Lithium ion battery

Abstract

The present invention provides a lithium ion battery which comprises a novel electrolyte or the like. A lithium ion battery according to the present invention is provided with: a positive electrode active material that contains nickel, cobalt and manganese; and an electrolyte that contains a fluorinated cyclic carbonate and a fluorinated chain carbonate. With respect to this lithium ion battery, the discharge capacity that is obtained by charging a half cell, which comprises the positive electrode active material and the electrolyte, with a constant current at a rate of 0.1 C until the voltage reaches 4.5 V, then charging the half cell with a constant voltage of 4.5 V until the current value falls to 0.05 C in an environment with an ambient temperature of 25°C, and subsequently discharging the half cell with a constant current at a rate of 0.1 C until the voltage falls to 2.5 V in an environment with an ambient temperature of -40°C is equal to or greater than 50% of the discharge capacity that is obtained by arranging the half cell in an environment with an ambient temperature of 25°C.

Description

Lithium-ion battery

[0001] One aspect of the present invention relates to a lithium-ion battery.

[0002] One embodiment of the present invention is not limited to the above fields, and relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, a vehicle, and a manufacturing method thereof. The above-described semiconductor device, display device, light-emitting device, power storage device, lighting device, electronic device, and vehicle can use the lithium-ion battery of one embodiment of the present invention as a necessary power source. For example, the above-described electronic device includes an information terminal device equipped with a lithium-ion battery. Furthermore, the above-described power storage device includes a stationary power storage device.

[0003] A lithium ion battery (sometimes referred to as a lithium ion secondary battery) refers to a battery that uses lithium ions as carrier ions. A lithium ion battery is a secondary battery that can be used repeatedly by charging and discharging. However, the carrier ions of the present invention are not limited to lithium ions, and alkali metal ions or alkaline earth metal ions can also be used as carrier ions. Specifically, sodium ions or magnesium ions can be used. In this case, the present invention can be understood by replacing lithium ions with sodium ions or magnesium ions. Furthermore, when the carrier ions are not limited, the battery may be referred to as a secondary battery.

[0004] Lithium-ion batteries are expected to be able to charge and discharge over a wide temperature range depending on the application. Therefore, research and development into enabling lithium-ion batteries to be charged and discharged at high and low temperatures is actively underway. Patent Document 1 proposes a fluorinated chain carboxylic acid ester as a non-aqueous electrolyte to suppress the decrease in battery capacity at high temperatures. Non-Patent Document 1 proposes a 9:1 mixture of methyl 3,3,3-trifluoropropionate (MTFP):fluoroethylene carbonate (FEC) as an electrolyte to improve output characteristics below freezing. Non-Patent Document 2 reports the crystal structure of a positive electrode active material.

[0005] JP 2009-289414 A

[0006] John, H. et al, “An All-Fluorinated Ester Electrolyte for Stable High-Voltage Li Metal Batteries Capable of Ultra-Low-Temperature Operation”, ACE Energy LETTERS, 2020, 5, 1438-1447 Zhaohui Chen et al, “Staging Phase Transitions in LixCoO▲2▼”, Journal of The Electrochemical Society, 2002, 149 (12) A1604-A1609

[0007] In Example 1 of Patent Document 1, lithium cobalt oxide (LiCoO ２ ) in which Al and Mg are respectively dissolved and Zr is applied to the particle surface, is used as the positive electrode active material, and 4-fluoroethylene carbonate (4-FEC) and CF ３ CH ２ COOCH ３ The document describes the use of a mixture of 2:8 by volume of the organic solvent for the non-aqueous electrolyte. However, Patent Document 1 does not consider battery characteristics below freezing point.

[0008] In Non-Patent Document 1, in order to examine battery characteristics below freezing point, it is described that a mixture of methyl 3,3,3-trifluoropropionate (MTFP) and fluoroethylene carbonate (FEC) in a ratio of 9:1 is used as an organic solvent for a non-aqueous electrolyte, and in cycle tests, NMC811 is used as a positive electrode active material. However, Non-Patent Document 1 did not examine battery characteristics at high temperatures.

[0009] In view of the above, an object of the present invention is to provide a lithium-ion battery having an electrolyte containing a new organic solvent and a new positive electrode active material, which enables charging and discharging over a wide temperature range, including from below freezing to high temperatures.

[0010] Note that the above-mentioned problems do not preclude the existence of other problems. Furthermore, the above-mentioned problems are considered to be independent of each other, and it is not necessary to solve all of the above-mentioned problems by one embodiment of the present invention. Furthermore, problems other than those mentioned above can be extracted from the description in the specification, drawings, and claims (these will be referred to as the "present specification, etc.").

[0011] In order to solve the above problems, one aspect of the present invention is a lithium-ion battery including a positive electrode having a positive electrode active material and an electrolyte, wherein the positive electrode active material includes lithium cobalt oxide containing Mg, F, Ni, and Al, and the electrolyte includes a fluorinated cyclic carbonate and a fluorinated chain carbonate.

[0012] Another aspect of the present invention is a lithium-ion battery including a positive electrode having a positive electrode active material and an electrolyte, wherein the positive electrode active material includes lithium cobalt oxide containing Mg, F, Ni, and Al, and the electrolyte includes fluoroethylene carbonate and methyl trifluoropropionate, and when the total content of the fluoroethylene carbonate and methyl trifluoropropionate is 100 vol %, the volume ratio of the fluoroethylene carbonate to the methyl trifluoropropionate is x:100−x (where 5≦x≦30).

[0013] The lithium cobalt oxide of one embodiment of the present invention is LixCoO ２ and LixCoO ２ When x is 1, lithium cobaltate has a layered rock-salt type crystal structure belonging to the space group R-3m, LixCoO ２ In the formula, when x satisfies 0.1<x≦0.24, that is, when the lithium cobaltate is in a charged state, it is preferable that the lithium cobaltate has a crystal structure of space group P2 / m, lattice constants a=0.488±0.001 nm, lattice constant b=0.282±0.001 nm, lattice constant c=0.484±0.001 nm, α=90°, β=109.58±0.01°, and γ=90°.

[0014] Another embodiment of the lithium cobalt oxide of the present invention is LixCoO ２ and LixCoO ２ When x is 1, lithium cobaltate has a layered rock-salt type crystal structure belonging to the space group R-3m, LixCoO２ When x satisfies the formula (1), 0.1<x≦0.24, i.e., when the lithium cobalt oxide is in a charged state, it is preferable that the lithium cobalt oxide has diffraction peaks at least at 2θ=19.37° or more and 19.57° or less and at 2θ=45.57° or more and 45.67° or less when analyzed by X-ray diffraction.

[0015] In another embodiment of the present invention, the lithium cobalt oxide preferably has a median diameter (D50) of 10 μm or more and 14 μm or less.

[0016] In another embodiment of the present invention, the lithium cobalt oxide preferably has a median diameter (D50) of 5 μm to 9 μm.

[0017] Another aspect of the present invention is a lithium ion battery comprising a positive electrode active material containing lithium cobalt oxide containing Mg, F, Ni, and Al, and an electrolyte containing a fluorinated cyclic carbonate and a fluorinated chain carbonate. A half-cell is prepared, comprising a positive electrode containing the positive electrode active material, the electrolyte, and a counter electrode of lithium metal. The half-cell is placed at an ambient temperature of 25° C. and is charged at a constant current of 0.1 C (where 1 C=200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.6 V is reached, and the constant current is maintained at 4.6 V until the current value reaches 0.05 C. The lithium-ion battery has a discharge capacity value of 50% or more when the same half-cell is placed at an ambient temperature of 25°C and subjected to constant current charging at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.6 V is reached, and then placed at an ambient temperature of -40°C and subjected to constant current discharging at a rate of 0.1 C until a voltage of 2.5 V is reached. The discharge capacity value is then determined by placing the same half-cell at an ambient temperature of 25°C and constant current charging at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.6 V is reached, and then constant voltage charging at 4.6 V until the current value reaches 0.05 C. The lithium-ion battery has a discharge capacity value of 50% or more when the same half-cell is placed at an ambient temperature of -40°C and constant current discharging at a rate of 0.1 C until a voltage of 2.5 V is reached.

[0018] Another aspect of the present invention is a lithium ion battery comprising a positive electrode having a positive electrode active material including lithium cobalt oxide containing Mg, F, Ni, and Al, and an electrolyte, wherein a half-cell is prepared comprising a positive electrode having the positive electrode active material, the electrolyte, and a counter electrode of lithium metal, and the half-cell is placed at an ambient temperature of 25°C and is subjected to constant current charging at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.6 V is reached, followed by constant voltage charging at 4.6 V until the current value reaches 0.05 C, and then charging at 2. This lithium-ion battery has a discharge capacity value of 50% or more of the value of the discharge capacity obtained by discharging at a constant current at a rate of 0.1 C until a voltage of 5 V is reached, and the same half-cell is placed at an ambient temperature of 25°C, charged at a constant current at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.6 V is reached, charged at a constant voltage of 4.6 V until the current value reaches 0.05 C, and then placed at an ambient temperature of -40°C and discharged at a constant current at a rate of 0.1 C until a voltage of 2.5 V is reached.

[0019] Another aspect of the present invention is a lithium ion battery comprising a positive electrode active material containing nickel, cobalt, and manganese, and an electrolyte containing a fluorinated cyclic carbonate and a fluorinated chain carbonate. A half-cell comprising a positive electrode having the positive electrode active material, the electrolyte, and a lithium metal counter electrode is placed at an ambient temperature of 25°C, and is charged at a constant current of 0.1C (where 1C = 200mA / g (current per weight of positive electrode active material is 200mA / g)) until a voltage of 4.5V is reached, and then charged at a constant voltage of 4.5V until the current value reaches 0.05C, and then charged at a constant voltage of -40°C. The lithium-ion battery has a discharge capacity value determined by placing the half-cell at an ambient temperature and discharging at a constant current at a rate of 0.1 C until the voltage reaches 2.5 V, and the value satisfies 50% or more of the discharge capacity value determined by placing the half-cell at an ambient temperature of 25°C, charging at a constant current at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until the voltage reaches 4.5 V, charging at a constant voltage of 4.5 V until the current value reaches 0.05 C, and then discharging at a constant current at a rate of 0.1 C until the voltage reaches 2.5 V.

[0020] Another aspect of the present invention is a lithium ion battery including a positive electrode active material containing nickel, cobalt, and manganese, and an electrolyte, the electrolyte including fluoroethylene carbonate and methyl trifluoropropionate, wherein the volume ratio of the fluoroethylene carbonate and methyl trifluoropropionate is x:100-x (where 5≦x≦30) when the total content of the fluoroethylene carbonate and methyl trifluoropropionate is 100 vol %, and a half cell including a positive electrode having the positive electrode active material, the electrolyte, and a lithium metal counter electrode is placed at an ambient temperature of 25° C., and a current of 0.1 C (where 1 C=200 mA / g (weight of the positive electrode active material)) is applied until a voltage of 4.5 V is reached. The lithium-ion battery is characterized in that the half-cell is subjected to constant current charging at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.5 V is reached, constant voltage charging at 4.5 V until the current value reaches 0.05 C, and then placed at an ambient temperature of −40° C. and discharged at a constant current at a rate of 0.1 C until a voltage of 2.5 V is reached, and the discharge capacity obtained by placing the half-cell at an ambient temperature of 25° C., constant current charging at a rate of 0.1 C (where 1 C = 200 mA / g (current per weight of positive electrode active material is 200 mA / g)) until a voltage of 4.5 V is reached, constant voltage charging at 4.5 V until the current value reaches 0.05 C, and then ....

[0021] In another aspect of the present invention, it is preferable that the ratio of nickel:cobalt:manganese in the positive electrode active material is 8:1:1 or close to that ratio.

[0022] In another aspect of the present invention, it is preferable that the proportion of nickel in the positive electrode active material is higher than the proportion of cobalt and the proportion of manganese.

[0023] In another aspect of the present invention, the median diameter (D50) of the positive electrode active material is preferably 4 μm or more and 7 μm or less.

[0024] In another aspect of the present invention, the separator of the half cell preferably comprises polyimide.

[0025] In another aspect of the present invention, the separator of the half cell preferably comprises polypropylene.

[0026] According to one embodiment of the present invention, a lithium-ion battery including an electrolyte containing a novel organic solvent and a novel positive electrode active material can be provided. According to one embodiment of the present invention, a lithium-ion battery capable of being charged and discharged over a wide temperature range, including from below freezing to high temperatures, can be provided.

[0027] Note that the above-described effects do not preclude the existence of other effects. Furthermore, the above-described effects are considered to be independent of each other, and one embodiment of the present invention does not necessarily exhibit all of the above-described effects. Furthermore, effects other than those described above can be extracted from the description of this specification and the like.

[0028] FIGS. 1A and 1B are diagrams illustrating a lithium ion battery of one embodiment of the present invention. FIGS. 2A to 2C are diagrams illustrating a method for fabricating a positive electrode of one embodiment of the present invention. FIGS. 3A to 3F are diagrams illustrating a positive electrode active material of one embodiment of the present invention. FIG. 4 is a diagram illustrating a crystal structure of a positive electrode active material of one embodiment of the present invention. FIG. 5 is a diagram illustrating a crystal structure of a positive electrode active material. FIG. 6 is a diagram illustrating diffraction peaks of a positive electrode active material. FIG. 7 is a diagram illustrating diffraction peaks of a positive electrode active material. FIGS. 8A and 8B are diagrams illustrating diffraction peaks of a positive electrode active material. FIG. 9 is a diagram illustrating a method for fabricating a positive electrode of one embodiment of the present invention. FIG. 10 is a diagram illustrating a method for fabricating a positive electrode of one embodiment of the present invention. FIG. 11 is a diagram illustrating a method for fabricating a positive electrode of one embodiment of the present invention. FIG. 12 is a diagram illustrating a method for fabricating a positive electrode of one embodiment of the present invention. FIG. 13 is a diagram illustrating a method for fabricating a positive electrode of one embodiment of the present invention. FIGS. 14A to 14D are diagrams illustrating a positive electrode of one embodiment of the present invention. FIGS. 15A to 15D are diagrams illustrating a positive electrode of one embodiment of the present invention. FIGS. 16A and 16B are diagrams illustrating a lithium-ion battery of one embodiment of the present invention. FIGS. 17A to 17C are diagrams illustrating a lithium-ion battery of one embodiment of the present invention. FIGS. 18A to 18D are diagrams illustrating a lithium-ion battery and a power storage system of one embodiment of the present invention. FIGS. 19A to 19C are diagrams illustrating a lithium-ion battery of one embodiment of the present invention. FIGS. 20A to 20C are diagrams illustrating a lithium-ion battery of one embodiment of the present invention. FIGS. 21A to 21C are diagrams illustrating an electric vehicle of one embodiment of the present invention. FIGS. 22A to 22D are diagrams illustrating a transportation vehicle of one embodiment of the present invention. FIGS. 23A to 23C are diagrams illustrating a two-wheeled vehicle or the like of one embodiment of the present invention. FIGS. 24A to 24D are diagrams illustrating electronic devices or the like of one embodiment of the present invention. FIGS. 25A to 25D are diagrams illustrating an example of space equipment. FIGS. 26A and 26B illustrate NMR spectra of an organic solvent of an electrolyte of one embodiment of the present invention. 27A and 27B are diagrams showing NMR of an organic solvent of an electrolyte according to one embodiment of the present invention. 28A and 28B are diagrams showing NMR of an organic solvent of an electrolyte according to one embodiment of the present invention. 29A and 29B are diagrams illustrating AC impedance measurement.FIGS. 30A to 30C show the results of AC impedance measurement of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 31A to 31C show the results of AC impedance measurement of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 32A and 32B show the charge capacity and discharge capacity of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 33A and 33B show the charge capacity and discharge capacity of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 34A and 34B show the charge capacity and discharge capacity of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 35A and 35B show the charge curve and discharge curve of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 36A to 36C show the cycle characteristics of a sample including an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIG. 37 shows the viscosity of an organic solvent for an electrolyte according to one embodiment of the present invention. 38A and 38B are graphs showing the charge capacity and discharge capacity of a sample having an electrolyte and a positive electrode active material according to one embodiment of the present invention. FIGS. 39A and 39B are graphs showing DSC results for an organic liquid. FIG. 40 is a diagram showing a model used for calculations. FIG. 41 is a graph showing the activation barrier for lithium ion diffusion. FIGS. 42A and 42B are graphs showing charge / discharge curves (25°C) of a full cell. FIGS. 43A and 43B are graphs showing the cycle characteristics (25°C) of a full cell.

[0029] The embodiments will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications in form and detail can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same parts or parts having similar functions will be denoted by the same reference numerals in different drawings, and their repeated description may be omitted.

[0030] Ordinal numbers such as "first" and "second" used in this specification are used to avoid confusion between components, and do not indicate the order or ranking of processes or stacking order. Furthermore, even if a term is not used in this specification, an ordinal number may be used in the claims to avoid confusion between components. Furthermore, even if a term is used in this specification, a different ordinal number may be used in the claims. Furthermore, even if a term is used in this specification, the ordinal number may be omitted in the claims.

[0031] In this specification, the term "positive electrode active material" refers to a compound containing a transition metal and oxygen, capable of inserting and desorbing carrier ions. Compounds containing oxygen are sometimes called oxides or composite oxides. Lithium ions are typically used as carrier ions, but sodium ions, magnesium ions, or the like may also be used. Carbonates and hydroxyl groups adsorbed after the preparation of the positive electrode active material are not included in the positive electrode active material. Furthermore, electrolytes, organic solvents, binders, conductive materials, or compounds derived from these materials attached to the positive electrode active material are also not included in the positive electrode active material.

[0032] In this specification, "homogeneous" refers to a phenomenon in which a certain element (e.g., A) is distributed with similar characteristics in specific regions in a solid composed of multiple elements (e.g., A, B, C). Specifically, the concentration of the element in the specific regions needs to be substantially the same. For example, the difference in element concentration between the specific regions needs to be within 10%, which may be referred to as the concentration being substantially the same. Examples of the specific region include the surface, protrusions, recesses, or interior of the active material. If the concentration of the element in the protrusions and interior is substantially the same, it can be said that the element is present homogeneously in the protrusions and interior.

[0033] In this specification, the term "segregation" refers to a phenomenon in which a certain element (e.g., B) is spatially non-uniformly distributed in a solid composed of multiple elements (e.g., A, B, C), and refers to the fact that a certain element (e.g., B) has a different concentration in a specific region from another specific region. "Segregation" is synonymous with uneven distribution, precipitation, non-uniformity, bias, or the mixture of regions with high concentration and regions with low concentration.

[0034] In this specification and the like, the term "particle" used in reference to active material particles and the like is not limited to referring only to spherical shapes (circular cross-sectional shapes). For example, particles may have cross-sectional shapes such as elliptical and asymmetric shapes, and further, the individual particles do not need to be uniform and may have irregular shapes.

[0035] In this specification, "below freezing" refers to 0° C. or below, "high temperature" refers to 25° C. or above, and room temperature refers to a temperature higher than 0° C. and lower than 25° C. In this specification, "a temperature range including from below freezing to high temperatures" includes the above-mentioned room temperature.

[0036] A "nonaqueous electrolyte" refers to a liquid electrolyte containing an organic solvent exhibiting carrier ion conductivity, but is not limited to a liquid state in the present invention. Therefore, in this specification, the concept of a nonaqueous electrolyte is referred to as an "electrolyte." That is, the electrolyte of one embodiment of the present invention is not limited in any way to its state, and includes, for example, an electrolyte whose viscosity has increased from a liquid state as a result of viscosity adjustment. Furthermore, the electrolyte also includes a solid or semi-solid state. A semi-solid state refers to a state intermediate between a liquid and a solid state. Specific examples of semi-solid states include flexible solids, typically gels. Semi-solid electrolytes are generally referred to as semi-solid electrolytes, and the electrolyte of one embodiment of the present invention also includes a semi-solid electrolyte. The liquid, solid, or semi-solid state, or viscosity, is determined when the lithium-ion battery is placed at 25°C.

[0037] In this specification and the like, the term "carbonate" refers to a compound having at least one carbonate ester in its molecular structure, and unless otherwise specified, includes "cyclic carbonates" and "chain carbonates." Furthermore, "chain" includes both linear and branched chains.

[0038] In this specification and the like, a value that indicates the magnitude of viscosity is called viscosity, and "appropriate viscosity" means that the viscosity is appropriate for a lithium ion battery.

[0039] In this specification and the like, the phrase "having A and / or B" may be used, which means having A, having B, or having A and B.

[0040] In this specification, a full cell refers to a battery cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a battery cell assembled with lithium metal as the negative electrode (counter electrode).

[0041] Embodiment 1 In this embodiment, a lithium-ion battery which is one embodiment of the present invention will be described with reference to the drawings.

[0042] <Lithium-ion battery> A lithium-ion battery according to one embodiment of the present invention has an electrolyte that allows charging and discharging over a wide temperature range, including at least sub-zero temperatures and even high temperatures. In addition to the electrolyte, the lithium-ion battery also includes a negative electrode, a positive electrode, a separator between the negative electrode and the positive electrode, and an exterior covering that surrounds the negative electrode and the positive electrode. Depending on the shape of the exterior, the battery is called a laminated lithium-ion battery, a coin-cell lithium-ion battery, or a cylindrical lithium-ion battery, but the present invention is not limited to the shape of the exterior. The separator can be omitted if the electrolyte is solid or semi-solid.

[0043] 1A illustrates an example of the configuration of a lithium-ion battery 100. In a cross-sectional view, the lithium-ion battery 100 can be seen to have a negative electrode 106, a separator 108, and a positive electrode 107. In the lithium-ion battery 100, the electrolyte 109 is liquid, and the electrolyte 109 is present throughout the negative electrode 106, the separator 108, and the positive electrode 107. As described above, the electrolyte 109 is not limited to being liquid.

[0044] The negative electrode 106 includes a negative electrode current collector 101 and a negative electrode active material layer 102. The negative electrode active material layer 102 includes at least a negative electrode active material and may include a conductive material and / or a binder. A known material can be used for the negative electrode active material, and details of the material will be described later. The positive electrode 107 includes a positive electrode current collector 105 and a positive electrode active material layer 104. The positive electrode active material layer 104 includes at least a positive electrode active material and may include a conductive material and / or a binder. Although a known material may be used for the positive electrode active material, the use of a positive electrode active material of one embodiment of the present invention allows the battery to withstand high-voltage charging and increase the discharge capacity of a lithium-ion battery. The positive electrode active material of one embodiment of the present invention will be described later.

[0045] The conductive material has a function of supporting a current path between positive electrode active materials and / or between a positive electrode active material and a current collector. The conductive material also has a function of supporting a current path between negative electrode active materials and / or between a negative electrode active material and a current collector. Known materials can be used as the conductive material, and details will be described later. The binder is also called a binding agent, and has a function of supporting adhesion between positive electrode active materials and / or between a positive electrode active material and a current collector. The binder also has a function of supporting adhesion between negative electrode active materials and / or between a negative electrode active material and a current collector. Known materials can be used as the binder, and details will be described later.

[0046] 1B illustrates a lithium ion battery 100 that does not have the negative electrode active material layer 102, unlike the lithium ion battery 100 in FIG. 1A. The negative electrode active material layer 102 can be eliminated depending on the material of the negative electrode current collector 101. The other configurations of the lithium ion battery 100 in FIG. 1B are the same as those of the lithium ion battery 100 in FIG. 1A, and therefore will not be described again.

[0047] In this embodiment, a lithium ion battery having excellent discharge characteristics and / or excellent charge characteristics can be realized over a wide temperature range, including temperatures from below freezing to high temperatures. In particular, a lithium ion battery having excellent discharge characteristics and / or excellent charge characteristics can be realized at any temperature below freezing (e.g., 0°C or below, -20°C or below, preferably -30°C or below, more preferably -40°C or below, even more preferably -50°C or below, and even more preferably -60°C or below). The following description focuses on the configuration of a lithium ion battery required for this purpose. Specifically, the description focuses on the positive electrode active material and electrolyte. Details of the configuration of a lithium ion battery other than the positive electrode active material and electrolyte will be described in embodiment 3 and subsequent embodiments.

[0048] <Positive Electrode Active Material> The positive electrode active material has the function of incorporating and / or releasing lithium ions, which are carrier ions, during charging and discharging. The positive electrode active material used in one embodiment of the present invention is capable of charging and discharging even at a high charging voltage (hereinafter also referred to as "high charging voltage") at least below freezing point, and can be made of a material that exhibits little deterioration (or little increase in resistance) during charging and discharging. Unless otherwise specified in this specification, the "charging voltage" is expressed based on the potential of lithium metal. Furthermore, in this specification, the "high charging voltage" refers to a charging voltage of, for example, 4.4 V or higher, preferably 4.5 V or higher, and more preferably 4.6 V or higher.

