Non-aqueous electrolyte secondary batteries

Abstract

A non-aqueous electrolyte secondary battery characterized by comprising: a non-aqueous electrolyte that includes a sulfonyl imide compound represented by general formula (1), LiN(RSO2)(FSO2), and an amide compound having a C3-6 branched or linear alkyl group; and a positive electrode that includes a positive electrode active material represented by general formula (2), LivNixCoyMnzO2+w.

Description

[Technical Field]

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

[0002] In order to improve the battery performance of secondary batteries such as lithium ion secondary batteries, various non-aqueous electrolyte solutions and materials thereof for use in secondary batteries have been studied. The applicant has found through his studies that a non-aqueous electrolyte solution containing a sulfonylimide compound such as lithium bis(fluorosulfonyl)imide as an electrolyte salt improves the battery performance of lithium ion secondary batteries, such as high-temperature durability and charge / discharge cycle performance.

[0003] For example, in Patent Document 1, the present applicant has proposed a nonaqueous electrolyte solution containing LiN(FSO2)2 as an electrolyte and at least one of a silicon atom-containing compound, a boron atom-containing compound, a carbon atom-containing compound, a sulfur atom-containing compound, and a phosphorus atom-containing compound as an additive. This nonaqueous electrolyte solution containing a sulfonylimide compound uses a specific compound to suppress battery self-discharge and reduce charge transfer resistance (impedance) and battery direct current resistance (DCR), thereby improving battery performance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 239807 Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, a non-aqueous electrolyte containing a sulfonylimide compound and LiNi, which is generally used as a positive electrode active material, 1 / 3 Co 1 / 3 Mn 1 / 3In a secondary battery having a positive electrode containing a lithium composite oxide containing Ni (nickel), such as O2(NCM111), the use of an additive may actually increase resistance. That is, depending on the type of positive electrode active material, the additive in the nonaqueous electrolyte may cause a decrease in battery performance.

[0006] Furthermore, conventional secondary batteries using NCM111 have difficulty meeting the energy density requirements of recent electric vehicle (EV) batteries, and therefore there is a demand for the development of secondary batteries with higher energy densities than those of conventional technologies. One possible method for improving the energy density of secondary batteries is to use, for example, a high-Ni lithium composite oxide, which has a high Ni content among transition metals, as the positive electrode active material. However, secondary batteries using high-Ni lithium composite oxides experience a large increase in resistance during battery use, so a technology to reduce the resistance is required.

[0007] In secondary batteries, three types of resistance affect battery performance: the initial resistance when the battery is completed after being charged and discharged under specified conditioning conditions after manufacture, the resistance associated with battery use, and the resistance after high-temperature storage. From the perspective of improving battery performance, it is desirable for all resistance values ​​to be small.

[0008] The present disclosure has been made in view of the above points, and an object of the present disclosure is to reduce the resistance of a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte containing a sulfonylimide compound and a high Ni-based positive electrode containing a high Ni-containing lithium composite oxide. [Means for solving the problem]

[0009] As a result of intensive studies to achieve the above object, the present inventor has found that by using, as an additive for a non-aqueous electrolyte containing a sulfonylimide compound, an amide compound having a branched or linear alkyl group with a specific number of carbon atoms, and a "high-Ni-containing ternary cathode active material" as a high-Ni-containing lithium composite oxide (cathode active material) in which the content ratio of Ni among three transition metals of Ni, Co (cobalt), and Mn (manganese) is 50% or more on a molar basis, all of the above three types of resistances of the secondary battery are reduced. The present disclosure is specifically as follows.

[0010] The non-aqueous electrolyte secondary battery of the present disclosure · General formula (1): LiN(RSO2)(FSO2) (where R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms) ··· (1) A non-aqueous electrolyte containing a sulfonylimide compound represented by the formula and an amide compound having a branched alkyl group with 3 to 6 carbon atoms, · General formula (2): Li v Ni x Co y Mn z O 2+w (0.2 ≦ v ≦ 1.2, 0.5 ≦ x ≦ 0.9, 0 < y ≦ 0.2, 0 < z ≦ 0.4, x + y + z = 1, -0.2 ≦ w ≦ 0.2 (where v is the molar ratio of Li, x is the molar ratio of Ni, y is the molar ratio of Co, z is the molar ratio of Mn, and w is the molar ratio of O)) ··· (2) It is characterized by comprising a cathode containing a cathode active material represented by the formula.

[0011] Further, the non-aqueous electrolyte of the present disclosure · A non-aqueous electrolyte containing a sulfonylimide compound represented by the above general formula (1) and an amide compound having a linear alkyl group with 3 to 6 carbon atoms, · It is characterized by comprising a cathode containing a cathode active material represented by the above general formula (2).

[0012] In the nonaqueous electrolyte secondary battery of the present disclosure, the content of the amide compound relative to the sulfonylimide compound may be 10 ppm by mass or more. The sulfonylimide compound may contain LiN(FSO2)2. [Effects of the Invention]

[0013] According to the present disclosure, it is possible to reduce the resistance of a nonaqueous electrolyte secondary battery that includes a nonaqueous electrolyte containing a sulfonylimide compound and a high Ni-based positive electrode that contains a high Ni lithium composite oxide. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present embodiment will be described in detail below. The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the present invention, its applications, or its uses.

[0015] <Nonaqueous electrolyte secondary battery> The nonaqueous electrolyte secondary battery is a secondary battery that includes a nonaqueous electrolyte. The nonaqueous electrolyte secondary battery according to this embodiment includes a nonaqueous electrolyte, a positive electrode, and a negative electrode.

