Coated positive electrode active material and method for producing same

Abstract

To provide a cathode active material capable of suppressing gas generation owing to the oxidative decomposition of a nonaqueous electrolyte in a lithium ion secondary battery arranged by use of a cathode active material that operates at a high electric potential, and a manufacturing method thereof.SOLUTION: A coated cathode active material comprises a cathode active material coated with a lithium ion-conducting oxide. The cathode active material is a lithium-conducting active material of which the operation voltage is equal to or higher than 4.5 V of a melting-deposition potential reference of lithium metal. The lithium ion-conducting oxide is amorphous below 600°C. The coated cathode active material is a transparent sol comminuted to a particle size of 10 nm or less according to a small angle X-ray scattering method.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coated positive electrode active material for a lithium ion secondary battery and a method for producing the same, and more particularly to a coated positive electrode active material for a lithium ion secondary battery in which gas generation during operation at high potential is suppressed, and a method for producing the same. [Background technology]

[0002] Research and development of lithium-ion secondary batteries is actively being conducted for applications in portable devices, hybrid vehicles, electric vehicles, and home energy storage. Lithium-ion secondary batteries used in these fields require high safety, long-term cycle stability, and high energy density.

[0003] In recent years, lithium-ion secondary batteries using lithium titanate as the negative electrode active material have been proposed from the viewpoints of high safety and long-term cycle stability. Because the operating potential of lithium titanate is higher than that of common negative electrode active materials such as graphite, lithium deposition is less likely to occur, improving safety, but this is disadvantageous from the viewpoint of energy density. Meanwhile, materials that operate at a high potential of 4.5 V or more relative to the deposition potential of Li have been proposed for the positive electrode active material (e.g., Patent Document 1).

[0004] The reduction in energy density due to the high operating potential of lithium titanate is expected to be improved by combining lithium titanate with a positive electrode active material that operates at a high potential, as shown in the aforementioned Patent Document 1. On the other hand, in conventional lithium-ion secondary batteries that use graphite as the negative electrode active material, gas is generated by oxidative decomposition of the non-aqueous electrolyte on the surface of the positive electrode active material, but the problem of gas generation described above becomes more pronounced in secondary batteries in which the operating potential of the positive electrode active material is higher than in conventional secondary batteries.

[0005] Therefore, a method for suppressing gas generation by forming a coating layer on the surface of a positive electrode material that operates at a high potential has been reported, as in Patent Document 2. However, gas generation was still confirmed due to factors such as the large particle size of the coating material, and there was room for improvement. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-185148 [Patent Document 2] International Publication No. 2020 / 049843 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a positive electrode active material that further suppresses gas generation in a lithium ion secondary battery that uses a positive electrode active material that operates at a high potential. [Means for solving the problem]

[0008] In view of the above circumstances, the present inventors have investigated means for suppressing the above-mentioned gas generation, and as a result have found that gas generation can be effectively suppressed in a coated positive electrode active material in which a positive electrode active material is coated with a lithium ion conductive oxide, if the positive electrode active material has an operating voltage of 4.5 V or more, which is the standard dissolution and deposition potential of lithium metal, and the lithium ion conductive oxide is amorphous at temperatures below 600°C, and is a transparent sol that has been pulverized to a particle size of 10 nm or less as measured by small-angle X-ray scattering, thereby completing the present invention.

[0009] The present invention also relates to a transparent sol comprising a compound represented by the following formula (A): Li 1+p+q (Al,Ga) p (Ti,Ge) 2-p Si q P 3-q O 12 (A) (In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.) and a pulverized product of a lithium ion conductive oxide represented by the formula (I), and the content of the pulverized product is preferably 15 to 30 wt % of the total weight. Furthermore, the positive electrode active material is represented by the following formula (1): Li 1+x M y Mn 2-x-y O4···(1) (In formula (1), x and y respectively satisfy 0 ≦ x ≦ 0.2 and 0 < y ≦ 0.8, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.) It is preferably a substituted lithium manganese compound represented by this formula. In another aspect, the present invention includes a step of pulverizing a lithium ion conductive oxide until the particle size by small-angle X-ray scattering method becomes 10 nm or less, a step of adding the pulverized lithium ion conductive oxide powder to alcohol to make a transparent sol state, a step of coating the transparent sol to which the lithium ion conductive oxide powder is added on a positive electrode active material by a mechanochemical method, and a step of heat-treating the positive electrode active material coated with the transparent sol at 100°C or higher and 500°C or lower. In the step of making the transparent sol state, the powder of the lithium ion conductive oxide is added at a ratio of 15 to 30% by weight based on the total weight of the transparent sol. The present invention relates to a method for producing a coated positive electrode active material characterized by this. In the above production method, the lithium ion conductive oxide is represented by the following formula (A): Li 1+p+q (Al,Ga) p (Ti,Ge) 2-p Si q P 3-q O 12 ···(A) (In formula (A), p and q respectively satisfy 0 ≦ p ≦ 1 and 0 ≦ q ≦ 1.) It is preferably represented by this. Also, it is preferably a method for producing a positive electrode active material in which the positive electrode active material is a substituted lithium manganese compound represented by the following formula (1). Li 1+x M y Mn 2-x-y O4···(1) (In formula (1), x and y satisfy 0 ≦ x ≦ 0.2 and 0 < y ≦ 0.8 respectively, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.) [Advantages of the Invention]

