Positive Electrode Active Material for Lithium Ion Secondary Battery and Method for Manufacturing Lithium Ion Secondary Battery

Abstract

To provide a method for manufacturing a positive electrode active material, by which gas generation owing to oxidative decomposition of a nonaqueous electrolyte can be further suppressed in a lithium ion secondary battery arranged by use of a positive electrode active material that operates at a high electric potential.SOLUTION: A positive electrode active material for a lithium ion battery is arranged to use a positive electrode active material operating at a high electric potential equal to or higher than a lithium metal standard electrode electric potential of 4.5 V. The positive electrode active material is coated with an oxide-based solid electrolyte represented by Li1+p+q(Al,Ga)p(Ti,Ge)2-pSiqP3-qO12 (0≤p≤1 and 0≤q≤1). With the oxide-based solid electrolyte, a peak intensity proportion in a range of 0 to -20 ppm in P peak in measurement by solid NMR is no less than 50% of a total peak area.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery.

Background Art

[0002] Research and development of lithium ion secondary batteries are actively carried out in a wide range of applications such as portable devices, hybrid vehicles, electric vehicles, and home power storage. Lithium ion secondary batteries used in these fields are required to have high safety, long-term cycle stability, high energy density, and the like.

[0003] In recent years, from the viewpoints of high safety and long-term cycle stability, lithium ion secondary batteries using lithium titanate (LTO) as a negative electrode active material have been proposed. Since the operating potential of lithium titanate is higher than that of graphite, which is a general negative electrode active material for lithium ion batteries, precipitation of lithium is less likely to occur and safety is improved, but it is disadvantageous from the viewpoint of energy density. On the other hand, regarding positive electrode active materials, materials that operate at a high potential of 4.5 V or more with respect to the precipitation potential of Li have been proposed (for example, Patent Document 1).

[0004] A decrease in energy density due to the high operating potential of lithium titanate is expected to be improved by combining a positive electrode active material that operates at a high potential as shown in Patent Document 1. On the other hand, in a conventional lithium ion secondary battery using graphite as a negative electrode active material, gas is generated by oxidative decomposition of the non-aqueous electrolyte on the surface of the positive electrode active material. However, in the case of a secondary battery having a higher operating potential of the positive electrode active material than that of a conventional secondary battery, the above-mentioned problem of gas generation becomes more prominent.

[0005] Therefore, in a conventional lithium ion secondary battery, means for suppressing gas generation by forming a film on the positive electrode surface by adding, for example, an additive to the non-aqueous electrolyte has been adopted. Although the same principle can be applied to a high-potential positive electrode active material, since the film requires higher oxidation resistance, the effect is considered to be insufficient.

[0006] On the other hand, in a lithium-ion secondary battery using a positive electrode active material that operates at a high potential, a method for producing a positive electrode active material that suppresses the generation of gas due to oxidative decomposition of a non-aqueous electrolyte is also disclosed (Patent Document 2).

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in any of the conventional technologies, there is still room for improvement in suppressing gas generation when using a high-potential positive electrode active material. Therefore, an object of the present invention is to provide a positive electrode active material with further suppressed gas generation.

Means for Solving the Problems

[0009] [1] In view of the above circumstances, as a result of further studies on the means for suppressing the gas generation described above, the present inventors coated the surface of a positive electrode active material that operates at a high potential with an oxide-based solid electrolyte under various conditions and examined it. As a result, the positive electrode active material has the following formula (1): Li 1+p+q (Al,Ga) p (Ti,Ge) 2-p Si q P 3-q O 12 (0 ≦ p ≦ 1, 0 ≦ q ≦ 1) ··(1) It is coated with an oxide-based solid electrolyte represented by , and when the solid electrolyte is measured by solid NMR, when the integrated intensity ratio of the peak whose peak top exists at 0 to -20 ppm in the peak corresponding to P (phosphorus) is 50% or more with respect to the total peak area, it was first found that the above problems can be solved, and it was confirmed that gas generation can be remarkably suppressed at that time, and the present invention has been completed.

