Non-aqueous electrolyte secondary battery

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 a nitrile 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

Non-aqueous electrolyte secondary battery

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

[0002] In order to improve the battery performance of secondary batteries such as lithium ion secondary batteries, various non-aqueous electrolyte solutions and materials therefor have been investigated. The present applicant has found through his previous 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 method for manufacturing a lithium nitride battery using LiN(FSO ２ ) ２ and at least one additive selected from the group consisting 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. This sulfonylimide compound-containing nonaqueous electrolyte suppresses battery self-discharge by using a specific compound, and reduces charge transfer resistance (impedance) and battery direct current resistance (DCR), thereby improving battery performance.

[0004] International Publication No. 2022 / 239807

[0005] By the way, a non-aqueous electrolyte containing a sulfonylimide compound and a LiNi, which is generally used as a positive electrode active material, １／３ Co １／３ Mn １／３ O ２ In a secondary battery having a positive electrode containing a lithium composite oxide containing Ni (nickel), such as (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 non-aqueous electrolyte may cause a decrease in battery performance.

[0006] Furthermore, conventional secondary batteries using NCM111 have difficulty meeting the energy density requirements of batteries for recent electric vehicles (EVs), 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, and therefore require a technology to reduce the resistance.

[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 predetermined 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.

[0009] As a result of intensive research to achieve the above object, the present inventors have found that all three types of resistance of a secondary battery can be reduced by using, as an additive to a non-aqueous electrolyte containing a sulfonylimide compound, a nitrile compound having a branched or linear alkyl group with a carbon number within a specific range, and, as a Ni-rich lithium composite oxide (cathode active material), a "Ni-rich ternary cathode active material" in which the Ni content of the three transition metals, Ni, Co (cobalt), and Mn (manganese), is 50% or more on a molar basis.

[0010] The nonaqueous electrolyte secondary battery of the present disclosure comprises a nonaqueous electrolyte solution containing a cation exchange material having a general formula (1): 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) and a nitrile compound having a branched alkyl group having 3 to 6 carbon atoms; and a non-aqueous electrolyte solution containing a sulfonylimide compound represented by the following 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 (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)

[0011] The nonaqueous electrolyte secondary battery of the present disclosure is characterized by comprising: a nonaqueous electrolyte containing a sulfonylimide compound represented by the general formula (1) above and a nitrile compound having a linear alkyl group having 3 to 6 carbon atoms; and a positive electrode containing a positive electrode active material represented by the general formula (2) above.

[0012] In the nonaqueous electrolyte secondary battery of the present disclosure, the content of the nitrile compound relative to the sulfonylimide compound may be 10 mass ppm or more. ２ ) ２ may also include:

[0013] According to the present disclosure, it is possible 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.

[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] <Non-aqueous electrolyte secondary battery> A non-aqueous electrolyte secondary battery is a secondary battery that includes a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery according to this embodiment includes a non-aqueous electrolyte, a positive electrode, and a negative electrode.

[0016] [Non-aqueous Electrolyte] (Electrolyte Salt) The non-aqueous electrolyte according to this embodiment contains an electrolyte salt represented by the general formula (1): [Chemical Formula 1] LiN(RSO ２ ) (FSO ２ The non-aqueous electrolyte solution contains a sulfonylimide compound (1) as an essential component (fluorine-containing sulfonylimide salt) represented by the following formula:

[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 alkyl groups having 1 to 6 carbon atoms include methyl, ethyl, propyl, isopropyl, butyl, pentyl, and hexyl. Among alkyl groups having 1 to 6 carbon atoms, linear or branched alkyl groups having 1 to 6 carbon atoms are preferred, and linear alkyl groups having 1 to 6 carbon atoms are 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(FSO ２ ) ２, LiFSI), lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide, lithium (fluorosulfonyl) (heptafluoropropylsulfonyl) imide, etc. 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 one obtained by synthesis by a conventionally known method.

