Lithium ion secondary battery

Abstract

The present invention provides a lithium ion secondary battery having excellent discharge characteristics even in a low-temperature environment. The lithium ion secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode has a lithium cobalt oxide having a median diameter (D50) of 1-12 µm inclusive. The lithium cobalt oxide has magnesium on the surface layer. The negative electrode has graphite particles, silicon particles, and a polymer having a carboxy group. The electrolyte has a mixed solvent of a fluorinated cyclic carbonate and a fluorinated chain carbonate.

Description

Lithium-ion secondary battery

[0001] One embodiment of the present invention relates to a lithium-ion secondary battery. The present invention is not limited to the above fields, but also relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, a vehicle, and a manufacturing method thereof. The above-described semiconductor device, display device, light-emitting device, power storage device, lighting device, electronic device, and vehicle can use a lithium-ion secondary battery according to one embodiment of the present invention as a necessary power source. For example, the above-described electronic device includes an information terminal device equipped with a lithium-ion secondary battery. Furthermore, the above-described power storage device includes a stationary power storage device.

[0002] In recent years, there has been active development of various types of storage batteries, including lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. Demand for high-power, high-capacity lithium-ion secondary batteries has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0003] It is known that the discharge capacity of lithium-ion secondary batteries varies depending on the temperature during discharge, and therefore there is a demand for lithium-ion secondary batteries that have excellent battery characteristics even in low-temperature environments (see, for example, Patent Document 1).

[0004] Furthermore, in order to increase the capacity of lithium-ion secondary batteries at room temperature and to improve their charge-discharge cycle characteristics, various research and development efforts are being conducted on both the positive and negative electrodes. Lithium cobalt oxide, which has a stable crystal structure, has been investigated as a positive electrode active material (see, for example, Patent Document 2).

[0005] Furthermore, fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and the like, and their physical properties have been studied (for example, Non-Patent Document 1).

[0006] Furthermore, as a negative electrode active material, silicon-based materials are known to have a higher capacity than graphite-based materials, and negative electrodes using silicon-based materials are being investigated (see, for example, Patent Document 3).

[0007] JP 2015-026608 A, WO2020 / 026078 Pamphlet, JP 2019-165005 A

[0008] W. E. Counts, R. Roy, and E. F. Osborn, “Fluoride Model Systems: II, The Binary Systems CaF▲2▼-BeF▲2▼, MgF▲2▼-BeF▲2▼, and LiF-MgF▲2▼”, Journal of the American Ceramic Society, 36 [1] 12-17 (1953).

[0009] Patent Document 1 describes that a lithium ion secondary battery that can operate even in a low-temperature environment (for example, below 0° C.) has been realized by using the electrolyte solution described in Patent Document 1. However, even the lithium ion secondary battery described in Patent Document 1 does not have a large discharge capacity when discharged in a low-temperature environment at the time of filing, and further improvement is desired.

[0010] Furthermore, in order to realize a lithium-ion secondary battery that has excellent discharge characteristics even in a low-temperature environment, it is necessary to develop not only an electrolyte but also a positive electrode and a negative electrode that are suitable for a lithium-ion secondary battery that can operate in a low-temperature environment.

[0011] In view of the above, an object of one embodiment of the present invention is to provide a lithium-ion secondary battery having excellent discharge characteristics even in a low-temperature environment, specifically, to provide a positive electrode, a negative electrode, an electrolyte, and the like that can be used in a lithium-ion secondary battery that has a large discharge capacity even when discharged in a low-temperature environment.

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

[0013] One embodiment of the present invention is a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte solution. The positive electrode includes lithium cobalt oxide having a median diameter (D50) of 1 μm or more and 12 μm or less, and the lithium cobalt oxide includes magnesium in a surface layer portion. The negative electrode includes graphite particles, silicon particles, and a polymer having a carboxy group. The electrolyte solution includes a mixed solvent of a fluorinated cyclic carbonate and a fluorinated chain carbonate.

[0014] In one embodiment of the present invention, the average particle size of the silicon particles is preferably less than 1 μm.

[0015] In one embodiment of the present invention, the average particle size of the graphite particles is preferably 5 μm or more.

[0016] In one embodiment of the present invention, the average particle size of the silicon particles is preferably smaller than the average particle size of the graphite particles.

[0017] In one embodiment of the present invention, it is preferable that the weight ratio of the silicon particles is smaller than the weight ratio of the graphite particles.

[0018] In one embodiment of the present invention, the polymer having a carboxy group may be polyglutamic acid.

[0019] In one embodiment of the present invention, the lithium cobalt oxide has a layered rock-salt crystal structure belonging to the space group R-3m, and the surface layer portion has a basal region having a surface parallel to the (00l) plane of the crystal structure and an edge region having a surface parallel to a plane other than the (00l) plane, and it is preferable that, when EDX-ray analysis is performed in the depth direction of the lithium cobalt oxide, the basal region has a higher magnesium concentration than the edge region.

[0020] Another aspect of the present invention is a lithium-ion secondary battery including a positive electrode, a negative electrode, and an electrolyte solution, in which the positive electrode includes lithium cobalt oxide having a median diameter (D50) of 1 μm or more and 12 μm or less, and the lithium cobalt oxide includes magnesium and nickel in a surface layer portion thereof, the negative electrode includes graphite particles, silicon particles, and a polymer having a carboxy group, in which the average particle diameter of the silicon particles is larger than the average particle diameter of the graphite particles, and the electrolyte solution includes a mixed solvent of a fluorinated cyclic carbonate and a fluorinated chain carbonate.

[0021] Another aspect of the present invention is a lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises lithium cobalt oxide having a median diameter (D50) of 1 μm or more and 12 μm or less, and the lithium cobalt oxide comprises magnesium and nickel in a surface layer portion thereof, the negative electrode comprises graphite particles, silicon particles, and a polymer having carboxy groups, the average particle diameter of the silicon particles being larger than the average particle diameter of the graphite particles, and the electrolyte contains fluoroethylene carbonate and methyl trifluoropropionate, and when the total content of the fluoroethylene carbonate and methyl trifluoropropionate is 100 vol %, the volume ratio of the fluoroethylene carbonate to the methyl trifluoropropionate is x:100−x (where 5≦x≦30).

[0022] In another embodiment of the present invention, the lithium cobalt oxide has a layered rock-salt crystal structure belonging to the space group R-3m, and the surface layer portion has a basal region having a surface parallel to the (00l) plane of the crystal structure and an edge region having a surface intersecting with the (00l) plane, and when line analysis by STEM-EDX, i.e., EDX line analysis in the depth direction, is performed on the lithium cobalt oxide, it is preferable that the edge region has a region where the distribution of magnesium and the distribution of nickel overlap.

[0023] In another embodiment of the present invention, the lithium cobalt oxide has a layered rock-salt crystal structure belonging to the space group R-3m, and the surface layer portion has a basal region having a surface parallel to the (00l) plane of the crystal structure and an edge region having a surface intersecting with the (00l) plane, and the lithium cobalt oxide preferably contains substantially no nickel in the basal region when subjected to line analysis by STEM-EDX, i.e., EDX line analysis in the depth direction.

[0024] According to one embodiment of the present invention, a lithium-ion secondary battery having excellent discharge characteristics even in a low-temperature environment can be provided, specifically, a positive electrode, a negative electrode, an electrolyte solution, and the like that can be used in a lithium-ion secondary battery having a high discharge capacity and / or a high discharge energy density even when discharged in a low-temperature environment can be provided.

[0025] FIG. 1A is a cross-sectional view illustrating the internal structure of a lithium-ion secondary battery, and FIG. 1B is a cross-sectional view illustrating a positive electrode active material, an electrolyte, and the like of the lithium-ion secondary battery. FIGS. 2A and 2B are cross-sectional views illustrating a positive electrode active material. FIGS. 3A1 to 3B3 are cross-sectional views illustrating a positive electrode active material. FIG. 4 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 5 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIG. 6 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 7 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIGS. 8A to 8D are diagrams illustrating a method for preparing a positive electrode active material. FIG. 9 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 10A to 10C are diagrams illustrating a method for preparing a positive electrode active material. FIG. 11 is a phase diagram showing the relationship between the composition of lithium fluoride and magnesium fluoride and temperature. FIG. 12 is a diagram illustrating the results of DSC analysis. FIG. 13 is a diagram illustrating a method for preparing a negative electrode active material. FIG. 14A is an exploded perspective view of a coin-type secondary battery, FIG. 14B is a perspective view of the coin-type secondary battery, and FIG. 14C is a cross-sectional perspective view thereof. FIG. 15A shows an example of a cylindrical secondary battery. FIG. 15B shows an example of a cylindrical secondary battery. FIG. 15C shows an example of a plurality of cylindrical secondary batteries. FIG. 15D shows an example of a power storage system including a plurality of cylindrical secondary batteries. FIGS. 16A and 16B are diagrams illustrating an example of a secondary battery, and FIG. 16C is a diagram illustrating the internal state of the secondary battery. FIGS. 17A to 17C are diagrams illustrating an example of a secondary battery. FIGS. 18A and 18B are diagrams illustrating the appearance of a secondary battery. FIGS. 19A to 19C are diagrams illustrating a manufacturing method of a secondary battery. FIG. 20A is a perspective view of a battery pack illustrating one embodiment of the present invention, FIG. 20B is a block diagram of the battery pack, and FIG. 20C is a block diagram of a vehicle including the battery pack. FIGS. 21A to 21D are diagrams illustrating an example of a transportation vehicle. Fig. 21E is a diagram illustrating an example of an artificial satellite. Figs. 22A and 22B are diagrams illustrating a power storage device according to one embodiment of the present invention. Fig. 23A is a diagram illustrating an electric bicycle, Fig. 23B is a diagram illustrating a secondary battery of the electric bicycle, and Fig. 23C is a diagram illustrating a scooter. Figs. 24A to 24D are diagrams illustrating examples of electronic devices.FIG. 25A shows an example of a wearable device, FIG. 25B shows a perspective view of a wristwatch-type device, and FIG. 25C is a diagram illustrating a side view of the wristwatch-type device. FIG. 26A is an SEM image of Sample 1, and FIG. 26B is a schematic diagram. FIGS. 27A and 27B are graphs showing the results of a low-temperature cycle test described in the Examples. FIGS. 28A to 28C are graphs showing XRD analysis of a high-voltage charged state described in the Examples. FIGS. 29A and 29B are graphs showing STEM-EDX analysis described in the Examples. FIGS. 30A to 30C are graphs showing STEM-EDX analysis described in the Examples. FIGS. 31A to 31C are graphs showing STEM-EDX analysis described in the Examples. FIG. 32 is a graph showing the results of a low-temperature cycle test described in the Examples. FIG. 33 is an SEM image of Sample 2.

[0026] The following description will explain the embodiments of the present invention with reference to the accompanying drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes can be made in form and detail without departing from the spirit and scope of the present invention. Therefore, in the embodiments described below, the same reference numerals are used in different drawings to indicate the same objects.

[0027] Furthermore, in each of the embodiments and examples described below, unless otherwise specified, it is possible to implement the embodiments and examples described in this specification and the like in appropriate combinations.

[0028] In this specification, a low-temperature environment refers to 0° C. or below, and 0° C. or below may be referred to as below freezing. When referring to a low-temperature environment in this specification, any temperature below 0° C. can be selected. For example, when referring to a low-temperature environment in this specification, one selected from 0° C. or below, −10° C. or below, −20° C. or below, −30° C. or below, −40° C. or below, −50° C. or below, −60° C. or below, −80° C. or below, and −100° C. or below can be selected.

[0029] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. Individual planes indicating crystal planes are expressed using ( ). In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice to facilitate understanding of the structure, and Miller indices such as (hkl) and (hkil) may be used. Here, i is −(h+k).

[0030] Any integer greater than or equal to 1 may be represented by letters such as h, k, i, l, etc. For example, (00l) includes (001), (003), and (006).

[0031] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0032] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. ２ The theoretical capacity of LiNiO is 274 mAh / g per weight. ２ The theoretical capacity of LiMn is 275mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0033] In addition, the amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ CoO ２In the case of a positive electrode active material of a lithium ion secondary battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a lithium ion secondary battery using as a positive electrode active material is charged at 219.2 mAh / g per weight of the positive electrode active material, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ The state where x is small is, for example, x≦0.24, and in consideration of the practical range when used as a lithium ion secondary battery, it is assumed that 0.1<x≦0.24, for example.

[0034] When the lithium cobalt oxide satisfies the stoichiometric ratio, LiCoO ２ and x = 1. In addition, the lithium ion secondary battery after discharge is also LiCoO ２ In other words, x=1. ２ In a lithium-ion secondary battery using a lithium cobalt oxide battery, the discharge voltage drops sharply before it reaches 2.5 V. For this reason, in this specification and the like, the state in which the voltage reaches 2.5 V (counter electrode is lithium) at a current of 100 mA / g or less per weight of positive electrode active material is regarded as the end of discharge, and x = 1. Therefore, to obtain lithium cobalt oxide when x = 0.2, for example, charging at 219.2 mAh / g per weight of positive electrode active material is sufficient.

[0035] Li ｘ CoO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above calculation be measured under conditions where there is no or little influence of short circuit and / or decomposition of the electrolyte. For example, it is not preferable to use data from a lithium ion secondary battery that has experienced a sudden change in voltage that is considered to be a short circuit in the calculation of x.

[0036] In this specification and the like, the term "carbonate" refers to a compound having at least one carbonate ester in its molecular structure, and unless otherwise specified, includes "cyclic carbonates" and "chain carbonates." Furthermore, "chain" includes both linear and branched chains.

[0037] In this specification and the like, the phrase "having A and / or B" may be used, which means having A, having B, or having A and B.

[0038] In this specification, a full cell refers to a battery cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a battery cell assembled with lithium metal as the negative electrode (counter electrode).

[0039] In this specification and the like, a lithium ion secondary battery is sometimes called a lithium ion battery, and refers to a battery that uses lithium ions as carrier ions. However, the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or alkaline earth metal ions can be used as carrier ions of the present invention, and specifically, sodium ions can be used. In this case, the present invention can be understood by replacing lithium ions with sodium ions. Furthermore, when describing a configuration in which there is no limitation on the carrier ions, the battery may be referred to as a secondary battery.

[0040] Embodiment 1 In this embodiment, a lithium ion secondary battery having excellent discharge characteristics even in a low temperature environment will be described.

[0041] [Lithium-ion secondary battery] A lithium-ion secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte solution. A separator is also included between the positive electrode and the negative electrode. The separator is not necessary when a solid electrolyte or a semi-solid electrolyte is used instead of the electrolyte solution. The battery may further include an exterior housing that houses the positive electrode, the negative electrode, the electrolyte solution, and the like.

[0042] In this embodiment, the description will focus on the configuration of a lithium ion secondary battery required to realize a lithium ion secondary battery having excellent discharge characteristics even in a low temperature environment (for example, 0° C. or lower, −20° C. or lower, preferably −30° C. or lower, more preferably −40° C. or lower, even more preferably −50° C. or lower, and most preferably −60° C. or lower). Specifically, the description will focus on the positive electrode active material contained in the positive electrode, the negative electrode active material layer, and the electrolyte solution.

[0043] In this specification and the like, excellent discharge characteristics in a low-temperature environment may refer to a state in which the discharge capacity in a low-temperature environment (e.g., 0°C or lower, -20°C or lower, preferably -30°C or lower, more preferably -40°C or lower, even more preferably -50°C or lower, and most preferably -60°C or lower) decreases at a smaller rate than the discharge capacity at 25°C.

[0044] 1A is a cross-sectional schematic diagram illustrating the internal structure of a lithium-ion secondary battery 10. The lithium-ion secondary battery 10 has a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11 has a positive electrode current collector 21 and a positive electrode active material layer 22 on the positive electrode current collector 21, and the negative electrode 12 has a negative electrode current collector 31 and a negative electrode active material layer 32. As shown in the figure, the positive electrode active material layer 22 and the negative electrode active material layer 32 face each other with the separator 13 interposed therebetween. Although not shown in FIG. 1A , an electrolyte solution is impregnated into voids in the positive electrode active material layer 22, voids in the separator, and voids in the negative electrode active material layer 32.

[0045] FIG. 1B shows an enlarged view of the portion A enclosed by the dashed line in FIG. 1A.

[0046] The positive electrode active material layer 22 includes a positive electrode active material 100 and a conductive material 41. Although not shown, the positive electrode active material layer 22 may include a binder in addition to the positive electrode active material 100 and the conductive material 41.

[0047] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with the electrolyte solution 51 as shown in the figure. For example, it is preferable that 60% or more of the voids in the positive electrode active material layer 22 be filled with the electrolyte solution 51, more preferably 70% or more of the voids, still more preferably 70% or more of the voids, still more preferably 80% or more of the voids, still more preferably 90% or more of the voids, still more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the positive electrode active material layer 22 refer to regions in the positive electrode active material layer 22 other than the solid components (such as the positive electrode active material and the conductive material).

[0048] Although detailed description will be omitted, similarly to the description of the positive electrode active material layer 22 above, the voids in the negative electrode active material layer 32 may also be filled with the electrolyte solution 51. For example, it is preferable that 60% or more of the voids in the negative electrode active material layer 32 be filled with the electrolyte solution 51, more preferably 70% or more of the voids, more preferably 80% or more of the voids, more preferably 90% or more of the voids, more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the negative electrode active material layer 32 refer to regions in the negative electrode active material layer 32 other than those filled with solid components (negative electrode active material, conductive material, etc.).

[0049] In this way, by filling every corner of the positive electrode active material layer 22 and the negative electrode active material layer 32 with the electrolyte 51, it is possible to widen the area where the positive electrode active material and the negative electrode active material come into contact with the electrolyte, thereby making it possible to obtain a lithium ion secondary battery with excellent charge and discharge characteristics in a low-temperature environment.

[0050] Furthermore, during charging in a low-temperature environment, the energy barrier for desorbing lithium ions from the positive electrode active material tends to be high. In other words, the lower the temperature of the charging environment, the greater the overvoltage required to desorb lithium ions from the positive electrode active material. In other words, the positive electrode active material may be exposed to a high voltage (a potential higher than the lithium potential) during charging in a low-temperature environment. In other words, if the positive electrode active material is not exposed to a high voltage during charging in a low-temperature environment, the charge capacity may be reduced.

[0051] Therefore, it is preferable to use a positive electrode active material that can withstand high voltage and obtain a high charge capacity during charging in a low-temperature environment as a positive electrode active material for a lithium-ion secondary battery that has excellent charge and discharge characteristics even in a low-temperature environment.

[0052] Furthermore, it is preferable that the electrolyte of a lithium ion secondary battery that has excellent charge and discharge characteristics even in a low-temperature environment uses a material that has excellent lithium ion conductivity even when charging and / or discharging (charging and discharging) in a low-temperature environment (e.g., 0°C, -20°C, preferably -30°C, more preferably -40°C).

[0053] A preferred positive electrode active material and electrolyte for a lithium ion secondary battery having excellent charge and discharge characteristics even in a low-temperature environment will be described in detail below.

[0054] [Positive Electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a positive electrode active material, and may further have at least one of a conductive material and a binder.

[0055] <Positive Electrode Active Material> The positive electrode active material has the function of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material used in one embodiment of the present invention can be a material that exhibits minimal deterioration (or minimal increase in resistance) during charge and / or discharge (hereinafter also referred to as "charge and discharge") in a low-temperature environment, even at high charge voltages (unless otherwise specified, this refers to a voltage value based on lithium metal; hereinafter, also referred to as "high charge voltage"). Specifically, it is preferable to use a positive electrode active material (composite oxide) obtained by the preparation method described in embodiment 2 and having a particle size (strictly speaking, a median diameter (D50)) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less). Of course, a positive electrode active material with a particle size greater than 12 μm and 20 μm or less may also be used. This positive electrode active material contains one or more of additive element X, additive element Y, and additive element Z. The additive element X, additive element Y, and additive element Z will be described in detail in the <Containing Elements> section.

[0056] The particle size can be measured using a particle size distribution analyzer using a laser diffraction / scattering method. The median diameter (D50) is the particle diameter when the cumulative amount in the cumulative particle amount curve of the particle size distribution measurement result accounts for 50%. The measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may be measured by analysis using a scanning electron microscope (hereinafter referred to as SEM) or a transmission electron microscope (hereinafter referred to as TEM). Note that, as a method for measuring the median diameter (D50) using analysis using SEM or TEM, for example, 20 or more particles are measured, a cumulative particle amount curve is created, and the particle diameter when the cumulative amount accounts for 50% can be taken as the median diameter (D50).

[0057] As one evaluation of low-temperature characteristics, it is preferable that the discharge capacity value in a low-temperature environment (e.g., 0° C., −20° C., preferably −30° C., more preferably −40° C.) is 50% or more (preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and most preferably 90% or more) of the discharge capacity value at 20° C. Note that the above numerical values ​​should be obtained assuming that the measurement conditions other than the environmental temperature are the same.

[0058] Alternatively, even at a high charging voltage, by using a material that is less prone to deterioration (or a material that is less prone to increase in resistance) during charging and discharging as the positive electrode active material, it is possible to achieve a large discharge capacity even at low temperatures (for example, 0°C, −20°C, preferably −30°C, more preferably −40°C).

[0059] More specifically, the discharge capacity when charging and discharging at -40°C is preferably 60% or more, more preferably 65% ​​or more, more preferably 70% or more, and even more preferably 75% or more, of the discharge capacity when charging and discharging at 25°C. Note that although -40°C is used, this temperature may be any low temperature and may be interpreted as other low temperatures such as -20°C or -30°C. For example, the discharge condition may be discharge at a current rate of 0.1C (where 1C = 200mA / g (per weight of positive electrode active material)). In the evaluation of low-temperature characteristics as described above, evaluation may be performed at a low rate as long as the measurement conditions other than the environmental temperature are consistent.

[0060] As another evaluation of low-temperature characteristics, it is preferable that the discharge energy density value in a low-temperature environment (e.g., 0°C, -20°C, preferably -30°C, more preferably -40°C) is 50% or more (preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and most preferably 90% or more) of the discharge energy density value at 25°C.

[0061] As used herein, the term "ambient temperature" refers to the temperature of a lithium-ion secondary battery. When measuring battery characteristics using a thermostatic chamber, the ambient temperature can be considered to be the temperature set in the thermostatic chamber. Therefore, after placing a battery (e.g., a test battery or half-cell) to be measured in the thermostatic chamber, it is advisable to wait a sufficient amount of time (e.g., one hour or more) until the test cell reaches the same temperature as the thermostatic chamber before starting measurement, but this method is not necessarily limited to this.

[0062] A cathode active material 100 according to one embodiment of the present invention will be described with reference to Fig. 2 and Fig. 3. The cathode active material 100 exhibits little deterioration due to repeated charging and discharging at a high voltage relative to lithium metal, and therefore can provide sufficient battery characteristics even in a low-temperature environment. In this embodiment, the high voltage is 4.6 V, preferably 4.65 V, and more preferably 4.7 V, relative to lithium metal.

[0063] 2A and 2B are cross-sectional views of a positive electrode active material 100 according to one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 2B are shown in FIGS. 3A1 to 3A3. Enlarged views of the vicinity of C-D in FIG. 2B are shown in FIGS. 3B1 to 3B3.

[0064] 2A, the positive electrode active material 100 has a surface layer portion 100a and an interior portion 100b. In these drawings, the boundary between the surface layer portion 100a and the interior portion 100b is indicated by a dashed line, but the boundary is not clearly defined.

[0065] The surface layer 100a of the positive electrode active material 100 refers to, for example, a region extending from the surface toward the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm, perpendicular or approximately perpendicular from the surface toward the interior. A narrow region extending from the surface toward the interior, specifically within 20 nm, is called a shell. Note that "approximately perpendicular" includes perpendicular, specifically, 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface or near-surface region.

[0066] The region of the positive electrode active material deeper than the surface layer 100a is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0067] Furthermore, when positive electrode active material 100 has a layered rock salt crystal structure of space group R-3m, surface layer portion 100a has edge region 100a1 and basal region 100a2 as shown in FIG. 2B.

[0068] 2A and 2B, the straight line marked (00l) represents the (00l) plane. The basal region 100a2 has a surface parallel or approximately parallel to the (00l) plane. The (00l) plane is called the basal plane, and the region having the (00l) plane is called the basal region 100a2. When lithium cobalt oxide is applied to the positive electrode active material 100, lithium ions can be inserted and removed from the basal plane. Furthermore, a surface other than the (00l) plane is called an edge plane, and a region having a surface other than the (00l) plane is called the edge region 100a1.

[0069] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the inner portion 100b. ２ O ３ This does not include metal oxides having no lithium sites that can contribute to charging and discharging, such as carbonates and hydroxyl groups that are chemically adsorbed after the preparation of the positive electrode active material. Note that the attached metal oxides refer to, for example, metal oxides whose crystal orientation does not match that of the interior 100b.

[0070] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, and the like. The determination can also be made based on the FFT pattern of a TEM image, the FFT pattern of a STEM image, etc. Furthermore, XRD (X-ray diffraction), neutron diffraction, etc. can also be used as materials for the determination.

[0071] The positive electrode active material does not include the electrolytic solution, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom that are attached to the positive electrode active material 100. In other words, the electrolytic solution, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material are removed.

[0072] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present may be the surface of the positive electrode active material. Therefore, surfaces resulting from slippage, cracks, and / or fissures are also included in the surface of the positive electrode active material. When analyzing the positive electrode active material, a protective film may be applied to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, or the like.

[0073] <Containing Elements> The positive electrode active material 100 contains lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 contains lithium cobalt oxide (LiCoO ２However, the positive electrode active material 100 of one embodiment of the present invention may have a crystal structure described later. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0074] The additive elements contained in the positive electrode active material 100 are preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium.

[0075] The additive element is preferably present in the form of a solid solution in the positive electrode active material 100. The additive element further stabilizes the crystal structure of the positive electrode active material 100, as will be described later.

[0076] The additive elements do not necessarily have to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium.

[0077] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, etc., are further enhanced. The weight of manganese contained in the cathode active material 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0078] The surface layer 100a, especially the edge region having the edge surface, is the region from which lithium ions are first desorbed during charging, and is the region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, in the surface layer 100a from which lithium ions are desorbed, especially in the edge region, some of the bonds of the atoms on the surface of the positive electrode active material 100 are broken. Therefore, the surface layer 100a is likely to become unstable, and is the region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 100a, especially the edge region, can be sufficiently stabilized, Li ｘ CoO ２Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of octahedrons of cobalt and oxygen can be made less likely to break. Furthermore, if the surface layer portion 100a, particularly the edge region, can be made sufficiently stable, it is possible to suppress displacement of the layer made of octahedrons of cobalt and oxygen in the inner portion 100b.

[0079] To provide the surface layer 100a with a stable composition and crystal structure, the surface layer 100a preferably contains the above-described additive elements, and more preferably contains a plurality of additive elements. The surface layer 100a preferably has a higher concentration of one or more selected from the additive elements than the interior 100b. The edge region 100a1 preferably has a higher concentration of one or more selected from the additive elements than the basal region 100a2.

[0080] 3A1 to 3A3 are enlarged views of the vicinity of A-B in FIG. 2B and are views illustrating the edge region 100a1 of the positive electrode active material 100. Also, FIGS. 3B1 to 3B3 are enlarged views of the vicinity of C-D in FIG. 2B and are views illustrating the basal region 100a2 of the positive electrode active material 100.

[0081] For example, some of the additive elements, such as magnesium, fluorine, and titanium, preferably have a concentration gradient that increases from the interior 100b toward the surface. In Figures 3A1 and 3B1, the concentration gradient is expressed using the density of the hatching. An additive element having such a concentration gradient is referred to as additive element X. However, the concentrations of magnesium, fluorine, titanium, and the like may be higher in the edge region 100a1 than in the basal region 100a2.

[0082] Another additive element, such as aluminum, preferably has a concentration gradient and a concentration peak in a region deeper than the additive element X shown in FIGS. 3A2 and 3B2. In FIGS. 3A2 and 3B2, the concentration gradient and peak region are expressed using the density of the hatching. The concentration peak may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. For example, it is preferable that the peak be present in a region of 5 nm to 30 nm from the surface toward the interior. An additive element having such a concentration gradient will be referred to as additive element Y. However, the concentration of aluminum, etc., may be higher in the edge region 100a1 than in the basal region 100a2.

[0083] Another additive element, such as nickel, may be clearly present in the edge region 100a1 but substantially absent in the basal region 100a2, as indicated by the presence or absence of hatching and the density of the hatching in Figures 3A3 and 3B3. The concentration of nickel or the like may be higher in the edge region 100a1 than in the basal region 100a2. Note that "clearly present" here refers to a case in which a characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 100. An additive element having such a distribution will be referred to as additive element Z.

[0084] Furthermore, "substantially free" refers to a case where the characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 100. This also refers to the element being below the lower limit of detection in the STEM-EDX analysis. In this case, it also refers to the element being below the lower limit of detection in the STEM-EDX analysis.

[0085] For example, magnesium, which is one of the additive elements X, is divalent, and magnesium is more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, so it is easy to enter the lithium site. When magnesium is present at an appropriate concentration at the lithium site in the surface layer 100a, it becomes easier to maintain the layered rock-salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers.ｘ CoO ２ When x is, for example, 0.24 or less, the desorption of oxygen from the periphery of magnesium can be suppressed.

[0086] At appropriate concentrations, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying cobalt sites in addition to lithium sites. Furthermore, excess magnesium compounds (e.g., oxides or fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become a resistive component in lithium-ion secondary batteries. Furthermore, as the magnesium concentration in the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium available for charging and discharging.

[0087] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.001 to 0.1 times the number of cobalt atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0088] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Aluminum also suppresses the elution of surrounding cobalt and improves continuous charging durability. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when used in lithium-ion secondary batteries. Furthermore, a positive electrode active material 100 can be obtained whose crystal structure is less likely to collapse even with repeated charging and discharging. On the other hand, excessive aluminum may adversely affect lithium insertion and extraction.

[0089] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0090] Nickel, which is one of the additional elements Z, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower oxidation-reduction potential than cobalt, which leads to an increase in discharge capacity, which is preferable.

[0091] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the volume change caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site can also be prevented from shifting to CoO ２This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0092] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also adversely affect lithium insertion and extraction.

[0093] Therefore, it is preferable that the entire positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 100 is preferably greater than 0% and not greater than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably greater than 0% and not greater than 4%. Alternatively, it is preferably greater than 0% and not greater than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0094] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When a portion of the oxygen in the surface layer portion 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the valence of cobalt ions changes with lithium desorption (from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine), resulting in different oxidation-reduction potentials. Therefore, when a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 100 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine occurs more smoothly. Therefore, when used in a lithium-ion secondary battery, charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. Furthermore, as will be described in a later embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of another additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source.

[0095] Furthermore, titanium oxide, which is one of the additive elements X, is known to have superhydrophilicity. Therefore, by providing the cathode active material 100 with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 has good wettability with highly polar solvents. When used in a secondary battery, this improves the contact at the interface between the cathode active material 100 and a highly polar electrolyte, which may suppress an increase in internal resistance.

[0096] Furthermore, when the surface layer 100a shown in FIG. 3A1 has magnesium and the surface layer 100a shown in FIG. 3A3 has nickel, that is, when both magnesium and nickel are present, there is a possibility that divalent nickel can exist more stably near the divalent magnesium. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0097] 3A3 and 3B3, the additive element Z may be contained in a large amount in the edge region 100a1 (also referred to as being contained preferentially or selectively), which is preferable because it improves the stability of the crystal structure of the edge region 100a1 where lithium ions enter and exit the positive electrode active material 100 during charging and discharging of the lithium ion secondary battery. Furthermore, when the additive element Z has the above-described distribution, for example, when the positive electrode active material 100 is lithium cobalt oxide, it is preferable because it can minimize the effects of adding the additive element Z, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0098] As described above, when multiple additive elements are present, the effects of each additive element are synergistic, which can contribute to further stabilization of the surface layer portion 100a. In particular, the presence of magnesium, nickel, and aluminum is highly effective in achieving a stable composition and crystal structure, making it preferable. In particular, it is preferable that the surface layer portion 100a of the positive electrode active material 100 has a region where magnesium is distributed closer to the surface than aluminum. Furthermore, in addition to the region where magnesium and aluminum are distributed, it is most preferable that the surface layer portion 100a of the positive electrode active material 100 has a region where the nickel distribution and the magnesium distribution overlap in the edge region 100a1.

[0099] <Crystalline Structure> One aspect of the present invention is to provide a lithium-ion secondary battery with improved battery characteristics in a low-temperature environment. However, XRD measurements and the like for identifying the crystalline structure and the like were performed at room temperature.

[0100] <Li ｘ CoO ２ When x is 1 in the positive electrode active material 100 according to one embodiment of the present invention, the positive electrode active material 100 is in a discharged state, that is, Li ｘ CoO ２ In the case where x = 1 in the formula (I), it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. The layered rock-salt type composite oxide has a high discharge capacity, has two-dimensional lithium ion diffusion paths, is suitable for lithium ion insertion / extraction reactions, and is excellent as a positive electrode active material for lithium ion secondary batteries. Therefore, it is preferable that the inner portion 100b, which occupies the majority of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure.

[0101] On the other hand, the surface layer portion 100a of the cathode active material 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b so that the layered structure of octahedra of cobalt and oxygen in the inner portion 100b is not destroyed even when lithium is released from the cathode active material 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and the inner portion 100b of the cathode active material 100, such as oxygen release, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0102] To provide a reinforcing function, the surface layer portion 100a may have a different crystal structure from the interior portion 100b. For example, the surface layer portion 100a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100 of one embodiment of the present invention may have a rock salt crystal structure. Alternatively, the surface layer portion 100a may have both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a may have characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0103] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0104] Furthermore, it is preferable that some of the added elements, especially magnesium, have a higher concentration in the surface layer 100a than in the interior 100b, and that they are present randomly and dilutely in the interior 100b. Furthermore, when aluminum is present at an appropriate concentration at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it can suppress the deviation of the layered structure consisting of octahedra of cobalt and oxygen, as described above. Furthermore, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, and a synergistic effect of suppressing magnesium elution can be expected.

[0105] Furthermore, it is preferable that the crystal structure continuously changes from the interior 100b toward the surface due to the magnesium concentration gradient described above, or that the crystal orientation of the surface layer 100a and the interior 100b roughly coincide.

[0106] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted lattice structure, and the orientation of the crystals may be roughly the same.

[0107] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0108] The rock salt crystal structure does not distinguish between cation sites, but the layered rock salt crystal structure has two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by the transition metal M. The layered structure, in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged, is the same for both the rock salt and layered rock salt structures.

[0109] The layered rock salt crystal structure and the anions in the rock salt crystal structure form a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3' crystal, which will be described later, also form a cubic close-packed structure. Therefore, when the layered rock salt crystal structure and the rock salt crystal structure come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0110] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0111] However, the space group of the layered rock salt type crystal structure and the O3' type crystal structure described later is R-3m, which is different from the space group Fm-3m of the rock salt type crystal structure (the space group of a general rock salt type crystal), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal structure, the O3' type crystal structure, and the rock salt type crystal structure. In this specification, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt type crystal structure, the O3' type crystal structure, and the rock salt type crystal structure, it may be said that the crystal orientations are approximately the same.

[0112] <Li ｘ CoO ２ The positive electrode active material 100 according to one embodiment of the present invention has the above-described magnesium distribution and / or crystal structure, and therefore, is in a discharged state (Li ｘ CoO ２ The crystal structure in the state where x is small (where x is small) is different from that of conventional positive electrode active materials.

[0113] 4 to 7, Li ｘ CoO ２The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0114] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 5. The conventional positive electrode active material shown in FIG. 5 is a lithium cobalt oxide (LiCoO ２ )

[0115] In Figure 5, R-3m O3 is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in the figure shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0116] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0117] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0118] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２It can also be said that this crystal structure is a structure in which the structure of and the structure of are stacked alternately. For this reason, this crystal structure is sometimes called an H1-3 crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 5 and other parts of this specification, to make it easier to compare with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the size of the unit cell.

[0119] As an example of the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that has a small GOF (goodness of fit) value, specifically one that is close to 1.

[0120] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0121] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 5, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0122] Furthermore, the difference in volume between these two crystal structures is large: per equivalent number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0123] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type.２ A structure with continuous layers is likely to be unstable.

[0124] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0125] On the other hand, in the positive electrode active material 100 according to one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less, specifically, x is 0.2 (which may be referred to as 0% Li existence probability), is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ CoO ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the value of x is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the lithium ion secondary battery is further improved, which is preferable.

[0126] Li ｘ CoO ２ The crystal structure of the inner portion 100b of the positive electrode active material 100 when x is approximately 1 or 0.2 is shown in FIG. 4. The inner portion 100b occupies the majority of the volume of the positive electrode active material 100 and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0127] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0128] However, the positive electrode active material 100 has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.12, which results in an H1-3 type crystal structure.

[0129] The positive electrode active material 100 according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The layer symmetry is the same as that of O3. Therefore, this crystal structure is referred to as an O3'-type crystal structure. Furthermore, although the positive electrode active material 100 of one embodiment of the present invention when x = approximately 0.2 does not have a spinel structure, a pattern similar to a spinel structure may appear in the XRD pattern, and this crystal structure may be referred to as a pseudo-spinel structure. This crystal structure is shown in Figure 4 with the notation R-3m O3'.

[0130] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ nm). The c-axis is 13.681≦c≦13.881 (×10 −１ nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0131] In the O3' type crystal structure, ions of cobalt, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0132] As shown by the dotted line in FIG. 4, the difference between R-3m(O3) in the discharged state and the O3′-type crystal structure is ２ There is almost no layer misalignment.

[0133] The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0134] As described above, in the positive electrode active material 100 according to one embodiment of the present invention, Li ｘ CoO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the positive electrode active material 100 suppresses the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because it can stably utilize more lithium than conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a lithium-ion secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0135] The positive electrode active material 100 is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is estimated that even when x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. However, the crystal structure is ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0136] Therefore, the positive electrode active material 100 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the entire interior 100b of the positive electrode active material 100 does not have to have the O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0137] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage.

[0138] In other words, the positive electrode active material 100 of one embodiment of the present invention is preferable because it can maintain a crystal structure having the symmetry of R-3m O3 even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it is preferable because it can adopt an O3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C.

[0139] In some cases, the H1-3 type crystal is finally observed when the charge voltage is further increased, even in the positive electrode active material 100. Furthermore, as described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher and lower than 4.6 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt the O3′ type crystal structure.

[0140] In addition, when graphite is used as the negative electrode active material in a lithium ion secondary battery, the voltage of the lithium ion secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a lithium ion secondary battery using graphite as the negative electrode active material, the battery has a similar crystal structure at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0141] In addition, in O3' of FIG. 4, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, in the monoclinic O1 (Li ０．５ CoO ２ The lithium distribution can be analyzed, for example, by neutron diffraction.

[0142] In order to form an O3'-type crystal structure, it is preferable that the magnesium concentration gradient be similar at multiple locations in the surface layer 100a of the positive electrode active material 100. In other words, it is preferable that reinforcement derived from magnesium is uniformly present in the surface layer 100a. Even if a portion of the surface layer 100a is reinforced, if there is a portion without reinforcement, stress may be concentrated in the unreinforced portion. If stress is concentrated in a portion of the positive electrode active material 100, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity. However, it is not necessary for magnesium to have a similar concentration gradient throughout the entire surface layer 100a of the positive electrode active material 100.

[0143] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ It can be said that the structure is one in which layers and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane. Again, the (001) plane is called the basal plane, and the planes other than the (001) plane where the diffusion path of lithium ions is exposed are called edge planes.

[0144] CoO ２ The layer is relatively stable, so CoO ２ The (001) plane, on which the layer exists, is relatively stable. The (001) plane does not expose the main diffusion path of lithium ions during charge and discharge.

[0145] On the other hand, the diffusion paths of lithium ions are exposed on surfaces other than the (001) plane. Therefore, the surfaces other than the (001) plane and the surface layer portion 100a having such surfaces are important regions for maintaining the diffusion paths of lithium ions, and at the same time, they are the regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surfaces other than the (001) plane and the surface layer portion 100a having such surfaces is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0146] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100 of one embodiment of the present invention has an O3′-type crystal structure by Liｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x in the positive electrode active material 100 using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. Among XRD methods, powder XRD is preferred because it can provide diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0147] Furthermore, even in the positive electrode active material 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the positive electrode active material 100 of one embodiment of the present invention is the positive electrode active material 100, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0148] Furthermore, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, it may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0149] Furthermore, whether or not the distribution of the additive elements contained in the positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0150] The crystal structure of the surface layer 100 a and the grain boundaries can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0151] <Charging Method> Charging for determining whether a composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed, for example, by preparing a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) using a lithium counter electrode and charging the coin cell.

[0152] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0153] The counter electrode can be made of lithium metal.

[0154] The lithium salt was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7 is used as the electrolyte, and vinylene carbonate (VC) can be used as an additive mixed in an amount of 2 wt % with respect to the mixed solvent.

[0155] The separator may be a 25 μm thick porous polypropylene film.

[0156] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0157] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less per weight of positive electrode active material. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less per weight of positive electrode active material. Charging at such a low current value is desirable to observe the phase change of the positive electrode active material. The temperature is set to 25°C. After charging in this manner, the coin cell can be disassembled in a glove box under an argon atmosphere and the positive electrode removed to obtain a positive electrode active material with the desired charge capacity. When performing various subsequent analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed in a sealed container under an argon atmosphere. After the charging is completed, the positive electrode is preferably taken out and analyzed promptly, preferably within 1 hour, more preferably within 30 minutes.

[0158] Furthermore, when analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge / discharge. For example, charging may be performed by constant current charging up to an arbitrary voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V) at a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material, followed by constant voltage charging until the current value becomes 2 mA / g or more and 10 mA / g or less per weight of the positive electrode active material, and then discharging by constant current discharging at 2.5 V at a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material.

[0159] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at, for example, 2.5 V, with a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material.

[0160] <XRD> The XRD measurement apparatus and conditions are not particularly limited as long as appropriate adjustment and calibration are performed. For example, measurements can be performed using the following apparatus and conditions: XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray source: Cu Output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan 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: 15 rpm The standard sample used for adjustment and calibration can be, for example, NIST (National Institute of Standards and Technology) standard aluminum oxide sintered plate SRM 1976.

[0161] If the measurement sample is a powder, it can be set by placing it on a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0162] The characteristic X-rays may be monochromated using a filter or by using XRD data analysis software after obtaining an XRD pattern. For example, the CuKα ２ Excluding the peak due to the line, CuKα １ It is possible to extract only the line peaks. The software can also be used to remove background noise.

[0163] In this specification, when the 2θ value of a certain diffraction peak is mentioned, it means the 2θ value at which the peak top of the diffraction peak appears in the XRD pattern after fitting with a calculation model. The crystal structure analysis software used for fitting is not particularly limited, but for example, TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker) can be used.

[0164] The ideal powder XRD patterns calculated from the O3' type crystal structure and the H1-3 type crystal structure model using CuKα1 radiation are shown in Figures 6 and 7. For comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structure of LiCoO3 and the trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD (Inorganic Crystal Structure Database). The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The XRD pattern of the H1-3 type crystal structure was created in the same manner as above, based on the information on the H1-3 type crystal structure shown in Fig. 7. The XRD pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material of one embodiment of the present invention, fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating an XRD pattern in the same manner as the others.

[0165] As shown in FIG. 6, in the O3′ type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less).

[0166] However, as shown in FIG. 7, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and 19.37° or less) and 2θ = 45.47 ± 0.10° (45.37° or more and 45.57° or less) when x is small can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0167] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0168] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２When x in the formula is small, the material has an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0169] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ=43° or more and 46° or less, the half-width is, for example, preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. A narrow half-width and high crystallinity contribute to the stabilization of the crystal structure after charging. On the other hand, in conventional LiCoO ２ In this case, even if a part of the crystal structure is similar to the O3' type crystal structure, the crystallite size will be small and the peak will be broad and small.

[0170] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements. The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, depending on the element.

[0171] The concentration of the added element may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and the like chemisorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more, more preferably 0.500 or more, more preferably 0.600 or more, more preferably 0.700 or more, more preferably 0.800 or more, more preferably 0.900 or more, and more preferably 1.000 or more. Furthermore, Mg / Co is preferably 2.000 or less, more preferably 1.500 or less, more preferably 1.400 or less, preferably 1.300 or less, or preferably 1.200 or less.

[0172] The ratio of the number of atoms of nickel to cobalt, Ni / Co, as determined by XPS analysis, is preferably 0.05 or more, more preferably 0.06 or more, more preferably 0.07 or more, more preferably 0.08 or more, and even more preferably 0.09 or more. Ni / Co is preferably 0.200 or less, preferably 0.150 or less, preferably 0.140 or less, preferably 0.130 or less, preferably 0.120 or less, or preferably 0.110 or less.

[0173] Furthermore, the ratio of the number of fluorine atoms to the number of cobalt atoms, F / Co, as determined by XPS analysis, is preferably 0.100 or more, more preferably 0.200 or more, more preferably 0.300 or more, more preferably 0.400 or more, more preferably 0.500 or more, more preferably 0.600 or more, and more preferably 0.700 or more. Furthermore, F / Co is preferably 1.500 or less, preferably 1.200 or less, preferably 1.100 or less, preferably 1.000 or less, and preferably 0.900 or less.

[0174] The above range indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100, but are widely distributed at preferred concentrations in the surface layer portion 100a of the positive electrode active material 100. In other words, as a result of the XPS analysis of the positive electrode active material 100, the above range indicates that the crystalline structure is less likely to collapse even when charging and discharging are repeated so that x is 0.24 or less, and excellent cycle characteristics can be achieved.

[0175] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0176] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0177] <EDX> It is preferable that one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0178] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0179] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the additive element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material 100. Furthermore, EDX analysis can analyze the concentration distribution and maximum value of the additive element. Furthermore, analysis after thinning the sample using FIB or the like is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0180] Therefore, when EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, it can be confirmed that the concentration of each additional element, particularly the additional element X, in the surface layer portion 100a is higher than that in the interior portion 100b.

[0181] In STEM-EDX ray analysis or the like, the element profile does not change sharply in principle or due to measurement errors, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX ray analysis or the like, it is important to consider whether the transition metal M is greater than the average value M of the amount detected inside. ＡＶＥ and the average background value M ＢＧ The point where the oxygen concentration is 50% of the sum of the two, or the oxygen concentration is the average value of the internal detection amount O ＡＶＥ and the average background value O ＢＧ The reference point is the point where the sum of the internal and background is 50%. If the transition metal M and oxygen are different from each other in terms of the 50% point of the sum of the internal and background, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen adhering to the surface. Therefore, the average value M of the internal detected amount of the transition metal M is used. ＡＶＥ and the average background value M ＢＧIn the case of a positive electrode active material having a plurality of transition metals M, the M of the element with the largest count in the inner portion 100b can be used. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0182] The average value of the cobalt background M ＢＧ can be obtained by averaging the outer range of 2 nm or more, preferably 3 nm or more, avoiding the vicinity where the amount of cobalt detected begins to increase. ＡＶＥ The average value of the oxygen background O can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably 50 nm or more, from the region where the cobalt and oxygen counts are saturated and stable, for example, the region where the detected amount of cobalt begins to increase. ＢＧ and the average value of the amount of oxygen detected inside O ＡＶＥ can also be found in the same way.

[0183] The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between the region where an image derived from the crystalline 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 derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are observed. Alternatively, it is the intersection of the tangent line drawn to the brightness profile from the surface toward the bulk of the STEM image and the axis in the depth direction. The surface in a STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0184] In addition, a peak in STEM-EDX-ray analysis refers to the detected intensity in each element profile or the maximum value of the characteristic X-rays for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0185] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, the magnesium concentration in the surface layer 100a is preferably higher than the magnesium concentration in the interior 100b. Furthermore, when EDX analysis is performed, the magnesium concentration peak in the surface layer 100a preferably exists within a depth of 3 nm from the surface toward the center of the cathode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Alternatively, it is preferably within ±1 nm from the surface. Furthermore, the magnesium concentration preferably decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it preferably decays to 30% or less of the peak at a depth of 2 nm from the peak top. Note that the concentration peak here refers to the maximum concentration. Due to the influence of spatial resolution in EDX analysis, the position where the magnesium concentration peak exists may take a negative value as a depth from the surface toward the interior.

[0186] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0187] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer 100a preferably exists within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Alternatively, it is preferable that the fluorine concentration peak exists slightly closer to the surface than the magnesium concentration peak, as this increases resistance to hydrofluoric acid. For example, it is more preferable that the fluorine concentration peak be 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferable that it be 1.5 nm or more closer to the surface.

[0188] Furthermore, in the positive electrode active material 100 containing nickel as an additive element, the nickel concentration peak in the surface layer 100a preferably exists within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Alternatively, it is preferable that it exists within ±1 nm from the surface. Furthermore, in the positive electrode active material 100 containing magnesium and nickel, the nickel distribution preferably overlaps with the magnesium distribution. For example, the difference in depth between the nickel concentration peak and the magnesium concentration peak is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0189] Furthermore, when the cathode active material 100 contains aluminum as an additive element, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100 a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present at a depth of 0.5 nm to 50 nm, more preferably 3 nm to 30 nm, from the surface to the center of the cathode active material 100.

[0190] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.01 or more and 0.6 or less, more preferably 0.05 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less, more preferably 0.05 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0191] <Washing> Various analyses have been described, but before subjecting the sample to analysis, such as the positive electrode active material and the positive electrode active material layer, may be washed to remove the electrolyte, binder, conductive material, or compounds derived from these adhering to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0192] <Electrolyte> As one form of the electrolyte, an electrolyte solution having a solvent and an electrolyte dissolved in the solvent can be used.The solvent is preferably an aprotic solvent, and for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc. can be used alone or in any combination and ratio of two or more of these.When two or more kinds are used, it may be referred to as a mixed solvent.

[0193] As another form of electrolyte, one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) can be used as the solvent. In this case, even if the internal temperature of the power storage device rises due to an internal short circuit or overcharging, the power storage device can be prevented from exploding or catching fire. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0194] The electrolyte (also called lithium salt) dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (Li(C) ２ O ４ ) ２ , LiBOB), or two or more of these can be used in any combination and ratio.

[0195] An additive may be mixed into the mixed solvent containing the lithium salt. Examples of the additive include vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), and dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % of the mixed solvent containing the lithium salt.

[0196] <Electrolyte Solution Example 1> As the mixed solvent used in one embodiment of the present invention, a material having excellent lithium ion conductivity can be used even when charging and / or discharging (charging and discharging) in a low-temperature environment (e.g., 0°C, −20°C, preferably −30°C, more preferably −40°C).

[0197] An example of the electrolyte solution will be described below. The electrolyte solution described in this embodiment is a mixed solvent in which a lithium salt is dissolved, and the mixed solvent is liquid at room temperature. The mixed solvent is not limited to being liquid at room temperature, and a solid electrolyte that becomes solid at room temperature can also be used. Alternatively, a semi-solid electrolyte that contains both liquid and solid at room temperature can also be used. The semi-solid electrolyte includes a gel-like electrolyte.

[0198] The mixed solvent of the electrolyte solution according to one embodiment of the present invention may contain two or more selected from fluorinated cyclic carbonates (also referred to as fluorinated cyclic carbonates) and fluorinated chain carbonates (also referred to as fluorinated chain carbonates).

[0199] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. Since all of these fluorinated cyclic carbonates have electron-withdrawing substituents, they are believed to have low solvation energies for lithium ions.

[0200] The following structural formula (H10) is the structural formula of FEC: In FEC, the electron-withdrawing substituent is an F group.

[0201]

[0202] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The following structural formula (H22) is the structural formula of methyl 3,3,3-trifluoropropionate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0203]

[0204] An example of a fluorinated chain carbonate is trifluoromethyl 3,3,3-trifluoropropionate. The following structural formula (H23) is the structural formula of trifluoromethyl 3,3,3-trifluoropropionate. The electron-withdrawing substituent is CF ３ It is the base.

[0205]

[0206] An example of a fluorinated chain carbonate is trifluoromethyl propionate. The following structural formula (H24) is the structural formula of trifluoromethyl propionate. The electron-withdrawing substituent is CF ３ It is the base.

[0207]

[0208] An example of a fluorinated chain carbonate is methyl 2,2-difluoropropionate. The following structural formula (H25) is the structural formula of methyl 2,2-difluoropropionate. The electron-withdrawing substituent is CF ２ It is the base.

[0209]

[0210] <FEC and MTFP> The mixed solvent described in this embodiment preferably contains FEC and MTFP. The reason for this will be described below.

[0211] FEC is a cyclic carbonate with a high dielectric constant, and therefore, when used in a mixed solvent, it has the effect of promoting the dissociation of lithium salts. Furthermore, since FEC has electron-withdrawing substituents, it easily bonds with lithium ions through Coulomb forces, etc. Specifically, FEC has a lower solvation energy than ethylene carbonate (abbreviated as "EC"), which does not have electron-withdrawing substituents, and therefore the bond between lithium ions and the solvent is easily separated, i.e., it is easily desolvated. Furthermore, FEC is thought to have a deep highest occupied molecular orbital (HOMO) level, and a deep HOMO level makes it less susceptible to oxidation and improves oxidation resistance. On the other hand, FEC has a high viscosity, and when FEC is used alone as a solvent, it is difficult to use below freezing. Therefore, the mixed solvent specifically described as one embodiment of the present invention further contains MTFP in addition to FEC. MTFP is a type of chain carbonate that has the effect of reducing or maintaining the viscosity of the electrolyte. MTFP also has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, but it may still solvate with lithium ions.

[0212] The measured values ​​of the HOMO level, solvation energy, and melting point are summarized in the table below.

[0213]

[0214] FEC and MTFP having such physical properties can be mixed and used in a volume ratio of x:100-x (where 5≦x≦30, preferably 10≦x≦20), assuming the total content of these two mixed solvents to be 100 vol%. That is, the mixed solvent should contain more MTFP than FEC. Note that the above volume ratio may be the volume ratio measured before mixing the mixed solvent, and the ambient air temperature when mixing the mixed solvent may be room temperature (typically 25°C). A mixed solvent containing FEC and MTFP is preferable because it exhibits a viscosity that allows it to operate as a lithium-ion secondary battery and maintains an appropriate viscosity even in a low-temperature environment.

[0215] Because typical solvents used in lithium ion secondary batteries freeze at around −20° C., it is difficult to fabricate a lithium ion secondary battery that can be charged and discharged at −30° C., preferably −40° C. However, the mixed solvent described as an example in this embodiment allows the freezing point to be −30° C. or lower, preferably −40° C. or lower, making it possible to realize a lithium ion secondary battery that can be charged and discharged even in a low-temperature environment. As a result, it is possible to realize a lithium ion secondary battery that can be charged and discharged over a wide temperature range, including at least a low-temperature environment.

[0216] Although FEC has been described above as a representative example, any of the organic compounds described as fluorinated cyclic carbonates has the effect of promoting the dissociation of lithium salts, has small solvation energy so that the bond between the lithium ion and the solvent is easily broken, and has high viscosity, making it difficult to use it alone below freezing point.

[0217] Although MTFP has been described as a representative example above, any of the organic compounds described as fluorinated chain carbonates can be said to have the effect of reducing or maintaining the viscosity of the electrolyte solution of one embodiment of the present invention. Therefore, as long as the mixed solvent of one embodiment of the present invention contains a fluorinated cyclic carbonate and a fluorinated chain carbonate, it is possible to provide a lithium ion secondary battery that can be charged and discharged in a low-temperature environment.

[0218] <Electrolyte Solution Example 2> A mixed solvent for an electrolyte solution according to another embodiment of the present invention may contain ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and the volume ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65) when the total content of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is 100 vol%. More specifically, a mixed solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 may be used. Note that the above volume ratio may be the volume ratio before the mixed solvent is mixed, and the ambient air temperature when the mixed solvent is mixed may be room temperature (typically, 25°C).

[0219] EC is a cyclic carbonate with a high dielectric constant, which promotes the dissociation of lithium salts. However, EC has a high viscosity and a high freezing point (melting point) of 38°C, making it difficult to use EC alone as a solvent in low-temperature environments. Therefore, a solvent specifically described as one embodiment of the present invention further contains EMC and DMC, rather than EC alone. EMC is a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −54°C. DMC is also a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −43°C. An electrolyte prepared using a mixed solvent containing EC, EMC, and DMC with such physical properties, in a volume ratio of x:y:100−x−y (where 5≦x≦35 and 0<y<65), assuming a total content of the three mixed solvents as 100 vol%, is characterized by a freezing point of −40°C or lower.

[0220] A typical electrolyte solution used in lithium ion secondary batteries solidifies at a temperature of at least −20° C., making it difficult to fabricate a battery that can be charged and discharged at −40° C. The electrolyte solution described as an example in this embodiment has a freezing point of −40° C. or lower, making it possible to realize a lithium ion secondary battery that can be charged and discharged even in an extremely low temperature environment of −40° C.

[0221] The lithium salt dissolved in the solvent may be, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ At least one lithium salt selected from the group consisting of lithium bis(oxalate)borate (LiBOB) and lithium bis(oxalate)borate (LiBOB) can be used in any combination and ratio. The lithium salt dissolved in the solvent is preferably 0.5 mol / L or more and 1.5 mol / L or less, more preferably 0.7 mol / L or more and 1.3 mol / L or less, and more preferably 0.8 mol / L or more and 1.2 mol / L or less, relative to the volume of the solvent. A specific example of use is when LiPF ６ is preferably 0.5 mol / L or more and 1.5 mol / L or less, more preferably 0.7 mol / L or more and 1.3 mol / L or less, and even more preferably 0.8 mol / L or more and 1.2 mol / L or less.

[0222] Furthermore, the mixed solvent is preferably highly purified, with a low content of granular dust or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0223] Furthermore, for the purpose of improving safety, etc., an additive such as vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or a dinitrile compound such as succinonitrile or adiponitrile may be added to the electrolyte to form a coating (solid electrolyte interphase film) at the interface between the electrode (active material layer) and the electrolyte. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % relative to the solvent.

[0224] In the electrolyte solution example 2, the lithium salt may be the same as that described in the electrolyte solution example 1. Also, the additive may be the same as that described in the electrolyte solution example 1.

[0225] As described above, examples of the electrolyte solution that can be used in the lithium-ion secondary battery of one embodiment of the present invention have been described, but the electrolyte solution that can be used in the lithium-ion secondary battery of one embodiment of the present invention is not limited to this example. Other materials can also be used as long as they have excellent lithium-ion conductivity even during charge and discharge in a low-temperature environment.

[0226] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector, and the negative electrode active material layer has a negative electrode active material.

[0227] <Binder> A polymer having a carboxy group is preferably used as the binder for the negative electrode according to one embodiment of the present invention. The carboxy group can be said to have two basic oxygen atoms, one acidic hydrogen atom, and one electrophilic carbon atom. The carboxy group can also be said to have a polar group, having a hydroxyl group (OH) and a carbonyl group (C=O). When the binder has a polar group such as a carboxy group, it is expected to interact with lithium ions, which are carrier ions. For example, the binder may attract lithium ions, thereby assisting the insertion of lithium ions into the negative electrode active material. The carboxy group can be identified using FT-IR or the like.

[0228] Examples of polymers having carboxy groups include polyglutamic acid (sometimes referred to as PGA), polyacrylic acid (sometimes referred to as PAA), and alginic acid (sometimes referred to as polysaccharide). Polyamino acids may also be used as polymers having carboxy groups, and specifically, polyornithine and polysarcosine may be used as binders. Furthermore, polyaspartic acid may also be used as a binder for polymers having ketone groups. Furthermore, binary copolymers (copolymers) may also be used as polymers having ketone groups, and copolymers of acrylic acid and maleic acid, or copolymers of acrylic acid and sulfonic acid may also be used as binders. Using these as binders for the negative electrode also has the effect of reducing the amount of binder mixed in the negative electrode.

[0229] Among the above polymers, polyglutamic acid or polyacrylic acid is particularly preferable as a binder for use in a negative electrode. The structural formula of polyglutamic acid is shown below.

[0230]

[0231] As is clear from the structural formula, polyglutamic acid contains nitrogen in addition to carboxyl groups, and since the nitrogen has an unshared electron pair, it is expected to interact with lithium ions, which are carrier ions. For example, the unshared electron pair may attract lithium ions and assist their insertion into the negative electrode active material.

[0232] Furthermore, as is clear from the structural formula, polyglutamic acid has a carbonyl group, C=O, in addition to the carbonyl group. If the binder has a polar group such as a carbonyl group, it is expected to interact with lithium ions, which are carrier ions, and may, for example, assist the insertion and desorption of lithium ions in the negative electrode active material.

[0233] Either linear γ-polyglutamic acid or cross-linked γ-polyglutamic acid may be used as the binder. These are collectively referred to as a structure mainly composed of γ-polyglutamic acid. Cross-linked γ-polyglutamic acid is more suitable for binders because it has a network structure. Furthermore, the molecular weight of polyglutamic acid should be 1 million or more, preferably 3 million or more, and more preferably 10 million to 50 million.

[0234] Depending on the method for preparing polyglutamic acid, it can be said that the structure is mainly composed of γ-glutamic acid containing other elements (e.g., Ca, Al, Na, Mg, Fe, Si, S). That is, polyglutamic acid may be neutralized with alkali metal ions, such as lithium ions or sodium ions.

[0235] Such polyglutamic acid is hydrophilic, so deionized water can be used as a solvent, which is suitable for forming a slurry.

[0236] The structural formula of polyacrylic acid is shown below.

[0237]

[0238] As is clear from the structural formula, polyacrylic acid has a carboxy group.

[0239] A material obtained by cross-linking polyacrylic acid may also be used. This is preferable because it can form a cross-linked structure, i.e., a network structure, which may enhance the function as a binder.

[0240] <Negative Electrode Active Material> A negative electrode according to one embodiment of the present invention includes both carbon particles and silicon particles as negative electrode active materials. Examples of the carbon particles include graphite, carbon having a layer structure similar to graphite, amorphous carbon, hard carbon, and carbon fiber. Specifically, graphite particles are preferably used as the carbon particles used in this specification.

[0241] The graphite particles according to one embodiment of the present invention preferably have an average particle size of 1 μm or more, preferably 5 μm or more, preferably 10 μm or more, more preferably 20 μm or more. The graphite particles may be mixed with silicon particles before use in the negative electrode.

[0242] The average particle size of graphite particles can be measured using a particle size distribution analyzer or the like using a laser diffraction / scattering method. In this specification and the like, the average particle size of graphite particles can be determined as the median diameter (D50). The median diameter (D50) is the particle diameter at which the cumulative amount in the cumulative particle amount curve of the particle size distribution measurement results accounts for 50%. Measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may also be measured by analysis such as SEM or TEM. Note that, as a method for measuring the median diameter (D50) using analysis such as SEM or TEM, for example, 20 or more particles can be measured, a cumulative particle amount curve can be created, and the particle diameter at which the cumulative amount accounts for 50% can be taken as the median diameter (D50).

[0243] The specific surface area of ​​the graphite particles is 0.5 m ２ / g or more 3m ２ / g or less. The specific surface area can be measured by the BET method. The specific surface area by the BET method is a value measured by the BET single-point method using nitrogen gas adsorption, and can be measured using an automatic specific surface area / pore distribution measuring device, Tristar II 3020 (manufactured by Shimadzu Corporation).

[0244] Silicon particles with an average particle size of 100 nm or thereabouts are preferably used, and these are sometimes called nanosilicon particles. Silicon has a capacity of 4200 mAh / g per weight, which is more than 10 times the capacity of graphite (372 mAh / g per weight of active material). However, silicon has the problem of rapid cycle degradation due to expansion and contraction during charge and discharge. Therefore, to improve cycle degradation, nanosilicon particles, in which silicon is refined to the above average particle size, are suitable.

[0245] The average particle size of silicon particles can be measured using a particle size distribution meter or the like using a laser diffraction / scattering method. In this specification, the average particle size of silicon particles can be determined as the median diameter (D50). The median diameter (D50) is the particle diameter when the cumulative amount in the cumulative particle amount curve of the particle size distribution measurement result accounts for 50%. The measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may also be measured by analysis such as SEM or TEM. Note that, as a method for measuring the median diameter (D50) by analysis such as SEM or TEM, for example, 20 or more particles are measured, a cumulative particle amount curve is created, and the particle diameter when the cumulative amount accounts for 50% can be taken as the median diameter (D50).

[0246] The silicon particles are preferably prepared by pulverizing silicon raw materials and adjusting the particle size to a uniform size. Through this adjustment, silicon particles having an average particle size of less than 1 μm can be obtained. Note that if the average particle size is large, the negative electrode active material layer may become thick, so it is preferable that the average particle size is less than 1 μm. The silicon particles may be any silicon-based material, and specifically, may include at least one of silicon, silicon oxide, and silicon alloy.

[0247] The specific surface area of ​​silicon particles is 10 m ２ / g or more 35m ２ / g or less, preferably 10m ２ / g or more 15m ２ / g or less. The specific surface area can be measured by the BET method. The specific surface area by the BET method is a value measured by the BET single-point method using nitrogen gas adsorption, and can be measured using an automatic specific surface area / pore distribution measuring device, Tristar II 3020 (manufactured by Shimadzu Corporation).

[0248] In one embodiment of the present invention, the negative electrode active material contains both graphite particles and silicon particles, thereby realizing a lithium ion secondary battery with a high discharge capacity. Furthermore, since the average particle diameter of the graphite particles is different from that of the silicon particles, mixing these particles and using them in the negative electrode can increase the amount of the negative electrode active material supported. In this specification, the amount of support refers to the weight of the negative electrode active material per unit surface area of ​​the negative electrode current collector. The amount of the negative electrode active material supported can be determined according to the capacity of the positive electrode. A low support amount can improve the output characteristics of the lithium ion secondary battery, but a low support amount reduces the discharge capacity. Therefore, the amount of the negative electrode active material supported is set to 1.5 mg / cm. ２ The above is preferable.

[0249] In one embodiment of the present invention, the weight ratio of the graphite particles in the negative electrode active material layer is preferably greater than the weight ratio of the silicon particles, for example, the weight ratio of the graphite particles is preferably 5 to 15 times the weight ratio of the silicon particles, in other words, the weight ratio of silicon to the total weight of the powder material constituting the negative electrode active material is preferably 7.5 wt % to 37.5 wt %.

[0250] Furthermore, a conductive material may be added when forming the negative electrode active material layer.

[0251] In a lithium-ion secondary battery, a negative electrode active material layer can be formed on one or both sides of a negative electrode current collector. The negative electrode active material layer is completed by applying a slurry onto the negative electrode current collector and then drying it.

[0252] In this specification, the weight ratio of each raw material may be regarded as the blending ratio of each raw material when preparing the slurry. That is, the weight ratio of the negative electrode active material is the blending ratio (wt%) of the negative electrode active material to the total weight of the negative electrode active material and binder in the slurry, or to the total weight of the negative electrode active material, binder, and conductive material. The weight ratio and blending ratio can be understood by replacing the negative electrode active material with the binder.

[0253] The weight ratio of the binder is preferably smaller than the weight ratio of the graphite particles, and in order to exert the effect as a binder, the weight ratio of the binder is preferably greater than 5 wt %.

[0254] <Method for Producing Negative Electrode Active Material Layer> Here, a method for producing a negative electrode active material layer will be described. A negative electrode slurry according to one embodiment of the present invention may be prepared by mixing graphite particles, silicon particles, and a binder having a carboxy group, followed by adding a solvent and mixing. In the slurry according to one embodiment of the present invention, the graphite particles, silicon particles, and the binder having a carboxy group can be mixed simultaneously, which is preferable because it can shorten the process. Furthermore, when preparing the slurry, the graphite particles, silicon particles, the binder having a carboxy group, and the solvent can also be mixed simultaneously. Furthermore, when preparing the slurry, a conductive material can also be mixed simultaneously. Specific examples of the conductive material will be described later, but acetylene black (hereinafter referred to as AB) is preferably used.

[0255] The slurry thus obtained is applied to one or both surfaces of a negative electrode current collector, dried, and pressed to form a negative electrode active material layer. By using the negative electrode active material layer according to one embodiment of the present invention, a lithium ion secondary battery having excellent cycle characteristics can be provided.

[0256] It is advisable to prevent the silicon particles from being oxidized. For example, when preparing a slurry, it is preferable to carry out a mixing process so that the silicon particles are not oxidized.

[0257] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0258] Embodiment 2 In this embodiment, a method for manufacturing a positive electrode active material applicable to a lithium ion secondary battery having excellent discharge characteristics even in a low-temperature environment will be described with reference to FIGS.

[0259] 8A to 8D , an example of a method for manufacturing a positive electrode active material that can be used as one embodiment of the present invention (Example 1 of a method for manufacturing a positive electrode active material) will be described. Note that in Example 1 of a method for manufacturing a positive electrode active material, the additive elements described as the additive element X, the additive element Y, and the additive element Z in Embodiment 1 are collectively referred to as the additive element A.

[0260] First, in step S10, lithium cobalt oxide is prepared as a starting material. The lithium cobalt oxide starting material can have a particle size (strictly speaking, a median diameter (D50)) of 12 μm or less (preferably 10 μm or less, and more preferably 8 μm or less). The lithium cobalt oxide having a median diameter (D50) of 12 μm or less may be a known or publicly used (in short, commercially available) lithium cobalt oxide, or may be a lithium cobalt oxide prepared through steps S11 to S14 shown in FIG. 8B. A representative example of a commercially available lithium cobalt oxide having a median diameter (D50) of 12 μm or less is lithium cobalt oxide manufactured by Nippon Chemical Industry Co., Ltd. (product name: "Cellseed C-5H"). The lithium cobalt oxide manufactured by Nippon Chemical Industry Co., Ltd. (product name: "Cellseed C-5H") has a median diameter (D50) of approximately 7 μm. A manufacturing method for obtaining lithium cobalt oxide having a median diameter (D50) of 12 μm or less through steps S11 to S14 will be described below.

[0261] <Step S11> In step S11 shown in FIG. 8B, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0262] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0263] As the cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide. The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased, and the reliability of the secondary battery is improved.

[0264] <Step S12> Next, in step S12 shown in FIG. 8B , the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed in a dry or wet manner. Wet pulverization and mixing allows for smaller pulverization, which is preferable for obtaining lithium cobalt oxide with a median diameter (D50) of 10 μm or less as a starting material. When performing the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). However, it is preferable to use an aprotic solvent that does not react easily with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the transition metal source in dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix the mixture. Using dehydrated acetone with the above purity reduces the amount of impurities that may be present.

[0265] <Step S13> Next, in step S13 shown in FIG. 8B , the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably approximately 950°C (1000°C or lower). If the temperature is too low, the decomposition and melting of the lithium source and transition metal source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, resulting in defects. For example, cobalt may change from trivalent to divalent, causing oxygen defects.

[0266] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. Therefore, the heating time should be 1 hour or more and 100 hours or less, preferably 2 hours or more and 20 hours or less, and more preferably 2 hours or more and 10 hours or less.

[0267] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0268] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0269] The heating atmosphere is preferably an atmosphere containing oxygen. For example, there is a method of continuously introducing dry air into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flow through the reaction chamber is called flow.

[0270] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing the oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen to prevent the oxygen from entering or leaving the reaction chamber may be used, which is called purging. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0271] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0272] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0273] The container used for heating is preferably an aluminum oxide crucible or an aluminum oxide sheath. An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, an aluminum oxide sheath with a purity of 99.9% is used. Note that it is preferable to place a lid on the crucible or sheath before heating, as this prevents the material from volatilizing.

[0274] After the heating is completed, the mixture may be crushed and sieved as necessary. Note that the same heating conditions as those in step S13 can be applied to heating steps other than step S13, which will be described later.

[0275] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized. ２ ) is an oxide containing multiple metal elements in its structure, and therefore can be called a composite oxide. In this specification and the like, the term "composite oxide" refers to an oxide containing multiple metal elements in its structure. After step S13, a crushing step and a classification step are carried out to adjust the particle size distribution, and then lithium cobalt oxide (LiCoO ２ ) may be obtained.

[0276] Although the composite oxide is produced by the solid phase method in steps S11 to S14, the composite oxide may be produced by a coprecipitation method or a hydrothermal method.

[0277] Through steps S11 to S14, lithium cobalt oxide can be obtained as a starting material for obtaining a positive electrode active material that can be used in lithium ion secondary batteries and has excellent discharge characteristics even in low-temperature environments. Specifically, lithium cobalt oxide having a median diameter (D50) of 10 μm or less can be obtained as the starting lithium cobalt oxide.

[0278] 8A, the lithium cobalt oxide starting material is heated. The heating in step S15 is sometimes referred to as initial heating in this specification, etc., because it is the first heating of the lithium cobalt oxide. Alternatively, because it is performed before step S31 described below, it is sometimes referred to as preheating or pretreatment.

[0279] The initial heating causes lithium compounds and the like unintentionally remaining on the surface of the lithium cobalt oxide to be desorbed. It is also expected to have the effect of enhancing the internal crystallinity. Although impurities may be present in the lithium source and / or cobalt source prepared in step S11, etc., the initial heating can reduce the impurities from the lithium cobalt oxide starting material. The effect of enhancing the internal crystallinity is, for example, the effect of alleviating distortion, misalignment, and the like resulting from differential shrinkage of the lithium cobalt oxide prepared in step S14.

[0280] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. Initial heating also has the effect of mitigating cracks, crystal defects, and the like that the lithium cobalt oxide has. In this specification and elsewhere, a "smooth" surface refers to a surface with few irregularities, a rounded overall surface, and rounded corners. Alternatively, a state in which there is little foreign matter attached to the surface is also referred to as "smooth." Foreign matter is considered to be a cause of irregularities, so it is preferable not to allow it to adhere to the surface.

[0281] In this initial heating, it is not necessary to separately prepare a material that functions as a lithium compound source, an additive element source, or a flux.

[0282] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. An appropriate heating time range can be selected, for example, from the heating conditions described in step S13. The heating temperature in step S15 is preferably lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in step S15 is preferably shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating is preferably performed at a temperature of 700°C or higher and 1000°C or lower (more preferably, 800°C or higher and 900°C or lower) for 1 hour or higher and 20 hours or lower (more preferably, 1 hour or higher and 5 hours or lower).

[0283] The heating in step S13 can cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference can induce a shrinkage difference. It is also thought that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is thought that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in the lithium cobalt oxide is relaxed. This smooths the surface of the lithium cobalt oxide. Alternatively, it can be said that the surface is improved. In other words, by going through step S15, the shrinkage difference that occurred in the lithium cobalt oxide is alleviated, resulting in a smooth surface for the composite oxide.

[0284] Furthermore, the shrinkage difference may cause microscopic misalignment, such as crystal misalignment, in the lithium cobalt oxide. To reduce this misalignment, it is preferable to perform step S15. By performing step S15, it is possible to equalize the misalignment of the composite oxide (alleviate the misalignment of crystals, etc., that has occurred in the composite oxide, or align the crystal grains). As a result, the surface of the composite oxide becomes smooth.

[0285] As described above, in step S10, pre-synthesized lithium cobalt oxide having a median diameter (D50) of 12 μm or less, preferably 10 μm or less, and more preferably 8 μm or less may be used. In this case, steps S11 to S13 can be omitted. It is useful to perform step S15 on pre-synthesized lithium cobalt oxide, and this is a preferred step because it allows lithium cobalt oxide with a smooth surface to be obtained.

[0286] Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0287] <Step S20> Next, details of step S20 for preparing the additional element A as the A source will be described with reference to FIGS. 8C and 8D.

[0288] <Step S21> Step S20 shown in FIG. 8C includes steps S21 to S23. In step S21, an additive element A is prepared. Specific examples of the additive element A include one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. Alternatively, one or more elements selected from bromine and beryllium may be used. FIG. 8C illustrates a case where a magnesium source (Mg source) and a fluorine source (F source) are prepared. In step S21, a lithium source may be separately prepared in addition to the additive element A.

[0289] When magnesium is selected as the additional element A, the source of the additional element A can be called a magnesium source. As the magnesium source, magnesium fluoride (MgF ２ ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ２ ), or magnesium carbonate (MgCO ３ ) etc. A plurality of magnesium sources may be used.

[0290] When fluorine is selected as the additional element A, the source of the additional element A can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0291] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Other lithium sources that can be used in step S21 include lithium carbonate.

[0292] The fluorine source may also be a gas, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２F) or the like may be used and mixed into the atmosphere in the heating step described below. A plurality of fluorine sources may be used.

[0293] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and magnesium source. ２ ) is prepared. In addition, when the melting point of a fluorine compound (sometimes called a fluoride) such as lithium fluoride is lower than the melting point of the other additive element source, the fluorine compound can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source. ２ As shown in FIG. 11 (quoted and added from FIG. 5 of Non-Patent Document 1), ２ Since the eutectic point of the alloy is around 742°C (T1), it is preferable to set the heating temperature to 742°C or higher in the heating step (step S33 described later, etc.) after mixing the additive element.

[0294] Here, differential scanning calorimetry (DSC) measurements of fluorine compounds and mixtures will be described with reference to Fig. 12. The mixture in Fig. 12 contains lithium cobalt oxide as the lithium oxide, LiF and MgF as the fluorine compounds. ２ It is a mixture of LiCoO ２ :LiF:MgF ２ The fluorine compounds in FIG. 12 were LiF and MgF ２ More specifically, the mixture is LiF:MgF ２ The mixture was mixed so that the molar ratio was 1:3.

[0295] As shown in Fig. 12, an endothermic peak is observed in the vicinity of 735°C for the fluorine compound. Also, an endothermic peak is observed in the vicinity of 830°C for the mixture. Therefore, the heating temperature after mixing the additive element (e.g., step S33 described later) is preferably 742°C or higher, more preferably 830°C or higher. Alternatively, it may be 800°C (T2 in Fig. 11) or higher, which is between these temperatures.

[0296] Lithium fluoride and magnesium fluoride are also known as LiF:MgF ２When the ratio of lithium fluoride to magnesium fluoride is about 65:35, the effect of lowering the melting point is maximized. In addition, if the ratio of lithium fluoride is too high, there is a concern that the lithium will be excessive, which may deteriorate the cycle characteristics. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or thereabouts) is more preferable. In this specification and the like, "nearby" means a value that is greater than 0.9 times and smaller than 1.1 times the value, unless otherwise specified.

[0297] 8C, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0298] 8C, the pulverized and mixed materials are collected to obtain a source of the additional element A. The source of the additional element A shown in step S23 includes a plurality of starting materials and can also be called a mixture.

[0299] The particle size of the mixture is preferably such that the median diameter (D50) is 100 nm or more and 10 μm or less, more preferably 300 nm or more and 5 μm or less. Even when a single material is used as the source of the additive element A, the median diameter (D50) is preferably 100 nm or more and 10 μm or less, more preferably 300 nm or more and 5 μm or less.

[0300] The mixture (including the case where only one type of additive element is contained) pulverized in step S22 is likely to be uniformly adhered to the surface of the lithium cobalt oxide when mixed with the lithium cobalt oxide in a later step. If the mixture is uniformly adhered to the surface of the lithium cobalt oxide, the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating, which is preferable.

[0301] <Step S21> A process different from that shown in Fig. 8C will be described with reference to Fig. 8D. Step S20 shown in Fig. 8D includes steps S21 to S23.

[0302] In step S21 shown in Fig. 8D, four types of additive element A sources to be added to lithium cobalt oxide are prepared. That is, Fig. 8D differs from Fig. 8C in the types of additive element A sources. In addition to the additive element A sources, a lithium source may also be prepared separately.

[0303] As sources of four types of additive element A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described with reference to FIG. 8C . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0304] <Step S22> and <Step S23> Next, step S22 and step S23 shown in FIG. 8D are the same as step S22 and step S23 described with reference to FIG. 8C.

[0305] <Step S31> Next, in step S31 shown in FIG. 8A , the lithium cobalt oxide that has undergone step S15 (initial heating) is mixed with an additive element A source (Mg source). Here, the ratio of the number of cobalt atoms Co in the lithium cobalt oxide that has undergone step S15 to the number of magnesium atoms Mg in the additive element A is preferably Co:Mg = 100:y (0.1≦y≦6), and more preferably Co:Mg = 100:y (0.3≦y≦3). Note that adding the additive element A to the lithium cobalt oxide that has undergone initial heating allows the additive element A to be added evenly. For this reason, it is preferable to add the additive element A after initial heating (step S15), rather than adding the additive element A and then performing initial heating (step S15).

[0306] Furthermore, when nickel is selected as the additional element A, it is preferable to perform the mixing in step S31 so that the number of nickel atoms in the nickel source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. Furthermore, when aluminum is selected as the additional element A, it is preferable to perform the mixing in step S31 so that the number of aluminum atoms in the aluminum source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0307] The mixing in step S31 is preferably performed under milder conditions than the pulverization and mixing in step S12 so as not to destroy the shape of the lithium cobalt oxide. For example, it is preferable to perform the mixing under conditions of a lower rotation speed or a shorter time than in step S12. It can also be said that the dry method provides milder conditions than the wet method. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium.

[0308] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0309] <Step S32> Next, in step S32 of Fig. 8A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0310] <Step S33> Next, in step S33 shown in FIG. 8A, the mixture 903 is heated. The heating in step S33 is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 800°C or higher and 950°C or lower, and even more preferably 850°C or higher and 900°C or lower. The heating time in step S33 may be 1 hour or higher and 100 hours or lower, and preferably 1 hour or higher and 10 hours or lower. The lower limit of the heating temperature in step S33 needs to be a temperature at which the reaction between the lithium cobalt oxide and the additive element A source proceeds. The temperature at which the reaction proceeds may be a temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element A source occurs, and may be lower than the melting temperature of these materials. For example, taking an oxide as an example, the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs, so the heating temperature in step S33 may be 500° C. or higher.

[0311] The reaction is more likely to proceed when the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF and MgF are used as the source of the additive element A. ２ As described above, when ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0312] Also, LiCoO ２ :LiF:MgF ２ As described above, the mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830°C in differential scanning calorimetry (DSC measurement). Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

[0313] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0314] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

[0315] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0316] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, for example, 742° C. to 950° C., and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0317] Incidentally, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF may volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. In this case, the function as a flux will be weakened. Therefore, it is preferable to heat the mixture while suppressing the volatilization or sublimation of LiF.

[0318] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization or sublimation of LiF in the mixture 903.

[0319] Furthermore, the heating in this step is preferably performed so as not to stick together the mixture 903. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the added element (for example, fluorine) diffuses is blocked, which may result in a poor distribution of the added element (for example, magnesium and fluorine) in the surface layer portion.

[0320] Furthermore, when the additive element (e.g., fluorine) is uniformly distributed in the surface layer portion, a smooth cathode active material with few irregularities can be obtained. Therefore, in this process, in order to maintain or further smooth the surface by heating in step S15, it is preferable that the mixture 903 does not stick to each other.

[0321] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 903 .

[0322] <Step S34> Next, in step S34 shown in FIG. 8A , the heated material is recovered to obtain the positive electrode active material 100. At this time, the material may be crushed as necessary, and it is preferable to further sieve the recovered positive electrode active material 100. Through the above steps, a positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less (preferably 10 μm or less, more preferably 8 μm or less) can be produced. The positive electrode active material 100 contains the additive element A.

[0323] 9 and 10 , another example of a method for producing a cathode active material that can be used as one embodiment of the present invention (Example 2 of Method for Producing a Cathode Active Material) will be described. Example 2 of Method for Producing a Cathode Active Material differs from Example 1 of Method for Producing a Cathode Active Material described above in the number of times the additive element is added and the mixing method. However, the other descriptions in Example 1 of Method for Producing a Cathode Active Material can be applied. In Example 2 of Method for Producing a Cathode Active Material, the additive element X described in Embodiment 1 is referred to as additive element A1. The additive element Y and additive element Z described in Embodiment 1 are collectively referred to as additive element A2.

[0324] 9, steps S10 and S15 are performed in the same manner as in FIG. 8A to prepare lithium cobalt oxide that has undergone initial heating. Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0325] <Step S20a> Next, as shown in step S20a, a first additive element A1 source (A1 source) is prepared. Details of step S20a will be described with reference to FIG.

[0326] <Step S21> In step S21 shown in Fig. 10A, a first additive element A1 source (A1 source) is prepared. The A1 source can be selected from the additive elements A described in step S21 shown in Fig. 8C and used. For example, the additive element A1 can be one or more selected from magnesium, fluorine, and calcium. Fig. 10A illustrates an example in which a magnesium source (Mg source) and a fluorine source (F source) are used as the additive element A1.

[0327] Steps S21 to S23 shown in Fig. 10A can be performed under the same conditions as steps S21 to S23 shown in Fig. 8C. As a result, an additional element A1 source (A1 source) can be obtained in step S23.

[0328] Steps S31 to S33 shown in FIG. 9 can be performed under the same conditions as steps S31 to S33 shown in FIG. 8A.

[0329] <Step S34a> Next, the material heated in step S33 is recovered to obtain lithium cobalt oxide containing the additional element A1. Here, this is also referred to as the second composite oxide to distinguish it from the lithium cobalt oxide (first composite oxide) that has been subjected to step S15.

[0330] <Step S40> In step S40 shown in Fig. 9, a source of the second additional element A2 (A2 source) is prepared. Step S40 will be described with reference to Figs. 10B and 10C.

[0331] <Step S41> In step S40 shown in Figure 10B, a second additive element A2 source (A2 source) is prepared. The A2 source can be selected from the additive elements A described in step S20 shown in Figure 8C and used. For example, the additive element A2 can be any one or more selected from nickel, titanium, boron, zirconium, and aluminum. Figure 10B illustrates an example in which a nickel source and an aluminum source are used as the additive element A2.

[0332] Steps S41 to S43 shown in Fig. 10B can be performed under the same conditions as steps S21 to S23 shown in Fig. 8C. As a result, an additional element A2 source (A2 source) can be obtained in step S43.

[0333] Steps S41 to S43 shown in Figure 10C are a modification of Step S40 shown in Figure 10B. In Step S41 shown in Figure 10C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in Step S42a, they are independently pulverized. As a result, in Step S43, a plurality of second additional element A2 sources (A2 sources) are prepared. Thus, Step S40 in Figure 10C differs from Step S40 in Figure 10B in that the additional element sources are independently pulverized in Step S42a.

[0334] <Steps S51 to S53> Next, steps S51 to S53 shown in Fig. 9 can be performed under the same conditions as steps S31 to S34 shown in Fig. 8A. The conditions for step S53 related to the heating step are preferably a lower temperature and / or a shorter time than those for step S33 shown in Fig. 9. Specifically, the heating temperature is preferably 800°C or higher and 950°C or lower, more preferably 820°C or higher and 870°C or lower, and even more preferably 850°C ± 10°C. Furthermore, the heating time is preferably 0.5 hours or higher and 8 hours or lower, and more preferably 1 hour or higher and 5 hours or lower.

[0335] When nickel is selected as the additional element A2, it is preferable to perform the mixing in step S51 so that the number of nickel atoms in the nickel source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. When aluminum is selected as the additional element A2, it is preferable to perform the mixing in step S51 so that the number of aluminum atoms in the aluminum source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0336] <Step S54> Next, in step S54 shown in FIG. 9 , the heated material is recovered to obtain a positive electrode active material 100. The recovered material may be crushed as necessary. Through the above steps, a positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less (preferably 10 μm or less, more preferably 8 μm or less) can be produced. Alternatively, a positive electrode active material 100 applicable to a lithium ion secondary battery having excellent discharge characteristics even in a low-temperature environment can be produced. The positive electrode active material 100 contains the additive element A1 and the additive element A2.

[0337] In the above-described example 2 of the preparation method, as shown in FIGS. 9 and 10 , the additive element is introduced into the lithium cobalt oxide separately as a first additive element A1 and a second additive element A2. By introducing the additive elements separately, the profile of each additive element can be changed. For example, the first additive element can be profiled so that its concentration is higher in the surface layer than in the interior, and the second additive element can be profiled so that its concentration is higher in the interior than in the surface layer. The positive electrode active material 100 prepared through the steps shown in FIGS. 8A and 8D has the advantage of being able to be prepared at low cost because multiple types of additive element A sources are added at once. On the other hand, the positive electrode active material 100 prepared through the steps shown in FIGS. 9 and 10 is preferable because multiple types of additive element A sources are added in multiple steps, resulting in relatively high preparation costs. However, this method allows for more accurate control of the depth profile of each additive element A.

[0338] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0339] Embodiment 3 In this embodiment, an example of a manufacturing flow of a negative electrode active material layer used in a lithium ion secondary battery is shown in FIG.

[0340] First, there are prepared graphite particles 200, silicon particles 201, a binder 202, and a conductive material 203. A polymer having a carboxy group is used as the binder.

[0341] <Step S60> The above-mentioned raw materials are each weighed, and the first mixing is performed in step S60 of Figure 13. Specifically, the weight ratio of silicon particles 201 to the total weight of the powders mixed in the first mixing ranges from 7.5 wt% to 37.5 wt%, and the weight ratio of binder 202 to the total weight ranges from 10 wt% to 50 wt%. Furthermore, the weight ratio of conductive material 203 to the total weight ranges from 0 wt% to 20 wt%. It is recommended to use AB as the conductive material 203 that satisfies the above weight ratios.

[0342] For example, the silicon particles 201, the graphite particles 200, the binder 202, and the conductive material 203 are weighed out so that the weight ratio is 3:5:1:1. Alternatively, without using the conductive material, the silicon particles 201, the graphite particles 200, and the binder 202 are weighed out so that the weight ratio is 3:5:1. Alternatively, the graphite particles 200, the silicon particles 201, and the binder 202 may be weighed out so that the weight ratio is 9:1:1.

[0343] <Mixing of mixture 204 and solvent 205> In one embodiment of the present invention, in step S60, all raw materials are powders, so they are mixed before adding the solvent to obtain mixture 204. Mixing the powders together allows for a uniform mixture. After that, solvent 205 is preferably added. Deionized water is preferably used as solvent 205.

[0344] <Step S61> After adding the solvent 205, the second mixing is performed in step S61 of Fig. 13 to prepare a slurry 206. The second mixing is sometimes called slurry preparation.

[0345] The slurry 206 is a material liquid used to form an active material layer on a current collector, and contains at least an active material, a binder, and a solvent, and may further contain a conductive material. The slurry is sometimes called an electrode slurry or an active material slurry.

[0346] Then, in step S62 of FIG. 13, the slurry 206 is applied onto the negative electrode current collector 207. Thereafter, in step S63 of FIG. 13, drying is performed. As drying conditions, preliminary drying and main drying may be performed. That is, two drying steps are performed, with the first drying step being performed under milder conditions. For example, the slurry may be dried in a dryer at 40° C. to 60° C. for 10 minutes to 1 hour, which may be referred to as preliminary drying. Next, as main drying, the slurry may be dried in a dryer at 60° C. to 90° C. for 30 minutes to 1.5 hours. Pressing may be performed simultaneously with drying.

[0347] After drying, a pressing process is performed as step S64 in FIG. 13. A roll press can be used for the pressing process, and the upper and lower rollers can be heated to a temperature of 100°C or higher and 150°C or lower. In other words, heating can be performed simultaneously with the pressing process. The linear pressure during pressing should be 0.3 MPa or higher and 1 MPa or lower. Of course, the lithium ion secondary battery can be operated even if the pressing process is omitted.

[0348] Through the above steps, the negative electrode 208 having the negative electrode active material layer on the negative electrode current collector 207 can be manufactured.

[0349] A lithium ion secondary battery using the negative electrode 208 thus obtained has a large discharge capacity and exhibits excellent cycle characteristics.

[0350] <Attachment to Separator> When the negative electrode 208 described in this embodiment is assembled into a lithium ion secondary battery, it is attached to a separator. At this time, it is preferable to drip a solvent such as deionized water onto the negative electrode 208 or the separator to allow the adhesiveness of the binder, specifically the adhesiveness of the PGA, to be exhibited at the attachment surface.

[0351] <Semi-Solid Battery> The negative electrode 208 described in this embodiment is expected to be applied to a semi-solid battery. For example, by gelling a mixed solvent used for an electrolyte solution with a gelling agent or the like, a semi-solid battery having a binder, specifically, a PGA, which is one embodiment of the present invention, can be obtained.

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

[0353] Fourth Embodiment In this embodiment, each of the elements constituting a lithium ion secondary battery will be described.

[0354] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include at least one of a conductive material and a binder. The positive electrode active material described in Embodiment 1 can be used.

[0355] <Positive electrode current collector> For example, a metal foil can be used as the positive electrode current collector. The positive electrode can be formed by applying a slurry onto a metal foil and drying it. Note that pressing may be performed after drying. The positive electrode is formed by forming an active material layer on a positive electrode current collector 21.

[0356] The current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. The material used for the positive electrode current collector preferably does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. The current collector preferably has a thickness of 5 μm to 30 μm.

[0357] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may further have a conductive material and a binder.

[0358] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0359] In addition, the negative electrode active material can be an element capable of undergoing a charge-discharge reaction through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g per weight. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, compounds containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0360] In this specification, "SiO" refers to, for example, silicon monoxide. ｘHere, x preferably has a value of 1 or close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0361] The carbon material may be graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, or the like.

[0362] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0363] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion secondary batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety than metallic lithium.

[0364] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and other oxides can be used.

[0365] In addition, as the negative electrode active material, a nitride of lithium and a transition metal, Li３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0366] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, is used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0367] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ This also occurs with fluorides such as

[0368] <Negative electrode current collector> The negative electrode current collector can be made of the same material as the positive electrode current collector, as well as copper. Note that metals that alloy with lithium ions, such as aluminum, cannot be used for the negative electrode current collector.

[0369] <Binder (binding agent)> The positive electrode and the negative electrode each contain a binder. The binders that exhibit unique effects and are used in the negative electrode are as described in Embodiments 1 to 3. Of course, the binders that exhibit unique effects may also be used in the positive electrode.

[0370] As a binder other than the binders described in the first to third embodiments, it is preferable to use rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, fluororubber can be used as a binder.

[0371] Furthermore, as the binder, it is preferable to use materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate (PMMA)), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0372] It is also preferable to use a thickener in addition to the binder. For example, a water-soluble polymer is preferably used as the thickener. For example, a polysaccharide can be used as the water-soluble polymer. For example, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or starch can be used as the polysaccharide.

[0373] The binder described above may be used as a comparative example for the binders that exhibit the unique effects described in the first to third embodiments.

[0374] When the binder covers the surface of the active material, or when the binder in contact with the surface forms a film, it can serve as a passive film and is expected to have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0375] <Conductive Material> The conductive material used in the positive electrode and negative electrode is also called a conductivity imparting agent or conductive material, and is a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0376] As the conductive material, for example, one or more of carbon black such as acetylene black (AB) and furnace black, graphite such as artificial graphite and natural graphite, carbon fiber such as carbon nanofiber and carbon nanotube, graphene, and graphene compounds can be used.

[0377] It is difficult for AB to come into surface contact with other active materials, and point contact is likely to occur. Therefore, when an active material and AB are mixed, it is conceivable to use a large amount of AB to reduce contact resistance, but this reduces the proportion of active material, resulting in a decrease in the discharge capacity of the secondary battery. In addition, AB is a material that easily aggregates, so it is preferable to form a slurry using a dispersant or the like to uniformly disperse it.

[0378] In view of these considerations, the weight ratio of AB in the negative electrode is preferably less than or equal to the weight ratio of silicon particles used in the negative electrode active material. In other words, by satisfying this weight ratio, AB can be mixed to exhibit high dispersibility without reducing the proportion of silicon particles. Therefore, the discharge capacity of the secondary battery can be increased.

[0379] As the carbon fiber, for example, mesophase pitch-based carbon fiber, isotropic pitch-based carbon fiber, or the like can be used. Furthermore, as the carbon fiber, carbon nanofiber or carbon nanotube can be used. Carbon nanotube can be produced by, for example, vapor phase growth method. VGCF (registered trademark) can also be used as the carbon fiber.

[0380] The above-mentioned graphene includes graphene, multilayer graphene, multigraphene, etc. Furthermore, the above-mentioned graphene compounds include graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene refers to a substance containing carbon, having a shape such as a plate or sheet, and having a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be referred to as a carbon sheet. Furthermore, graphene is preferably rigid and has a curved shape. Graphene compounds may have holes in the carbon rings or may have more than six rings, and may also have functional groups. Furthermore, because graphene compounds are soft, they may be rolled up into, for example, carbon nanofibers.

[0381] Since graphene or a graphene compound can be in surface contact with an active material, a smaller amount of graphene or a graphene compound is required than a conventional conductive material. This allows the proportion of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the secondary battery.

[0382] Although carbon fibers are in surface contact with active materials, their long axis is longer than their short axis, allowing for an appropriate electrical path between active materials that are spaced apart. This allows for a smaller amount of carbon fiber than a typical conductive material. This allows for a larger proportion of active material in the active material layer, thereby increasing the discharge capacity of the secondary battery.

[0383] [Electrolyte] Both the positive electrode and the negative electrode contain an electrolyte. The electrolyte described in the first embodiment can be used.

[0384] [Separator] A separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0385] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0386] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0387] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0388] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0389] [Exterior Body] The exterior body of the secondary battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0390] Embodiment 5 In this embodiment, examples of shapes of a secondary battery that can include the positive electrode active material according to one embodiment of the present invention will be described.

[0391] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 14A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 14B is an external view, and Fig. 14C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0392] 14A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding, and therefore, FIGS. 14A and 14B are not completely identical corresponding views.

[0393] In Fig. 14A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. These are sealed with a negative electrode can 302 and a positive electrode can 301 by a gasket. Note that the gasket for sealing is not shown in Fig. 14A. The spacer 322 and the washer 312 are used to protect the inside or to fix the position inside the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 is made of stainless steel or an insulating material. The washer 312 is made of a conductive material.

[0394] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0395] FIG. 14B is a perspective view of the completed coin-type secondary battery.

[0396] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be formed of lithium metal foil or a lithium-aluminum alloy foil.

[0397] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0398] The positive electrode can 301 and the negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0399] These negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte solution, and as shown in FIG. 14C , the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303, thereby producing a coin-type secondary battery 300.

[0400] By applying the configurations described in the above embodiments to the negative electrode, positive electrode, electrolyte solution, and the like, a coin-type secondary battery having an excellent discharge capacity even in a low-temperature environment can be obtained.

[0401] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 15A. As shown in Fig. 15A, a cylindrical secondary battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0402] 15B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 15B has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0403] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal, such as nickel, aluminum, or titanium, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel), which is corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat the battery can 602 with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The electrolyte can be the same as that used in coin-type secondary batteries.

[0404] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector.

[0405] By applying the configurations described in the above embodiments to the negative electrode, positive electrode, electrolyte, etc., a cylindrical secondary battery having an excellent discharge capacity even in a low-temperature environment can be obtained.

[0406] A positive electrode terminal (positive electrode current collecting lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collecting lead) 607 is connected to the negative electrode 606. The positive electrode terminal 603 can be made of a metal material such as aluminum. The negative electrode terminal 607 can be made of a metal material such as copper. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases when the temperature rises, and the increase in resistance limits the amount of current to prevent abnormal heat generation.３ )-based semiconductor ceramics, etc. can be used.

[0407] 15C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. The positive electrodes of the respective secondary batteries are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of the respective secondary batteries are electrically connected to the control circuit 620 via wiring 626. The control circuit 620 may be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0408] 15D shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0409] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0410] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0411] 15D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0412] [Another Example of Secondary Battery Structure] An example of the structure of a secondary battery will be described with reference to FIGS. 16 and 17. FIG.

[0413] The secondary battery 913 shown in FIG. 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. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 due to the use of an insulating material or the like. Note that in FIG. 16A , the housing 930 is shown separated for convenience, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. Considering gas permeability, the housing 930 can be made of a metal material (e.g., aluminum) or a resin material in addition to a metal material. A resin material may be used for the sealing portion of the housing 930.

[0414] 16B, the housing 930 shown in Fig. 16A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 16B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0415] Considering gas permeability, the housing 930a can be made of a metal material (e.g., aluminum) or a metal material plus an organic resin. Considering gas permeability, the housing 930b can be made of a metal material (e.g., aluminum) or a metal material plus an organic resin.

[0416] 16C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0417] Alternatively, a secondary battery 913 may be provided that has a wound body 950a as shown in Fig. 17A. The wound body 950a shown in Fig. 17A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0418] By applying the configurations described in the above embodiments to the negative electrode, positive electrode, electrolyte, and the like, a secondary battery having an excellent discharge capacity even in a low-temperature environment can be obtained.

[0419] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0420] 17B , the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0421] 17C , the wound body 950 a and the electrolyte are covered by the housing 930 to form the secondary battery 913. It is preferable to provide the housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0422] As shown in Fig. 17B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a secondary battery 913 with a larger discharge capacity. For other elements of the secondary battery 913 shown in Figs. 17A and 17B, refer to the descriptions of the secondary battery 913 shown in Figs. 16A to 16C.

[0423] 18A and 18B show examples of external views of a laminated secondary battery, which includes a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0424] 19A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and a negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. Note that the area or shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 19A .

[0425] By applying the configurations described in the above embodiments to the negative electrode, positive electrode, electrolyte, etc., a laminated secondary battery having an excellent discharge capacity even in a low-temperature environment can be obtained.

[0426] <Method of Manufacturing Laminated Secondary Battery> An example of a method of manufacturing the laminated secondary battery whose external view is shown in FIG. 18A will be described with reference to FIGS. 19B and 19C.

[0427] First, the negative electrode 506, separator 507, and positive electrode 503 are stacked. FIG. 19B shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. This can also be called a laminate consisting of a negative electrode, a separator, and a positive electrode. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrodes 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0428] Next, the negative electrode 506 , the separator 507 , and the positive electrode 503 are arranged on the outer casing 509 .

[0429] Next, as shown in Fig. 19C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0430] Next, the electrolyte solution is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0431] Embodiment 6 In this embodiment, an example of a vehicle including a secondary battery of one embodiment of the present invention will be described.

[0432] The secondary battery can be applied to a typical vehicle, such as an automobile. Examples of the automobile include next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHEVs or PHVs). The secondary battery can be used as one of the power sources mounted on the automobile. The vehicle is not limited to an automobile. Examples of the vehicle include trains, monorails, ships, submersibles (deep-sea exploration vessels, unmanned submersibles), aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, rockets, and artificial satellites), electric bicycles, and electric motorcycles. The secondary battery of one embodiment of the present invention can be applied to these vehicles.

[0433] The electric vehicle is equipped with first batteries 1301a and 1301b as main driving secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0434] The internal structure of the first battery 1301a may be a wound type shown in Fig. 16C or 17A or a stacked type shown in Fig. 18A or 18B. The first battery 1301a may use the all-solid-state battery of Embodiment 6. Use of the all-solid-state battery of Embodiment 6 for the first battery 1301a enables a high capacity, improved safety, and reduction in size and weight.

[0435] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple secondary batteries, it is possible to extract large amounts of power. The multiple secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of secondary batteries is also called a battery pack.

[0436] In addition, in a secondary battery for vehicle use, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a in order to cut off power from multiple secondary batteries.

[0437] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0438] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0439] Next, the first battery 1301a will be described with reference to FIG. 20A.

[0440] FIG. 20A shows an example in which nine prismatic secondary batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (such as from the road surface), it is preferable to fix multiple secondary batteries using the fixing portions 1413, 1414, the battery housing box, etc. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0441] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS).

[0442] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, a metal oxide such as In-M-Zn oxide (wherein the element M is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, etc.) may be used as the metal oxide. In particular, the In-M-Zn oxide that can be used as the metal oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In—Ga oxide or an In—Zn oxide may be used as the metal oxide. The CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis oriented in a specific direction. The specific direction is the thickness direction of the CAAC-OS film, the normal direction to the surface on which the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. The crystalline regions are regions in which the atomic arrangement is periodic. When the atomic arrangement is considered as a lattice arrangement, the crystalline regions are also regions in which the lattice arrangement is aligned.

[0443] Note that "CAC-OS" has a mosaic structure in which a material is separated into a first region and a second region, and the first region is distributed throughout the film (hereinafter, also referred to as a cloud structure). That is, CAC-OS is a composite metal oxide having a structure in which the first region and the second region are mixed. However, it may be difficult to observe a clear boundary between the first region and the second region.

[0444] For example, in the case of CAC-OS in an In—Ga—Zn oxide, EDX mapping obtained using energy dispersive X-ray spectroscopy (EDX) can confirm that the CAC-OS has a structure in which a region containing In as a main component (first region) and a region containing Ga as a main component (second region) are unevenly distributed and mixed.

[0445] When a CAC-OS is used in a transistor, the conductivity due to the first region and the insulating property due to the second region act complementarily, thereby providing the CAC-OS with a switching function (a function of turning on / off). That is, a CAC-OS has a conductive function in a part of the material and an insulating function in a part of the material, and functions as a semiconductor as a whole. By separating the conductive function and the insulating function, both functions can be maximized. Therefore, by using a CAC-OS in a transistor, a high on-current (I ｏｎ ), high field-effect mobility (μ), and good switching behavior can be achieved.

[0446] Oxide semiconductors have a variety of structures, each of which has different characteristics. The oxide semiconductor of one embodiment of the present invention may include two or more of an amorphous oxide semiconductor, a polycrystalline oxide semiconductor, an a-like OS, a CAC-OS, an nc-OS, and a CAAC-OS.

[0447] Furthermore, because the control circuit unit 1320 can be used in high-temperature environments, it is preferable to use a transistor using an oxide semiconductor. To simplify the process, the control circuit unit 1320 may be formed using a unipolar transistor. Transistors using an oxide semiconductor in the semiconductor layer have a wider operating ambient temperature range than single-crystal Si, from −40° C. to 150° C., and exhibit smaller characteristic changes than single-crystal Si transistors even when the secondary battery is overheated. The off-current of a transistor using an oxide semiconductor is below the lower limit of measurement regardless of temperature, even at 150° C., whereas the off-current characteristics of single-crystal Si transistors 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 is not sufficiently large. The control circuit unit 1320 can improve safety. Furthermore, combining the positive electrode active material 100 obtained in Embodiments 1 and 2 with a secondary battery using the positive electrode can achieve a synergistic effect in terms of safety. The secondary battery and control circuit unit 1320 using the positive electrode active material 100 obtained in the first or second embodiment or the like for the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0448] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for a secondary battery to address 10 causes of instability, such as micro-short circuits. The functions for addressing the 10 causes of instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of degradation, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for a secondary battery can be miniaturized.

[0449] Furthermore, a "micro-short" refers to a tiny short circuit inside a secondary battery, which is not so small that the positive and negative electrodes of the secondary battery are short-circuited and render it unable to be charged or discharged, but rather refers to a phenomenon in which a small short-circuited part allows a small amount of short-circuit current to flow. Even if the short-circuit occurs in a relatively short period of time and in a small location, a large voltage change occurs, and this abnormal voltage value may affect subsequent estimations.

[0450] One of the causes of micro-short circuits is said to be that multiple charge and discharge cycles cause uneven distribution of the positive electrode active material, resulting in localized current concentration in parts of the positive electrode and negative electrode, causing parts of the separator to stop functioning, or the generation of by-products due to side reactions, resulting in micro-short circuits.

[0451] In addition to detecting micro-shorts, the control circuit 1320 can also be said to detect the terminal voltage of the secondary battery and manage the charge / discharge state of the secondary battery. For example, to prevent overcharging, it can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0452] Next, an example of a block diagram of the battery pack 1415 shown in FIG. 20A is shown in FIG. 20B.

[0453] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the secondary battery used and limits the upper limit of the current from the outside or the upper limit of the output current to the outside. The range between the lower limit and the upper limit of the secondary battery's voltage is within the recommended voltage range, and when the secondary battery falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and / or overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0454] The switch portion 1324 can be configured by combining n-channel transistors or p-channel transistors. The switch portion 1324 is not limited to a switch having a Si transistor using single crystal silicon. For example, the switch portion 1324 may be formed using a power transistor having Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), or GaOx (gallium oxide; x is a real number greater than 0). Furthermore, memory elements using OS transistors can be freely arranged by stacking them on circuits using Si transistors, and therefore integration can be easily achieved. Furthermore, OS transistors can be manufactured using the same manufacturing equipment as Si transistors, and therefore can be manufactured at low cost. That is, the control circuit portion 1320 using OS transistors can be stacked on the switch portion 1324 and integrated into a single chip. The volume occupied by the control circuit section 1320 can be reduced, which allows for miniaturization.

[0455] The first batteries 1301a and 1301b primarily supply power to 42V (high-voltage) in-vehicle devices, while the second battery 1311 supplies power to 14V (low-voltage) in-vehicle devices. Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Lead-acid batteries have the disadvantage of being more self-discharged than lithium-ion secondary batteries and being prone to deterioration due to a phenomenon called sulfation. Using a lithium-ion secondary battery as the second battery 1311 offers the advantage of being maintenance-free, but after prolonged use, e.g., three years or more, there is a risk of abnormalities occurring that are difficult to identify during manufacturing. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b still have remaining capacity, the motor cannot be started. To prevent this, if the second battery 1311 is a lead-acid battery, power is supplied from the first battery to the second battery, and the second battery is constantly charged to maintain a fully charged state.

[0456] In this embodiment, excellent discharge characteristics can be exhibited even in a low-temperature environment when the lithium-ion secondary battery according to one embodiment of the present invention is used for both the first battery 1301a and the second battery 1311. Note that the second battery 1311 may be a lead-acid battery, an all-solid-state battery, or an electric double-layer capacitor.

[0457] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 via the motor controller 1303 or the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0458] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the secondary battery used, and can perform rapid charging.

[0459] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a and 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a and 1301b via the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU includes a microcomputer. The ECU uses a CPU or a GPU.

[0460] External chargers installed at charging stations and the like include 100V-200V outlets, or three-phase 200V and 50kW. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0461] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0462] Furthermore, by using graphene as a conductive material, a secondary battery with significantly improved electrical characteristics can be realized, as a synergistic effect of suppressing capacity decline and maintaining high capacity even when the electrode layer is thickened and the amount of graphene supported is increased. This is particularly effective for secondary batteries used in vehicles, and it is possible to provide a vehicle with a long driving range, specifically a driving distance of 500 km or more per charge, without increasing the ratio of the weight of the secondary battery to the total weight of the vehicle.

[0463] Next, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0464] 15D, 17C, and 20A can be installed in a vehicle to realize next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Furthermore, the secondary battery can also be installed in agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft. The secondary battery of one embodiment of the present invention can be a high-capacity secondary battery. Therefore, the secondary battery of one embodiment of the present invention is suitable for miniaturization and weight reduction and can be suitably used in transportation vehicles.

[0465] 21A to 21D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 illustrated in FIG. 21A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a secondary battery is installed in a vehicle, an example of the secondary battery described in Embodiment 4 is installed in one or more locations. The automobile 2001 illustrated in FIG. 21A includes a battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. When the lithium-ion secondary battery of one embodiment of the present invention is used as the secondary battery, excellent discharge characteristics can be exhibited even in a low-temperature environment. Furthermore, a charge control device electrically connected to the secondary battery module is preferably provided.

[0466] Furthermore, the automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging may be performed using a predetermined charging method or connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The charging device may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge the power storage device installed in the automobile 2001 using external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0467] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0468] 21B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 has, for example, a four-cell unit, each of which has a nominal voltage of 3.0 V to 5.0 V, and 48 cells connected in series to achieve a maximum voltage of 170 V. Apart from the number of secondary batteries constituting the secondary battery module of the battery pack 2201, the secondary battery module has the same functions as those shown in FIG. 21A , and therefore a description thereof will be omitted. When the lithium-ion secondary battery of one embodiment of the present invention is used as the secondary battery in FIG. 21B , excellent discharge characteristics can be exhibited even in a low-temperature environment.

[0469] FIG. 21C illustrates, as an example, a large transport vehicle 2003 having an electrically controlled motor. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, which is obtained by connecting in series one hundred or more secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less. Therefore, secondary batteries with minimal variation in characteristics are required. By using a secondary battery using the positive electrode active material 100 described in Embodiments 1 and 2, etc., as a positive electrode, a secondary battery with stable battery characteristics can be manufactured, enabling mass production at low cost from the viewpoint of yield. Furthermore, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2202, the secondary battery module has the same functions as those shown in FIG. 21A , and therefore further description thereof will be omitted. When the lithium-ion secondary battery of one embodiment of the present invention is used as the secondary battery in FIG. 21C , excellent discharge characteristics can be exhibited even in a low-temperature environment.

[0470] 21D shows an example of an aircraft 2004 having an engine that burns fuel. The aircraft 2004 shown in FIG. 21D has wheels for takeoff and landing, and therefore can be considered part of a transportation vehicle. It has a battery pack 2203 in which a secondary battery module is formed by connecting multiple secondary batteries and includes a secondary battery module and a charge control device. When the lithium-ion secondary battery of one embodiment of the present invention is used as the secondary battery in FIG. 21D , excellent discharge characteristics can be exhibited even in a low-temperature environment.

[0471] The secondary battery module of the aircraft 2004 is, for example, eight 4 V secondary batteries connected in series to produce a maximum voltage of 32 V. Other than the number of secondary batteries constituting the secondary battery module of the battery pack 2203, the secondary battery module has the same functions as those shown in Fig. 21A, and therefore a description thereof will be omitted.

[0472] 21E illustrates an example of a satellite 2005 equipped with a secondary battery 2204. The satellite 2005 is used in space at extremely low temperatures, and therefore preferably includes the secondary battery 2204 of one embodiment of the present invention, which has excellent low-temperature resistance. It is more preferable that the secondary battery 2204 be mounted inside the satellite 2005 while being covered with a heat-insulating member. When the lithium-ion secondary battery of one embodiment of the present invention is used as the secondary battery in FIG. 21E, excellent discharge characteristics can be exhibited even in a low-temperature environment.

[0473] Embodiment 7 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted in a building will be described with reference to FIGS. 22A and 22B.

[0474] The house illustrated in FIG. 22A includes a power storage device 2612 including a secondary battery of one embodiment of the present invention and a solar panel 2610. The power storage device 2612 is electrically connected to the solar panel 2610 via a wiring 2611 or the like. The power storage device 2612 may also be electrically connected to a ground-mounted charging device 2604. The power obtained by the solar panel 2610 can be charged to the power storage device 2612. The power stored in the power storage device 2612 can be charged to a secondary battery included in the vehicle 2603 via the charging device 2604. The power storage device 2612 is preferably installed in an underfloor space. By installing the power storage device 2612 in the underfloor space, the space above the floor can be effectively utilized. Alternatively, the power storage device 2612 may be installed on the floor. When the lithium-ion secondary battery of one embodiment of the present invention is used for the power storage device in FIG. 22A , excellent discharge characteristics can be exhibited even in a low-temperature environment.

[0475] The power stored in the power storage device 2612 can also be supplied to other electronic devices in the house. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the power storage device 2612 of one embodiment of the present invention can be used as an uninterruptible power supply, enabling the use of electronic devices.

[0476] 22B illustrates an example of a power storage device according to one embodiment of the present invention. As illustrated in FIG. 22B , a power storage device 791 according to one embodiment of the present invention is installed in an underfloor space of a building 799. A control circuit may be provided in the power storage device 791, thereby improving safety.

[0477] A control device 790 is installed in the power storage device 791, and the control device 790 is electrically connected to the distribution board 703, the power storage controller 705 (also called the control device), the display 706, and the router 709 by wiring.

[0478] Electric power is sent from commercial power source 701 to distribution board 703 via service line attachment portion 710. Electric power is also sent to distribution board 703 from power storage device 791 and commercial power source 701, and distribution board 703 supplies the sent electric power to general load 707 and power storage load 708 via outlets (not shown).

[0479] The general load 707 is, for example, an electronic device such as a television or a personal computer, and the power storage load 708 is, for example, an electronic device such as a microwave oven, a refrigerator, or an air conditioner.

[0480] The power storage controller 705 includes a measurement unit 711, a prediction unit 712, and a planning unit 713. The measurement unit 711 has a function of measuring the amount of power consumed by the general load 707 and the power storage load 708 during a day (e.g., from midnight to midnight). The measurement unit 711 may also have a function of measuring the amount of power of the power storage device 791 and the amount of power supplied from the commercial power source 701. The prediction unit 712 has a function of predicting the amount of power demand to be consumed by the general load 707 and the power storage load 708 during the next day based on the amount of power consumed by the general load 707 and the power storage load 708 during the previous day. The planning unit 713 has a function of creating a plan for charging and discharging the power storage device 791 based on the amount of power demand predicted by the prediction unit 712.

[0481] The amount of power consumed by the general load 707 and the power storage load 708 measured by the measurement unit 711 can be confirmed on the display 706. It can also be confirmed on an electronic device such as a television or a personal computer via the router 709. It can also be confirmed on a mobile electronic device such as a smartphone or a tablet via the router 709. The amount of power demand for each time period (or each hour) predicted by the prediction unit 712 can also be confirmed on the display 706, the electronic device, or the mobile electronic device.

[0482] Embodiment 8 In this embodiment, an example in which a lithium-ion secondary battery according to one embodiment of the present invention is mounted on a motorcycle or a bicycle will be described as an example in which a secondary battery is mounted on a vehicle.

[0483] 23A illustrates an example of an electric bicycle using a power storage device of one embodiment of the present invention. By applying the lithium-ion secondary battery of one embodiment of the present invention to the electric bicycle 8700 illustrated in FIG. 23A, the electric bicycle 8700 has excellent discharge characteristics even in a low-temperature environment. The power storage device of one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

[0484] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state detached from the bicycle in FIG. 23B . The power storage device 8702 has excellent discharge characteristics even in a low-temperature environment by using the lithium-ion secondary battery of one embodiment of the present invention. Furthermore, the remaining battery charge of the power storage device 8702 can be displayed on a display unit 8703. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery. The control circuit 8704 is electrically connected to the positive and negative electrodes of the storage battery 8701.

[0485] 23C illustrates an example of a two-wheeled vehicle using a power storage device of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 23C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. By using the lithium-ion secondary battery of one embodiment of the present invention for the power storage device 8602, the power storage device 8602 has excellent discharge characteristics even in a low-temperature environment.

[0486] 23C , a power storage device 8602 can be stored in an under-seat storage 8604. Even if the under-seat storage 8604 is small, the power storage device 8602 can be stored in the under-seat storage 8604. By using the lithium-ion secondary battery of one embodiment of the present invention for the power storage device 8602, the power storage device 8602 has excellent discharge characteristics even in a low-temperature environment.

[0487] Embodiment 9 In this embodiment, an example of mounting a secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0488] 24A illustrates an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 includes a secondary battery 2107. By using the lithium-ion secondary battery of one embodiment of the present invention as the secondary battery 2107, the mobile phone has excellent discharge characteristics even in a low-temperature environment.

[0489] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0490] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0491] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0492] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0493] Furthermore, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a body temperature sensor or other human body sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0494] 24B illustrates an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is also called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 of one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. By using the lithium-ion secondary battery of one embodiment of the present invention as the secondary battery mounted in the unmanned aerial vehicle 2300, the unmanned aerial vehicle 2300 has excellent discharge characteristics even in a low-temperature environment.

[0495] Fig. 24C shows an example of a robot. A robot 6400 shown in Fig. 24C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0496] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0497] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0498] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0499] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in an internal region thereof. By using the lithium-ion secondary battery according to one embodiment of the present invention as the secondary battery 6409 mounted on the robot 6400, the robot 6400 has excellent discharge characteristics even in a low-temperature environment.

[0500] 24D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, operation buttons 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0501] The cleaning robot 6300 can analyze an image captured by the camera 6303 and determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected by image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. By using the lithium-ion secondary battery of one embodiment of the present invention as the secondary battery 6306 included in the cleaning robot 6300, the cleaning robot 6300 has excellent discharge characteristics even in a low-temperature environment. Furthermore, the cleaning robot 6300 includes a semiconductor device or an electronic component.

[0502] 25A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0503] For example, a secondary battery according to one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 25A , and the device has excellent discharge characteristics even in a low-temperature environment. The eyeglasses-type device 4000 includes a frame 4000 a and a display unit 4000 b. By mounting a secondary battery on temples of the curved frame 4000 a, the eyeglasses-type device 4000 can be lightweight, well-balanced in weight, and has a long continuous use time. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2 as a positive electrode has a high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0504] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b or the earphone unit 4001c. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2 as a positive electrode has high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0505] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for a positive electrode has high energy density, and a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0506] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for its positive electrode has high energy density, and a structure that can accommodate space saving due to miniaturization of the housing can be realized.

[0507] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted in an inner region of the belt portion 4006a. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for its positive electrode has high energy density and can realize a configuration that can accommodate space saving associated with miniaturization of the housing.

[0508] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided in the display portion 4005a or the belt portion 4005b. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1, 2, and the like for its positive electrode has high energy density, and a structure that can accommodate space saving due to miniaturization of the housing can be realized.

[0509] The display unit 4005a can display not only the time but also various other information such as incoming emails or phone calls.

[0510] Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0511] FIG. 25B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0512] 25C shows a side view of the battery 913. The lithium-ion secondary battery 913, which is one embodiment of the present invention, can be used as the secondary battery 913, resulting in excellent discharge characteristics even in a low-temperature environment.

[0513] In this example, a test battery including a positive electrode active material, a negative electrode active material, and the like according to one embodiment of the present invention was fabricated, and a charge-discharge cycle test was performed.

[0514] <Preparation of Positive Electrode Active Material> The steps for preparing the positive electrode active material used in this example will be described with reference to the preparation method shown in FIGS.

[0515] LiCoO in step S14 of FIG. ２ Commercially available lithium cobalt oxide (CellSeed C-5H, manufactured by Nippon Chemical Industry Co., Ltd.) containing cobalt as the transition metal M and no additional elements was prepared and sieved using an automatic sieving machine. The particle size distribution of CellSeed C-5H was measured using a laser diffraction particle size distribution analyzer SALD-2200, revealing a median diameter (D50) of 7.0 μm. For the initial heating step S15, the lithium cobalt oxide was placed in a sheath, covered with a lid, and heated at 850°C for 2 hours using a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.). Air (compressed air, thoroughly dried) was flowed through the furnace at a rate of 10 L / min. The flow rate, specifically the opening width of the exhaust port, was adjusted so that the furnace differential pressure gauge read 5 Pa, creating a positive pressure inside the furnace. After the initial heating, the inside of the furnace was cooled at a rate of 200°C / hour, and the air flow was not stopped until the temperature reached 200°C.

[0516] In this example, Mg and F were added separately as additive elements according to step S20a shown in Fig. 10A. First, according to step S21 shown in Fig. 10A, LiF was prepared as the F source, and MgF was prepared as the Mg source. ２ LiF:MgF was prepared. ２ The mixture was weighed out so that the molar ratio was 1:3, and mixed in dehydrated acetone at a rotation speed of 500 rpm for 20 hours. The mixture was sieved through a sieve with 300 μm openings to prepare an additive element source (A1 source) with a uniform particle size.

[0517] Next, in step S31 shown in Fig. 9, magnesium in the Al source was weighed out so that it accounted for 1 mol % of cobalt in the lithium cobalt oxide. The Al source and the initially heated lithium cobalt oxide were stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron Co., Ltd.), yielding a mixture 903 (step S32). A Nobilta rotor was used for the Picobond. Prior to the next step S33, the mixture 903 was sieved using an automatic sieving machine.

[0518] Next, in step S33, the mixture 903 was heated. The heating conditions were 850°C and 10 hours. During heating, the mixture 903 was placed in a sagger and a lid was placed on it. The sagger was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed in the furnace at a rate of 10 L / min (O ２ Flow). The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a positive pressure inside the furnace. After the initial 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, a composite oxide containing Mg and F was obtained (step S34a).

[0519] Next, in step S40, a composite oxide and an additive element source (A2 source) were prepared. First, according to step S41 shown in FIG. 10C, nickel hydroxide that had undergone a pulverization process was prepared as the nickel source, and aluminum hydroxide that had undergone a pulverization process was prepared as the aluminum source, and these were used as the additive element source (A2 source). In the pulverization process, nickel hydroxide and aluminum hydroxide were mixed in dehydrated acetone at a rotation speed of 500 rpm for 20 hours. Then, the mixture was sieved through a sieve with 300 μm openings.

[0520] As the A2 source, nickel hydroxide was weighed so that the nickel content of the nickel hydroxide was 0.5 mol % of the cobalt content, and aluminum hydroxide was weighed so that the aluminum content of the aluminum hydroxide was 0.5 mol % of the cobalt content. These and a composite oxide containing Mg and F were stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron) to obtain a mixture 904 (step S52). A Nobilta was used as the rotor for the Picobond. Before the next step S53, the mixture 904 was sieved using an automatic sieving machine.

[0521] Next, in step S53, the mixture 904 was heated. The heating conditions were 850°C and 2 hours. During heating, the mixture 904 was placed in a sagger and a lid was placed on it. The sagger was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Company Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed in the furnace at 10 L / min (O ２The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a 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.

[0522] In this way, lithium cobalt oxide containing Mg, F, Ni, and Al was obtained (step S54). The lithium cobalt oxide thus obtained was used as a sample positive electrode active material. The median diameter (D50) of the positive electrode active material was 1 μm or more and 12 μm or less.

[0523] <Preparation of Positive Electrode> The above-mentioned lithium cobalt oxide was prepared as the positive electrode active material, acetylene black (AB) was prepared as the conductive material, and polyvinylidene fluoride (PVDF) was prepared as the binder. PVDF was prepared by dissolving it in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After the slurry was applied to the positive electrode current collector, the solvent was evaporated.

[0524] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process was performed using a roll press machine. The pressing process was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press machine were both set at 120°C.

[0525] By the above steps, the positive electrode samples were obtained. The amount of the positive electrode active material supported was 10 mg / cm. ２ 11mg / cm or more ２ The values ​​were adjusted to the following ranges:

[0526] <Fabrication of Negative Electrode> The negative electrode used in this example will be described with reference to the fabrication method shown in FIG.

[0527] Graphite particles having an average particle size of 20 μm (Formula BT1520T, manufactured by Superior Graphite) were prepared in accordance with Fig. 13. The graphite was obtained by spheroidizing natural graphite and then coating it with low-crystalline carbon.

[0528] According to FIG. 13, the silicon particles of this example have a specific surface area of ​​12.7715 m2 measured by the BET method. ２ Silicon particles (manufactured by Aldrich: product number 633097) having an average particle size of 100 nm and a molecular weight of 1000 kJ / g were prepared. The 100 nm silicon particles are called nanosilicon particles.

[0529] 13, polyglutamic acid (manufactured by Nippon Polyglu Co., Ltd., referred to as PGA) or polyacrylic acid (20CLPAH manufactured by Fujifilm Wako Chemical Co., Ltd., referred to as PAA) was used as the binder in this example. In this example, to vary the binder conditions, the full cell using PGA is called Sample 1, and the full cell using PAA (crosslinking degree 20CL) is called Sample 2.

[0530] According to FIG. 13, AB was prepared as the conductive material in this example.

[0531] 13, a mixture 204 was prepared by mixing graphite particles, silicon particles, AB, and PGA in a weight ratio of 72:8:6:14 as sample 1. A mixture 204 was also prepared by mixing graphite particles, silicon particles, AB, and PAA in a weight ratio of 72:8:6:14 as sample 2. It is recommended to mix in powder form in step S60. The amount of negative electrode active material supported was 3.8 mg / cm. ２ 4.2mg / cm or more ２ The values ​​were adjusted to the following ranges:

[0532] According to FIG. 13, deionized water was prepared as the solvent.

[0533] 13, deionized water was added to the mixture 204 corresponding to sample 1 and mixed to obtain a slurry 206. Deionized water was also added to the mixture 204 corresponding to sample 2 and mixed to obtain a slurry 206.

[0534] Comparative Example: A negative electrode used as a comparative example of this example used graphite particles as the negative electrode active material, carbon fiber (VGCF (registered trademark) manufactured by Showa Denko K.K.) as the conductive material, and CMC and SBR as the binder. The mixture ratio was graphite particles:VGCF:CMC:SBR = 97:1:1:1 (weight ratio). The conditions for the negative electrodes of Sample 1, Sample 2, and the comparative example are shown in the table below.

[0535]

[0536] According to FIG. 13, copper foil was prepared as a negative electrode current collector, and the above slurry was applied to the copper foil according to step S62.

[0537] The negative electrode 208 was obtained by drying according to step S63 in FIG. 13 and pressing according to step S64. The drying was performed in two stages: first drying (pre-drying) and second drying (main drying). The pre-drying was performed by clamping the electrode between a hot plate heated to 50°C for 30 minutes. The main drying was then performed in a ventilated dryer at 80°C for 45 minutes. A roll press was used for pressing, and the comparative example was pressed at a linear pressure of 0.5 MPa using a roll heated to 120°C. However, Samples 1 and 2 were not pressed.

[0538] <SEM Observation of Samples 1 and 2> After drying in accordance with step S63, Sample 1 was observed under an SEM. The S4800 manufactured by Hitachi High-Technologies Corporation was used as the SEM. The acceleration voltage was set to 5 kV, and images were acquired at 2000x magnification. FIG. 26A shows an SEM image of the top surface of Sample 1, and FIG. 26B shows a schematic diagram of the SEM image. In FIG. 26B, graphite particles 200 are shown surrounded by a solid line. FIG. 33 shows an SEM image of the top surface of Sample 2.

[0539] 26A and 26B , when viewed from above, a portion of graphite particle 200 is exposed from binder 202 containing polyglutamic acid. In other words, graphite particle 200 having an area not covered with binder 202a containing polyglutamic acid is considered to have an area where lithium can be inserted and extracted. Furthermore, binder 202a containing polyglutamic acid is located between graphite particles 200, and this polyglutamic acid enables the graphite particles to bond to each other.

[0540] Furthermore, silicon particles 201 can be seen in the region where the binder 202a containing polyglutamic acid is located, and it is also possible to confirm that the silicon particles 201 are aggregated. Furthermore, AB 203a, which is a conductive material, can be seen in the region where the binder 202a containing polyglutamic acid is located, and it is also possible to confirm that the AB 203a is aggregated.

[0541] There is a concern that silicon particles may slide off due to expansion and contraction during charge and discharge, but in Sample 1, the silicon particles 201 are wrapped in the binder 202a containing polyglutamic acid, which is thought to prevent sliding off even after repeated charge and discharge cycles. Furthermore, the binder 202a containing polyglutamic acid wraps the silicon particles 201 so that the AB 203a is located near the silicon particles 201, which is thought to make it easier to ensure a conductive path.

[0542] In this way, the negative electrode preferably has regions where the graphite particles 200 are not covered with a binder and regions where the silicon particles 201 are aggregated and covered with a binder. Furthermore, in the negative electrode, a conductive material is preferably located near the silicon particles 201, and the conductive material is preferably covered with a binder. Note that if the conductive material is in the form of carbon fiber, it does not need to be covered with a binder.

[0543] <Electrolyte> Next, an electrolyte was prepared. The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF ) in a mixed solvent containing FEC (fluoroethylene carbonate) and MTFP (methyl 3,3,3-trifluoropropionate) in a volume ratio of FEC:MTFP=20:80 at a concentration of 1 mol / L. ６ An electrolyte solution was prepared by dissolving the above-mentioned ammonium hydroxide in the electrolyte solution. No additives were used.

[0544] <Separator> Next, a separator was prepared. A porous polypropylene film (PP) was used as the separator.

[0545] <Assembly of Test Battery> The positive electrode, negative electrode, and separator prepared above were placed in a battery can, and the electrolyte prepared above was added dropwise to form a test battery. The test battery of this example may be referred to as a full cell. Because the assembled test batteries differed in the conditions, such as the binder of the negative electrode, as described above, the test battery containing Sample 1 will be referred to as Full Cell 1, the test battery containing Sample 2 will be referred to as Full Cell 2, and the test battery containing the comparative example will be referred to as a comparative cell.

[0546] <Charge-Discharge Cycle Test 1> Next, a charge-discharge cycle test was conducted on Full Cell 1, Full Cell 2, and the comparative cell. First, an initial charge-discharge test was performed at 25°C, followed by a charge-discharge cycle test at -40°C, and then the temperature was returned to 25°C to continue the charge-discharge cycle test. The 25°C and -40°C temperatures refer to the temperatures of the thermostatic chamber in which each test battery was placed. A charge-discharge cycle test is a test in which charging and discharging are performed at a predetermined temperature, and repeated charging and discharging are sometimes referred to as a cycle. The initial charge-discharge test is sometimes referred to as aging or conditioning, and corresponds to the first cycle for the test battery. The measurement conditions for the charge-discharge cycle test in this example are summarized in the table below, with different rate conditions used at each temperature.

[0547]

[0548] Here, the rate of the charge-discharge cycle test conditions will be explained. The rate during discharge is called the discharge rate, and this discharge rate is the relative ratio of the current during discharge to the battery capacity, and is expressed in units of C. For a battery with a rated capacity of X (Ah), the current equivalent to 1 C is X (A). When discharged at a current of 2X (A), it is said to have been discharged at 2C, and when discharged at a current of X / 2 (A), it is said to have been discharged at 0.5C. The rate during charging is called the charge rate. Similarly to the charge rate, when charged at a current of 2X (A), it is said to have been charged at 2C, and when charged at a current of X / 2 (A), it is said to have been charged at 0.5C. The charge rate and discharge rate are sometimes collectively referred to as the C rate. In the charge-discharge cycle test conditions of this example, 1C = 200 mA / g (per weight of positive electrode active material).

[0549] The conditions for the charge-discharge cycle test described above, other than the C rate, were as follows: during charging, the test battery was subjected to constant current charging (CC charging) until the upper limit voltage reached 4.5 V, and then constant voltage charging (CV charging) until the C rate reached 1 / 10 (except 1 / 5 at −40° C.). During discharging, the battery was subjected to constant current discharging (CC discharging) until the lower limit voltage reached 2.5 V. A rest period of 5 to 15 minutes may be provided between charging and discharging.

[0550] 27A and 27B show the discharge capacity as a result of the charge-discharge cycle test. Fig. 27A shows the discharge capacity per weight of lithium cobalt oxide, which is the positive electrode active material, and Fig. 27B shows the discharge capacity per total weight of graphite particles and silicon particles, which is the mass of the negative electrode active material. Therefore, no comparative examples are shown in Fig. 27B.

[0551] The table below shows the discharge capacity X at -40°C or below (discharge capacity at the 14th cycle in Fig. 27A), the discharge capacity Y at 25°C (discharge capacity at the 15th cycle in Fig. 27A), and the value corresponding to discharge capacity X / discharge capacity Y x 100 for each test battery shown in Fig. 27A. For both full cell 1 and full cell 2, the discharge capacity X / discharge capacity Y x 100 was 50% or more, preferably 60% or more, and it was found that these batteries exhibited better discharge characteristics in low-temperature environments than the comparative cell.

[0552]

[0553] The table below shows the discharge capacity X at −40° C. or below (discharge capacity at the 14th cycle in FIG. 27B ), the discharge capacity Y at 25° C. (discharge capacity at the 15th cycle in FIG. 27B ), and the value corresponding to discharge capacity X / discharge capacity Y × 100 for each test battery shown in FIG. 27B . For both full cell 1 and full cell 2, the discharge capacity X / discharge capacity Y × 100 was 50% or more, preferably 60% or more, demonstrating excellent discharge characteristics in a low-temperature environment.

[0554]

[0555] FIG. 27A and Table 4 show that Full Cell 1 and Full Cell 2 have better cycle characteristics and higher discharge capacity at low temperatures than the comparative cell. FIG. 27B and Table 5 show that Full Cell 1 and Full Cell 2 have better cycle characteristics and higher discharge capacity at low temperatures. That is, at low temperatures such as -40°C, it is preferable to use a negative electrode active material containing a mixture of graphite particles and silicon particles, and it is also preferable to apply a binder containing PGA or PAA. In addition to the above, this example also suggests that a binder containing PAA has better low-temperature characteristics than a binder containing PGA.

[0556] For charging and discharging at low temperatures, it is preferable to use lithium cobalt oxide containing at least Mg as the positive electrode active material, and for the electrolyte at low temperatures, it is preferable to use a mixed solvent of a fluorinated cyclic carbonate and a fluorinated chain carbonate.

[0557] <Comparison of charge-discharge characteristics due to differences in separator and PAA crosslinking degree> Next, test batteries were prepared with different separators for full cells in which the PAA (crosslinking degree 20 CL) of Full Cell 2 was changed to PAA (crosslinking degree 10 CL), and a charge-discharge cycle test was performed. The battery using a PP separator is designated Full Cell 2_PP, the battery using glass fiber (GFC) as the separator is designated Full Cell 2_G, the battery using a single polyimide layer as the separator is designated Full Cell 2_PI1, the battery using two polyimide layers as the separator is designated Full Cell 2_PI2, and the battery using three polyimide layers as the separator is designated Full Cell 2_PI3. The conditions for the above-mentioned full cells are shown in the table below.

[0558]

[0559] <Charge-Discharge Cycle Test 2> A charge-discharge cycle test was performed on Full Cell 2_PP, Full Cell 2_G, Full Cell 2_PI1, Full Cell 2_PI2, and Full Cell 2_PI3. The conditions for the charge-discharge cycle test, including the rate conditions, were all the same as those for the above-mentioned Charge-Discharge Cycle Test 1.

[0560] The discharge capacity as a result of the charge-discharge cycle test is shown in Figure 32. Figure 32 shows the discharge capacity per weight of lithium cobalt oxide, which is the positive electrode active material.

[0561] The table below shows the discharge capacity X (discharge capacity at the 14th cycle in FIG. 32) at −40° C. or below, the discharge capacity Y (discharge capacity at the 15th cycle in FIG. 32) at 25° C., and the value corresponding to discharge capacity X / discharge capacity Y×100 for each test battery shown in FIG. 32. With the exception of Full Cell 2_G, all of the batteries had a discharge capacity X / discharge capacity Y×100 of 50% or more, preferably 60% or more, demonstrating superior discharge characteristics in low-temperature environments compared to the comparative cells. It was also found that using polyimide or polypropylene for the separator is preferable in low-temperature environments.

[0562]

[0563] In this example, XPS analysis, XRD analysis in a high-voltage charged state, and STEM-EDX analysis were performed on lithium cobalt oxide containing Mg, F, Ni, and Al.

[0564] <Method of Producing LCO 1> In this example, it will be described that lithium cobalt oxide 1 having a median diameter (D50) of 12 μm or less can be obtained based on the description in Embodiment 1 and FIGS.

[0565] The starting material lithium cobalt oxide (LiCoO ２ As the lithium cobalt oxide (CellSeed C-5H, manufactured by Nippon Chemical Industry Co., Ltd.), which does not contain any additional elements, was prepared. Hereinafter, in the present specification, this will be referred to simply as "C-5H." C-5H has a median diameter (D50) of approximately 7.0 μm, and satisfies the condition that the median diameter (D50) is 10 μm or less.

[0566] Next, in the heating step S15, C-5H was placed in a sheath (container), and after the sheath was covered, it was heated in a muffle furnace at 850°C for 2 hours. After the inside of the muffle furnace was made into an oxygen atmosphere, no flow occurred (O ２ (Purge) When C-5H was placed in the sheath, the powder height (also called bulk height) was adjusted to 10 mm or less and flat inside the sheath.

[0567] Next, according to step S20a shown in FIG. 10A, a source of the additional element A1 was prepared. First, lithium fluoride (LiF) was prepared as the F source, and magnesium fluoride (MgF ２ ) were prepared. LiF and MgF ２ The ratio of LiF:MgF ２ were weighed out so that the molar ratio was 1:3. Next, LiF and MgF were dissolved in dehydrated acetone. ２ The mixture was mixed and stirred at a rotation speed of 500 rpm for 20 hours. A ball mill was used for mixing, and zirconium oxide balls were used as grinding media. 20 mL of dehydrated acetone, 22 g of zirconium oxide balls (1 mm diameter), and a total of approximately 9 g of the additive element A1 source were added to a 45 mL capacity container of the mixing ball mill and mixed. The mixture was then sieved through a sieve with 300 μm openings to obtain the additive element A1.

[0568] Next, according to step S31 shown in FIG. 9 , the lithium cobalt oxide obtained by heating in step S15 (lithium cobalt oxide after initial heating) was mixed with the additive element A1 source obtained in step S20a. Specifically, magnesium atoms were weighed out so that the number of magnesium atoms was 1 atomic % relative to the number of cobalt atoms in the lithium cobalt oxide, and then the lithium cobalt oxide after initial heating and the additive element A1 source were dry mixed. This was done by stirring at a rotation speed of 150 rpm for 1 hour. The mixture was then sieved through a sieve with 300 μm openings to obtain a mixture 903 (step S32).

[0569] Next, in step S33, the mixture 903 was heated. The heating conditions were 900°C and 5 hours. During heating, a lid was placed on the scabbard containing the mixture 903. An oxygen-containing atmosphere was created inside the scabbard, and the inflow and outflow of oxygen was blocked (purging). By heating, a composite oxide containing Mg and F (lithium cobalt oxide containing Mg and F) was obtained (step S34a).

[0570] Next, according to step S40 shown in FIG. 10C, a source of the additional element A2 was prepared. First, nickel hydroxide (Ni(OH) ２ ) was prepared, and aluminum hydroxide (Al(OH) ３) was prepared. Next, nickel hydroxide and aluminum hydroxide were separately stirred in dehydrated acetone at a rotation speed of 500 rpm for 20 hours. A ball mill was used for mixing, and zirconium oxide balls were used as the grinding media. Approximately 10 g of nickel hydroxide and aluminum hydroxide were placed in separate containers, along with 20 mL of dehydrated acetone and 22 g of zirconium oxide balls (1 mm diameter), for a mixing ball mill container capacity of 45 mL, and stirred. Each was then sieved through a sieve with 300 μm openings to obtain a source of the additional element A2.

[0571] Next, in step S51, the composite oxide containing Mg and F and the additive element A2 source were dry-mixed. Specifically, the mixture was stirred at a rotation speed of 150 rpm for 1 hour. The mixing ratio was such that the nickel hydroxide and aluminum hydroxide contained in the additive element A2 source accounted for 0.5 atomic % of each of the cobalt atoms contained in the lithium cobalt oxide. A ball mill was used for mixing, and zirconium oxide balls were used as the grinding medium. Approximately 7.5 g of the Ni source, Al source, and the composite oxide (lithium cobalt oxide containing Mg and F) obtained in step S34 were mixed with 22 g of zirconium oxide balls (1 mm diameter) in a 45 mL capacity mixing ball mill. Finally, the mixture was sieved through a 300 μm mesh sieve to obtain a mixture 904 (step S52).

[0572] Next, in step S53, the mixture 904 was heated. The heating conditions were 850°C and 2 hours. During heating, a lid was placed on the sheath containing the mixture 904, and the mixture was heated in a muffle furnace. After the muffle furnace was filled with an oxygen atmosphere, no flow occurred (O ２ (Purge). By heating, lithium cobalt oxide (composite oxide) containing Mg, F, Ni, and Al was obtained (step S54). In this way, lithium cobalt oxide 1 (referred to as LCO1) was obtained.

[0573] <Method of Producing LCO2> Lithium cobalt oxide 2 (referred to as LCO2) was produced under conditions different from those for LCO1. In the method of producing LCO2, in step S33, the heating conditions for heating mixture 903 were 900°C and 20 hours, and in step S53, the heating conditions for heating mixture 904 were 850°C and 10 hours. Except for the heating temperature, LCO2 was produced in the same manner as LCO1. In this manner, LCO2 was obtained.

[0574] <XRD Analysis of High-Voltage Charged State> Experiments were carried out to investigate the crystal structure of LCO1 in a high-voltage charged state.

[0575] First, a half-cell containing LCO1 was assembled. LCO1 was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. The PVDF was dissolved in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry.

[0576] Next, the slurry was applied to a positive electrode current collector, and the solvent was then evaporated to form a positive electrode active material layer on the positive electrode current collector.

[0577] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process was performed using a roll press machine. The pressing process was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press machine were both set at 120°C.

[0578] The electrolyte used in the half cell was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, to which 2 wt% vinylene carbonate (VC) was added as an additive. The electrolyte (lithium salt) contained in the electrolyte was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) was used.

[0579] The separator was a porous polypropylene film, and the negative electrode (counter electrode) was lithium metal.

[0580] <Charge-Discharge Test of Half Cell> The half cell was charged and discharged. Charging was performed by constant current charging at 0.2 C up to 4.50 V, followed by constant voltage charging until the current value reached 0.05 C. Furthermore, discharging was performed by constant current discharging at 0.2 C down to 3.0 V. Note that, as in Example 1, 1 C was set to 200 mA / g (per weight of positive electrode active material).

[0581] Next, charging was performed to a high voltage charging state by charging at a constant current of 0.2 C up to 4.60 V, and then by constant voltage charging until the current value reached 0.02 C.

[0582] The half-cell was disassembled within one hour after the above charging was completed. During disassembly, the positive electrode containing LCO1 was removed while still in a high-voltage charged state, so the disassembly was carried out carefully using insulating tools to avoid short-circuiting. The disassembly was carried out in a glove box filled with argon, the dew point and oxygen concentration of which were controlled. The dew point of the glove box was preferably −70° C. or lower, and the oxygen concentration was preferably 5 ppm or lower. Furthermore, since the crystalline structure of the positive electrode active material may change due to self-discharge if a long period of time has passed since the above charging, it is preferable to disassemble and analyze the cell as soon as possible. Disassembly is preferably carried out within 15 minutes, more preferably within 5 minutes, of the high-voltage charged state.

[0583] The LCO1 obtained by disassembling the half cell was set on a sealable stage for XRD measurement within the glove box, thereby obtaining LCO1 sealed with argon on the stage for XRD measurement.

[0584] Within 15 minutes, XRD measurement was started. The XRD device and conditions were as follows: XRD device: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 75° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0585] 28A to 28C show the XRD measurement data of LCO1 in the high-voltage charged state measured above. In FIG. 28A to 28C, the reference profile of the O3′ structure (O3′), the reference profile of the H1-3 structure (H1-3), and the CoO ２ Reference profile (CoO ２ ) are also shown.

[0586] Fig. 28A shows the range of 2θ in XRD measurement from 15° to 75° C. Fig. 28B and Fig. 28C are enlarged views of Fig. 28A with the vertical axis of the measurement data of LCO1 partially enlarged.

[0587] 28A to 28C, Sample 1 in a high-voltage charged state of 4.6 V has a diffraction peak at 2θ=19.30° within the range of 2θ=19.25±0.12° (19.13° to 19.37°), and a diffraction peak at 2θ=45.52° within the range of 2θ=45.47±0.10° (45.37° to 45.57°). In other words, it was confirmed that Sample 1 has an O3′ structure.

[0588] <XPS Analysis> Next, XPS analysis was carried out on LCO1 and LCO2.

[0589] The XPS measurement conditions are as follows: Measurement device: Quantera II manufactured by PHI Corporation X-ray source: Monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: Approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0590] The XPS analysis results were analyzed, and the XPS analysis results shown in Table 8 were obtained. In Table 8, the number of atoms of each element is shown as a percentage, with the total number of atoms of Li, Co, Ni, Al, O, Mg, F, C, Ca, Na, S, Cl, and Ti in each sample being taken as 100%. Note that, because the values ​​after analysis were rounded off for table presentation, the total amount shown in Table 8 may be 100.1% or 99.9%, but for the purposes of this XPS analysis, the total number of atoms was calculated as 100.0%.

[0591]

[0592] In LCO1 and LCO2, large amounts of Mg and Ni were detected, and small amounts of Li and Co were detected. This result is thought to suggest that the surface layer 100a described in the second embodiment is formed in LCO1 and LCO2.

[0593] Comparing LCO1 and LCO2, in LCO1, Ni, Mg and F were detected in large amounts, while Li and Co were detected in small amounts.

[0594] The difference between the preparation conditions for LCO1 and LCO2 is that the heating time after mixing the A1 source and the heating time after mixing the A2 source are both longer for LCO2. Looking at the F in Table 8, the number of F atoms in LCO1 is significantly greater than that in LCO2. In other words, the number of F atoms detected on the surface of LCO2 is significantly lower than that on the surface of LCO1. This is thought to be due to Ni and Mg diffusing from the surface layer to the interior of the positive electrode active material.

[0595] Based on the results of the XPS analysis shown in Table 8, the number of Ni atoms relative to the number of Co atoms (Ni / Co), the number of Mg atoms relative to the number of Co atoms (Mg / Co), and the number of F atoms relative to the number of Co atoms (F / Co) were calculated, and the results shown in Table 9 were obtained.

[0596]

[0597] In LCO1, the ratio of Ni atoms to Co atoms (Ni / Co) was 0.099, the ratio of Mg atoms to Co atoms (Mg / Co) was 1.092, and the ratio of F atoms to Co atoms (F / Co) was 0.794. In LCO2, the ratio of Ni atoms to Co atoms (Ni / Co) was 0.048, the ratio of Mg atoms to Co atoms (Mg / Co) was 0.396, and the ratio of F atoms to Co atoms (F / Co) was 0.021.

[0598] That is, in XPS analysis, the ratio of Ni atoms to Co atoms (Ni / Co) of LCO1 was 0.090 or more, the ratio of Mg atoms to Co atoms (Mg / Co) was 1.000 or more, and the ratio of F atoms to Co atoms (F / Co) was 0.700 or more. Note that when LCO1 contains excessive amounts of Ni, Mg, and F, for example, amounts exceeding approximately twice the amounts detected in the above-mentioned LCO1, it is believed that the charge / discharge capacity of the positive electrode active material decreases.

[0599] From the above results, it can be said that in XPS analysis of lithium cobalt oxide having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less), the ratio of Ni atoms to Co atoms (Ni / Co) is preferably 0.05 or more, more preferably 0.06 or more, more preferably 0.07 or more, even more preferably 0.08 or more, and even more preferably 0.09 or more. It can also be said that Ni / Co is preferably 0.200 or less, preferably 0.150 or less, preferably 0.140 or less, preferably 0.130 or less, preferably 0.120 or less, or preferably 0.110 or less.

[0600] In addition, in XPS analysis of lithium cobalt oxide having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less), the ratio of Mg atoms to Co atoms (Mg / Co) is preferably 0.400 or more, more preferably 0.500 or more, more preferably 0.600 or more, more preferably 0.700 or more, more preferably 0.800 or more, more preferably 0.900 or more, and more preferably 1.000 or more. Furthermore, it can be said that Mg / Co is preferably 2.000 or less, preferably 1.500 or less, preferably 1.400 or less, preferably 1.300 or less, or preferably 1.200 or less.

[0601] In addition, in XPS analysis of lithium cobalt oxide having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less), the ratio of F atoms to Co atoms (F / Co) is preferably 0.100 or more, more preferably 0.200 or more, more preferably 0.300 or more, more preferably 0.400 or more, more preferably 0.500 or more, more preferably 0.600 or more, and more preferably 0.700 or more. Furthermore, F / Co is preferably 1.500 or less, preferably 1.200 or less, preferably 1.100 or less, preferably 1.000 or less, and preferably 0.900 or less.

[0602] Due to the above-mentioned characteristics, LCO1 is capable of high-voltage charging and is thought to have excellent charge / discharge characteristics in a minus 40°C environment.

[0603] <STEM-EDX Analysis> Next, LCO1 was subjected to line analysis by STEM-EDX.

[0604] As a pretreatment before analysis, LCO1 was sliced ​​into thin sections by the FIB method (μ-sampling method).

[0605] The STEM and EDX were performed using the following equipment and conditions.

[0606] <STEM observation> Scanning transmission electron microscope: Hitachi High-Tech HD-2700 Observation conditions Acceleration voltage: 200 kV Magnification accuracy: ±3%

[0607] EDX Analysis method: Energy dispersive X-ray spectroscopy (EDX) Scanning transmission electron microscope: Hitachi High-Tech HD-2700 Acceleration voltage: 200 kV Beam diameter: Approximately 0.2 nmφ Elemental analyzer: Two Octane T Ultra W instruments installed (also known as a two-barreled analyzer) X-ray detector: Si drift detector Energy resolution: Approximately 130 eV X-ray take-off angle: 25° Solid angle: 2 sr Number of captured pixels: 512 x 400

[0608] 29A, 30A, 30B, and 30C show profiles (counts) of STEM-EDX ray analysis in the basal region of LCO1. Also, FIGS. 29B, 31A, 31B, and 31C show profiles (counts) of STEM-EDX ray analysis in the edge region of LCO2. The data at each measurement point in the profiles shown in FIGS. 29A to 31C was subjected to a smoothing process to obtain the average value of five points, including the four adjacent points. Since the measurement points are spaced approximately 0.2 nm apart, the above five-point average can also be considered as the average value of a region of approximately 0.8 nm.

[0609] 30A, 30B, and 30C are graphs in which the vertical axis of FIG. 29A is enlarged, with FIG. 30A showing the cobalt and magnesium profiles (counts), FIG. 30B showing the cobalt and aluminum profiles (counts), and FIG. 30C showing the cobalt and nickel profiles (counts). In the energy spectrum in the basal region of LCO1, no peaks derived from the characteristic X-rays of nickel were observed. In other words, it can be said that Sample 1 is substantially free of nickel in the basal region. Therefore, the nickel profile shown in FIG. 30C is thought to be derived not from the characteristic X-rays of nickel, but from the characteristic X-rays of cobalt, which is close to nickel in the energy spectrum.

[0610] From the profile in Figure 29A, the reference point was estimated to be a point at a distance of 44.3 nm on the x-axis. Specifically, the area avoiding the area where the detected amount of cobalt begins to increase was set to a distance of 10 to 20 nm in Figure 29A. The area where the cobalt count stabilized was set to a distance of 94 to 98 nm. From the cobalt profile, M ＡＶＥ and M ＢＧ Calculating the 50% point of the sum of these values ​​gave a value of 276.8 Counts, and estimating the surface by calculating a regression line gave a value of 44.3 nm.

[0611] 30A, 30B, and 30C, the peak positions of the additive elements were -0.3 nm for Mg and 3.9 nm for Al, respectively, with the particle interior direction being the positive direction based on the standard estimated above. Furthermore, the ratio of the detected intensity of the additive element at the peak position to the average value of the detected intensity of cobalt in the region where the cobalt count was stable was Mg / Co = 0.05 and Al / Co = 0.06 in the basal region. The half-width of the magnesium distribution was 2.6 nm.

[0612] 31A, 31B, and 31C are graphs in which the vertical axis of FIG. 29B is enlarged, with FIG. 31A showing the cobalt and magnesium profiles (counts), FIG. 31B showing the cobalt and aluminum profiles (counts), and FIG. 31C showing the cobalt and nickel profiles (counts). In the energy spectrum in the edge region of LCO1, a peak derived from the characteristic X-rays of nickel was clearly observed.

[0613] From the profile in Figure 29B, the reference plane was estimated to be a point at a distance of 50.5 nm. Specifically, the area avoiding the area where the detected amount of cobalt begins to increase was set to a distance of 10 to 20 nm in Figure 29B. The area where the cobalt count stabilized was set to a distance of 97 to 100 nm. From the cobalt profile, M ＡＶＥ and M ＢＧ Calculating the 50% point of the sum of these values ​​gave 610.2 Counts, and estimating the surface by finding a regression line gave a value of 50.5 nm.

[0614] 31A, 31B, and 31C, the peak positions of the additional elements were -0.9 nm for Mg, 4.9 nm for Al, and 1.9 nm for Ni, with the particle interior direction being the positive direction based on the reference position estimated above. Furthermore, the ratio of the detection intensity of the additional element at the peak position to the average value of the detection intensity of cobalt in the region where the cobalt count was stable was Mg / Co = 0.11, ALCO = 0.05, and Ni / Co = 0.05 in the edge region. The half-width of the magnesium distribution was 4.5 nm, and the half-width of the nickel distribution was 8.1 nm.

[0615] As described above, it was confirmed that in lithium cobalt oxide 1, both the basal region and the edge region have a region where magnesium is distributed closer to the surface of the positive electrode active material than aluminum. It was also confirmed that in the edge region, there is a region where magnesium and nickel are distributed closer to the surface of the positive electrode active material than aluminum. It was also confirmed that in the edge region, the peak position of magnesium and the peak position of nickel are close to each other, and there is a region where the magnesium distribution overlaps with the nickel distribution.

[0616] 100a: surface layer, 100b: interior, 100: positive electrode active material, 200: graphite particles, 201: silicon particles, 202a: binder, 202: binder, 203a: AB

Claims

1. It has a positive electrode, a negative electrode, and an electrolyte, The positive electrode has positive electrode active material particles having a median diameter (D50) of 1 μm or more and 12 μm or less. The positive electrode active material particles have lithium cobalt oxide. The positive electrode active material particles have magnesium in their surface layer, The negative electrode comprises graphite particles, silicon particles, and a polymer having a carboxyl group. The electrolyte comprises a cyclic fluoride carbonate and a chain-like fluoride carbonate. Lithium-ion rechargeable battery.

2. In claim 1, The average particle diameter of the aforementioned silicon particles is less than 1 μm. Lithium-ion rechargeable battery.

3. In claim 1, The average particle size of the graphite particles is 5 μm or larger. Lithium-ion rechargeable battery.

4. In claim 1, The average particle diameter of the silicon particles is smaller than the average particle diameter of the graphite particles. Lithium-ion rechargeable battery.

5. In claim 1, In the negative electrode, the weight ratio of the silicon particles is smaller than the weight ratio of the graphite particles. Lithium-ion rechargeable battery.

6. In claim 1, The polymer having the carboxyl group is polyglutamic acid. Lithium-ion rechargeable battery.

7. In claim 1, The positive electrode active material particles have a layered rock salt type crystalline structure belonging to space group R-3m. The surface portion has a basal region having a surface parallel to the (00l) plane of the crystal structure, and an edge region. When linear analysis is performed on the positive electrode active material particles using STEM-EDX, the magnesium concentration in the edge region is detected to be higher than the magnesium concentration in the basal region. Lithium-ion rechargeable battery.

8. It has a positive electrode, a negative electrode, and an electrolyte, The positive electrode has positive electrode active material particles having a median diameter (D50) of 1 μm or more and 12 μm or less. The positive electrode active material particles have lithium cobalt oxide. The positive electrode active material particles have magnesium and nickel in their surface layer. The negative electrode comprises graphite particles, silicon particles, and a polymer having a carboxyl group. The average particle diameter of the silicon particles is smaller than the average particle diameter of the graphite particles. The electrolyte comprises a cyclic fluoride carbonate and a chain-like fluoride carbonate. Lithium-ion rechargeable battery.

9. It has a positive electrode, a negative electrode, and an electrolyte, The positive electrode has positive electrode active material particles having a median diameter (D50) of 1 μm or more and 12 μm or less. The positive electrode active material particles have lithium cobalt oxide. The lithium cobalt oxide has magnesium and nickel in its surface layer, The negative electrode comprises graphite particles, silicon particles, and a polymer having a carboxyl group. The average particle diameter of the silicon particles is greater than the average particle diameter of the graphite particles. The electrolyte comprises fluoroethylene carbonate and methyl 3,3,3-trifluoropropionate, and when the total content of fluoroethylene carbonate and methyl 3,3,3-trifluoropropionate is 100 vol%, the volume ratio of fluoroethylene carbonate and methyl 3,3,3-trifluoropropionate is x:100-x (where 5 ≤ x ≤ 30). Lithium-ion rechargeable battery.

10. In claim 8 or claim 9, The positive electrode active material particles have a layered rock salt type crystalline structure belonging to space group R-3m. The surface portion has a basal region having a surface parallel to the (00l) plane of the crystal structure, and an edge region. When line analysis is performed on the positive electrode active material particles using STEM-EDX, it is found that in the edge region, there is a region where the distribution of magnesium overlaps with the distribution of nickel. Lithium-ion rechargeable battery.

11. In claim 8 or claim 9, The positive electrode active material particles have a layered rock salt type crystalline structure belonging to space group R-3m. The surface portion has a basal region having a surface parallel to the (00l) plane of the crystal structure, and an edge region. Linear analysis of the positive electrode active material particles by STEM-EDX shows that the basal region substantially does not contain nickel. Lithium-ion rechargeable battery.