[0010] [2] In the present invention, it is preferable that the positive electrode active material is a positive electrode active material in which the average particle diameter of the solid electrolyte is refined to 10 nm or less. [3] In the present invention, it is preferable that the positive electrode active material is the positive electrode active material according to any one of [1] to [3], which is a substituted lithium manganese compound represented by the following formula (2). Li 1+x M y Mn 2-x-y O4 ··· (2) In the formula (2), x and y each 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. [4] As another invention of the present application, a method for manufacturing a non-aqueous lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein a positive electrode mixture containing the coated positive electrode active material according to any one of [1] to [4] above is applied to a positive electrode current collector. A method for manufacturing a lithium ion secondary battery, comprising the step of

Effects of the Invention

[0011] According to the present invention, when a positive electrode active material operating at a high potential is used in a lithium ion secondary battery, it is possible to suppress the generation of gas due to oxidative decomposition of the non-aqueous electrolyte more than any conventional technology.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0013] One embodiment of the present invention will be described below, but the present invention is not limited thereto.

[0014] The positive electrode active material of the present invention is coated with an oxide-based solid electrolyte.

[0015] Generally, a non-aqueous electrolyte is used in a lithium-ion secondary battery. Although details will be described later, a liquid non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent is used. On the other hand, there is a solid electrolyte in a solid state that has both functions of a non-aqueous solvent and a lithium salt. Since the solid electrolyte has higher oxidation resistance than the liquid non-aqueous electrolyte, oxidative decomposition at a high potential can be suppressed. However, since the lithium ion conductivity of a solid is lower than that of a liquid, if all of the electrolyte is replaced with a solid electrolyte, the performance of the battery will significantly decrease.

[0016] Therefore, by coating only the surface of the positive electrode active material at a high potential with a solid electrolyte, generation of gas can be suppressed even if the non-aqueous electrolyte remains conventional. The coating method is not particularly limited, but means capable of uniform coating such as spray coating and mechanical coating are preferred. Spray coating is preferred because the solid electrolyte is dispersed in a solvent, and adhesiveness and spreadability are imparted by the solvent, making it possible to coat uniformly. Further, when the solvent is volatilized by heat treatment, the heat treatment temperature is adjusted, preferably the particle size of the solid electrolyte and the mixing ratio with the positive electrode active material are controlled, so that the positive electrode active material can be coated with the solid electrolyte without increasing the resistance of the positive electrode active material and without degrading the battery performance.

[0017] Since the solid electrolyte is in a solid state, a certain amount of significant energy is required to coat the solid cathode active material. Therefore, a mechanical coating method that can apply shear force and compressive force is preferred. By coating the cathode active material with the solid electrolyte, contact between the non-aqueous electrolyte and the cathode active material, which was a conventional problem, can be reduced, and gas generation can be suppressed. Further, although details will be described later, similar to spray coating, by adjusting the heat treatment temperature, preferably controlling the particle size of the solid electrolyte and the mixing ratio with the cathode active material, the solid electrolyte can be coated on the cathode active material without increasing the resistance of the cathode active material and without degrading the battery performance.

[0018] Regardless of the coating method, since the solid electrolyte coats the entire surface of the cathode material, the area in contact with the electrolytic solution becomes smaller, suppressing gas generation. Further, when the electrolytic solution or additive partially decomposes, the decomposition products fill the gaps in the coating by the solid electrolyte to form a good film, making it possible to further suppress the decomposition of the electrolytic solution.

[0019] The evaluation of the solid electrolyte in the cathode active material after coating is performed by solid NMR measurement, particularly 31 evaluated by the spectrum of P-NMR. In the formula of Li 1+p+q (Al,Ga) p (Ti,Ge) 2-p Si q P 3-q O 12 (0 ≦ p ≦ 1, 0 ≦ q ≦ 1) ··· Equation (1), phosphorus usually takes only a single-bonded state, and only a peak with a peak top at a chemical shift of -20 to -30 ppm is observed. However, due to the change in the bonding state caused by micronization, particularly the atoms bonded to the oxygen bonded to phosphorus change, and a peak with a peak top at 0 to -20 ppm is mixed or only that peak is observed. When the ratio of the integrated intensity of the peak at 0 to -20 ppm in the total peak intensity of the phosphorus spectrum is 50% or more and 100% or less, more preferably 70% or more and 100% or less, gas generation can be effectively suppressed.

[0020] <Spray Coating Method> Spray coating means that the base material is wetted by the mist of the spray liquid sprayed from the spray nozzle, and at the same time, the solid components contained in the spray liquid adhere to the surface of the base material, dry and solidify, and a coating layer is formed on the surface of the base material. In the present invention, the positive electrode active material corresponds to the base material, and the coating agent corresponds to the solid electrolyte. The device to be used is not particularly limited, but for example, a fluidized bed coating device, a centrifugal rolling coating device, or a rolling fluidized bed coating device can be preferably used.

[0021] In the treatment by the spray coating method, the solvent to be used is not particularly limited, and water or an organic solvent can be used. As the organic solvent, for example, alcohols such as ethanol can be used. By mixing a polymer material such as polyethylene glycol or polyvinyl alcohol with the solvent, aggregation of the solid electrolyte fine particles can be suppressed, and it becomes possible to uniformly coat the surface of the positive electrode material. The concentration of the solid electrolyte in the slurry when the solid electrolyte is dispersed in the solvent and spray-coated in a slurry state is, for example, 10 to 25% by mass.

[0022] The treatment temperature of the spray coating is preferably 5 to 100 °C, more preferably 8 to 80 °C, and even more preferably 10 to 50 °C. The treatment time depends on the size of the device, but is preferably 5 to 90 minutes, more preferably 10 to 60 minutes. The treatment atmosphere is not particularly limited, and an inert gas atmosphere or an air atmosphere may be used.

[0023] <Mechanical Coating Method> Mechanical coating refers to a means of mixing a base material and a coating agent and coating the surface of the base material by mechanically contacting the base material and the coating agent while applying at least one type of energy, such as shear force, compressive force, impact force, and centrifugal force, to the base material and / or the coating agent (it is preferable to be able to apply shear force and compressive force, and more preferably to be able to apply shear force, compressive force, and impact force). In the present invention, the positive electrode active material corresponds to the base material, and the coating agent corresponds to the oxide-based solid electrolyte. The apparatus to be used is not particularly limited, but for example, a grinding mill typified by Nobilta manufactured by Hosokawa Micron Corporation or a planetary ball mill (for example, manufactured by Fritsch) can be preferably used. Among these, from the viewpoints that the operation is simple, there is no need to separate balls after treatment like a ball mill, coating proceeds preferentially over particle aggregation, and it is easy to obtain surface smoothness, a grinding mill is preferable.

[0024] In the production method of the present invention, it is preferable to provide a bottomed cylindrical container and a rotor having tip vanes, provide a predetermined clearance between the tip vanes 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 oxide-based solid electrolyte to perform mechanical coating.

[0025] The treatment by the mechanical coating method may be dry or wet. In the wet case, the solvent to be used is not particularly limited, and water or an organic solvent can be used. As the organic solvent, for example, an alcohol such as ethanol can be used. The addition timing of the solvent in the wet case is not particularly limited, but the oxide-based solid electrolyte may be dispersed in the solvent and used in the mechanical coating method in a slurry state. The concentration of the oxide-based solid electrolyte in the slurry is, for example, 10 to 25% by mass.

[0026] The processing temperature of the mechanical coating is preferably 5 to 100 °C, more preferably 8 to 80 °C, still more preferably 10 to 50 °C, and the processing time is preferably 5 to 90 minutes, more preferably 10 to 60 minutes. The processing atmosphere is not particularly limited, and an inert gas atmosphere or an air atmosphere may be used.

[0027] Although it is possible to use the sample as it is after spray coating or mechanical coating, it is preferable to perform heat treatment. This improves the adhesion between the positive electrode active material and the oxide-based solid electrolyte, suppresses the peeling of the oxide-based solid electrolyte from the positive electrode active material even after repeated charge and discharge, and improves the long-term reliability of the battery. If the heat treatment temperature is too high, the crystal structure of the oxide-based solid electrolyte may change, and the Li ion conductivity may decrease, resulting in abnormal charge and discharge of the battery. Therefore, the heat treatment temperature is preferably 600 °C or lower, more preferably 500 °C or lower. The heat treatment time is preferably 30 minutes or longer, more preferably 1 hour or longer, and the upper limit is not particularly limited, but for example, it is 3 hours or shorter.

[0028] <Positive electrode active material> The positive electrode active material used in the production method of the present invention has an average potential of lithium desorption and insertion of Li + / Li, that is, with respect to the deposition potential of Li (vs. Li + / Li, which may be expressed as such) is 4.5 V or more and 5.0 V or less. The potential of the lithium ion insertion / desorption reaction (hereinafter also referred to as voltage) (vs. Li + / Li) can be obtained, for example, by measuring the charge and discharge characteristics of a working electrode using the positive electrode active material and a lithium metal counter electrode half-cell and reading the voltage values at the start and end of the plateau. When there are two or more plateaus, it is sufficient that the plateau with the lowest voltage value is 4.5 V (vs. Li + / Li) or more, and the plateau with the highest voltage value is 5.0 V (vs. Li + / Li) or less.

