Nonaqueous electrolyte storage element and method for manufacturing same

Abstract

The non-aqueous electrolyte power storage element according to one aspect of the present invention is provided with a negative electrode, a positive electrode, and a non-aqueous electrolyte, the negative electrode comprises a negative electrode active material layer that includes graphite and an acrylic resin, and the non-aqueous electrolyte contains at least lithium difluorooxalatoborate or lithium difluorophosphate.

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element and a method for producing the same. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium-ion non-aqueous electrolyte secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc., due to their high energy density. The non-aqueous electrolyte secondary batteries generally include an electrode assembly having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double layer capacitors are also widely used as energy storage elements.

[0003] Lithium-ion nonaqueous electrolyte secondary batteries with rapid charging capabilities are required as energy sources for the above-mentioned automobiles, etc. For example, Patent Document 1 proposes a technology that enables rapid charging by using, as a positive electrode active material, a manganese-containing oxide having a specific composition and a spinel structure, and a nickel-containing oxide having a specific composition and a layered structure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-076997 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in recent years, the demand for lithium ion secondary batteries as an energy source has rapidly increased, and further improvements in rapid charging performance are required.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a non-aqueous electrolyte electricity storage element having excellent rapid charging characteristics and a method for producing the same. [Means for solving the problem]

[0007] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a negative electrode, a positive electrode, and a non-aqueous electrolyte solution, wherein the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, and the non-aqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate.

[0008] A method for manufacturing a nonaqueous electrolyte storage element according to one aspect of the present invention includes housing a negative electrode, a positive electrode, and a nonaqueous electrolyte solution in a case, wherein the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, and the nonaqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate. [Effects of the Invention]

[0009] A nonaqueous electrolyte storage element according to one aspect of the present invention has excellent rapid charging characteristics.

[0010] A method for producing a nonaqueous electrolyte electricity storage element according to one aspect of the present invention can produce a nonaqueous electrolyte electricity storage element having excellent rapid charging characteristics. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an external perspective view showing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an electricity storage device constructed by assembling a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a negative electrode, a positive electrode, and a non-aqueous electrolyte solution, wherein the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, and the non-aqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate.

[0013] Graphite, which has a large charge / discharge capacity per unit mass at a base potential close to that of lithium, is widely used as the negative electrode material for nonaqueous electrolyte storage elements. Styrene-butadiene rubber, which can be used in relatively small amounts, is widely used as the negative electrode binder. However, the present inventors have discovered that when the negative electrode active material layer of a nonaqueous electrolyte storage element contains graphite as the negative electrode active material and an acrylic resin as the binder, and the nonaqueous electrolyte contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate, the nonaqueous electrolyte storage element exhibits excellent rapid charging performance. While the reason for this is unclear, it is believed to be as follows: When styrene-butadiene rubber is used as the negative electrode binder, it is distributed relatively largely on the edge surfaces of the graphite, which is the negative electrode active material. In contrast, when an acrylic resin is used as the negative electrode binder, it is distributed uniformly around the graphite. Furthermore, when the non-aqueous electrolyte contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate, a high-quality coating of lithium difluorooxalatoborate or lithium difluorophosphate is formed on the edge surfaces of the graphite, which is believed to result in improved rapid charging performance of the non-aqueous electrolyte storage element.

[0014] The content of lithium difluorooxalatoborate or lithium difluorophosphate in the non-aqueous electrolyte solution is preferably 0.2% by mass or more and 2.0% by mass or less. When the content of lithium difluorooxalatoborate or lithium difluorophosphate is within the above range, the rapid charging performance of the non-aqueous electrolyte storage element can be further improved.

[0015] The positive electrode preferably contains a positive electrode active material containing nickel, cobalt, and manganese. When the positive electrode contains a positive electrode active material containing nickel, cobalt, and manganese, the energy density of the nonaqueous electrolyte storage element can be improved.

