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, it may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0283] However, even if handled in an argon atmosphere, the crystal structure of the positive electrode may change over time after the secondary battery is disassembled. Therefore, it is preferable to adopt a method that allows for rapid measurement as needed.

[0284] In the case of XRD, the detector and / or measurement start angle can be appropriately changed in order to quickly observe the main peak. For example, it is preferable to measure the main peak within 13 minutes from the start of measurement.

[0285] Whether the distribution of the additive elements contained in the positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, EPMA (Electron Probe Micro Analyzer), or the like.

[0286] The crystal structure of the surface layer 100 a and the grain boundaries can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0287] <Charging Method> Charging for determining whether a composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed by fabricating a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) using the composite oxide as a positive electrode and lithium metal as a counter electrode. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0288] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0289] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0290] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0291] The separator may be a 25 μm thick porous polypropylene film.

[0292] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0293] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material, charging at such a low current value is desirable. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. When performing various subsequent analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed in a sealed container under an argon atmosphere. After charging is completed, the positive electrode is preferably removed and analyzed promptly. Specifically, within 1 hour, and more preferably within 30 minutes, after charging is completed.

[0294] Furthermore, when analyzing the crystal structure in the charged state after multiple charge / discharge cycles, for example, charging can be performed by constant current charging at a current value of 20 mA / g to 100 mA / g up to a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V), followed by constant voltage charging until the current value reaches 2 mA / g to 10 mA / g, and then discharging at a constant current of 20 mA / g to 100 mA / g at 2.5 V. Alternatively, discharging can be performed by constant current discharging at 3.0 V at a current value of 20 mA / g to 200 mA / g.

[0295] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed, for example, at 2.5 V and a current value of 20 mA / g or more and 200 mA / g or less, or at 3.0 V and a current value of 20 mA / g or more and 200 mA / g or less.

[0296] <XRD> The XRD measurement apparatus and conditions are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions: XRD apparatus: D8 ADVANCE manufactured by Bruker Corporation; X-ray: CuKα1; Output: 40 kV, 40 mA; Slit width: Div. Slit, 0.5°; Detector: LynxEye; Scanning method: 2θ / θ continuous scan; Measurement range (2θ): 15° to 90°; Step width (2θ): 0.01°; Set counting time: 1 second / step; Sample stage rotation: 15 rpm.

[0297] If the measurement sample is a powder, it can be placed in a glass sample holder or sprinkled on a greased silicone anti-reflective plate. If the measurement sample is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device. If the measurement sample is a positive electrode, the positive electrode can be attached to a substrate of appropriate thickness with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device. If the positive electrode active material layer is higher than the measurement surface required by the device, the diffraction pattern will be shifted overall to higher angles; if it is lower, the diffraction pattern will be shifted overall to lower angles. In this case, it is preferable to correct the diffraction pattern shift using crystal structure analysis software, etc.

[0298] The ideal powder XRD patterns calculated from the O3' type crystal structure and the H1-3 type crystal structure model using CuKα1 radiation are shown in Figures 11 and 12. For comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structure of LiCoO3 and the trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD. The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The XRD pattern of the H1-3 type crystal structure was created by the same method as above, based on the information on the H1-3 type crystal structure shown in FIG. 10. The XRD pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material of one embodiment of the present invention, fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating an XRD pattern in the same manner as the others.

[0299] As shown in FIG. 11, in the O3′ type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less).

[0300] However, as shown in FIG. 12, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and 19.37° or less) and 2θ = 45.47 ± 0.10° (45.37° or more and 45.57° or less) when x is small can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0301] 11, for example, when charging is performed with an upper limit of the charging voltage set to a voltage slightly lower than 4.60 V (4.56 V, 4.57 V, 4.58 V, or 4.59 V), the above peak appears shifted to a lower angle. For example, when charging is performed with an upper limit of the charging voltage set to 4.58 V, the positive electrode active material 100 has diffraction peaks at 2θ = 18.85 ± 0.20 ° and 2θ = 45.15 ± 0.10 °, which are diffraction peaks derived from the O3'-type crystal structure.

[0302] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0303] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２ When x in the formula is small, the material has an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0304] Furthermore, even after 5 or more, 30 or more, 50 or more, or 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3'-type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0305] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ = 43° or more and 46° or less, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0306] The crystallite size of the O3'-type crystal structure of the positive electrode active material 100 is ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the graph is small, a clear peak of the O3' type crystal structure can be confirmed. ２ In this case, even if a part of the crystal structure resembles the O3'-type crystal structure, the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0307] <XPS> In XPS (X-ray Photoelectron Spectroscopy), in the case of inorganic oxides, if monochromatic aluminum Kα rays are used as X-rays, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so that the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be performed to analyze the bonding state of the elements. Note that the quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, depending on the element.

[0308] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer 100a than in the interior 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer 100a measured by XPS or the like is preferably higher than the average magnesium concentration throughout the cathode active material 100. Furthermore, the nickel concentration in at least a portion of the surface layer 100a is preferably higher than the average nickel concentration throughout the cathode active material 100. It is also preferable that the aluminum concentration in at least a part of the surface layer portion 100a is higher than the average aluminum concentration in the entire positive electrode active material 100. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 100a is higher than the average fluorine concentration in the entire positive electrode active material 100.

[0309] Note that the surface and surface layer 100a of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 100. Furthermore, the surface of the positive electrode active material 100 also does not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0310] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0311] The concentration of the added element may be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and the like that are chemically adsorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more and 2.000 or less, more preferably 0.500 or more and 1.500 or less, more preferably 0.600 or more and 1.400 or less, more preferably 0.700 or more and 1.300 or less, and even more preferably 0.800 or more and 1.200 or less.

[0312] Furthermore, the ratio of the number of atoms of nickel to cobalt, Ni / Co, as determined by, for example, XPS analysis, is preferably 0.05 or more and 0.200 or less, more preferably 0.06 or more and 0.150 or less, even more preferably 0.07 or more and 0.140 or less, even more preferably 0.08 or more and 0.130 or less, and even more preferably 0.09 or more and 0.120 or less.

[0313] Furthermore, the ratio of the number of fluorine atoms to the number of cobalt atoms, F / Co, as determined by, for example, XPS analysis, is preferably 0.100 or more and 1.500 or less, more preferably 0.200 or more and 1.200 or less, even more preferably 0.300 or more and 1.100 or less, even more preferably 0.400 or more and 1.000 or less, and even more preferably 0.500 or more and 0.900 or less.

[0314] The above range indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100, but are widely distributed at preferred concentrations in the surface layer portion 100a of the positive electrode active material 100. In other words, as a result of the XPS analysis of the positive electrode active material 100, the above range indicates that the crystalline structure is less likely to collapse even when charging and discharging are repeated so that x is 0.24 or less, and excellent cycle characteristics can be achieved.

[0315] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-rays. Furthermore, it is preferable to use an XPS apparatus with an energy resolution such that the half-width of the Ag3d5 / 2 peak (112 eV) in the XPS spectrum of an Ag sample is 1.0 eV±0.1 eV. The take-off angle can be, for example, 45°. For example, the following XPS apparatus and measurement conditions can be used for the measurement. Measurement apparatus: PHI Quantera II X-ray: Monochromated Al Kα (1486.6 eV) Energy resolution: Half-width of the Ag3d5 / 2 peak is 1.0 eV±0.1 eV Detection area: 100 μmφ Detection depth: Approximately 4-5 nm (take-off angle 45°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0316] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0317] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0318] <EDX> It is preferable that one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, electron probe microanalysis (EPMA), or the like.

[0319] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0320] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0321] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, etc.