[0049] The positive electrode active material is not limited to one type, and two or more materials with different median diameters (D50) may be mixed, or two or more materials with different compositions may be mixed, as long as they are materials that undergo little deterioration during charging and discharging at least at sub-zero temperatures and high charging voltages. In this specification, "different compositions" includes not only cases where the compositions of elements contained in the materials are different, but also cases where the ratios of elements contained in the materials are different even if the compositions of elements contained in the materials are the same.

[0050] As mentioned above, in this specification, a "high charging voltage" is defined as 4.5 V or higher based on the potential when the negative electrode is made of lithium metal, but when the potential when the negative electrode is made of a carbon material (e.g., graphite) is used as the reference, a "high charging voltage" is defined as 4.4 V or higher. In short, in the case of a half cell using lithium metal as the negative electrode, a charging voltage of 4.5 V or higher is defined as a high charging voltage, and in the case of a full cell using a carbon material (e.g., graphite) as the negative electrode, a charging voltage of 4.4 V or higher is defined as a high charging voltage.

[0051] By using a material that undergoes little deterioration (or little increase in resistance) with charge and discharge even at a high charge voltage at any sub-zero temperature as the positive electrode active material, a lithium ion battery with large charge and / or discharge capacities can be realized even at sub-zero temperatures. Specifically, a lithium ion battery can be realized in which the charge and / or discharge capacity at any sub-zero temperature is 50% or more (preferably 60% or more, more preferably 70% or more, and most preferably 80% or more) of the charge and / or discharge capacity at 25°C.

[0052] The temperature during charging or discharging described in this specification refers to the temperature of the environment in which the lithium-ion battery is placed (hereinafter, sometimes referred to as "environmental temperature" in this specification). In measuring battery characteristics, a thermostatic chamber stabilized at a desired temperature is used, so the environmental temperature is equal to the temperature of the thermostatic chamber. After placing a test cell (e.g., a full cell or a half cell) to be measured in the thermostatic chamber, measurement can be started after a sufficient time (e.g., one hour or more) has elapsed until the test cell reaches the same temperature as the thermostatic chamber, but measurement of battery characteristics is not necessarily limited to this method.

[0053] <Electrolyte> The electrolyte used in one embodiment of the present invention can be a material that has excellent lithium ion conductivity at any temperature below freezing (e.g., 0°C, −20°C, preferably −30°C, more preferably −40°C, and even more preferably −50°C or −60°C).

[0054] The electrolyte contains an organic solvent, but the organic solvent of the electrolyte according to one embodiment of the present invention is not limited to being liquid at 25° C. and may be solid or semi-solid at 25° C. Note that the organic solvent of the electrolyte according to one embodiment of the present invention is preferably liquid at any sub-zero temperature (e.g., 0° C., −20° C., preferably −30° C., more preferably −40° C., and even more preferably −50° C. or −60° C.), but is not limited thereto. The organic solvent of the electrolyte according to one embodiment of the present invention may be liquid, solid, or semi-solid at any sub-zero temperature.

[0055] <Organic Solvent> The organic solvent described in this embodiment may contain a fluorinated cyclic carbonate (sometimes referred to as a fluorinated cyclic carbonate) or a fluorinated chain carbonate (sometimes referred to as a fluorinated chain carbonate). Furthermore, the organic solvent preferably contains both a fluorinated cyclic carbonate and a fluorinated chain carbonate. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have electron-withdrawing substituents, and have lower solvation energy of lithium ions than organic compounds without electron-withdrawing substituents. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable organic solvents.

[0056] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluorinated ethylene carbonate, fluoroethylene carbonate, FEC, and F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. Since all of these fluorinated cyclic carbonates have electron-withdrawing substituents, they are believed to have low solvation energies for lithium ions.

[0057] The following structural formula (H10) is the structural formula of FEC: In FEC, the electron-withdrawing substituent is an F group.

[0058]

[0059] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The following structural formula (H22) is the structural formula of methyl 3,3,3-trifluoropropionate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0060]

[0061] An example of a fluorinated chain carbonate is trifluoromethyl 3,3,3-trifluoropropionate. The following structural formula (H23) is the structural formula of trifluoromethyl 3,3,3-trifluoropropionate. The electron-withdrawing substituent is CF ３ It is the base.

[0062]

[0063] An example of a fluorinated chain carbonate is trifluoromethyl propionate. The following structural formula (H24) is the structural formula of trifluoromethyl propionate. The electron-withdrawing substituent is CF ３ It is the base.

[0064]

[0065] An example of a fluorinated chain carbonate is methyl 2,2-difluoropropionate. The following structural formula (H25) is the structural formula of methyl 2,2-difluoropropionate. The electron-withdrawing substituent is CF ２ It is the base.

[0066]

[0067] The organic solvent of the electrolyte according to one embodiment of the present invention may include at least one selected from the fluorinated cyclic carbonates and at least one selected from the fluorinated chain carbonates. For example, the organic solvent described in this embodiment may include FEC and MTFP. The reason for this will be described below.

[0068] <FEC and MTFP> FEC is a cyclic carbonate and has a high dielectric constant. Therefore, when used in an organic solvent, it has the effect of promoting the dissociation of lithium salts. Furthermore, since FEC has an electron-withdrawing substituent, it easily bonds with lithium ions through Coulomb force, etc. Specifically, since FEC has a smaller solvation energy than ethylene carbonate (abbreviated as "EC"), which does not have an electron-withdrawing substituent, it can be said that it easily forms solvates with lithium ions. Furthermore, FEC is thought to have a deep highest occupied molecular orbital (HOMO), and a deep HOMO makes it less susceptible to oxidation and improves oxidation resistance. On the other hand, FEC has a high viscosity, and when FEC is used alone as an organic solvent, it is difficult to use below freezing. Therefore, the organic solvent specifically described as one embodiment of the present invention further contains MTFP in addition to FEC. MTFP is a type of chain carbonate that has the effect of reducing or maintaining the viscosity of the electrolyte. Of course, MTFP also has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, so it may form a solvate with lithium ions.

[0069] FEC and MTFP having such physical properties can be mixed and used in a volume ratio of x:100-x (where 5≦x≦30, preferably 10≦x≦20), assuming the total content of these two organic solvents to be 100 vol%. The organic solvents can be mixed so that there is more MTFP than FEC. The volume ratio may be the volume ratio measured before mixing the organic solvents, and the outside air may be room temperature (typically 25°C) when mixing the organic solvents. An organic solvent mixed with FEC and MTFP is preferred because it exhibits a viscosity that allows it to operate as a lithium-ion battery and maintains an appropriate viscosity even below freezing.

[0070] Because typical organic solvents used in lithium ion batteries freeze at around −20° C., it is difficult to fabricate a lithium ion battery that can be charged and discharged at −40° C., preferably −50° C. or −60° C. However, the organic solvent described as an example in this embodiment allows the freezing point to be −40° C. or lower, preferably −50° C. or lower, thereby realizing a lithium ion battery that can be charged and discharged even in a sub-freezing environment. As a result, a lithium ion battery that can be charged and discharged over a wide temperature range, including at least sub-freezing, can be realized.

[0071] While FEC has been described above as a representative, any of the organic compounds described as fluorinated cyclic carbonates has the effect of promoting dissociation of lithium salts, has a low solvation energy and is prone to solvation with lithium ions, and has a high viscosity, making it difficult to use at sub-zero temperatures when used alone. Furthermore, although MTFP has been described above as a representative, any of the organic compounds described as fluorinated chain carbonates has the effect of reducing or maintaining the viscosity of the electrolyte, which is an embodiment of the present invention. Therefore, if the organic solvent, which is an embodiment of the present invention, contains a fluorinated cyclic carbonate and a fluorinated chain carbonate, it is possible to provide a lithium-ion battery that can be charged and discharged over a wide temperature range, including at least sub-zero temperatures.

[0072] The organic solvent is preferably highly purified, with a low content of particulate dust or elements other than the constituent elements of the organic solvent (hereinafter simply referred to as "impurities," including water or moisture). Specifically, the ratio of impurities to the organic solvent is preferably 1 mol % or less, preferably 0.1 mol % or less, and more preferably 0.01 mol % or less.

[0073] <Lithium Salt> The lithium salt dissolved in the organic solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０Cl 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 ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (LiBOB), or any combination of the above-mentioned lithium salts in any ratio may be used. The lithium salt is one of the components contained in the electrolyte of one embodiment of the present invention, but is not necessarily contained.

[0074] <Additives> For the purpose of improving safety, an additive may be mixed into the organic solvent to form a coating at the interface between the active material and the electrolyte. The additive may be one or more selected from vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), SUN (suberonitrile), or a dinitrile compound such as succinonitrile or adiponitrile. The concentration of the additive may be, for example, 0.1 wt% to 5 wt% of the total organic solvent. While the additive is one of the components of the electrolyte of one embodiment of the present invention, it is not necessarily required. Furthermore, it is preferable to select a material different from the organic solvent as the additive.

[0075] As described above, an example of an electrolyte that can be used in the lithium-ion battery of one embodiment of the present invention has been described, but the electrolyte that can be used in the lithium-ion battery of one embodiment of the present invention is not limited to this example. Other materials can also be used as long as they have appropriate viscosity even below freezing and excellent lithium-ion conductivity.

[0076] The lithium-ion battery of one embodiment of the present invention includes at least the above-described positive electrode active material and an electrolyte, and thus can be a lithium-ion battery that can be charged and discharged over a wide temperature range including at least temperatures below freezing.

[0077] Embodiment 2 In this embodiment, a positive electrode active material that can be used in a lithium-ion battery according to one embodiment of the present invention (hereinafter also referred to as a "positive electrode active material that can be used as one embodiment of the present invention") and a manufacturing method thereof will be described with reference to FIGS. 2A to 2C .

[0078] As described above in Embodiment 1, any material that undergoes little deterioration due to charge and discharge at a high charge voltage can be used as the positive electrode active material that can be used in the lithium-ion battery of one embodiment of the present invention. Therefore, the positive electrode active material that can be used in the lithium-ion battery disclosed in this specification and the like does not need to be interpreted as being limited to the specific materials described in this embodiment and the like. Materials that are known as materials that undergo little deterioration due to charge and discharge even at a high charge voltage (for example, 4.5 V or higher) at the time of filing the present application can also be used.

[0079] An example of a method for producing a cathode active material that can be used as one embodiment of the present invention will be described below. In this embodiment, a case where a cathode active material is produced using a solid-phase method will be described. However, a cathode active material produced using a coprecipitation method, a hydrothermal method, or the like, other than the solid-phase method, can also be applied to the lithium-ion battery of the present invention. Note that the flow used to explain the production method and the like in this embodiment indicates the order of elements connected by lines, and does not indicate the order of elements not connected by lines.

[0080] [Method for Producing Positive Electrode Active Material] An example of a flow for producing the positive electrode active material 10 will be described with reference to FIGS. 2A to 2C.

[0081] <Step S11> In step S11 shown in FIG. 2A , a lithium source (Li source) and a transition metal M source (M source) are prepared as starting materials for lithium and transition metal M, respectively. A lithium-containing compound is preferably used as the lithium source, and examples of such compounds include lithium carbonate, lithium hydroxide, lithium nitrate, and lithium fluoride. The transition metal M may be, for example, one or more of manganese, cobalt, and nickel. When lithium cobalt oxide (LCO) is prepared as the positive electrode active material, cobalt is used as the transition metal M. When nickel-cobalt-manganese composite oxide (NCM) is prepared as the positive electrode active material, cobalt, manganese, and nickel are used as the transition metal M. Aluminum may also be used in addition to the transition metal M.

[0082] As the transition metal M source, it is preferable to use a compound containing the transition metal M, and for example, an oxide or hydroxide of a metal exemplified as the transition metal M can be used. As a cobalt source, cobalt oxide, cobalt hydroxide, etc. can be used. As a manganese source, manganese oxide, manganese hydroxide, etc. can be used. As a nickel source, nickel oxide, nickel hydroxide, etc. can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used. Furthermore, when two or more transition metal M sources are used, it is preferable to prepare the two or more transition metal M sources in a ratio (mixing ratio) that allows the two or more transition metal M sources to form a layered rock salt type crystal structure.

[0083] <Step S12> Next, in step S12 shown in FIG. 2A , the Li source and the M source are mixed while being pulverized to prepare a mixed material. The mixing while being pulverized can be performed by either a dry method or a wet method. When performed by a wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). Acetone with a water content of 10 ppm or less and a purity of 99.5% or higher is referred to as "dehydrated acetone," and it is preferable to use dehydrated acetone as the solvent.

[0084] <Step S13> Next, in step S13 shown in FIG. 2A , the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, and more preferably 900°C or higher and 1000°C or lower. If the temperature is too low, decomposition and melting of the Li source and M source may be insufficient. On the other hand, if the temperature is too high, lithium may sublimate from the Li source and / or the transition metal used as the M source may be excessively reduced. If the heating time is too short, a composite oxide containing lithium and the transition metal M may not be synthesized, but if it is too long, productivity may decrease. Therefore, the heating time is preferably 1 hour or higher and 100 hours or lower, and preferably 2 hours or higher and 20 hours or lower. The temperature rise rate depends on the heating temperature reached, but is preferably 80°C / h or higher and 250°C / h or lower, more preferably 100°C / h or higher and 250°C / h or lower.

[0085] The heating atmosphere is preferably an atmosphere with little water. The low water atmosphere can be defined by the dew point, and for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less, is preferably used as the heating atmosphere. Furthermore, an atmosphere containing oxygen, such as dry air, is preferred as the heating atmosphere. For example, there is a method of continuously introducing oxygen into the reaction chamber. In this case, the flow rate of oxygen is preferably 5 L / min or more and 15 L / min or less. The state in which oxygen is continuously introduced into the reaction chamber and flows through the reaction chamber is called "flow."

[0086] In addition to the above-described flow, the heating atmosphere may be changed to an oxygen-containing atmosphere by, for example, reducing the pressure in the reaction chamber, introducing oxygen, and then controlling the oxygen so that it does not enter or leave the reaction chamber. This is called "purging." For example, the reaction chamber may be reduced in pressure to -970 hPa, and then oxygen may be introduced to 50 hPa, and the oxygen entry and exit may be stopped. This state may be referred to as filling the reaction chamber with oxygen.

[0087] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0088] The heating in this step may be performed using a rotary kiln or a roller hearth kiln. A rotary kiln is a rotary calcination device, which is preferred because it allows the raw materials to be heated while being stirred. A roller hearth kiln is a calcination device in which the raw materials are transported by rollers, which is preferred because the raw materials can be passed through a heating zone, a cooling zone, etc. continuously. Of course, a batch-type calcination device may also be used for the heating in this step.

[0089] The container (specifically, a container called a crucible or a sheath) in which the mixed material is placed during heating is preferably made of a highly heat-resistant material such as aluminum oxide (hereinafter referred to as alumina), mullite / cordierite, magnesia, zirconia, etc. Furthermore, during heating, it is preferable to place a lid on the crucible or the sheath, as the placement of the lid can prevent the raw materials from sublimating.

[0090] After heating, the mixed material may be transferred from the crucible or scabbard to a mortar, where it is crushed and recovered. The mortar is preferably made of a highly heat-resistant material such as alumina, mullite / cordierite, magnesia, or zirconia. Heating conditions equivalent to those of step S13 can also be applied to heating steps other than step S13, which will be described later.

[0091] After heating, the recovered mixture may be agglomerated. To break down the agglomerated state, the recovered mixture may be crushed. To further break down the agglomerated state, the recovered mixture may be sieved. Sieving may be performed after crushing, sieving may be performed while crushing is being performed, or sieving may be performed instead of crushing.

[0092] <Step S14> By the above steps, a composite oxide containing lithium and a transition metal M (LiMO) is obtained in step S14 shown in FIG. 2A. ２ The composite oxide can be obtained by ２ However, the composition is not strictly limited to Li:M:O=1:1:2. When cobalt is used as the transition metal M, it is called a composite oxide containing cobalt, and is represented by LiCoO ２ However, the composition is not strictly limited to Li:Co:O=1:1:2.

[0093] Although the example of producing the composite oxide by the solid phase method in steps S11 to S14 has been shown, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0094] <Step S15> Next, in step S15 shown in Fig. 2A, the composite oxide is heated. Because this is the first heating of the composite oxide, the heating in step S15 may be referred to as initial heating. Alternatively, because this heating is performed before step S20 described below, it may be referred to as preheating or pretreatment.

[0095] The heating temperature in step S15 is preferably 500°C or higher and 1000°C or lower, more preferably 500°C or higher and 950°C or lower, and even more preferably 500°C or higher and 900°C or lower. It is also preferably 700°C or higher and 1000°C or lower, or 700°C or higher and 950°C or lower, and even more preferably 700°C or higher and 900°C or lower. It is also preferably 800°C or higher and 1000°C or lower, or 800°C or higher and 950°C or lower, and even more preferably 800°C or higher and 900°C or lower. The heating temperature in step S15 is preferably lower than that in step S13.

[0096] The initial heating may cause lithium to be released from a portion of the composite oxide. It is also expected to have the effect of increasing the crystallinity of the composite oxide. Furthermore, although the Li source and / or metal M source prepared in step S11 may contain impurities, the initial heating can reduce the amount of impurities in the composite oxide.

[0097] Furthermore, initial heating has the effect of smoothing the surface of the composite oxide. A smooth surface means that there are few irregularities, the surface of the composite oxide is rounded overall, and the corners are also rounded. A surface with little foreign matter adhering to it is also sometimes called smooth.

[0098] For this initial heating, it is not necessary to prepare a raw material such as a Li source, or a material that functions as a flux (a material added to facilitate melting).

[0099] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. Regarding the heating conditions, the heating temperature in this step should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in this step should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0100] The effect of increasing the crystallinity of the composite oxide also includes, for example, the effect of alleviating distortion resulting from differential shrinkage caused by heating the composite oxide, and the effect of alleviating deviation resulting from the differential shrinkage.

[0101] The heating in step S13 may cause a temperature difference between the surface and the interior of the composite oxide. The temperature difference may induce a contraction difference. It is also thought that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a contraction difference. The energy associated with the contraction difference causes a difference in internal stress in the composite oxide, which causes distortion. This energy is sometimes called strain energy. The internal stress is removed by the initial heating in step S15; in other words, the strain energy is thought to be reduced by the initial heating in step S15. When the strain energy is reduced, the distortion of the composite oxide is alleviated. Therefore, the surface of the composite oxide may become smoother after step S15. In other words, it is thought that the contraction difference that occurred in the composite oxide is alleviated after step S15, resulting in a smoother surface of the composite oxide. Step S15 may also be called tempering or annealing of the composite oxide.

[0102] Furthermore, the difference in shrinkage may cause microscopic misalignment in the composite oxide, for example, misalignment in the crystal plane. This step is preferably carried out in order to reduce this misalignment. This step can reduce the misalignment of the composite oxide. If the misalignment is uniformed, the surface of the composite oxide may become smooth. This is also referred to as the alignment of crystal grains. In other words, it is believed that, after step S15, the misalignment of the crystals and the like that has occurred in the composite oxide is alleviated, and the surface of the composite oxide becomes smooth.

[0103] The smooth surface of a composite oxide can be said to have a surface roughness of at least 10 nm or less when surface irregularity information is quantified from measurement data at a cross section of the composite oxide. The cross section is, for example, a cross section obtained when observing with a scanning transmission electron microscope (referred to as STEM).

[0104] When a composite oxide with a smooth surface is used as a positive electrode active material, deterioration during charging and discharging of a lithium ion battery is reduced and cracking of the positive electrode active material can be prevented.

[0105] In step S14, a composite oxide containing lithium and a transition metal M that has been synthesized in advance may be used. In this case, steps S11 to S13 can be omitted. By performing step S15 on a composite oxide that has been synthesized in advance, a composite oxide with a smooth surface can be obtained.

[0106] It is conceivable that the lithium in the composite oxide may be reduced by the initial heating. This reduction in lithium may make it easier for the additive elements, which will be described in the next step S20_1, to enter the composite oxide. Furthermore, if the additive elements are added to a composite oxide with a smooth surface, the additive elements can be added evenly, so it is preferable to add the additive elements after the initial heating. The step of adding the additive elements will be described using FIG. 2B.

[0107] <Step S20_1> and <Step S21_1> Details of step S20_1 shown in Fig. 2A are shown in Fig. 2B. In step S21_1 of Fig. 2B, an additive element A1 source (A1 source) to be added to the composite oxide is prepared. In order to compensate for the lithium lost by the initial heating, a lithium source may be prepared together with the additive element A1 source.

[0108] The additional element A1 can be one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic.

[0109] When magnesium is selected as the additional element A1, the source of the additional element A1 can be called a magnesium source (Mg source). As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0110] When fluorine is selected as the additional element A1, the source of the additional element A1 can be called a fluorine source (F source). Examples of the fluorine source that can be used include lithium fluoride, magnesium fluoride, aluminum fluoride, titanium fluoride, cobalt fluoride, nickel fluoride, zirconium fluoride, vanadium fluoride, manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride, calcium fluoride, sodium fluoride, potassium fluoride, barium fluoride, cerium fluoride, lanthanum fluoride, and sodium aluminum hexafluoride. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating step described below. Note that FIG. 2B shows an example in which a Mg source and a F source are used as the source of the additional element A1.

[0111] 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_1 is lithium carbonate.

[0112] In this embodiment, lithium fluoride is prepared as the fluorine source, and magnesium fluoride is prepared as the fluorine source and magnesium source. Lithium fluoride and magnesium fluoride are prepared in the form of LiF:MgF ２ When the molar ratio of lithium fluoride and magnesium fluoride is about LiF:MgF = 65:35, the effect of lowering the melting point is maximized. On the other hand, if the amount of lithium fluoride is too large, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, in this embodiment, the molar ratio of lithium fluoride and magnesium fluoride is LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x:1 (x = 0.33 or its vicinity) is more preferable. Note that "in the vicinity" refers to a value that is greater than 0.9 times and less than 1.1 times the value.

[0113] At the same time, the amount of magnesium added is ２ , typically LiCoO ２ The content of magnesium relative to the number of Co atoms is preferably greater than 0.1 atomic % and less than 3 atomic %, more preferably greater than 0.5 atomic % and less than 2 atomic %, and even more preferably greater than 0.5 atomic % and less than 1 atomic %. If the amount of magnesium added is 0.1 atomic % or less, the initial discharge capacity is high, but repeated high-voltage charging can cause a rapid decrease in discharge capacity. If the amount of magnesium added exceeds 0.1 atomic %, that is, greater than 0.1 atomic % and less than 3 atomic %, both the initial discharge characteristics and charge-discharge cycle characteristics are good even after repeated high-voltage charging. On the other hand, if the amount of magnesium added exceeds 3 atomic %, the initial discharge capacity is low, and the charge-discharge cycle characteristics tend to gradually deteriorate.

[0114] 2B, the magnesium source and the fluorine source are mixed while being pulverized. This step can be performed under the pulverization conditions and the mixing conditions selected from those described in step S12.

[0115] If necessary, a heating step may be performed after step S22_1. In this case, the heating step can be performed under heating conditions selected from those described in step S13. The heating time is preferably 2 hours or more, and the heating temperature is preferably 800° C. or higher and 1100° C. or lower.

[0116] 2B, the pulverized and mixed material is recovered to obtain an additional element A1 source (A1 source). Note that the additional element A1 source shown in step S23_1 may contain multiple raw materials, such as an Mg source and an F source, and in this case, the A1 source may be called a mixture.

[0117] The particle size of the mixture is preferably such that the median diameter (D50) is 600 nm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less. Even when a single material is used as the source of the additional element A1, the median diameter (D50) is preferably 600 nm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less.

[0118] A mixture with such a median diameter (D50) (including cases where the additive element A1 is one type) can easily adhere the mixture uniformly to the surface of the composite oxide when mixed with the composite oxide in a later step. Uniform adhesion of the mixture to the surface of the composite oxide is preferable because it facilitates uniform distribution or diffusion of fluorine and / or magnesium in the surface layer portion of the composite oxide after heating. The region where fluorine and / or magnesium are distributed can also be referred to as the surface layer portion. If there is a region in the surface layer that does not contain fluorine and / or magnesium, it may be difficult to obtain the O3'-type crystal structure described below in the charged state. Although fluorine has been used in the description, fluorine may also be chlorine, and can be interpreted as including these and therefore as halogen.