[0022] Among the sulfonylimide compounds (1), LiN(FSO ２ ) ２ , lithium(fluorosulfonyl)(trifluoromethylsulfonyl)imide and lithium(fluorosulfonyl)(pentafluoroethylsulfonyl)imide are preferred, LiN(FSO ２ ) ２ In other words, in the non-aqueous electrolyte, LiN(FSO ２ ) ２ Preferably, it contains:

[0023] The concentration (content, total content when two or more types 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 the sulfonylimide compound (1) in the non-aqueous electrolyte solution 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 solution. The upper limit of the content is 100 mol%. That is, the electrolyte salt contained in the non-aqueous electrolyte solution may contain the 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. 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 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 lithium (ions) in sulfonylimide compound (1) are substituted with cations other than lithium ions). Examples of salts in which a cation other than lithium ions is substituted include alkali metal salts such as sodium salts, potassium salts, rubidium salts, and cesium salts; alkaline earth metal salts such as beryllium salts, magnesium salts, calcium salts, strontium salts, and barium salts; aluminum salts; ammonium salts; and phosphonium salts. The other sulfonylimide compounds may be used alone or in combination of two or more. Furthermore, commercially available products may be used as the other sulfonylimide compounds, or compounds synthesized by conventional methods may be used.

[0028] Examples of the non-imide salt include salts of non-imide anions and cations (lithium ions and the cations exemplified above). 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 the general formula (4): [Chemical formula 3] LiBF ｂ (C ｎ F ２ｎ+１ ) ４－ｂ (b: 0≦b≦4, n: 1≦n≦4) (4) (hereinafter referred to as "fluoroborate compound (4)"), lithium hexafluoroarsenate (LiAsF ６ ), LiSbF ６ , LiClO ４ , LiSCN, LiAlF ４ , C.F. ３ SO ３ Li, LiC [(CF ３ SO ２ ) ３ ], LiN(NO ２ ), LiN[(CN)２ Examples of the non-lithium salt include salts in which the lithium (ion) in these lithium salts is replaced with the cations listed above (for example, NaBF ４ , NaPF ６ , NaPF ３ (CF ３ ) ３ The non-imide salts may be used alone or in combination of two or more. In addition, the non-imide salts may be commercially available products or may be synthesized by a conventional method.

[0029] Among other electrolytes, non-imide salts are preferred from the viewpoints of ionic conductivity, cost, etc., and fluorophosphate compounds (3), fluoroboric acid compounds (4), and LiAsF ６ is preferred, and the fluorophosphate compound (3) is more preferred.

[0030] The fluorophosphate compound (3) is LiPF ６ , LiPF ３ (CF ３ ) ３ , LiPF ３ (C ２ F ５ ) ３ , LiPF ３ (C ３ F ７ ) ３ , LiPF ３ (C ４ F ９ ) ３ Among the fluorophosphate compounds (3), LiPF ６ and LiPF ３ (C ２ F ５ ) ３ is preferred, and LiPF ６ is more preferred.

[0031] The fluoroboric acid compound (4) is LiBF ４ , LiBF(CF ３ ) ３ , LiBF(C ２ F ５ ) ３ , LiBF(C３ F ７ ) ３ Among the fluoroboric acid compounds (4), LiBF ４ , and LiBF(CF ３ ) ３ is preferred, and LiBF ４ 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 the sulfonylimide compound (1), or an electrolyte salt having a mixed salt composition containing the 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 the sulfonylimide compound (1) and the fluorophosphate compound (3) is preferred, and LiN(FSO ２ ) ２ and LiPF ６ An electrolyte salt having a mixed salt composition containing the following is more preferred.

[0034] When an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and other electrolytes is used, 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, still 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] (Nitrile Compound Having a Chain Alkyl Group of 3 to 6 Carbons) The nonaqueous electrolyte solution according to this embodiment contains, as an additive, a nitrile compound having a branched alkyl group of 3 to 6 carbon atoms (hereinafter also referred to as a "branched alkyl nitrile compound") or a nitrile compound having a linear alkyl group of 3 to 6 carbon atoms (hereinafter also referred to as a "linear alkyl nitrile compound") as an essential component. The "linear alkyl nitrile compound" is distinguished from the "branched alkyl nitrile compound" in that the linear alkyl group having 3 to 6 carbon atoms does not have a branched structure. The "branched alkyl nitrile compound" and the "linear alkyl nitrile compound" may be used alone or in combination of two or more types. The "branched alkyl nitrile compound" and the "linear alkyl nitrile compound" are collectively referred to as the "linear alkyl nitrile compound."