[0322] In STEM-EDX-ray analysis or the like, in principle or due to measurement errors, the graph of the detected amount of characteristic X-rays of the element does not change sharply, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX-ray analysis or the like, the detected amount of characteristic X-rays of the transition metal M is the average value M of the detected amount of characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the detected amount of oxygen characteristic X-rays and the average value O ＡＶＥ and the average value O of the detected amount of characteristic X-rays of oxygen in the background ＢＧ The reference point is the point where the detected amount of the characteristic X-rays of the transition metal M is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M and the average value of the detected amount of the characteristic X-rays of the background transition metal M. If the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen is different from the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen attached to the surface, and therefore, the detected amount of the characteristic X-rays of the transition metal M is different from the point where ... oxygen is different from the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ In the case of a positive electrode active material containing a plurality of transition metals M, the M of the transition metal element with the largest amount of characteristic X-rays detected inside can be used as the reference point. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0323] The average value M of the detected amount of characteristic X-rays of the transition metal M in the background ＢＧ can be obtained by averaging, for example, a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＡＶＥcan be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the detected amount of characteristic X-rays of the transition metal M and oxygen becomes saturated and stable, for example, the region where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＢＧ and the average value O of the detected amount of characteristic X-rays of oxygen inside ＡＶＥ can also be found in the same way.

[0324] Furthermore, the surface of the positive electrode active material 100 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystalline structure of the positive electrode active material is observed and an area where an image is not observed, and is the outermost area of ​​an area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are confirmed.

[0325] In addition, a peak in STEM-EDX-ray analysis refers to a convex maximum value that appears in a graph of the characteristic X-ray intensity for each element, or the maximum value of the characteristic X-ray for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0326] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured by two scans can be used as the detection value for each element. The number of scans is not limited to two, and more scans can be performed and the average can be used as the detection value for each element.

[0327] The STEM-EDX analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material. For example, carbon can be vapor-deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0328] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0329] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W EDX detector. An example of conditions for EDX ray analysis using the Hitachi High-Tech HD-2700 is to set the emission current of the STEM device to 6 μA or more and 10 μA or less, and measure a portion of the thinned sample with minimal depth and unevenness. The magnification is, for example, approximately 150,000 times. The conditions for EDX ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0330] In order to increase the spatial resolution in STEM-EDX ray analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX ray analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, in order to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used in STEM-EDX ray analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0331] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. For example, the difference in depth between the peak of the fluorine concentration or detection amount and the peak of the magnesium concentration or detection amount is preferably within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and even more preferably within 0.5 nm.

[0332] In addition, in the positive electrode active material 100 containing nickel as an additive element, the peak of the nickel concentration or detectable amount in the surface layer 100a is preferably present at the surface of the positive electrode active material 100 or at a depth of up to 3 nm from the reference point toward the center, and more preferably at a depth of up to 1 nm. In addition, in the positive electrode active material 100 containing magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the nickel concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, and more preferably within 1 nm.

[0333] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that, when EDX-ray analysis is performed, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 100a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 100a is located more inward than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material 100 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0334] Here, how to express the positional relationship of element distribution when EDX-ray analysis is performed will be explained using Figures 13A to 13G. Figures 13A to 13F are schematic diagrams showing the concentration distribution or detection amount distribution of a first element e1 and a second element e2. Figure 13G is a schematic diagram showing the concentration distribution or detection amount distribution of a first element e1, a second element e2, and a third element e3.

[0335] For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 13A, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 13B, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 13C, the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 13D, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 13E, the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 13F, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum.

[0336] The expression "having an overlapping region" will be explained using an example in which the concentration distributions or detection amount distributions of a first element e1, a second element e2, and a third element e3 have the positional relationship shown in Figure 13G. In this specification and the like, "having an overlapping region" of the distributions of two elements means, for example, that the position of maximum value in the concentration distribution or detection amount distribution of at least one element is located in a range in which the concentration or detection amount in the concentration distribution or detection amount distribution of the other element is 1 / 5 or more of the maximum value. Note that when the background detection intensity in EDX-ray analysis is equal to or greater than the above-mentioned "1 / 5 of the maximum value," the "1 / 5 of the maximum value" in the above sentence shall be referred to as the "background detection intensity (also referred to as the detection limit)."

[0337] 13G, the position (P2) at which the second element e2 has a maximum value in its concentration distribution or detectable amount distribution is located in the range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution, and therefore the first element e1 and the second element e2 have an overlapping distribution area. Also, the position (P3) at which the third element e3 has a maximum value in its concentration distribution or detectable amount distribution is not located in the range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution, and therefore the first element e1 and the third element e3 do not have an overlapping distribution area.

[0338] 13G, the distribution of the second element e2 and the distribution of the third element e3 can be said to be located more inward than the distribution of the first element e1. Alternatively, the distribution of the second element e2 and the distribution of the third element e3 can be said to be biased more inward than the distribution of the first element e1.

[0339] <Powder Resistivity Measurement> The positive electrode active material 100 according to one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge. The positive electrode active material 100 having the above-described excellent properties is characterized in that, in the above-described <XRD>, Li ｘ CoO ２ It has been explained that when x in the formula (I) is small, the cathode active material 100 has an O3'-type and / or monoclinic O1(15)-type crystal structure. Furthermore, in the above <EDX>, a preferred abundance distribution of the additive element A (e.g., Mg, Al, Ni) when subjected to STEM-EDX analysis of the cathode active material 100 has been explained. Furthermore, in the above <XPS>, a preferred abundance ratio of the additive element A (e.g., Mg, Al, Ni) when subjected to XPS analysis of the cathode active material 100 has been explained. Furthermore, the cathode active material 100 of one embodiment of the present invention is also characterized by the volume resistivity of the powder.

[0340] A feature of the positive electrode active material 100 according to one embodiment of the present invention is that the volume resistivity of the powder of the positive electrode active material 100 is 1.0×10 ８ Ω・cm or more 1.0×10 １０ Preferably, the volume resistivity is Ω·cm or less. The positive electrode active material 100 having the above volume resistivity has a stable crystal structure even at high voltages. Therefore, the volume resistivity of the powder of the positive electrode active material 100 being within the above range can be used as an indicator that the surface layer portion 100 a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been successfully formed.

[0341] The volume resistivity of the powder of the positive electrode active material 100 is 1.0×10 ８ Ω・cm or more 1.0×10 １０ When the dielectric constant is 5.0×10 Ω cm or less, favorable cycle characteristics are exhibited in a charge-discharge cycle test under high voltage conditions. ８ Ω・cm or more 1.5×10 ９ When the electrical resistance is Ω·cm or less, more preferable cycle characteristics are exhibited in a charge / discharge cycle test under voltage conditions.

[0342] A method for measuring the volume resistivity of the powder of the positive electrode active material 100 according to one embodiment of the present invention will be described.

[0343] The measurement of the volume resistivity of a powder preferably includes an instrument having a terminal for resistance measurement and a mechanism for applying pressure to the powder to be measured. The terminal for resistance measurement preferably has four terminals (also called four-probe). For example, the MCP-PD600 manufactured by Nitto Seiko Analytech Co., Ltd. can be used as a measuring device having a terminal for resistance measurement and a mechanism for applying pressure to the powder (sample) to be measured. The Loresta GXII or Hiresta UX can be used as the instrument for the four-probe method. The Loresta GXII can be used to measure low-resistivity samples, and the Hiresta UX can be used to measure high-resistivity samples. The measurement environment is preferably a stable environment such as a dry room. The dry room environment is preferably, for example, a temperature environment of 25°C and a dew point environment of -40°C or below.

[0344] Measurement of the volume resistivity of powder using the measuring device shown above will be described. First, a powder sample is set in the measuring unit. The measuring unit is structured so that the powder sample and a terminal for resistance measurement are in contact with each other and can apply pressure to the powder sample. The measuring unit also has a structure for measuring the volume of the powder sample. Specifically, the measuring unit has a cylindrical space in which the powder sample is set. The structure for measuring the volume of the powder sample described above can measure the volume occupied by the powder at that time by measuring the height of the powder set in the space.

[0345] In measuring the volume resistivity of a powder, the electrical resistance of the powder and the volume of the powder are measured while pressure is applied to the powder. The pressure applied to the powder can be measured under a variety of conditions. For example, the electrical resistance and volume of the powder can be measured under pressure conditions of 13 MPa, 25 MPa, 38 MPa, 51 MPa, and 64 MPa. The volume resistivity of the powder can be calculated from the measured electrical resistance and volume of the powder.

