Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode is provided with a complex oxide containing lithium and a transition metal, and an additive that covers at least a portion of the surface of the complex oxide. The additive contains a metal oxide, and a phosphoric acid ester compound, the phosphoric acid ester compound having at least one alkenyl group within a single molecule.

Description

[Technical field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, have high energy density and high output, and are considered promising as power sources for mobile devices such as smartphones, power sources for vehicles such as electric vehicles, storage devices for natural energy such as solar power, etc. A composite oxide containing lithium and a transition metal is used as the positive electrode active material of non-aqueous electrolyte secondary batteries.

[0003] Patent Document 1 proposes forming a coating layer containing a metal oxide and a compound containing Li and P on the surface of a composite oxide containing lithium and a transition metal, which is a positive electrode active material for a non-aqueous electrolyte secondary battery. The metal oxide contains at least one metal element selected from the group consisting of Groups 3 and 13 of the periodic table and lanthanoids (hereinafter referred to as lanthanoids, etc.). Examples of compounds containing Li and P include Li3PO4, Li4P2O7, and Li3PO3 (hereinafter referred to as Li3PO4, etc.). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 047877 Brochure Summary of the Invention

[0005] The coating layer is formed by contacting the composite oxide with raw material solutions (a first aqueous solution containing lanthanoids, etc. and a second aqueous solution containing P) by a liquid phase method, followed by heat treatment, and the coating layer containing oxides of lanthanoids, etc. and Li3PO4, etc. is formed in an island shape. This is due to the influence of the density difference between the raw material and the product during the heat treatment, the sintering of the product, and the gas generation accompanying the decomposition reaction of the raw material in the raw material solution.

[0006] If the coating layer is formed in an island shape, the coating of the composite oxide becomes insufficient, and the non-aqueous electrolyte may come into contact with the composite oxide and decompose, resulting in a decrease in cycle characteristics.

[0007] In view of the above, one aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery including a composite oxide containing lithium and a transition metal, and an additive covering at least a part of a surface of the composite oxide, the additive including a metal oxide and a phosphate compound, and the phosphate compound having at least one alkenyl group in one molecule.

[0008] According to the present disclosure, the cycle characteristics of a non-aqueous electrolyte secondary battery can be improved. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view, with a portion cut away, of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a composite oxide (positive electrode active material) containing lithium and a transition metal, and an additive that covers at least a part of the surface of the composite oxide. The additive includes a metal oxide and a phosphate compound having at least one alkenyl group in one molecule (hereinafter also referred to as compound A).

[0011] When the additive that coats the surface of the composite oxide contains a metal oxide and compound A, the surface of the composite oxide is sufficiently and stably coated with the additive, which suppresses decomposition of the non-aqueous electrolyte due to contact with the composite oxide, improving cycle characteristics.

[0012] The surface of the composite oxide is covered with the metal oxide in an island shape, and has a region that is not covered with the metal oxide. The region that is not covered with the metal oxide is covered with compound A. By covering and filling the region that is not covered with the metal oxide with compound A, the coverage of the composite oxide surface by the additive is improved, and contact between the non-aqueous electrolyte and the composite oxide is sufficiently suppressed.

[0013] The composite oxide surface can be coated with a thin layer of metal oxide (e.g., 1 nm or more and 5 nm or less in thickness) and further coated with compound A to enhance the coverage of the composite oxide surface by the additive. This makes it possible to avoid the inconvenience of using a large amount of metal oxide to enhance coverage, which increases the thickness of the metal oxide coating layer and thus increases resistance. The surface of the composite oxide can be thinly and efficiently coated with a small amount of metal oxide and compound A. This makes it easy to obtain a battery with low internal resistance and excellent cycle characteristics.

[0014] Although the detailed reason why the use of compound A improves the surface coverage of the complex oxide is unclear, it is speculated that one of the factors improving the surface coverage is the interaction between the alkenyl group (carbon-carbon double bond) of compound A and the transition metal in the complex oxide.

[0015] (Compound A) Compound A is a phosphoric acid ester (organic phosphoric acid) having at least one alkenyl group in one molecule, and can be easily incorporated into the non-aqueous electrolyte used in the battery by dissolving Compound A. The non-aqueous electrolyte containing Compound A is prepared during battery production, and the surface of the composite oxide can be easily coated with Compound A using the non-aqueous electrolyte, which is advantageous in terms of productivity.

[0016] From the viewpoint of interaction with the transition metal in the composite oxide, the carbon-carbon double bond of the alkenyl group is preferably closer to the tip of the alkenyl group. From the viewpoints of easy dissolution of compound A in the non-aqueous electrolyte, easy attachment of compound A to the area of ​​the composite oxide surface not covered with metal oxide, and easy adjustment of the viscosity of the non-aqueous electrolyte containing compound A to a moderately low level, the number of carbon atoms of the alkenyl group is preferably, for example, 2 or more and 5 or less. From the same viewpoint, the alkenyl group is preferably linear. When compound A has multiple alkenyl groups, the multiple alkenyl groups may be the same or different from each other.

[0017] Specifically, the alkenyl group may include at least one selected from the group consisting of a vinyl group, a 1-propenyl group, a 2-propenyl group (allyl group), an isopropenyl group, a 1-butenyl group, a 2-butenyl group, and a 3-butenyl group. From the viewpoints of ease of dissolving compound A in the non-aqueous electrolyte, ease of attachment of compound A to the region of the composite oxide surface that is not covered with the metal oxide, ease of adjusting the viscosity of the non-aqueous electrolyte containing compound A to an appropriate low level, etc., among them, the alkenyl group is preferably an allyl group or a 3-butenyl group, and more preferably an allyl group.

[0018] Compound A has, for example, a structure represented by the following formula (I):

[0019] [ka]

[0020] In formula (I), R 1 , R 2 and R 3 At least one of R is an alkenyl group. 1 , R 2 and R 3are preferably all alkenyl groups. When compound A represented by formula (I) has a plurality of alkenyl groups, the plurality of alkenyl groups may be the same as or different from each other. A portion of the hydrogen atoms contained in the alkenyl group may be substituted with a halogen atom such as a chlorine atom. The number of carbon atoms in the alkenyl group is, for example, 2 or more and 5 or less. The alkenyl group may be linear or branched. The alkenyl group is preferably CH2=CH-(CH2) n n may be 0 or more and 3 or less, and n=1 is more preferable.

[0021] In formula (I), R 1 , R 2 and R 3 One or two of the may be a hydrocarbon group other than an alkenyl group. The hydrocarbon group other than an alkenyl group includes an alkyl group, etc. A part of the hydrogen atoms contained in the hydrocarbon group other than an alkenyl group (alkyl group, etc.) may be substituted with a halogen atom, such as a chlorine atom. When the compound A represented by formula (I) has two hydrocarbon groups other than an alkenyl group, the hydrocarbon groups other than an alkenyl group may be the same or different from each other. The number of carbon atoms of the alkyl group is, for example, 2 or more and 5 or less. The alkyl group may be linear or branched. The alkyl group includes a methyl group, an ethyl group, a propyl group, etc.

[0022] Compound A includes a phosphoric acid monoester, a phosphoric acid diester, and a phosphoric acid triester, and among them, a phosphoric acid triester is preferred. The phosphoric acid triester preferably includes triallyl phosphate. With a small amount of triallyl phosphate, the resistance of the positive electrode can be suppressed to a low level while the coverage of the composite oxide whose surface is covered with a metal oxide can be efficiently increased. In addition, triallyl phosphate is easily dissolved in a non-aqueous electrolyte, and a non-aqueous electrolyte with low viscosity can be easily prepared.

[0023] The content of compound A in the non-aqueous electrolyte may be 2% by mass or less, 0.25% by mass or more and 2% by mass or less, or 0.25% by mass or more and 1.25% by mass or less, based on the total amount of the non-aqueous electrolyte. For example, the content of compound A may be within the above range when the non-aqueous electrolyte is prepared (before being poured into the battery). In this case, the area of ​​the composite oxide surface that is not covered with the metal oxide can be sufficiently covered with compound A, and the cycle characteristics are easily improved.

[0024] When the content of compound A during preparation of the non-aqueous electrolyte is 2% by mass or less, the content of compound A in the non-aqueous electrolyte in the initial battery (for example, after injection of the non-aqueous electrolyte or after several charging and discharging) may be, for example, 1% by mass or less, 100 ppm or less, or a trace amount close to the detection limit. If the presence of compound A can be confirmed in the non-aqueous electrolyte in the battery, it is estimated that compound A derived from the non-aqueous electrolyte is attached to the composite oxide to a certain extent, and the corresponding improvement effect of the cycle characteristics is recognized. The content of compound A in the non-aqueous electrolyte is determined by gas chromatography mass spectrometry (GC / MS) or the like.

[0025] (Metal oxides) The metal oxide coating the surface of the composite oxide is different from the composite oxide used as a positive electrode active material, and does not play a role as a positive electrode active material, but has lithium ion conductivity. The metal oxide includes a metal Mc. The metal Mc may include at least one selected from the group consisting of aluminum, silicon, titanium, magnesium, zirconium, niobium, germanium, calcium, and strontium. More specifically, the metal oxide may include at least one selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, magnesium oxide, zirconium oxide, niobium oxide, germanium oxide, calcium oxide, and strontium oxide. The aluminum oxide includes alumina (Al2O3) and the like. The silicon oxide includes silica (SiO2) and the like. The titanium oxide includes TiO2 and the like. The magnesium oxide includes MgO and the like. The zirconium oxide includes ZrO2 and the like. The metal oxide may also include silica alumina (a composite oxide including aluminum and silicon).

[0026] Among these, from the viewpoints of cost advantage and excellent lithium ion conductivity, chemical stability, and thermal stability, it is preferable that the metal oxide includes at least one selected from the group consisting of aluminum oxide, silicon oxide, and silica alumina.

[0027] From the viewpoint of improving the coverage of the composite oxide surface by compound A, the amount of metal oxide coverage (thickness of the coverage layer) can be reduced. The thickness of the metal oxide coverage layer can be reduced, for example, in the range of 1 nm or more and 5 nm or less. When the thickness of the metal oxide coverage layer is 5 nm or less, the movement of lithium ions between the composite oxide and the non-aqueous electrolyte via the metal oxide coverage layer is easily performed smoothly, and high capacity and excellent cycle characteristics are easily obtained. From the viewpoint of the amount of composite oxide contained in the positive electrode (positive electrode capacity), the thickness of the metal oxide coverage layer may be 1 nm or more and 3 nm or less.

[0028] When the surface of a nickel-based composite oxide represented by general formula (2) described below is covered with Al2O3, the atomic ratio of Al derived from the metal oxide to Ni derived from the composite oxide at the outermost surface of the composite oxide, Al / Ni, is, for example, not more than 2. In this case, it is presumed that a thin layer of the metal oxide (e.g., having a thickness of 1 nm or more and 3 nm or less) is distributed in an island shape on the surface of the composite oxide.

[0029] The distribution state of metal Mc derived from the metal oxide and P derived from compound A can be confirmed by performing elemental analysis (elemental mapping) on ​​a cross-section of the positive electrode mixture layer or the composite oxide using an electron probe microanalyzer (EPMA) or an energy dispersive X-ray (EDX) analyzer.

[0030] (composite oxide) The positive electrode active material includes a composite oxide containing lithium and a metal other than lithium, Me. The metal Me includes at least a transition metal. The transition metal may include at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), copper (Cu), chromium (Cr), titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V), tantalum (Ta), and molybdenum (Mo).

[0031] The composite oxide is synthesized by using a coprecipitation method or the like, for example, by mixing a lithium compound with a compound containing a metal other than lithium, Me, obtained by the coprecipitation method or the like, and firing the resulting mixture under a predetermined condition. The composite oxide usually forms secondary particles in which a plurality of primary particles are aggregated. The average particle size (D50) of the composite oxide particles is, for example, 3 μm or more and 25 μm or less. The average particle size (D50) of the composite oxide particles means the particle size (volume average particle size) at which the volume integrated value is 50% in the volume-based particle size distribution measured by the laser diffraction scattering method.

[0032] The metal Me may include a metal other than a transition metal. The metal other than a transition metal may include at least one selected from the group consisting of aluminum (Al), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn) and silicon (Si). In addition, the composite oxide may further include boron (B) and the like in addition to the metal.

[0033] From the viewpoint of increasing capacity, the transition metal preferably contains at least Ni. The metal Me may contain Ni and at least one selected from the group consisting of Co, Mn, Al, Ti and Fe. From the viewpoint of increasing capacity and output, the metal Me preferably contains Ni and at least one selected from the group consisting of Co, Mn and Al, and more preferably contains Ni, Co, Mn and / or Al. When the metal Me contains Co, the phase transition of the composite oxide containing Li and Ni is suppressed during charging and discharging, the stability of the crystal structure is improved, and the cycle characteristics are easily improved. When the metal Me contains Mn and / or Al, the thermal stability is improved.

[0034] From the viewpoint of facilitating the attainment of high capacity, the atomic ratio of Ni to metal Me in the composite oxide: Ni / Me may be 0.3 or more and less than 1, preferably 0.5 or more and less than 1, and more preferably 0.75 or more and less than 1.

[0035] From the viewpoint of improving cycle characteristics and increasing output, the positive electrode active material may contain a composite oxide having a layered rock-salt crystal structure and containing Ni and / or Co, or may contain a composite oxide having a spinel crystal structure and containing Mn. Among them, from the viewpoint of increasing capacity, the positive electrode active material may be a composite oxide having a layered rock-salt crystal structure, containing Ni, and having an atomic ratio of Ni to metal Me: Ni / Me of 0.3 or more (hereinafter also referred to as nickel-based composite oxide).

[0036] Nickel-based composite oxides have a relatively unstable crystal structure and are prone to degradation due to elution of Ni and the like caused by contact with non-aqueous electrolytes at high-potential positive electrodes, and the cycle characteristics are likely to deteriorate. Therefore, in the case of nickel-based composite oxides, the effect of improving the cycle characteristics by coating the surface of the composite oxide with an additive containing compound A and a metal oxide is remarkably obtained. By the above coating, the high capacity of the nickel-based composite oxide can be fully utilized.

[0037] The composite oxide has a layered rock salt-type crystal structure and has a general formula (1): LiNi α M 1-α O2 (satisfying 0.3 ≦ α < 1, and M is at least one element selected from the group consisting of Co, Mn, Al, Ti, and Fe).) may have a composition represented by. When α is in the above range, the effect of Ni and the effect of element M can be obtained in a well-balanced manner. α may be 0.5 or more, and may be 0.75 or more.

[0038] From the viewpoints of improving cycle characteristics, increasing capacity, and increasing output power, the composite oxide has a layered rock salt-type crystal structure and has a general formula (2): LiNi x Co y M 1-x-y O2, and in the general formula (2), 0.3 ≦ x < 1, 0 < y ≦ 0.5, and 0 < 1 - x - y ≦ 0.35 are satisfied, and M is at least one selected from the group consisting of Al and Mn. The x value may be in the range of 0.5 ≦ x < 1. The y value may be in the range of 0 < y ≦ 0.35.

[0039] (Method for coating the surface of the composite oxide with an additive) The method for coating the surface of a composite oxide with an additive includes, for example, a first step of coating the surface of the composite oxide with a metal oxide, and a second step of coating the surface of the composite oxide coated with the metal oxide with a compound A. In the first step, the surface of the composite oxide is covered with the metal oxide in an island shape, and the surface of the composite oxide has a region that is not covered with the metal oxide. In the second step, the region of the surface of the composite oxide that is not covered with the metal oxide is covered with the compound A. A part of the metal oxide may be thinly covered with the compound A. Thus, the surface of the composite oxide is sufficiently covered with the additive containing the metal oxide and the compound A, and contact between the composite oxide and the non-aqueous electrolyte is sufficiently suppressed.

[0040] (1st step) In the first step, the surface of the composite oxide can be coated with a metal oxide by a liquid phase method or a gas phase method. Examples of the liquid phase method include a spray coating method and a dip coating method. Examples of the gas phase method include a chemical vapor deposition (CVD) method and an atomic layer deposition (ALD) method.

[0041] The first step includes, for example, a step (1A) of forming a positive electrode mixture layer containing a composite oxide on the surface of a positive electrode current collector, and a step (1B) of coating the surface of the positive electrode mixture layer with a metal oxide to obtain a positive electrode intermediate.

[0042] In step (1A), for example, a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium is applied to the surface of a positive electrode current collector, and then dried to form a positive electrode mixture layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains at least a composite oxide, and may further contain a binder, a conductive agent, etc. As the dispersion medium, N-methyl-2-pyrrolidone (NMP) or the like is used.

[0043] Examples of the binder include resin materials such as fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, etc. Examples of the fluororesins include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc. One type of binder may be used alone, or two or more types may be used in combination.

[0044] Examples of the conductive agent include carbon blacks such as acetylene black, conductive fibers such as carbon fibers and metal fibers, and carbon fluoride. The conductive agent may be used alone or in combination of two or more.

[0045] The positive electrode current collector may be, for example, a metal foil. Examples of metals constituting the positive electrode current collector include aluminum, titanium, alloys containing these metal elements, and stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 3 to 50 μm.

[0046] In step (1B), a thin layer of metal oxide is preferably formed on the surface of the positive electrode mixture layer by a gas phase method. The gas phase method is preferably an ALD method. By forming a thin layer of metal oxide, the composite oxide can smoothly absorb and release lithium ions. Depending on the temperature during film formation by the ALD method, the amount of metal oxide deposited, and the like, the metal oxide coating layer can be formed thin and in an island shape. Even if the metal oxide coating layer is formed in an island shape, the surface coverage of the composite oxide is enhanced by compound A in the second step.

[0047] The ALD method is a film formation method in which a source gas containing metal Mc (such as Al) and an oxidant are alternately supplied to a reaction chamber in which a target object is placed, to form a layer containing an oxide of metal Mc on the surface of the target object. In the ALD method, a self-limiting effect works, so that deposition occurs on the surface of the target object in atomic layer units. Therefore, the thickness of the metal oxide layer is controlled by the number of cycles, which is one cycle consisting of supply of source gas → exhaust (purging) of source gas → supply of oxidant → exhaust (purging) of oxidant. In other words, the ALD method can easily control the thickness of the metal oxide layer to be formed.

[0048] In addition, while CVD is generally performed at a temperature of 400 to 900°C, the ALD method can be performed at a temperature of 100 to 400°C. In other words, the ALD method is superior in that it can suppress thermal damage to the electrodes. Examples of oxidizing agents used in the ALD method include water, oxygen, and ozone. The oxidizing agent may be supplied to the reaction chamber as plasma using the oxidizing agent as a raw material.

[0049] Al and the like are supplied to the reaction chamber as a precursor gas containing Al and the like. The precursor is, for example, an organometallic compound containing Al and the like, which makes it easier for Al and the like to be chemically adsorbed to the target object. As the precursor, various organometallic compounds that have been used in the conventional ALD method can be used. For example, precursors containing Al include trimethylaluminum ((CH3)3Al) and triethylaluminum ((C2H5)3Al).

[0050] The first step may also include a step (1a) of coating the surface of the composite oxide with a metal oxide, and a step (1b) of forming a positive electrode mixture layer containing the composite oxide whose surface is coated with the metal oxide on the surface of a positive electrode current collector to obtain a positive electrode intermediate.

[0051] In step (1a), for example, a liquid phase method is used. Step (1a) includes, for example, step (1a-1) of attaching a raw material solution to the surface of the composite oxide, and step (1a-2) of heating and drying the composite oxide with the raw material solution attached to the surface. In step (1a-1), for example, the composite oxide is added to the raw material solution and dispersed by stirring. Step (1a-2) combines a step of removing the dispersion medium attached to the surface of the composite oxide by heating and a step of reacting the raw material attached to the surface of the composite oxide to generate a metal oxide. For example, an aqueous solution containing a raw material containing metal Mc is used as the raw material solution. For the raw material containing metal Mc, a compound that can generate a metal oxide by decomposition reaction by heating can be used, and examples of the raw material containing metal Mc include metal Mc salts of organic acids such as citric acid, maleic acid, and lactic acid, and organometallic complexes containing metal Mc. In step (1a-2), a coating layer of metal oxide can be formed thinly and in an island shape. Even if the metal oxide coating layer is formed in an island shape, the coverage of the surface of the composite oxide is enhanced by compound A in the second step.

[0052] In step (1b), for example, a positive electrode slurry in which a positive electrode mixture containing a composite oxide whose surface is coated with a metal oxide is dispersed in a dispersion medium is applied to the surface of a positive electrode current collector and dried. The positive electrode mixture may further contain a binder, a conductive agent, etc. The binder, the conductive agent, the dispersion medium, and the positive electrode current collector may be those exemplified in step (1A).

[0053] (2nd process) The second step preferably includes a step (2A) of preparing a non-aqueous electrolyte containing compound A, and a step (2B) of contacting the non-aqueous electrolyte containing compound A with a composite oxide whose surface is coated with a metal oxide. In the step (2B), the area of ​​the composite oxide surface that is not coated with the metal oxide is coated with compound A. Since compound A is an organic phosphoric acid, compound A can be easily dissolved in the non-aqueous electrolyte and incorporated therein. By using a non-aqueous electrolyte containing compound A in the manufacturing process of a battery, the surface of the composite oxide can be easily coated with compound A, which is advantageous in terms of improving productivity. In the step (2B), for example, an electrode group including a positive electrode intermediate obtained in the step (1B) or the step (1b), a negative electrode, and a separator disposed between the positive electrode intermediate and the negative electrode may be formed, and the electrode group may be made to contain a non-aqueous electrolyte. For example, the electrode group may be housed in a battery case, the non-aqueous electrolyte may be injected into the battery case containing the electrode group, and the opening of the battery case may be closed with a sealing plate.

[0054] In the case of the positive electrode intermediate obtained in step (1B), the surface of the positive electrode mixture layer coated with the metal oxide can be further coated with compound A in step (2B). In the case of the positive electrode intermediate obtained in step (1b), the surface of the composite oxide coated with the metal oxide can be further coated with compound A in step (2B). Since the coating with compound A is performed after the formation of the positive electrode mixture layer, contact points between the composite oxide particles are easily formed without the intervention of compound A, and a conductive network between the composite oxide particles is easily secured.

[0055] The configuration of the nonaqueous electrolyte secondary battery will be described in detail below.

[0056] (positive electrode) The positive electrode may include, for example, a positive electrode current collector, a positive electrode mixture layer supported on the surface of the positive electrode current collector, and a positive electrode mixture layer containing at least a complex oxide. The positive electrode mixture layer may further include the above-mentioned conductive agent and binder. The surface of the complex oxide contained in the positive electrode mixture layer may be coated with an additive containing a metal oxide and compound A. The surface of the positive electrode mixture layer containing a complex oxide may be coated with an additive containing a metal oxide and compound A.

[0057] (Negative electrode) The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium to the surface of the negative electrode current collector and drying it. The coating film after drying may be rolled as necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector or on both surfaces. For example, water or NMP is used as the dispersion medium.

[0058] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a conductive agent, a thickener, and the like as optional components. As the binder and conductive agent, those exemplified for the positive electrode can be used. In addition, a rubber material such as styrene-butadiene copolymer rubber (SBR) may be used as the binder. As the thickener, for example, carboxymethylcellulose (CMC) and its modified form (Na salt, etc.) can be mentioned.

[0059] The negative electrode active material may contain a carbon material that absorbs and releases lithium ions. Examples of the carbon material that absorbs and releases lithium ions include graphite (natural graphite, artificial graphite), graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among them, graphite is preferable because it has excellent charge / discharge stability and a small irreversible capacity.

[0060] The negative electrode active material may include an alloy-based material. The alloy-based material is a material containing at least one metal capable of forming an alloy with lithium, and examples thereof include silicon, tin, a silicon alloy, a tin alloy, and a silicon compound. As the silicon compound, a composite material having a lithium ion conductive phase and silicon particles dispersed in the phase may be used. As the lithium ion conductive phase, a silicate phase such as a lithium silicate phase, a silicon oxide phase having 95% by mass or more of silicon dioxide, a carbon phase, or the like may be used.

[0061] The negative electrode active material may be a combination of an alloy material and a carbon material. In this case, the proportion of the carbon material in the total of the alloy material and the carbon material is, for example, preferably 80 mass % or more, and more preferably 90 mass % or more.

[0062] The shape and thickness of the negative electrode current collector can be selected from the shape and range corresponding to those of the positive electrode current collector. Examples of metals constituting the negative electrode current collector include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements.

[0063] (Non-aqueous electrolyte) The non-aqueous electrolyte includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte may include a compound A. The compound A contained in the non-aqueous electrolyte may be attached to the surface of a composite oxide, and the surface of the composite oxide may be coated with the compound A. The concentration of the lithium salt in the non-aqueous electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ion conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0064] As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylate, a chain carboxylate, etc. are used. Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), etc. The cyclic carbonate may include a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), or a cyclic carbonate having an unsaturated carbon-carbon bond such as vinylene carbonate (VC), vinylethylene carbonate, etc. Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. Examples of the cyclic carboxylate include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of the chain carboxylate include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, etc. The non-aqueous solvent may be used alone or in combination of two or more kinds.

[0065] As the lithium salt, known lithium salts can be used. Preferred lithium salts include, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10, lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bistrifluoromethanesulfonate imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF3SO2)(C4F9SO2)), lithium bispentafluoroethanesulfonate imide (LiN(C2F5SO2)2), etc. The lithium salts may be used alone or in combination of two or more.

[0066] (Separator) Generally, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator may contain a non-aqueous electrolyte containing compound A. The separator has high ion permeability and has appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used. As the material of the separator, polyolefin such as polypropylene and polyethylene is preferable.

[0067] An example of the structure of the non-aqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a non-aqueous electrolyte are housed in an exterior body. Alternatively, instead of a wound type electrode group, an electrode group of another form, such as a stacked type electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be applied. The non-aqueous electrolyte secondary battery may be in any form, such as a cylindrical type, a square type, a coin type, a button type, a laminate type, or the like.

[0068] FIG. 1 is a schematic perspective view, with a portion cut away, of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure.

[0069] The battery includes a bottomed prismatic battery case 4, and an electrode group 1 and a non-aqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0070] One end of the negative electrode lead 3 is attached to the negative electrode collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of the positive electrode lead 2 is attached to the positive electrode collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4 which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 and the sealing plate 5, and separates the negative electrode lead 3 and the battery case 4. The periphery of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitting portion is laser welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed with a sealing plug 8 .

[0071] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0072] Examples 1 to 4 [Preparation of positive electrode intermediate] N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture and stirred to prepare a positive electrode slurry. The positive electrode mixture was a mixture of the positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF). The positive electrode active material was a layered rock salt type LiNi 0.35 Co 0.35 Mn 0.30The composite oxide particles (average particle size (D50) 4 μm) having the composition of (NCM) were used. In the positive electrode mixture, the mass ratio of the positive electrode active material, AB, and PVDF was 100:2:2.

[0073] A positive electrode slurry was applied to the surface of the aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer. The positive electrode mixture layer was formed on both sides of the aluminum foil. Furthermore, the surface of the positive electrode mixture layer was covered with Al2O3 by the ALD method (temperature: 120°C, precursor: trimethylaluminum, oxidizing agent: H2O, pressure: several Torr, 10 cycles). In this way, a positive electrode intermediate was obtained. The atomic ratio Al / Ni at the outermost surface of the positive electrode intermediate, determined by the above-mentioned method, was 2 or less.

[0074] [Preparation of negative electrode] Water was added to the negative electrode mixture and stirred to prepare a negative electrode slurry. A mixture of artificial graphite (average particle size 20 μm), styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) was used for the negative electrode mixture. In the negative electrode mixture, the mass ratio of the artificial graphite, SBR, and CMC-Na was 100:1:1. The negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then rolled to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the copper foil.

[0075] [Preparation of non-aqueous electrolyte] LiPF6 was dissolved in a mixed solvent (volume ratio 2:8) of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC), and triallyl phosphate (TP) was further added to obtain a non-aqueous electrolyte. The concentration of LiPF6 in the non-aqueous electrolyte was 1 mol / L. The content of TP in the non-aqueous electrolyte (mass ratio to the entire non-aqueous electrolyte) was the value shown in Table 1.

[0076] [Preparation of non-aqueous electrolyte secondary battery] An Al positive electrode lead was attached to the positive electrode intermediate obtained above. A Ni negative electrode lead was attached to the negative electrode obtained above. In an inert gas atmosphere, the positive electrode intermediate and the negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween to prepare a wound-type electrode group. The electrode group was housed in a bag-shaped exterior body formed of a laminate sheet having an Al layer, and the nonaqueous electrolyte was injected, and then the exterior body was sealed to prepare a nonaqueous electrolyte secondary battery. When the electrode group was housed in the exterior body, a part of the positive electrode lead and the negative electrode lead were exposed to the outside from the exterior body. In addition, the positive electrode intermediate was brought into contact with a nonaqueous electrolyte (TP) in the battery, and the surface of the positive electrode mixture layer was further covered with TP to obtain a positive electrode. In Table 1, the batteries of Examples 1 to 4 are A1 to A4, respectively.

[0077] Comparative Example 1 In the preparation of the positive electrode intermediate, both sides of the positive electrode mixture layer were not covered with Al2O3. In the preparation of the non-aqueous electrolyte, TP was not contained in the non-aqueous electrolyte. Other than the above, a battery B1 was prepared in the same manner as the battery A1 of Example 1.

[0078] Comparative Example 2 Battery B2 was produced in the same manner as Battery A1 in Example 1, except that in the preparation of the nonaqueous electrolyte, TP was not contained in the nonaqueous electrolyte.

[0079] Comparative Example 3 Battery B3 was produced in the same manner as Battery A2 in Example 2, except that in the preparation of the positive electrode intermediate, both sides of the positive electrode mixture layer were not covered with Al2O3.

[0080] The batteries A1 to A4 of Examples 1 to 4 and the batteries B1 to B3 of Comparative Examples 1 to 3 were evaluated as follows.

[0081] [Evaluation 1: Capacity retention at 151 cycles] (1) First charge / discharge The battery was charged at a constant current of 0.2 C until the voltage reached 4.5 V, and then charged at a constant voltage of 4.5 V until the current reached 0.05 C. It was then discharged at a constant current of 0.2 C until the voltage reached 2.5 V. The rest time between charging and discharging was 60 minutes. The charging and discharging were performed in an environment of 25°C.

[0082] (2)Second charge / discharge The battery was charged at a constant current of 0.3 C until the voltage reached 4.5 V, and then discharged at a constant current of 0.5 C until the voltage reached 2.5 V. The rest time between charging and discharging was 10 minutes. The charging and discharging were performed in an environment of 25°C.

[0083] (3) Measurement of capacity retention at 151 cycles Six sets of a process in which the first charge / discharge described in (1) above was performed for one cycle and then the second charge / discharge described in (2) above was performed for 24 cycles were performed as one set. That is, the first charge / discharge described in (1) above was performed at the 1st, 26th, 51st, 76th, 101st, 126th, and 151st cycles. At the other cycles, the second charge / discharge described in (2) above was performed. The ratio of the discharge capacity of the first charge / discharge in the 151st cycle to the discharge capacity of the first charge / discharge in the 1st cycle was calculated as the capacity retention rate at the 151st cycle.

[0084] [Evaluation 2: Internal resistance at 101st cycle] After the first charge / discharge in the 101st cycle, battery A3 and batteries B1 to B3 were charged to 50% of the full charge amount. Then, constant current discharge was performed at a current I of 0.3 C for 30 seconds, and the voltage drop ΔV from the start of discharge to 30 seconds after the start of discharge was measured, and ΔV / I was calculated as the internal resistance.

[0085] The evaluation results are shown in Table 1.

[0086] [Table 1]

[0087] The batteries A1 to A4 had higher capacity retention rates than the batteries B1 to B3. In the battery B1, the surface of the positive electrode mixture layer was not coated with either Al2O3 or TP, so the non-aqueous electrolyte came into contact with the complex oxide, resulting in a decrease in the capacity retention rate. In the battery B2, the surface of the positive electrode mixture layer was coated with Al2O3 but not with TP, so the coating of the positive electrode mixture layer was insufficient, so the non-aqueous electrolyte came into contact with the complex oxide, resulting in a decrease in the capacity retention rate. In the battery B3, the surface of the positive electrode mixture layer was coated with TP but not with Al2O3, so the internal resistance (resistance of the positive electrode) increased and the capacity retention rate decreased. In the battery A3, the TP content was higher than in the battery B3, but the surface of the positive electrode mixture layer was coated with Al2O3, so the internal resistance (resistance of the positive electrode) was lower than that of the battery B3. [Industrial Applicability]

[0088] The nonaqueous electrolyte secondary battery according to the present disclosure is suitable for use, for example, as a power source for mobile devices such as smartphones, a power source for vehicles such as electric cars, and a storage device for natural energy such as solar power. [Explanation of symbols]

[0089] 1 electrode group 2 Positive Lead 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing

Claims

1. A positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises a composite oxide containing lithium and a transition metal, a conductive agent, and an additive that coats the composite oxide and the conductive agent; The additive includes a metal oxide and a phosphoric acid ester compound, The phosphoric acid ester compound has at least one alkenyl group in one molecule.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the alkenyl group includes at least one selected from the group consisting of a vinyl group, a 1-propenyl group, a 2-propenyl group, an isopropenyl group, a 1-butenyl group, a 2-butenyl group, and a 3-butenyl group.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the phosphate ester compound comprises triallyl phosphate.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the metal oxide contains at least one element selected from the group consisting of aluminum, silicon, titanium, magnesium, zirconium, niobium, germanium, calcium, and strontium.

5. The composite oxide has a layered rock salt type crystal structure, and General formula: LiNi ｘ Co ｙ M １－ｘ－ｙ O ２ wherein, in the general formula, 0.3≦x<1, 0<y≦0.5, and 0<1-x-y≦0.35 are satisfied, and M is at least one selected from the group consisting of Al and Mn.

6. The metal oxide is Al ２ O ３ Including, 6. The nonaqueous electrolyte secondary battery according to claim 5, wherein, at an outermost surface of the composite oxide whose surface is covered with the additive, an atomic ratio of Al derived from the metal oxide to Ni derived from the composite oxide: Al / Ni is 2 or less.

7. 7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte contains the phosphate ester compound in an amount of 2 mass % or less based on the total amount of the nonaqueous electrolyte.

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