[0038] Examples of branched alkyl nitrile compounds include mononitrile compounds such as isobutyronitrile (isopropyl cyanide) and isovaleronitrile (isobutyl cyanide). The branched alkyl nitrile compounds may be used alone or in combination of two or more. Among these, isobutyronitrile is preferred from the viewpoint of improving battery performance (particularly reducing resistance).

[0039] Examples of linear alkyl nitrile compounds include mononitrile compounds such as butyronitrile (propyl cyanide) and valeronitrile (butyl cyanide). The linear alkyl nitrile compounds may be used alone or in combination of two or more. Among these, butyronitrile is preferred from the viewpoint of improving battery performance (particularly reducing resistance).

[0040] From the viewpoint of improving battery performance (particularly reducing resistance), the content of the linear alkyl nitrile 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 linear alkyl nitrile 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 of the sulfonylimide compound (1) to the linear alkyl nitrile compound within the above range, the resistance of a secondary battery including this nonaqueous electrolyte can be reduced.

[0041] (Additives) In addition to the linear alkylnitrile compound, the non-aqueous electrolyte may contain additives for improving various properties of the lithium ion secondary battery. The additives 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; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, and trimethylene glycol sulfate. Sulfur-containing compounds such as esters; 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 erythritan carbonate; sulfamic acid (amidosulfuric acid, H ３ NSO ３ ); sulfamates (alkali metal salts such as lithium salts, sodium salts, potassium salts, etc.; alkaline earth metal salts such as calcium salts, strontium salts, barium salts, etc.; other metal salts such as manganese salts, copper salts, zinc salts, iron salts, cobalt salts, nickel salts, etc.; ammonium salts; guanidine salts, etc.); lithium fluorosulfonate (LiFSO ３ ), sodium fluorosulfonate (NaFSO ３ ), potassium fluorosulfonate (KFSO ３ ), magnesium fluorosulfonate (Mg(FSO ３ ) ２ fluorosulfonic acid compounds such as lithium monofluorophosphate (Li ２ P.O. ３ F), lithium difluorophosphate (LiPO ２ F ２and fluorooxalato compounds such as lithium salts having an oxalic acid skeleton, such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorooxalatophosphate (LIDFOP), lithium tetrafluorooxalatophosphate (LITFOP), lithium difluorobis(oxalato)phosphate (LiDFBOP), and lithium tris(oxalato)phosphate. 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 the 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 having a high dielectric constant, a high solubility for the electrolyte, a boiling point of 60° C. or higher, and a wide electrochemical stability range. An organic solvent with a low water content is more preferable. 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 erythritan 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, and the like. fluorine-containing cyclic carbonate solvents such as methyl benzoate and trifluoropropylene carbonate; aromatic carboxylic acid ester solvents such as methyl benzoate and ethyl benzoate; lactone solvents such as γ-butyrolactone, γ-valerolactone and δ-valerolactone; phosphate ester solvents such as trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate and triethyl phosphate; nitrile solvents such as acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile and 2-methylglutaronitrile tolyl-based solvents; sulfur compound-based solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane; aromatic nitrile-based 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-based 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 formed into a film by a conventionally known method, thereby impregnating and supporting 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, which is then impregnated with a solvent (these are referred to as gel electrolytes); a method in which a non-aqueous electrolyte in which an electrolyte salt has been dissolved in an organic solvent is mixed with a polymer, which is then 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 polyether polymers such as polyethylene oxide (PEO), which is a homopolymer or copolymer of an epoxy compound (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), polypropylene oxide, etc., 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 the sulfonylimide compound (1) and a branched alkylnitrile compound or a linear alkylnitrile compound (a chain alkylnitrile compound), and optionally contains other components such as an electrolyte salt, an electrolyte solvent, various additives (other than the chain alkylnitrile 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, as a positive electrode active material, a transition metal oxide (high-Ni-containing ternary positive electrode active material), such as a lithium composite oxide containing nickel (Ni), in which the Ni content is 50% or more relative to 100% of the total molar amount of the three transition metals Ni, cobalt (Co), and manganese (Mn) contained in the lithium composite oxide. Because the high-Ni-containing ternary positive electrode active material has a higher Ni content in the transition metals than conventional positive electrode active materials (approximately 33% in NCM111), nonaqueous electrolyte secondary batteries using such materials have a high energy density that can meet 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-containing ternary positive electrode active material (including a high-Ni-based positive electrode).