[0016] A nonaqueous electrolyte electricity storage element according to one embodiment of the present invention will be described in detail below. <Non-aqueous electrolyte energy storage element> A nonaqueous electrolyte storage element according to one embodiment of the present invention comprises a negative electrode, a positive electrode, and a nonaqueous electrolyte solution. Hereinafter, a nonaqueous electrolyte secondary battery will be described as an example of a nonaqueous electrolyte storage element. The positive electrode and negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a case, and the case is filled with a nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. The case may be a known metal case, resin case, or the like, that is typically used as a case for a nonaqueous electrolyte secondary battery.

[0017] [Negative electrode] The negative electrode includes a negative electrode substrate and a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer is laminated along at least one surface of the negative electrode substrate directly or via an intermediate layer.

[0018] (negative electrode substrate) The negative electrode substrate is a substrate having electrical conductivity. Metals such as copper, nickel, stainless steel, and nickel-plated steel, or alloys thereof, are used as the material of the negative electrode substrate, with copper or a copper alloy being preferred. The negative electrode substrate may be in the form of a foil, a vapor-deposited film, or the like, with foil being preferred from the standpoint of cost. That is, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil. Having "electrical conductivity" refers to a material having a volume resistivity of 1×10 as measured in accordance with JIS-H-0505 (1975). 7 "Non-conductive" means that the volume resistivity is 1×10 7 This means that the resistance is greater than Ω·cm.

[0019] (Negative electrode active material layer) The negative electrode active material layer is laminated directly or via an intermediate layer along at least one surface of the negative electrode substrate. The negative electrode active material layer is formed from a so-called negative electrode mixture containing a negative electrode active material. The negative electrode active material layer contains graphite and an acrylic resin.

[0020] The negative electrode active material is usually a material capable of absorbing and releasing lithium ions. The nonaqueous electrolyte storage element contains graphite as the negative electrode active material. Examples of graphite include natural graphite and artificial graphite.

[0021] The negative electrode active material layer may contain other negative electrode active materials, such as other carbon materials such as non-graphitizable carbon (hard carbon) or easily graphitizable carbon (soft carbon), semimetals such as Si, metals such as Sn, oxides of these semimetals or metals, or composites of these semimetals or metals with carbon materials. These materials may be used alone or in appropriate combinations of two or more. Among these, it is preferable to contain non-graphitizable carbon. By containing non-graphitizable carbon, it is possible to minimize expansion of the negative electrode during charging. Furthermore, it is possible to more stably maintain the shape of the negative electrode active material layer over a long period of time.

[0022] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0023] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002) is 0.34 nm or more and 0.42 nm or less. Non-graphitizable carbon includes non-graphitizable carbon and graphitizable carbon. Non-graphitizable carbon includes, for example, resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, alcohol-derived materials, etc. "Non-graphitizable carbon" refers to a carbon material having the above d 002 The term "easily graphitizable carbon" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less. 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0024] Here, the "discharged state" refers to a state in which an open circuit voltage is 0.7 V or higher in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as a working electrode and metallic Li as a counter electrode. Since the potential of the metallic Li counter electrode in the open circuit state is approximately equal to the redox potential of Li, the open circuit voltage in the single-electrode battery is approximately equal to the potential of the negative electrode containing the carbon material relative to the redox potential of Li. In other words, an open circuit voltage of 0.7 V or higher in the single-electrode battery means that lithium ions capable of being absorbed and released during charging and discharging have been sufficiently released from the carbon material, which is the negative electrode active material.

[0025] The lower limit of the graphite content in the negative electrode active material is preferably 60 mass %, more preferably 70 mass %, and even more preferably 80 mass %, while the upper limit of this content is preferably 99 mass %, more preferably 95 mass %.

[0026] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but the lower limit is preferably 50 mass %, more preferably 80 mass %, and even more preferably 90 mass %, while the upper limit is preferably 99 mass %, more preferably 98 mass %.

[0027] (binder) The negative electrode mixture of the nonaqueous electrolyte storage element contains an acrylic resin as a binder. "Acrylic resin" refers to a resin formed from a monomer whose main component is acrylic acid, methacrylic acid, or a derivative thereof. "Mainly composed" means that the content of structural units derived from acrylic acid, methacrylic acid, or a derivative thereof in the acrylic resin is 50% by mass or more. The lower limit of the content of structural units derived from acrylic acid, methacrylic acid, or a derivative thereof in the acrylic resin is 50% by mass, preferably 60% by mass, more preferably 70% by mass, and even more preferably 75% by mass. Examples of the acrylic resin include polyacrylic acid, methyl polyacrylate, polyacrylamide, copolymers containing acrylic acid, and alkali metal salts of polyacrylic acid. Polyacrylic acid, methyl polyacrylate, polyacrylamide, and alkali metal salts of polyacrylic acid are preferred, and polyacrylic acid is more preferred. Note that polyacrylonitrile and copolymers containing acrylonitrile are not included in the acrylic resin.

[0028] The negative electrode mixture may contain other binders, such as thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic acid, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers. From the viewpoint of input characteristics, the binder preferably contains a small amount of resin having a butadiene-derived structural unit, such as styrene-butadiene rubber (SBR), and more preferably does not contain substantially any resin having a butadiene-derived structural unit. Specifically, the upper limit of the mass ratio of the styrene-butadiene rubber (SBR) to the acrylic resin is preferably 2.3, more preferably 1.5, even more preferably 1.0, and particularly preferably 0.5. Similarly, in a copolymer of acrylic acid-derived structural units and butadiene-derived structural units, it is preferable that the content of butadiene-derived structural units is low. Specifically, the upper limit of the content of butadiene-derived structural units in a copolymer containing acrylic acid-derived structural units and butadiene-derived structural units is, for example, preferably 50% by mass, more preferably 40% by mass, even more preferably 30% by mass, and even more preferably 25% by mass. The acrylic resin may not contain butadiene-derived structural units from the viewpoint of improving output performance, or may contain butadiene-derived structural units from the viewpoint of adhesion of the negative electrode active material layer. The lower limit of the content of butadiene-derived structural units in a copolymer containing acrylic acid-derived structural units and butadiene-derived structural units may be, for example, 1% by mass, and in some cases 2%, 5%, or 10% by mass is preferred.

[0029] The content of the acrylic resin in the binder is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 99% by mass or more, and may be 100% by mass.

[0030] The lower limit of the binder content in the negative electrode active material layer is preferably 0.2 mass %, more preferably 0.5 mass %, and even more preferably 1 mass %, while the upper limit of this content is preferably 10 mass %, more preferably 5 mass %.

[0031] The content of the binder in the negative electrode active material layer is preferably 0.2% by mass to 10% by mass, more preferably 0.5% by mass to 5% by mass. By setting the binder content within this range, the active material can be stably maintained.

[0032] (Other optional ingredients) The negative electrode mixture may contain optional components such as a conductive agent, a thickener, and a filler as needed.

[0033] The conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include carbonaceous materials, metals, conductive ceramics, etc. Carbonaceous materials include graphitized carbon, non-graphitized carbon, graphene-based carbon, etc. Non-graphitized carbon includes carbon nanofiber, pitch-based carbon fiber, carbon black, etc. Carbon black includes furnace black, acetylene black, ketjen black, etc. Graphene-based carbon includes graphene, carbon nanotubes (CNT), fullerene, etc. The conductive agent may be in the form of powder, fiber, etc. As the conductive agent, one of these materials may be used alone, or two or more may be mixed and used. These materials may also be used as a composite. For example, a composite material of carbon black and CNT may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0034] Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. In addition, when the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like.

[0035] The filler is not particularly limited, and examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, and glass.

[0036] (middle class) The intermediate layer is a coating layer on the surface of the negative electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the negative electrode substrate and the negative electrode active material layer. As with the positive electrode, the configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

[0037] [Positive electrode] The positive electrode includes a positive electrode substrate and a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer is laminated along at least one surface of the positive electrode substrate directly or via an intermediate layer.

[0038] (Positive electrode substrate) The positive electrode substrate is a substrate having electrical conductivity. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum and aluminum alloys are preferred in terms of the balance between potential resistance, high electrical conductivity, and cost. Examples of the form of the positive electrode substrate include foil and vapor-deposited film, with foil being preferred in terms of cost. In other words, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085 and A3003 as specified in JIS-H4000 (2014).

[0039] (Cathode active material layer) The positive electrode active material layer is formed from a so-called positive electrode mixture containing a positive electrode active material. The positive electrode active material can be appropriately selected from, for example, known positive electrode active materials. As the positive electrode active material for a lithium ion secondary battery, a material capable of absorbing and releasing lithium ions is usually used. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal oxides having a spinel type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. As the lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, for example, Li[Li x Ni 1-x ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Examples of lithium transition metal oxides with spinel crystal structures include Li x Mn2O4,Li x Ni γ Mn (2-γ)O4, etc. Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. Among these, the lithium transition metal composite oxides are preferred as the positive electrode active material from the viewpoint of achieving high energy density, and nickel-cobalt-manganese-containing lithium transition metal composite oxides containing nickel, cobalt, and manganese as constituent elements in addition to Li are more preferred.

[0040] The surface of the materials listed above as the positive electrode active material may be coated with another material. In the positive electrode active material layer, one of these materials may be used alone, or two or more of them may be used in combination.

[0041] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but the lower limit is preferably 50 mass %, more preferably 80 mass %, and even more preferably 90 mass %, while the upper limit is preferably 99 mass %, more preferably 98 mass %.

[0042] (Other optional ingredients) The positive electrode mixture may contain optional components such as a binder, a conductive agent, a thickener, and a filler, as required.

[0043] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic acid, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0044] Optional components such as a conductive agent, a thickener, and a filler can be selected from the materials exemplified for the negative electrode above.

[0045] (middle class) The intermediate layer is a coating layer on the surface of the positive electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

[0046] [Non-aqueous electrolyte] The nonaqueous electrolyte typically contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. In the nonaqueous electrolyte storage element according to this embodiment, the nonaqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate as an additive.

[0047] (non-aqueous solvent) The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0048] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0049] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0050] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0051] (electrolyte salt) The electrolyte salt may be any known electrolyte salt commonly used in non-aqueous electrolytes for general energy storage elements, including lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts, with lithium salts being preferred.

[0052] Examples of the lithium salt include inorganic lithium salts such as LiPF, LiPOF, LiBF, and LiClO, and lithium salts having a hydrocarbon group in which hydrogen is substituted with fluorine, such as LiSOCF, LiC(SOCF), and LiC(SOCF) Among these, inorganic lithium salts are preferred, and LiPF is more preferred.

[0053] The lower limit of the content of the electrolyte salt in the non-aqueous solution is 0.1 mol / dm 3 is preferred, and 0.3 mol / dm 3 More preferably, 0.5 mol / dm 3 is more preferably 0.7 mol / dm 3 On the other hand, the upper limit is not particularly limited, but is preferably 2.5 mol / dm 3 is preferred, and 2 mol / dm 3 More preferably, 1.5 mol / dm 3 The non-aqueous solution means a state in which an electrolyte salt is dissolved in a non-aqueous solvent, and refers to the state before dissolving an additive such as a boron-containing oxalato complex salt.

[0054] (additives) The non-aqueous electrolyte contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate. By containing at least one of lithium difluorooxalatoborate and lithium difluorophosphate in the non-aqueous electrolyte, the non-aqueous electrolyte storage element has excellent rapid charging characteristics.

[0055] The lower limit of the content of lithium difluorooxalatoborate or lithium difluorophosphate in the non-aqueous electrolyte is preferably 0.05% by mass, more preferably 0.2% by mass, even more preferably 0.3% by mass, and even more preferably 0.5% by mass. Meanwhile, the upper limit of this content is preferably 2.0% by mass, and even more preferably 1.5% by mass. By having the content of lithium difluorooxalatoborate or lithium difluorophosphate within the above range, the effect of suppressing the increase in internal resistance after charge-discharge cycles can be further improved. Here, the content of lithium difluorooxalatoborate or lithium difluorophosphate refers to the mass of lithium difluorooxalatoborate or lithium difluorophosphate relative to the mass of the non-aqueous solution.

[0056] The non-aqueous electrolyte may contain both lithium difluorooxalatoborate and lithium difluorophosphate. When the non-aqueous electrolyte contains both lithium difluorooxalatoborate and lithium difluorophosphate, the lower limit of the sum of the contents of lithium difluorooxalatoborate and lithium difluorophosphate is preferably 0.05% by mass, more preferably 0.2% by mass, even more preferably 0.3% by mass, and even more preferably 0.4% by mass. On the other hand, the upper limit of this content is preferably 4.0% by mass, more preferably 3.0% by mass, even more preferably 2.0% by mass, and even more preferably 1.5% by mass. When the sum of the contents of lithium difluorooxalatoborate and lithium difluorophosphate is within the above range, the effect of suppressing the increase in internal resistance after charge-discharge cycling can be further improved. Here, the sum of the contents of lithium difluorooxalatoborate and lithium difluorophosphate means the sum of the masses of lithium difluorooxalatoborate and lithium difluorophosphate relative to the mass of the non-aqueous solution.

[0057] The non-aqueous electrolyte may contain additives other than lithium difluorooxalatoborate and lithium difluorophosphate, such as lithium bis(fluorosulfonyl)imide (LiFSI), lithium fluorosulfonate, and lithium tetrafluorooxalatophosphate.

[0058] The non-aqueous electrolyte can be obtained by dissolving the electrolyte salt, and at least one of lithium difluorooxalatoborate and lithium difluorophosphate, in the non-aqueous solvent.

[0059] [Separator] The separator may be, for example, a woven fabric, a nonwoven fabric, or a porous resin film. Among these, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of nonaqueous electrolyte retention. As the main component of the separator, a polyolefin such as polyethylene or polypropylene is preferred from the viewpoint of strength, and a polyimide or aramid is preferred from the viewpoint of resistance to oxidative decomposition. These resins may also be combined.

[0060] An inorganic layer may be disposed between the separator and the electrode (usually a positive electrode). This inorganic layer is a porous layer also called a heat-resistant layer. Alternatively, a separator having an inorganic layer formed on one side of a porous resin film may be used. The inorganic layer is usually composed of inorganic particles and a binder, and may contain other components.

[0061] [Specific configuration of the energy storage element] FIG. 1 is a schematic diagram of a rectangular nonaqueous electrolyte storage element 1 (nonaqueous electrolyte secondary battery) that is one embodiment of the nonaqueous electrolyte storage element according to the present invention. The figure is a see-through view of the inside of the case. The nonaqueous electrolyte storage element 1 shown in FIG. 1 includes an electrode assembly 2 housed in a case 3. The electrode assembly 2 is formed by winding a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer with a separator interposed therebetween. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode current collector 4′, and the negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode current collector 5′. A nonaqueous electrolyte is poured into the case 3.

[0062] The configuration of the nonaqueous electrolyte electricity storage element according to the present invention is not particularly limited, and examples include a cylindrical battery, a prismatic battery (a rectangular battery), and a flat battery.

[0063] [Method of manufacturing nonaqueous electrolyte energy storage element] A method for manufacturing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes housing the negative electrode, a positive electrode, and a nonaqueous electrolyte containing at least one of lithium difluorooxalatoborate and lithium difluorophosphate in a case. The positive electrode can be obtained by laminating the positive electrode active material layer directly onto a positive electrode substrate or via an intermediate layer. The positive electrode active material layer is laminated by applying a positive electrode mixture paste to the positive electrode substrate. Similarly to the positive electrode, the negative electrode can be obtained by laminating the negative electrode active material layer directly onto a negative electrode substrate or via an intermediate layer. The negative electrode active material layer is laminated by applying a negative electrode mixture paste containing graphite and an acrylic resin to the negative electrode substrate. The positive electrode mixture paste and the negative electrode mixture paste may contain a dispersion medium. Examples of the dispersion medium include aqueous solvents such as water and mixed solvents mainly composed of water; and organic solvents such as N-methylpyrrolidone and toluene.

[0064] The method for manufacturing the nonaqueous electrolyte storage element further includes, as another step, stacking the negative electrode and the positive electrode with a separator interposed therebetween to form an electrode assembly.

[0065] The negative electrode, positive electrode, non-aqueous electrolyte, etc. can be housed in a case by a known method. After the housing, the housing opening is sealed to obtain a non-aqueous electrolyte storage element. Details of each element constituting the non-aqueous electrolyte storage element obtained by the above manufacturing method are as described above.

[0066] [Other embodiments] The nonaqueous electrolyte electricity storage element of the present invention is not limited to the above-described embodiment.

[0067] In the above-described embodiment, the nonaqueous electrolyte storage element is mainly a nonaqueous electrolyte secondary battery, but other nonaqueous electrolyte storage elements may also be used. Examples of other nonaqueous electrolyte storage elements include capacitors (electric double layer capacitors, lithium ion capacitors), etc. Examples of nonaqueous electrolyte secondary batteries include lithium ion nonaqueous electrolyte secondary batteries.

[0068] Furthermore, although a wound electrode body is used in the above embodiment, a laminated electrode body formed from a laminate of a plurality of sheets each including a positive electrode, a negative electrode, and a separator may be used.

[0069] The present invention can also be realized as an electricity storage device including a plurality of the above-described nonaqueous electrolyte electricity storage elements. In this case, the technology of the present invention may be applied to at least one of the nonaqueous electrolyte electricity storage elements included in the electricity storage device. Furthermore, a battery pack can be formed by using one or more nonaqueous electrolyte electricity storage elements (cells) of the present invention, and this battery pack can be used to form an electricity storage device. The electricity storage device can be used as a power source for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Furthermore, the electricity storage device can be used in various power supply devices such as engine starting power supplies, auxiliary power supplies, and uninterruptible power supplies (UPS).

[0070] 2 shows an example of an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, and a bus bar (not shown) that electrically connects two or more energy storage units 20. The energy storage units 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements. [Example]

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0072] [Examples 1 to 2 and Comparative Examples 1 to 8] (Negative electrode) A coating liquid (negative electrode mixture paste) containing graphite and non-graphitizable carbon as negative electrode active materials, a binder listed in Table 1, and carboxymethyl cellulose (CMC) as a thickener, with water as a dispersion medium, was prepared. The mass ratio of graphite to non-graphitizable carbon in the negative electrode active material was 85:15. The mixing ratio of the negative electrode active material, binder, and thickener was 96:2:2 by mass. The coating liquid was applied to both sides of a 10 μm-thick copper foil substrate and dried to form negative electrode active material layers, thereby obtaining negative electrodes of the examples and comparative examples. (non-aqueous electrolyte) LiPF6 was added to a non-aqueous solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 at 1.4 mol / dm 3 The resulting solution was used as a non-aqueous solution, and additives were dissolved in the amounts shown in Table 1 (content ratios when the non-aqueous solution was taken as 100% by mass) to obtain non-aqueous electrolytes. The additives used were lithium difluorooxalatoborate (LiFOB), lithium difluorophosphate (LiDFP), vinylene carbonate (VC), lithium difluorobisoxalatophosphate (LiFOP), and lithium bisoxalatoborate (LiBOB). In Table 1, "-" indicates that the respective component was not used. (positive electrode) NCM (LiNi) with α-NaFeO2 type crystal structure 0.5 Mn 0.3 Co 0.2 A positive electrode containing 02) as the positive electrode active material was fabricated. The positive electrode contained the above positive electrode active material, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive agent. A coating liquid (positive electrode mixture paste) was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The mixing ratio of the positive electrode active material, binder, and conductive agent was 93:4:3 by mass. The coating liquid was applied to both sides of a positive electrode substrate, dried, and pressed to form a positive electrode active material layer. Aluminum foil with a thickness of 15 μm was used as the positive electrode substrate.

[0073] (Fabrication of non-aqueous electrolyte energy storage element) Next, the positive electrode and the negative electrode were stacked with a separator made of a polyethylene substrate and an inorganic layer formed on the polyethylene substrate interposed therebetween to prepare an electrode assembly. This electrode assembly was housed in an aluminum rectangular battery case, and a positive electrode terminal and a negative electrode terminal were attached. The nonaqueous electrolyte was poured into this case (rectangular battery case), which was then sealed to obtain nonaqueous electrolyte storage elements of Examples and Comparative Examples.