[0016] [Non-aqueous electrolyte] (electrolyte salt) The nonaqueous electrolyte according to this embodiment contains an electrolyte salt represented by the general formula (1): [C1] LiN(RSO2)(FSO2)···(1) The non-aqueous electrolyte contains a sulfonylimide compound (1) as an essential component (hereinafter referred to as a "sulfonylimide compound (1)" and a fluorine-containing sulfonylimide salt).

[0017] In the general formula (1), R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms.

[0018] Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Among the alkyl groups having 1 to 6 carbon atoms, a linear or branched alkyl group having 1 to 6 carbon atoms is preferred, and a linear alkyl group having 1 to 6 carbon atoms is more preferred.

[0019] Examples of the fluoroalkyl group having 1 to 6 carbon atoms include alkyl groups having 1 to 6 carbon atoms in which some or all of the hydrogen atoms have been substituted with fluorine atoms. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, and a pentafluoroethyl group. In particular, the fluoroalkyl group may be a perfluoroalkyl group.

[0020] The substituent R is preferably a fluorine atom or a perfluoroalkyl group (for example, a perfluoroalkyl group having 1 to 6 carbon atoms, such as a trifluoromethyl group, a pentafluoroethyl group, or a heptafluoropropyl group), more preferably a fluorine atom, a trifluoromethyl group, or a pentafluoroethyl group, still more preferably a fluorine atom or a trifluoromethyl group, and still more preferably a fluorine atom.

[0021] Specific examples of the sulfonylimide compound (1) include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2, LiFSI), lithium (fluorosulfonyl)(methylsulfonyl)imide, lithium (fluorosulfonyl)(ethylsulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium (fluorosulfonyl)(pentafluoroethylsulfonyl)imide, and lithium (fluorosulfonyl)(heptafluoropropylsulfonyl)imide. The sulfonylimide compounds may be used alone or in combination of two or more. The sulfonylimide compound (1) may be a commercially available product or may be synthesized by a conventional method.

[0022] Among the sulfonylimide compounds (1), from the viewpoint of improving battery performance, LiN(FSO2)2, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, and lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide are preferred, with LiN(FSO2)2 being more preferred. In other words, among nonaqueous electrolytes, those containing LiN(FSO2)2 as the sulfonylimide compound (1) are preferred.

[0023] The concentration (content, total content when two or more kinds are used) of the sulfonylimide compound (1) in the nonaqueous electrolyte is preferably 0.2 mol / L or more, more preferably 0.3 mol / L or more, and even more preferably 0.5 mol / L or more from the viewpoint of improving battery performance (particularly reducing resistance). Moreover, the concentration is preferably 5 mol / L or less, more preferably 3 mol / L or less, and even more preferably 2 mol / L or less from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte.

[0024] From the viewpoint of improving battery performance, the content of sulfonylimide compound (1) in the non-aqueous electrolyte is preferably 10 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, even more preferably 50 mol% or more, and even more preferably more than 50 mol% based on the total 100 mol% of the electrolyte salt contained in the non-aqueous electrolyte. The upper limit of the content is 100 mol%. That is, the electrolyte salt contained in the non-aqueous electrolyte may contain sulfonylimide compound (1) alone.

[0025] The content of sulfonylimide compound (1) in the non-aqueous electrolyte is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, based on the total amount of the components contained in the non-aqueous electrolyte (100% by mass), from the viewpoint of improving battery performance. Also, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte, the concentration is preferably 70% by mass or less, more preferably 50% by mass or less, even more preferably 30% by mass or less, and even more preferably 20% by mass or less.

[0026] The electrolyte salt (lithium salt) may contain the sulfonylimide compound (1), but may also contain other electrolyte salts (electrolyte salts other than the sulfonylimide compound (1)). Examples of other electrolytes include imide salts and non-imide salts.

[0027] Examples of imide salts include fluorine-containing sulfonylimide salts other than sulfonylimide compound (1) (hereinafter referred to as "other sulfonylimide compounds"). Examples of other sulfonylimide compounds include non-lithium salts of the fluorine-containing sulfonylimides listed as sulfonylimide compound (1) (for example, salts in which the lithium (ion) in sulfonylimide compound (1) is substituted with a cation other than lithium ion). Examples of salts in which a cation other than lithium ion is substituted include alkali metal salts such as sodium salt, potassium salt, rubidium salt, and cesium salt; alkaline earth metal salts such as beryllium salt, magnesium salt, calcium salt, strontium salt, and barium salt; aluminum salt; ammonium salt; and phosphonium salt. The other sulfonylimide compounds may be used alone or in combination of two or more. In addition, commercially available products may be used as the other sulfonylimide compounds, or those synthesized by conventionally known methods may be used.

[0028] Examples of the non-imide salt include salts of non-imide anions and cations (lithium ions and the above-mentioned cations). [C2] LiPF a (C m F 2m+1 ) 6-a (a:0≦a≦6, m:1≦m≦4)···(3) (hereinafter referred to as "fluorophosphate compound (3)"), a compound represented by general formula (4): [C3] LiBF b (C n F 2n+1 ) 4-b (b:0≦b≦4, n:1≦n≦4) (4) Examples of the non-lithium salt include a compound represented by the formula (hereinafter referred to as "fluoroborate compound (4)"), lithium hexafluoroarsenate (LiAsF), LiSbF, LiClO, LiSCN, LiAlF, CFSOLi, LiC[(CFSO)], LiN(NO), and LiN[(CN)]; and non-lithium salts. Examples of the non-lithium salt include salts in which the lithium (ion) in these lithium salts is substituted with one of the cations listed above (e.g., NaBF, NaPF, NaPF(CF)). The non-imide salts may be used alone or in combination of two or more. Furthermore, commercially available non-imide salts may be used, or those synthesized by conventional methods may be used.

[0029] Among the other electrolytes, non-imide salts are preferred from the viewpoints of ionic conductivity, cost, etc., and fluorophosphate compound (3), fluoroborate compound (4) and LiAsF6 are preferred, with fluorophosphate compound (3) being more preferred.

