WO2025182300A1 - Non-aqueous electrolytic solution and lithium ion secondary battery - Google Patents

Abstract

This non-aqueous electrolytic solution for a lithium ion secondary battery comprises a compound represented by formula (1), a cyclic sulfonic acid ester, a compound represented by formula (2), and a non-aqueous solvent.　(1): LiN(R1SO2)(R2SO2) [In formula (1), R1 and R2 each independently represent a fluorine atom or an alkyl group which has 1-6 carbon atoms and which may be substituted with a fluorine atom.]　(2): FSO2NHR3 [In formula (2), R3 represents a hydrogen atom or a hydrocarbon group which has 1-18 carbon atoms and which may be substituted.]

Description

Nonaqueous electrolyte and lithium ion secondary battery

The present invention relates to a non-aqueous electrolyte and a lithium-ion secondary battery.

Lithium-ion secondary batteries are used as power sources for smartphones, personal computers, and other electronic devices, as well as for automobiles. Research into the batteries used in these applications is being conducted with the aim of improving various battery characteristics, such as higher output, higher energy density, cycle characteristics, and rate characteristics.

Various additives have been investigated to improve battery characteristics. For example, Patent Documents 1 to 3 each propose adding a cyclic sulfonate ester, such as a cyclic disulfonate ester, a cyclic compound containing an —SO ₂ — bond, or a γ-sultone compound, as an additive to a non-aqueous electrolyte solution to improve battery characteristics.

JP 2012-094454 A Special Publication No. 2021-532531 Japanese Patent Application Laid-Open No. 2000-235866

However, as a result of extensive research by the present inventors, it became clear that conventional lithium-ion secondary batteries containing cyclic sulfonate esters as additives suffer from problems such as a large increase in resistance during high-temperature storage and cycle charge/discharge, resulting in significant degradation of battery performance.

The present disclosure therefore aims to provide a nonaqueous electrolyte solution containing a cyclic sulfonic acid ester that can sufficiently suppress resistance increases during high-temperature storage and cyclic charge/discharge, and a lithium-ion secondary battery equipped with the same.

The present disclosure relates to, for example, the following [1] to [11].
[1] A non-aqueous electrolyte solution for a lithium ion secondary battery, comprising a compound represented by the following formula (1), a cyclic sulfonic acid ester, a compound represented by the following formula (2), and a non-aqueous solvent:
LiN(R ¹ SO ₂ ) (R ² SO ₂ )...(1)
[In formula (1), R ¹ and R ² each independently represent a fluorine atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom.]
FSO ₂ NHR ³ …(2)
[In formula (2), ^R3 represents a hydrogen atom or an optionally substituted hydrocarbon group having 1 to 18 carbon atoms.]
[2] The nonaqueous electrolyte solution according to [1], wherein the compound represented by the formula (1) includes at least one selected from the group consisting of LiN(FSO ₂ ) ₂ and LiN(CF ₃ SO ₂ ) ₂ .
[3] The nonaqueous electrolyte solution according to [1] or [2], wherein the compound represented by the formula (1) includes LiN(FSO ₂ ) ₂ .
[4] The nonaqueous electrolyte solution according to any one of [1] to [3], wherein the cyclic sulfonate ester comprises at least one selected from the group consisting of a compound represented by the following formula (3) and a compound represented by the following formula (4):
[In formulas (3) and (4), R ⁴ to R ⁶ each independently represent a divalent saturated hydrocarbon group having 1 to 10 carbon atoms or a divalent unsaturated hydrocarbon group having 2 to 10 carbon atoms.]
[5] The nonaqueous electrolyte solution according to [4], wherein the cyclic sulfonate ester contains a compound represented by the formula (3).
[6] The nonaqueous electrolyte solution according to [5], wherein the cyclic sulfonate ester comprises a compound represented by the following formula (3-1):
[In formula (3-1), R ¹¹ to R ¹⁶ each independently represent a hydrogen atom or a methyl group.]
[7] The nonaqueous electrolyte solution according to [6], wherein in formula (3-1), at least one of R ¹¹ to R ¹⁶ is a methyl group.
[8] The nonaqueous electrolyte solution according to [5], wherein the cyclic sulfonate ester comprises a compound represented by the following formula (3-2):
[In formula (3-2), R ¹⁷ to R ²⁴ each independently represent a hydrogen atom or a methyl group. The total number of carbon atoms contained in R ¹⁷ to R ²⁴ is 0 to 6.]
[9] The non-aqueous electrolyte solution according to any one of [1] to [8], wherein the content of the cyclic sulfonate ester in the total amount of the non-aqueous electrolyte solution is 0.05% by mass or more and 5% by mass or less.
[10] The non-aqueous electrolyte solution according to any one of [1] to [9], wherein the content of the compound represented by formula (2) is 0.005 mass% or more and 15 mass% or less in the total amount of the non-aqueous electrolyte solution.
[11] The nonaqueous electrolyte solution according to any one of [1] to [10],
a positive electrode having a positive electrode mixture layer and a positive electrode current collector;
a negative electrode having a negative electrode mixture layer and a negative electrode current collector,
The positive electrode comprises at least one positive electrode active material selected from the group consisting of a positive electrode active material represented by the following formula (A) and a positive electrode active material represented by the following formula (B):
Li _v Ni _x Co _{y Mnz} O _(2+w) …(A)
[In formula (A), 0.2≦v≦1.2, 0.6≦x≦0.9, 0<y≦0.3, 0<z<0.4, x+y+z=1, and −0.2≦w≦0.2.]
LiMPO ₄ ...(B)
[In formula (B), M represents Ni, Mn, Co, or Fe.]