[0010] According to the present invention, a positive electrode active material that operates at a high potential and can suppress the generation of gas due to oxidative decomposition of a non-aqueous electrolyte can be provided. [Embodiments for Carrying Out the Invention]

[0011] One embodiment of the present invention will be described below, but the present invention is not limited thereto.

[0012] The coated positive electrode active material of the present invention is a coated positive electrode active material in which the positive electrode active material is coated with a lithium ion conductive oxide, The above positive electrode active material is a lithium conductive active material having an operating voltage of 4.5 V or more based on the dissolution and deposition potential of lithium metal, The above lithium ion conductive oxide is an amorphous transparent sol at less than 600 °C and pulverized to a particle size of 10 nm or less by small-angle X-ray scattering method. The above coated positive electrode active material is, for example, a step of pulverizing the lithium ion conductive oxide until the particle size by small-angle X-ray scattering method becomes 10 nm or less, a step of adding the pulverized powder of the lithium ion conductive oxide to alcohol to form a transparent sol state, a step of coating the transparent sol added with the powder of the lithium ion conductive oxide on the positive electrode active material by a mechanochemical method, a step of heat-treating the positive electrode active material coated with the transparent sol at 100 °C or more and 500 °C or less, and In the step of forming the transparent sol state, the powder of the lithium ion conductive oxide is added at a ratio of 15 to 30% by weight based on the total weight of the transparent sol, and is obtained by the manufacturing method.

[0013] Generally, lithium-ion secondary batteries use non-aqueous electrolytes. These are liquid non-aqueous electrolytes, which are made by dissolving lithium salts in a non-aqueous solvent (see below for details). On the other hand, there are solid electrolytes that combine the functions of both a non-aqueous solvent and a lithium salt. Solid electrolytes have higher oxidation resistance than liquid non-aqueous electrolytes, so oxidative decomposition at high potentials is suppressed. However, because lithium ion conductivity is lower in solids than in liquids, replacing the entire electrolyte with a solid electrolyte significantly reduces battery performance.

[0014] Therefore, by covering only the surface of the high-potential positive electrode active material with a lithium ion conductive oxide, which is a solid electrolyte, gas generation can be suppressed even if the non-aqueous electrolyte remains the same. The coating method is not particularly limited, but a method capable of achieving uniform coating, such as mechanical coating, is preferred. Mechanical coating is preferred because, in the present invention, a lithium ion conductive oxide in a transparent sol state is used, and the solvent provides adhesiveness and ductility, enabling uniform coating. Furthermore, when volatilizing the solvent by heat treatment, the heat treatment temperature is adjusted, and preferably the particle size of the lithium ion conductive oxide and the mixing ratio with the positive electrode active material are controlled, so that the lithium ion conductive oxide can be coated on the positive electrode active material without increasing the resistance of the positive electrode active material or reducing battery performance. The lithium ion conductive oxide used in the present invention is in a transparent sol state, but a certain amount of energy is required to coat the solid positive electrode active material. Therefore, a mechanical coating method using a mechanochemical method that can apply shearing force and compressive force is preferred. By coating the positive electrode active material with a solid electrolyte, contact between the conventional nonaqueous electrolyte and the positive electrode active material can be reduced, and gas generation can be suppressed. Furthermore, as will be described in detail later, by adjusting the heat treatment temperature and preferably controlling the particle size of the lithium ion conductive oxide and the mixing ratio with the positive electrode active material, it is possible to coat the positive electrode active material with the lithium ion conductive oxide without increasing the resistance of the positive electrode active material or reducing the battery performance.

