Secondary battery

Abstract

To provide an electrolyte solution capable of suppressing ignition or the like, and a secondary battery having the electrolyte solution.SOLUTION: A secondary battery has a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte solution. The electrolyte solution has a mixed solvent and a lithium salt. In the electrolyte solution, the concentration of the lithium salt is at least more than 1 mol per liter of the mixed solvent. The mixed solvent has a fluorinated chain carbonate and a fluorinate cyclic carbonate. In DSC measurement of the electrolyte solution, the peak of a heat flow rate in the exoergic reaction in a range of 180°C or more and 300°C or less is 200 mW / g or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a secondary battery. However, the technical field of the present invention is not limited to secondary batteries, and examples of the technical field of the present invention include semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, vehicles, and methods for manufacturing these devices. For example, the secondary battery of the present invention can be applied as a necessary power source in semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, and vehicles. The above-mentioned electronic devices include information terminal devices equipped with a secondary battery, and the power storage devices include stationary power storage devices. [Background technology]

[0002] In recent years, demand for high-power, high-capacity lithium-ion secondary batteries has rapidly expanded in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0003] When a lithium-ion secondary battery is heated externally, the positive electrode, negative electrode, and electrolyte react independently or mutually, resulting in an exothermic reaction. Generally, when the temperature of a lithium-ion secondary battery approaches 100°C, the negative electrode begins to disintegrate, generating heat. Above 100°C, a reduction reaction of the electrolyte occurs at the negative electrode, generating heat as well. It is known that when the temperature of a lithium-ion secondary battery approaches 180°C, thermal decomposition of the electrolyte occurs, leading to oxygen release and thermal decomposition at the positive electrode, resulting in thermal runaway. Furthermore, the resulting heat generation can melt the separator. Melting the separator can cause an internal short circuit in the lithium-ion secondary battery, which can then cause Joule heat to lead to thermal runaway.

[0004] The heat generation described above causes the lithium ion secondary battery to generate gases such as hydrogen, carbon monoxide, carbon dioxide, or hydrocarbons. These gases are organic solvents used in the electrolyte or thermal decomposition products of organic solvents, and since they contain flammable gases, there is a risk that the lithium ion secondary battery may catch fire.

[0005] In order to suppress the thermal runaway reaction, Patent Document 1 proposes a configuration in which a flame retardant is blended into the positive electrode composite or the negative electrode composite. Furthermore, Patent Document 2 proposes a configuration having a container that houses a laminate in which positive electrodes and negative electrodes are alternately stacked with separators interposed therebetween, an electrolyte solution stored in the container, and a high thermal conductivity gas filled in the container.

[0006] Furthermore, various research and development efforts are being made on the reliability and safety of lithium-ion secondary batteries. For example, Non-Patent Document 1 describes the thermal stability of positive electrode active materials and electrolyte solutions. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2009-16106 A [Patent Document 2] Patent Publication No. 2010-262792 [Non-patent literature]

[0008] [Non-Patent Document 1] Nobuo Eda 2-4 Heat Generation Mechanism Learning from Data Li-ion Battery Charging and Discharging Technology CQ Publishing Published April 4, 2020 P.68-72 Summary of the Invention [Problem to be solved by the invention]

[0009] To suppress thermal runaway or ignition in a secondary battery, an improved electrolyte solution is necessary. However, Patent Documents 1 and 2 do not discuss the electrolyte solution. Therefore, an object of one embodiment of the present invention is to provide an electrolyte solution with high thermal stability in order to suppress at least ignition or thermal runaway in a secondary battery. Another object of one embodiment of the present invention is to provide an electrolyte solution in which gas generation at temperatures higher than 25° C. is suppressed in order to suppress at least ignition or thermal runaway in a secondary battery. Another object of one embodiment of the present invention is to provide a separator with high heat resistance and good wettability with an electrolyte solution in order to suppress at least ignition or thermal runaway in a secondary battery. Another object of one embodiment of the present invention is to provide a secondary battery in which at least ignition or thermal runaway is suppressed.

[0010] Note that the description of these problems does not preclude the existence of other problems. Furthermore, one embodiment of the present invention does not necessarily solve all of these problems. Furthermore, problems other than these can be extracted from the description of this specification, drawings, claims, etc. [Means for solving the problem]

[0011] In view of the above, one embodiment of the present invention is a secondary battery comprising: a positive electrode; a negative electrode; a separator positioned between the positive electrode and the negative electrode; and an electrolyte solution, the electrolyte solution comprising a mixed solvent and a lithium salt, the concentration of the lithium salt in the electrolyte solution being higher than 1 mol per liter of the mixed solvent, the mixed solvent comprising a fluorinated chain carbonate and a fluorinated cyclic carbonate, and wherein, in Differential Scanning Calorimetry (DSC) measurement of the electrolyte solution, the peak heat flow of an exothermic reaction in the temperature range of 180°C to 300°C is 200 mW / g or less.

[0012] Another embodiment of the present invention is a secondary battery comprising: a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode; and an electrolyte solution, the electrolyte solution comprising a mixed solvent and a lithium salt, the concentration of the lithium salt in the electrolyte solution being higher than 1 mol per liter of the mixed solvent, the mixed solvent comprising a fluorinated chain carbonate and a fluorinated cyclic carbonate, and wherein DSC measurement of the electrolyte solution shows that the peak heat flow of an exothermic reaction in the temperature range of 180°C to 300°C is 100 mW / g or less.

[0013] Another aspect of the present invention is a secondary battery comprising: a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode; and an electrolyte solution, the electrolyte solution comprising a mixed solvent and a lithium salt, wherein the concentration of the lithium salt in the electrolyte solution is greater than 1 mol per liter of the mixed solvent, and the mixed solvent comprises a fluorinated chain carbonate and a fluorinated cyclic carbonate, and in DSC measurement of the electrolyte solution, the peak heat flow of an exothermic reaction in the temperature range of 180°C to 300°C is 200 mW / g or less, and the separator comprises an imide compound in a region in contact with the electrolyte solution.

[0014] Another aspect of the present invention is a secondary battery comprising: a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode; and an electrolyte solution, the electrolyte solution comprising a mixed solvent and a lithium salt, wherein the concentration of the lithium salt in the electrolyte solution is greater than 1 mol per liter of the mixed solvent, and the mixed solvent comprises a fluorinated chain carbonate and a fluorinated cyclic carbonate, and in DSC measurement of the electrolyte solution, the peak heat flow of an exothermic reaction in the temperature range of 180°C to 300°C is 100 mW / g or less, and the separator comprises an imide compound in a region in contact with the electrolyte solution.

[0015] In the present invention, the lithium salt is LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, Li2B 10 Cl 10 , Li2B 12 Cl 12, LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), and LiN(C2F5SO2)2 are preferred.

[0016] In the present invention, the fluorinated chain carbonate is preferably FEC.

[0017] In the present invention, the fluorinated cyclic carbonate is preferably MTFP.

[0018] In the present invention, the imide compound is preferably a polyimide. [Effects of the Invention]

[0019] According to one embodiment of the present invention, an electrolyte solution with high thermal stability can be provided. According to another embodiment of the present invention, an electrolyte solution in which gas generation at temperatures higher than 50° C. is suppressed can be provided. According to another embodiment of the present invention, a separator with high heat resistance and good wettability with respect to the electrolyte solution can be provided. According to another embodiment of the present invention, a secondary battery in which ignition or thermal runaway is suppressed can be provided. [Brief explanation of the drawings]

[0020] [Figure 1] 1A to 1D illustrate a separator included in a secondary battery of one embodiment of the present invention. [Figure 2] 2A and 2B illustrate a secondary battery of one embodiment of the present invention. [Figure 3] FIG. 3A illustrates a secondary battery of one embodiment of the present invention, and FIG. 3B illustrates a positive electrode active material layer of one embodiment of the present invention. [Figure 4] FIG. 4 is a diagram illustrating an electrolyte injection device. [Figure 5] 5(A) and 5(B) are diagrams illustrating a bendable secondary battery according to one embodiment of the present invention. [Figure 6]FIG. 6 is a diagram illustrating a method for producing a positive electrode active material. [Figure 7] 7(A) to 7(C) are diagrams illustrating a method for producing a positive electrode active material. [Figure 8] 8(A) and 8(B) are diagrams illustrating the positive electrode active material. [Figure 9] 9A to 9F are cross-sectional views illustrating the positive electrode active material. [Figure 10] FIG. 10 is a diagram illustrating the crystal structure of the positive electrode active material. [Figure 11] FIG. 11 is a diagram illustrating the crystal structure of a conventional positive electrode active material. [Figure 12] FIG. 12 shows the XRD pattern calculated from the crystal structure. [Figure 13] FIG. 13 shows an XRD pattern calculated from the crystal structure. [Figure 14] 14(A) to 14(G) are diagrams for explaining the positional relationship of distribution in EDX line analysis. [Figure 15] FIG. 15 is a diagram illustrating a method for producing a negative electrode active material layer. [Figure 16] FIG. 16 is a diagram illustrating a TMA test device. [Figure 17] FIG. 17 is a diagram illustrating a tensile testing machine. [Figure 18] 18(A) and 18(B) are diagrams illustrating the nail penetration test. [Figure 19] FIG. 19 is a diagram illustrating the nail penetration operation. [Figure 20] FIG. 20 is a graph showing changes when the internal temperature rises in a secondary battery in which an internal short circuit has occurred. [Figure 21] FIG. 21 is a graph showing changes when the internal temperature of the secondary battery increases. [Figure 22] 22(A) is an exploded perspective view of a coin-type secondary battery, FIG. 22(B) is a perspective view of the coin-type secondary battery, and FIG. 22(C) is a cross-sectional perspective view thereof. [Figure 23]Fig. 23(A) shows an example of a cylindrical secondary battery. Fig. 23(B) shows an example of the internal structure of a cylindrical secondary battery. Fig. 23(C) shows an example of multiple cylindrical secondary batteries. Fig. 23(D) shows an example of a power storage system having multiple cylindrical secondary batteries. [Figure 24] 24(A) and 24(B) are diagrams for explaining an example of a secondary battery, and FIG. 24(C) is a diagram showing the internal structure of the secondary battery. [Figure 25] 25(A) to 25(C) are diagrams illustrating examples of secondary batteries. [Figure 26] 26(A) and 26(B) are diagrams showing examples of secondary batteries. [Figure 27] FIG. 27(A) shows an example of the configuration of an automobile, FIG. 27(B) shows a battery pack, and FIG. 27(C) shows an example of the configuration of an electric automobile and a battery pack. [Figure 28] 28(A) to 28(D) are diagrams showing an example of space equipment. [Figure 29] FIG. 29 shows the results of DSC measurement of the electrolyte solution. [Figure 30] FIG. 30 is a graph showing the vapor pressure of the electrolyte solution versus the temperature. [Figure 31] FIG. 31 shows the results of Raman spectroscopic analysis of the electrolyte solution. [Figure 32] FIG. 32 shows the results of DSC measurement of the separator. [Figure 33] Figures 33(A) and 33(B) show the results of TMA analysis of the separator. [Figure 34] 34(A) and 34(B) are images of the nail penetration test. DETAILED DESCRIPTION OF THE INVENTION

[0021] The following description will explain the embodiments of the present invention with reference to the accompanying drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes can be made in form and detail without departing from the spirit and scope of the present invention. Therefore, in the embodiments described below, the same reference numerals are used in different drawings to indicate the same objects.

[0022] In this specification, ordinal numbers such as "first" and "second" are used for convenience and do not limit the number of components or the order of the components. The order of the components includes, for example, the order of processes or the order of stacking. In other words, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the claims. Furthermore, the ordinal numbers used in the examples of this specification may not match the ordinal numbers used in the claims. Furthermore, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the examples of this specification.

[0023] In this specification and the like, a lithium ion secondary battery may be referred to as a lithium ion battery, and refers to a secondary battery that uses lithium ions as carrier ions. However, the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or alkaline earth metal ions may be used as carrier ions of the present invention, and specifically, sodium ions may be used. In this case, the present invention can be understood by replacing lithium ions with sodium ions. Furthermore, when describing a configuration in which there is no limitation on the carrier ions, the term "secondary battery" or "battery" may be used.

[0024] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode substance, a positive electrode material for secondary batteries, a positive electrode material for lithium ion secondary batteries, or the like.

[0025] In this specification and the like, an electrolytic solution may be referred to as an electrolyte. An electrolytic solution means that the solution is liquid at 25°C. This also means that the electrolyte is not limited to the state at 25°C.

[0026] In this specification, space groups are represented using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are represented using Miller indices. While space groups, crystal planes, and crystal directions are represented by superscript bars in crystallography, in this specification, due to formatting constraints, they may be represented by a minus sign (-) before the number instead of a bar above it. Individual orientations indicating directions within a crystal are represented by [ ], collective orientations indicating all equivalent directions are represented by < >, individual planes indicating crystal planes are represented by ( ), and collective planes with equivalent symmetry are represented by {}. For ease of understanding the structure, trigonal crystals represented by the space group R-3m are generally represented by a hexagonal composite hexagonal lattice. Unless otherwise specified, the space group R-3m will be represented by a composite hexagonal lattice. Miller indices may also be (hkil) rather than (hkl). Here, i is -(h+k).

[0027] In this specification, etc., the space group of the positive electrode active material etc. is identified by XRD, electron beam diffraction, neutron beam diffraction, etc. Therefore, in this specification, etc., "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0028] In this specification, if the anions have a structure in which three layers are stacked with a mutually offset, such as ABCABC, it is referred to as a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. Furthermore, since real crystals always have defects, analytical results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, if the deviation between the theoretical position and orientation is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0029] In this specification, the (001) plane and the (003) plane may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C-plane, the basal plane, or the like. In addition, lithium in lithium cobalt oxide has a two-dimensional diffusion path. In other words, it can be said that the lithium diffusion path exists along the (00l) plane. In this specification, the surface where the lithium diffusion path is exposed, that is, the surface where lithium is inserted and extracted (specifically, a surface other than the (00l) plane), may be referred to as the edge plane.

[0030] In this specification and the like, the cross-sectional shape of a particle is not limited to a circle, i.e., a sphere. The cross-sectional shape of a particle includes an ellipse, a rectangle, a trapezoid, a triangle, a square with rounded corners, an asymmetric shape, etc. When there are multiple particles, the cross-sectional shapes of the particles may be different from each other.

[0031] In this specification, etc., when describing the characteristics of individual particles of a positive electrode active material in embodiments, etc., it is not necessary for all particles to have that characteristic. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of a positive electrode active material have that characteristic, it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material and a secondary battery containing it.

[0032] In this specification, particle size can be measured using a particle size distribution analyzer (laser diffraction particle size distribution analyzer) that uses a laser diffraction / scattering method. In this specification, median diameter (D50) can be used as the average particle size. D50 is the particle size at which the cumulative amount accounts for 50% on the cumulative curve of the particle size distribution measurement results.

[0033] In this specification, the measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the long diameter of the particle cross section may be measured by analysis such as SEM (Scanning Electron Microscope) or TEM. In this specification, the maximum particle size can be the particle size that can be confirmed in a cross section of a positive electrode measuring 100 μm square. Furthermore, as a method for measuring D50 by analysis such as SEM or TEM, for example, 20 or more particles are measured, a cumulative curve is created, and the particle diameter at which the cumulative amount accounts for 50% can be taken as D50.

[0034] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable lithium contained in the positive electrode active material is deintercalated. For example, the theoretical capacity of LiCoO2 is 274 mAh / g per weight, the theoretical capacity of LiNiO2 is 275 mAh / g per weight, and the theoretical capacity of LiMn2O4 is 148 mAh / g per weight.

[0035] In this specification and the like, the amount of lithium remaining in the positive electrode active material that can be inserted and removed is determined by x in the composition formula, for example, Li x It is sometimes represented by x in MO2. Note that M represents a transition metal, and in this specification and the like, unless otherwise specified, M is cobalt and / or nickel. In the case of a positive electrode active material in a lithium ion secondary battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, when a lithium ion secondary battery equipped with a positive electrode active material containing lithium cobalt oxide is charged at 219.2 mAh / g, Li 0.2 CoO2 or x=0.2. x For example, x in MO2 is small, for example, 0.1 <x≦0.24をいう。

[0036] In this specification and the like, when properly synthesized lithium cobalt oxide before use in a positive electrode approximately satisfies the stoichiometric ratio, it is LiCoO2, and x = 1. It can also be said that lithium cobalt oxide contained in a lithium ion secondary battery after discharge is also LiCoO2, and x = 1. The state after discharge (discharged state) here refers to a state in which, for example, the current is 100 mA / g or less and the voltage is 3.0 V or less or 2.5 V or less.

[0037] In this specification, Li x The charge capacity and / or discharge capacity used to calculate x in MO2 should preferably be measured under conditions that are free of or minimally affected by short circuits and / or thermal decomposition of the electrolyte. For example, data from a secondary battery that has experienced a sudden change in capacity that may be due to a short circuit should not be used to calculate x.

[0038] In this specification, the distribution of a certain element refers to a region in which the element is continuously detected within a range that is not a noise by a certain analytical method. The region in which the element is continuously detected within a range that is not a noise can also be referred to as a region in which the element is detected at or above the lower detection limit.

[0039] Unless otherwise specified, in this specification, the materials (positive electrode, negative electrode, electrolyte, separator, etc.) contained in a secondary battery are described in their pre-degradation state. Note that a decrease in discharge capacity due to aging and burn-in treatments during secondary battery manufacturing is not considered to be degradation. For example, a secondary battery consisting of a single cell or a battery pack can be said to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. For secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other secondary batteries, the rated capacity conforms to not only the above JIS standard but also to various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0040] In this specification, etc., secondary particles refer to particles formed by aggregation of primary particles. Also, in this specification, etc., primary particles refer to particles that do not have grain boundaries on their appearance. Also, in this specification, etc., primary particles may be referred to as single particles. In this specification, etc., grain boundaries may be referred to as the interface between two crystals that are in contact with each other.

[0041] Here, we will explain the flow of electrons and lithium ions during charging in a secondary battery. When a charger is connected and charging of a secondary battery begins, electrons are released from the positive electrode, causing an oxidation reaction, and electrons are supplied to the negative electrode, causing a reduction reaction. Lithium ions are then released from the positive electrode into the electrolyte, and the lithium ions move to the negative electrode. During discharge, a reduction reaction occurs at the positive electrode, and an oxidation reaction occurs at the negative electrode. In other words, in a secondary battery, the anode (positive electrode) and cathode (negative electrode) are interchanged during discharge and charging, and the oxidation reaction and reduction reaction are interchanged. Therefore, the electrode with a higher reaction potential is called the positive electrode, and the electrode with a lower reaction potential is called the negative electrode. Therefore, in this specification, whether during charging or discharging, the positive electrode is called the "positive electrode" or "+ electrode (plus electrode)," and the negative electrode is called the "negative electrode" or "- electrode (minus electrode)."

[0042] In this specification, a full cell refers to a cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a cell assembled with lithium metal as the negative electrode (counter electrode).

[0043] In this specification, unless otherwise specified, the charge voltage is expressed based on the potential of lithium metal. Furthermore, in this specification, a high charge voltage is, for example, a charge voltage of 4.6 V or higher, preferably 4.65 V or higher, more preferably 4.7 V or higher, even more preferably 4.75 V or higher, and most preferably 4.8 V or higher. In other words, in the case of a half cell using lithium metal as the counter electrode, a charge voltage of 4.6 V or higher is referred to as a high charge voltage.

[0044] In this specification, a high charging voltage is defined as a charging voltage of 4.5 V or higher, based on the potential when the negative electrode is made of a carbon material (e.g., graphite). In other words, in the case of a full cell in which a carbon material (e.g., graphite) is used as the negative electrode, a charging voltage of 4.5 V or higher is referred to as a high charging voltage.

[0045] In this specification and the like, carbonate refers to a compound having at least one carbonate ester in its molecular structure, and includes cyclic carbonates and chain carbonates unless otherwise specified. Furthermore, chain carbonates include both linear and branched chain carbonates.

[0046] In this specification and the like, a mixed solvent refers to a mixture of two or more solvents.

[0047] In this specification, the porosity can be a value calculated from the volume, density, and mass. In this specification, the porosity can also be calculated based on an observation image of an object that has voids, is filled with an organic material, and then processed into a thin film. For processing, a focused ion beam (FIB) or ion milling can be used.

[0048] In this specification and the like, flexibility refers to the property that an object is soft and deformable. In this specification, an object having flexibility refers to at least a part of the object having flexibility. In other words, a flexible object may have a part that is not flexible (also called a hard part).

[0049] In this specification, a secondary battery that can be deformed in accordance with a deformable electronic device is referred to as a deformable secondary battery, a flexible secondary battery, or a flexible battery. In this specification, "deformable" means that the shape of an object changes, and includes the shape of an object being deformed in response to an external force applied to the object. In this specification, "the shape of an object being deformed in response to an external force" means that the shape of an object can be deformed by the hand of an average adult without requiring excessive force.

[0050] In this specification, the shape of an object deformed in response to an external force includes a shape of an object bent in response to an external force. In this specification, a secondary battery that can be bent to follow a bendable electronic device is referred to as a bendable secondary battery, foldable battery, bendable battery, or the like. For a foldable battery, the shape of a deformed object in response to an external force includes a folded shape.

[0051] In this specification, bendable electronic devices and bendable secondary batteries include a state in which they are bent and fixed, and also include a state in which they are repeatedly bent and straightened. In this specification, a state in which they are repeatedly bent and straightened includes a state in which they are repeatedly bent and straightened. In this specification, a state in which they are unbent includes a straight state.

[0052] In this specification, ignition in a nail penetration test refers to the observation of a flame outside the exterior of the cell within one minute of the nail being inserted into the cell, or the occurrence of thermal runaway in the secondary battery. For example, thermal runaway is said to have occurred if thermal decomposition products of the positive and / or negative electrodes are observed at a location 2 cm or more away from the point of insertion after the nail penetration test is completed. If smoke is generated when a nail is inserted into the cell but no flame is observed, the cell is considered to have not ignited.

[0053] In this specification and the like, "A and / or B" may be stated, which means "A", "B", or "A and B".

[0054] (Embodiment 1) A secondary battery according to one embodiment of the present invention includes an electrolyte solution, and the electrolyte solution includes at least a solvent and a lithium salt.

[0055] [solvent] The solvent contained in the electrolyte solution according to one embodiment of the present invention will be described. The electrolyte solution preferably contains a mixed solvent as the solvent. Furthermore, it is preferable to use a mixed solvent containing a fluorinated cyclic carbonate (sometimes referred to as a fluorinated cyclic carbonate) and a fluorinated chain carbonate (sometimes referred to as a fluorinated chain carbonate). In this specification and the like, a mixed solvent of a fluorinated cyclic carbonate and a fluorinated chain carbonate may be referred to as a fluoride mixed solvent. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have a substituent that exhibits electron-withdrawing properties, and tend to have low solvation energy with lithium ions, which are carrier ions. However, they are capable of forming solvation in a secondary battery, making them preferable as mixed solvents.