[0346] <EPMA> The concentration of the additive element in the positive electrode active material 100 can be analyzed using EDX, but it can also be analyzed using EPMA. EPMA has a higher detection capability (also referred to as a lower detection limit) than EDX when analyzing elements present in trace amounts in a sample. Therefore, it is preferable to use EPMA when analyzing a region where a trace amount of the additive element is present.

[0347] In the EPMA analysis, a cross section of the positive electrode active material 100 is exposed by mechanical polishing, ion polishing, FIB, etc., and the cross section is analyzed. As an EPMA device, for example, an electron probe microanalyzer JXA-iHP200F manufactured by JEOL Ltd. can be used.

[0348] EPMA uses a wavelength-dispersive detector, so it has a higher ability to detect trace elements than EDX, which uses an energy-dispersive detector. On the other hand, the spatial resolution of EPMA analysis is inferior to that of EDX (especially STEM-EDX). Therefore, STEM-EDX is suitable for analysis focusing on the detailed distribution of added elements in the surface layer 100a of the positive electrode active material 100, while EPMA analysis is suitable for analysis of trace amounts of added elements in the interior 100b. Note that EPMA and EDX use different analytical methods, so the concentration values ​​obtained when analyzing the same region using each analytical method may not match.

[0349] <Microelectron Beam Diffraction Pattern> As with Raman spectroscopy, it is preferable that the characteristics of the rock salt-type crystal structure are observed in the microelectron beam diffraction pattern as well as the layered rock salt crystal structure. However, in the STEM image and the microelectron beam diffraction pattern, taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock salt-type crystal structure are not too strong in the surface layer portion 100a, particularly in the outermost surface (for example, 1 nm deep from the surface). This is because, rather than the outermost surface being covered with a rock salt-type crystal structure, it is preferable that an additive element such as magnesium is present in the lithium layer while maintaining the layered rock salt-type crystal structure, which can ensure a lithium diffusion path and has a stronger function of stabilizing the crystal structure.

[0350] Therefore, for example, when a micro-electron beam diffraction pattern is obtained from a region having a depth of 1 nm or less from the surface and a micro-electron beam diffraction pattern from a region having a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constant calculated from these patterns is small.

[0351] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is 0.1 × 10 for the a-axis. −１０ m or less, and the c-axis is preferably 1.0 × 10 −１０ m or less. −１０ m or less, and the c-axis is preferably 0.6×10 −１０ It is more preferable that the a-axis is 0.04×10 −１０ m or less, and the c-axis is more preferably 0.3 × 10 −１０ It is more preferable that the length is m or less.

[0352] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0353] Embodiment Mode 3 In this embodiment mode, each element constituting a battery will be described.

[0354] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include at least one of a conductive material and a binder. The positive electrode active material may be the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2.

[0355] The positive electrode current collector can be, for example, a metal foil. The positive electrode can be formed by applying a slurry to a metal foil and drying it. Pressing may be performed after drying. The positive electrode is formed by forming an active material layer on the positive electrode current collector.

[0356] The term "slurry" refers to a material liquid used to form an active material layer on a positive electrode current collector, and contains an active material, a binder, and a solvent, preferably further mixed with a conductive material. Note that the slurry is also called an electrode slurry or an active material slurry, and is sometimes called a positive electrode slurry when forming a positive electrode active material layer and a negative electrode slurry when forming a negative electrode active material layer.

[0357] The positive electrode active material has a function of taking in and releasing lithium ions during charging and discharging. The carbon-coated positive electrode active material 120 according to one embodiment of the present invention described above can be made of a material that is less susceptible to deterioration during charging and discharging even at a high charging voltage.

[0358] <Binder> As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0359] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0360] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0361] The binder may be used in combination with two or more of the above.

[0362] For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with other materials. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0363] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials or other components when preparing electrode slurries. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0364] Water-soluble polymers stabilize viscosity by dissolving in water, allowing active materials and other materials combined as binders, such as styrene-butadiene rubber, to be stably dispersed in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0365] When the binder covering or contacting the surface of the active material forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0366] <Conductive Material> The conductive material is also called a conductivity imparting agent or a conductivity aid, and is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0367] The active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably contain a conductive material.

[0368] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fibers such as carbon nanofibers and carbon nanotubes, and graphene compounds can be used.

[0369] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0370] In this specification and the like, graphene compounds include graphene, multilayer graphene, multi-graphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, etc. Graphene compounds refer to compounds that contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be referred to as a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0371] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0372] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the battery.

[0373] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the spaces between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. The battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and is stable, making it effective as an in-vehicle battery.

[0374] <Positive Electrode Current Collector> The current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable to use a current collector with a thickness of 5 μm to 30 μm.

[0375] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may further have a conductive material and a binder.

[0376] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0377] In addition, the negative electrode active material can be an element capable of undergoing a charge-discharge reaction through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, compounds containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements capable of undergoing charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0378] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0379] The carbon material may be graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, or the like.

[0380] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0381] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety compared to lithium metal.

[0382] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten oxide (WO ２ ), molybdenum oxide (MoO ２ ) and other oxides can be used.

[0383] In addition, as the negative electrode active material, a composite nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0384] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the composite nitride of lithium and a transition metal be used as the negative electrode active material, even when a material containing lithium ions is used as the positive electrode active material, by first removing the lithium ions contained in the positive electrode active material.

[0385] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ Fluorides such as:

[0386] Another example of a negative electrode is one that does not have a negative electrode active material at the end of the battery fabrication. A negative electrode that does not have a negative electrode active material can be, for example, a negative electrode that has only a negative electrode current collector at the end of the battery fabrication, in which lithium ions released from the positive electrode active material upon charging the battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

[0387] When a negative electrode without a negative electrode active material is used, a film for uniforming lithium deposition may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniforming lithium deposition. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as films for uniforming lithium deposition because they are relatively easy to form uniformly on the negative electrode current collector. Furthermore, for example, a metal film that forms an alloy with lithium can be used as the film for uniforming lithium deposition. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Lithium and magnesium form a solid solution over a wide composition range, making them suitable as films for uniforming lithium deposition.

[0388] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0389] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0390] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0391] [Electrolyte] The secondary battery has an electrolyte containing carrier ions. In this specification and the like, the electrolyte is not limited to an electrolyte containing an organic solvent that is liquid at room temperature, but also includes a solid electrolyte, and also includes an electrolyte containing both an organic solvent that is liquid at room temperature and a solid electrolyte that is solid at room temperature (semi-solid electrolyte). Note that a lithium salt dissolved in an organic solvent that is liquid at room temperature may be referred to as an electrolyte solution.

[0392] <Organic solvent that is liquid at room temperature> An example of an organic solvent that is liquid at room temperature will be described below.

[0393] The organic solvent that is liquid at room temperature is preferably an aprotic organic solvent, and for example, one or more of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate (PP), methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc. can be used.

[0394] By using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as an organic solvent that is liquid at room temperature, it is possible to prevent the battery cell from exploding or catching fire even if the internal temperature rises due to an internal short circuit or overcharging of the battery cell. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the organic solvent include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the organic solvent include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0395] The lithium salt dissolved in the organic solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４, LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), and LiN(C ２ F ５ SO ２ ) ２ One or more selected from the above can be used.

[0396] <Additives> The organic solvent may contain an additive. The additive can suppress reactive decomposition of the electrolyte that may occur on the positive electrode surface or the negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives that can be used include vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate)borate (LiBOB). LiBOB is particularly preferred because it easily forms a good coating. VC or FEC is preferred because it can form a good coating on the negative electrode during aging of the secondary battery or during charging in the early stages of use, thereby improving the cycle characteristics.

[0397] The compound represented by the following general formula (G1) may be contained as an additive: The compound represented by the following general formula (G1) has two cyano groups and can be called a dinitrile compound.

[0398]

[0399] In the above general formula (G1), R represents a hydrocarbon group having 1 to 5 carbon atoms.

[0400] Specific examples of the general formula (G1) include succinonitrile, glutaronitrile, and adiponitrile (ADN). Furthermore, the additive may include a compound having an ether group at the R position in the general formula (G1), such as ethylene glycol bis(propionitrile) ether (EGBE).

[0401] The structural formula (H1) of succinonitrile is shown below.

