Secondary battery

Abstract

To provide a lithium-ion battery having an electrolyte that allows charging and discharging over a wide temperature range, including from sub-freezing to elevated temperatures.SOLUTION: A battery includes a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode. The electrolyte is methyl 2,2,2-trifluoroethyl carbonate, the content of lithium salt in the electrolyte is 1.5 mol / L or more and less than 3.0 mol / L, and propane sultone is further added to the electrolyte.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

[0003] A lithium-ion secondary battery (sometimes referred to as a lithium-ion battery) is a battery that uses lithium ions as the carrier ion. A lithium-ion secondary battery is a secondary battery that can be used repeatedly by charging and discharging. [Background technology]

[0004] In recent years, there has been active development of various types of energy storage devices, such as 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.

[0005] It is known that the discharge capacity of lithium ion secondary batteries varies depending on the temperature during discharge. 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).

[0006] 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). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-026608 [Patent Document 2] WO2020 / 026078 Brochure Summary of the Invention [Problem to be solved by the invention]

[0008] Lithium ion secondary batteries still have room for improvement in various aspects, such as discharge capacity, cycle characteristics, reliability, safety, and cost.

[0009] Another object of one embodiment of the present invention is to provide a lithium ion secondary battery using a new electrolyte solution, specifically, an organic solvent containing a lithium salt, so that the battery can be charged and discharged over a wide temperature range, including from below freezing to high temperatures.

[0010] Another object is to provide a novel secondary battery.

[0011] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description of the specification, drawings, and claims. [Means for solving the problem]

[0012] The electrolyte solution contains an organic solvent and an electrolyte salt dissolved in the organic solvent. The electrolyte solution acts as a medium for transporting carrier ions, typically lithium ions. Because the driving voltage of lithium ion secondary batteries is 4 V or higher, an organic solvent that does not decompose even when a voltage of 4 V or higher is applied is used.

[0013] As the organic solvent of the electrolyte used in the secondary battery according to one embodiment of the present invention, a fluorinated chain carbonate, specifically methyl 2,2,2-trifluoroethyl carbonate (also referred to as FEMC) having a freezing point of −80° C. or lower, is used.

[0014] FEMC can be represented by the following structural formula:

[0015] [ka]

[0016] FEMC is a type of fluorine-containing carbonate ester compound with a short carbon chain and two asymmetric substituents, one of which is a fluoroalkyl group. Therefore, using FEMC as a solvent for electrolytes can prevent secondary batteries from exploding or catching fire in harsh environments. This will lead to the realization of secondary batteries using flame-retardant electrolytes that, even if the electrolyte ignites, will quickly extinguish or not sustain combustion. Furthermore, it is important that the solvent used to dissolve the electrolyte salt does not solidify at sub-zero temperatures.

[0017] Examples of electrolyte salts include lithium salts, sodium salts, and potassium salts. Among these, it is preferable to use lithium salts. In particular, lithium hexafluorophosphate (LiPF6) is used as the lithium salt. LiPF6 can be represented by the following structural formula:

[0018] [ka]

[0019] The inventors have found that a flame-retardant electrolyte solution can be obtained when the volume molar concentration of lithium hexafluorophosphate in FEMC is higher than 1 mol / L (1M), preferably 1.5 mol / L (1.5M) or higher. The upper limit of the volume molar concentration is set to a volume molar concentration at which no residue remains when FEMC and lithium hexafluorophosphate are mixed, specifically, less than 3.0 mol / L (3M). While FEMC alone cannot be considered flame-retardant, a volume molar concentration of lithium hexafluorophosphate higher than 1.0 mol / L (1M), preferably 1.5 mol / L (1.5M) or higher, can be considered flame-retardant.

[0020] In this specification and the like, "flame retardancy" refers to the property that, in an ignition test, a flame does not persist and the flame goes out or becomes undetectable by visual inspection.

[0021] Furthermore, when another lithium salt, lithium bis(fluorosulfonyl)imide (LiFSI), was used, an ignition test confirmed that the battery continued to burn even at a high concentration of 1.0 mol / L or more and 2.5 mol / L or less by volume molar concentration. Therefore, the use of LiFSI as a lithium salt in the electrolyte solution of one embodiment of the present invention is excluded.

[0022] The invention disclosed in this specification is a secondary battery including a positive electrode, a separator, a negative electrode, and an electrolyte solution, the electrolyte solution including a fluorinated chain carbonate as an organic solvent and lithium hexafluorophosphate as a solute, the content (volume molar concentration) of lithium salt in the electrolyte solution being 1.5 mol / L or more and less than 3.0 mol / L, and the electrolyte solution having fluidity in the temperature range of 0°C to -40°C (temperature range of 0°C or less and -40°C or less).

[0023] In this specification, "fluidity" means that the behavior changes depending on the conditions at the time. An electrolyte in a liquid state can be said to have fluidity. On the other hand, an electrolyte frozen into a solid state can be said to have lost fluidity. Fluidity can be easily confirmed visually by placing 3 ml or less in a transparent container (a 5 ml glass bottle) and tilting it. For temperatures below 0°C, simply place the glass bottle in a chilled ethanol solution, remove it, and visually inspect it. The degree of fluidity can also be confirmed by measuring the viscosity of the electrolyte using, for example, a viscometer (TV-35 manufactured by Toki Sangyo Co., Ltd.). An electrolyte with fluidity can be measured at least at 0°C with a viscometer. Note that depending on the object being measured, fluidity may be exhibited regardless of whether the viscosity is high or low.

[0024] When the electrolyte freezes and becomes solid (also called solidification), its ionic conductivity is impaired. To charge and discharge a secondary battery in the temperature range of 0°C to minus 40°C, it is preferable for the electrolyte to have a freezing point lower than the operating temperature range. Furthermore, at low temperatures, the solubility of lithium salts decreases, and if they precipitate, there is a risk of short-circuiting during operation of the secondary battery.

[0025] The electrolyte solution disclosed in this specification does not precipitate lithium salts when kept in a temperature range of 0°C to -40°C for 10 or 20 hours, and remains in a liquid state, although its fluidity is reduced compared to that at room temperature.

[0026] Furthermore, it is preferable to add an additive to the electrolyte to improve cycle characteristics. The concentration of the additive is, for example, 0.1 wt% to 5 wt% of the total electrolyte. The additive can suppress reactive decomposition of the electrolyte that may occur on the positive electrode surface or negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives that can be used include propane sultone (PS), vinylene carbonate (VC), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate)borate (LiBOB). PS is particularly preferable. PS forms a good coating on the positive electrode during charge and discharge, improving cycle characteristics.

[0027] The additive may be one or more dinitrile compounds, such as succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

[0028] Fluorobenzene may also be added to the organic solvent. EGBE is preferred because it forms a good coating on the positive electrode during charge and discharge, improving cycle characteristics. Dinitrile compounds are preferred because the nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving voltage resistance. Furthermore, dinitrile compounds are preferred because they can prevent copper dissolution during overdischarge when a copper-containing current collector is used in the negative electrode. Considering the use of secondary batteries at high voltages, it is preferable to add a nitrile compound.

[0029] Furthermore, electrolytes with a volume molar concentration of lithium hexafluorophosphate in FEMC of 1.5 mol / L or more are fluid below freezing point, specifically in the temperature range of 0°C to -40°C, and their viscosity can be measured using a viscometer.

[0030] In this specification and the like, a value that indicates the magnitude of viscosity is called viscosity, and "appropriate viscosity" means that the viscosity is appropriate for a lithium ion secondary battery.

[0031] Further, the positive electrode of the secondary electrode is manufactured by applying a slurry containing a positive electrode active material and a binder onto a current collector and then drying it. The slurry may also be referred to as an electrode slurry or an active material slurry. When forming a positive electrode active material layer, it may be referred to as a positive electrode slurry, and when forming a negative electrode active material layer, it may be referred to as a negative electrode slurry. A conductive aid may be added to the slurry. A typical carbon material used as a conductive aid is acetylene black (also referred to as AB).

[0032] The positive electrode active material is not particularly limited as long as it is a lithium oxide, and any one or more of lithium cobalt oxide (also referred to as LiCoO2: LCO), lithium iron phosphate (also referred to as LiFePO4: LFP), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium manganese phosphate (LiMnPO4), and lithium iron manganese phosphate (LiFe a Mn b PO4: a + b is 1 or less, 0 < a < 1, 0 < b < 1) can be used. When using lithium iron phosphate, it is preferable to use a carbon-coated one.

[0033] Particularly, when using lithium cobalt oxide having magnesium, titanium, aluminum, and nickel in the surface layer portion as the positive electrode active material, the low-temperature characteristics are improved, which is preferable.

[0034] In this specification and the like, the surface layer portion of the positive electrode active material refers to, for example, a region within 50 nm from the surface toward the inside, more preferably within 35 nm from the surface toward the inside, still more preferably within 20 nm from the surface toward the inside, and most preferably within 10 nm from the surface toward the inside perpendicular or substantially perpendicular to the surface. Note that substantially perpendicular means 80° or more and 100° or less. A surface generated by a crack and / or a crack may also be referred to as a surface. The surface layer portion is synonymous with the vicinity of the surface, the region near the surface, or the shell.

[0035] In addition, when the carrier ion is an alkali metal ion other than a lithium ion, an alkaline earth metal ion, a beryllium ion, or a magnesium ion, an alkali metal (e.g., sodium, potassium, etc.), an alkaline earth metal (e.g., calcium, strontium, barium, etc.), beryllium, or magnesium may be used as the positive electrode active material instead of lithium.

[0036] The negative electrode current collector used in the negative electrode is preferably not alloyed with carrier ions such as lithium. Specifically, copper or a copper alloy is used as the material for the negative electrode current collector. Carbon particles or a silicon-based material is used as the negative electrode active material.

[0037] As the carbon particles, graphite, carbon having a layer structure like graphite, amorphous carbon, or hard carbon may be used. Carbon fibers may also be used instead of carbon particles. The carbon particles used in this specification specifically refer to graphite particles, which are abundant in nature and therefore inexpensive, making them preferable as an active material for a negative electrode.

[0038] A lithium metal foil can also be used as the negative electrode.

[0039] To prevent short circuits, a separator is placed between the positive and negative electrodes. The separator is a porous film with many tiny pores filled with an electrolyte, allowing lithium ions moving between the positive and negative electrodes to pass through the tiny pores in the separator. The separator may have a laminated structure, for example, a two-layer structure with one side (front) and the other side (back) made of different materials. Materials that can be used for the separator include polyimide, polypropylene, polyester, or polyacrylonitrile. Glass fiber may also be used for the separator.

[0040] In this specification, "below freezing" refers to 0°C or below, "high temperature" refers to 25°C or above, and room temperature refers to a temperature higher than 0°C but lower than 25°C. In this specification, "a temperature range including below freezing to high temperatures" includes the above-mentioned room temperature. In this specification, "low temperature environment" refers to 0°C or below, and 0°C or below may also 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. [Effects of the Invention]

[0041] According to one embodiment of the present invention, a flame-retardant electrolyte solution that allows charging and discharging over a wide temperature range, including from below freezing to high temperatures, can be realized, and a highly safe secondary battery can be manufactured.