[0015] <Mechanical coating method> Mechanical coating refers to a method of applying at least one type of energy selected from shear force, compression force, impact force, and centrifugal force to a base material and / or coating agent (preferably, shear force and compression force can be applied, and more preferably, shear force, compression force, and impact force can be applied) while mechanically contacting the base material and coating agent, thereby mixing the base material and coating agent and coating the surface of the base material with the coating agent. In the present invention, the positive electrode active material corresponds to the base material, and the coating agent corresponds to the lithium ion conductive oxide in a transparent sol state. The apparatus used is not particularly limited, but suitable examples include a grinding mill, such as the Nobilta manufactured by Hosokawa Micron Corporation, and a planetary ball mill (e.g., manufactured by Fritsch). Among these, a grinding mill is preferred from the viewpoints of simple operation and the absence of the need to separate the balls after processing, as is the case with a ball mill.

[0016] In the production method of the present invention, it is preferable to provide a bottomed cylindrical container and a rotor with tip blades, provide a predetermined clearance between the tip blades and the inner periphery of the container, and rotate the rotor to apply compressive force and shear force to the mixture containing the positive electrode active material and the lithium ion conductive oxide in a transparent sol state, thereby performing mechanical coating.

[0017] In the case of wet treatment by mechanical coating, the solvent used is not particularly limited, and water or an organic solvent can be used. Examples of the organic solvent include alcohols such as ethanol. The timing of adding the solvent in the case of wet treatment is not particularly limited, and the lithium ion conductive oxide in a transparent sol state may be diluted with a solvent before use in the mechanical coating method. The concentration of the lithium ion conductive oxide in a transparent sol state in the slurry is not particularly limited, and is, for example, 10 to 30 wt %, preferably 15 to 30 wt %, and more preferably 15 to 25 wt %, based on the total weight.

[0018] The treatment temperature for mechanical coating is preferably 5 to 100°C, more preferably 8 to 80°C, and even more preferably 10 to 50°C, and the treatment time is preferably 5 to 90 minutes, more preferably 10 to 60 minutes. The treatment atmosphere is not particularly limited, and may be an inert gas atmosphere or an air atmosphere.

[0019] Although the sample after mechanical coating can be used as is, it is preferable to perform heat treatment. This improves the adhesion between the positive electrode active material and the lithium ion conductive oxide, preventing the lithium ion conductive oxide from peeling off from the positive electrode active material even during repeated charge and discharge, thereby improving the long-term reliability of the battery. If the heat treatment temperature is too high, the crystal structure of the lithium ion conductive oxide may change, reducing Li-ion conductivity and preventing normal battery charge and discharge. Therefore, the heat treatment temperature is preferably 500°C or less, more preferably 100°C or more and 500°C or less. The heat treatment time is preferably 30 minutes or more, more preferably 1 hour or more. There is no particular upper limit, but it is, for example, 3 hours or less.

[0020] <Cathode active material> The positive electrode active material used in the manufacturing method of the present invention is not particularly limited, but it is preferable that the average potential of lithium deintercalation and intercalation is Li + / Li, i.e., with respect to the deposition potential of Li (vs. Li + The potential (hereinafter also referred to as voltage) (vs. Li / Li) of the lithium ion insertion / extraction reaction is preferably 4.5 V or more, and more preferably 4.5 V or more and 5.0 V or less. + / Li) can be determined by measuring the charge / discharge characteristics of a half-cell with a working electrode using a positive electrode active material and a lithium metal counter electrode, and reading the voltage values ​​at the start and end of the plateau. If there are two or more plateaus, the plateau with the lowest voltage value is 4.5 V (vs. Li + / Li) or higher, and the highest voltage plateau is 5.0V (vs. Li + / Li) or less is sufficient.

[0021] The cathode active material in which the insertion / desorption reaction of lithium ions proceeds at 4.5 V or more and 5.0 V or less with respect to the deposition potential of Li is not particularly limited, but a substituted lithium manganese compound represented by the following formula (1) has been conventionally studied and is preferable.

[0022] Li 1+x M y Mn 2-x-y O4···(1) In the above formula (1), x and y satisfy 0 ≦ x ≦ 0.2 and 0 < y ≦ 0.8, respectively, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.

[0023] Among the above formula (1), a Ni-substituted lithium manganese compound in which M is Ni is preferable, and particularly x = 0, y = 0.5, and M = Ni, that is, LiNi 0.5 Mn 1.5 O4 is particularly preferable because of its high stability effect in the charge / discharge cycle.