[0053] In the nonaqueous electrolyte secondary battery according to this embodiment, the positive electrode is a nonaqueous electrolyte secondary battery according to the general formula (2): ｖ Ni ｘ Co ｙ Mn ｚ O２＋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)

[0054] In the high Ni-containing ternary positive electrode active material (2), the content ratio of Ni ("x" in general formula (2)) relative to the total amount of transition metals (100% by mole) is 50% or more (0.5≦x), preferably 55% or more (0.55≦x), and more preferably 70% or more (0.7≦x). The upper limit of this content ratio is 90% or less (x≦0.9), preferably less than 85% (x<0.85), and more preferably 80% or less (x≦0.8). The content ratios of the components other than Ni in the high Ni-containing ternary positive electrode active material (2) ("v", "y", "z", and "w" (2+w) in general formula (2)) may be appropriately adjusted within the ranges of the above molar ratios.

[0055] The high Ni-containing ternary positive electrode active material (2) may be used alone or in combination of two or more kinds. The high Ni-containing ternary positive electrode active material (2) may be a commercially available product or may be synthesized by a conventionally known method. Specific examples of the high Ni-containing ternary positive electrode active material (2) include, for example, LiNi ０．５ Co ０．２ Mn ０．３ O ２ (NCM523), LiNi ０．６ Co ０．２ Mn ０．２ O ２ (NCM622), LiNi ０．８ Co ０．１ Mn ０．１ O ２ (NCM811) and the like.

[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 the other positive electrode active material include lithium cobalt oxide, lithium nickel oxide, LiNi １／３ Co １／３ Mn １／３ O ２ 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 (NCM111); LiAPO ４ (A: Ni, Mn, Co) or other phosphate-based positive electrode active materials having an olivine structure; LiNi ｐ Mn １－ｐ O ２ (0.5≦p≦1);Li ２ NiPO ４ Positive electrode active material having a fluorinated olivine structure such as F: LiFePO ４ , LiFe ０．９９５ Mn ０．００５ P.O. ４ Iron phosphate-based positive electrode active materials with an olivine structure such as; solid solution materials incorporating multiple transition metals (electrochemically inactive layered Li ２ MnO ３ and electrochemically active layered LiMO ２ (M=solid solution with transition metal such as Co, Ni); LiCo ｘ Mn １－ｑ O ２ (0≦q≦1);Li ２ APO ４ Compounds having a fluorinated olivine structure such as F (A: Fe, Mn, Co); LiMn ２．０ O ４ , LiNi ０．５ Mn １．５ O ４ 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 (the total content when a plurality of positive electrode active materials is 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 the 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. From the viewpoint of improving the output characteristics and electrical characteristics of lithium-ion secondary batteries, the content of the conductive additive in the non-volatile content of the positive electrode composite is preferably 1 to 20 mass %, and more preferably 1.5 to 10 mass %.

[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 necessary, such as (meth)acrylic polymers, nitrile polymers, non-fluorinated polymers such as diene polymers, polymers such as fluorinated polymers such as polytetrafluoroethylene, emulsifiers such as anionic emulsifiers, nonionic emulsifiers, cationic emulsifiers, etc.; dispersants such as styrene-maleic acid copolymers and polymer dispersants such as polyvinylpyrrolidone, thickeners such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers, preservatives, etc. The content of other components in the non-volatile 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 a 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 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 nonaqueous electrolyte secondary battery may include a separator. The separator is disposed to separate the positive electrode and 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 (nonaqueous 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 and the like, the above-exemplified materials may be used alone or in combination of two or more.

[0074] (Battery Exterior Material) 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 deterioration, 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, prismatic, laminated, coin, large, etc. Furthermore, when used as a high-voltage power source (several tens of volts to several hundreds of volts) to be mounted on electric vehicles, hybrid electric vehicles, etc., a battery module can be formed by connecting individual batteries 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, etc., 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 uses, as its constituent materials, a nonaqueous electrolyte containing a sulfonylimide compound (1) and a branched alkylnitrile compound or a linear alkylnitrile compound (a chain alkylnitrile compound), and a high-Ni-based positive electrode containing a high-Ni-containing ternary positive electrode active material (2) in which the Ni content in the transition metal is 50 mol % or more (0.5≦x). This nonaqueous electrolyte secondary battery, due to the above-mentioned configuration, not only reduces all three types of resistance, i.e., initial resistance, resistance associated with battery use, and resistance after high-temperature storage, but also has a high energy density that can meet the performance requirements of EV batteries.