[0402]

[0403] The structural formula (H2) of glutaronitrile is shown below.

[0404]

[0405] The structural formula (H3) of adiponitrile is shown below.

[0406]

[0407] The structural formula (H4) of ethylene glycol bis(propionitrile) ether is shown below.

[0408]

[0409] As the additive, one or more dinitrile compounds can be used.

[0410] Fluorobenzene may also be added to the organic solvent. The concentration of the additive may be, for example, 0.1 wt% or more and 5 wt% or less relative to the total electrolyte solution. PS or EGBE are preferred because they form a good coating on the positive electrode during charge and discharge, improving cycle characteristics. FB is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes. Dinitrile compounds are preferred because their nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving high-voltage resistance. Furthermore, dinitrile compounds are preferred because they can prevent copper dissolution during overdischarge when a copper-containing current collector is used on the negative electrode. Considering the use of secondary batteries at high voltages, adding a nitrile compound is preferred.

[0411] The organic solvent does not need to be liquid at room temperature; a semi-solid material called a polymer gel electrolyte can be used instead. Using a polymer gel electrolyte increases safety against leakage and other issues. It also allows for thinner and lighter battery cells.

[0412] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0413] Examples of polymers that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0414] [Separator] When the electrolyte contains an electrolytic solution, a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those formed from cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or porous films made of nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, polyimide, or polyurethane. It is preferable that the separator be processed into a bag shape and disposed so as to enclose either the positive electrode or the negative electrode.

[0415] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene may be coated with a ceramic material, hydroxide material, fluorine material, polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles, silicon oxide particles, and magnesium oxide. Examples of hydroxide materials include magnesium hydroxide and aluminum hydroxide. Examples of fluorine materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid and para-aramid).

[0416] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0417] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0418] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0419] [Exterior Body] The exterior body of the battery can be made of a metal material such as aluminum, stainless steel, or titanium, or a resin material. A film-like exterior body can also be used. Examples of films include a three-layer structure in which a highly flexible metal thin film or metal foil such as aluminum, stainless steel, titanium, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the metal thin film as the outer surface of the exterior body. Such a multilayer structure film can be called a laminate film. In this case, the laminate film may be referred to as an aluminum (aluminum) laminate film, a stainless steel laminate film, a titanium laminate film, a copper laminate film, a nickel laminate film, or the like, using the name of the material of the metal layer.

[0420] The material or thickness of the metal layer of the laminate film may affect the flexibility of the battery. For example, an aluminum laminate film having a polypropylene layer, an aluminum layer, and a nylon layer is preferably used as an exterior body for batteries that prioritize flexibility or lightweight design. Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the aluminum layer is thinner than 10 μm, there is a concern that pinholes in the aluminum layer may reduce the gas barrier properties, so the thickness of the aluminum layer is preferably 10 μm or more.

[0421] For example, a stainless steel laminate film having a polypropylene layer, a stainless steel layer, and a nylon layer is preferably used as an exterior body for batteries where physical strength or safety is important. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the stainless steel layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the stainless steel layer is thinner than 10 μm, there is a concern that pinholes in the stainless steel layer may reduce the gas barrier property, so the thickness of the stainless steel layer is preferably 10 μm or more. In this specification, stainless steel refers to steel (an alloy of iron and carbon) containing approximately 12% or more chromium, and can be broadly classified into martensitic, ferritic, and austenitic types based on composition. It also includes stainless steels to which one or more elements selected from Ti, Nb, Mo, Cu, Ni, or Si are added.

[0422] Alternatively, for example, a titanium laminate film having a polypropylene layer, a titanium layer, and a nylon layer is preferably used. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the titanium layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the titanium layer is thinner than 10 μm, there is a concern that pinholes in the titanium layer may reduce the gas barrier properties, so the thickness of the titanium layer is preferably 10 μm or more.

[0423] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0424] Embodiment Mode 4 In this embodiment mode, examples of shapes of a secondary battery having a positive electrode manufactured by the manufacturing method described in the previous embodiment mode will be described.

[0425] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 14A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 14B is an external view, and Fig. 14C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0426] 14A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding, and therefore, FIGS. 14A and 14B are not completely corresponding views.

[0427] In Fig. 14A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. These are sealed with a negative electrode can 302 and a positive electrode can 301 by a gasket. Note that the gasket for sealing is not shown in Fig. 14A. The spacer 322 and the washer 312 are used to protect the inside or to fix the position inside the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 and the washer 312 are made of stainless steel or an insulating material.

[0428] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0429] FIG. 14B is a perspective view of the completed coin-type secondary battery.

[0430] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be formed of lithium metal foil or a lithium-aluminum alloy foil.

[0431] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0432] The positive electrode can 301 and the negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0433] These negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte solution, and as shown in FIG. 14C , the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303, thereby producing a coin-type secondary battery 300.

[0434] By having the above-described configuration, the coin-type secondary battery 300 can have a high discharge capacity and excellent cycle characteristics.

[0435] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 15A. As shown in Fig. 15A, a cylindrical secondary battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0436] 15B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 15B has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0437] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal, such as nickel, aluminum, or titanium, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel), which is corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat the battery can 602 with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The nonaqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0438] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector.

[0439] By using the carbon-coated positive electrode active material 120 described in embodiments 1 and 2, etc., for the positive electrode 604, a cylindrical secondary battery 616 can be obtained that has a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0440] A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature increases, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element is made of barium titanate (BaTiO ３ )-based semiconductor ceramics, etc. can be used.

[0441] 15C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. The positive electrodes of the respective secondary batteries are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of the respective secondary batteries are electrically connected to the control circuit 620 via wiring 626. The control circuit 620 may be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0442] 15D shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0443] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0444] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0445] 15D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0446] [Another Example of Secondary Battery Structure] An example of the structure of a secondary battery will be described with reference to FIGS. 16 and 17. FIG.

[0447] A secondary battery 913 shown in FIG. 16A has a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is immersed in an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in FIG. 16A , for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a resin material.

[0448] 16B, the housing 930 shown in Fig. 16A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 16B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0449] The housing 930a can be made of an insulating material such as organic resin. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to suppress shielding of the electric field by the secondary battery 913. Note that if the shielding of the electric field by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0450] 16C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0451] Alternatively, a secondary battery 913 may be provided that has a wound body 950a as shown in Fig. 17A. The wound body 950a shown in Fig. 17A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0452] By using the carbon-coated positive electrode active material 120 described in the first and second embodiments, etc., for the positive electrode 932, a secondary battery 913 can be obtained that has a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0453] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0454] 17B , the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0455] 17C , the wound body 950 a and the electrolyte are covered by the housing 930 to form the secondary battery 913. It is preferable to provide the housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0456] As shown in Fig. 17B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a secondary battery 913 with a larger discharge capacity. For other elements of the secondary battery 913 shown in Figs. 17A and 17B, refer to the descriptions of the secondary battery 913 shown in Figs. 16A to 16C.

[0457] 18A and 18B show examples of external views of a laminated secondary battery, which includes a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0458] 19A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and a negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. Note that the area or shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 19A .

[0459] <Method of Manufacturing Laminated Secondary Battery> An example of a method of manufacturing the laminated secondary battery whose external view is shown in FIG. 18A will be described with reference to FIGS. 19B and 19C.

[0460] First, the negative electrode 506, separator 507, and positive electrode 503 are stacked. FIG. 19B shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. This can also be called a laminate consisting of a negative electrode, a separator, and a positive electrode. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrodes 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0461] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0462] Next, as shown in Fig. 19C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0463] Next, the electrolyte solution is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0464] By using the carbon-coated positive electrode active material 120 described in the first and second embodiments, etc., for the positive electrode 503, a secondary battery 500 can be obtained that has a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0465] [Example of Battery Pack] An example of a secondary battery pack according to one embodiment of the present invention, which can be wirelessly charged using an antenna, will be described with reference to FIG.

[0466] Fig. 20A is a diagram showing the appearance of secondary battery pack 531, which has a thin rectangular parallelepiped shape (also called a thick flat plate shape). Fig. 20B is a diagram illustrating the configuration of secondary battery pack 531. Secondary battery pack 531 has circuit board 540 and secondary battery 513. Label 529 is affixed to secondary battery 513. Circuit board 540 is fixed with sticker 515. Secondary battery pack 531 also has antenna 517.