[0042] Note that the above-described effects do not preclude the existence of other effects. Furthermore, the above-described effects are considered to be independent of each other, and one embodiment of the present invention does not necessarily exhibit all of the above-described effects. Furthermore, effects other than those described above can be extracted from the description of this specification and the like. [Brief explanation of the drawings]

[0043] [Figure 1] FIG. 1A is a schematic cross-sectional view illustrating an internal structure of a secondary battery according to one embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view illustrating a structure of a positive electrode. [Figure 2] 2A and 2B illustrate a lithium-ion secondary battery of one embodiment of the present invention. [Figure 3] 3A to 3C illustrate a lithium-ion secondary battery of one embodiment of the present invention. [Figure 4] 4A to 4D illustrate a lithium-ion secondary battery and a power storage system according to one embodiment of the present invention. [Figure 5]5A to 5C illustrate a lithium-ion secondary battery of one embodiment of the present invention. [Figure 6] 6A to 6C illustrate a lithium-ion secondary battery of one embodiment of the present invention. [Figure 7] 7A to 7C are diagrams illustrating an electric vehicle according to one embodiment of the present invention. [Figure 8] 8A to 8D are diagrams illustrating a transportation vehicle according to one embodiment of the present invention. [Figure 9] 9A to 9C are diagrams illustrating a two-wheeled vehicle and the like according to one embodiment of the present invention. [Figure 10] 10A to 10G illustrate electronic devices and the like according to one embodiment of the present invention. [Figure 11] 11(A) to 11(D) are diagrams showing an example of space equipment. [Figure 12] FIG. 12 is a cross-sectional view of the positive electrode active material. [Figure 13] FIG. 13 is a diagram illustrating a method for producing a positive electrode active material. [Figure 14] 14A to 14C are diagrams illustrating a method for manufacturing a positive electrode active material. [Figure 15] 15(A) and 15(B) are diagrams showing the charge-discharge characteristics of a half cell. [Figure 16] FIG. 16 shows the cycle results. [Figure 17] FIG. 17 shows the results of CV measurement. DETAILED DESCRIPTION OF THE INVENTION

[0044] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0045] (Embodiment 1) In this embodiment, a lithium-ion secondary battery according to one embodiment of the present invention will be described with reference to the drawings.

[0046] <Lithium-ion secondary battery> A lithium-ion secondary battery according to one embodiment of the present invention includes an electrolyte solution that allows charging and discharging over a wide temperature range, including temperatures at least below freezing and even up to high temperatures. In addition to the electrolyte solution, the lithium-ion secondary battery includes a negative electrode, a positive electrode, a separator between the negative electrode and the positive electrode, and an exterior covering the negative electrode and the positive electrode. Depending on the shape of the exterior, the battery is called a laminated lithium-ion secondary battery, a coin-cell lithium-ion secondary battery, or a cylindrical lithium-ion secondary battery. However, the present invention is not limited to the shape of the exterior. The separator can be omitted if a solid electrolyte layer is provided on the negative electrode side.

[0047] 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 charge / discharge characteristics even in a low temperature environment (for example, 0° C. or lower, preferably −20° C. or lower, more preferably −30° C. or lower, more preferably −40° C. or lower). Specifically, the description will focus on the electrolyte.

[0048] FIG. 1(A) is a cross-sectional schematic diagram illustrating the internal structure of a battery 10. The 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. 1(A), the battery 10 contains an electrolyte in the voids of the positive electrode active material layer 22, the voids of the separator, and the voids of the negative electrode active material layer 32.

[0049] The negative electrode active material layer 32 may be made of a known material, such as carbon particles or a silicon-based material. The negative electrode current collector 31 may be made of copper or a copper alloy. The negative electrode 12 may be made of lithium metal.

[0050] 1(B) shows the positive electrode active material layer 22, which 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.

[0051] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with the electrolyte solution 60. 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 60, more preferably 70% or more of the voids, 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 positive electrode active material layer 22 refer to regions in the positive electrode active material layer 22 other than those containing solid components (such as the positive electrode active material and conductive material).

[0052] Although detailed description will be omitted, similar to the description of the positive electrode active material layer 22 above, the voids in the negative electrode active material layer 32 are preferably filled with the electrolyte. 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, 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 containing solid components (negative electrode active material, conductive material, etc.).

[0053] 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 solution 60, it is possible to increase the area where the positive electrode active material and the negative electrode active material come into contact with the electrolyte solution 60. In other words, it is possible to provide a lithium ion secondary battery with excellent charge and discharge characteristics in a low-temperature environment.

[0054] Furthermore, it is preferable that the electrolyte solution 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 (for example, 0°C or lower, preferably -20°C or lower, more preferably -30°C or lower, more preferably -40°C or lower).

[0055] In this embodiment, methyl 2,2,2-trifluoroethyl carbonate, a type of fluorinated chain carbonate, is used as the electrolyte solution 60. Furthermore, lithium hexafluorophosphate is used as the lithium salt, and the content of the lithium salt in the electrolyte solution is higher than 1.0 mol / L, preferably 1.5 mol / L or more and less than 3.0 mol / L. An electrolyte solution having a concentration within this range is fluid in the temperature range of 0°C to -40°C, and can therefore maintain its function as an electrolyte solution. Furthermore, an electrolyte solution having a concentration within this range can be a flame-retardant electrolyte solution that, even if ignited, will quickly extinguish or not sustain combustion.

[0056] If the content of lithium salt in the electrolyte is 3.0 mol / L or more, the excess lithium salt may not dissolve in the electrolyte and may precipitate due to temperature changes. Also, if the content of lithium salt in the electrolyte is 1.0 mol / L or less, the electrolyte may ignite but not be extinguished in a short time, or the combustion may continue, increasing the risk of explosion or fire when used in a secondary battery.

[0057] Furthermore, the separator 13 preferably has a low contact angle with the electrolyte at room temperature and good wettability, which improves the impregnation of the separator 13 with the electrolyte. A sheet-like separator 13 made of a porous material is used for the separator 13. If the contact angle of the electrolyte with the separator 13 is greater than 30°, the electrolyte cannot sufficiently penetrate the separator, resulting in partial reduction in ion permeability and variations in battery characteristics. The separator 13 may be made of a material that has a low contact angle with the electrolyte. Separators made of polyimide are particularly preferred, as they have a contact angle of less than 10° even when the lithium salt content is 1.5 mol / L or more.

[0058] 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 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.

[0059] Therefore, it is preferable to use a positive electrode active material that can withstand high voltage and obtain a high charge capacity when 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.

[0060] The positive electrode active material has the function of absorbing and releasing lithium ions during charging and discharging. The positive electrode active material used in one embodiment of the present invention is a material that undergoes little deterioration (or little increase in resistance) during charging and / or discharging (hereinafter also referred to as "charging and discharging") in a low-temperature environment, even at high charging voltages (unless otherwise specified, this refers to a voltage value based on lithium metal, hereinafter also referred to as "high charging voltage"). Specifically, it is preferable to use a positive electrode active material (composite oxide) with a particle size (strictly speaking, median diameter (D50)) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less).

[0061] Particle size can be measured using a particle size distribution analyzer that uses a laser diffraction / scattering method. The median diameter (D50) is the particle diameter at which the cumulative volume in the cumulative curve of the particle size distribution measurement results accounts for 50%. Particle size measurement is not limited to laser diffraction particle size distribution measurement; the long diameter of the particle cross section can also be measured by analysis using a scanning electron microscope (hereinafter referred to as SEM) or a transmission electron microscope (hereinafter referred to as TEM). For example, a method for measuring the median diameter (D50) using analysis using SEM or TEM can involve measuring 20 or more particles, creating a cumulative curve, and determining the particle diameter at which the cumulative volume accounts for 50% as the median diameter (D50).

[0062] As an evaluation of low-temperature characteristics, it is preferable that the discharge capacity value in a low-temperature environment 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 values ​​should be obtained assuming that the measurement conditions other than the environmental temperature are the same.

[0063] Alternatively, even at high charging voltages, by using a material that is less prone to deterioration during charging and discharging (or a material that is less prone to increase in resistance) as the positive electrode active material, it is possible to obtain a large discharge capacity even at low temperature environments.

[0064] The environmental temperature described in this specification refers to the temperature of a lithium-ion secondary battery. When measuring battery characteristics using a thermostatic chamber, the environmental temperature can be considered to be the temperature set in the thermostatic chamber. Therefore, after placing the battery to be measured (e.g., a test battery or half-cell) in the thermostatic chamber, it is preferable to wait a sufficient 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.

[0065] As described above, a flame-retardant electrolyte solution has been realized that allows charging and discharging over a wide temperature range, including temperatures from below freezing to high temperatures. Therefore, battery 10, which includes this electrolyte solution, positive electrode 11, separator 13, and negative electrode 12, is a highly safe lithium-ion secondary battery.

[0066] (Embodiment 2) In this embodiment, an example of a lithium ion secondary battery will be described.

[0067] [Laminated lithium-ion secondary battery] 2(A) and 2(B) show examples of a laminated lithium-ion secondary battery 500. FIGS. 2(A) and 2(B) are external views, and the lithium-ion secondary battery 500 includes the electrolyte and separator (not shown in FIG. 2) described in Embodiment 1 above, a negative electrode 506, and a positive electrode 507. In the lithium-ion secondary battery 500, the negative electrode 506 preferably has a larger area than the positive electrode 507. The lithium-ion secondary battery 500 further includes a negative electrode lead electrode 510 electrically connected to the negative electrode 506 and a positive electrode lead electrode 511 electrically connected to the positive electrode 507. The electrolyte, the negative electrode 506, and the positive electrode 507 are housed in an outer casing 509, and a portion of the negative electrode lead electrode 510 and a portion of the positive electrode lead electrode 511 protrude from the outer casing 509. The outer casing 509 has an adhesive region 508 on a portion of its outer periphery. 2(A) shows an example in which a negative electrode lead electrode 510 and a positive electrode lead electrode 511 protrude from the same side of an exterior body 509, and adhesive regions 508 are located at least on the side from which each lead electrode protrudes and on two sides adjacent to that side. Also, FIG. 2(B) shows an example in which the side from which the negative electrode lead electrode 510 protrudes from the exterior body 509 and the side from which the positive electrode lead electrode 511 protrudes from the exterior body 509 face each other, and adhesive regions 508 are located at least on the two sides from which each lead electrode protrudes and on one side sandwiched between those two sides. In FIGS. 2(A) and 2(B), the sides on which adhesive regions 508 are not located preferably correspond to the folded sides of the exterior body 509.

[0068] By using the electrolyte solution of one embodiment of the present invention in the laminated lithium-ion secondary battery 500, the secondary battery can be highly safe.

[0069] [Coin-type lithium-ion secondary battery] An example of a coin-type lithium ion secondary battery will be described. FIG. 3(A) is an exploded perspective view of a coin-type (single-layer flat) lithium ion secondary battery, FIG. 3(B) is an external view, and FIG. 3(C) is a cross-sectional view thereof. Coin-type lithium ion secondary batteries are mainly used in small electronic devices. In this specification, coin-type lithium ion secondary batteries include button-type lithium ion secondary batteries.

[0070] In order to make it easier to understand, Figure 3(A) is a schematic diagram that shows the overlapping of components (vertical relationship and positional relationship). Therefore, Figure 3(A) and Figure 3(B) are not completely corresponding drawings.