[0119] <Step S31> Next, in step S31 shown in FIG. 2A, the composite oxide and the additive element Al source (Al source) are mixed. The number of atoms A of the transition metal M in the composite oxide containing lithium and the transition metal M is Ｍ and the number of magnesium atoms A contained in the additive element A1. Ｍｇ The ratio of A Ｍ : AＭｇ = 100:y (0.1 ≦ y ≦ 6), and A Ｍ : A Ｍｇ It is more preferable that y=100:y (0.3≦y≦3).

[0120] The mixing in step S31 is preferably performed under milder conditions than the conditions for pulverization and mixing in step S12 so as not to destroy the composite oxide. For example, it is preferable to perform the mixing under conditions with a lower rotation speed or a shorter time than in step S12. Furthermore, a dry method is more preferable than a wet method because the conditions are milder.

[0121] The mixing is preferably carried out in an atmosphere having a dew point of −100° C. or higher and −10° C. or lower. For example, the mixing can be carried out in a dry room. The atmosphere of the dry room preferably contains dry air.

[0122] <Step S32> Next, in step S32 of Fig. 2A, the mixed materials are recovered to obtain a mixture 903. When recovering, the mixture 903 may be crushed to break up any agglomerated material. The mixture 903 may be sieved to further break up any agglomerated material. Sieving may be performed after crushing, or sieving may be performed while crushing, or sieving may be performed only instead of crushing.

[0123] In this embodiment, a method of adding lithium fluoride as a fluorine source and magnesium fluoride as a magnesium source to a composite oxide that has undergone initial heating has been described, but the present invention is not limited to the above method. In step S11, that is, in the stage of starting materials for the composite oxide, a magnesium source, a fluorine source, etc. can be prepared together with a Li source and an M source, and the process can proceed to step S12. Thereafter, in step S13, heating is performed to obtain LiMO with added magnesium and fluorine. ２ In this case, it is not necessary to separate the steps S11 to S14 from the steps S21_1 to S23_1. This method can be said to be simple and highly productive.

[0124] Alternatively, a composite oxide to which magnesium and fluorine have been added in advance may be used. If a composite oxide to which magnesium and fluorine have been added is used, the processes of steps S11 to S32 and step S20_1 can be omitted. This method can be said to be simple and highly productive.

[0125] Alternatively, a magnesium source and a fluorine source may be further added to a composite oxide to which magnesium and fluorine have been added in advance in step S20_1. Instead of or in addition to the magnesium source and the fluorine source, a nickel source and an aluminum source may be added.

[0126] 2A, the mixture 903 is heated. The heating conditions can be selected from those described in step S13. The heating time is preferably 2 hours or more.

[0127] Here, a supplementary note about the heating temperature will be given. The lower limit of the heating temperature in step S33 is ２ The temperature at which the reaction between the composite oxide and the additive element A1 source proceeds must be equal to or higher than the temperature at which the reaction between the composite oxide and the additive element A1 source proceeds. The temperature at which the reaction proceeds may be lower than the melting point of these materials as long as the elements contained in the composite oxide and the additive element A1 source can diffuse into each other. An oxide will be used as an example to explain the temperature at which the reaction proceeds. ｄ (Melting point T ｍ It is known that solid-phase diffusion occurs at a temperature above which the temperature is calculated as 0.757 times the normal temperature. Therefore, the heating temperature in step S33 should be 500° C. or higher.

[0128] Of course, the reaction proceeds more easily when the temperature is equal to or higher than the temperature at which at least a part of the mixture 903 melts. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0129] Also, LiCoO ２ :LiF:MgF ２A mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830°C in differential scanning calorimetry (DSC). Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

[0130] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0131] The upper limit of the heating temperature is set to be lower than the decomposition temperature of the composite oxide. For example, LiCoO ２ In this case, the temperature is set to be lower than the decomposition temperature of 1130°C. At temperatures close to the decomposition temperature, a small amount of LiMO ２ There is concern about the decomposition of LiMO. ２ When decomposition occurs, unnecessary reaction products are generated in the composite oxide, typically Co in the case of lithium cobalt oxide. ３ O ４ Therefore, the temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 920°C or lower.

[0132] In consideration of these, the heating temperature in step S33 is preferably 500°C or higher and lower than 1130°C, more preferably 500°C or higher and lower than 1000°C, even more preferably 500°C or higher and lower than 950°C, even more preferably 500°C or higher and lower than 920°C, and even more preferably 500°C or higher and lower than 900°C. Also, it is preferably 742°C or higher and lower than 1130°C, more preferably 742°C or higher and lower than 1000°C, even more preferably 742°C or higher and lower than 950°C, even more preferably 742°C or higher and lower than 920°C, and even more preferably 742°C or higher and lower than 900°C. Also, it is preferably 800°C or higher and lower than 1130°C, more preferably 800°C or higher and lower than 1000°C, even more preferably 800°C or higher and lower than 950°C, even more preferably 800°C or higher and lower than 920°C, and even more preferably 800°C or higher and lower than 900°C. Furthermore, the temperature is preferably 830°C or higher and lower than 1130°C, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, even more preferably 830°C or higher and 920°C or lower, and even more preferably 830°C or higher and 900°C or lower.

[0133] The heating temperature in step S33 is preferably lower than that in step S13. ２ The heating temperature in step S33 is preferably higher than that in step S15.

[0134] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride originating from a fluorine source or the like in the processing chamber or the crucible within an appropriate range. In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be controlled within a range of 1000 saturation points. ２ ) can be lowered to a temperature lower than the decomposition temperature, for example, 742°C or higher and 950°C or lower, and additive elements A1 such as magnesium can be distributed in the surface layer portion, thereby producing a positive electrode active material with good characteristics.

[0135] However, LiF may sublimate when heated, and since LiF has a lower specific gravity in gaseous state than oxygen, it is conceivable that the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the sublimation of LiF. Even if LiF is not used as a fluorine source, etc., LiMO ２ There is also a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may then sublime. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress sublimation.

[0136] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF in the processing chamber is high. Such heating can suppress sublimation of LiF in the mixture 903. In addition, by placing a lid on the container as described above, it is also possible to suppress sublimation of LiF in the mixture 903.

[0137] The heating in this step is preferably performed so as not to stick together the mixture 903. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases and the path along which the additional element A1 (e.g., magnesium and / or fluorine) diffuses is blocked, which may make it difficult for the additional element A1 (e.g., magnesium and / or fluorine) to diffuse.

[0138] Furthermore, it is believed that uniform distribution of the additive element A1 (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the mixture 903 does not stick to itself.

[0139] The heating time is determined by the heating temperature, the LiMO ２ It changes depending on the size and composition of LiMO. ２ When the median diameter (D50) is small, a lower temperature or a shorter time may be more preferable than when the median diameter (D50) is large. The median diameter (D50) can be determined using a laser diffraction particle size distribution analyzer.

[0140] LiMO in step S14 of FIG. ２ When lithium cobalt oxide is used as the catalyst, and the median diameter (D50) of the lithium cobalt oxide is about 12 μm, such as 10 μm or more and 14 μm or less, the heating temperature in step S33 is preferably, for example, 800° C. or more and 920° C. or less, and more preferably 850° C. or more and 920° C. or less. The heating time in step S33 is, for example, more preferably 10 hours or more, even more preferably 20 hours or more, and may be 60 hours or more. When the median diameter (D50) is large, the composite oxide (LiMO ２) increases in volume, and therefore, internal stress is relaxed or removed in the bulk layer of the composite oxide, and therefore, a longer heating time may be required. If the median diameter (D50) is large, it takes time for the additive elements A1, including magnesium, to be uniformly distributed in the surface layer portion, and as described above, the heating time may be longer. The median diameter (D50) of lithium cobalt oxide may increase after heat treatment, but it is preferable that the median diameter (D50) be 10 μm or more and 14 μm or less even after heat treatment. In other words, it is preferable that the median diameter (D50) of the positive electrode active material be 10 μm or more and 14 μm or less.

[0141] LiMO in step S14 ２ When lithium cobalt oxide is used as the sintering agent, and the median diameter (D50) of the lithium cobalt oxide is about 7 μm, such as 5 μm or more and 9 μm or less, the heating temperature in step S33 is preferably in the same range as the heating temperature when the median diameter (D50) is about 12 μm. On the other hand, the heating time in step S33 can be shorter than the heating time when the median diameter (D50) is about 12 μm. For example, a heating time of 1 hour or more and 10 hours or less is preferable, and 5 hours or more and 10 hours or less is more preferable. When the median diameter (D50) is small, the time for the additional element A1, including magnesium, to be distributed in the surface layer portion is shortened, and therefore the heating time can be shortened as described above. When the median diameter (D50) is small, the composite oxide (LiMO ２ ) becomes smaller, the time required to temper or anneal the bulk layer of the composite oxide can be shortened. Although the median diameter (D50) of lithium cobalt oxide may increase after heat treatment, it is preferable that the median diameter (D50) remains 5 μm or more and 9 μm or less even after heat treatment. In other words, it is preferable that the median diameter (D50) of the positive electrode active material remains 5 μm or more and 9 μm or less.

[0142] After step S33, a step of further adding an additional element different from the additional element A1 may be provided. This step will be described with reference to FIG. 2C.

[0143] <Step S20_2> and <Step S21_2> Details of step S20_2 shown in Fig. 2A are shown in Fig. 2C. In step S21_2 of Fig. 2C, an additive element A2 source (A2 source) to be added to the composite oxide is prepared. A lithium source may be prepared together with the additive element A2 source.

[0144] The additive element A2 can be one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic. The additive element A2 should preferably be at least an element not selected as the additive element A1, but may also contain an element selected as the additive element A1. Note that FIG. 2C shows an example in which a Ni source and an Al source are used as the source of the additive element A2.

[0145] In this embodiment, nickel hydroxide is prepared as the nickel source, and aluminum hydroxide is prepared as the aluminum source. Nickel oxide or nickel carbonate may be used instead of nickel hydroxide. Aluminum oxide or aluminum carbonate may be used instead of aluminum hydroxide.

[0146] <Step S22_2> Next, in step S22_2 shown in Fig. 2C, the nickel source is mixed while being pulverized, and further the aluminum source is mixed while being pulverized. This step can be performed under the pulverization conditions and mixing conditions selected from those described in step S12. Note that this step may also be performed by combining the nickel source and the aluminum source and then mixing them while being pulverized, as in step S22_1 in Fig. 2B.

[0147] If necessary, a heating step may be performed after step S22_2. In this case, the heating step can be performed under the heating conditions selected from those described in step S13.

[0148] <Step S23_2> Next, in step S23_2 shown in FIG. 2C, the material crushed or mixed as described above is recovered to obtain a source of the additional element A2 (A2 source).

[0149] <Step S34> Next, in step S34 shown in FIG. 2A, the composite oxide that has been heated in step S33 is mixed with an additive element A2 source (A2 source). As described above, a plurality of additive element A2 sources (A2 sources) may be prepared. The number of atoms A of the transition metal M in the composite oxide is Ｍ and the number of nickel atoms A contained in the additive element A2. Ｎｉ The ratio of A Ｍ : A Ｎｉ = 100:y (0.1 ≤ y ≤ 3), and A Ｍ : A Ｎｉ It is more preferable that the number of atoms of the transition metal M in the composite oxide is 100:y (0.3≦y≦1). Ｍ and the number of aluminum atoms A contained in the additive element A2. Ａｌ The ratio of A Ｍ : A Ａｌ = 100:y (0.1 ≤ y ≤ 3), and A Ｍ : A Ａｌ It is more preferable that y=100:y (0.3≦y≦1).

[0150] The mixing conditions in step S34 can be selected from the mixing conditions described in step S31.

[0151] <Step S35> Next, in step S35 of Fig. 2A, the mixed materials are recovered to obtain a mixture 904. When recovering, the mixture 904 may be crushed to break up any agglomerated materials. The mixture 904 may also be sieved to further break up any agglomerated materials. Sieving may be performed after crushing, sieving may be performed while crushing is being performed, or sieving alone may be performed instead of crushing.

[0152] 2A, the mixture 904 is heated. The heating conditions can be selected from those described in step S33. The heating time is preferably 2 hours or more.

[0153] The heating temperature in step S36, similar to step S33, is preferably 500°C to 1130°C, more preferably 500°C to 1000°C, even more preferably 500°C to 950°C, and even more preferably 500°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 800°C to 1100°C, or 830°C to 1130°C, more preferably 830°C to 1000°C, even more preferably 830°C to 950°C, and even more preferably 830°C to 900°C. The heating temperature in step S36 is preferably lower than that in step S13. The heating temperature in step S36 is preferably lower than that in step S33.

[0154] The heating in this step is preferably performed so as not to stick together the mixture 904. If the mixture 904 sticks together during heating, the contact area with oxygen in the atmosphere will decrease and the path along which the additional element A2 diffuses will be blocked, which may result in a poor distribution of the additional element A2.

[0155] <Step S37> Next, in step S37 shown in FIG. 2A , 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 positive electrode active material 10. Through the above steps, the positive electrode active material 10 of one embodiment of the present invention can be produced. The positive electrode active material of one embodiment of the present invention has a smooth surface. The above is an example of a method for producing a positive electrode active material.

[0156] [Positive Electrode Active Material] A cross section of a positive electrode active material 10 obtainable by the above-described manufacturing method will be described using FIG. 3A. Also, unlike FIG. 3A, FIG. 3B shows a cross section of the positive electrode active material 10, with the grain boundaries 15 clearly indicated by dashed lines. Both the positive electrode active material 10 shown in FIGS. 3A and 3B have a surface layer portion 10a and an interior portion (the interior portion will be referred to as the "bulk portion") 10b, with the boundary between them indicated by a dashed line. It is more preferable that the surface layer portion 10a covers 90% or more of the bulk portion 10b. The dashed lines in FIGS. 3A and 3B are examples, and the dashed line in FIG. 3B is also an example, and the proportion of the surface layer portion covered is also an example.

[0157] The surface layer portion 10a does not have to cover the entire bulk portion 10b. Figures 3A and 3B show a cathode active material 10 in which the surface layer portion 10a covers 50% or more, specifically 65% ​​to 75%, of the outer periphery of the bulk portion 10b. For example, as shown in Figure 3B, the cathode active material 10 may have a region in which the bulk portion 10b is exposed.

[0158] The crystal grain boundary 15 shown in FIG. 3B refers to, for example, a portion where the positive electrode active material 10 is adhered to itself, a portion where the crystal orientation changes within the positive electrode active material 10, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, or a portion where the crystal structure is disordered. Crystal defects refer to defects that can be observed in a cross-sectional TEM (transmission electron microscope) image, a cross-sectional STEM image, or the like, and such defects can also be called a structure in which other elements have entered between lattices, or a cavity, etc. In other words, the crystal grain boundary 15 can be said to be one of the planar defects. The vicinity of the crystal grain boundary 15 refers to a region within 20 nm, preferably within 10 nm, of the center of the crystal grain boundary 15, and the vicinity of the grain boundary exists both inside and outside the particle. These can be distinguished by indicating the vicinity of the grain boundary inside the particle or the vicinity of the grain boundary outside the particle.

[0159] Because the positive electrode active material 10 contains a composite oxide containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between the region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and the region where it is not present can be considered the "surface" of the positive electrode active material. The region where the transition metal M is not present may contain an additive element. Surfaces newly formed by slippage, cracks, and / or fractures may also be considered the surface of the positive electrode active material.

[0160] Again, in this specification, the surface layer portion 10a refers to, for example, a region within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm from the surface toward the interior. Alternatively, the surface layer portion may be referred to as the near-surface, near-surface region, or shell. The region within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm from the surface toward the interior refers to the depth direction along a perpendicular or approximately perpendicular direction from the surface. Perpendicular or approximately perpendicular to the surface refers to a direction that forms an angle of 80° or more and 100° or less with a tangent to the surface. Furthermore, the bulk portion 10b refers to a region deeper than the surface layer portion 10a. The bulk portion 10b may be referred to as the interior, or may be referred to as the core. The bulk portion 10b may include the central portion of the positive electrode active material.

[0161] The region of the positive electrode active material 10 where lithium is inserted and removed may be referred to as the "surface." Therefore, the "surface" can be considered to be a region of the positive electrode active material 10 that comes into contact with the electrolyte. For example, the surface of the positive electrode active material 10 includes the surface of the surface layer portion 10a, and in a region where the bulk portion 10b is exposed, the bulk portion 10b may be the surface.

[0162] Carbonate groups, hydroxyl groups, and the like that are chemically adsorbed after preparation of the positive electrode active material 10 are considered to be regions where lithium cannot be inserted or removed, and these do not constitute the surface of the positive electrode active material 10. Similarly, the electrolyte, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 10 do not constitute the surface of the positive electrode active material 10 either.

[0163] Furthermore, the "surface" of the positive electrode active material 10 in a cross-sectional STEM (scanning transmission electron microscope) image or the like refers to the boundary between an area where an electron beam combined image is observed and an area where it is not observed, and can be the outermost area where bright spots originating from the atomic nuclei of metal elements having atomic numbers larger than that of lithium are observed. The surface in a cross-sectional STEM image or the like may be determined in conjunction with the results of an analysis with higher spatial resolution, such as electron energy loss spectroscopy (EELS).

[0164] The positive electrode active material must contain a transition metal capable of oxidation and reduction to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 10 of one embodiment of the present invention primarily uses cobalt as the transition metal M responsible for the oxidation and reduction reaction. However, 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 M contained therein, as this has many advantages, such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0165] When nickel is used as the transition metal M in the positive electrode active material 10 at 33 atomic % or more, preferably at 60 atomic % or more, and more preferably at 80 atomic % or more, the raw material may be cheaper than when cobalt is used in large amounts, and the discharge capacity per weight may increase, which is preferable.

[0166] The additive element A (additive element A1 and additive element A2) contained in the positive electrode active material 10 will be listed again. It is preferable to use one or more elements selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic.

[0167] That is, the positive electrode active material 10 can be called lithium cobalt oxide to which the additive element A has been added. As will be described later, the additive element A can further stabilize the crystal structure of the positive electrode active material 10.

[0168] The additional element A does not necessarily have to contain one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic.

[0169] For example, if positive electrode active material 10 is substantially free of manganese as additive element A, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, are further enhanced. The weight of manganese contained in positive electrode active material 10 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less. The weight of manganese can be analyzed using, for example, a glow discharge mass spectrometer (GD-MS).

[0170] <Crystal Structure> Using Figs. 4 to 8, LixCoO ２ The change in crystal structure accompanying the change in x in the positive electrode active material 10 usable as one embodiment of the present invention will be described by comparing it with a conventional positive electrode active material. The crystal structure of the positive electrode active material 10 usable as one embodiment of the present invention is shown in FIG. 4, and the crystal structure of the conventional positive electrode active material is shown in FIG. 5. The conventional positive electrode active material shown in FIG. 5 is a lithium cobalt oxide (LiCoO ２ In this specification, "not containing any particular added elements" refers to a case where the amount of added elements is below the lower limit of detection when measured using analytical means, or, if the amount is at the very edge of the lower limit of detection, to a level that does not affect the presence or absence of an action or effect.

[0171] <Li ｘ CoO ２ When x is 1, the positive electrode active material 10 usable as one embodiment of the present invention is Li ｘ CoO ２ In the case where x=1, that is, in the discharged state, it is preferable that the material has a layered rock-salt type crystal structure belonging to the space group R-3m. 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 refers to a structure in which an octahedral structure in which cobalt is six-coordinated with oxygen is continuous on a plane in an edge-sharing state. ２ The layer is sometimes referred to as a layer consisting of cobalt and oxygen octahedra. ｘ CoO ２The crystal structure when x is 1 is designated as R-3m(O3). ｘ CoO ２ When x is 1 in the formula, the crystal structure is the same as the crystal structure shown in FIG. 4, and is similarly denoted by R-3m(O3).

[0172] The layered rock-salt composite oxide has a high discharge capacity, two-dimensional lithium ion diffusion paths, and is suitable for lithium ion insertion / extraction reactions, making it an excellent positive electrode active material for lithium ion batteries. Therefore, it is preferable that the bulk portion 10b, which accounts for the majority of the positive electrode active material 10, has a layered rock-salt crystal structure.

[0173] The crystal structure of the surface layer 10a of the cathode active material 10 usable as one embodiment of the present invention does not have to be a layered rock salt type. The surface layer 10a preferably has a function of reinforcing the bulk portion 10b, which is made up of octahedra of cobalt and oxygen, so that the layered structure of the bulk portion 10b is not destroyed even when lithium is removed from the cathode active material 10 upon charging. Alternatively, the surface layer 10a preferably functions as a barrier film for the cathode active material 10. Alternatively, the surface layer 10a, which is the outer periphery of the cathode active material 10, preferably reinforces the cathode active material 10. Here, "reinforcement" refers to suppressing structural changes in the surface layer 10a and bulk portion 10b of the cathode active material 10, such as oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 10.

[0174] 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 bulk portion 10b. In addition, when lithium ions are released, some of the bonds of the atoms on the surface of the positive electrode active material 10 in the surface layer 10a are broken. Therefore, the surface layer 10a is likely to become unstable, and is a region where the crystal structure is likely to change, that is, where deterioration is likely to begin. On the other hand, if the surface layer 10a can be made sufficiently stable, Li ｘ CoO ２ When x is small, for example, even if x is 0.24 or less, CoO ２ Furthermore, the CoO ２ The misalignment of the layers can be suppressed.

[0175] In order to give the surface layer portion 10a a stable composition and 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 bulk 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. The concentration peak here refers to the maximum concentration value in the surface layer portion 10a or within 50 nm from the surface.

[0176] Here, the above concentration gradient and concentration peak will be explained with reference to the conceptual diagrams of FIGS. 3C to 3F.

[0177] 3C and 3D show enlarged views of the vicinity of A-B in FIG. 3A. FIGS. 3C and 3D are cross-sectional views of the surface layer having the (001) plane (hereinafter referred to as the (001) plane, and may also be referred to as the c-plane or basal plane), that is, cross-sectional views of the region oriented in (001). In the layered rock salt type crystal structure, cations are arranged parallel to the (001) plane. This is due to the CoO ２ It can be said that the structure is one in which the CoO layer and the lithium layer are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions exists parallel to the (001) plane. ２ The layer is relatively stable, so CoO ２ The (001) plane on which the layer is present is relatively stable, and the main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0178] FIG. 3C shows the distribution of magnesium as an example of the added element A. The shading in FIG. 3C corresponds to the change in magnesium concentration. The (001) plane is CoO ２Since the layer is relatively stable, the additional element A may not be detected. When the additional element A is detected, FIG. 3C shows an example of the distribution of the additional element A, in which the additional element A is present at the highest concentration at or near the surface of the surface layer portion 10a, and the concentration of the additional element A decreases toward the bulk portion 10b. It can be said that there is a concentration peak of magnesium or the like at the position showing the highest concentration. The decrease in concentration may also be referred to as a concentration gradient. In this specification, the additional element A that shows a distribution as shown in FIG. 3C on the (001) plane will be referred to as the additional element X.

[0179] FIG. 3D shows the distribution of aluminum as another example of the added element A. The shading in FIG. 3D corresponds to the change in aluminum concentration. The (001) plane is CoO ２ Because the layer is relatively stable, the additional element A may not be detected. When the additional element A is detected, FIG. 3D shows an example of the distribution of the additional element A, in which the additional element A is present at the highest concentration at a position deeper than the surface or near the surface in the surface layer portion 10a, and the concentration of the additional element A decreases toward the surface and bulk portion 10b. It can be said that there is an aluminum concentration peak at the position showing the highest concentration, and the aluminum concentration peak position may be located slightly deeper than the concentration peak positions of the magnesium and other elements. The decrease in concentration may also be referred to as a concentration gradient. In this specification, the additional element A that exhibits a distribution as shown in FIG. 3D on the (001) plane will be referred to as the additional element Y.

[0180] Depending on the element of the additional element A, a distribution like that of the additional element X or a distribution like that of the additional element Y may be exhibited, and the distributions may differ from each other. Furthermore, depending on the element of the additional element A, a concentration peak position like that of the additional element X or a concentration peak position like that of the additional element Y may be exhibited, and the concentration peak positions may differ from each other.

[0181] Figures 3E and 3F are enlarged views of the vicinity of C-D in Figure 3A. Figures 3E and 3F can be said to be cross-sectional views of a surface layer having a plane other than the (001) plane (hereinafter, sometimes referred to as an ab plane or an edge plane), and in a layered rock-salt crystal structure, a plane other than the (001) plane is a plane where a diffusion path for lithium ions exists.