[0056] Fluoride mixed solvents are preferred as electrolytes because they have low viscosity at room temperature (e.g., 25°C). In particular, fluorinated chain carbonates are expected to have low viscosity at low temperatures (e.g., 0°C), so mixed solvents containing fluorinated chain carbonates are suitable for use in secondary batteries at low temperatures (including below freezing).

[0057] Furthermore, an electrolyte solution containing a fluoride mixed solvent is preferable because it can suppress exothermic reactions. The exothermic reaction can be measured using the heat flow determined by differential scanning calorimetry (DSC). Suppressing an exothermic reaction includes keeping the peak of the heat release (heat flow) low. Suppressing an exothermic reaction also includes setting the onset temperature of the exothermic reaction higher. A secondary battery containing a fluoride mixed solvent as an electrolyte solution can prevent thermal runaway and / or fire of the secondary battery because the exothermic reaction is suppressed even if the internal temperature of the secondary battery rises due to an internal short circuit.

[0058] Furthermore, since the vapor pressure of the fluoride mixed solvent is low at temperatures higher than 50° C., evaporation at these temperatures can be suppressed. Therefore, in a secondary battery having a fluoride mixed solvent as an electrolyte, even if the internal temperature of the secondary battery rises due to an internal short circuit, generation of flammable gas from the electrolyte is suppressed, and thermal runaway and / or fire of the secondary battery can be prevented.

[0059] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, FEC), difluoroethylene carbonate (DFEC, FEC), trifluoroethylene carbonate (FEC), and tetrafluoroethylene carbonate (FEC). DFEC includes isomers such as cis-4,5 and trans-4,5. Since all of these fluorinated cyclic carbonates have electron-withdrawing substituents, they have low solvation energy for lithium ions and are therefore preferred as mixed solvents.

[0060] The structural formula of FEC is as shown below in structural formula (H10): In FEC, the electron-withdrawing substituent is an F group.

[0061] [ka]

[0062] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." The structural formula of MTFP is shown below in structural formula (H22). In MTFP, the electron-withdrawing substituent is a CF3 group.

[0063] [ka]

[0064] An example of a fluorinated chain carbonate is trifluoromethyl 3,3,3-trifluoropropionate. The structural formula of trifluoromethyl 3,3,3-trifluoropropionate is shown below in structural formula (H23). In trifluoromethyl 3,3,3-trifluoropropionate, the electron-withdrawing substituent is a CF3 group.

[0065] [ka]

[0066] An example of a fluorinated chain carbonate is trifluoromethyl propionate. The structural formula of trifluoromethyl propionate is shown below in structural formula (H24). In trifluoromethyl propionate, the electron-withdrawing substituent is a CF3 group.

[0067] [ka]

[0068] Methyl 2,2-difluoropropionate is an example of a fluorinated chain carbonate. The structural formula of methyl 2,2-difluoropropionate is shown below in structural formula (H25). In methyl 2,2-difluoropropionate, the electron-withdrawing substituent is a CF2 group.

[0069] [ka]

[0070] An example of a fluorinated chain carbonate is 2,2,2-trifluoroethylmethyl carbonate. The structural formula of 2,2,2-trifluoroethylmethyl carbonate is shown below in structural formula (H26). In 2,2,2-trifluoroethylmethyl carbonate, the electron-withdrawing substituent is a CF3 group.

[0071] [ka]

[0072] The mixed solvent according to one embodiment of the present invention preferably contains one or more fluorinated cyclic carbonates and one or more fluorinated chain carbonates. For example, the mixed solvent more preferably contains FEC and MTFP. The reason for this will be explained below.

[0073] [FEC and MTFP] FEC is a fluorinated cyclic carbonate with a high dielectric constant, which promotes the dissociation of lithium salts when used in an electrolyte. Furthermore, because FEC contains electron-withdrawing fluorine as a substituent, it facilitates desolvation of lithium ions compared to ethylene carbonate (EC), which does not contain electron-withdrawing substituents. Specifically, FEC has a lower solvation energy for lithium ions than ethylene carbonate (EC). Therefore, FEC is preferred because it facilitates the release of lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery.

[0074] Furthermore, FEC is preferable because it is resistant to oxidation due to its deep highest occupied molecular orbital (HOMO) level and has high oxidation resistance. However, one concern with FEC is its high viscosity. Therefore, it is preferable to use a mixed solvent of FEC and MTFP in the electrolyte. MTFP is a chain carbonate that can reduce the viscosity of the electrolyte. Specifically, MTFP can maintain low viscosity even at low temperatures (e.g., 0°C). Normally, solvation does not occur when the solvation energy is low. However, because the solvation energy of MTFP varies depending on the molecular configuration, it may solvate with lithium ions when the solvation energy is high. In other words, both FEC and MTFP are preferable because they can form solvates with lithium ions.

[0075] The table below shows the HOMO levels, solvation energies, and boiling points for FEC, MTFP, EC, and MP. The HOMO levels and solvation energies were calculated using quantum chemical calculations, and the higher the solvation energy value in the table below, the easier it is to solvate.

[0076] [Table 1]

[0077] The mixed solvent used in the electrolyte preferably contains a larger proportion of fluorinated chain carbonate than fluorinated cyclic carbonate. For example, when the total content of the mixed solvent is 100 vol%, the volume ratio (vol%) of the fluorinated cyclic carbonate FEC to the fluorinated chain carbonate MTFP is preferably x:100-x (where 5≦x≦30, preferably 10≦x≦20). When MTFP is contained in a larger proportion than FEC, the viscosity of the electrolyte can be reduced, which is preferable for the electrolyte.

[0078] Furthermore, it is preferable that each solvent constituting the mixed solvent has almost no peaks due to impurities that can be confirmed by NMR (Nuclear Magnetic Resonance) measurement, etc. "Almost no peaks can be confirmed" means that the ratio of the integrated area of ​​the peak due to the impurity to the integrated area of ​​the peak due to the main component (simply referred to as "integral ratio") is 0.005 or less, preferably 0.002 or less.

[0079] For example, in the case of MTFP, it is known that when 1H-NMR is measured using acetonitrile-d3 solvent, four peaks appear at δ between 3.29 ppm and 3.43 ppm. However, if other peaks appear in this vicinity, for example, if a peak appears at δ between 3.24 ppm and 3.29 ppm, the peak is considered to be derived from impurities. Therefore, if the ratio (integral ratio) of the peak area between 3.24 ppm and 3.29 ppm to the peak area between 3.29 ppm and 3.43 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks due to impurities are almost impossible to detect.

[0080] The device used for NMR measurement is not particularly limited, but for example, Bruker's AVANCE III 400 can be used. In addition, in 1H-NMR measurement, the central peak of the five peaks derived from acetonitrile-d3 used as a diluent solvent can be set to 1.94 ppm.

[0081] The mixed solvent preferably has a low water (HO) or moisture content and is highly purified. Specifically, the water (HO) or moisture content in the mixed solvent is 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. For example, the moisture content can be measured by Karl Fischer titration.

[0082] [Solvent that can be added] Another solvent may be added to the mixed solvent described above. Examples of the other solvent that can be added to the mixed solvent include one or more of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate (MP), ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone.

[0083] [Lithium salt] Next, lithium salts will be described. Examples of lithium salts include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B 12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), and LiN(C2F5SO2)2 can be used alone or in any combination and ratio of two or more of these. For example, the use of fluorides LiPF6 and LiBF4 improves the safety of lithium-ion secondary batteries, so a combination of LiPF6 and LiBF4 is preferred as the lithium salt. Note that the concentration of the lithium salt in the electrolyte is greater than 1 mol and less than 3.0 mol per liter of mixed solvent, preferably greater than 1 mol and less than 2.0 mol. "Per liter of mixed solvent" refers to 1 liter of the total amount of mixed solvent.

[0084] For example, when a fluoride-mixed solvent is used as an electrolyte solution containing more than 1 mol and less than 3.0 mol, preferably more than 1 mol and less than 2.0 mol, of lithium salt per liter of the fluoride-mixed solvent, thermal runaway and / or fire of the secondary battery can be prevented even if the internal temperature rises due to an internal short circuit of the secondary battery. This is because dissolving more than 1 mol and less than 3.0 mol, preferably more than 1 mol and less than 2.0 mol, of lithium salt per liter of the fluoride-mixed solvent in the fluoride-mixed solvent results in an electrolyte solution with high thermal stability. Furthermore, dissolving more than 1 mol and less than 3.0 mol, preferably more than 1 mol and less than 2.0 mol, of lithium salt per liter of the fluoride-mixed solvent in the fluoride-mixed solvent results in an electrolyte with a lower vapor pressure at temperatures above 50°C than an electrolyte solution containing less than 1 mol of lithium salt per liter of the fluoride-mixed solvent. Therefore, gas generation can be suppressed even if the internal temperature rises due to an internal short circuit of the secondary battery and exceeds 50°C.

[0085] [DSC measurement] The electrolyte solution used in the secondary battery according to one embodiment of the present invention preferably has a heat flow (heat generation) peak of 200 mW / g or less, preferably 100 mW / g or less, in the temperature range of 180°C to 300°C. If there are two or more peaks in the temperature range, the maximum peak is preferably 200 mW / g or less, preferably 100 mW / g or less. Furthermore, if there are two or more peaks in the temperature range, it is more preferable that all peaks are 200 mW / g or less, preferably 100 mW / g or less. Surprisingly, the heat flow (heat generation) obtained by DSC measurement does not depend on the lithium salt concentration, and does not vary significantly depending on the lithium salt concentration. In other words, by using a fluoride-containing mixed solvent, the heat flow (heat generation) peak in the temperature range of 180°C to 300°C can be 200 mW / g or less, preferably 100 mW / g or less. For heat flow values, see the Examples described below. A secondary battery having a fluoride mixed solvent as an electrolyte can prevent thermal runaway and / or fire of the secondary battery because the exothermic reaction is suppressed even if the internal temperature of the secondary battery rises due to an internal short circuit.

[0086] The DSC measurement device and conditions are not particularly limited, but in this embodiment, it is preferable to perform the measurement using the following device and conditions. DSC equipment: Rigaku EVO2 DSC8271 Heating rate: 5℃ / min to 10℃ / min Temperature range: Room temperature (25°C) to 500°C The measurement results were analyzed using the Thermo prus EVO software for background correction. Heat flow was calculated as the heat flow rate per sample weight.

[0087] [Combustion test] The non-flammability and thermal stability are preferably confirmed by a combustion test, which is consistent with the DSC measurement. The electrolyte used in the secondary battery according to one embodiment of the present invention is preferably non-flammable.

[0088] [Vapor pressure] The vapor pressure is a value that depends on temperature, and surprisingly, the vapor pressure of the electrolyte also depends on the concentration of the lithium salt. Regarding the vapor pressure of the electrolyte used in one embodiment of the present invention, when the lithium salt concentration is higher than 1.0 mol per liter of the fluoride mixed solvent, the vapor pressure of the electrolyte can be 0.03 MPa or less at 50°C. When the lithium salt concentration is higher than 1.0 mol per liter of the fluoride mixed solvent, the vapor pressure of the electrolyte can be 0.08 MPa or less at 75°C. When the lithium salt concentration is higher than 1.0 mol per liter of the fluoride mixed solvent, the vapor pressure of the electrolyte can be 0.2 MPa or less at 100°C. When the lithium salt concentration is higher than 1.0 mol per liter of the fluoride mixed solvent, the vapor pressure of the electrolyte can be 0.3 MPa or less at 125°C. Furthermore, when the concentration of the lithium salt is higher than 1.0 mol per liter of the fluoride mixed solvent, the vapor pressure of the electrolyte can be 0.4 MPa or less at 150° C. For the value of the vapor pressure, see the examples described later.

[0089] [Additives] The electrolyte solution of a secondary battery according to one embodiment of the present invention may contain an additive, so long as it has the above-described configuration. The additive will now be described. The organic materials listed above as examples of the mixed solvent can be used as the additive. Other organic materials that can be used as the additive include one or more selected from vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), lithium bis(oxalato)borate (LiBOB), and dinitrile compounds such as succinonitrile or adiponitrile. The concentration of the additive is preferably 0.1 wt% to 10 wt% based on the total weight of the mixed solvent and the lithium salt. FEC, VC, or LiBOB are preferred additives because they readily form a good coating. When FEC is used as the fluoride mixed solvent, FEC is not required as an additive.

[0090] Among the additives mentioned above, 1,3-propane sultone (PS) has HOMO and LUMO levels equivalent to those of ethylene carbonate (EC) and diethyl carbonate (DEC), making it less susceptible to oxidation and reduction even when high cutoff voltages are used during charge and discharge. Furthermore, PS tends to form polymers when decomposed on the surface of the positive electrode active material, reducing the likelihood of gasification. The electrolyte preferably contains 0.25 wt% to 7.5 wt% PS based on the combined weight of the mixed solvent and lithium salt. By incorporating additives into the electrolyte, gas generation at temperatures above 25°C can be suppressed.

[0091] [Separator] The separator according to one embodiment of the present invention preferably has high wettability with the electrolyte solution containing the above-described mixed solvent. A specific compound that has high wettability with the electrolyte solution containing the above-described mixed solvent is an imide compound, and polyimide is typically preferred. Therefore, it is preferred that the separator contain an imide compound in the region that comes into contact with the electrolyte. Since imide compounds contain negatively polarized oxygen, the polarized oxygen interacts with the hydrogen in the fluoride mixed solvent, typically the hydrogen in FEC and the hydrogen in MTFP, which is thought to result in high wettability. In this specification, wettability can be evaluated by contact angle. The contact angle is preferably measured according to JIS R3257, for example, in an environment of 25°C, after 30 to 60 seconds have elapsed since 10 μL to 25 μL of electrolyte solution was dropped onto the separator member. The contact angle can be measured from an image observed from a horizontal direction. The contact angle is preferably the average value of measurements taken at multiple locations. In this specification, good wettability is defined as a contact angle of less than 30 degrees, preferably less than 20 degrees, and more preferably less than 10 degrees.

[0092] Specific examples of the separator will be described with reference to FIGS. 1(A) to 1(D).

[0093] As shown in FIG. 1A, the separator 105 can have a single-layer structure including a member 15. The member 15 preferably has a contact angle with the electrolyte of less than 30 degrees, preferably less than 20 degrees, and more preferably less than 10 degrees. It is preferable that the member 15 located on the surface of the separator 105 exhibits high wettability with the electrolyte, since this improves the injection of the electrolyte into the exterior body. Furthermore, it is preferable that the member 15 located on the surface of the separator 105 exhibits high wettability with the electrolyte, since this ensures the retention of the electrolyte in the separator 105 even when the electrodes expand and contract during charging and discharging. An imide compound is preferably used as a material that satisfies the above contact angle. Examples of imide compounds include polyimide and polyamic acid (a polyimide precursor). When the above-described DSC measurement is performed on a separator material, those that do not exhibit a peak that is considered to be due to an endothermic reaction between 25°C and 500°C, preferably between 25°C and 350°C, have high heat resistance, and secondary batteries using such separator materials can be considered to be highly safe. Examples of separator materials that do not exhibit endothermic reactions in the above temperature range include imide compounds, specifically polyimides, but materials other than polyimides may also be used as long as they do not exhibit endothermic reactions in the above temperature range.

[0094] As shown in FIG. 1(B), separator 105 can have a structure in which member 17, member 16, and member 15 are laminated in this order. Members 15 and 17 each preferably have a contact angle with the electrolyte of less than 30 degrees, preferably less than 20 degrees, and more preferably less than 10 degrees. It is preferable that members 15 and 17 located on the surface of separator 105 each exhibit high wettability with the electrolyte, since this improves the injection of the electrolyte into the exterior body. Furthermore, it is preferable that members 15 and 17 located on the surface of separator 105 each exhibit high wettability with the electrolyte, since this ensures the retention of the electrolyte in separator 105 even when the electrodes expand and contract during charge and discharge. An imide compound is preferably used as a material that satisfies the above contact angle. Imide compounds include polyimides and polyamic acids (polyimide precursors), etc.

[0095] A porous substrate is preferably used for the member 16. The porous substrate is preferably made of an insulating material, and one or more insulating materials selected from organic and inorganic materials can be used. The porous substrate is preferably made of a thermoplastic resin as the organic material to provide the separator 105 with a shutdown function. The shutdown function is a function that closes the pores of the separator 105 when the secondary battery experiences abnormal heat generation. Thermoplastic resins soften when heated, thereby closing the pores. Using a thermoplastic resin in this way can provide the separator 105 with a shutdown function. A material with a softening point or melting point of less than 200°C is preferably used as the thermoplastic resin, and typically one or more selected from polypropylene (PP), polyethylene (PE), acrylic, and polyamide (PA) can be used. Polypropylene has higher heat resistance than polyethylene, with a softening point of 140°C or higher and a melting point of 164°C to 170°C. Because polypropylene's softening point is close to the temperature at which abnormal heat generation occurs, polypropylene is considered a material with good shutdown function. When the separator material is subjected to the above-mentioned DSC measurement, a good shutdown effect is considered to be achieved when a peak believed to be an endothermic reaction is detected between 100° C. and 200° C., preferably between 160° C. and 180° C. Materials other than thermoplastic resins may be used for the separator as long as such a peak is detected.

[0096] Materials other than polypropylene (PP), polyethylene (PE), acrylic, and polyamide (PA) can also be used for member 16. Materials having insulating properties that can be used for member 16 include cellulose-containing fibers, nonwoven fabrics, glass fibers, ceramics, and synthetic fibers made from nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. While the above-mentioned materials may have a lower shutdown function than polypropylene, because separator 105 includes members 15 and 17, the options for materials that can be used for member 16 can be expanded.

[0097] Furthermore, in separator 105, one or more materials selected from cellulose-containing fibers, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane may be provided between member 16 and member 15.

[0098] Furthermore, in separator 105, one or more materials selected from cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane may be provided between member 16 and member 17.

[0099] As shown in FIG. 1(C), the separator 105 has a structure in which the member 17, the member 16, and the member 15 are laminated in this order, and a coating layer 18 can be formed on the surface of the member 15. The coating layer 18 can be made of a ceramic material, a fluorine-based material, a polyamide-based material, or a mixture thereof. Examples of the ceramic material include aluminum oxide particles and silicon oxide particles. Examples of the fluorine-based material include PVDF and polytetrafluoroethylene. Examples of the polyamide-based material include nylon and aramid (meta-aramid, para-aramid). In the separator 105, high wettability with the electrolyte can be ensured by the member 17. Furthermore, when high wettability with the electrolyte is ensured by the member 15, it is preferable to selectively form the coating layer 18 so that a portion of the member 15 in the separator 105 is exposed from the coating layer 18.

[0100] As shown in FIG. 1(D), separator 105 has a structure in which member 17, member 16, and member 15 are laminated in this order, and may have coating layer 18 on the surface of member 15 and further have coating layer 19 on the surface of member 17. Coating layer 19 can be selected from the materials described for coating layer 18 and may be the same material as coating layer 18 or a different material from coating layer 18. In order to ensure high wettability of member 15 with the electrolyte in separator 105, it is preferable to selectively form coating layer 18 so that a portion of member 15 is exposed from coating layer 18 in separator 105. In order to ensure high wettability of member 17 with the electrolyte in separator 105, it is preferable to selectively form coating layer 19 so that a portion of member 17 is exposed from coating layer 19 in separator 105.

[0101] Next, the thickness of the separator 105 will be described. The thickness of the separator 105 is preferably 10 μm or more and 80 μm or less, and more preferably 20 μm or more and 60 μm or less. Applying a laminated structure to the separator 105 allows components with different physical properties to be combined, making it possible to reduce the thickness of the entire separator. Typically, even when the thickness is 20 μm or more and 40 μm or less, the safety of the secondary battery can be maintained. This allows the ratio of the positive electrode and the negative electrode to be increased, and the capacity per volume of the secondary battery can be increased. The thickness of the separator 105 can be, for example, a value measured at the center of a cross-sectional observation image of a secondary battery including the separator 105.

[0102] Next, the thicknesses of members 15 to 17 will be described. Member 16 is a member that imparts a shutdown function to separator 105, and is preferably thicker than members 15 and 17. The polyimide used for members 15 and 17 can satisfy a porosity of 75% or more and 85% or less, and therefore can easily be made thinner than member 16. Members 15 and 17 are members that ensure wettability with the electrolyte, and can still function adequately as separator 105 even if they are made thinner than member 16. The thicknesses of members 15 to 17 can be, for example, values ​​measured at the center of a cross-sectional observation image of a secondary battery including members 15 to 17.

[0103] Furthermore, when the member 15 is disposed so as to be close to the negative electrode side of the separator 105, it is preferable that the thickness of the member 15 is greater than the thickness of the member 17. This can prevent an internal short circuit of the secondary battery caused by dendrites that may occur in the negative electrode. Furthermore, when the coating layer 18 of the separator 105 is disposed so as to be close to the negative electrode side, it can also prevent an internal short circuit of the secondary battery caused by dendrites.

[0104] Furthermore, member 15 may have recesses on its surface. A recess is a region where the thickness is reduced in a cross-sectional observation image, and it is also possible to form a recess by removing a part of member 15. It is preferable to arrange the recesses in a stripe pattern. Similarly, member 17 may have recesses on its surface. A recess is a region where the thickness is reduced compared to other regions in a cross-sectional observation image, and it is also possible to form a recess by removing member 17. It is preferable to arrange the recesses in a stripe pattern. Having recesses makes it easier to form coating layer 18 and coating layer 19. Having recesses makes it possible to selectively form coating layer 18 and coating layer 19.

[0105] The shape of separator 105 is not limited, and it can be, for example, a sheet. Separator 105 may also be in a bag shape, and a form in which either the positive electrode or the negative electrode is housed in the bag is also suitable for separator 105.

[0106] [Thermomechanical analysis (TMA)] TMA is a method for measuring the degree of deformation of a sample as a function of temperature or time when a non-oscillating load such as compression, tension or bending is applied while changing the temperature of the sample.

[0107] FIG. 16 shows a simplified diagram of a measuring device (TMA tester) used for thermomechanical analysis. The TMA tester includes a load generator 701, a probe 702, and a heating furnace 705. The load generator 701 applies a constant tensile load to the sample 703 via the probe 702, while the heating furnace 705 is used to change the temperature of the sample 703. The TMA tester also includes a thermocouple 706, and the temperature of the sample 703 can be obtained by detecting a temperature signal from the thermocouple 706. When deformation such as thermal expansion or softening occurs in the sample 703 in response to a temperature change, the resulting displacement is measured by a position detector 707 as the position change of the probe 702, and this is output as a signal. In this way, it is possible to measure deformation with respect to temperature while applying a non-oscillating load (constant load).