[0024] The particle size of the cathode active material is not particularly limited, but if the particle size is too small, the difference from the particle size of the oxide-based solid electrolyte described later becomes small and coating becomes difficult. Therefore, the median diameter d50 is preferably 5 μm or more, more preferably 10 μm or more, and still more preferably 20 μm or more. Also, the median diameter d50 is preferably 100 μm or less, more preferably 80 μm or less, and still more preferably 50 μm or less. Considering the thickness range during processing into an electrode, the d50 is preferably 10 to 50 μm, and more preferably 20 to 50 μm.

[0025] <Transparent sol-state lithium ion conductive oxide> The transparent sol-state lithium ion conductive oxide used in the present invention is not particularly limited, but it is preferable to use an oxide-based solid electrolyte in consideration of chemical stability. Oxide-based solid electrolytes include an inverse fluorite type, a NASICON type, a perovskite type, a garnet type, etc. depending on the crystal structure, but are not particularly limited. Examples of the oxide-based solid electrolyte include the following formula (A): Li 1+p+q(Al,Ga) p (Ti,Ge) 2-p Si q P 3-q O 12 (A) (In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.) LATP represented by the following formula (A1): Li 1+p Al p Ti 2-p P3O 12 ···(A1) (In formula (A1), p satisfies 0≦p≦1.) An oxide-based solid electrolyte represented by the following formula is preferred.

[0026] The lithium ion conductive oxide in the transparent sol state is pulverized until it becomes amorphous and transparent, so that it inevitably becomes fine particles. Specifically, it is desirable to microparticle the BET specific surface area equivalent diameter (dBET) to preferably 10 nm or less, more preferably 8 nm or less, and even more preferably 6 nm or less. As a method for microparticulation treatment, known means such as a ball mill, a bead mill, etc. can be used. The BET specific surface area equivalent diameter (dBET) is a particle diameter calculated by determining the nitrogen adsorption BET specific surface area by the nitrogen adsorption single-point method according to the method specified in JIS-Z8830 (2013), and then calculating dBET = 6 / (density × BET specific surface area).

[0027] The ratio of the median diameter d50 of the positive electrode active material to the BET specific surface area equivalent diameter dBET of the lithium ion conductive oxide in the transparent sol state is preferably 10,000:1 to 100:1, more preferably 5,000:1 to 300:1, even more preferably 2,000:1 to 500:1, and particularly preferably 1,000:1 to 500:1.

[0028] The ratio of the lithium ion conductive oxide in a transparent sol state (solid content when used as a slurry) to 100 parts by mass of the positive electrode active material is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, even more preferably 2 parts by mass or more, and is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 4 parts by mass or less. The ratio is preferably 1 part by mass or more and 5 parts by mass or less (i.e., the mass ratio of the positive electrode active material to the lithium ion conductive oxide in a transparent sol state is 100:1 to 20:1), and also preferably 2 parts by mass or more and 4 parts by mass or less (i.e., the mass ratio of the positive electrode active material to the lithium ion conductive oxide in a transparent sol state is 50:1 to 25:1).

[0029] <Lithium-ion secondary battery> Lithium-ion secondary batteries are primarily composed of a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode is fabricated, for example, by applying a positive electrode mixture containing a positive electrode active material, a conductive additive, a binder, etc. to a positive electrode current collector. The negative electrode is fabricated, for example, by applying a negative electrode mixture containing a negative electrode active material, a conductive additive, a binder, etc. to a negative electrode current collector. The coated positive electrode active material obtained by the manufacturing method of the present invention is suitable for use as the positive electrode active material of a lithium-ion secondary battery. Specifically, a positive electrode can be fabricated by applying a positive electrode mixture containing the coated positive electrode active material obtained by the manufacturing method of the present invention to a positive electrode current collector. After applying the positive electrode mixture to the positive electrode current collector and the negative electrode mixture to the negative electrode current collector, they can be dried at approximately 100 to 200°C.

[0030] The configuration of a lithium ion secondary battery using the coated positive electrode active material, the materials used other than the coated positive electrode active material, and the manufacturing apparatus and conditions for the lithium ion secondary battery may be any conventionally known ones and are not particularly limited.

[0031] <Negative electrode active material> As described above, lithium titanate is preferably used as the negative electrode active material because it is less susceptible to lithium deposition and improves safety. Among lithium titanates, lithium titanate with a spinel structure is particularly preferred because it exhibits small expansion and contraction of the active material during the lithium ion insertion and desorption reactions. Lithium titanate may contain trace amounts of elements other than lithium and titanium, such as Nb.