[0074] [evaluation] (Measurement of initial discharge capacity) Each of the obtained nonaqueous electrolyte storage elements was subjected to constant current / constant voltage charging at 0.18 A for 7 hours at a temperature of 25°C, with a charge cut-off voltage of 4.25 V. After charging, the element was subjected to constant current discharging at a current value of 0.18 A with a discharge cut-off voltage of 2.75 V. After a 10-minute pause, the element was subjected to constant current / constant voltage charging at 0.9 A for 3 hours at a temperature of 25°C, with a charge cut-off voltage of 4.25 V. After a 10-minute pause, the element was subjected to constant current discharging at a current value of 0.9 A. This discharge capacity was designated the "initial discharge capacity."

[0075] (Quick charging performance) After measuring the initial discharge capacity, each of the nonaqueous electrolyte storage elements was evaluated for rapid charging performance under the following conditions. At 25°C, the battery was charged at a constant current of 2.2 A up to 4.25 V, and then charged at a constant voltage of 4.25 V. The charging termination condition was a charging time of 3 hours. A 10-minute rest period was then allowed. Subsequently, the battery was discharged at a constant current of 2.2 A down to 2.75 V, followed by a 10-minute rest period. This charge / discharge cycle was repeated 10 times. The obtained value indicates the capacity loss after 10 cycles, and the smaller the value, the better the fast charging performance. The results are shown in Table 1 below.

[0076] [Table 1]

[0077] As shown in Table 1, Examples 1 and 2, in which the negative electrode active material layer contained an acrylic resin as a binder and the nonaqueous electrolyte solution contained at least one of lithium difluorooxalatoborate and lithium difluorophosphate as an additive, had good rapid charging performance.

[0078] Comparative Examples 1 and 2, in which the non-aqueous electrolyte solution did not contain any additives, exhibited inferior fast charging performance compared to the Examples, regardless of whether the binder was an acrylic resin or a styrene-butadiene copolymer. Furthermore, Comparative Examples 3 and 4, in which the non-aqueous electrolyte solution contained at least one of lithium difluorooxalatoborate and lithium difluorophosphate as an additive, but the binder was a styrene-butadiene copolymer, failed to improve fast charging performance. Furthermore, Comparative Examples 5 to 7, in which the non-aqueous electrolyte solution contained vinylene carbonate (VC), lithium difluorobisoxalatophosphate (LiFOP), or lithium bisoxalatoborate (LiBOB) as an additive, failed to improve fast charging performance, even though the binder was an acrylic resin.

[0079] On the other hand, the results of Comparative Examples 1 and 8 show that when the acrylic resin used as the binder contains a butadiene monomer unit, the rapid charging performance is reduced.

[0080] As described above, the nonaqueous electrolyte electricity storage element was shown to have excellent rapid charging characteristics. [Industrial Applicability]

[0081] The present invention is suitably used as a nonaqueous electrolyte storage element, including a nonaqueous electrolyte secondary battery used as a power source for electronic devices such as personal computers and communication terminals, automobiles, and the like. [Explanation of symbols]

[0082] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 Cases 4 Positive terminal 4' Positive electrode current collector 5 Negative terminal 5' Negative electrode current collector 20 Energy storage unit 30 Energy storage device

Claims

1. A battery comprising a negative electrode, a positive electrode, and a non-aqueous electrolyte solution, the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, the non-aqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate, The nonaqueous electrolyte storage element, wherein the acrylic resin is a resin having a content of structural units derived from acrylic acid of 75% by mass or more.

2. 2. The nonaqueous electrolyte storage element according to claim 1, wherein the content of lithium difluorooxalatoborate or lithium difluorophosphate in the nonaqueous electrolyte is 0.2% by mass or more and 2.0% by mass or less.

3. 3. The nonaqueous electrolyte storage element according to claim 1, wherein the positive electrode contains a positive electrode active material containing nickel, cobalt, and manganese.

4. a negative electrode, a positive electrode, and a non-aqueous electrolyte solution housed in a case; the negative electrode has a negative electrode active material layer containing graphite and an acrylic resin, the non-aqueous electrolyte solution contains at least one of lithium difluorooxalatoborate and lithium difluorophosphate, The method for producing a nonaqueous electrolyte storage element, wherein the acrylic resin is a resin having a content of structural units derived from acrylic acid of 75 mass % or more.

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