[0108] When a separator material is subjected to thermomechanical analysis, it is preferable that the separator material elongate under load without shrinking or breaking even when the temperature is increased. Specifically, when a graph is created with the sample's elongation (expansion or contraction length [μm] / temperature [°C]) on the vertical axis and temperature [°C] on the horizontal axis, a separator material is preferable if the elongation is in the range of 0.2 [μm / °C] to 3.0 [μm / °C] between 150°C and 300°C. Furthermore, a separator material is even more preferable if the elongation is in the range of 0.6 [μm / °C] to 2.0 [μm / °C] between 150°C and 300°C.

[0109] [Tensile test] A tensile test can be performed as a separate measurement from TMA to obtain the mechanical strength characteristics of the separator material. A precision universal testing machine can be used for the tensile test, and the tensile strength is increased at a set rate while the temperature is kept constant, allowing the amount of change in the separator material to be obtained. The difference from TMA is that the temperature is kept constant during the tensile test.

[0110] A simplified diagram of a tensile tester is shown in Figure 17. A test sample 712 is attached to a first jig 711a and a second jig 711b of the tester, and the test is performed by pulling the sample 712 at a set speed. The test ends when the sample 712 breaks. The temperature can be set at any temperature during the tensile test, for example, 25°C, which simulates room temperature, or 250°C, the temperature at which a secondary battery experiences thermal runaway. The specified pulling speed can typically be between 45 [mm / min] and 70 [mm / min], and the separator material is evaluated based on the maximum test force [N] and maximum stress [MPa].

[0111] In the case of separator materials, in a tensile test at 25°C, the maximum test force is preferably 0.2 [N] or more, more preferably 1.0 [N] or more. In the same tensile test, the maximum stress is preferably 20 [MPa] or more, more preferably 30 [MPa] or more. In the case of separator materials, in a tensile test at 250°C, the maximum test force is preferably 0.1 [N] or more, more preferably 0.5 [N] or more. In the same tensile test, the maximum stress is preferably 10 [MPa] or more, more preferably 20 [MPa] or more. When evaluating separator materials, tensile testers and conditions other than those described in this embodiment may be used.

[0112] [Secondary battery] Next, a secondary battery having the above-described electrolyte solution and separator 105 will be described with reference to Fig. 2(A) to Fig. 3(B). Fig. 2(A) shows a state in which the components of the secondary battery 100 are overlapped, and Fig. 2(B) shows the components of the secondary battery 100 separated from each other. Fig. 3(A) shows a cross-sectional view of the secondary battery 100, and Fig. 3(B) shows the positive electrode active material layer 22 of the secondary battery 100.

[0113] The secondary battery 100 has multiple positive electrodes. In FIG. 2B, a first positive electrode 103a and a second positive electrode 103b are illustrated as the multiple positive electrodes. The first positive electrode 103a and the second positive electrode 103b are collectively referred to as the positive electrode 103. However, the number of positive electrodes in the secondary battery 100 is not limited to two layers, and the secondary battery 100 may have a single layer or three or more layers.

[0114] The secondary battery 100 has multiple negative electrodes. In FIG. 2B, the multiple negative electrodes are illustrated as a first negative electrode 106a, a second negative electrode 106b, and a third negative electrode 106c. The first negative electrode 106a, the second negative electrode 106b, and the third negative electrode 106c are collectively referred to as the negative electrode 106. However, the number of negative electrodes in the secondary battery 100 is not limited to three layers, and may be a single layer, two layers, or four or more layers.

[0115] The secondary battery 100 has a separator between the negative electrode and the positive electrode. In FIG. 2(B), the multiple separators, a first separator 105a, a second separator 105b, a third separator 105c, and a fourth separator 105d, are indicated by dashed lines. The first separator 105a, the second separator 105b, the third separator 105c, and the fourth separator 105d are collectively referred to as separator 105. However, the number of separator layers in the secondary battery 100 is not limited to four, and may be a single layer, two layers, three layers, or five or more layers. The multiple separators may be independent sheets as shown in FIG. 2(B), or a continuous separator may also be used. A continuous separator is a separator that is prepared with an area larger than that of the positive electrode and the negative electrode, and is folded appropriately to arrange the separator portions at positions corresponding to the first separator 105a to the fourth separator 105d. Since more separators are arranged than the positive electrode or the negative electrode, the overall thickness of the separators is reduced, which allows the capacity per volume of the secondary battery to be increased.

[0116] Fig. 3(A) is an example of a cross-sectional view of the secondary battery 100 taken along the dashed line AB in Fig. 2(B). Fig. 3(A) will be described using the positive electrode 103, separator 105, negative electrode 106, protrusion 31t, etc.

[0117] The positive electrode 103 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is a layer containing positive electrode active material particles and has a region in contact with the positive electrode current collector 21. The manufacturing process of the positive electrode 103 includes a press process, and in a positive electrode that has undergone this press process, recesses into which the positive electrode active material particles are pressed may be formed in part of the positive electrode current collector 21. As shown in FIG. 3(A), the positive electrode active material layer 22 can be formed on both sides of the positive electrode current collector 21. This is called a double-sided coated structure. Although not shown, the positive electrode active material layer 22 can also be formed on only one side of the positive electrode current collector 21. This is called a single-sided coated structure.

[0118] The protrusion 21t shown in FIG. 2(A) is a part of the positive electrode current collector 21. In other words, the protrusion 21t is a region of the positive electrode current collector 21 where the positive electrode active material layer 22 is not provided. As shown in FIG. 2(A), in the secondary battery 100, the multiple protrusions 21t overlap each other to form an assembly. The assembly of the protrusions 21t is called a positive electrode tab. The positive electrode tab is joined to the positive electrode lead 107a at the joint 109a. Ultrasonic bonding can be used for the joining. As a result of the joining, they are electrically connected to each other. The positive electrode lead 107a can be made of a material selected from aluminum, nickel, titanium, or an alloy thereof. An insulating seal may be placed to surround the joint 109a and / or the positive electrode lead 107a. Kapton tape can be used as the insulating seal.

[0119] 3(B) shows an example of a cross-sectional view of the positive electrode active material layer 22. The positive electrode active material layer 22 has at least a positive electrode active material 10. The positive electrode active material layer 22 may have a second positive electrode active material 20. The positive electrode active material layer 22 may have a conductive material 41. The positive electrode active material layer 22 has an electrolyte 108. Although not shown, the positive electrode active material layer 22 may have a binder. The positive electrode active material layer 22 may not have the second positive electrode active material 20. The positive electrode active material layer 22 may not have the conductive material 41. The positive electrode active material layer 22 may not have a binder.

[0120] The positive electrode active material 10 preferably has an average particle size of 9 μm or more and less than 20 μm, and a maximum particle size of less than 30 μm, and is therefore large in diameter (also referred to as a large particle size). Secondary particles may be used as the positive electrode active material 10, and the secondary particles preferably have an average particle size of 9 μm or more and less than 20 μm, and a maximum particle size of less than 30 μm. The positive electrode active material 10 may be a composite oxide having a layered rock salt crystal structure and represented by LiM1O2 (where M1 is one or more selected from Fe, Ni, Co, Mn, and Al), and typically lithium cobalt oxide may be used. Lithium cobalt oxide, which has excellent high-voltage charging characteristics, will be described in the second and subsequent embodiments.

[0121] Furthermore, the positive electrode active material 10 preferably has an average particle size of 5 μm or less, preferably 0.1 μm to 5 μm, and a maximum particle size of less than 9 μm. It is also preferable to use an active material that satisfies the small diameter (also called small particle size). For the positive electrode active material 10, LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, and Mn) having an olivine-type crystal structure can be used. Examples of LiMPO4 include LiFePO4, LiNiPO4, LiCoPO4, LiMnPO4, and LiFe a Ni b PO4, LiFe a Co b PO4, LiFe a Mn b PO4, LiNi a Co b PO4, LiNi a Mn b PO4(a+b is less than 1, 0 <a<1、0<b<1)、LiFec Ni d Co e PO4, LiFe c Ni d Mn e PO4, LiNi c Co d Mn e PO4 (c + d + e is 1 or less, 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe f Ni g Co h Mn i PO4 (f + g + h + i is 1 or less, 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), etc. are available, and typically lithium iron phosphate can be used. Also, it is preferable that the particle surface of the positive electrode active material 10 has a carbon layer.

[0122] As the second positive electrode active material 20 included in the positive electrode active material layer 22, it is preferable that the average particle diameter is 5 μm or less, preferably 0.1 μm or more and 5 μm or less, and the maximum particle diameter is less than 9 μm, and it is preferable to use an active material that satisfies a small diameter (also referred to as a small particle diameter). For the second positive electrode active material 20, LiM2PO4 (M2 is one or more selected from Fe, Ni, Co, and Mn) having an olivine-type crystal structure can be used. Examples of LiMPO4 include LiFePO4, LiNiPO4, LiCoPO4, LiMnPO4, LiFe a Ni b PO4, LiFe a Co b PO4, LiFe a Mn b PO4, LiNi a Co b PO4, LiNi a Mn b PO4 (a + b is 1 or less, 0 < a < 1, 0 < b < 1), LiFe c Ni d Co e PO4, LiFe c Ni d Mn e PO4, LiNi c Co d Mn e PO4 (c + d + e is 1 or less, 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe f Nig Co h Mn i There are, for example, PO4 (where f + g + h + i is 1 or less, 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), and typically lithium iron phosphate can be used. Also, it is preferable that the particle surface of the second positive electrode active material 20 has a carbon layer.

[0123] The negative electrode 106 has a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode active material layer 32 is a layer having negative electrode active material particles and has a region in contact with the negative electrode current collector 31. In the manufacturing process of the negative electrode 106, there is a pressing process, and in the negative electrode after this pressing process, a recess may be formed in a part of the negative electrode current collector 31 where the negative electrode active material particles are pressed in. As shown in FIG. 3(A), the negative electrode active material layer 32 can be formed on only one side of the negative electrode current collector 31. In the negative electrode disposed on the outermost layer of the secondary battery 100, since the insertion and extraction of carrier ions do not occur or are difficult to occur in the negative electrode active material layer that is not disposed facing the positive electrode, it is not necessary to form the negative electrode active material layer. That is, single-sided coating is often used for the outermost layer negative electrode. Although not shown, a double-sided coating structure in which the negative electrode active material layer 32 is formed on both sides of the negative electrode current collector 31 may also be used. Preparing all the negative electrodes as a double-sided coating structure is highly productive and preferable. In this case, a negative electrode with a double-sided coating structure can also be disposed in the outermost layer.

[0124] Furthermore, the negative electrode current collector 31 has a protruding portion 31t. The protruding portion 31t is a region where the negative electrode active material layer 32 is not provided. In FIG. 2(B), the first protruding portion 31ta, the second protruding portion 31tb, and the third protruding portion 31tc are shown as a plurality of protruding portions. The first protruding portion 31ta, the second protruding portion 31tb, and the third protruding portion 31tc are collectively referred to as the protruding portion 31t. The protruding portion 31t is also shown in FIG. 2(A). The plurality of protruding portions 31t overlap with each other to form an aggregate. The aggregate of the protruding portions 31t is called a negative electrode tab. In FIG. 3(A), the protruding portion 31t is shown separated from other protruding portions.

[0125] As shown in FIG. 2(A), the negative electrode tab (protrusion 31t) is joined to the negative electrode lead 107b at the joint 109b. Ultrasonic bonding can be used for the joining. As a result of the joining, they are electrically connected to each other. The negative electrode lead 107b can be made of a material selected from nickel, copper, titanium, or an alloy thereof. An insulating seal can be placed so as to surround the joint 109b and / or the negative electrode lead 107b. Kapton tape can be used as the insulating seal.

[0126] The negative electrode active material layer 32 may further include a binder. The negative electrode active material layer 32 may further include a conductive material. Of course, the negative electrode active material layer 32 does not necessarily have to include a binder or a conductive material. The binder and the conductive material will be described later.

[0127] In this specification and the like, a structure in which a plurality of positive electrodes, a plurality of negative electrodes, and a plurality of separators are stacked as shown in FIG. 2(B) is referred to as a stacked electrode.

[0128] FIG. 4 shows a liquid injection device 280 for the electrolyte 180. The liquid injection device 280 includes a processing chamber 281, a liquid injection nozzle 282 located on the upper surface of the processing chamber 281, a pump 283 for sending the electrolyte 180 to the liquid injection nozzle 282, and a tank 284 for storing the electrolyte 180. The pump 283 preferably has a function for adjusting the liquid delivery speed. The processing chamber 281 includes a fixing device 285 for fixing the secondary battery 100 (however, the secondary battery 100 includes a stacked battery housed in an exterior body, and a portion of the exterior body is thermocompression bonded). Although not shown, the processing chamber 281 preferably includes a vacuum pump or the like, and liquid injection is performed in a vacuum atmosphere. The vacuum pump may be a dry pump, turbomolecular pump, oil rotary pump, cryopump, or mechanical booster pump. The vacuum atmosphere includes an atmosphere reduced in pressure so that a differential pressure gauge included in the processing chamber 281 indicates a pressure of -0.1 MPa or more but less than -0.08 MPa. It is preferable that a heating mechanism 286 is provided in a portion of the injection nozzle 282 that is close to the processing chamber 281. The viscosity of the electrolytic solution 180 can be controlled by the heating mechanism 286. When the injection of the electrolytic solution 180 is completed, the injection nozzle 282 moves up, and thermocompression bonding of the exterior body can be performed in the processing chamber 281.

[0129] Since the separator 105 has a larger area than the positive electrode and the negative electrode, when the electrolyte is poured, the separator 105 is the first to come into contact with the electrolyte. Therefore, it is preferable to use a separator 105 that has good wettability with the electrolyte, as this makes it easier to pour the electrolyte.

[0130] In the separator 105, when stripe-shaped recesses are provided in the members 15 and 17, it is preferable that the stripe-shaped recesses start from the injection nozzle 282 side and extend toward the opposing side.

[0131] [Exterior body] Although not shown, the secondary battery 100 has an outer casing, and the laminated electrodes are housed in the outer casing. The outer casing of the secondary battery 100 can be made of a metal material such as aluminum, stainless steel, or titanium, or a resin material. Alternatively, a film-like outer casing can be used. Examples of films that can be used include a three-layer structure in which a flexible metal thin film or metal foil such as aluminum, stainless steel, titanium, copper, or nickel is provided on a membrane 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 outer casing. Such a multilayer structure film can be called a laminate film. In this case, the laminate film may be referred to as an aluminum (aluminum) laminate film, a stainless steel laminate film, a titanium laminate film, a copper laminate film, a nickel laminate film, or the like, using the name of the material of the metal layer.

[0132] The material or thickness of the metal layer of the laminate film may affect the flexibility, i.e., the bendability, of the secondary battery 100. For example, an aluminum laminate film having a polypropylene layer, an aluminum layer, and a nylon layer is preferably used as an exterior body for a secondary battery 100 that prioritizes flexibility or lightweight design. Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the aluminum layer is thinner than 10 μm, there is a concern that pinholes in the aluminum layer may reduce the gas barrier properties, so the thickness of the aluminum layer is desirably 10 μm or more.

[0133] For example, a stainless steel laminate film having a polypropylene layer, a stainless steel layer, and a nylon layer is preferably used as an exterior body for a secondary battery 100 that prioritizes physical strength or safety. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the stainless steel layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Note that if the stainless steel layer is thinner than 10 μm, there is a concern that pinholes in the stainless steel layer may reduce the gas barrier property, so the thickness of the stainless steel layer is desirably 10 μm or more. Note that, in this specification, stainless steel refers to steel (an alloy of iron and carbon) containing approximately 12% or more chromium, and can be broadly classified into martensitic, ferritic, and austenitic types based on composition. Furthermore, stainless steel also includes stainless steels containing one or more elements selected from Ti, Nb, Mo, Cu, Ni, and Si.

[0134] For example, it is preferable to use a titanium laminate film having a polypropylene layer, a titanium layer, and a nylon layer. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the titanium layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the titanium layer is thinner than 10 μm, there is a concern that pinholes in the titanium layer may reduce the gas barrier properties, so the thickness of the titanium layer is preferably 10 μm or more.

[0135] A secondary battery that uses a film as an exterior body is called a laminated secondary battery. Although not shown in the present embodiment, a can case may be used as the exterior body. For example, a secondary battery that uses a circular case is called a coin-type secondary battery. A secondary battery that uses a cylindrical case is called a cylindrical secondary battery.

[0136] [Bendable secondary battery] Next, a bendable secondary battery 100x will be described with reference to FIGS. 5(A) and 5(B). FIG. 5(A) is a cross-sectional view of the secondary battery 100x, and FIG. 5(B) is a perspective view of the secondary battery 100x. Like the secondary battery 100, the secondary battery 100x includes a positive electrode 103, a separator 105, and a negative electrode 106. The positive electrode 103, the separator 105, and the negative electrode 106 are preferably made of flexible materials. Regarding the positive electrode 103, the separator 105, and the negative electrode 106, descriptions of components similar to those of the secondary battery 100 will be omitted. As a difference from the secondary battery 100, the bendable secondary battery 100x may include two single-sided coated positive electrodes 103, with the positive electrode current collectors in contact with each other. A structure in which the positive electrode current collectors are in contact with each other is called a current collector overlapping structure. Similarly, two single-sided coated negative electrodes 106 may be prepared to prepare a negative electrode having a current collector overlapping structure in which the negative electrode current collectors are in contact with each other. In a secondary battery having a current collector overlapping structure, the contacting current collectors are likely to come off when the secondary battery is bent. Therefore, a current collector overlapping structure is suitable for a bendable secondary battery 100x.

[0137] Furthermore, in a bendable secondary battery 100x, the protrusion 31t may wrinkle, potentially causing the protrusion 31t to break. To prevent the protrusion 31t from wrinkling, FIGS. 5(A) and 5(B) show a secondary battery 100x bent so that the ends of the stacked battery on the protrusion 31t side are aligned. As a result, in the secondary battery 100x, misalignment occurs in the stacked battery on the side facing the protrusion 31t, and the misalignment increases toward the opposite side. In this case, using a current collector mating structure allows appropriate misalignment to occur, since the current collectors that are in contact with each other are likely to be misaligned. In a secondary battery 100x bent so that the ends of the stacked battery on the protrusion 31t side are aligned, there is a possibility that separators other than the outermost layer may come into contact with the exterior body.

[0138] In the case where the protrusion 31t is wrinkled and breaks, it is preferable to bend the secondary battery 100x (i.e., apply an external force) at a position farther from the protrusion 31t than the center of the stacked electrode. Of course, the bend position of the secondary battery 100x (i.e., apply an external force) may be at the center of the stacked electrode.

[0139] In this specification, a secondary battery 100x having a curved region (curved region) as shown in Figures 5(A) and 5(B) may be referred to as a curved secondary battery. A laminated secondary battery is preferably used as the secondary battery 100x. The exterior body used in a laminated secondary battery is flexible, which is preferable because it easily follows the deformation of the secondary battery, specifically the bending of the secondary battery. The secondary battery 100x can be bent and fixed as shown in Figures 5(A) and 5(B) and charged and discharged.

[0140] The bendable secondary battery 100x also includes a secondary battery that can be repeatedly changed between the straight state shown in FIG. 3(A) and the bent state shown in FIG. 5(A) and FIG. 5(B). The secondary battery 100x can be discharged while changing from a straight state to a bent state or from a bent state to a straight state. Charging instead of discharging is also possible.

[0141] The separator 105 of the bendable secondary battery 100x is preferably the separator 105 described above with reference to FIGS. 1(A) to 1(D). However, in the bendable secondary battery 100x, it is preferable that no adhesive layer is provided on the outermost surface of the separator 105, and that the separator 105 has a region in contact with the positive electrode 103. This allows the separator 105 and the positive electrode 103 to be appropriately misaligned. Similarly, it is preferable that no adhesive layer is provided, and that the separator 105 has a region in contact with the negative electrode 106. This allows the separator 105 and the negative electrode 106 to be appropriately misaligned. Specifically, a preferred configuration is one in which a member 15 is positioned on one of the outermost surfaces of the separator 105, and this surface is not coated with a ceramic material, a fluorine-based material, a polyamide-based material, or a mixture thereof. Similarly, it is preferable that member 17 is positioned on the outermost surface of separator 105 and that this surface is not coated with a ceramic material, a fluorine material, a polyamide material, a mixture of these, etc. Note that members 15 and 17 may be coated with a ceramic material, a fluorine material, a polyamide material, or a mixture of these that does not exhibit adhesive properties.

[0142] The bendable secondary battery 100x has a separator with good wettability with the electrolyte, which allows for good injection of the electrolyte into the exterior body. This is also preferable because it ensures the amount of electrolyte held by the separator 105 even when the electrodes expand and contract during charging and discharging of the bendable secondary battery 100x. Furthermore, the use of a separator with a multilayer structure allows for a large capacity per volume of the bendable secondary battery 100x.

[0143] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0144] (Embodiment 2) A positive electrode active material according to one embodiment of the present invention will be described.

[0145] <<Method for producing positive electrode active material>> A method for manufacturing the positive electrode active material 10 will be described with reference to FIGS.

[0146] <Step S11> In step S11 shown in FIG. 6, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0147] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and for example, it is recommended to use a material with a purity of 99.99% or higher.

[0148] As the cobalt source, it is preferable to use a compound containing cobalt, for example, cobalt oxide such as tricobalt tetroxide, cobalt hydroxide, or the like.

[0149] The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased and / or the reliability of the secondary battery is improved.

[0150] In addition, the cobalt source preferably has high crystallinity, for example, single crystal grains. The crystallinity of the cobalt source can be evaluated using TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. The above-mentioned methods for evaluating crystallinity can be applied not only to cobalt sources but also to the evaluation of the crystallinity of other sources.

[0151] <Step S12> Next, in step S12 shown in FIG. 6, the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller fragments. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the cobalt source in dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0152] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as media. Zirconium oxide balls are preferred because they emit less impurities. When using a ball mill, a bead mill, or the like, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0153] <Step S13> Next, in step S13 shown in FIG. 6, the mixed material is heated. Heating is preferably performed at a temperature of 800°C to 1100°C, more preferably 900°C to 1000°C, and even more preferably around 950°C. If the temperature is too low, the decomposition and melting of the lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to lithium evaporation from the lithium source and / or excessive reduction of cobalt. For example, cobalt may change from trivalent to divalent, causing oxygen defects.

[0154] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0155] The temperature rise rate depends on the heating temperature reached, but is preferably 80°C / h to 250°C / h. For example, when heating at 1000°C for 10 hours, the temperature rise rate is preferably 200°C / h.