[0032] <Conductive additive> The conductive additive is not particularly limited, but is preferably a carbon material for both the positive electrode and the negative electrode. Examples of carbon materials include natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotubes, acetylene black, ketjen black, and furnace black. These carbon materials may be used alone or in combination. The amount of conductive additive contained in the positive electrode is preferably 1 to 30 parts by weight, more preferably 2 to 15 parts by weight, per 100 parts by weight of the positive electrode active material. Within this range, the conductivity of the positive electrode is ensured. Furthermore, adhesion with the binder described below is maintained, and sufficient adhesion with the current collector can be obtained. The amount of conductive additive contained in the negative electrode is preferably 1 to 30 parts by weight, more preferably 2 to 15 parts by weight, per 100 parts by weight of the negative electrode active material.

[0033] <Binder> The binder is not particularly limited, but for both the positive electrode and the negative electrode, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof can be used. The binder is preferably dissolved or dispersed in a nonaqueous solvent or water for ease of fabrication of the positive electrode and the negative electrode. The nonaqueous solvent is not particularly limited, but examples include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. A dispersant or thickener may be added to these. The amount of binder contained in the positive electrode of the present invention is preferably 1 to 30 parts by weight, more preferably 2 to 15 parts by weight, per 100 parts by weight of the positive electrode active material. Within the above range, adhesion between the positive electrode active material and the conductive additive is maintained, and sufficient adhesion to the current collector can be obtained. The amount of binder contained in the negative electrode is preferably 1 part by weight to 30 parts by weight, more preferably 2 parts by weight to 15 parts by weight, per 100 parts by weight of the negative electrode active material.

[0034] <Current collector> The positive electrode current collector and the negative electrode current collector are not particularly limited, but are preferably aluminum or an aluminum alloy. Because aluminum or an aluminum alloy is stable in the positive and negative electrode reaction atmosphere, high-purity aluminum, such as JIS standards 1030, 1050, 1085, 1N90, and 1N99, is preferred. The thickness of the current collector is not particularly limited, but is preferably 10 μm to 100 μm. Within this range, it is easy to balance the handling during battery fabrication, cost, and resulting battery characteristics. Current collectors made of metals other than aluminum (copper, SUS, nickel, titanium, and alloys thereof) coated with a metal that does not react with the positive and negative electrode potentials can also be used.

[0035] <Non-aqueous electrolyte> The non-aqueous electrolyte is not particularly limited, but examples thereof include a non-aqueous electrolytic solution in which a solute is dissolved in a non-aqueous solvent, and a gel electrolyte in which a polymer is impregnated with a non-aqueous electrolytic solution in which a solute is dissolved in a non-aqueous solvent.

[0036] The non-aqueous solvent preferably includes, for example, a cyclic aprotic solvent and / or a chain aprotic solvent. Examples of cyclic aprotic solvents include cyclic carbonates, cyclic esters, cyclic sulfones, and cyclic ethers. Examples of chain aprotic solvents include chain carbonates, chain carboxylic acid esters, chain ethers, and acetonitrile, which are commonly used as solvents for non-aqueous electrolytes. More specifically, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyl lactone, 1,2-dimethoxyethane, sulfolane, dioxolane, and methyl propionate can be used. These solvents may be used alone or in combination. However, it is preferable to use a mixture of two or more solvents, considering the ease of dissolving the solute and the high lithium ion conductivity described below.

[0037] When two or more types are mixed, a mixture of one or more types of chain carbonates exemplified by dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, and methyl propyl carbonate with one or more types of cyclic compounds exemplified by ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone is preferred, as this provides high stability at high temperatures and high lithium conductivity at low temperatures, and a mixture of one or more types of chain carbonates exemplified by dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate with one or more types of cyclic carbonates exemplified by ethylene carbonate, propylene carbonate, and butylene carbonate is particularly preferred.

[0038] The solute is not particularly limited, but examples thereof include LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiBOB (Lithium Bis(Oxalato)Borate), and LiN(SO2CF3)2, which are preferred because they are easily soluble in the solvent. The concentration of the solute contained in the nonaqueous electrolyte is preferably 0.5 mol / L or more and 2.0 mol / L or less. When the concentration is 0.5 mol / L or more, the desired lithium ion conductivity is easily exhibited, while when the concentration is 2.0 mol / L or less, the solute is easily dissolved.