[0182] In FIG. 3E , which shows a plane other than the (001) plane, the distribution of magnesium or the like is shown as an example of the additional element X. Compared to the (001) plane in FIG. 3C , the concentration of the additional element X may be higher in the plane other than the (001) plane in FIG. 3E . The concentration peak of magnesium or the like may be located on or near the surface of the surface layer 10 a, and may exhibit a higher intensity than the concentration peak in the (001) plane in FIG. 3C . Furthermore, the additional element X may be distributed over a wide range in the plane other than the (001) plane in FIG. 3E .

[0183] 3F shows the distribution of aluminum and the like as an example of the added element Y. The aluminum concentration peak is preferably located in a region of 5 nm to 50 nm from the surface toward the inside, whether on the (001) plane in FIG. 3D or on a plane other than the (001) plane in FIG. 3F. Depending on the heat treatment conditions, the aluminum concentration peak may be deeper on a plane other than the (001) plane in FIG. 3F than on the (001) plane in FIG. 3D.

[0184] In this way, the distribution of the additive element may differ depending on the surface direction of the positive electrode active material.

[0185] Again, the diffusion paths of lithium ions exist on surfaces other than the (001) plane, and the diffusion paths of lithium ions are exposed on surfaces other than the (001) plane. Therefore, as shown in FIGS. 3E and 3F, the surface layer 10a corresponding to the surfaces other than the (001) plane is an important region for maintaining the diffusion paths of lithium ions, but it is also the region from which lithium ions are first desorbed and therefore prone to instability. Therefore, in order to maintain the crystal structure of the entire positive electrode active material 10, it is advisable to preferentially reinforce the surfaces other than the (001) plane and the corresponding surface layer 10a. In other words, it is advisable for the additive element A to be preferentially present on the surfaces other than the (001) plane and the corresponding surface layer 10a.

[0186] As described in the above manufacturing method, LiCoO formed through initial heating ２ In the case of a manufacturing method in which the additive element is mixed and heated, the additive element spreads through the diffusion path of lithium ions, and therefore, it is easy to make the distribution of the additive element in the planes other than the (001) plane and the surface layer portion 10a corresponding to those planes fall within a preferred range, as shown in FIGS.

[0187] Although it has been stated that the surface of the positive electrode active material 10 is preferably smooth and has few irregularities, the entire positive electrode active material 10 does not necessarily have to be smooth. For example, the positive electrode active material 10 may have irregularities due to slips occurring on a surface parallel to the (001) plane, for example, on a surface where lithium is arranged. Slips are also called stacking faults. For example, pressing is performed when preparing the positive electrode, and the pressing causes LiCoO ２ The deformation can occur along the lattice fringe direction (the ab plane direction), and this deformation is also included in slip. Deformation can include the lattice fringes shifting back and forth. When the lattice fringes shift back and forth, a step occurs on the surface in the direction perpendicular to the lattice fringes (the c-axis direction).

[0188] Furthermore, the surface resulting from the slip and its surface layer 10a are often the (001) plane, and the surface layer 10a corresponding to the (001) plane may not contain any added elements or may contain elements below the detection limit. As mentioned above, the (001) plane does not expose the diffusion path of lithium ions and is relatively stable, so there is almost no problem even if the added elements are not present or are below the detection limit.

[0189] The composition is LiCoO ２ In the composite oxide having a layered rock salt type crystal structure of R-3m, cobalt is arranged parallel to the (001) plane. ２ Among these, cobalt, which has the largest atomic number, has the highest brightness. Therefore, in a HAADF-STEM image, the arrangement of bright atoms can be considered to be the arrangement of cobalt. The repetition of this bright arrangement is synonymous with crystal fringes or lattice fringes.

[0190] Next, the additive elements will be described. Magnesium, which is one of the additive elements X, is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer portion 10a makes it easier to reinforce the layered rock-salt crystal structure of the bulk portion 10b, etc. This is because magnesium present at the lithium site is easily absorbed by CoO２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２ When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 10. Furthermore, a high magnesium concentration in the surface layer portion 10a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0191] At an appropriate concentration, magnesium does not adversely affect the intercalation and deintercalation of lithium during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect the intercalation and deintercalation of lithium. Furthermore, 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. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) 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 lithium-ion 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.

[0192] 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 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.

[0193] Nickel, which is one of the additional elements X, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, its oxidation-reduction potential is lower than that of cobalt, which is preferable as it leads to an increase in discharge capacity.

[0194] Furthermore, when nickel is present at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the volume change caused by charge and discharge is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel present at the lithium site and CoO ２ It is thought that this is because they function as pillars supporting the layers.

[0195] 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.

[0196] Therefore, it is preferable that the amount of nickel contained in the entire positive electrode active material 10 is appropriate. For example, the number of nickel atoms contained in the positive electrode active material 10 is preferably greater than 0% and less than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably greater than 0% and less than 4%. Alternatively, it is preferably greater than 0% and less than 2%. Alternatively, it is preferably 0.05% to less than 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to less than 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be a value obtained by performing elemental analysis of the entire positive electrode active material using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0197] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Furthermore, aluminum has the effect of suppressing the elution of surrounding cobalt and improving continuous charging durability. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, a positive electrode active material containing aluminum as an additive element can improve safety when used in a lithium-ion battery. Furthermore, a positive electrode active material 10 can be obtained whose crystal structure is less likely to collapse even after repeated charging and discharging.

[0198] Furthermore, it is preferable that aluminum, which is one of the additional elements Y, is present at a position slightly deeper than the surface (specifically, that the aluminum concentration peak is in a region deeper than the concentration peak of the additional element X). Alternatively, it is preferable that the presence of aluminum, which is one of the additional elements Y, is confirmed in a region deeper than the deepest position where the presence of the additional element X is confirmed. This is because, if aluminum were to be substituted for a lithium site on the surface, lithium present in the vicinity of the aluminum substituted for the lithium site would be fixed, and this could hinder the diffusion path of lithium.

[0199] 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.

[0200] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When a portion of the oxygen in the surface layer portion 10a is substituted with fluorine, the lithium desorption energy decreases. This is because the valence of the cobalt ion changes with lithium desorption (from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine), resulting in different oxidation-reduction potentials. Therefore, when a portion of the oxygen in the surface layer portion 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 used in a lithium-ion battery, charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 10a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. Furthermore, as will be described in a later embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of another additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source.

[0201] Furthermore, when the surface layer 10a contains both magnesium and nickel, there is a possibility that divalent magnesium can exist more stably near divalent nickel. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer 10a.

[0202] Furthermore, having additive elements with different distributions, such as additive element X and additive element Y, 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 both magnesium and nickel as the additive element X and aluminum as the additive element Y, it can stabilize the crystal structure in a wider region than when it has only one of the additive element X and the additive element Y. In this way, when the positive electrode active material 10 has both additive element X and additive element Y, the additive element Y, such as aluminum, is not essential to the surface because the surface stabilization can be sufficiently achieved by the additive element X, such as magnesium. Rather, it is preferable for aluminum to be widely distributed in a deep region, for example, a region with a depth from the surface of 5 nm to 50 nm, because this can stabilize the crystal structure in a wider region.

[0203] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 10 a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0204] However, if the surface layer 10a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 10a to be occupied only by MgO, a structure in which MgO and NiO(II) are solid-solved, and / or a structure in which MgO and CoO(II) are solid-solved. Therefore, it is preferable that the surface layer 10a contains at least cobalt and also contains lithium in a discharged state, so that a path for insertion and extraction of lithium is secured.

[0205] In order to ensure sufficient paths for lithium insertion and desorption, the surface layer 10a preferably has a higher cobalt concentration than magnesium. The surface layer 10a preferably has a higher cobalt concentration than nickel. The surface layer 10a preferably has a higher cobalt concentration than aluminum. The surface layer 10a preferably has a higher cobalt concentration than fluorine.

[0206] Furthermore, since there is a risk that an excessive amount of nickel may hinder the diffusion of lithium, it is preferable that the concentration of magnesium in the surface layer portion 10a is higher than that of nickel.

[0207] Furthermore, it is preferable that some of the additive elements, particularly magnesium and nickel, be present at a higher concentration in the surface layer portion 10a than in the bulk portion 10b, and that they also be present randomly and in a sparse manner in the bulk portion 10b. It is also preferable that aluminum, which is one of the additive elements, is also present randomly and in a sparse manner in the bulk portion 10b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the bulk 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 bulk portion 10b, it can suppress the shift in the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0208] Furthermore, due to the concentration gradient of the added element, it is preferable that the crystal structure of the lithium cobalt oxide continuously changes from the bulk portion 10b toward the surface portion 10a. In this case, it is preferable that the surface portion 10a has a composition and crystal structure that are more stable at room temperature (25°C) than the bulk portion 10b. For example, it is preferable that at least a portion of the surface portion 10a of the positive electrode active material 10 usable as one embodiment of the present invention has a rock salt type crystal structure. Alternatively, it is preferable that the surface 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 portion 10a has characteristics of both a layered rock salt type crystal structure and a rock salt type crystal structure.

[0209] Furthermore, due to the concentration gradient of the added element, it is preferable that the crystal orientation of the surface layer portion 10a and the bulk portion 10b is approximately the same, or that the surface layer portion 10a and the bulk portion 10b are topotaxy.

[0210] In this specification, topotaxis refers to a three-dimensional structural similarity in which the crystal orientations roughly coincide, or to a crystallographically identical orientation, while epitaxy refers to a two-dimensional interface structural similarity.

[0211] The topotactic relationship between the surface layer portion 10 a and the bulk portion 10 b can reduce distortion of the crystal structure and / or misalignment of the atoms, thereby suppressing the cause of pits. In this specification, pits refer to holes formed by the progression of defects in the positive electrode active material.

[0212] <Li ｘ CoO ２ When x is small in the positive electrode active material 10 usable as one embodiment of the present invention, due to the distribution of the additive element and / or the crystal structure as described above, ｘ CoO ２ The crystal structure when x is small differs from that of conventional positive electrode active materials. Again, in this specification, "small x" means 0.1<x≦0.24.

[0213] First, conventional Li ｘ CoO ２ In the case of lithium cobalt oxide where x=0.5, which corresponds to a slightly small value of x, the symmetry of lithium is increased as shown in FIG. 5, and the lithium cobalt oxide has a crystal structure belonging to the monoclinic space group P2 / m. This structure has CoO in the unit cell. ２ There is one layer. Therefore, it is sometimes called monoclinic O1 type or O1 type. In Figure 5, the crystal structure with x = 0.5 is labeled P2 / m (monoclinic O1).

[0214] In addition, conventional Li ｘ CoO ２ In the above formula, when x=0, lithium cobalt oxide has a crystal structure of the trigonal space group P-3m1 as shown in FIG. 5, and CoO ２ There is one layer. Therefore, this crystal structure is sometimes called trigonal O1 type or O1 type. In addition, the trigonal crystal is sometimes converted into a composite hexagonal lattice and called hexagonal O1 type. In Figure 5, the crystal structure with x = 0 is labeled P-3m1 (trigonal O1).

[0215] In addition, conventional Li ｘ CoO ２In the above formula, when x is small (x=0.12), lithium cobalt oxide has a crystal structure of the space group R-3m as shown in FIG. 5. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m(O3). 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. Therefore, 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 Figure 5 and other figures in this specification, to facilitate comparison with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell. In Figure 5, the crystal structure with x = 0.12 is labeled R-3m1 (H1-3).

[0216] In 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 express a certain crystal structure can be determined, for example, by Rietveld analysis using X-ray diffraction (abbreviated as XRD). In the Rietveld analysis, the unit cell that results in the smallest GOF (goodness of fit) value can be adopted.

[0217] When x is small, the corresponding Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.12 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m(O3) structure in the discharged state, as shown in Figure 5.

[0218] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in the crystal structure labeled R-3m1 (H1-3) in FIG. 5, in the H1-3 type crystal structure, CoO ２The layer is significantly different from the R-3m(O3) in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0219] Furthermore, the two crystal structures that undergo dynamic structural changes also have a large difference in volume: When compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3m(O3) crystal structure is greater than 3.5%, typically 3.9% or more.

[0220] 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.

[0221] Therefore, when charging and discharging are repeated so that x is 0.12 or less, the crystalline structure of conventional lithium cobalt oxide collapses. This collapse of the crystalline structure causes deterioration of cycle characteristics. This is because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult to insert and extract lithium. Note that not only when charging and discharging are repeated so that x is 0.12 or less, but also when x is 0.24 or less, collapse of the crystalline structure often occurs, causing deterioration of cycle characteristics. For this reason, in practical use, conventional lithium cobalt oxide is repeatedly charged and discharged in lithium-ion batteries while controlling x to a value greater than 0.24.

[0222] On the other hand, the positive electrode active material 10 shown in FIG. 4 that can be used as one embodiment of the present invention is Li ｘ CoO ２ The change in the crystal structure between the discharged state where x is 1 and the state where x is 0.24 or less, or the state where x is 0.15, is smaller than that of conventional positive electrode active materials. Specifically, the positive electrode active material 10 has a smaller change in the crystal structure between the discharged state where x is 1 and the state where x is 0.24 or less, and a smaller change in the crystal structure of CoO ２ The layer misalignment can be reduced. Furthermore, the positive electrode active material 10 can reduce the change in volume per cobalt atom. Therefore, the positive electrode active material 10 is resistant to breakdown of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and can achieve excellent cycle characteristics. Furthermore, the positive electrode active material 10 has a low Li ｘCoO ２ When x is 0.24 or less, the positive electrode active material 10 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the state where x is 0.24 or less is maintained, a short circuit is unlikely to occur, and the safety of the lithium ion battery is improved.

[0223] 4 shows the crystal structures of positive electrode active material 10 when x = 0.2 and x = 0.15, where 0.1 < x ≦ 0.24. When x = 0.2 and x = 0.15, positive electrode active material 10 has a crystal structure that differs from the H1-3 type crystal structure of conventional lithium cobalt oxide.

[0224] Specifically, when x=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. For this reason, in this specification and the like, this crystal structure will be referred to as an "O3'-type crystal structure." In FIG. 4, R-3m(O3)' is added to the crystal structure where x = 0.2. Although the O3'-type crystal structure possessed by the positive electrode active material 10 has been described as a case where x = 0.2, if x is about 0.2, it can also be an O3'-type crystal structure. An x of about 0.2 can be expressed, for example, as 0.18≦x≦0.24, typically 0.18≦x≦0.22.

[0225] In the O3'-type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant a of the unit cell is preferably 0.2797≦a≦0.2837 (nm), more preferably 0.2807≦a≦0.2827 (nm), and typically a=0.2817 (nm). The lattice constant c is preferably 1.368≦c≦1.388 (nm), more preferably 1.375≦c≦1.381 (nm), and typically c=1.378 (nm).

[0226] As an example of 0.1<x≦0.24, when x=0.15, the positive electrode active material 10 has a crystal structure belonging to the monoclinic space group P2 / m. ２There is one layer. For this reason, in this specification and the like, this crystal structure will be referred to as a "monoclinic O1(15) type crystal structure." In FIG. 4, the crystal structure with x = 0.15 is labeled P2 / m (monoclinic O1(15)). Although the monoclinic O1(15) type crystal structure of the positive electrode active material 10 has been described as a case where x = 0.15, if x is about 0.15, it can become a monoclinic O1(15) type crystal structure. An x of about 0.15 can be expressed, for example, as 0.13≦x≦0.24, typically 0.13≦x≦0.18.

[0227] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ ≦0.71. The lattice constant a of the unit cell is a = 0.488 ± 0.001 (nm), the lattice constant b is b = 0.282 ± 0.001 (nm), and c = 0.484 ± 0.001 (nm). The angles indicating a monoclinic crystal are α = 90°, β = 109.58 ± 0.01°, and γ = 90°.

[0228] This crystal structure can also be fitted to the space group R-3m if a certain degree of error is allowed. In this case, the coordinates of the oxygen atoms of cobalt in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constant a of the unit cell is a=0.2817±0.002 (nm), and the lattice constant c is c=1.368±0.002 (nm).

[0229] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy six oxygen coordination positions, although light elements such as lithium may occupy four oxygen coordination positions.

[0230] In Figure 4, CoO ２ As shown by the dotted lines along the edge of the layer, the R-3m(O3) in the discharged state, the O3'-type crystal structure, and the monoclinic O1(15)-type crystal structure show a significant difference in the CoO ２ There is almost no layer misalignment. 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, and typically 1.8%. The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the monoclinic O1(15)-type crystal structure is 3.3% or less, more specifically 3.0% or less, and typically 2.5%.

[0231] The positive electrode active material 10 is Li ｘ CoO ２ It can be seen that when x is small, that is, when a large amount of lithium is released, the change in crystal structure is suppressed more than with conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, it can be seen that the crystal structure of the positive electrode active material 10 is less likely to collapse even when repeatedly charged and discharged such that x is 0.24 or less, and the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than with 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 lithium-ion battery with a high discharge capacity per weight and per volume can be fabricated.

[0232] The positive electrode active material 10 is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be formed, and it is estimated that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is formed. ｘ CoO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically 0.13 or more and less than 0.18, the monoclinic O1(15) type crystal structure may be obtained. However, the crystal structure is similar to that of Li ｘ CoO ２The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0233] Therefore, the positive electrode active material 10 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the bulk portion 10b of the positive electrode active material 10 may have only the O3'-type crystal structure, only the monoclinic O1(15)-type crystal structure, or both. Furthermore, the bulk portion 10b of the positive electrode active material 10 does not necessarily have to have the O3'-type and / or the monoclinic O1(15)-type crystal structure. It may contain other crystal structures, or a portion may be amorphous.

[0234] Also, Li ｘ CoO ２ To make the x in the lithium secondary battery small, it is generally necessary to charge the battery at a high charging voltage. ｘ CoO ２ The state where x is small can be rephrased as a state charged at a high charging voltage. The positive electrode active material 10 is preferable because it can maintain the R-3m(O3) crystal structure even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25°C. The positive electrode active material 10 is also preferable because it can adopt an O3'-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25°C. The positive electrode active material 10 is also preferable because it can adopt a monoclinic O1(15)-type crystal structure when charged at an even higher charging voltage, for example, a voltage of 4.7 V or higher and 4.8 V or lower at 25°C.

[0235] Even with positive electrode active material 10, if the charge voltage is further increased, H1-3 type crystals may finally be observed. Furthermore, as described above, since the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, electrolyte, etc., when the charge voltage is lower, for example, even if the charge voltage is 4.5 V or more and less than 4.6 V at 25° C., positive electrode active material 10 usable as one embodiment of the present invention may be able to adopt an O3′ type crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.7 V or less at 25° C., it may be able to adopt a monoclinic O1(15) type crystal structure.

[0236] In addition, when graphite is used as the negative electrode active material in a lithium ion battery, the voltage of the lithium ion battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.01 V to 0.7 V relative to the potential of lithium metal. Therefore, in the case of a lithium ion 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.

[0237] In addition, although the O3'-type crystal structure and the monoclinic O1(15)-type crystal structure in Figure 4 are shown as having lithium present at all lithium sites with equal probability, this is not limited thereto. Lithium may be present disproportionately at some lithium sites. The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0238] The O3' type crystal structure and the monoclinic O1(15) type crystal structure have random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0239] Furthermore, it is preferable that the concentration gradient of the additive element be the same 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 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. However, it is not necessary that the additive element have the same concentration gradient throughout the entire surface layer portion 10a of the positive electrode active material 10.

[0240] <Grain Boundaries> In addition to the distribution described above, it is more preferable that at least a portion of the additive elements contained in positive electrode active material 10 that can be used as one embodiment of the present invention is unevenly distributed in and near grain boundaries 15 as shown in FIG. 3B .

[0241] For example, the magnesium concentration at and near the crystal grain boundaries 15 of the positive electrode active material 10 is preferably higher than that in other regions of the bulk portion 10b. The fluorine concentration at and near the crystal grain boundaries 15 is also preferably higher than that in other regions of the bulk portion 10b. The nickel concentration at and near the crystal grain boundaries 15 is also preferably higher than that in other regions of the bulk portion 10b. The aluminum concentration at and near the crystal grain boundaries 15 is also preferably higher than that in other regions of the bulk portion 10b.

[0242] Since the grain boundaries 15 are one type of planar defect, they are prone to become unstable like the surface, and changes in the crystal structure are likely to begin at these boundaries. Therefore, by increasing the concentration of the added element at and near the grain boundaries 15, such changes in the crystal structure can be more effectively suppressed.

[0243] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries 15, even if cracks occur along the grain boundaries 15 of the positive electrode active material 10 that can be used as one embodiment of the present invention, the magnesium concentration and fluorine concentration become high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the cracks occur.

[0244] <Analysis method> In a certain positive electrode active material, Li ｘ CoO ２ When x in the formula (I) is small, whether the positive electrode active material 10 has an O3′ type and / or a monoclinic O1(15) type crystal structure and can be used as one embodiment of the present invention can be determined by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material in a charged state in which x is small using XRD, electron beam diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0245] 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 lithium ion battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the bulk portion 10b of the positive electrode active material 10, which occupies the majority of the volume of the positive electrode active material 10.

[0246] When the measurement sample is a powder, this is sometimes called the powder XRD measurement, and the powder can be set by placing it in a glass sample holder, sprinkling the sample on a greased silicone anti-reflection plate, etc. When the measurement sample is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0247] Furthermore, even in the case of cathode active material 10 that can be used as one embodiment of the present invention, if x 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 formed. Therefore, to determine whether or not cathode active material 10 can be used as one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charging capacity or the charging voltage are required.

[0248] Furthermore, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, the crystal structure may change from O3'-type and monoclinic O1(15)-type to 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.

[0249] Furthermore, whether or not the distribution of the added elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0250] The crystal structure of the surface layer 10 a, the grain boundaries 15 , etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 10 .

[0251] <Evaluation Conditions> An example of an evaluation condition for determining whether a certain composite oxide is a cathode active material 10 that can be used as an embodiment of the present invention is to prepare a coin cell (e.g., CR2032 type, diameter 20 mm, height 3.2 mm) having lithium metal as a counter electrode and charge it under predetermined conditions. An example of an evaluation condition for determining whether a certain electrolyte is an electrolyte that can be used as an embodiment of the present invention is to prepare a coin cell (e.g., CR2032 type, diameter 20 mm, height 3.2 mm) having lithium metal as a counter electrode and charge it under predetermined conditions.

[0252] The procedure 1 described below is an example for confirming the physical properties of the positive electrode active material 10 that can be used as one embodiment of the present invention. Therefore, the electrolyte has a different configuration from that of the lithium ion battery that is one embodiment of the present invention.

[0253] Evaluation Procedure 1: A lithium-ion battery is disassembled, and the electrolyte-impregnated positive electrode is removed. The positive electrode is then punched out to a size that fits into a prepared coin cell. The positive electrode contains a conductive material and a binder in addition to the positive electrode active material. The electrolyte and other materials are removed before punching out the positive electrode. For example, after removing the positive electrode, the positive electrode may be washed using an organic solvent or the like.

[0254] The coin cell has lithium metal as the counter electrode. However, materials other than lithium metal may be used as the counter electrode. Unless otherwise specified, the potential in this specification is the potential of the positive electrode when the counter electrode is lithium metal.

[0255] The coin cell was prepared using an electrolyte containing 1 mol / L lithium hexafluorophosphate (LiPF) in an organic solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 and 2 wt% vinylene carbonate (VC). ６ ) may be dissolved in the water.

[0256] The coin cell has a 25 μm thick porous polypropylene film as a separator.

[0257] The coin cell uses stainless steel (SUS) for the positive electrode can and stainless steel (SUS) for the negative electrode can.

[0258] In this way, a coin cell A for evaluation is prepared.

[0259] The coin cell A for evaluation prepared under the above conditions is subjected to constant current charging (also called CC charging) at a current value of 10 mA / g (equivalent to 0.05 C when 1 C is 200 mA / g of positive electrode active material weight) up to a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, or 4.65 V, 4.7 V, 4.75 V, or 4.8 V). Charging at such a small current value is desirable for observing the phase change of the positive electrode active material.

[0260] The temperature during charging of the evaluation coin cell A can be 25° C. The temperature during charging may be the temperature of the thermostatic bath in which the coin cell A is placed.

[0261] After charging under these conditions, coin cell A is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with a desired charge capacity. When various analyses are performed thereafter, it is preferable to seal the cell in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to remove and analyze the positive electrode promptly after charging is completed. Specifically, it is preferable to do so within one hour, and more preferably within 30 minutes, after charging is completed.