[0029] 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 (2) has been conventionally studied and is preferable.

[0030] Li 1+x M y Mn 2-x-y O4···(2) In the above formula (2), 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.

[0031] Among the above formula (2), 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 on the charge / discharge cycle.

[0032] The particle size of the cathode active material is not particularly limited. However, if the particle size is too small, the difference from the particle size of the oxide-based solid electrolyte described later becomes small, making it difficult to coat. Therefore, the median diameter d 50 is preferably 5 μm or more, more preferably 10 μm or more. Also, the median diameter d 50 is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 30 μm or less. Considering the thickness range during processing into an electrode, the above d 50 is preferably 10 to 50 μm, more preferably 10 to 30 μm.

[0033] <Solid electrolyte> The solid electrolyte used in the present invention contains aluminum as an element, and it is preferable to use an oxide-based solid electrolyte in consideration of chemical stability. Oxide-based solid electrolytes include an inverse fluorite type, NASICON type, perovskite type, garnet type, etc. depending on the crystal structure, but are not particularly limited. As the oxide-based solid electrolyte, for example, the solid electrolyte Li 1+p+q (Al,Ga) p (Ti,Ge)2-p Si q P 3-q O 12 (0 ≤ p ≤ 1, 0 ≤ q ≤ 1) ··· It is possible to use LATP represented by formula (1), especially Li 1+p Al p Ti 2-p P3O 12 (0 ≤ p ≤ 1) is preferable.

[0034] Regarding the particle size of the above solid electrolyte, in order to uniformly coat the surface of the above positive electrode active material, the particle size in terms of BET specific surface area (d BET ) is made into fine particles up to 10 nm or less. As a method for the fine particle treatment, known means such as a ball mill and a bead mill can be used. The particle size in terms of BET specific surface area (d BET ) is obtained by the single-point method of nitrogen adsorption according to the method specified in JIS-Z8830 (2013) to obtain the nitrogen adsorption BET specific surface area, and d BET = 6 / (density × BET specific surface area) is the particle size obtained by this formula.

[0035] The ratio of the median diameter d 50 of the above positive electrode active material to the particle size in terms of BET specific surface area d BET of the above solid electrolyte is preferably 10000:1 to 1000:1, more preferably 8000:1 to 1500:1, and particularly preferably 5000:1 to 2000:1. The difference between the median diameter d 50 of the positive electrode active material and the particle size in terms of BET specific surface area d BET of the above solid electrolyte is preferably as large as possible. When the difference is small, aggregation of the solid electrolytes and formation of aggregates of the positive electrode active material and the solid electrolyte become dominant rather than the coating of the solid electrolyte on the positive electrode active material, and the intended effect is not exhibited.

[0036] Further, the ratio of the solid electrolyte (solid content when used in slurry) to 100 parts by mass of the positive electrode active material is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, still more preferably 2 parts by mass or more, and preferably 10 parts by mass or less, more preferably 5 parts by mass or less, still 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 (that is, the mass ratio of the positive electrode active material to the solid electrolyte is 100:1 to 20:1), and preferably 2 parts by mass or more and 4 parts by mass or less (that is, the mass ratio of the positive electrode active material to the solid electrolyte is 50:1 to 25:1).

[0037] <Lithium Ion Secondary Battery> A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode is usually produced by applying a positive electrode mixture containing a positive electrode active material, a conductive assistant, a binder, etc. to a positive electrode current collector. The negative electrode is usually produced by applying a negative electrode mixture containing a negative electrode active material, a conductive assistant, a binder, etc. to a negative electrode current collector. The coated positive electrode active material obtained by the production method of the present invention can be suitably used as the positive electrode active material of a lithium ion secondary battery. Specifically, a positive electrode can be produced by applying a positive electrode mixture containing the coated positive electrode active material obtained by the production method of the present invention to a positive electrode current collector. After applying the positive electrode mixture to the positive electrode current collector and after applying the negative electrode mixture to the negative electrode current collector, it may be dried at about 100 to 200°C.

[0038] For 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, the manufacturing apparatus and conditions of the lithium ion secondary battery, those conventionally known can be applied and are not particularly limited.

[0039] <Negative Electrode Active Material> As the negative electrode active material, as described above, it is preferable to use lithium titanate from the viewpoint of being less likely to cause lithium precipitation and improving safety. Among lithium titanates, lithium titanate having a spinel structure is particularly preferable because the expansion and contraction of the active material in the reaction of insertion and desorption of lithium ions are small. Lithium titanate may contain a trace amount of an element other than lithium and titanium, such as Nb.

[0040] <Conductive aid> The conductive aid is not particularly limited, but a carbon material is preferable. Examples thereof include natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotube, acetylene black, ketjen black, and furnace black. These carbon materials may be used alone or in combination of two or more. The amount of the conductive aid contained in the positive electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less, based on 100 parts by weight of the positive electrode active material. If it is within the above range, the conductivity of the positive electrode is ensured. In addition, the adhesiveness with the binder described later is maintained, and the adhesiveness with the current collector can be sufficiently obtained. The amount of the conductive aid contained in the negative electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less, based on 100 parts by weight of the negative electrode active material.

[0041] <Binder> The binder is not particularly limited, and for either the positive electrode or the negative electrode, for example, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and their derivatives can be used. The binder is preferably dissolved or dispersed in a non-aqueous solvent or water for ease of preparation of the positive and negative electrodes. The non-aqueous solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. A dispersant and a thickener may be added thereto. The amount of the binder contained in the positive electrode of the present invention is preferably 1 part by weight or more and 10 parts by weight or less, more preferably 2 parts by weight or more and 5 parts by weight or less, based on 100 parts by weight of the positive electrode active material. Within the above range, the adhesiveness between the positive electrode active material and the conductive auxiliary material can be maintained, and sufficient adhesiveness with the current collector can be obtained. The amount of the binder contained in the negative electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less, based on 100 parts by weight of the negative electrode active material.

[0042] <Current collector> Both the positive electrode current collector and the negative electrode current collector are preferably aluminum or an aluminum alloy. Aluminum or an aluminum alloy is not particularly limited because it is stable in the positive and negative electrode reaction atmospheres, but is preferably high-purity aluminum represented by JIS standards 1030, 1050, 1085, 1N90, 1N99, etc. The thickness of the current collector is not particularly limited, but is preferably 10 μm or more and 100 μm or less. Within this range, it is easy to achieve a balance in terms of handleability during battery production, cost, and the battery characteristics obtained. In addition, as the current collector, those obtained by coating the surface of a metal other than aluminum (copper, SUS, nickel, titanium, and their alloys) with a metal that does not react at the potentials of the positive and negative electrodes can also be used.

[0043] <Non-aqueous electrolyte> The non-aqueous electrolyte is not particularly limited, and examples thereof include a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, a gel electrolyte in which a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent is impregnated in a polymer, and the like.

[0044] The non-aqueous solvent preferably contains a cyclic aprotic solvent and / or a chain aprotic solvent. Examples of the cyclic aprotic solvent include cyclic carbonates, cyclic esters, cyclic sulfones, and cyclic ethers. As the chain aprotic solvent, solvents generally used as solvents for non-aqueous electrolytes such as chain carbonates, chain carboxylic acid esters, chain ethers, and acetonitrile may be used. More specifically, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, sulfolane, dioxolane, methyl propionate, and the like can be used. These solvents may be used alone or in combination of two or more. However, from the viewpoint of ease of dissolving the solute described below and high lithium ion conductivity, it is preferable to use a mixed solvent of two or more kinds.

[0045] When mixing two or more kinds, due to high stability at high temperatures and high lithium conductivity at low temperatures, one or more of the chain carbonates exemplified by dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, and methyl propyl carbonate, and one or more of the cyclic compounds exemplified by ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone are preferably mixed. Particularly preferred is a mixture of one or more of the chain carbonates exemplified by dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, and one or more of the cyclic carbonates exemplified by ethylene carbonate, propylene carbonate, and butylene carbonate.

[0046] The solute is not particularly limited, but for example, LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiBOB (Lithium Bis (Oxalato) Borate), LiN(SO2CF3)2, etc. are preferred because they are easily soluble in the solvent. The concentration of the solute contained in the non-aqueous electrolyte is preferably 0.5 mol / L or more and 2.0 mol / L or less. If it is less than 0.5 mol / L, the desired lithium ion conductivity may not be exhibited. On the other hand, if it is higher than 2.0 mol / L, the solute may not dissolve any more.

[0047] The amount of the non-aqueous electrolyte used in the lithium ion secondary battery of the present invention is not particularly limited, but it is preferably 0.1 mL or more and 10 mL or less per 1 Ah of battery capacity. With this amount, the conduction of lithium ions accompanying the electrode reaction can be ensured, and the desired battery performance can be exhibited.

[0048] The non-aqueous electrolyte may be included in the positive electrode, negative electrode and separator in advance, or may be added after winding or laminating a structure in which a separator is disposed between the positive electrode side and the negative electrode side.

[0049] In addition to the above-described configuration, the lithium ion secondary battery usually further includes a separator and an exterior material.

[0050] (Separator) The separator is installed between the positive electrode and the negative electrode, and any structure may be used as long as it is insulating and can contain the non-aqueous electrolyte described below. For example, nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and woven fabrics, non-woven fabrics, microporous membranes, etc. made by combining two or more of them can be mentioned. Because of the excellent stability of the cycle characteristics, it is preferably a non-woven fabric of nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and those obtained by combining two or more of them.

[0051] The separator may contain various plasticizers, antioxidants, and flame retardants, or may be coated with metal oxides or the like. The thickness of the separator is not particularly limited, but is preferably 10 μm or more and 100 μm or less. Within this range, it is possible to prevent the positive and negative electrodes from short-circuiting while suppressing an increase in the resistance of the battery. From the viewpoints of economy and handling, it is more preferably 15 μm or more and 50 μm or less.

[0052] The porosity of the separator is preferably 30% or more and 90% or less. If it is less than 30%, the diffusivity of lithium ions decreases, resulting in a significant deterioration in cycle characteristics. On the other hand, if it is higher than 90%, there is a very high risk that the unevenness of the electrodes will penetrate the separator and cause a short circuit. From the viewpoint of ensuring the diffusivity 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 because the above balance is particularly excellent.

[0053] (Exterior material) The exterior material is a member that encloses a laminate formed by alternately laminating or winding a positive electrode, a negative electrode, and a separator, and terminals for electrically connecting the laminate. As the exterior material, a composite film in which a thermoplastic resin layer for heat sealing is provided on a metal foil, or one having a metal layer formed by vapor deposition or sputtering is used. Also, rectangular, elliptical, cylindrical, coin-shaped, button-shaped, or sheet-shaped metal cans are preferably used.

[0054] Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and it is of course possible to appropriately modify and implement it within a range that conforms to the above and the following gists, and all of them are included in the technical scope of the present invention.

[0055] The batteries obtained in the following examples and comparative examples were evaluated by the following methods.

[0056] (Gas generation amount) The gas generation amount of the lithium-ion secondary battery before and after the cycle characteristic evaluation in the examples and comparative examples was evaluated using the Archimedes' method, that is, by using the buoyancy of the lithium-ion secondary battery. The evaluation was carried out as follows.

[0057] First, the weight of the lithium-ion secondary battery was measured with an electronic balance. Next, the weight in water was measured using a hydrometer (manufactured by Alpha Mirage Co., Ltd., product number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. The volume of the lithium-ion secondary battery was calculated by dividing this buoyancy by the density of water (1.0 g / cm 3 ). By comparing the volume after aging with the volume after the cycle characteristic evaluation, the amount of gas generated was calculated. Those with a gas generation amount of less than 10 ml were judged to be good.

[0058] (Cycle Characteristic Evaluation of Lithium-Ion Secondary Battery) The lithium-ion secondary battery prepared in the example or comparative example was connected to a charge-discharge device (HJ1005SD8, manufactured by Hokuto Denko Co., Ltd.) and subjected to cycle operation. In an environment of 60 °C, constant current charging was performed at a current value equivalent to 1.0C until the battery voltage reached the end voltage of 3.4V, and the charging was stopped. Subsequently, constant current discharging was performed at a current value equivalent to 1.0C, and the discharging was stopped when the battery voltage reached 2.5V. This was regarded as one cycle, and the charge-discharge was repeated. The stability of the cycle characteristics was evaluated as the discharge capacity maintenance rate (%) of the 500th discharge capacity when the first discharge capacity was set to 100. Those with a discharge capacity maintenance rate of 90% or more for the 500th time were considered good, and those less than 90% were considered bad.

[0059] Synthesis Example 1 (Preparation of Solid Electrolyte) As the solid electrolyte, Li 1.3 Al 0.3 Ti 1.7(PO4)3 was prepared. As starting materials, a predetermined amount of Li2CO3, AlPO4, TiO2, NH4H2PO4, and ethanol as a solvent were mixed, and planetary ball milling treatment was performed at 150 G for 1 hour using zirconia balls with a diameter of 3 mm. After removing the zirconia balls from the treated mixture with a sieve, it was dried at 120 °C to remove ethanol. Then, it was treated at 800 °C for 2 hours to obtain LATP powder.

[0060] A predetermined amount of ethanol as a solvent was mixed with the obtained LATP powder, and planetary ball milling treatment was performed for 1 to 3 hours using zirconia balls with a diameter of 0.5 mm. After removing the zirconia balls from the treated mixture with a sieve, it was dried at 120 °C to remove ethanol. As a result, LATP fine powder with d BET of 3 to 12 nm was obtained. Next, the above LATP fine powder and ethanol were mixed to obtain an ethanol dispersion slurry in which the LATP fine powder was 16.4% by weight.

[0061] Example 1 (i) Preparation of positive electrode As the active material of the positive electrode, spinel-type lithium nickel manganate (LiNi 0.5 Mn 1.5 O4, hereinafter also referred to as LNMO) with a median diameter of 12 μm was used.

[0062] 40 g of LNMO was put into a attritor mill (Nobilta, manufactured by Hosokawa Micron Corporation), and while rotating at a clearance of 0.6 mm, a rotor load power of 1.5 kW, and 2600 rpm, 6.1 g of an ethanol dispersion slurry of LATP fine powder pulverized for 1 hour in Synthesis Example 1 was added in two portions. The particle size of the pulverized LATP was 6 nm, 31 and the peak intensity ratio of 0 to -20 ppm in P-NMR measurement was 80%. NMR measurement was performed using VNMRS600 manufactured by VARIAN. The NMR measurement results are shown in Fig. 1. Then, while maintaining the above rotor rotation speed in the range of 2600 to 3000 rpm, it was treated at room temperature for 10 minutes in an air atmosphere to obtain LNMO coated with LATP on the surface. The obtained LNMO coated with LATP on the surface was heat-treated at 350 °C for 1 hour.

[0063] A mixture containing the obtained surface-coated LNMO, acetylene black as a conductive aid, and polyvinylidene fluoride (PVdF) as a binder, in amounts of 90 parts by weight, 6 parts by weight, and 4 parts by weight in terms of solid content concentration, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The above binder was adjusted to an N-methyl-2-pyrrolidone (NMP) solution with a solid content concentration of 5 wt%, and NMP was further added to adjust the viscosity for easier coating as described later.

[0064] After coating the above slurry on a 20-μm aluminum foil, it was dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, a positive electrode was fabricated by further vacuum drying at 170°C.

[0065] (ii) Fabrication of the negative electrode As the negative electrode active material, spinel-type lithium titanate (Li4Ti5O 12 , hereinafter also referred to as LTO) was used. A mixture containing the above LTO, acetylene black as a conductive auxiliary material, and PVdF as a binder, in amounts of 100 parts by weight, 5 parts by weight, and 5 parts by weight in terms of solid content concentration, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The above binder was adjusted to an NMP solution with a solid content concentration of 5 wt%, and NMP was further added to adjust the viscosity for easier coating as described later.

[0066] After coating the above slurry on a 20-μm aluminum foil, it was dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, a negative electrode was fabricated by further vacuum drying at 170°C.

[0067] (iii) Fabrication of the lithium-ion secondary battery Using the positive electrode and negative electrode prepared in (i) and (ii) above and a 20-μm polypropylene separator, a battery was fabricated according to the following procedure. First, the positive electrode and negative electrode 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. The outermost layers were both separators. Next, aluminum tabs were vibration-welded to the positive and negative electrodes at both ends.

[0068] Two aluminum laminate films to be used as the exterior material were prepared. After forming a depression for the battery part and a depression for the gas collection part by pressing, the above electrode laminate was placed inside. The outer periphery leaving a space for injecting the non-aqueous electrolyte was heat-sealed at 180 °C for 7 seconds. From the unsealed part, ethylene carbonate, propylene carbonate, and ethyl methyl carbonate were mixed in a solvent at a volume ratio of ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70, and a non-aqueous electrolyte in which LiPF6 was dissolved at a ratio of 1 mol / L was put in. Then, while under reduced pressure, the unsealed part was heat-sealed at 180 °C for 7 seconds. The obtained battery was subjected to constant-current charging at a current value equivalent to 0.2C until the battery voltage reached the end voltage of 3.4V, and the charging was stopped. Thereafter, it was left standing in an environment of 60 °C for 24 hours, and then constant-current discharging was performed at a current value equivalent to 0.2C. Discharging was stopped when the battery voltage reached 2.5V. After stopping the discharging, the gas accumulated in the gas collection part was extracted, and resealing was performed. By the above operations, a lithium-ion secondary battery for evaluation was fabricated.

[0069] Example 2 In the fabrication of the positive electrode, the pulverization time of LATP was 3 hours, and the particle size of LATP was 3 nm. 31 A lithium-ion secondary battery for evaluation was fabricated by performing the same operations as in Example 1 except that surface-coated LNMO was fabricated by joining those having an integration ratio of the peaks present at 0 to -20 ppm in 31P-NMR of 100%. The NMR measurement results are shown in Figure 2.

[0070] Comparative Example 1 In the fabrication of the positive electrode, the pulverization time was 30 minutes, and the particle size of LATP was 9 nm.31 Except for using those in which the integral ratio of the peaks present at 0 to -20 ppm in 31P-NMR was 47%, the same operations as in Example 1 were carried out to fabricate a lithium-ion secondary battery for evaluation.

[0071] Comparative Example 2 Except for using LNMO without surface coating, the same operations as in Example 1 were carried out to fabricate a lithium-ion secondary battery for evaluation.

[0072] The evaluation results of the examples and comparative examples are shown in Table 1.

[0073]

Table 1

[0074] For the lithium-ion secondary batteries of Examples 1 and 2, the amount of gas generated by the cycle performance evaluation was significantly less than that of the prior art, and the capacity retention rate was also high. It is considered that by making the solid electrolyte into fine particles compared with the prior art (for example, the above Patent Document 2), the bonding state of phosphorus changed, leading to an effect of suppressing gas generation and an improvement in cycle performance.

[0075] On the other hand, Comparative Example 1, in which the particle size was 12 nm, larger than that of Examples 1 and 2, 31 and the peak ratio present at -20 to 0 ppm in 31P-NMR was 47%, had a large amount of gas generation and a low capacity retention rate. This is presumably because the pulverization of the LATP particles was insufficient and the bonding state of phosphorus did not have a structure that suppresses gas generation.

[0076] From the above results, it was clarified that a lithium-ion secondary battery using a positive electrode active material coated with a solid electrolyte containing at least phosphorus on the surface of a positive electrode active material operating at a high potential has a small amount of gas generation even when charged and discharged at a high potential and also has good cycle characteristics.

Industrial Applicability

[0077] The coated positive electrode active material of the present invention is suitably used as a positive electrode active material of a lithium ion secondary battery.

Claims

1. A positive electrode active material for a lithium ion secondary battery using a positive electrode active material that operates at a high potential of 4.5 V or more of the standard electrode potential of lithium metal, wherein the positive electrode active material is coated with an oxide-based solid electrolyte represented by the following formula (1), Li １＋ｐ＋ｑ (Al, Ga) ｐ (Ti, Ge) ２－ｐ Si ｑ P ３－ｑ O １２ (0 ≦ p ≦ 1, 0 ≦ q ≦ 1) ··· (1) The oxide-based solid electrolyte has an integrated intensity ratio of peaks from 0 to -20 ppm at the P peak measured by solid NMR of 50% or more of the total peak area. A positive electrode active material for a lithium ion secondary battery characterized by this.

2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the oxide-based solid electrolyte has an average particle size of 10 nm or less.

3. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material is a substituted lithium manganese compound represented by the following formula (2). Li １＋ｘ M ｙ Mn ２－ｘ－ｙ O ４ (x, y each satisfy 0 ≦ x ≦ 0.2, 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) ··· (2)

4. A method for manufacturing a non-aqueous lithium ion secondary battery, comprising a step of applying a positive electrode mixture containing the positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3 to a positive electrode current collector. A method for manufacturing a lithium ion secondary battery characterized by this.

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