Secondary battery

Abstract

To provide a secondary battery capable of withstanding at least high temperature by devising a structure of the secondary battery.SOLUTION: In the secondary battery, a positive electrode active material obtained by a manufacturing method including a first step in which magnesium fluoride, lithium fluoride, a nickel source, and an aluminum source are each pulverized and then mixed with lithium cobalt oxide powder to form a first mixture and a second step in which heating is performed at a temperature lower than the upper temperature limit of lithium cobalt oxide to form a second mixture is used, and LiBOB is added to an electrolytic solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a secondary battery, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a secondary battery, a power storage device, and a manufacturing method thereof.

[0002] In this specification, the term "secondary battery" or "power storage device" generally refers to elements and devices having a power storage function. [Background technology]

[0003] In recent years, there has been active development of various power storage devices, including lithium-ion secondary batteries, lithium-ion capacitors, and air batteries, as well as all-solid-state batteries. Demand for high-power, high-energy-density lithium-ion secondary batteries has rapidly expanded alongside the development of the semiconductor industry, and they are now essential to the modern information society as a rechargeable energy source, thanks to their high output and high energy density. These batteries are used in a variety of applications, including mobile phones, smartphones, tablets, and notebook computers, as well as portable music players, digital cameras, medical devices, and next-generation clean-energy vehicles, including hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs).

[0004] In addition, electric vehicles (EVs) are vehicles that are driven solely by electric motors, but there are also hybrid vehicles that have both an internal combustion engine and an electric motor. Multiple secondary batteries used in the vehicle are grouped into a single battery pack, and multiple sets of battery packs are placed under the vehicle.

[0005] As described above, lithium ion secondary batteries are used in a variety of fields and applications, and among these, the characteristics required of lithium ion secondary batteries include high energy density, high cycle characteristics, and safety in various operating environments.

[0006] Patent Document 1 discloses a secondary battery in which lithium bis(oxalato)borate (LiBOB) is added to the electrolyte.

[0007] Furthermore, fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and the like, and their physical properties have been studied (Non-Patent Document 1). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2019-179758 [Non-patent literature]

[0009] [Non-Patent Document 1] WE Counts, R. Roy, and EF Osborn, "Fluoride Model Systems: II, The Binary Systems CaF2-BeF2, MgF2-BeF2, and LiF-MgF2," Journal of the American Ceramic Society, 36 [1] 12-17 (1953). Summary of the Invention [Problem to be solved by the invention]

[0010] Electric vehicles are prone to temperature changes depending on their operating conditions and the environment, so temperature safety measures are necessary. Among the components installed in electric vehicles, secondary batteries play the most important role as the power source for the electric vehicle. However, there is a problem in that the temperature range in which they can operate normally is narrowly tolerable in the environment in which the electric vehicle is used.

[0011] If the temperature environment deviates from the normal range, it can have a significant impact on the charge / discharge performance and lifespan of the secondary battery, so it is desirable to use it in a constant temperature environment as much as possible. In addition to the surrounding environment, the temperature of the secondary battery itself rises when a large amount of current flows during charging and discharging.

[0012] In addition, organic solvents are used in the electrolytes used to construct secondary batteries. However, organic solvents are volatile and have low flash points, and when these organic solvents are used in lithium-ion secondary batteries, there is a risk of the lithium-ion secondary battery exploding or catching fire due to an increase in the internal temperature of the lithium-ion secondary battery caused by an internal short circuit or overcharging. In addition, some of the electrolytes (lithium salts) generate hydrofluoric acid through hydrolysis, which corrodes metals, raising concerns about the reliability of the battery.

[0013] Therefore, one of the objectives is to realize a secondary battery that can withstand at least high temperatures by devising a configuration for the secondary battery.

[0014] In addition, since electric vehicles are equipped with large-capacity secondary batteries, it may take a long time to fully charge the batteries after the capacity is low. For rapid charging, secondary batteries that can withstand high-voltage charging are required. An object of one embodiment of the present invention is to provide a secondary battery that can be charged at a high voltage.

[0015] Another object of one embodiment of the present invention is to provide a power storage device that is less susceptible to deterioration at high temperatures or at high charging voltage, or to provide a novel power storage device, electronic device, or the like. [Means for solving the problem]

[0016] If the charging voltage applied to a secondary battery can be increased, the time that it can be charged at a high voltage will be extended, the amount of charge per unit time will increase, and the charging time will be shortened. In the field of electrochemical cells, such as lithium-ion secondary batteries, high voltages exceeding 4.5 V will cause battery degradation.

[0017] Increasing the charging voltage applied to a secondary battery can cause side reactions, significantly reducing battery performance. Side reactions include the formation of reactants or oxidation caused by chemical reactions between the active material or electrolyte, or the promotion of decomposition of the electrolyte. Decomposition of the electrolyte can also lead to gas generation and volume expansion.

[0018] One embodiment of the present invention is a secondary battery in which a boron-based additive is added to an electrolyte solution. As the boron-based additive, LiBOB or lithium difluorooxalatoborate (LiDFOB) can also be used.

[0019] In one embodiment of the present invention, positive electrode active material particles containing fluorine are used.

[0020] A method for producing fluorine-containing positive electrode active material particles includes a first step of placing a container containing lithium oxide and fluoride in a heating furnace, and a second step of heating the heating furnace in an oxygen-containing atmosphere, the heating temperature in the second step being 750° C. to 950° C. The heating temperature in the second step may be any temperature at which interdiffusion of elements contained in the lithium oxide and fluoride occurs. When the fluoride contains LiF and MgF2, the eutectic point P of LiF and MgF2 is around 742° C. (T1) as shown in FIG. 13, so the heating temperature in the second step is preferably 742° C. or higher.

[0021] In the above, the heating temperature is preferably 775°C or higher and 925°C or lower, and more preferably 800°C or higher and 900°C or lower.

[0022] Furthermore, the above method preferably includes a step of covering the container before or during heating, and the fluoride is lithium fluoride. By covering the container during heating to maintain a constant or non-decreasing concentration of the gasified fluoride in the space within the container, fluorine can be incorporated into the surface layer of the particles. By using a lid, the positive electrode active material can be annealed in a fluoride-containing atmosphere simply and inexpensively. In this specification, the "surface layer" refers to the region from the surface of the positive electrode active material to a depth of about 10 nm. The surface may also refer to a surface formed by cracks and / or fissures. The region deeper than the surface layer of the positive electrode active material is referred to as the "interior." The surface layer of the positive electrode active material is also sometimes referred to as the "near-surface" region.

[0023] It is preferable that the composite oxide containing lithium, a transition metal (cobalt, nickel, manganese, etc.), and oxygen has a layered rock-salt crystal structure with few defects and distortion. Therefore, it is preferable that the composite oxide contains few impurities. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or distortion.

[0024] In order to prevent impurities from being mixed, it is preferable to modify the surface of the positive electrode active material by heating with a lid on after mixing with the fluoride. The timing of the lid can be any one of placing the lid on the container before heating and then placing it in the heating furnace, placing the lid on the container after placing it in the heating furnace, or placing the lid on during heating before the fluoride melts.

[0025] By using the above-mentioned manufacturing method, the positive electrode active material particles contain fluorine, which improves the wettability of the positive electrode active material surface, making it homogenous and flat. The combination of the positive electrode active material particles and LiBOB obtained in this way is less likely to collapse in crystal structure during repeated high-voltage charge and discharge, and the secondary battery having the combination of the positive electrode active material particles and LiBOB obtained in this way has significantly improved cycle characteristics.

[0026] Furthermore, if the amount of LiBOB added is too large, there is a risk of the initial capacity decreasing, so the proportion of LiBOB in the electrolyte is preferably more than 0.1 wt % and less than 3 wt %.

[0027] In addition, the positive electrode active material particles have a layered structure, and aluminum or magnesium is added to prevent the elution of transition metals, specifically cobalt, making the region including the outer surface of the positive electrode active material particles (the surface layer of the particles) mechanically and chemically strong. In addition, the addition of manganese to the outside of the positive electrode active material particles can also prevent the elution of transition metals, specifically nickel or cobalt.

[0028] The secondary battery contains at least a positive electrode, a negative electrode, a conductive material, a separator, an electrolyte, and a lithium salt.

[0029] Examples of lithium salts include lithium chloride (LiCl), lithium fluoride (LiF), lithium perchlorate (LiClO), lithium borofluoride (LiBF), LiAsF, LiPF, Li(CFSO), Li(FSO)N (so-called LiFSA), and Li(CFSO)N (so-called LiTFSA).

[0030] The role of lithium salts is to facilitate the movement of Li ions in the electrolyte. The use of LiPF6 is preferable due to its compatibility with the aluminum used in the electrodes and cost. However, LiPF6 is unstable at high temperatures, and its decomposition at high temperatures can generate hydrofluoric acid, which can cause deterioration of secondary batteries.

[0031] The electrolyte is made of a material that allows the migration of carrier ions. An aprotic organic solvent is preferable for the electrolyte solvent. Typical examples of aprotic organic solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran, and one or more of these can be used. Furthermore, using a gelatinizable polymer material as the electrolyte solvent enhances safety against leakage and other issues. Furthermore, the storage battery can be made thinner and lighter. Typical examples of gelatinizable polymer materials include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0032] Among the electrolytes, ethylene carbonate (EC) and diethyl carbonate (DEC) are preferred because of their high heat resistance.