[0262] <Evaluation Procedure 2> A lithium-ion battery is disassembled, the positive electrode impregnated with the electrolyte is removed, and the electrolyte is analyzed by nuclear magnetic resonance (e.g. １ Measurements using H NMR are carried out to identify at least the organic solvent. Nuclear magnetic resonance spectroscopy can also identify the mixing ratio (volume ratio) of the organic solvent. It is also possible to identify other components, such as the lithium salt and additives contained in the electrolyte.

[0263] The positive electrode is then punched out to a size that fits into a coin cell. The positive electrode contains a conductive material and a binder in addition to the positive electrode active material. The electrolyte and other components are removed after the nuclear magnetic resonance measurement but before punching out the positive electrode. For example, after removing the positive electrode, the positive electrode may be washed with an organic solvent or the like.

[0264] The coin cell has a lithium metal counter electrode, although materials other than lithium metal may be used as the counter electrode.

[0265] The coin cell is provided with an electrolyte identified using nuclear magnetic resonance spectroscopy, which is an electrolyte according to one embodiment of the present invention.

[0266] The coin cell has a 25 μm thick porous polypropylene film as a separator.

[0267] The coin cell uses stainless steel (SUS) for the positive electrode can and stainless steel (SUS) for the negative electrode can.

[0268] In this way, a coin cell B for evaluation is prepared.

[0269] The coin cell B for evaluation prepared under the above conditions is charged to a desired voltage (for example, 4.5 V, 4.55 V, 4.6 V, or 4.65 V, 4.7 V, 4.75 V, or 4.8 V) and then discharged. The charging conditions may be referred to in the Examples below. The discharging conditions may be referred to in the Examples below.

[0270] The temperature during charging of the evaluation coin cell B can be set to 25°C and below freezing, and it can be confirmed how the charge / discharge capacity at below freezing is compared to the charge / discharge capacity at 25°C.

[0271] <XRD> The XRD measurement performed on the coin cell A and the like can be performed using the following equipment and conditions. However, the equipment and conditions for the XRD measurement are not limited to those described below. XRD equipment: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １ Line output: 40KV, 40mA

[0272] <Powder XRD pattern> CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model. １ Ideal XRD patterns of the lines are shown in Figures 6, 7, 8A and 8B. ｘ CoO ２ LiCoO where x=1 ２ 8A and 8B show the XRD patterns of the O3′ type, the monoclinic O1(15) type, and the H1-3 type, with Fig. 8A showing an enlarged view of the region where 2θ is between 18° and 21°, and Fig. 8B showing an enlarged view of the region where 2θ is between 42° and 46°. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database). The 2θ range was 15° to 75°, Step = 0.01°, and wavelength λ = 1.540562 × 10 −１０ The monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 2. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of a positive electrode active material usable as one embodiment of the present invention, and fitted using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same manner as the others.

[0273] As shown in Figures 6, 8A, and 8B, when the O3' type crystal structure is analyzed by X-ray diffraction, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0274] Furthermore, in the monoclinic O1(15) type crystal structure, X-ray diffraction analysis reveals diffraction peaks at 2θ = 19.47 ± 0.10° (19.37° to 19.57°) and 2θ = 45.62 ± 0.05° (45.57° to 45.67°).

[0275] On the other hand, as shown in Figures 7, 8A and 8B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of peaks at 19.13° or more and less than 19.37° and / or 19.37° or more and 19.57° or less, and at 45.37° or more and less than 45.57° and / or 45.57° or more and 45.67° or less when x is small can be said to be a characteristic of positive electrode active material 10 that can be used as one embodiment of the present invention.

[0276] It can also be said that the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, it can be said that the difference in 2θ between the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ values ​​of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0277] The positive electrode active material 10 that can be used in one embodiment of the present invention is Li ｘ CoO ２ When x in the formula is small, the material has an O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the material has an O3'-type and / or monoclinic O1(15)-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0278] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0279] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ = 43° or more and 46° or less, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Therefore, it contributes to the stabilization of the crystalline structure after charging sufficiently.

[0280] The crystallite size of the O3'-type and monoclinic O1(15) crystal structures 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 ｘ CoO ２ When x in the graph is small, a clear peak of the O3' type crystal structure can be confirmed. ２ In this case, even if a part of the crystal structure resembles the O3'-type crystal structure, the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0281] <Surface Roughness and Specific Surface Area> The cathode active material 10 that can be used as one embodiment of the present invention preferably has a smooth surface with few irregularities. A smooth surface with few irregularities indicates that the effect of the flux described below is fully exerted and the additive element source and the surface of the lithium cobalt oxide are fused (formed a solid solution). Therefore, this is one factor indicating that the additive element is well distributed in the surface layer portion 10 a.

[0282] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional scanning electron microscope (SEM) image or cross-sectional TEM image of the positive electrode active material 10, the specific surface area of ​​the positive electrode active material 10, or the like.

[0283] <Additional Features> The positive electrode active material 10 may have a coating portion on the outside of the surface layer portion 10 a. The coating portion does not have to cover the entire positive electrode active material. The coating portion may be an inorganic compound formed during the preparation of the positive electrode active material, or may be formed by the accumulation of decomposition products of the electrolyte and the organic electrolyte solution during charge and discharge.

[0284] When the coating portion contains an electrolyte and a decomposition product of an organic electrolyte solution, it preferably contains carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as part of the electrolyte solution, a high-quality coating is easily obtained. Therefore, a coating portion containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating and is therefore preferred.

[0285] This embodiment mode can be freely combined with other embodiment modes.

[0286] (Embodiment 3) In this embodiment, a method for manufacturing a cathode active material that can be used in the lithium-ion battery of the present invention is described. In this embodiment, a case where a cathode active material is manufactured using a coprecipitation method is described. However, in addition to the coprecipitation method, cathode active materials manufactured using a solid-phase method, a hydrothermal method, or the like can also be applied to the lithium-ion battery of the present invention. Note that the flowchart used to explain the manufacturing method in this embodiment shows the order of elements connected by lines, but does not show the order of elements not connected by lines. Furthermore, in this embodiment, when an oxide is manufactured as the cathode active material, a precursor to the oxide, for example, a hydroxide, is referred to as a precursor.

[0287] <Production Method 1> In this production method 1, a method including a heating step, which is one embodiment of the present invention, will be described.

[0288] <Step S201: Preparation of Raw Materials> First, in step S201 of FIG. 9 , raw materials are prepared according to the type of positive electrode active material. In this production method 1, an aqueous solution in which at least a transition metal salt is dissolved is prepared. The aqueous solution in which the transition metal salt is dissolved can be referred to as a transition metal source. Furthermore, when the pH value of the aqueous solution in which the transition metal salt is dissolved is less than 7, preferably a pH value of 1 or more and 6 or less, the aqueous solution is acidic and can be referred to as an acidic aqueous solution.

[0289] The transition metal will now be described. In the present invention, one or more transition metals selected from manganese, cobalt, and nickel can be used. Specifically, the transition metal may be cobalt alone, nickel alone, cobalt and manganese, cobalt and nickel, or cobalt, manganese, and nickel.

[0290] A composite oxide obtained using three elements, nickel, cobalt, and manganese, is sometimes referred to as a nickel-cobalt-manganese composite oxide. Nickel-cobalt-manganese composite oxide refers to lithium-nickel-cobalt-manganese composite oxide before lithium is mixed in. In this specification, "lithium-nickel-cobalt-manganese composite oxide" is referred to as "NCM." NCM has the chemical formula LiNi ｘ Co ｙ Mn ｚ O ２ (x>0, y>0, 0.8<x+y+z<1.2). In NCM, Li is not limited to 1 based on the above chemical formula, but Li can be 0.58 or more and 1.10 or less, preferably 0.90 or more and 1.05 or less, and preferably 0.92 or more and 1.01 or less. Note that NCM may further contain elements other than nickel, cobalt, and manganese.

[0291] LiNi of the above chemical formula ｘ Co ｙ Mn ｚ O ２In the formula, the relationship between the values ​​that x, y, and z can take may, for example, satisfy 0.1x<y<8x and 0.1x<z<8x. Specific values ​​that x, y, and z can take preferably satisfy x:y:z=1:1:1 or a value close thereto. As another specific example of values, x, y, and z preferably satisfy x:y:z=5:2:3 or a value close thereto. As another specific example of values, x, y, and z preferably satisfy x:y:z=8:1:1 or a value close thereto. As another specific example of values, x, y, and z preferably satisfy x:y:z=9:0.5:0.5 or a value close thereto. As another specific example of values, x, y, and z preferably satisfy x:y:z=6:2:2 or a value close thereto. As another specific example of values, x, y, and z preferably satisfy x:y:z=1:4:1 or a value close thereto. In this paragraph, "in the vicinity of" includes a range of ±10% of the numerical value. For example, when describing "in the vicinity of x:y:z=8:1:1," x is 7.2 or more and 8.8 or less, and y and z are 0.9 or more and 1.1 or more, respectively.

[0292] The above numerical values ​​of x, y, and z are sometimes referred to as the mixing ratio of nickel, cobalt, and manganese, and the mixing ratio is at least the proportion of each element used when weighing the raw materials. When the mixing ratios described above as x, y, and z are satisfied, it is preferable because a layered rock salt type crystal structure can be obtained.

[0293] When NCM is analyzed by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICP-MS), or energy dispersive X-ray spectroscopy (TEM-EDX), the ratio of each element obtained, i.e., the values ​​corresponding to x, y, and z, is called the ratio of nickel, cobalt, and manganese, and the ratio does not have to be the same as the mixing ratio. For example, if unreacted raw materials remain during the manufacturing process, the mixing ratio may differ from the ratio. If some of the nickel raw material remains unreacted, the ratio of nickel will be smaller than the mixing ratio.

[0294] A high proportion of nickel among the transition metals is preferable because it allows for an inexpensive positive electrode active material to be provided and also allows for a positive electrode active material with a high potential or high capacity. For example, when the sum of the numbers of nickel, cobalt, and manganese atoms in the positive electrode active material is taken as 100, the number of nickel atoms is preferably 33 or more, more preferably 50 or more, and even more preferably 80 or more. However, if the proportion of nickel is too high, chemical stability and heat resistance may be reduced. Therefore, when the sum of the numbers of nickel, cobalt, and manganese atoms in the positive electrode active material is taken as 100, the number of nickel atoms is preferably 95 or less.

[0295] The presence of cobalt as a transition metal is preferable because it results in a high average discharge voltage and improves cycle characteristics and reliability of the secondary battery, as the cobalt contributes to stabilizing the layered rock-salt structure. However, because cobalt is more expensive and unstable than nickel and manganese, if the proportion of cobalt is too high, the manufacturing cost may increase. Therefore, for example, when the sum of the numbers of nickel, cobalt, and manganese atoms in the positive electrode active material is taken as 100, the number of cobalt atoms is preferably 2.5 to 34.

[0296] The presence of manganese as a transition metal is preferable because it improves heat resistance and chemical stability. However, if the proportion of manganese is too high, the discharge voltage and discharge capacity tend to decrease. Therefore, for example, when the sum of the numbers of nickel, cobalt, and manganese atoms contained in the positive electrode active material is taken as 100, the number of manganese atoms is preferably 2.5 to 33.

[0297] An aqueous solution containing a transition metal salt will now be described. In the present invention, the aqueous solution containing a transition metal salt may be an aqueous solution containing the nickel salt or a water-soluble salt of nickel. A typical example is an aqueous solution containing nickel sulfate, nickel nitrate, or the like dissolved in water. In this aqueous solution, nickel ions may be present, and nickel may exist as a complex. In the present invention, the aqueous solution containing a transition metal salt may be an aqueous solution containing a cobalt salt or a water-soluble salt of cobalt. A typical example is an aqueous solution containing cobalt sulfate, cobalt nitrate, or the like dissolved in water. In this aqueous solution, cobalt ions may be present, and cobalt may exist as a complex. In the present invention, the aqueous solution containing a transition metal salt may be an aqueous solution containing a manganese salt or a water-soluble salt of manganese. A typical example is an aqueous solution containing manganese sulfate, manganese nitrate, or the like dissolved in water. In this aqueous solution, manganese ions may be present, and manganese may exist as a complex.

[0298] The aqueous solution containing the dissolved transition metal salt preferably has high purity, and pure water is preferably used for the aqueous solution. The concentration of the transition metal ions in the aqueous solution containing the dissolved transition metal salt is 1 mol / L or more and 5 mol / L or less, preferably 2 mol / L or more and 3 mol / L or less. When the aqueous solution contains multiple transition metal salts, it is sufficient that the total concentration of the transition metal ions satisfies the above range.

[0299] In the present invention, when three transition metals, i.e., cobalt, manganese, and nickel, are used as the transition metals, an aqueous solution containing a cobalt salt, a manganese salt, and a nickel salt can be used as the aqueous solution containing the transition metal salts dissolved therein. Typically, an aqueous solution containing nickel sulfate, cobalt sulfate, and manganese sulfate can be used as the aqueous solution containing the transition metal salts dissolved therein.

[0300] Furthermore, in step S201 of FIG. 9 , an alkaline aqueous solution (referred to as alkaline aqueous solution) is prepared. The alkaline aqueous solution refers to an aqueous solution with a pH value greater than 7, preferably a pH value of 8 or greater. In the present invention, the alkaline aqueous solution may be an aqueous solution containing sodium hydroxide, potassium hydroxide, lithium hydroxide, or ammonia. For example, an aqueous solution in which sodium hydroxide, potassium hydroxide, lithium hydroxide, or ammonia is dissolved in water may be used. An aqueous solution in which multiple elements selected from sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonia are dissolved in water may also be used. It is preferable to use pure water as the water. The alkali concentration of the alkaline aqueous solution is 1 mol / L or more and 10 mol / L or less, preferably 3 mol / L or more and 7 mol / L or less. When the aqueous solution contains multiple alkalis, the total alkali concentration should be within the above range.

[0301] The pure water used for the aqueous solution containing a dissolved transition metal salt and the alkaline aqueous solution preferably has a resistivity of 1 MΩ cm or more, more preferably 10 MΩ cm or more, and even more preferably 15 MΩ cm or more. Water satisfying this resistivity requirement has high purity and contains very few impurities.

[0302] <Step S203: Mixing Step> Next, in step S203 of FIG. 9 , the two aqueous solutions are mixed to produce a mixed aqueous solution (referred to as a mixed solution or a coprecipitated mixed solution). In this step, pure water may be prepared separately from the two aqueous solutions, and the mixed aqueous solution may be produced in the pure water. In this step, the aqueous solution containing the dissolved transition metal salt can be reacted with an alkaline aqueous solution. This reaction may be referred to as a neutralization reaction, an acid-base reaction, or a co-precipitation reaction. As the reaction progresses in this step, a coprecipitate precipitates. In this specification, the product of the reaction (reaction product) is referred to as a coprecipitate. When the aqueous solution containing the dissolved transition metal salt is mixed with an alkaline aqueous solution, a hydroxide is formed as a coprecipitate. To promote this reaction, the temperature and pH of the mixed solution may be kept constant, and the mixed solution may be stirred. The temperature may be 40°C or higher and 90°C or lower, preferably 45°C or higher and 70°C or lower. The pH value is preferably in the range of 9.0 to 13.0, and more preferably 10.5 to 11.5. The stirring speed is preferably 800 to 1200 rpm, and more preferably 900 to 1100 rpm.

[0303] Again, in this step, a coprecipitate precipitates in the mixed solution as a reaction product. The coprecipitate may settle in the mixed solution and is sometimes referred to as a precipitate. When the coprecipitate begins to precipitate, the mixed solution may become a suspension. A suspension refers to a liquid in which coprecipitate particles are dispersed in a liquid.

[0304] <Step S205: Filtration Step> Next, in step S205 of FIG. 9 , the mixed solution is filtered to obtain a coprecipitate from the mixed solution. Specifically, the coprecipitate is extracted from the mixed solution. Suction filtration is preferably used for the filtration. The coprecipitate has a size (major axis) of 1 μm or more and 20 μm or less. The coprecipitate obtained by filtration may be assigned an ordinal number to distinguish it from the coprecipitate in the mixed solution, and may also be referred to as filtered powder.

[0305] A hydroxide containing a transition metal is obtained as a coprecipitate. When an aqueous solution containing a nickel salt, a cobalt salt, and a manganese salt is used as the aqueous solution containing the transition metal, a hydroxide containing cobalt, manganese, and nickel (hereinafter referred to as a composite hydroxide containing cobalt, manganese, and nickel) is obtained as a coprecipitate. The coprecipitate obtained by filtration, typically a hydroxide, contains impurities such as water.

[0306] The hydroxide obtained as a coprecipitate may become secondary particles formed by aggregation of primary particles. The term "primary particle" refers to the smallest particle (clump) that can be observed under observation at 20,000 magnification using a scanning electron microscope (SEM) or the like. In other words, primary particles are the smallest particle. The term "secondary particle" refers to particles that are not easily separated (particles that are independent of other particles) because the primary particles aggregate to share part of the grain boundary (such as the outer periphery of the primary particles).

[0307] <Step S207: Washing Step> Next, in step S207 of FIG. 9 , the coprecipitate is washed to obtain hydroxide from which impurities have been removed. Washing using water can be used in this step. Washing using water is sometimes referred to as water washing. Note that water washing can be performed once or multiple times. Water washing can remove impurities and the like from the coprecipitate to a certain extent. Distilled water or pure water is preferably used as the water. For details about pure water, see the description of step S201. In this step, suction filtration may be performed after the coprecipitate is washed with water. Furthermore, when water washing is repeated multiple times, suction filtration is preferably performed after water washing.

[0308] Furthermore, washing with an organic solvent can be applied to the washing in the above step. Washing with an organic solvent can be performed once or multiple times. Washing with an organic solvent can be followed by a drying treatment of the coprecipitate. Drying treatment includes removing water or moisture that has adhered by previous water washing or the like. As the organic solvent, acetone or an alcohol such as isopropanol (typically isopropyl alcohol) can be used. In this step, suction filtration can be performed after washing the coprecipitate with an organic solvent. Furthermore, when washing with an organic solvent is repeated multiple times, suction filtration can be performed after washing with the organic solvent.

[0309] The cleaning step can be a combination of washing with water and washing with an organic solvent. Suction filtration is preferably used. For example, a step of washing with water followed by suction filtration can be performed, followed by washing with an organic solvent followed by suction filtration. In this case, it is preferable to wash with water more times than the number of times washing with an organic solvent is performed.

[0310] <Step S209: Heating Step> Step S209 in FIG. 9 is a step of heating the coprecipitate, which sufficiently removes impurities. However, this step may be omitted if the amount of impurities is small. Specifically, this step can remove hydrogen and oxygen from the coprecipitate as water. Because removing hydrogen and oxygen as water is called dehydration, the heating step in this step includes a dehydration step. This step can also remove water or moisture contained in the coprecipitate. Because removing water or moisture is called drying, the heating step in this step includes a drying step. In addition to water or moisture, the heating step in this step can also remove impurities by gasification. For example, the organic solvent used in step S207 can also be removed by the heating step in this step.

[0311] A supplementary note about the temperature in this step: The upper limit of the heating temperature in this step should be lower than the temperature at which the hydroxide, which is the coprecipitate, starts to change into an oxide. In other words, it is preferable not to change the hydroxide into an oxide in this step. The temperature at which the hydroxide changes into an oxide can be determined by thermogravimetry-differential thermal analysis (TG-DTA). When Ni is used as the hydroxide, ０．８ Co ０．１ Mn ０．１ (OH) ２ When using the above formula, in the region where the curve showing TG shows weight loss, the curve showing DTA starts to decline from 210° C. to 230° C., typically at or near 220° C., and a maximum endothermic peak is observed at or near 260° C. From this result, 220° C. is derived as the temperature at which hydroxide begins to decompose, dehydrate, or reduce, i.e., the temperature at which hydroxide begins to change to oxide, and 220° C. can be set as the upper limit temperature for heat treatment.

[0312] It is preferable that the temperature of the heat treatment is high, since the treatment proceeds more easily, the treatment time is shorter, and productivity is high.

[0313] The lower limit of the temperature for the heat treatment may be any temperature at which the water or moisture in the hydroxide can be removed. Removal of water or moisture is also called drying.

[0314] Taking the above into consideration, the specific temperature for the heat treatment is 130°C or higher and 220°C or lower, preferably 150°C or higher and 220°C or lower, and more preferably 180°C or higher and 220°C or lower.

[0315] The heat treatment time in this step is 3 hours or more and 15 hours or less, preferably 8 hours or more and 15 hours or less, preferably 10 hours or more and 13 hours or less, and more preferably 11 hours or more and 12 hours or less.

[0316] Furthermore, the atmosphere for the heating treatment in this step is preferably an oxygen-free atmosphere. An oxygen-free atmosphere is referred to as a non-oxygen atmosphere. Examples of the non-oxygen atmosphere include a dry atmosphere, a vacuum atmosphere, and an inert atmosphere (typically, a nitrogen atmosphere or an argon atmosphere). When heating in a dry atmosphere, the dew point inside the container should be −40°C or lower, preferably −80°C or lower. When heating in a vacuum atmosphere, a bell-jar-type vacuum device can be used, which has a container (referred to as a bell jar) whose interior can be evacuated to a vacuum and a vacuum pump connected to the bell jar. When heating in a vacuum atmosphere, a vacuum drying furnace can also be used, which has a vacuum pump connected to the drying furnace. The vacuum pump in the bell-jar-type vacuum device and vacuum drying furnace can be a dry pump, turbomolecular pump, oil rotary pump, cryopump, or mechanical booster pump. The vacuum atmosphere in the bell-jar-type vacuum device and vacuum drying furnace includes an atmosphere reduced in pressure so that the differential pressure gauge of each device indicates −0.1 MPa or higher but less than −0.08 MPa. When heating is performed in a nitrogen atmosphere, a gas containing nitrogen may be flowed into the container of the bell jar type vacuum device and the vacuum drying furnace.

[0317] Furthermore, the heat treatment in this step may be performed in multiple stages. For example, it may be performed at a first temperature for a first time, and then at a second temperature for a second time. The second temperature should be within the temperature range for the heat treatment described above. The first temperature is lower than the second temperature, for example, a temperature in the range of 80°C or higher and lower than 90°C. The second time should be within the time range for the heat treatment described above. The first time is shorter than the second time, for example, 0.5 hours or higher and 1 hour or lower. Multi-stage treatment is preferable because it makes it easier to remove impurities from the hydroxide.

[0318] <Step S210: Preparing a Lithium Source> Next, in step S210 of FIG. 9, a lithium source is prepared. The ratio of the lithium source to the hydroxide (lithium source / hydroxide) is set to 0.90 or more and 1.05 or less, preferably 0.92 or more and 1.01 or less. In the present invention, a lithium compound can be used as the lithium source. Examples of lithium compounds include lithium hydroxide, lithium carbonate, and lithium nitrate. A high purity lithium source is preferred. Furthermore, it is preferable to pulverize the lithium source to facilitate the solid-state reaction.

[0319] Furthermore, lithium hydroxide has a melting point of 462°C, which is low among lithium compounds. When producing a positive electrode active material with a high nickel content, heating must be performed at a low temperature to suppress cation mixing. Therefore, lithium compounds with low melting points, such as lithium hydroxide, are preferably used when producing a positive electrode active material with a high nickel content.

[0320] <Step S211: Mixing Step> Next, in step S211 of Fig. 9, a mixture is produced by mixing a hydroxide with a lithium source. In this specification and the like, the stage before the oxide is formed is called a precursor. To distinguish this mixing step from the previous mixing step, an ordinal number may be assigned to this mixing step. In the present invention, the mixing in this step may be performed by a dry method or a wet method.

[0321] A ball mill, a bead mill, a kneader, etc. can be used as a means for mixing, etc. When a ball mill is used, it is preferable to use, for example, zirconia balls as media.

[0322] <Step S213: Heating Step> Next, in step S213 of Fig. 9, the mixture is heated. Since the mixture becomes an oxide after this heating step, the mixture is called a precursor. Note that an ordinal number may be assigned to this heating step to distinguish it from the previous heating step.

[0323] The heating conditions for this step are preferably heating at a first temperature and then heating at a second temperature. Heating at the first temperature may be referred to as the first baking, and heating at the second temperature may be referred to as the second baking. Note that in the present invention, the second baking may be performed without performing the first baking. In other words, this step may be performed in one baking.

[0324] In this step, the second temperature is preferably higher than the first temperature. In this case, heating at the first temperature may be referred to as pre-baking, and heating at the second temperature may be referred to as main baking. In the present invention, main baking may be performed without pre-baking. However, when lithium hydroxide is used as the lithium source, it is preferable to perform pre-baking.

[0325] In this step, the first temperature is preferably higher than the melting point of the lithium source. Typically, the first temperature is preferably 500° C. or higher and 700° C. or lower. In this step, the second temperature is preferably higher than 500° C. and 1050° C. or lower, and when the second temperature is higher than the first temperature, the second temperature is preferably higher than 700° C. and 1050° C. or lower.

[0326] In this step, the heating time at the first temperature and the heating time at the second temperature are each preferably 1 hour or more and 20 hours or less. The heating time at the first temperature may be equal to, longer than, or shorter than the heating time at the second temperature.

[0327] In the present invention, heating at the first temperature and heating at the second temperature are preferably carried out in an oxygen atmosphere, and particularly preferably while supplying oxygen. For example, the flow rate is 2 L / min to 15 L / min, preferably 5 L / min to 10 L / min, per 1 L of the furnace volume. The heating atmosphere at the first temperature may be the same as or different from the heating atmosphere at the second temperature.

[0328] In the present invention, the calcining apparatus used for heating at the first temperature and the second temperature may be an electric furnace or a rotary kiln. The calcining apparatus used for heating at the first temperature may be the same as or different from the calcining apparatus used for heating at the second temperature.

[0329] In the present invention, the mixture is preferably placed in a crucible or sheath during heating. The crucible or sheath is preferably made of a highly heat-resistant material such as alumina (aluminum oxide), mullite / cordierite, magnesia, or zirconia. Among these materials, aluminum oxide is preferred as it is less likely to be contaminated with impurities. For example, a crucible or sheath made of alumina with a purity of 99% or more, preferably 99.5% or more, is preferably used. In this embodiment, a crucible made of aluminum oxide with a purity of 99.9% is used. Furthermore, it is preferable to heat the crucible or sheath with a lid, which can prevent sublimation of the materials contained in the mixture. The lid may be arranged to isolate the interior of the crucible from the air inside the furnace, or it may be arranged to be partially open so that the interior of the crucible can be exposed to the air inside the furnace.

[0330] In the present invention, it is preferable to pulverize or disintegrate the mixture in a mortar between the heating step at the first temperature and the heating step at the second temperature. The adhering state of the mixture or the agglomerated state of the mixture can be loosened by pulverization or disintegration. If the mixture adheres to each other during heating, the contact area with oxygen in the atmosphere may decrease, so it is preferable to pulverize or disintegrate the mixture as described above. Furthermore, after pulverization or disintegration, the mixture may be classified using a sieve.

[0331] When recovering the material after heating, it is preferable to transfer it from the crucible to a mortar and then recover it, since this prevents impurities from being mixed into the material. The mortar is also preferably made of a material that does not easily release impurities. Specifically, it is preferable to use a mortar made of alumina with a purity of 90% or more, preferably 99% or more.

[0332] In this way, a lithium composite oxide can be produced as a positive electrode active material. According to Production Method 1, if the prepared raw materials are used, an NCM can be obtained as a positive electrode active material. The use of the obtained NCM is preferable because it can achieve a high discharge capacity in terms of battery characteristics. Furthermore, the median diameter (D50) of the NCM is 3 μm or more and 13 μm or less, or 4 μm or more and 10 μm or less. Since a smaller median diameter (D50) increases the discharge capacity, the median diameter (D50) of the NCM is preferably 4 μm or more and 7 μm or less.

[0333] <Production Method 2> In this Production Method 2, a case where a complexing agent, typically a chelating agent, is prepared in the preparation of raw materials will be described. Since this Production Method 2 has the same steps as Production Method 1, it will be described using the flowchart in Figure 9.

[0334] <Step S201: Preparation of Raw Materials> A complexing agent may be added to the raw materials prepared in step S201 of Fig. 9. A complexing agent is a compound that can form a complex with transition metal ions in an aqueous solution. Examples of complexing agents include ammonia and ammonium salts. An aqueous solution obtained by dissolving these in water, for example, pure water, becomes the complexing agent. When ammonia is used, it can be referred to as an aqueous ammonia solution.

[0335] Furthermore, a chelating agent, which is a complexing agent for producing a chelate compound, may be prepared as a raw material prepared in step S201 of FIG. Examples of chelating agents include glycine, oxine, 1-nitroso-2-naphthol, 2-mercaptobenzothiazole, and EDTA (ethylenediaminetetraacetic acid). It is also possible to use multiple types selected from glycine, oxine, 1-nitroso-2-naphthol, and 2-mercaptobenzothiazole. An aqueous solution of these agents dissolved in water, for example, pure water, serves as the chelating agent. When glycine is used, the solution can be referred to as a glycine aqueous solution.

[0336] In this step, it is preferable to mix an aqueous solution containing a transition metal salt with a chelating agent. That is, it is preferable to prepare an aqueous solution containing a transition metal and glycine. In the aqueous solution containing a transition metal and glycine, the glycine concentration is preferably 0.05 mol / L or more and 0.15 mol / L or less, and more preferably 0.07 mol / L or more and 0.12 mol / L or less.

[0337] The use of a chelating agent suppresses the unnecessary generation of crystal nuclei, allowing hydroxides with a good particle size distribution to be obtained. The use of a chelating agent also slows down the acid-base reaction, allowing the reaction to proceed gradually, resulting in secondary particles that are nearly spherical. For this reason, a general complexing agent, such as an aqueous ammonia solution, is more preferable as the chelating agent.

[0338] Furthermore, in step S201 of FIG. 9 , a complexing agent, specifically a chelating agent, may be prepared. This complexing agent, specifically a chelating agent, may be placed in a reaction vessel. To distinguish this complexing agent or chelating agent from the complexing agent or chelating agent mixed with the aqueous solution containing the dissolved transition metal salt, an ordinal number may be assigned to this complexing agent or chelating agent. Materials that can be used as the complexing agent or chelating agent are as described above. When preparing a glycine aqueous solution as the chelating agent, the glycine concentration in the glycine aqueous solution may be 0.05 mol / L or more and 0.15 mol / L or less, preferably 0.07 mol / L or more and 0.12 mol / L or less. The glycine concentration in this glycine aqueous solution may be equal to the glycine concentration mixed with the aqueous solution containing the dissolved transition metal salt.

[0339] Thereafter, steps S203 and thereafter are carried out in the same manner as in manufacturing method 1.

[0340] The coprecipitation synthesis apparatus 170 used in Production Method 2 will now be described with reference to FIG. 10. The coprecipitation synthesis apparatus 170 can be installed in a draft chamber and has at least a reaction tank 171. A reaction vessel can be used for the reaction tank 171. A separable flask can be used for the lower part of the reaction vessel, and a separable cover can be used for the upper part. The separable flask can be either cylindrical or round. In the case of a cylindrical type, the separable flask has a flat bottom. The separable cover has multiple inlets, for example, four inlets.

[0341] The atmosphere in the reaction tank 171 can be controlled using at least one inlet of the separable cover. For example, it is preferable to control the atmosphere so that it contains nitrogen. In this case, it is preferable to control the nitrogen flow in the reaction tank 171. At this time, it is preferable that the airflow be an amount necessary to discharge the gas generated by the thermal decomposition reaction. Furthermore, nitrogen may be bubbled in the aqueous solution 203 to be placed in the reaction tank 171. The coprecipitation synthesis apparatus 170 may be equipped with a reflux condenser connected to another inlet of the separable cover, and the reflux condenser can return water to the reaction tank 171 while discharging the atmospheric gas, such as nitrogen, in the reaction tank 171.

[0342] The chelating agent described above is placed in the aqueous solution 203. An aqueous solution that is initially placed in the reaction tank 171, such as the aqueous solution 203, may be referred to as a feed solution. The feed solution may also be referred to as an adjustment solution. The feed solution and adjustment solution refer to the aqueous solution before the reaction, that is, the aqueous solution in the initial state.

[0343] An aqueous solution containing a transition metal salt dissolved therein is prepared as the aqueous solution 201. The aqueous solution 201 is placed in the first tank 180. An alkaline aqueous solution is prepared as the aqueous solution 202. The alkaline aqueous solution is used to maintain a constant pH value, and is therefore sometimes referred to as a pH adjusting solution. The aqueous solution 202 is placed in the second tank 186. Nitrogen may be bubbled through the first tank 180 and the second tank 186 to remove oxygen from each aqueous solution. Furthermore, a tank separate from the first tank 180 and the second tank 186 may be prepared and contain the above-mentioned chelating agent.

[0344] The first tank 180 has a pump 182 and a tube (also referred to as a pipe) 181 connected to the pump 182. The pipe 181 is fixed to the inlet of the separable cover, and an aqueous solution 201 can be dripped from the tip of the pipe 181 into the reaction tank 171. The second tank 186 has a pump 188 and a pipe 187 connected to the pump 188. The pipe 187 is fixed to the inlet of the separable cover, and an aqueous solution 202 is dripped from the tip of the pipe 187 into the reaction tank 171. The tips of the pipes 181 and 187 may be immersed in an aqueous solution 203. In this case, it is also described that the aqueous solutions 201 and 202 are dripped into the reaction tank 171. In this way, according to step S201, each raw material is prepared in the reaction tank 171, the first tank 180, and the second tank 186.

[0345] <Step S203: Mixing Step> Next, the mixing step of step S203 will be described. First, the conditions for the coprecipitation method in this step will be described.

[0346] <Conditions for Coprecipitation Method> The pH value of the aqueous solution 203 in the reaction tank 171 is set to a range of 9.0 to 13.0, preferably 10.5 to 11.5. The temperature of the aqueous solution 203 in the reaction tank 171 is set to a range of 40°C to 90°C, preferably 45°C to 70°C. The water temperature can also be controlled according to the temperature in the reaction tank 171. The temperature in the reaction tank 171 may be equal to the water temperature or may differ by less than 5°C, preferably less than 2°C. Therefore, the temperature in the reaction tank 171 is set to a range of 35°C to 95°C, preferably 40°C to 75°C. The rotation speed for stirring the aqueous solution 203 in the reaction tank 171 is set to a range of 800 rpm to 1200 rpm, preferably 900 rpm to 1100 rpm. The concentration of transition metal ions in the aqueous solution 201 is set to a range of 1 mol / L to 5 mol / L, preferably 2 mol / L to 3 mol / L. When multiple transition metals are present, the total concentration of the transition metal ions satisfies the above range. The dripping rate of the aqueous solution 201 is 0.03 mL / min to 1.0 mL / min, preferably 0.03 mL / min to 0.5 mL / min. A slow dripping rate can reduce the size of the hydroxide, i.e., the median diameter (D50), while a fast dripping rate can increase the size of the hydroxide, i.e., the median diameter (D50). If the size of the hydroxide is small, the positive electrode active material also becomes small, and if the size of the hydroxide is large, the positive electrode active material also becomes large. The hydroxide, which affects the size of the positive electrode active material, which is a lithium composite oxide, can also be called a precursor.

[0347] The alkali concentration of the aqueous solution 202 is 1 mol / L or more and 10 mol / L or less, preferably 3 mol / L or more and 7 mol / L or less. The chelating agent concentration of the aqueous solution 203 is 0.05 mol / L or more and 0.15 mol / L or less, preferably 0.07 mol / L or more and 0.12 mol / L or less.

[0348] The configuration of a coprecipitation synthesis apparatus 170 capable of obtaining a coprecipitate according to the above conditions will be described with reference to FIG. 10 . The reaction tank 171 shown in FIG. 10 includes a stirring unit 172. The stirring unit 172 can stir the aqueous solution 203 in the reaction tank 171 and further includes a stirring motor 173 as a power source for rotating the stirring unit 172. The stirring unit 172 has paddle-shaped stirring blades (referred to as paddle blades). The paddle blades may have two to six blades, each of which may be inclined at an angle of 40 degrees to 70 degrees. The blades may be movable up and down in the stirring unit 172. In the mixing process of step S203, the rotation speed of the stirring unit 172, specifically the rotation speed of the paddle blades, may be set to 800 rpm to 1200 rpm, preferably 900 rpm to 1100 rpm. Although not shown, a baffle plate may be installed in the reaction tank 171.

[0349] A thermometer 174 is provided to measure the temperature of the reaction tank 171 or the temperature of the aqueous solution 203. The temperature of the reaction tank 171 can be controlled using a thermoelectric element so that the temperature of the aqueous solution 203 is constant. An example of a thermoelectric element is a Peltier element. When measuring the temperature of the aqueous solution 203, the tip of the thermometer 174 is preferably immersed in the aqueous solution 203. In the mixing process of step S203, the aqueous solution 203 is preferably heated to a temperature of 40°C or higher and 90°C or lower, and preferably 45°C or higher and 70°C or lower. The temperature of the reaction tank 171 may be controlled using the thermometer 174 to control the temperature of the aqueous solution 203.

[0350] The coprecipitation synthesis apparatus 170 is equipped with a control device 190 and the like to control the dropping conditions from each pump or the stirring conditions. The control device 190 can control the rotation speed of the stirring unit 172, the dropping amount of each aqueous solution, and the like based on information obtained from the thermometer 174. In the mixing process of step S203, for example, the dropping rate of the aqueous solution 201 is preferably 0.05 mL / min to 1.0 mL / min, and more preferably 0.08 mL / min to 0.5 mL / min. The concentration of the transition metal ions in the aqueous solution 201 is preferably 1 mol / L to 5 mol / L, and more preferably 2 mol / L to 3 mol / L. When multiple transition metals are present, the total concentration of the transition metal ions should preferably fall within the above range.

[0351] Although not shown, a pH meter is also placed in the reaction tank 171 to measure the pH of the aqueous solution 203. In the mixing process of step S203, the pH value is set to a range of 9.0 to 13.0, preferably 10.5 to 11.5. The concentration of the chelating agent in the aqueous solution 203 is set to a range of 0.05 mol / L to 0.15 mol / L, preferably 0.07 mol / L to 0.12 mol / L.

[0352] When the pH value deviates from the desired value, the aqueous solution 202 is dropped. The alkali concentration of the aqueous solution 202 is set to 1 mol / L or more and 10 mol / L or less, preferably 3 mol / L or more and 7 mol / L or less.

[0353] After the mixing step of step S203, a reaction product is precipitated in the reaction tank 171. The reaction product is a coprecipitate, specifically, a hydroxide.

[0354] The subsequent steps, specifically, step S205 and thereafter are the same as those in manufacturing method 1, and therefore the description thereof will be omitted.

[0355] In this way, a positive electrode active material can be produced. By using Production Method 2, an NCM having a good particle size distribution can be obtained as a positive electrode active material. The use of the obtained NCM is preferable because it increases the discharge capacity in terms of battery characteristics. Furthermore, the use of the obtained NCM is preferable because it reduces the variation in discharge capacity in terms of battery characteristics.

[0356] <Production Method 3> In the present invention, the NCM may contain one or more elements selected from calcium and aluminum at a concentration of 0.1 atm% or more and 5 atm% or less relative to the NCM. Calcium and aluminum at the above concentrations may be referred to as "additive elements." The additive elements are often located in the surface layer of the active material, and the "surface layer" refers to a region up to 50 nm from the surface of the active material, preferably up to 30 nm, and more preferably up to 10 nm. The "surface layer" can be considered to be located in the same way whether the active material is a primary particle or a secondary particle. The "surface layer" refers to a region up to 50 nm from the surface of the primary particle or the surface of the secondary particle, preferably up to 30 nm, and more preferably up to 10 nm. The "surface layer" refers to the interface between a region where a transition metal (e.g., Co, Ni, Mn, Fe, etc.) that oxidizes and reduces upon lithium insertion / extraction is present and a region where it is not present.

[0357] The NCM containing aluminum as the main component is sometimes referred to as NCMA, which is a lithium composite oxide containing Ni, Co, Mn, and Al.

[0358] Furthermore, a lithium composite oxide containing Ni and Co as a main component and containing aluminum may be referred to as an NCA, or a lithium composite oxide containing Ni, Co, and Al.

[0359] In this manufacturing method 3, the case where the additive element source is added in the same step as the raw material prepared in step S201, i.e., simultaneously, will be described with reference to Fig. 11. In this manufacturing method 3 shown in Fig. 11, steps S203 to S213 are the same as those in manufacturing method 1, but step S215 is newly added.

[0360] <Step S215: Preparation of Additive Element Source> In step S215 of FIG. 11 , an additive element source is prepared. As the additive element source, an aqueous solution in which a salt of the additive element source is dissolved can be used. As the aqueous solution, an aqueous solution in which aluminum sulfate, aluminum chloride, aluminum nitrate, calcium oxide, calcium carbonate, calcium hydroxide, or calcium sulfate is dissolved can be used. The additive element source is weighed out so that the additive element is 0.1 atm % or more and 5 atm % or less of the NCM. Multiple additive elements may be included. When multiple additive elements are included, it is sufficient that the total concentration of the additive elements satisfies the range of 0.1 atm % or more and 5 atm % or less of the NCM.

[0361] 11, an aqueous solution containing a transition metal salt, an alkaline aqueous solution, and an aqueous solution containing a salt of an additive element source are mixed to produce a mixed solution. The mixing in this step is similar to step S203 in production method 1.

[0362] The subsequent steps, specifically, step S205 and thereafter are the same as those in manufacturing method 1, and therefore the description thereof will be omitted.

[0363] In this way, a positive electrode active material can be produced. By using Production Method 3, an NCM containing an additive element can be obtained as a positive electrode active material. The additive element is preferably located in the surface layer of the NCM. By using Production Method 3, an NCMA or NCA can also be obtained as a positive electrode active material.

[0364] <Manufacturing Method 4> In this manufacturing method 4, the case where the additive element source is added in the same step as the lithium source prepared in step S210, i.e., simultaneously, will be described with reference to Fig. 12. In this manufacturing method 4 shown in Fig. 12, steps S201 to S210 are the same as those in manufacturing method 1, but step S215 is newly added. The order of step S215 differs from that in manufacturing method 3.

[0365] <Step S215: Preparation of Additive Element Source> In step S215 of Fig. 12, an additive element source is prepared. As the additive element source, aluminum sulfate, aluminum chloride, aluminum nitrate, calcium oxide, calcium carbonate, calcium hydroxide, or calcium sulfate can be used. The additive element source is weighed out so that the additive element is 0.1 atm% or more and 5 atm% or less of the NCM. Multiple additive elements may be included. When multiple additive elements are included, it is sufficient that the total concentration of the additive elements satisfies the range of 0.1 atm% or more and 5 atm% or less of the NCM.

[0366] 12, a hydroxide, a lithium source, and an additive element source are mixed to produce a mixture. The mixing in this step is the same as in step S211 in production method 1.

[0367] The subsequent steps, specifically, step S213 and thereafter are the same as those in manufacturing method 1, and therefore the description thereof will be omitted.

[0368] In this way, a positive electrode active material can be produced. By using Production Method 4, an NCM containing an additive element can be obtained as a positive electrode active material. The additive element is preferably located in the surface layer of the NCM. By using Production Method 4, an NCMA or NCA can also be obtained as a positive electrode active material.

[0369] <Manufacturing Method 5> In this manufacturing method 5, a case in which the additive element source is added after the heating step of step S213 will be described with reference to Fig. 13. In this manufacturing method 5 shown in Fig. 13, steps S201 to S213 are the same as those of manufacturing method 1, but steps S215 to S217 are newly added. The order of step S215 differs from that of manufacturing methods 3 and 4.

[0370] <Step S215: Preparation of Additive Element Source> In step S215 of Fig. 13, an additive element source is prepared. As the additive element source, aluminum sulfate, aluminum chloride, aluminum nitrate, calcium oxide, calcium carbonate, calcium hydroxide, or calcium sulfate can be used. The additive element source is weighed out so that the additive element is 0.1 atm% or more and 5 atm% or less of the NCM. Multiple additive elements may be included. When multiple additive elements are included, it is sufficient that the total concentration of the additive elements satisfies the range of 0.1 atm% or more and 5 atm% or less of the NCM.

[0371] 13, the composite oxide and the additive element source are mixed to produce a mixture. The mixing in this step is the same as in step S211 in production method 1.

[0372] 13, the mixture is heated in step S217. The heating in this step is similar to step S213 in production method 1.

[0373] In this manner, a positive electrode active material can be produced. By using Production Method 5, an NCM containing an additive element can be obtained as a positive electrode active material. The additive element is preferably located in the surface layer of the NCM. By using Production Method 5, an NCMA or NCA can also be obtained as a positive electrode active material.

[0374] This embodiment mode can be freely combined with other embodiment modes.

[0375] Embodiment 4 In this embodiment, a configuration other than the positive electrode active material and the electrolyte contained in a lithium ion battery will be described.

[0376] <Positive electrode 1> The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include at least one of a conductive additive and a binder. The positive electrode active material described in Embodiment 1 can be used.

[0377] FIG. 14A shows an example of a schematic cross-sectional view of a positive electrode.

[0378] The current collector 550 can be, for example, a metal foil. The positive electrode can be formed by applying a slurry to a metal foil and drying it. Note that pressing may be performed after drying. The positive electrode is formed by forming an active material layer on the current collector 550.

[0379] The slurry is a material liquid used to form an active material layer on the current collector 550, and refers to a material containing an active material, a binder, and a solvent, and preferably further mixed with a conductive additive. The slurry is also called an electrode slurry or an active material slurry, and is sometimes called a positive electrode slurry when forming a positive electrode active material layer, and a negative electrode slurry when forming a negative electrode active material layer.

[0380] The positive electrode active material 561 has the function of taking in and / or releasing lithium ions during charging and discharging. The positive electrode active material 561 used in one embodiment of the present invention can be a material that exhibits little deterioration during charging and discharging, even at high charging voltages. Unless otherwise specified, the charging voltage is expressed based on the potential of lithium metal in this specification. Furthermore, in this specification, a high charging voltage is, for example, a charging 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.

[0381] The positive electrode active material 561 used in one embodiment of the present invention can be any material that does not deteriorate much due to charging and discharging even at a high charging voltage, and can be any material described in Embodiment 1 or 2. Note that the positive electrode active material 561 can be two or more materials with different particle sizes as long as the material does not deteriorate much due to charging and discharging even at a high charging voltage.

[0382] The conductive additive is also called a conductivity-imparting agent or a conductive material, and a carbon material can be used. By attaching the conductive additive between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, the term "attachment" does not only refer to physical adhesion between the active material and the conductive additive, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, the conductive additive covers part of the surface of the active material, the conductive additive is embedded in the surface irregularities of the active material, and electrical connection even when not in contact with each other.

[0383] Specific examples of carbon materials that can be used as the conductive additive include carbon black (furnace black, acetylene black, graphite, etc.).

[0384] FIG. 14A illustrates carbon black 553 as a conductive additive.

[0385] A binder (resin) may be mixed to bond the current collector 550, such as a metal foil, and the active material to form the positive electrode of a lithium-ion battery. The binder is also called a binding agent. The binder is a polymeric material, and adding a large amount of binder reduces the proportion of active material in the positive electrode, thereby reducing the discharge capacity of the lithium-ion battery. Therefore, it is preferable to mix the binder in a minimum amount. In FIG. 14A , the areas not filled with the positive electrode active material 561, the second active material 562, and the carbon black 553 represent voids or binders.

[0386] 14A shows an example in which the positive electrode active material 561 is illustrated as a sphere, but this is not particularly limited. For example, the cross-sectional shape of the positive electrode active material 561 may be an ellipse, a rectangle, a trapezoid, a cone, a polygon with rounded corners, or an asymmetric shape. For example, FIG. 14B shows an example in which the positive electrode active material 561 has a polygonal shape with rounded corners.

[0387] 14B, graphene 554 is used as a carbon material used as a conductive additive in the positive electrode. In FIG. 14B, a positive electrode active material layer including a positive electrode active material 561, graphene 554, and carbon black 553 is formed over a current collector 550.

[0388] In the step of mixing the graphene 554 and the carbon black 553 to obtain electrode slurry, the weight of the carbon black to be mixed is preferably 1.5 to 20 times, more preferably 2 to 9.5 times, that of the graphene.

[0389] Furthermore, when the mixture of graphene 554 and carbon black 553 is within the above range, the dispersion stability of carbon black 553 is excellent and agglomerations are less likely to occur during slurry preparation. Furthermore, when the mixture of graphene 554 and carbon black 553 is within the above range, a higher electrode density can be achieved than a positive electrode using only carbon black 553 as a conductive additive. Increasing the electrode density can increase the capacity per unit weight. Specifically, the density of the positive electrode active material layer measured gravimetrically can be 3.5 g / cc or more.

[0390] Although the electrode density is lower than that of a positive electrode using only graphene as a conductive additive, by mixing the first carbon material (graphene) and the second carbon material (acetylene black) within the above range, it is possible to accommodate rapid charging, and therefore it is particularly effective when used as an in-vehicle lithium-ion battery.

[0391] 14C illustrates an example of a positive electrode in which carbon fiber 555 is used instead of graphene. Fig. 14C shows an example different from Fig. 14B. The use of carbon fiber 555 can prevent aggregation of carbon black 553 and improve dispersibility.

[0392] In FIG. 14C, the regions not filled with the positive electrode active material 561, the carbon fibers 555, and the carbon black 553 indicate voids or binders.

[0393] 14D shows another example of a positive electrode. Fig. 14C shows an example in which carbon fibers 555 are used in addition to graphene 554. Using both graphene 554 and carbon fibers 555 can prevent aggregation of carbon black such as carbon black 553 and further improve dispersibility.

[0394] In FIG. 14D , regions that are not filled with the positive electrode active material 561 , the carbon fibers 555 , the graphene 554 , and the carbon black 553 indicate voids or binders.

[0395] A lithium ion battery can be produced by using any one of the positive electrodes shown in FIGS. 14A to 14D , stacking a separator on the positive electrode, placing the stacked body obtained by stacking the negative electrode on the separator in a container (such as an exterior body or a metal can) and filling the container with an electrolyte.

[0396] <Positive electrode 2> The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a positive electrode active material and may further have a conductive material. The positive electrode active material layer may further have a binder. The positive electrode active material layer may further have a conductive material and a binder.

[0397] <Positive Electrode Active Material> Fig. 15A shows an example of a positive electrode active material 10. The materials described in the second embodiment and the like can be used as the positive electrode active material. The positive electrode active material produced according to the second embodiment and the like is composed of secondary particles 12 having primary particles 11. Voids 13 may be observed between the aggregated primary particles 11. Furthermore, interfaces 14 exist between adjacent primary particles 11.

[0398] [Distribution of Additional Element] The positive electrode active material produced by manufacturing methods 3 to 5 of the third embodiment above can contain an additional element. Figure 15B shows the concentration distribution of the additional element in the E-F cross section indicated by the dashed dotted line in Figure 15A. Because the E-F cross section passes through interface 14, Figure 15B shows the concentration distribution of the additional element from interface 14 to the inside of the primary particle. In this specification, the concentration distribution may be referred to as a concentration gradient, and the vicinity of the interface refers to a region of the primary particle less than 10 nm from the interface, preferably a region of the primary particle less than 8 nm from interface 14. The region of the primary particle less than 10 nm from the interface may be referred to as the surface layer of the primary particle.

[0399] The horizontal axis of FIG. 15B corresponds to the distance along the E-F cross section, and the vertical axis corresponds to the concentration of the additive element. The concentration peak of the additive element should be located at a position overlapping with interface 14. Calcium is an example of an additive element. A positive electrode active material containing an additive element having such a concentration peak is preferable because it suppresses cycle deterioration. Furthermore, when a positive electrode active material is manufactured using multiple additive elements, the concentration peak position may differ depending on the additive element.

[0400] In primary particles 11, it is preferable that the concentration of the additive element decreases from the surface toward the inside, as shown in FIG. 15B. That is, in primary particles, it is preferable that the concentration of the additive element is higher in the surface layer portion than in the inside. For example, it is preferable that the concentration of calcium, which is one example of an additive element, decreases from the surface toward the inside, as shown in FIG. 15B. Furthermore, when a positive electrode active material is produced using multiple additive elements, the shape of the concentration distribution may differ depending on the additive element.

[0401] 15C shows an example of a cross-sectional view of the positive electrode 107. The positive electrode 107 has a positive electrode current collector 105 and a positive electrode active material layer 104.

[0402] <Positive Electrode Current Collector> The positive electrode current collector 105 can be, for example, a metal foil. The positive electrode can be formed by applying a slurry onto a metal foil and drying it. Note that pressing may be performed after drying. The positive electrode is formed by forming a positive electrode active material layer 104 on the positive electrode current collector 105.

[0403] The metal foil can be made of a highly conductive material, such as stainless steel, gold, platinum, aluminum, titanium, or an alloy thereof. The material used for the positive electrode current collector preferably does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. The current collector preferably has a thickness of 5 μm to 30 μm.

[0404] The slurry is a material liquid used to form the positive electrode active material layer 104 on the positive electrode current collector 105, and refers to a material containing an active material, a binder, and a solvent, and preferably further mixed with a conductive material. The slurry is also called an electrode slurry or an active material slurry, and is sometimes called a positive electrode slurry when forming a positive electrode active material layer and a negative electrode slurry when forming a negative electrode active material layer.

[0405] The positive electrode active material layer 104 includes secondary particles 12a and secondary particles 12b as the positive electrode active material. The secondary particles 12a have a different median diameter (D50) from the secondary particles 12b.

[0406] <Conductive Material> Next, the conductive material will be described. The conductive material is also called a conductivity-imparting agent or a conductivity aid, and a carbon material can be used. By attaching a conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, 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 fits into the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0407] Specific examples of carbon materials that can be used as the conductive material include carbon black (furnace black, acetylene black, graphite, etc.), graphene, graphene compounds, and carbon fibers.

[0408] [Graphene and graphene compounds] In this specification and the like, graphene has carbon, has a shape such as a plate or sheet, and has a two-dimensional structure formed by six-membered carbon rings. In addition to graphene, it includes multi-layer graphene, multi-graphene, etc. Graphene may also be rolled up into a shape like a carbon nanofiber. Graphene having a two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet.

[0409] In this specification and the like, graphene compounds have a shape such as a plate or a sheet, and include 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 may also be rolled into nanofibers. Furthermore, graphene compounds may have a functional group, and the functional group is preferably an epoxy group, a carboxy group, or a hydroxy group.

[0410] In this specification and the like, reduced graphene oxide, reduced multilayer graphene oxide, or reduced multi-graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % or more and 15 atomic % or less. By achieving such carbon and oxygen concentrations, even a small amount can function as a highly conductive material. Furthermore, reduced graphene oxide, reduced multilayer graphene oxide, or reduced multi-graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. By achieving such an intensity ratio, even a small amount can function as a highly conductive material. Furthermore, by reducing graphene oxide, multilayer graphene oxide, or multi-graphene oxide, holes can be formed in the graphene compound in some cases.

[0411] 15C illustrates carbon black 553 as a conductive material. Because carbon black 553 is a fine particle, it often aggregates. Carbon black 553 can be located between secondary particles 12a, etc., and can serve as a current path between adjacent secondary particles 12a. Carbon black 553 can also be located between positive electrode current collector 105 and the secondary particles, and can serve as a current path between positive electrode current collector 105 and the secondary particles.

[0412] <Binder> In order to bond the positive electrode current collector 105 and the active material to each other in the positive electrode of the lithium ion battery, a binder (resin) may be mixed. The binder is also called a binding agent.

[0413] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0414] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include one or more of cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, and starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0415] The binder may be used in combination with two or more of the above.

[0416] For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with another material. For example, while rubber materials have excellent adhesive strength and / or elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with a particularly excellent viscosity adjusting effect, the aforementioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, and starch may be used.

[0417] The binder is a polymer material, and if a large amount of binder is added, the ratio of active material in the positive electrode or negative electrode decreases, resulting in a decrease in the discharge capacity of the lithium-ion battery. Therefore, it is preferable to mix the amount of binder to a minimum.

[0418] 15C, the secondary particles 12a and 12b are illustrated as spherical (having a circular cross-sectional shape), but this is not particularly limited. In many cases, irregularities that are part of the primary particles can be seen on the surface of the cross-sectional shape of the secondary particles formed by agglomeration of primary particles.

[0419] 15D shows a positive electrode in which, as the carbon material used as the conductive material, carbon black 553 and graphene 554 are used. Note that in the step of mixing graphene 554 and carbon black 553 to obtain electrode slurry, the weight of the carbon black to be mixed is preferably 1.5 to 20 times, more preferably 2 to 9.5 times, the weight of the graphene.

[0420] Furthermore, when the mixture of graphene 554 and carbon black 553 is within the above range, dispersion stability of carbon black 553 is excellent during slurry preparation, which is preferable. Furthermore, when the mixture of graphene 554 and carbon black 553 is within the above range, a higher electrode density can be achieved than a positive electrode using only carbon black 553 as a conductive material. Increasing the electrode density can increase the capacity per unit weight. Specifically, the density of the positive electrode active material layer measured gravimetrically can be 3.5 g / cc or more.

[0421] Although the electrode density is lower than that of a positive electrode using only graphene as a conductive material, rapid charging is possible by mixing the graphene 554 and the carbon black 553 within the above range. Therefore, mixing the graphene 554 and the carbon black 553 is effective for in-vehicle lithium-ion batteries.

[0422] Although not shown, carbon fiber (carbon nanotube) may be used instead of the graphene. When carbon fiber is used as the conductive material, aggregation of the carbon black 553 can be prevented.

[0423] 15C and 15D , a separator is placed on the positive electrode, and the stack of the separator and the negative electrode is placed in a container (such as an exterior body or a metal can) that contains the stack, and the container is filled with an electrolyte, thereby producing a lithium ion battery. That is, the void regions in FIGS. 15C and 15D are impregnated with the electrolyte.

[0424] <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.

[0425] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material and / or a carbon material can be used.

[0426] 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, one or more materials selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Compounds containing these elements may also be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements 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.

[0427] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0428] The carbon material may be one or more selected from graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, and the like.

[0429] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0430] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety compared to lithium metal.

[0431] Titanium dioxide (TiO ２ ), lithium titanium composite 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 the like can be used.

[0432] 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 (per weight of negative electrode active material), 1890 mAh / cm ３ ) and is preferred.

[0433] 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.

[0434] 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 This also occurs with fluorides such as

[0435] 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.

[0436] <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.

[0437] [Electrolyte] The electrolyte described in the first embodiment and the like can be used.

[0438] [Separator] When the electrolyte contains an electrolytic solution, a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polypropylene (referred to as PP), polyimide (referred to as PI), polyester, acrylic, polyolefin, and polyurethane. The porosity of the separator can be 35% to 90%, preferably 60% to 85%. Separators using polypropylene can have a porosity of 35% to 45%. Separators using polyimide can have a porosity of 75% to 85%. The thickness of the separator is preferably 10 μm to 80 μm, more preferably 20 μm to 60 μm. Separators using polyimide can have a high porosity and can be made thicker (typically, a thickness of 50 μm to 60 μm).

[0439] The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0440] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0441] By using a multilayer separator, the safety of the lithium-ion battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the lithium-ion battery can be increased.

[0442] [Exterior Body] The exterior body of a lithium-ion battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, 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 thin metal film as the outer surface of the exterior body.

[0443] <Evaluation Conditions> A coin cell (e.g., CR2032 type, 20 mm diameter, 3.2 mm height) having lithium metal as the counter electrode is fabricated and charged / discharged under predetermined conditions to evaluate whether the cell contains an electrolyte according to one embodiment of the present invention.

[0444] <Evaluation Procedure> A lithium-ion battery is disassembled, the positive electrode impregnated with the electrolyte is removed, and the electrolyte is analyzed by nuclear magnetic resonance (e.g. １ Measurements using H NMR are carried out to identify at least the organic solvent. Nuclear magnetic resonance spectroscopy can also identify the mixing ratio (volume ratio) of the organic solvent. It is also possible to identify other components, such as the lithium salt and additives contained in the electrolyte.

[0445] The positive electrode is then punched out to a size that fits into a coin cell. The positive electrode contains a conductive material and a binder in addition to the positive electrode active material. The electrolyte and other components are removed after the nuclear magnetic resonance measurement but before punching out the positive electrode. For example, after removing the positive electrode, the positive electrode may be washed with an organic solvent or the like.

[0446] The coin cell has a lithium metal counter electrode, although materials other than lithium metal may be used as the counter electrode.

[0447] An electrolyte identified using nuclear magnetic resonance spectroscopy is prepared as the electrolyte for the coin cell. This electrolyte is an embodiment of the present invention.

[0448] The coin cell has a 25 μm thick porous polypropylene film as a separator.

[0449] The coin cell uses stainless steel (SUS) for the positive electrode can and stainless steel (SUS) for the negative electrode can.

[0450] In this way, a coin cell for evaluation is prepared.

[0451] The coin cell for evaluation prepared under the above conditions is charged to a desired voltage (for example, 4.4 V, 4.5 V, or 4.6 V) and then discharged. The charging conditions can be found in the Examples below. The discharging conditions can be found in the Examples below.

[0452] The charging temperature of the evaluation coin cell can be set to 25°C and below freezing, and it can be confirmed how the charge / discharge capacity at below freezing is compared to the charge / discharge capacity at 25°C.

[0453] This embodiment can be used in combination with other embodiments.

[0454] Embodiment 5 In this embodiment, an example of a lithium ion battery will be described.

[0455] [Laminated Lithium-ion Battery] An example of a laminated lithium-ion battery 100 is shown in Figures 16A and 16B. Figures 16A and 16B are external views, and the lithium-ion battery 100 includes the electrolyte and separator (not shown in Figure 16) described in the above embodiment, a negative electrode 106, and a positive electrode 107. In the lithium-ion battery 100, the negative electrode 106 preferably has a larger area than the positive electrode 107. The lithium-ion battery 100 further includes a negative electrode lead electrode 510 electrically connected to the negative electrode 106 and a positive electrode lead electrode 511 electrically connected to the positive electrode 107. The electrolyte, the negative electrode 106, and the positive electrode 107 are housed in an outer casing 509, and a portion of the negative electrode lead electrode 510 and a portion of the positive electrode lead electrode 511 protrude from the outer casing 509. An adhesive region 508 is provided on a portion of the outer periphery of the outer casing 509. Fig. 16A shows an example in which the negative lead electrode 510 and the positive lead electrode 511 protrude from the same side of the exterior body 509, and the adhesive region 508 is located at least on the side from which each lead electrode protrudes and two sides adjacent to that side. Fig. 16B shows an example in which the side from which the negative lead electrode 510 protrudes from the exterior body 509 and the side from which the positive lead electrode 511 protrudes from the exterior body 509 face each other, and the adhesive region 508 is located at least on the two sides from which each lead electrode protrudes and one side sandwiched between those two sides. In Figs. 16A and 16B, the sides on which the adhesive region 508 is not located may correspond to the sides along which the exterior body 509 is folded.

[0456] When the organic solvent and positive electrode active material of the present invention are used in the laminated lithium ion battery 100, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0457] [Coin-type lithium-ion battery] An example of a coin-type lithium-ion battery will be described. Fig. 17A is an exploded perspective view of a coin-type (single-layer flat) lithium-ion battery, Fig. 17B is an external view, and Fig. 17C is a cross-sectional view thereof. Coin-type lithium-ion batteries are mainly used in small electronic devices. In this specification, coin-type lithium-ion batteries include button-type lithium-ion batteries.

[0458] 17A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding, and therefore, FIGS. 17A and 17B are not completely identical corresponding views.

[0459] 17A shows a state in which the positive electrode 304, negative electrode 307, spacer 342, and washer 332 are stacked and sealed with the negative electrode can 302 and positive electrode can 301. Note that the electrolyte and separator described in the above embodiment are not shown in FIG. 17A. The spacer 342 and washer 332 are used to protect the interior or fix the position within the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 342 or washer 332 is made of stainless steel or an insulating material.

[0460] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0461] FIG. 17B is a perspective view of the completed coin-type lithium-ion battery 100.

[0462] In the coin-type lithium-ion battery 100, 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, may be 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 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.

[0463] Note that the positive electrode 304 and the negative electrode 307 used in the coin-type lithium ion battery 100 each only need to have an active material layer formed on one side.

[0464] As shown in FIG. 17C , the positive electrode can 301 is placed downward, and the positive electrode 304, the negative electrode 307, and the negative electrode can 302 are stacked in this order, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303 to produce a coin-shaped lithium ion battery 100.

[0465] When the organic solvent and positive electrode active material of the present invention are used in the coin-type lithium ion battery 100, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0466] [Cylindrical Lithium-ion Battery] An example of a cylindrical lithium-ion battery will be described with reference to Fig. 18A. As shown in Fig. 18A, a cylindrical lithium-ion battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0467] Fig. 18B is a schematic diagram showing the cross section of a cylindrical lithium-ion battery. The cylindrical lithium-ion battery shown in Fig. 18B has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0468] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched therebetween. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. Inside the battery can 602, the wound battery element of the positive electrode, negative electrode, and separator is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte (not shown) of one embodiment of the present invention is injected into the battery can 602 in which the battery element is provided.

[0469] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector. While the lithium-ion battery 616 shown in Figures 18A to 18D has a cylinder whose height is greater than its diameter, this is not limiting. A lithium-ion battery whose diameter is greater than its height may also be used. This configuration, for example, can reduce the size of the lithium-ion battery.

[0470] A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature increases, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element is made of barium titanate (BaTiO ３ )-based ceramic materials, etc. can be used.

[0471] 18C shows an example of a power storage system 615. The power storage system 615 has multiple lithium ion batteries 616 and is sometimes called a battery pack. The positive electrodes of each lithium ion battery 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 each lithium ion battery are electrically connected to the control circuit 620 via wiring 626. A protection circuit or the like that prevents overcharging or overdischarging can be used as the control circuit 620.

[0472] 18D shows an example of a power storage system 615. The power storage system 615 has a plurality of lithium ion batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of lithium ion batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of lithium ion 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 lithium ion batteries 616, a large amount of power can be extracted.

[0473] A plurality of lithium ion batteries 616 may be connected in parallel and then further connected in series.

[0474] A temperature control device may be provided between the plurality of lithium ion batteries 616. When the lithium ion batteries 616 are overheated, they can be cooled by the temperature control device, and when the lithium ion batteries 616 are too cold, they can be heated by the temperature control device. This makes the performance of the power storage system 615 less susceptible to the influence of the outside air temperature.

[0475] 18D , 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 lithium ion batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of lithium ion batteries 616 via a conductive plate 614.

[0476] When the electrolyte and positive electrode active material according to one embodiment of the present invention are used in the cylindrical lithium ion battery 100, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0477] [Another Example of the Structure of the Lithium-Ion Battery] An example of the structure of the lithium-ion battery will be described with reference to FIGS. 19 and 20. FIG.

[0478] A lithium ion battery 913 shown in FIG. 19A has a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolyte according to one embodiment of the present invention inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that for convenience, the housing 930 is shown separated in FIG. 19A , but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a resin material.

[0479] 19B, the housing 930 shown in Fig. 19A may be formed using a plurality of materials. For example, in the lithium ion battery 913 shown in Fig. 19B, a housing 930a and a housing 930b are bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0480] The housing 930a can be made of an insulating material such as organic resin. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to prevent the lithium ion battery 913 from blocking the electric field. Note that if the electric field blocking by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0481] 19C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0482] 20A to 20C may be used as a lithium ion battery 913 having a wound body 950a. The wound body 950a shown in Fig. 20A includes a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 includes a negative electrode active material layer 931a. The positive electrode 932 includes a positive electrode active material layer 932a.

[0483] 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.

[0484] 20B, the negative electrode 931 is electrically connected to a terminal 951. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952. The terminal 952 is electrically connected to a terminal 911b.

[0485] 20C , wound body 950 a is covered with housing 930 to form lithium ion battery 913. Housing 930 is preferably provided with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0486] As shown in Fig. 20B, the lithium ion battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a lithium ion battery 913 with a larger charge / discharge capacity. For other elements of the lithium ion battery 913 shown in Figs. 20A and 20B, refer to the descriptions of the lithium ion battery 913 shown in Figs. 19A to 19C.

[0487] When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the lithium ion battery 913 having the wound body, excellent charge and discharge characteristics can be expected in a wide temperature range including temperatures from below freezing to high temperatures.

[0488] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0489] Sixth Embodiment In this embodiment, an example of application to an electric vehicle (EV) will be described with reference to FIG.

[0490] 21A , the electric vehicle is equipped with first batteries 1301 a and 1301 b as main driving lithium ion batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. When the electrolyte and positive electrode active material according to one embodiment of the present invention are used in the first batteries 1301 a and 1301 b, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0491] The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have a 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.

[0492] The internal structure of the first battery 1301a may be a wound type or a stacked type. The first battery 1301a may use the all-solid-state battery described in Embodiment 5. By using the all-solid-state battery described in Embodiment 5 for the first battery 1301a, a high capacity can be achieved, safety can be improved, and the battery can be made smaller and lighter.

[0493] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple lithium ion batteries, it is possible to extract large amounts of power. The multiple lithium ion batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of lithium ion batteries is also called a battery pack.

[0494] In addition, in an in-vehicle lithium-ion battery, in order to cut off power from multiple lithium-ion batteries, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0495] 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.

[0496] 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.

[0497] The first battery 1301a will be described with reference to FIG. 21B.

[0498] FIG. 21B shows an example in which nine prismatic lithium-ion batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic lithium-ion batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (e.g., from the road surface), it is preferable to fix multiple lithium-ion batteries using the fixing portions 1413 and 1414 and the battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0499] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS).

[0500] 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 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, and magnesium) may be used as the oxide. In particular, the In-M-Zn oxide that can be used as the 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 oxide. A CAAC-OS is an oxide semiconductor having multiple crystalline regions, each with its c-axis aligned in a specific direction. The specific direction refers to the thickness direction of the CAAC-OS film, the normal direction to the surface where the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. A crystalline region is a region with periodic atomic arrangement. If the atomic arrangement is considered as a lattice arrangement, a crystalline region is also a region with a uniform lattice arrangement. Furthermore, a CAAC-OS has a region where multiple crystalline regions are connected in the a-b plane direction, and the region may have distortion. Note that distortion refers to a portion where the lattice arrangement changes between a region with a uniform lattice arrangement and a region with another uniform lattice arrangement in a region where multiple crystalline regions are connected. In other words, a CAAC-OS is an oxide semiconductor with a c-axis aligned and no clear orientation in the a-b plane direction. CAC-OS is a material in which elements constituting a metal oxide are unevenly distributed in a size of 0.5 nm to 10 nm, preferably 1 nm to 3 nm, or in the vicinity thereof. Note that hereinafter, a state in which one or more metal elements are unevenly distributed in a metal oxide and regions containing the metal elements are mixed in a size of 0.5 nm to 10 nm, preferably 1 nm to 3 nm, or in the vicinity thereof, is also referred to as a mosaic or patch state.

[0501] Furthermore, since the control circuit unit 1320 can be used in low-temperature environments, it is preferable to use a transistor using an oxide semiconductor. To simplify the process, the control circuit unit 1320 may be formed using a unipolar transistor. A transistor using an oxide semiconductor for the semiconductor layer has a wider operating ambient temperature range than single-crystal Si, from −40° C. to 150° C., and its characteristics change less when the lithium-ion battery is heated than single-crystal Si. The off-current of a transistor using an oxide semiconductor is below the lower limit of measurement regardless of temperature, even at 150° C., whereas the off-current characteristics of a single-crystal Si transistor are highly temperature-dependent. For example, at 150° C., the off-current of a single-crystal Si transistor increases, and the current on / off ratio does not become sufficiently large. The control circuit unit 1320 can improve safety.

[0502] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for lithium-ion batteries to address 10 causes of instability, such as micro-short circuits. The functions for addressing the 10 causes of instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of degradation, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for lithium-ion batteries can be miniaturized.

[0503] Micro-short circuits are tiny short circuits that occur inside lithium-ion batteries. One of the causes of micro-short circuits is said to be local current concentration in parts of the positive electrode and negative electrode due to uneven distribution of the positive electrode active material caused by multiple charge and discharge cycles, or the generation of by-products due to side reactions, which causes micro-short circuits.

[0504] In addition to detecting micro-short circuits, the control circuit 1320 can also be said to detect the terminal voltage of the lithium-ion battery and manage the charge / discharge state of the lithium-ion battery. For example, to prevent overcharging, it can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0505] FIG. 21C shows an example of a block diagram of the battery pack 1415 shown in FIG. 21B.

[0506] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the lithium-ion battery used and limits the upper limit of the external current and the upper limit of the output current. The range between the lower and upper voltage limits for the lithium-ion battery is within the recommended voltage range. If the voltage falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0507] The switch unit 1324 can be configured by combining an n-channel transistor and a p-channel transistor. The switch unit 1324 is not limited to a switch having a Si transistor using single crystal silicon, and may be, for example, Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), GaO ｘThe switch portion 1324 may be formed using a power transistor having gallium oxide (x is a real number greater than 0). Furthermore, a memory element using an OS transistor can be freely arranged by stacking it on a circuit using a Si transistor, and thus integration can be easily achieved. By stacking the control circuit portion 1320 using an OS transistor on the switch portion 1324 and integrating them, it is possible to form it into a single chip, thereby enabling miniaturization.

[0508] The first batteries 1301a and 1301b mainly supply power to on-board equipment in the 42V system (high voltage system), while the second battery 1311 supplies power to on-board equipment in the 14V system (low voltage system). Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Using a lithium-ion battery as the second battery 1311 offers the advantage of being maintenance-free, but over long periods of use, such as three years or more, there is a risk of abnormalities occurring that cannot be detected at the time of manufacture. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b have remaining capacity, the second battery 1311 is charged to maintain a full charge state by supplying power from the first battery to the second battery.

[0509] In this embodiment, an example in which lithium ion batteries are used for both the first battery 1301 a and the second battery 1311 is shown, but a lead-acid battery, an all-solid-state battery, or an electric double layer capacitor may be used for the second battery 1311. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the above-described lithium ion battery, excellent charge / discharge characteristics can be expected in a wide temperature range including temperatures from below freezing to high temperatures.

[0510] 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 from the motor controller 1303 and 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.

[0511] 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 lithium ion batteries used, and can perform rapid charging.

[0512] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a, 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, 1301b via the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. 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 includes a microcomputer. The ECU uses a CPU or a GPU.

[0513] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0514] Next, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0515] Furthermore, installing lithium-ion batteries in vehicles will enable next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), plug-in hybrid vehicles (PHVs), etc. Lithium-ion batteries can also be installed in transportation vehicles such as agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

[0516] 22A to 22D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 22A is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor and an engine as a power source for traveling. When a lithium-ion battery is installed in a vehicle, an example of the lithium-ion battery described in the above embodiment is installed in one or more locations. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in a lithium-ion battery installed in a vehicle, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0517] 22A includes a battery pack 2200, which includes a battery module to which a plurality of lithium-ion batteries are connected. The battery pack 2200 further preferably includes a charge control device electrically connected to the battery module.

[0518] Furthermore, the automobile 2001 can charge its lithium-ion battery by receiving power from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging can be performed using a predetermined charging method and connector standards, such as CHAdeMO (registered trademark) or Combo, as appropriate. The lithium-ion battery may be charged at a charging station installed in a commercial facility or from a home power source. For example, plug-in technology can be used to charge the power storage device installed in the automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0519] Furthermore, although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the lithium-ion battery while the vehicle is stopped and while moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0520] Figure 22B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The battery module of transport vehicle 2002 is, for example, a four-cell unit of lithium-ion batteries with a nominal voltage of 3.0 V to 5.0 V, with 48 cells connected in series to achieve a maximum voltage of 170 V. Other than the number of lithium-ion batteries in battery pack 2201, the battery pack 2201 has the same functions as those shown in Figure 21B, and therefore a description thereof will be omitted. When the lithium-ion batteries of battery pack 2201 use an electrolyte and a positive electrode active material according to one embodiment of the present invention, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0521] 22C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The battery module of the transport vehicle 2003 has, for example, one hundred or more lithium ion batteries with a nominal voltage of 3.0 V to 5.0 V connected in series to produce a maximum voltage of 600 V. Furthermore, except for the number of lithium ion batteries constituting the battery module of the battery pack 2202, the battery module has the same functions as that shown in FIG. 22B , and therefore a description thereof will be omitted. When the lithium ion batteries of the module use an electrolyte and a positive electrode active material according to one embodiment of the present invention, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0522] Fig. 22D shows, as an example, an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 22D has wheels for takeoff and landing, it can also be said to be part of a transportation vehicle, and has a battery pack 2203 that includes a battery module formed by connecting multiple lithium ion batteries and includes the battery module and a charge control device.

[0523] The battery module of the aircraft 2004 is, for example, eight 4 V lithium ion batteries connected in series, with a maximum voltage of 32 V. Other than the number of lithium ion batteries constituting the battery module of the battery pack 2203, the battery module has the same functions as those shown in Fig. 21B, and therefore a description thereof will be omitted.

[0524] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0525] Embodiment 7 In this embodiment, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted on a vehicle such as a motorcycle or a bicycle will be described.

[0526] 23A illustrates an example of an electric bicycle using the lithium-ion battery of one embodiment of the present invention. The lithium-ion battery of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 23A. The lithium-ion battery of one embodiment of the present invention may include a protection circuit.

[0527] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state removed from the bicycle in FIG. 23B . The power storage device 8702 includes a plurality of lithium-ion batteries 8701 of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit 8703. When the lithium-ion battery 8701 includes an electrolyte and a positive electrode active material of one embodiment of the present invention, excellent charge and discharge characteristics can be expected in a wide temperature range, including temperatures from below freezing to high temperatures.

[0528] The power storage device 8702 also includes a control circuit 8704 capable of controlling charging or detecting an abnormality of the lithium ion battery, an example of which is shown in Embodiment 7. The control circuit 8704 is electrically connected to the positive electrode and the negative electrode of the lithium ion battery 8701. This can greatly contribute to eliminating accidents such as fires caused by lithium ion batteries.

[0529] 23C illustrates an example of a two-wheeled vehicle using a lithium-ion battery of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 23C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. When the electrolyte and the positive electrode active material of one embodiment of the present invention are used in a lithium-ion battery, excellent charge and discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0530] 23C can store a power storage device 8602 in an under-seat storage space 8604. The power storage device 8602 can be stored in the under-seat storage space 8604 even if the under-seat storage space 8604 is small.

[0531] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0532] (Embodiment 8) In this embodiment, an example of mounting a lithium-ion battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a lithium-ion battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet terminals, e-book readers, and mobile phones.

[0533] 24A shows an example of a mobile phone. The mobile phone 2100 includes a display unit 2102 built into a housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. The mobile phone 2100 also includes a lithium ion battery 2107. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the lithium ion battery, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0534] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0535] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0536] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0537] The mobile phone 2100 also includes an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0538] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0539] 24B shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a lithium-ion battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. When the electrolyte and positive electrode active material according to one embodiment of the present invention are used in a lithium-ion battery, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0540] Fig. 24C shows an example of a robot. A robot 6400 shown in Fig. 24C includes a lithium ion battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0541] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0542] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0543] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0544] The robot 6400 includes a lithium-ion battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the lithium-ion battery, excellent charge and discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0545] 24D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a lithium-ion battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0546] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, if an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a lithium-ion battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the lithium-ion battery, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0547] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0548] Embodiment 9 In this embodiment, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted in space equipment will be described.

[0549] 25A shows an artificial satellite 6800 as an example of space equipment. The artificial satellite 6800 has a body 6801, a solar panel 6802, an antenna 6803, and a lithium-ion battery 6805. The solar panel may be called a solar cell module.

[0550] When sunlight is irradiated onto the solar panel 6802, the power required for the operation of the satellite 6800 is generated. However, for example, in a situation where sunlight is not irradiated onto the solar panel or in a situation where the amount of sunlight irradiating the solar panel is small, the generated power is small. Therefore, there is a possibility that the power required for the operation of the satellite 6800 will not be generated. In order to operate the satellite 6800 even in a situation where the generated power is small, it is preferable to provide a lithium ion battery 6805 in the satellite 6800. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the lithium ion battery, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures.

[0551] The satellite 6800 can generate a signal. The signal is transmitted via the 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, the position of the receiver that received the signal can be measured, for example. As described above, the satellite 6800 can constitute, for example, a satellite positioning system.

[0552] 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.

[0553] FIG. 25B shows a probe 6900 with a solar sail (also called a sun sail) as an example of space equipment. The probe 6900 includes a body 6901, a solar sail 6902, and a lithium-ion battery 6905. When the electrolyte and positive electrode active material of one embodiment of the present invention are used in the lithium-ion battery, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures. 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.

[0554] 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 25B.

[0555] FIG. 25C shows a spacecraft 6910 as an example of space equipment. The spacecraft 6910 has a body 6911, a solar panel 6912, and a lithium-ion battery 6913. When the electrolyte and positive electrode active material according to one embodiment of the present invention are used in the lithium-ion battery, excellent charge / discharge characteristics can be expected over a wide temperature range, including temperatures from below freezing to high temperatures. The body 6911 can have, for example, a pressurized compartment and an unpressurized compartment. The pressurized compartment may be designed to accommodate a crew member. Electricity generated by sunlight irradiating the solar panel 6912 can be used to charge the lithium-ion battery 6913.

[0556] 25D shows a rover 6920 as an example of space equipment. The rover 6920 includes a body 6921 and a lithium-ion battery 6923. When the electrolyte and the positive electrode active material according to one embodiment of the present invention are used in the lithium-ion battery, excellent charge / discharge characteristics are expected over a wide temperature range, including temperatures from below freezing to high temperatures. The rover 6920 may also include a solar panel 6922.

[0557] The rover 6920 may be designed to accommodate a crew member. The lithium ion battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or the lithium ion battery 6923 may be charged with electricity generated by other power sources, such as a fuel cell, a radioisotope thermoelectric converter, or the like.

[0558] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0559] <NMR Measurement> In this example, FEC, MTFP, and a mixture of FEC:MTFP=2:8 (volume ratio) according to one embodiment of the present invention were mixed with 1 mol / L of LiPF as a lithium salt. ６ Nuclear magnetic resonance analysis of mixed solution A ( １ The results of these measurements will be explained below.

[0560] FEC was mixed in a deuterated acetonitrile solution and analyzed by nuclear magnetic resonance spectroscopy ( １The measurement was performed by H-NMR. Charts of the measurement results are shown in Figures 26A and 26B. Note that Figure 26A shows the range of δ = 0-10 ppm, and Figure 26B shows the range of δ = 3-7 ppm as an enlarged view. From Figures 26A and 26B, the peak position of FEC can be read. For example, FEC is １ When analyzed by H-NMR, it can be seen that there are peaks at 3 ppm or more and 7 ppm or less.

[0561] Next, MTFP is mixed with a deuterated acetonitrile solution, １ The measurement was performed by H-NMR. Charts of the measurement results are shown in Figures 27A and 27B. Note that Figure 27A shows the range of δ = 0-10 ppm, and Figure 27B shows the range of δ = 3-4 ppm as an enlarged view. From Figures 27A and 27B, the peak position of MTFP can be read. For example, MTFP is １ When analyzed by H-NMR, it can be seen to have a peak between 3 ppm and 4 ppm.

[0562] Next, FEC and MTFP were mixed in a volume ratio of 2:8, which is an embodiment of the present invention, and LiPF was added to the mixture so that the lithium salt concentration was 1 mol / L. ６ The mixture A containing the above is mixed with the deuterium acetonitrile solution, １ The measurements were performed by H-NMR. Charts of the measurement results are shown in Figures 28A and 28B. Note that Figure 28A shows the range of δ = 0-10 ppm, and Figure 28B shows the range of δ = 3-7 ppm as an enlarged view. From Figures 28A and 28B, the peak positions of the mixed liquids can be read. For example, mixed liquid A has １ When analyzed by H-NMR, it can be seen that there are peaks at 3 ppm or more and 4 ppm or less, and peaks at 4 ppm or more and 7 ppm or less.

[0563] Looking at the enlarged view of Figure 28B, both the peak corresponding to FEC confirmed in Figure 26B and the peak corresponding to MTFP confirmed in Figure 27B were confirmed. In other words, it is presumed that FEC and MTFP are not bonded, and that FEC, which forms a solvate with the lithium salt, and MTFP coexist in the mixed solution. It can be considered that MTFP is mixed in order to maintain an appropriate viscosity. MTFP may also form a solvate with the lithium salt.

[0564] In this example, FEC, MTFP, and a mixture of FEC and MTFP in a volume ratio of 2:8 according to one embodiment of the present invention were added with LiPF as a lithium salt to give a concentration of 1 mol / L. ６ It was possible to confirm the NMR of the mixed solution A to which the above was added.

[0565] <HOMO Level and Solvation Energy> In this example, the HOMO level and solvation energy were calculated. The solvation energy refers to the energy required for the organic solvent used in the electrolyte to bind with lithium ions through Coulomb force or the like. The binding is also called coordination. In this example, in order to compare with FEC and MTFP, which have electron-withdrawing substituents, the HOMO level and solvation energy were calculated for EC and MP as organic compounds without electron-withdrawing substituents. Here, the solvation energy is calculated as the energy required for stabilization when four molecules of the organic solvent used in the electrolyte are coordinated to one Li ion.

[0566] Density functional theory (DFT) was used to calculate the HOMO levels and solvation energy. In DFT, the total energy is expressed as the sum of potential energy, electrostatic energy between electrons, and exchange-correlation energy, which includes electron kinetic energy and complex electron interactions. DFT also approximates the exchange-correlation interaction with a single-electron potential functional (a function of a function) expressed in terms of electron density, allowing for fast and highly accurate calculations. In this study, the mixed functional B3LYP was used to define the weights of each parameter related to exchange and correlation energy. The 6-311G basis set (a triple-split valence basis set using three contraction functions for each valence orbital) was applied to all atoms. For example, the above-mentioned basis functions consider the 1s-3s orbitals for hydrogen atoms, and the 1s-4s and 2p-4p orbitals for carbon atoms. Furthermore, to improve the accuracy of the calculation, p-functions were added to hydrogen atoms and d-functions to atoms other than hydrogen atoms as polarization basis sets. The above calculation conditions are summarized in the table below.

[0567]

[0568] The calculation results of the HOMO level and solvation energy are shown in the table below, along with the measured melting points. In the table below, the solvation energies of the chain organic compounds MTFP and MP are shown as ranges because the solvation energies change due to the influence of the rotational coordinate system.

[0569]

[0570] Differences in HOMO levels were confirmed between organic compounds having electron-withdrawing substituents and organic compounds without such substituents. Specifically, FEC with the above substituents had a deeper HOMO level than EC without the above substituents. Furthermore, MTFP with the above substituents had a deeper HOMO level than MP without the above substituents. Organic compounds with a deeper HOMO are considered to be less susceptible to oxidation, and organic solvents using such organic compounds are thought to have improved oxidation resistance. Therefore, lithium-ion batteries using the above organic solvents as electrolytes are expected to have improved cycle characteristics.

[0571] In addition, differences in solvation energy were confirmed between organic compounds having electron-withdrawing substituents and organic compounds without such substituents. Specifically, FEC having the above substituents had a lower solvation energy than EC without the above substituents. Furthermore, it was suggested that MTFP having the above substituents tended to have a lower solvation energy than MP without the above substituents. Small solvation energy reduces the resistance between the electrolyte containing the organic solvent and the positive electrode, or between the electrolyte and the negative electrode. Therefore, lithium-ion batteries using the above organic solvents as electrolytes are expected to have improved battery performance over a wide temperature range, especially when placed below freezing.

[0572] Although the melting point of MTFP is unknown, considering that the melting point of MP is -87.5°C, it is believed that MTFP maintains an appropriate viscosity even when exposed to temperatures below freezing. Therefore, it is believed that by using a mixed solution of MTFP and FEC as an organic solvent, it is possible to provide an electrolyte that maintains an appropriate viscosity even below freezing. The ratio (specifically, volume ratio) of FEC and MTFP is as described in the first embodiment above.

[0573] By using organic compounds with electron-withdrawing substituents as organic solvents, lithium-ion batteries can be used over a wide temperature range, and furthermore, improved battery performance is expected when the lithium-ion battery is placed below freezing.

[0574] In this example, in order to measure AC impedance and evaluate charge / discharge characteristics, Sample 1, Sample 2, and Reference Example 1 were prepared. The conditions for producing each sample will be described.

[0575] <Conditions for preparing Sample 1> As the electrolyte for Sample 1, an organic solvent was prepared in which FEC:MTFP was mixed to satisfy a volume ratio of 2:8, and lithium hexafluorophosphate (LiPF ) was added as a lithium salt to the organic solvent at a concentration of 1 mol / L. ６ ) was added. A solvent containing two or more organic solvents is sometimes called a mixed solvent. Note that no additive was used in Sample 1.

[0576] The positive electrode active material of Sample 1 was LCO formed according to the solid phase method described in the second embodiment. The method for manufacturing the LCO will be described. First, steps S11 to S13 in FIG. 2A are omitted, and the LiMO ２ As the Al source in step S20_1, lithium cobalt oxide (manufactured by Nippon Chemical Industry Co., Ltd., product name: C-10N, hereinafter referred to as "C-10N") was prepared. The median diameter (D50) of C-10N was approximately 12.0 μm. In step S15, the lithium cobalt oxide was heated at 850° C. for 2 hours in a furnace to which oxygen had b...

Claims

1. A positive electrode comprising a positive electrode active material and an electrolyte, The positive electrode active material comprises Mg, F, Ni, Al, and lithium cobalt oxide. The electrolyte comprises a cyclic fluoride carbonate and a chain-like fluoride carbonate. The lithium cobalt oxide is represented as Li x CoO 2, When x in Li x CoO 2 is 1, the lithium cobalt oxide has a layered rock salt type crystal structure belonging to space group R-3m. When x in the Li x CoO 2 is 0.1 < x ≤ 0.24, the lithium cobalt oxide is space group P2 / m, Lattice constant a=0.488±0.001 nm, Lattice constant b=0.282±0.001 nm, Lattice constant c=0.484±0.001 nm, α＝９０°、 β＝１０９．５８±０．０１°、 A lithium-ion battery having a crystal structure with γ = 90°.

2. A positive electrode comprising a positive electrode having a positive electrode active material and an electrolyte, The positive electrode active material comprises Mg, F, Ni, Al, and lithium cobalt oxide. The electrolyte comprises a cyclic fluoride carbonate and a chain-like fluoride carbonate. The lithium cobalt oxide is represented as Li x CoO 2, When x in Li x CoO 2 is 1, the lithium cobalt oxide has a layered rock salt type crystal structure belonging to space group R-3m. When x in the Li x CoO 2 is 0.1 < x ≤ 0.24, the lithium cobalt oxide is analyzed by X-ray diffraction and at least 2θ = 19.37° or more and 19.57° or less, A lithium-ion battery having a diffraction peak at 2θ = 45.57° or more and 45.67° or less.

3. A positive electrode comprising a positive electrode active material and an electrolyte, The positive electrode active material comprises Mg, F, Ni, Al, and lithium cobalt oxide. The electrolyte comprises fluoroethylene carbonate and methyl trifluoropropionate, and when the total content of fluoroethylene carbonate and methyl trifluoropropionate is 100 vol%, the volume ratio of fluoroethylene carbonate and methyl trifluoropropionate is x:100-x (where 5≦x≦30). The lithium cobalt oxide is represented as Li x CoO 2, When x in Li x CoO 2 is 1, the lithium cobalt oxide has a layered rock salt type crystal structure belonging to space group R-3m. When x in the Li x CoO 2 is 0.1 < x ≤ 0.24, the lithium cobalt oxide is space group P2 / m, Lattice constant a=0.488±0.001 nm, Lattice constant b=0.282±0.001 nm, Lattice constant c=0.484±0.001 nm, α＝９０°、 β＝１０９．５８±０．０１°、 A lithium-ion battery having a crystal structure with γ = 90°.

4. A positive electrode comprising a positive electrode having a positive electrode active material and an electrolyte, The positive electrode active material comprises Mg, F, Ni, Al, and lithium cobalt oxide. The electrolyte comprises fluoroethylene carbonate and methyl trifluoropropionate, and when the total content of fluoroethylene carbonate and methyl trifluoropropionate is 100 vol%, the volume ratio of fluoroethylene carbonate and methyl trifluoropropionate is x:100-x (where 5≦x≦30), and the lithium cobaltate is represented as Li x CoO 2. When x in Li x CoO 2 is 1, the lithium cobalt oxide has a layered rock salt type crystal structure belonging to space group R-3m. When x in the Li x CoO 2 is 0.1 < x ≤ 0.24, the lithium cobalt oxide is analyzed by X-ray diffraction and at least 2θ = 19.37° or more and 19.57° or less, A lithium-ion battery having a diffraction peak at 2θ = 45.57° or more and 45.67° or less.

5. In any one of claims 1 to 4, A lithium-ion battery in which the median diameter (D50) of the positive electrode active material is 10 μm or more and 14 μm or less.

6. In any one of claims 1 to 4, A lithium-ion battery in which the median diameter (D50) of the positive electrode active material is 5 μm or more and 9 μm or less.

7. A lithium-ion battery comprising a positive electrode active material containing Mg, F, Ni, Al, and lithium cobalt oxide, and an electrolyte containing cyclic fluoride carbonate and chain fluoride carbonate, A half-cell is prepared comprising a positive electrode having the positive electrode active material, the electrolyte, and a lithium metal counter electrode. The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of the positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.6V. After charging with a constant voltage of 4.6V until the current value becomes 0.05C, the discharge capacity obtained by discharging with a constant current of 0.1C until the voltage reaches 2.5V is compared to the value obtained by this procedure. The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.6V. After charging at a constant voltage of 4.6V until the current value becomes 0.05C, it is placed at an ambient temperature of -40°C and discharged with a constant current of 0.1C until the voltage reaches 2.5V. The discharge capacity obtained by this method is 50% or more. Lithium-ion battery.

8. A lithium-ion battery comprising a positive electrode having a positive electrode active material containing Mg, F, Ni, Al, and lithium cobalt oxide, and an electrolyte, The electrolyte comprises fluoroethylene carbonate and methyl trifluoropropionate, and when the total content of fluoroethylene carbonate and methyl trifluoropropionate is 100 vol%, the volume ratio of fluoroethylene carbonate and methyl trifluoropropionate is x:100-x (where 5≦x≦30). A half-cell is prepared comprising a positive electrode having the positive electrode active material, the electrolyte, and a lithium metal counter electrode. The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of the positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.6V. After charging with a constant voltage of 4.6V until the current value becomes 0.05C, the discharge capacity obtained by discharging with a constant current of 0.1C until the voltage reaches 2.5V is compared to the value obtained by this procedure. The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.6V. After charging at a constant voltage of 4.6V until the current value becomes 0.05C, it is placed at an ambient temperature of -40°C and discharged with a constant current of 0.1C until the voltage reaches 2.5V. The discharge capacity obtained by this method is 50% or more. Lithium-ion battery.

9. A lithium-ion battery comprising a positive electrode active material having nickel, cobalt, and manganese, and an electrolyte containing a cyclic fluoride carbonate and a chain fluoride carbonate, A half-cell is prepared comprising a positive electrode having the positive electrode active material, the electrolyte, and a lithium metal counter electrode. The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of the positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.5V. After charging at a constant voltage of 4.5V until the current value becomes 0.05C, the half-cell is placed at an ambient temperature of -40°C and discharged with a constant current of 0.1C until the voltage reaches 2.5V. The value of the discharge capacity obtained by this process is: The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.5V, then charged at a constant voltage of 4.5V until the current value becomes 0.05C, and finally discharged at a constant current of 0.1C until the voltage reaches 2.5V. The discharge capacity obtained by this process is 50% or more of the value obtained. Lithium-ion battery.

10. A lithium-ion battery comprising a positive electrode active material having nickel, cobalt, and manganese, and an electrolyte, The electrolyte comprises fluoroethylene carbonate and methyl trifluoropropionate, and when the total content of fluoroethylene carbonate and methyl trifluoropropionate is 100 vol%, the volume ratio of fluoroethylene carbonate and methyl trifluoropropionate is x:100-x (where 5≦x≦30). A half-cell is prepared comprising a positive electrode having the positive electrode active material, the electrolyte, and a lithium metal counter electrode. The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of the positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.5V. After charging at a constant voltage of 4.5V until the current value becomes 0.05C, the half-cell is placed at an ambient temperature of -40°C and discharged with a constant current of 0.1C until the voltage reaches 2.5V. The value of the discharge capacity obtained by this process is: The half-cell is placed at an ambient temperature of 25°C and charged with a constant current of 0.1C (where 1C is the current value per unit weight of positive electrode active material, and 1C = 200mA / g) until the voltage reaches 4.5V, then charged at a constant voltage of 4.5V until the current value becomes 0.05C, and finally discharged at a constant current of 0.1C until the voltage reaches 2.5V. The discharge capacity obtained by this process is 50% or more of the value obtained. Lithium-ion battery.

11. In claim 9 or claim 10, In the positive electrode active material, the ratio of nickel:cobalt:manganese satisfies 8:1:1 or close to it. Lithium-ion battery.

12. In claim 9 or claim 10, In the positive electrode active material, the proportion of nickel is higher than the proportion of cobalt and higher than the proportion of manganese. Lithium-ion battery.

13. In claim 9 or claim 10, The positive electrode active material has a median diameter (D50) of 4 μm or more and 7 μm or less. Lithium-ion battery.

14. In any one of claims 7 to 10, The separator of the half cell has polyimide, Lithium-ion battery.

15. In any one of claims 7 to 10, The separator of the half cell is made of polypropylene. Lithium-ion battery.