[0039] The amount of nonaqueous electrolyte used in the lithium ion secondary battery of the present invention is not particularly limited, but is preferably 0.1 mL or more per 1 Ah of battery capacity, which ensures the conduction of lithium ions accompanying the electrode reaction and allows the desired battery performance to be achieved.

[0040] The non-aqueous electrolyte may be contained in the positive electrode, negative electrode and separator in advance, or may be added after wrapping or laminating the positive electrode side and negative electrode side with the separator disposed between them.

[0041] In addition to the above-described configuration, the lithium ion secondary battery usually further includes a separator and an exterior material.

[0042] (separator) The separator may be disposed between the positive electrode and the negative electrode, and may have any structure that is insulating and capable of containing the non-aqueous electrolyte described below, such as a woven fabric, nonwoven fabric, or microporous membrane made of nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, or a composite of two or more of these. Nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, or a nonwoven fabric made of a composite of two or more of these is preferred because it provides excellent stability in cycle characteristics.

[0043] The separator may contain various plasticizers, antioxidants, and flame retardants, and may be coated with metal oxides, etc. The thickness of the separator is not particularly limited, but is preferably 10 μm or more and 100 μm or less. Within this range, short-circuiting between the positive electrode and the negative electrode can be prevented while suppressing an increase in battery resistance. From the standpoint of economy and ease of handling, a thickness of 15 μm or more and 50 μm or less is even more preferable.

[0044] The porosity of the separator is preferably 30% or more and 90% or less. When it is 30% or more, the diffusibility of lithium ions is less likely to decrease, making it easier to maintain cycle characteristics, while when it is 90% or less, the risk of short circuits caused by electrode irregularities penetrating the separator can be effectively reduced. From the viewpoint of a balance between ensuring the diffusibility of lithium ions and preventing short circuits, it is more preferably 35% or more and 85% or less, and particularly preferably 40% or more and 80% or less, as this balance is particularly excellent.

[0045] (exterior materials) The exterior packaging material is a member that encloses a laminate formed by alternately stacking or winding positive electrodes, negative electrodes, and separators, as well as terminals that electrically connect the laminate. Suitable exterior packaging materials include composite films in which a thermoplastic resin layer for heat sealing is provided on a metal foil, metal layers formed by vapor deposition or sputtering, and metal cans in the shape of a square, oval, cylinder, coin, button, or sheet. [Example]

[0046] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can of course be practiced with appropriate modifications within the scope of the above and below-described aims, all of which are included in the technical scope of the present invention.

[0047] Lithium ion secondary batteries fabricated using the coated positive electrodes obtained in the following Examples and Comparative Examples were evaluated by the following methods.

[0048] (gas generation amount) The amount of gas generated from the lithium ion secondary battery before and after the cycle performance evaluation was evaluated using the Archimedes method, that is, the buoyancy of the lithium ion secondary battery. The evaluation was performed as follows.

[0049] First, the weight of the lithium-ion secondary battery was measured using an electronic balance. Next, the weight in water was measured using a hydrometer (manufactured by Alpha Mirage, product number: MDS-3000). The buoyancy was calculated by taking the difference between these weights. This buoyancy was calculated based on the density of water (1.0 g / cm 3 The volume of the lithium ion secondary battery was calculated by dividing the volume by the measured value. The volume after aging and the volume after cycle performance evaluation were compared to calculate the amount of gas generated. A gas generation amount of less than 20 ml was considered to be good. A gas generation amount of 15 ml or less is more preferable.

[0050] (Evaluation of cycle characteristics of lithium-ion secondary batteries) The fabricated lithium-ion secondary battery was connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Corporation) and cycled. Constant-current charging was performed at a current value equivalent to 1.0 C in a 60°C environment until the battery voltage reached a cutoff voltage of 3.4 V, at which point charging was stopped. Subsequently, constant-current discharging was performed at a current value equivalent to 1.0 C, and discharging was stopped when the battery voltage reached 2.5 V. This constituted one cycle, and charging and discharging were repeated. The stability of the cycle characteristics was evaluated as the discharge capacity retention rate (%), where the discharge capacity at the 500th cycle was taken as 100. A discharge capacity retention rate of 80% or more at the 500th cycle was considered good, and a rate of less than 80% was considered poor.

[0051] Synthesis Example 1: Preparation of lithium ion conductive oxide As a lithium ion conducting oxide, Li 1.3 A 10.3 Ti 1.7(PO4)3 (hereinafter referred to as LATP) was prepared. The starting materials, Li2CO3, AlPO4, TiO2, NH4H2PO4, and the solvent ethanol, were mixed in predetermined amounts and processed in a planetary ball mill at 150 G for 1 hour using 3 mm diameter zirconia balls. After the processing, the zirconia balls were removed from the mixture using a sieve, and the mixture was dried at 120 °C to remove the ethanol. The mixture was then processed at 800 °C for 2 hours to obtain LATP powder.

[0052] (Preparation of coated positive electrode active material) Example 1 (i) Grinding lithium ion conductive oxide and adding it to alcohol The obtained LATP powder was mixed with a predetermined amount of ethanol as a solvent and treated with a planetary ball mill using zirconia balls with a diameter of 0.5 mm for 3 to 6 hours. After the treatment, the zirconia balls were removed with a sieve, and ethanol was added to adjust the LATP solid content to 16.4 wt%. This resulted in LATP in a transparent sol state with a dBET of 3 to 10 nm and no peaks attributable to LATP in X-ray crystal diffraction measurements.

[0053] (ii) Coating treatment, heat treatment The positive electrode active material was a spinel-type lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4, hereafter also referred to as LNMO) was used.

[0054] 40 g of LNMO was placed in a grinding mill (Hosokawa Micron Corporation, Nobilta) with a clearance of 0.6 mm, a rotor load power of 1.5 kW, and rotation at 2600 rpm. 6.1 g of LATP, a transparent sol that had been milled for 3 hours, was then added in two batches. The rotor rotation speed was then maintained between 2600 and 3000 rpm, and the mixture was treated at room temperature for 10 minutes in an air atmosphere to obtain LNMO with a surface coated with LATP. The resulting surface-coated LNMO was then heat-treated at 350°C for 1 hour to obtain a coated cathode active material.

[0055] Example 2 A coated positive electrode active material was produced in the same manner as in Example 1, except that the amount of LATP coated in a transparent sol state was changed to 9.1 g.

[0056] Comparative Example 1 In preparing the coated positive electrode active material, the particle size of the LATP was the same, but the surface-coated LNMO obtained was heat-treated at 600°C for 1 hour. The same procedure as in Example 1 was carried out to prepare the coated positive electrode active material.

[0057] Comparative Example 2 In preparing the coated positive electrode active material, the particle size of the LATP was the same, but the surface-coated LNMO obtained was heat-treated at 50°C for 1 hour. The same procedure as in Example 1 was carried out to prepare the coated positive electrode active material.

[0058] Comparative Example 3 A coated positive electrode active material was prepared by carrying out the same operation as in Example 1, except that in the grinding of the lithium ion conductive oxide, the grinding time of LATP was set to 1 hour and that particles having a particle diameter of about 20 nm and having a peak derived from LATP in X-ray crystal diffraction measurement were used.

[0059] Comparative Example 4 A coated positive electrode active material was prepared by carrying out the same operations as in Example 1, except that in the grinding of the lithium ion conductive oxide, the grinding time of LATP was set to 2 hours, and that the particle size of LATP was about 15 nm, which did not have a peak derived from LATP in X-ray crystal diffraction measurement but was not in a transparent sol state.

[0060] Comparative Example 5 A positive electrode active material was produced in the same manner as in Example 1, except that LNMO without surface coating was used.

[0061] Using the positive electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 to 5, positive electrode mixtures, positive electrodes, negative electrodes, and lithium ion secondary batteries were prepared by the following procedures, and the gas generation amounts and cycle characteristics of the lithium ion secondary batteries were evaluated according to the evaluation methods described above. The evaluation results are shown in Table 1.

[0062] (Preparation of positive electrode) A mixture containing the resulting surface-coated LNMO, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder at solid concentrations of 90 parts by weight, 6 parts by weight, and 4 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture. The binder was prepared as an N-methyl-2-pyrrolidone (NMP) solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating, as described below.

[0063] The positive electrode mixture was applied to a 20 μm thick aluminum foil and then dried in an oven at 120° C. This operation was performed on both sides of the aluminum foil, and then the foil was further dried in a vacuum at 170° C. to prepare a positive electrode.

[0064] (Preparation of negative electrode) The negative electrode active material was spinel-type lithium titanate (Li4Ti5O 12 (hereinafter, also referred to as LTO) was used. A mixture containing the LTO, acetylene black as a conductive additive, and PVdF as a binder in solid concentrations of 100 parts by weight, 5 parts by weight, and 5 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode mixture. The binder was prepared as an NMP solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating, as described below.