[0033] By using LiBOB as an additive, a first coating is formed on the surface of the positive electrode active material, and a second coating is formed on the surface of the negative electrode active material, thereby preventing the elution of transition metals, the decomposition of LiPF6, and the decomposition of the electrolyte. Charging and discharging at high temperatures and high voltages of 4.5 V or higher may cause the elution of transition metals and the decomposition of LiPF6. The first and second coatings are hardly formed immediately after the secondary battery cell is fabricated, but are formed by the charge generated during the charging and discharging of the secondary battery. When a current is applied to release gas during the fabrication of the secondary battery cell, known as an aging treatment, the first and second coatings may be formed during the application of current.

[0034] Furthermore, when LiPF6 decomposes in trace amounts, hydrofluoric acid can contribute to the formation of a high-quality coating on the negative electrode interface. The fluoride ions generated by the decomposition of LiPF6 form a high-quality coating that prevents corrosion of the aluminum used in the positive electrode, especially pitting corrosion.

[0035] By combining these positive electrode active materials that can be charged at high voltages with LiBOB, stability at high temperatures is ensured even when using LiPF6, a lithium salt, and there is also a significant improvement in high-temperature cycle characteristics, resulting in a remarkable synergistic effect. [Effects of the Invention]

[0036] The cycle characteristics of the secondary battery can be improved at a charging voltage of 4.5 V and at 45°C or 60°C. Therefore, a power storage device can be realized that has good rapid charging cycle characteristics and is less susceptible to deterioration at high temperatures or high charging voltages. [Brief explanation of the drawings]

[0037] [Figure 1] 1A and 1B are graphs showing the cycle characteristics of a secondary battery. [Figure 2] FIG. 2 is a graph showing the relationship between the amount of additive and the discharge capacity. [Figure 3] FIG. 3 shows an example of a flow for producing a positive electrode active material according to one embodiment of the present invention. [Figure 4] FIG. 4 shows an example of a flow for producing a positive electrode active material according to one embodiment of the present invention. [Figure 5] 5A, 5B, and 5C are diagrams illustrating an example of fabricating a secondary battery. [Figure 6] 6A and 6B are diagrams illustrating a laminated secondary battery. [Figure 7] FIG. 7A is a top view of the positive electrode, FIG. 7B is a top view of the negative electrode, and FIG. 7C is a diagram illustrating the laminate. [Figure 8] FIG. 8A is a top view of a laminated secondary battery, and FIG. 8B is a cross-sectional view thereof. [Figure 9] 9A is a perspective view of a secondary battery, FIG. 9B is a cross-sectional perspective view thereof, FIG. 9C is a perspective view of a battery pack including a plurality of secondary batteries, and FIG. 9D is a top view thereof. [Figure 10] FIG. 10 is a diagram illustrating the crystal structure and magnetism of the positive electrode active material. [Figure 11]FIG. 11 is a diagram illustrating the crystal structure and magnetism of a conventional positive electrode active material. [Figure 12] 12A, 12B, 12C, 12D, and 12E are perspective views showing electronic devices. [Figure 13] FIG. 13 is a phase diagram showing the relationship between the composition of lithium fluoride and magnesium fluoride and the temperature. [Figure 14] FIG. 14A is a model diagram showing the state of the positive electrode active material inside the secondary battery, the electrolyte solution and additives disposed therearound, and the like, and FIG. 14B is a model diagram showing a conventional example. [Figure 15] FIG. 15 is a diagram showing a chemical reaction formula. [Figure 16] FIG. 16 is a diagram showing a chemical reaction formula. [Figure 17] FIG. 17A shows a chemical formula of a type of lithium salt, FIGS. 17B, 17C, and 17D show chemical formulas of an electrolyte, FIG. 17E shows a chemical formula of an additive, and FIGS. 17F and 17G show chemical formulas of an electrolyte. [Figure 18] FIG. 18 is a partially enlarged model diagram of a secondary battery showing one embodiment of the present invention. [Figure 19] FIG. 19 is a diagram showing the cycle characteristics of the secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0038] 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.

[0039] (Embodiment 1) The secondary battery of this embodiment includes a positive electrode active material containing lithium, a transition metal, magnesium, oxygen, and fluorine, and an electrolyte containing lithium bis(oxalato)borate (LiBOB). The transition metal is at least one of cobalt, nickel, and manganese. The positive electrode active material further includes aluminum. The electrolyte contains a lithium salt, diethyl carbonate, and ethylene carbonate that dissolve the lithium salt. The lithium salt is lithium hexafluorophosphate. The negative electrode active material is artificial graphite. A mixture may be used by adding a conductive material to the positive electrode active material. Examples of the conductive material include acetylene black (AB), VGCF (registered trademark), and graphene oxide compounds. Graphene oxide compounds are particularly preferred because they have a small surface area and can suppress decomposition of the electrolyte.

[0040] Graphene compounds may have excellent electrical properties, such as high electrical conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds also have a planar shape. Graphene compounds enable surface contact with low contact resistance. Even when thin, they can have very high electrical conductivity, allowing a small amount to efficiently form a conductive path within the active material layer. Therefore, using graphene compounds as a conductive additive is preferable because it can increase the contact area between the active material and the conductive additive. It is also preferable because it can reduce electrical resistance. Examples of graphene compounds include graphene, multilayer graphene, multi-graphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, and graphene quantum dots. Reduced graphene oxide is also called reduced graphene oxide (hereinafter, RGO). Here, RGO refers to a compound obtained by reducing graphene oxide (GO). When using active material particles with a small particle size, for example, 1 μm or less, the specific surface area of ​​the active material particles is large, requiring more conductive paths connecting the active material particles. In such cases, it is particularly preferable to use a graphene compound that can efficiently form conductive paths even in small amounts. Furthermore, in this specification, graphene oxide refers to a compound containing carbon and oxygen, having a sheet-like shape, and containing functional groups, particularly epoxy groups, carboxy groups, or hydroxy groups. Furthermore, by bonding multiple graphene compounds together, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed. When the graphene net covers the active material, the graphene net can also function as a binder that binds the active material together. Therefore, the amount of binder can be reduced or eliminated, thereby improving the ratio of active material to electrode volume or weight. This increases the capacity of the secondary battery.

[0041] 1A and 1B show the cycle characteristics of a secondary battery containing a positive electrode active material containing lithium, cobalt, nickel, aluminum, oxygen, and fluorine, and an electrolyte containing 1 wt % lithium bis(oxalato)borate. FIG. 1A shows the cycle characteristics under conditions of 45°C and charging at 4.5 V, and FIG. 1B shows the cycle characteristics under conditions of 60°C and charging at 4.5 V. FIG. 19 also shows the results when the horizontal axis of the charge cycle number is 800, indicating that 600 cycles corresponds to a retention rate of 80%. The portion of FIG. 19 showing the 300th cycle corresponds to FIG. 1A.

[0042] In Figure 1A, the cycle characteristics were evaluated at 45°C under cycle conditions of CCCV charging (0.5C, 4.5V, cut-off current 0.2C) and CC discharging (0.5C, 3.0V). In Figure 1B, the cycle characteristics were evaluated at 60°C under cycle conditions of CCCV charging (0.5C, 4.5V, cut-off current 0.2C) and CC discharging (0.5C, 3V). In Figure 1A, the initial discharge capacity of the secondary battery with additive was 191.4 mAh / g.

[0043] The electrolyte of these secondary batteries contains, in addition to LiBOB, lithium hexafluorophosphate, a lithium salt, diethyl carbonate to dissolve the lithium salt, and ethylene carbonate, with the ratio of ethylene carbonate to diethyl carbonate being 3:7.

[0044] The additive LiBOB is poorly soluble in the solvent, and as shown in Figure 2, increasing the amount of LiBOB reduces the discharge capacity. Figure 2 shows a graph with maximum discharge capacity plotted on the vertical axis for a secondary battery without additive, a secondary battery containing 1 wt% LiBOB, a secondary battery containing 1.5 wt% LiBOB, and a secondary battery containing 2 wt% LiBOB. Figure 2 shows the results of charging at 45°C between 3 and 4.5 V at 0.5 C and 0.2 C, and then discharging at 0.5 C once the cutoff voltage was reached. Furthermore, adding too much LiBOB can lead to precipitation of LiBOB at low temperatures. Therefore, the proportion of LiBOB in the electrolyte is preferably greater than 0.1 wt% and less than 3 wt%.

[0045] The positive electrode active material is also unique, containing lithium, cobalt, magnesium, aluminum, nickel, oxygen, and fluorine. Combining this positive electrode active material with an electrolyte and additives produces significant effects, as shown in Figure 1.

[0046] The production of the positive electrode active material will be described below using the production flow shown in FIG.

[0047] <Step S21> First, a halogen source such as a fluorine source or a chlorine source, a magnesium source, a nickel source, and an aluminum source are prepared as materials for the mixture 901. It is also preferable to prepare a lithium source.

[0048] Examples of fluorine sources that can be used include lithium fluoride and magnesium fluoride. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the annealing step described below. Examples of chlorine sources that can be used include lithium chloride and magnesium chloride. Examples of magnesium sources that can be used include magnesium fluoride, magnesium oxide, magnesium hydroxide, and magnesium carbonate. Examples of lithium sources that can be used include lithium fluoride and lithium carbonate. In other words, lithium fluoride can be used as both a lithium source and a fluorine source. Magnesium fluoride can be used as both a fluorine source and a magnesium source.

[0049] In this embodiment, lithium fluoride LiF is prepared as the fluorine source and the lithium source, and magnesium fluoride MgF2 is prepared as the fluorine source and the magnesium source (Step S21 in FIG. 3).