The nonaqueous electrolyte solution disclosed herein and the lithium-ion secondary battery containing it can sufficiently suppress resistance increases during high-temperature storage and cyclic charge/discharge. Furthermore, these nonaqueous electrolyte solutions and lithium-ion secondary batteries improve self-discharge during high-temperature storage and also improve low-temperature discharge capacity after high-temperature storage.

Below, modes for implementing the inventions included in this disclosure are explained, but the inventions included in this disclosure are not limited to the following embodiments. Note that when a numerical range is indicated as X to Y, it means X or more and Y or less. Furthermore, unless otherwise specified, the materials, components, or methods exemplified in this specification can be used alone or in combination of two or more.

<Non-aqueous electrolyte for lithium-ion secondary batteries>
One embodiment of the present disclosure is a nonaqueous electrolyte solution for a lithium ion secondary battery (hereinafter also simply referred to as "nonaqueous electrolyte solution") containing a compound represented by formula (1) described below, a cyclic sulfonic acid ester, a compound represented by formula (2) described below, and a nonaqueous solvent.

<Compound represented by formula (1)>
The non-aqueous electrolyte solution according to this embodiment contains a compound represented by the following formula (1): Hereinafter, the compound represented by the following formula (1) will also be referred to as the "compound of formula (1)."
LiN(R ¹ SO ₂ ) (R ² SO ₂ )...(1)

In formula (1), ^R1 and ^R2 each independently represent a fluorine atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom. The number of carbon atoms in the alkyl group may be 1 to 6, 1 to 5, 1 to 4, 1 to 3, 1 to 2, or 1. The number of fluorine atoms substituted in the alkyl group may be, for example, 1 to 13, 1 to 9, 1 to 8, 1 to 5, 1 to 3, 1 to 2, or 1. The alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom may be a perfluoroalkyl group having 1 to 6 carbon atoms.

For example, in formula (1), ^R1 and ^R2 may each independently represent a fluorine atom or a perfluoroalkyl group having 1 to 6 carbon atoms, may each independently represent a fluorine atom, trifluoromethyl, or pentafluoroethyl, may each independently represent a fluorine atom, trifluoromethyl, or a fluorine atom. The compound in which ^R1 and ^R2 in formula (1) are fluorine atoms is lithium bis(fluorosulfonyl)imide, also known as LiFSI.

The compound of formula (1) preferably contains at least one selected from the group consisting of LiN(FSO ₂ ) ₂ and LiN(CF ₃ SO ₂ ) ₂ , and more preferably contains LiN(FSO ₂ ) ₂ .

The content of the compound of formula (1) in the nonaqueous electrolyte may be, for example, 0.01 mol/L or more and 10.0 mol/L or less, 0.05 mol/L or more and 4.00 mol/L or less, 0.10 mol/L or more and 2.00 mol/L or less, 0.15 mol/L or more and 1.20 mol/L or less, 0.20 mol/L or more and 1.00 mol/L or less, or 0.40 mol/L or more and 0.80 mol/L or less. When the molar concentration of the compound of formula (1) is within the above range, self-discharge during high-temperature storage is less likely to occur.

<Cyclic sulfonate ester>
The non-aqueous electrolyte according to this embodiment contains a cyclic sulfonate ester, which is a compound having at least one sulfonate ester moiety (—S(═O) ₂ O—), such as compounds represented by the following formulas (3) to (7):

In formulas (3) to (7), R ⁴ to R ¹⁰ each independently represent a divalent saturated hydrocarbon group having 1 to 10 carbon atoms or a divalent unsaturated hydrocarbon group having 2 to 10 carbon atoms.

The compounds represented by formulas (3) to (7) may be, for example, cyclic sulfonic acid esters having a 5- to 10-membered ring, a 5- to 8-membered ring, or a 5- to 6-membered ring.

The divalent saturated hydrocarbon group having 1 to 10 carbon atoms may be a linear or branched saturated hydrocarbon group. The number of carbon atoms in the saturated hydrocarbon group may be, for example, 1 to 8, 1 to 5, 1 to 4, 1 to 3, 1 to 2, or 1.

Specific examples of divalent saturated hydrocarbon groups having 1 to 10 carbon atoms include methylene, ethylene, 1,2-propylene, n-propylene, 1,2-butylene, 1,3-butylene, n-butylene, 2,3-butylene, n-pentylene, and n-hexylene.