[0030] Examples of the fluorophosphate compound (3) include LiPF, LiPF(CF), LiPF(C,F), LiPF(C,F), LiPF(C,F), etc. Among the fluorophosphate compounds (3), LiPF and LiPF(C,F) are preferred, with LiPF being more preferred.

[0031] Examples of the fluoroboric acid compound (4) include LiBF, LiBF(CF), LiBF(C,F), LiBF(C,F) and LiBF(C,F), etc. Among the fluoroboric acid compounds (4), LiBF and LiBF(CF) are preferred, and LiBF is more preferred.

[0032] These electrolyte salts (sulfonylimide compound (1), other electrolyte salts, etc.) may be present (contained) in the form of ions in the non-aqueous electrolyte solution.

[0033] The electrolyte salt composition may be an electrolyte salt having a simple salt composition of sulfonylimide compound (1), or an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and another electrolyte. When an electrolyte salt having a mixed salt composition is used, an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and fluorophosphate compound (3) is preferred, and an electrolyte salt having a mixed salt composition containing LiN(FSO2)2 and LiPF6 is more preferred.

[0034] When using an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and other electrolytes, the concentration of the other electrolytes in the nonaqueous electrolyte (content, or the total content when two or more types are used in combination) is preferably 0.1 mol / L or more, more preferably 0.2 mol / L or more, even more preferably 0.5 mol / L or more, even more preferably 0.7 mol / L or more, and even more preferably 1 mol / L or more, from the viewpoint of improving battery performance. Furthermore, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte, the concentration is preferably 5 mol / L or less, more preferably 3 mol / L or less, even more preferably 2 mol / L or less, and even more preferably 1.5 mol / L or less.

[0035] The total concentration of the electrolyte salts in the nonaqueous electrolyte solution is preferably 0.8 mol / L or more, more preferably 1 mol / L or more, and even more preferably 1.2 mol / L or more from the viewpoint of improving battery performance, and is preferably 5 mol / L or less, more preferably 3 mol / L or less, and even more preferably 2 mol / L or less from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte solution.

[0036] From the viewpoint of improving battery performance, it is preferable to increase the concentration of sulfonylimide compound (1). The molar ratio of sulfonylimide compound (1) to other electrolytes (the molar ratio of sulfonylimide compound (1) concentration to other electrolyte concentration) is preferably 1:25 or more, more preferably 1:10 or more, even more preferably 1:8 or more, even more preferably 1:5 or more, even more preferably 1:2 or more, and particularly preferably 1:1 or more, with the upper limit being preferably 25:1 or less, more preferably 10:1 or less, even more preferably 5:1 or less, and even more preferably 2:1 or less.

[0037] (Amide compound having a chain alkyl group having 3 to 6 carbon atoms) Furthermore, the nonaqueous electrolyte according to this embodiment contains, as an additive, an amide compound having a branched alkyl group having 3 to 6 carbon atoms (hereinafter also referred to as a "branched alkylamide compound") or an amide compound having a linear alkyl group having 3 to 6 carbon atoms (hereinafter also referred to as a "linear alkylamide compound") as an essential component. The "linear alkylamide compound" is distinguished from the "branched alkylamide compound" in that the linear alkyl group having 3 to 6 carbon atoms does not have a branched structure. The "branched alkylamide compound" and the "linear alkylamide compound" may be used alone or in combination of two or more types. The "branched alkylamide compound" and the "linear alkylamide compound" are collectively referred to as the "linear alkylamide compound".

[0038] Examples of branched alkylamide compounds include isobutylamide (2-methylpropionamide) and N,N-dimethylisobutylamide. The branched alkylamide compounds may be used alone or in combination of two or more. Among these, isobutylamide is preferred from the viewpoint of improving battery performance (especially reducing resistance).

[0039] Examples of linear alkylamide compounds include n-butylamide and N,N-dimethylbutylamide. The linear alkylamide compounds may be used alone or in combination of two or more. Among these, n-butylamide is preferred from the viewpoint of improving battery performance (especially reducing resistance).

[0040] From the viewpoint of improving battery performance (particularly reducing resistance), the content of the chain alkylamide compound (the total amount when two or more types are used in combination) relative to the sulfonylimide compound (1) (the total amount when two or more types are used in combination) is preferably 10 ppm by mass or more, more preferably 20 ppm by mass or more, even more preferably 100 ppm by mass or more, even more preferably 500 ppm by mass or more, and even more preferably 1000 ppm by mass or more. The upper limit of the content is preferably 6000 ppm by mass or less, more preferably 5000 ppm by mass or less. The content of the chain alkylamide compound relative to the sulfonylimide compound (1) is preferably 10 ppm by mass or more and 6000 ppm by mass or less. By specifying the quantitative ratio between the sulfonylimide compound (1) and the chain alkylamide compound within the above range, the resistance of a secondary battery including this nonaqueous electrolyte can be reduced.

[0041] (additives) The non-aqueous electrolyte may contain, in addition to the chain alkylamide compound, an additive for improving various properties of the lithium ion secondary battery. The additive may be added to the non-aqueous electrolyte or may be added during the preparation process of the non-aqueous electrolyte.Examples of additives include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, and trimethylene glycol sulfate; Yellow compounds; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, and cycloheptane; carbonate compounds such as vinylene carbonate, fluoroethylene carbonate (FEC), trifluoropropylene carbonate, phenylethylene carbonate, and erythrityl carbonate; sulfamic acid (amidosulfuric acid, H3NSO3); sulfamate salts ( Alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as calcium salts, strontium salts, and barium salts; other metal salts such as manganese salts, copper salts, zinc salts, iron salts, cobalt salts, and nickel salts; ammonium salts; guanidine salts; fluorosulfonic acid compounds such as lithium fluorosulfonate (LiFSO3), sodium fluorosulfonate (NaFSO3), potassium fluorosulfonate (KFSO3), and magnesium fluorosulfonate (Mg(FSO3)2); lithium monofluorophosphate (Li2PO3F fluorophosphate compounds such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorooxalatophosphate (LIDFOP), lithium tetrafluorooxalatophosphate (LITFOP), lithium difluorobis(oxalato)phosphate (LiDFBOP), lithium tris(oxalato)phosphate, and other fluorooxalato compounds such as lithium salts having an oxalic acid skeleton. These additives may be used alone or in combination of two or more.