[0071] 3A shows a state in which the positive electrode 304, negative electrode 307, spacer 342, and washer 332 are stacked and sealed with the negative electrode can 302 and positive electrode can 301. Note that the electrolyte and separator described in the above embodiment are not shown in FIG. 3A. The spacer 342 and washer 332 are used to protect the inside or fix the position inside the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 342 or washer 332 is made of stainless steel or an insulating material.

[0072] 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 .

[0073] FIG. 3(B) is a perspective view of the completed coin-type lithium ion secondary battery 300.

[0074] In a coin-type lithium-ion 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, may be insulated and sealed with a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0075] The positive electrode 304 and the negative electrode 307 used in the coin-type lithium ion secondary battery 300 may each have an active material layer formed on only one side.

[0076] As shown in FIG. 3(C), a positive electrode 304, a negative electrode 307, and a 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 to produce a coin-shaped lithium ion secondary battery 300.

[0077] By using the electrolyte solution of one embodiment of the present invention for the coin-type lithium-ion secondary battery 300, a highly safe secondary battery can be realized.

[0078] [Cylindrical lithium-ion secondary battery] An example of a cylindrical lithium ion secondary battery will be described with reference to Fig. 4(A). As shown in Fig. 4(A), a cylindrical lithium ion 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.

[0079] Fig. 4(B) is a diagram showing a schematic cross section of a cylindrical lithium ion secondary battery. The cylindrical lithium ion secondary battery shown in Fig. 4(B) 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.

[0080] 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 an electrolyte solution 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. Inside the battery can 602, the wound battery element, in which the positive electrode, the negative electrode, and the separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte solution (not shown) of one embodiment of the present invention is poured into the battery can 602 with the battery element provided therein.

[0081] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector. While the lithium-ion secondary battery 616 shown in Figures 4A to 4D has a cylinder whose height is greater than its diameter, this is not limiting. A lithium-ion secondary battery whose diameter is greater than its height may also be used. This configuration, for example, can reduce the size of the lithium-ion secondary battery.

[0082] A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases with an increase in temperature. This increase in resistance limits the amount of current and prevents abnormal heat generation. The PTC element can be made of a barium titanate (BaTiO3)-based ceramic material or the like.

[0083] 4C shows an example of a power storage system 615. The power storage system 615 has a plurality of lithium-ion secondary batteries 616 and is sometimes called a battery pack. A positive electrode of each lithium-ion secondary battery is in contact with and electrically connected to a conductor 624 separated by an insulator 625. The conductor 624 is electrically connected to a control circuit 620 via a wiring 623. A negative electrode of each lithium-ion secondary battery is electrically connected to the control circuit 620 via a wiring 626. A protection circuit or the like that prevents overcharging or overdischarging can be used as the control circuit 620.

[0084] 4(D) shows an example of a power storage system 615. The power storage system 615 has a plurality of lithium ion secondary batteries 616, and the plurality of lithium ion secondary batteries 616 are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of lithium ion secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of lithium ion secondary batteries 616 may be connected in parallel, in series, or in series after being connected in parallel. By configuring the power storage system 615 to have a plurality of lithium ion secondary batteries 616, a large amount of power can be extracted.

[0085] A plurality of lithium ion secondary batteries 616 may be connected in parallel and then further connected in series.

[0086] A temperature control device may be provided between the plurality of lithium ion secondary batteries 616. When the lithium ion secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the lithium ion secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes the performance of the power storage system 615 less susceptible to the influence of the outside temperature.

[0087] 4(D), the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to positive electrodes of the plurality of lithium ion secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to negative electrodes of the plurality of lithium ion secondary batteries 616 via a conductive plate 614.

[0088] By using the electrolyte solution of one embodiment of the present invention for the cylindrical lithium-ion secondary battery 616, a highly safe secondary battery can be realized.

[0089] [Other examples of lithium-ion secondary battery structures] An example of the structure of a lithium ion secondary battery will be described with reference to FIGS.

[0090] A lithium ion secondary battery 913 shown in FIG. 5A has a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolyte solution of one embodiment of the present invention inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that for convenience, the housing 930 is shown separated in FIG. 5A, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a resin material.

[0091] 5(B), the housing 930 shown in Fig. 5(A) may be formed using a plurality of materials. For example, in a lithium ion secondary battery 913 shown in Fig. 5(B), a housing 930a and a housing 930b are bonded together, and a wound body 950 is provided in a region surrounded by the housing 930a and the housing 930b.

[0092] The housing 930a can be made of an insulating material such as organic resin. In particular, using a material such as organic resin on the surface on which the antenna is formed can prevent the lithium ion secondary battery 913 from blocking the electric field. Note that if the housing 930a does not block the electric field much, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0093] 5(C) shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and an electrolyte solution 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 electrolyte solution 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the electrolyte solution 933 may be stacked.

[0094] 6A to 6C may be used as a lithium ion secondary battery 913 having a wound body 950a. The wound body 950a shown in FIG. 6A includes a negative electrode 931, a positive electrode 932, and an electrolyte solution 933. The negative electrode 931 includes a negative electrode active material layer 931a. The positive electrode 932 includes a positive electrode active material layer 932a.

[0095] The electrolyte 933 has a width greater than that of the negative electrode active material layer 931a and the positive electrode active material layer 932a, and is wound so as to overlap with the negative electrode active material layer 931a and the positive electrode active material layer 932a. From the standpoint of safety, it is preferable that the width of the negative electrode active material layer 931a is greater than that of the positive electrode active material layer 932a. A wound body 950a having such a shape is preferable because of its high safety and productivity.

[0096] 6B, the negative electrode 931 is electrically connected to a terminal 951. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952. The terminal 952 is electrically connected to a terminal 911b.

[0097] 6(C), the wound body 950a is covered with the housing 930 to form the lithium ion secondary battery 913. The housing 930 is preferably provided with a safety valve, an overcurrent protection element, and the like. 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.

[0098] As shown in Figure 6(B), the lithium ion secondary battery 913 may have a plurality of wound bodies 950a. By using a plurality of wound bodies 950a, the lithium ion secondary battery 913 can have a larger charge / discharge capacity. For other elements of the lithium ion secondary battery 913 shown in Figures 6(A) and 6(B), the descriptions of the lithium ion secondary battery 913 shown in Figures 5(A) to 5(C) can be referred to.

[0099] By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery 913 having a wound body, a highly safe secondary battery can be realized.

[0100] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0101] (Embodiment 3) In this embodiment, an example of application to an electric vehicle (EV) will be shown with reference to FIG.

[0102] 7A, an electric vehicle is equipped with first batteries 1301a and 1301b as main driving lithium-ion secondary batteries and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. By using the electrolyte solution of one embodiment of the present invention for the first batteries 1301a and 1301b, a highly safe secondary battery can be realized.

[0103] 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.

[0104] The internal structure of the first battery 1301a may be a wound type or a stacked type.

[0105] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more may be connected in parallel. Also, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple lithium ion secondary batteries, it is possible to extract large amounts of power. The multiple lithium ion secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of lithium ion secondary batteries is also called a battery pack.

[0106] In addition, in an in-vehicle lithium ion secondary battery, 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 lithium ion secondary batteries.

[0107] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DC-DC circuit 1306. When a rear motor 1317 is provided on the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0108] Furthermore, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0109] The first battery 1301a will be described with reference to FIG. 7(B).

[0110] FIG. 7B shows an example in which nine prismatic lithium-ion secondary batteries 1300 are used as one battery pack 1415. Nine prismatic lithium-ion 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 assumed that a vehicle is subjected to vibration or shaking from the outside (such as a road surface), it is preferable to fix multiple lithium-ion secondary batteries using the fixing portions 1413 and 1414 and the battery housing box. One electrode is electrically connected to a control circuit unit 1320 by a wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by a wiring 1422.

[0111] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit unit 1320. A charge control circuit or a battery control system having a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor.

[0112] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, a metal oxide such as In-M-Zn oxide (wherein M is one or more elements selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium) can be used as the oxide. In particular, the In-M-Zn oxide that can be used as the oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In-Ga oxide or an In-Zn oxide may be used as the oxide. A CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis aligned in a specific direction. The specific direction may be the thickness direction of the CAAC-OS film, a normal direction to the surface on which the CAAC-OS film is formed, or a normal direction to the surface of the CAAC-OS film. A crystalline region is a region in which the atomic arrangement is periodic. Considering the atomic arrangement as a lattice arrangement, a crystalline region is also a region in which the lattice arrangement is aligned. Furthermore, the CAAC-OS has a region in which multiple crystalline regions are connected in the ab-plane direction, and the region may have distortion. The distortion refers to a location in the region in which multiple crystalline regions are connected, where the lattice arrangement changes direction between a region with a consistent lattice arrangement and a region with a different consistent lattice arrangement. In other words, the CAAC-OS is an oxide semiconductor that is c-axis aligned but not clearly aligned in the ab-plane direction.

[0113] Furthermore, because the control circuit unit 1320 can be used in low-temperature environments, it is preferable to use transistors using oxide semiconductors. To simplify the process, the control circuit unit 1320 may be formed using unipolar transistors. Transistors using oxide semiconductors in the semiconductor layer have a wider operating ambient temperature range than single-crystal Si, from -40°C to 150°C, and their characteristics change less when the lithium-ion secondary battery is heated than single-crystal Si. The off-current of transistors using oxide semiconductors 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 single-crystal Si transistors increases, and the current on / off ratio does not become sufficiently large. The control circuit unit 1320 can improve safety.

[0114] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for lithium-ion secondary batteries to address 10 causes of instability, such as micro-short circuits. The functions for addressing the 10 causes of instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of degradation, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for lithium-ion secondary batteries can be miniaturized.

[0115] Micro-short circuits are tiny short circuits that occur inside lithium-ion secondary batteries. One of the causes of micro-short circuits is said to be local current concentration in parts of the positive electrode and negative electrode due to uneven distribution of the positive electrode active material caused by repeated charge and discharge cycles, or the generation of by-products due to side reactions, which causes micro-short circuits.

[0116] In addition to detecting micro-shorts, the control circuit 1320 can also be said to detect the terminal voltage of the lithium-ion secondary battery and manage the charge / discharge state of the lithium-ion 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.

[0117] FIG. 7C shows an example of a block diagram of the battery pack 1415 shown in FIG. 7B.

[0118] The control circuit unit 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 unit 1320 sets upper and lower voltage limits for the lithium-ion secondary battery used and limits the upper limit of the external current and the upper limit of the output current. The range between the lower limit and the upper limit of the lithium-ion secondary battery's voltage is within the recommended voltage range. If the voltage falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit unit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. A PTC element may also be provided in the charge / discharge path to provide a function for cutting off the current in response to a rise in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0119] The switch unit 1324 can be configured by combining n-channel transistors and p-channel transistors. The switch unit 1324 is not limited to a switch having a Si transistor using single crystal silicon, and may be configured using, for example, Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), GaOx The switch unit 1324 may be formed using a power transistor having gallium oxide (x is a real number greater than 0). Furthermore, memory elements using OS transistors can be freely arranged by stacking them on a circuit using Si transistors, which facilitates integration. By stacking the control circuit unit 1320 using OS transistors on the switch unit 1324 and integrating them, it is possible to form them into a single chip, thereby enabling miniaturization.

[0120] The first batteries 1301a and 1301b mainly 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. Using a lithium-ion secondary battery as the second battery 1311 offers the advantage of being maintenance-free, but over extended use, such as three years or more, there is a risk of abnormalities occurring that cannot be detected at the time of manufacture. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b still have remaining capacity, the second battery 1311 is charged to maintain a full charge state by supplying power from the first battery to the second battery.

[0121] Although this embodiment shows an example in which lithium-ion secondary batteries are used for both the first battery 1301a and the second battery 1311, a lead-acid battery, an all-solid-state battery, or an electric double-layer capacitor may be used for the second battery 1311. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery, a highly safe secondary battery can be realized.

[0122] Furthermore, regenerated energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 from the motor controller 1303 and the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerated energy, it is desirable that the first batteries 1301a and 1301b be capable of being rapidly charged.

[0123] 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 according to the charging characteristics of the lithium ion secondary battery used, and can perform rapid charging.

[0124] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a, 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the function of the battery controller 1302 is not used, it is preferable to charge the first batteries 1301a, 1301b via a control circuit unit 1320 to prevent overcharging. In some cases, the connection cable or the charger's connection cable is provided with a control circuit. The control circuit unit 1320 is also called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU includes a microcomputer. The ECU uses a CPU or a GPU.

[0125] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging is also possible by receiving power from external charging equipment using methods such as contactless power supply.

[0126] Next, an example in which the lithium-ion secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0127] Furthermore, installing lithium-ion secondary batteries in vehicles will enable next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), plug-in hybrid vehicles (PHVs), etc. Lithium-ion secondary batteries can also be installed in transportation vehicles such as agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

[0128] 8A to 8D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 8A 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 lithium-ion secondary battery is installed in a vehicle, an example of the lithium-ion secondary battery described in the above embodiment is installed in one or more locations. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery installed in the vehicle, a highly safe secondary battery can be realized.

[0129] 8A includes a battery pack 2200, which includes a battery module to which a plurality of lithium-ion secondary batteries are connected. The battery pack 2200 preferably further includes a charge control device electrically connected to the battery module.

[0130] Furthermore, automobile 2001 can charge its lithium-ion secondary battery by receiving power supply from an external charging facility using a plug-in method, a contactless power supply method, or the like. Charging can be performed using a predetermined charging method and connector standards, such as CHAdeMO (registered trademark) or Combo, as appropriate. Lithium-ion secondary batteries are used in charging stations installed in commercial facilities. Lithium-ion secondary batteries are also used as home power sources. For example, plug-in technology can be used to charge an energy storage device installed in automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device such as an AC-DC converter.

[0131] Although not shown, a power receiving device can be mounted on a vehicle and can be charged by receiving power contactlessly from a ground-based power transmitting device. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or exterior wall, charging can be performed not only while the vehicle is stopped but also while it 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, and lithium-ion secondary batteries can be charged while the vehicle is stopped and moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0132] FIG. 8B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The battery module of the transport vehicle 2002 includes, for example, four lithium-ion secondary batteries with a nominal voltage of 3.0 V to 5.0 V, each of which is a cell unit, and 48 cells are connected in series to achieve a maximum voltage of 170 V. Aside from the number of lithium-ion secondary batteries in the battery pack 2201, the transport vehicle 2002 has the same functions as those in FIG. 7B, and therefore a description thereof will be omitted. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary batteries in the battery pack 2201, a highly safe secondary battery can be realized.

[0133] 8C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The battery module of the transport vehicle 2003 has a maximum voltage of 600 V, for example, in which one hundred or more lithium-ion secondary batteries with a nominal voltage of 3.0 V to 5.0 V are connected in series. Furthermore, except for the number of lithium-ion secondary batteries constituting the battery module of the battery pack 2202, the transport vehicle 2003 has the same functions as those of the transport vehicle 2003 shown in FIG. 7B, and therefore the description thereof will be omitted. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary batteries included in the module, a highly safe secondary battery can be realized.

[0134] Figure 8(D) shows, as an example, an aircraft 2004 having an engine that burns fuel. The aircraft 2004 shown in Figure 8(D) has wheels for takeoff and landing, and can therefore be considered part of a transportation vehicle, and has a battery pack 2203 that includes a battery module formed by connecting multiple lithium ion secondary batteries and includes the battery module and a charge control device.

[0135] The battery module of the aircraft 2004 is, for example, eight 4V lithium ion secondary batteries connected in series for a maximum voltage of 32V. Other than the number of lithium ion secondary batteries constituting the battery module of the battery pack 2203, it has the same functions as those in Fig. 7(B), and therefore a description thereof will be omitted.

[0136] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0137] (Fourth embodiment) In this embodiment, an example will be described in which a lithium-ion secondary battery according to one embodiment of the present invention is mounted on a vehicle such as a motorcycle or a bicycle.

[0138] 9A shows an example of an electric bicycle using the lithium-ion secondary battery of one embodiment of the present invention. The lithium-ion secondary battery of one embodiment of the present invention can be applied to an electric bicycle 8700 shown in FIG. 9A. The lithium-ion secondary battery of one embodiment of the present invention may include a protection circuit.

[0139] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state removed from the bicycle in FIG. 9B. The power storage device 8702 includes a plurality of lithium-ion secondary batteries 8701 of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit 8703. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery 8701, a highly safe secondary battery can be realized.

[0140] The power storage device 8702 also includes a control circuit 8704 capable of controlling charging or detecting an abnormality of the lithium ion secondary battery. The control circuit 8704 is electrically connected to the positive and negative electrodes of the lithium ion secondary battery 8701. This can greatly contribute to preventing accidents such as fires caused by lithium ion secondary batteries.

[0141] 9C shows an example of a two-wheeled vehicle using the lithium-ion secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 9C 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 electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery, a highly safe secondary battery can be realized.

[0142] 9C, a power storage device 8602 can be stored in an under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

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

[0144] (Embodiment 5) In this embodiment, an example of mounting a lithium-ion secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a lithium-ion 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.

[0145] 10A illustrates an example of a mobile phone. A mobile phone 7400 includes a display portion 7402 built into a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. The mobile phone 7400 also includes a secondary battery 7407. By using the electrolyte solution of one embodiment of the present invention for a lithium-ion secondary battery, a highly safe secondary battery can be realized.

[0146] FIG. 10(B) shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 provided inside is also bent. FIG. 10(C) shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. The secondary battery 7407 has a lead electrode electrically connected to a current collector.

[0147] FIG. 10D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display unit 7102, operation buttons 7103, and a secondary battery 7104. FIG. 10E shows a bent secondary battery 7104. When the secondary battery 7104 is worn on a user's arm in a bent state, the housing deforms, causing a change in the curvature of part or the entire secondary battery 7104. Note that the degree of curvature at any point on the curve, expressed as the radius of a corresponding circle, is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or the entire main surface of the housing or the secondary battery 7104 changes when the radius of curvature is 40 mm to 150 mm. High reliability can be maintained when the radius of curvature of the main surface of the secondary battery 7104 is 40 mm to 150 mm. By using the electrolyte solution of one embodiment of the present invention for the secondary battery 7104, a highly safe secondary battery can be realized.

[0148] 10F shows an example of a wristwatch-type portable information terminal 7200. The portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input / output terminal 7206, and the like.

[0149] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0150] The display surface of the display portion 7202 is curved, and a display can be performed along the curved display surface. The display portion 7202 is also provided with a touch sensor, and can be operated by touching the screen with a finger or a stylus. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0151] The operation button 7205 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 7205 can be freely set by an operating system incorporated in the mobile information terminal 7200.

[0152] The mobile information terminal 7200 is also capable of performing standardized short-range wireless communication. For example, hands-free conversation is also possible by communicating with a wirelessly enabled headset.

[0153] The portable information terminal 7200 also includes an input / output terminal 7206, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal 7206. Note that charging may be performed wirelessly without using the input / output terminal 7206.

[0154] The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a lightweight portable information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 10E can be incorporated into the housing 7201 in a curved state or into the band 7203 in a bendable state.

[0155] The portable information terminal 7200 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0156] 10G shows an example of an armband-type display device. The display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can also be provided with a touch sensor in the display portion 7304 and can function as a portable information terminal.

[0157] The display surface of the display portion 7304 is curved, and display can be performed along the curved display surface. The display state of the display device 7300 can be changed by short-range wireless communication according to a communication standard.

[0158] The display device 7300 also includes an input / output terminal, allowing direct data exchange with other information terminals via a connector. Charging can also be performed via the input / output terminal. Note that charging may be performed wirelessly without using the input / output terminal.

[0159] By using the electrolyte solution of one embodiment of the present invention for the secondary battery included in the display device 7300, a highly safe secondary battery can be realized.

[0160] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0161] (Embodiment 6) In this embodiment, an example in which a lithium-ion secondary battery according to one embodiment of the present invention is mounted on space equipment will be described.

[0162] 11A shows an artificial satellite 6800 as an example of space equipment. The artificial satellite 6800 includes a body 6801, a solar panel 6802, an antenna 6803, and a lithium-ion secondary battery 6805. The solar panel may be called a solar cell module.

[0163] When sunlight is irradiated onto the solar panel 6802, power required for operating the satellite 6800 is generated. However, for example, when sunlight is not irradiated onto the solar panel or when the amount of sunlight irradiating the solar panel is small, the generated power is small. Therefore, the power required for operating the satellite 6800 may not be generated. To operate the satellite 6800 even when the generated power is small, the satellite 6800 may be provided with a lithium-ion secondary battery 6805. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery, a highly safe secondary battery can be realized.

[0164] The satellite 6800 can generate a signal. The signal is transmitted via an antenna 6803, and can be received by, for example, a receiver on the ground or another satellite. By receiving the signal transmitted by the satellite 6800, for example, the position of the receiver that received the signal can be determined. As described above, the satellite 6800 can constitute, for example, a satellite positioning system.

[0165] Alternatively, the artificial satellite 6800 may be configured to include a sensor. For example, by including a visible light sensor, the artificial satellite 6800 may have the function of detecting sunlight reflected from an object on the ground. Alternatively, by including a thermal infrared sensor, the artificial satellite 6800 may have the function of detecting thermal infrared rays emitted from the earth's surface. As described above, the artificial satellite 6800 may function as, for example, an earth observation satellite.

[0166] FIG. 11B shows a probe 6900 having a solar sail (also called a solar sail) as an example of space equipment. The probe 6900 includes a body 6901, a solar sail 6902, and a lithium-ion secondary battery 6905. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery, a highly safe secondary battery can be realized. When photons emitted from the sun hit the surface of the solar sail 6902, momentum is transferred to the solar sail 6902. Therefore, it is preferable that the surface of the solar sail 6902 has a thin film with high reflectivity and faces the sun.

[0167] The solar sail 6902 may also be designed to be folded up small until it leaves the atmosphere, and then deployed into a large sheet outside the Earth's atmosphere (outer space) as shown in Figure 11(B).

[0168] FIG. 11C illustrates a spacecraft 6910 as an example of space equipment. The spacecraft 6910 includes a body 6911, a solar panel 6912, and a lithium-ion secondary battery 6913. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery, a highly safe secondary battery can be realized. The body 6911 can have, for example, a pressurized compartment and an unpressurized compartment. The pressurized compartment may be designed to accommodate a crew member. Electric power generated when sunlight is irradiated onto the solar panel 6912 can be charged into the lithium-ion secondary battery 6913.

[0169] 11D illustrates a rover 6920 as an example of space equipment. The rover 6920 includes a body 6921 and a lithium-ion secondary battery 6923. By using the electrolyte solution of one embodiment of the present invention for the lithium-ion secondary battery, a highly safe secondary battery can be realized. The rover 6920 may include a solar panel 6922.

[0170] The rover 6920 may be designed to accommodate a crew member. The lithium ion secondary battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or may be charged with electricity generated by other power sources, such as a fuel cell, a radioisotope thermoelectric converter, or the like.

[0171] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate. [Example]

[0172] An experiment was carried out on a flame-retardant electrolyte solution for use in a secondary battery, and the results are described below.

[0173] <Flame retardancy test method for electrolyte> The samples were prepared by dissolving a predetermined amount of lithium hexafluorophosphate (LiPF6) in an organic solvent, methyl 2,2,2-trifluoroethyl carbonate (FEMC). Three types of electrolyte samples were prepared, with lithium salt content of 0M, 1.0mol / L (1M), 1.5mol / L (1.5M), and 2.0mol / L (2M) in the organic solvent.

[0174] Glass fiber was impregnated with a sample of electrolyte, and in an atmospheric environment, a lighter flame was brought close to the sample from below, and the state of the flame was visually confirmed to evaluate the flame retardancy of each sample. The combustion behavior of the ignited flame was observed and evaluated. At the same time, ignition ability, flammability, and phenomena occurring during secondary ignition were also observed.

[0175] [Table 1]

[0176] As shown in Table 1, FEMC alone (0M) without added lithium salt and a lithium salt volume molar concentration of 1M resulted in sustained combustion, whereas samples with LiPF6 contents of 1.5M and 2M were confirmed to be flame retardant. The flames of the 1.5M and 2M samples were extinguished immediately when the lighter was removed.

[0177] Furthermore, as a comparative example, a sample using lithium bis(fluorosulfonyl)imide (LiFSI) as the lithium salt was prepared. When LiFSI was used, the 1M, 1.5M, and 2M samples all continued to burn. When LiFSI was used, the samples continued to burn even after the lighter was removed, and white smoke rose after the flame went out.

[0178] [Table 2]

[0179] These experimental results confirmed that electrolytes with LiPF6 contents of 1.5M and 2M for FEMC have flame-retardant properties. [Example]

[0180] In this example, a half-cell was fabricated using a flame-retardant electrolyte solution, and its electrical properties were measured. In this specification, a half-cell refers to a battery cell assembled using lithium metal as the negative electrode (counter electrode).

[0181] First, the positive electrode active material 100 used in the half cell will be described with reference to FIG.

[0182] FIG. 12 is a cross-sectional schematic diagram of a positive electrode active material 100, showing an example of the positive electrode active material 100 to which a coating portion 104 is attached. As shown in FIG. 12, the positive electrode active material 100 has a surface portion 100a and an interior portion 100b. In FIG. 12, a dashed line indicates the boundary between the surface portion 100a and the interior portion 100b. The surface portion 100a is synonymous with the near-surface, near-surface region, or shell.

[0183] 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.

[0184] Furthermore, when the cathode active material 100 has a layered rock-salt crystal structure of space group R-3m, the surface layer portion 100a has an edge region and a basal region. Here, the edge region has a surface exposed in a direction intersecting with the (001) plane (also referred to as a surface other than the (001)-oriented surface). The edge region is a region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or substantially perpendicular to the surface. Here, "intersects" refers to an angle between a perpendicular to the first plane (the (001) plane) and a normal to the second plane (the surface of the cathode active material 100) of 10 to 90 degrees, more preferably 30 to 90 degrees, and even more preferably 50 to 90 degrees.

[0185] The basal region has a surface parallel to the (001) plane (also referred to as a (001)-oriented surface), and the region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or nearly perpendicular from the surface to the interior is referred to as the basal region. Note that "parallel" here means that the angle between the perpendicular to the first surface (the (001) plane) and the normal to the second surface (the surface of the positive-electrode active material 100) is 0 degrees or more and less than 10 degrees, preferably 0 degrees or more and 5 degrees or less, and more preferably 0 degrees or more and 2.5 degrees or less.

[0186] The surface of the positive electrode active material 100 refers to the surface of the positive electrode active material including the surface layer 100a and the interior 100b. Therefore, the positive electrode active material 100 does not include metal oxides such as aluminum oxide that do not have lithium sites that can contribute to charge and discharge, carbonates that are chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. The attached metal oxide refers to, for example, a metal oxide whose crystal structure does not match that of the interior 100b.

[0187] Furthermore, the positive electrode active material 100 does not include the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 100.

[0188] Although not shown, the positive electrode active material 100 may have a crystal grain boundary. The crystal grain boundary refers to, for example, a portion where particles of the positive electrode active material 100 adhere to each other, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repeated bright and dark lines in a STEM image or the like become discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. Furthermore, the crystal defect refers to a defect that can be observed in a cross-sectional TEM (transmission electron microscope) image, a cross-sectional STEM image, etc., i.e., a structure in which other atoms have entered between lattices, a cavity, etc. A crystal grain boundary can be considered a type of planar defect. Furthermore, the vicinity of a crystal grain boundary refers to a region within 10 nm of the crystal grain boundary.

[0189] <Contained elements> The positive electrode active material 100 contains lithium, a transition metal M, oxygen, and an additive element. The transition metal M is at least one selected from cobalt, nickel, and manganese. Alternatively, the positive electrode active material 100 contains a composite oxide (LiMO2) of lithium and a transition metal M to which the additive element has been added. However, the positive electrode active material 100 of one embodiment of the present invention has a distribution or crystalline structure of the additive element described below. Therefore, the composition is not strictly limited to Li:M:O=1:1:2 (atomic ratio).

[0190] The positive electrode active material of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and extracted. The positive electrode active material 100 of one embodiment of the present invention preferably uses cobalt as the transition metal M responsible for the oxidation and reduction reaction. It is preferable that the positive electrode active material 100 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals M contained in the positive electrode active material 100, because of its many advantages, such as relatively easy synthesis and handling, and excellent charge-discharge cycle characteristics.

[0191] Furthermore, when cobalt accounts for 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metal M in the positive electrode active material 100, the stability when a large amount of lithium is desorbed during charging is superior to composite oxides in which nickel accounts for the majority of the transition metal, such as lithium nickel oxide (LiNiO2). This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides in which octahedral low-spin nickel(III) accounts for the majority of the transition metal M, such as lithium nickel oxide, are significantly affected by the Jahn-Teller effect, which makes the octahedral layers of nickel and oxygen more susceptible to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions, closer in size to lithium ions. Therefore, in layered rock-salt type composite oxides in which nickel accounts for the majority of the transition metal M, such as lithium nickel oxide, there is a problem in that cation mixing of nickel and lithium is likely to occur.

[0192] The additive elements contained in the positive electrode active material 100 are preferably one or more selected from magnesium, titanium, nickel, aluminum, fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0193] That is, the positive electrode active material 100 can include lithium cobalt oxide doped with magnesium and titanium, lithium cobalt oxide doped with magnesium, titanium, and fluorine, lithium cobalt oxide doped with magnesium, nickel, aluminum, and titanium, lithium cobalt oxide doped with magnesium, nickel, aluminum, titanium, and fluorine, and the like.

[0194] The additive element is preferably dissolved in the positive electrode active material 100. Therefore, for example, when performing a line analysis by STEM-EDX, the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the transition metal M detected increases, i.e., closer to the interior of the positive electrode active material 100.

[0195] In this specification, the depth at which the amount of a certain element detected increases in STEM-EDX line analysis refers to the depth at which measurement values ​​that can be determined not to be noise in terms of intensity, spatial resolution, etc. are continuously obtained.

[0196] These added elements further stabilize the crystal structure of the positive electrode active material 100, as will be described later.

[0197] The additive elements do not necessarily have to include all of fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0198] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent charge-discharge 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.

[0199] The positive electrode active material 100 has the above-described additive element in the surface layer portion 100a. It is more preferable that the positive electrode active material 100 has a plurality of additive elements. It is also preferable that the surface layer portion 100a has a higher concentration of one or more selected from the additive elements than the inner portion 100b. It is also preferable that the surface layer portion 100a has a stronger detected amount of one or more selected from the additive elements than the inner portion 100b. It is also preferable that the one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient.

[0200] It is also preferable that the distribution of the positive electrode active material 100 differs depending on the added element. For example, it is more preferable that the depth of the peak of the detected amount in the surface layer 100a from the surface or from a reference point in EDX ray analysis (described later) differs depending on the added element. The detected amount peak here refers to the maximum value of the detected amount at a depth of 50 nm or less from the surface in the surface layer 100a. The detected amount refers to, for example, the count in EDX ray analysis.

[0201] 〔titanium〕 The presence of titanium as an additive element in the surface layer portion 100a can promote the insertion and desorption of lithium ions into the positive electrode active material 100.

[0202] Although titanium lies stably in the octahedral oxygen-coordinated positions in the oxide, titanium oxide and lithium titanate cannot form a stable layered rock-salt crystal structure. For example, TiO2 has the most stable rutile crystal structure, and Li4Ti5O 12 has a spinel-type crystal structure. Therefore, when a small amount of titanium is dissolved in the surface layer 100a having a layered rock salt type or rock salt type crystal structure, defects occur in part of the crystal structure of the surface layer 100a.

[0203] Lithium cobalt oxide is in a discharged state (i.e., Li x It is known that when x in CoO2 is 1, the band gap is large and the electrical resistance is high. Therefore, by introducing titanium defects into a part of the surface layer portion 100a, the band gap can be narrowed and the resistance can be reduced to a desirable extent.

[0204] Cobalt is also present in the oxide. 3+ is the most stable, while titanium is Ti 4+ Therefore, the charge on the lithium ion around titanium is relatively lower than that around cobalt. 4+ Defects may be induced in the cation sites near titanium, and such defects reduce the diffusion resistance of cations, especially lithium ions. Therefore, the diffusion resistance of lithium ions is reduced around titanium. Note that some of the cobalt around titanium is also affected by the presence of titanium, resulting in the formation of Co 2+ It may be the case.

[0205] Therefore, the presence of titanium in the surface layer portion 100a can reduce the diffusion resistance of lithium ions at the interface between the electrolyte and the positive electrode active material 100. Furthermore, it is believed that the titanium present in the surface layer portion 100a does not significantly reduce the electrical resistance (e.g., powder resistance) of the positive electrode active material 100. A relatively high electrical resistance of the positive electrode active material 100 is preferable because it contributes to safety in the event of a short circuit in the secondary battery.

[0206] If the amount of titanium is too small, the effect of promoting the insertion and desorption of lithium ions described above will not be fully exhibited. On the other hand, if the amount of titanium is too large, there is a risk of forming a heterophase (such as MgTiO3 with an ilmenite-type crystal structure) with other added elements, including magnesium. Furthermore, there is a risk that magnesium will be lost in the heterophase formation, which will reduce the magnesium concentration in the surface layer 100a, whose crystal structure roughly matches that of the interior 100b. 4.6V (vs. Li / Li + For the cathode active material 100 of one embodiment of the present invention, which is intended for high-voltage charging exceeding 100 V, magnesium present at a high concentration in the surface layer 100a is essential for suppressing phase change. Therefore, the loss of magnesium due to the formation of a different phase is a major disadvantage. Furthermore, if there are too many titanium defects, there is a concern that oxygen may be more likely to be desorbed from the surface. Therefore, it is preferable that titanium, together with magnesium, fluorine, etc., be present at a lower concentration than magnesium in the surface and near-surface regions of the surface layer 100a.

[0207] Specifically, the Ti / Co ratio (atomic ratio) of the entire positive electrode active material is preferably 0.0001 or more and 0.005 or less, more preferably 0.0005 or more and 0.0025 or less.

[0208] Furthermore, in EDX analysis of the surface layer portion 100a, when the denominator is the sum of carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, gallium, and molybdenum, the maximum titanium concentration is preferably 0.3 atomic% or more and 3 atomic% or less, and more preferably 0.5 atomic% or more and 2 atomic% or less.

[0209] The titanium source is not particularly limited, but examples thereof include titanium oxides such as TiO and / or LiTiO, LiTiO, LiTiO, and LiTiO. 12 In particular, when a heating step is carried out after mixing the titanium source, it is possible that part of the lithium evaporates during heating, and therefore it is preferable to use the lithium titanate described above that contains lithium.

[0210] Furthermore, the element that can be added to promote the insertion and desorption of lithium ions is not limited to titanium. Other elements may also be added. When using elements other than titanium, it is preferable that they exist stably in the hexacoordinated octahedral oxygen positions in the oxide, but that the oxide of lithium and the element does not form a stable layered rock-salt type crystal structure. Furthermore, elements that are stable at a higher oxidation number than cobalt are preferable. For example, niobium, manganese, tungsten, chromium, molybdenum, rhenium, tantalum, etc. can be used.

[0211] 〔distribution〕 To achieve the above-mentioned effects, it is preferable that the concentration of at least magnesium, nickel, and titanium among the additive elements is higher in the surface layer portion 100a than in the interior portion 100b. Alternatively, it is preferable that the detectable amount in the surface layer portion 100a is greater than that in the interior portion 100b. Furthermore, it is preferable that the detectable amount peak is located closer to the surface of the surface layer portion 100a. For example, it is preferable that the detectable amount peak is located on the surface or within 3 nm from the reference point. It is also preferable that the distributions of magnesium and nickel overlap. The detectable amount peaks of magnesium and nickel may be at the same depth, or the magnesium peak may be closer to the surface, or the nickel peak may be closer to the surface. The difference in depth between the detectable amount peak of nickel and the detectable amount peak of magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detectable amount is narrow. In this specification, the half-width refers to the full width at half maximum.

[0212] Similarly, it is preferable that the distributions of magnesium and titanium overlap. The peaks of the detected amounts of magnesium and titanium may be at the same depth, or the magnesium peak may be closer to the surface, or the titanium peak may be closer to the surface. The difference in depth between the peak of detected amount of titanium and the peak of detected amount of magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount is narrow.

[0213] In other words, it is preferable that the distributions of magnesium, nickel, and titanium overlap. The difference in depth between the peaks of detected titanium, nickel, and magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amounts is narrow.

[0214] The above-mentioned region where the distributions of magnesium and nickel overlap, the region where the distributions of magnesium and titanium overlap, or the region where the distributions of magnesium, nickel, and titanium overlap is preferably located in the edge region of the surface layer portion 100a where lithium ions are inserted and desorbed. On the other hand, the above-mentioned overlapping region is not necessarily required in the basal region of the surface layer portion 100a.

[0215] Furthermore, the amount of nickel detected in the interior 100b may be very small compared to the surface layer 100a, or may not be detected, or may be 1 atomic % or less.

[0216] Although not shown, it is preferable that the detectable amount of fluorine in the surface layer 100a is greater than the detectable amount inside, as with magnesium or nickel. It is also preferable that the detectable amount peak in the surface layer 100a is closer to the surface. For example, it is preferable that the detectable amount peak is on the surface or within 3 nm from the reference point. Similarly, it is preferable that the detectable amount of titanium, silicon, phosphorus, boron, and / or calcium is greater than the detectable amount inside the surface layer 100a. It is also preferable that the detectable amount peak is on the surface or within 3 nm from the reference point.

[0217] Furthermore, it is preferable that at least aluminum, among the added elements, has a peak of detectable amount further inward than magnesium and titanium. The distributions of magnesium and aluminum may overlap, but the overlapping region may be minimal. The peak of detectable amount of aluminum 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 or the reference point toward the interior.

[0218] The distribution of aluminum as described above can further stabilize the layered rock-salt crystal structure of the positive electrode active material 100. For example, it is expected that the change from the layered rock-salt crystal structure to a spinel crystal structure in the surface layer 100a of the positive electrode active material 100 can be suppressed. The spinel crystal structure generated in the layered rock-salt crystal structure may move or expand due to the transfer of charge of the transition metal M. Furthermore, defects such as grain boundaries in the positive electrode active material 100 can serve as paths for lithium ion diffusion in the c-axis direction. Additional elements such as aluminum may be present near the defects. In other words, the presence of aluminum may facilitate lithium ion diffusion.

[0219] The reason why aluminum is distributed deeper than magnesium and titanium is thought to be because aluminum has a faster diffusion rate than magnesium. On the other hand, the reason why the amount of aluminum detected in the region closest to the surface is low is presumably because aluminum exists more stably in regions where magnesium and other elements are not present in solid solution at high concentrations than in regions where they are not.

[0220] 〔magnesium〕 Magnesium is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is presumably because the magnesium present at the lithium site functions as a pillar supporting the MO2 layers. In addition, the presence of magnesium makes it easier to maintain the Li x When x in CoO2 is, for example, 0.24 or less, the desorption of oxygen from around magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100.

[0221] 〔nickel〕 In the layered rock-salt crystal structure of LiMO2, nickel can exist in either the cobalt site or the lithium site. When nickel exists in the cobalt site, it has a lower redox potential than cobalt, so it is easier to release lithium and electrons during charging, for example. This can be expected to result in faster charge and discharge speeds. Therefore, even at the same charge voltage, a larger charge and discharge capacity can be obtained when the transition metal M is nickel than when it is cobalt.

[0222] 〔aluminum〕 Aluminum can also 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. Therefore, as described below, even if the positive electrode active material 100 experiences a force that causes it to expand and contract in the c-axis direction due to the insertion and extraction of lithium ions, i.e., even if a force that causes it to expand and contract in the c-axis direction due to changes in the charge depth or charge rate occurs, deterioration of the positive electrode active material 100 can be suppressed.

[0223] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, because the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen from the aluminum's surroundings. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0224] [Fluorine] Fluorine is a monovalent anion, and when some of the oxygen in the surface layer 100a is replaced by fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on whether or not fluorine is present. In other words, when fluorine is absent, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two. Therefore, when some of the oxygen in the surface layer 100a of the positive electrode active material 100 is replaced by fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, corrosion resistance to hydrofluoric acid can be effectively improved.

[0225] Furthermore, when the melting point of a fluoride such as lithium fluoride is lower than that of the other additive element sources, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element sources. When the fluoride contains LiF and MgF2, the eutectic point of LiF and MgF2 is around 742°C, so it is preferable to set the heating temperature in the heating step after mixing the additive elements to 742°C or higher.

[0226] [Other additive elements] When phosphorus is present in the surface layer 100a, Li x When x in CoO2 is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that CoO2 exists in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0227] When LiPF6 is mixed into the electrolyte, there is a risk of hydrogen fluoride being generated due to hydrolysis. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 can be suppressed.

[0228] When the positive electrode active material 100 contains phosphorus together with magnesium, Li xWhen x in CoO2 is small, the stability is extremely high, which is preferable.

[0229] 12, the coating portion 104 covering the surface of the positive electrode active material 100 is preferably formed by the accumulation of an ion-conductive polymer, an electrolyte solution, and decomposition products of the electrolyte solution, for example, during charge and discharge. The coating portion 104 preferably contains, for example, carbon, oxygen, and fluorine. The coating portion 104 does not have to cover the entire positive electrode active material 100. For example, it is sufficient for the coating portion 104 to cover 50% or more of the surface of the positive electrode active material 100, with 70% or more being more preferable and 90% or more being even more preferable.

[0230] The positive electrode active material 100 used in this example has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge.

[0231] The distribution of the additive elements contained in the positive electrode active material 100 can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0232] The crystal structure of the surface layer 100a, the grain boundaries, etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100, for example.

[0233] The positive electrode active material 100 used in this example is characterized in that the volume resistivity of the powder of the positive electrode active material 100 is 1.0×10 8 Ω cm or more 1.0×10 10 It is preferable that the resistance is 5.0×10 Ω·cm or less. 8 Ω cm or more 1.5×10 9 It is more preferable that the resistivity is Ω·cm or less.

[0234] The positive electrode active material 100 having the above volume resistivity has a stable crystal structure even at high voltages. Therefore, the volume resistivity of the powder of the positive electrode active material 100 being within the above range can be used as an indicator that the surface layer portion 100a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been successfully formed.

[0235] <Preparation of positive electrode active material> A method for producing the positive electrode active material 100 used in this example will be described with reference to the production method shown in FIGS.

[0236] In order to prepare the positive electrode active material 100 having the distribution, composition, and / or crystal structure of the additive elements as described above, the method of adding the additive elements is important. At the same time, it is also important that the crystallinity of the inner portion 100b is good.

[0237] Therefore, in the process of producing the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, and then mix in the additive element source and perform a heat treatment.

[0238] In the method of synthesizing lithium cobalt oxide containing an additive element by mixing an additive element source simultaneously with a cobalt source and a lithium source, it is difficult to increase the concentration of the additive element in the surface layer portion 100a. Furthermore, if the additive element source is simply mixed without heating after synthesizing lithium cobalt oxide, the additive element will simply adhere to the lithium cobalt oxide without dissolving in the lithium cobalt oxide. Without sufficient heating, it is difficult to achieve a good distribution of the additive element. Therefore, it is preferable to mix the additive element source after synthesizing lithium cobalt oxide and then perform a heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

[0239] However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that an added element, such as magnesium, will enter the cobalt site. Magnesium present in the cobalt site will react with Li xWhen x in CoO2 is small, it is not effective in maintaining the R-3m layered rock salt crystal structure. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as cobalt being reduced to divalent and lithium evaporating.

[0240] Therefore, it is preferable to mix a material that functions as a flux with the additive element source. Any material with a lower melting point than lithium cobalt oxide can function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting points of the additive element source and lithium cobalt oxide. Lowering the melting point makes it easier to distribute the additive elements well at a temperature where cation mixing is unlikely to occur.

[0241] <<Method for producing positive electrode active material>> A method for manufacturing the positive electrode active material 100 will be described with reference to FIGS.

[0242] <Step S11> In step S11 shown in FIG. 13(A), a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0243] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and for example, it is recommended to use a material with a purity of 99.99% or higher.

[0244] As the cobalt source, it is preferable to use a compound containing cobalt, for example, cobalt oxide such as tricobalt tetroxide, cobalt hydroxide, or the like.

[0245] 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 and / or reliability of the secondary battery are increased.

[0246] In addition, the cobalt source preferably has high crystallinity, for example, it preferably has single crystal grains. The crystallinity of the cobalt source can be evaluated using TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, etc., or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. Note that the above-mentioned methods for evaluating crystallinity can be applied not only to cobalt sources but also to the evaluation of the crystallinity of other sources.

[0247] <Step S12> Next, in step S12 shown in FIG. 13(A), the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller fragments. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the cobalt source in dehydrated acetone with a purity of 99.5% or higher, with a water content of 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0248] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as pulverizing media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0249] <Step S13> Next, in step S13 shown in FIG. 13(A), the mixed material is heated. Heating is preferably performed at a temperature between 800°C and 1100°C, more preferably between 900°C and 1000°C, and even more preferably around 950°C. If the temperature is too low, the decomposition and melting of the lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, 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.

[0250] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0251] The temperature rise rate depends on the heating temperature reached, but is preferably 80°C / h to 250°C / h. For example, when heating at 1000°C for 10 hours, the temperature rise rate is preferably 200°C / h.

[0252] Heating is preferably carried out in an atmosphere with little moisture, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, to suppress impurities that may be mixed into the material, it is preferable that the impurity concentrations of CH4, CO, CO2, and H2 in the heating atmosphere are each 5 ppb (parts per billion) or less.

[0253] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flow through the reaction chamber is called flow.

[0254] When the heating atmosphere is an atmosphere containing oxygen, a method of preventing oxygen flow can be used. For example, the reaction chamber can be depressurized and then filled with oxygen (or purged) to prevent oxygen from entering or leaving the reaction chamber. For example, the reaction chamber can be depressurized to -970 hPa and then filled with oxygen to 50 hPa.

[0255] After heating, the material is allowed to cool naturally, but it is preferable that the time required to cool from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary, as long as the material is cooled to a temperature acceptable for the next step.

[0256] The heating in this step can also be a step of heating using a rotary kiln or a roller hearth kiln. Heating using a rotary kiln can be performed while stirring, whether it is a continuous or batch type.

[0257] The container for accommodating the object to be heated during heating is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, a setter made of aluminum oxide with a purity of 99.9% is used. It is preferable to heat the crucible or setter with a lid, as this prevents the material from volatilizing. Mullite-cordierite may also be used as the material for the crucible and setter.

[0258] Furthermore, it is preferable to use a crucible that has been used multiple times rather than a new one. In this specification, a new crucible refers to one that has undergone two or fewer heating processes with materials containing lithium, transition metal M, and / or additive elements. A multiple-use crucible refers to one that has undergone three or more heating processes with materials containing lithium, transition metal M, and / or additive elements. This is because using a new crucible may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the sheath during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, may not fall within the desired range. On the other hand, this risk is less likely with a multiple-use crucible.

[0259] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use an aluminum oxide mortar as the mortar. Aluminum oxide mortars are made of a material that does not easily release impurities. Specifically, an aluminum oxide mortar with a purity of 90% or more, preferably 99% or more, is used. Note that heating conditions equivalent to those of step S13 can be applied to heating steps other than step S13, which will be described later.

[0260] <Step S14> By the above steps, lithium cobalt oxide (LiCoO2) can be synthesized as shown in step S14 in FIG. 13(A).

[0261] Although the composite oxide is produced by the solid phase method in steps S11 to S14, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0262] Note that lithium cobalt oxide synthesized in advance may be used in step S14, in which case steps S11 to S13 can be omitted.

[0263] <Step S20> Next, as shown in step S20, it is preferable to add an additive element to the lithium cobalt oxide. In the method for producing a positive electrode active material described in this embodiment, the additive element is added in multiple steps, so in the flow shown in Figure 13, the additive element added first will be referred to as A1, the additive element added second time will be referred to as A2, and the additive element added third time will be referred to as A3. The step of adding additive element A1 will be described with reference to Figure 14(A).

[0264] <Step S21> 14(A), an additive element source (Al source) to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the Al source.

[0265] The additive element A1 can be any of the additive elements described in the previous embodiments, specifically, one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0266] When magnesium is selected as the additive element, the source of the additive element can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0267] When fluorine is selected as the additive element, the source of the additive element can be called a fluorine source. Examples of the fluorine source that can be used include lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF3, CeF4), lanthanum fluoride (LaF3), and sodium aluminum hexafluoride (Na3AlF6). Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating step described below.

[0268] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0269] The fluorine source may be fluorine (F), carbon fluoride, sulfur fluoride, oxygen fluoride (OF, OF, OF, OF, OF, OF), nitrogen trifluoride (NF), or the like, which may be mixed into the atmosphere during the heating step described below. A plurality of the above-mentioned fluorine sources may also be used.

[0270] In the positive electrode active material preparation method 1 described in FIGS. 13 and 14(C), magnesium and fluorine are used as the additive element A1. Lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as the fluorine source and magnesium source. The melting point is most effectively lowered when lithium fluoride and magnesium fluoride are mixed in a molar ratio of approximately LiF:MgF2 = 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium content will be excessive, resulting in poor charge-discharge cycle performance. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably LiF:MgF2 = 1:x (2≦x≦4), more preferably LiF:MgF2 = 1:x (2.5≦x≦3.5), and even more preferably LiF:MgF2 = 1:x (x = 3 or nearby). In this specification, "nearby" refers to a value greater than 0.9 times but less than 1.1 times the value.

[0271] <Step S22> 14(A), the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0272] <Step S23> Next, in step S23 shown in Fig. 14(A), the pulverized and mixed materials are collected to obtain the A1 source. Note that the A1 source shown in step S23 contains multiple starting materials and can be called a mixture.

[0273] The particle size of the mixture is preferably such that D50 (median diameter) is 600 nm or more and 20 μm or less.

[0274] Such a finely powdered mixture (including the case where only one additive element is included) can be easily adhered uniformly to the surfaces of the lithium cobalt oxide particles when mixed with the lithium cobalt oxide in a later step. If the mixture is evenly adhered to the surfaces of the lithium cobalt oxide particles, it is preferable because the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating.

[0275] <Step S31> 13, lithium cobalt oxide and an Al source are mixed together. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the Al source is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably Co:Mg=100:y (0.3≦y≦3).

[0276] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0277] 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.

[0278] <Step S32> 13, the mixed materials are collected to obtain a mixture 901. When collecting the materials, they may be crushed and then sieved, if necessary.

[0279] <Step S33> Next, in step S33 shown in FIG. 13(A), the mixture 901 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may be higher than atmospheric pressure in order to increase the oxygen partial pressure in the heating atmosphere. This is because if the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and the like may be reduced, and the layered rock salt type crystal structure of lithium cobalt oxide and the like may not be able to be maintained.

[0280] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds is the temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and can be set to a temperature lower than the melting temperature of these materials. An oxide will be used as an example for explanation, and the melting temperature T m 0.757 times (Tanman temperature T d ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 is preferably 650° C. or higher.

[0281] Of course, the reaction proceeds more easily if the temperature is equal to or higher than the melting point of one or more of the materials contained in mixture 901. For example, when LiF and MgF2 are contained as the additive element sources, the eutectic point of LiF and MgF2 is around 742°C, so it is preferable to set the lower limit of the heating temperature in step S33 to 742°C.

[0282] The upper limit of the heating temperature is below the melting point of lithium cobalt oxide (1130°C). At temperatures near the melting point, decomposition of lithium cobalt oxide, even if slight, is a concern. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride, such as that from the fluorine source, within an appropriate range. If the temperature is too high, the fluoride will evaporate and decrease. For example, the vapor pressure of lithium fluoride increases rapidly above 900°C. Therefore, a temperature of 1000°C or less is preferable, and 950°C or less is even more preferable, and 850°C or less is even more preferable. Suppressing the evaporation of lithium fluoride allows the surface layer 100a to have high concentrations of fluorine and lithium. Having a sufficient amount of lithium in the surface layer 100a also has the advantage of making it less likely for a different phase (e.g., MgTiO) to form when titanium is added in a later process.

[0283] In consideration of these, the heating temperature in step S33 is preferably 650°C to 1130°C, more preferably 650°C to 1000°C, even more preferably 650°C to 950°C, and even more preferably 650°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 800°C to 1100°C, or 800°C to 1130°C, more preferably 800°C to 1000°C, even more preferably 800°C to 950°C, and even more preferably 800°C to 900°C.

[0284] Furthermore, when the mixture 901 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0285] In the fabrication method described in this embodiment, some materials, such as LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be lowered below the melting point of lithium cobalt oxide, for example, to 742°C or higher and 950°C or lower, and allows additive elements such as magnesium to be distributed in the surface layer, thereby producing a positive electrode active material with excellent characteristics.

[0286] However, because LiF has a lighter specific gravity in its gaseous state than oxygen, it may volatilize or sublime when heated. If it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, the Li on the LiCoO2 surface may react with the F fluorine source, producing LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress the volatilization.

[0287] Therefore, it is preferable to heat the mixture 901 in an atmosphere containing LiF, that is, to heat the mixture 901 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, the volatilization of LiF in the mixture 901 can be suppressed.

[0288] The heating in this step is preferably performed so as not to cause the particles of the mixture 901 to stick together. If the particles of the mixture 901 stick together during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the added elements (for example, fluorine) diffuse is blocked, which may result in poor distribution of the added elements (for example, magnesium and fluorine) in the surface layer.

[0289] It is also believed that uniform distribution of an additive element (e.g., fluorine) in the surface layer portion results in a smooth, less uneven cathode active material. Therefore, in order to maintain or further smooth the surface after heating in step S15 in this process, it is better for the particles of mixture 901 not to stick together.

[0290] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0291] When heating is performed using a roller hearth kiln, the mixture 901 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 901.

[0292] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of the lithium cobalt oxide in step S14, etc. When the lithium cobalt oxide is small, a lower temperature or a shorter heating time may be preferable than when the lithium cobalt oxide is large.

[0293] When the median diameter (D50) of the lithium cobalt oxide in step S14 of Fig. 13(A) is about 7 µm, the heating temperature is preferably, for example, 650°C or higher and 950°C or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 5 hours. The temperature reduction time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0294] <Step S34> Next, in step S34 shown in FIG. 13, the heated material is recovered and crushed as necessary to obtain a composite oxide 902.

[0295] <Step S40> Next, in step S40 shown in FIG. 13, an additive element source (A2 source) is prepared. The additive element A2 can be the additive element described in step S21. In method 1 for producing a positive electrode active material described in FIGS. 13 to 14(C), nickel and aluminum are used as the additive element A2. Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source. As shown in steps S41 to S43 in FIG. 14(B), the nickel source and the aluminum source can be crushed to obtain the A2 source. The crushing conditions can be determined by taking into account the conditions in step S22.

[0296] <Step S51> 13, the composite oxide 902 and the A2 source are mixed. The mixing conditions can be determined by referring to the description of step S31.

[0297] <Step S52> 13, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0298] <Step S53> 13, the mixture 903 is heated for two hours.

[0299] <Step S54> Next, in step S54 shown in FIG. 13, the heated material is recovered and crushed as necessary to obtain a composite oxide 904.

[0300] <Step S60> Next, in step S60 shown in FIG. 13, an additive element source (A3 source) is prepared. The additive element A3 can be the additive element described in step S21. In the method for producing a positive electrode active material described in FIGS. 13 to 14(C), titanium is used as the additive element A3. Examples of titanium sources that can be used include lithium titanate, titanium oxide, and titanium hydroxide. As shown in steps S61 to S63 of FIG. 14(C), the titanium source can be pulverized to obtain the A3 source. The pulverization conditions can be referenced to the conditions in step S22. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of titanium atoms Ti in the A3 source is preferably Co:Ti=100:z (0.01≦z≦0.5), and more preferably Co:Mg=100:z (0.05≦z≦0.25).

[0301] <Step S71> 13, the composite oxide 904 is mixed with the A3 source. The mixing conditions can be determined by referring to the description of step S31.

[0302] <Step S72> 13, the mixed materials are recovered to obtain a mixture 905. When recovering the materials, they may be crushed and then sieved, if necessary.

[0303] <Step S73> 13, the mixture 905 is heated for two hours.

[0304] <Step S74> Next, in step S74 shown in FIG. 13, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered particles. Through the above steps, the positive electrode active material 100 according to one embodiment of the present invention can be produced. The positive electrode active material of this example has a smooth surface.

[0305] Positive electrode active material 100 with a smooth surface may be more resistant to physical destruction due to pressure, etc. than positive electrode active materials that do not have a smooth surface. For example, positive electrode active material 100 is less likely to be destroyed in tests involving pressure, such as a nail penetration test, and as a result, safety may be improved.

[0306] <Preparation of positive electrode> A positive electrode active material 100 obtained according to the above-described method for producing a positive electrode active material was prepared. Acetylene black (AB) was prepared as a conductive material, and polyvinylidene fluoride (PVDF) was prepared as a 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 96:2:2 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.

[0307] 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. The pressing process was performed under a linear pressure of 1467 kN / m. The upper and lower rolls of the roll press were both set at 120°C.

[0308] By the above steps, a positive electrode having the positive electrode active material 100 was obtained. The amount of the positive electrode active material supported was 13 mg / cm. 2 More than 14mg / cm 2 The values ​​were adjusted to the following ranges:

[0309] Next, a half-cell was fabricated using the above positive electrode. As mentioned above, lithium metal was used as the counter electrode (negative electrode of the test battery). The aforementioned flame-retardant electrolyte (two types: 1.5M and 2.0M) was used as the electrolyte. The test battery was a coin-type battery (coin cell) with a CR2032 type (diameter 20mm, height 3.2mm).

[0310] The positive electrode can and the negative electrode can can be made of stainless steel (SUS). In this example, an aluminum foil is placed on the positive electrode can, and the positive electrode is placed on the aluminum foil.

[0311] The separator used was a polyimide separator manufactured by Tokyo Ohka Kogyo Co., Ltd. The polyimide separator is flame-retardant and has a contact angle of less than 10° with the electrolyte, making it preferable.

[0312] The charge-discharge rate characteristics at 25°C were measured using the test battery (half cell) prepared as described above. Figure 15(A) shows the characteristics of a half cell using a 1.5M electrolyte, and Figure 15(B) shows the characteristics of a half cell using a 2.0M electrolyte. The graphs show the first through fourth cycles. The first cycle was charged under CCCV (constant current, constant voltage) conditions (0.2C, approximately 4.6V, 0.05C cutoff) and discharged under CC conditions (0.2C, 3V cutoff). Note that 0.05C corresponds to a current value of 10mA / g (assuming 1C is 200mA / g of positive electrode active material). The rest time was 10 minutes.

[0313] Furthermore, when observing the phase change of the positive electrode active material, 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 the desired voltage is charged for a sufficient period of time. For example, when charging using CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV (constant voltage) charging can be terminated at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material, it is desirable to charge at such a low current value. On the other hand, if the current does not reach 2 mA / g or more and 10 mA / g or less even after long-term CV charging, it is likely that the current is being consumed for the decomposition of the electrolyte rather than for charging the positive electrode active material. Therefore, CV charging can be terminated after a sufficient time has elapsed. In this case, a sufficient time can be, for example, 1.5 hours or more and 3 hours or less. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell can be disassembled in an argon-filled glove box and the positive electrode removed to obtain a positive electrode active material with the desired charge capacity. When performing various analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container in an argon atmosphere. It is also preferable to remove the positive electrode promptly after charging is complete and subject it to analysis. Specifically, within 1 hour after charging is complete, and more preferably within 30 minutes.

[0314] 15(A) and 15(B) show that a secondary battery using powdered cathode active material 100 as the cathode active material and a flame-retardant electrolyte (using methyl 2,2,2-trifluoroethyl carbonate as the solvent and LiPF6 as the lithium salt) can be charged and discharged, and that the capacity per unit weight is large. The coin cell using the electrolyte (1.5M) in FIG. 15(A) exhibited a discharge capacity per unit weight of approximately 215 mAh / g, and the coin cell using the electrolyte (2M) in FIG. 15(B) exhibited a discharge capacity per unit weight of approximately 216 mAh / g. The combination of cathode active material 100 and the flame-retardant electrolyte is the reason for such high discharge capacity.

[0315] A half cell was also fabricated that was partially different from the half cell described above, and the results of the cycle test are shown in Figure 16. In Figure 16, the vertical axis represents the discharge capacity, and the horizontal axis represents the number of cycles.

[0316] The electrolyte used in the half cell was methyl 2,2,2-trifluoroethyl carbonate as a solvent, and LiPF6 as a lithium salt at a concentration of 2.0 M, to which 2 wt % of propane sultone was further added.

[0317] 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. 2 The pressing conditions when performing the pressing treatment using a roll press were a linear pressure of 210 kN / m.

[0318] The cycle evaluation conditions were CCCV charging (0.5C, 4.6V, 0.05C cutoff) and CC discharging (0.5C, 2.5V cutoff). The measurement environment was 25°C, and the rest time was 10 minutes.

[0319] The cycle test results shown in FIG. 16 are good, and this is due to the combination of the positive electrode active material 100, the flame-retardant electrolyte, and the additive propane sultone. [Example]

[0320] In this example, CV (cyclic voltammetry) measurements were carried out on a flame-retardant electrolyte solution.

[0321] The counter electrode was lithium, and the working electrode was aluminum foil (aluminum foil coated with AB:PVDF=1:1). The working electrode area was 1.130 cm 2The electrolyte for the reference electrode used methyl 2,2,2-trifluoroethyl carbonate as a solvent and LiPF6 as a lithium salt, with the concentration of the lithium salt in the electrolyte being 2.0 mol / L.

[0322] The scan rate was 0.5 mV / sec, and the voltage range was 2.5 V to 4.6 V. The CV measurement results of the electrolyte are shown in Figure 17. In Figure 17, the horizontal axis is Li / Li + The vertical axis shows the potential versus the current density. CV measurements were performed for 100 cycles, but Figure 17 shows only 50 cycles.

[0323] In Figure 17, no decomposition peaks were observed after the coating was formed in cycles 1, 2, 3, 4, and 5. Furthermore, no particular reaction was observed on the carbon electrode or the lithium negative electrode even after 50 cycles. [Explanation of symbols]

[0324] 10 batteries 11 Positive electrode 12 Negative electrode 13 Separator 21 Positive electrode current collector 22 Cathode active material layer 31 Negative electrode current collector 32 Negative electrode active material layer 41 Conductive materials 60 Electrolyte 100 Cathode active material 100a Surface layer 100b internal 104 Covering part 300 Lithium-ion secondary battery 301 Positive electrode can 302 Anode can 303 Gasket 304 Positive electrode 305 Positive electrode current collector 306 Positive electrode active material layer 307 Negative electrode 308 Negative electrode current collector 309 Negative electrode active material layer 332 Washer 342 Spacer 500 Lithium-ion secondary battery 506 negative electrode 507 Positive electrode 508 Adhesive area 509 Exterior body 510 Negative lead electrode 511 Positive lead electrode 601 Positive electrode cap 602 Battery can 603 Positive terminal 604 Positive electrode 605 Electrolyte 606 negative electrode 607 Negative terminal 608 Insulating plate 609 Insulating board 611 PTC element 613 Safety valve mechanism 614 Conductive Plate 615 Energy Storage System 616 Lithium-ion secondary battery 620 Control Circuit 621 Wiring 622 Wiring 623 Wiring 624 Conductors 625 Insulator 626 Wiring 627 Wiring 628 Conductive Plate 901 mixture 902 Complex oxides 903 mixture 904 Complex oxides 905 mixture 911a terminal 911b terminal 913 Lithium-ion secondary battery 930 chassis 930a housing 930b housing 931 negative electrode 931a Negative electrode active material layer 932 Positive electrode 932a Cathode active material layer 933 Electrolyte 950 Wound body 950a Wound body 951 terminal 952 terminals 1300 Prismatic lithium-ion secondary battery 1301a First Battery 1301b First Battery 1302 Battery Controller 1303 Motor Controller 1304 Motor 1305 Gear 1306 DC / DC circuit 1307 Electric power steering 1308 Heater 1309 Defogger 1310 DC / DC circuit 1311 Second Battery 1312 inverter 1313 Audio 1314 Power window 1315 Lamps 1316 Tires 1317 Rear motor 1320 Control circuit section 1321 Control circuit section 1322 control circuit 1324 Switch section 1325 External terminal 1326 External terminal 1413 Fixed part 1414 Fixed part 1415 Battery Pack 1421 Wiring 1422 Wiring 2001 Automobile 2002 Transport Vehicle 2003 Transport Vehicle 2004 aircraft 2200 battery pack 2201 Battery pack 2202 Battery Pack 2203 Battery Pack 6800 satellite 6801 aircraft 6802 Solar Panel 6803 Antenna 6805 Lithium-ion secondary battery 6900 Probe 6901 aircraft 6902 Solar Sail 6905 Lithium-ion secondary battery 6910 Spaceship 6911 aircraft 6912 Solar Panel 6913 Lithium-ion secondary battery 6920 Rover 6921 aircraft 6922 Solar Panel 6923 Lithium-ion secondary battery 7100 Portable display devices 7101 Housing 7102 Display section 7103 Operation button 7104 Secondary battery 7200 Personal Digital Assistant 7201 Case 7202 Display section 7203 Band 7204 Buckle 7205 Operation button 7206 Input / output terminal 7207 Icon 7300 display device 7304 Display section 7400 mobile phone 7401 Housing 7402 Display section 7403 Operation button 7404 External connection port 7405 Speaker 7406 Microphone 7407 Secondary battery 8600 Scooter 8601 Side mirror 8602 Energy storage devices 8603 Turn signal light 8604 Under-seat storage 8700 Electric Bicycle 8701 Lithium-ion secondary battery 8702 Energy storage devices 8703 Display section 8704 Control circuit

Claims

1. A secondary battery including a positive electrode, a separator, a negative electrode, and an electrolyte solution, The electrolyte solution contains a fluorinated chain carbonate as an organic solvent and lithium hexafluorophosphate as a solute, The content of lithium hexafluorophosphate in the electrolyte solution is 1.5 mol / L or more and less than 3.0 mol / L, The secondary battery has an electrolyte solution that is fluid in a temperature range of 0°C to -40°C.

2. 2. The secondary battery according to claim 1, wherein the fluorinated chain carbonate is methyl 2,2,2-trifluoroethyl carbonate.

3. 2. The secondary battery according to claim 1, wherein the separator comprises polypropylene, polyimide, or glass fiber.

4. 2. The secondary battery according to claim 1, wherein the electrolyte solution contains propane sultone as an additive.

5. 2. The secondary battery according to claim 1, wherein the positive electrode comprises a composite oxide containing lithium, cobalt, magnesium, and nickel.

6. 2. The secondary battery according to claim 1, wherein the positive electrode comprises a composite oxide containing lithium, cobalt, magnesium, nickel, aluminum, and titanium.

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