[0467] The interior of the secondary battery 513 may have a structure including a wound body or a laminated body.

[0468] 20B , the secondary battery pack 531 has a control circuit 590 on a circuit board 540. The circuit board 540 is electrically connected to the terminals 514. The circuit board 540 is also electrically connected to the antenna 517, one 551 of the positive and negative leads of the secondary battery 513, and the other 552 of the positive and negative leads.

[0469] Alternatively, as shown in FIG. 20C, the device may have a circuit system 590 a provided on the circuit board 540 and a circuit system 590 b electrically connected to the circuit board 540 via the terminals 514 .

[0470] The antenna 517 is not limited to a coil shape and may be, for example, a wire shape or a plate shape. Furthermore, antennas such as a planar antenna, an aperture antenna, a traveling wave antenna, an EH antenna, a magnetic field antenna, and a dielectric antenna may also be used. Alternatively, the antenna 517 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. In other words, the antenna 517 may function as one of the two conductors of a capacitor. This allows power to be exchanged not only by electromagnetic fields and magnetic fields but also by electric fields.

[0471] The secondary battery pack 531 includes a layer 519 between the antenna 517 and the secondary battery 513. The layer 519 has a function of, for example, shielding an electromagnetic field generated by the secondary battery 513. The layer 519 can be formed using, for example, a magnetic material.

[0472] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0473] Embodiment 5 In this embodiment, an example of a vehicle including a secondary battery of one embodiment of the present invention will be described.

[0474] The secondary battery can be applied to a typical vehicle, such as an automobile. Examples of the automobile include next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHEVs or PHVs). The secondary battery can be used as one of the power sources mounted on the automobile. The vehicle is not limited to an automobile. Examples of the vehicle include trains, monorails, ships, submersibles (deep-sea exploration vessels, unmanned submersibles), aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, rockets, and artificial satellites), electric bicycles, and electric motorcycles. The secondary battery of one embodiment of the present invention can be applied to these vehicles.

[0475] As shown in Fig. 21C , an electric vehicle is equipped with first batteries 1301a and 1301b as main driving secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0476] The internal structure of the first battery 1301a may be a wound type as shown in FIG. 16C or FIG. 17A, or a stacked type as shown in FIG. 18A or FIG. 18B.

[0477] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple secondary batteries, it is possible to extract large amounts of power. The multiple secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of secondary batteries is also called a battery pack.

[0478] In addition, in a secondary battery for vehicle use, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a in order to cut off power from multiple secondary batteries.

[0479] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0480] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0481] Next, the first battery 1301a will be described with reference to FIG. 21A.

[0482] FIG. 21A shows an example in which nine prismatic secondary batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (such as from the road surface), it is preferable to fix multiple secondary batteries using the fixing portions 1413 and 1414 and the battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0483] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS).

[0484] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, a metal oxide such as In-M-Zn oxide (wherein the element M is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, etc.) may be used as the metal oxide. In particular, the In-M-Zn oxide that can be used as the metal oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In—Ga oxide or an In—Zn oxide may be used as the metal oxide. The CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis oriented in a specific direction. The specific direction is the thickness direction of the CAAC-OS film, the normal direction to the surface on which the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. The crystalline regions are regions in which the atomic arrangement is periodic. When the atomic arrangement is considered as a lattice arrangement, the crystalline regions are also regions in which the lattice arrangement is aligned.

[0485] Note that "CAC-OS" has a mosaic structure in which a material is separated into a first region and a second region, and the first region is distributed throughout the film (hereinafter, also referred to as a cloud structure). That is, CAC-OS is a composite metal oxide having a structure in which the first region and the second region are mixed. However, it may be difficult to observe a clear boundary between the first region and the second region.

[0486] For example, in the case of a CAC-OS made of an In—Ga—Zn oxide, EDX mapping obtained using EDX confirms that the CAC-OS has a structure in which a region containing In as the main component (first region) and a region containing Ga as the main component (second region) are unevenly distributed and mixed.

[0487] When a CAC-OS is used in a transistor, the conductivity due to the first region and the insulating property due to the second region act complementarily, thereby providing the CAC-OS with a switching function (a function of turning on / off). That is, a CAC-OS has a conductive function in a part of the material and an insulating function in a part of the material, and functions as a semiconductor as a whole. By separating the conductive function and the insulating function, both functions can be maximized. Therefore, by using a CAC-OS in a transistor, a high on-current (I ｏｎ ), high field-effect mobility (μ), and good switching behavior can be achieved.

[0488] Oxide semiconductors have a variety of structures, each of which has different characteristics. The oxide semiconductor of one embodiment of the present invention may include two or more of an amorphous oxide semiconductor, a polycrystalline oxide semiconductor, an a-like OS, a CAC-OS, an nc-OS, and a CAAC-OS.

[0489] Furthermore, because the control circuit unit 1320 can be used in high-temperature environments, it is preferable to use transistors using oxide semiconductors. To simplify the process, the control circuit unit 1320 may be formed using unipolar transistors. Transistors using oxide semiconductors in the semiconductor layer have a wider ambient operating temperature range than single-crystal Si transistors, from −40° C. to 150° C., and their characteristics change less even when the secondary battery overheats than single-crystal Si transistors. The off-current of transistors using oxide semiconductors is below the lower limit of measurement even at 150° C., whereas the off-current characteristics of single-crystal Si transistors are highly temperature-dependent. For example, at 150° C., the off-current of single-crystal Si transistors increases, and the current on / off ratio is not sufficiently large. The control circuit unit 1320 can improve safety. Furthermore, combining the carbon-coated positive electrode active material 120 described in the first and second embodiments with a secondary battery using the positive electrode provides a synergistic effect in terms of safety. The secondary battery and control circuit unit 1320 using the carbon-coated positive electrode active material 120 described in the first and second embodiments as the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0490] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for the secondary battery to address causes of instability such as micro-short circuits. Functions for eliminating causes of instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of deterioration, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for the secondary battery can be made ultra-compact.

[0491] Furthermore, a "micro-short" refers to a minute short circuit within a secondary battery, which is not so severe that the positive and negative electrodes of the secondary battery are short-circuited and render it unable to be charged or discharged, but rather refers to a phenomenon in which a small amount of short-circuit current flows at the minute short-circuited part. Even if the short-circuit occurs in a relatively short period of time and in a small location, a large voltage change occurs, and this abnormal voltage value may affect subsequent estimations.

[0492] One of the causes of micro-short circuits is said to be that multiple charge and discharge cycles cause uneven distribution of the positive electrode active material, resulting in localized current concentration in parts of the positive electrode and negative electrode, causing parts of the separator to stop functioning, or the generation of by-products due to side reactions, resulting in micro-short circuits.

[0493] In addition to detecting micro-shorts, the control circuit 1320 can also be said to detect the terminal voltage of the secondary battery and manage the charge / discharge state of the secondary battery. For example, to prevent overcharging, it can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0494] Next, an example of a block diagram of the battery pack 1415 shown in FIG. 21A is shown in FIG. 21B.

[0495] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the secondary battery used and limits the upper limit of the current from the outside or the upper limit of the output current to the outside. The range between the lower limit and the upper limit of the secondary battery's voltage is within the recommended voltage range, and when the secondary battery falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and / or overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0496] The switch portion 1324 can be configured by combining n-channel transistors or p-channel transistors. The switch portion 1324 is not limited to a switch having a Si transistor using single crystal silicon. For example, the switch portion 1324 may be formed using a power transistor having Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), or GaOx (gallium oxide; x is a real number greater than 0). Furthermore, memory elements using OS transistors can be freely arranged by stacking them on circuits using Si transistors, and therefore integration can be easily achieved. Furthermore, OS transistors can be manufactured using the same manufacturing equipment as Si transistors, and therefore can be manufactured at low cost. That is, the control circuit portion 1320 using OS transistors can be stacked on the switch portion 1324 and integrated into a single chip. The volume occupied by the control circuit section 1320 can be reduced, which allows for miniaturization.