[0156] Heating is preferably carried out in an atmosphere with little moisture, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, to suppress impurities that may be mixed into the material, it is preferable that the impurity concentrations of CH4, CO, CO2, and H2 in the heating atmosphere be 5 ppb (parts per billion) or less.

[0157] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flow through the reaction chamber is called flow.

[0158] When the heating atmosphere is an atmosphere containing oxygen, a method that does not allow oxygen to flow is preferable. For example, a method of depressurizing the reaction chamber and then filling it with oxygen (or purging it) to prevent the oxygen from entering or leaving the reaction chamber may be preferable. For example, it is preferable to depressurize the reaction chamber to -970 hPa and then fill it with oxygen until the differential pressure gauge indicates 50 hPa.

[0159] After heating, the material can be cooled naturally, but it is preferable that the time required to cool the material from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary, as long as the material is cooled to a temperature acceptable for the next step.

[0160] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0161] The container for accommodating the object to be heated during heating is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, a setter made of aluminum oxide with a purity of 99.9% is used. It is preferable to heat the crucible or setter with a lid, as this prevents the material from volatilizing. Mullite-cordierite may also be used as the material for the crucible and setter.

[0162] Furthermore, it is preferable to use a crucible that has been used multiple times rather than a new one. In this specification, a new crucible refers to one that has undergone two or fewer heating steps with materials containing lithium, transition metal M, and / or additive elements. A multiple-use crucible refers to one that has undergone three or more heating steps with materials containing lithium, transition metal M, and / or additive elements. This is because using a new crucible may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the sheath during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, may not fall within the desired range. On the other hand, this risk is less likely with a multiple-use crucible.

[0163] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use an aluminum oxide mortar as the mortar. Aluminum oxide mortars are made of a material that does not easily release impurities. Specifically, an aluminum oxide mortar with a purity of 90% or more, preferably 99% or more, is used. Note that heating conditions equivalent to those of step S13 can be applied to heating steps other than step S13, which will be described later.

[0164] <Step S14> Through the above steps, lithium cobalt oxide (LiCoO2) can be synthesized as shown in step S14 in Fig. 6. The lithium cobalt oxide (LiCoO2) thus produced can be used as a starting material.

[0165] Although the composite oxide is produced by the solid phase method in steps S11 to S14, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0166] Note that pre-synthesized lithium cobalt oxide may be used in step S14. In this case, steps S11 to S13 can be omitted. By heating pre-synthesized lithium cobalt oxide, lithium cobalt oxide with a smooth surface can be obtained.

[0167] <Step S20> Next, as shown in step S20, it is preferable to add an additive element to the lithium cobalt oxide. In the method for producing a positive electrode active material described in this embodiment, the additive element is added in multiple steps, so the additive element added first in the flow shown in Figure 6 will be described as A1, the additive element added second time as A2, and the additive element added third time as A3. The step of adding additive element A1 will be described with reference to Figure 7(A).

[0168] <Step S21> 7(A), an additive element source (Al source) to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the Al source.

[0169] The additive element A1 can be any of the additive elements described in the previous embodiments, specifically, one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0170] When magnesium is selected as the additive element, the source of the additive element can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0171] When fluorine is selected as the additive element, the source of the additive element can be called a fluorine source. Examples of the fluorine source that can be used include lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF3, CeF4), lanthanum fluoride (LaF3), and sodium aluminum hexafluoride (Na3AlF6). Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating step described below.

[0172] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0173] The fluorine source is preferably a gas, such as fluorine (F), carbon fluoride, sulfur fluoride, oxygen fluoride (OF, OF, OF, OF, OF, OF), or nitrogen trifluoride (NF), which may be mixed into the atmosphere during the heating step described below. A plurality of the above-mentioned fluorine sources may also be used.

[0174] In the method for preparing a positive electrode active material described in FIGS. 6 and 7A, magnesium and fluorine are used as the additive element A1. Lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as the fluorine source and magnesium source. The melting point is most effectively lowered when lithium fluoride and magnesium fluoride are mixed in a molar ratio of approximately LiF:MgF2 = 65:35. On the other hand, excessive lithium fluoride may result in excessive lithium, which may deteriorate cycle characteristics. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably LiF:MgF2 = x:1 (0≦x≦1.9), more preferably LiF:MgF2 = x:1 (0.1≦x≦0.5), and even more preferably LiF:MgF2 = x:1 (x = 0.33 or thereabouts). In this specification, "nearby" refers to a value greater than 0.9 times but less than 1.1 times the value.

[0175] <Step S22> 7(A), the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0176] <Step S23> Next, in step S23 shown in Fig. 7(A), the pulverized and mixed materials are collected to obtain the A1 source. Note that the A1 source shown in step S23 contains multiple starting materials and can be called a mixture.

[0177] The particle size of the mixture is preferably a median diameter (D50) of 600 nm to 200 μm, more preferably 1 μm to 150 μm. Even when a single material is used as the additive element source, the median diameter (D50) is preferably 600 nm to 200 μm, more preferably 1 μm to 150 μm.

[0178] Such a finely powdered mixture (including the case where only one additive element is included) can be easily adhered to the surface of the lithium cobalt oxide particles when mixed with the lithium cobalt oxide in a later step. If the mixture is evenly adhered to the surface of the lithium cobalt oxide particles, it is preferable because the additive element can be easily distributed or diffused uniformly in the surface layer portion 10a of the composite oxide after heating.

[0179] <Step S31> 6, lithium cobalt oxide and an Al source are mixed together. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the Al source is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably Co:Mg=100:y (0.3≦y≦3).

[0180] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than those in step S12. Also, it can be said that dry mixing conditions are milder than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as media, for example.

[0181] In this embodiment, the materials may be mixed in a dry manner using a ball mill with zirconium oxide balls having a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of -100°C or higher and -10°C or lower.

[0182] In mixing the lithium cobalt oxide and the Al source in step S31, a surface treatment, precision mixing, or a composite treatment such as spheroidization may be used.

[0183] In the composite process, pressure and shear force are applied to a mixture of two or more raw materials, and a composite is produced in which one raw material is fused to the surface of the other raw material, that is, a composite in which the raw materials are bonded to each other.

[0184] A typical device for the compounding process is a Picoline (manufactured by Hosokawa Micron) with a Nobilta rotor. The stirring speed is preferably between 2000 rpm and 4000 rpm. The stirring time is preferably between 5 minutes and 1 hour. Furthermore, it is preferable to use cooling water during the compounding process to suppress heat generation in the stirring area. The compounding process is preferably carried out in a dry room with a dew point between -100°C and -10°C.

[0185] <Step S32> 6, the mixed materials are collected to obtain a mixture 901. When collecting the materials, they may be crushed and then sieved, if necessary.

[0186] <Step S33> Next, in step S33 shown in FIG. 6, the mixture 901 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may be higher than atmospheric pressure in order to increase the oxygen partial pressure in the heating atmosphere. This is because if the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and the like may be reduced, and the layered rock salt type crystal structure of lithium cobalt oxide and the like may not be able to be maintained.

[0187] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds is preferably the temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example to explain this, but the melting temperature T m 0.757 times (Tanman temperature T d ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 is preferably 650° C. or higher.

[0188] Of course, the reaction proceeds more easily if the temperature is equal to or higher than the melting point of one or more of the materials contained in mixture 901. For example, when LiF and MgF2 are contained as the additive element sources, the eutectic point of LiF and MgF2 is around 742°C, so the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0189] In addition, the mixture 903 obtained by mixing LiCoO2:LiF:MgF2=100:0.33:1 (molar ratio) had an initial melting temperature T im is 779℃, the melting peak temperature T pm is 815℃, and the melting end temperature T em The lower limit of the heating temperature was 826°C. Therefore, the lower limit of the heating temperature is more preferably 826°C or higher.

[0190] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0191] The upper limit of the heating temperature is below the melting point of lithium cobalt oxide (1130°C). At temperatures near the melting point, decomposition of lithium cobalt oxide, albeit slight, is a concern. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride, originating from the fluorine source, within an appropriate range. If the temperature is too high, fluoride decreases due to evaporation. For example, the vapor pressure of lithium fluoride increases rapidly from 900°C. Therefore, a temperature of 1000°C or less is preferable, 950°C or less is even more preferable, 900°C or less is even more preferable, and 850°C or less is even more preferable. Suppressing the evaporation of lithium fluoride allows the surface layer 10a to have high concentrations of fluorine and lithium. Having a sufficient amount of lithium in the surface layer 10a also has the advantage of making it less likely for a different phase (e.g., MgTiO) to form when titanium is added in a later process.

[0192] Considering these, the heating temperature in step S33 is preferably 650°C to 1130°C, more preferably 650°C to 1000°C, even more preferably 650°C to 950°C, and even more preferably 650°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 826°C to 1100°C, or 826°C to 1130°C, more preferably 826°C to 1000°C, even more preferably 826°C to 950°C, and even more preferably 826°C to 900°C. The heating temperature in step S33 is preferably lower than that in step S13.

[0193] Furthermore, when the mixture 901 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0194] In the fabrication method described in this embodiment, some materials, such as LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be lowered below the melting point of lithium cobalt oxide, for example, to 742°C or higher and 950°C or lower, and allows additive elements such as magnesium to be distributed in the surface layer, thereby producing a positive electrode active material with excellent characteristics.

[0195] However, because LiF has a lower specific gravity in its gaseous state than oxygen, it may volatilize or sublime when heated. If it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, the Li on the LiCoO2 surface may react with the F fluorine source, producing LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress the volatilization.

[0196] Therefore, it is preferable to heat the mixture 901 in an atmosphere containing LiF, that is, to heat the mixture 901 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, the volatilization of LiF in the mixture 901 can be suppressed.

[0197] The heating in this step is preferably performed so as not to cause the particles of the mixture 901 to stick together. If the particles of the mixture 901 stick together during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the added elements (for example, fluorine) diffuse is blocked, which may result in poor distribution of the added elements (for example, magnesium and fluorine) in the surface layer.

[0198] It is also believed that uniform distribution of an additive element (e.g., fluorine) in the surface layer portion results in a smooth, less uneven cathode active material. Therefore, in this step, to maintain or further smooth the surface, it is better for the particles of mixture 901 not to stick together.

[0199] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0200] When heating is performed using a roller hearth kiln, the mixture 901 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 901.

[0201] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of the lithium cobalt oxide in step S14, etc. When the lithium cobalt oxide is small, a lower temperature or a shorter heating time may be preferable than when the lithium cobalt oxide is large.

[0202] 6, when the median diameter (D50) of the lithium cobalt oxide is about 12 μm, the heating temperature is preferably, for example, 650° C. to 950° C. The heating time is preferably, for example, 3 hours to 60 hours, more preferably 10 hours to 30 hours, and even more preferably about 20 hours. The temperature reduction time after heating is preferably, for example, 10 hours to 50 hours.

[0203] On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 7 μm, the heating temperature is preferably, for example, 650° C. to 950° C. The heating time is preferably, for example, 1 hour to 10 hours, more preferably about 5 hours. The temperature reduction time after heating is preferably, for example, 10 hours to 50 hours.

[0204] <Step S34> Next, in step S34 shown in FIG. 6, the heated material is recovered and crushed as necessary to obtain a composite oxide 902.

[0205] <Step S40> Next, in step S40 shown in FIG. 6, an additive element source (A2 source) is prepared. The additive element A2 can be the additive element described in step S21. In the method for producing a positive electrode active material described in FIGS. 6 to 7(C), nickel and aluminum are used as the additive element A2. Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source. As shown in steps S41 to S43 of FIG. 7(B), the nickel source and the aluminum source can be pulverized to obtain the A2 source. The pulverization conditions can refer to the conditions in step S22.

[0206] <Step S51> 6, the composite oxide 902 is mixed with the A2 source. The mixing conditions can be found in the description of step S31.

[0207] <Step S52> 6, the mixed materials are recovered to obtain a mixture 903. When recovering the materials, they may be crushed and then sieved, if necessary.

[0208] <Step S53> 6, the mixture 903 is heated. The heating conditions can be found in the description of step S33.

[0209] <Step S54> Next, in step S54 shown in FIG. 6, the heated material is recovered and crushed as necessary to obtain a composite oxide 904.

[0210] <Step S60> Next, in step S60 shown in FIG. 6, an additive element source (A3 source) is prepared. The additive element A3 can be the additive element described in step S21. In the method for producing a positive electrode active material described in FIGS. 6 to 7(C), titanium is used as the additive element A3. Lithium titanate, titanium oxide, titanium hydroxide, etc. can be used as the titanium source. As shown in steps S61 to S63 of FIG. 7(C), the titanium source can be pulverized to obtain the A3 source. The pulverization conditions can refer to the conditions in step S22.

[0211] <Step S71> 6, the composite oxide 904 is mixed with the A3 source. The description of step S31 can be referred to for the mixing conditions.

[0212] <Step S72> 6, the mixed materials are recovered to obtain a mixture 905. When recovering the materials, they may be crushed and then sieved, if necessary.

[0213] <Step S73> 6, the mixture 905 is heated. For the heating conditions, see the description of step S33.

[0214] <Step S74> Next, in step S74 shown in FIG. 6, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 10. At this time, it is preferable to further sieve the recovered particles. Through the above steps, the positive electrode active material 10 according to one embodiment of the present invention can be produced. The positive electrode active material according to one embodiment of the present invention has a smooth surface.

[0215] Positive electrode active material 10 with a smooth surface may be more resistant to physical destruction due to pressure, etc. than positive electrode active material that does not have a smooth surface. For example, positive electrode active material 10 is less likely to be destroyed in a test involving pressure, such as a nail penetration test, and as a result, safety may be improved.

[0216] [Initial heating] In the above-described manufacturing method, it may be more preferable to perform heating after synthesizing lithium cobalt oxide and before mixing the additive element. This heating is called initial heating.

[0217] The initial heating causes lithium to be released from a part of the surface layer 10a of the lithium cobalt oxide, resulting in a more favorable distribution of the additive elements.

[0218] More specifically, it is believed that initial heating facilitates different distributions depending on the additive element through the following mechanism. First, lithium is released from part of the surface layer 10a by initial heating. Next, this lithium-deficient lithium cobalt oxide having the surface layer 10a is mixed with an additive element source, such as a nickel source, an aluminum source, or a magnesium source, and the mixture is heated. Of the additive elements, magnesium is a divalent typical element, and nickel is a transition metal, but it easily becomes a divalent ion. Therefore, Mg is released from part of the surface layer 10a. 2+ and Ni 2+ and Co reduced by lithium deficiency. 2+ However, since this phase is formed only in a part of the surface layer portion 10a, it may not be clearly observed in an electron microscope image such as a STEM or an electron beam diffraction pattern.

[0219] Of the additive elements, nickel is easily dissolved in lithium cobalt oxide having a layered rock-salt crystal structure when the positive electrode active material 10 is made of lithium cobalt oxide and diffuses into the interior of the positive electrode active material 10. However, when a portion of the surface layer of the positive electrode active material 10 has a rock-salt crystal structure, nickel tends to remain in the surface layer 10a. This allows nickel and other divalent additive elements to remain in the surface layer of the positive electrode active material 10. It is preferable that the concentration of nickel and other divalent additive elements be higher in the surfaces of the positive electrode active material 10 other than the (001) orientation and in the surface layer including the surfaces than the interior.

[0220] Furthermore, in these rock salt structures, the bond distance between metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt structure.

[0221] For example, rock salt Ni 0.5 Mg 0.5 The Me-O distance in the rock salt type MgO is 2.09 Å, and the Me-O distance in the rock salt type MgO is 2.11 Å. Even if a spinel type phase is formed in a part of the surface layer portion 10a, the Me-O distance in the spinel type NiAl2O4 is 2.0125 Å, and the Me-O distance in the spinel type MgAl2O4 is 2.02 Å. In both cases, the Me-O distance exceeds 2 Å. Note that 1 Å = 10 -10 m.

[0222] On the other hand, in layered rocksalt structures, the bond distances between metals other than lithium and oxygen are shorter than those mentioned above. For example, the Al-O distance in layered rocksalt LiAlO2 is 1.905 Å (Li-O distance is 2.11 Å). Also, the Co-O distance in layered rocksalt LiCoO2 is 1.9224 Å (Li-O distance is 2.0916 Å).

[0223] According to Shannon's ionic radii, the ionic radius of hexacoordinated aluminum is 0.535 Å, and the ionic radius of hexacoordinated oxygen is 1.4 Å, the sum of which is 1.935 Å.

[0224] From the above, it is considered that aluminum exists more stably in non-lithium sites in the layered rock-salt structure than in the rock-salt structure, and therefore aluminum is more likely to be distributed in the deeper region having the layered rock-salt structure and / or the interior 10b than in the region close to the surface having the rock-salt structure in the surface layer portion 10a.

[0225] Furthermore, the initial heating is expected to have the effect of increasing the crystallinity of the layered rock salt type crystal structure of the inner portion 10b.

[0226] However, initial heating is not necessarily required. In other heating processes, such as annealing, the atmosphere, temperature, time, etc. can be controlled to achieve Li x When x in CoO2 is small, a positive electrode active material 10 having an O3' type can sometimes be produced.

[0227] The characteristics of the positive electrode active material 10 produced through the above-described steps will be described with reference to FIGS. 8(A) to 14(F).

[0228] <Cathode active material 10> 8(A) and 8(B) are cross-sectional views of a positive electrode active material 10 according to one embodiment of the present invention. Enlarged views of the vicinity of AB in FIG. 8(B) are shown in FIGS. 9(A) to 9(C). Enlarged views of the vicinity of CD in FIG. 8(B) are shown in FIGS. 9(D) to 9(F).

[0229] 8(A), the positive electrode active material 10 has a surface layer portion 10a and an interior portion 10b. In FIGS. 8(A) and 8(B), a dashed line indicates an example of the boundary between the surface layer portion 10a and the interior portion 10b.

[0230] The surface layer 10a of the positive electrode active material 10 refers to, for example, a region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface to the interior. Note that "approximately perpendicular" refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 10a is synonymous with the near-surface, near-surface region, or shell.

[0231] The region of the positive electrode active material deeper than the surface layer portion 10a is referred to as the inner portion 10b, which is synonymous with the inner region or core.

[0232] Furthermore, when the positive electrode active material 10 has a layered rock-salt crystal structure of space group R-3m, the surface layer portion 10a has an edge region 10a1 and a basal region 10a2, as shown in FIG. 8(B). In FIGS. 8(A) and 8(B), the line labeled (00l) represents the (00l) plane. Here, the edge region 10a1 is a region intersecting with the (00l) plane, and is defined as a region extending from the surface of the edge region 10a1 to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or substantially perpendicular to the surface. Note that "intersecting with the (00l) plane" means that the angle between the perpendicular to the (00l) plane and the normal to the surface of the positive electrode active material 10 is between 10 and 90 degrees, more preferably between 30 and 90 degrees.

[0233] Basal region 10a2 has a surface parallel to the (00l) plane, and the region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface to the interior is referred to as basal region 10a2. Note that "parallel to the (00l) plane" means that the angle between the perpendicular to the (00l) plane and the normal to the surface of positive electrode active material 10 is 0 to 5 degrees, more preferably 0 to 2.5 degrees.

[0234] The surface of the positive electrode active material 10 refers to the surface of the composite oxide including the surface layer 10a and the interior 10b. Therefore, the positive electrode active material 10 does not include metal oxides attached to the surface, such as aluminum oxide (Al2O3) that does not have lithium sites that can contribute to charge and discharge, carbonates chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. Note that the attached metal oxide refers to, for example, metal oxides whose crystal orientation does not match that of the interior 10b.

[0235] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM images, ABF-STEM images, electron beam diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and STEM images, etc. XRD, neutron beam diffraction, etc. can also be used as materials for determination.

[0236] It also does not include the electrolyte solution, decomposition products of the electrolyte solution, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 10.

[0237] Since the positive electrode active material 10 is a compound 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 oxygen is present and the region where oxygen is not present is defined as the surface of the positive electrode active material. Surfaces created by slips, cracks, and / or fissures may also be considered the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multi-layer film of carbon, metal, oxide, resin, etc.

[0238] <Contained elements> The positive electrode active material 10 contains lithium, cobalt, oxygen, and an additive element A. The positive electrode active material 10 may also contain lithium cobalt oxide (LiCoO) to which the additive element A has been added. However, the positive electrode active material 10 of one embodiment of the present invention preferably has a crystal structure described below, and the composition of the lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0239] 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 extracted. The positive electrode active material 10 of one embodiment of the present invention preferably uses cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or more selected from nickel and manganese may also be used. It is preferable that the positive electrode active material 10 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals contained therein, because of many advantages such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0240] Furthermore, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metals in the positive electrode active material 10, the Li content is higher than that of a composite oxide in which nickel accounts for the majority of the transition metals, such as lithium nickel oxide (LiNiO). x CoO2 exhibits superior stability when x is small. This is thought to be because the Jahn-Teller effect has a smaller effect on distortion in cobalt than in nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the transition metal's d orbital. Layered rock-salt composite oxides, such as lithium nickelate, in which octahedral low-spin nickel(III) occupies the majority of the transition metal, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This raises concerns about the collapse of the crystal structure during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions, closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickelate, in which nickel occupies the majority of the transition metal, are prone to cation mixing between nickel and lithium.

[0241] The additive element A contained in the positive electrode active material 10 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium.

[0242] That is, the positive electrode active material 10 can be one or more of lithium cobalt oxide containing magnesium, lithium cobalt oxide containing magnesium and aluminum, lithium cobalt oxide containing magnesium and nickel, lithium cobalt oxide containing magnesium, aluminum, and nickel, lithium cobalt oxide containing magnesium and fluorine, lithium cobalt oxide containing magnesium, fluorine, and nickel, lithium cobalt oxide containing magnesium, fluorine, nickel, and aluminum, and the like.

[0243] It can also be said that the positive electrode active material 10 can be one or more of a positive electrode active material having cobalt, oxygen, and magnesium, a positive electrode active material having cobalt, oxygen, magnesium, and aluminum, a positive electrode active material having cobalt, oxygen, magnesium, and nickel, a positive electrode active material having cobalt, oxygen, magnesium, aluminum, and nickel, a positive electrode active material having cobalt, oxygen, magnesium, and fluorine, a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and aluminum, a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and nickel, and a positive electrode active material having cobalt, oxygen, magnesium, fluorine, nickel, and aluminum.

[0244] The additive element A is preferably present as a solid solution in the positive electrode active material 10. For example, when performing a line analysis using STEM-EDX, the rising position at which the additive element A is detected in the depth direction is preferably located deeper than the rising position at which the transition metal M is detected, i.e., closer to the interior of the positive electrode active material 10.

[0245] These additional elements A further stabilize the crystal structure of the positive electrode active material 10, as will be described later.

[0246] The additional element A does not necessarily have to include magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, or beryllium.