[0065] The negative electrode mixture was applied to a 20 μm thick aluminum foil and then dried in an oven at 120° C. This operation was performed on both sides of the aluminum foil, and then the foil was further dried in a vacuum at 170° C. to prepare a negative electrode.

[0066] (Fabrication of lithium-ion secondary batteries) A battery was fabricated using the positive and negative electrodes prepared above and a 20 μm polypropylene separator according to the following procedure. First, the positive and negative electrodes were dried under reduced pressure at 80°C for 12 hours. Next, 15 positive electrodes and 16 negative electrodes were stacked in the order of negative electrode / separator / positive electrode. Both outermost layers were separators. Next, aluminum tabs were vibration-welded to both ends of the positive and negative electrodes.

[0067] Two sheets of aluminum laminate film were prepared as the exterior material. A depression for the battery section and a depression for the gas collection section were formed by pressing, and the electrode stack was then inserted. The outer periphery, leaving a space for nonaqueous electrolyte injection, was heat-sealed at 180°C for 7 seconds. A nonaqueous electrolyte was then inserted from the unsealed portion. The nonaqueous electrolyte was then added. The solvent was a mixture of ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of 15 / 15 / 70, with LiPF6 dissolved at a ratio of 1 mol / L. The unsealed portion was then heat-sealed at 180°C for 7 seconds while the pressure was reduced. The resulting battery was charged at a constant current equivalent to 0.2 C until the battery voltage reached a cut-off voltage of 3.4 V, after which charging was stopped. The battery was then left to stand at 60°C for 24 hours, after which a constant current discharge was performed at a current equivalent to 0.2 C. Discharge was stopped when the battery voltage reached 2.5 V. After the discharge was stopped, the gas trapped in the gas trap was removed and the battery was resealed. A lithium ion secondary battery for evaluation was fabricated by the above-described operations.

[0068] [Table 1]

[0069] The lithium ion secondary batteries of Examples 1 and 2 were found to generate a small amount of gas in the cycle characteristic evaluation and also had a high capacity retention rate.

[0070] On the other hand, Comparative Example 1, in which the heat treatment temperature was 600°C, had a particle size equivalent to that of Examples 1 and 2, but the amount of gas generated was large and the capacity retention rate was low. This is thought to be due to recrystallization of LATP.

[0071] Furthermore, although the particle size was similar to Examples 1 and 2, Comparative Example 2, in which the heat treatment temperature was set to 50°C, also generated a large amount of gas and had a low capacity retention rate. This is thought to be due to residual solvent remaining during spray coating.

[0072] Comparative Example 3, in which the LATP crushing time was 1 hour and which had a peak derived from LATP in the X-ray crystal diffraction measurement, also resulted in a large amount of gas being generated and a low capacity retention rate.

[0073] Comparative Example 4, in which the LATP was crushed for 2 hours and did not have a peak derived from LATP in the X-ray crystal diffraction measurement but was not in a sol state, also produced a large amount of gas and had a low capacity retention rate.

[0074] From the above results, it was revealed that a lithium ion secondary battery using a positive electrode active material that operates at a high potential and that has a surface coated with a lithium ion conductive oxide that has been pulverized until it becomes an amorphous and transparent sol has low gas generation even when charged and discharged at a high potential, and also has good cycle characteristics. [Industrial Applicability]

[0075] The coated positive electrode active material of the present invention is suitably used as a positive electrode active material for lithium ion secondary batteries.

Claims

1. A coated positive electrode active material in which the positive electrode active material is coated with a lithium ion conductive oxide, The positive electrode active material is a lithium conductive active material having an operating voltage of 4.5 V or more based on the dissolution and deposition potential of lithium metal, The coated positive electrode active material is characterized in that the lithium ion conductive oxide is amorphous and is in the form of fine particles that have been pulverized to have a particle diameter of 3 nm or less as calculated by BET specific surface area.

2. The fine particles are represented by the following formula (A): Li １＋ｐ＋ｑ (A-,1) ｐ (Ti,Ge) ２－ｐ Si ｑ P ３－ｑ O １２ ・・・(A) (In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.) The coated positive electrode active material according to claim 1 , comprising a pulverized lithium ion conductive oxide represented by the formula:

3. The positive electrode active material is a compound represented by the following formula (1): Li １＋ｘ M ｙ Mn ２－ｘ－ｙ O ４ ・・・(1) (In formula (1), x and y satisfy 0≦x≦0.2 and 0<y≦0.8, respectively, and M is at least one element selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.) 3. The coated positive electrode active material according to claim 1, which is a substituted lithium manganese compound represented by the formula:

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