[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) A mixed solvent (EC / EMC = 30 / 70 (vol%)) containing ethylene carbonate (EC, commercially available) and ethyl methyl carbonate (EMC, commercially available) was dissolved in LiPF as an electrolyte salt. ６ (commercially available product) and LiFSI (manufactured by Nippon Shokubai, sulfonylimide compound) were each dissolved to a concentration of 0.6 mol / L to prepare reference electrolytes (Comparative Examples 1 to 3). To the reference electrolyte, isobutyronitrile (IBN, commercially available product) shown in Table 1 as a nitrile compound having a branched alkyl group having 3 to 6 carbon atoms (branched alkyl nitrile compound) or butyronitrile (BN, commercially available product) shown in Table 1 as a nitrile compound having a linear alkyl group having 3 to 6 prime numbers (linear alkyl nitrile compound) was added and dissolved to the content shown in Table 1 (the content of linear alkyl nitrile compound relative to LiFSI), to prepare a "nitrile compound-containing electrolyte." In the table, branched alkyl nitrile compounds and linear alkyl nitrile compounds are collectively referred to as "linear alkyl nitrile compounds."

[0082] (Positive electrode) As a ternary positive electrode active material, "NCM111" (LiNi １／３ Co １／３ Mn１／３ O ２ , manufactured by Umicore), "NCM523" (LiNi ０．５ Co ０．２ Mn ０．３ O ２ , manufactured by Beijing Toben Co., Ltd.) or "NCM811" (LiNi ０．８ Co ０．１ Mn ０．１ O ２ , manufactured by Beijing Toben Co., Ltd.), acetylene black (Denka Black, manufactured by Denka), graphite (SP270, manufactured by Nippon Graphite Co., Ltd.), and polyvinylidene fluoride (PVdF, #1120, commercially available product) were weighed in a mass ratio of 100:3:3:3 and dispersed in N-methyl-2-pyrrolidone (NMP, commercially available product) to prepare a slurry. The prepared slurry was coated on one side of an aluminum foil (NCM111: coating weight 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 After drying, the mixture was roll-pressed 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): carboxymethyl cellulose (CMC, commercially available) in a mass ratio of 100:2:1:1 was prepared and coated on 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. They were then placed facing each other with a 25 μm polyethylene (PE) separator and sealed on three sides with a laminate exterior. The above electrolyte was poured into the unsealed side, 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 degas the battery. After degassing, the battery was charged and discharged under the following conditioning conditions to complete the evaluation battery. (Conditioning Conditions) 1st Cycle: Charge: 3 mA, constant current / constant voltage charge at 4.2 V, 0.3 mA termination ⇒ Discharge: 6 mA discharge, 2.75 V termination. 2nd Cycle: Charge: 15 mA, constant current / constant voltage charge at 4.2 V, 0.6 mA termination ⇒ Discharge: 6 mA discharge, 2.75 V termination. 3rd cycle: Charge: 15mA, 4.2V constant current / constant voltage charge, terminated at 0.6mA ⇒ Discharge: 30mA discharge, terminated at 2.75V. 4th cycle: Charge: 15mA, 4.2V constant current / constant voltage charge, terminated at 0.6mA ⇒ Discharge: 60mA discharge, terminated at 2.75V.

[0085] (2) Evaluation of Characteristics of 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] <DCR Decrease Rate> (Initial DCR) Using a charge / discharge tester, the evaluation battery was charged at a constant current and constant voltage of 30 mA (1 C), 4.2 V, and a 0.6 mA cutoff, to a fully charged state. The DCR (DCR before cycle testing) was measured at 25°C from the fully charged state. For DCR measurement, the battery was waited 30 minutes after full charge and then discharged at 6 mA (0.2 C) for 10 seconds. After waiting 30 minutes, the battery was discharged at 30 mA (1 C) for 10 seconds. Finally, after waiting 30 minutes, the battery was discharged at 90 mA (3 C) for 10 seconds. An IV line was created from the relationship between the difference in voltage immediately before and 10 seconds after the start of discharge at each discharge current and the current, and the slope of the line was calculated as the DCR (initial DCR). The reduction rate of the initial DCR was calculated using an evaluation battery having the same positive electrode active material as a comparison battery according to the following formula (1): [Mathematical Formula 1] Reduction rate of initial DCR (%) = (initial DCR of "nitrile compound-containing electrolyte solution") / (initial DCR of "reference electrolyte solution") × 100 (1) A smaller reduction rate of the initial DCR means a greater reduction in the initial DCR of the battery.