[0247] For example, if the cathode active material 10 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, as described above, will be enhanced. The weight of manganese contained in the cathode active material 10 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0248] 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 interior 10b. In addition, the atoms on the surface of the positive electrode active material 10 in the surface layer 10a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 10a is likely to become unstable, and can be said to be a region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 10a can be made sufficiently stable, Li x Even when x in CoO2 is small, for example, 0.24 or less, the layered structure of the inner 10b composed of cobalt and oxygen octahedra can be made less likely to break.Furthermore, the displacement of the inner 10b composed of cobalt and oxygen octahedra can be suppressed.

[0249] To give the surface layer portion 10a a stable crystal structure, the surface layer portion 10a preferably contains an additional element A, and more preferably contains a plurality of additional elements A. Furthermore, the surface layer portion 10a preferably has a higher concentration of one or more selected from the additional elements A than the interior portion 10b. Furthermore, the one or more selected from the additional elements A contained in the positive electrode active material 10 preferably have a concentration gradient. Furthermore, it is more preferable that the distribution of the positive electrode active material 10 differs depending on the additional element A. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additional element A. Here, the concentration peak refers to the maximum value of the concentration in the surface layer portion 10a or within 50 nm from the surface.

[0250] [distribution] The distribution of the additive element A will be described. Figures 9(A) to 9(C) are diagrams illustrating the edge region 10a1 of the positive electrode active material 10. Figures 9(D) to 9(F) are diagrams illustrating the basal region 10a2 of the positive electrode active material 10.

[0251] For example, some of the additive elements A, such as magnesium, fluorine, silicon, phosphorus, titanium, boron, and calcium, preferably have a concentration gradient that increases from the interior 10b toward the surface, as shown by the gradation in Figures 9(A) and 9(D). The additive element A having such a concentration gradient will be referred to as additive element X. Although additive element X often corresponds to additive element A1, it does not necessarily have to correspond to additive element A1. The concentration gradient shown by the gradation in Figures 9(A) and 9(D) can be achieved depending on the diffusion rate rather than the timing of addition.

[0252] Another additive element A, such as aluminum or manganese, preferably has a concentration gradient and a concentration peak in a region deeper than the additive element X shown in FIGS. 9(A) and 9(D), as shown by the hatch density in FIGS. 9(B) and 9(E). The concentration peak may be present in the surface layer portion 10a or may be deeper than the surface layer portion 10a. For example, it is preferable that the peak be present in a region of 5 nm to 30 nm from the surface toward the interior. An additive element having such a concentration gradient will be referred to as additive element Y. Although additive element Y often corresponds to additive element A2, it does not necessarily correspond to additive element A2. As shown by the hatch density in FIGS. 9(B) and 9(E), depending on the diffusion rate rather than the timing of addition, a concentration gradient is formed such that the concentration is higher at the boundary between the surface layer portion 10a and the interior portion 10b.

[0253] As shown by the presence or absence of hatching and the density of the hatching in FIGS. 9(C) and 9(F), other additive elements, such as nickel and barium, may be clearly present in the edge region 10a1 but substantially absent in the basal region 10a2. Here, "clearly present" refers to a case in which the characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 10. "Substantially absent" refers to a case in which the characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 10. This also refers to the element being below the detection limit in STEM-EDX analysis. An additive element having such a distribution is referred to as additive element Z. While additive element Z often corresponds to additive element A2, it does not necessarily correspond to additive element A2. The concentration gradient shown by the density of the hatching in Figures 9(B) and 9(E) is determined depending on the diffusion rate rather than the timing of addition.

[0254] For example, magnesium ions, which are one of the added elements X, are 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 10a makes it easier to maintain the layered rock salt crystal structure. This is presumably because the magnesium present at the lithium site functions as a pillar supporting the CoO2 layers. In addition, the presence of magnesium makes it easier to maintain the Li x When x in CoO2 is, for example, 0.24 or less, the desorption of oxygen from around magnesium can be suppressed. The presence of magnesium is also expected to increase the density of the positive electrode active material 10. Furthermore, a high magnesium concentration in the surface layer portion 10a is also expected to improve corrosion resistance to hydrogen fluoride produced by decomposition of the electrolyte.

[0255] 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, it may reduce the effect of stabilizing the crystal structure. This is thought to be due to magnesium occupying the cobalt site in addition to the lithium site. In addition, excess magnesium compounds (e.g., oxides or fluorides) that do not substitute for either the lithium or cobalt site may segregate on the surface of the positive electrode active material and become a resistance component in the secondary battery. 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 site, reducing the amount of lithium contributing to charging and discharging.

[0256] Therefore, it is preferable that the total amount of magnesium contained in the 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 positive electrode active material 10 as a whole 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 used in the production of the positive electrode active material 10.

[0257] 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. Aluminum also suppresses the elution of surrounding cobalt, improving cycle performance. Furthermore, because the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as the additive element Y can improve the safety of the positive electrode active material 10 when used in a secondary battery. Furthermore, the positive electrode active material 10 can be made to have a crystal structure that is less likely to collapse even after repeated charging and discharging.

[0258] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0259] Therefore, it is preferable that the total amount of aluminum contained in the cathode active material 10 is appropriate. For example, the number of aluminum atoms contained in the entire cathode 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%. The above numerical range is preferably 0.05% to 2%. The above numerical range is preferably 0.1% to 4%. The amount contained in the entire cathode active material 10 referred to here may be, for example, a value obtained by performing elemental analysis of the entire cathode 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 cathode active material 10.

[0260] Nickel, which is one of the additive elements Z, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, its redox potential is lower than that of cobalt, which leads to an increase in discharge capacity, which is preferable.

[0261] Furthermore, when nickel is present at the lithium site, it can suppress the shift in the layered structure consisting of octahedra of cobalt and oxygen. It also suppresses the volume change during charging and discharging. It also increases the elastic modulus, meaning the battery becomes harder. This is presumably because the nickel present at the lithium site also functions as a pillar supporting the CoO2 layers. Therefore, it is expected that the crystal structure will be more stable, especially at high temperatures, such as 45°C or higher, during charging.

[0262] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also have a negative effect on lithium insertion and extraction.

[0263] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 10 is appropriate. For example, the number of nickel atoms in the positive electrode active material 10 is preferably greater than 0% and less than or equal to 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%. Furthermore, the above numerical range is preferably greater than 0% and less than or equal to 4%. Furthermore, the above numerical range is preferably greater than 0% and less than or equal to 2%. Furthermore, the above numerical range is preferably 0.05% to 7.5%. Furthermore, the above numerical range is preferably 0.05% to 2%. Furthermore, the above numerical range is preferably 0.1% to 7.5%. Furthermore, the above numerical range 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 raw material composition during the production process of the positive electrode active material.

[0264] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When some of the oxygen atoms in the surface layer portion 10a are 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 redox potentials. Therefore, when some of the oxygen atoms in the surface layer portion 10a of the positive electrode active material 10 are substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine atoms occurs smoothly. Therefore, when the positive electrode active material 10 is used in a secondary battery, the 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 hydrogen fluoride. Furthermore, as described in the first embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than that of the other additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source.

[0265] Furthermore, as shown in Figures 9(A) and 9(C), when the surface layer 10a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. x Even when x in CoO2 is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 10a.

[0266] Furthermore, the presence of additive elements with different distributions, such as additive element X, additive element Y, and additive element Z, is preferable because it stabilizes the crystal structure over a wider region. For example, when positive electrode active material 10 contains magnesium, which is one of the additive elements X, aluminum, which is one of the additive elements Y, and nickel, which is one of the additive elements Z, it can stabilize the crystal structure over a wider region than when it contains only one or two of additive elements X, Y, and Z. In this way, when positive electrode active material 10 contains additive element X, additive element Y, and additive element Z, additive element Y, such as aluminum, is not required on the surface because surface stabilization is sufficiently achieved by additive element X, such as magnesium, and additive element Z, such as nickel. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable for aluminum to be continuously detected in a region from the surface to a depth of 1 nm to 25 nm. This widespread distribution of aluminum is preferable because it stabilizes the crystal structure over a wider region.

[0267] 9(C) and 9(F), the additive element Z is preferably contained in a larger amount in the edge region 10a1 than in the basal region 10a2 (also referred to as being preferentially contained or selectively contained), because this improves the stability of the crystal structure of the edge region 10a1, where lithium ions enter and exit the positive electrode active material 10 during charging and discharging of the lithium-ion secondary battery. Furthermore, when the additive element Z has the above-described distribution, for example, when the positive electrode active material 10 is lithium cobalt oxide, this is preferable because it minimizes the effects of adding the additive element Z, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0268] When multiple additive elements are included as described above, the effects of each additive element are synergistic, which can contribute to further stabilization of the surface layer portion 10a. In particular, the inclusion of magnesium, nickel, and aluminum is highly effective in achieving a stable composition and crystal structure, and is therefore preferred. In particular, it is preferable that the surface layer portion 10a of the positive electrode active material 10 has a region where magnesium is distributed closer to the surface than aluminum. Furthermore, in addition to the region where magnesium and aluminum are distributed, it is most preferable that the surface layer portion 10a of the positive electrode active material 10 has a region where the nickel distribution and the magnesium distribution overlap in the edge region 10a1.

[0269] <Crystal structure> <Li x When x in CoO2 is 1> The positive electrode active material 10 according to one embodiment of the present invention is in a discharged state, i.e., Li x When x = 1 in CoO2, it preferably has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner part 10b, which occupies the majority of the volume of the positive electrode active material 10, has a layered rock-salt type crystal structure.

[0270] On the other hand, the surface layer portion 10a of the cathode active material 10 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 10b, which is made up of an octahedron of the transition metal M and oxygen, so that the layered structure formed by the octahedron of the transition metal M and oxygen is not broken even when lithium is released from the cathode active material 10 upon charging. Furthermore, the surface layer portion 10a preferably functions as a barrier film for the cathode active material 10. Furthermore, the surface layer portion 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 portion 10a and inner portion 10b of the cathode active material 10, such as oxygen release, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 10.

[0271] Therefore, it is preferable that the surface layer portion 10a has a different crystal structure from the interior portion 10b. It is also preferable that the surface layer portion 10a has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 10b. For example, it is preferable that at least a portion of the surface layer portion 10a of the positive electrode active material 10 of one embodiment of the present invention has a rock salt crystal structure. It is also preferable that the surface layer portion 10a has both a layered rock salt crystal structure and a rock salt crystal structure. It is also preferable that the surface layer portion 10a has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0272] Furthermore, although it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer portion 10a than in the interior portion 10b, they are also preferably present randomly and in a sparse manner in the interior portion 10b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior portion 10b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior portion 10b, it can also suppress the deviation of the layered structure consisting of octahedra of the transition metal M and oxygen, as described above. Furthermore, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, which is expected to have a synergistic effect of suppressing magnesium elution.

[0273] It is also preferable that the crystal structure continuously changes from the interior 10b toward the surface due to the concentration gradient of the added element A. It is also preferable that the crystal orientation of the surface layer portion 10a and the interior 10b roughly coincide.

[0274] For example, it is preferable that the crystal structure continuously change from the layered rock salt type interior 10b toward the surface and surface layer portion 10a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure. It is also preferable that the crystal orientation of the surface layer portion 10a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure is roughly the same as that of the layered rock salt type interior 10b.

[0275] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal M such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as vacancies of cations or anions may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted rock-salt crystal lattice, which may result in a lower symmetry than the rock-salt crystal structure.

[0276] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects are also acceptable.

[0277] The presence of both the layered rock salt type crystal structure and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0278] While the rock-salt type has no distinction between cation sites, the layered rock-salt type has two types of cation sites in its crystal structure: one is mostly occupied by lithium and the other by a transition metal (M). Both the rock-salt type and the layered rock-salt type share a layered structure, with alternating two-dimensional planes of cations and two-dimensional planes of anions. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, if the central spot (transmitted spot) is taken as the origin (000), the bright spot closest to the central spot would be, for example, the (111) plane in an ideal rock-salt type, and, for example, the (003) plane in a layered rock-salt type. For example, when comparing the electron diffraction patterns of rock-salt MgO and layered rock-salt LiCoO2, the bright spots on the (003) plane of LiCoO2 are observed at a distance approximately half the distance between the bright spots on the (111) plane of MgO. Therefore, if the analysis area contains two phases, for example, rock-salt MgO and layered rock-salt LiCoO2, the electron diffraction pattern will show plane orientations in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock-salt and layered rock-salt types will have strong brightness, while bright spots occurring only in the layered rock-salt type will have weak brightness.

[0279] Furthermore, when a layered rock-salt crystal structure is observed perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternating. This characteristic is not seen in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternating in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in part of the low-brightness layers, i.e., the lithium layers.

[0280] The anions in layered rock salt crystals and rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3' crystals described below also have a cubic close-packed structure. Therefore, when layered rock salt crystals and rock salt crystals come into contact, there are crystal faces where the cubic close-packed structure composed of anions is aligned.

[0281] It can also be explained as follows: Anions on the {111} plane of the cubic crystal structure have a triangular lattice. Layered rock salt has a space group of R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice of the cubic {111} plane has the same atomic arrangement as the hexagonal lattice of the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structure.

[0282] However, the space group of the layered rock salt type crystal and the O3' type crystal is R-3m, which is different from the space group Fm-3m of the rock salt type crystal (the space group of a general rock salt type crystal), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal and the O3' type crystal and the rock salt type crystal. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt type crystal, the O3' type, and the rock salt type crystal are aligned, it may be said that the crystal orientations are approximately the same.

[0283] <Li x When x in CoO2 is small> The positive electrode active material 10 of one embodiment of the present invention has the above-described distribution of the additive element A and / or the crystal structure in a discharged state, and therefore, x The crystal structure when x in CoO2 is small differs from conventional positive electrode active materials in that magnesium present in the lithium site maintains the R-3m layered rock salt type crystal structure. <x≦0.24をいうこととする。

[0284] Using Figs. 10 to 13, Li x The change in the crystal structure accompanying the change in x in CoO2 will be described by comparing a conventional positive electrode active material with a positive electrode active material 10 according to one embodiment of the present invention.

[0285] The change in the crystal structure of a conventional positive electrode active material is shown in Figure 11. The conventional positive electrode active material shown in Figure 11 is lithium cobalt oxide (LiCoO2) that does not contain any additional element A.

[0286] Figure 11 shows R-3m O3 and Li x This shows the crystal structure of lithium cobalt oxide with x=1 in CoO2. In this crystal structure, lithium occupies octahedral sites, and there are three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is coordinated with six oxygen atoms, and these structures are connected in a plane with edge sharing. This is sometimes called a layer consisting of octahedra of cobalt and oxygen.

[0287] Conventional lithium cobalt oxide is known to have a crystal structure that belongs to the monoclinic space group P2 / m, where the symmetry of lithium increases when x is around 0.5. This structure has one CoO2 layer in the unit cell. For this reason, it is sometimes called the O1 type or monoclinic O1 type.

[0288] When x = 0, the positive electrode active material has a trigonal space group P-3m1 crystal structure, with one CoO2 layer in each unit cell. Therefore, this crystal structure is sometimes called the O1 type or trigonal O1 type. The trigonal structure may also be converted to a composite hexagonal lattice, which is sometimes called the hexagonal O1 type.

[0289] Furthermore, conventional lithium cobalt oxide (LiCOO) with x = 0.12 or so has a crystal structure of the space group R-3m. This structure can be thought of as a structure in which a trigonal O1-type CoO2 structure and an R-3m O3-type LiCoO2 structure are alternately stacked. For this reason, this crystal structure is sometimes referred to as the H1-3-type crystal structure. In reality, the H1-3-type crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in Figure 11 and other parts of this specification, the c-axis of the H1-3-type crystal structure is shown as half the unit cell to facilitate comparison with other crystal structures.

[0290] As an example of an 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 oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, the unit cell with the smallest GOF (goodness of fit) value can be used.

[0291] Li x When conventional lithium cobalt oxide is repeatedly charged and discharged so that x in CoO2 becomes 0.24 or less, the crystal structure changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0292] However, these two crystal structures have a large misalignment of the CoO2 layers. As shown by the dotted lines and arrows in Figure 11, in the H1-3 crystal structure, the CoO2 layers are significantly misaligned from those in the R-3m O3 discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0293] Furthermore, the difference in volume between these two crystal structures is large: 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 exceeds 3.5%, typically 3.9% or more.

[0294] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers like the trigonal O1 type, is likely to be unstable.

[0295] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide breaks down. This break in the crystal structure causes a deterioration in cycle characteristics. This is because the number of sites where lithium can exist stably decreases and it becomes difficult for lithium to be inserted and extracted.

[0296] On the other hand, in the positive electrode active material 10 according to one embodiment of the present invention shown in FIG. x The change in the crystal structure between the discharge state where x in CoO2 is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the deviation of the CoO2 layer between the state where x is 1 and the state where x is 0.24 or less can be reduced. Furthermore, the change in volume compared per cobalt atom can be reduced. Therefore, the positive electrode active material 10 of one embodiment of the present invention is resistant to collapse of the crystal structure even when repeated charge and discharge in which x is 0.24 or less is performed, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 10 of one embodiment of the present invention has a low crystalline structure even when Li is charged and discharged. x When x in CoO2 is 0.24 or less, the positive electrode active material 10 according to one embodiment of the present invention can have a more stable crystal structure than conventional positive electrode active materials. x When x in CoO2 is kept at 0.24 or less, the safety of the secondary battery is further improved, which is preferable.

[0297] Li x 10 shows the crystal structure of the interior 10b of the positive electrode active material 10 when x in CoO2 is approximately 1 and 0.2. The interior 10b occupies the majority of the volume of the positive electrode active material 10 and is the part that contributes greatly to charge and discharge, so it can be said that the displacement of the CoO2 layer and changes in volume are the most problematic part.

[0298] When x=1, the positive electrode active material 10 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0299] However, when x is 0.24 or less, for example, about 0.2 or 0.12, which is the value at which conventional lithium cobalt oxide has an H1-3 type crystal structure, the positive electrode active material 10 has a crystal structure that is different from this.

[0300] When x is approximately 0.2, the positive electrode active material 10 of one embodiment of the present invention has a crystal structure belonging to the trigonal space group R-3m. This has the same symmetry as the CoO2 layer of O3. Therefore, this crystal structure is referred to as an O3'-type crystal structure. This crystal structure is shown in Figure 10, labeled R-3m O3'.

[0301] The O3' type crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), with the range of 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837(×10 -1 nm), and 2.807≦a≦2.827(×10 -1 nm) is more preferable, and typically a=2.817(×10 -1 nm). The c-axis is 13.681≦c≦13.881(×10 -1 nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 -1 nm).

[0302] In the O3' crystal structure, ions of cobalt, nickel, magnesium, etc. occupy six oxygen coordination positions, while light elements such as lithium may occupy four oxygen coordination positions.

[0303] As shown by the dotted line in Figure 10, there is almost no deviation in the CoO2 layer between the R-3m(O3) in the discharged state and the O3'-type crystal structure.

[0304] 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%.

[0305] As described above, in the positive electrode active material 10 according to one embodiment of the present invention, Li xWhen x in CoO2 is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the crystal structure of positive electrode active material 10 is resistant to collapse even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, positive electrode active material 10 has a large discharge capacity per weight and per volume. Therefore, by using positive electrode active material 10, secondary batteries with high discharge capacity per weight and per volume can be fabricated.

[0306] The positive electrode active material 10 is Li x It has been confirmed that when x in CoO2 is between 0.15 and 0.24, it may have an O3' type crystal structure, and it is also presumed that when x is between 0.24 and 0.27, it also has an O3' type crystal structure. However, the crystal structure is Li x Since it is affected not only by x in CoO2 but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it is not necessarily limited to the above range of x.

[0307] Therefore, the positive electrode active material 10 is Li x When x in CoO2 is greater than 0.1 and equal to or less than 0.24, the entire interior 10b of the positive electrode active material 10 does not have to have an O3'-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0308] Also Li x To make the x in CoO2 small, it is generally necessary to charge at a high charging voltage. x A state where x in CoO2 is small can be said to be a state where the battery is charged at a high charging voltage. For example, when a battery is charged at a constant current / constant voltage (CC / CV) at a voltage of 4.6 V or higher relative to the potential of lithium metal at 25°C, the H1-3 crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be said to be a high charging voltage.

[0309] Therefore, in other words, the positive electrode active material 10 of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it is 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.

[0310] In some cases, the H1-3 crystal structure is finally observed when the charge voltage is further increased, even in the positive electrode active material 10. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, at a charge voltage of 4.5 V or higher but lower than 4.6 V at 25°C, the positive electrode active material 10 of one embodiment of the present invention may be able to adopt the O3' crystal structure.

[0311] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the amount of the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystalline structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0312] In addition, in the O3' type crystal structure of FIG. 10, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, for example, in the monoclinic O1(Li 0.5 The lithium distribution can be analyzed by neutron diffraction, for example.

[0313] The O3' type crystal structure can also be said to be similar to the CdCl2 type crystal structure, although it has random lithium between the layers. This CdCl2 type-like crystal structure is similar to the CdCl2 type crystal structure, but the O3' type crystal structure has random lithium between the layers. 0.06Although the crystal structure is similar to that when charged to NiO2, it is known that pure lithium cobaltate or layered rock salt type positive electrode active materials containing a large amount of cobalt do not usually adopt a CdCl2 type crystal structure.

[0314] Furthermore, it is preferable that the concentration gradient of the additive element A is similar at multiple locations in the surface layer portion 10a of the positive electrode active material 10. In other words, it is preferable that the reinforcement derived from the additive element A is uniformly present in the surface layer portion 10a. Even if a portion of the surface layer portion 10a is reinforced, if there is a portion without reinforcement, stress may be concentrated in the unreinforced portion. If stress is concentrated in a portion of the positive electrode active material 10, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0315] However, the additive element A does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 10a of the positive electrode active material 10. It is preferable that the additive element A has the distribution of the additive element X shown in FIG. 9(A) in the edge region 10a1 and the distribution of the additive element Y shown in FIG. 9(E) in the basal region 10a2.

[0316] Here, the area near the CD has an R-3m layered rock-salt crystal structure, and the surface has a (00l) orientation. The (00l)-oriented surface may have a different distribution of the additional element A than the other surfaces. For example, the (00l)-oriented surface and its surface layer 10a may have a distribution of one or more concentration peaks selected from the additional element A limited to a shallower portion from the surface compared to surfaces other than the (00l)-oriented surface. Furthermore, the (00l)-oriented surface and its surface layer 10a may have a lower concentration of one or more elements selected from the additional element A compared to the other orientations. Furthermore, the (00l)-oriented surface and its surface layer 10a may have one or more elements selected from the additional element A below the lower detection limit.