[0050] When lithium fluoride (LiF) and magnesium fluoride (MgF2) are mixed in a molar ratio of approximately LiF:MgF2 = 65:35, the melting point is most effectively lowered. However, if the amount of lithium fluoride is too high, there is a concern that the lithium content will be excessive, resulting in poor cycle performance. Therefore, the molar ratio of lithium fluoride (LiF) and magnesium fluoride (MgF2) is preferably LiF:MgF2 = x:1 (0≦x≦1.9), more preferably LiF:MgF2 = x:1 (0.1≦x≦0.5), and even more preferably LiF:MgF2 = x:1 (x = approximately 0.33).

[0051] As the nickel source, for example, nickel hydroxide (Ni(OH)2) can be used. In this case, it is preferable that the nickel source is finely powdered. For example, finely powdered nickel hydroxide can be obtained by mixing and pulverizing nickel hydroxide using acetone as a solvent using a ball mill, a bead mill, or the like.

[0052] As the aluminum source, for example, aluminum hydroxide (Al(OH)3) can be used. The aluminum source is preferably in a finely powdered form. For example, finely powdered aluminum hydroxide can be obtained by mixing and pulverizing aluminum hydroxide using acetone as a solvent using a ball mill, bead mill, or the like.

[0053] If the subsequent mixing and grinding steps are performed wet, 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, acetone is used (see step S21 in FIG. 3).

[0054] <Step S22> Next, the materials for the mixture 901 are mixed and pulverized (step S22 in FIG. 3). Mixing can be performed by either a dry or wet method, but a wet method is preferred because it allows for finer pulverization. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the medium. It is preferable to thoroughly perform this mixing and pulverization process to finely pulverize the mixture 901.

[0055] The mixing means is preferably a blender, a mixer, or a ball mill.

[0056] <Steps S23 and S24> The mixed and crushed materials are collected (step S23 in FIG. 3) to obtain a mixture 901 (step S24 in FIG. 3).

[0057] The median diameter (D50) of the mixture 901 is preferably, for example, 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. Such a finely pulverized mixture 901 facilitates uniform adhesion of the mixture 901 to the surface of the composite oxide particles when mixed with a composite oxide containing lithium, a transition metal, and oxygen in a subsequent process. Uniform adhesion of the mixture 901 to the surface of the composite oxide particles is preferable because it facilitates thorough distribution of halogen and magnesium throughout the surface layer of the composite oxide particles after heating. If there are regions in the surface layer that do not contain halogen and magnesium, it may be difficult to form the pseudospinel crystal structure described above in a charged state.

[0058] <Step S25> In step S25, a composite oxide containing lithium, a transition metal, and oxygen that has been synthesized in advance is used.

[0059] When using a pre-synthesized composite oxide containing lithium, transition metals, and oxygen, it is preferable to use one with few impurities. In this specification, the main components of the composite oxide containing lithium, transition metals, and oxygen, and the positive electrode active material, are lithium, cobalt, nickel, manganese, aluminum, and oxygen, and elements other than the main components are considered impurities. For example, when analyzed by glow discharge mass spectrometry, the total impurity concentration is preferably 10,000 ppm wt or less, more preferably 5000 ppm wt or less. In particular, the total impurity concentration of transition metals such as titanium and arsenic is preferably 3000 ppm wt or less, more preferably 1500 ppm wt or less.

[0060] For example, lithium cobalt oxide particles (product name: Cellseed C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used as pre-synthesized lithium cobalt oxide. This lithium cobalt oxide has a median diameter (D50) of approximately 12 μm, and impurity analysis by glow discharge mass spectrometry (GD-MS) shows that the magnesium and fluorine concentrations are 50 ppm wt or less, the calcium, aluminum, and silicon concentrations are 100 ppm wt or less, the nickel concentration is 150 ppm wt or less, the sulfur concentration is 500 ppm wt or less, the arsenic concentration is 1100 ppm wt or less, and the concentrations of other elements other than lithium, cobalt, and oxygen are 150 ppm wt or less.

[0061] The composite oxide containing lithium, a transition metal, and oxygen in step S25 preferably has a layered rock-salt crystal structure with few defects and strain. Therefore, a composite oxide with few impurities is preferred. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or strains.

[0062] <Step S31> Next, the mixture 901 is mixed with a composite oxide containing lithium, a transition metal, and oxygen (step S31 in FIG. 3). The ratio of the number of transition metal atoms TM in the composite oxide containing lithium, a transition metal, and oxygen to the number of magnesium atoms MgMix1 in the mixture 902 is preferably TM:MgMix1=1:y (0.005≦y≦0.05), more preferably TM:MgMix1=1:y (0.007≦y≦0.04), and even more preferably about TM:MgMix1=1:0.02.

[0063] The mixing conditions in step S31 are preferably milder than those in step S22 so as not to destroy the composite oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than those in step S22. It can also be said that dry mixing conditions are milder than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media.

[0064] The mixed materials are collected (step S32 in FIG. 3) to obtain a mixture 903 (step S33 in FIG. 3).

[0065] Next, the mixture 903 is heated (step S34 in FIG. 3). This step is sometimes called annealing or calcination. The annealing generates LiMO2. Therefore, the conditions for performing step S34, such as the temperature, time, atmosphere, and weight of the mixture 903 to be annealed, are important. In this specification, annealing also refers to heating the mixture 903 or at least heating a heating furnace containing the mixture 903. In this specification, a heating furnace is a facility used to heat-treat (anneal) a substance or mixture, and has a heater, an atmosphere containing fluoride, and an inner wall that can withstand at least 600°C. The heating furnace may also be equipped with a pump that can reduce or increase the pressure inside the furnace. For example, pressure may be applied during the annealing step S34.

[0066] The annealing temperature of S34 must be higher than the temperature at which the reaction between lithium cobalt oxide (S25) and fluoride proceeds. The temperature at which the reaction proceeds here is the temperature at which the interdiffusion of elements contained in lithium cobalt oxide and fluoride occurs. Therefore, it can be lower than the melting temperature of these materials. For example, in the case of oxides, the melting temperature T m 0.757 times (Tanman temperature T d ) solid-state diffusion occurs. Therefore, for example, a temperature of 500°C or higher is sufficient.

[0067] However, the reaction proceeds more easily if the annealing temperature is equal to or higher than the temperature at which at least a portion of the mixture 903 melts. Therefore, the annealing temperature is preferably equal to or higher than the eutectic point of the fluoride. When the fluoride contains LiF and MgF2, the eutectic point P of LiF and MgF2 is around 742°C (T1) as shown in Figure 13 (quoted and added from Non-Patent Document 1, Figure 1471-A), so it is preferable to set the annealing temperature of S34 to 742°C or higher.

[0068] The higher the annealing temperature of S34, the easier the reaction will proceed, the shorter the annealing time will be, and the higher the productivity will be, which is preferable.

[0069] However, the annealing temperature must be below the decomposition temperature of LiCoO2 (1130°C). Although the decomposition temperature of LiCoO2 is 1130°C, there is concern that a small amount of LiCoO2 may decompose at temperatures around that temperature. Therefore, the annealing temperature is preferably 1130°C or below, more preferably 1000°C or below, even more preferably 950°C or below, and even more preferably 900°C or below.

[0070] Therefore, the annealing temperature is preferably 500°C to 1130°C, more preferably 500°C to 1000°C, even more preferably 500°C to 950°C, and even more preferably 500°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 800°C to 1130°C, more preferably 800°C to 1000°C, even more preferably 800°C to 950°C, and most preferably 800°C (T2) to 900°C (T3) (range L). Also, the temperature is preferably 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, and even more preferably 830°C or higher and 900°C or lower.

[0071] More specifically, by using LiF as the fluoride and annealing S34 with a lid on, a positive electrode active material with good cycle characteristics can be produced. In addition, it is thought that using LiF and MgF2 as the fluoride promotes the reaction with LiCoO2, resulting in the production of LiMO2.

[0072] In addition, in this embodiment, it is believed that LiF, a fluoride, functions as a flux. Therefore, since the volume inside the heating furnace is larger than the volume of the container and is lighter than oxygen, it is expected that LiF will volatilize, and if the amount of LiF in the mixture 903 decreases, the generation of LiMO2 will be suppressed. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Furthermore, even if LiF is not used, Li and F on the surface of the lithium cobalt oxide may react to produce 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 volatilization.

[0073] Therefore, by heating the mixture 903 in an atmosphere containing LiF, that is, by heating the mixture 903 in a heating furnace under a high LiF partial pressure, the volatilization of LiF in the mixture 903 is suppressed. By annealing with a lid using a fluoride (LiF or MgF) that forms a eutectic mixture, the annealing temperature can be lowered to below the decomposition temperature of LiCoO2 (1130°C), specifically, to between 742°C and 1000°C, allowing the formation of LiMO2 to proceed efficiently. As a result, a positive electrode active material with good characteristics can be produced, and the annealing time can also be shortened.

[0074] The annealing in step S34 is preferably performed at an appropriate temperature and time. The appropriate temperature and time vary depending on conditions such as the size and composition of the lithium cobalt oxide (S25) particles. If the particles are small, a lower temperature or shorter time may be more preferable than if the particles are large. After the annealing in step S34, a step of removing the lid is included.

[0075] For example, when the median diameter (D50) of the particles of lithium cobalt oxide (S25) is about 12 μm, the annealing time is preferably, for example, 3 hours or more, and more preferably 10 hours or more.

[0076] On the other hand, when the median particle diameter (D50) of the lithium cobalt oxide (S25) particles is about 5 μm, the annealing time is preferably, for example, from 1 hour to 10 hours, and more preferably about 2 hours.

[0077] The temperature drop time after annealing is preferably, for example, 10 hours or more and 50 hours or less.

[0078] The annealed material is collected (Step S35 in FIG. 3). Preferably, the particles are then sieved. Through the above steps, positive electrode active material 200A of one embodiment of the present invention can be produced (Step S36 in FIG. 3).

[0079] Furthermore, the positive electrode active material is not limited to the above-described configuration, and even if a positive electrode active material does not use nickel or aluminum, a significant effect can be obtained by combining this positive electrode active material with an electrolyte and an additive.

[0080] Another example of the production of a positive electrode active material that does not use nickel or aluminum will be described below using the production flow shown in FIG.

[0081] As shown in step S11 of Figure 4, first, lithium fluoride, which is a fluorine source, and magnesium fluoride, which is a magnesium source, are prepared as materials for the mixture 902. Lithium fluoride is preferable because it has a relatively low melting point of 848°C and is easily melted in the annealing step described below. Lithium fluoride can be used as both a lithium source and a fluorine source. Magnesium fluoride can also be used as both a fluorine source and a magnesium source.

[0082] 4, lithium fluoride LiF is prepared as the fluorine source and lithium source, and magnesium fluoride MgF2 is prepared as the fluorine source and magnesium source (Step S11 in FIG. 4). The molar ratio of lithium fluoride LiF to magnesium fluoride MgF2 is preferably LiF:MgF2=x:1 (0≦x≦1.9), more preferably LiF:MgF2=x:1 (0.1≦x≦0.5), and even more preferably LiF:MgF2=x:1 (x=nearly 0.33).

[0083] If the subsequent mixing and grinding steps are performed wet, 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, acetone is used (see step S11 in FIG. 4).

[0084] Next, the materials for the mixture 902 are mixed and pulverized (step S12 in FIG. 4). Mixing can be performed by either a dry or wet method, but a wet method is preferred because it allows for finer pulverization. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the medium. It is preferable to thoroughly perform this mixing and pulverization process to finely pulverize the mixture 902.

[0085] The mixed and crushed materials are collected (step S13 in FIG. 4) to obtain a mixture 902 (step S14 in FIG. 4).

[0086] The D50 of the mixture 902 is preferably, for example, 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. Such a finely pulverized mixture 902 facilitates uniform adhesion of the mixture 902 to the surface of the composite oxide particles when mixed with a composite oxide containing lithium, a transition metal, and oxygen in a subsequent process. Uniform adhesion of the mixture 902 to the surface of the composite oxide particles is preferable because it facilitates thorough distribution of halogen and magnesium throughout the surface layer of the composite oxide particles after heating. If there are regions in the surface layer that do not contain halogen and magnesium, it may be difficult to form the pseudospinel crystal structure described above in a charged state.

[0087] Next, a lithium source is prepared as shown in step S25. In step S25, a composite oxide containing lithium, a transition metal, and oxygen that has been synthesized in advance is used.

[0088] For example, lithium cobalt oxide particles (product name: Cellseed C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used as pre-synthesized lithium cobalt oxide. This lithium cobalt oxide has a median diameter (D50) of approximately 12 μm, and impurity analysis by glow discharge mass spectrometry (GD-MS) shows that the magnesium and fluorine concentrations are 50 ppm wt or less, the calcium, aluminum, and silicon concentrations are 100 ppm wt or less, the nickel concentration is 150 ppm wt or less, the sulfur concentration is 500 ppm wt or less, the arsenic concentration is 1100 ppm wt or less, and the concentrations of other elements other than lithium, cobalt, and oxygen are 150 ppm wt or less.

[0089] The composite oxide containing lithium, a transition metal, and oxygen in step S25 preferably has a layered rock-salt crystal structure with few defects and strain. Therefore, a composite oxide with few impurities is preferred. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or strains.

[0090] Next, the mixture 902 is mixed with a composite oxide containing lithium, a transition metal, and oxygen (step S31 in FIG. 4). The ratio of the number of transition metal atoms TM in the composite oxide containing lithium, a transition metal, and oxygen to the number of magnesium atoms MgMix1 in the mixture 902 is preferably TM:MgMix1=1:y (0.005≦y≦0.05), more preferably TM:MgMix1=1:y (0.007≦y≦0.04), and even more preferably about TM:MgMix1=1:0.02.

[0091] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the composite 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 conditions are milder than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media.

[0092] The mixed materials are collected (step S32 in FIG. 4) to obtain mixture B (step S33 in FIG. 4).

[0093] Next, the mixture B is heated (step S34 in FIG. 4).

[0094] The annealing is preferably performed at an appropriate temperature and time. The appropriate temperature and time vary depending on conditions such as the size and composition of the composite oxide particles containing lithium, transition metal, and oxygen in step S25. If the particles are small, a lower temperature or shorter time may be more preferable than if the particles are large.

[0095] For example, when the median particle diameter (D50) of the particles in step S25 is about 12 μm, the annealing temperature is preferably, for example, 600° C. or more and 950° C. or less. The annealing time is, for example, preferably 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more.

[0096] On the other hand, when the median diameter (D50) of the particles in step S25 is about 5 μm, the annealing temperature is preferably, for example, 600° C. to 950° C. The annealing time is preferably, for example, 1 hour to 10 hours, more preferably about 2 hours.

[0097] The temperature drop time after annealing is preferably, for example, 10 hours or more and 50 hours or less.

[0098] When mixture B is annealed, it is thought that the material with a low melting point in mixture B (for example, lithium fluoride, melting point 848°C) melts first and distributes in the surface layer of the composite oxide particles. Next, the presence of this molten material lowers the melting points of other materials, which then melts them. For example, magnesium fluoride (melting point 1263°C) melts and distributes in the surface layer of the composite oxide particles.

[0099] The diffusion of elements contained in Mixture B is faster in the surface layer and near the grain boundaries than in the interior of the composite oxide particles. Therefore, magnesium and halogens are concentrated at higher concentrations in the surface layer and near the grain boundaries than in the interior. As will be described later, a high magnesium concentration in the surface layer and near the grain boundaries can more effectively suppress changes in the crystal structure.

[0100] The annealed material is collected (Step S35 in FIG. 4) to obtain positive electrode active material 200B (Step S36 in FIG. 4).

[0101] If an electrolyte solution containing LiBOB is used in a secondary battery using the positive electrode active material 200B obtained in this way, good results can be obtained in terms of cycle characteristics at 45° C. or higher.

[0102] (Embodiment 2) An example of a method for producing a laminated secondary battery will be described with reference to FIGS. 5B and 5C.

[0103] First, the negative electrode 506, the separator 507 and the positive electrode 503 are laminated.

[0104] FIG. 5A shows the external appearance of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. A slurry of a positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) mixed in a weight ratio of positive electrode active material:AB:PVDF = 95:3:2 is applied to the positive electrode current collector 501 and pressed at 120°C with a linear pressure of 120 kN / m to form the positive electrode active material layer 502. AB is used as a conductive material (also called a conductive additive). The active material, AB, and 40% of the polyvinylidene fluoride (PVDF) are first mixed and kneaded until homogeneous. Then, the remaining 60% of the PVDF is added, and NMP is added to adjust the viscosity to prepare a slurry. After coating, the coating is dried in a ventilated oven at 80°C for 30 minutes.

[0105] Furthermore, the positive electrode 503 has a region (hereinafter referred to as a tab region) where the positive electrode current collector 501 is partially exposed. The negative electrode 506 has a negative electrode current collector 504, and the negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. The area and shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 5A.

[0106] FIG. 5B shows a stack of negative electrodes 506, separators 507, and positive electrodes 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and a positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrodes 506 are joined together, and a negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0107] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are placed on the exterior body 509 .

[0108] Next, as shown in Fig. 5C, exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of exterior body 509 is joined. For example, thermocompression bonding may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of exterior body 509 so that electrolyte 508 can be introduced later.

[0109] Next, electrolyte 508 is introduced into the inside of exterior body 509 through an inlet provided in exterior body 509. Introduction of electrolyte 508 is preferably performed under a reduced pressure atmosphere or an inert atmosphere. In this embodiment, 1 mol / L LiPF6 is used as the lithium salt, EC:DEC is used as the solvent in a volume ratio of 3:7, and LiBOB is used as the additive, and a total of 600 μL is introduced through the inlet. Finally, the inlet is joined. In this manner, secondary battery 500, which is a laminated secondary battery, can be fabricated.

[0110] By using the positive electrode active material particles described in the previous embodiment and LiBOB for the positive electrode 503, the secondary battery 500 can be made less susceptible to deterioration and highly safe.

[0111] Next, we will explain aging after fabrication of a secondary battery. It is preferable to perform aging after fabrication of a secondary battery. An example of aging conditions is described below. First, charging is performed at a rate of 0.001 C or more and 0.2 C or less. The temperature may be, for example, above room temperature and below 60°C. Here, if the reaction potential of the positive electrode or negative electrode exceeds the potential window range of the electrolyte 508, decomposition of the electrolyte may occur during charging and discharging of the secondary battery. If gas is generated by decomposition of the electrolyte, this gas may accumulate in the cell, creating areas where the electrolyte cannot contact the electrode surface. In other words, the effective reaction area of ​​the electrode is reduced, which corresponds to an increase in effective resistance.

[0112] Furthermore, excessively high resistance can cause a drop in the negative electrode potential, resulting in lithium insertion into the graphite and, at the same time, lithium deposition on the graphite surface. This lithium deposition can lead to a decrease in capacity. For example, if a coating or the like grows on the surface after lithium deposition, the lithium deposited on the surface cannot be re-eluted, resulting in an increase in lithium that does not contribute to capacity. Furthermore, if the deposited lithium physically collapses and loses electrical continuity with the electrode, lithium that does not contribute to capacity will also be generated. Therefore, it is preferable to remove the gas before the potential of the negative electrode reaches the lithium potential due to an increase in charging voltage.

[0113] After degassing, the battery may be held in a charged state at a temperature higher than room temperature, preferably 30°C to 60°C, more preferably 35°C to 50°C, for example, for 1 hour to 100 hours. During the initial charging, the electrolyte decomposed on the surface forms a coating on the graphite surface. Therefore, for example, by holding the battery at a temperature higher than room temperature after degassing, the formed coating may become denser.

[0114] In some cases, excess electrolyte is removed after degassing. However, since the amount is so small, it is considered to have almost no effect on the weight of the battery.

[0115] An example of a laminated secondary battery will be described with reference to FIGS. 6A and 6B.

[0116] Fig. 6A shows an example of an external view of a laminated secondary battery 500. Fig. 6B shows another example of an external view of a laminated secondary battery 500.

[0117] 6A and 6B show a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0118] The laminated secondary battery 500 has a plurality of rectangular positive electrodes 503, a separator 507, and a plurality of rectangular negative electrodes 506.

[0119] 5 shows an example of a laminate, but a wound body may also be used. In that case, a negative electrode 506 and a positive electrode 503 are stacked on top of each other with a separator 507 sandwiched between them, and the laminate sheet is wound up.

[0120] 7A shows a positive electrode having an L-shaped positive electrode current collector 701 and a positive electrode active material layer 702. The positive electrode also has a region where the positive electrode current collector 701 is partially exposed (hereinafter referred to as a tab region). FIG. 7B shows a negative electrode having an L-shaped negative electrode current collector 704 and a negative electrode active material layer 705. The negative electrode has a region where the negative electrode current collector 704 is partially exposed, i.e., a tab region.

[0121] 7C shows a perspective view of four layers of positive electrodes 703 and four layers of negative electrodes 706. For simplicity, the separator provided between positive electrodes 703 and negative electrodes 706 is shown by dotted lines in FIG.

[0122] 8A includes a positive electrode 703 having an L-shaped positive electrode current collector 701 and a positive electrode active material layer 702, a negative electrode 706 having an L-shaped negative electrode current collector 704 and a negative electrode active material layer 705, a separator 707, an electrolyte 708, and an exterior body 709. The separator 707 is disposed between the positive electrode 703 and the negative electrode 706 provided within the exterior body 709. The interior of the exterior body 709 is filled with the electrolyte 708.

[0123] 8A , the positive electrode current collector 701 and the negative electrode current collector 704 also serve as terminals for electrical contact with the outside. Therefore, the positive electrode current collector 701 and the negative electrode current collector 704 may be arranged so as to be partially exposed to the outside from the exterior body 709. Alternatively, the positive electrode current collector 701 and the negative electrode current collector 704 may not be exposed to the outside from the exterior body 709, but may be exposed to the outside by using a lead electrode and ultrasonically bonding the lead electrode to the positive electrode current collector 701 or the negative electrode current collector 704.

[0124] In a laminated secondary battery, the exterior body 709 can be a three-layer laminate film having a highly flexible metal thin film made of aluminum, stainless steel, copper, nickel, or the like on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and further having an insulating synthetic resin film made of polyamide-based resin, polyester-based resin, or the like on the metal thin film as the outer surface of the exterior body.

[0125] An example of the cross-sectional structure of a laminated secondary battery is shown in Fig. 8B. Although omitted in Fig. 8A for simplicity, the battery is actually made up of multiple electrode layers.

[0126] In FIG. 8B, the number of electrode layers is 16 as an example. FIG. 8B shows a structure with a total of 16 layers, including eight layers of negative electrode current collectors 704 and eight layers of positive electrode current collectors 701. Note that FIG. 8B shows a cross section of the positive electrode lead-out portion cut along the chain line in FIG. 8A, in which eight layers of negative electrode current collectors 704 are ultrasonically bonded. Of course, the number of electrode layers is not limited to 16 and may be more or less. When the number of electrode layers is large, a secondary battery with a larger capacity can be obtained. Furthermore, when the number of electrode layers is small, the battery can be made thinner.

[0127] 9A to 9D, an example of a cylindrical secondary battery will be described. As shown in Fig. 9A, a cylindrical secondary battery 600 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.

[0128] FIG. 9B is a schematic diagram showing the cross section of a cylindrical secondary battery. Inside a hollow cylindrical battery can 602, a battery element is provided, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a center pin. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal that is corrosion-resistant to the electrolyte, such as nickel, aluminum, or titanium, or an alloy of these metals or alloys of these metals with other metals (e.g., stainless steel). Furthermore, a coating of nickel, aluminum, or the like is preferable to prevent corrosion by the electrolyte. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The non-aqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0129] Because the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form active materials on both sides of the current collector. 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 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 612 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature rises, and the increased resistance limits the amount of current to prevent abnormal heat generation. Barium titanate (BaTiO3) based semiconductor ceramics or the like can be used for the PTC element.

[0130] 9C , a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between conductive plates 613 and 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in series after being connected in parallel. By configuring a module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0131] FIG. 9D is a top view of module 615. For clarity, conductive plate 613 is shown with dotted lines. As shown in FIG. 9D, module 615 may have conductive wires 616 that electrically connect multiple secondary batteries 600. A conductive plate can be superimposed on the conductive wires 616. Furthermore, a temperature control device 617 may be provided between multiple secondary batteries 600. When a secondary battery 600 becomes overheated, it can be cooled by the temperature control device 617, and when a secondary battery 600 becomes too cold, it can be heated by the temperature control device 617. This makes it less likely that the performance of module 615 will be affected by the outside temperature.

[0132] By using the positive electrode active material manufactured by the manufacturing method described in the above embodiment for the positive electrode 604, the cylindrical secondary battery 600 can be one that is less susceptible to deterioration and has high safety.

[0133] (Embodiment 3) [Positive electrode active material structure] Materials with a layered rock-salt crystal structure, such as lithium cobalt oxide (LiCoO2), are known to have high discharge capacity and are excellent as positive electrode active materials for secondary batteries. An example of a material with a layered rock-salt crystal structure is a composite oxide represented by LiMO2. Examples of the element M include one or more selected from Co, Ni, and Mn. Examples of the element M include one or more selected from Co, Ni, and Mn, as well as one or more selected from Al and Mg.

[0134] It is known that 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.

[0135] In compounds containing nickel, distortion may occur due to the Jahn-Teller effect. Therefore, when LiNiO2 is charged and discharged at high voltages, there is a concern that the crystal structure may collapse due to distortion. In LiCoO2, the influence of the Jahn-Teller effect is suggested to be small, and LiCoO2 may have better durability against charge and discharge at high voltages, which is preferable.

[0136] The positive electrode active material will be described with reference to Figures 10 and 11. Figures 10 and 11 describe the case where cobalt is used as the transition metal contained in the positive electrode active material.

[0137] <Conventional positive electrode active materials> The positive electrode active material shown in Figure 11 is lithium cobalt oxide (LiCoO2) to which no halogen or magnesium is added. The crystal structure of the lithium cobalt oxide shown in Figure 11 changes depending on the depth of charge.

[0138] As shown in Figure 11, lithium cobalt oxide at a depth of charge of 0 (discharged state) has a region with a crystal structure of space group R-3m, in which lithium occupies octahedral sites and there are three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer refers to a structure in which an octahedral structure in which oxygen is six-coordinated to cobalt is connected in an edge-sharing manner to form a continuous plane.

[0139] At a charge depth of 1, the crystal structure is of the space group P-3m1, with one CoO2 layer in the unit cell. Therefore, this crystal structure is sometimes called an O1-type crystal structure.

[0140] Furthermore, lithium cobalt oxide at a charge depth of approximately 0.8 has a crystal structure of the space group R-3m. This structure can be described as a structure in which a CoO2 structure such as P-3m1(O1) and a LiCoO2 structure such as R-3m(O3) are alternately stacked. Therefore, this crystal structure is sometimes referred to as an H1-3 crystal structure. In reality, the H1-3 crystal structure has twice the number of cobalt atoms per unit cell as other structures. However, in Figure 11 and other parts of this specification, for ease of comparison with other structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0141] As an example, the coordinates of cobalt and oxygen in the unit cell of the H1-3 crystal structure can be expressed as Co(0,0,0.42150±0.00016), O1(0,0,0.27671±0.00045), and O2(0,0,0.11535±0.00045). O1 and O2 are each an oxygen atom. Thus, the H1-3 crystal structure is expressed by a unit cell using one cobalt and two oxygen atoms. On the other hand, as described below, the O3' crystal structure of one embodiment of the present invention is preferably expressed by a unit cell using one cobalt and one oxygen atom. This indicates that the symmetry between cobalt and oxygen differs between the O3' crystal structure and the H1-3 structure, and that the O3' crystal structure exhibits smaller changes from the O3 structure than the H1-3 structure. The unit cell that is more preferably used to represent the crystal structure of the positive electrode active material may be selected, for example, so that the GOF (good of fitness) value is smaller in Rietveld analysis of XRD.

[0142] When lithium cobalt oxide is repeatedly charged and discharged at a high voltage of 4.6 V or higher, based on the redox potential of lithium metal, or at a deep charge depth of 0.8 or higher, the crystal structure of the lithium cobalt oxide changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 crystal structure and the R-3m(O3) structure in the discharged state.

[0143] However, these two crystal structures have a large deviation in the CoO2 layers. As shown by the dotted lines and arrows in Figure 11, in the H1-3 crystal structure, the CoO2 layers are significantly deviated from the R-3m(O3) structure. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0144] Furthermore, the difference in volume is large: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the O3 crystal structure in the discharged state is more than 3.0%.

[0145] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers such as P-3m1(O1), is likely to be unstable.

[0146] Therefore, repeated high-voltage charging and discharging causes the crystalline structure of lithium cobalt oxide to collapse, which leads to a deterioration in cycle characteristics. This is thought to be because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0147] <Positive Electrode Active Material of One Embodiment of the Present Invention> The positive electrode active material 904 manufactured according to one embodiment of the present invention can reduce the displacement of the CoO layer during repeated high-voltage charge and discharge. Furthermore, the change in volume can be reduced. Therefore, the compound can achieve excellent cycle characteristics. Furthermore, the compound can have a stable crystal structure in a high-voltage charged state. Therefore, the compound may be less likely to short circuit when maintained in a high-voltage charged state. In such cases, safety is further improved, which is preferable.

[0148] In the positive electrode active material of one embodiment of the present invention, the change in crystal structure and the difference in volume per the same number of transition metal atoms between a fully discharged state and a high-voltage charged state are small.

[0149] 10 shows the crystal structures of a positive electrode active material 904 of one embodiment of the present invention before and after charge and discharge. The positive electrode active material 904 is a composite oxide containing lithium, cobalt as a transition metal, and oxygen. In addition to the above, the positive electrode active material 904 preferably contains magnesium as an additional element. It also preferably contains a halogen such as fluorine or chlorine as an additional element.

[0150] The crystal structure at a charge depth of 0 (discharged state) in FIG. 10 is the same as that in FIG. 11, R-3m(O3). On the other hand, when the cathode active material 904 is fully charged, it has a crystal structure different from the H1-3 crystal structure. This structure is in the space group R-3m. Although it is not a spinel crystal structure, ions such as cobalt and magnesium occupy six oxygen coordination positions, and the cation arrangement has a symmetry similar to that of a spinel structure. The symmetry of the CoO2 layers in this structure is the same as that of an O3 type. Therefore, this structure is referred to herein as an O3' type crystal structure or a pseudo-spinel type crystal structure. Therefore, the O3' type crystal structure may also be referred to as a pseudo-spinel type crystal structure. In the diagram of the O3' type crystal structure shown in FIG. 10, lithium is omitted to explain the symmetry of the cobalt atoms and the oxygen atoms. However, in reality, lithium is present between the CoO2 layers at, for example, 20 atomic % or less relative to the cobalt. In both the O3-type and O3'-type crystal structures, magnesium is preferably present in a dilute form between the CoO2 layers, i.e., at the lithium sites, and halogens such as fluorine are preferably present randomly and dilutely at the oxygen sites.

[0151] In the O3'-type crystal structure, light elements such as lithium may occupy the oxygen tetracoordination site, and in this case too, the ion arrangement has a symmetry similar to that of the spinel type.

[0152] It can also be said that the O3' type crystal structure is similar to the CdCl2 type crystal structure, although it has random Li between the layers. This CdCl2 type-like crystal structure was observed when lithium nickel oxide was charged to a charge depth of 0.94 (Li 0.06The crystal structure is similar to that of lithium cobaltate (NiO2), but it is known that pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt do not usually adopt this crystal structure.

[0153] In the positive electrode active material 904 of one embodiment of the present invention, when a large amount of lithium is released by charging at a high voltage, the change in the crystal structure is suppressed more than in conventional positive electrode active materials. For example, as shown by the dotted line in FIG. 10, there is almost no displacement of the CoO layer in these crystal structures.

[0154] More specifically, the cathode active material 904 of one embodiment of the present invention has high structural stability even at high charge voltages. For example, conventional cathode active materials adopt an H1-3 crystal structure at a charge voltage of about 4.6 V relative to the potential of lithium metal. However, the cathode active material 904 of one embodiment of the present invention can maintain the R-3m(O3) crystal structure even at a charge voltage of about 4.6 V. Even at higher charge voltages, for example, at a voltage of about 4.65 V to 4.7 V relative to the potential of lithium metal, the cathode active material 904 of one embodiment of the present invention can adopt an O3' crystal structure. When the charge voltage is further increased above 4.7 V, the H1-3 crystal structure may finally be observed in the cathode active material 904 of one embodiment of the present invention. Furthermore, even at lower charge voltages (for example, at a charge voltage of 4.5 V or higher but lower than 4.6 V relative to the potential of lithium metal), the cathode active material 904 of one embodiment of the present invention may adopt an O3' crystal structure.

[0155] Note that when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above-mentioned value by the potential of the graphite. The potential of graphite is approximately 0.05 V to 0.2 V relative to the potential of lithium metal. Therefore, even when the voltage of a secondary battery using graphite as the negative electrode active material is 4.3 V or higher and 4.5 V or lower, the positive electrode active material 904 of one embodiment of the present invention can maintain the R-3m(O3) crystal structure. Furthermore, even when the charge voltage is higher, for example, when the voltage of the secondary battery is higher than 4.5 V and lower than 4.6 V, the positive electrode active material 904 of one embodiment of the present invention can also have the O3'-type crystal structure. Furthermore, even when the charge voltage is lower, for example, when the voltage of the secondary battery is 4.2 V or higher and lower than 4.3 V, the positive electrode active material 904 of one embodiment of the present invention can sometimes have the O3'-type crystal structure.

[0156] Therefore, the crystal structure of the positive electrode active material 904 of one embodiment of the present invention is not easily destroyed even when repeatedly charged and discharged at a high voltage.

[0157] In addition, in the positive electrode active material 904, the difference in volume per unit cell between the O3 type crystal structure at a charge depth of 0 and the O3' type crystal structure at a charge depth of 0.8 is 2.5% or less, more specifically 2.2% or less.

[0158] In addition, the O3' type crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25.

[0159] An additive element, such as magnesium, randomly and dilutely present between CoO layers, i.e., at the lithium sites, has the effect of suppressing the misalignment of the CoO layers. Therefore, the presence of magnesium between CoO layers tends to result in an O3'-type crystal structure. Therefore, magnesium is preferably distributed throughout the particles of the positive electrode active material 904 of one embodiment of the present invention. Furthermore, to distribute magnesium throughout the particles, heat treatment is preferably performed during the manufacturing process of the positive electrode active material 904 of one embodiment of the present invention.

[0160] However, if the heat treatment temperature is too high, cation mixing occurs, increasing the possibility that added elements, such as magnesium, will enter the cobalt site. Magnesium present in the cobalt site is ineffective in maintaining the R-3m structure during high-voltage charging. Furthermore, if the heat treatment temperature is too high, there are concerns that adverse effects such as cobalt being reduced to a divalent state and lithium evaporating may occur.

[0161] Therefore, it is preferable to add a halogen compound such as a fluorine compound to the lithium cobalt oxide before the heat treatment to distribute magnesium throughout the particles. The addition of the halogen compound lowers the melting point of the lithium cobalt oxide. Lowering the melting point makes it easier to distribute magnesium throughout the particles at a temperature where cation mixing is unlikely to occur. Furthermore, the presence of a fluorine compound is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0162] However, if the magnesium concentration is increased above a desired value, the effect on stabilizing the crystal structure may be reduced. This is thought to be due to the incorporation of magnesium into the cobalt site in addition to the lithium site. The number of magnesium atoms in the positive electrode active material of one embodiment of the present invention is preferably 0.001 to 0.1 times the number of transition metal atoms, more preferably greater than 0.01 and less than 0.04, and even more preferably approximately 0.02. The magnesium concentration shown here may be, for example, a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using ICP-MS or the like, or may be based on the value of the raw material composition during the production process of the positive electrode active material.

[0163] Lithium cobalt oxide may contain, as an additive element, one or more metals other than cobalt, selected from nickel, aluminum, manganese, titanium, vanadium, and chromium, with nickel and aluminum being particularly preferred. Manganese, titanium, vanadium, and chromium may be stable and easily tetravalent, which may contribute significantly to structural stability. Addition of an additive element may result in a more stable crystal structure in the positive electrode active material of one embodiment of the present invention, for example, in a charged state at a high voltage. In the positive electrode active material of one embodiment of the present invention, the additive element is preferably added at a concentration that does not significantly alter the crystallinity of the lithium cobalt oxide. For example, the amount is preferably such that the Jahn-Teller effect, etc., described above, is not exhibited.

[0164] As shown in the legend in Figure 10, transition metals such as nickel and manganese and aluminum are preferably present at the cobalt site, but some may be present at the lithium site. Magnesium is also preferably present at the lithium site. Oxygen may be partially substituted with fluorine.

[0165] As the magnesium concentration in the positive electrode active material of one embodiment of the present invention increases, the capacity of the positive electrode active material may decrease. For example, this may be due to magnesium entering the lithium site, which may reduce the amount of lithium contributing to charge and discharge. Excessive magnesium may also produce magnesium compounds that do not contribute to charge and discharge. When the positive electrode active material of one embodiment of the present invention contains nickel as an additive element in addition to magnesium, the capacity per weight and per volume may be increased. When the positive electrode active material of one embodiment of the present invention contains aluminum as an additive element in addition to magnesium, the capacity per weight and per volume may be increased. When the positive electrode active material of one embodiment of the present invention contains nickel and aluminum as an additive element in addition to magnesium, the capacity per weight and per volume may be increased.

[0166] Hereinafter, the concentration of an element such as magnesium contained in a positive electrode active material of one embodiment of the present invention will be expressed in terms of the number of atoms.

[0167] The number of nickel atoms in the positive electrode active material of one embodiment of the present invention is preferably 10% or less of the number of cobalt atoms, more preferably 7.5% or less, even more preferably 0.05% to 4%, and particularly preferably 0.1% to 2%. The nickel concentration shown here may be a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0168] If the battery is charged at a high voltage for a long period of time, transition metals may leach out of the positive electrode active material into the electrolyte, causing the crystal structure to collapse. However, by including nickel in the above proportion, it may be possible to suppress the leach-out of transition metals from the positive electrode active material 904.

[0169] The number of aluminum atoms in the positive electrode active material of one embodiment of the present invention is preferably 0.05% to 4%, more preferably 0.1% to 2%, of the number of cobalt atoms. The aluminum concentration shown here may be a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0170] The positive electrode active material of one embodiment of the present invention preferably contains an additional element X, and phosphorus is preferably used as the additional element X. Furthermore, the positive electrode active material of one embodiment of the present invention more preferably contains a compound containing phosphorus and oxygen.

[0171] When the positive electrode active material of one embodiment of the present invention contains a compound containing an additional element X, a short circuit may be less likely to occur when the positive electrode active material is maintained in a charged state at a high voltage.

[0172] When the positive electrode active material of one embodiment of the present invention contains phosphorus as the additional element X, hydrogen fluoride generated by decomposition of the electrolyte solution may react with the phosphorus, resulting in a decrease in the hydrogen fluoride concentration in the electrolyte solution.

[0173] When the electrolyte contains LiPF6, hydrogen fluoride may be generated by hydrolysis. Hydrogen fluoride may also be generated by the reaction of PVDF, which is used as a component of the positive electrode, with alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating may be suppressed. Furthermore, gelation and / or insolubilization of PVDF may be suppressed, which may reduce adhesion.

[0174] When the positive electrode active material of one embodiment of the present invention contains magnesium in addition to the additive element X, the stability in a high-voltage charged state is extremely high. When the additive element X is phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and even more preferably 0.7% to 4%. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS, or may be based on values ​​obtained by combining raw materials in the process of producing the positive electrode active material.

[0175] When the positive electrode active material has cracks, the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the cracks may inhibit the progression of the cracks.

[0176] As is clear from the oxygen atoms indicated by the arrows in Figure 10, the symmetry of the oxygen atoms is slightly different between the O3 and O3' crystal structures. Specifically, in the O3 crystal structure, the oxygen atoms are aligned along the (-1 0 2) plane indicated by the dotted line, whereas in the O3' crystal structure, the oxygen atoms are not strictly aligned along the (-1 0 2) plane. This is because, in the O3' crystal structure, as lithium decreases, tetravalent cobalt increases, increasing Jahn-Teller distortion and distorting the octahedral structure of CoO6. Another factor is that as lithium decreases, the repulsion between oxygen atoms in the CoO2 layer becomes stronger.

[0177] Magnesium is preferably distributed throughout the particles of the positive electrode active material 904 of one embodiment of the present invention, and in addition, the magnesium concentration in the surface layer is preferably higher than the average throughout the particles. For example, the magnesium concentration in the surface layer measured by XPS or the like is preferably higher than the average magnesium concentration throughout the particles measured by ICP-MS or the like.

[0178] In addition, when the positive electrode active material 904 of one embodiment of the present invention contains an element other than cobalt, such as one or more metals selected from nickel, aluminum, manganese, iron, and chromium, the concentration of the metal in the particle surface layer is preferably higher than the average concentration of the entire particle. For example, the concentration of the element other than cobalt in the surface layer measured by XPS or the like is preferably higher than the average concentration of the element in the entire particle measured by ICP-MS or the like.

[0179] The particle surface is essentially a crystal defect, and because lithium is released from the surface during charging, the lithium concentration is likely to be lower than in the interior. This makes the surface more unstable and prone to the collapse of the crystal structure. If the magnesium concentration in the surface layer is high, changes in the crystal structure can be more effectively suppressed. Furthermore, a high magnesium concentration in the surface layer is expected to improve corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte.

[0180] Furthermore, the concentration of halogen such as fluorine in the surface layer of the positive electrode active material 904 of one embodiment of the present invention is preferably higher than the average concentration in the entire particle. The presence of halogen in the surface layer, which is the region in contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid.

[0181] As described above, the surface portion of the positive electrode active material 904 of one embodiment of the present invention preferably has a different composition from the interior portion, that is, the concentration of additive elements, such as magnesium and fluorine, is higher than that of the interior portion. Furthermore, the composition preferably has a stable crystal structure at room temperature. Therefore, the surface portion may have a different crystal structure from the interior portion. For example, at least a part of the surface portion of the positive electrode active material 904 of one embodiment of the present invention may have a rock-salt crystal structure. Furthermore, when the surface portion and the interior portion have different crystal structures, the crystal orientations of the surface portion and the interior portion preferably roughly match.

[0182] The anions in layered rock salt crystals and rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in O3'-type crystals also have a cubic close-packed structure. When they contact, there is a crystal plane where the cubic close-packed structure formed by the anions is oriented in the same direction. However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space groups Fm-3m (the space group of general rock salt crystals) and Fd-3m (the space group of rock salt crystals with the simplest symmetry) of rock salt crystals. Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and O3'-type crystals and rock salt crystals. In this specification, when the cubic close-packed structure formed by the anions is oriented in the same direction in layered rock salt crystals, O3'-type crystals, and rock salt crystals, it may be said that the crystal orientations are approximately the same.

[0183] The general alignment of the crystal orientations of the two regions can be determined from TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, and ABF-STEM (annular bright-field scanning transmission electron microscope) images. X-ray diffraction (XRD), electron diffraction, and neutron diffraction can also be used for this determination. If the crystal orientations are generally aligned, the difference in the orientation of the alternating linear array of cations and anions can be observed in TEM images, etc., to be 5 degrees or less, more preferably 2.5 degrees or less. Light elements such as oxygen and fluorine may not be clearly visible in TEM images, but in such cases, the alignment of the orientations can be determined from the arrangement of metal elements.

[0184] However, if the surface layer is only MgO or only a solid solution of MgO and CoO(II), it becomes difficult to insert and extract lithium. Therefore, the surface layer must contain at least cobalt, and in the discharged state, it must also contain lithium, providing a path for lithium insertion and extraction. It is also preferable that the concentration of cobalt is higher than that of magnesium.

[0185] The additional element X is preferably located in the surface layer portion of the particles of the positive electrode active material 904 of one embodiment of the present invention. For example, the positive electrode active material 904 of one embodiment of the present invention may be covered with a coating containing the additional element X.

[0186] <Grain Boundary> The additional element X contained in the positive electrode active material 904 of one embodiment of the present invention may be present randomly and dilutely within the positive electrode active material 904, but it is more preferable that a part of the additional element X segregates at the grain boundary.

[0187] In other words, the concentration of the additional element X in and around the grain boundaries of the positive electrode active material 904 of one embodiment of the present invention is preferably higher than that in other regions inside the grain boundaries.

[0188] Like particle surfaces, grain boundaries are also planar defects. This makes them prone to instability and facilitates changes in the crystal structure. Therefore, if the concentration of the added element X at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0189] Furthermore, when the concentration of the additional element X is high at and near the grain boundary, even if cracks occur along the grain boundary of particles of the positive electrode active material 904 of one embodiment of the present invention, the concentration of the additional element X becomes high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the cracks occur.

[0190] In this specification and the like, the vicinity of the grain boundary refers to the region up to about 10 nm from the grain boundary.

[0191] <Particle size> If the particle size of the positive electrode active material 904 is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector occur. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte occur. Therefore, the average particle size (D50: also referred to as median diameter) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less.

[0192] <Analysis method> Whether a certain positive electrode active material exhibits the O3'-type crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. XRD is particularly preferred because it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, and it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery.

[0193] As described above, the positive electrode active material 904 of one embodiment of the present invention is characterized by minimal change in crystal structure between a high-voltage charged state and a discharged state. Materials in which a crystal structure that exhibits a significant change between a high-voltage charged state and a discharged state occupies 50 wt% or more of a chargeable material are undesirable because they cannot withstand high-voltage charge and discharge. It should be noted that the desired crystal structure may not be achieved simply by adding an additive element. For example, even if both materials share the common feature of being lithium cobalt oxide containing magnesium and fluorine, there are cases in which the O3'-type crystal structure occupies 60 wt% or more of a chargeable material in a high-voltage charged state, and cases in which the H1-3-type crystal structure occupies 50 wt% or more of a chargeable material in a high-voltage charged state. Furthermore, at a certain voltage, the O3'-type crystal structure may be nearly 100 wt%, and further increasing the voltage may result in the H1-3-type crystal structure. Therefore, crystal structure analysis, such as XRD, is required to determine whether a material is the positive electrode active material 904 of one embodiment of the present invention.

[0194] However, when positive electrode active materials are charged or discharged at high voltage, their crystal structure may change when exposed to air. For example, they may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is recommended that all samples be handled in an inert atmosphere such as an argon atmosphere.

[0195] (Fourth embodiment) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted in an electronic device or a mobile object will be described.