[0497] The first batteries 1301a and 1301b primarily supply power to onboard equipment in the 42V system (high-voltage HV system), while the second battery 1311 supplies power to onboard equipment in the 14V system (low-voltage LV system). Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Lead-acid batteries have the disadvantage of being more self-discharged than lithium-ion batteries and being prone to deterioration due to a phenomenon called sulfation. Using a lithium-ion battery as the second battery 1311 offers the advantage of being maintenance-free, but after prolonged use, e.g., three years or more, there is a risk of abnormalities occurring that are difficult to identify during manufacturing. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b have remaining capacity, the second battery 1311 is charged to maintain a full charge state by supplying power from the first battery to the second battery.

[0498] In this embodiment, an example is shown in which lithium ion batteries are used as both the first battery 1301a and the second battery 1311. The second battery 1311 may be a lead storage battery, an all-solid-state battery, or an electric double layer capacitor.

[0499] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 via the motor controller 1303 or the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0500] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the secondary battery used, and can perform rapid charging.

[0501] Although not shown, when the electric vehicle is connected to an external charger, the charger plug or charger connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a and 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a and 1301b via a control circuit unit 1320 to prevent overcharging. The charger plug or charger connection cable may also be provided with a control circuit. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU also includes a microcomputer. The ECU uses a CPU or a GPU.

[0502] External chargers installed at charging stations and the like include 100V-200V outlets, or three-phase 200V and 50kW. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0503] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0504] Furthermore, by using graphene as a conductive material, a secondary battery with significantly improved electrical characteristics can be realized, as a synergistic effect of suppressing capacity decline and maintaining high capacity even when the electrode layer is thickened and the amount of graphene supported is increased. This is particularly effective for secondary batteries used in vehicles, and it is possible to provide a vehicle with a long driving range, specifically a driving distance of 500 km or more per charge, without increasing the ratio of the weight of the secondary battery to the total weight of the vehicle.

[0505] In particular, the secondary battery of the present embodiment described above can increase the operating voltage of the secondary battery by using the carbon-coated positive electrode active material 120 described in Embodiments 1, 2, etc., and can increase the usable capacity as the charging voltage increases. Furthermore, by using the carbon-coated positive electrode active material 120 described in Embodiments 1, 2, etc., in the positive electrode, a secondary battery for a vehicle having excellent cycle characteristics can be provided.

[0506] Next, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0507] 15D, 17C, and 21A can be installed in a vehicle to realize next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Furthermore, the secondary battery can also be installed in agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft. The secondary battery of one embodiment of the present invention can be a high-capacity secondary battery. Therefore, the secondary battery of one embodiment of the present invention is suitable for miniaturization and weight reduction and can be suitably used in transportation vehicles.

[0508] 22A to 22D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 22A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a secondary battery is installed in a vehicle, an example of the secondary battery described in Embodiment 4 is installed in one or more locations. The automobile 2001 shown in FIG. 22A includes a battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. It is preferable that the automobile further include a charge control device electrically connected to the secondary battery module.

[0509] Furthermore, automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging may be performed using a predetermined charging method or connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The charging facility may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge an electric storage device mounted on automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0510] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0511] 22B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 is, for example, a four-cell unit of secondary batteries with a nominal voltage of 3.0 V to 5.0 V, with 48 cells connected in series for a maximum voltage of 170 V. Apart from the number of secondary batteries constituting the secondary battery module of the battery pack 2201, the transport vehicle 2002 has the same functions as those shown in FIG. 22A, and therefore a description thereof will be omitted.

[0512] FIG. 22C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, which is obtained by connecting in series one hundred or more secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less. Therefore, a secondary battery with minimal variation in characteristics is required. By using a secondary battery that uses the carbon-coated positive electrode active material 120 described in embodiments 1 and 2 as a positive electrode, a secondary battery with stable battery characteristics can be manufactured, enabling mass production at low cost from the standpoint of yield. Furthermore, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2202, the battery pack 2202 has the same functions as those shown in FIG. 22A , and therefore a description thereof will be omitted.

[0513] Fig. 22D shows, as an example, an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 22D has wheels for takeoff and landing, it can also be considered a type of transport vehicle, and has a battery pack 2203 that includes a secondary battery module formed by connecting multiple secondary batteries and the secondary battery module and a charge control device.

[0514] The secondary battery module of the aircraft 2004 has, for example, eight 4 V secondary batteries connected in series, with a maximum voltage of 32 V. Other than the number of secondary batteries constituting the secondary battery module of the battery pack 2203, the secondary battery module has the same functions as those in Fig. 22A, and therefore a description thereof will be omitted.

[0515] 22E shows, as an example, a satellite 2005 equipped with a secondary battery 2204. It is preferable that the secondary battery 2204 is mounted inside the satellite 2005 while being covered with a heat insulating material.

[0516] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0517] Embodiment 6 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted in a building will be described with reference to FIGS. 23A and 23B.

[0518] The house illustrated in FIG. 23A includes a power storage device 2612 including a secondary battery of one embodiment of the present invention, and a solar panel 2610. The power storage device 2612 is electrically connected to the solar panel 2610 via wiring 2611 or the like. The power storage device 2612 may also be electrically connected to a ground-mounted charging device 2604. The power obtained by the solar panel 2610 can be charged to the power storage device 2612. The power stored in the power storage device 2612 can be charged to a secondary battery included in the vehicle 2603 via the charging device 2604. The power storage device 2612 is preferably installed in an underfloor space. By installing the power storage device 2612 in the underfloor space, the space above the floor can be effectively utilized. Alternatively, the power storage device 2612 may be installed on the floor.

[0519] The power stored in the power storage device 2612 can also be supplied to other electronic devices in the house. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the power storage device 2612 of one embodiment of the present invention can be used as an uninterruptible power supply, enabling the use of electronic devices.

[0520] 23B illustrates an example of a power storage device according to one embodiment of the present invention. As illustrated in FIG. 23B , a power storage device 791 according to one embodiment of the present invention is installed in an underfloor space 796 of a building 799. Furthermore, by using a secondary battery including the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2, etc., in its positive electrode, as the power storage device 791, a synergistic effect on safety can be obtained. The secondary battery including the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2, etc., in its positive electrode can significantly contribute to preventing accidents such as fires caused by the power storage device 791 including the secondary battery.

[0521] A control device 790 is installed in the power storage device 791, and the control device 790 is electrically connected to the distribution board 703, the power storage controller 705 (also called the control device), the display 706, and the router 709 by wiring.

[0522] Electric power is sent from commercial power source 701 to distribution board 703 via service line attachment portion 710. Electric power is also sent to distribution board 703 from power storage device 791 and commercial power source 701, and distribution board 703 supplies the sent electric power to general load 707 and power storage load 708 via outlets (not shown).

[0523] The general load 707 is, for example, an electronic device such as a television or a personal computer, and the power storage load 708 is, for example, an electronic device such as a microwave oven, a refrigerator, or an air conditioner.

[0524] The power storage controller 705 includes a measurement unit 711, a prediction unit 712, and a planning unit 713. The measurement unit 711 has a function of measuring the amount of power consumed by the general load 707 and the power storage load 708 during a day (e.g., from midnight to midnight). The measurement unit 711 may also have a function of measuring the amount of power of the power storage device 791 and the amount of power supplied from the commercial power source 701. The prediction unit 712 has a function of predicting the amount of power demand to be consumed by the general load 707 and the power storage load 708 during the next day based on the amount of power consumed by the general load 707 and the power storage load 708 during the previous day. The planning unit 713 has a function of creating a plan for charging and discharging the power storage device 791 based on the amount of power demand predicted by the prediction unit 712.

[0525] The amount of power consumed by the general load 707 and the power storage load 708 measured by the measurement unit 711 can be confirmed on the display 706. It can also be confirmed on an electronic device such as a television or a personal computer via the router 709. It can also be confirmed on a mobile electronic device such as a smartphone or a tablet via the router 709. The amount of power demand for each time period (or each hour) predicted by the prediction unit 712 can also be confirmed on the display 706, the electronic device, or the mobile electronic device.

[0526] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0527] Embodiment 7 In this embodiment, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted on a motorcycle or a bicycle will be described as an example in which a secondary battery is mounted on a vehicle.

[0528] 24A illustrates an example of an electric bicycle using the power storage device of one embodiment of the present invention. The power storage device of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 24A. The power storage device of one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

[0529] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state detached from the bicycle in FIG. 24B . The power storage device 8702 includes a plurality of built-in storage batteries 8701, which are included in the power storage device of one embodiment of the present invention, and a display unit 8703 can display the remaining battery charge and other information. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery. The control circuit 8704 is electrically connected to the positive electrode and the negative electrode of the storage battery 8701. A synergistic effect in terms of safety can be obtained by combining the power storage device 8702 with a secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2, etc., as its positive electrode. The secondary battery and control circuit 8704 using the carbon-coated positive electrode active material 120 described in the first and second embodiments and the like for the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0530] 24C illustrates an example of a two-wheeled vehicle using the power storage device of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 24C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. The power storage device 8602 includes a plurality of secondary batteries each using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 for a positive electrode. This power storage device 8602 can have a high capacity and contribute to miniaturization.

[0531] 24C can store a power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

[0532] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0533] Embodiment 8 In this embodiment, an example of mounting a secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0534] 25A shows an example of a mobile phone. The mobile phone 2100 includes a display unit 2102 built into a housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. The mobile phone 2100 also includes a secondary battery 2107. By including the secondary battery 2107 using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode, a high capacity can be achieved, and a configuration that can accommodate space saving due to the miniaturization of the housing can be realized.

[0535] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0536] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0537] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0538] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0539] Furthermore, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a body temperature sensor or other human body sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0540] FIG. 25B illustrates an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is sometimes referred to as a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has a high energy density and is highly safe. Therefore, the secondary battery can be used safely for a long period of time and is suitable as a secondary battery to be mounted on the unmanned aerial vehicle 2300.

[0541] Fig. 25C shows an example of a robot. A robot 6400 shown in Fig. 25C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0542] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0543] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0544] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0545] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has a high energy density and is highly safe, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as the secondary battery 6409 to be mounted on the robot 6400.

[0546] 25D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, operation buttons 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0547] The cleaning robot 6300 can analyze images captured by the camera 6303 and determine the presence or absence of obstacles such as walls, furniture, or steps. Furthermore, if an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2, etc., as a positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery 6306 is suitable for use in the cleaning robot 6300.

[0548] 26A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used in daily life or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0549] For example, the secondary battery according to one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 26A . The eyeglasses-type device 4000 includes a frame 4000 a and a display unit 4000 b. Mounting the secondary battery on the temples of the curved frame 4000 a makes it possible to provide the eyeglasses-type device 4000 with a lightweight design, a good weight balance, and a long continuous use time. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has a high energy density and can realize a configuration that can accommodate space saving associated with a miniaturized housing.

[0550] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b or the earphone unit 4001c. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has a high energy density, and can realize a configuration that can accommodate space saving associated with a miniaturized housing.

[0551] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0552] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0553] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and a secondary battery can be mounted in an internal region of the belt portion 4006a. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has a high energy density, and can realize a configuration that can accommodate space saving associated with a miniaturized housing.

[0554] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided in the display portion 4005a or the belt portion 4005b. A secondary battery using the carbon-coated positive electrode active material 120 described in Embodiments 1 and 2 as a positive electrode has high energy density, and a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0555] The display unit 4005a can display not only the time but also various other information such as incoming emails or phone calls.

[0556] Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0557] FIG. 26B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0558] 26C shows a side view of the display portion 4005a. Fig. 26C shows a state in which a secondary battery 913 is built in the internal region. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913 is provided in a position overlapping with the display portion 4005a, and can have high density and high capacity, and is small and lightweight.

[0559] Since the wristwatch-type device 4005 is required to be small and lightweight, by using the carbon-coated positive electrode active material 120 described in embodiments 1 and 2, etc., in the positive electrode of the secondary battery 913, it is possible to obtain a high-energy density and small-sized secondary battery 913.

[0560] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0561] In this example, a carbon-coated positive electrode active material according to one embodiment of the present invention was prepared, and the powder was analyzed and its charge / discharge characteristics were evaluated.

[0562] [Preparation of Positive Electrode Active Material] The LCO sample prepared in this example will be described with reference to the preparation method shown in FIGS.

[0563] LiCoO in step S14 of FIG. ２ As the starting material, commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) containing cobalt as the transition metal M and no additional elements was prepared and sieved using an automatic sieving machine. For the initial heating in step S15, the lithium cobalt oxide was placed in a scabbard, covered, and heated at 850°C for 2 hours using a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.). Air (compressed air, thoroughly dried) was flowed through the furnace at a rate of 10 L / min. The flow rate was adjusted so that the differential pressure gauge indicated 5 Pa. The furnace was cooled at a rate of 200°C / hour, and the air flow was not stopped until the temperature reached 200°C.

[0564] In this example, Mg, F, Ni, and Al were added separately as additive elements according to step S21 and step S41 shown in Figures 5A and 5C. First, according to step S21 shown in Figure 5A, LiF was prepared as the F source, and MgF was prepared as the Mg source. ２ LiF:MgF was prepared. ２ were weighed out so as to have a molar ratio of 1:3, and sieved in an automatic sieving machine according to step S22. Next, LiF and MgF ２ The above was mixed in dehydrated acetone at a rotation speed of 500 rpm for 20 hours to prepare an additive element source (A1 source).

[0565] Next, in step S31, magnesium as the Al source was weighed out so as to be 1 mol % of cobalt, and the resulting mixture was dry-mixed with the initially heated lithium cobalt oxide. The mixture was stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron Co., Ltd.), and then sieved with an automatic sieving machine to obtain a mixture 903 (step S32).

[0566] Next, in step S33, the mixture 903 was heated. The heating conditions were 900° C. and 20 hours. During heating, the scabbard containing the mixture 903 was covered with a lid. By providing the lid, the inside of the scabbard was made into an oxygen-containing atmosphere, and the inflow and outflow of the oxygen could be blocked (O ２ The sheath was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed through the furnace at a rate of 10 L / min (O ２ Flow). The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge read 5 Pa. When the inside of the furnace was cooled, it was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C. In this way, a composite oxide containing Mg and F was obtained (step S34a).

[0567] Next, in step S51, the composite oxide and an additive element source (A2 source) were mixed. According to step S41 shown in FIG. 5C, nickel hydroxide that had undergone a pulverization process was prepared as the nickel source, and aluminum hydroxide that had undergone a pulverization process was prepared as the aluminum source, and these were used as the additive element source (A2 source). The nickel hydroxide and aluminum hydroxide were weighed so that the nickel content was 0.5 mol % of the cobalt, and the aluminum content was 0.5 mol % of the cobalt, and then dry-mixed with the composite oxide. Using a Picobond (manufactured by Hosokawa Micron), the mixture was stirred at a rotation speed of 3000 rpm for 10 minutes, yielding a mixture 904 (step S52).

[0568] Next, in step S53, the mixture 904 was heated. The heating conditions were 850°C and 10 hours. During heating, the sachet containing the mixture 904 was covered with a lid. The sachet was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Company Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed through the furnace at a rate of 10 L / min (O ２ The flow rate was adjusted so that the differential pressure gauge indicated 5 Pa. The furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C. In this manner, lithium cobalt oxide containing Mg, F, Ni, and Al was obtained (step S54). The positive electrode active material thus obtained was used as an LCO sample.

[0569] [Preparation of Carbon Quantum Dot Colloid Solution] The CQDs sample prepared in this example will be described with reference to the preparation method shown in FIG.

[0570] 2, commercially available 1,3,6-trinitropyrene (ANGENE INTERNATIONAL) was prepared. In step S141, pure water was prepared. In step S142, commercially available sodium sulfite (Kishida Chemical) was prepared.

[0571] Next, in step S151 of Fig. 2, the components were mixed and stirred so that the molar ratio of 1,3,6-trinitropyrene:water:sodium sulfite was 1:186862:21.

[0572] Thereafter, in step S152 of FIG. 2 , the mixture and a stirrer were placed in an autoclave having a glass inner cylinder and sealed. Next, the mixture was heated at 200°C for 12 hours while stirring with the stirrer. The pressure inside the inner cylinder during heating was in the range of 1.2 MPa to 1.5 MPa at 200°C. After heating, the mixture was left to cool, and the synthesized product inside the inner cylinder was filtered and the filtrate was collected (step S153). The carbon quantum dot colloidal solution prepared in this way in step S160 was used as a CQDs sample. The autoclave used was a Minireactor MS200-C manufactured by OM Labotec Co., Ltd.

[0573] [Preparation of Carbon-Coated Positive Electrode Active Material] The carbon-coated positive electrode active material prepared in this example will be described with reference to the preparation method shown in FIG.

[0574] 1, the CQDs sample prepared above was prepared in step S101. Furthermore, pure water was prepared in step S102. In step S103, the CQDs sample and pure water were mixed and stirred to prepare a CQDs dispersion in step S111.

[0575] 1, the LCO sample prepared as described above was prepared. In step S121, the LCO sample and the CQDs dispersion were mixed and stirred so that the weight ratio of the LCO sample to the CQDs sample was LCO sample:CQDs sample = 99.61:0.39.

[0576] Thereafter, in step S122, the sample was dried to remove water, and then heated in step S123. The heating conditions were 300°C for 3 hours. After drying, the sample was crushed (step S124) to obtain a carbon-coated cathode active material (step S130). The carbon-coated active material thus prepared was used as a carbon-coated LCO sample. Based on the weight of the residue obtained by drying the CQDs dispersion, the weight ratio of the dried material derived from the CQDs dispersion contained in the carbon-coated LCO sample was calculated to be 0.39%.

[0577] In Example 2, a discharge rate test and a charge-discharge cycle test were performed using the carbon-coated LCO sample prepared in Example 1 and an LCO sample (comparative example). In addition, as an XRD analysis during charging, changes in the crystal structure of the positive electrode active material during charging in the charge-discharge cycle test were investigated.

[0578] <Preparation of Positive Electrode> The above carbon-coated LCO sample was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. PVDF was previously dissolved in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, each positive electrode active material was mixed in a weight ratio of positive electrode active material:AB:PVDF = 96:2:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After the slurry was applied to the positive electrode current collector, the solvent was evaporated.

[0579] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process was performed using a roll press. The pressing process was performed under the conditions of a first pressing (linear pressure 210 kN / m) followed by a second pressing (linear pressure 1467 kN / m). The upper and lower rolls of the roll press were both set at 120°C.

[0580] A positive electrode was obtained by the above steps. The amount of the positive electrode active material supported per area of ​​the positive electrode was 14 mg / cm. ２ 15mg / cm or more ２ By this preparation method, a carbon-coated LCO positive electrode having a carbon-coated LCO sample was prepared.

[0581] A comparative positive electrode (LCO positive electrode) was also fabricated using the LCO sample in the same manner as above. The amount of the positive electrode active material carried per area of ​​the positive electrode was 14 mg / cm. ２ 15mg / cm or more ２ It was as follows.

[0582] <Fabrication of Half Cell> A coin-shaped half cell was fabricated using the above-mentioned positive electrode, lithium metal foil, separator, electrolyte, a coin cell positive electrode can, and a coin cell negative electrode can. The coin-shaped half cell had a CR2032 type shape (diameter 20 mm, height 3.2 mm).

[0583] The electrolyte was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 1 mol / L of lithium hexafluorophosphate (LiPF ６ ) was dissolved in a solution to which 2 wt % of vinylene carbonate (VC) was added as an additive.

[0584] A porous polypropylene film was used as the separator.

[0585] In this way, a carbon-coated LCO half-cell was fabricated using the carbon-coated LCO positive electrode. Similarly, an LCO half-cell was fabricated using the LCO positive electrode.

[0586] <Discharge Rate Test> Discharge rate tests were carried out on the carbon-coated LCO half-cell prepared above and the LCO (uncoated) half-cell of the comparative example.

[0587] The conditions for the discharge rate test are as follows: In the first charge / discharge cycle, the battery was charged at a constant current of 0.2 C to approximately 4.6 V, then charged at a constant voltage until the current reached 0.05 C, and then discharged at a constant current of 0.2 C to 3.0 V. In the second charge / discharge cycle, the battery was charged at a constant current of 0.5 C to approximately 4.6 V, then charged at a constant voltage until the current reached 0.05 C, and then discharged at a constant current of 0.1 C to 3.0 V. In the third charge / discharge cycle, the battery was charged at a constant current of 0.5 C to approximately 4.6 V, then charged at a constant voltage until the current reached 0.05 C, and then discharged at a constant current of 1.0 C to 3.0 V. In the fourth charge / discharge cycle, the battery was charged at a constant current of 0.5 C to approximately 4.6 V, then charged at a constant voltage until the current reached 0.05 C, and then discharged at a constant current of 2.0 C to 3.0 V. The current was 200 mA / g at 1 C. The temperature of the measurement environment was 25° C. The post-charge rest time from the completion of charge to the start of discharge and the post-discharge rest time from the completion of discharge to the start of charge were both 10 minutes.

[0588] The results of the discharge rate tests for the carbon-coated LCO half-cell and the LCO (uncoated) half-cell are shown in Figure 27. In the graph, the horizontal axis shows the measurement conditions (charge rate / discharge rate), and the vertical axis shows the discharge capacity under each measurement condition. The discharge capacity is calculated by dividing the discharge capacity of each half-cell by the weight of the positive electrode active material contained in each half-cell. Under all conditions, the carbon-coated LCO half-cell showed a higher discharge capacity than the LCO (uncoated) half-cell.

[0589] <Charge-Discharge Cycle Test> A charge-discharge cycle test was carried out using the carbon-coated LCO half-cell prepared above.

[0590] The conditions for the charge-discharge cycle test were a constant current charge of 0.5 C up to approximately 4.6 V, followed by a constant voltage charge until the current value reached 0.05 C. The discharge was a constant current discharge of 1 C down to 3.0 V. 1 C was set to 200 mA / g. The temperature of the measurement environment was 45°C. The post-charge rest time from the completion of charge to the start of discharge, and the post-discharge rest time from the completion of discharge to the start of charge, were each 10 minutes. In addition to the termination conditions described above...

Claims

A first step of mixing lithium cobalt oxide having magnesium, aluminum, and nickel with a carbon quantum dot dispersion to prepare a first mixture; a second step of drying the first mixture at a temperature of 90° C. or higher and 200° C. or lower for a time of 0.5 hours or higher and 3 hours or lower; a third step of heating the first mixture that has been subjected to the second step at a temperature of 200° C. or higher and 500° C. or lower for a time period of 1 hour or higher and 10 hours or lower; A method for producing a positive electrode active material. 　 In claim 1, Step 1a: mixing 1,3,6-trinitropyrene, water, and sodium sulfite to form mixture 1a; Step 2a: placing the mixture of step 1a in a sealed container and heating it at a temperature of 180°C or higher and 220°C or lower for a time period of 10 hours or higher and 24 hours or lower; Step 3a of filtering the mixture of step 1a that has been subjected to step 2a; The carbon quantum dot dispersion liquid prepared by a method including the steps of: collecting the filtrate obtained through the step 3a; and mixing the filtrate with water (step 4a). A method for producing a positive electrode active material. 　 In claim 1, Step 1b: heating the lithium cobalt oxide at a temperature of 850° C. or higher and 950° C. or lower for a time period of 2 hours or higher and 10 hours or lower; a step 2b of mixing a magnesium source with the lithium cobalt oxide that has been subjected to the step 1b to prepare a mixture 1b; a step 3b of heating the mixture 1b at a temperature of 850°C or more and 950°C or less for a time period of 2 hours or more and 60 hours or less; a step 4b of mixing a nickel source and an aluminum source with the mixture 1b that has been subjected to the step 3b to prepare a mixture 2b; and (5b) heating the mixture (2b) at a temperature of 800° C. or higher and 900° C. or lower for a time period of 2 hours or higher and 20 hours or lower. A method for producing a positive electrode active material.

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