[0042] The additive is preferably used in an amount of 0.1% by mass to 10% by mass, more preferably 0.2% by mass to 8% by mass, and even more preferably 0.3% by mass to 5% by mass, relative to 100% by mass of the total amount of components contained in the non-aqueous electrolyte. If the amount of additive used is too small, it may be difficult to obtain the effects derived from the additive. On the other hand, even if a large amount of additive is used, it may be difficult to obtain effects commensurate with the amount added, and the viscosity of the non-aqueous electrolyte may increase, resulting in a decrease in conductivity.

[0043] (Electrolyte solvent) The non-aqueous electrolyte may contain an electrolyte solvent. The electrolyte solvent is not particularly limited as long as it can dissolve and disperse the electrolyte salt. Examples of the electrolyte solvent include non-aqueous solvents, polymers used in place of electrolyte solvents, polymer gels, and other media, and any solvent generally used in batteries can be used.

[0044] The non-aqueous solvent is preferably a solvent that has a high dielectric constant, high solubility for the electrolyte, a boiling point of 60° C. or higher, and a wide electrochemical stability range, and more preferably an organic solvent with a low water content.Examples of such organic solvents include ether solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; chain carbonate ester (carbonate) solvents such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), diphenyl carbonate, and methyl phenyl carbonate; saturated cyclic carbonate ester solvents such as ethylene carbonate (EC), propylene carbonate (PC), 2,3-dimethyl ethylene carbonate, 1,2-butylene carbonate, and erythrityl carbonate; cyclic carbonate ester solvents having an unsaturated bond such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate; fluoroethylene carbonate, 4,5-difluoroethylene carbonate, trifluoropropylene carbonate, and propylene carbonate; fluorine-containing cyclic carbonate solvents such as benzoate, ethyl benzoate, and other aromatic carboxylic acid ester solvents; γ-butyrolactone, γ-valerolactone, δ-valerolactone, and other lactone solvents; trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, triethyl phosphate, and other phosphate ester solvents; acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, isobutyronnitrile Examples of suitable solvents include nitrile solvents such as tolyl; sulfur compound solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane; aromatic nitrile solvents such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, and 3-methyl-2-oxazolidinone; and chain ester solvents such as ethyl acetate, butyl acetate, and propyl propionate. These solvents may be used alone or in combination of two or more.

[0045] Among the electrolyte solvents, carbonate solvents such as chain carbonate ester solvents and cyclic carbonate ester solvents, lactone solvents, ether solvents, and chain ester solvents are preferred, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, γ-butyrolactone, and γ-valerolactone are more preferred, and carbonate solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate are even more preferred.

[0046] When a polymer or polymer gel is used instead of the electrolyte solvent, the following methods may be employed: a method in which a solution of an electrolyte salt dissolved in a solvent is dropped onto a polymer film formed by a conventionally known method to impregnate and support the electrolyte salt and non-aqueous solvent; a method in which a polymer and an electrolyte salt are melted and mixed at a temperature equal to or higher than the melting point of the polymer, and then a film is formed, and the film is impregnated with a solvent (these are referred to as gel electrolytes); a method in which a non-aqueous electrolyte solution in which an electrolyte salt has been dissolved in an organic solvent is mixed with a polymer, and then the mixture is formed into a film by a casting method or a coating method, and the organic solvent is volatilized; a method in which a polymer and an electrolyte salt are melted at a temperature equal to or higher than the melting point of the polymer, mixed, and molded (true polymer electrolyte), etc.

[0047] Examples of polymers that can be used in place of the electrolyte solvent include polyethylene oxide (PEO), which is a homopolymer or copolymer of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), polyether polymers such as polypropylene oxide, methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), fluorine-based polymers such as polyvinylidene fluoride (PVdF) and polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof. These polymers may be used alone or in combination of two or more.

[0048] As described above, the nonaqueous electrolyte according to this embodiment essentially comprises a sulfonylimide compound (1) and a branched alkylamide compound or a linear alkylamide compound (a chain alkylamide compound), and optionally contains other components such as an electrolyte salt, an electrolyte solvent, various additives (other than the chain alkylamide compound), etc. The nonaqueous electrolyte can be prepared, for example, by mixing these components in a predetermined composition ratio.

[0049] [Positive electrode] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer, and the positive electrode mixture layer is formed on the positive electrode current collector and is usually formed into a sheet shape.

[0050] Examples of metals used for the positive electrode current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. Among these, aluminum is preferred. The shape and dimensions of the positive electrode current collector are not particularly limited.

[0051] The positive electrode mixture layer is formed from a positive electrode mixture (positive electrode composition) that contains a positive electrode active material, a conductive additive, a binder, a solvent for dispersing these components, and the like.

[0052] The positive electrode according to this embodiment includes a transition metal oxide (high-Ni ternary positive electrode active material) as a positive electrode active material, such as a lithium composite oxide containing nickel (Ni), in which the Ni content is 50% or more relative to the total molar amount of the three transition metals (Ni, cobalt (Co), and manganese (Mn)) contained in the lithium composite oxide. Because the Ni content of the high-Ni ternary positive electrode active material is higher than that of conventional positive electrode active materials (approximately 33% in NCM111), nonaqueous electrolyte secondary batteries using such materials have a high energy density that meets the performance requirements of EV batteries. Thus, the nonaqueous electrolyte secondary battery according to this embodiment is based on the use of a high-Ni ternary positive electrode active material (including a high-Ni positive electrode).

[0053] In the nonaqueous electrolyte secondary battery according to this embodiment, the positive electrode is a nonaqueous electrolyte secondary battery represented by the general formula (2): [Chemical Formula 4] Li v Ni x Co y Mn z O 2+w (0.2 ≤ v ≤ 1.2, 0.5 ≤ x ≤ 0.9, 0 < y ≤ 0.2, 0 < z ≤ 0.4, x + y + z = 1, -0.2 ≤ w ≤ 0.2 (v represents the molar ratio of Li, x represents the molar ratio of Ni, y represents the molar ratio of Co, z represents the molar ratio of Mn, and w(2 + w) represents the molar ratio of O) ··· (2) It contains a high-Ni content ternary cathode active material represented by the following (hereinafter referred to as "high-Ni content ternary cathode active material (2)").

[0054] In the high-Ni content ternary cathode active material (2), the content ratio of Ni ( "x" in the general formula (2)) with respect to the total amount of 100% (100 mol%) based on the molar basis of transition metals is 50% or more (0.5 ≤ x), preferably 55% or more (0.55 ≤ x), more preferably 70% or more (0.7 ≤ x). The upper limit of the content ratio is 90% or less (x ≤ 0.9), preferably less than 85% (x < 0.85), more preferably 80% or less (x ≤ 0.8). In addition, the respective content ratios of components other than Ni in the high-Ni content ternary cathode active material (2) ( "v", "y", "z", "w(2 + w)" in the general formula (2)) may be appropriately adjusted within the ranges of the above respective molar ratios.

[0055] The high-Ni content ternary cathode active material (2) may be used alone or in combination of two or more kinds. In addition, a commercially available product may be used as the high-Ni content ternary cathode active material (2), or one obtained by synthesizing by a conventionally known method may be used. Specific examples of the high-Ni content ternary cathode active material (2) include, for example, LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), etc.

[0056] The positive electrode may contain a positive electrode active material other than the high Ni-containing ternary positive electrode active material. The other positive electrode active material may be any material capable of absorbing and releasing lithium ions, and may be, for example, a positive electrode active material used in a conventionally known secondary battery (lithium ion secondary battery). Examples of other positive electrode active materials include lithium cobalt oxide, lithium nickel oxide, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Layered rock salt type positive electrode active materials including ternary positive electrode active materials other than high Ni-containing ternary positive electrode active materials such as O2 (NCM111); phosphate type positive electrode active materials with an olivine structure such as LiAPO4 (A: Ni, Mn, Co); LiNi p Mn 1-p O2 (0.5≦p≦1); Positive electrode active materials with fluorinated olivine structure such as Li2NiPO4F; LiFePO4, LiFe 0.995 Mn 0.005 Positive electrode active materials with iron phosphates having an olivine structure such as PO4; solid solution materials incorporating multiple transition metals (solid solutions of electrochemically inactive layered Li2MnO3 and electrochemically active layered LiMO2 (M = transition metals such as Co and Ni)); LiCo x Mn 1-q O2 (0≦q≦1); compounds with fluoride olivine structure such as Li2APO4F (A: Fe, Mn, Co); LiMn 2.0 O4, LiNi 0.5 Mn 1.5 Positive electrode active materials having a spinel structure such as O4; sulfur, etc. The other positive electrode active materials may be used alone or in combination of two or more.

[0057] From the viewpoint of improving the output characteristics and electrical characteristics of the secondary battery, the content of the positive electrode active material (total content when multiple positive electrode active materials are included) is preferably 75% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the total amount of components included in the positive electrode composite, and is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 95% by mass or less.

[0058] Conductive additives are used to improve the output of lithium-ion secondary batteries. Conductive carbon is mainly used as the conductive additive. Examples of conductive carbon include carbon black, fibrous carbon (carbon fiber), and graphite. Each conductive additive may be used alone, or two or more types may be used in combination. Among conductive additives, carbon black is preferred. Examples of carbon black include ketjen black and acetylene black. The content of the conductive additive in the non-volatile matter of the positive electrode mixture is preferably 1 to 20 mass %, more preferably 1.5 to 10 mass %, from the viewpoint of improving the output characteristics and electrical characteristics of the lithium-ion secondary battery.

[0059] Examples of binders include fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber (SBR) and nitrile butadiene rubber; polyamide-based resins such as polyamideimide; polyolefin-based resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; and cellulose-based resins such as carboxymethyl cellulose (CMC). Each binder may be used alone, or two or more types may be used in combination. Furthermore, the binder may be dissolved in a solvent or dispersed in a solvent when used.

[0060] Examples of the solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetonitrile, acetone, ethanol, ethyl acetate, and water. Each of the solvents may be used alone, or two or more of them may be used in combination. The amount of the solvent used is not particularly limited and may be determined appropriately depending on the production method and the materials used.

[0061] The positive electrode mixture may contain other components as needed, such as polymers such as non-fluorinated polymers (e.g., (meth)acrylic polymers, nitrile polymers, and diene polymers), and fluorinated polymers (e.g., polytetrafluoroethylene), emulsifiers (e.g., anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers), dispersants (e.g., polymer dispersants (e.g., styrene-maleic acid copolymers and polyvinylpyrrolidone), thickeners (e.g., carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salts), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers), preservatives, etc. The content of other components in the nonvolatile content of the positive electrode mixture is preferably 0 to 15% by mass, more preferably 0 to 10% by mass.

[0062] The positive electrode mixture can be prepared, for example, by mixing a positive electrode active material, a conductive additive, a binder, a solvent, and other components as necessary, and dispersing the mixture using a bead mill, a ball mill, an agitator mixer, or the like.

[0063] The method for forming the positive electrode (coating method) is not particularly limited, and examples thereof include: (1) a method in which a positive electrode composite is applied to a positive electrode current collector by a conventional coating method (e.g., a doctor blade method, etc.) (and then dried); (2) a method in which a positive electrode current collector is immersed in the positive electrode composite (and then dried); (3) a method in which a sheet formed from the positive electrode composite is bonded to a positive electrode current collector (e.g., bonded via a conductive adhesive) and pressed (and then dried); (4) a method in which a positive electrode composite to which a liquid lubricant has been added is applied or cast onto a positive electrode current collector, formed into a desired shape, and then the liquid lubricant is removed (and then stretched in uniaxial or multiaxial directions); and (5) a method in which a positive electrode composite (or a solid content forming a positive electrode composite layer) is slurried with an electrolyte, transferred in a semi-solid state to a current collector (positive electrode current collector), and used as an electrode (positive electrode) without drying.

[0064] The positive electrode mixture layer may be dried or pressed after being formed or coated (applied), as needed.

[0065] [Negative electrode] The negative electrode includes a negative electrode current collector and a negative electrode mixture layer, and the negative electrode mixture layer is formed on the negative electrode current collector and is usually formed into a sheet shape.

[0066] Examples of metals used for the negative electrode current collector include iron, copper, aluminum, nickel, stainless steel (SUS), titanium, tantalum, gold, and platinum. Among these, copper is preferred. The shape and dimensions of the negative electrode current collector are not particularly limited.

[0067] The negative electrode mixture layer is formed from a negative electrode mixture (negative electrode composition) that contains a negative electrode active material, a conductive additive, a binder, a solvent for dispersing these components, and the like.

[0068] The negative electrode active material may be any conventionally known negative electrode active material used in various batteries (e.g., lithium secondary batteries), as long as it is capable of absorbing and releasing various ions (e.g., lithium ions). Specific negative electrode active materials that can be used include graphite materials (graphite) such as artificial graphite and natural graphite, mesophase sintered bodies made from coal and petroleum pitch, carbon materials such as non-graphitizable carbon, Si-based negative electrode materials such as Si, Si alloys, and SiO, Sn-based negative electrode materials such as Sn alloys, lithium metal, and lithium alloys such as lithium-aluminum alloys. The negative electrode active materials may be used alone or in combination of two or more.

[0069] The negative electrode mixture may further contain a conductive additive (conductive substance), a binder, a solvent, etc. The conductive additive, binder, solvent, etc. may be the same components as those described above. The proportions used are also the same as those described above.

[0070] The negative electrode may be manufactured by the same method as the positive electrode.

[0071] (separator) The non-aqueous electrolyte secondary battery may include a separator. The separator is disposed to separate the positive electrode from the negative electrode. There are no particular limitations on the separator, and any conventionally known separator can be used in the present disclosure. Specific examples of the separator include porous sheets made of polymers capable of absorbing and retaining an electrolyte (non-aqueous electrolyte) (e.g., polyolefin-based microporous separators, cellulose-based separators, etc.), nonwoven fabric separators, porous metal bodies, etc.

[0072] Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

[0073] Examples of materials for the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, and glass. Depending on the required mechanical strength, the above-mentioned materials may be used alone or in combination of two or more.

[0074] (battery exterior materials) A battery element including a positive electrode, a negative electrode, and a non-aqueous electrolyte (and a separator) is usually housed in a battery exterior material to protect the battery element from external impacts during battery use, environmental degradation, etc. The material of the battery exterior material is not particularly limited, and any conventionally known exterior material can be used.

[0075] If necessary, the battery exterior may contain expanded metal, an overcurrent prevention element such as a fuse or a PTC element, lead plates, etc. to prevent pressure buildup inside the battery and overcharging and discharging.

[0076] The shape of the battery (lithium ion secondary battery, etc.) is not particularly limited, and any of the conventionally known shapes of batteries (lithium ion secondary batteries, etc.) can be used, such as cylindrical, square, laminated, coin, large, etc. Furthermore, when used as a high-voltage power source (several tens to several hundreds of volts) to be mounted on electric vehicles, hybrid electric vehicles, etc., it can also be made into a battery module consisting of individual batteries connected in series.

[0077] The rated charging voltage of a nonaqueous electrolyte secondary battery (such as a lithium ion secondary battery) is not particularly limited, but when the secondary battery has a positive electrode containing the above-described ternary positive electrode active material as a main component, it may be 3.6 V or higher, preferably 4.0 V or higher, more preferably 4.1 V or higher, and even more preferably 4.2 V or higher. The higher the rated charging voltage, the higher the energy density can be, but from the viewpoint of safety, the rated charging voltage may be 4.6 V or lower (for example, 4.5 V or lower).

[0078] A non-aqueous electrolyte secondary battery can be easily produced, for example, by stacking a positive electrode and a negative electrode (with a separator interposed therebetween as necessary), placing the resulting laminate in a battery exterior material, injecting a non-aqueous electrolyte into the battery exterior material, and sealing the battery exterior material.

[0079] As described above, the nonaqueous electrolyte secondary battery according to this embodiment includes the following constituent materials: a non-aqueous electrolyte solution containing a sulfonylimide compound (1) and a branched alkylamide compound or a linear alkylamide compound (a chain alkylamide compound); The battery is used in combination with a high-Ni positive electrode containing a high-Ni ternary positive electrode active material (2) in which the Ni content in the transition metals is 50 mol% or more (0.5≦x). This nonaqueous electrolyte secondary battery, with the above-mentioned configuration, not only reduces all three types of resistance—initial resistance, resistance associated with battery use, and resistance after high-temperature storage—but also has a high energy density that meets the performance requirements of EV batteries. [Example]

[0080] The present disclosure will be described below based on examples. Note that the present disclosure is not limited to the following examples, and the following examples can be modified or changed based on the spirit of the present disclosure, and such modifications are not excluded from the scope of the present disclosure.

[0081] (1) Preparation of evaluation battery (Non-aqueous electrolyte) Reference electrolytes (Comparative Examples 1 to 3) were prepared by dissolving LiPF (commercially available) and LiFSI (a sulfonylimide compound manufactured by Nippon Shokubai Co., Ltd.) as electrolyte salts at 0.6 mol / L in a mixed solvent (EC / EMC = 30 / 70 (vol%)) containing ethylene carbonate (EC, commercially available) and ethyl methyl carbonate (EMC, commercially available). Isobutylamide (IBA, commercially available) shown in Table 1, which is an amide compound having a branched alkyl group with 3 to 6 carbon atoms (branched alkylamide compound), or n-butylamide (BA, commercially available) shown in Table 1, which is an amide compound having a linear alkyl group with 3 to 6 carbon atoms (linear alkylamide compound), was added and dissolved in the reference electrolytes to the amounts shown in Table 1 (the amount of linear alkylamide compound relative to LiFSI), to prepare "amide compound-containing electrolytes." In the table, branched alkylamide compounds and linear alkylamide compounds are collectively referred to as "linear alkylamide compounds."

[0082] (positive electrode) As a ternary positive electrode active material, "NCM111" (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, manufactured by Umicore), "NCM523" (LiNi 0.5 Co 0.2 Mn 0.3 O2, manufactured by Beijing Toben Co., Ltd.) or "NCM811" (LiNi 0.8 Co 0.1 Mn 0.1O2 (manufactured by Beijing Toben Co., Ltd.), acetylene black (manufactured by Denka, Denka Black), graphite (Nippon Graphite, SP270), and polyvinylidene fluoride (PVdF, #1120, commercially available) were weighed in a mass ratio of 100:3:3:3 and dispersed in N-methyl-2-pyrrolidone (NMP, commercially available) to prepare a slurry. The prepared slurry was coated on one side of aluminum foil (NCM111 with a coating weight of 19.7 mg / cm). 2 , NCM523 has a coating weight of 19.5 mg / cm 2 , NCM811 has a coating weight of 15.7 mg / cm 2 ) and dried, followed by roll pressing to prepare a positive electrode.

[0083] (Negative electrode) An aqueous slurry containing graphite (O-MAC, manufactured by Osaka Gas Chemicals Co., Ltd.), carbon fiber (VGCF, manufactured by Showa Denko K.K.), styrene butadiene rubber (SBR, commercially available), and carboxymethyl cellulose (CMC, commercially available) in a mass ratio of 100:2:1:1 was prepared and applied to one side of a copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to prepare a negative electrode.

[0084] (Evaluation battery) The resulting positive and negative electrodes were cut, and the polarity leads were ultrasonically welded. A 25 μm polyethylene (PE) separator was placed between the electrodes, and the electrodes were sealed on three sides with a laminate exterior. The above electrolyte was poured into the unsealed side, and the battery was vacuum sealed and charged at a constant current of 3 mA at 25°C for 3 hours. The battery was then left at room temperature for 2 days, and one piece of the laminate exterior was cleaved and vacuum sealed again to allow for degassing. After degassing, the battery was charged and discharged under the following conditioning conditions to complete the evaluation battery. (Conditioning conditions) 1st cycle: 3mA, constant current / constant voltage charge at 4.2V, terminated at 0.3mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 2nd cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 3rd cycle charging: Constant current and constant voltage charging at 15 mA and 4.2 V, termination at 0.6 mA ⇒ Discharge: Discharge at 30 mA, termination at 2.75 V. 4th cycle charging: Constant current and constant voltage charging at 15 mA and 4.2 V, termination at 0.6 mA ⇒ Discharge: Discharge at 60 mA, termination at 2.75 V.

[0085] (2) Characteristics evaluation of the evaluation battery Using the evaluation battery obtained in (1) above, the reduction rates of DCR (direct current resistance) and impedance (charge transfer resistance) were evaluated by the following method. The results are shown in Table 1.

[0086] <Reduction rate of DCR> (Initial DCR) · The evaluation battery was subjected to constant current and constant voltage charging at 30 mA (1C), 4.2 V, and termination at 0.6 mA using a charge-discharge test device to obtain a fully charged state. · The DCR (DCR before the cycle test) was measured at 25 °C from the fully charged state. For the DCR measurement, after waiting for 30 minutes after the completion of full charge, a discharge was performed at 6 mA (0.2C) for 10 seconds. Subsequently, after waiting for 30 minutes, a discharge was performed at 30 mA (1C) for 10 seconds. Finally, after waiting for 30 minutes, a discharge was performed at 90 mA (3C) for 10 seconds. An I-V straight line was created from the relationship between the difference in voltage and current immediately before the start of discharge and 10 seconds later at each discharge current, and the slope was calculated as the DCR (initial DCR). · The reduction rate of the initial DCR was calculated using the following formula (1) with an evaluation battery having the same positive electrode active material as the comparison target: [Equation 1] Reduction rate of initial DCR (%) = (Initial DCR of "electrolyte containing amide compound") / (Initial DCR of "reference electrolyte") × 100 ··· (1) The smaller the reduction rate of the initial DCR, the more the initial DCR of the battery decreases.