[0317] In the layered rock-salt crystal structure of R-3m, cations are arranged parallel to the (00l) plane. This can be said to be a structure in which CoO2 layers and lithium layers are alternately stacked parallel to the (00l) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (00l) plane.

[0318] Since the CoO2 layer is relatively stable, the surface of the positive electrode active material 10 is more stable when oriented in the (00l) direction. The main diffusion path of lithium ions during charge and discharge is not exposed on the (00l) plane.

[0319] On the other hand, the diffusion paths of lithium ions are exposed on surfaces other than the (00l) orientation. Therefore, the surfaces and the surface layer portion 10a other than the (00l) orientation are important regions for maintaining the diffusion paths of lithium ions, and at the same time, they are prone to instability because they are the regions from which lithium ions are first desorbed. Therefore, reinforcing the surfaces and the surface layer portion 10a other than the (00l) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 10.

[0320] <Grain boundary> In addition to the distribution described above, the additional element A contained in the positive electrode active material 10 of one embodiment of the present invention is more preferably at least partially distributed unevenly in and near the grain boundaries, that is, present at a high concentration.

[0321] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of high-concentration and low-concentration regions.

[0322] For example, the magnesium concentration at and near the grain boundaries of the positive electrode active material 10 is preferably higher than that in other regions of the interior 10b. The fluorine concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 10b. The nickel concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 10b. The aluminum concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 10b.

[0323] Grain boundaries are a type of planar defect. Therefore, like surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of added element A at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0324] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material 10 of one embodiment of the present invention, the magnesium concentration and fluorine concentration become high near the cracked surface. Therefore, the corrosion resistance to hydrogen fluoride can be improved even after the cracks occur in the positive electrode active material.

[0325] <Analysis method> A certain positive electrode active material is Li x When x in CoO2 is small, whether the positive electrode active material 10 of one embodiment of the present invention has an O3'-type crystal structure or not can be determined by Li x This can be determined by analyzing a positive electrode having a positive electrode active material with a small x in CoO2 using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0326] XRD is particularly preferred in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, can compare the level of crystallinity and crystal orientation, can analyze lattice periodic distortion and crystallite size, and can obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 10b of the positive electrode active material 10, which occupies the majority of the volume of the positive electrode active material 10.

[0327] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the size.

[0328] As described above, the positive electrode active material 10 according to one embodiment of the present invention is Li xA characteristic of this material is that there is little change in the crystal structure when x in CoO2 is 1 and when it is 0.24 or less. When charged at high voltage, a positive electrode active material in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50 wt% or more is not suitable because it cannot withstand repeated high-voltage charging and discharging.

[0329] It should also be noted that simply adding an additional element A may not result in an O3'-type crystal structure. For example, even if lithium cobalt oxide with magnesium and fluorine or lithium cobalt oxide with magnesium and aluminum has something in common, depending on the concentration and distribution of the additional element A, the structure may be different from Li x There are cases where x in CoO2 is 0.24 or less and the O3' type crystal structure accounts for 60 wt% or more of the H1-3 type crystal structure and the O3' type crystal structure, and cases where the H1-3 type crystal structure accounts for 50 wt% or more of the H1-3 type crystal structure and the O3' type crystal structure.

[0330] Furthermore, even in the case of cathode active material 10 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be formed if x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the cathode active material is cathode active material 10 of one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0331] However, positive electrode active materials with a small x value may undergo a change in crystal structure when exposed to air. For example, the crystal structure may change from an O3'-type to an H1-3-type. Therefore, it is recommended that all samples used for crystal structure analysis be handled in an inert atmosphere such as an argon atmosphere.

[0332] Furthermore, whether the distribution of the additive element A in the positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, EPMA (Electron Probe Micro Analyzer), or the like.

[0333] The crystal structure, such as the grain boundaries, can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 10 .

[0334] <Charging method> To determine whether a composite oxide is the positive electrode active material 10 of one embodiment of the present invention, a coin cell (CR2032 type, 20 mm in diameter and 3.2 mm in height) can be fabricated using the composite oxide as a positive electrode and lithium metal as a counter electrode, and then charged. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can. The coin cell used to determine whether the composite oxide is the positive electrode active material 10 does not necessarily include the separator and electrolyte of one embodiment of the present invention.

[0335] More specifically, the positive electrode can be prepared by coating an aluminum foil positive electrode current collector with a slurry containing a positive electrode active material, a conductive material, and a binder. The term "slurry" used here refers to a liquid material used to form an active material layer on the positive electrode current collector, and includes an active material, a binder, and a solvent, and preferably further includes a conductive material.

[0336] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the voltage value of the secondary battery and the potential value of the positive electrode will differ. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0337] The electrolyte can be prepared as a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC) added to the mixed solvent. The lithium salt contained in the electrolyte can be 1 mol of lithium hexafluorophosphate (LiPF6) per liter of the mixed solvent containing vinylene carbonate.

[0338] The separator can be a 25 μm thick porous polypropylene film, but materials other than polypropylene may also be used for the separator.

[0339] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0340] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging using CC / CV, the CC charging current can be 20 mA / g or more and 100 mA / g or less. CV charging can be completed at 2 mA / g or more and 10 mA / g or less. Charging at such a low current value is desirable to observe the phase change of the positive electrode active material. The temperature is 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. When performing various subsequent analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed in a sealed container under an argon atmosphere. After the charging is completed, it is preferable to quickly remove the positive electrode and subject it to analysis, preferably within 1 hour, more preferably within 30 minutes after the charging is completed.

[0341] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, for example, charging can be performed by constant current charging at a current value of 20 mA / g or more and 100 mA / g or less up to a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V), followed by constant voltage charging until the current value reaches 2 mA / g or more and 10 mA / g or less, and discharging can be performed by constant current discharging at a current value of 20 mA / g or more and 100 mA / g or less down to 2.5 V. Alternatively, discharging can be performed by constant current discharging at a current value of 20 mA / g or more and 200 mA / g or less down to 3.0 V.

[0342] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed, for example, at 2.5 V and a current value of 20 mA / g or more and 200 mA / g or less, or at 3.0 V and a current value of 20 mA / g or more and 200 mA / g or less.

[0343] <xrd> The XRD measurement apparatus and conditions are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions: XRD apparatus: D8 ADVANCE manufactured by Bruker AXS, X-ray: CuKα1, output: 40 kV, 40 mA, slit width: Div. Slit, 0.5°, detector: LynxEye, scan method: 2θ / θ continuous scan, measurement range (2θ): 15° to 90°, step width (2θ): 0.01°, set counting time: 1 second / step, sample stage rotation: 15 rpm.

[0344] If the measurement sample is a powder, it can be set by placing it in a glass sample holder or by sprinkling the sample on a greased silicone non-reflective plate. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0345] The ideal powder XRD patterns calculated from the O3'-type crystal structure and H1-3-type crystal structure model using CuKα1 radiation are shown in Figures 12 and 13. For comparison, Li x The ideal XRD patterns calculated from the crystal structure of LiCoO2(O3) with x = 1 in CoO2 and trigonal O1 with x = 0 are also shown. The patterns of LiCoO2(O3) and CoO2(O1) were created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD. The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 = 1.540562 × 10 -10 m and λ2 were not set, and the monochromator was set to single. The XRD pattern of the H1-3 type crystal structure was created in the same manner as above, based on the information on the H1-3 type crystal structure shown in Figure 11. The XRD pattern of the O3' type crystal structure was estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, and fitting was performed using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker), and an XRD pattern was created in the same manner as the others.

[0346] As shown in FIG. 12, in the O3' type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less).

[0347] However, as shown in Figure 13, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. x When x in CoO2 is small, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less), which can be said to be a characteristic of the positive electrode active material 10 of one embodiment of the present invention.

[0348] 12, for example, when charging is performed with the upper limit of the charging voltage set to a voltage slightly lower than 4.60 V (4.56 V, 4.57 V, 4.58 V, or 4.59 V). For example, when charging is performed with the upper limit of the charging voltage set to 4.58 V, positive electrode active material 10 exhibits diffraction peaks at 2θ=18.85±0.20° and 2θ=45.15±0.10°, which are diffraction peaks attributable to the O3′-type crystal structure.

[0349] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures with x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures with x = 1 and x ≦ 0.24 that appear at 2θ angles of 42° to 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0350] The positive electrode active material 10 according to one embodiment of the present invention is Li x When x in CoO2 is small, the material has an O3'-type crystal structure, but not all of the material needs to have an O3'-type crystal structure. The cathode active material 10 of one embodiment of the present invention may contain other crystal structures, or may be partially amorphous. Furthermore, the plurality of cathode active materials 10 may contain cathode active materials having crystal structures other than the O3'-type crystal structure. However, when Rietveld analysis is performed on the XRD patterns, the proportion of the O3'-type crystal structure among the crystal structures of the plurality of cathode active materials 10 is preferably 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more. Excellent cycle characteristics can be exhibited when the proportion of the O3'-type crystal structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more.

[0351] Furthermore, even after 5 or more, 30 or more, 50 or more, or 100 or more charge / discharge cycles have been performed since the start of measurement, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3'-type crystal structure accounts for 35 wt% or more of the crystal structures possessed by the plurality of positive electrode active materials 10, more preferably 40 wt% or more, and even more preferably 43 wt% or more.

[0352] Furthermore, the sharpness of diffraction peaks in an XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., has a narrow half-width. 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 between 2θ = 43° and 46°, 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 that crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after charging.

[0353] In addition, the crystallite size of the O3'-type crystal structure of the positive electrode active material 10 is reduced to only about 1 / 20 of that of LiCoO2(O3) in the discharged state. x When x in CoO2 is small, a clear peak of the O3'-type crystal structure can be confirmed. On the other hand, with conventional LiCoO2, even if a portion of the material has a structure similar to the O3'-type crystal structure, the crystallite size becomes small and the peak becomes broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0354] <xps> In XPS (X-ray Photoelectron Spectroscopy), in the case of inorganic oxides, using monochromatic aluminum Kα rays as the X-rays allows analysis of a region from the surface to a depth of approximately 2 to 8 nm (usually 5 nm or less), making it possible to quantitatively analyze the concentration of each element in a region approximately half the depth of the surface layer 10a. Furthermore, narrow scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often approximately ±1 atomic %, and the lower detection limit is approximately 1 atomic %, depending on the element.

[0355] In the cathode active material 10 according to one embodiment of the present invention, the concentration of one or more selected from the additive elements A is preferably higher in the surface layer portion 10a than in the interior portion 10b. This is equivalent to saying that the concentration of one or more selected from the additive elements A in the surface layer portion 10a is preferably higher than the average concentration of the entire cathode active material 10. Therefore, for example, it can be said that the concentration of one or more selected from the additive elements A in the surface layer portion 10a measured by XPS or the like is preferably higher than the average concentration of the additive elements A in the entire cathode active material 10 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer portion 10a measured by XPS or the like is preferably higher than the average magnesium concentration in the entire cathode active material 10. Furthermore, the nickel concentration in at least a portion of the surface layer portion 10a is preferably higher than the average nickel concentration in the entire cathode active material 10. Furthermore, the aluminum concentration in at least a portion of the surface layer portion 10a is preferably higher than the average aluminum concentration in the entire cathode active material 10. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 10 a is higher than the average fluorine concentration in the entire positive electrode active material 10 .

[0356] The surface and surface layer 10a of the positive electrode active material 10 according to one embodiment of the present invention are assumed to be free of carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 10. The surface of the positive electrode active material 10 is also assumed to be free of electrolyte, binder, conductive material, and compounds derived therefrom that are attached to the surface of the positive electrode active material 10. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0357] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom adhering to the surface of the positive electrode active material. At this time, lithium may dissolve in the solvent used for washing, but even in this case, the added element A is unlikely to dissolve, and therefore the atomic ratio of the added element A is not affected.

[0358] The concentration of the additive element A may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances that are chemically adsorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 to 1.20, more preferably 0.500 to 1.00, even more preferably 0.500 to 0.900, and even more preferably 0.500 to 0.700.

[0359] Furthermore, the ratio of the number of atoms of nickel to cobalt (Ni / Co) as determined by, for example, XPS analysis is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, even more preferably 0.050 or more and 0.100 or less, and even more preferably 0.050 or more and 0.070 or less.

[0360] Furthermore, the ratio of the number of aluminum atoms to the number of cobalt atoms (Al / Co) as determined by XPS analysis is preferably 0.010 or more and 0.100 or less, more preferably 0.010 or more and 0.050 or less, and even more preferably 0.010 or more and 0.040 or less.

[0361] Furthermore, the ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg), as determined by XPS analysis, is preferably 0.100 or more and 1.00 or less, more preferably 0.100 or more and 0.800 or less, more preferably 0.100 or more and 0.500 or less, more preferably 0.100 or more and 0.300 or less, and more preferably 0.100 or more and 0.200 or less.

[0362] The above range indicates that the additive element A is not attached to a narrow area on the surface of the positive electrode active material 10, but is widely distributed at a preferred concentration in the surface layer portion 10a of the positive electrode active material 10. In other words, as a result of XPS analysis of the positive electrode active material 10, the above range indicates that the crystalline structure is not easily broken even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, good lithium insertion and extraction can be achieved in the positive electrode active material 10, and excellent rate characteristics can be achieved.

[0363] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. Furthermore, it is recommended to use an XPS apparatus with an energy resolution such that the half-width of the Ag3d5 / 2 peak (112 eV) in the XPS spectrum of an Ag sample is 1.0 eV±0.1 eV. The take-off angle can be, for example, 45°. For example, measurements can be performed using the following XPS apparatus and measurement conditions: Measurement equipment: PHI Quantera II X-ray: Monochromated Al Kα (1486.6 eV) Energy resolution: Half width of Ag3d5 / 2 peak is 1.0 eV ± 0.1 eV Detection area: 100 μmφ Detection depth: Approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: Wide scan, narrow scan for each detected element

[0364] When positive electrode active material 10 according to one embodiment of the present invention is analyzed by XPS, the peak (Mg1s peak) showing the bond energy between magnesium and other elements is preferably equal to or greater than 1303.0 eV and less than 1305.0 eV, and more preferably about 1304.0 eV, which is different from the bond energy of magnesium fluoride, 1306.0 eV, and is closer to the bond energy of magnesium oxide.

[0365] In the XPS analysis of the positive electrode active material 10 according to one embodiment of the present invention, the measured XPS spectrum is preferably corrected so that the C1s peak is aligned with the reference value (284.8 eV), i.e., the entire spectrum is preferably shifted, which can reduce the influence of differences in the XPS apparatus, differences in measurement conditions, etc. on the XPS measurement.

[0366] Furthermore, in an XPS analysis of the positive electrode active material 10 of one embodiment of the present invention, when the Mg1s peak is analyzed to analyze the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond, it is preferable that the positive electrode active material 10 has a peak component derived from the O-Mg-O bond. Furthermore, the positive electrode active material 10 may contain a peak component derived from the "O-Mg-F" bond, but the peak component preferably accounts for 30% or less of the total of the three peak components, more preferably 20% or less, even more preferably 20% or less, and preferably 10% or less. Furthermore, the positive electrode active material 10 may contain a peak component derived from the "F-Mg-F" bond, but the peak component preferably accounts for 10% or less of the total.

[0367] That is, when analyzing the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond in an XPS analysis of the positive electrode active material 10 of one embodiment of the present invention, the peak component derived from the "O-Mg-O" bond is preferably 70% or more, more preferably 80% or more, still more preferably 90% or more, and particularly preferably 100%.

[0368] A method for analyzing the Mg1s peak of an XPS spectrum in XPS analysis will now be described. In analyzing the Mg1s peak, it is preferable to define the peak component derived from the O-Mg-O bond as fit peak 1, the peak component derived from the O-Mg-F bond as fit peak 2, and the peak component derived from the F-Mg-F bond as fit peak 3, synthesize these three fit peaks, and calculate the area ratios of fit peak 1, fit peak 2, and fit peak 3 in the synthesized peak so that the difference from the Mg1s peak of the XPS spectrum obtained by XPS analysis is minimized. The analysis results can be output assuming that the area ratios of fit peak 1, fit peak 2, and fit peak 3 represent the proportions of O-Mg-O bonds, O-Mg-F bonds, and F-Mg-F bonds.

[0369] In the above-described XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also referred to as the peak top) of fit peak 1 can refer to the energy value at the maximum value of the Mg1s peak when measured separately using MgO-coated LiCoO2 as a standard sample. Furthermore, the energy value (Ep3) at the maximum value of fit peak 3 can refer to the energy value at the maximum value of the Mg1s peak when measured separately using magnesium fluoride (MgF2, Kojundo Chemical Research Institute MGH18XB, purity 99.9% (3N up)) as a standard sample. Furthermore, the energy value (Ep2) at the maximum value of fit peak 2 can be set to a value intermediate between Ep1 and Ep3. EP1 is located on the lower energy side compared to EP3. The energy value at the maximum value of a peak is also referred to as the peak position.

[0370] In an XPS analysis of the positive electrode active material 10 according to one embodiment of the present invention, the half width of the Mg1s peak is preferably 1.0 eV to 3.0 eV, more preferably 1.0 eV to 2.8 eV, and particularly preferably 1.0 eV to 2.6 eV. Note that the peak position of the Mg1s peak is located on the lower energy side than the maximum energy of the Mg1s peak measured separately using magnesium fluoride as a standard sample.

[0371] <edx> It is preferable that one or more selected from the additive elements A contained in the positive electrode active material 10 have a concentration gradient. When two or more additive elements A are used, it is more preferable that the depth from the surface of the concentration peak of each additive element A is different. For example, the concentration gradient, concentration peak, etc. of the additive element A can be evaluated by exposing a cross section of the positive electrode active material 10 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, EPMA (electron probe microanalysis), etc.

[0372] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area in two dimensions. Linear analysis is performed by scanning a line to evaluate the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also sometimes used to extract data from a linear area of ​​EDX area analysis. Point analysis is performed by measuring an area without scanning.

[0373] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the additive element A in the surface layer 10a, the interior 10b, and near the grain boundaries of the positive electrode active material 10. Furthermore, EDX line analysis can analyze the concentration distribution and maximum value of the additive element A. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by elements present in the depth direction of the sample.

[0374] Since the positive electrode active material 10 is a compound 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 oxygen is present and the region where oxygen is not present is defined as the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multi-layer film of carbon, metal, oxide, resin, etc.

[0375] The reference point in STEM-EDX ray analysis, etc., is the average value M of the detected amount (including intensity) of characteristic X-rays of the transition metal M1 inside the positive electrode active material. AVE The reference point is defined as the point where the detected amount of characteristic X-rays of the transition metal M1 to the left of the reference point is 50% of the detected amount of characteristic X-rays of the transition metal M1 in the background. When the left side of the reference point is described as the outside of the positive electrode active material and the right side of the reference point is described as the inside of the positive electrode active material, the reference point may be referred to as the surface position of the positive electrode active material. In addition, in STEM-EDX ray analysis, if the detected amount of characteristic X-rays of the transition metal M1 does not decrease sufficiently to the left of the reference point, the detected amount of characteristic X-rays of the transition metal M1 to the left of the reference point is called background, and the average value M of the detected amount of characteristic X-rays of the transition metal M1 in the background is used. BG and the average amount of transition metal M1 detected inside AVE The reference point may be the point where the sum of the detected amount of characteristic X-rays of oxygen inside the positive electrode active material is 50% of the sum of the detected amount of characteristic X-rays of the transition metal M1. The reference point can be determined by replacing the transition metal M1 with oxygen. However, since oxygen is an element that is easily affected by the outside of the positive electrode active material, the reference point is the average value M of the detected amount of characteristic X-rays of the transition metal M1. AVE It is preferable to calculate it from 50% of the average value M of the detected amount of characteristic X-rays of the transition metal M1. AVE 50% of the detected amount of oxygen characteristic X-rays, M AVE If the difference is not 50% of the value of the average M of the detected amount of characteristic X-rays of the transition metal M1, it is considered to be due to the influence of metal oxides and carbonates containing oxygen that adhere to the surface of the positive electrode active material. AVE In the case of a positive electrode active material having a plurality of transition metals M1, it is preferable to adopt the average value M of the transition metals with the largest amount of characteristic X-rays detected inside. AVE The reference point can be found using

[0376] The average detected amount of characteristic X-rays of the internal transition metal M1, M AVE can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 20 nm or more, preferably 30 nm or more, from the region where the detected amount of characteristic X-rays of the transition metal M1 saturates and stabilizes, for example, the region where the detected amount of characteristic X-rays of the transition metal M1 starts to increase. BG can be obtained by averaging the amount of detected characteristic X-rays of the transition metal M1 over a range of 2 nm or more, preferably 3 nm or more, outside the area where the amount of detected characteristic X-rays of the transition metal M1 begins to increase. AVE and the average amount of characteristic X-rays of oxygen detected in the background. BG can also be found in the same way.

[0377] Furthermore, the surface of the positive electrode active material 10 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystalline structure of the positive electrode active material is observed and an area where it is not observed, and is the outermost area of ​​an area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are confirmed.

[0378] In STEM-EDX analysis, a peak refers to a convex maximum value that appears in a graph of the characteristic X-ray intensity for each element, or the maximum value of the characteristic X-ray for each element. Note that noise in STEM-EDX analysis can be measured values ​​with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0379] In STEM-EDX analysis, a peak refers to a convex maximum value that appears in a graph of the characteristic X-ray intensity for each element, or the maximum value of the characteristic X-ray for each element. Note that noise in STEM-EDX analysis can be measured values ​​with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0380] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured after two scans can be used as the detection value for each element. The number of scans is not limited to two, and more scans can be performed, and the integrated value can be used as the detection value for each element.

[0381] STEM-EDX ray analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material. For example, carbon can be vapor-deposited using a carbon coating unit of an ion sputtering device (MC1000 manufactured by Hitachi High-Tech).

[0382] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using a FIB-SEM device (Hitachi High-Tech XVision200TBS). The pickup is performed using an MPS (micro-probing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0383] STEM-EDX analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) with an EDAX Octane T Ultra W (Dual EDS) EDX detector. For example, the conditions for EDX analysis using the Hitachi High-Tech HD-2700 are as follows: the acceleration voltage of the STEM device is set to 200 kV, the emission current is set to 6 μA or more and 10 μA or less, and a portion of the thinned sample with minimal depth and unevenness is measured. The magnification is, for example, approximately 150,000 times. The conditions for EDX analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and a frame count of 6 or more.

[0384] In order to increase the spatial resolution in STEM-EDX line analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX line analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, in order to increase the analytical sensitivity in STEM-EDX line analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used for STEM-EDX line analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0385] In addition, in the positive electrode active material 10 having magnesium and fluorine as the additive element A, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration or detection amount and the peak of the magnesium concentration or detection amount is preferably within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and even more preferably within 0.5 nm.

[0386] In addition, in a positive electrode active material 10 containing nickel as the additive element A, the peak of the nickel concentration or detectable amount in the surface layer 10a preferably exists at a depth of 3 nm, more preferably 1 nm, from the surface or reference point toward the center of the positive electrode active material 10. In addition, in a positive electrode active material 10 containing magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the nickel concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, more preferably within 1 nm.

[0387] Furthermore, when the positive electrode active material 10 contains aluminum as the additive element A, it is preferable that, upon EDX analysis, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 10a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 10a is located more inward than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material 10 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0388] Here, how to express the positional relationship of element distribution when EDX line analysis is performed will be explained using Figures 14(A) to 14(G). Figures 14(A) to 14(F) are schematic diagrams showing the concentration distribution or detection amount distribution of a first element e1 and a second element e2. Also, Figure 14(G) is a schematic diagram showing the concentration distribution or detection amount distribution of a first element e1, a second element e2, and a third element e3.

[0389] For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 14(A), the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 14(B), the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 14(C), the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 14(D), the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 14(E), the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 14(F), the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum.

[0390] The expression "having an overlapping region" will be explained using an example in which the concentration distributions or detection amount distributions of a first element e1, a second element e2, and a third element e3 have the positional relationship shown in Figure 14(G). In this specification and the like, "having an overlapping region" means, for example, that the position of maximum value in the concentration distribution or detection amount distribution of at least one element is located in a range in which the concentration or detection amount in the concentration distribution or detection amount distribution of the other element is 1 / 5 or more of the maximum value.

[0391] 14(G), the position (P2) where the second element e2 has a maximum value in its concentration distribution or detectable amount distribution is located in the range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution. Therefore, the first element e1 and the second element e2 have an overlapping distribution area. Furthermore, the position (P3) where the third element e3 has a maximum value in its concentration distribution or detectable amount distribution is not located in the range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution. Therefore, the first element e1 and the third element e3 do not have an overlapping distribution area.

[0392] 14(G), the distribution of the second element e2 and the distribution of the third element e3 are located more inward than the distribution of the first element e1. Also, the distribution of the second element e2 and the distribution of the third element e3 are located more inward than the distribution of the first element e1.

[0393] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0394] (Embodiment 3) In this embodiment, each of the elements constituting the secondary battery will be described.

[0395] [Positive electrode] The secondary battery has a positive electrode, which is as described in the first and second embodiments.

[0396] <Positive electrode current collector> The positive electrode has a positive electrode current collector. The positive electrode current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. A coating layer may be provided on the surface of the material. The material used for the positive electrode current collector is preferably one that 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 positive electrode 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 positive electrode current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. The thickness of the positive electrode current collector is preferably 5 μm to 30 μm.

[0397] <Binder> The positive electrode preferably contains a binder. Examples of the binder include rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluorine-containing rubber can also be used as the binder.

[0398] As the binder, it is preferable to use, for example, a water-soluble polymer. As the water-soluble polymer, for example, polysaccharides can be used. As the polysaccharide, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, etc., or starch can be used. Furthermore, it is more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0399] As the binder, it is preferable to use materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0400] The binder may be a combination of two or more of the above binders. For example, a material with a particularly excellent viscosity adjusting effect may be combined with another material. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as a water-soluble polymer with a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0401] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials or other components when preparing electrode slurry. In this specification, cellulose and cellulose derivatives used as electrode binders also include their salts.

[0402] Water-soluble polymers stabilize viscosity by dissolving in water, allowing active materials and other materials combined as binders, such as styrene-butadiene rubber, to be stably dispersed in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0403] When the binder covers the surface of the active material or contacts the surface and forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the secondary battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0404] <Conductive material> The positive electrode preferably contains a conductive material. The conductive material is also called a conductivity imparting agent or a conductivity aid, and is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the active material is electrically connected even when not in contact with each other.

[0405] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fiber such as carbon nanofiber and carbon nanotube, and graphene compound can be used.

[0406] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0407] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0408] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0409] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the secondary battery.

[0410] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the regions between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased and an excellent conductive path can be formed. A secondary battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and is stable, making it effective as an in-vehicle secondary battery.

[0411] [Negative electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, and may further include a conductive material and a binder.

[0412] <Negative electrode active material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0413] The negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, materials containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium 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, silicon is preferred as the negative electrode active material. Compounds containing these elements can also be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. 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.

[0414] In this specification, "SiO" refers to, for example, silicon monoxide. Alternatively, SiO may refer to SiO 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.

[0415] The carbon material may be graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, or the like.

[0416] 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 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.

[0417] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a low potential similar to that of metallic lithium (0.05 V to 0.3 V vs. Li / Li + ) This allows lithium-ion secondary batteries using graphite to exhibit high operating voltages. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0418] In addition, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten dioxide (WO2), molybdenum dioxide (MoO2), and other oxides can be used.

[0419] In addition, the negative electrode active material is a nitride of lithium and a transition metal, Li3N-type Li 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 (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0420] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, and therefore it is preferable to combine it with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0421] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 , sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorides such as FeF3 and BiF3.

[0422] Another example of a negative electrode is one that does not have a negative electrode active material at the end of the secondary battery fabrication. A negative electrode that does not have a negative electrode active material can be, for example, a negative electrode that has only a negative electrode current collector at the end of the secondary battery fabrication, in which lithium ions released from the positive electrode active material upon charging the secondary battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A secondary battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) secondary battery, a negative electrode-less (anode-less) secondary battery, or the like.

[0423] When a negative electrode without a negative electrode active material is used, a film for uniforming lithium deposition may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniforming lithium deposition. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as films for uniforming lithium deposition because they are relatively easy to form uniformly on the negative electrode current collector. Furthermore, for example, a metal film that forms an alloy with lithium can be used as the film for uniforming lithium deposition. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Lithium and magnesium form a solid solution over a wide composition range, making them suitable as films for uniforming lithium deposition.

[0424] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0425] 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.

[0426] <Negative electrode current collector> The negative electrode current collector is preferably made of a material that does not alloy with carrier ions such as lithium. Specifically, the negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper in addition to the same material. Alternatively, a structure in which a coating layer is provided on the surface of the above-mentioned material may be used.

[0427] <Method of manufacturing the negative electrode> The method for producing the negative electrode will be described with reference to FIG.

[0428] <Step S160> As shown in step S160, a negative electrode active material is prepared. Furthermore, in this step, a binder, a thickener, a conductive material, and a solvent are prepared. The binder, the thickener, and the conductive material are not limited, and the compounding ratios thereof are not limited. The proportion of the negative electrode active material in the negative electrode active material layer is 95 wt% or more and 99 wt% or less. Water can be used as the solvent.

[0429] <Steps S161 and S162> As shown in step S161, the prepared materials are mixed in a solvent to form a slurry as shown in step S162. The slurry here refers to a material liquid used to form an active material layer on a negative electrode current collector (described later), and refers to a mixture containing an active material, a binder, and a solvent, and preferably further containing a conductive material. Furthermore, as step S162, a negative electrode current collector is prepared.

[0430] <Steps S165 and S166> As shown in step S165, the slurry is applied to the negative electrode current collector, and as shown in step S166, the slurry is dried to remove the solvent.

[0431] <Step S167, Step S169> As shown in step S167, the negative electrode active material layer is pressed together with the negative electrode current collector, and as shown in step S169, the negative electrode is completed.

[0432] [Separator] The secondary battery has a separator, which is as described in the first embodiment and the like.

[0433] [Exterior body] The secondary battery has an exterior body, which is as described in the first embodiment and the like.

[0434] [Electrolyte] The secondary battery has an electrolyte solution containing carrier ions, as described in the first embodiment and the like.

[0435] [Nail penetration test] The nail penetration test is one of the tests to verify the safety of secondary batteries. The secondary battery is fully charged, and a nail with a specified diameter selected from 2 mm to 10 mm is thrust into the secondary battery at a specified speed.

[0436] <Nail penetration test device> First, the nail penetration test device will be described. FIG. 18(A) shows a side view of the nail penetration test device 1000, and FIG. 18(B) shows a perspective view of the stage of the nail penetration test device 1000. The nail penetration test device 1000 shown in FIG. 18(A) includes a stage 1001, a drive unit 1002, a nail 1003, a voltage measuring device 1015, a temperature measuring device 1016, and a control unit 1018. The drive unit 1002 includes a drive mechanism 1012 that moves the nail 1003 in the direction of the arrow in the figure. The drive mechanism 1012 operates so that the nail 1003 penetrates a secondary battery 1004 placed on the stage 1001. At this time, the secondary battery 1004 is in a fully charged state (State of Charge: a state equivalent to SOC 100%), and this operation is called a nail penetration operation. The dashed line in FIG. 18(A) indicates a recess in the stage 1001 that is provided to accommodate the nail 1003 after penetration during the nail insertion operation.

[0437] Information relating to the voltage of the secondary battery during the nail penetration operation is transmitted from the voltage measuring device 1015 to the control unit 1018. Specifically, the amount of voltage change and the like are transmitted to the control unit 1018. Furthermore, information relating to the temperature during the nail penetration operation is transmitted from the temperature measuring device 1016 to the control unit 1018. When controlling the operating conditions of the nail 1003, the control unit 1018 can transmit a control signal to the driving unit 1002.

[0438] FIG. 18(B) is a perspective view illustrating the vicinity of the upper portion of the stage 1001 of the nail penetration test device 1000. A secondary battery 1004 placed on the stage 1001 is electrically connected to wires 1005a and 1005b. The wires 1005a and 1005b belong to a voltage measuring device 1015, and the wires 1005a and 1005b are electrically connected to the positive and negative tabs of the secondary battery 1004, respectively, to measure the voltage of the secondary battery 1004. The voltage of the secondary battery 1004 is simply referred to as voltage, voltage value between the positive and negative electrodes, battery voltage, cell voltage, or open-circuit voltage. When a temperature sensor is used as the temperature measuring device 1016, the temperature sensor is provided so as to be in contact with the surface of the exterior body of the secondary battery 1004.

[0439] 18B shows an example in which a first temperature sensor 1006a and a second temperature sensor 1006b are arranged, but one or three or more temperature sensors may be arranged. In FIG. 18B, the first temperature sensor 1006a is arranged on a side where the wiring 1005a and the wiring 1005b are not arranged, and the second temperature sensor 1006b is arranged on a side where the wiring 1005a and the wiring 1005b are arranged. Arranging two or more temperature sensors is preferable because even if one temperature sensor becomes unusable due to expansion of the exterior body or the like, the other temperature sensors can be used.

[0440] Furthermore, there is a welded region on the side where the wiring 1005a and the wiring 1005b are arranged, but there is no welded region on the side where the wiring 1005a and the wiring 1005b are not arranged because the exterior body is folded back. Therefore, even if the exterior body expands, expansion on the side where the wiring 1005a and the wiring 1005b are not arranged is suppressed, and the first temperature sensor 1006a is more unlikely to peel off than the second temperature sensor 1006b, which is preferable.

[0441] The dashed ellipse shown in FIG. 18(B) is the area where the nail 1003 penetrates the secondary battery 1004 during the nail penetration operation. The first temperature sensor 1006a and the second temperature sensor 1006b should be provided in an area equidistant from the area where the nail 1003 penetrates. Typically, the first temperature sensor 1006a and the second temperature sensor 1006b should be provided within 5 cm, preferably within 2 cm, of the area where the nail 1003 penetrates. This is preferable because it allows for the detection of temperature changes in the vicinity of the area where the nail 1003 penetrates. When two or more temperature sensors are provided, it is preferable to start the nail penetration operation after confirming that the difference in temperature indicated by the temperature sensors is within ±5°C, preferably within ±2°C.

[0442] <Secondary battery in nail penetration test> Next, the state of the secondary battery in the nail penetration test will be described again with reference to FIG. 19 and other figures. The nail penetration test involves inserting a nail 1003 having a predetermined diameter (2 mm to 10 mm) into a fully charged secondary battery 1004 at a predetermined speed. FIG. 19 shows a cross-sectional view of the secondary battery 1004 with the nail 1003 inserted. The secondary battery 1004 has a structure in which a positive electrode 503, a separator 508, a negative electrode 506, and an electrolyte 530 are housed in an outer casing 541. The positive electrode 503 has a positive electrode current collector 501 and positive electrode active material layers 502 formed on both sides of the positive electrode current collector 501. It is preferable to apply the configuration described in the above embodiment to the positive electrode active material layer. The negative electrode 506 has a negative electrode current collector 521 and negative electrode active material layers 512 formed on both sides of the negative electrode current collector 521.

[0443] As shown in FIG. 19, when a nail 1003 is stuck into a secondary battery 1004, specifically, the nail 1003 penetrates the positive electrode 503 and the negative electrode 506, causing an internal short circuit. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode 506, and electrons (e - ) flows to the positive electrode 503, and Joule heat is generated at the internal short circuit point and its vicinity. In addition, due to the internal short circuit, carrier ions, typically lithium ions (Li + ) are released into the electrolyte as indicated by the white arrow. However, before all the lithium ions are released from the negative electrode, the battery temperature rises rapidly due to Joule heat generated by an internal short circuit, causing the electrolyte to begin to reduce and decompose on the surface of the negative electrode. This is an electrochemical reaction, and is called the reduction reaction of the electrolyte by the negative electrode.

[0444] Furthermore, if the temperature of the secondary battery 1004 rises due to Joule heat, when lithium cobalt oxide is used as the positive electrode active material, the lithium cobalt oxide may undergo a phase change (i.e., a structural change) to an H1-3 type crystal structure or an O1 type crystal structure, which may result in further heat generation.

[0445] The electrons (e - ), the tetravalent Co in the charged lithium cobalt oxide is reduced to trivalent or divalent Co. This reduction reaction releases oxygen from the lithium cobalt oxide, and the electrolyte 530 is decomposed by an oxidation reaction caused by the oxygen. This is an electrochemical reaction, and is called the oxidation reaction of the electrolyte by the positive electrode. The rate at which current flows into a positive electrode active material such as lithium cobalt oxide varies slightly depending on the insulating properties of the positive electrode active material, and it is thought that the rate at which current flows affects the electrochemical reaction.

[0446] As mentioned above, when an internal short circuit occurs in a secondary battery, the temperature is thought to change as shown in the graph in Figure 20. Figure 20 is a partially modified version of the graph shown on page 70 (Figures 2-12) of Non-Patent Document 1, showing the temperature (specifically, the internal temperature) of the secondary battery versus time. When an internal short circuit occurs at (P0), the temperature of the secondary battery rises over time. As shown in (P1), when the temperature of the secondary battery rises to around 100°C due to Joule heat generated by the internal short circuit, it may exceed the reference temperature (Ts), which is the limit temperature at which the secondary battery does not experience thermal runaway. Then, at (P2), the negative electrode (if graphite is used, the negative electrode becomes C6Li) reduces the electrolyte and generates heat. At (P3), the positive electrode oxidizes the electrolyte and generates heat. At (P4), heat is generated due to thermal decomposition of the electrolyte. The secondary battery then experiences thermal runaway and may catch fire.

[0447] At this time, the electrons that suddenly flow into the positive electrode active material cause a reaction in which cobalt is reduced from Co4 to Co2, releasing oxygen from the positive electrode active material. Because this reaction is exothermic, it accelerates thermal runaway. In other words, if this reaction can be suppressed, a safe secondary battery can be made that is less susceptible to thermal runaway.

[0448] In order to suppress the above reaction, for example, the surface layer of the positive electrode active material preferably contains an additive element X that does not easily release oxygen, and further, the concentration of the additive element X is preferably higher than that of the interior. If oxygen is not released from the positive electrode active material, the above reduction reaction (for example, the reaction in which Co4 becomes Co2) is also suppressed. Examples of additive elements X that do not easily release oxygen include magnesium and aluminum. Magnesium is suitable as an additive element X that does not easily release oxygen because the closer the oxygen is to the magnesium, the greater the energy required to desorb it. Nickel is also thought to have the effect of suppressing oxygen release when present at the lithium site.

[0449] Furthermore, even if cobalt or the like is reduced, if lithium ions can be inserted into the positive electrode active material before oxygen is released, electroneutrality is maintained and oxygen release does not occur. Therefore, even if electrons suddenly flow into the positive electrode active material, it is sufficient as long as the crystalline structure of the positive electrode active material is kept stable until the lithium ions are inserted into the positive electrode active material from the negative electrode via the electrolyte.

[0450] To prevent smoking, heat generation, and other issues during a nail penetration test, it is considered desirable to suppress temperature rise in the secondary battery and ensure that the anode, cathode, and / or electrolyte have stable properties at high temperatures. Specifically, it is preferable for the first cathode active material 10x to have a stable structure that does not release oxygen, especially when exposed to high temperatures. Alternatively, it is preferable for the first cathode active material 10x to have a structure that slows the rate of current flow to the cathode active material. This is expected to have significant effects, such as reducing thermal runaway and fire. As will be described later, the first cathode active material 10x, which is one embodiment of the present invention, can have both the stable structure and a structure that slows the rate of current flow.

[0451] <Thermal runaway of secondary batteries> Figure 21 shows the mechanism of thermal runaway in secondary batteries, partially revised from the graph shown on page 69 (Figures 2-11) of Non-Patent Document 1. When the temperature (specifically, the internal temperature) of a secondary battery described above rises during charging, it passes through several states before thermal runaway occurs. Figure 21 shows a graph of the temperature of a secondary battery over time. For example, when the temperature of the secondary battery reaches or approaches 100°C, (1) the SEI (Solid Electrolyte Interphase) of the negative electrode collapses and heat is generated. Furthermore, when the temperature of the secondary battery exceeds 100°C, (2) the negative electrode (when graphite is used, the negative electrode becomes C6Li) reduces the electrolyte and generates heat. (3) At or near 150°C, the positive electrode oxidizes the electrolyte and generates heat. Furthermore, when the temperature of the secondary battery reaches or near 180°C, (4) the electrolyte thermally decomposes, and (5) oxygen is released from the positive electrode and the positive electrode thermally decomposes (including structural changes in the positive electrode active material). If the temperature of the secondary battery subsequently exceeds 200°C, (6) decomposition of the negative electrode occurs, and finally, (7) direct contact between the positive and negative electrodes occurs. Through these states, particularly states (5), (6), or (7), the secondary battery reaches thermal runaway.

[0452] To prevent thermal runaway, it is considered effective to suppress the temperature rise of the secondary battery and to ensure that the separator, negative electrode, positive electrode and / or electrolyte have stable properties at high temperatures.

[0453] <Characteristics of secondary batteries in nail penetration tests> The characteristics of a secondary battery including a positive electrode having the positive electrode active material and separator described in the above embodiments and the like when a nail penetration test is performed will be described.

[0454] The temperature rise of the secondary battery during a nail penetration test, i.e., the difference between the temperature before the nail penetration test and the maximum temperature reached after the nail penetration (also referred to as the temperature rise ΔT), is preferably 100°C or less, more preferably 70°C or less, and even more preferably 50°C or less. The temperature is measured within 5 cm, preferably within 2 cm, of the nail hole, and specifically, the value is output by a temperature sensor placed within 5 cm, preferably within 2 cm, of the nail hole. The temperature sensor should be installed so as to be in contact with the exterior body of the secondary battery.

[0455] The maximum temperature during the nail penetration test is preferably 250° C. or lower, more preferably 200° C. or lower, and even more preferably 180° C. or lower, and more preferably lower than the temperature at which oxygen is released from the positive electrode and thermal decomposition of the positive electrode occurs.

[0456] Furthermore, the maximum temperature during the nail penetration test is preferably 150°C or lower, more preferably 100°C or lower, and even more preferably 80°C or lower. It is more preferable that the maximum temperature is lower than the temperature at which oxidation of the electrolyte occurs due to the positive electrode. It is even more preferable that the maximum temperature is lower than the flash point of the mixed solvent used in the electrolyte. If the flash point of the mixed solvent is unknown, the flash point of each solvent can be used as a reference. It is even more preferable that the maximum temperature is lower than the softening point of the separator. For example, the softening point of polypropylene that can be used for the separator is approximately 155°C.

[0457] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0458] (Fourth embodiment) In this embodiment, a secondary battery having an electrolyte solution and a separator according to one embodiment of the present invention will be described. A secondary battery having an electrolyte solution and a separator is preferable because it can prevent thermal runaway and / or fire. It is also preferable because it can ensure the amount of electrolyte solution retained in the separator even when the electrodes expand and contract during charging and discharging of the secondary battery.

[0459] [Coin-type secondary battery] An example of a coin-type secondary battery will be described. Fig. 22(A) is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 22(B) is an external view, and Fig. 22(C) is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0460] In addition, in order to make it easier to understand, Fig. 22(A) is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships), and therefore Fig. 22(A) and Fig. 22(B) are not completely corresponding drawings.

[0461] In FIG. 22A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. A separator according to one embodiment of the present invention can be used for the separator 310. These components are sealed with a negative electrode can 302 and a positive electrode can 301 by a gasket. Note that the gasket for sealing is not shown in FIG. 22A. The spacer 322 and the washer 312 are used to protect the interior or to fix the position within the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 and the washer 312 are made of stainless steel or an insulating material.

[0462] A positive electrode 304 has a stacked structure in which a positive electrode active material layer 306 is formed over a positive electrode current collector 305. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 306.

[0463] FIG. 22(B) is a perspective view of the completed coin-type secondary battery.

[0464] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be made of lithium metal foil or a lithium-aluminum alloy foil.

[0465] The positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 can each have an active material layer formed on only one surface.

[0466] Positive electrode can 301 and negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., it is preferable to coat them with nickel, aluminum, or the like. Positive electrode can 301 is electrically connected to positive electrode 304, and negative electrode can 302 is electrically connected to negative electrode 307.

[0467] 22(C) , the positive electrode 304, the separator 310, the negative electrode 307, and the negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downward, and the positive electrode can 301 and the negative electrode can 302 are pressure-bonded together via a gasket 303 to produce a coin-shaped secondary battery 300. The mixed solvent of one embodiment of the present invention is preferably used as the solvent for the electrolyte.

[0468] [Cylindrical secondary battery] An example of a cylindrical secondary battery will be described with reference to Fig. 23(A). As shown in Fig. 23(A), a cylindrical secondary battery 616 has a positive electrode cap (secondary battery lid) 601 on the top surface, and a secondary battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the secondary battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0469] Fig. 23(B) is a diagram showing a schematic cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in Fig. 23(B) has a positive electrode cap (secondary battery lid) 601 on the top surface, and a secondary battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and secondary battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0470] A secondary battery element is provided inside a hollow cylindrical secondary battery can 602. The secondary battery element includes a strip-shaped positive electrode 604 and a strip-shaped negative electrode 606 wound with a separator 605 sandwiched therebetween. A separator according to one embodiment of the present invention can be used for the separator 605. Although not shown, the secondary battery element is wound around a central axis. The secondary battery can 602 is closed at one end and open at the other end. The secondary battery can 602 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to an electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, the secondary battery can 602 is preferably coated with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the secondary battery can 602, the wound secondary battery element including the positive electrode, negative electrode, and separator is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte (not shown) is poured into the interior of the secondary battery can 602 in which the secondary battery element is provided. The electrolyte may be the same as that used in a coin-type secondary battery.

[0471] Since the positive and negative electrodes used in cylindrical secondary batteries are wound up, it is preferable to form active materials on both sides of the current collector.

[0472] Lithium cobalt oxide, which is one embodiment of the present invention, can be used as a positive electrode active material of the positive electrode 604, and the cylindrical secondary battery 616 can have favorable high-voltage charging characteristics.

[0473] 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. The positive electrode terminal 603 can be made of a metal material such as aluminum. The negative electrode terminal 607 can be made of a metal material such as copper. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the secondary battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the secondary battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature increases, and the increased resistance limits the amount of current to prevent abnormal heat generation. Barium titanate (BaTiO3) based semiconductor ceramics and the like can be used for the PTC element.

[0474] 23C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. A positive electrode of each secondary battery is in contact with and electrically connected to a conductor 624 separated by an insulator 625. The conductor 624 is electrically connected to a control circuit 620 via a wiring 623. A negative electrode of each secondary battery is electrically connected to the control circuit 620 via a wiring 626. The control circuit 620 can be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0475] 23(D) shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, and the plurality of secondary batteries 616 are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in series after being connected in parallel. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0476] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0477] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside temperature.

[0478] 23(D), the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0479] [Other examples of secondary battery structures] An example of the structure of the secondary battery will be described with reference to FIGS. 24 and 25. FIG.

[0480] A secondary battery 913 shown in FIG. 24A includes a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is immersed in an electrolyte solution inside the housing 930. The mixed solvent of one embodiment of the present invention is preferably used as a solvent for the electrolyte solution. The terminal 952 is in contact with the housing 930, and the terminal 951 is not in contact with the housing 930 because an insulating material or the like is used. Note that for convenience, the housing 930 is shown separated in FIG. 24A; however, 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 (for example, aluminum) or a laminate of a metal material and a resin material.

[0481] 24(B), the housing 930 shown in Fig. 24(A) may be formed using a plurality of materials. For example, a secondary battery 913 shown in Fig. 24(B) has housings 930a and 930b bonded together, and a wound body 950 is provided in the area surrounded by the housings 930a and 930b.

[0482] The housing 930a can be made of a metal material (such as aluminum) or a laminate of a metal material and a resin material. The resin material can be an organic resin or the like. In particular, using a material such as organic resin on the surface on which the antenna is formed can prevent the secondary battery 913 from blocking the electric field. If the electric field blocking effect of the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of a metal material (such as aluminum) or a laminate of a metal material and a resin material.

[0483] 24C shows the structure of a wound body 950. The wound body 950 includes a negative electrode 931, a positive electrode 932, and a separator 933. The separator 933 can be a separator according to one embodiment of the present invention. The wound body 950 is formed by stacking the negative electrode 931 and the positive electrode 932 with the separator 933 sandwiched therebetween, and then winding the laminated sheet. Note that a plurality of stacks of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0484] Alternatively, a secondary battery 913 may be provided that has a wound body 950a as shown in Fig. 25. A wound body 950a shown in Fig. 25(A) has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0485] Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 932a.

[0486] The separator 933 has a width wider than the negative electrode active material layer 931a and the positive electrode active material layer 932a, and is wound so as to overlap the negative electrode active material layer 931a and the positive electrode active material layer 932a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931a be wider than the positive electrode active material layer 932a. A wound body 950a having such a shape is preferable because of its high safety and productivity.

[0487] 25(B), the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0488] 25(C), the wound body 950a and the electrolyte are covered with a housing 930 to form a secondary battery 913. It is preferable to provide a safety valve, an overcurrent protection element, etc. in the housing 930. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure, and can prevent the secondary battery from exploding.

[0489] 25(B), the secondary battery 913 may have a plurality of wound bodies 950a. By using a plurality of wound bodies 950a, the secondary battery 913 can have a larger discharge capacity. For other elements of the secondary battery 913 shown in FIGS. 25(A) and 25(B), refer to the descriptions of the secondary battery 913 shown in FIGS. 24(A) to 24(C).

[0490] <Laminated secondary battery> Next, examples of external views of a laminated secondary battery are shown in Fig. 26(A) and Fig. 26(B). Fig. 26(A) and Fig. 26(B) include a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511. The separator 507 can be a separator according to one embodiment of the present invention. Although not shown, the electrolyte solution is preferably an electrolyte solution according to one embodiment of the present invention, particularly a mixed solvent.

[0491] FIG. 26(A) shows an external view of a positive electrode 103 and a negative electrode 106. The positive electrode 103 has a positive electrode current collector 21, and a positive electrode active material layer 22 is formed on the surface of the positive electrode current collector 21. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 22. The positive electrode 103 has a region where the positive electrode current collector 21 is partially exposed (hereinafter referred to as a tab region). The negative electrode 106 has a negative electrode current collector 31, and the negative electrode active material layer 32 is formed on the surface of the negative electrode current collector 31. The negative electrode 106 also has a region where the negative electrode current collector 31 is partially exposed, i.e., a tab region. Note that the area or shape of the tab regions of the positive electrode and the negative electrode are not limited to the example shown in FIG. 26(A).

[0492] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0493] (Embodiment 5) In this embodiment, an example of a vehicle equipped with a secondary battery having an electrolyte solution and a separator according to one embodiment of the present invention will be described. Examples of vehicles include automobiles, trains, airplanes, and buses. A secondary battery having an electrolyte solution and a separator is preferable because it can prevent thermal runaway and / or fire. It is also preferable because it can ensure the amount of electrolyte retained in the separator even when the electrodes expand and contract during charging and discharging of the secondary battery.

[0494] 27(A) is an electric vehicle that uses an electric motor as a power source for traveling. It is also a hybrid vehicle that can select and use either an electric motor or an engine as a power source for traveling. The vehicle 2001 has a secondary battery pack 2200, and the secondary battery pack preferably has a secondary battery module to which multiple secondary batteries are connected and a charge control device electrically connected to the secondary battery module.

[0495] Next, the secondary battery pack 2200 will be described with reference to FIG. 27(B). FIG. 27(B) shows an example in which one secondary battery pack 2200 has nine prismatic secondary batteries 1300. The nine prismatic secondary batteries 1300 are connected in series, with one electrode group fixed by a fixing portion 1413 made of an insulator and the other electrode group fixed by a fixing portion 1414 made of an insulator. Instead of the fixing portions 1413 and 1414, the electrode groups may be fixed by a configuration in which they are housed in a secondary battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (such as from the road surface), it is preferable to fix multiple prismatic secondary batteries 1300 using the fixing portions 1413, 1414, the secondary battery housing box, etc. One electrode group is electrically connected to a control circuit unit 1320 by wiring 1421. The other electrode group is electrically connected to the control circuit unit 1320 by wiring 1422.

[0496] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit unit 1320. A charge control circuit or a secondary battery control system having 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.

[0497] 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 M is one or more elements selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, etc.) can be used. In particular, the In-M-Zn oxide that can be used as the metal oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In-Ga oxide or an In-Zn oxide may be used. A CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis aligned in a specific direction. The specific direction is 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. The crystalline region is a region having a periodic atomic arrangement. If the atomic arrangement is considered as a lattice arrangement, the crystalline region is also a region with a regular lattice arrangement.

[0498] In addition, "CAC-OS" has a mosaic structure in which the material is separated into a first region and a second region, and the first region is distributed throughout the film (hereinafter also referred to as a cloud structure). In other words, CAC-OS is a composite metal oxide having a structure in which the first region and the second region are mixed. However, it may be difficult to observe a clear boundary between the first region and the second region.

[0499] Oxide semiconductors have a variety of structures, each with different characteristics. The oxide semiconductor of one embodiment of the present invention may include two or more of an amorphous oxide semiconductor, a polycrystalline oxide semiconductor, an a-like OS, a CAC-OS, an nc-OS, and a CAAC-OS.

[0500] Furthermore, transistors using oxide semiconductors in the semiconductor layer have a wider operating ambient temperature range than single-crystal Si transistors, from -40°C to 150°C, and their characteristics change less even when the secondary battery overheats than single-crystal Si transistors. The off-current of transistors using oxide semiconductors is below the lower measurement limit regardless of temperature, even at 150°C, whereas the off-current characteristics of single-crystal Si transistors are highly temperature-dependent. For example, at 150°C, the off-current of single-crystal Si transistors increases, and the current on / off ratio is not sufficiently large. The control circuit 1320 using transistors using oxide semiconductors can improve safety. The secondary battery and control circuit 1320 can significantly contribute to eliminating accidents such as fires caused by secondary batteries.

[0501] Next, FIG. 27(C) shows an example of a block diagram of the automobile 2001 shown in FIG. 27(A) and the secondary battery pack 2200 shown in FIG. 27(B).

[0502] As shown in Fig. 27(C), an electric vehicle is equipped with first batteries 1301a and 1301b as main driving secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0503] The internal structure of the first battery 1301a may be a wound type shown in FIG. 24(C) or FIG. 25(A), or may be a stacked type shown in FIG. 26(A) or FIG. 26(B).

[0504] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more may be connected in parallel. Also, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a secondary battery pack having multiple secondary batteries, it is possible to extract large amounts of power. The multiple secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of secondary batteries is also called a secondary battery pack.

[0505] In addition, in the case of a secondary battery for vehicle, in order to cut off power from a plurality of secondary batteries, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0506] 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 DC-DC circuit 1306. When a rear motor 1317 is provided on the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0507] Furthermore, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0508] The first batteries 1301a and 1301b primarily supply power to 42V (high-voltage HV) in-vehicle devices, while the second battery 1311 supplies power to 14V (low-voltage LV) in-vehicle devices. Lead-acid batteries are often used for the second battery 1311 due to their cost advantages. Lead-acid batteries have a higher self-discharge rate than lithium-ion secondary batteries and are prone to deterioration due to a phenomenon called sulfation. Using a lithium-ion secondary battery as the second battery 1311 offers the advantage of being maintenance-free, but after prolonged use (e.g., three years or more), there is a risk of abnormalities that are difficult to identify during manufacturing. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b still have remaining capacity, the motor cannot be started. To prevent this, if the second battery 1311 is a lead-acid battery, power is supplied from the first battery to the second battery, and the second battery is constantly charged to maintain a fully charged state.

[0509] In this embodiment, an example is shown in which lithium ion secondary batteries are used for both the first battery 1301a and the second battery 1311. The second battery 1311 may be a lead battery, an all-solid-state secondary battery, or an electric double layer capacitor.

[0510] Furthermore, regenerated 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 or the battery controller 1302 via the control circuit unit 1321. The first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. The first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerated energy, it is desirable that the first batteries 1301a and 1301b be capable of being quickly charged.

[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 secondary battery used, and can perform rapid charging.

[0512] Although not shown, when the electric vehicle is connected to an external charger, the charger plug or the charger connection cable is electrically connected to the battery controller 1302. The 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 function of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a, 1301b via a control circuit unit 1320 to prevent overcharging. In some cases, the charger plug or the charger connection cable is provided with a control circuit. The control circuit unit 1320 is also 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 are available in 100V-200V outlets, or three-phase 200V and 50kW. Charging is also possible by receiving power from external charging equipment using a wireless power supply system, etc.

[0514] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0515] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0516] (Sixth embodiment) In this embodiment, a space device equipped with a secondary battery having an electrolyte and a separator according to one embodiment of the present invention will be described. A secondary battery having an electrolyte and a separator is preferable because it can prevent thermal runaway and / or fire. It is also preferable because it can ensure the amount of electrolyte held by the separator even when the electrodes expand and contract during charging and discharging of the secondary battery.

[0517] 28A shows an example of space equipment, namely, an artificial satellite 6800. The artificial satellite 6800 includes a body 6801, a solar panel 6802, an antenna 6803, and a secondary battery 6805.

[0518] When sunlight is irradiated onto the solar panel 6802, the power required for the operation of the satellite 6800 is generated. However, for example, when sunlight is not irradiated onto the solar panel or when the amount of sunlight irradiating the solar panel is low, 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. To operate the satellite 6800 even under conditions where the generated power is low, it is recommended to provide a secondary battery 6805 in the satellite 6800. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting good low-temperature characteristics can be obtained.

[0519] The satellite 6800 can generate a signal. The signal is transmitted via an antenna 6803, and can be received by, for example, a receiver on the ground or another satellite. By receiving the signal transmitted by the satellite 6800, for example, the position of the receiver that received the signal can be measured. As described above, the satellite 6800 can constitute, for example, a satellite positioning system.

[0520] The artificial satellite 6800 can also be configured to include a sensor. For example, by including a visible light sensor, the artificial satellite 6800 can have the function of detecting sunlight reflected from an object on the ground. Furthermore, by including a thermal infrared sensor, the artificial satellite 6800 can have the function of detecting thermal infrared rays emitted from the earth's surface. As described above, the artificial satellite 6800 can function as, for example, an earth observation satellite.

[0521] Figure 28(B) shows a probe 6900 with a solar sail (also called a sun sail) as an example of space equipment. The probe 6900 has a body 6901, a solar sail 6902, and a secondary battery 6905. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting excellent low-temperature characteristics can be obtained. When photons emitted from the sun hit 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 thin film with high reflectivity and preferably faces the sun.

[0522] The solar sail 6902 may also be designed to be folded up small until it leaves the atmosphere, and then unfold into a large sheet once outside the Earth's atmosphere (outer space) as shown in Figure 28(B).

[0523] FIG. 28(C) shows a spacecraft 6910 as an example of space equipment. The spacecraft 6910 has a body 6911, a solar panel 6912, and a secondary battery 6913. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting excellent low-temperature characteristics can be obtained. The body 6911 can have, for example, a pressurized compartment and a non-pressurized compartment. The pressurized compartment may be designed to accommodate a crew member. Electric power generated by sunlight irradiating the solar panel 6912 can be charged into the secondary battery 6913.

[0524] 28(D) shows a rover 6920 as an example of space equipment. The rover 6920 has a body 6921 and a secondary battery 6923. By using the secondary battery of the present invention as the secondary battery, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention as the secondary battery, a secondary battery exhibiting excellent low-temperature characteristics can be obtained. The rover 6920 may also have a solar panel 6922.

[0525] The rover 6920 may be designed to accommodate a crew member. The secondary battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or may be charged with electricity generated by other power sources, such as a fuel cell, a radioisotope thermoelectric converter, or the like.

[0526] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate. [Example]

[0527] In this example, DSC measurements, combustion tests, vapor pressure calculations, Raman spectroscopy, etc. were performed on several electrolyte solutions shown in the table below. Electrolytes A and B contained more than 1 mole of the lithium salt LiFP6 per liter of the fluoride mixed solvent, electrolyte C contained 1 mole or less of the lithium salt LiFP6 per liter of the fluoride mixed solvent, electrolyte D did not contain any lithium salt, and electrolyte E contained 1 mole or less of the lithium salt LiFP6 per liter of the mixed solvent.

[0528] [Table 2]

[0529] <DSC measurement of electrolyte> DSC measurements were performed to confirm the thermal stability of the electrolytes. First, 15 mL of electrolytes B, C, and E were prepared. Each electrolyte was sealed in a SUS container in an Ar-filled glove box, and DSC measurements were initiated. The measurement equipment and conditions for the DSC measurements were as follows: DSC equipment: Rigaku EVO2 DSC8271 Heating rate: 5℃ / min Temperature range: Room temperature (25°C) to 350°C The measurement results were subjected to background correction using the analysis software Thermo prus EVO.

[0530] FIG. 29 shows the results of DSC measurements. In FIG. 29, the horizontal axis represents temperature (°C) and the vertical axis represents heat flow (mW / g). Heat flow corresponds to the heat flow per sample weight. Electrolytes B and C had smaller heat flows than electrolyte E. This confirms that electrolytes B and C, which contain fluoride-containing mixed solvents, are more thermally stable than electrolyte E. In particular, electrolytes B and C exhibited peak heat flows (calorific values) of 200 mW / g or less, preferably 100 mW / g or less, in the temperature range of 180°C to 300°C. Furthermore, there was no significant difference in the thermal stability of electrolytes B and C, and it was found that there was no significant difference in heat flow even when the LiPF concentration increased. The DSC measurements in this example revealed that electrolytes B and C, which contain fluoride-containing mixed solvents, exhibited higher thermal stability than electrolyte E. It was found that secondary batteries containing electrolyte solution B or electrolyte solution C tend to be able to suppress thermal runaway and / or fire.

[0531] <Combustion test> A combustion test was conducted to confirm non-flammability and thermal stability. First, glass fibers (18 mm in diameter) were prepared and impregnated with 2.8 mL of each of electrolytes A, B, C, to prepare four samples each. In this example, the four samples are identified by the designations n1, n2, n3, and n4. A test flame was brought close to each sample, and the burning time of the ignited flame was measured. The evaluation criterion of non-flammability refers to the case where the test flame did not ignite at all even when ignited. The evaluation criterion of combustion refers to the case where an ignited flame was confirmed.

[0532] The table below shows the evaluation results of the combustion test.

[0533] [Table 3]

[0534] All four samples of electrolyte A were rated as non-flammable. Two samples of electrolyte B were rated as flammable, but two were rated as non-flammable. All four samples of electrolyte C were rated as flammable. Therefore, the combustion test showed that electrolyte A had the highest thermal stability. It was found that electrolytes A and B, which have a lithium salt LiPF6 concentration higher than 1 mole per liter of fluoride mixed solvent, are less likely to burn than electrolyte C, which has a LiPF6 concentration of 1 mole or less per liter of fluoride mixed solvent. It was found that secondary batteries containing electrolytes A or B can suppress thermal runaway and / or fire.

[0535] The results of this combustion test were consistent with the results of the DSC measurements. Furthermore, while the DSC measurements did not reveal any significant difference in heat flow with increasing LiPF6 concentration, the combustion test showed that the higher the LiPF6 concentration, the more non-flammable the electrolyte became. Specifically, because electrolyte C burned, this test demonstrated that a LiPF6 concentration higher than 1 mole per liter of fluoride-mixed solvent is preferable. It was found that electrolytes with a specified concentration of lithium salt and a fluoride-mixed solvent have high thermal stability, and secondary batteries containing such electrolytes can suppress thermal runaway and / or fire.

[0536] <Verification of gas generation due to vaporization> It is believed that the reason why the electrolyte burns or continues to burn is because the vaporized molecules of the electrolyte are burning. Therefore, in this example, the vaporization tendency of electrolytes A to D was calculated by classical molecular dynamics calculation.

[0537] This section explains the calculations for determining the initial coordinates of each molecule and the charge of each atom used in the classical molecular dynamics calculations. - The initial coordinates of each molecule and the charge assigned to each atom were calculated using quantum chemical calculations using Gaussian 16 software as follows: First, geometry optimization was performed in vacuum at the B3LYP / 6-31G(d) level. Then, single-point calculations were performed in vacuum at the HF / 6-31G(d) level to calculate the ESP charge of each atom. The calculated ESP charge was then added to the PF6 - In the case of Li, the charge assigned to each atom is multiplied by 0.8. + The charge of was set to 0.8. The coordinates after the above structure optimization were used as the initial coordinates of the molecule in the classical molecular dynamics calculation.

[0538] Next, models were prepared in which electrolytes A to D were aggregated as droplets. The cell size was a cube of 10 nm x 10 nm x 10 nm. The number of molecules in electrolytes A to D was as shown in the table below. The volume ratio of FEC to MTFP was approximately 2:8.

[0539] [Table 4]

[0540] Classical molecular dynamics calculations to determine the vapor pressure of the electrolytes were performed using Gromacs (version 2024.1). GAFF2 was used for the intramolecular potential, and OPLS for the intermolecular potential. The particle mesh Ewald (PME) method with a cutoff distance of 1.0 nm was used to calculate Coulomb interactions. The cutoff method with a cutoff distance of 1.0 nm was used to calculate van der Waals forces. NVT was used for ensemble analysis, and the v-rescale method was used for temperature control. Classical molecular dynamics calculations were performed for each model of electrolytes A to D at constant temperatures (51.85°C, 76.85°C, 101.85°C, 126.85°C, and 151.85°C). The time step was set to 0.5 fs, and calculations were performed up to 50 nsec. The vapor pressure was calculated as the average pressure...

Claims

1. a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the electrolyte solution includes a mixed solvent and a lithium salt; In the electrolyte solution, the concentration of the lithium salt is greater than 1 mole per liter of the mixed solvent; the mixed solvent contains a fluorinated chain carbonate and a fluorinated cyclic carbonate, In a DSC measurement of the electrolyte solution, the peak of the heat flow rate of the exothermic reaction in the range of 180°C to 300°C is 200 mW / g or less. Secondary battery.

2. a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the electrolyte solution includes a mixed solvent and a lithium salt; In the electrolyte solution, the concentration of the lithium salt is greater than 1 mole per liter of the mixed solvent; the mixed solvent contains a fluorinated chain carbonate and a fluorinated cyclic carbonate, In a DSC measurement of the electrolyte solution, the peak heat flow of an exothermic reaction in the range of 180°C to 300°C is 100 mW / g or less. Secondary battery.

3. a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the electrolyte solution includes a mixed solvent and a lithium salt; In the electrolyte solution, the concentration of the lithium salt is greater than 1 mole per liter of the mixed solvent; the mixed solvent contains a fluorinated chain carbonate and a fluorinated cyclic carbonate, In a DSC measurement of the electrolyte solution, the peak of the heat flow rate of an exothermic reaction in the range of 180°C or higher and 300°C or lower is 200 mW / g or lower; the separator has an imide compound in a region that comes into contact with the electrolyte solution; Secondary battery.

4. a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the electrolyte solution includes a mixed solvent and a lithium salt; In the electrolyte solution, the concentration of the lithium salt is greater than 1 mole per liter of the mixed solvent; the mixed solvent contains a fluorinated chain carbonate and a fluorinated cyclic carbonate, In a DSC measurement of the electrolyte solution, the peak of the heat flow rate of an exothermic reaction in the range of 180°C or higher and 300°C or lower is 100 mW / g or lower; the separator has an imide compound in a region that comes into contact with the electrolyte solution; Secondary battery.

5. In any one of claims 1 to 4, The lithium salt is LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ One or more selected from Secondary battery.

6. In any one of claims 1 to 4, The secondary battery, wherein the fluorinated chain carbonate is FEC.

7. In any one of claims 1 to 4, The secondary battery, wherein the fluorinated cyclic carbonate is MTFP.

8. In any one of claims 3 and 4, The secondary battery, wherein the imide compound is a polyimide.

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