[0196] 12A to 12E show examples of electronic devices incorporating the secondary battery described in part of embodiment 3. Examples of electronic devices to which the secondary battery is applied include television devices (also called televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0197] Furthermore, the secondary battery can be applied to a mobile object, typically an automobile. Examples of the automobile include next-generation clean energy automobiles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs), and the secondary battery can be applied as one of the power sources mounted on the automobile. The mobile object is not limited to an automobile. Examples of the mobile object include trains, monorails, ships, aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, and rockets), electric bicycles, and electric motorcycles, and the secondary battery of one embodiment of the present invention can be applied to these mobile objects.

[0198] Furthermore, the secondary battery of this embodiment may be applied to a ground-mounted charging device installed in a house or a charging station installed in a commercial facility.

[0199] 12A shows an example of a mobile phone. Mobile phone 2100 includes a display unit 2102 built into housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, and a microphone 2106. Mobile phone 2100 also includes a secondary battery 2107.

[0200] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0201] The operation button 2103 can be provided with various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system built into the mobile phone 2100.

[0202] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0203] The mobile phone 2100 also has an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Charging may also be performed by wireless power supply without using the external connection port 2104.

[0204] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, or other human body sensor, a touch sensor, a pressure sensor, or an acceleration sensor.

[0205] FIG. 12B shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. The secondary battery according to one embodiment of the present invention is highly safe and can be used safely for a long period of time, making it suitable as a secondary battery to be installed in the unmanned aerial vehicle 2300.

[0206] As shown in FIG. 12C, a secondary battery 2602 including a plurality of secondary batteries 2601 of one embodiment of the present invention may be mounted in a hybrid electric vehicle (HEV), an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), or other electronic devices.

[0207] FIG. 12D shows an example of a vehicle equipped with a secondary battery 2602. The vehicle 2603 is an electric vehicle that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid vehicle that can appropriately select and use an electric motor or an engine as a power source for traveling. The vehicle 2603 using an electric motor has multiple ECUs (Electronic Control Units), and the ECUs perform engine control and the like. The ECUs include a microcomputer. The ECUs are connected to a Controller Area Network (CAN) provided in the electric vehicle. CAN is one of the serial communication standards used for in-vehicle LANs. By using the secondary battery of one embodiment of the present invention, the vehicle can function as a power source for the ECU, thereby achieving a highly safe vehicle with a long cruising range.

[0208] The secondary battery can not only drive an electric motor (not shown) but also supply power to light-emitting devices such as headlights, room lights, etc. The secondary battery can also supply power to display devices and semiconductor devices such as a speedometer, a tachometer, and a navigation system that the vehicle 2603 has.

[0209] The vehicle 2603 can charge the secondary battery of the secondary battery 2602 by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like.

[0210] FIG. 12E shows a state in which a vehicle 2603 is being charged via a cable from a ground-mounted charging device 2604. Charging may be performed using a predetermined charging method, connector specifications, or the like, such as CHAdeMO (registered trademark) or Combo. For example, plug-in technology can be used to charge a secondary battery 2602 mounted on the vehicle 2603 using external power supply. Charging can be performed by converting AC power to DC power via a conversion device such as an AC-DC converter. The charging device 2604 may be installed in a home as shown in FIG. 12E, or may be a charging station installed in a commercial facility.

[0211] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between vehicles. Furthermore, a solar cell can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0212] 12E includes a power storage system 2612 including a secondary battery which is one embodiment of the present invention, and a solar panel 2610. The power storage system 2612 is electrically connected to the solar panel 2610 via a wiring 2611 or the like. The power storage system 2612 may be electrically connected to a ground-mounted charging device 2604. The power obtained by the solar panel 2610 can be charged to the power storage system 2612. The power stored in the power storage system 2612 can be charged to a secondary battery 2602 included in a vehicle 2603 via the charging device 2604.

[0213] The power stored in the power storage system 2612 can also be supplied to other electronic devices in the house. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the power storage system 2612 of one embodiment of the present invention can be used as an uninterruptible power supply, enabling the use of electronic devices.

[0214] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0215] (Embodiment 5) In this embodiment, the relationship between the positive electrode active material particles, the electrolyte, the additives, etc. will be described below.

[0216] 14A is a model diagram showing a plurality of positive electrode active materials and the electrolyte solution, additives, etc. disposed therearound inside the secondary battery of Embodiment 1 or 2. The left side of FIG. 14A shows a model diagram of a plurality of particles, and the right side shows an enlarged view of one particle.

[0217] As shown in FIG. 14A, Li + Ions are shown migrating into the particles of positive electrode active material 200A or into the electrolyte.

[0218] The figure shows that Mg, Al, and Ni are unevenly distributed in the surface layer of the particles of positive electrode active material 200A, and that at least a partial coating of additives is formed on the surface. The additive coating is formed when the boron (B) portion of LiBOB adheres to a portion of the particles of positive electrode active material 200A. The areas where Mg, Al, and Ni are unevenly distributed also contain fluorine, which prevents the transition metals contained in the particles, typically cobalt (or manganese, nickel, etc.), from dissolving into the electrolyte. The coating also prevents the transition metals from dissolving into the electrolyte. The coating also prevents side reactions with the electrolyte. The presence of the areas where Mg, Al, and Ni are unevenly distributed and the synergistic effect of the coating significantly improve reliability.

[0219] As a comparative example, FIG. 14B is a model diagram showing the state of multiple LiCoO particles inside a conventional secondary battery during charge and discharge. The left side of FIG. 14B shows a model diagram of multiple particles, and the right side shows an enlarged view of a single particle. FIG. 18 is an example of a model showing the relationship between positive electrode active material particles 101 and additive 103. As shown in FIG. 18, adding LiBOB to the electrolyte creates areas on the surface layer 102 of the positive electrode active material particles 101 where the LiBOB is in contact and areas where it is not in contact. During charge and discharge, Li moves in and out of the gaps between the areas where the LiBOB is in contact. Adding an appropriate amount of LiBOB to the electrolyte can suppress the elution of nickel, manganese, and other elements contained in the positive electrode active material particles 101. The optimal amount of LiBOB to be added to the electrolyte is preferably such that it is in contact with the surface layer 102 of the positive electrode active material particles 101.

[0220] As shown in FIG. 14B, Li + The image shows ions moving into the LiCoO2 particles or into the electrolyte. It also shows that transition metals contained in the LiCoO2 particles, typically cobalt (or manganese, nickel, etc.), dissolve into the electrolyte during charging and discharging. The dissolving cobalt adheres to the negative electrode of the secondary battery, accelerating its degradation.

[0221] In the secondary battery of embodiment 1 or 2, LiPF6 shown in Fig. 17A is used as the lithium salt in the electrolyte, and diethyl carbonate (DEC) shown in Fig. 17B and ethylene carbonate (EC) shown in Fig. 17C are used as the electrolyte.

[0222] The chemical reactions that occur inside these secondary batteries are shown in Figure 15 or Figure 16.

[0223] Other possible solvents for the electrolyte solution include propylene carbonate (PC) shown in Figure 17D, ethyl methyl carbonate (EMC) shown in Figure 17F, and dimethyl carbonate (DMC) shown in Figure 17G. Other possible solvents for the electrolyte solution include butylene carbonate, chloroethylene carbonate, γ-butyrolactone, γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone, and any combination and ratio of two or more of these may be used. In this embodiment, the additive is not limited to LiBOB, and other additives (such as vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), dinitrile compounds such as succinonitrile and adiponitrile) may also be added. Note that vinylene carbonate (VC) shown in FIG. 17E is an additive.

[0224] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Explanation of symbols]

[0225] 200A: positive electrode active material, 200B: positive electrode active material, 500: secondary battery, 501: positive electrode current collector, 502: positive electrode active material layer, 503: positive electrode, 504: negative electrode current collector, 505: negative electrode active material layer, 506: negative electrode, 507: separator, 508: electrolyte, 509: exterior body, 510: positive electrode lead electrode, 511: negative electrode lead electrode, 600: secondary battery, 601: positive electrode cap, 602: battery can, 603: positive electrode terminal, 604: positive electrode, 605: separator, 606: negative electrode, 607: negative electrode terminal, 608: insulating plate, 609: insulating plate, 611: PTC element, 612: safety valve mechanism, 613: Conductive plate, 614: Conductive plate, 615: Module, 616: Conductive wire, 617: Temperature control device, 902: Mixture, 903: Mixture, 2100: Mobile phone, 2101: Housing, 2102: Display unit, 2103: Operation button, 2104: External connection port, 2105: Speaker, 2106: Microphone, 2107: Secondary battery, 2300: Unmanned aerial vehicle, 2301: Secondary battery, 2302: Rotor, 2303: Camera, 2601: Secondary battery, 2602: Secondary battery, 2603: Vehicle, 2604: Charging device, 2610: Solar panel, 2611: Wiring, 2612: Energy storage system

Claims

1. a positive electrode active material containing lithium, cobalt, nickel, aluminum, oxygen, and fluorine; an electrolyte containing lithium bis(oxalato)borate; and a negative electrode active material that is graphite; The electrolyte solution contains a lithium salt, diethyl carbonate in which the lithium salt is dissolved, and ethylene carbonate, the positive electrode active material has a layered rock salt crystal structure, A secondary battery, wherein the proportion of the lithium bis(oxalato)borate in the electrolyte solution is more than 0.1 wt % and less than 3 wt %.

2. 2. The secondary battery according to claim 1, wherein the lithium salt is lithium hexafluorophosphate.

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