[0087] (DCR after 200 cycle tests) After measuring the initial DCR, the battery was subjected to 200 cycles of 45°C cycling. The cycle conditions were: charge: 4.2V, 30mA (1C), terminated at 0.6mA (0.05C), 10-minute rest; discharge: 30mA (1C), terminated at 2.75V, 10-minute rest. After the cycling test, the "DCR after 200 cycles" was measured and calculated at 25°C in the same manner as above. The DCR reduction rate after 200 cycles was calculated in the same way as above, except that "initial DCR" was replaced with "DCR after 200 cycles" in formula (1). The smaller the DCR reduction rate after 200 cycles, the more the DCR decreases with battery use.

[0088] (DCR after 28 days at 60°C (high temperature storage)) After measuring the initial DCR, the battery was fully charged in the same manner as above, stored at 60°C for 28 days, and then left to stand at 25°C for 4 hours. The "DCR after 28 days at 60°C" was then measured and calculated in the same manner as above. The DCR decrease rate after 28 days at 60°C was calculated in the same manner as above, except that "initial DCR" was replaced with "DCR after 28 days at 60°C" in formula (1). The smaller the DCR decrease rate after 28 days at 60°C, the greater the decrease in the DCR of the battery after high-temperature storage.

[0089] <Impedance reduction rate> (initial impedance) The test battery was fully charged in the same manner as above. Next, the impedance of this evaluation battery was measured at 25°C using an impedance analyzer (Bio Logic, model number: VSP-300) over a frequency range from 1 GHz to 1 mHz. The real axis resistance (interface resistance) was calculated from the frequency at which the arc of the measured values ​​diverged. The frequency at which the arc diverged is the frequency between 1 kHz and 0.001 Hz at which the imaginary axis value reached a minimum. Specifically, the real axis resistance at which the imaginary axis resistance became zero was defined as the bulk resistance, and the value obtained by subtracting the bulk resistance from the real axis resistance at which the imaginary axis resistance reached its maximum in the low-frequency range below 1 kHz was defined as the "initial impedance." The rate of decrease in initial impedance was calculated in the same manner as above, except that "initial DCR" was replaced with "initial impedance" in formula (1). The smaller the rate of decrease in initial impedance, the greater the decrease in the initial impedance of the battery.

[0090] (Impedance after 200 cycles) After the initial impedance measurement, the battery was subjected to 200 cycles of a 45°C cycle test in the same manner as above. After the cycle test, the "impedance after 200 cycles test" was measured at 25°C in the same manner as above. The impedance reduction rate after 200 cycles was calculated in the same manner as above, except that in formula (1) "initial DCR" was replaced with "impedance after 200 cycles." The smaller the impedance reduction rate (%) after 200 cycles, the more the impedance decreases with battery use.

[0091] [Table 1]

[0092] (Summary of Examples) From Table 1, it was confirmed that in nonaqueous electrolyte secondary batteries having a high Ni-based positive electrode containing a “high Ni-containing ternary positive electrode active material” (NCM523, NCM811) in which the Ni content ratio among the transition metals is 50 mol % or more, each Example having an “amide compound-containing electrolyte” containing a branched alkylamide compound or a linear alkylamide compound together with sulfonylimide compound (1) exhibited a decrease in all three types of resistance, i.e., initial resistance, resistance associated with battery use, and resistance after high-temperature storage, compared to each Comparative Example (with the same positive electrode) having a “reference electrolyte” not containing a chain alkylamide compound. On the other hand, in nonaqueous electrolyte secondary batteries equipped with a low-Ni-based positive electrode containing a low-Ni-containing ternary positive electrode active material (NCM111) in which the Ni content in the transition metals is less than 50 mol%, it was confirmed that Comparative Examples 4 to 13 equipped with an "amide compound-containing electrolyte" exhibited increased resistances for all three types of resistance compared to Comparative Example 3 equipped with a "reference electrolyte" (same positive electrode). The chain alkylamide compound added to the nonaqueous electrolyte is believed to be the cause of this. In other words, in nonaqueous electrolyte secondary batteries equipped with a low-Ni-based positive electrode, the chain alkylamide compound added to the nonaqueous electrolyte is believed to actually reduce battery performance.

Claims

1. a non-aqueous electrolyte solution containing a sulfonylimide compound represented by general formula (1) and an amide compound having a branched alkyl group having 3 to 6 carbon atoms; and a positive electrode containing a positive electrode active material represented by general formula (2). LiN (RSO ２ ) (FSO ２ ) (R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms) ... (1) Li ｖ Ni ｘ Co ｙ Mn ｚ O ２＋ｗ (0.2≦v≦1.2, 0.5≦x≦0.9, 0<y≦0.2, 0<z≦0.4, x+y+z=1, −0.2≦w≦0.2 (v represents the molar ratio of Li, x represents the molar ratio of Ni, y represents the molar ratio of Co, z represents the molar ratio of Mn, and w represents the molar ratio of O)) (2)

2. a non-aqueous electrolyte solution containing a sulfonylimide compound represented by general formula (1) and an amide compound having a linear alkyl group having 3 to 6 carbon atoms; and a positive electrode containing a positive electrode active material represented by general formula (2). LiN (RSO ２ ) (FSO ２ ) (R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms) ... (1) Li ｖ Ni ｘ Co ｙ Mn ｚ O ２＋ｗ (0.2≦v≦1.2, 0.5≦x≦0.9, 0<y≦0.2, 0<z≦0.4, x+y+z=1, −0.2≦w≦0.2 (v represents the molar ratio of Li, x represents the molar ratio of Ni, y represents the molar ratio of Co, z represents the molar ratio of Mn, and w represents the molar ratio of O)) (2)

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the amide compound relative to the sulfonylimide compound is 10 ppm by mass or more.

4. The sulfonylimide compound is LiN(FSO ２ ) ２ 3. The nonaqueous electrolyte secondary battery according to claim 1, further comprising:

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