Secondary battery and electrolyte solution

Abstract

Provided is a secondary battery or an electrolyte solution that exhibits excellent battery characteristics below the ice point. According to the present invention, a secondary battery includes an electrolyte solution that includes at least a lithium salt and a mixed solvent that includes a cyclic carbonate and a chain carbonate, the molar ratio (CH / CY) of the chain carbonate (CH) to the cyclic carbonate (CY) being greater than 1.5, and the concentration of the lithium salt per liter of the mixed solvent being 0.25–1 mol.

Description

Secondary batteries and electrolytes

[0001] One embodiment of the present invention relates to a secondary battery and an electrolyte. However, the technical field of the present invention is not limited to the secondary battery and the electrolyte. The technical field of the present invention can be any one of a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, and a vehicle. Furthermore, the technical field of the present invention can be a manufacturing method of any of the above. For example, the secondary battery of the present invention can be applied as a power source required for a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, and a vehicle. The electronic device includes an information terminal device equipped with the secondary battery, and the power storage device includes a stationary power storage device.

[0002] In recent years, there has been an increasing demand for higher performance lithium-ion secondary batteries. For example, when lithium-ion secondary batteries are used to power electric vehicles, they must be operated in low-temperature environments of around −20° C., and high levels of battery performance are required below freezing (0° C. or below). For example, to ensure ionic conductivity below freezing, the solvent of the electrolyte solution must have low viscosity.

[0003] To improve battery characteristics below freezing point, the self-diffusion coefficient of the electrolyte solvent at -20°C is 1.4 × 10 −１０ m ２ / sec or more 2.0×10 −１０ m ２ A structure having a chain carbonate with a self-diffusion coefficient of 1 / sec or less has been proposed (see Patent Document 1). The self-diffusion coefficient is a parameter that correlates with ionic conductivity.

[0004] To improve the low-temperature characteristics, lithium hexafluorophosphate (LiPF) was added to a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 15:85 or 20:80). ６) to a concentration of 1 mol / L, and then mixed with hexamethylcyclotrisiloxane in an amount of 0.3 mass% relative to the mixed solution (see Patent Document 2). Furthermore, although not related to battery characteristics below freezing, in order to improve high-temperature storage characteristics, LiPF5 was added to a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 30:70). ６ It has been proposed to prepare a basic electrolyte solution by adding difluoroacetic acid 2-propynyl ester to the basic electrolyte solution so that the concentration of difluoroacetic acid becomes 1.0 mol / liter (see Patent Document 3).

[0005] Although not related to battery characteristics below freezing, a mixed solvent of EC / EMC / TFETFPE = 30 / 40 / 30 (mass ratio) has been proposed as a solvent for the electrolyte to suppress corrosion of the aluminum-containing positive electrode current collector (see Patent Document 4). EC stands for ethylene carbonate, EMC stands for methyl ethyl carbonate, and TFETFPE stands for 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0006] Although not related to battery characteristics below freezing, an initial electrolyte solution containing cations with an ionic radius larger than that of lithium ions has been proposed to improve output characteristics (see Patent Document 5). Also, although not related to battery characteristics below freezing, a configuration has been proposed in which a carbonaceous material-containing negative electrode forms an intercalation compound with lithium and one or more alkali metals and / or alkaline earth metals with an atomic radius larger than that of lithium in order to provide a lithium ion secondary battery with high output characteristics and excellent high-temperature durability (see Patent Document 6).

[0007] It is important to take van der Waals forces into account when calculating graphite. Corrections related to dispersion forces such as van der Waals forces in computational science are reported in, for example, Non-Patent Document 1.

[0008] Although it is not related to the battery characteristics at sub-zero temperatures, in order to suppress gas generation during high-temperature storage, a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and fluorobenzene (volume ratio 2:7:1) was used as the electrolyte solvent, and LiPF was added to the mixed solvent of 0.1% by mass of bis(vinylsulfonyl)methane and 2% by mass of vinylene carbonate. ６ In addition, as a comparative example, a solution of LiPF in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and fluorobenzene in a volume ratio of 3:6:1 has been proposed as an electrolyte for lithium ion secondary batteries that has excellent storage characteristics at high temperatures, although it is not related to battery characteristics below freezing point. ６ A solution containing 1.15M of the above has been proposed (see Patent Document 8).

[0009] JP 2015-079747 A JP 2007-149656 A WO2019 / 150896 A JP 2023-63507 A JP 2005-302630 A JP 2018-056025 A JP 2011-138759 A JP 2010-118355 A

[0010] S. Grimme “Semiempirical GGA-type density functional constructed with a long-range dispersion correction.” Journal of Computational Chemistry 27.15 (2006): 1787-1799.

[0011] Although a solvent for an electrolyte solution has been proposed as in Patent Document 1, further improvement of the solvent is considered necessary to exhibit excellent battery characteristics below freezing. Patent Document 3 does not disclose the exhibiting of excellent battery characteristics below freezing, and the volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) in Patent Document 3 needs to be improved in order to exhibit excellent battery characteristics below freezing. Furthermore, Patent Documents 2 and 3 use additives such as hexamethylcyclotrisiloxane or 2-propynyl ester of difluoroacetic acid, and the low-temperature characteristics are not improved by the electrolyte solvent alone. Patent Documents 4 to 6 do not disclose the exhibiting of excellent battery characteristics below freezing. In view of these circumstances, an object of one embodiment of the present invention is to provide a novel mixed solvent as a solvent for an electrolyte solution to exhibit excellent battery characteristics below freezing.

[0012] Note that the description of these problems does not preclude the existence of other problems. Furthermore, one embodiment of the present invention does not necessarily solve all of these problems. Furthermore, problems other than these can be extracted from the description of this specification, drawings, claims, etc.

[0013] In view of the above problems, one embodiment of the present invention is a secondary battery having an electrolytic solution, the electrolytic solution including at least a mixed solvent and a lithium salt, the mixed solvent including a cyclic carbonate and a chain carbonate, a molar ratio (CH / CY) of the chain carbonate (CH) to the cyclic carbonate (CY) being greater than 1.5, and a concentration of the lithium salt being 0.25 mol or more and 1 mol or less per liter of the mixed solvent.

[0014] In one embodiment of the present invention, it is preferable that the cyclic carbonate is ethylene carbonate, the chain carbonate is ethyl methyl carbonate, and the molar ratio of ethyl methyl carbonate (EMC) to ethylene carbonate (EC) (EMC / EC) is 3 or more and 83 or less.

[0015] In one embodiment of the present invention, it is preferable that the cyclic carbonate is ethylene carbonate, the chain carbonate is ethyl methyl carbonate, and the molar ratio of ethyl methyl carbonate (EMC) to ethylene carbonate (EC) (EMC / EC) is 3 or more and 33 or less.

[0016] Another aspect of the present invention is a secondary battery having an electrolytic solution, the electrolytic solution having at least a mixed solvent and a lithium salt, the mixed solvent having a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, and the proportion of the cyclic carbonate is less than 29 mol% of the entire mixed solvent.

[0017] In another embodiment of the present invention, the mixed solvent preferably contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0018] Another embodiment of the present invention is a secondary battery including a negative electrode active material having a carbon material and an electrolyte solution, wherein the electrolyte solution includes at least a mixed solvent, a first salt, and a second salt, the first salt including a lithium salt, and the second salt including a cation having an ionic radius larger than that of a lithium ion, the mixed solvent including a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, and the proportion of the cyclic carbonate relative to the mixed solvent is 2 mol % to 25 mol %.

[0019] Another embodiment of the present invention is a secondary battery including a negative electrode active material having a carbon material and an electrolyte solution. The electrolyte solution includes at least a mixed solvent, a first salt, and a potassium salt. The first salt includes a lithium salt. The mixed solvent includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE). The proportion of EC relative to the mixed solvent is 2 mol % to 17 mol %.

[0020] In another embodiment of the present invention, the concentration of the lithium salt in the electrolyte is preferably higher than the concentration of the potassium salt.

[0021] Another aspect of the present invention is a secondary battery having a negative electrode active material containing a carbon material and an electrolyte solution, the electrolyte solution containing at least a mixed solvent, LiPF ６ and KFSI, and a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), in which the proportion of EC is 2 mol % or more and 17 mol % or less with respect to the mixed solvent.

[0022] In another embodiment of the present invention, LiPF in the electrolyte ６ The concentration of is preferably higher than the concentration of KFSI.

[0023] Another aspect of the present invention is a secondary battery having an electrolytic solution, the electrolytic solution including at least a mixed solvent and a lithium salt, the mixed solvent including a cyclic carbonate, a chain carbonate, and a fluorine-containing aromatic compound, and the proportion of the fluorine-containing aromatic compound relative to the total amount of the mixed solvent is 33 mol % or less.

[0024] In another embodiment of the present invention, the mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and fluorobenzene, and the proportion of fluorobenzene is preferably 11 mol % or more and 33 mol % or less based on the entire mixed solvent.

[0025] Another embodiment of the present invention further includes a positive electrode active material. The positive electrode active material preferably contains lithium cobalt oxide, magnesium, fluorine, aluminum, and nickel, and a surface layer portion of the positive electrode active material preferably contains magnesium and fluorine.

[0026] Another aspect of the present invention is an electrolyte solution comprising at least a mixed solvent and a lithium salt, wherein the mixed solvent comprises a cyclic carbonate and a chain carbonate, the molar ratio (CH / CY) of the chain carbonate (CH) to the cyclic carbonate (CY) is greater than 1.5, and the concentration of the lithium salt is 0.25 mol or more and 1 mol or less per liter of the mixed solvent.

[0027] In another embodiment of the present invention, it is preferable that the cyclic carbonate is ethylene carbonate, the chain carbonate is ethyl methyl carbonate, and the molar ratio of ethyl methyl carbonate (EMC) to ethylene carbonate (EC) (EMC / EC) is 3 or more and 83 or less.

[0028] In another embodiment of the present invention, it is preferable that the cyclic carbonate is ethylene carbonate, the chain carbonate is ethyl methyl carbonate, and the molar ratio of ethyl methyl carbonate (EMC) to ethylene carbonate (EC) (EMC / EC) is 3.7 or more and 33 or less.

[0029] In another embodiment of the present invention, the concentration of the lithium salt is preferably 0.5 mol or more and 1 mol or less per liter of the mixed solvent.

[0030] Another aspect of the present invention is an electrolyte solution comprising at least a mixed solvent and a lithium salt, the mixed solvent comprising a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, and the proportion of the cyclic carbonate is less than 29 mol% of the total mixed solvent.

[0031] In another embodiment of the present invention, the mixed solvent preferably contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0032] In another embodiment of the present invention, the mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and the proportion of ethylene carbonate is preferably 2 mol % or more and 9 mol % or less based on the entire mixed solvent.

[0033] In another embodiment of the present invention, the mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and the proportion of ethylene carbonate is preferably 2 mol % or more and 5 mol % or less based on the entire mixed solvent.

[0034] Another embodiment of the present invention is an electrolytic solution comprising at least a mixed solvent, a first salt, and a second salt, wherein the first salt comprises a lithium salt, and the second salt comprises a cation having an ionic radius larger than that of a lithium ion, the mixed solvent comprises a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, and the proportion of the cyclic carbonate is 2 mol % to 25 mol % relative to the mixed solvent.

[0035] Another embodiment of the present invention is an electrolyte solution comprising at least a mixed solvent, a lithium salt, and a potassium salt, wherein the mixed solvent comprises EC, EMC, and TTE, and the proportion of EC is 2 mol % to 17 mol % relative to the mixed solvent.

[0036] In another embodiment of the present invention, the concentration of the lithium salt in the electrolyte is preferably higher than the concentration of the potassium salt.

[0037] Another aspect of the present invention is a method for producing a sol- vent containing at least a mixed solvent, LiPF ６ and KFSI, and the mixed solvent contains EC, EMC, and TTE, and the proportion of EC is 2 mol % or more and 17 mol % or less relative to the mixed solvent.

[0038] In another embodiment of the present invention, LiPF in the electrolyte ６ The concentration of is preferably higher than the concentration of KFSI.

[0039] Another aspect of the present invention is an electrolyte solution comprising at least a mixed solvent and a lithium salt, wherein the mixed solvent comprises a cyclic carbonate, a chain carbonate, and a fluorine-containing aromatic compound, and the proportion of the fluorine-containing aromatic compound is 33 mol % or less relative to the total mixed solvent.

[0040] In another embodiment of the present invention, the mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and fluorobenzene, and the proportion of fluorobenzene is preferably 11 mol % or more and 33 mol % or less based on the entire mixed solvent.

[0041] In another embodiment of the present invention, the amount of the lithium salt is preferably 0.25 mol or more and less than 1 mol per liter of the mixed solvent.

[0042] In another embodiment of the present invention, the mixed solvent contains ethylene carbonate, and the proportion of ethylene carbonate is preferably 25 mol % or less based on the entire mixed solvent.

[0043] According to one embodiment of the present invention, a secondary battery or an electrolyte solution exhibiting excellent battery characteristics at least below freezing point can be provided. Also, according to one embodiment of the present invention, a secondary battery or an electrolyte solution exhibiting excellent battery characteristics at room temperature and below freezing point can be provided.

[0044] 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.

[0045] FIGS. 1A and 1B are diagrams illustrating a secondary battery according to one embodiment of the present invention. FIG. 2A is a diagram illustrating a secondary battery according to one embodiment of the present invention, and FIG. 2B is a diagram illustrating a cathode active material layer according to one embodiment of the present invention. FIG. 3 is a diagram illustrating a method for manufacturing a cathode active material. FIGS. 4A and 4B are diagrams illustrating a method for manufacturing a cathode active material. FIGS. 5A and 5B are cross-sectional views illustrating a cathode active material. FIGS. 6A, 6B, 6C, 6D, 6E, and 6F are cross-sectional views illustrating a cathode active material. FIG. 7 is a diagram illustrating the crystal structure of a cathode active material. FIG. 8 is a diagram illustrating the crystal structure of a conventional cathode active material. FIG. 9 is a diagram illustrating an XRD profile calculated from the crystal structure. FIG. 10 is a diagram illustrating an XRD profile calculated from the crystal structure. FIGS. 11A, 11B, 11C, 11D, 11E, 11F, and 11G are diagrams illustrating EDX-ray analysis distributions of two or more elements. FIG. 12A is an exploded perspective view of a coin-type secondary battery, FIG. 12B is a perspective view of the coin-type secondary battery, and FIG. 12C is a cross-sectional perspective view thereof. FIG. 13A shows an example of a cylindrical secondary battery. FIG. 13B shows an example of the internal structure of a cylindrical secondary battery. FIG. 13C shows an example of a plurality of cylindrical secondary batteries. FIG. 13D shows an example of a power storage system having a plurality of cylindrical secondary batteries. FIGS. 14A and 14B are diagrams illustrating an example of a secondary battery, and FIG. 14C is a diagram illustrating the internal structure of a secondary battery. FIGS. 15A, 15B, and 15C are diagrams illustrating an example of a secondary battery. FIGS. 16A and 16B are diagrams illustrating an example of a secondary battery. FIG. 17A shows an example of the configuration of a vehicle, FIG. 17B shows a battery pack, and FIG. 17C shows an example of the configuration of an electric vehicle and battery pack. FIGS. 18A, 18B, 18C, and 18D are diagrams illustrating an example of space equipment. 19A and 19B are graphs showing the battery characteristics of an example at room temperature (25°C) and below freezing (-30°C). FIG. 20 is a graph showing the battery characteristics of an example at room temperature (25°C) and below freezing (-30°C). FIG. 21 is a graph showing the battery characteristics of an example at room temperature (25°C). FIG. 22 is a graph showing the battery characteristics of an example at room temperature (25°C). FIG. 23 is a graph showing the battery characteristics of an example at room temperature (25°C) and below freezing (-30°C).FIG. 24 is a graph showing the battery characteristics of an example at room temperature (25°C) and below freezing (-30°C). FIGS. 25A and 25B are diagrams explaining a calculation model. FIGS. 26A and 26B are diagrams explaining the calculation model. FIGS. 27A and 27B are diagrams explaining the calculation model. FIGS. 28A and 28B are graphs explaining calculation results. FIGS. 29A and 29B are graphs explaining calculation results. FIG. 30 is a graph showing the battery characteristics of an example at below freezing (-30°C). FIG. 31 is a graph showing the battery characteristics of an example at below freezing (-30°C). FIG. 32 is a graph showing the battery characteristics of an example at below freezing (-30°C). FIG. 33 is a graph showing the battery characteristics of an example at room temperature (25°C).

[0046] The following description will explain the embodiments of the present invention with reference to the accompanying drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes can be made in form and detail without departing from the spirit and scope of the present invention. Therefore, in the embodiments described below, the same reference numerals are used in different drawings to indicate the same objects.

[0047] In this specification, ordinal numbers such as "first" and "second" are used for convenience and do not limit the number of components or the order of the components. The order of the components includes, for example, the order of processes or the order of stacking. In other words, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the claims. Furthermore, the ordinal numbers used in the examples of this specification may not match the ordinal numbers used in the claims. Furthermore, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the examples of this specification.

[0048] In this specification, the term "lithium ion secondary battery" may be referred to as a "lithium ion battery," and refers to a secondary battery that uses lithium ions as carrier ions. However, the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or Group 2 element ions may be used as carrier ions of the present invention, and specifically, sodium ions may be used. In this case, the present invention can be understood by replacing "lithium ions" with "sodium ions." Furthermore, when describing a configuration in which there is no limitation on the carrier ions, the term "secondary battery" or "battery" may be used.

[0049] In this specification etc., the electrolytic solution may be referred to as an electrolytic solution for a secondary battery. In this specification etc., the electrolytic solution may be referred to as an electrolyte. The electrolytic solution means that the solution is liquid at 25°C. The electrolyte also means that there is no limitation on the state at 25°C.

[0050] 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 placing a superscript bar above the number. However, due to formatting constraints, in this specification, instead of placing a bar above the number, a minus sign (-) may be placed before the number. 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).

[0051] In this specification and the like, the space group of a positive electrode active material or the like is identified by X-ray diffraction (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."

[0052] In this specification, if the anions have a structure in which three layers are stacked with a skew, such as ABCABC, it is referred to as a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. Furthermore, 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.

[0053] In this specification, the (001) plane and the (003) plane may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may be referred to as the C-plane, the basal plane, or the like. Furthermore, 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 (00l) plane. In this specification, the surface on which the lithium diffusion path is exposed, that is, the surface on which lithium is inserted and extracted (specifically, a surface other than the (00l) plane), may be referred to as the edge plane.

[0054] In this specification and the like, the cross-sectional shape of a particle is not limited to a circle, i.e., a sphere. The cross-sectional shape of a particle includes an ellipse, a rectangle, a trapezoid, a triangle, a square with rounded corners, an asymmetric shape, and the like. When there are multiple particles, the cross-sectional shapes of the particles may be different from each other.

[0055] In this specification, etc., when describing the characteristics of individual particles of a positive electrode active material in 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 a 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 including the same.

[0056] In this specification, particle size can be measured using a particle size distribution analyzer (laser diffraction particle size distribution analyzer) using a laser diffraction / scattering method. In this specification, median diameter (D50) can be used as the average particle size. D50 is the particle size at which the cumulative frequency accounts for 50% in the cumulative curve of the particle size distribution measurement results.

[0057] In this specification, 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. In this specification, the maximum particle size can be a particle size that can be confirmed on a cross section of a positive electrode measuring 100 μm square. Furthermore, 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 diameter at which the cumulative frequency accounts for 50% can be taken as D50.

[0058] In this specification, 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. 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 positive electrode active material is 148 mAh / g. Unless otherwise specified in this specification, the theoretical capacity is a value per weight of the positive electrode active material.

[0059] In this specification, the amount of lithium remaining in the positive electrode active material that can be inserted or removed is determined by Xα in the composition formula, for example, Li Ｘα CoO ２In the case of a positive electrode active material in a lithium ion secondary battery, Xα can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, when a lithium ion secondary battery equipped with a positive electrode active material containing lithium cobalt oxide is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or, it can be said that Xα=0.2. Ｘα CoO ２ The expression "Xα is small" means, for example, that 0.1<Xα≦0.24.

[0060] In this specification and the like, when properly synthesized lithium cobalt oxide before use in a positive electrode approximately satisfies the stoichiometric ratio, LiCoO ２ and Xα=1. The lithium cobalt oxide contained in the lithium ion secondary battery after discharge is also LiCoO ２ It can be said that Xα = 1. The state where discharge is completed (discharged state) here refers to a state where the voltage is 3.0 V or less or 2.5 V or less at a current of 100 mA / g or less. Unless otherwise specified in this specification, etc., the current indicates a value per weight of the positive electrode active material.

[0061] In this specification, Li Ｘα CoO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate Xα in the above should be measured under conditions where there is little or no influence of short circuit and / or thermal decomposition and / or electrolysis of the electrolyte. 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 Xα.

[0062] In this specification, the distribution of a certain element refers to a region in which the element is continuously detected within a range that is not noise. This region begins at a position above the detection limit. "Continuous detection within a range that is not noise" means that the element continues to be detected above the detection limit.

[0063] 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.

[0064] In this specification and the like, the peak refers to the maximum peak unless otherwise specified.

[0065] Unless otherwise specified, in this specification, the materials (positive electrode, negative electrode, electrolyte, separator, etc.) of a secondary battery are described in their pre-degradation state. Note that a decrease in discharge capacity due to pre-shipment inspection (sometimes referred to as aging treatment) 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.

[0066] Here, we will explain the flow of electrons and lithium ions during charging in a lithium-ion secondary battery. When a charger is connected and charging of the secondary battery begins, electrons are released from the positive electrode, causing an oxidation reaction, and electrons are supplied to the negative electrode, causing a reduction reaction. Lithium ions are then released from the positive electrode into the electrolyte, and the lithium ions move to the negative electrode. During discharge, a reduction reaction occurs at the positive electrode, and an oxidation reaction occurs at the negative electrode. In other words, in a secondary battery, the anode (positive electrode) and cathode (negative electrode) are interchanged during discharge and charging, and the oxidation reaction and reduction reaction alternate. Therefore, the electrode with a higher reaction potential is called the positive electrode, and the electrode with a lower reaction potential is called the negative electrode. Therefore, in this specification, the positive electrode is called the "positive electrode" and the negative electrode is called the "negative electrode" whether during charging or discharging.

[0067] As used herein, a full cell refers to a cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a cell assembled with lithium metal as the negative electrode (counter electrode).

[0068] Unless otherwise specified, in this specification, the charge voltage is expressed based on the potential of lithium metal. Furthermore, in this specification, a high charge voltage is, for example, a charge voltage of 4.6 V or higher, preferably 4.65 V or higher, more preferably 4.7 V or higher, even more preferably 4.75 V or higher, and most preferably 4.8 V or higher. In other words, in the case of a half cell using lithium metal as the counter electrode, a charge voltage of 4.6 V or higher is referred to as a high charge voltage.

[0069] In this specification, a high charging voltage is defined as a charging voltage of 4.5 V or higher, based on the potential when the negative electrode is made of a carbon material (e.g., graphite). That is, in the case of a full cell in which a carbon material (e.g., graphite) is used as the negative electrode, a charging voltage of 4.5 V or higher is referred to as a high charging voltage.

[0070] In this specification and the like, carbonate refers to a compound having at least one carbonate ester bond (—O—C(═O)—O—) in its molecular structure, and includes cyclic carbonates and chain carbonates unless otherwise specified. Furthermore, chain carbonates include both linear and branched chain carbonates.

[0071] In this specification and the like, a mixed solvent refers to a mixture of two or more solvents.

[0072] In this specification and the like, the phrase "A and / or B" may be used, which means "A," "B," or "A and B."

[0073] Embodiment 1 A secondary battery which is one embodiment of the present invention includes an electrolyte solution, and the electrolyte solution includes at least a mixed solvent and a lithium salt.

[0074] [Mixed Solvent] The mixed solvent contained in the electrolyte solution according to one embodiment of the present invention will be described. The mixed solvent preferably contains a cyclic carbonate and a chain carbonate. Both the cyclic carbonate and the chain carbonate have high dielectric constants, so when used in the mixed solvent, they can efficiently dissociate the lithium salt and further solvate the lithium ions to stabilize the electrolyte solution.

[0075] <Cyclic Carbonate> The cyclic carbonate refers to a carbonate having a ring structure. Specific examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, and vinylene carbonate (VC). Among these, EC and PC, which have high dielectric constants, are preferred as solvents for the electrolyte. Furthermore, in the case of a battery using graphite as the negative electrode active material, EC is more preferred. In addition, in the present invention, two or more types of cyclic carbonates may be used in combination.

[0076] <Chain carbonate> The chain carbonate refers to a carbonate that does not have a ring structure. Specific examples of the chain carbonate include ethyl methyl carbonate (EMC), diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, methyl butyl carbonate, and ethyl propyl carbonate. In the present invention, two or more types of chain carbonates may be used in combination.

[0077] <Fluorine-Containing Ether Compound> Specific examples of fluorine-containing chain ethers include 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (hereinafter referred to as TTE). These are all chain ether compounds. The coordination energy of the above-mentioned fluorine-containing ether compounds may be lower than that of cyclic carbonates, thereby achieving excellent battery performance below freezing. The coordination energy of the fluorine-containing ether compounds may also be lower than that of chain carbonates, thereby achieving excellent battery performance below freezing. Among the above-mentioned fluorine-containing ether compounds, TTE will be used in the examples described below. The coordination energy of TTE is lower than that of EC and lower than that of EMC, thereby achieving excellent battery performance below freezing.

[0078] <Ether Compound> Specific examples of ether compounds excluding those containing two or more ether bonds include dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethyl propyl ether, and ethyl tert-butyl ether. The coordination energy of the above-mentioned ether compounds may be lower than that of cyclic carbonates, and excellent battery characteristics can be obtained below freezing point. Furthermore, the coordination energy of ether compounds may be lower than that of chain carbonates, and excellent battery characteristics can be obtained below freezing point.

[0079] <Fluorine-Containing Aromatic Compound> The fluorine-containing aromatic compound is preferably a fluorine-containing aromatic compound having 9 or less carbon atoms. Specific examples of fluorine-containing aromatic compounds having 9 or less carbon atoms include fluorobenzene (FB), 1,2-difluorobenzene, 1,3-difluorobenzene, and 1,4-difluorobenzene. In addition, in the present invention, two or more fluorine-containing aromatic compounds having 9 or less carbon atoms may be used in combination. The coordination energy of the above-mentioned fluorine-containing aromatic compounds having 9 or less carbon atoms may be lower than that of cyclic carbonates, thereby achieving excellent battery characteristics below freezing points. The coordination energy of the fluorine-containing aromatic compounds having 9 or less carbon atoms may be lower than that of chain carbonates, thereby achieving excellent battery characteristics below freezing points. Among the above-mentioned fluorine-containing aromatic compounds having 9 or less carbon atoms, FB will be used in the examples described below. The coordination energy of FB is lower than that of EC and lower than that of EMC, thereby achieving excellent battery characteristics below freezing points.

[0080] [Mixing Ratio 1] When the mixing ratio of cyclic carbonate (CY):chain carbonate (CH) is within a certain range, excellent battery characteristics are exhibited below freezing. When the molar ratio is used as the mixing ratio, when the cyclic carbonate (CY):chain carbonate (CH) ratio satisfies 1:Z (where Z is greater than 1.5, preferably 3≦Z≦83, more preferably 5≦Z≦83), in other words, when the molar ratio of chain carbonate (CH) to cyclic carbonate (CY) (CH / CY) is greater than 1.5, more preferably 3 or more and 83 or less, more preferably 5 or more and 83 or less, excellent battery characteristics are exhibited below freezing, as can be seen from the examples described below.

[0081] When the battery characteristics at 25°C are considered in addition to the above-mentioned battery characteristics at sub-zero temperatures, it is preferable that the cyclic carbonate (CY):chain carbonate (CH) ratio satisfies 1:Z (3≦Z≦33, preferably 3.7≦Z≦16), in other words, the molar ratio of the chain carbonate (CH) to the cyclic carbonate (CY) (CH / CY) satisfies 3.7 or more and 33 or less, more preferably 3.7 or more and 16 or less. When the above numerical range for Z is satisfied, it is preferable to use ethylene carbonate (EC) as the cyclic carbonate and ethyl methyl carbonate (EMC) as the chain carbonate.

[0082] [Mixing ratio 2] The ratio of the cyclic carbonate in the mixed solvent is preferably less than 29 mol%, more preferably 2 mol% to 17 mol%, more preferably 2 mol% to 9 mol%, and even more preferably 2 mol% to 5 mol%. Satisfying this ratio is preferred because it results in excellent battery characteristics below freezing. This will be demonstrated in the examples below.

[0083] [Mixing ratio 3] The proportion of the fluorine-containing aromatic compound in the mixed solvent is preferably 33 mol% or less relative to the total mixed solvent. A more preferred range for the proportion of the fluorine-containing aromatic compound will be shown in the Examples below. When the above ratio is satisfied, excellent battery characteristics are exhibited below freezing point and also at 25°C, which is preferable. This will be shown in the Examples below.

[0084] To achieve excellent battery performance below freezing, it is desirable for the mixed solvent to have low viscosity below freezing, particularly below −20°C. The viscosity of the mixed solvent is related to the viscosity of each solvent. For example, cyclic carbonates tend to be more viscous than linear carbonates, or their viscosity below freezing tends to be higher than that at room temperature (25°C). Therefore, to achieve even better battery performance below freezing, the proportion of cyclic carbonate relative to the entire mixed solvent must be extremely low. However, without cyclic carbonate, the battery performance at room temperature (25°C) and below freezing is poor. Therefore, specifically, the proportion of cyclic carbonate is preferably greater than 0 and less than 6 mol%, and preferably greater than 0 and less than 3 mol%. Furthermore, because cyclic carbonates tend to have a higher freezing point than linear carbonates, as described above, it is preferable to make the proportion of cyclic carbonate extremely low relative to the entire mixed solvent, specifically greater than 0 and less than 6 mol%, and preferably greater than 0 and less than 3 mol%.

[0085] [Solvents that can be added] Another solvent may be added to the above-mentioned mixed solvent. Any aprotic solvent can be added as the other solvent that can be added to the mixed solvent. Specific examples include one or more selected from methyl formate, methyl acetate, ethyl acetate, methyl propionate (MP), ethyl propionate, propyl propionate, methyl butyrate, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, 1,3-dioxane, 1,4-dioxane, benzonitrile, tetrahydrofuran, sulfolane, and sultone.

[0086] The mixed solvent may further contain one or more fluorinated cyclic carbonates (sometimes referred to as fluorinated cyclic carbonates) and fluorinated chain esters (sometimes referred to as fluorinated chain esters). Both the fluorinated cyclic carbonates and the fluorinated chain esters have electron-withdrawing substituents, which lower the coordination energy, making them preferable.

[0087] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. Examples of fluorinated chain esters include methyl 3,3,3-trifluoropropionate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP."

[0088] The mixed solvent described above preferably has a low content of molecules other than organic molecules (hereinafter simply referred to as "impurities," including oxygen and water) and is highly purified. It is also preferable that the reaction by-products produced during synthesis are suppressed through appropriate purification. Specifically, the impurities are 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The water content can be detected by Karl Fischer titration.

[0089] [Lithium Salt] Next, the lithium salt of the electrolyte will be described. 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 For example, LiPF 5 , which is a fluoride, can be used in any proportion. ６ and LiBF ４ Since the safety of lithium-ion secondary batteries is improved by using LiPF as the lithium salt, ６ and LiBF ４ It is preferable to combine

[0090] The concentration of the lithium salt in the electrolyte is 0.25 mol to 1 mol, preferably 0.5 mol to 1 mol, preferably 0.25 mol to less than 1 mol, and preferably 0.5 mol to less than 1 mol per liter of the mixed solvent when the temperature of the mixed solvent is 25°C ± 5°C. "Per liter of mixed solvent" refers to 1 liter of the total amount of the mixed solvent. A high lithium salt concentration leads to a decrease in lithium ion conductivity due to an increase in viscosity in a low-temperature environment, so the above concentration range is preferred. Furthermore, if the lithium salt concentration is too low, there is a concern that the number of lithium ions in the electrolyte will decrease, so the above concentration range is preferred.

[0091] [Salts Other Than Lithium Salts] The electrolyte solution of the present invention may contain salts other than lithium salts. The salts other than lithium salts (second salts) will now be described. It is preferable to use a salt having a cation with an ionic radius larger than that of lithium ions as the second salt. The ionic radius can be determined by reference to the ionic radius values ​​listed in the Chemistry Handbook (edited by the Chemical Society of Japan). Specific examples of cations with an ionic radius larger than that of lithium ions include potassium, rubidium, cesium, calcium, strontium, barium, and lanthanum (or lanthanoids). Based on Example 1 described below, it is preferable to use one or more selected from potassium and barium as the cation with an ionic radius larger than that of lithium ions. Based on Example 4, it can be seen that the use of potassium exhibits excellent battery characteristics below freezing.

[0092] The anion of the second salt is not limited in any way, and may be, for example, bis(fluorosulfonyl)imide (FSI − ), bis(trifluoromethanesulfonyl)imide (TFSI − ), P.F. ６ − , ClO ４ − , B.F. ４ − , and SCN − An anion selected from the group consisting of:

[0093] When a potassium salt is used as the second salt, potassium bis(fluorosulfonyl)imide (KFSI), potassium bis(trifluoromethanesulfonyl)imide (KTFSI), KPF ６ , KClO ４ , K.B.F. ４ One or more potassium salts selected from KFSI, KFSI, and KSCN can be used. Based on Example 4 described later, it is found that KFSI is preferably used as the second salt in order to exhibit excellent battery characteristics below freezing point.

[0094] In the electrolyte, the total concentration of the lithium salt and the second salt is preferably 0.15 mol or more and less than 1.5 mol, and more preferably 0.2 mol or more and less than 1 mol, per liter of the mixed solvent when the temperature of the mixed solvent is 25°C ± 5°C. "Per liter of mixed solvent" refers to per liter of the total (total) amount of the mixed solvent. A high total concentration of the lithium salt and the second salt may lead to a decrease in lithium ion conductivity due to an increase in viscosity in a low-temperature environment, so the above concentration range is preferred. If the lithium salt concentration is too low, there is a concern that the number of lithium ions in the electrolyte may decrease, so the lithium salt concentration is preferably higher than the second salt concentration.

[0095] [Additives] The electrolyte solution according to one embodiment of the present invention may contain additives as long as it has the above-described configuration. The additives will now be described. The organic materials listed above as examples of the mixed solvent can be used as additives. Other organic materials that can be used as additives include one or more selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), tert-butylbenzene (TBB), lithium bis(oxalate)borate (LiBOB), and dinitrile compounds such as succinonitrile, adiponitrile, or suberonitrile. The concentration of the additive is preferably 0.1 wt % to 10 wt % of the total weight of the mixed solvent and the lithium salt. FEC, VC, or LiBOB are preferred additives because they are likely to form a good coating.

[0096] Among the additives mentioned above, 1,3-propane sultone (PS) is preferred because it has the same HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) levels as ethylene carbonate (EC), making it less susceptible to oxidation and reduction even when a high cutoff voltage is used as a charge / discharge condition. Furthermore, PS has the advantage of being less likely to gasify when decomposed on the surface of the positive electrode active material. The electrolyte preferably contains 0.25 wt % to 7.5 wt % of PS based on the combined weight of the mixed solvent and lithium salt.

[0097] [Ionic Conductivity] The electrolyte solution according to one embodiment of the present invention has an ionic conductivity of 1×10 at −30° C. −７ The ionic conductivity of the electrolyte solution here is the characteristic of a solution in which a lithium salt is dissolved in a mixed solvent. When a mixed solvent in which a lithium salt and a potassium salt are dissolved is used, the ionic conductivity of the electrolyte solution is the characteristic of a solution in which a lithium salt and a potassium salt are dissolved in a mixed solvent. When an additive is contained, the ionic conductivity is the characteristic of a state in which the additive is mixed. When the ionic conductivity is 1×10 −７ If the ionic conductivity is less than 10 S / cm, the secondary battery containing the electrolyte solution of the present invention may not exhibit sufficient battery characteristics below freezing point. Of course, there is no limitation on the ionic conductivity as long as the battery characteristics below freezing point are sufficient. The ionic conductivity is more preferably 5×10 −６ S / cm or more, and more preferably 1×10 −６ S / cm or more, and particularly preferably 1×10 −５ The method for measuring the ionic conductivity is not particularly limited, but a method for measuring by a complex impedance method is preferred.

[0098] [Viscosity] The viscosity of the electrolyte solution according to one embodiment of the present invention at 25°C is preferably 320 mPa·s or less. The viscosity of the electrolyte solution at 25°C refers to the characteristics of a lithium salt dissolved in a mixed solvent. When a mixed solvent containing a lithium salt and a potassium salt is used, the viscosity of the electrolyte solution at 25°C refers to the characteristics of a lithium salt and a potassium salt dissolved in the mixed solvent. When an additive is used, the viscosity refers to the characteristics of the electrolyte solution containing the additive. If the viscosity exceeds 320 mPa·s, the ionic conductivity may be insufficient, and a secondary battery containing the electrolyte solution of the present invention may not exhibit sufficient battery characteristics below freezing. Of course, the viscosity at 25°C is not particularly limited as long as the battery characteristics below freezing are sufficient. The viscosity at 25°C is more preferably 220 mPa·s or less, even more preferably 120 mPa·s or less, and most preferably 50 mPa·s or less. There are no particular limitations on the method for measuring the viscosity, but a method using a viscometer at 25°C is preferred.

[0099] The electrolyte solution of one embodiment of the present invention includes one injected into the interior of a secondary battery, specifically one impregnated into a separator, as described below. The electrolyte solution includes one that is already completed as an electrolyte solution when injected into a secondary battery, and one that is completed by injecting components of the electrolyte solution (e.g., mixed solvent, lithium salt) separately into a secondary battery and mixing them inside the secondary battery. Furthermore, the electrolyte solution of one embodiment of the present invention includes one recovered from a secondary battery that has undergone pre-shipment inspection.

[0100] Another embodiment of the electrolyte solution of the present invention includes an electrolyte solution that is already completed when injected into a secondary battery, and an electrolyte solution that is completed by injecting the components of the electrolyte solution (e.g., mixed solvent, lithium salt, second salt) separately into a secondary battery and mixing them inside the secondary battery. Furthermore, an embodiment of the electrolyte solution of the present invention includes an electrolyte solution recovered from a secondary battery that has undergone pre-shipment inspection, and also includes cases in which the second salt (cation) is not detected in the recovered electrolyte solution. The cation of the second salt may not be detected in the recovered electrolyte solution during pre-shipment inspection. This is thought to be because the cation may form a graphite intercalation compound (GIC) or a solid electrolyte interface (SEI). It is also thought that the cation may be below detection level due to the small amount of second salt added.

[0101] [Separator] The secondary battery according to one embodiment of the present invention is not limited to any particular separator, but one having high wettability with the above-described mixed solvent or with an electrolyte solution containing the mixed solvent is preferred. A specific compound having high wettability with the above-described mixed solvent electrolyte is an imide compound, and polyimide is a typical example. In this specification, wettability can be evaluated by contact angle. The contact angle is preferably measured according to JIS R3257. For example, the value measured at 25°C after 30 to 60 seconds of dropping 1 μL to 25 μL of electrolyte onto the separator member can be used. The contact angle can be measured from an image observed from a horizontal direction. The contact angle is preferably the average value of measurements at three or more points. In this specification, good wettability is defined as a contact angle of less than 30 degrees, preferably less than 20 degrees, and more preferably less than 10 degrees.

[0102] Furthermore, the separator can be made of one or more materials selected from cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polypropylene, polyester, acrylic, polyolefin, and polyurethane. One or more of these materials can be combined with polyimide. Polypropylene is particularly preferred because it can exhibit a shutdown function. The shutdown function occurs when a secondary battery generates abnormal heat, and a molten material (typically polypropylene) blocks the pores in the separator through which carrier ions pass.

[0103] The separator may have a laminated structure (referred to as a laminated structure) of any of the above-mentioned materials. The laminated structure includes two or more layers of the same material, such as two or more layers of polyimide.

[0104] The separator preferably has a coating layer on its surface. In order to maintain the wettability of the separator to the electrolyte, it is preferable that at least a portion of the separator's surface is exposed from the coating layer. Therefore, it is preferable to provide the coating layer at a selective position on the separator. The coating layer preferably contains an inorganic material, typically magnesium oxide, silicon oxide, or aluminum oxide, which may be in the form of particles. In other words, a layer in which particulate inorganic material is attached to the surface of the separator is included in the coating layer. It is also preferable to use a coating layer containing carbon, which can have a higher conductivity than the separator.

[0105] The separator may have recesses on its surface. A recess refers to a region where the thickness is reduced in a cross-sectional observation image. The recesses are preferably arranged in a striped pattern when viewed from above the separator. The coating layer can be provided only on the recesses. Alternatively, the coating layer can be provided only on the protrusions formed together with the recesses. The presence of recesses can improve the degree of electrolyte absorption, making them a preferable separator configuration.

[0106] The thickness of the separator is preferably 10 μm to 80 μm, more preferably 20 μm to 60 μm. The thickness of the separator can be measured at the center of a cross-sectional image of a secondary battery including the separator, for example. The thickness of the recessed portion is preferably 10 μm to 50 μm.

[0107] The shape of the separator is not limited, and may be, for example, a sheet. The separator may also be in the form of a bag, and a form in which either the positive electrode or the negative electrode is housed in the bag is also suitable as a separator.

[0108] [Secondary Battery] Next, a secondary battery having the above-described electrolyte and separator will be described with reference to Figures 1A to 2B. Figure 1A shows a state in which the components of the secondary battery 100 are stacked, and Figure 1B shows the components of the secondary battery 100 separated from each other. Figure 2A shows a cross-sectional view of the secondary battery 100, and Figure 2B shows the positive electrode active material layer 22 of the secondary battery 100.

[0109] The secondary battery 100 has a plurality of positive electrodes. In FIG. 1B , a first positive electrode 103 a and a second positive electrode 103 b are illustrated as the plurality of positive electrodes. The first positive electrode 103 a and the second positive electrode 103 b are collectively referred to as the positive electrode 103. However, the number of positive electrodes in the secondary battery 100 is not limited to two layers, and the secondary battery 100 may have a single layer or three or more layers.

[0110] The secondary battery 100 has a plurality of negative electrodes. In FIG. 1B , a first negative electrode 106 a, a second negative electrode 106 b, and a third negative electrode 106 c are illustrated as the plurality of negative electrodes. The first negative electrode 106 a, the second negative electrode 106 b, and the third negative electrode 106 c are collectively referred to as the negative electrode 106. However, the number of negative electrodes in the secondary battery 100 is not limited to three layers, and the secondary battery 100 may have a single layer, two layers, or four or more layers.

[0111] The secondary battery 100 has a separator between the negative electrode and the positive electrode. In FIG. 1B , the multiple separators, a first separator 105a, a second separator 105b, a third separator 105c, and a fourth separator 105d, are indicated by dashed lines. The first separator 105a, the second separator 105b, the third separator 105c, and the fourth separator 105d are collectively referred to as separator 105. However, the number of separators in the secondary battery 100 is not limited to four layers, and may be a single layer, two or three layers, or five or more layers. The multiple separators may be prepared as independent sheets as shown in FIG. 1B , or a continuous separator may also be used. The continuous separator is formed by preparing a separator having a larger area than the positive electrode and the negative electrode, and folding the separator appropriately to arrange the separator portions at positions corresponding to the first separator 105 a to the fourth separator 105 d. Since the separator is arranged in greater numbers than the positive electrode or the negative electrode, the thickness of the entire separator becomes thinner, which increases the capacity per volume of the secondary battery.

[0112] 2A is an example of a cross-sectional view of the secondary battery 100 taken along the dashed line AB in FIG. 1B. In FIG. 2A, the positive electrode 103, the separator 105, the negative electrode 106, the protrusion 31t, and the like will be used for explanation.

[0113] The positive electrode 103 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is a layer containing positive electrode active material particles and has a region in contact with the positive electrode current collector 21. The manufacturing process of the positive electrode 103 includes a press process, and in a positive electrode that has undergone this press process, recesses into which the positive electrode active material particles are pressed may be formed in a portion of the positive electrode current collector 21. As shown in FIG. 2A , the positive electrode active material layer 22 can be formed on both sides of the positive electrode current collector 21. This is called a double-sided coated structure. Furthermore, although not shown, the positive electrode active material layer 22 can be formed on only one side of the positive electrode current collector 21. This is called a single-sided coated structure.

[0114] The protrusion 21t shown in FIG. 1A is a part of the positive electrode current collector 21. In other words, the protrusion 21t is a region of the positive electrode current collector 21 where the positive electrode active material layer 22 is not provided. As shown in FIG. 1A, in the secondary battery 100, multiple protrusions 21t overlap each other to form an assembly. The assembly of the protrusions 21t is called a positive electrode tab. The positive electrode tab is joined to the positive electrode lead 107a at the joint 109a. Ultrasonic bonding can be used for the joining. As a result of the joining, they are electrically connected to each other. The positive electrode lead 107a can be made of a material selected from aluminum, nickel, titanium, or an alloy thereof. An insulating seal may be placed to surround the joint 109a and / or the positive electrode lead 107a. Kapton tape can be used as the insulating seal.

[0115] 2B shows an example of a cross-sectional view of the positive electrode active material layer 22. The positive electrode active material layer 22 has at least a positive electrode active material 10. The positive electrode active material layer 22 has an electrolyte solution 108. The electrolyte solution 108 has at least a mixed solvent and a lithium salt. The positive electrode active material layer 22 may have a second positive electrode active material 20. The positive electrode active material layer 22 may have a conductive material 41. Although not shown, the positive electrode active material layer 22 may have a binder. The positive electrode active material layer 22 may not have the second positive electrode active material 20. The positive electrode active material layer 22 may not have the conductive material 41. The positive electrode active material layer 22 may not have a binder.

[0116] The positive electrode active material 10 is preferably an active material having an average particle size of 5 μm or more and less than 10 μm and a maximum particle size of less than 20 μm, and it is preferable that the material satisfy the large diameter (also referred to as large particle size) requirement. Secondary particles may be used as the positive electrode active material 10, and it is preferable that the secondary particles satisfy the average particle size of 5 μm or more and less than 10 μm and a maximum particle size of less than 20 μm. The positive electrode active material 10 is preferably a LiMO having a layered rock salt type crystal structure. ２ (M is one or more selected from Fe, Ni, Co, Mn, and Al) can be used, and lithium cobalt oxide can be used as a representative example. Lithium cobalt oxide, which has good high-voltage charging characteristics, will be described in the second embodiment and onward.

[0117] The second positive electrode active material 20 contained in the positive electrode active material layer 22 preferably has an average particle size of 5 μm or less, more preferably 0.1 μm or more and 5 μm or less, and it is preferable to use an active material that satisfies the small diameter (also referred to as small particle size). The second positive electrode active material 20 is LiM2PO4 having an olivine-type crystal structure. ４ (M2 is one or more selected from Fe, Ni, Co, and Mn). ４ As an example, LiFePO ４ , LiNiPO ４ , LiCoPO ４ , LiMnPO ４ , LiFe ａ Ni ｂ P.O. ４ , LiFe ａ Co ｂ P.O. ４ , LiFe ａ Mn ｂ P.O. ４ , LiNi ａ Co ｂ P.O. ４ , LiNi ａ Mn ｂ P.O. ４ (a+b is 1 or less, 0<a<1, 0<b<1), LiFe ｃ Ni ｄ Co ｅ P.O. ４ , LiFe ｃ Ni ｄ Mn ｅ P.O. ４ , LiNiｃ Co ｄ Mn ｅ P.O. ４ (c+d+e is 1 or less, 0<c<1, 0<d<1, 0<e<1), LiFe ｆ Ni ｇ Co ｈ Mn ｉ P.O. ４ (f+g+h+i is 1 or less, 0<f<1, 0<g<1, 0<h<1, 0<i<1), etc., and lithium iron phosphate can be used as a representative example. In addition, it is preferable that the particle surfaces of second positive electrode active material 20 have a carbon layer.

[0118] The negative electrode 106 includes a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode active material layer 32 is a layer containing negative electrode active material particles and has a region in contact with the negative electrode current collector 31. The manufacturing process for the negative electrode 106 includes a press process, and in a negative electrode that has undergone this press process, recesses into which the negative electrode active material particles are pressed may be formed in a portion of the negative electrode current collector 31. As shown in FIG. 2A , the negative electrode active material layer 32 can be formed on only one side of the negative electrode current collector 31. In a negative electrode disposed in the outermost layer of the secondary battery 100, the negative electrode active material layer that is not disposed facing the positive electrode does not need to have a negative electrode active material layer because insertion and desorption of carrier ions is not performed or is difficult to perform. In other words, a single-sided coating is often used for the outermost negative electrode. Alternatively, a double-sided coating structure in which the negative electrode active material layer 32 is formed on both sides of the negative electrode current collector 31 may be used, although this is not shown. Preparing all negative electrodes with a double-sided coating structure is preferable for high productivity. In this case, a negative electrode having a double-sided coating structure can also be disposed on the outermost layer.

[0119] Furthermore, the negative electrode current collector 31 has a protrusion 31t. The protrusion 31t is a region where the negative electrode active material layer 32 is not provided. In FIG. 1B, a first protrusion 31ta, a protrusion 31tb, and a third protrusion 31tc are illustrated as multiple protrusions. The first protrusion 31ta, the protrusion 31tb, and the third protrusion 31tc are collectively referred to as the protrusion 31t. The protrusion 31t is also illustrated in FIG. 1A. The multiple protrusions 31t overlap each other to form an assembly. The assembly of the protrusions 31t is called a negative electrode tab. Note that in FIG. 2A, the protrusion 31t is shown separated from the other protrusions.

[0120] As shown in FIG. 1A , the negative electrode tab (protrusion 31t) is joined to the negative electrode lead 107b at a joint 109b. Ultrasonic bonding can be used for the joining. As a result of the joining, they are electrically connected to each other. The negative electrode lead 107b can be made of a material selected from nickel, copper, titanium, or an alloy thereof. An insulating seal may be placed so as to surround the joint 109b and / or the negative electrode lead 107b. Kapton tape can be used as the insulating seal.

[0121] The negative electrode active material layer 32 may further contain a binder. The negative electrode active material layer 32 may further contain a conductive material. Of course, the negative electrode active material layer 32 does not necessarily have to contain a binder or a conductive material. The binder and the conductive material will be described later.

[0122] In this specification and the like, a structure in which a plurality of positive electrodes, a plurality of negative electrodes, and a plurality of separators are stacked as shown in FIG. 1B is referred to as a stacked electrode.

[0123] [Exterior Body] Although not shown, the secondary battery 100 has an exterior body, in which the positive electrode, negative electrode, etc. are housed. The exterior body of the secondary battery 100 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 that can be used include a three-layer structure in which a 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.

[0124] For example, an aluminum laminate film having a polypropylene layer, an aluminum layer, and a nylon layer is preferably used as the exterior body. 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 more preferably 20 μm or less. Note that 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 desirably 10 μm or more.

[0125] For example, a laminate film having a polypropylene layer, a stainless steel layer, and a nylon layer, preferably laminated in this order, with the nylon layer being the outermost layer, is preferably used as an exterior body for a secondary battery 100 that prioritizes physical strength or safety. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer, with the polyethylene terephthalate layer preferably being the outermost layer. Furthermore, a stainless steel layer may be provided on the nylon layer, with the stainless steel layer preferably being the outermost layer. 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. Note that 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 properties, so the thickness of the stainless steel layer is preferably 10 μm or more. Note that, in this specification, stainless steel refers to steel (an alloy of iron and carbon) containing approximately 12% or more of chromium, and can be broadly classified into martensitic, ferritic, and austenitic types based on their composition. Furthermore, stainless steel also includes stainless steel to which one or more elements selected from Ti, Nb, Mo, Cu, Ni, and Si are added.

[0126] For example, it is preferable to use a titanium laminate film having a polypropylene layer, a titanium layer, and a nylon layer as the exterior. 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.

[0127] A secondary battery that uses a film as an exterior body is called a laminated secondary battery. Although not shown in the present embodiment, a can case may be used as the exterior body. For example, a secondary battery that uses a circular case is called a coin-type secondary battery. A secondary battery that uses a cylindrical case is called a cylindrical secondary battery.

[0128] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0129] Embodiment 2 A positive electrode active material according to one embodiment of the present invention will be described.

[0130] <<Method for Producing Positive Electrode Active Material>> Lithium cobalt oxide, which has good high-voltage charging characteristics, can be used for the positive electrode active material 10. A method for producing such positive electrode active material 10 will be described with reference to FIGS.

[0131] <Step S14> As shown in step S14, lithium cobalt oxide (LiCoO ２ ) is prepared. Pre-synthesized lithium cobalt oxide can be used. The median diameter (D50) of the lithium cobalt oxide is preferably 5 μm or more and 10 μm or less.

[0132] <Step S15> Next, as shown in step S15, the lithium cobalt oxide is heated. The heating in this step is called initial heating. By performing initial heating on lithium cobalt oxide synthesized in advance, lithium cobalt oxide with a smooth surface can be obtained. A state in which there is little foreign matter attached to the surface is called smooth.

[0133] Lithium cobalt oxide is placed in a container, which is then covered and heated. Heating conditions are preferably, for example, at a temperature of 750°C to 950°C for 1 hour to 3 hours. It is also preferable to flow oxygen through the heating furnace at a rate of 8 L / min to 12 L / min, creating a positive pressure within the heating furnace. After the initial heating, the heating furnace is cooled or naturally cooled at a rate of 180°C / hr to 220°C / hr.

[0134] Initial heating may cause lithium to be released from some of the lithium cobalt oxide, and initial heating is expected to have the effect of increasing the crystallinity of the lithium cobalt oxide.

[0135] In this initial heating, it is not necessary to prepare a lithium source, an additive element source, or a material that functions as a flux.

[0136] However, initial heating is not necessarily required, in which case this step can be omitted, thereby improving productivity.

[0137] <Step S20> Next, as shown in step S20, it is preferable to add an additive element to the lithium cobalt oxide. In this embodiment, the additive element is added in a plurality of steps. The additive element added first, as in step S20, is denoted as A1, and the additive element added second, as in step S40 described below, is denoted as A2. The step of adding additive element A1 will be described with reference to FIG. 4A.

[0138] 4A, an additive element source (Al source) to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the Al source.

[0139] The additional element A1 can be one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0140] When magnesium is selected as the additive element, the source of the additive element can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0141] When fluorine is selected as the additive element, the source of the additive element can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and 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.

[0142] 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.

[0143] The fluorine source may also be a gas, for example, fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F), nitrogen trifluoride (NF ３ ) or the like may be used and mixed in the atmosphere during the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0144] In the method for producing a positive electrode active material described with reference to FIGS. 3 and 4A, magnesium and fluorine are used as the additive element A1. Lithium fluoride (LiF) is prepared as a fluorine source, and magnesium fluoride (MgF ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２ The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ = γ:1 (0≦γ≦1.9), and LiF:MgF ２ = γ:1 (0.1≦γ≦0.5) is more preferable, and LiF:MgF ２ It is more preferable that γ=γ:1 (γ=0.33 or its vicinity). 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.

[0145] <Step S22> Next, in step S22 shown in FIG. 4A, the magnesium source and the fluorine source are pulverized and mixed. A wet method is preferably used for this step. When using the wet method, the magnesium source and the fluorine source are added to a solvent such as dehydrated acetone along with a medium, and pulverized and mixed at a rotation speed of 400 rpm to 600 rpm for 15 hours to 25 hours. The solvent is then removed by drying or the like to obtain a mixture containing the magnesium source and the fluorine source. To break down agglomerations in the mixture, it is preferable to sieve the mixture using a sieve with a mesh size of 200 μm to 400 μm.

[0146] 4A, the mixture is collected to prepare an Al source. The Al source shown in step S23 includes a plurality of starting materials and can be called a mixture.

[0147] The particle size of the mixture is preferably a median diameter (D50) of 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less. Such a finely powdered mixture makes it easy to uniformly adhere the mixture to the surfaces of lithium cobalt oxide particles when mixed with lithium cobalt oxide in a later step. Uniform adhesion of the mixture to the surfaces of lithium cobalt oxide particles is preferable because it makes it easy to uniformly distribute or diffuse the added elements in the lithium cobalt oxide after heating.

[0148] <Step S31> Next, in step S31 shown in FIG. 3 , the Al source and the initially heated lithium cobalt oxide are mixed. Mixing is preferably performed in a dry room with a dew point of −100° C. or higher and −10° C. or lower. To prevent the lithium cobalt oxide particles from being destroyed, the mixture is preferably stirred for 5 to 20 minutes at a rotation speed of 2000 to 4000 rpm using, for example, a Picobond (manufactured by Hosokawa Micron). Furthermore, a Nobilta rotor is preferably used for the Picobond. Furthermore, the ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the Al source is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably Co:Mg=100:y (0.3≦y≦3).

[0149] 3, the mixed materials are recovered to obtain a mixture 901. When recovering the materials, the materials may be crushed as needed, and then sieved using media having a particle size of 40 μm or more and 60 μm or less.

[0150] <Step S33> Next, in step S33 shown in Fig. 3, the mixture 901 is placed in a container, which is then covered and heated. Heating conditions are, for example, a temperature of 750°C to 950°C, preferably 830°C to 950°C, for 8 hours to 12 hours. It is also preferable to flow oxygen through the heating furnace at a rate of 8 L / min to 12 L / min, creating a positive pressure within the heating furnace. After heating, the heating furnace is cooled or naturally cooled at a rate of 180°C / hr to 220°C / hr.

[0151] Here, a supplementary note will be made regarding the heating temperature in this step. 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 is preferably the temperature at which interdiffusion between the lithium cobalt oxide and the additive element 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 is preferably 750° C. or higher.

[0152] 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 901. For example, LiF and MgF are used as the additive element source. ２ When LiF and MgF ２ The eutectic point of the alloy is around 742° C. Therefore, the lower limit of the heating temperature in step S33 is preferably 742° C. or higher, and typically 750° C.

[0153] Also, LiCoO ２ :LiF:MgF ２ The mixture 903 obtained by mixing the two components in a molar ratio of 100:0.33:1 had an initial melting temperature T ｉｍ is 779 ° C, the melting peak temperature T ｐｍ is 815°C, and the melting end temperature T ｅｍ Therefore, the lower limit of the heating temperature is more preferably 826°C or higher, and typically can be set to 830°C.

[0154] The upper limit of the heating temperature is set to be below the melting point of lithium cobalt oxide (1130°C). At temperatures near the melting point, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range. If the temperature is too high, the fluoride will evaporate and decrease. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, a temperature of 950°C or lower is more preferable.

[0155] In the fabrication method described in this embodiment, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be lowered below the melting point of lithium cobalt oxide, for example, to 750°C or higher and 950°C or lower, allowing additive elements such as magnesium to be distributed in the surface layer, thereby producing a positive electrode active material with excellent characteristics. If LiF evaporates, the flux effect may decrease. Therefore, it is preferable to cover the container to suppress the evaporation of LiF.

[0156] A further note on 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. When the lithium cobalt oxide is small, a lower temperature or a shorter heating time may be preferable than when it is large. When the median diameter (D50) of the lithium cobalt oxide in step S14 is 5 μm or more and 10 μm or less, the heating temperature is preferably, for example, 750° C. or more and 950° C. or less. The heating time is preferably, for example, 8 hours or more and 12 hours or less.

[0157] <Step S34> Next, in step S34 shown in FIG. 3, the heated material is recovered and crushed as necessary to obtain a composite oxide 902.

[0158] <Step S40> Next, in step S40 shown in FIG. 3 , an additive element source (A2 source) is prepared. The additive element A2 can be the additive element described in step S21. In method 1 for producing a positive electrode active material described in FIGS. 3 to 4B , nickel and aluminum are used as the additive element A2. Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source. As shown in steps S41 to S43 of FIG. 4B , the nickel source and the aluminum source can be pulverized to obtain the A2 source. The pulverization conditions can refer to the conditions in step S22.

[0159] 3, the composite oxide 902 and the A2 source are mixed. Mixing is preferably performed in a dry room with a dew point of −100° C. or higher and −10° C. or lower. To prevent the shape of the lithium cobalt oxide particles from being destroyed, the mixture is preferably stirred for 5 minutes to 20 minutes at a rotation speed of 2000 rpm to 4000 rpm using, for example, a Picobond (manufactured by Hosokawa Micron Co., Ltd.). It is also preferable to use a Nobilta rotor for the Picobond.

[0160] 3, the mixed materials are recovered to obtain a mixture 903. When recovering the materials, the materials may be crushed as needed, and then sieved using media having a particle size of 40 μm or more and 60 μm or less.

[0161] 3, the mixture 903 is heated. The heating conditions can be found in the description of step S33.

[0162] 3, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 10. At this time, it is preferable to further sieve the recovered particles. By the above steps, the positive electrode active material 10 can be produced.

[0163] The positive electrode active material 10 has a smooth surface. A positive electrode active material 10 with a smooth surface may be more resistant to physical damage caused by pressure or the like than a positive electrode active material that does not have a smooth surface.

[0164] The characteristics of the positive electrode active material 10 produced through the above-described steps will be described with reference to FIGS. 5A to 11G.

[0165] <Positive Electrode Active Material 10> Figures 5A and 5B are cross-sectional views of a positive electrode active material 10 according to one embodiment of the present invention. Figures 6A to 6C show enlarged views of the vicinity of A-B in Figure 5B. Figures 6D to 6F show enlarged views of the vicinity of C-D in Figure 5B.

[0166] 5A, the positive electrode active material 10 has a surface layer portion 10a and an inner portion 10b. In FIGS. 5A and 5B, a dashed line indicates an example of the boundary between the surface layer portion 10a and the inner portion 10b.

[0167] The surface layer 10a of the positive electrode active material 10 refers to a region within 20 nm, preferably within 10 nm, from the surface of the particle. Newly generated surfaces of the particle due to cracks and / or fissures may also be referred to as the surface. The surface layer 10a is synonymous with the near-surface, near-surface region, or shell.

[0168] The region deeper than the surface layer 10a of the positive electrode active material is referred to as the inner portion 10b, which is synonymous with the inner region or core.

[0169] Furthermore, when the positive electrode active material 10 has a layered rock salt crystal structure of space group R-3m, as shown in FIG. 5B , the surface layer portion 10a has an edge region 10a1 and a basal region 10a2. In FIGS. 5A and 5B , the line labeled (00l) represents the (00l) plane. The edge region 10a1 is a region having a plane that intersects with the (00l) plane. "Intersecting with the (00l) plane" means that the angle between the perpendicular to the (00l) plane and the normal to the surface of the positive electrode active material 10 is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less. The basal region 10a2 is a region having a surface parallel to the (00l) plane. Here, being parallel to the (00l) plane means that the angle formed between the perpendicular to the (00l) plane and the normal to the surface of the positive electrode active material 10 is 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0170] The surface of the positive electrode active material 10 refers to the surface of the composite oxide including the surface layer portion 10a and the inner portion 10b. Therefore, the positive electrode active material 10 is made of aluminum oxide (Al ２ O ３ ) and silicon oxide (SiO ２ The term "adhered metal oxide" does not include metal oxides attached to the surface of the positive electrode active material, such as the inner surface 10b, 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 inner surface 10b.

[0171] 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 the FFT pattern of a TEM image and the FFT pattern of a STEM image, etc. XRD, neutron beam diffraction, etc. can also be used as materials for determination. For example, in a HAADF-STEM image, if the angle between the bright line of the first crystal in the two regions and the bright line of the second crystal is 5 degrees or less, it can be said that the orientations of the respective crystals roughly coincide. Although it is possible to use the dark lines in a HAADF-STEM image, it is preferable to use bright lines that are easily visible when determining whether the orientations coincide.

[0172] The positive electrode active material 10 does not include any attached electrolyte solution, decomposition products of the electrolyte solution, organic solvent, binder, conductive material, or compounds derived from these.

[0173] <Containing Elements> The positive electrode active material 10 contains lithium, cobalt, oxygen, and an additional element A. That is, the positive electrode active material 10 contains lithium cobalt oxide (LiCoO ２ ) and the additional element A. The positive electrode active material 10 preferably has a crystal structure described below, and the composition of the lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0174] The positive electrode active material 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 10 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 for the positive electrode active material 10 to contain 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.

[0175] Furthermore, when the cobalt content of the transition metals in the positive electrode active material 10 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, Ｘα CoO ２ When Xα in the compound is small, the stability is better.

[0176] The additive element A contained in the positive electrode active material 10 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium. These additive elements A further stabilize the crystal structure of the positive electrode active material 10, as described below.

[0177] The additional element A does not necessarily have to contain magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, or beryllium.

[0178] For example, if the cathode active material 10 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, etc., are further enhanced. The weight amount of manganese contained in the cathode active material 10 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0179] The additive element A is preferably present as a solid solution in the positive electrode active material 10. For example, when performing a line analysis using STEM-EDX, the rising position of the peak at which the additive element A is detected in the depth direction is preferably located deeper than the rising position of the peak at which the transition metal M is detected, i.e., closer to the interior of the positive electrode active material 10.

[0180] The surface layer 10a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 10b. In addition, the atoms on the surface of the positive electrode active material 10 in the surface layer 10a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 10a is likely to become unstable, and can be said to be a region where deterioration of the positive electrode active material due to changes in the crystal structure is likely to begin. On the other hand, if the surface layer 10a can be made sufficiently stable, Li Ｘα CoO ２ Even when Xα in the inner portion 10b is small, for example, even when Xα is 0.24 or less, the layered structure of cobalt and oxygen octahedra in the inner portion 10b can be made less likely to break.Furthermore, displacement of the layers of cobalt and oxygen octahedra in the inner portion 10b can be suppressed.

[0181] In order to give the surface layer portion 10a a stable crystal structure, the surface layer portion 10a preferably contains an additive element A, and more preferably contains a plurality of additive elements A. Furthermore, the surface layer portion 10a preferably has a higher concentration of one or more selected from the additive elements A than the interior portion 10b. Furthermore, it is preferable that the one or more selected from the additive elements A contained in the positive electrode active material 10 have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive element A in the positive electrode active material 10 differs depending on the additive element A. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additive element A. Here, the concentration peak refers to the maximum value of the concentration in the surface layer portion 10a or within 20 nm from the surface.

[0182] [Distribution] The distribution of the additional element A will be described. Figures 6A to 6C are diagrams illustrating the edge region 10a1 of the positive electrode active material 10. Figures 6D to 6F are diagrams illustrating the basal region 10a2 of the positive electrode active material 10.

[0183] For example, some of the additive elements A, such as magnesium, fluorine, silicon, phosphorus, titanium, boron, and calcium, preferably have a concentration gradient that increases from the interior 10b toward the surface, as shown by the gradation in Figures 6A and 6D. The additive element A having such a concentration gradient will be referred to as the additive element Xa. The additive element Xa often corresponds to the additive element A1, but does not necessarily correspond to the additive element A1. The concentration gradient shown by the gradation in Figures 6A and 6D can be achieved depending on the diffusion rate rather than the timing of addition.

[0184] Another additive element A, such as aluminum or manganese, preferably has a concentration gradient and a concentration peak in a region deeper than the additive element Xa shown in FIGS. 6A and 6D, as shown by the hatch density in FIGS. 6B and 6E. The concentration peak may be present in the surface layer 10a or may be deeper than the surface layer 10a. For example, it is preferable that the peak be in a region of 5 nm to 50 nm from the surface toward the interior. An additive element having such a concentration gradient will be referred to as additive element Ya. Although additive element Ya often corresponds to additive element A2, it does not necessarily correspond to additive element A2. The concentration gradient shown by the hatch density in FIGS. 6B and 6E depends on the diffusion rate rather than the timing of addition.

[0185] As shown by the presence or absence of hatching and the density of the hatching in FIGS. 6C and 6F , another additive element, such as nickel or barium, may be clearly present in the edge region 10a1 but substantially absent in the basal region 10a2. Here, "clearly present" refers to a case in which the characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 10. "Substantially absent" refers to a case in which the characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 10. This also refers to the element being below the detection limit in STEM-EDX analysis. An additive element having such a distribution is referred to as additive element Za. While additive element Za often corresponds to additive element A2, it does not necessarily correspond to additive element A2. The concentration gradient shown by the density of the hatching in FIGS. 6B and 6E is determined depending on the diffusion rate rather than the timing of addition.

[0186] For example, magnesium ions, which are one of the additive elements Xa, are divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, and therefore more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 10a makes it easier to maintain the layered rock-salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. Ｘα CoO ２ When Xα is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, if the magnesium concentration in the surface layer portion 10a is high, it can be expected that the corrosion resistance against hydrogen fluoride produced by decomposition of the electrolyte will be improved.

[0187] 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 resistance components in secondary batteries. Furthermore, as the magnesium concentration in the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0188] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 10 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 10 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 10 using, for example, glow discharge mass spectrometry (GD-MS) or inductively coupled plasma mass spectrometry (ICP-MS), or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 10.

[0189] Furthermore, aluminum, one of the additive elements Ya, 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 suppresses the elution of surrounding cobalt, improving cycle characteristics. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as the additive element Ya can improve safety when the positive electrode active material 10 is used in a secondary battery. Furthermore, the positive electrode active material 10 can be made to have a crystal structure that is less likely to collapse even with repeated charging and discharging.

[0190] On the other hand, excessive aluminum may adversely affect lithium insertion and desorption. Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 10 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 10 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 10 here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 10 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 10.

[0191] Nickel, which is one of the additive elements Za, 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.

[0192] 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 change in volume 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 CoO ２It is thought that this is because they function as pillars supporting the layers.

[0193] 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.

[0194] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 10 is appropriate. For example, the number of nickel atoms in the positive electrode active material 10 is preferably greater than 0% and not greater than 7.5% of the total number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably greater than 0% and not greater than 4%. Alternatively, it is preferably greater than 0% and not greater than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0195] Furthermore, fluorine, one of the additive elements Xa, is a monovalent anion. When a portion of the oxygen in the surface layer 10a 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 10a of the positive electrode active material 10 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine occurs more smoothly. Therefore, when the positive electrode active material 10 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 10a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrogen fluoride. Furthermore, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of other additive element sources, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element sources.

[0196] 6A and 6C, when the surface layer 10a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. Ｘα CoO ２ Even when Xα in the surface layer 10a is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer 10a.

[0197] Furthermore, having additive elements with different distributions, such as additive element Xa, additive element Ya, and additive element Za, in combination is preferable because it can stabilize the crystal structure in a wider region. For example, when the positive electrode active material 10 has magnesium, which is one of the additive elements Xa, aluminum, which is one of the additive elements Ya, and nickel, which is one of the additive elements Za, it can stabilize the crystal structure in a wider region than when it has only one or two of the additive elements Xa, Ya, and Za. In this way, when the positive electrode active material 10 has additive elements Xa, Ya, and Za in combination, the surface stabilization can be sufficiently achieved by additive element Xa such as magnesium and additive element Za such as nickel, so additive element Ya 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.

[0198] 6C and 6F , it is preferable that the additive element Za is contained in a larger amount in the edge region 10a1 than in the basal region 10a2 (also referred to as being contained preferentially or selectively), because this improves the stability of the crystal structure of the edge region 10a1, where lithium ions enter and exit the positive electrode active material 10 during charging and discharging of the lithium ion secondary battery. Furthermore, when the additive element Za has the above-described distribution, for example, when the positive electrode active material 10 is lithium cobalt oxide, it is preferable because the effects of adding the additive element Za, such as a decrease in discharge voltage or a decrease in discharge capacity, can be minimized.

[0199] 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 10a. In particular, the presence of magnesium, nickel, and aluminum is highly effective in achieving a stable composition and crystal structure, and is therefore preferred. In particular, it is preferable that the surface layer portion 10a of the positive electrode active material 10 has a region where magnesium is distributed closer to the surface than aluminum. Furthermore, in addition to the region where magnesium and aluminum are distributed, it is most preferable that the surface layer portion 10a of the positive electrode active material 10 has a region where the nickel distribution and the magnesium distribution overlap in the edge region 10a1.

[0200] <Crystal structure> <Li Ｘα CoO ２ When Xα is 1, the positive electrode active material 10 is in a discharged state, that is, Li Ｘα CoO ２ In the case where Xα=1 in the above formula, 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 10b, which occupies the majority of the volume of the positive electrode active material 10, has a layered rock-salt type crystal structure.

[0201] On the other hand, the surface layer 10a of the positive electrode active material 10 preferably has a function of reinforcing the inner portion 10b, which is made up of an octahedron of the transition metal M and oxygen, so that the layered structure of the inner portion 10b is not destroyed even when lithium is released from the positive electrode active material 10 upon charging. Alternatively, the surface layer 10a preferably functions as a barrier film for the positive electrode active material 10. Alternatively, the surface layer 10a, which is the outer periphery of the positive electrode active material 10, preferably reinforces the positive electrode active material 10. Here, "reinforcement" refers to suppressing structural changes in the surface layer 10a and inner portion 10b of the positive electrode active material 10, such as oxygen release, and / or suppressing oxidative decomposition of the electrolyte on the surface of the positive electrode active material 10.

[0202] Therefore, it is preferable that the surface layer portion 10a has a different crystal structure from the inner portion 10b. Furthermore, it is preferable that the surface layer portion 10a has a composition and crystal structure that are more stable at room temperature (25°C) than the inner portion 10b. For example, it is preferable that at least a portion of the surface layer portion 10a of the positive electrode active material 10 has a rock salt type crystal structure. Alternatively, it is preferable that the surface layer portion 10a has both a layered rock salt type crystal structure and a rock salt type crystal structure. Alternatively, it is preferable that the surface layer portion 10a has characteristics of both a layered rock salt type crystal structure and a rock salt type crystal structure.

[0203] 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 portion 10a than in the interior portion 10b, they are also preferably present randomly and dilutely in the interior portion 10b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior portion 10b, 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 portion 10b, 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, and a synergistic effect of suppressing magnesium elution can be expected.

[0204] It is also preferable that the crystal structure continuously changes from the interior 10b toward the surface due to the concentration gradient of the added element A. Alternatively, it is preferable that the crystal orientation of the surface layer 10a and the interior 10b are substantially the same.

[0205] For example, it is preferable that the crystal structure continuously changes from the layered rock salt type interior 10b toward the surface and surface layer portion 10a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer portion 10a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure and the layered rock salt type interior 10b are approximately the same.

[0206] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal M such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted rock-salt crystal lattice structure, which may result in a lower symmetry than the rock-salt crystal structure.

[0207] 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.

[0208] The presence of both the layered rock salt type crystal structure and the rock salt type crystal structure can be determined by electron beam diffraction, TEM images, cross-sectional STEM images, and the like.

[0209] 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 set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the rock salt type in an ideal state, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO 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. ２In 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.

[0210] 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.

[0211] The layered rock salt crystal and the anions in the rock salt crystal have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3'-type crystal, which will be described later, also have a cubic close-packed structure. Therefore, when the layered rock salt crystal and the rock salt crystal come into contact with each other, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0212] 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.

[0213] 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.

[0214] <Li Ｘα CoO ２ The positive electrode active material 10 has the above-described distribution of the additive element A and / or the crystal structure in the discharged state, and therefore, Ｘα CoO ２ The crystal structure with a small Xα in the lithium ion-containing layer differs from that of conventional positive electrode active materials in that the magnesium present in the lithium site maintains the R-3m layered rock salt crystal structure. Note that a small Xα here means that 0.1<Xα≦0.24.

[0215] 7 to 10, Li Ｘα CoO ２ The change in the crystal structure accompanying the change in Xα in the positive electrode active material 10 will be described by comparing a conventional positive electrode active material with the positive electrode active material 10.

[0216] The change in the crystal structure of the conventional positive electrode active material is shown in FIG. 8. The conventional positive electrode active material shown in FIG. 8 is a lithium cobalt oxide (LiCoO ２ )

[0217] In Figure 8, R-3m O3 is added to Li Ｘα CoO ２ This shows the crystal structure of lithium cobalt oxide with Xα=1 in the figure. In this crystal structure, 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.

[0218] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when Xα is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO in the unit cell. ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0219] Furthermore, when Xα=0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0220] Furthermore, when Xα 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 this crystal structure is a structure in which the structure of and the structure of 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 8, 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.

[0221] As an example of the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, a unit cell with a small GOF (goodness of fit) value can be adopted.

[0222] Li Ｘα CoO ２When conventional lithium cobalt oxide is repeatedly charged and discharged so that Xα in the charge-discharge cycle becomes 0.24 or less, the crystal structure of the lithium cobalt oxide repeatedly changes (i.e., undergoes a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0223] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 8, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0224] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0225] 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.

[0226] Therefore, when conventional lithium cobalt oxide is repeatedly charged and discharged so that Xα becomes 0.24 or less, the crystal structure of the 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.

[0227] On the other hand, in the positive electrode active material 10 shown in FIG. Ｘα CoO ２ The change in the crystal structure between the discharged state where Xα is 1 and the state where Xα is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Also, the volume change per cobalt atom can be reduced. Therefore, the positive electrode active material 10 is resistant to breakdown of its crystal structure even when repeatedly charged and discharged so that Xα is 0.24 or less, and excellent cycle characteristics can be achieved. In addition, the positive electrode active material 10 has a low ionic strength and a high ionic strength. ＸαCoO ２ In the state where Xα is 0.24 or less, the positive electrode active material 10 can have a more stable crystal structure than conventional positive electrode active materials. Ｘα CoO ２ When Xα is maintained at 0.24 or less, the safety of the secondary battery is further improved, which is preferable.

[0228] Li Ｘα CoO ２ The crystal structure of the inner portion 10b of the positive electrode active material 10 when Xα is about 1 and 0.2 is shown in FIG. 8. The inner portion 10b occupies the majority of the volume of the positive electrode active material 10 and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0229] When Xα = 1, positive electrode active material 10 has the same R-3m O3 crystal structure as conventional lithium cobalt oxide. However, when Xα is 0.24 or less, for example, about 0.2 or 0.12, which is the value at which conventional lithium cobalt oxide has an H1-3 type crystal structure, positive electrode active material 10 has a different crystal structure.

[0230] When Xα is about 0.2, the positive electrode active material 10 has a crystal structure belonging to the trigonal space group R-3m. ２ The layer symmetry 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 7 with the notation R-3m O3'.

[0231] The O3' type crystal structure has the coordinates of cobalt and oxygen in the unit cell as Co(0,0,0.5), O(0,0,x Ｏ ), 0.20≦x Ｏ The lattice constant of the unit cell can be expressed in the range of 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 −１nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0232] 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.

[0233] As shown by the dotted line in FIG. 7, the difference between R-3m(O3) in the discharged state and the O3′-type crystal structure is ２ There is almost no layer misalignment.

[0234] 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%.

[0235] In this way, in the positive electrode active material 10, Li Ｘα CoO ２ When Xα is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the crystal structure of the positive electrode active material 10 is resistant to collapse even when repeatedly charged and discharged so that Xα is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 10 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 10, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0236] The positive electrode active material 10 is Li Ｘα CoO ２ It has been confirmed that when Xα 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 Xα is greater than 0.24 and less than 0.27, it has an O3' type crystal structure. However, the crystal structure is Ｘα CoO ２ Since Xα is affected by not only Xα in the battery but also the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., Xα is not necessarily limited to the above range.

[0237] Therefore, the positive electrode active material 10 is Li Ｘα CoO ２ When Xα is greater than 0.1 and equal to or less than 0.24, the entire interior 10b of the positive electrode active material 10 does not have to have the O3′-type crystal structure, but may contain other crystal structures, or may be partly amorphous.

[0238] Also Li Ｘα CoO ２ In order to make the Xα in the lithium ion battery small, it is generally necessary to charge the battery at a high charging voltage. Ｘα CoO ２ The state where Xα is small can be said to be a state where the battery is charged at a high charging voltage. For example, when a conventional positive electrode active material is charged at a constant current / constant voltage (CC / CV) at a voltage of 4.6 V or higher relative to the potential of lithium metal in a 25°C environment, an H1-3 crystal structure appears. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be said to be a high charging voltage.

[0239] Therefore, positive electrode active material 10 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.

[0240] In some cases, the H1-3 crystal structure is finally observed when the charge voltage is further increased, even in positive electrode active material 10. As described above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, temperature, 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 but lower than 4.6 V at 25° C., positive electrode active material 10 of one embodiment of the present invention may be able to adopt the O3′ crystal structure.

[0241] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0242] In addition, in O3' of FIG. 7, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, in the monoclinic O1 (Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0243] 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 are usually CdCl ２ It is known that it does not have a typical crystal structure.

[0244] Furthermore, it is preferable that the concentration gradient of the additive element A is similar at multiple locations in the surface layer portion 10a of the positive electrode active material 10. In other words, it is preferable that the reinforcement derived from the additive element A is uniformly present in the surface layer portion 10a. Even if a portion of the surface layer portion 10a 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 10, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0245] However, the additional element A does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 10a of the positive electrode active material 10. It is preferable that the additional element A has the distribution of the additional element Xa shown in Fig. 6D and the distribution of the additional element Ya shown in Fig. 6E.

[0246] Here, the vicinity of C-D has an R-3m layered rock salt type crystal structure, and the surface has a (00l) orientation. The (00l)-oriented surface may have a different distribution of the additional element A than the other surfaces. For example, the (00l)-oriented surface and its surface layer 10a may have a distribution of the concentration of one or more elements selected from the additional element A limited to a shallower portion from the surface compared to surfaces other than the (00l) orientation. Alternatively, the (00l)-oriented surface and its surface layer 10a may have a lower concentration of one or more elements selected from the additional element A compared to other orientations. Alternatively, the (00l)-oriented surface and its surface layer 10a may have one or more elements selected from the additional element A below the lower detection limit.

[0247] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is such that the lithium ion diffusion path is parallel to the (00l) plane.

[0248] CoO ２ Since the layer is relatively stable, the surface of the positive electrode active material 10 is more stable when it is oriented in the (00l) direction. The main diffusion path of lithium ions during charge and discharge is not exposed on the (00l) plane.

[0249] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (00l) orientation. Therefore, the surface and the surface layer portion 10a other than the (00l) 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 portion 10a other than the (00l) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 10.

[0250] <Grain Boundary> In addition to the distribution as described above, it is more preferable that at least a portion of the additive element A contained in the positive electrode active material 10 is unevenly distributed, i.e., present at a high concentration, in and around the grain boundary. The vicinity here refers to a region up to 10 nm inward from the grain boundary.

[0251] For example, the magnesium concentration at and near the grain boundaries of the positive electrode active material 10 is preferably higher than that in other regions of the interior 10b. The fluorine concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 10b. The nickel concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 10b. The aluminum concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 10b.

[0252] Grain boundaries are one type of planar defect. Therefore, like surfaces, they are prone to become unstable and changes in the crystal structure are likely to occur. 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.

[0253] 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 10, the magnesium concentration and fluorine concentration become high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrogen fluoride can be improved even in the positive electrode active material after the cracks occur.

[0254] <Analysis method> A certain positive electrode active material is Ｘα CoO ２ When Xα in the positive electrode active material 10 is small, it can be determined whether the positive electrode active material 10 has an O3′-type crystal structure. Ｘα CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small Xα therein using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0255] In particular, XRD is preferred 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 an XRD profile that reflects the crystalline structure of the interior 10b of the positive electrode active material 10, which occupies the majority of the volume of the positive electrode active material 10.

[0256] 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.

[0257] As described above, the positive electrode active material 10 is Li Ｘα CoO ２ A characteristic of this material is that there is little change in the crystal structure when Xα is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at a high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0258] It should also be noted that simply adding an additional element A may not result in an O3'-type crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additional element A, Li Ｘα CoO ２ In some cases, Xα 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.

[0259] Furthermore, even in the case of positive electrode active material 10, if Xα is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9 V, an H1-3 type or trigonal O1 type crystal structure may be generated. Therefore, to determine whether or not a material is positive electrode active material 10, analysis of the crystal structure, such as XRD, and information such as the charging capacity or the charging voltage are required.

[0260] A positive electrode active material with a small Xα may undergo a change in crystal structure when exposed to air. For example, the crystal structure may change from an O3'-type to an H1-3-type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0261] Whether the distribution of the additive element A 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.

[0262] The crystal structure, such as the grain boundaries, can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 10 .

[0263] <Charging Method> Charging for determining whether a composite oxide is the positive electrode active material 10 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. The coin cell for determining whether the composite oxide is the positive electrode active material 10 does not need to include the electrolyte and separator of one embodiment of the present invention.

[0264] 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.

[0265] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the voltage value of the secondary battery and the potential value of the positive electrode will differ. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0266] The electrolyte solution was prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, and lithium hexafluorophosphate (LiPF ６ ) is prepared. The lithium hexafluorophosphate should be 1 mol per liter of mixed solvent when the temperature of the mixed solvent is 25°C ± 5°C. Vinylene carbonate (VC) may also be prepared as an additive. It is preferable to add the additive at 2 wt% of the electrolyte (total amount of mixed solvent and lithium salt).

[0267] The separator may be a 25 μm thick porous polypropylene film. The separator for checking the state of charge of the positive electrode active material may be made of a material other than polypropylene.

[0268] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0269] 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 using CC / CV, 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.

[0270] 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 or more and 100 mA / g or less 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 or more and 10 mA / g or less, and discharging can be performed by constant current discharging at 20 mA / g or more and 100 mA / g or less up to 2.5 V. Alternatively, discharging can be performed by constant current discharging at a current value of 20 mA / g or more and 200 mA / g or less up to 3.0 V.

[0271] 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.

[0272] <XRD> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS, X-ray: CuKα1, output: 40 kV, 40 mA, slit width: Div. Slit, 0.5°, detector: LynxEye, scan method: 2θ / θ continuous scan, measurement range (2θ): 15° to 90°, step width (2θ): 0.01°, set counting time: 1 second / step, sample stage rotation: 15 rpm.

[0273] If the measurement sample is a powder, it can be set by placing it in a glass sample holder or by sprinkling the sample on a greased silicone non-reflective plate. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0274] CuKα calculated from the O3' type crystal structure and the H1-3 type crystal structure model １ The ideal powder XRD profiles are shown in Figures 9 and 10. For comparison, Li Ｘα CoO ２ LiCoO with Xα=1 ２ The ideal XRD profiles calculated from the crystal structure of LiCoO3 and the trigonal O1 with Xα = 0 are also shown. ２ (O3) and CoO ２ The XRD profile 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 in the same manner as above based on the information on the H1-3 type crystal structure shown in Figure 8. The XRD pattern of the O3' type crystal structure was estimated from the XRD pattern of the positive electrode active material 10, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and an XRD profile was created in the same manner as the others.

[0275] As shown in FIG. 9, 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).

[0276] However, as shown in FIG. 10, no diffraction peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. Ｘα CoO ２ It can be said that 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 Xα in the positive electrode active material 10 is small is a characteristic of the positive electrode active material 10.

[0277] 9, for example, when charging is performed with a voltage slightly lower than 4.60 V (4.56 V, 4.57 V, 4.58 V, or 4.59 V) as the upper limit of the charging voltage, the above diffraction peak appears shifted to a lower angle. For example, when charging is performed with a charging voltage upper limit of 4.58 V, positive electrode active material 10 has diffraction peaks at 2θ = 18.85 ± 0.20 ° and 2θ = 45.15 ± 0.10 ° as diffraction peaks derived from the O3'-type crystal structure.

[0278] This can also be said to be because the positions at which the XRD profiles appear are close between the crystal structures of Xα = 1 and Xα ≦ 0.24. More specifically, for the diffraction peaks appearing at 2θ of 42° to 46° in the main XRD profiles of the crystal structures of Xα = 1 and Xα ≦ 0.24, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0279] The positive electrode active material 10 is Li Ｘα CoO ２ When Xα in the O3'-type crystal structure is small, not all of the O3'-type crystal structure may be O3'-type. Other crystal structures may be included, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD profile, the O3'-type crystal structure is preferably 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more. If the O3'-type crystal structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0280] 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 wt% or more, more preferably 40 wt% or more, and even more preferably 43% wt% or more.

[0281] Furthermore, the sharpness of the diffraction peaks in the XRD profile 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 diffraction peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for diffraction 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 diffraction peaks necessarily meet this requirement. If some diffraction peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after charging.

[0282] The crystallite size of the O3'-type crystal structure of the positive electrode active material 10 is approximately equal to that of LiCoO ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the Ｘα CoO ２ When Xα in the LiCoO is small, a clear XRD profile 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 diffraction peak will be broad and small. The crystallite size can be determined from the half-width of the diffraction peak.

[0283] <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 10a. 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.

[0284] In the positive electrode active material 10, the concentration of one or more selected from the additive elements A is preferably higher in the surface layer portion 10a than in the interior portion 10b. This is equivalent to saying that the concentration of one or more selected from the additive elements A in the surface layer portion 10a is preferably higher than the average concentration of the additive elements A in the entire positive electrode active material 10. Therefore, for example, it can be said that the concentration of one or more selected from the additive elements A in the surface layer portion 10a measured by XPS or the like is preferably higher than the average concentration of the additive elements A in the entire positive electrode active material 10 measured by ICP-MS, GD-MS, or the like. For example, the magnesium concentration in at least a portion of the surface layer portion 10a measured by XPS or the like is preferably higher than the average magnesium concentration in the entire positive electrode active material 10. Furthermore, the nickel concentration in at least a portion of the surface layer portion 10a is preferably higher than the average nickel concentration in the entire positive electrode active material 10. Furthermore, the aluminum concentration in at least a portion of the surface layer portion 10a is preferably higher than the average aluminum concentration in the entire positive electrode active material 10. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 10 a is higher than the average fluorine concentration in the entire positive electrode active material 10 .

[0285] The surface and surface layer 10a of the positive electrode active material 10 do not include carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 10. The surface also does not include the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 10. 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.

[0286] Furthermore, before being subjected to various analyses, samples such as the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. At this time, lithium may dissolve in the solvent used for washing, but even in this case, the additional element A is unlikely to dissolve, and therefore the atomic ratio of the additional element A is not affected.

[0287] The concentration of the additive element A may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparison 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 the number of cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more and 1.20 or less, more preferably 0.500 or more and 1.00 or less, even more preferably 0.500 or more and 0.900 or less, and even more preferably 0.500 or more and 0.700 or less.

[0288] Furthermore, for example, the ratio of the number of atoms of nickel to cobalt (Ni / Co) determined by XPS analysis is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, even more preferably 0.050 or more and 0.100 or less, and even more preferably 0.050 or more and 0.070 or less.

[0289] Furthermore, the ratio of the number of aluminum atoms to the number of cobalt atoms (Al / Co) determined by XPS analysis, for example, is preferably 0.010 or more and 0.100 or less, more preferably 0.010 or more and 0.050 or less, and even more preferably 0.010 or more and 0.040 or less.

[0290] Furthermore, the ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg) as determined by, for example, XPS analysis is preferably 0.100 or more and 1.00 or less, more preferably 0.100 or more and 0.800 or less, still more preferably 0.100 or more and 0.500 or less, still more preferably 0.100 or more and 0.300 or less, and still more preferably 0.100 or more and 0.200 or less.

[0291] The above range indicates that the additive element A is not attached to a narrow area on the surface of the positive electrode active material 10, but is widely distributed at a preferred concentration in the surface layer 10a of the positive electrode active material 10. In other words, as a result of XPS analysis of the positive electrode active material 10, the above range indicates that the crystalline structure is less likely to collapse even when repeatedly charged and discharged so that Xα is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, good lithium insertion and desorption is possible in the positive electrode active material 10, and excellent rate characteristics can be achieved.

[0292] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. It is also recommended 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 measurement can be performed using the following XPS apparatus and measurement conditions: 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 angle (detection depth): Take-off angle 45° (approximately 4 nm to 5 nm) Measurement spectrum: Wide scan, narrow scan of each detected element

[0293] When the positive electrode active material 10 is analyzed by XPS, the peak (Mg1s peak) showing the bond energy between magnesium and other elements is preferably 1303.0 eV or more and less than 1305.0 eV, and more preferably about 1304.0 eV, which is a different value from the bond energy of magnesium fluoride, 1306.0 eV, and is close to the bond energy of magnesium oxide.

[0294] In the XPS analysis of the positive electrode active material 10, it is preferable to correct the measured XPS spectrum so that the C1s peak is aligned with the reference value (284.8 eV), i.e., to shift the entire spectrum, which can reduce the influence of differences in the XPS apparatus, differences in measurement conditions, etc. on the XPS measurement.

[0295] Furthermore, when analyzing the Mg1s peak in an XPS analysis of the positive electrode active material 10 to analyze a peak component derived from an "O-Mg-O" bond, a peak component derived from an "O-Mg-F" bond, and a peak component derived from an "F-Mg-F" bond, it is preferable that the positive electrode active material 10 has a peak component derived from an O-Mg-O bond. Furthermore, the positive electrode active material 10 may contain a peak component derived from an "O-Mg-F" bond, but it preferably accounts for 30% or less of the total of the three peak components, more preferably 20% or less, still more preferably 20% or less, and preferably 10% or less. Furthermore, the positive electrode active material 10 may contain a peak component derived from an "F-Mg-F" bond, but it preferably accounts for 10% or less of the total.

[0296] That is, in an XPS analysis of the positive electrode active material 10, when the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond are analyzed, the peak component derived from the "O-Mg-O" bond preferably accounts for 70% or more, more preferably 80% or more, still more preferably 90% or more, and particularly preferably 100%.

[0297] A method for analyzing the Mg1s peak of an XPS spectrum in XPS analysis will be described. In analyzing the Mg1s peak, it is preferable to define the peak component derived from the O-Mg-O bond as fit peak 1, the peak component derived from the O-Mg-F bond as fit peak 2, and the peak component derived from the F-Mg-F bond as fit peak 3, synthesize these three fit peaks, and calculate the peak synthesis ratio that minimizes the difference from the Mg1s peak of the XPS spectrum obtained by XPS analysis. The analysis results can be output by assuming that the area ratio of fit peak 1, fit peak 2, and fit peak 3 is the ratio of the O-Mg-O bond, the O-Mg-F bond, and the F-Mg-F bond.

[0298] In the above-mentioned XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also called peak top) of fit peak 1 is determined by the MgO-coated LiCoO ２The energy value at the maximum value of the Mg1s peak when measured separately as a standard sample can be referenced. The energy value at the maximum value of fit peak 3 (Ep3) can be referenced to the energy value at the maximum value of magnesium fluoride (MgF ２ The energy value at the maximum value of the Mg1s peak when a standard sample (99.9% purity (3N) up) is separately measured can be referenced. The energy value at the maximum value of fit peak 2 (Ep2) can be set to an intermediate value between Ep1 and Ep3. EP1 is located on the lower energy side compared to EP3. The energy value at the maximum value of a peak is also referred to as the peak position.

[0299] In the XPS analysis of the positive electrode active material 10, the half width of the Mg1s peak is preferably 1.0 eV to 3.0 eV, more preferably 1.0 eV to 2.8 eV, and particularly preferably 1.0 eV to 2.6 eV. Note that the peak position of the Mg1s peak is on the lower energy side than the energy value of the maximum value of the Mg1s peak when magnesium fluoride is separately measured as a standard sample.

[0300] <EDX> It is preferable that one or more selected from the additive elements A contained in the positive electrode active material 10 have a concentration gradient. Furthermore, when two or more additive elements A are used, it is more preferable that the depth from the surface of the concentration peak of each additive element A is different. For example, the concentration gradient, concentration peak, etc. of the additive element A can be evaluated by exposing a cross section of the positive electrode active material 10 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, EPMA (electron probe microanalysis), or the like.

[0301] 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.

[0302] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the additive element A in the surface layer 10a, the interior 10b, and near the grain boundaries of the positive electrode active material 10. EDX ray analysis can also analyze the concentration distribution and maximum value of the additive element A. 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 elements present in the depth direction of the sample.

[0303] Since the positive electrode active material 10 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 multi-layer film of carbon, metal, oxide, resin, etc.

[0304] The reference point in the STEM-EDX-ray analysis is the average value M of the detected amount of characteristic X-rays of the transition metal M inside the positive electrode active material. ＡＶＥ When the left side of the reference point is described as the outside of the positive electrode active material and the right side of the reference point is described as the inside of the positive electrode active material, the reference point may be referred to as the position of the surface of the positive electrode active material. In addition, in STEM-EDX-ray analysis, if the detected amount of characteristic X-rays of the transition metal M does not decrease sufficiently to the left of the reference point, the detected amount of characteristic X-rays of the transition metal M to the left of the reference point is called background, and the average value M of the detected amount of characteristic X-rays of the transition metal M in the background is used. ＢＧ and the average amount of detected transition metal M inside ＡＶＥ The reference point may be the point where the sum of the detected amount of characteristic X-rays of oxygen inside the positive electrode active material is 50% of the sum of the detected amount of characteristic X-rays of the transition metal M. The reference point can be determined by replacing the transition metal M with oxygen. However, since oxygen is an element that is easily affected by the outside of the positive electrode active material, the reference point is the average value M of the detected amount of characteristic X-rays of the transition metal M. ＡＶＥ It is preferable to calculate it from 50% of the average value M of the detected amount of characteristic X-rays of the transition metal M.ＡＶＥ 50% of the detected amount of characteristic X-rays of oxygen, M ＡＶＥ If the difference is not 50% of the value of the average M of the detected amount of characteristic X-rays of the transition metal M, it is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen that adhere to the surface of the positive electrode active material. ＡＶＥ In the case of a positive electrode active material containing a plurality of transition metals M, it is preferable to adopt the point where the amount of characteristic X-rays detected inside the material is 50% of the amount of the transition metal M. ＡＶＥ The reference point can be found using

[0305] The average value M of the detected amount of characteristic X-rays of the internal transition metal M ＡＶＥ can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 20 nm or more, preferably 30 nm or more, from a region where the detected amount of characteristic X-rays of the transition metal M becomes saturated and stable, for example, a region where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＢＧ can be obtained by averaging the amount of characteristic X-rays of the transition metal M within a range of 2 nm or more, preferably 3 nm or more, avoiding the area where the amount of detected characteristic X-rays of the transition metal M begins to increase. ＢＧ and the average value of the detected amount of characteristic X-rays of oxygen in the background O ＡＶＥ can also be found in the same way.

[0306] Furthermore, the surface of the positive electrode active material 10 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystal 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.

[0307] Furthermore, 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-ED X-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, less than R / 2.

[0308] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured after 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 integrated value can be used as the detection value for each element.

[0309] 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 a carbon coating unit of an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0310] 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.

[0311] 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 (Dual EDS) EDX detector. An example of the conditions for EDX ray analysis using the Hitachi High-Tech HD-2700 is to set the acceleration voltage of the STEM device to 200 kV and the emission current 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.

[0312] 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.

[0313] In addition, in the positive electrode active material 10 having magnesium and fluorine as the additive element A, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. For example, the difference in the depth direction 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.

[0314] Furthermore, in the positive electrode active material 10 having nickel as the additive element A, the peak of the nickel concentration or detectable amount in the surface layer 10a is preferably present at a depth of 3 nm, more preferably at a depth of 1 nm, from the surface or reference point toward the center of the positive electrode active material 10. Furthermore, in the positive electrode active material 10 having 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, more preferably within 1 nm.

[0315] Furthermore, when the positive electrode active material 10 contains aluminum as the additive element A, it is preferable that, when EDX-ray analysis is performed, the peak of the concentration or detected amount of magnesium, nickel, or fluorine is closer to the surface than the peak of the concentration or detected amount of aluminum in the surface layer portion 10a. In other words, it is preferable that the peak of the concentration or detected amount of aluminum in the surface layer portion 10a is located more inward than the peak of the concentration or detected amount of magnesium, nickel, or fluorine. For example, it is preferable that the peak of the concentration or detected amount of aluminum exists on the surface of the positive electrode active material 10, 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.

[0316] Here, how to express the positional relationship of element distribution when EDX-ray analysis is performed will be explained using Figures 11A to 11G. Figures 11A to 11F are schematic diagrams showing the detected amount profiles of the first element e1 and the second element e2. Figure 11G is a schematic diagram showing the detected amount profiles of the first element e1, the second element e2, and the third element e3. The detected amount includes concentration and intensity.

[0317] For example, when the detection amount profile of the first element e1 and the second element e2 has a shape as shown in FIG. 11A , the position where the detection amount of the second element e2 is maximum is said to be located more inward than the position where the detection amount of the first element e1 is maximum. For example, when the detection amount profile of the first element e1 and the second element e2 has a shape as shown in FIG. 11B , the position where the detection amount of the second element e2 is maximum is said to be located more inward than the position where the detection amount of the first element e1 is maximum. For example, when the detection amount profile of the first element e1 and the second element e2 has a shape as shown in FIG. 11C , the position where the detection amount of the first element e1 is maximum is said to be located more inward than the position where the detection amount of the second element e2 is maximum. For example, when the detection amount profile of the first element e1 and the second element e2 has a shape as shown in FIG. 11D , the position where the detection amount of the second element e2 is maximum is said to be located more inward than the position where the detection amount of the first element e1 is maximum. For example, when the detection amount profiles of the first element e1 and the second element e2 have a shape as shown in Fig. 11E, the position where the detection amount of the first element e1 is maximum is said to be located more inward than the position where the detection amount of the second element e2 is maximum. For example, when the detection amount profiles of the first element e1 and the second element e2 have a shape as shown in Fig. 11F, the position where the detection amount of the second element e2 is maximum is said to be located more inward than the position where the detection amount of the first element e1 is maximum.

[0318] The expression "having an overlapping region of distributions" will be explained using an example in which the detected amount profiles of a first element e1, a second element e2, and a third element e3 have the positional relationship shown in Fig. 11G. In this specification and the like, "having an overlapping region of distributions of two elements" means, for example, that the position where the detected amount of at least one element is maximum is located in a range where the detected amount of the other element is 1 / 5 or more of the maximum detected amount.

[0319] 11G, the position (P2) where the second element e2 has a maximum value in the detection amount profile is located in the range (hatched area in the figure) where the detection amount is equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the detection amount profile of the first element e1, and therefore the first element e1 and the second element e2 have an overlapping distribution area. Also, the position (P3) where the third element e3 has a maximum value in the detection amount profile is not located in the range (hatched area in the figure) where the detection amount is equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the detection amount profile of the first element e1, and therefore it cannot be said that the first element e1 and the third element e3 have an overlapping distribution area.

[0320] 11G, it can be said that the distribution of the second element e2 and the distribution of the third element e3 are located more inward than the distribution of the first element e1. Alternatively, it can be said that the distribution of the second element e2 and the distribution of the third element e3 are biased more inward than the distribution of the first element e1.

[0321] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0322] Embodiment 3 In this embodiment, elements constituting a secondary battery will be described.

[0323] [Positive Electrode] The secondary battery has a positive electrode, which is as described in the first and second embodiments.

[0324] <Positive Electrode Current Collector> The positive electrode has a positive electrode current collector. Materials with high conductivity, such as metals such as stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof, can be used as the positive electrode current collector. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can also be used. The positive electrode current collector may also be formed from 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 may be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable that the thickness of the current collector be 5 μm or more and 30 μm or less.

[0325] <Binder> The positive electrode preferably contains a binder. As the binder, it is preferable to use a material such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, or nitrocellulose.

[0326] As another 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, fluororubber can be used as the binder.

[0327] As the other binder, it is preferable to use, for example, a water-soluble polymer. For example, a polysaccharide or the like can be used as the water-soluble polymer. As the polysaccharide, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, or starch can be used. Furthermore, it is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0328] The binder may be a combination of two or more of the above binders. For example, a material with a particularly excellent viscosity adjusting effect may be combined with another material. 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 a water-soluble polymer 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.

[0329] 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 slurry. In this specification, cellulose and cellulose derivatives used as electrode binders also include their salts.

[0330] 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.

[0331] When the binder covers the surface of the active material or contacts the surface and 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 secondary battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0332] <Conductive Material> The positive electrode preferably contains a conductive material. The conductive material is also called a conductivity imparting agent or a conductive auxiliary, 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 active material is electrically connected even when not in contact with each other.

[0333] 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 fiber such as carbon nanofiber and carbon nanotube, and graphene compound can be used.

[0334] 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.

[0335] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0336] 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.

[0337] 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 proportion of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the secondary battery.

[0338] 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 regions between multiple active materials. By combining a carbon-containing compound that tends to fill microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. A secondary battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and is stable, making it effective as an in-vehicle secondary battery.

[0339] [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.

[0340] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0341] Furthermore, 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 (per weight of negative electrode active material) 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, CeSb３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0342] In this specification, "SiO" refers to, for example, silicon monoxide. β Here, β preferably has a value of 1 or close to 1. For example, β is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0343] Examples of the carbon material that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, and carbon black.

[0344] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite that can be used include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. The artificial graphite may have a carbon coating layer, which is a low-crystalline layer. Since the shape of the artificial graphite is spherical, it is called spherical graphite. For example, MCMB is one of the preferred materials for spherical graphite. Furthermore, it is relatively easy to reduce the specific surface area of ​​MCMB. If the specific surface area is large, the decomposition reaction with the electrolyte on the surface of the negative electrode active material may become large, and good cycle characteristics may not be obtained. In order to suppress the decomposition reaction, the specific surface area of ​​the carbon is 0.8 m ２ / g or more 8m ２ / g, preferably 1m ２ / g or more 2m ２ / g is preferably satisfied. Typically, it is preferable that the spherical graphite has the specific surface area described above as a powder characteristic. The specific surface area can be measured by the BET method (Brunauer Emmett Teller method). The BET method is an analytical technique that extends the Langmuir theory to multilayer adsorption of adsorbed gas molecules, and is the most common method for calculating the specific surface area. The specific surface area by the BET method can be measured using an automatic specific surface area measuring device, Tristar 2 3020.

[0345] Examples of natural graphite include flake graphite, spheroidized natural graphite, etc. The natural graphite may have a carbon coating layer which is a low-crystalline layer.

[0346] If the particle size or median diameter (D50) is small, the negative electrode active material becomes bulky and may inhibit improvement of electrode density. Therefore, the particle size or median diameter (D50) of the negative electrode active material should be 3 μm or more and 20 μm or less, preferably 7 μm or more and 12 μm or less. Typically, the median diameter (D50) should be in the above range as a powder characteristic of graphite.

[0347] 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 secondary 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 than metallic lithium.

[0348] 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 dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and other oxides can be used.

[0349] In addition, as the negative electrode active material, a 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 per weight of negative electrode active material) ３ ) and is preferred.

[0350] When a 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, is used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0351] 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:

[0352] Another example of the negative electrode is a negative electrode that does not have a negative electrode active material at the end of the secondary battery production. 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 secondary battery production, in which lithium ions released from the positive electrode active material upon charging the secondary battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A secondary battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) secondary battery, a negative electrode-less (anode-less) secondary battery, or the like.

[0353] 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.

[0354] 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.

[0355] 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.

[0356] <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.

[0357] [Separator] The secondary battery has a separator, which is as described in the first embodiment and the like.

[0358] [Exterior Body] The secondary battery has an exterior body, which is as described in the first embodiment and the like.

[0359] [Electrolyte] The secondary battery has an electrolyte containing carrier ions. The electrolyte is as described in the first embodiment and the like.

[0360] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0361] In this embodiment, a secondary battery having an electrolyte solution and a separator according to one embodiment of the present invention will be described. A secondary battery having a separator is preferable because it can prevent thermal runaway and / or fire. Furthermore, it is preferable because it can ensure the amount of electrolyte solution retained in the separator even when the electrodes expand and contract during charging and discharging of the secondary battery.

[0362] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 12A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 12B is an external view, and Fig. 12C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0363] 12A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding, and therefore, FIGS. 12A and 12B are not completely identical corresponding views.

[0364] In Fig. 12A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. A separator according to one embodiment of the present invention can be used for the separator 310. 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. 12A. The spacer 322 and the washer 312 are used to protect the interior or to fix the position within 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.

[0365] A positive electrode 304 has a stacked structure in which a positive electrode active material layer 306 is formed over a positive electrode current collector 305. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 306.

[0366] FIG. 12B is a perspective view of the completed coin-type secondary battery.

[0367] 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.

[0368] Note that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 can each have an active material layer formed on only one surface.

[0369] 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.

[0370] 12C , the positive electrode 304, the separator 310, the negative electrode 307, and the 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 pressure-bonded together via a gasket 303 to produce a coin-shaped secondary battery 300. The mixed solvent of one embodiment of the present invention is preferably used as the solvent for the electrolyte.

[0371] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 13A. As shown in Fig. 13A, a cylindrical secondary battery 616 has a positive electrode cap (secondary battery lid) 601 on the top surface and a secondary battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the secondary battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0372] 13B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 13B has a positive electrode cap (secondary battery lid) 601 on the top surface and a secondary battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and secondary battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0373] A secondary battery element is provided inside a hollow cylindrical secondary battery can 602, in which a strip-shaped positive electrode 604 and a strip-shaped negative electrode 606 are wound with a separator 605 sandwiched therebetween. A separator according to one embodiment of the present invention can be used for the separator 605. Although not shown, the secondary battery element is wound around a central axis. One end of the secondary battery can 602 is closed and the other end is open. Metals such as nickel, aluminum, and titanium, which are corrosion-resistant to an electrolyte solution, or alloys of these metals and other metals (e.g., stainless steel) can be used for the secondary battery can 602. Furthermore, to prevent corrosion by the electrolyte solution, the secondary battery can 602 is preferably coated with nickel, aluminum, or the like. Inside the secondary battery can 602, the wound secondary 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. An electrolyte (not shown) is poured into the secondary battery can 602 in which the secondary battery element is provided. The electrolyte may be the same as that used in a coin-type secondary battery.

[0374] Since the positive and negative electrodes used in cylindrical secondary batteries are wound up, it is preferable to form active materials on both sides of the current collector.

[0375] Lithium cobalt oxide, which is one embodiment of the present invention, can be used as a positive electrode active material of the positive electrode 604, and the cylindrical secondary battery 616 can have favorable high-voltage charging characteristics.

[0376] A positive electrode terminal (positive electrode current collecting lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collecting lead) 607 is connected to the negative electrode 606. The positive electrode terminal 603 can be made of a metal material such as aluminum. The negative electrode terminal 607 can be made of a metal material such as copper. 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 secondary 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 secondary battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases when the temperature rises, and the increase in resistance limits the amount of current to prevent abnormal heat generation. ３ )-based semiconductor ceramics, etc. can be used.

[0377] 13C 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.

[0378] 13D 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.

[0379] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0380] 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.

[0381] 13D , 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.

[0382] [Another Example of Secondary Battery Structure] An example of the structure of a secondary battery will be described with reference to FIGS. 14 and 15. FIG.

[0383] A secondary battery 913 shown in FIG. 14A includes a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is immersed in an electrolyte solution inside the housing 930. The mixed solvent of one embodiment of the present invention is preferably used as a solvent for the electrolyte solution. The terminal 952 is in contact with 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 for convenience, the housing 930 is shown separated in FIG. 14A , 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 laminate of a metal material and a resin material.

[0384] 14B, the housing 930 shown in Fig. 14A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 14B 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.

[0385] The housing 930a can be made of a metal material (such as aluminum) or a laminate of a metal material and a resin material. An organic resin or the like can be used as the resin material. In particular, by using a material such as organic resin on the surface on which the antenna is formed, the electric field generated by the secondary battery 913 can be suppressed. Note that if the electric field shielding by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of a metal material (such as aluminum) or a laminate of a metal material and a resin material.

[0386] 14C shows the structure of a wound body 950. The wound body 950 includes a negative electrode 931, a positive electrode 932, and a separator 933. The separator 933 can be a separator according to one embodiment of the present invention. The wound body 950 is formed by winding a laminate sheet in which the negative electrode 931 and the positive electrode 932 are stacked with the separator 933 sandwiched therebetween. Note that a plurality of stacks of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0387] Alternatively, a secondary battery 913 may be provided that has a wound body 950a as shown in Fig. 15A. The wound body 950a shown in Fig. 15A 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.

[0388] Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 932a.

[0389] 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.

[0390] 15B , 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.

[0391] 15C , the wound body 950a 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, and can prevent the secondary battery from exploding.

[0392] As shown in Fig. 15B, 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. 15A and 15B, refer to the descriptions of the secondary battery 913 shown in Figs. 14A to 14C.

[0393] <Laminated Secondary Battery> Next, examples of external views of an example of a laminated secondary battery are shown in FIGS. 16A and 16B . Each of FIGS. 16A and 16B 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. The separator 507 can be a separator according to one embodiment of the present invention. Although not shown, the mixed solvent according to one embodiment of the present invention is preferably used as a solvent for the electrolyte solution.

[0394] FIG. 16A shows an external view of a positive electrode 103 and a negative electrode 106. The positive electrode 103 has a positive electrode current collector 21, and a positive electrode active material layer 22 is formed on the surface of the positive electrode current collector 21. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 22. The positive electrode 103 also has a region where the positive electrode current collector 21 is partially exposed (hereinafter referred to as a tab region). The negative electrode 106 has a negative electrode current collector 31, and the negative electrode active material layer 32 is formed on the surface of the negative electrode current collector 31. The negative electrode 106 also has a region where the negative electrode current collector 31 is partially exposed, i.e., a tab region. Note that the area or shape of the tab region of the positive electrode and the negative electrode is not limited to the example shown in FIG. 16A .

[0395] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0396] (Embodiment 5) In this embodiment, an example of a vehicle equipped with a secondary battery having an electrolyte solution and a separator according to one embodiment of the present invention will be described. Examples of the vehicle include automobiles, trains, airplanes, and buses. A secondary battery having a separator is preferable because it can prevent thermal runaway and / or fire. Furthermore, it is preferable because it can ensure the amount of electrolyte solution retained in the separator even when the electrodes expand and contract during charging and discharging of the secondary battery.

[0397] 17A is an electric vehicle that uses an electric motor as a power source for running, or a hybrid vehicle that can select and use an electric motor or an engine as a power source for running. The vehicle 2001 has a secondary battery pack 2200, and the secondary battery pack preferably has a secondary battery module to which multiple secondary batteries are connected, and a charge control device electrically connected to the secondary battery module.

[0398] Next, the secondary battery pack 2200 will be described using FIG. 17B . FIG. 17B illustrates an example in which one secondary battery pack 2200 includes nine prismatic secondary batteries 1300. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode group fixed by a fixing portion 1413 made of an insulator and the other electrode group fixed by a fixing portion 1414 made of an insulator. Instead of the fixing portions 1413 and 1414, the electrode groups may be fixed using a configuration in which the secondary battery is housed in a secondary battery housing box (also referred to as a housing). Because vehicles are expected to be subjected to external vibrations or shaking (e.g., from the road surface), it is preferable to fix multiple prismatic secondary batteries 1300 using the fixing portions 1413, 1414, the secondary battery housing box, etc. Furthermore, one electrode group is electrically connected to the control circuit unit 1320 by wiring 1421. Furthermore, the other electrode group is electrically connected to the control circuit unit 1320 by wiring 1422.

[0399] 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 secondary 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).

[0400] 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.

[0401] 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.

[0402] 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.

[0403] Furthermore, transistors using oxide semiconductors in the semiconductor layer have a wider operating ambient 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 transistors. The off-current of transistors using oxide semiconductors is extremely low regardless of temperature, 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 does not become sufficiently large. The control circuit 1320 can improve safety. The secondary battery and control circuit 1320 can significantly contribute to eliminating accidents such as fires caused by secondary batteries.

[0404] Next, an example of a block diagram of the automobile 2001 shown in FIG. 17A and the secondary battery pack 2200 shown in FIG. 17B is shown in FIG. 17C.

[0405] 17C , 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.

[0406] The internal structure of the first battery 1301a may be a wound type as shown in FIG. 14C or FIG. 15A, or a stacked type as shown in FIG. 16A or FIG. 16B.

[0407] 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 secondary 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 secondary battery pack.

[0408] In addition, in a secondary battery for vehicle use, in order to cut off power from multiple secondary batteries, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0409] 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.

[0410] 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.

[0411] 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 secondary batteries and being prone to deterioration due to a phenomenon called sulfation. Using a lithium-ion secondary 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 that are difficult to identify during manufacturing. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, the motor may not be able to start even if the first batteries 1301a and 1301b have remaining capacity. If the second battery 1311 is a lead-acid battery, the first battery supplies power to the second battery, and the battery is charged to maintain a full charge state at all times, preventing the motor from being unable to operate as described above.

[0412] In this embodiment, an example is shown in which lithium ion secondary batteries are used as both the first battery 1301a and the second battery 1311. The second battery 1311 may be a lead battery, an all-solid-state secondary battery, or an electric double layer capacitor.

[0413] 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.

[0414] 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.

[0415] Although not shown, when the electric vehicle is connected to an external charger, the charger plug or the 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 the 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.

[0416] 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.

[0417] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0418] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0419] In this embodiment, a space device equipped with a secondary battery having an electrolyte and a separator according to one embodiment of the present invention will be described. A secondary battery having a separator is preferable because it can prevent thermal runaway and / or fire. Furthermore, it is preferable because it can ensure the amount of electrolyte held by the separator even when the electrodes expand and contract during charging and discharging of the secondary battery.

[0420] 18A shows an example of space equipment, namely, an artificial satellite 6800. The artificial satellite 6800 has a body 6801, a solar panel 6802, an antenna 6803, and a secondary battery 6805.

[0421] When sunlight is irradiated onto the solar panel 6802, the power required for the satellite 6800 to operate is generated. However, for example, in a situation where sunlight is not irradiated onto the solar panel or where the amount of sunlight irradiating the solar panel is low, the generated power is reduced. Therefore, there is a possibility that the power required for the satellite 6800 to operate will not be generated. In order to operate the satellite 6800 even in a situation where the generated power is low, it is preferable to provide a secondary battery 6805 in the satellite 6800. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting good low-temperature characteristics can be obtained.

[0422] The satellite 6800 can generate a signal. The signal is transmitted via an antenna 6803, and can be received by, for example, a receiver installed on the ground or another satellite. By receiving the signal transmitted by the satellite 6800, for example, the position of the receiver that received the signal can be measured. As described above, the satellite 6800 can constitute, for example, a satellite positioning system.

[0423] Alternatively, the artificial satellite 6800 may be configured to include a sensor. For example, by including a visible light sensor, the artificial satellite 6800 may have the function of detecting sunlight reflected from an object on the ground. Alternatively, by including a thermal infrared sensor, the artificial satellite 6800 may have the function of detecting thermal infrared rays emitted from the earth's surface. As described above, the artificial satellite 6800 may function as, for example, an earth observation satellite.

[0424] FIG. 18B shows a probe 6900 with a solar sail (also called a sun sail) as an example of space equipment. The probe 6900 has a body 6901, a solar sail 6902, and a secondary battery 6905. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting excellent low-temperature characteristics can be obtained. When photons emitted from the sun strike the surface of the solar sail 6902, momentum is transferred to the solar sail 6902. Therefore, the surface of the solar sail 6902 preferably has a highly reflective thin film and preferably faces the sun.

[0425] The solar sail 6902 may also be designed to be folded up small until it leaves the atmosphere, and then deployed into a large sheet shape outside the Earth's atmosphere (outer space) as shown in Figure 18B.

[0426] FIG. 18C shows a spacecraft 6910 as an example of space equipment. The spacecraft 6910 has a body 6911, a solar panel 6912, and a secondary battery 6913. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting good low-temperature characteristics can be obtained. The body 6911 can have, for example, a pressurized compartment and a non-pressurized compartment. The pressurized compartment may be designed to accommodate a crew member. Electricity generated by sunlight irradiating the solar panel 6912 can be charged into the secondary battery 6913.

[0427] 18D shows a rover 6920 as an example of space equipment. The rover 6920 has a body 6921 and a secondary battery 6923. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting good low-temperature characteristics can be obtained. The rover 6920 may also have a solar panel 6922.

[0428] The rover 6920 may be designed to allow a crew member to ride in. The secondary battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or the secondary battery 6923 may be charged with electricity generated by other power sources, such as a fuel secondary cell, a radioisotope thermoelectric converter, or the like.

[0429] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0430] In this example, samples were prepared in which the ratio of cyclic carbonate to chain carbonate in the mixed solvent used in the electrolyte was varied, and battery characteristics were measured at sub-zero and room temperature (25°C). The samples were coin-type secondary batteries (coin cells, CR2032 type, diameter 20 mm, height 3.2 mm). Each component of the coin cell will be described.

[0431] [Positive Electrode] The coin cell has a positive electrode. First, a method for producing a positive electrode active material will be described. As shown in step S14 of FIG. 3, pre-synthesized lithium cobalt oxide particles (Cellseed 5H manufactured by Nippon Chemical Industry Co., Ltd.) were prepared. The lithium cobalt oxide particles had a D50 of approximately 7 μm.

[0432] The lithium cobalt oxide particles were placed in a sheath, which was then covered, and heated in a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.). The heating conditions were 850°C and 2 hours. Oxygen was flowed through the heating furnace at 10 L / min, and the flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the heating furnace reached 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at 200°C / hour, and the oxygen flow continued until the temperature reached 200°C.

[0433] Next, lithium fluoride and magnesium fluoride were prepared as additive element sources (Al sources). They were weighed so that the lithium fluoride:magnesium fluoride ratio (molar ratio) was 1:3, and mixed by a wet method. Using dehydrated acetone as a solvent, the mixture was mixed at a rotation speed of 500 rpm for 20 hours. The acetone was removed, and the mixture was sieved with a 300 μm mesh to break up any agglomerations.

[0434] The lithium cobalt oxide particles after the initial heating were mixed with the Al source. The raw materials were prepared under mixing conditions such that the ratio of the number of cobalt atoms in the lithium cobalt oxide (Co) to the number of magnesium atoms in the Al source (Mg) was Co:Mg = 100:1, and the mixture was stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron). A Nobilta rotor was used for the Picobond. Mixture A was obtained in which the Al source adhered to the lithium cobalt oxide particles.

[0435] Mixture A was placed in a sagger, the sagger was covered, and the mixture was heated in a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.). The heating conditions were 850°C and 10 hours. Oxygen was flowed through the heating furnace at 10 L / min, and the flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the heating furnace reached 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C.

[0436] Next, nickel hydroxide and aluminum hydroxide were prepared as the additive element source (A2 source). The nickel hydroxide was pulverized by a wet method, and the aluminum hydroxide was pulverized by a wet method. The pulverized nickel hydroxide and aluminum hydroxide were mixed to prepare the A2 source. The wet pulverization conditions were the same as those for the A1 source.

[0437] Lithium cobalt oxide particles to which the A1 source had been added were mixed with the A2 source. The mixing conditions were as follows: the ratio of the number of cobalt atoms in the lithium cobalt oxide (Co) to the number of nickel atoms in the nickel hydroxide (Ni) was Co:Ni = 100:0.5, and the ratio of the number of cobalt atoms (Co) to the number of aluminum atoms in the aluminum hydroxide (Al) was Co:Al = 100:0.5. The mixture was stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron). A Nobilta rotor was used for the Picobond. Mixture B was obtained, in which the A2 source adhered to the lithium cobalt oxide particles to which the A1 source had been added.

[0438] Mixture B was placed in a sagger, the sagger was covered, and the mixture was heated in a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.). The heating conditions were 850°C and 2 hours. Oxygen was flowed through the heating furnace at 10 L / min, and the flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the heating furnace read 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C.

[0439] Through the above-described steps, a positive electrode active material containing Mg, F, Ni, Al, and lithium cobalt oxide was obtained, which was used as the positive electrode active material in this example.

[0440] The positive electrode was prepared by coating a positive electrode current collector (aluminum foil) with a slurry of a positive electrode active material, acetylene black (AB), and PVDF in a weight ratio of 95:3:2 using N-methyl-2-pyrrolidone (NMP) as the solvent.

[0441] After the slurry was applied to the current collector, the solvent was evaporated, and then the current collector was pressed with a roll press at a linear pressure of 210 kN / m. A positive electrode having the positive electrode active material prepared above was obtained. The amount of the active material carried on the positive electrode was 10.0 mg / cm. ２ More than 10.7mg / cm ２ The positive electrode was circular with a diameter of 15.96 mm.

[0442] [Negative Electrode] A carbon material was used as the negative electrode active material. Specifically, the negative electrode active material had a median diameter (D50) of 20.9 μm and a specific surface area of ​​1.01 m ２ Formula BT SLC1520T (manufactured by Superior Graphite) with a 1000 vol. / g content was used. A slurry was prepared by mixing the negative electrode active material, VGCF (registered trademark), CMC, and SBR in a weight ratio of 97:1:1:1. Water was used as the solvent for the slurry. The slurry was applied to a copper negative electrode current collector (copper foil), and the solvent was then evaporated.

[0443] Thereafter, in order to increase the density of the negative electrode active material layer, a pressure treatment was performed using a roll press after the solvent had evaporated. The pressure treatment was performed under a linear pressure of 28 kN / m, which was lower than that of the positive electrode. A negative electrode was obtained through the above process. The amount of active material carried on the negative electrode was 6.8 mg / cm. ２ 7.4mg / cm or more ２ The negative electrode was circular with a diameter of 16.16 mm.

[0444] [Separator] The separator used was a laminate of three polyimide layers.

[0445] [Electrolyte] As a mixed solvent for the electrolyte, ethylene carbonate (EC) was prepared as a cyclic carbonate, ethyl methyl carbonate (EMC) was prepared as a chain carbonate, and LiPF 6 was prepared as a lithium salt. ６ In this example, LiPF ６ The concentration of LiPF was measured relative to 1 liter of the mixed solvent when the temperature of the mixed solvent was 25° C.±5° C. (Electrolyte A) A mixed solvent with EC:EMC=1:1.5 (mol ratio), i.e., a molar ratio of EMC to EC (EMC / EC) of 1.5, was prepared, and the concentration of LiPF was 0.7 (mol) relative to 1 liter of the mixed solvent. ６ The resulting solution was used as electrolyte A. The volume ratio of the mixed solvent for electrolyte A was equivalent to EC:EMC = 3:7. (Electrolyte B) A mixed solvent with EC:EMC = 1:5 [molar ratio], i.e., a molar ratio of EMC to EC (EMC / EC) of 5, was prepared, and LiPF was dissolved in 0.7 [mol] of LiPF per 1 liter of the total mixed solvent. ６The resulting solution was used as electrolyte solution B. (Electrolyte solution C) A mixed solvent with EC:EMC=1:7.5 (molar ratio), i.e., a molar ratio of EMC to EC (EMC / EC) of 7.5, was prepared, and LiPF 6 was dissolved in 0.7 (mol) of LiPF 6 per liter of the total mixed solvent. ６ The resulting solution was used as electrolyte C. (Electrolyte D) A mixed solvent with EC:EMC=1:16 [molar ratio], i.e., a molar ratio of EMC to EC (EMC / EC) of 16, was prepared, and LiPF was dissolved in 0.7 [mol] of LiPF per 1 liter of the mixed solvent. ６ The resulting solution was used as electrolyte solution D. (Electrolyte solution E) A mixed solvent with EC:EMC = 1:33 [molar ratio], i.e., a molar ratio of EMC to EC (EMC / EC) of 33, was prepared, and LiPF 6 was dissolved in 0.7 [mol] of LiPF 6 per liter of the total mixed solvent. ６ The electrolyte solution E was prepared by dissolving the above in an EC:EMC ratio of 1:83 (molar ratio), i.e., a mixed solvent in which the molar ratio of EMC to EC (EMC / EC) was 83, and LiPF was dissolved in the mixed solvent at a concentration of 0.7 mol per liter of the total mixed solvent. ６ The electrolyte solution A to F was prepared by dissolving LiPF in the mixed solvent. ６ (dissolved) LiPF ６ The conditions for the electrolyte solutions A to F are summarized in Table 1 below. In the table below, the mixing ratio of the mixed solvent is EC:EMC=Y:Z [molar ratio], and the amount of LiPF per 1 L of the mixed solvent is ６ was defined as X [mol].

[0446]

[0447] (Electrolyte Solution G) Furthermore, a mixed solvent with EC:EMC=1:1.5 [molar ratio], that is, a molar ratio of EMC to EC (EMC / EC) of 1.5 was prepared, and LiPF 6 was added at 1 [mol] per 1 liter of the mixed solvent. ６The electrolyte solution G was prepared by dissolving the above in an EC:EMC=1:3 [molar ratio], i.e., a mixed solvent in which the molar ratio of EMC to EC (EMC / EC) was 3, and LiPF was dissolved in the mixed solvent at a concentration of 1 [mol] per liter of the total mixed solvent. ６ The resulting solution was used as electrolyte solution H. (Electrolyte solution I) A mixed solvent with EC:EMC=1:3.7 [molar ratio], i.e., a molar ratio of EMC to EC (EMC / EC) of 3.7, was prepared, and LiPF 6 was added at 1 [mol] per liter of the total mixed solvent. ６ The electrolyte solution I was prepared by dissolving the above in an EC:EMC ratio of 1:5 (molar ratio), i.e., a mixed solvent in which the molar ratio of EMC to EC (EMC / EC) was 5, was prepared. ６ The solution was used as electrolyte solution J. (Electrolyte solution K) As a comparative example, a solvent containing only EMC was prepared, and LiPF 6 was dissolved in the solvent at a concentration of 1 mol per liter. ６ The electrolyte solution K was prepared by dissolving LiPF in the mixed solvent. No additives were added to the electrolyte solutions G to K in this example. ６ (dissolved) LiPF ６ The concentration was 1.06 mol / L. Table 1 above shows the conditions for the electrolyte solutions G to K.

[0448] Coin cells were assembled using the above-described positive electrodes, negative electrodes, and electrolytes. The coin cell using electrolyte solution A was designated coin cell A, the coin cell using electrolyte solution B was designated coin cell B, the coin cell using electrolyte solution C was designated coin cell C, the coin cell using electrolyte solution D was designated coin cell D, the coin cell using electrolyte solution E was designated coin cell E, the coin cell using electrolyte solution F was designated coin cell F, the coin cell using electrolyte solution G was designated coin cell G, the coin cell using electrolyte solution H was designated coin cell H, the coin cell using electrolyte solution I was designated coin cell I, the coin cell using electrolyte solution J was designated coin cell J, and the coin cell using electrolyte solution K was designated coin cell K.

[0449] [Battery Characteristics 1 Below Freezing Point] A thermostatic chamber was adjusted to 25°C, and coin cells A to F were placed inside. Charge-discharge cycles were performed under the following conditions. In this example, 1C = 200 [mAh / g] (per weight of positive electrode active material). Condition 1 (25°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 2 (-30°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 1 was repeated three times, and condition 2 was repeated ten times. Under conditions 1 and 2, a 10-minute rest period was provided between each charge and discharge. The battery characteristics of coin cells A to G at -30°C were confirmed. The results are shown in Figure 19A. The results of enlarging the vertical axis of Fig. 19A are shown in Fig. 19B. The vertical axis of Fig. 19A and Fig. 19B represents discharge capacity [mAh / g], and the horizontal axis represents the number of cycles [times], with Condition 2 corresponding to 4 to 13 times on the horizontal axis.

[0450] As a result of the measurement, when EC:EMC = 1:Z [molar ratio], coin cells B to F, in which Z was greater than 1.5, specifically satisfying 5≦Z≦83, exhibited higher discharge capacities at −30° C. compared to coin cell A, in which Z was outside the above range, i.e., 1.5. Therefore, in order to exhibit excellent battery characteristics below freezing point, such as −30° C., when EC:EMC = 1:Z [molar ratio], it was found that Z is preferably greater than 1.5, specifically satisfying 5≦Z≦83.

[0451] [Battery Characteristics 2 Below Freezing Point] The thermostatic chamber was adjusted to 25°C, and coin cells G to K were placed therein. Charge-discharge cycles were performed under the same conditions as those for Battery Characteristics 1 Below Freezing Point. The results are shown in Fig. 20. The vertical axis of Fig. 20 represents the discharge capacity [mAh / g], and the horizontal axis represents the number of cycles [times]. Condition 2 corresponds to 4 to 13 times on the horizontal axis.

[0452] As a result of the measurements, regardless of the lithium salt concentration, when EC:EMC = 1:Z [molar ratio], coin cells H to J, in which Z was greater than 1.5, specifically satisfying 3≦Z≦5, exhibited higher discharge capacities at −30°C than coin cell G, in which Z was outside the above range, i.e., 1.5. Furthermore, it was found that when electrolyte K (solvent consisting only of EMC) without EC was used, high discharge capacities were not exhibited at −30°C. Therefore, in order to exhibit excellent battery characteristics below freezing points, such as −30°C, when EC:EMC = 1:Z [molar ratio], it was found that, regardless of the lithium salt concentration, Z is preferably greater than 1.5, specifically satisfying 3≦Z≦5.

[0453] [Battery Characteristics at Room Temperature (25°C)] Next, the above coin cells A, D, G, H, I, and K were newly prepared and subjected to charge-discharge cycles under the following conditions: Condition 3 (25°C): Charge: CCCV charge, 0.5C rate, 4.5V, 0.05C cutoff; Discharge: CC discharge, 0.5C rate, 2.5V cutoff. Condition 3 was repeated 100 times, with a 10-minute rest period between each charge and discharge. The results are shown in Figure 21. The vertical axis of Figure 21 represents discharge capacity [mAh / g], and the horizontal axis represents the number of cycles [cycles]. Furthermore, the results of the discharge capacity values ​​at each cycle relative to the maximum discharge capacity (referred to as discharge capacity retention [%]) are shown in Figure 22. The vertical axis of FIG. 22 represents the discharge capacity retention rate [%], and the vertical axis represents the number of cycles [times], and the results for coin cell A, coin cell D, coin cell G, coin cell H, coin cell I, and coin cell K are shown.

[0454] As a result of the measurements, it was found that in order to exhibit excellent battery characteristics below freezing point and also at 25°C, when EC:EMC=1:Z [molar ratio], Z should be greater than 1.5, specifically, satisfy 3≦Z≦33, and preferably 3.7≦Z≦16.

[0455] In this example, samples were prepared in which the proportion of cyclic carbonate was varied in a mixed solvent containing TTE, a fluorine-containing ether compound, and battery characteristics were measured at sub-zero temperatures. Specifically, several samples were prepared in which the proportion of cyclic carbonate was less than 29 mol% of the total mixed solvent, and several samples were prepared in which the proportion of cyclic carbonate was greater than 29 mol%. Coin cells (CR2032 type, diameter 20 mm, height 3.2 mm) were prepared as samples. Each component of the coin cell will be described.

[0456] [Positive Electrode] A positive electrode was prepared under the same conditions as in Example 1.

[0457] [Negative Electrode] A negative electrode was prepared under the same conditions as in Example 1.

[0458] [Separator] A separator was prepared under the same conditions as in Example 1.

[0459] [Electrolyte Solution] Seven mixed solvents containing TTE were prepared with different concentrations of EC, a cyclic carbonate, as shown in the table below. In the table below, the mixed solvent ratio was EC:EMC:TTE=X:3:2 [molar ratio]. Specifically, electrolyte solutions TA to TD were prepared in which the EC ratio was less than 29 mol% per liter of the total mixed solvent, and electrolyte solutions TE to TG were prepared in which the EC ratio was higher than 29 mol%. The lithium salt of the electrolyte solution was LiPF6, which was 0.7 [mol] per liter of the total mixed solvent when the temperature of the mixed solvent was 25°C ± 5°C. ６ No additives were added to the electrolyte.

[0460]

[0461] Coin cells were assembled using the above components. The coin cell using electrolyte solution TA was designated coin cell TA, the coin cell using electrolyte solution TB was designated coin cell TB, the coin cell using electrolyte solution TC was designated coin cell TC, the coin cell using electrolyte solution TD was designated coin cell TD, the coin cell using electrolyte solution TE was designated coin cell TE, the coin cell using electrolyte solution TF was designated coin cell TF, and the coin cell using electrolyte solution TG was designated coin cell TG.

[0462] [Battery Characteristics T1 Below Freezing Point] Coin cells TA to TG were placed in a thermostatic chamber adjusted to 25°C. Charge-discharge cycles were performed under the following conditions. In this example, 1C = 200 mAh / g (per weight of positive electrode active material). Condition 1 (25°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 2 (-30°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 1 was repeated three times, condition 2 ten times, and condition 1 three times, with a 10-minute rest period between charge and discharge under conditions 1 and 2. The battery characteristics of coin cells TA to TG at -30°C were confirmed. The results are shown in Figure 23. The vertical axis of FIG. 23 represents the discharge capacity (mAh / g), and the horizontal axis represents the number of cycles (times), with condition 2 corresponding to 4 to 13 times on the horizontal axis.

[0463] Measurements showed that coin cells TA to TD, which contained less than 29 mol% EC (cyclic carbonate) relative to the total solvent mixture, exhibited higher discharge capacities at -30°C compared to coin cells TE to TG, which contained higher EC concentrations. Therefore, it was found that a solvent mixture containing less than 29 mol% EC (cyclic carbonate) relative to the total solvent mixture is effective for achieving excellent battery performance below freezing points, such as -30°C. The specific EC ratio relative to the total solvent mixture can be calculated using the values ​​in the table above. It was found that a ratio of EC between 2 mol% and 17 mol%, more preferably between 2 mol% and 9 mol%, more preferably between 2 mol% and 5 mol%, and even more preferably 2 mol% or near 2 mol% (meaning 0.8 to 1.2 times the value). It was also found that varying the EC concentration while maintaining a fixed EMC:TTE molar ratio of 3:2 demonstrated excellent battery performance below freezing points. That is, the ratio of EMC and TTE was not limited in any way to exhibit excellent battery characteristics below freezing point in this example. The EC / EMC / TFETFPE = 30 / 40 / 30 (mass ratio) described in Patent Document 4 is 4 / 3.5 / 1.8 (molar ratio), which is close to the ratio of the electrolyte TG, and it was found that the mixed solvent described in Patent Document 4 has poor battery characteristics below freezing point.

[0464] [Battery Characteristics T2 Below Freezing Point] Next, as an electrolyte solution with an adjusted lithium salt concentration, LiPF 5 was added to 1 liter of the mixed solvent at a concentration of Y mol. ６ As shown in the table below, five solvents with different Y values ​​were prepared. In the table below, the mixing ratio of the mixed solvent was EC:EMC:TTE = 1:3:2 [molar ratio], and the amount of LiPF per liter of the total mixed solvent was ６ was defined as Y [mol]. In other words, the mixed solvent was fixed at EC:EMC:TTE = 1:3:2 [molar ratio], and no additives were added. The electrolyte solution TD used in Battery Characteristics 1 above is the same as the electrolyte solution TD in the table below.

[0465]

[0466] The above components were used to assemble coin cells. The coin cell using electrolyte solution TD1 was designated coin cell TD1, the coin cell using electrolyte solution TD2 was designated coin cell TD2, the coin cell using electrolyte solution TD was designated coin cell TD, the coin cell using electrolyte solution TD3 was designated coin cell TD3, and the coin cell using electrolyte solution TD4 was designated coin cell TD4.

[0467] [Battery Characteristics T2 Below Freezing Point] A thermostatic chamber was adjusted to 25°C, and coin cells TD1, TD2, TD, TD3, and TD4 were placed therein. Charge-discharge cycles were performed under the same conditions as for battery characteristics T1. The results are shown in Figure 24. In Figure 24, the vertical axis represents discharge capacity (mAh / g), and the horizontal axis represents the number of cycles (cycles), with condition 2 corresponding to 4 to 13 cycles on the horizontal axis.

[0468] As a result of the measurement, coin cell TD1, coin cell TD2, coin cell TD, and coin cell TD3 all showed high discharge capacities at -30°C. In particular, coin cell TD2 and coin cell TD showed high discharge capacities. Therefore, in order to show even higher discharge capacities at -30°C, the lithium salt LiPF ６ However, when the temperature of the mixed solvent is 25°C ± 5°C, it is preferable that the concentration is 0.25 mol or more and 1 mol or less per liter of the mixed solvent, more preferably greater than 0.25 mol and less than 1 mol, and even more preferably 0.5 mol% or thereabouts (nearby means 0.8 times or more and 1.2 times or less than that value).

[0469] In this example, elements suitable as intercalants were determined by calculation.

[0470] <Calculation 1> Using a model assuming bulk graphite, the formation energy of GIC for each intercalant and the interlayer spread of graphite were calculated using first-principles molecular dynamics calculations. A calculation was performed assuming the insertion of one intercalant between graphite layers. In this calculation, the intercalants lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), aluminum (Al), scandium (Sc), yttrium (Y), and lanthanum (La: La is used as a representative lanthanoid) were calculated for each element. Table 4 shows the specific calculation conditions for the first-principles quantum molecular dynamics calculation. The models used in the calculations are shown in Figures 25A to 27B.

[0471]

[0472] The conditions for van der Waals forces in the density functional theory (DFT-D2) calculations are as follows: IVDW: 1 VDW_RADIUS: 30 VDW_S6: 0.75 VDW_D: 20 VDW_C6: Values ​​corresponding to each element listed in Non-Patent Document 1 (Table 1) were used (however, the value of 38.54 was used for Cs, Ba, and La) VDW_R0: Values ​​corresponding to each element listed in Non-Patent Document 1 (Table 1) were used (however, the values ​​of 1.771 were used for Cs, 1.738 for Ba, and 1.716 for La)

[0473] The perspective view including the a-axis to the c-axis in Fig. 25A and the plan view seen from the c-axis in Fig. 25B show a calculation model of graphite in which no intercalant is inserted between the crystal layers. The perspective view including the a-axis to the c-axis in Fig. 26A and the plan view seen from the c-axis in Fig. 26B show a calculation model of graphite in which an intercalant element M is inserted between the layers. The perspective view including the a-axis to the c-axis in Fig. 27A and the plan view seen from the c-axis in Fig. 27B show an example of a calculation model in which the intercalant exists as a metal (here, the crystalline structure of lithium). The parallelepipeds shown by dashed lines in Figs. 25A, 26A, and 27A represent lattices, which are repeating units of the calculation model, and the squares shown by dashed lines in Figs. 25B, 26B, and 27B also correspond to lattices, which are repeating units of the calculation model.

[0474] The calculation model of Figures 25A and 25B shows 144 carbon atoms (C), with two layers of 72 carbon atoms each stacked. The calculation model of Figures 26A and 26B shows a structure in which one intercalant atom (metal M) is inserted between the layers in the calculation model of Figures 25A and 25B. The calculation model of Figures 27A and 27B shows a structure in which 128 atoms (metal M) used as intercalants are arranged in the calculation model of Figures 26A and 26B.

[0475] The results of calculations performed under the above conditions are shown in Figures 28A, 28B, Tables 5 and 6. Figure 28A and Table 2 show the amount of change in energy ΔE caused by inserting an intercalant between graphite layers.

[0476]

[0477] The calculation method of ΔE shown in Fig. 28A and Table 5 will be described. The energy value obtained by performing a structural relaxation calculation (also called a structural optimization calculation) on the calculation model shown in Figs. 25A and 25B is defined as Eg, and the energy value obtained by performing a structural relaxation calculation on the calculation model shown in Figs. 26A and 26B is defined as E ＧＩＣ（Ｍ） The energy value obtained by performing structural relaxation calculations on the calculation model shown in FIGS. 27A and 27B is divided by 128 (the number of atoms) to obtain E ＭＥＴＡＬ（Ｍ） Then, ΔE = E ＧＩＣ（Ｍ） −(Eg + EＭＥＴＡＬ（Ｍ） For example, when the intercalant is lithium (Li), ΔE = E ＧＩＣ（Ｌｉ） −(Eg + E ＭＥＴＡＬ（Ｌｉ） In the structural relaxation calculations, the lattice size and atomic coordinates were optimized for each calculation model.

[0478] When ΔE is a negative value, there is a possibility that the intercalant will enter between the graphite layers. As shown in FIG. 28A and the calculation results in Table 5, elements that can enter between the graphite crystal layers are lithium, potassium, rubidium, cesium, strontium, and barium.

[0479] FIG. 28B and Table 6 show the change in the c-axis length of the lattice, ΔD, due to the insertion of an intercalant between the crystal layers of graphite.

[0480]

[0481] The calculation method of ΔD shown in FIG. 28B and Table 6 will be described. The c-axis length of the lattice after structural relaxation calculation of the calculation model shown in FIGS. 25A and 25B is defined as D. Ｇ The c-axis length of the lattice after structural relaxation calculation of the calculation model shown in FIGS. 26A and 26B is D ＧＩＣ（Ｍ） When this is done, the change in the c-axis length of the lattice ΔD is ΔD Ｍ =D ＧＩＣ（Ｍ） -D ＧＩＣ（Ｌｉ） For example, when the intercalant is lithium (Li), ΔD Ｌｉ =D ＧＩＣ（Ｌｉ） -D ＧＩＣ（Ｌｉ） Note that 1 Å = 10 −１０ m.

[0482] The metal M is the change in the c-axis length of the lattice when the metal M is inserted between the layers of graphite. Ｍ The change in the c-axis length of the lattice when lithium is inserted between the graphite layers is ΔD Ｌｉ It is preferable that the element has a larger ΔD Ｌｉ ΔD greater than ＭWhen graphite has a region where metal M is inserted between layers, it is expected that the region where metal M is inserted will reduce the reaction resistance when lithium is inserted between the layers of graphite.

[0483] Therefore, ΔD Ｌｉ ΔD greater than Ｍ When graphite has a region where metal M is intercalated between layers, it is believed that lithium can be intercalated between the graphite layers in the region where metal M is intercalated, even in a low-temperature environment. In other words, the charge-discharge characteristics of the battery can be improved in a low-temperature environment.

[0484] As shown in FIG. 28B and the calculation results in Table 6, it is predicted that all of the elements calculated this time can widen the gap between the crystal layers of graphite more than when lithium is inserted.

[0485] <Calculation 2> Next, Calculation 2 was performed in which some of the calculation conditions were changed from Calculation 1.

[0486] 25A to 27B was used as the calculation model, as in Calculation 1. Table 7 shows specific calculation conditions for the first-principles molecular dynamics calculation.

[0487]

[0488] The IVDW was set to 12 as a condition for van der Waals force interactions in the density functional theory (DFT-D3BJ) calculation.

[0489] The results of calculations performed under the same conditions as those shown in Calculation 1, except for the conditions related to the functionals, are shown in Figures 29A, 29B, Tables 8 and 9. Figure 29A and Table 8 show the amount of change in energy ΔE caused by inserting an intercalant between the crystal layers of graphite.

[0490]

[0491] Elements with a negative ΔE value have the potential to intercalate between the crystal layers of graphite as intercalants. As shown in FIG. 29A and the calculation results in Table 8, elements that have the potential to intercalate between the layers of graphite are lithium, potassium, rubidium, cesium, calcium, strontium, barium, and lanthanum. Note that, as a result of Calculation 2, lanthanum has the potential to intercalate between the layers of graphite, and therefore, it is believed that not only lanthanum but also lanthanoids other than lanthanum are elements that have the potential to intercalate between the layers of graphite.

[0492] FIG. 29B and Table 9 show the change ΔD in the c-axis length of the lattice due to the insertion of an intercalant between the crystal layers of graphite.

[0493]

[0494] As shown in FIG. 29B and the calculation results in Table 9, it is predicted that all of the elements calculated this time can widen the gap between graphite layers more than when lithium is inserted.

[0495] Calculation 1 and Calculation 2 are calculations in which the correction conditions for the van der Waals forces are different. The calculation conditions for Calculation 1 are KC ８ The calculation conditions for Calculation 2 are the conditions under which a calculated value (0.523 to 0.530 nm) close to the measured value (0.535 nm) of the interlayer distance of LiC ６ These are the conditions under which the calculated value (0.359 nm to 0.360 nm) of the interlayer distance is close to the measured value (0.372 nm). In this way, in calculations dealing with layered materials such as graphite, the calculation results may differ depending on the correction conditions for van der Waals forces, so calculations were performed under multiple correction conditions.

[0496] Based on the results of Calculation 1 and Calculation 2 shown above, elements that can be used as the metal M will be considered. In at least one of Calculation 1 and Calculation 2, an element that has a negative energy change when inserted between the crystal layers of graphite can be used as the metal M. In other words, the metal M can be any one or more of potassium, rubidium, cesium, calcium, strontium, barium, and lanthanum (or lanthanoids). Among these, it has been found that it is more preferable to use either one or both of potassium and barium as the metal M because of its high safety.

[0497] As the anion of the metal M salt, for example, bis(fluorosulfonyl)imide (FSI − ), bis(trifluoromethanesulfonyl)imide (TFSI − ), P.F. ６ − , ClO ４ − , B.F. ４ − , and SCN − At least one anion among the above can be used.

[0498] When potassium is used as the metal M, potassium bis(fluorosulfonyl)imide (KFSI), potassium bis(trifluoromethanesulfonyl)imide (KTFSI), KPF ６ , KClO ４ , K.B.F. ４ For an electrolyte that exhibits excellent battery characteristics in a low-temperature environment, it is preferable to use KFSI as the second lithium salt.

[0499] In this example, to confirm the effect of potassium ions, which are cations with a larger ionic radius than lithium ions in the electrolyte, samples using lithium salt and potassium salt, and samples using only lithium salt, were prepared and battery characteristics were measured below freezing point. Coin cells (CR2032 type, diameter 20 mm, height 3.2 mm) were prepared as samples. Each component of the coin cell will be described.

[0500] [Positive Electrode] A positive electrode was prepared under the same conditions as in Example 1.

[0501] [Negative Electrode] A negative electrode was prepared under the same conditions as in Example 1.

[0502] [Separator] A separator was prepared under the same conditions as in Example 1.

[0503] [Electrolyte] In this example, as shown in Table 10, samples with different conditions for the salt contained in the electrolyte were prepared, including electrolytes containing lithium salt and potassium salt (KA, KD) and electrolytes containing only lithium salt (KB, KC, KE). In the table below, two types of mixed solvents were prepared, and the amount of lithium salt (LiPF) per 1 L of the mixed solvent was ６ The concentrations of the sucrose and sucrose (one or more of sucrose and KFSI) were varied.

[0504] The electrolyte KA is a lithium salt, LiPF ６ The electrolyte KB contains 0.7 mol of KFSI per liter of the total mixed solvent, and 0.2 mol of KFSI, a potassium salt, per liter of the total mixed solvent (EC:EMC:TTE = 1:3:2 [mol ratio], where the EC ratio is 17 [mol %]). ６ The electrolyte solution KC contains only LiPF , a lithium salt, in a mixed solvent similar to the electrolyte solution KA at a concentration of 1 mol per liter of the total mixed solvent. ６ The electrolyte solution KD contains only LiPF , a lithium salt, in a mixed solvent similar to the electrolyte solution KA, at a concentration of 0.7 mol per liter of the total mixed solvent. ６ The electrolyte KE contains 1 mol of FEC per liter of the total mixed solvent, and 0.2 mol of KFSI, a potassium salt, per liter of the total mixed solvent (FEC:MTFP = 2.8:7.2 [mol ratio]). ６Only 1 mol of lithium salt was contained in a mixed solvent similar to that of electrolyte solution KD at a concentration of 1 mol per liter of the total mixed solvent. The concentrations of lithium salt in electrolyte solutions KA to KE were higher than that of potassium salt. The concentrations of lithium salt in electrolyte solutions KA to KE were values ​​per liter of mixed solvent when the temperature of the mixed solvent was 25°C ± 5°C, and the concentrations of potassium salt in electrolyte solutions KA and KD were values ​​per liter of mixed solvent when the temperature of the mixed solvent was 25°C ± 5°C. No additives were added to the electrolyte solutions of this example.

[0505]

[0506] Coin cells were assembled using the above components. As shown in Table 10, the coin cell using electrolyte solution KA was designated coin cell KA, the coin cell using electrolyte solution KB was designated coin cell KB, the coin cell using electrolyte solution KC was designated coin cell KC, the coin cell using electrolyte solution KD was designated coin cell KD, and the coin cell using electrolyte solution KE was designated coin cell KE.

[0507] [Battery Characteristics K1 at Subzero Temperatures] Coin cell KA, coin cell KB, coin cell KD, and coin cell KE were placed in a thermostatic chamber adjusted to 25°C. Charge-discharge cycles were performed under the following conditions. In this example, 1C = 200 mAh / g (per weight of positive electrode active material). Condition 1 (25°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 2 (-30°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 1 was repeated three times, condition 2 ten times, and condition 1 three times, with a 10-minute rest period between each charge and discharge. The results are shown in Figure 30. The vertical axis of FIG. 30 represents the discharge capacity (mAh / g), and the horizontal axis represents the number of cycles (times), with condition 2 corresponding to 4 to 13 times on the horizontal axis.

[0508] The measurement results showed that coin cell KA exhibited superior battery performance at sub-zero temperatures (-30°C) compared to coin cell KB. It appears that cycle degradation was particularly suppressed in coin cell KA. Furthermore, coin cell KD exhibited superior battery performance at sub-zero temperatures (-30°C) compared to coin cell KE. These results indicate that electrolytes containing lithium salt and a second salt, specifically, lithium salt and potassium salt, exhibit superior battery performance at sub-zero temperatures compared to electrolytes containing only lithium salt. This is likely due to the formation of GBIC (Graphite Bi-Intercalation Compound) (sometimes referred to as GBC), in which potassium and lithium are co-intercalated into graphite, resulting in superior discharge capacity and suppressed cycle degradation at sub-zero temperatures. While potassium salt as the second salt is believed to be effective for improving battery performance at sub-zero temperatures, the results of Example 3 indicate that barium and other salts also form GIC, suggesting that the addition of barium salt also exhibits superior battery performance at sub-zero temperatures.

[0509] Based on the results of coin cell KA, coin cell KB, coin cell KD, and coin cell KE, it was found that the mixed solvent is not particularly limited in terms of exhibiting excellent battery characteristics below freezing. Furthermore, based on the results of coin cell KA and coin cell KB, it was found that the mixed solvent preferably contains 17 mol% or less of cyclic carbonate EC. It was also found that the EC content range is preferably 2 mol% or more and 17 mol% or less.

[0510] [Battery Characteristics K2 Below Freezing Point] Next, the coin cell KA and coin cell KC described above were newly prepared, and Condition 1 used in [Battery Characteristics K1 Below Freezing Point] was repeated three times, followed by Condition 2 57 times. A 10-minute rest period was provided between each charge and discharge. The results are shown in Figure 31. The vertical axis of Figure 31 represents the discharge capacity (mAh / g), and the horizontal axis represents the number of cycles.

[0511] The measurement results showed that coin cell KA exhibited better cycle characteristics at sub-zero temperatures than coin cell KC. The maximum discharge capacity of coin cell KA at sub-zero temperatures was 106 mAh / g at the 15th cycle. The discharge capacity retention rate of coin cell KA was 68%, and the electrolyte solution of this example achieved a discharge capacity retention rate of 60% or more at -30°C.

[0512] In this example, samples were prepared in which the proportion of fluorine-containing aromatic compounds having 9 or less carbon atoms in the mixed solvent used in the electrolyte was varied, and battery characteristics below freezing were obtained. Specifically, several samples were prepared in which the proportion of fluorine-containing aromatic compounds having 9 or less carbon atoms was 33 mol% or less relative to the total mixed solvent, and samples containing fluorine-containing aromatic compounds at a higher proportion were also prepared. Coin cells (CR2032 type, 20 mm diameter, 3.2 mm height) were prepared as samples. Each component of the coin cell will be described.

[0513] [Positive Electrode] A positive electrode was prepared under the same conditions as in Example 1.

[0514] [Negative Electrode] A negative electrode was prepared under the same conditions as in Example 1.

[0515] [Separator] A separator was prepared under the same conditions as in Example 1.

[0516] [Electrolyte Solution] As shown in the table below, three mixed solvents for the electrolyte solution were prepared, each with a different concentration ratio of FB, a fluorine-containing aromatic compound having 9 or less carbon atoms. In the table below, the mixed solvent ratio was EC:EMC:FB=Y:3:Z (molar ratio). Specifically, electrolyte solutions FA and FB were prepared, each containing FB at a ratio of 33 mol% or less relative to the total mixed solvent. Electrolyte solution FC was also prepared, containing FB at a ratio greater than 33 mol%. Furthermore, electrolyte solution FD, which did not contain FB, was also prepared. In electrolyte solutions FA to FC, the lithium salt was LiPF in an amount of 0.7 mol per liter of the mixed solvent when the temperature of the mixed solvent was 25°C ± 5°C. ６ In the electrolyte FD, the lithium salt is LiPF in an amount of 1 mol per liter of the total mixed solvent when the temperature of the mixed solvent is 25°C ± 5°C. ６No additive was added to the electrolyte solution in this example.

[0517]

[0518] The above components were used to assemble coin cells. The coin cell using electrolyte solution FA was designated as coin cell FA, the coin cell using electrolyte solution FB was designated as coin cell FB, the coin cell using electrolyte solution FC was designated as coin cell FC, and the coin cell using electrolyte solution FD was designated as coin cell FD.

[0519] [Battery Characteristics F1 at Subzero Temperatures] Coin cells FA to FD were placed in a thermostatic chamber adjusted to 25°C. Charge-discharge cycles were performed under the following conditions. In this example, 1C = 200 mAh / g (per weight of positive electrode active material). Condition 1 (25°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 2 (-30°C): Charge: CCCV charge, 0.1C rate, 4.5V, 0.01C cutoff. Discharge: CC discharge, 0.1C rate, 2.5V cutoff. Condition 1 was repeated three times, condition 2 ten times, and condition 1 three times, with a 10-minute rest period between each charge and discharge under conditions 1 and 2. The battery characteristics of coin cells FA to FD at -30°C were confirmed. The results are shown in Figure 32. The vertical axis of FIG. 32 represents the discharge capacity (mAh / g), and the horizontal axis represents the number of cycles (times), with condition 2 corresponding to 4 to 13 times on the horizontal axis.

[0520] As a result of the measurements, coin cells FA and FB, which contained 33 mol% or less of FB, a fluorine-containing aromatic compound with 9 or less carbon atoms, relative to the total mixed solvent, showed higher discharge capacities at −30°C than coin cell FC, which contained a higher proportion of FB. Therefore, it was found that in order to exhibit excellent battery characteristics below freezing points such as −30°C, it is best to keep the proportion of FB, a fluorine-containing aromatic compound with 9 or less carbon atoms, relative to the total mixed solvent to 33 mol% or less. The specific proportion of FB relative to the total mixed solvent can be shown using the values ​​in the table above, and it was found that it is best to have 33 mol% or less, more preferably 11 mol% to 33 mol%. Coin cell FD, which did not contain FB, had lower discharge capacities than coin cells FA and FB. It is estimated that battery characteristics at an even lower temperature of −40°C are best achieved when the proportion of FB is 11 mol% or less (nearby, meaning 0.9 to 1.1 times that value), as in coin cell FB.

[0521] Furthermore, the coin cell FA showed a smaller change in discharge capacity at -30°C than the coin cell FB, demonstrating superior battery characteristics. Specifically, the coin cell FA had a discharge capacity of 130 mAh / g at the fourth cycle and 145 mAh / g at the thirteenth cycle, demonstrating a suppressed decrease in discharge capacity during cycling at -30°C. In other words, the coin cell FA suppressed the change in discharge capacity during cycling tests at -30°C to within 15%. Furthermore, the coin cell FA had a discharge capacity of 193 mAh / g at the fourteenth cycle, a suppressed decrease compared to the discharge capacity of 203 mAh / g at the third cycle. In other words, the coin cell FA suppressed the change in discharge capacity during cycling tests at 25°C to within 5%. It was found that the coin cell FA exhibited the best electrical characteristics.

[0522] Next, the proportion of EC was examined. The proportions of EC in each coin cell were 22 mol% for coin cell FA, 17 mol% for coin cell FB, 8 mol% for coin cell FC, and 40 mol% for coin cell FD. Furthermore, the mixed solvent of ethylene carbonate, ethyl methyl carbonate, and fluorobenzene (volume ratio 2:7:1) described in Example 17 of Patent Document 7 had a molar ratio of 3:6.9:1.1, with an ethylene carbonate content of 27 mol% or more, which is considered to be an electrolyte similar to that of coin cell FD. Furthermore, the mixed solvent of ethylene carbonate, ethyl methyl carbonate, and fluorobenzene in a volume ratio of 3:6:1 described in Comparative Example 2 of Patent Document 8 had a molar ratio of 4:5.2:0.95, with an ethylene carbonate content of 39 mol% or more, which is considered to be an electrolyte similar to that of coin cell FD. Since the coin cell FD has poor battery characteristics at −30° C., it is thought that Patent Document 7 and Patent Document 8 also have poor battery characteristics at −30° C. From this result, in order to exhibit excellent battery characteristics below freezing, the proportion of EC in the mixed solvent is preferably 25 mol % or less. It is estimated that a proportion of EC of 25 mol % or less will exhibit appropriate viscosity for exhibiting excellent battery characteristics below freezing.

[0523] [Battery Characteristics F at 25°C] Next, the coin cells FA to FC were newly prepared and subjected to charge-discharge cycles under the following conditions. Condition 3 (25°C): Charge: CCCV charge, 0.5C rate, 4.5V, 0.05C cutoff. Discharge: CC discharge, 0.5C rate, 2.5V cutoff. Condition 3 was repeated 100 times, with a 10-minute rest period between each charge and discharge. The results are shown in Figure 33. The vertical axis of Figure 33 represents discharge capacity (mAh / g), and the horizontal axis represents the number of cycles.

[0524] Measurements revealed that in order to achieve excellent battery performance both below freezing and at 25°C, the ratio of FB, a fluorine-containing aromatic compound with 9 or less carbon atoms, to the total mixed solvent should be 33 mol% or less. The specific ratio of FB to the total mixed solvent can be shown using the values ​​in the table above, and it was found that it is preferable to set it to 33 mol% or less, more preferably 11 mol% to 33 mol%. The maximum discharge capacity at the 12th cycle was 194 mAh / g for coin cell FA and 190 mAh / g for coin cell FB. The discharge capacity retention rate was 94% for coin cell FA and 98% for coin cell FB. The electrolyte of this example achieved a discharge capacity retention rate of 90% or more.

[0525] 10: positive electrode active material, 10a: surface layer portion, 10b: interior, 20: second positive electrode active material, 21: positive electrode current collector, 21t: protrusion portion, 22: positive electrode active material layer, 31: negative electrode current collector, 31t: protrusion portion, 31ta: first protrusion portion, 31tb: protrusion portion, 31tc: third protrusion portion, 32: negative electrode active material layer, 41: conductive material, 100: secondary battery, 103: positive electrode, 103a: first positive electrode, 103b: second positive electrode, 105: separator, 105a: first separator, 105b: second separator, 105c: third separator, 105d: fourth separator, 106: negative electrode, 106a: second 1 negative electrode, 106b: second negative electrode, 106c: third negative electrode, 107a: positive electrode lead, 107b: negative electrode lead, 108: electrolyte, 109a: joint, 109b: joint, 300: secondary battery, 301: positive electrode can, 302: negative electrode can, 303: gasket, 304: positive electrode, 305: positive electrode current collector, 306: positive electrode active material layer, 307: negative electrode, 308: negative electrode current collector, 309: negative electrode active material layer, 310: separator, 312: washer, 322: spacer, 503: positive electrode, 506: negative electrode, 507: separator, 509: exterior body, 510: positive electrode lead electrode, 511: negative electrode lead Electrode, 601: positive electrode cap, 602: secondary battery can, 603: positive electrode terminal, 604: positive electrode, 605: separator, 606: negative electrode, 607: negative electrode terminal, 608: insulating plate, 609: insulating plate, 611: PTC element, 613: safety valve mechanism, 614: conductive plate, 615: power storage system, 616: secondary battery, 620: control circuit, 621: wiring, 622: wiring, 623: wiring, 624: conductor, 625: insulator, 626: wiring, 627: wiring, 628: conductive plate, 901: mixture, 902: composite oxide, 903: mixture, 911a: terminal, 911b: terminal, 913: secondary battery , 930: housing, 930a: housing, 930b: housing, 931: negative electrode, 931a: negative electrode active material layer, 932: positive electrode, 932a: positive electrode active material layer, 933: separator, 950: wound body, 950a: wound body, 951: terminal, 952: terminal, 1300: prismatic secondary battery, 1301a: first battery, 1301b: first battery, 1302: battery controller, 1303: motor controller, 1304: motor, 1305: gear, 1306: DCDC circuit, 1307: electric power steering, 1308: heater, 1309: defogger, 1310: DCDC circuit,1311: Second battery, 1312: Inverter, 1313: Audio, 1314: Power window, 1315: Lamps, 1316: Tire, 1317: Rear motor, 1320: Control circuit section, 1321: Control circuit section, 1413: Fixing section, 1414: Fixing section, 1421: Wiring, 1422: Wiring, 2001: Automobile, 2200: Secondary battery pack, 6800: Artificial Satellite, 6801: Airframe, 6802: Solar panel, 6803: Antenna, 6805: Secondary battery, 6900: Probe, 6901: Airframe, 6902: Solar sail, 6905: Secondary battery, 6910: Spacecraft, 6911: Airframe, 6912: Solar panel, 6913: Secondary battery, 6920: Rover, 6921: Airframe, 6922: Solar panel, 6923: Secondary battery,

Claims

A secondary battery having an electrolyte, the electrolyte solution contains at least a mixed solvent and a lithium salt; the mixed solvent contains a cyclic carbonate and a chain carbonate, a molar ratio (CH / CY) of the chain carbonate (CH) to the cyclic carbonate (CY) is greater than 1.5; the concentration of the lithium salt is 0.25 mol or more and 1 mol or less per liter of the mixed solvent; Secondary battery. 　 In claim 1, The cyclic carbonate is ethylene carbonate, The chain carbonate is ethyl methyl carbonate, a molar ratio (EMC / EC) of the ethyl methyl carbonate (EMC) to the ethylene carbonate (EC) is 3 or more and 83 or less; Secondary battery. 　 In claim 1, The cyclic carbonate is ethylene carbonate, The chain carbonate is ethyl methyl carbonate, a molar ratio (EMC / EC) of the ethyl methyl carbonate (EMC) to the ethylene carbonate (EC) is 3 or more and 33 or less; Secondary battery. 　 A secondary battery having an electrolyte, the electrolyte solution contains at least a mixed solvent and a lithium salt; the mixed solvent contains a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, The proportion of the cyclic carbonate is less than 29 mol% based on the total amount of the mixed solvent. Secondary battery. 　 In claim 4, The mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. Secondary battery. 　 A secondary battery having a negative electrode active material containing a carbon material and an electrolyte solution, the electrolyte solution includes at least a mixed solvent, a first salt, and a second salt; the first salt comprises a lithium salt; the second salt has a cation with an ionic radius larger than that of a lithium ion; the mixed solvent contains a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, The ratio of the cyclic carbonate to the mixed solvent is 2 mol % or more and 25 mol % or less. Secondary battery. 　 A secondary battery having a negative electrode active material containing a carbon material and an electrolyte solution, the electrolyte solution includes at least a mixed solvent, a first salt, and a potassium salt; the first salt comprises a lithium salt; the mixed solvent contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), The ratio of the EC to the mixed solvent is 2 mol% or more and 17 mol% or less. Secondary battery. 　 In claim 7, A secondary battery, wherein the concentration of the lithium salt in the electrolyte is higher than the concentration of the potassium salt. 　 A secondary battery having a negative electrode active material containing a carbon material and an electrolyte solution, The electrolyte solution contains at least a mixed solvent, LiPF ６ and KFSI, the mixed solvent contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), The ratio of the EC to the mixed solvent is 2 mol% or more and 17 mol% or less. Secondary battery. 　 In claim 9, The LiPF in the electrolyte ６ The concentration of the secondary battery is higher than the concentration of the KFSI. 　 A secondary battery having an electrolyte, the electrolyte solution contains at least a mixed solvent and a lithium salt; the mixed solvent contains a cyclic carbonate, a chain carbonate, and a fluorine-containing aromatic compound, the proportion of the fluorine-containing aromatic compound is 33 mol % or less based on the total amount of the mixed solvent; Secondary battery. 　 In claim 11, the mixed solvent comprises ethylene carbonate, ethyl methyl carbonate, and fluorobenzene; The ratio of the fluorobenzene to the total amount of the mixed solvent is 11 mol % or more and 33 mol % or less. Secondary battery. 　 The secondary battery according to any one of claims 1 to 12 further comprises a positive electrode active material, the positive electrode active material comprises lithium cobalt oxide, magnesium, fluorine, aluminum, and nickel; the surface layer portion of the positive electrode active material contains magnesium and fluorine; Secondary battery. 　 comprising at least a mixed solvent and a lithium salt; the mixed solvent contains a cyclic carbonate and a chain carbonate, a molar ratio (CH / CY) of the chain carbonate (CH) to the cyclic carbonate (CY) is greater than 1.5; the concentration of the lithium salt is 0.25 mol or more and 1 mol or less per liter of the mixed solvent; Electrolyte. 　 In claim 14, The cyclic carbonate is ethylene carbonate, The chain carbonate is ethyl methyl carbonate, a molar ratio (EMC / EC) of the ethyl methyl carbonate (EMC) to the ethylene carbonate (EC) is 3 or more and 83 or less; Electrolyte. 　 In claim 14, The cyclic carbonate is ethylene carbonate, The chain carbonate is ethyl methyl carbonate, a molar ratio (EMC / EC) of the ethyl methyl carbonate (EMC) to the ethylene carbonate (EC) is 3.7 or more and 33 or less; Electrolyte. 　 In claim 14, the concentration of the lithium salt is 0.5 mol or more and 1 mol or less per liter of the mixed solvent; Electrolyte. comprising at least a mixed solvent and a lithium salt; the mixed solvent contains a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, The proportion of the cyclic carbonate is less than 29 mol% based on the total amount of the mixed solvent. Electrolyte. 　 In claim 18, The mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. Electrolyte. 　 In claim 18, the mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether; The proportion of the ethylene carbonate is 2 mol % or more and 9 mol % or less with respect to the entire mixed solvent. Electrolyte. 　 In claim 18, the mixed solvent contains ethylene carbonate, ethyl methyl carbonate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether; The ratio of the ethylene carbonate to the total amount of the mixed solvent is 2 mol % or more and 5 mol % or less. Electrolyte. 　 comprising at least a mixed solvent, a first salt, and a second salt; the first salt comprises a lithium salt; the second salt has a cation with an ionic radius larger than that of a lithium ion; the mixed solvent contains a cyclic carbonate, a chain carbonate, and a fluorine-containing ether compound, The ratio of the cyclic carbonate to the mixed solvent is 2 mol % or more and 25 mol % or less. Electrolyte. comprising at least a mixed solvent, a lithium salt, and a potassium salt; the mixed solvent comprises EC, EMC, and TTE; The ratio of the EC to the mixed solvent is 2 mol% or more and 17 mol% or less. Electrolyte. 　 In claim 23, The concentration of the lithium salt in the electrolyte is higher than the concentration of the potassium salt. At least a mixed solvent, LiPF ６ and KFSI, the mixed solvent comprises EC, EMC, and TTE; The ratio of the EC to the mixed solvent is 2 mol% or more and 17 mol% or less. Electrolyte.

26. In claim 25, The LiPF in the electrolyte ６ The concentration of the electrolyte is higher than the concentration of the KFSI. 　 comprising at least a mixed solvent and a lithium salt; the mixed solvent contains a cyclic carbonate, a chain carbonate, and a fluorine-containing aromatic compound, the proportion of the fluorine-containing aromatic compound is 33 mol % or less based on the total amount of the mixed solvent; Electrolyte.

28. In claim 27, the mixed solvent comprises ethylene carbonate, ethyl methyl carbonate, and fluorobenzene; The ratio of the fluorobenzene to the total amount of the mixed solvent is 11 mol % or more and 33 mol % or less. Electrolyte.

28. In claim 27, The lithium salt is present in an amount of 0.25 mol or more and less than 1 mol per liter of the mixed solvent. Electrolyte.

28. In claim 27, the mixed solvent comprises ethylene carbonate, The proportion of the ethylene carbonate is 25 mol % or less based on the total amount of the mixed solvent. Electrolyte.

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