[0087] (DCR after 200 cycle test) - After measuring the initial DCR, the battery was subjected to 200 cycles of a 45°C cycle test. The cycle conditions were: charge: 4.2 V, 30 mA (1 C), terminated at 0.6 mA (0.05 C), and rested for 10 minutes; discharge: 30 mA (1 C), terminated at 2.75 V, and rested for 10 minutes. After the cycle test, the "DCR after 200 cycle test" was measured and calculated at 25°C in the same manner as above. - The rate of decrease in DCR after 200 cycle test was calculated in the same manner as above, except that in formula (1), "initial DCR" was replaced with "DCR after 200 cycle test." A smaller rate of decrease in DCR after 200 cycle test indicates a greater decrease in DCR with battery use.

[0088] (DCR after 28 days at 60°C (after high-temperature storage)) - The battery after initial DCR measurement 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, and then the "DCR after 28 days at 60°C" was measured and calculated at 25°C in the same manner as above. - The rate of decrease in DCR after 28 days at 60°C was calculated in the same manner as above, except that in formula (1) "initial DCR" was changed to "DCR after 28 days at 60°C". A smaller rate of decrease in DCR after 28 days at 60°C means that the DCR of the battery after high-temperature storage is more reduced.

[0089] <Impedance Decrease Rate> (Initial Impedance) The evaluation battery was fully charged using the same method as above. Subsequently, the evaluation battery was subjected to impedance measurement at frequencies from 1 GHz to 1 mHz at 25°C using an impedance analyzer (manufactured by Bio Logic, product number: VSP-300). The real axis resistance (interface resistance) was calculated from the frequency at which the arc of the obtained measured value diverged. The frequency at which the arc diverged refers to the frequency at which the imaginary axis value reached a minimum between 1 kHz and 0.001 Hz. 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 was maximized in the low-frequency region below 1 kHz was defined as the "initial impedance." The initial impedance decrease rate was calculated in the same manner as in the above formula (1), except that "initial DCR" was replaced with "initial impedance." A smaller decrease rate of initial impedance indicates a greater decrease in the initial impedance of the battery.

[0090] (Impedance after 200 cycle test) 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 cycle test" was measured at 25°C in the same manner as above. The rate of decrease in impedance after 200 cycle test was calculated in the same manner as above, except that in formula (1), "initial DCR" was changed to "impedance after 200 cycle test." A smaller rate of decrease (%) in impedance after 200 cycle test indicates a greater decrease in impedance with battery use.

[0091]

[0092] (Summary of Examples) From Table 1, it was confirmed that in nonaqueous electrolyte secondary batteries equipped with a high Ni-based positive electrode containing a "high Ni-containing ternary positive electrode active material" (NCM523, NCM811) in which the Ni content in the transition metal is 50 mol % or more, each Example equipped with a "nitrile compound-containing electrolyte" containing a branched alkyl nitrile compound or a linear alkyl nitrile compound together with sulfonylimide compound (1) exhibited reduced three types of resistance, i.e., initial resistance, resistance associated with battery use, and resistance after high-temperature storage, compared to each Comparative Example (having the same positive electrode) equipped with a "reference electrolyte" not containing a chain alkyl nitrile 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 in Comparative Examples 4 to 13 equipped with a "nitrile compound-containing electrolyte," all of the above three types of resistance were equivalent (not decreased) or increased, compared to Comparative Example 3 equipped with a "reference electrolyte" (same positive electrode). This is thought to be due to the chain alkyl nitrile compound added to the nonaqueous electrolyte. That is, in nonaqueous electrolyte secondary batteries equipped with a low Ni-based positive electrode, it is thought that the chain alkyl nitrile compound added to the nonaqueous electrolyte does not exhibit its added effect or, conversely, reduces battery performance.

Claims

1. a non-aqueous electrolyte solution containing a sulfonylimide compound represented by general formula (1) and a nitrile 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 a nitrile 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 nitrile 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: