Secondary battery and electrolyte solution

Abstract

Provided is a secondary battery or an electrolyte solution that exhibits excellent battery characteristics below zero. The secondary battery has an electrolyte solution, wherein: the electrolyte solution has a first organic compound, a second organic compound, and a lithium salt; the electrolyte solution has a freezing point of lower than -30°C; the lithium salt has LiPF6; the lithium ion coordination energy of the first organic compound or the lithium ion coordination energy of the second organic compound is -35[kJ / mol] or less; and the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less.

Description

Secondary batteries and electrolytes

[0001] One aspect of the present invention relates to a secondary battery and an electrolyte. However, the technical field of the present invention is not limited to secondary batteries and electrolytes, and may include any one of semiconductor devices, display devices, light-emitting devices, energy storage devices, lighting devices, electronic devices, and vehicles. 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, energy storage devices, lighting devices, electronic devices, and vehicles. Furthermore, methods for manufacturing the above-mentioned items can be cited as a technical field of the present invention.

[0002] In recent years, there has been a growing demand for higher performance lithium-ion secondary batteries. For example, when lithium-ion secondary batteries are used to power electric vehicles, excellent battery characteristics at sub-zero temperatures (below 0°C) are required. Therefore, the electrolyte must ensure ionic conductivity at sub-zero temperatures, and furthermore, it must not solidify at sub-zero temperatures and maintain low viscosity.

[0003] To address the problem of electrolytes freezing below freezing point, the use of freezing point depressants has been investigated (see Patent Document 1). A porous body having pores with an average pore diameter of 3 nm to 100 nm has been proposed as a freezing point depressant, and it effectively suppresses freezing below freezing point for various solvents impregnated within these pores.

[0004] To improve the battery performance below freezing point, the solvent of the electrolyte has a self-diffusion coefficient of 1.4 × 10 at -20°C. −１０ I understand ２ / sec or more 2.0×10 −１０ I understand ２ A configuration having chain-like molecules with a self-diffusion coefficient of less than / sec has been proposed (see Patent Document 2). The self-diffusion coefficient is a parameter that correlates with ionic conductivity.

[0005] Japanese Patent Publication No. 2020-129536 Japanese Patent Publication No. 2015-079747

[0006] Patent Document 1 uses a freezing point depressant and does not improve the battery characteristics below freezing point using only the electrolyte solvent. Furthermore, although an electrolyte solvent such as that in Patent Document 2 has been proposed, it is thought that further improvement of the above solvent is necessary to exhibit excellent battery characteristics below freezing point. In view of these circumstances, one aspect of the present invention aims to provide a novel mixed solvent as an electrolyte solvent in order to exhibit excellent battery characteristics below freezing point.

[0007] Furthermore, the description of these problems does not preclude the existence of other problems. Also, one aspect of the present invention does not need to solve all of these problems. It is also possible to extract other problems from the description in this specification, drawings, claims, etc.

[0008] In view of the above problems, one aspect of the present invention is a secondary battery having an electrolyte, wherein the electrolyte comprises a first organic compound, a second organic compound, and a lithium salt, the freezing point of the electrolyte is less than -30°C, and the lithium salt is LiPF4 ６ This is a secondary battery having the following characteristics: the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -35 [kJ / mol] or less, and the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less.

[0009] Another aspect of the present invention is a secondary battery having a negative electrode and an electrolyte, wherein the electrolyte comprises a first organic compound, a second organic compound and a lithium salt, the freezing point of the electrolyte is less than -30°C, and the lithium salt is LiPF4 ６ The secondary battery has the following characteristics: the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -35 [kJ / mol] or less, the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less, and the negative electrode is made of graphite.

[0010] In another aspect of the present invention, it is preferable that the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each −37 [kJ / mol] or less.

[0011] In another aspect of the present invention, it is preferable that the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each −39 [kJ / mol] or less.

[0012] Another aspect of the present invention includes a first organic compound, a second organic compound, and a lithium salt, and has a freezing point of lower than −30°C, wherein the lithium salt is LiPF ６ , wherein the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each −35 [kJ / mol] or less, and the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less, which is an electrolytic solution.

[0013] Another aspect of the present invention includes a first organic compound, a second organic compound, and a lithium salt, and has a freezing point of lower than −30°C, wherein the lithium salt is LiPF ６ , wherein the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each −37 [kJ / mol] or less, and the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less, which is an electrolytic solution.

[0014] Another aspect of the present invention includes a first organic compound, a second organic compound, and a lithium salt, and has a freezing point of lower than −30°C, wherein the lithium salt is LiPF ６ , wherein the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each −39 [kJ / mol] or less, and the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less, which is an electrolytic solution.

[0015] In another embodiment of the present invention, the concentration of the lithium salt is preferably 0.2 [mol] or more and less than 1 [mol] per liter of the mixed solvent containing the first organic compound and the second organic compound.

[0016] According to one aspect of the present invention, it is possible to provide a secondary battery or electrolyte that exhibits excellent battery characteristics at least below freezing point.

[0017] Furthermore, the description of these effects does not preclude the existence of other effects. Moreover, one aspect of the present invention does not necessarily have to possess all of these effects. Other effects will naturally become apparent from the description in the specification, drawings, and claims, and it is possible to extract other effects from the description in the specification, drawings, and claims.

[0018] Figures 1A and 1B are model diagrams for determining the coordination energy of organic compounds. Figures 2A and 2B are diagrams illustrating the conditions that the organic compounds used in the electrolyte must satisfy. Figures 3A and 3B are diagrams illustrating a secondary battery according to one embodiment of the present invention. Figure 4A is a diagram illustrating a secondary battery according to one embodiment of the present invention, and Figure 4B is a diagram illustrating a positive electrode active material layer according to one embodiment of the present invention. Figure 5 is a diagram illustrating a method for manufacturing the positive electrode active material. Figures 6A and 6B are diagrams illustrating a method for manufacturing the positive electrode active material. Figures 7A and 7B are cross-sectional views illustrating the positive electrode active material. Figures 8A, 8B, 8C, 8D, 8E, and 8F are cross-sectional views illustrating the positive electrode active material. Figure 9 is a diagram illustrating the crystal structure of the positive electrode active material. Figure 10 is a diagram illustrating the crystal structure of a conventional positive electrode active material. Figure 11 is a diagram showing the XRD profile calculated from the crystal structure. Figure 12 is a diagram showing the XRD profile calculated from the crystal structure. Figures 13A, 13B, 13C, 13D, 13E, 13F, and 13G illustrate the distribution of EDX radiation analysis for two or more elements. Figure 14A is an exploded perspective view of a coin-type secondary battery, Figure 14B is a perspective view of a coin-type secondary battery, and Figure 14C is a cross-sectional perspective view thereof. Figure 15A shows an example of a cylindrical secondary battery. Figure 15B shows an example of the internal structure of a cylindrical secondary battery. Figure 15C shows an example of multiple cylindrical secondary batteries. Figure 15D shows an example of an energy storage system having multiple cylindrical secondary batteries. Figures 16A and 16B illustrate examples of secondary batteries, and Figure 16C shows the internal structure of a secondary battery. Figures 17A, 17B, and 17C illustrate examples of secondary batteries. Figures 18A and 18B show examples of secondary batteries. Figure 19A shows an example of the configuration of an electric vehicle, Figure 19B shows a battery pack, and Figure 19C shows an example of the configuration of both the electric vehicle and the battery pack. Figures 20A, 20B, 20C, and 20D show an example of space equipment. Figure 21 is a graph showing the battery characteristics of the embodiment at sub-zero temperatures (-30°C) and room temperature (25°C). Figure 22 is a graph showing the battery characteristics of the embodiment at sub-zero temperatures (-30°C, -40°C, -50°C), room temperature (25°C), and high temperatures (40°C, 50°C, 60°C).Figure 23 is a graph showing the battery characteristics of the example at sub-zero temperatures (-30°C) and room temperature (25°C). Figure 24 is a graph showing the battery characteristics of the example at sub-zero temperatures (-30°C) and room temperature (25°C).

[0019] Embodiments of the present invention will be described with appropriate use of the drawings. However, it will be readily apparent to those skilled in the art that the present invention is not limited to the following description, and that its form and details can be modified in various ways without departing from the spirit and scope of the present invention. Accordingly, in the embodiments of the present invention shown below, the same reference numerals will be common to different drawings.

[0020] In this specification, the ordinal numbers "first," "second," etc., are used for convenience only and do not limit the number or order of the components. The order of the components includes, for example, the order of processes or the order of stacking. That is, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the claims. Also, the ordinal numbers used in the examples of this specification may not match the ordinal numbers used in the claims. Also, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the examples of this specification.

[0021] In this specification, 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 in this invention are not limited to lithium ions. For example, alkali metal ions or group 2 element ions can be used as carrier ions in this invention, and specifically, sodium ions, etc., can be applied. In this case, lithium ions can be read as sodium ions, etc., and the invention can be understood accordingly. Furthermore, when describing a configuration in which there are no limitations on the carrier ions, the term secondary battery or battery may be used.

[0022] In this specification, the term "electrolyte" may be used interchangeably with "electrolyte." "Electrolyte" means that the solution is liquid at 25°C. Furthermore, "electrolyte" is not limited in any way to its state at 25°C.

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

[0024] In this specification, the space group of positive electrode active material, etc., is identified by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. Therefore, in this specification, "belonging to a certain space group," "being part of a certain space group," or "being a certain space group" can be rephrased as "being identified to a certain space group."

[0025] In this specification, if an anion has a structure in which three layers are stacked with a slight offset from each other, such as ABCABC, it will be referred to as a cubic close-packed structure. Therefore, the anion does not have to be strictly a cubic lattice. Also, since real crystals always have defects, the analytical results do not necessarily conform to theory. For example, in FFT (Fast Fourier Transform) patterns such as electron diffraction patterns or TEM (Transmission Electron Microscope) images, spots may appear at positions slightly different from the theoretical positions.

[0026] In this specification, the (001) plane and the (003) plane, etc., may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C plane, basal plane, etc. In lithium cobalt oxide having a layered rock salt crystal structure, the lithium diffusion pathway exists along the (00l) plane. In this specification, the plane through which lithium enters and exits the above diffusion pathway may be referred to as the edge plane.

[0027] In this specification, the term "particle" is not limited to having a circular or spherical cross-sectional shape. The cross-sectional shapes of particles include elliptical, rectangular, trapezoidal, triangular, rounded quadrilateral, and asymmetrical shapes. If there are multiple particles, their cross-sectional shapes may differ.

[0028] In this specification, when describing the characteristics of positive electrode active material particles in embodiments, it is not necessarily required that all particles possess those characteristics. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected positive electrode active material particles possess those characteristics, it can be said that this sufficiently improves the properties of the positive electrode active material and the secondary battery having it.

[0029] In this specification, particle size can be measured using a particle size analyzer (laser diffraction particle size distribution analyzer) or the like. In this specification, the median diameter (D50) can be used as the average particle size. The median diameter (D50) is the particle size at which its cumulative frequency accounts for 50% in the cumulative curve of the particle size distribution measurement results.

[0030] In this specification, particle size measurement is not limited to laser diffraction particle size distribution measurement, but may also be measured by measuring the major axis of the particle cross-section using analysis such as SEM (Scanning Electron Microscope) or TEM. In this specification, the maximum particle size can be defined as the particle size that can be confirmed in one cross-section of a 100 μm square positive electrode. Furthermore, as a method for measuring the median diameter (D50) from analysis such as SEM or TEM, for example, 20 or more particles can be measured, a cumulative curve can be created, and the particle size at which the cumulative frequency accounts for 50% can be defined as the median diameter (D50).

[0031] In this specification, etc., LiCoO ２ Its theoretical capacity is 274 mAh / g, LiNiO ２ Its theoretical capacity is 275 mAh / g, LiMn ２ O ４ The theoretical capacity is 148 mAh / g. Unless otherwise specified in this specification, the theoretical capacity is given per unit weight of the positive electrode active material.

[0032] In this specification, the proportion of lithium that can be inserted into and removed from the positive electrode active material is expressed as x in the composition formula. α For example, Lix α CoO ２ x inside α This is sometimes shown. In the case of the positive electrode active material in a lithium-ion secondary battery, x α = (Theoretical capacity - Charge capacity) / Theoretical capacity. For example, when a lithium-ion secondary battery equipped with lithium cobalt oxide as the positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ or x α It can be said that this equals 0.2. α CoO ２ x inside α For example, 0.1 < x α This refers to a value of ≤0.24.

[0033] In this specification, when appropriately synthesized lithium cobalt oxide, before use as the positive electrode, approximately satisfies the stoichiometric ratio, LiCoO ２ and x α= 1. Also, lithium cobalt oxide contained in a lithium-ion secondary battery after discharge is also LiCoO ２ and x α It can be said that this equals 1. The state in which discharge is completed (discharge state) as used here refers to a state in which the battery voltage is 3.0V or less or 2.5V or less with a current of 100mA / g or less. Unless otherwise specified in this specification, the current is given as a value per unit weight of the positive electrode active material.

[0034] In this specification, etc., Lix α CoO ２ x inside α The charging capacity and / or discharging capacity used in the calculation should preferably be measured under conditions where there is little or no influence from short circuits and / or thermal decomposition and / or electrolysis of the electrolyte. For example, data from a lithium-ion secondary battery in which a sudden change in capacity, which appears to be due to a short circuit, is x α It must not be used in the calculation of [the specified value].

[0035] In this specification, the distribution of an element refers to a region in which the element is continuously detected within a non-noise range. This region begins at a position above the detection limit. Continuous detection within a non-noise range means that the element continues to be detected above the detection limit within that region.

[0036] In this specification, "non-uniformity" refers to a situation where the concentration of an element in one region differs from that in other regions, and is synonymous with segregation, precipitation, heterogeneity, and bias. Particles having non-uniform regions include particles with regions of high and low elemental concentrations.

[0037] In this specification, unless otherwise specified, "peak" refers to the maximum peak.

[0038] Unless otherwise specified in this specification, the materials of a secondary battery (positive electrode, negative electrode, electrolyte, separator, etc.) will be described in their state before degradation. A decrease in discharge capacity due to pre-shipment inspection (sometimes called aging treatment) during the secondary battery manufacturing process is not considered degradation. For example, a secondary battery consisting of a single cell or a battery pack can be considered in its state before degradation if it has a discharge capacity of 97% or more of its rated capacity. The rated capacity for secondary batteries for portable devices conforms to JIS C 8711:2019. For other secondary batteries, the rated capacity conforms to various JIS and IEC standards, including those for electric vehicle propulsion and industrial use, in addition to the above JIS standard.

[0039] Here, we will explain the flow of electrons and lithium ions during charging in a lithium-ion secondary battery. When a charger is connected and charging of the secondary battery begins, electrons are released at the positive electrode and an oxidation reaction occurs, while electrons are supplied at the negative electrode and a reduction reaction occurs. Then, lithium ions are released from the positive electrode into the electrolyte and 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 and cathode are swapped during discharge and charging, and the oxidation and reduction reactions are swapped. Therefore, the electrode with the higher reaction potential is called the positive electrode, and the electrode with the lower reaction potential is called the negative electrode. Accordingly, in this specification, whether during charging or discharging, the positive electrode will be called the "positive electrode" and the negative electrode will be called the "negative electrode".

[0040] In this specification, a full cell means a cell assembled so that different electrodes are located, such as a positive electrode / negative electrode unit cell. In this specification, a half cell means a cell assembled using lithium metal as the negative electrode (counter electrode).

[0041] In this specification, unless otherwise specified, the charging voltage shall be expressed with respect to the potential of the lithium metal. Furthermore, in this specification, a high charging voltage is defined as a charging voltage of 4.6V or higher, preferably 4.65V or higher, more preferably 4.7V or higher, even more preferably 4.75V or higher, and most preferably 4.8V or higher. In other words, in the case of a half-cell using lithium metal as the counter electrode, a charging voltage of 4.6V or higher shall be referred to as a high charging voltage.

[0042] In this specification, a high charging voltage is defined as a charging voltage of 4.5V 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 4.5V or higher is referred to as a high charging voltage.

[0043] In this specification, "carbonate" refers to a compound having at least one carbonate ester bond (−O−C(=O)−O−) in its molecular structure, and unless otherwise specified, includes both cyclic and linear molecules. Furthermore, "linear" includes both linear and branched chains.

[0044] In this specification, a mixed solvent refers to a mixture of two or more organic compounds.

[0045] In this specification, the phrase "A and / or B" may be used, but this refers to "A," "B," or "A and B."

[0046] (Embodiment 1) The electrolyte of a secondary battery according to one aspect of the present invention will be described.

[0047] [Electrolyte] The electrolyte contains a lithium salt and an organic solvent, and may further contain additives. The main component of the electrolyte is the organic solvent, which accounts for approximately 80 vol% or more of the electrolyte. The organic solvent may consist of multiple organic compounds, and this is sometimes called a mixed solvent. The electrolyte is impregnated into the separator and active material described later. When the secondary battery is charged, carrier ions (typically lithium ions) released from the positive electrode move from the positive electrode to the negative electrode through the electrolyte. When the secondary battery is discharged, the carrier ions move from the negative electrode to the positive electrode through the electrolyte. Examples of organic compounds used in the organic solvent include cyclic molecules and chain molecules. Since both have high dielectric constants, when used in a mixed solvent, they can efficiently dissociate the lithium salt and further stabilize the electrolyte by solvating the carrier ions. A cyclic molecule is an organic compound having a ring structure, and a chain molecule is an organic compound not having a ring structure. In one embodiment of the present invention, the mixed solvent has a freezing point of less than -30°C, preferably -30°C or lower, preferably -40°C or lower, and more preferably -50°C or lower. By having the mixed solvent, which is the main component of the electrolyte, have the above freezing point, excellent battery characteristics can be achieved below freezing point.

[0048] [Lithium-ion coordination energy] Organic compounds used in the organic solvent of the electrolyte can coordinate to lithium ions. The stabilization energy when an organic compound coordinates to a lithium ion is called the lithium-ion coordination energy (hereinafter referred to as coordination energy). The inventors have found that by focusing on the coordination energy and selecting the organic compounds that make up the mixed solvent, it is possible to exhibit excellent battery characteristics below freezing point. Here, we will explain how to determine the coordination energy. The coordination energy can be determined by density functional theory (B3LYP / 6-311++G(d,p)). Density functional theory is a method of calculating the energy of an electron system from the electron density, and can be calculated using commercially available software such as Gaussian 16 (Gaussian is a registered trademark of Gaussian Ltd.). The calculation level in density functional theory is selected appropriately according to the molecular weight of the molecule to be calculated or the capabilities of the computer. In this invention, the dielectric constant of acetone is set to ε = 20.493 using B3LYP / 6-311++G(d,p) and the SMD method. SMD stands for Solvation Model Density. Furthermore, the coordination energy is set to a value corrected for zero-point energy (ZPE).

[0049] Figure 1A shows a model for determining the coordination energy in which one 1,4,7,10-tetraoxacyclododecane (abbreviated as 12-Crown-4) is coordinated to a lithium ion. In this model, the four oxygen atoms in 12-Crown-4 are assumed to be coordinated to the lithium ion. Using this model, the coordination energy of the organic compound when it is coordinated to the lithium ion at the position furthest from 12-Crown-4 is calculated using the formula described later. Figure 1B shows ethylene carbonate (abbreviated as EC) as an example of the organic compound, illustrating the state in which EC is coordinated to the lithium ion at the position furthest from 12-Crown-4, and the coordination energy of EC at this point is calculated. In Figures 1A and 1B, coordinated states are indicated by dashed lines.

[0050] The formula for calculating the coordination energy is as follows: E ｃ = E - (E ｅ +E Ｌｉ＋１２Ｃｒｏｗｎ４ ) E ｃ : Coordination energy E: Energy of a lithium ion coordinated with 12-Crown-4, where one organic compound is coordinated E ｅ : Energy E of one organic compound Ｌｉ＋１２Ｃｒｏｗｎ４ : Energy of 12-Crown-4 in the state coordinated to the lithium ion

[0051] Calculations based on a model in which 12-Crown-4, which is not present in actual electrolytes, is coordinated to the lithium ion, eliminate the need for calculations based on models that assume a solvation structure not expected in electrolytes where the lithium salt concentration is less than 1 mol / L. In other words, it becomes possible to perform calculations based on the solvation structure expected in electrolytes where the lithium salt concentration is less than 1 mol / L. Furthermore, calculations based on a model in which 12-Crown-4 is coordinated to the lithium ion can reduce computational costs compared to calculations based on models in which two or more organic compounds are coordinated to the lithium ion.

[0052] Focusing on these coordination energies, the inventors have discovered that using organic compounds that satisfy coordination energy conditions 1 and 2 as the electrolyte results in excellent battery characteristics below freezing point.

[0053] Coordination energy condition 1 is that, as shown in Figure 2A, there is at least one first organic compound having a coordination energy of -35 [kJ / mol] or less. More preferably, the coordination energy of the first organic compound is -37 [kJ / mol] or less, and even more preferably -39 [kJ / mol] or less. Furthermore, it is preferable that the coordination energy of the first organic compound is -80 [kJ / mol] or more, more preferably -45 [kJ / mol] or more, even more preferably -43 [kJ / mol] or more, and most preferably -42 [kJ / mol] or more. The presence of a first organic compound with a low coordination energy allows for sufficient dissociation of the lithium salt, making it easier to maintain the lithium ion state and resulting in excellent battery characteristics below freezing point. Therefore, it is preferable to use a cyclic molecule as the first organic compound that satisfies coordination energy condition 1, but a chain molecule may also be used as the first organic compound.

[0054] Coordination energy condition 2 is that, as shown in Figure 2A, the second organic compound further contains a second organic compound that satisfies the condition that the difference (ΔE) between the coordination energy of the first organic compound and the second organic compound is 5 [kJ / mol] or less. This condition indicates that the coordination energy of the second organic compound is close to that of the first organic compound, and it is thought that the diffusion barrier of lithium ions coordinated by the first organic compound is reduced, resulting in excellent battery characteristics below freezing point. Furthermore, it is more preferable that the second organic compound satisfies condition 1 above, but the second organic compound does not have to satisfy condition 1 above. For this reason, it is preferable to use a chain-like molecule as the second organic compound, but a cyclic molecule may also be used as the second organic compound.

[0055] If these conditions are met, the electrolyte of the present invention may contain a third organic compound in addition to the first and second organic compounds, as shown in Figure 2B. In other words, the electrolyte of the present invention may contain a mixed solvent having three or more organic compounds. Furthermore, the coordination energy of the third organic compound is not limited in any way, but it is preferable for obtaining good battery characteristics below freezing point if the difference in coordination energy between the third organic compound and the second organic compound satisfies condition 2 above.

[0056] <Cyclic Molecules> Examples of cyclic molecules that can be used in the mixed solvent of the present invention while satisfying the above conditions include ethylene carbonate (abbreviated as EC), propylene carbonate (abbreviated as PC), butylene carbonate, chloroethylene carbonate, and vinylene carbonate (abbreviated as VC). Of these, the coordination energy of EC and the coordination energy of PC can satisfy coordination energy condition 1 and are preferable to use in the mixed solvent. Other cyclic molecules that are difficult to satisfy coordination energy condition 1 (for example, gamma-butyrolactone (abbreviated as GBL)) can be used in the mixed solvent as the second or third organic compound described above. Furthermore, EC and PC, which have high dielectric constants, are particularly preferred as mixed solvents, and in the case of a battery using graphite as the negative electrode active material, it is preferable to use EC as the mixed solvent.

[0057] Furthermore, among the organic compounds that can be used in the electrolyte of the present invention while satisfying the above conditions, fluorocyclic molecules (sometimes referred to as fluorinated cyclic molecules) are mentioned as cyclic molecules. Fluorcyclic molecules have substituents that exhibit electron-withdrawing properties, and their coordination energy is higher than that of EC, making them preferable as they easily satisfy coordination energy condition 2. Examples of fluorocyclic molecules include fluoroethylene carbonate (fluoroethylene carbonate (abbreviated as FEC), difluoroethylene carbonate (abbreviated as DFEC), trifluoroethylene carbonate (abbreviated as F3EC), or tetrafluoroethylene carbonate (abbreviated as F4EC)). Note that DFEC has isomers such as cis-4,5 and trans-4,5. However, the coordination energy of fluorocyclic molecules such as FEC is not sufficient to satisfy coordination energy condition 1, and they can be used in the mixed solvent as the second or third organic compound mentioned above.

[0058] In the mixed solvent of the present invention, there may be two or more cyclic molecules. In other words, the mixed solvent may be one in which a substance that satisfies coordination energy condition 1 is added to a substance that does not satisfy coordination energy condition 1.

[0059] <Chain Molecules> Among the organic compounds that can be used in the electrolyte of the present invention while satisfying the above conditions, chain molecules include ethyl propionate (abbreviated as EP), methyl propionate (abbreviated as MP), propyl propionate (abbreviated as PP), methyl formate, methyl acetate, ethyl acetate, methyl butyrate, ethyl methyl carbonate (abbreviated as EMC), diethyl carbonate (abbreviated as DEC), dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, methyl butyl carbonate, or ethyl propyl carbonate. Of these, the coordination energy of EP can satisfy coordination energy condition 1 and is preferable for use in the mixed solvent. Other chain molecules that do not easily satisfy coordination energy condition 1 can be used in the mixed solvent as the second or third organic compound described above.

[0060] Furthermore, among the organic compounds that can be used in the electrolyte of the present invention while satisfying the above conditions, examples of chain-like molecules include fluorinated chain molecules (sometimes referred to as fluorinated chain molecules) or fluorinated chain carboxylic acid esters (sometimes referred to as fluorinated chain carboxylic acid esters). Fluorinated chain molecules have substituents that exhibit electron-withdrawing properties, resulting in a high coordination energy, which is preferable as it easily satisfies coordination energy condition 2. As fluorinated chain carboxylic acid esters, methyl 3,3,3-trifluoropropionate (abbreviated as MTFP) can be used. The coordination energy of MTFP makes it difficult to satisfy coordination energy condition 1, and it can be used in the mixed solvent as the second or third organic compound described above.

[0061] The mixed solvent of the present invention may contain two or more types of chain molecules. In other words, it may be a mixed solvent in which a substance that satisfies coordination energy condition 1 is added to a substance that does not satisfy coordination energy condition 1.

[0062] Other organic compounds that can be used in the electrolyte of the present invention while satisfying the above conditions include one or more selected from 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, or 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (abbreviated as TTE), dimethoxyethane (abbreviated as DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, 1,3-dioxane, 1,4-dioxane, benzonitrile, tetrahydrofuran, sulfolane, and sultone. Of these, those that are difficult to satisfy coordination energy condition 1 can be used in the mixed solvent as the second or third organic compound described above.

[0063] Other organic compounds that can be used in the mixed solvent of the present invention while satisfying the above conditions include fluorine-containing aromatic compounds. Fluorine-containing aromatic compounds with 9 or fewer carbon atoms are preferred. Specific examples of fluorine-containing aromatic compounds with 9 or fewer carbon atoms include fluorobenzene (abbreviated as FB), 1,2-difluorobenzene, 1,3-difluorobenzene, and 1,4-difluorobenzene.

[0064] The coordination energy of the fluorine-containing aromatic compounds with 9 or fewer carbon atoms mentioned above may be higher than that of cyclic molecules, and is therefore preferable because it can satisfy coordination energy condition 2. Those of these compounds that do not easily satisfy coordination energy condition 1 can be used in the mixed solvent as the second or third organic compound mentioned above.

[0065] [Mixing Ratio] When the above coordination energy conditions 1 and 2 are met, the mixing ratio of organic compounds in the mixed solvent is not limited. However, since cyclic molecules tend to have higher freezing points than chain molecules, it is preferable to lower the proportion of cyclic molecules relative to the entire mixed solvent in order to obtain even better battery characteristics below freezing point. If the mixed solvent does not contain cyclic molecules, it is difficult to exhibit good battery characteristics below freezing point. Specifically, it is preferable that the proportion of cyclic molecules be higher than 0 and 50 mol% or less, preferably higher than 0 and 30 mol% or less, and more preferably 20 mol% or less, relative to the entire mixed solvent.

[0066] The mixed solvent described above preferably contains a low amount of molecules other than organic molecules (hereinafter also simply referred to as "impurities," which include oxygen, water, etc.) and is highly purified. It is also preferable that reaction by-products during synthesis are suppressed through appropriate purification. Specifically, the impurity content should be 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The water content can be detected by Karl Fischer titration.

[0067] [Lithium Salts] Next, we will explain lithium salts for electrolytes. Lithium compounds that are soluble in organic solvents can be used as lithium salts. Examples of lithium salts include LiPF ６ LiClO ４ LiAsF ６ LiBF ４ LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ Li ２ B １０ Cl １０ Li ２ B １２ Cl １２ LiCF ３ SO ３ LiC ４ F ９ SO ３ LiC (CF ３ SO ２ ) ３ LiC(C ２ F ５ SO ２ ) ３,LiN(CF ３ SO ２ ) ２ ,LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ) and LiN(C ２ F ５ SO ２ ) ２ One or more selected from the above can be used in any proportion. For example, LiPF4 fluoride ６ and LiBF ４ Using this method is preferable because it improves the safety of lithium-ion secondary batteries.

[0068] In the electrolyte, the concentration of lithium salt is preferably in the range of 0.2 mol to 1 mol per liter of mixed solvent, more preferably 0.2 mol to less than 1 mol, and more preferably 0.2 mol to less than 0.8 mol, when the temperature of the mixed solvent is 25°C ± 5°C. "Per liter of mixed solvent" refers to the total amount (total volume) of the mixed solvent per liter. The above concentration range is preferred because a higher lithium salt concentration leads to an increase in viscosity or solidification in low-temperature environments, resulting in a decrease in lithium ion conductivity. Furthermore, the above concentration range is preferred because a lithium salt concentration that is too low may lead to a decrease in the number of lithium ions in the electrolyte.

[0069] The freezing point of the electrolyte, in which the lithium salt is dissolved in the mixed solvent described above, is less than -30°C, preferably -30°C or lower, preferably -40°C or lower, and more preferably -50°C or lower. By having the electrolyte at the above freezing point, excellent battery characteristics can be achieved below freezing point.

[0070] [Additives] An electrolyte according to one aspect of the present invention may contain additives insofar as it has the above-described configuration. The additives will now be described. The additives can be the organic materials listed above as mixed solvents. Other organic materials that can be used as additives are preferably one or more selected from vinylene carbonate (abbreviated as VC), 1,3-propanesultone (abbreviated as PS), tert-butylbenzene (abbreviated as TBB), lithium bis(oxalate)borate (abbreviated as LiBOB), lithium difluorooxalatoborate (abbreviated as LiODFB), and dinitrile compounds such as succinonitrile, adiponitrile, or suberonitrile. The concentration of the additive is preferably 0.1 wt% to 10 wt% with respect to the total weight of the mixed solvent and lithium salt. FEC, VC, LiODFB, or LiBOB are preferred as additives because they easily form a good coating.

[0071] Of the additives mentioned above, 1,3-propanesultone (abbreviated as PS) is preferable because it has HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) levels equivalent to ethylene carbonate (abbreviated as EC), making it suitable for high charge-discharge conditions and less susceptible to oxidation and reduction even when using a wide cutoff voltage. Furthermore, PS has the advantage of being less likely to gasify when decomposed on the surface of the positive electrode active material. The electrolyte preferably contains 0.25 wt% to 7.5 wt% PS relative to the total weight of the mixed solvent and lithium salt.

[0072] The freezing point of the electrolyte, in which the lithium salt and additives are dissolved in the mixed solvent described above, is also set to less than -30°C, preferably -30°C or lower, preferably -40°C or lower, and more preferably -50°C or lower. By setting the electrolyte to the above freezing point, excellent battery characteristics can be achieved below freezing point.

[0073] [Ionic Conductivity] An electrolyte according to one aspect of the present invention has an ionic conductivity of 1 × 10 at -30°C. −７It is preferable that the ionic conductivity is 1 S / cm or higher. The ionic conductivity of the electrolyte referred to here is the characteristic of the lithium salt dissolved in the mixed solvent. If additives are present, the characteristic is the characteristic of the state in which the additives are mixed. The ionic conductivity is 1 × 10⁻⁶ −７ If the ionic conductivity is less than S / cm, the secondary battery having the electrolyte of the present invention may not be able to exhibit sufficient battery characteristics below freezing point. Of course, if the battery characteristics below freezing point are sufficient, the ionic conductivity is not limited in any way. The ionic conductivity at -30°C is more preferably 5 × 10⁻⁶. −６ It is S / cm or more, and more preferably 1 × 10 −６ The S / cm is greater than or equal to 1 × 10⁻⁶. −５ The ionic conductivity is greater than or equal to S / cm. There are no particular limitations on the method for measuring the above ionic conductivity, but the electrochemical impedance method is preferred.

[0074] [Viscosity] In one embodiment of the present invention, the electrolyte preferably has a viscosity of 320 mPa·s or less at 25°C. The viscosity of the electrolyte at 25°C referred to here is the characteristic of the lithium salt dissolved in the mixed solvent. If additives are present, the viscosity refers to the characteristic of the state in which the additives are mixed. If it exceeds 320 mPa·s, the ionic conductivity may not be sufficient, and the secondary battery having the electrolyte of the present invention may not be able to exhibit sufficient battery characteristics below freezing point. Of course, if the battery characteristics below freezing point are sufficient, the viscosity at 25°C is not limited in any way. The viscosity at 25°C is more preferably 220 mPa·s or less, even more preferably 120 mPa·s or less, and most preferably 50 mPa·s or less. There are no particular limitations on the method of measuring the above viscosity, but a method of measuring it using a viscometer at 25°C is preferred.

[0075] One embodiment of the present invention includes electrolytes injected into the casing of a secondary battery, and specifically includes electrolytes impregnated into the separator described later. The electrolytes include those that are already completed as an electrolyte when injected into the secondary battery, and those in which the components of the electrolyte (mixed solvent, lithium salt, etc.) are individually injected into the secondary battery and mixed within the battery to complete the electrolyte. Furthermore, one embodiment of the present invention includes electrolytes recovered from secondary batteries that have undergone pre-shipment inspection.

[0076] [Separator] The secondary battery according to one aspect of the present invention is not limited in any way to the separator, but it is preferable that the separator has high wettability to the mixed solvent described above, or high wettability to the electrolyte containing the mixed solvent. A specific compound that has high wettability to the electrolyte containing the mixed solvent described above is an imide compound, and it is preferable to use polyimide as a representative example. In this specification, wettability can be evaluated by the contact angle. The contact angle is preferably measured according to JIS R3257, but for example, the value measured when 1 μL to 25 μL of electrolyte is dropped onto the separator member at 25°C and 30 seconds to 60 seconds have elapsed can be adopted. The contact angle can be measured from an image observed from the horizontal direction. It is also preferable that the contact angle be the average value of measurements taken at three or more points. In this specification, good wettability means that the contact angle is less than 30 degrees, preferably less than 20 degrees, and more preferably less than 10 degrees.

[0077] Furthermore, the separator can be made of one or more materials selected from cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polypropylene, polyester, acrylic, polyolefin, and polyurethane. One or more of these can be combined with polyimide. Polypropylene is particularly preferred because it can exhibit a shutdown function. The shutdown function is that when the secondary battery overheats abnormally, the material (typically polypropylene) melts, blocking the pores through which the carrier ions of the separator pass, and thereby stopping the oxidation-reduction reaction inside the battery.

[0078] The separator may have a structure (referred to as a laminated structure) in which the above-mentioned materials are arbitrarily selected and laminated. The laminated structure includes a structure in which two or more layers of the same material are laminated, for example, a configuration in which two or more layers of polyimide are laminated.

[0079] The separator preferably has a coating layer on its surface. To maintain the wettability of the separator to the electrolyte, it is preferable that at least a portion of the separator's surface is exposed from the coating layer. Therefore, it is preferable that the coating layer be provided at a selective position relative to the separator. The coating layer preferably has an inorganic material, typically magnesium oxide, silicon oxide, or aluminum oxide, and these materials may be in the form of parts. That is, a layer of particulate inorganic material adhering to the surface of the separator is included in the coating layer. It is also preferable to use a coating layer containing carbon.

[0080] The separator may have recesses on its surface. A recess refers to a region where the thickness is reduced in a cross-sectional view. Preferably, the recesses are arranged in a stripe pattern when viewed from above. The coating layer can be applied only to the recesses. Alternatively, the coating layer can be applied only to the protrusions formed together with the recesses. The presence of recesses may improve the electrolyte content, making it a preferable configuration for the separator.

[0081] The separator film thickness is preferably 10 μm to 80 μm, and more preferably 20 μm to 60 μm. The separator film thickness can be determined, for example, by measuring the length at the center of a cross-sectional image of a secondary battery including the separator. The film thickness of the recess is preferably 10 μm to 50 μm.

[0082] The shape of the separator is not limited and can be, for example, a sheet. The separator may also be in the form of a bag, and a form in which either the positive or negative electrode is housed in the bag is also suitable as a separator.

[0083] [Secondary Battery] Next, a secondary battery having the electrolyte and separator described above will be explained using Figures 3A to 4B. Figure 3A shows the components of the secondary battery 100 stacked on top of each other, and Figure 3B shows the components of the secondary battery 100 separated from each other. Figure 4A shows a cross-sectional view of the secondary battery 100, and Figure 4B shows the positive electrode active material layer 22 of the secondary battery 100.

[0084] The secondary battery 100 has multiple positive electrodes. In Figure 3B, the multiple positive electrodes are shown as a first positive electrode 103a and a second positive electrode 103b. The first positive electrode 103a and the second positive electrode 103b are collectively referred to as positive electrode 103. However, the number of positive electrodes in the secondary battery 100 is not limited to two layers; it may be one layer or have three or more layers.

[0085] The secondary battery 100 has multiple negative electrodes. In Figure 3B, the multiple negative electrodes are shown 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 negative electrode 106. However, the number of negative electrodes in the secondary battery 100 is not limited to three layers; it may be one, two, or four or more layers.

[0086] The secondary battery 100 has a separator between the negative electrode and the positive electrode. In Figure 3B, the first separator 105a, the second separator 105b, the third separator 105c, and the fourth separator 105d are shown 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, in the secondary battery 100, the number of separators is not limited to four layers; it may be one, two, three, or five or more layers. As for the multiple separators, they may be separate sheets as shown in Figure 3B, but it is also possible to use a continuous separator. A continuous separator is prepared by using a separator with a larger surface area than the positive and negative electrodes, and by bending the separator as appropriate, the separator portion is positioned to correspond to the first separator 105a to the fourth separator 105d. Since there are more separators than positive or negative electrodes, the overall film thickness of the separator becomes thinner, which increases the capacity per unit volume of the secondary battery.

[0087] Figure 4A is an example of a cross-sectional view of a secondary battery 100 along the dashed line AB in Figure 3B. In Figure 4A, the positive electrode 103, separator 105, negative electrode 106, and protrusion 31t are used for explanation.

[0088] 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 having 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 involves press working, and in the positive electrode that has undergone this press working, a recess may be formed in a part of the positive electrode current collector 21 in which positive electrode active material particles have been pressed. As shown in Figure 4A, 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 coating 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 coating structure.

[0089] The protrusion 21t shown in Figure 3A is part of the positive electrode current collector 21. In other words, the protrusion 21t is a region of the positive electrode current collector 21 in which the positive electrode active material layer 22 is not provided. As shown in Figure 3A, in the secondary battery 100, multiple protrusions 21t overlap and form an assembly. The assembly of protrusions 21t is called the 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 around the joint 109a and / or the positive electrode lead 107a. Kapton tape can be used as the insulating seal.

[0090] Furthermore, Figure 4B 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 positive electrode active material 10. The positive electrode active material layer 22 has an electrolyte 108. The electrolyte 108 has at least a mixed solvent and a lithium salt. The positive electrode active material layer 22 may have a second positive electrode active material 20. The positive electrode active material layer 22 may have a conductive material 41. Although not shown, the positive electrode active material layer 22 may also have a binder. The positive electrode active material layer 22 does not have a second positive electrode active material 20. The positive electrode active material layer 22 does not have a conductive material 41. The positive electrode active material layer 22 does not have a binder.

[0091] The positive electrode active material 10 preferably has an average particle size of 5 μm or more and less than 10 μm, and a maximum particle size of less than 20 μm, and it is desirable that it satisfies the requirement of a large diameter (also called a large particle size). Secondary particles may also be used as the positive electrode active material 10, and it is desirable that the secondary particles satisfy the requirement of an average particle size of 5 μm or more and less than 10 μm, and a maximum particle size of less than 20 μm. The positive electrode active material 10 may have a layered rock salt type crystal structure, LiMO ２ A composite oxide represented by (where M is one or more selected from Fe, Ni, Co, Mn, and Al) can be used, and lithium cobalt oxide can be used as a typical example. Lithium cobalt oxide with good high-voltage charging characteristics will be described in Embodiment 2 and later.

[0092] As the second cathode active material 20 included in the cathode 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 it is preferable to use an active material that satisfies a small diameter (also referred to as small particle size). The second cathode active material 20 includes LiM having an olivine-type crystal structure ２ PO ４ (wherein M2 is one or more selected from Fe, Ni, Co, and Mn) can be used. LiMPO ４ , as examples, include LiFePO ４ , LiNiPO ４ , LiCoPO ４ , LiMnPO ４ , LiFe ａ Ni ｂ PO ４ , LiFe ａ Co ｂ PO ４ , LiFe ａ Mn ｂ PO ４ , LiNi ａ Co ｂ PO ４ , LiNi ａ Mn ｂ PO ４ (where a + b ≤ 1, 0 < a < 1, 0 < b < 1), LiFe ｃ Ni ｄ Co ｅ PO ４ , LiFe ｃ Ni ｄ Mn ｅ PO ４ , LiNi ｃ Co ｄ Mn ｅ PO ４ (where c + d + e ≤ 1, 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe ｆ Ni ｇ Co ｈ Mn ｉ PO ４ (where f + g + h + i ≤ 1, 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), etc., and lithium iron phosphate can be typically used. Further, it is preferable that the particle surface of the second cathode active material 20 has a carbon layer.

[0093] 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. The manufacturing process of the negative electrode 106 involves press working, and in the negative electrode that has undergone this press working, a recess may be formed in a part of the negative electrode current collector 31 in which the negative electrode active material particles are pressed. As shown in Figure 4A, 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 placed in the outermost layer of the secondary battery 100, the negative electrode active material layer that is not placed facing the positive electrode does not have carrier ion insertion and removal, or insertion and removal is difficult, so it is not necessary to form a negative electrode active material layer. In other words, single-sided coating is often used for the outermost 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. It is preferable to prepare all negative electrodes as a double-sided coating structure as it is more productive. In this case, a negative electrode with a double-sided coating structure can be placed even in the outermost layer.

[0094] Furthermore, the negative electrode current collector 31 has a protrusion 31t. The protrusion 31t is a region where the negative electrode active material layer 32 is not provided. In Figure 3B, a first protrusion 31ta, a protrusion 31tb, and a third protrusion 31tc are shown as multiple protrusions. The first protrusion 31ta, the protrusion 31tb, and the third protrusion 31tc are collectively referred to as the protrusion 31t. The protrusion 31t is also shown in Figure 3A. Multiple protrusions 31t overlap and form an aggregate. The aggregate of protrusions 31t is called the negative electrode tab. Note that in Figure 4A, the protrusion 31t is shown separated from the other protrusions.

[0095] As shown in Figure 3A, the negative electrode tab (protrusion 31t) is joined to the negative electrode lead 107b at the joint 109b. Ultrasonic bonding can be used for bonding. As a result of bonding, they are electrically connected to each other. The negative electrode lead 107b can be made of a material selected from nickel, copper, titanium, or an alloy thereof. An insulating seal may be placed around the joint 109b and / or the negative electrode lead 107b. Kapton tape can be used as the insulating seal.

[0096] The negative electrode active material layer 32 may further contain a binder. The negative electrode active material layer 32 may also further contain a conductive material. Of course, the negative electrode active material layer 32 does not necessarily have to contain a binder or conductive material. The binder and conductive material will be described later.

[0097] In this specification, a structure in which multiple positive electrodes, multiple negative electrodes, and multiple separators are stacked, as shown in Figure 3B, is referred to as a stacked electrode.

[0098] [Outer Covering] Although not shown in the diagram, the secondary battery 100 has an outer covering, and the positive electrode, negative electrode, etc., are housed in the outer covering. For example, the outer covering 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 covering can be used. As a film, for example, a three-layer film can be used in which a highly flexible metal thin film or metal foil made of aluminum, stainless steel, titanium, copper, nickel, etc. is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is provided on the metal thin film as the outer surface of the outer covering. Such a multilayer film can be called a laminate film. In this case, the name of the material of the metal layer in the laminate film may be used to refer to it, such as aluminum laminate film, stainless steel laminate film, titanium laminate film, copper laminate film, nickel laminate film, etc.

[0099] For example, it is preferable to use an aluminum laminate film having a polypropylene layer, an aluminum layer, and a nylon layer as the outer casing. Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and more preferably 20 μm or less. If the aluminum layer is thinner than 10 μm, there is a concern that the gas barrier properties will decrease due to pinholes in the aluminum layer, so it is desirable that the thickness of the aluminum layer be 10 μm or more.

[0100] For a secondary battery 100 where physical strength or safety is important, it is preferable to use a laminate film for the outer casing that has, for example, a polypropylene layer, a stainless steel layer, and a nylon layer, preferably laminated in this order, with the nylon layer being the outermost layer. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer, in which case it is preferable that the polyethylene terephthalate layer be the outermost layer. Furthermore, a stainless steel layer may be provided on the nylon layer, in which case it is preferable that the stainless steel layer be the outermost layer. The thickness of the stainless steel layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and more preferably 20 μm or less. If the stainless steel layer is thinner than 10 μm, there is a concern that the gas barrier properties may decrease due to pinholes in the stainless steel layer, so it is desirable that the thickness of the stainless steel layer be 10 μm or more. In this specification, stainless steel refers to steel (an alloy of iron and carbon) containing about 12% or more chromium, and can be broadly classified into martensitic, ferritic, or austenitic types in terms of composition. Furthermore, stainless steel also includes stainless steel to which one or more elements selected from Ti, Nb, Mo, Cu, Ni, or Si have been added.

[0101] Furthermore, it is preferable to use a titanium laminate film having, for example, a polypropylene layer, a titanium layer, and a nylon layer as the outer casing. In addition, a polyethylene terephthalate layer may be present 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 more preferably 20 μm or less. If the titanium layer is thinner than 10 μm, there is a concern that the gas barrier properties will decrease due to pinholes in the titanium layer, so it is desirable that the thickness of the titanium layer be 10 μm or more.

[0102] A secondary battery that uses film as its outer casing is called a laminated secondary battery. Although not shown in this embodiment, a tin case may also be used as the outer casing. For example, a secondary battery using a circular case is called a coin-type secondary battery. A secondary battery using a cylindrical case is called a cylindrical secondary battery.

[0103] The contents of this embodiment can be appropriately combined with the contents of other embodiments.

[0104] (Embodiment 2) A positive electrode active material, which is one aspect of the present invention, will be described.

[0105] 《Method for preparing positive electrode active material》 Lithium cobalt oxide, which has good high-voltage charging characteristics, can be used as the positive electrode active material 10. A method for preparing such a positive electrode active material 10 will be explained with reference to Figures 5 to 6B.

[0106] <Step S14> Prepare lithium cobalt oxide as shown in Step S14. Pre-synthesized lithium cobalt oxide can be used. The median diameter (D50) of the lithium cobalt oxide is preferably 5 μm or more and 10 μm or less.

[0107] <Step S15> Next, as shown in step S15, the lithium cobalt oxide is heated. This heating step is called initial heating. By performing initial heating on the pre-synthesized lithium cobalt oxide, a lithium cobalt oxide with a smooth surface can be obtained. A smooth surface is defined as a state in which there are few foreign substances adhering to the surface.

[0108] Lithium cobalt oxide is placed in a container, covered, and the container is heated. The heating conditions are preferably, for example, a temperature of 750°C to 950°C for 1 to 3 hours. It is also preferable to flow oxygen into the heating furnace at a rate of 8 L / min to 12 L / min and to maintain positive pressure inside the heating furnace. After initial heating, the heating furnace is cooled at a rate of 180°C / hour to 220°C / hour or allowed to cool naturally.

[0109] Initial heating may cause some lithium to detach from lithium cobalt oxide. Furthermore, initial heating can be expected to enhance the crystallinity of lithium cobalt oxide.

[0110] In this initial heating stage, it is not necessary to prepare a lithium source. Furthermore, in the initial heating stage, it is not necessary to prepare an additive element source. Also, in the initial heating stage, it is not necessary to prepare a material that functions as a flux.

[0111] However, initial heating is not always necessary. In that case, this step can be omitted, which improves productivity.

[0112] <Step S20> Next, as shown in step S20, it is preferable to add an additive element to lithium cobalt oxide. In this embodiment, the additive element is added in multiple steps. The additive element added first, as in step S20, is denoted as A1, and the additive element added second, as in step S40 described later, is denoted as A2. The step of adding additive element A1 will be explained using Figure 6A.

[0113] <Step S21> In step S21 shown in Figure 6A, a source of additive elements (source A1) to be added to lithium cobalt oxide is prepared. A lithium source may also be prepared together with source A1.

[0114] As additive element A1, one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used.

[0115] When magnesium is selected as the additive element, the additive element source can be called a magnesium source. Suitable magnesium sources include magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate. Multiple magnesium sources may also be used.

[0116] When fluorine is chosen as the additive element, the additive element source can be called a fluorine source. Examples of such fluorine sources include lithium fluoride (LiF) and magnesium fluoride (MgF). ２ ), aluminum fluoride (AlF ３ ), Titanium Fluoride (TiF ４ ), cobalt fluoride (CoF ２ CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF) ２ ), cerium fluoride (CeF ３ CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ ) and the like can be used. Among them, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating process described later.

[0117] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Another lithium source used in step S21 is lithium carbonate.

[0118] Furthermore, a gaseous fluorine source can also be used, for example, fluorine (F ２ ), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F), Nitrogen trifluoride (NF ３ Alternatively, a mixture of the above-mentioned fluorine sources may be used and mixed into the atmosphere during the heating process described later.

[0119] In the method for preparing the positive electrode active material described in Figures 5 and 6A, a magnesium source and a fluorine source (labeled Mg source and F source in Figure 6A) are used as additive elements A1. Lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is used as the fluorine source and magnesium source. ２ Prepare the following: Lithium fluoride and magnesium fluoride are LiF:MgF ２Mixing lithium fluoride and magnesium fluoride in a molar ratio of approximately 65:35 yields the greatest effect in lowering the melting point. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be in excess and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride should be LiF:MgF ２ Preferably, γ: 1 (0 ≤ γ ≤ 1.9), and LiF: MgF ２ = γ: 1 (0.1 ≤ γ ≤ 0.5) is more preferable, and LiF: MgF ２ A more preferable value is γ:1 (γ = 0.33 or its vicinity). In this specification, "nearby" means a value greater than 0.9 times the value and less than 1.1 times the value.

[0120] <Step S22> Next, in step S22 shown in Figure 6A, the magnesium source and fluorine source are crushed and mixed. A wet method is preferred for this step. When using a wet method, the magnesium source and fluorine source are added together with the media to a solvent such as dehydrated acetone, and crushed and mixed at a rotational speed of 400 rpm to 600 rpm for 15 to 25 hours. After that, the solvent is removed by drying or the like to obtain a mixture containing the magnesium source and fluorine source. To dissolve the agglomeration of the mixture, it is preferable to sift it using a sieve with a mesh size of 200 μm to 400 μm.

[0121] <Step S23> Next, in step S23 shown in Figure 6A, the above mixture is recovered and used as source A1. The source A1 shown in step S23 has multiple starting materials and can be called a mixture.

[0122] The particle size of the above mixture is preferably such that the median diameter (D50) is 600 nm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less. When the mixture is finely powdered in this way, it is easier to uniformly adhere the mixture to the surface of the lithium cobalt oxide particles when it is mixed with lithium cobalt oxide in a later step. When the mixture is uniformly adhered to the surface of the lithium cobalt oxide particles, the added elements are more easily distributed or diffused uniformly in the heated lithium cobalt oxide, which is preferable.

[0123] <Step S31> Next, in step S31 shown in Figure 5, the A1 source and the lithium cobalt oxide after initial heating are mixed. The mixing is preferably carried out in a dry room with a dew point of -100°C or higher and -10°C or lower. To avoid destroying the shape of the lithium cobalt oxide particles, it is preferable to stir the mixture for 5 to 20 minutes at a rotational speed of 2000 rpm to 4000 rpm using, for example, a Picobond (manufactured by Hosokawa Micron). It is also preferable to use a Novilta rotor for the Picobond. Furthermore, the ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the A1 source is preferably Co:Mg = 100:y (0.1 ≤ y ≤ 6), and more preferably Co:Mg = 100:y (0.3 ≤ y ≤ 3).

[0124] <Step S32> Next, in step S32 of Figure 5, the materials mixed above are recovered to obtain mixture 901. When recovering, if necessary, the materials may be crushed and then sieved using media of 40 μm to 60 μm.

[0125] <Step S33> Next, in step S33 shown in Figure 5, the mixture 901 is placed in a container, covered, and heated. The heating conditions are, for example, a temperature of 750°C to 950°C, preferably 830°C to 950°C, for 8 to 12 hours. It is also preferable to flow oxygen into the heating furnace at a rate of 8 L / min to 12 L / min and to maintain positive pressure inside the heating furnace. After heating, the heating furnace is cooled at a rate of 180°C / hour to 220°C / hour or allowed to cool naturally.

[0126] Here, we will add some information regarding the heating temperature in this step. The lower limit of the heating temperature in step S33 must be above the temperature at which the reaction between lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds is preferably the temperature at which interdiffusion between lithium cobalt oxide and the additive element occurs, and may be lower than the melting temperature of these materials. We will explain using oxides as an example, but the melting temperature T ｍ 0.757 times (Tammann temperature T) ｄ It is known that solid-phase diffusion occurs from ). For this reason, the heating temperature in step S33 is preferably 750°C or higher.

[0127] Of course, the reaction proceeds more easily if the temperature is above the melting point of one or more of the materials selected from the mixture 901. For example, LiF and MgF can be used as additive element sources. ２ If LiF and MgF are present, ２ The eutectic point is around 742°C. Therefore, the lower limit of the heating temperature in step S33 is preferably 742°C or higher, and can typically be 750°C.

[0128] Also, lithium cobalt oxide (LiCoO ２ ): LiF: MgF ２ The mixture 901 obtained by mixing in a molar ratio of 100:0.33:1 had an initial melting temperature T ｉｍ The temperature is 779°C, and the melting peak temperature is T. ｐｍ The temperature is 815°C, and the melting end temperature is T. ｅｍ The temperature was 826°C. Therefore, the lower limit of the heating temperature is more preferably 826°C or higher, and can typically be 830°C.

[0129] The upper limit of the heating temperature should be below the melting point of lithium cobalt oxide (1130°C). At temperatures near the melting point, there is a concern that lithium cobalt oxide may decompose, albeit in small amounts. Furthermore, when heating mixture 901, it is preferable to control the partial pressure of fluorine or fluoride, which may be due to a fluorine source, within an appropriate range. If the temperature is too high, the fluoride will decrease due to evaporation. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, it is more preferable to keep the temperature below 950°C.

[0130] In the manufacturing method described in this embodiment, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be lowered to below the melting point of lithium cobalt oxide, for example, between 750°C and 950°C, enabling the distribution of additive elements, including magnesium, to the surface layer and producing a positive electrode active material that exhibits good battery characteristics. If LiF evaporates, the flux effect may decrease. Therefore, in this step, it is preferable to cover the container to suppress the evaporation of LiF.

[0131] Let me add some information about the heating time. The heating time varies depending on conditions such as the heating temperature, the size of the lithium cobalt oxide in step S14, and its composition. When the lithium cobalt oxide is small, a lower temperature or shorter time may be preferable than when it is large. When the median diameter (D50) of the lithium cobalt oxide in step S14 is 5 μm or more and 10 μm or less, the heating temperature is preferably 750°C or more and 950°C or less. The heating time is preferably 8 hours or more and 12 hours or less.

[0132] <Step S34> Next, in step S34 shown in Figure 5, the heated material is recovered and crushed as necessary to obtain the composite oxide 902.

[0133] <Step S40> Next, in step S40 shown in Figure 5, an additive element source (A2 source) is prepared. As the additive element A2, the additive elements described in step S21 can be used. In the method for producing positive electrode active material 1 described in Figures 5 to 6B, nickel and aluminum are used as the additive element A2. As the nickel source, nickel oxide, nickel hydroxide, etc. can be used. As the aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used. As shown in steps S41 to S43 in Figure 6B, the nickel source and the aluminum source can be crushed to make the A2 source. The conditions for crushing can be found in the conditions of step S22.

[0134] <Step S51> Next, in step S51 shown in Figure 5, the composite oxide 902 and the A2 source are mixed. The mixing is preferably carried out in a dry room with a dew point of -100°C or higher and -10°C or lower. In order not to destroy the shape of the lithium cobalt oxide particles, it is preferable to stir the mixture for 5 to 20 minutes at a rotational speed of 2000 rpm to 4000 rpm using, for example, a picobond (manufactured by Hosokawa Micron). It is also preferable to use a Novilta rotor for the picobond.

[0135] <Step S52> Next, in step S52 shown in Figure 5, the materials mixed above are recovered to obtain a mixture 903. When recovering, if necessary, the materials may be crushed and then sieved using a media of 40 μm to 60 μm.

[0136] <Step S53> Next, in step S53 shown in Figure 5, the mixture 903 is heated. The heating conditions can be found in the description of step S33.

[0137] <Step S74> Next, in step S74 shown in Figure 5, 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 sift the recovered particles. The positive electrode active material 10 can be produced by the above steps.

[0138] The positive electrode active material 10 has a smooth surface. A positive electrode active material 10 with a smooth surface may be more resistant to physical damage caused by pressure, etc., than a positive electrode active material with a less smooth surface.

[0139] The characteristics of the positive electrode active material 10 produced through the process described above will be explained with reference to Figures 7A to 13G.

[0140] <Positive electrode active material 10> Figures 7A and 7B are cross-sectional views of a positive electrode active material 10 according to one embodiment of the present invention. Figures 8A to 8C show enlarged views of the area around A-B in Figure 7B. Figures 8D to 8F show enlarged views of the area around C-D in Figure 7B.

[0141] As shown in Figure 7A, the positive electrode active material 10 has a surface layer 10a and an interior 10b. In Figures 7A and 7B, a dashed line is shown as an example of the boundary between the surface layer 10a and the interior 10b.

[0142] The surface layer 10a of the positive electrode active material 10 refers to the region within 20 nm, preferably within 10 nm, of the surface of the particle. Surfaces newly formed on the particle due to cracks and / or fissures may also be considered the surface. The surface layer 10a is synonymous with the vicinity of the surface, the vicinity of the surface region, or the shell.

[0143] Furthermore, the region deeper than the surface layer 10a of the positive electrode active material is called the interior 10b. The interior 10b is synonymous with the interior region or core.

[0144] Furthermore, when the positive electrode active material 10 has a layered rock salt type crystalline structure of space group R-3m, the surface layer 10a has an edge region 10a1 and a basal region 10a2, as shown in Figure 7B. In Figures 7A and 7B, the line labeled (00l) represents the (00l) plane. The edge region 10a1 is a region having a plane that intersects with the (00l) plane. Intersecting with the (00l) plane means that the angle between the perpendicular to the (00l) plane and the normal to the surface of the positive electrode active material 10 is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less. The basal region 10a2 is a region having a surface parallel to the (00l) plane. Furthermore, "parallel to 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 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0145] 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 is aluminum oxide (Al) which does not have lithium sites that can contribute to charging and discharging. ２ O ３ ) and silicon oxide (SiO ２ This excludes metal oxides adhering to surfaces such as the ) and carbonates and hydroxyl groups chemically adsorbed after the production of the positive electrode active material. Adhered metal oxides refer to, for example, metal oxides whose crystal orientation does not match that of the interior 10b.

[0146] The general agreement of crystal orientations between two regions can be determined from TEM images, STEM images, HAADF-STEM images, ABF-STEM images, electron diffraction patterns, etc. It can also be determined from the FFT patterns of TEM images and STEM images, etc. Furthermore, XRD and neutron diffraction can also be used as criteria for determination. For example, in a HAADF-STEM image, if the angle between the bright fringes of the first crystal in the two regions and the bright fringes of the second crystal is 5 degrees or less, it can be said that the crystal orientations of the two regions are general agreement. It is also possible to use the dark fringes in HAADF-STEM images, but when determining orientation agreement, it is preferable to use the bright fringes that are easier to see.

[0147] Furthermore, the positive electrode active material 10 does not contain any attached electrolyte, electrolyte decomposition products, organic solvents, binders, conductive materials, or compounds derived therefrom.

[0148] <Elements contained> The positive electrode active material 10 contains lithium, cobalt, oxygen, and additive element A. In other words, the positive electrode active material 10 is lithium cobalt oxide (LiCoO ２ Also known as LCO, the composition of lithium cobalt oxide is not strictly limited to an atomic ratio of Li:Co:O = 1:1:2, and the atomic ratio of Li may be between 0.8 and 1.2) and can have additive element A. The positive electrode active material 10 is preferably one having the crystalline structure described later.

[0149] The positive electrode active material needs to contain a redox-capable transition metal in order to maintain charge neutrality even when lithium ions are inserted and removed. It is preferable that the positive electrode active material 10 primarily uses cobalt as the transition metal responsible for the redox reaction. In addition to cobalt, at least one or more selected from nickel and manganese may be used. It is preferable that the positive electrode active material 10 contains 75 atomic percent or more, preferably 90 atomic percent or more, of cobalt, as this offers many advantages, such as being relatively easy to synthesize, easy to handle, and possessing excellent cycle characteristics.

[0150] Furthermore, if the cobalt content among the transition metals of the positive electrode active material 10 is 75 atomic percent or more, preferably 90 atomic percent or more, and more preferably 95 atomic percent or more, then lithium nickelate (LiNiO) ２ Compared to composite oxides in which nickel, such as ), makes up the majority of the transition metals, Lix α CoO ２ x inside α Stability is better when the value is small.

[0151] It is preferable to use one or more of the following as the additive element A in the positive electrode active material 10: magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium. These additive elements A further stabilize the crystal structure of the positive electrode active material 10, as will be described later.

[0152] Additive 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.

[0153] For example, if the positive electrode active material 10 is substantially free of manganese, the above advantages such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics become even greater. The weight of manganese contained in the positive electrode active material 10 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0154] It is preferable that the additive element A is solid-dissolved in the positive electrode active material 10. For example, when performing STEM-EDX line analysis, it is preferable that the position of the rising peak where the additive element A is detected in the depth direction is deeper than the position of the rising peak where the transition metal Me is detected, i.e., located inside the positive electrode active material 10.

[0155] The surface layer 10a is the region where lithium ions first desorb during charging, and it is a region where the lithium concentration tends to be lower than that of the interior 10b. Furthermore, the atoms on the surface of the positive electrode active material 10 in the surface layer 10a can be described as having some of their bonds broken. Therefore, the surface layer 10a is prone to instability, and it is a region where degradation of the positive electrode active material due to changes in crystal structure is likely to begin. On the other hand, if the surface layer 10a can be made sufficiently stable, Lix α CoO ２ x inside α Even when x is small, for example, α Even if the ratio is 0.24 or less, the layered structure of the internal 10b, consisting of octahedrons of cobalt and oxygen, can be made less prone to breakage. Furthermore, the displacement of the layers of the internal 10b, consisting of octahedrons of cobalt and oxygen, can be suppressed.

[0156] To ensure a stable crystalline structure in the surface layer 10a, it is preferable that the surface layer 10a contains additive element A, and more preferably that it contains multiple additive element A. Furthermore, it is preferable that the surface layer 10a has a higher concentration of one or more selected additive element A than the interior 10b. It is also preferable that the one or more selected additive element A in the positive electrode active material 10 have a concentration gradient. Moreover, it is more preferable that the distribution of the positive electrode active material 10 differs depending on the additive element A. For example, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element A. Here, "concentration peak" refers to the maximum concentration in the surface layer 10a or within 20 nm from the surface.

[0157] [Distribution] The distribution of additive element A will now be explained. Figures 8A to 8C illustrate the edge region 10a1 of the positive electrode active material 10. Figures 8D to 8F illustrate the basal region 10a2 of the positive electrode active material 10.

[0158] 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 towards the surface, as shown by the gradient in Figures 8A and 8D. Additive elements A having such a concentration gradient will be called additive elements Xa. Additive elements Xa often correspond to additive elements A1, but do not necessarily have to correspond to additive elements A1. The concentration gradient shown by the gradient in Figures 8A and 8D can be obtained depending on the diffusion rate, not the timing of addition.

[0159] Another additive element A, such as aluminum or manganese, preferably has a concentration gradient as shown by the density of the hatches in Figures 8B and 8E, and has a concentration peak in a region deeper than that of additive element Xa shown in Figures 8A and 8D. The concentration peak may be located in the surface layer 10a or deeper than the surface layer 10a. For example, it is preferable to have a peak in a region of 5 nm to 50 nm from the surface inward. An additive element having such a concentration gradient will be called additive element Ya. Additive element Ya often corresponds to additive element A2, but does not necessarily have to correspond to additive element A2. The concentration gradient shown by the density of the hatches in Figures 8B and 8E depends on the diffusion rate, not the timing of addition.

[0160] 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, as shown by the presence or absence and density of hatches in Figures 8C and 8F. Here, "clearly present" means that the characteristic X-ray energy spectrum of the element is detected in the cross-sectional STEM-EDX analysis of the positive electrode active material 10. "Substantially absent" means that the characteristic X-ray energy spectrum of the element is not detected in the cross-sectional STEM-EDX analysis of the positive electrode active material 10. This can also be said to mean that the element is below the detection limit in the STEM-EDX analysis. Additive elements with such a distribution will be called additive element Za. Additive element Za often corresponds to additive element A2, but does not necessarily have to correspond to additive element A2. The concentration gradient shown by the density of the hatches in Figures 8B and 8E depends not on the timing of addition, but on the diffusion rate.

[0161] For example, magnesium ions, one of the doping elements Xa, are divalent, and since magnesium ions are more stable in lithium sites than in cobalt sites in the layered rock salt crystal structure, they readily enter the lithium sites. The presence of magnesium at an appropriate concentration in the lithium sites of the surface layer 10a makes it easier to maintain the layered rock salt crystal structure. This is because the magnesium present in the lithium sites is CoO ２ It is presumed that this is because it functions as a pillar to support the layers. Also, the presence of magnesium is due to Lix α CoO ２ x inside α For example, when the concentration is 0.24 or less, the desorption of oxygen around the magnesium can be suppressed. Furthermore, if the magnesium concentration in the surface layer 10a is high, it can be expected that the corrosion resistance to hydrogen fluoride produced by the decomposition of the electrolyte will improve.

[0162] At appropriate concentrations, magnesium does not adversely affect lithium insertion and removal during charging and discharging, and the above benefits can be enjoyed. However, excessive magnesium may adversely affect lithium insertion and removal. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters not only lithium sites but also cobalt sites. In addition, excess magnesium compounds (oxides or fluorides, etc.) that do not substitute for lithium or cobalt sites may segregate on the surface of the positive electrode active material and become a resistive component of the secondary battery. Moreover, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the lithium sites, reducing the amount of lithium that contributes to charging and discharging.

[0163] Therefore, it is preferable that the total amount of magnesium in the positive electrode active material 10 is appropriate. For example, the number of magnesium atoms is preferably 0.001 times or more and 0.1 times or less the number of cobalt atoms, more preferably greater than 0.01 times and less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium in the total positive electrode active material 10 referred to here may be the value obtained by elemental analysis of the entire positive electrode active material 10 using, for example, Glow Discharge Mass Spectrometry (GD-MS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), etc., or it may be based on the value of the raw material composition in the process of manufacturing the positive electrode active material 10.

[0164] Furthermore, aluminum, one of the additive elements Ya, can be present in the cobalt site of a layered rock salt-type crystal structure. Since aluminum is a trivalent typical element and its valency does not change, lithium around the aluminum does not easily move during charging and discharging. Therefore, the aluminum and the surrounding lithium can function as pillars, suppressing changes in the crystal structure. In addition, aluminum suppresses the dissolution of surrounding cobalt, improving cycle characteristics. Moreover, since 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, having aluminum as an additive element Ya can improve the safety when using the positive electrode active material 10 in a secondary battery. Furthermore, it is possible to create a positive electrode active material 10 whose crystal structure is less likely to collapse even after repeated charging and discharging.

[0165] On the other hand, an excess of aluminum may adversely affect the insertion and removal of lithium. Therefore, it is preferable that the total amount of aluminum in the positive electrode active material 10 is appropriate. For example, the total number of aluminum atoms in the positive electrode active material 10 is preferably 0.05% to 4%, more preferably 0.1% to 2%, and more preferably 0.3% to 1.5% of the total number of cobalt atoms. Alternatively, the above is preferably 0.05% to 2%, or more preferably 0.1% to 4%. The total amount of aluminum in the positive electrode active material 10 referred to here may be, for example, the value obtained by elemental analysis of the entire positive electrode active material 10 using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material 10.

[0166] Furthermore, nickel, one of the additive elements Za, can be present in both cobalt and lithium sites. When present in the cobalt site, its oxidation-reduction potential is lower compared to cobalt, which is preferable as it leads to an increase in discharge capacity.

[0167] Furthermore, when nickel is present in the lithium site, the displacement of the layered structure consisting of octahedra of cobalt and oxygen can be suppressed. Also, volume changes associated with charging and discharging are suppressed. Additionally, the elastic modulus increases, meaning it becomes harder. This is because nickel present in the lithium site also contributes to the CoO ２ It is presumed that this is because they function as pillars to support the layers together.

[0168] On the other hand, an excess of nickel is undesirable because it intensifies the distortion caused by the Jahn-Teller effect. Furthermore, an excess of nickel may adversely affect the insertion and removal of lithium.

[0169] 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% of the number of cobalt atoms and 7.5% or less, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferable that the above is greater than 0% and 4% or less. Alternatively, it is preferable that the above is greater than 0% and 2% or less. Alternatively, it is preferable that the above is 0.05% to 7.5%. Alternatively, it is preferable that the above is 0.05% to 2%. Alternatively, it is preferable that the above is 0.1% to 7.5%. Alternatively, it is preferable that the above is 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition in the process of manufacturing the positive electrode active material.

[0170] Furthermore, fluorine, one of the additive elements Xa, is a monovalent anion, and if some of the oxygen in the surface layer 10a is replaced by fluorine, the lithium desorption energy decreases. This is because the change in the valence of cobalt ions accompanying lithium desorption is from trivalent to tetravalent when fluorine is absent, and from divalent to trivalent when fluorine is present, resulting in different oxidation-reduction potentials. Therefore, if some of the oxygen in the surface layer 10a of the positive electrode active material 10 is replaced by fluorine, the desorption and insertion of lithium ions near the fluorine can occur more smoothly. As a result, when the positive electrode active material 10 is used in a secondary battery, the charge-discharge characteristics and high-current characteristics can be improved. In addition, the presence of fluorine in the surface layer 10a, which is the part that comes into contact with the electrolyte, can effectively improve corrosion resistance to hydrogen fluoride. Furthermore, if the melting point of fluorides, including lithium fluoride, is lower than the melting point of other additive element sources, it can function as a flux (also called a fluxing agent) to lower the melting point of other additive element sources.

[0171] Furthermore, as shown in Figures 8A and 8C, when the surface layer 10a contains both magnesium and nickel, divalent nickel may be able to exist more stably near divalent magnesium. Therefore, Lix α CoO ２ x inside αEven when the amount is small, magnesium leaching can be suppressed. Therefore, it can contribute to the stabilization of the surface layer 10a.

[0172] Furthermore, it is preferable to have a combination of additive elements with different distributions, such as additive elements Xa, Ya, and Za, as this allows for stabilization of the crystal structure over a wider area. For example, if the positive electrode active material 10 contains magnesium (one of the additive elements Xa), aluminum (one of the additive elements Ya), and nickel (one of the additive elements Za), it can stabilize the crystal structure over a wider area than when it contains only one or two of the additive elements Xa, Ya, and Za. Thus, when the positive electrode active material 10 contains a combination of additive elements Xa, Ya, and Za, surface stabilization can be sufficiently achieved by additive elements Xa such as magnesium and additive elements Za such as nickel, so additive elements Ya such as aluminum are not essential for the surface. Rather, it is preferable for aluminum to be widely distributed in deeper regions. For example, it is preferable for aluminum to be continuously detected in the region from the surface to a depth of 1 nm or more and 25 nm or less. A wider distribution of aluminum is preferable because it allows for stabilization of the crystal structure over a wider area.

[0173] Furthermore, as shown in Figures 8C and 8F, when the additive element Za is present in greater quantities in the edge region 10a1 compared to the basal region 10a2 (also referred to as preferentially present or selectively present), it is preferable because it 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. In addition, when the additive element Za has the above-described distribution, for example, when the positive electrode active material 10 is lithium cobalt oxide, it is preferable because it can minimize the effects of adding the additive element Za, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0174] As described above, having multiple additive elements allows the effects of each additive element to synergistically contribute to further stabilization of the surface layer 10a. In particular, the presence of magnesium, nickel, and aluminum is highly preferable as it is effective in achieving a stable composition and crystal structure. Among these, it is preferable that the surface layer 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 regions where magnesium and aluminum are distributed as described above, it is most preferable that the surface layer 10a of the positive electrode active material 10 has a region in the edge region 10a1 where the distribution of nickel and magnesium overlap.

[0175] <Crystal structure> <Lix α CoO ２ x inside α When it is 1 > The positive electrode active material 10 is in a discharge state, i.e., Lix α CoO ２ x inside α When = 1, it is preferable to have a layered rock salt type crystal structure belonging to space group R-3m. Layered rock salt type composite oxides have high discharge capacity, possess a two-dimensional lithium ion diffusion pathway, are suitable for lithium ion insertion / desorption reactions, and are excellent as positive electrode active materials for secondary batteries. For this reason, it is particularly preferable that the interior 10b, which accounts for most of the volume of the positive electrode active material 10, has a layered rock salt type crystal structure.

[0176] On the other hand, it is preferable that the surface layer 10a of the positive electrode active material 10 has a function to reinforce the layered structure of the interior 10b, which consists of the transition metal Me and an octahedron of oxygen, so that it does not break down even if lithium is removed from the positive electrode active material 10 due to charging. Alternatively, it is preferable that the surface layer 10a functions as a barrier film for the positive electrode active material 10. Alternatively, it is preferable that the surface layer 10a, which is the outer periphery of the positive electrode active material 10, reinforces the positive electrode active material 10. Reinforcement here means suppressing structural changes of the surface layer 10a and interior 10b of the positive electrode active material 10, including the desorption of oxygen, and / or suppressing the oxidative decomposition of the electrolyte on the surface of the positive electrode active material 10.

[0177] Therefore, it is preferable that the surface layer 10a has a different crystal structure from the interior 10b. It is also preferable that the surface layer 10a has a composition and crystal structure that is more stable at room temperature (25°C) than the interior 10b. For example, it is preferable that at least a part of the surface layer 10a of the positive electrode active material 10 has a rock salt type crystal structure. Alternatively, it is preferable that the surface layer 10a has both layered rock salt type and rock salt type crystal structures. Alternatively, it is preferable that the surface layer 10a has characteristics of both layered rock salt type and rock salt type crystal structures.

[0178] Furthermore, while it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, are present at higher concentrations in the surface layer 10a than in the interior 10b, it is also preferable that they be present randomly and dilutely in the interior 10b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites of the interior 10b, it has the effect of making it easier to maintain a layered rock salt-type crystal structure, similar to the above. Also, when nickel is present at an appropriate concentration in the interior 10b, the displacement of the layered structure consisting of octahedra of the transition metal Me and oxygen can be suppressed, similar to the above. In addition, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, so a synergistic effect of suppressing magnesium leaching can be expected.

[0179] Furthermore, due to the concentration gradient of the added element A as described above, it is preferable that the crystal structure changes continuously from the interior 10b toward the surface. Alternatively, it is preferable that the crystal orientations of the surface layer 10a and the interior 10b are roughly the same.

[0180] For example, it is preferable that the crystal structure changes continuously from the interior 10b of the layered rock salt mold toward the surface and surface layer 10a which have a rock salt crystal structure, or a rock salt crystal structure and a layered rock salt crystal structure. Alternatively, it is preferable that the orientation of the crystals in the surface layer 10a which has a rock salt crystal structure, or a rock salt crystal structure and a layered rock salt crystal structure, and the crystal orientation in the interior 10b of the layered rock salt mold are roughly the same.

[0181] In this specification, the layered rock salt crystal structure belonging to space group R-3m, which is found in composite oxides containing lithium and transition metals such as cobalt and Me, refers to a crystal structure that has a rock salt-type ionic arrangement in which cations and anions are arranged alternately, and in which the transition metal Me and lithium are regularly arranged to form a two-dimensional plane, thereby enabling two-dimensional diffusion of lithium. Defects such as vacancies in cations or anions may be present. Furthermore, strictly speaking, the layered rock salt crystal structure may have a distorted lattice structure compared to the rock salt crystal structure, and its symmetry may be lower than that of the rock salt crystal structure.

[0182] Furthermore, a rock salt-type crystal structure refers to a cubic crystal structure, including crystal structures belonging to the space group Fm-3m, in which cations and anions are arranged alternately. It is also acceptable for there to be vacancies in the cations or anions.

[0183] Furthermore, the presence of both a layered rock salt crystal structure and a rock salt crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc.

[0184] In rock salt type MgO, there is no distinction in the cation sites, but in layered rock salt type MgO, there are two types of cation sites in the crystal structure: one is mostly occupied by lithium, and the other by the transition metal Me. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately is the same for both rock salt type and layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmission spot) is taken as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the ideal state of rock salt type, and for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO, ２ The bright spots on the (003) plane are observed at a distance of approximately half the distance between the bright spots on the (111) plane of MgO. Therefore, in the analysis region, for example, rock salt type MgO and layered rock salt type LiCoO ２When there are two phases, the electron diffraction pattern shows a plane orientation in which bright spots of high and low brightness are arranged alternately. Bright spots common to both rock salt and layered rock salt types have high brightness, while bright spots occurring only in the layered rock salt type have low brightness.

[0185] Furthermore, in cross-sectional STEM images, when a layered rock salt crystal structure is observed from a direction perpendicular to the c-axis, layers with high brightness and layers with low brightness are observed alternately. This characteristic is not seen in rock salt crystals because there is no distinction in the sites of cations. In the case of a crystal structure that has characteristics of both rock salt and layered rock salt crystals, when observed from a specific crystal orientation, layers with high brightness and layers with low brightness are observed alternately in cross-sectional STEM images, and furthermore, a metal with an atomic number greater than lithium is present in a part of the low-brightness layer, i.e., the lithium layer.

[0186] The anions in layered rock salt crystals and rock salt crystals adopt a cubic close-packed structure (face-centered cubic lattice structure). It is also presumed that the anions in the O3' type crystals, which will be discussed later, adopt a cubic close-packed structure. Therefore, when layered rock salt crystals and rock salt crystals are in contact, there exists a crystal plane in which the orientation of the cubic close-packed structure composed of anions is aligned.

[0187] Alternatively, it can be explained as follows: The anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt type has a space group R-3m and a rhombohedral structure, but to facilitate understanding of the structure, it is generally represented by a composite hexagonal lattice, and the (0001) plane of the layered rock salt type has a hexagonal lattice. The triangular lattice of the {111} plane of the cubic crystal has a similar atomic arrangement to the hexagonal lattice of the (0001) plane of the layered rock salt type. The consistency between the two lattices can be described as the orientation of the cubic close-packed structure being aligned.

[0188] However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group Fm-3m of rock salt crystals (the space group of typical rock salt crystals). Therefore, the Miller indices of crystal planes that satisfy the above conditions are different for layered rock salt crystals and O3'-type crystals and for rock salt crystals. In this specification, it is sometimes said that the crystal orientations are roughly the same when the orientations of the cubic close-packed structure composed of anions are aligned in layered rock salt crystals, O3'-type crystals, and rock salt crystals.

[0189] <Lix α CoO ２ x inside α When Lix is ​​small > The positive electrode active material 10 has the above-described distribution of additive element A and / or crystal structure in the discharge state, α CoO ２ x inside α Unlike conventional cathode active materials, the crystal structure in the small state of x is due to the effect of the magnesium present at the lithium site maintaining a layered rock salt-type crystal structure of R-3m. α Small means 0.1 < x α This means ≤ 0.24.

[0190] Using Figures 9 to 12, Lix α CoO ２ x inside α The changes in crystal structure associated with these changes will be explained by comparing the conventional positive electrode active material with the positive electrode active material 10.

[0191] Figure 10 shows the changes in the crystal structure of a conventional positive electrode active material. The conventional positive electrode active material shown in Figure 10 is lithium cobalt oxide (LiCoO2) which does not contain any additive element A. ２ )

[0192] Figure 10 shows R-3m O3 attached to Lix α CoO ２ x inside α This shows the crystal structure of lithium cobalt oxide with a cobalt content of 1. In this crystal structure, lithium occupies octahedral sites, and CoO is present in the unit cell. ２ Three layers are present. Therefore, this crystal structure is sometimes called an O3 type crystal structure. Note that CoO ２A layer is defined as a structure in which octahedral structures, each with six oxygen atoms coordinated to cobalt, are continuous on a plane with shared edges. This is sometimes referred to as a layer composed of octahedra of cobalt and oxygen.

[0193] Furthermore, conventional lithium cobalt oxide is x α When the ratio is around 0.5, the symmetry of lithium increases, and it is known to have a crystal structure that belongs to the monoclinic space group P2 / m. This structure has CoO in the unit cell. ２ It has one layer. Therefore, it is sometimes called O1 type or monoclinic O1 type.

[0194] Also x α When = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO in the unit cell. ２ One layer is present. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. Alternatively, the trigonal structure can be converted to a composite hexagonal lattice and called hexagonal O1 type.

[0195] Also x α Conventional lithium cobalt oxide with a cobalt content of around 0.12 has a crystal structure of space group R-3m. This structure is similar to that of trigonal O1 type CoO ２ The structure and LiCoO such as R-3m O3 ２ This structure can be described as a structure in which the and structures are alternately stacked. For this reason, this crystal structure is sometimes called 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 this specification, including Figure 10, the H1-3 type crystal structure is shown with the c-axis halved to represent half the unit cell to facilitate comparison with other crystal structures.

[0196] As an example, the H1-3 type crystal structure can be represented by the coordinates of cobalt and oxygen in the unit cell as follows: Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are oxygen atoms, respectively. The unit cell that should 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 that produces a smaller GOF (goodness of fit) value can be adopted.

[0197] Lix α CoO ２ x inside α When the lithium is repeatedly charged and discharged in a manner that brings the R-3 O3 level below 0.24, the conventional lithium cobalt oxide undergoes repeated changes in its crystal structure (i.e., non-equilibrium phase transitions) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0198] However, these two crystal structures are CoO ２ The layer displacement is large. As shown by the dotted lines and arrows in Figure 10, in the H1-3 type crystal structure, CoO ２ The layer deviates significantly from the discharged state R-3m O3. Such dynamic structural changes can negatively affect the stability of the crystal structure.

[0199] Furthermore, these two crystal structures also have a large volume difference. When compared per the same number of cobalt atoms, the volume difference between the H1-3 type crystal structure and the R-3m O3 type crystal structure in the discharged state exceeds 3.5%, and is typically 3.9% or more.

[0200] In addition, the H1-3 crystal structure has a CoO structure similar to the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0201] Therefore, x αRepeated charging and discharging cycles that cause the lithium charge level to fall below 0.24 will cause the conventional lithium cobalt oxide crystal structure to break down. This breakdown of the crystal structure leads to a deterioration of the cycle characteristics. This is because the breakdown of the crystal structure reduces the number of sites where lithium can exist stably, and also makes it more difficult for lithium to be inserted and removed.

[0202] On the other hand, in the positive electrode active material 10 shown in Figure 9, Lix α CoO ２ x inside α Discharge state is 1, and x α The change in crystal structure when x is 0.24 or less is less than that of conventional positive electrode active materials. More specifically, x α When is 1, and x α CoO in a state where is 0.24 or less ２ The layer displacement can be reduced. Also, the change in volume when comparing per cobalt atom can be reduced. Therefore, the positive electrode active material 10 is x α Even when repeated charging and discharging cycles occur such that the ratio falls below 0.24, the crystal structure is less likely to collapse, enabling the realization of excellent cycle characteristics. Furthermore, the positive electrode active material 10 is Lix α CoO ２ x inside α When Lix is ​​0.24 or less, it can adopt a more stable crystal structure than conventional positive electrode active materials. Therefore, the positive electrode active material 10 is Lix α CoO ２ x inside α When the value is maintained at 0.24 or less, the safety of the secondary battery is further improved, which is preferable.

[0203] Lix α CoO ２ x inside α Figure 10 shows the crystal structure of the interior 10b of the positive electrode active material 10 when the CoO2 is approximately 1 and 0.2. The interior 10b occupies most of the volume of the positive electrode active material 10 and is a part that greatly contributes to charging and discharging, therefore CoO2 ２ The shifting of layers and changes in volume are the most problematic aspects.

[0204] The positive electrode active material 10 is x αWhen = 1, it has the same R-3m O3 crystal structure as conventional lithium cobalt oxide. However, the positive electrode active material 10 has x such that conventional lithium cobalt oxide has an H1-3 type crystal structure. α When the value is 0.24 or less, for example, around 0.2 or 0.12, it has a crystal with a different structure.

[0205] x α When the value is approximately 0.2, the positive electrode active material 10 has a crystal structure that belongs to the trigonal space group R-3m. This is CoO ２ The layer symmetry is the same as that of O3. Therefore, this crystal structure will be called the O3' type crystal structure. This crystal structure is shown in Figure 9, labeled R-3m O3'.

[0206] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,x Ｏ ), 0.20 ≤ x Ｏ It can be shown within the range of ≤0.25. Also, the lattice constant of the unit cell is 2.797 ≤ a ≤ 2.837 (×10) for the a-axis. −１ (nm) is preferred, and 2.807 ≤ a ≤ 2.827 (×10 −１ nm) is more preferable, and typically a = 2.817 (×10 −１ The value is (nm). The c-axis is 13.681 ≤ c ≤ 13.881 (×10). −１ c = 13.781 (nm) is preferred, 13.751 ≤ c ≤ 13.811 is more preferred, and typically c = 13.781 (×10) −１ It is (nm).

[0207] In the O3' type crystal structure, ions such as cobalt, nickel, and magnesium occupy the six-coordinate positions to oxygen. Lighter elements such as lithium may occupy the four-coordinate positions to oxygen.

[0208] As shown by the dotted line in Figure 9, the CoO is different between the discharged state R-3m(O3) and the O3' type crystal structure. ２ There is almost no misalignment of the layers.

[0209] Furthermore, the difference in volume per unit number of cobalt atoms between the discharged state R-3m(O3) and the O3'-type crystal structure is 2.5% or less, more specifically 2.2% or less, and typically 1.8%.

[0210] Thus, in the positive electrode active material 10, Lix α CoO ２ x inside α When x is small, that is, when a large amount of lithium is desorbed, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Also, the change in volume when compared per the same number of cobalt atoms is suppressed. Therefore, positive electrode active material 10 is x α The crystal structure is less likely to collapse even when repeated charging and discharging cycles occur, such as when the lithium ratio falls below 0.24. Therefore, the positive electrode active material 10 suppresses the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because it can stably utilize more lithium than conventional positive electrode active materials, the positive electrode active material 10 has a large discharge capacity per unit weight and per unit volume. Therefore, by using the positive electrode active material 10, secondary batteries with high discharge capacity per unit weight and per unit volume can be manufactured.

[0211] The positive electrode active material 10 is Lix α CoO ２ x inside α It has been confirmed that when the value is between 0.15 and 0.24, it may have an O3' type crystal structure. α It is estimated that even when the value is greater than 0.24 and less than or equal to 0.27, it has an O3' type crystal structure. However, the crystal structure is Lix α CoO ２ x inside α Furthermore, it is affected by factors such as the number of charge / discharge cycles, charge / discharge current, temperature, and electrolyte, so it is not necessarily x α This is not limited to the above range.

[0212] Therefore, the positive electrode active material 10 is Lix α CoO ２ x inside α When the value is greater than 0.1 and less than or equal to 0.24, the entire interior 10b of the positive electrode active material 10 does not have to have an O3' type crystal structure. It may contain other crystal structures, or a portion may be amorphous.

[0213] Also Lix α CoO ２ x inside α To reduce the value, it is generally necessary to charge with a high charging voltage. Therefore, Lix α CoO２ x inside α A state where this value is small can be rephrased as a state where the device is charged at a high charging voltage. For example, when constant current / constant voltage charging (CC / CV charging) is performed at a voltage of 4.6V or higher relative to the potential of lithium metal in an environment of 25°C, the H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6V or higher relative to the potential of lithium metal can be said to be a high charging voltage.

[0214] Therefore, the positive electrode active material 10 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 4.6V or higher at 25°C. It is also preferable because it can adopt an O3' type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65V or higher and 4.7V or lower at 25°C.

[0215] Even with the positive electrode active material 10, an H1-3 type crystal structure may only be observed when the charging voltage is further increased. Furthermore, as mentioned above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., so even at lower charging voltages, for example, when the charging voltage is 4.5V or more and less than 4.6V at 25°C, the positive electrode active material 10 of one embodiment of the present invention may take on an O3' type crystal structure.

[0216] Furthermore, when graphite is used as the negative electrode active material in a secondary battery, for example, the voltage of the secondary battery will decrease by the amount of the graphite's potential compared to the above. The potential of graphite is approximately 0.05V to 0.2V relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is observed when the voltage obtained by subtracting the graphite's potential from the above voltage is obtained.

[0217] Furthermore, while Figure 9 shows O3' with lithium present at all lithium sites with equal probability, this is not the only way. It may be concentrated at some lithium sites, or, for example, as shown in Figure 10, monoclinic O1(Li ０．５ CoO ２ It may have symmetries such as ). The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0218] Furthermore, the O3' type crystal structure has CdCl randomly placed lithium between the layers. ２ It can also be said that this CdCl has a crystal structure similar to that of the type. ２ A crystal structure similar to this type is lithium nickelate Li ０．０６ NiO ２ The crystal structure is similar to that when charged to this level, but in pure lithium cobalt oxide, or layered rock salt type cathode active materials containing a large amount of cobalt, CdCl is usually used. ２ It is known that it does not adopt a specific crystal structure.

[0219] Furthermore, it is preferable that the concentration gradient of additive element A is similar at multiple locations on the surface layer 10a of the positive electrode active material 10. In other words, it is preferable that the reinforcement derived from additive element A is uniformly present on the surface layer 10a. Even if there is reinforcement in a part of the surface layer 10a, if there are parts without reinforcement, stress may concentrate in those parts. If stress concentrates in a part of the positive electrode active material 10, defects such as cracks may occur from there, which may lead to cracking of the positive electrode active material and a decrease in discharge capacity.

[0220] However, it is not necessarily required that the additive element A has a similar concentration gradient across the entire surface layer 10a of the positive electrode active material 10. Preferably, the additive element A has the distribution of additive element Xa shown in Figure 8D and the distribution of additive element Ya shown in Figure 8E.

[0221] Here, the region near C-D has a layered rock salt type crystal structure of R-3m, and the surface is (00l) oriented. The (00l) oriented surface may have a different distribution of additive 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 concentrations selected from additive element A that is limited to a shallower portion from the surface compared to surfaces other than the (00l) oriented surface. Alternatively, the (00l) oriented surface and its surface layer 10a may have lower concentrations of one or more elements selected from additive element A compared to the other orientations. Alternatively, the (00l) oriented surface and its surface layer 10a may have one or more elements selected from additive element A below the detection limit.

[0222] In the layered rock salt crystal structure of R-3m, cations are arranged parallel to the (00l) plane. This is CoO ２ It can be said that the structure consists of layers and lithium layers stacked alternately parallel to the (00l) plane. Therefore, the diffusion pathways of lithium ions also exist parallel to the (00l) plane.

[0223] CoO ２ Since the layer is relatively stable, the surface of the positive electrode active material 10 is more stable when it is oriented in the (00l) direction. The main diffusion pathway of lithium ions during charging and discharging is not exposed on the (00l) plane.

[0224] On the other hand, the lithium ion diffusion pathways are exposed on surfaces other than those oriented in the (00l) direction. Therefore, the surfaces and surface layer 10a other than those oriented in the (00l) direction are important regions for maintaining the lithium ion diffusion pathways, but at the same time, they are prone to instability because they are the regions where lithium ions first desorb. For this reason, reinforcing the surfaces and surface layer 10a other than those oriented in the (00l) direction is extremely important for maintaining the overall crystal structure of the positive electrode active material 10.

[0225] <Grain Boundaries> In addition to the distribution described above, it is more preferable that at least a portion of the additive element A present in the positive electrode active material 10 is concentrated at and near the grain boundaries, i.e., at a high concentration. Here, "nearby" refers to the region from the grain boundary inward to 10 nm.

[0226] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material 10 is higher than that of other regions of the interior 10b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that of other regions of the interior 10b. Furthermore, it is preferable that the nickel concentration at and near the grain boundaries is higher than that of other regions of the interior 10b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that of other regions of the interior 10b.

[0227] Grain boundaries are a type of surface defect. Therefore, like surfaces, they tend to be unstable and prone to initiating changes in crystal structure. For this reason, increasing the concentration of additive element A at and near the grain boundaries can more effectively suppress changes in crystal structure.

[0228] Furthermore, if the magnesium and fluorine concentrations are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material 10, the magnesium and fluorine concentrations will be high near the surface created by the cracks. Therefore, the corrosion resistance to hydrogen fluoride can be improved even in the positive electrode active material after cracks have occurred.

[0229] <Analysis Method> A certain positive electrode active material is Lix α CoO ２ x inside α When Lix is ​​small, whether or not the positive electrode active material 10 has an O3' type crystal structure is determined by Lix α CoO ２ x inside α A positive electrode with a small positive electrode active material can be identified by analyzing it using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0230] XRD is particularly favored because it can analyze the symmetry of transition metals such as cobalt in the positive electrode active material with high resolution, compare the crystallinity and crystal orientation, analyze the periodic distortion of the lattice and the crystallite size, and obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling a secondary battery. Among XRD methods, powder XRD provides an XRD profile that reflects the crystal structure of the interior 10b of the positive electrode active material 10, which occupies most of the volume of the positive electrode active material 10.

[0231] When analyzing crystallite size using powder XRD, it is preferable to measure while excluding the influence of the orientation of positive electrode active material particles due to pressure, etc. For example, it is preferable to extract the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, and measure it after obtaining a powder sample.

[0232] The positive electrode active material 10 is Lix α CoO ２ x inside α A characteristic feature is that there is little change in the crystal structure when the value is 1 and when it is 0.24 or less. When charged at high voltage, materials in which the crystal structure changes significantly and where the crystal structure accounts for 50% or more are undesirable because they cannot withstand repeated high-voltage charging and discharging.

[0233] It should also be noted that merely adding additive element A may not result in an O3'-type crystal structure in some cases. For example, even if lithium cobalt oxide containing magnesium and fluorine, or lithium cobalt oxide containing magnesium and aluminum, are common in this respect, depending on the concentration and distribution of additive element A, Lix α CoO ２ x in α has two cases: when x is 0.24 or less, the O3'-type crystal structure accounts for 60% or more; and when the H1-3-type crystal structure accounts for 50% or more.

[0234] Also, even for the positive electrode active material 10, when x α is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9 V, an H1-3-type or trigonal O1-type crystal structure may be formed in some cases. Therefore, to determine whether a material is the positive electrode active material 10, analysis of the crystal structure including XRD and information such as charging capacity or charging voltage are required.

[0235] x α A positive electrode active material in a state where x is small may cause a change in crystal structure when exposed to the atmosphere. For example, it may change from an O3'-type crystal structure to an H1-3-type crystal structure in some cases. Therefore, it is preferable to handle all samples to be subjected to crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0236] In addition, whether the distribution of the additive element A contained in the positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, EPMA (Electron Probe Micro Analyzer), or the like.

[0237] In addition, the crystal structure such as grain boundaries can be analyzed by electron diffraction of a cross-section of the positive electrode active material 10 or the like.

[0238] <Charging Method> To determine whether the composite oxide is a positive electrode active material 10, charging can be performed, for example, by producing a coin cell (CR2032 type, 20 mm in diameter, 3.2 mm in height) using the composite oxide for the positive electrode and lithium metal for the counter electrode, and performing charging. The coin cell includes an electrolytic solution, a separator, a positive electrode can, and a negative electrode can. The coin cell for determining whether the composite oxide is the positive electrode active material 10 does not need to include the electrolytic solution and the separator according to one embodiment of the present invention.

[0239] More specifically, the positive electrode can be formed by applying a slurry obtained by mixing a positive electrode active material, a conductive material, and a binder onto an aluminum foil positive electrode current collector.

[0240] Lithium metal can be used for the counter electrode. Note that 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 are different. Unless otherwise specified, voltage and potential in the present specification and the like refer to the potential of the positive electrode.

[0241] For the electrolytic solution, ethylene carbonate (abbreviated as EC) and diethyl carbonate (abbreviated as DEC) are prepared as a mixed solvent at EC:DEC = 3:7 (volume ratio), and lithium hexafluorophosphate (LiPF ６ ) is prepared. Lithium hexafluorophosphate is adjusted to be 1 mol per liter of the mixed solvent when the temperature of the mixed solvent is 25°C±5°C. Vinylene carbonate (abbreviated as VC) may also be prepared as an additive. It is preferable that the additive is added in an amount of 2 wt% with respect to the electrolytic solution (the total amount of the mixed solvent and the lithium salt).

[0242] A 25-μm-thick porous polypropylene film can be used for the separator. A material other than polypropylene may be used for the separator for checking the charged state of the positive electrode active material.

[0243] The positive electrode can and the negative electrode can can be formed of stainless steel (SUS).

[0244] The coin cell prepared under the above conditions is charged with an arbitrary voltage (e.g., 4.50V, 4.55V, 4.58V, 4.60V, 4.62V, 4.65V, 4.70V, 4.75V, or 4.80V). The charging method is not particularly limited as long as it can be charged at any voltage for a sufficient amount of time. For example, when charging with CC / CV, the current in CC charging can be 20mA / g or more and 100mA / g or less. CV charging can be completed at 2mA / g or more and 10mA / g or less. It is desirable to charge with such small current values ​​in order to observe the phase change of the positive electrode active material. The temperature should be 25°C or 45°C. After charging in this way, the coin cell can be disassembled in a glove box with an argon atmosphere and the positive electrode can be removed to obtain a positive electrode active material with an arbitrary charging capacity. When performing various analyses afterward, it is preferable to seal it in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the device in a sealed container under an argon atmosphere. It is also preferable to remove the positive electrode as soon as possible after charging is complete and subject it to analysis. Specifically, it is preferable to do so within one hour after charging is complete, and more preferably within 30 minutes.

[0245] Furthermore, when analyzing the crystal structure of the charged state after multiple charge-discharge cycles, for example, charging can be performed with a constant current of 20 mA / g to 100 mA / g up to any 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), then with a constant voltage charge until the current is between 2 mA / g and 10 mA / g, and discharging can be performed with a constant current of 20 mA / g to 100 mA / g until the voltage reaches 2.5 V. Alternatively, discharging can be performed with a constant current of 20 mA / g to 200 mA / g until the voltage reaches 3.0 V.

[0246] Furthermore, when analyzing the crystal structure of the discharged state after multiple charge-discharge cycles, constant current discharge can be performed at, for example, 2.5V and a current value of 20mA / g to 200mA / g. Alternatively, constant current discharge can be performed at 3.0V and a current value of 20mA / g to 200mA / g.

[0247] <XRD> The equipment and conditions for XRD measurement are not particularly limited. For example, measurements can be taken with the following equipment and conditions: XRD equipment: Bruker AXS, D8 ADVANCE; X-ray: CuK α 1. Output: 40kV, 40mA, Slit width: Div. Slit, 0.5°, Detector: LynxEye, Scanning method: 2θ / θ continuous scan, Measurement range (2θ): 15° to 90°, Step width (2θ): 0.01°, Setting counting time: 1 second / step, Sample stage rotation: 15rpm.

[0248] If the sample to be measured is a powder, it can be set up by placing it in a glass sample holder or by sprinkling the sample onto a grease-coated silicone anti-reflective plate. If the sample to be measured is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set up to match the measurement surface required by the device.

[0249] CuK calculated from the O3' type crystal structure and the H1-3 type crystal structure model. α１ The ideal powder XRD profile using lines is shown in Figures 11 and 12. For comparison, Lix α CoO ２ x inside α = 1 LiCoO ２ O3 and x α The ideal XRD profile calculated from the trigonal O1 crystal structure with =0 is also shown. ２ (O3) and CoO ２ The XRD profile for (O1) was created using Reflex Powder Diffraction, one of the modules in Materials Studio (BIOVIA), based on crystal structure information obtained from ICSD. The range of 2θ was set to 15° to 75°, Step size = 0.01, and wavelength λ1 = 1.540562 × 10⁻¹⁰. −１０m and λ² were left unset, and the Monochromator was set to single. The XRD pattern for the H1-3 type crystal structure was created using the same method as above, based on the information of the H1-3 type crystal structure shown in Figure 10. For the O3' type crystal structure, the crystal structure was estimated from the XRD pattern of the positive electrode active material 10, fitted using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and the XRD profile was created in the same manner as the others.

[0250] As shown in Figure 11, in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° to 19.37°) and 2θ = 45.47 ± 0.10° (45.37° to 45.57°).

[0251] However, as shown in Figure 12, diffraction peaks do not appear at these positions in the H1-3 type crystal structure and trigonal O1. Therefore, Lix α CoO ２ x inside α The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° to 19.37°) and 2θ = 45.47 ± 0.10° (45.37° to 45.57°) when the value is small can be said to be a characteristic of the positive electrode active material 10.

[0252] Furthermore, the O3' type crystal structure is represented by the x in the XRD profile shown in Figure 11. α If the value is slightly greater than this, for example, when charging is performed with a voltage slightly lower than 4.60V (4.56V, 4.57V, 4.58V, or 4.59V) as the upper limit of the charging voltage, the above diffraction peaks will appear shifted to the lower angle side. For example, when charging is performed with a charging voltage of 4.58V as the upper limit of the charging voltage, the positive electrode active material 10 has diffraction peaks at 2θ = 18.85 ± 0.20° and 2θ = 45.15 ± 0.10° as diffraction peaks originating from the O3' type crystal structure.

[0253] This is x α = 1 and x α It can also be said that the positions where the XRD profile appears are close together in crystal structures with a x of ≤0.24. More specifically, x α = 1 and x αFor diffraction peaks appearing in the main XRD profiles of crystal structures with a coefficient of ≤0.24 where 2θ is between 42° and 46°, the difference in 2θ can be said to be 0.7° or less, more preferably 0.5° or less.

[0254] Note that the positive electrode active material 10 is Lix α CoO ２ x inside α When the O3' crystal structure is small, not all of the O3'-type crystal structure is required. Other crystal structures may be present, and some may be amorphous. However, when Rietveld analysis is performed on the XRD profile, it is preferable that the O3'-type crystal structure accounts for 50 wt% or more of the total crystal structure in the measurement range, more preferably 60 wt% or more, and even more preferably 66 wt% or more. As described above, if the O3'-type crystal structure accounts for 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, a cathode active material with sufficiently excellent cycle characteristics can be obtained.

[0255] Furthermore, even after 5 or more charge-discharge cycles, 30 or more charge-discharge cycles, 50 or more charge-discharge cycles, or 100 or more charge-discharge cycles from the start of measurement, it is preferable that the O3' type crystal structure accounts for 35 wt% or more of the total crystal structure in the measurement range when Rietveld analysis is performed, more preferably 40 wt% or more, and even more preferably 43 wt% or more.

[0256] Furthermore, the sharpness of diffraction peaks in the XRD profile indicates high crystallinity. Therefore, it is preferable for each diffraction peak after charging to be sharp, i.e., have a narrow full width at half maximum (FWHM). The FWHM varies depending on the XRD measurement conditions or the value of 2θ, even for diffraction peaks originating from the same crystalline phase. Under the measurement conditions described above, for diffraction peaks observed at 2θ = 43° to 46°, the FWHM is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. It is not necessary for all diffraction peaks to satisfy this requirement. If some diffraction peaks satisfy this requirement, it can be said that the crystal phase has high crystallinity. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0257] Furthermore, the crystallite size of the O3' type crystal structure of the positive electrode active material 10 is the same as that of LiCoO in the discharged state. ２ It only drops to about 1 / 20th of (O3). Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, Lix α CoO ２ x inside α When the value is small, a clear XRD profile of the O3' type crystal structure can be observed. On the other hand, conventional LiCoO ２ Even if a portion of the crystal structure resembles the O3' type, the crystallite size will be smaller, and the diffraction peaks will be broad and small. The crystallite size can be determined from the full width at half maximum of the diffraction peak.

[0258] <XPS> In XPS (X-ray Photoelectron Spectroscopy), in the case of inorganic oxides, monochromatic aluminum K is used as the X-ray. α Using a line, analysis is possible in areas from the surface to a depth of approximately 2 nm to 8 nm (usually 5 nm or less), allowing for quantitative analysis of the concentration of each element in an area 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 generally around ±1 atomic percent, and the detection limit is approximately 1 atomic percent, although this varies depending on the element.

[0259] In the positive electrode active material 10, it is preferable that the concentration of one or more elements A selected from the additive elements A is higher in the surface layer 10a than in the interior 10b. This is equivalent to saying that it is preferable that the concentration of one or more elements A selected from the additive elements A in the surface layer 10a is higher than the average concentration of the positive electrode active material 10 as a whole. For example, it can be said that it is preferable that the concentration of one or more elements A selected from the surface layer 10a, as measured by XPS, is higher than the average concentration of additive elements A of the positive electrode active material 10 as a whole, as measured by ICP-MS or GD-MS. For example, it is preferable that the concentration of magnesium in at least a part of the surface layer 10a, as measured by XPS, is higher than the average magnesium concentration of the positive electrode active material 10 as a whole. It is also preferable that the concentration of nickel in at least a part of the surface layer 10a is higher than the average nickel concentration of the positive electrode active material 10 as a whole. Furthermore, it is preferable that the concentration of aluminum in at least a part of the surface layer 10a is higher than the average aluminum concentration of the positive electrode active material 10 as a whole. Furthermore, it is preferable that the fluorine concentration in at least a portion of the surface layer 10a is higher than the average fluorine concentration of the entire positive electrode active material 10.

[0260] Furthermore, the surface and surface layer 10a of the positive electrode active material 10 do not contain carbonates, hydroxyl groups, etc., that have been chemically adsorbed after the positive electrode active material 10 was manufactured. Also, electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material 10 are not included. Therefore, when quantifying the elements present in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc., which can be detected by surface analysis such as XPS. For example, XPS allows for the separation of bond types through analysis, and corrections may be made to exclude C-F bonds derived from the binder.

[0261] Furthermore, before subjecting the sample to various analyses, the positive electrode active material and positive electrode active material layer may be washed to remove electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve into the solvent used for washing, but even in that case, the additive element A is unlikely to dissolve, so it will not affect the atomic ratio of additive element A.

[0262] Furthermore, the concentration of additive element A may be compared in ratio to cobalt. Using the ratio to cobalt is preferable because it reduces the influence of carbonates and the like that chemically adsorbed after the production of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more and 1.20 or less, more preferably 0.500 or more and 1.00 or less, more preferably 0.500 or more and 0.900 or less, and more preferably 0.500 or more and 0.700 or less.

[0263] Furthermore, for example, the ratio of nickel to cobalt atoms (Ni / Co) determined by XPS analysis is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, more preferably 0.050 or more and 0.100 or less, and more preferably 0.050 or more and 0.070 or less.

[0264] Furthermore, for example, the ratio of aluminum to cobalt atoms (Al / Co) 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.

[0265] Furthermore, for example, the ratio of fluorine to magnesium atoms (F / Mg) 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.

[0266] The fact that the range is as described above indicates that these additive elements A are not attached to a narrow area on the surface of the positive electrode active material 10, but are widely distributed at a favorable concentration in the surface layer 10a of the positive electrode active material 10. In other words, the fact that the range is as described above as a result of the XPS analysis of the positive electrode active material 10 indicates that x αEven when repeated charging and discharging cycles occur, resulting in a voltage of 0.24 or less, the crystal structure remains stable, enabling excellent cycle characteristics. Furthermore, good lithium insertion and deinsertion are possible in the positive electrode active material 10, resulting in excellent rate characteristics.

[0267] When performing XPS analysis, for example, monochromatic aluminum K is used as the X-ray. α X-rays can be used. Furthermore, as for energy resolution, it is preferable to use an XPS instrument with an energy resolution such that the full width at half maximum of the Ag3d5 / 2 peak (112 eV) in the XPS spectrum of an Ag sample is 1.0 eV ± 0.1 eV. The extraction angle can be, for example, 45°. For example, measurements can be performed using the following XPS instrument and measurement conditions: Measurement instrument: PHI Quantera II; X-ray: Monochromatic Al K α (1486.6 eV) Energy resolution: Full width at half maximum of Ag3d5 / 2 peak is 1.0 eV ± 0.1 eV Detection area: 100 μmφ Detection angle (detection depth): Extraction angle 45° (approximately 4 nm to 5 nm) Measurement spectrum: Wide scan, narrow scan of each detected element

[0268] When the positive electrode active material 10 is subjected to XPS analysis, the peak indicating the bond energy between magnesium and other elements (Mg1s peak) is preferably 1303.0 eV or more and less than 1305.0 eV, and more preferably around 1304.0 eV. This value is different from the bond energy of magnesium fluoride, which is 1306.0 eV, and is close to the bond energy of magnesium oxide.

[0269] In XPS analysis of the positive electrode active material 10, it is preferable to correct the measured XPS spectrum so that the C1s peak matches a reference value (284.8 eV), that is, to shift the entire spectrum. This reduces the influence of differences in XPS instruments, measurement conditions, etc., on the XPS measurement.

[0270] Furthermore, in the XPS analysis of the positive electrode active material 10, by analyzing the Mg 1s peak, when analyzing 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 has a peak component derived from the O-Mg-O bond. Further, the peak component derived from the "O-Mg-F" bond may be included, but it is preferably 30% or less of the total of the above three peak components, more preferably 20% or less, still more preferably 10% or less. Further, the peak component derived from the "F-Mg-F" bond may be included, but it is preferably 10% or less of the total.

[0271] That is, in the XPS analysis of the positive electrode active material 10, when analyzing 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, 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%.

[0272] A method for analyzing the Mg 1s peak in an XPS spectrum in XPS analysis will be described. In the analysis of the Mg 1s peak, the peak component derived from the O-Mg-O bond is defined as fit peak 1, the peak component derived from the O-Mg-F bond is defined as fit peak 2, and the peak component derived from the F-Mg-F bond is defined as fit peak 3. It is preferable to synthesize these three fit peaks and calculate the peak synthesis ratio that minimizes the difference from the Mg 1s peak in the XPS spectrum obtained by XPS analysis. The area ratio of fit peak 1, fit peak 2, and fit peak 3 at this time can be assumed to be the abundance ratio of the O-Mg-O bond, O-Mg-F bond, and F-Mg-F bond, and an analysis result can be output.

[0273] Note that in the above analysis method for the XPS spectrum, the energy value (Ep1) at the maximum value (also referred to as peak top) of fit peak 1 is for MgO-coated LiCoO ２The energy value at the maximum value of the Mg1s peak when measured separately using the standard sample can be referenced. Furthermore, the energy value at the maximum value of the fitted peak 3 (Ep3) is the same as that of magnesium fluoride (MgF ２ The energy value at the maximum value of the Mg1s peak when a standard sample with a purity of 99.9% (3N) up was measured separately can be referenced. Furthermore, the energy value at the maximum value of fit peak 2 (Ep2) can be an intermediate value between Ep1 and Ep3. Also, EP1 is located on the lower energy side compared to EP3. The energy value at the maximum value of a peak is also called the peak position.

[0274] In the XPS analysis of the positive electrode active material 10, the full width at half maximum 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. In the above, the peak position of the Mg1s peak is on the lower energy side than the energy value at the maximum value of the Mg1s peak when magnesium fluoride is measured separately as a standard sample.

[0275] <EDX> It is preferable that one or more of the additive elements A present in the positive electrode active material 10 have a concentration gradient. Furthermore, when two or more additive elements A are used, it is more preferable that the depth from the surface of the concentration peaks of each additive element A is different. For example, the concentration gradient and concentration peaks of the additive elements A can be evaluated by exposing the cross-section of the positive electrode active material 10 using FIB (Focused Ion Beam) or the like, and analyzing the cross-section using EDX, EPMA (Electron Probe Microanalysis), or the like.

[0276] In EDX measurements, the method of scanning within a region to evaluate it in two dimensions is called EDX surface analysis. The method of scanning linearly to evaluate the distribution of atomic concentration within the positive electrode active material is called line analysis. Furthermore, sometimes the extraction of linear region data from EDX surface analysis is also called line analysis. Finally, measuring a region without scanning is called point analysis.

[0277] EDX surface analysis (e.g., elemental mapping) allows for quantitative analysis of the concentration of additive element A in the surface layer 10a, interior 10b, and near grain boundaries of the positive electrode active material 10. EDX radiation analysis allows for the analysis of the concentration distribution and maximum value of additive element A. Furthermore, analysis using a thinned sample, such as STEM-EDX, is preferable because it allows for the analysis of 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 in the depth direction of the sample.

[0278] Since the positive electrode active material 10 is a compound containing a transition metal and oxygen that allows for lithium insertion and removal, the interface between the region where the transition metal Me (e.g., Co, Ni, Mn, Fe, etc.) and oxygen, which undergo oxidation and reduction during lithium insertion and removal, are present and the region where they are absent, 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 applied to the surface, but the protective film is not included in the positive electrode active material. As the protective film, a single-layer or multi-layer film of carbon, metal, oxide, resin, etc. may be used.

[0279] In STEM-EDX radiation analysis, the reference point is the average value M of the amount of characteristic X-rays detected of the transition metal Me inside the positive electrode active material. ＡＶＥ This point is defined as the point where the value reaches 50%. When describing the area to the left of the above reference point as the outside of the positive electrode active material and the area to the right of the reference point as the inside of the positive electrode active material, the reference point is sometimes referred to as the position on the surface of the positive electrode active material. Furthermore, in STEM-EDX radiation analysis, if the amount of characteristic X-rays detected of the transition metal Me does not decrease sufficiently to the left of the reference point, the amount of characteristic X-rays detected of the transition metal Me to the left of the reference point is called the background, and the average value M of the amount of characteristic X-rays detected of the above transition metal Me in the background is called the background. ＢＧ and the average value M of the amount of transition metal Me detected inside ＡＶＥ The point at which the sum of these values ​​is 50% may be used as the reference point. Instead of the transition metal Me, the amount of characteristic X-rays detected of oxygen inside the positive electrode active material may be used, and the reference point can be determined by replacing the transition metal Me with oxygen. However, since oxygen is an element that is easily affected by external influences of the positive electrode active material, the reference point is the average value M of the amount of characteristic X-rays detected of the transition metal Me. ＡＶＥIt is preferable to calculate it from 50% of the value. Also, the average value M of the detection amount of characteristic X-rays of the transition metal Me. ＡＶＥ 50% of the average value of the detected amount of characteristic X-rays of oxygen M ＡＶＥ If the point differs from the point where it becomes 50%, it is thought to be due to the influence of oxygen-containing metal oxides, carbonates, etc. adhering to the surface of the positive electrode active material, and therefore the average value M of the characteristic X-ray detection amount of the transition metal Me mentioned above. ＡＶＥ It is preferable to adopt the point that is 50% of the value. Also, in the case of a positive electrode active material having multiple transition metals Me, the element M that has the highest amount of characteristic X-rays detected inside is preferred. ＡＶＥ The reference point can be determined using this method.

[0280] The average value M of the detected characteristic X-rays of the internal transition metal Me. ＡＶＥ This can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, from a depth of 20 nm or more, preferably beyond 30 nm, in the region where the detection amount of characteristic X-rays of the transition metal Me saturates and stabilizes, for example, from the region where the detection amount of characteristic X-rays of the transition metal Me begins to increase. The average value M of the detection amount of the above transition metal Me characteristic X-rays in the background ＢＧ For example, the average value of the detected characteristic X-rays of the transition metal Me can be determined by averaging the range of 2 nm or more, preferably 3 nm or more, outside the area where the detection amount of characteristic X-rays of the transition metal Me begins to increase. ＢＧ and the average value of the amount of characteristic X-rays detected from the internal oxygen O ＡＶＥ This can be calculated in a similar manner.

[0281] Furthermore, the surface of the positive electrode active material 10 in cross-sectional STEM images, etc., is defined as the boundary between the region where an image originating from the crystal structure of the positive electrode active material is observed and the region where it is not observed, and is the outermost region where atomic columns originating from the nuclei of metal elements with atomic numbers greater than lithium among the metal elements constituting the positive electrode active material are confirmed.

[0282] Furthermore, in STEM-EDX analysis, a peak refers to the maximum value of a convex shape appearing on the graph of characteristic X-ray intensity for each element, or the maximum value of the characteristic X-ray for each element. Noise in STEM-EDX analysis can include measurements with a width at half maximum (FWHM) below the spatial resolution (R), for example, R / 2 or less.

[0283] Scanning the same location multiple times under the same conditions can reduce the effects of noise. For example, the cumulative value measured by two scans can be used as the detected value for each element. The number of scans is not limited to two; more scans can be performed, and the cumulative value can be used as the detected value for each element.

[0284] STEM-EDX radiation analysis can be performed, for example, as follows: First, a protective film is deposited on the surface of the positive electrode active material. For example, carbon can be deposited using the carbon coating unit of an ion sputtering apparatus (Hitachi High-Tech MC1000).

[0285] Next, the positive electrode active material is thinned to prepare a STEM cross-sectional sample. For example, thinning can be performed using a FIB-SEM device (Hitachi High-Tech XVision 200TBS). In this case, pickup is performed using an MPS (microprobing system), and the finishing conditions can be set to, for example, an acceleration voltage of 10kV.

[0286] STEM-EDX ray analysis can be performed using, for example, a STEM apparatus (Hitachi High-Tech HD-2700), with an EDAX Octane T Ultra W (Dual EDS) EDX detector. An example of EDX ray analysis conditions using the Hitachi High-Tech HD-2700 is to set the STEM apparatus's acceleration voltage to 200 kV and the emission current to 6 μA to 10 μA, measuring areas of the thinned sample with minimal depth and surface irregularities. The magnification can be, for example, around 150,000x. The EDX ray analysis conditions can include drift correction, a line width of 42 nm, a pitch of 0.2 nm, and 6 or more frames.

[0287] To achieve high spatial resolution in STEM-EDX ray analysis, it is preferable to have a small electron beam diameter (also called beam diameter, probe diameter, or probe diameter). In STEM-EDX ray analysis, the beam diameter is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the electron beam current (also called probe current). Therefore, it is preferable that the apparatus used for STEM-EDX ray analysis be equipped with a spherical aberration correction device (Cs collector) that can reduce the beam diameter and increase the beam current.

[0288] Furthermore, in a positive electrode active material 10 having magnesium and fluorine as additive element A, it is preferable that the distribution of fluorine has a region that overlaps with the distribution of magnesium. For example, it is preferable that the difference in depth direction between the peak of fluorine concentration or detected amount and the peak of magnesium concentration or detected amount is within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and still more preferably within 0.5 nm.

[0289] Furthermore, in a positive electrode active material 10 having nickel as additive element A, the peak of nickel concentration or detected amount in the surface layer 10a is preferably located on the surface of the positive electrode active material 10, or within a depth of 3 nm from the reference point toward the center, and more preferably within a depth of 1 nm. Furthermore, in a positive electrode active material 10 having magnesium and nickel, the distribution of nickel is preferably in a region that overlaps with the distribution of magnesium. For example, the difference in depth between the peak of nickel concentration or detected amount and the peak of magnesium concentration or detected amount is preferably within 3 nm, and more preferably within 1 nm.

[0290] Furthermore, when the positive electrode active material 10 contains aluminum as additive element A, when EDX radiation analysis is performed, it is preferable that the peaks of concentration or detection amount of magnesium, nickel, or fluorine are closer to the surface than the peaks of concentration or detection amount of aluminum in the surface layer 10a. In other words, it is preferable that the peak of concentration or detection amount of aluminum in the surface layer 10a is located further inward than the peaks of concentration or detection amount of magnesium, nickel, or fluorine. For example, it is preferable that the peak of concentration or detection amount of aluminum is located on the surface of the positive electrode active material 10, or at a depth of 0.5 nm to 50 nm from a reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0291] Here, we will explain how to represent the positional relationship of elemental distribution when performing EDX radiation analysis, using Figures 13A to 13G. Figures 13A to 13F are schematic diagrams showing the detection amount profiles of the first element e1 and the second element e2. Figure 13G is a schematic diagram showing the detection amount profiles of the first element e1, the second element e2, and the third element e3. Note that the detection amount includes concentration and intensity.

[0292] For example, if the detection profiles of the first element e1 and the second element e2 are as shown in Figure 13A, the position where the detection amount of the second element e2 is maximum is located further inside than the position where the detection amount of the first element e1 is maximum. Also, for example, if the detection profiles of the first element e1 and the second element e2 are as shown in Figure 13B, the position where the detection amount of the second element e2 is maximum is located further inside than the position where the detection amount of the first element e1 is maximum. Also, for example, if the detection profiles of the first element e1 and the second element e2 are as shown in Figure 13C, the position where the detection amount of the first element e1 is maximum is located further inside than the position where the detection amount of the second element e2 is maximum. Also, for example, if the detection profiles of the first element e1 and the second element e2 are as shown in Figure 13D, the position where the detection amount of the second element e2 is maximum is located further inside than the position where the detection amount of the first element e1 is maximum. For example, if the detection profiles of the first element e1 and the second element e2 are shaped as shown in Figure 13E, the position where the detection amount of the first element e1 is maximum is located further inside than the position where the detection amount of the second element e2 is maximum. For example, if the detection profiles of the first element e1 and the second element e2 are shaped as shown in Figure 13F, the position where the detection amount of the second element e2 is maximum is located further inside than the position where the detection amount of the first element e1 is maximum.

[0293] The expression "having a region where the distributions overlap" will be explained using the example of the positional relationship shown in Figure 13G for the detection amount profiles of the first element e1, the second element e2, and the third element e3. In this specification, "having a region where the distributions of two elements overlap" means, for example, that the position where the detection amount of at least one element is at its maximum is located within a range where the detection amount of the other element is at least 1 / 5 of the maximum value.

[0294] For example, in the positional relationship shown in Figure 13G, the position (P2) where the detection amount profile of the second element e2 is at its maximum value is located within the range (hatched area in the figure) where the detection amount in the detection amount profile of the first element e1 is at least 1 / 5 of the maximum value (or the lower detection limit). Therefore, the first element e1 and the second element e2 have a region where their distributions overlap. On the other hand, the position (P3) where the detection amount profile of the third element e3 is at its maximum value is not located within the range (hatched area in the figure) where the detection amount in the detection amount profile of the first element e1 is at least 1 / 5 of the maximum value (or the lower detection limit). Therefore, the first element e1 and the third element e3 cannot be said to have a region where their distributions overlap.

[0295] Furthermore, in the case of the positional relationship shown in Figure 13G, it can be said that the distribution of the second element e2 and the distribution of the third element e3 are located more inward than the distribution of the first element e1. Alternatively, it can be said that the distribution of the second element e2 and the distribution of the third element e3 are biased more towards the inward side than the distribution of the first element e1.

[0296] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0297] (Embodiment 3) In this embodiment, each element constituting the secondary battery will be described.

[0298] [Positive electrode] The secondary battery has a positive electrode. The positive electrode is as described in Embodiments 1 and 2, etc.

[0299] <Positive Electrode Current Collector> The positive electrode has a positive electrode current collector. As the positive electrode current collector, materials with high conductivity such as stainless steel, gold, platinum, aluminum, titanium, and alloys thereof can be used. Furthermore, it is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. In addition, aluminum alloys to which elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, have been added can be used. Alternatively, it may be formed from a metallic element that reacts with silicon to form a silicide. Metallic elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the shape of foil, plate, sheet, mesh, perforated metal, expanded metal, etc. as appropriate. The current collector should preferably have a thickness of 5 μm or more and 30 μm or less.

[0300] <Binder> The positive electrode preferably has a binder. Preferably, the binder is made of materials such as polystyrene, methyl polyacrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, or nitrocellulose.

[0301] As alternative binders, it is preferable to use rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluororubber can also be used as a binder.

[0302] As another binder, it is preferable to use, for example, a water-soluble polymer. As a water-soluble polymer, for example, polysaccharides can be used. As polysaccharides, cellulose derivatives such as carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, diacetylcellulose, and regenerated cellulose, or starch can be used. Furthermore, it is even preferable to use these water-soluble polymers in combination with the rubber material mentioned above.

[0303] The binder may be a combination of several of the above. For example, a material with particularly excellent viscosity-modifying properties may be used in combination with other materials. For example, rubber materials have excellent adhesive and elastic properties, but viscosity adjustment can be difficult when mixed with a solvent. In such cases, it is preferable to mix them with a material with particularly excellent viscosity-modifying properties. As a material with particularly excellent viscosity-modifying properties, for example, a water-soluble polymer may be used. As a water-soluble polymer with particularly excellent viscosity-modifying properties, the aforementioned polysaccharides, such as carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, and diacetylcellulose, cellulose derivatives such as regenerated cellulose, or starch can be used.

[0304] Furthermore, cellulose derivatives such as carboxymethylcellulose can be made more soluble by using salts such as sodium or ammonium salts of carboxymethylcellulose, thereby increasing their effectiveness as viscosity modifiers. Increased solubility also improves the dispersibility with active materials or other components when preparing electrode slurries. In this specification, cellulose and cellulose derivatives used as electrode binders include their salts.

[0305] Water-soluble polymers stabilize viscosity by dissolving in water, allowing for stable dispersion of active materials and other materials used as binders, such as styrene-butadiene rubber, in aqueous solutions. Furthermore, their functional groups are expected to facilitate stable adsorption to the surface of the active material. Additionally, cellulose derivatives such as carboxymethylcellulose often possess functional groups such as hydroxyl or carboxyl groups, and these functional groups allow the polymers to interact with each other, resulting in a broad coverage of the active material surface.

[0306] When a binder covers or is in contact with the surface of the active material, it is expected to act as a passivation film, suppressing the decomposition of the electrolyte. Here, a "passivation film" is a film that does not conduct electricity, or has extremely low electrical conductivity. For example, when a passivation film is formed on the surface of the active material, the decomposition of the electrolyte can be suppressed at the reaction potential of the secondary battery. Furthermore, it is even more desirable for the passivation film to suppress electrical conductivity while still allowing lithium ions to conduct.

[0307] <Conductive Material> The positive electrode preferably has a conductive material. The conductive material is also called a conductivity imparting agent or conductivity enhancer, and carbon materials are used. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, and conductivity is increased. Note that "attachment" does not only refer to the physical close contact between the active materials and the conductive material, but also includes cases where covalent bonding occurs, bonding occurs by van der Waals forces, the conductive material covers a part of the surface of the active material, the conductive material fits into the surface irregularities of the active material, and cases where they are electrically connected even if they are not in contact with each other.

[0308] As conductive materials, one or more of the following can be used: carbon black such as acetylene black and furnace black; graphite such as artificial graphite and natural graphite; carbon fibers such as carbon nanofibers and carbon nanotubes (CNTs); and graphene compounds.

[0309] As carbon fibers, for example, mesophase pitch carbon fibers and isotropic pitch carbon fibers can be used. Alternatively, carbon nanofibers or carbon nanotubes can be used. Carbon nanotubes can be fabricated, for example, by vapor deposition.

[0310] In this specification, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multigraphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multigraphene oxide, graphene quantum dots, etc. A graphene compound is defined as a material having carbon, having a plate-like or sheet-like shape, and possessing a two-dimensional structure formed by a six-membered carbon ring. This two-dimensional structure formed by a six-membered carbon ring may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a bent shape. Graphene compounds may also be rolled up to resemble carbon nanofibers.

[0311] The content of conductive material relative to the total amount of active material layer is preferably 1 wt% to 10 wt%, and more preferably 1 wt% to 5 wt%.

[0312] 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. Therefore, a smaller amount of graphene compound can improve the electrical conductivity between the granular active material and the graphene compound compared to conventional conductive materials. Consequently, the proportion of the active material in the active material layer can be increased. This, in turn, can increase the discharge capacity of the secondary battery.

[0313] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, readily penetrate minute spaces. These minute spaces refer, for example, to regions between multiple active materials. By using a carbon-containing compound that readily penetrates minute spaces in combination with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the electrode density can be increased, and excellent conductive paths can be formed. A secondary battery obtained by the manufacturing method according to one embodiment of the present invention can have high capacity density and stability, making it effective as a secondary battery for automotive use.

[0314] [Negative electrode] The negative electrode comprises a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also contain a negative electrode active material, a conductive material, and a binder.

[0315] <Negative electrode active material> As the negative electrode active material, for example, alloy materials or carbon materials can be used.

[0316] Furthermore, the negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc., can be used. Such elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity (per weight of negative electrode active material) of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Compounds containing these elements may also be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ Mg ２ Sn, SnS ２ , V ２ Sn ３ FeSn ２ CoSn ２ Ni ３ Sn ２ ,Cd ６ Sn ５ Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb３ LaSn ３ La ３ Co ２ Sn ７ CoSb ３ Examples include InSb and SbSn. Here, elements capable of undergoing charge-discharge reactions through alloying and de-alloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloying materials.

[0317] In this specification, "SiO" refers to silicon monoxide, for example. Alternatively, SiO refers to SiO β It can also be expressed as follows. Here, β is preferably 1 or a value close to 1. For example, β is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0318] Carbon materials that can be used include graphite, easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), carbon fibers (carbon nanotubes), graphene, and carbon black.

[0319] 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. Artificial graphite may have a carbon coating layer, which is a low-crystallinity layer. Since artificial graphite is spherical in shape, it is called spheroidal graphite. For example, MCMB is a preferred material for spheroidal graphite. Furthermore, it is relatively easy to reduce the specific surface area of ​​MCMB. If the specific surface area is large, the decomposition reaction with the electrolyte on the surface of the negative electrode active material becomes large, and good cycle characteristics may not be obtained. To suppress the above decomposition reaction, the specific surface area of ​​carbon should be 0.8 m². ２ / g or more 8m ２ / g or less, preferably 1m ２ / g or more 2m ２It is preferable that the value is less than or equal to / g. Typically, it is preferable that the powder has the specific surface area described above as a characteristic of spheroidal graphite. The specific surface area can be measured by the BRENAUER-EMMETTT-TELLER method. The BRENAUER-TELLER method is an analytical technique that extends Langmuir theory to multilayer adsorption of adsorbed gas molecules, and is the most common method for calculating specific surface area. The specific surface area by the BRENAUER method can be measured using the automatic specific surface area measuring device TriStar 2 3020.

[0320] Examples of natural graphite include flaky graphite or spheroidized natural graphite. Natural graphite may have a carbon coating layer, which is a low-crystallinity layer.

[0321] If the particle size or median diameter (D50) is small, the bulkiness increases, which can hinder the improvement of electrode density. Therefore, the particle size or median diameter (D50) of the negative electrode active material should be between 3 μm and 20 μm, preferably between 7 μm and 12 μm. Typically, the median diameter (D50) of graphite powder is within the above range.

[0322] Graphite exhibits a potential as low as lithium metal (0.05V to 0.3V vs. Li / Li) when lithium ions are inserted into it (during the formation of lithium-graphite intercalation compounds). ＋ This allows lithium-ion secondary batteries using graphite to exhibit a high operating voltage. Furthermore, graphite is preferable 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.

[0323] Furthermore, titanium dioxide (TiO) is used as the negative electrode active material. ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ Oxides such as those listed above can be used.

[0324] Furthermore, as the negative electrode active material, lithium and a nitride of a transition metal, Li ３ Li with an N-type structure ３−ｘ M ｘ N (M = Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large discharge capacity (900 mAh / g and 1890 mAh / cm² per weight of negative electrode active material). ３ ) indicates a preference.

[0325] When lithium and transition metal nitrides are used, lithium ions are contained in the negative electrode active material, so the positive electrode active material does not contain lithium ions. ２ O ５ , Cr ３ O ８ It is preferable that it be combined with materials such as the above. Furthermore, even when a material containing lithium ions is used as the positive electrode active material, lithium and a nitride of a transition metal can be used as the negative electrode active material by desorbing the lithium ions contained in the positive electrode active material beforehand.

[0326] Furthermore, 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), may be used as the negative electrode active material. As for materials that undergo a conversion reaction, Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ Oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ ,Cd ３ N, Ge ３ N ４ Nitrides such as NiP ２ FeP ２ CoP ３ Phosphates such as FeF ３ BiF ３ Examples of fluorides include the following.

[0327] Furthermore, as another form of negative electrode, a negative electrode may be one that does not have negative electrode active material at the end of the secondary battery manufacturing process. For example, a negative electrode without negative electrode active material may be one that has only a negative electrode current collector at the end of the secondary battery manufacturing process, in which lithium ions that detach from the positive electrode active material during charging of the secondary battery deposit as lithium metal on the negative electrode current collector, forming 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 or a negative electrode-less (anode-less) secondary battery.

[0328] When using a negative electrode without a negative electrode active material, a film may be provided on the negative electrode current collector to homogenize the deposition of lithium. As a film to homogenize the deposition of lithium, for example, a solid electrolyte having lithium ion conductivity can be used. As a solid electrolyte, sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes can be used. Among these, polymer-based solid electrolytes are suitable as a film to homogenize the deposition of lithium because it is relatively easy to form a uniform film on the negative electrode current collector. Alternatively, as a film to homogenize the deposition of lithium, for example, a metal film that forms an alloy with lithium can be used. As a metal film that forms an alloy with lithium, for example, a magnesium metal film can be used. Since lithium and magnesium form a solid solution over a wide composition range, it is suitable as a film to homogenize the deposition of lithium.

[0329] Furthermore, when using a negative electrode without negative electrode active material, a negative electrode current collector with irregularities can be used. When using a negative electrode current collector with irregularities, the recesses in the negative electrode current collector become cavities where lithium can easily be deposited, thus suppressing the formation of dendrite-like shapes when lithium is deposited.

[0330] The conductive material and binder that the negative electrode active material layer can have can be the same materials as the conductive material and binder that the positive electrode active material layer can have.

[0331] <Negative Electrode Current Collector> In addition to the same materials as the positive electrode current collector, copper and other materials can also be used for the negative electrode current collector. It is preferable to use a material for the negative electrode current collector that does not alloy with carrier ions such as lithium.

[0332] [Separator] The secondary battery has a separator. The separator is as described in Embodiment 1, etc.

[0333] [Outer casing] The secondary battery has an outer casing. The outer casing is as described in Embodiment 1, etc.

[0334] [Electrolyte] The secondary battery has an electrolyte containing carrier ions. The electrolyte is as described in Embodiment 1, etc.

[0335] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0336] (Embodiment 4) This embodiment describes a secondary battery having an electrolyte and a separator, which is one aspect of the present invention. A secondary battery having a separator is preferable because it can prevent thermal runaway and / or ignition. 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.

[0337] [Coin-type rechargeable battery] An example of a coin-type rechargeable battery is described below. Figure 14A is an exploded perspective view of a coin-type (single-layer flat type) rechargeable battery, Figure 14B is an external view, and Figure 14C is a cross-sectional view thereof. Coin-type rechargeable batteries are mainly used in small electronic devices.

[0338] Note that Figure 14A is a schematic diagram to show the overlapping (upper and lower relationships and positional relationships) of the components for clarity. Therefore, Figures 14A and 14B are not perfectly identical corresponding diagrams.

[0339] In Figure 14A, the positive electrode 304, separator 310, negative electrode 307, spacer 322, and washer 312 are stacked. A separator according to one embodiment of the present invention can be applied to the separator 310. These are sealed with a negative electrode can 302, a positive electrode can 301, and a gasket. Note that the gasket for sealing is not shown in Figure 14A. The spacer 322 and washer 312 are used to protect the inside or fix their position within the can when the positive electrode can 301 and the negative electrode can 302 are pressed together. The spacer 322 and washer 312 are made of stainless steel or an insulating material.

[0340] The positive electrode 304 is a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305. Lithium cobalt oxide, one embodiment of the present invention, can be applied as the positive electrode active material of the positive electrode active material layer 306.

[0341] Figure 14B is a perspective view of the completed coin-type rechargeable battery.

[0342] The coin-type secondary battery 300 has a positive electrode casing 301, which also serves as the positive electrode terminal, and a negative electrode casing 302, which also serves as the negative electrode terminal, which 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 it. 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 it. Furthermore, the negative electrode 307 is not limited to a laminated structure, and lithium metal foil or a lithium-aluminum alloy foil may be used.

[0343] Furthermore, the positive electrode 304 and negative electrode 307 used in the coin-type secondary battery 300 can each have the active material layer formed on only one side.

[0344] The positive electrode can 301 and the negative electrode can 302 can be made of metals such as nickel, aluminum, or titanium, or alloys thereof, or alloys of these with other metals (e.g., stainless steel), which are corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat them with nickel or aluminum to prevent corrosion caused by the electrolyte. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0345] The negative electrode 307, positive electrode 304, and separator 310 are immersed in the electrolyte, and as shown in Figure 14C, the positive electrode 304, separator 310, negative electrode 307, and negative electrode 302 are stacked in this order with the positive electrode 301 at the bottom, and the positive electrode 301 and negative electrode 302 are pressed together via a gasket 303 to manufacture a coin-type secondary battery 300. It is preferable to use a mixed solvent, which is one embodiment of the present invention, as the solvent for the electrolyte.

[0346] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Figure 15A. As shown in Figure 15A, the cylindrical secondary battery 616 has a positive electrode cap (secondary battery cover) 601 on the top surface and a secondary battery casing (outer casing) 602 on the sides and bottom. The positive electrode cap 601 and the secondary battery casing (outer casing) 602 are insulated by a gasket (insulating packing) 610.

[0347] Figure 15B is a schematic diagram showing a cross-section of a cylindrical secondary battery. The cylindrical secondary battery shown in Figure 15B has a positive electrode cap (secondary battery cover) 601 on the top surface and a secondary battery casing (outer casing) 602 on the sides and bottom. The positive electrode cap and the secondary battery casing (outer casing) 602 are insulated from each other by a gasket (insulating packing) 610.

[0348] Inside the hollow cylindrical secondary battery casing 602, a secondary battery element is provided, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 in between. A separator according to one embodiment of the present invention can be applied to the separator 605. Although not shown, the secondary battery element is wound around a central axis. The secondary battery casing 602 is closed at one end and open at the other end. The secondary battery casing 602 can be made of a metal such as nickel, aluminum, or titanium, which is corrosion-resistant to the electrolyte, or an alloy thereof, or an alloy of these with other metals (for example, stainless steel). Furthermore, it is preferable to coat the secondary battery casing 602 with nickel and aluminum, etc., to prevent corrosion by the electrolyte. Inside the secondary battery casing 602, the secondary battery element in which the positive electrode, negative electrode, and separator are wound is sandwiched between a pair of opposing insulating plates 608 and insulating plate 609. Furthermore, the inside of the secondary battery casing 602, which houses the secondary battery elements, is filled with an electrolyte (not shown). The electrolyte can be the same as that used in coin-type secondary batteries.

[0349] Since the positive and negative electrodes used in cylindrical secondary batteries are wound, it is preferable to form the active material on both sides of the current collector.

[0350] Lithium cobalt oxide, one embodiment of the present invention, can be applied as the positive electrode active material of the positive electrode 604, resulting in a cylindrical secondary battery 616 with good high-voltage charging characteristics.

[0351] 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 the 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 disconnects the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the rise in the internal pressure of the secondary battery exceeds a predetermined threshold. Furthermore, the PTC element 611 is a thermal resistance element whose resistance increases when the temperature rises, and it prevents abnormal heat generation by limiting the amount of current through the increase in resistance. The PTC element contains barium titanate (BaTiO2). ３ ) semiconductor ceramics and the like can be used.

[0352] Figure 15C shows an example of an energy storage system 615. The energy storage system 615 has a plurality of secondary batteries 616. The positive electrode of each secondary battery is in contact with a conductor 624 separated by an insulator 625 and is electrically connected. The conductor 624 is electrically connected to a control circuit 620 via wiring 623. The negative electrode of each secondary battery is also electrically connected to the control circuit 620 via 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 over-discharging.

[0353] Figure 15D shows an example of an energy storage system 615. The energy storage system 615 has multiple secondary batteries 616, which are sandwiched between conductive plates 628 and 614. The multiple secondary batteries 616 are electrically connected to the conductive plates 628 and 614 by wiring 627. The multiple secondary batteries 616 may be connected in parallel, in series, or connected in parallel and then in series. By configuring an energy storage system 615 with multiple secondary batteries 616, a large amount of power can be extracted.

[0354] Multiple secondary batteries 616 may be connected in parallel and then further connected in series.

[0355] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When a secondary battery 616 overheats, it can be cooled by the temperature control device, and when a secondary battery 616 becomes too cold, it can be heated by the temperature control device. This makes the performance of the energy storage system 615 less susceptible to the influence of ambient temperature.

[0356] Furthermore, in Figure 15D, the energy storage system 615 is electrically connected to the control circuit 620 via wiring 621 and wiring 622. Wiring 621 is electrically connected to the positive terminals of the multiple secondary batteries 616 via conductive plate 628, and wiring 622 is electrically connected to the negative terminals of the multiple secondary batteries 616 via conductive plate 614.

[0357] [Other structural examples of secondary batteries] Structural examples of secondary batteries will be explained using Figures 16 and 17.

[0358] The secondary battery 913 shown in Figure 16A has a wound body 950 with terminals 951 and 952 provided inside a housing 930. The wound body 950 is immersed in an electrolyte inside the housing 930. It is preferable to use a mixed solvent, which is one embodiment of the present invention, as the solvent for the electrolyte. Terminal 952 is in contact with the housing 930, while terminal 951 is not in contact with the housing 930 by using an insulating material or the like. In Figure 16A, the housing 930 is shown separated for convenience, but in reality, the wound body 950 is covered by the housing 930, and terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a laminate of a metal material and a resin material.

[0359] Furthermore, as shown in Figure 16B, the housing 930 shown in Figure 16A may be formed from multiple materials. For example, in the secondary battery 913 shown in Figure 16B, housing 930a and housing 930b are bonded together, and a winding body 950 is provided in the area surrounded by housing 930a and housing 930b.

[0360] The housing 930a can be made of a metal material (e.g., aluminum) or a laminate of a metal material and a resin material. An organic resin can be used as the resin material. In particular, using an organic resin on the surface where the antenna is formed can suppress the electric field generated by the secondary battery 913. If the shielding of the electric field by the housing 930a is small, the antenna may be placed inside the housing 930a. The housing 930b can be made of a metal material (e.g., aluminum) or a laminate of a metal material and a resin material.

[0361] Furthermore, the structure of the wound body 950 is shown in Figure 16C. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. A separator according to one aspect of the present invention can be applied to the separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 in between, and the stacked sheets are wound up. Note that multiple stacks of the negative electrode 931, positive electrode 932, and separator 933 may be stacked further.

[0362] Alternatively, the secondary battery 913 may have a wound body 950a as shown in Figure 17. The wound body 950a shown in Figure 17A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0363] Lithium cobalt oxide, according to one aspect of the present invention, can be applied as the positive electrode active material in the positive electrode active material layer 932a.

[0364] The separator 933 has a wider width 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. Furthermore, it is preferable from a safety standpoint that the negative electrode active material layer 931a is wider than the positive electrode active material layer 932a. A wound body 950a of this shape is also preferable due to its good safety and productivity.

[0365] As shown in Figure 17B, the negative electrode 931 is electrically connected to terminal 951 by ultrasonic bonding, welding, or crimping. Terminal 951 is electrically connected to terminal 911a. The positive electrode 932 is electrically connected to terminal 952 by ultrasonic bonding, welding, or crimping. Terminal 952 is electrically connected to terminal 911b.

[0366] As shown in Figure 17C, the coiled body 950a and electrolyte are covered by the housing 930, forming 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, thereby preventing the secondary battery from rupturing.

[0367] As shown in Figure 17B, the secondary battery 913 may have multiple windings 950a. By using multiple windings 950a, a secondary battery 913 with a larger discharge capacity can be made. Other elements of the secondary battery 913 shown in Figures 17A and 17B can be found by referring to the description of the secondary battery 913 shown in Figures 16A to 16C.

[0368] <Laminated Secondary Battery> Next, an example of an external view of a laminated secondary battery is shown in Figures 18A and 18B. Figures 18A and 18B show 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. A separator according to one embodiment of the present invention can be applied to the separator 507. Although not shown, it is preferable to apply a mixed solvent according to one embodiment of the present invention as the solvent for the electrolyte.

[0369] Figure 18A shows the external view of the positive electrode 103 and the negative electrode 106. The positive electrode 103 has a positive electrode current collector 21, and the positive electrode active material layer 22 is formed on the surface of the positive electrode current collector 21. Lithium cobalt oxide, one embodiment of the present invention, can be applied as the positive electrode active material of the positive electrode active material layer 22. The positive electrode 103 also has a region where the positive electrode current collector 21 is partially exposed (hereinafter referred to as the 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., the tab region. Note that the area or shape of the tab regions of the positive and negative electrodes are not limited to the example shown in Figure 18A.

[0370] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0371] (Embodiment 5) This embodiment shows an example of a vehicle equipped with a secondary battery having an electrolyte and a separator, which is one aspect of the invention. The vehicle includes automobiles, trains, airplanes, and buses. A secondary battery having a separator is preferable because it can prevent thermal runaway and / or ignition. 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.

[0372] The automobile 2001 shown in Figure 19A is an electric vehicle that uses an electric motor as a power source for driving, or a hybrid vehicle that can appropriately select and use an electric motor and an engine as power sources for driving. The automobile 2001 has a secondary battery pack 2200, and preferably the secondary battery pack has a secondary battery module in which a plurality of secondary batteries are connected and a charging control device electrically connected to the secondary battery module.

[0373] Next, the secondary battery pack 2200 will be described using Figure 19B. Figure 19B shows an example in which one secondary battery pack 2200 has nine prismatic secondary batteries 1300. In this example, the nine prismatic secondary batteries 1300 are connected in series, with one electrode group fixed by a fixing part 1413 made of an insulator, and the other electrode group fixed by a fixing part 1414 made of an insulator. Instead of fixing parts 1413 and 1414, the electrode groups may be fixed by housing them in a secondary battery housing box (also called a casing). Since vehicles are expected to be subjected to vibration or shaking from the outside (road surface, etc.), it is preferable to fix multiple prismatic secondary batteries 1300 with fixing parts 1413, 1414 and a secondary battery housing box. In addition, one electrode group is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode group is electrically connected to the control circuit unit 1320 by wiring 1422.

[0374] Furthermore, the control circuit unit 1320 may also use a memory circuit that includes a transistor made of an oxide semiconductor. A charging control circuit or secondary battery control system having a memory circuit that includes a transistor made of an oxide semiconductor may be referred to as BTOS (Battery operating system or Battery oxide semiconductor).

[0375] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, as the metal oxide, it is preferable to use a metal oxide such as In-M-Zn oxide (where element M is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, or magnesium). In particular, it is preferable that the In-M-Zn oxide applicable as the metal oxide is CAAC-OS (C-Axis Aligned Crystal Oxide Semiconductor) or CAC-OS (Cloud-Aligned Composite Oxide Semiconductor). Alternatively, In-Ga oxide or In-Zn oxide may also be used as the metal oxide. CAAC-OS is an oxide semiconductor having multiple crystalline regions, the c-axis of which is oriented in a specific direction. This specific direction is the thickness direction of the CAAC-OS film, the normal direction to the surface on which the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. Furthermore, a crystalline region is a region with periodicity in its atomic arrangement. If the atomic arrangement is considered as a lattice arrangement, then a crystalline region is also a region with a aligned lattice arrangement.

[0376] Furthermore, "CAC-OS" is a mosaic-like structure formed by the separation of the material into a first region and a second region, with the first region distributed within the film (hereinafter also referred to as a cloud-like 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.

[0377] Oxide semiconductors can take on diverse structures, each possessing different properties. One embodiment of the present invention may include two or more of amorphous oxide semiconductors, polycrystalline oxide semiconductors, a-like OS, CAC-OS, nc-OS, and CAAC-OS.

[0378] 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 exhibit smaller characteristic changes even when the secondary battery overheats compared to single crystals. The off-current of an oxide semiconductor transistor is extremely low regardless of temperature, even at 150°C, but the off-current characteristics of a single-crystal Si transistor are highly temperature-dependent. For example, at 150°C, the off-current of a single-crystal Si transistor increases, and the current on / off ratio does not become sufficiently large. The control circuit unit 1320 can improve safety. The secondary battery and the control circuit unit 1320 can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0379] Next, Figure 19C shows an example of a block diagram for the automobile 2001 shown in Figure 19A and the secondary battery pack 2200 shown in Figure 19B.

[0380] As shown in Figure 19C, electric vehicles are equipped with a first battery 1301a, 1301b as the main secondary battery for driving, and a second battery 1311 that supplies power to the inverter 1312 that starts the motor 1304. The second battery 1311 is also called a cranking battery (or starter battery). The second battery 1311 only needs to be able to output power, and a large capacity is not particularly necessary, so the capacity of the second battery 1311 is smaller than that of the first batteries 1301a, 1301b.

[0381] The internal structure of the first battery 1301a may be a wound type as shown in Figure 16C or Figure 17A, or a stacked type as shown in Figure 18A or Figure 18B.

[0382] 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 not be necessary. By configuring a secondary battery pack having multiple secondary batteries, a large amount of power can be extracted. Multiple secondary batteries may be connected in parallel, in series, or connected in parallel and then in series. Multiple secondary batteries are also called a secondary battery pack.

[0383] Furthermore, the vehicle-mounted secondary battery has a service plug or circuit breaker that can interrupt high voltage without the use of tools in order to interrupt power from multiple secondary batteries, and is provided on the first battery 1301a.

[0384] Furthermore, the power from the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V onboard components (electric power steering 1307, heater 1308, defogger 1309, etc.) via the DC-DC circuit 1306. In the case where there is a rear motor 1317 on the rear wheels, the first battery 1301a is used to rotate the rear motor 1317.

[0385] Furthermore, the second battery 1311 supplies power to 14V automotive components (audio system 1313, power windows 1314, lights 1315, etc.) via the DC-DC circuit 1310.

[0386] The first batteries 1301a and 1301b primarily supply power to 42V (high-voltage HV) onboard equipment, while the second battery 1311 supplies power to 14V (low-voltage LV) onboard equipment. Lead-acid batteries are often used for the second battery 1311 due to cost advantages. Lead-acid batteries have the disadvantage of higher self-discharge and are prone to degradation due to a phenomenon called sulfation compared to lithium-ion secondary batteries. Using a lithium-ion secondary battery for the second battery 1311 offers the advantage of being maintenance-free, but after long-term use, for example more than three years, there is a risk of malfunctions occurring that are difficult to detect at the time of manufacture. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, the motor may not start even if the first batteries 1301a and 1301b have remaining capacity. If the second battery 1311 is a lead-acid battery, power is supplied from the first battery to the second battery, and it is charged to always maintain a fully charged state, so the above-mentioned inability to operate the motor will not occur.

[0387] This embodiment shows an example 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-acid battery, an all-solid-state secondary battery, or an electric double-layer capacitor.

[0388] Furthermore, the regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305 and charged to the second battery 1311 via the control circuit unit 1321 from the motor controller 1303 or the battery controller 1302. Alternatively, it is charged to the first battery 1301a via the control circuit unit 1320 from the battery controller 1302. Alternatively, it is charged to the first battery 1301b via the control circuit unit 1320 from the battery controller 1302. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b are capable of rapid charging.

[0389] 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 the charging conditions according to the charging characteristics of the secondary battery being used and enable rapid charging.

[0390] Although not shown in the diagram, when an electric vehicle is connected to an external charger, the charger's plug or connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger charges the first batteries 1301a and 1301b via the battery controller 1302. In some cases, a control circuit is provided in the charger, and the functions of the battery controller 1302 are not used, but it is preferable to charge the first batteries 1301a and 1301b via the control circuit unit 1320 to prevent overcharging. In some cases, the charger's plug or connection cable may also have a control circuit. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) installed in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU also includes a microcomputer. Furthermore, the ECU uses either a CPU or a GPU.

[0391] External chargers installed at charging stations and other locations may have a 100V-200V outlet, or a 3-phase 200V and 50kW output. Additionally, it is possible to charge by receiving power from external charging equipment using contactless power supply methods.

[0392] For rapid charging, a rechargeable battery capable of withstanding high-voltage charging is desired to achieve short charging times.

[0393] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0394] (Embodiment 6) This embodiment describes space equipment equipped with a secondary battery having an electrolyte and a separator, which is one aspect of the invention. A secondary battery having a separator is preferable because it can prevent thermal runaway and / or ignition. 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.

[0395] Figure 20A shows an example of space equipment, specifically a satellite 6800. The satellite 6800 comprises a body 6801, a solar panel 6802, an antenna 6803, and a secondary battery 6805.

[0396] When sunlight shines on the solar panel 6802, the power necessary for the satellite 6800 to operate is generated. However, in situations where, for example, sunlight does not shine on the solar panel, or when the amount of sunlight shining on the solar panel is low, the amount of power generated will decrease. Therefore, there is a possibility that the power necessary for the satellite 6800 to operate will not be generated. To operate the satellite 6800 even under conditions where the amount of power generated is low, it is advisable to provide the satellite 6800 with a secondary battery 6805. By using the secondary battery of the present invention, a highly reliable secondary battery can be obtained. Furthermore, by using the secondary battery of the present invention, a secondary battery exhibiting good low-temperature characteristics can be obtained.

[0397] The satellite 6800 can generate a signal. This signal is transmitted via antenna 6803, and can be received by, for example, a receiver on the ground or another satellite. By receiving the signal transmitted by satellite 6800, the position of the receiver that received the signal can be measured. Thus, satellite 6800 can constitute, for example, a satellite positioning system.

[0398] Alternatively, the satellite 6800 can be configured to include sensors. For example, by configuring it to include a visible light sensor, the satellite 6800 can have the function of detecting sunlight reflected off an object on the ground. Alternatively, by configuring it to include a thermal infrared sensor, the satellite 6800 can have the function of detecting thermal infrared radiation emitted from the Earth's surface. Thus, the satellite 6800 can function, for example, as an Earth observation satellite.

[0399] Figure 20B shows a spacecraft 6900 equipped with a solar sail (also called a solar sail) as an example of space equipment. The spacecraft 6900 comprises a body 6901, a solar sail 6902, and a secondary battery 6905. By using the secondary battery of the present invention, a highly reliable secondary battery can be made. Furthermore, by using the secondary battery of the present invention, a secondary battery exhibiting good low-temperature characteristics can be made. When photons emitted from the sun strike the surface of the solar sail 6902, momentum is transferred to the solar sail 6902. Therefore, it is preferable that the surface of the solar sail 6902 has a thin film with high reflectivity, and more preferably that it faces the direction of the sun.

[0400] Furthermore, the solar sail 6902 may be designed to remain folded compactly until it leaves the Earth's atmosphere (in outer space), and then unfold into a large sheet-like shape as shown in Figure 20B.

[0401] Figure 20C 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, a highly reliable secondary battery can be made. Furthermore, by using the secondary battery of the present invention, a secondary battery exhibiting good low-temperature characteristics can be made. The body 6911 may have, for example, a pressurized chamber and an unpressurized chamber. The pressurized chamber may be designed to accommodate a crew. The electricity generated when sunlight is shone on the solar panel 6912 can be used to charge the secondary battery 6913.

[0402] Figure 20D 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, a highly reliable secondary battery can be made. Furthermore, by using the secondary battery of the present invention, a secondary battery exhibiting good low-temperature characteristics can be made. The rover 6920 may also have a solar panel 6922.

[0403] The rover 6920 may be designed to accommodate a crew. The secondary battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or it may be charged with electricity generated by other power sources, such as a fuel secondary battery or a radioisotope thermoelectric converter.

[0404] The contents of this embodiment can be appropriately combined with the contents of other embodiments.

[0405] In this example, samples were prepared with different ratios of cyclic and linear molecules in the mixed solvent used as the electrolyte, and the battery characteristics were obtained at sub-zero, room temperature (25°C), and high temperatures. A coin-type secondary battery (coin cell) was used as the sample. The individual components of the coin cell will now be described.

[0406] [Positive Electrode] The coin cell has a positive electrode. First, the method for preparing the positive electrode active material will be explained. As shown in step S14 of Figure 5, pre-synthesized lithium cobalt oxide particles (Cellseed 5H manufactured by Nippon Chemical Industrial Co., Ltd.) were prepared. The median diameter (D50) of these lithium cobalt oxide particles was approximately 7 μm.

[0407] Lithium cobalt oxide particles were placed in a container (a sheath), the sheath was sealed, and heated in a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.). The heating conditions were 850°C for 2 hours. Oxygen was flowed into the furnace at a rate of 10 L / min, and the flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace read 5 Pa, thereby creating positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was continued until the temperature reached 200°C. In this way, lithium cobalt oxide after initial heating was obtained.

[0408] Next, lithium fluoride and magnesium fluoride were prepared as the additive element source (source A1). The lithium fluoride and magnesium fluoride were weighed to satisfy a molar ratio of 1:3 and mixed by a wet method. Dehydrated acetone was used as the solvent, and the mixture was mixed at a rotation speed of 500 rpm for 20 hours. After removing the acetone, the mixture was sieved through a sieve with a mesh size of 300 μm to dissolve any agglomeration.

[0409] The lithium cobalt oxide particles, after initial heating, were mixed with the A1 source. The mixing conditions were such that the ratio of cobalt atoms (Co) in the lithium cobalt oxide to magnesium atoms (Mg) in the A1 source was Co:Mg = 100:1. The mixture was stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron). A Novilta rotor was used for the Picobond. A mixture A was obtained in which the A1 source was attached to the lithium cobalt oxide particles.

[0410] Mixture A was placed in a casing, the casing was covered, and heated in a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.). The heating conditions were 850°C for 10 hours. Oxygen was flowed into the furnace at a rate of 10 L / min, and the flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace read 5 Pa, thereby creating positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C. In this way, lithium cobalt oxide with source A1 added was obtained.

[0411] Next, nickel hydroxide and aluminum hydroxide were prepared as the additive element source (A2 source). Nickel hydroxide was pulverized by a wet process, and aluminum hydroxide was pulverized by a wet process. The wet pulverization conditions were the same as for the A1 source. The pulverized nickel hydroxide and aluminum hydroxide were mixed to form the A2 source, which was then mixed with lithium cobalt oxide particles to which the A1 source had been added. The mixing conditions were such that the ratio of the number of cobalt atoms (Co) in lithium cobalt oxide to the number of nickel atoms (Ni) in nickel hydroxide was Co:Ni = 100:0.5, and the ratio of the number of cobalt atoms (Co) in lithium cobalt oxide to the number of aluminum atoms (Al) in aluminum hydroxide was Co:Al = 100:0.5. The raw materials were prepared in this manner, and stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron). A Novilta rotor was used for the Picobond. A mixture B was obtained in which the A2 source was attached to the lithium cobalt oxide particles to which the A1 source had been added.

[0412] Mixture B was placed in a casing, the casing was covered, and heated in a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.). The heating conditions were 850°C for 2 hours. Oxygen was flowed into the furnace at a rate of 10 L / min, and the flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace read 5 Pa, thereby creating positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C.

[0413] Through the process described above, a positive electrode active material having Mg, F, Ni, Al, and lithium cobalt oxide was obtained. This was used as the positive electrode active material in this embodiment.

[0414] The positive electrode was prepared by coating a positive electrode current collector (aluminum foil) with a slurry of positive electrode active material, acetylene black (AB), and PVDF in a weight ratio of positive electrode active material:AB:PVDF = 95:3:2. N-methyl-2-pyrrolidone (NMP) was used as the solvent for the slurry.

[0415] After coating the current collector with slurry, the solvent was evaporated, and then pressurized with a linear pressure of 210 kN / m using a roll press. A positive electrode containing the positive electrode active material prepared as described above was obtained. The positive electrode was then punched out into a circular shape with a diameter of 15.96 mm. The amount of active material carried in the positive electrode was 10.0 mg / cm². ２ 10.7mg / cm or more ２ The range was as follows:

[0416] [Negative electrode] The negative electrode active material has a median diameter (D50) of 20.9 μm and a specific surface area of ​​1.01 m². ２ Formula BT SLC1520T (manufactured by Superior Graphite) was used in a quantity of 1 / g. A slurry was prepared by mixing the negative electrode active materials VGCF (registered trademark), CMC, and SBR in a ratio of 97:1:1:1 (by weight). Water was used as the solvent for the slurry. The slurry was coated onto a copper negative electrode current collector (copper foil), and then the solvent was evaporated.

[0417] Subsequently, to increase the density of the negative electrode active material layer, a pressurized treatment using a roll press was performed after solvent evaporation. The pressurized treatment conditions were a linear pressure of 28 kN / m, which was lower than that of the positive electrode. Then, the negative electrode was punched out into a circular shape with a diameter of 16.16 mm. The negative electrode was obtained through the above process. The amount of active material supported in the negative electrode was 6.8 mg / cm³. ２ 7.4mg / cm or more ２ The range was as follows:

[0418] [Separator] A three-layer polyimide structure was used as the separator.

[0419] [Electrolyte] The coordination energy of the organic compounds used in the mixed solvent of the electrolyte in this embodiment was determined from the formula shown in the above embodiment. The coordination energy was rounded to the first decimal place and is shown in Table 1 below.

[0420]

[0421] (Electrolyte A) As the mixed solvent for the electrolyte, ethylene carbonate (abbreviated as EC) is prepared as a cyclic molecule, ethyl propionate (abbreviated as EP) is prepared as a chain molecule, and LiPF is prepared as the lithium salt. ６Prepared a mixed solvent satisfying the EC:EP = 1:5 [mol ratio], and added LiPF so that it was 0.7 [mol] per liter of the total volume of the mixed solvent. ６ The solution obtained by dissolving LiPF was used as electrolyte A. In this example, LiPF ６ The concentrations were defined as values ​​per liter of the total mixed solvent at a temperature of 25°C ± 5°C. The coordination energy of EC was -40 [kJ / mol], and the coordination energy of EP was -39 [kJ / mol]. The coordination energy of EC, which had the lowest coordination energy, was at least -35 [kJ / mol] or less, and the coordination energy of EP was also -35 [kJ / mol] or less. That is, the coordination energies of the cyclic and chain molecules satisfied coordination energy condition 1. Furthermore, the difference in coordination energies between EC and EP (ΔE) was 1 [kJ / mol], satisfying coordination energy condition 2 of 5 [kJ / mol] or less.

[0422] (Electrolyte B) As the mixed solvent for the electrolyte, ethylene carbonate (abbreviated as EC) is prepared as a cyclic molecule, ethyl propionate (abbreviated as EP) as the first chain molecule, ethyl methyl carbonate (abbreviated as EMC) as the second chain molecule, and LiPF as the lithium salt. ６ Prepared a mixed solvent satisfying the ratio EC:EP:EMC = 1:1:10 [mol ratio], and added LiPF so that it was 0.7 [mol] per liter of the total volume of the mixed solvent. ６ The solution obtained by dissolving the following was designated as electrolyte B. The coordination energy of EC was -40 [kJ / mol], the coordination energy of EP was -39 [kJ / mol], and the coordination energy of EMC was -37 [kJ / mol]. The coordination energy of EC, which had the lowest coordination energy, was at least -35 [kJ / mol] or less, the coordination energy of EP, which had the next lowest coordination energy, was also -35 [kJ / mol] or less, and the coordination energy of EMC was also -35 [kJ / mol] or less. In other words, the coordination energies of the cyclic and chain molecules satisfied coordination energy condition 1. Furthermore, the difference (ΔE) between the coordination energy of EC, which had the lowest coordination energy, and EP, which had the next lowest, was 1 [kJ / mol], satisfying coordination energy condition 2 of 5 [kJ / mol] or less.

[0423] (Electrolyte C) As the mixed solvent for the electrolyte, fluoroethylene carbonate (abbreviated as FEC) is prepared as a cyclic molecule, ethyl propionate (abbreviated as EP) is prepared as a chain molecule, and LiPF is prepared as the lithium salt. ６ Prepared a mixed solvent satisfying the FEC:EP = 1:5 [mol ratio], and added LiPF so that it was 0.7 [mol] per 1 liter of the total volume of the mixed solvent. ６ The solution obtained by dissolving the substances was called electrolyte C. The coordination energy of FEC was -33 [kJ / mol], and the coordination energy of EP was -39 [kJ / mol]. The coordination energy of EP was -35 [kJ / mol] or less, but the coordination energy of FEC was higher than -35 [kJ / mol]. In other words, the coordination energy of the chain molecules satisfied coordination energy condition 1. However, the difference in coordination energies (ΔE) between FEC and EP was 6 [kJ / mol], which did not satisfy coordination energy condition 2, which is 5 [kJ / mol] or less.

[0424] (Electrolyte D) As the mixed solvent for the electrolyte, fluoroethylene carbonate (FEC) is prepared as a cyclic molecule, methyl 3,3,3-trifluoropropionate (MTFP) is prepared as a chain molecule, and LiPF is prepared as the lithium salt. ６ Prepared a mixed solvent satisfying the ratio FEC:MTFP = 2.8:7.3 [mol ratio], and added LiPF so that it was 0.7 [mol] per liter of the total volume of the mixed solvent. ６ The solution obtained by dissolving the substances was defined as electrolyte D. The coordination energy of FEC was -33 [kJ / mol], and the coordination energy of MTFP was -34 [kJ / mol]. The coordination energies of both FEC and MTFP were higher than -35 [kJ / mol]. In other words, the coordination energies of the cyclic and chain molecules did not satisfy coordination energy condition 1. Furthermore, the difference in coordination energies (ΔE) between FEC and MTFP was 1 [kJ / mol], and this difference satisfied coordination energy condition 2, which is 5 [kJ / mol] or less.

[0425] (Electrolyte E) As a mixed solvent for the electrolyte, gamma-butyrolactone (abbreviated as GBL) is prepared as the first cyclic molecule, ethylene carbonate (abbreviated as EC) as the second cyclic molecule, ethyl propionate (abbreviated as EP) as the first chain molecule, ethyl methyl carbonate (abbreviated as EMC) as the second chain molecule, and LiPF as the lithium salt. ６ Prepared a mixed solvent satisfying the ratio GBL:EC:EP:EMC = 1:1:1:15 [mol ratio], and added LiPF so that it was 0.7 [mol] per liter of the total volume of the mixed solvent. ６ The solution obtained by dissolving the substances was defined as electrolyte E. The coordination energy of GBL was -44 [kJ / mol], the coordination energy of EC was -40 [kJ / mol], the coordination energy of EP was -39 [kJ / mol], and the coordination energy of EMC was -37 [kJ / mol]. The coordination energy of GBL, which had the lowest coordination energy, was at least -35 [kJ / mol] or less, the coordination energy of EC, which had the next lowest coordination energy, was also -35 [kJ / mol] or less, the coordination energy of EP, which had the next lowest coordination energy, was also -35 [kJ / mol] or less, and the coordination energy of EMC was also -35 [kJ / mol] or less. In other words, the coordination energies of the cyclic and chain molecules satisfied coordination energy condition 1. Furthermore, the difference (ΔE) between the coordination energy of the lowest GBL and the next lowest EC was 4 [kJ / mol], satisfying coordination energy condition 2 of 5 [kJ / mol] or less.

[0426] In this embodiment, no additives were added to electrolytes A through E. The conditions for the mixed solvents of electrolytes A through E are summarized in Table 2 below.

[0427]

[0428] Coin cells were assembled using the positive electrode, negative electrode, and electrolyte described above. A coin cell using electrolyte A was designated as coin cell A, a coin cell using electrolyte B was designated as coin cell B, a coin cell using electrolyte C was designated as coin cell C, a coin cell using electrolyte D was designated as coin cell D, and a coin cell using electrolyte E was designated as coin cell E.

[0429] [Observation below freezing point] The temperature of the constant temperature bath was adjusted to -30°C, and electrolytes A to D were placed in the bath. After 24 hours, electrolytes A to D were observed and visually determined to see if they were frozen or not. Electrolytes A to D were not frozen, and it was found that the freezing points of all electrolytes A to D are below -30°C.

[0430] [Battery Characteristics 1] The constant temperature chamber was adjusted to 25°C, and coin cells A and C were placed inside. A charge-discharge cycle was performed under the following cycle conditions. A charge-discharge cycle is a test in which the battery is repeatedly charged and discharged, and under the following cycle conditions, 1C = 200 [mA / g] (current value per unit weight of positive electrode active material). Cycle Condition 1 (Constant temperature chamber temperature 25°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Cycle Condition 2 (Constant temperature chamber temperature -30°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Figure 21 shows the results of repeating the cycle under Condition 1 three times, the cycle under Condition 2 ten times, and the cycle under Condition 1 three times. When changing from Condition 1 to Condition 2, a 2-hour rest period was provided when the temperature of the constant temperature bath was changed. Furthermore, in both Condition 1 and Condition 2, a 10-minute rest period was provided between charging and discharging. In Figure 21, the vertical axis represents the discharge capacity [mAh / g], and the horizontal axis represents the number of cycles [times]. The battery characteristics at -30°C can be confirmed in Condition 2, which specifically corresponds to 4 to 13 cycles on the horizontal axis of Figure 21.

[0431] Although the freezing points of electrolytes A and C were both below -30°C, differences were observed in the charge-discharge cycle test results. Specifically, electrolyte A, in which the coordination energy of EC and EP were both -35 [kJ / mol] or less, and the difference between the coordination energy of EC and EP was 5 [kJ / mol] or less, showed a higher discharge capacity below freezing point (-30°C) compared to electrolyte C, which used FEC and EP that did not meet coordination energy condition 2. The lowest coordination energy value for EC was -37 [kJ / mol] or less, and -35 [kJ / mol] or less. Thus, it was found that the lowest coordination energy value satisfies coordination energy condition 1, resulting in good battery characteristics below freezing point. Coin cell A showed a discharge capacity retention rate of 110%, calculated as discharge capacity at cycle 13 / discharge capacity at cycle 4 × 100, indicating a high discharge capacity below freezing point. On the other hand, coin cell C had a discharge capacity retention rate of 75%, which is calculated as the discharge capacity at cycle number 13 / the discharge capacity at cycle number 4 × 100. This was unfavorable compared to coin cell A.

[0432] Furthermore, coin cell A showed good cycle characteristics, with a discharge capacity retention rate of 92%, calculated as (discharge capacity at 14 cycles / discharge capacity at 3 cycles) × 100. On the other hand, coin cell C showed a discharge capacity retention rate of 71%, calculated as (discharge capacity at 14 cycles / discharge capacity at 3 cycles) × 100, which was unfavorable compared to coin cell A. It is thought that deterioration occurred in coin cell C during cycling at -30°C.

[0433] The presence of a first organic compound with a low coordination energy in electrolyte A allows for sufficient ionization of the lithium salt, making it easier to maintain the lithium ion state. Furthermore, the coordination energy of the second organic compound is close to that of the first organic compound, which lowers the diffusion barrier of the lithium ions coordinated to the first organic compound. This is thought to be the reason why electrolyte A exhibits excellent battery characteristics below freezing point.

[0434] From this, it was found that in order to exhibit excellent battery characteristics below freezing point, in addition to the freezing point of the electrolyte, it is preferable that in the mixed solvent containing the first organic compound and the second organic compound, the coordination energy of the first organic compound is -35 [kJ / mol] or less, and the difference between the coordination energy of the first organic compound and the coordination energy of the second organic compound is 5 [kJ / mol] or less.

[0435] [Battery Characteristics 2] The constant temperature bath was adjusted to 25°C, and coin cell B was placed inside. A charge-discharge cycle was performed under the following cycle conditions. Under the following cycle conditions, 1C = 200 [mA / g] (current value per unit weight of positive electrode active material). Cycle Condition 1 (Constant temperature bath temperature 25°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Cycle Condition 2 (Constant temperature bath temperature -30°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Cycle Condition 3 (Constant temperature bath temperature -40°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Cycle Condition 4 (Constant temperature bath temperature -50°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Cycle Condition 5 (Constant temperature bath temperature 40°C) Charging: CCCV charging, 0.5C rate, 4.5V, 0.05C cutoff Discharge: CC discharge, 0.5C rate, 2.5V cutoff Cycle condition 6 (temperature of constant temperature chamber 50°C) Charging: CCCV charging, 0.5C rate, 4.5V, 0.05C cutoff Discharge: CC discharge, 0.5C rate, 2.5V cutoff Cycle condition 7 (temperature of constant temperature chamber 60°C) Charging: CCCV charging, 0.5C rate, 4.5V, 0.05C cutoff Discharge: CC discharge, 0.5C rate, 2.5V cutoff The results of repeating the cycle of condition 1 three times, condition 2 ten times, condition 1 four times, condition 3 ten times, condition 1 four times, condition 4 ten times, condition 1 four times, condition 5 ten times, condition 1 four times, condition 6 ten times, condition 1 four times, condition 7 ten times, and condition 1 three times are shown in Figure 22. When the conditions were changed from condition 1 to condition 7, a 2-hour rest period was provided at each set temperature, and a 10-minute rest period was provided between charging and discharging in each of conditions 1 to 7. The vertical axis of Figure 22 is discharge capacity [mAh / g], and the horizontal axis is number of cycles [times].The battery characteristics at -30°C can be confirmed under condition 2, specifically corresponding to the 4th to 13th cycles on the horizontal axis of Figure 22; the battery characteristics at -40°C can be confirmed under condition 3, specifically corresponding to the 18th to 27th cycles on the horizontal axis of Figure 22; the battery characteristics at -50°C can be confirmed under condition 4, specifically corresponding to the 32nd to 41st cycles on the horizontal axis of Figure 22; the battery characteristics at 40°C can be confirmed under condition 5, specifically corresponding to the 46th to 55th cycles on the horizontal axis of Figure 22; the battery characteristics at 50°C can be confirmed under condition 6, specifically corresponding to the 60th to 70th cycles on the horizontal axis of Figure 22; and the battery characteristics at 60°C can be confirmed under condition 7, specifically corresponding to the 74th to 83rd cycles on the horizontal axis of Figure 22.

[0436] Electrolyte B, in which the coordination energy of EC is -35 [kJ / mol] or less, and the difference between the coordination energy of the lowest EC and the next lowest EP is 5 [kJ / mol] or less, was found to exhibit high discharge capacity at sub-zero temperatures such as -30°C, -40°C, and -50°C. The coordination energy of EC, which shows the lowest value, was -37 [kJ / mol] or less, and -35 [kJ / mol] or less. Thus, it was found that the lowest value of coordination energy satisfies both coordination energy condition 1 and coordination energy condition 2, resulting in good battery characteristics at sub-zero temperatures. It is thought that the reason why electrolyte B exhibits excellent battery characteristics at sub-zero temperatures is that the presence of a first organic compound with a low coordination energy allows for sufficient ionization of the lithium salt and makes it easier to maintain the state of lithium ions, and the coordination energy of the second organic compound is close to that of the first organic compound, which lowers the diffusion barrier of lithium ions coordinated to the first organic compound. From these results, it can be concluded that satisfying coordination energy condition 2 for the difference between the coordination energy of the third organic compound and the second organic compound, or the difference between the coordination energy of the third organic compound and the first organic compound, as shown in Figure 2B, resulted in excellent battery characteristics below freezing point.

[0437] Furthermore, it was found that electrolyte B, which satisfies the above coordination energy conditions, exhibits high discharge capacity and high cycle characteristics at room temperature of 25°C, as well as at high temperatures of 40°C, 50°C, and 60°C. Conventionally, electrolytes that exhibit excellent battery characteristics below freezing point were selected based on organic compounds that take into account viscosity below freezing point, but it was difficult to achieve excellent battery characteristics at high temperatures. On the other hand, the inventors focused on coordination energy and selected organic compounds constituting the mixed solvent, which enabled them to conceive of an electrolyte that exhibits excellent battery characteristics below freezing point and at high temperatures.

[0438] [Battery Characteristics 3] The constant temperature chamber was adjusted to 25°C, and coin cells B and D were placed inside. A charge-discharge cycle was performed under the following cycle conditions. A charge-discharge cycle is a test in which the battery is repeatedly charged and discharged, and under the following cycle conditions, 1C = 200 [mA / g] (current value per unit weight of positive electrode active material). Cycle condition 1 (constant temperature chamber temperature 25°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Cycle condition 2 (constant temperature chamber temperature -30°C) Charging: CCCV charging, 0.1C rate, 4.5V, 0.01C cutoff Discharge: CC discharge, 0.1C rate, 2.5V cutoff Figure 23 shows the results of repeating the cycle under condition 1 three times, the cycle under condition 2 ten times, and the cycle under condition 1 three times. When changing from Condition 1 to Condition 2, a 2-hour rest period was provided when the temperature of the constant temperature bath was changed. Furthermore, in both Condition 1 and Condition 2, a 10-minute rest period was provided between charging and discharging. In Figure 23, the vertical axis represents the discharge capacity [mAh / g], and the horizontal axis represents the number of cycles [times]. The battery characteristics at -30°C can be confirmed in Condition 2, which specifically corresponds to 4 to 13 cycles on the horizontal axis of Figure 23.

[0439] Although the freezing points of electrolytes B and D were both below -30°C, differences were observed in the charge-discharge cycle test results. Specifically, 0.7 [mol / L] LiPF ６In the case where the FEC coordination energy and MTFP coordination energy are not -35 [kJ / mol] or less, electrolyte D exhibited less favorable sub-zero battery characteristics compared to electrolyte B. Electrolyte B showed a discharge capacity retention rate of 107%, calculated as the discharge capacity at cycle 13 / discharge capacity at cycle 4 × 100, indicating high discharge capacity below freezing point. On the other hand, electrolyte D showed a discharge capacity retention rate of 39%, calculated as the discharge capacity at cycle 13 / discharge capacity at cycle 4 × 100, which was less favorable compared to electrolyte B.

[0440] Furthermore, electrolyte B showed good cycle characteristics, with a discharge capacity retention rate of 96%, calculated as (discharge capacity at 14 cycles / discharge capacity at 3 cycles) × 100. On the other hand, electrolyte D showed a discharge capacity retention rate of 51%, calculated as (discharge capacity at 14 cycles / discharge capacity at 3 cycles) × 100, which was unfavorable compared to electrolyte B. It is thought that electrolyte D deteriorated during cycling at -30°C. Therefore, the coordination energy of the MTF used in electrolyte D was set as the reference value for coordination energy condition 1, and was defined as -35 [kJ / mol] or less.

[0441] A new coin cell E was assembled. The battery characteristics of coin cell E at sub-zero temperatures were confirmed.

[0442] [Battery Characteristics 4] The constant temperature bath was adjusted to 25°C, and coin cell E was placed inside. A charge-discharge cycle was performed under the same conditions as in [Battery Characteristics Below Freezing Point 1] above, and the results are shown in Figure 24. In Figure 24, the vertical axis represents the discharge capacity [mAh / g], and the horizontal axis represents the number of cycles [times]. The battery characteristics at -30°C can be confirmed with 4 to 13 cycles on the horizontal axis, which corresponds to condition 2. Coin cell E did not show superior charge-discharge cycle test results compared to coin cell B. In particular, the charge-discharge cycle test results below freezing point for coin cell E were unfavorable compared to coin cell B. Therefore, the coordination energy of GBL used in electrolyte E was set as the reference value for coordination energy condition 1, and the coordination energy of the first organic compound was set to be -42 [kJ / mol] or higher.

[0443] 10: Positive electrode active material, 10a: Surface layer, 10b: Interior, 20: Second positive electrode active material, 21: Positive electrode current collector, 21t: Protrusion, 22: Positive electrode active material layer, 31: Negative electrode current collector, 31t: Protrusion, 31ta: First protrusion, 31tb: Protrusion, 31tc: Third protrusion, 32: Negative electrode active material layer, 41: Conductive material, 100: Secondary battery, 103: Positive electrode, 103a: First positive electrode, 103b: Second positive electrode, 105: Separator, 105a: First separator, 105b: Second separator, 105c: Third separator, 105d: Fourth separator, 106: Negative electrode, 106a: 1: Negative electrode, 106b: Secondary negative electrode, 106c: Third negative electrode, 107a: Positive electrode lead, 107b: Negative electrode lead, 108: Electrolyte, 109a: Junction, 109b: Junction, 300: Secondary battery, 301: Positive electrode can, 302: Negative electrode can, 303: Gasket, 304: Positive electrode, 305: Positive electrode current collector, 306: Positive electrode active material layer, 307: Negative electrode, 308: Negative electrode current collector, 309: Negative electrode active material layer, 310: Separator, 312: Washer, 322: Spacer, 503: Positive electrode, 506: Negative electrode, 507: Separator, 509: Outer casing, 510: Positive electrode lead, 511: Negative electrode lead Electrode, 601: Positive electrode cap, 602: Secondary battery casing, 603: Positive electrode terminal, 604: Positive electrode, 605: Separator, 606: Negative electrode, 607: Negative electrode terminal, 608: Insulating plate, 609: Insulating plate, 611: PTC element, 613: Safety valve mechanism, 614: Conductive plate, 615: Energy storage system, 616: Secondary battery, 620: Control circuit, 621: Wiring, 622: Wiring, 623: Wiring, 624: Conductor, 625: Insulator, 626: Wiring, 627: Wiring, 628: Conductive plate, 901: Mixture, 902: Composite oxide, 903: Mixture, 911a: Terminal, 911b: Terminal, 913: Secondary battery , 930: Housing, 930a: Housing, 930b: Housing, 931: Negative electrode, 931a: Negative electrode active material layer, 932: Positive electrode, 932a: Positive electrode active material layer, 933: Separator, 950: Winding body, 950a: Winding body, 951: Terminal, 952: Terminal, 1300: Rectangular secondary battery, 1301a: First battery, 1301b: First battery, 1302: Battery controller, 1303: Motor controller, 1304: Motor, 1305: Gear, 1306: DC-DC circuit, 1307: Electric power steering, 1308: Heater, 1309: Defogger, 1310: DC-DC circuit,1311: Second battery, 1312: Inverter, 1313: Audio, 1314: Power windows, 1315: Lights, 1316: Tires, 1317: Rear motor, 1320: Control circuit unit, 1321: Control circuit unit, 1413: Fixing unit, 1414: Fixing unit, 1421: Wiring, 1422: Wiring, 2001: Automobile, 2200: Secondary battery pack, 6800: Artificial Satellite, 6801: Body, 6802: Solar panel, 6803: Antenna, 6805: Rechargeable battery, 6900: Probe, 6901: Body, 6902: Solar sail, 6905: Rechargeable battery, 6910: Spacecraft, 6911: Body, 6912: Solar panel, 6913: Rechargeable battery, 6920: Rover, 6921: Body, 6922: Solar panel, 6923: Rechargeable battery,

Claims

A secondary battery having an electrolyte, The electrolyte comprises a first organic compound, a second organic compound, and a lithium salt. The freezing point of the electrolyte is less than -30°C. The lithium salt is LiPF ６ It has, The lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -35 [kJ / mol] or less, and the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less. Secondary battery. 　 A secondary battery having a negative electrode and an electrolyte, The electrolyte comprises a first organic compound, a second organic compound, and a lithium salt. The freezing point of the electrolyte is less than -30°C. The lithium salt is LiPF ６ It has, The lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -35 [kJ / mol] or less. The difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less. The negative electrode has graphite, Secondary battery. 　 In claim 1 or claim 2, A secondary battery in which the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -37 [kJ / mol] or less. 　 In claim 1 or claim 2, A secondary battery in which the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -39 [kJ / mol] or less. 　 In claim 1 or claim 2, A secondary battery in which the concentration of the lithium salt is 0.2 mol or more and less than 1 mol per liter of a mixed solvent containing the first organic compound and the second organic compound. 　 The present invention comprises a first organic compound, a second organic compound, and a lithium salt. The freezing point is below -30°C. The lithium salt is LiPF ６ It has, The lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -35 [kJ / mol] or less. An electrolyte in which the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less. 　 The present invention comprises a first organic compound, a second organic compound, and a lithium salt. The freezing point is below -30°C. The lithium salt is LiPF ６ It has, The lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -37 [kJ / mol] or less. An electrolyte in which the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less. 　 The present invention comprises a first organic compound, a second organic compound, and a lithium salt. The freezing point is below -30°C. The lithium salt is LiPF ６ It has, The lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound are each -39 [kJ / mol] or less. An electrolyte in which the difference between the lithium ion coordination energy of the first organic compound and the lithium ion coordination energy of the second organic compound is 5 [kJ / mol] or less. 　 In any one of claims 6 to 8, The electrolyte is such that the concentration of the lithium salt is 0.2 mol or more and less than 1 mol per liter of a mixed solvent containing the first organic compound and the second organic compound.

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