The divalent unsaturated hydrocarbon group having 2 to 10 carbon atoms may be a linear or branched unsaturated hydrocarbon group. The number of carbon atoms in the unsaturated hydrocarbon group may be, for example, 2 to 8, 2 to 5, 2 to 4, or 2 to 3.

Specific examples of divalent unsaturated hydrocarbon groups having 2 to 10 carbon atoms include vinylene groups, propenylene groups, and methylvinylene groups.

Specific examples of compounds represented by formula (3) include 1,3-propane sultone (PS), 2-methyl-1,3-propane sultone (2Me-PS), 1,3-butane sultone (1,3-BS), 2,4-butane sultone (2,4-BS), 1,4-butane sultone (1,4-BS), 1-propene-1,3-sultone (PRS), and 3H-1,2-oxathiol-2,2-dioxide.

Specific examples of compounds represented by formula (4) include methylenemethane disulfonate (MMDS), dimethylenemethane disulfonate, trimethylenemethane disulfonate, etc.

Specific examples of compounds represented by formula (5) include 1,2,4-oxadithietane-2,2,4,4-tetraoxide, 1,2,5-oxadithiane-2,2,5,5-tetraoxide, 1,2,5-oxadithiol-2,2,5,5-tetraoxide, and 1,2,6-oxadithiane-2,2,6,6-tetraoxide.

Specific examples of compounds represented by formula (6) include 1,2,4-oxadithiolane-2,2,4,4-tetraoxide and 1,2,4-oxadithiane-2,2,4,4-tetraoxide.

Specific examples of compounds represented by formula (7) include 1,3,2,4-dioxadithiolane-2,2,4,4-tetraoxide, 1,3,2,4-dioxadithiane-2,2,4,4-tetraoxide, and 1,3,2,4-dioxadithiin-2,2,4,4-tetraoxide.

The cyclic sulfonate ester preferably contains at least one compound selected from the group consisting of compounds represented by formula (3) and compounds represented by formula (4), and more preferably contains a compound represented by formula (3).

It is further preferable that the cyclic sulfonate ester contains a compound represented by the following formula (3-1):

In formula (3-1), R ¹¹ to R ¹⁶ each independently represent a hydrogen atom or a methyl group. Preferably, at least one of R ¹¹ to R ¹⁶ is a methyl group, and more preferably, at least one of R ¹¹ R ¹³ R ¹⁵ is a methyl group.

It is also more preferable that the cyclic sulfonate ester includes a compound represented by the following formula (3-2):

In formula (3-2), R ¹⁷ to R ²⁴ each independently represent a hydrogen atom or a methyl group, and the total number of carbon atoms contained in R ¹⁷ to R ²⁴ is 0 to 6.

The content of the cyclic sulfonate ester in the non-aqueous electrolyte may be 0.005% by mass or more and 15% by mass or less, 0.01% by mass or more and 10% by mass or less, or 0.05% by mass or more and 5% by mass or less. When the content of the cyclic sulfonate ester is within the above range, self-discharge during high-temperature storage is less likely to occur.

<Compound represented by formula (2)>
The non-aqueous electrolyte solution according to this embodiment contains a compound represented by the following formula (2): Hereinafter, the compound represented by the following formula (2) will also be referred to as the "compound of formula (2)."
FSO ₂ NHR ³ …(2)

In formula (2), ^R3 represents a hydrogen atom or an optionally substituted hydrocarbon group having 1 to 18 carbon atoms. The hydrocarbon group may be an aliphatic hydrocarbon group or an aromatic hydrocarbon group, or may be an aliphatic hydrocarbon group. The hydrocarbon group may be a chain or cyclic hydrocarbon group, or may be a chain hydrocarbon group, or may be a chain aliphatic hydrocarbon group. The chain hydrocarbon group may be a linear or branched hydrocarbon group, or may be a linear hydrocarbon group, or may be a linear aliphatic hydrocarbon group. The hydrocarbon group may be a saturated hydrocarbon group or an unsaturated hydrocarbon group, or may be a saturated hydrocarbon group. The number of carbon atoms in the hydrocarbon group may be, for example, 1 to 18, 1 to 8, 1 to 5, 1 to 4, 1 to 3, 1 to 2, or 1.

In formula (2), the number of substituents on the hydrocarbon group in R ³ may be, for example, 1 to 18, 1 to 6, 1 to 4, 1 to 3, 1 to 2, or 1.

In formula (2), the substituents of the hydrocarbon group in ^R3 may each independently be, for example, a group selected from the group consisting of a halogen atom, a hydroxy group, a nitro group, a cyano group, an aryl group, an alkoxy group, an acyl group, an alkoxycarbonyl group, and a carbamoyl group.

^R3 may be a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms (i.e., a hydrocarbon group having no substituents). ^R3 may be a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or an alkenyl group having 2 to 18 carbon atoms. ^R3 may be a hydrogen atom or an alkyl group having 1 to 18, 1 to 8, 1 to 5, 1 to 4, 1 to 3, 1 to ² , or 1 carbon atom. ^R3 may be a hydrogen atom, methyl, ethyl, propyl, isopropyl, cyclopropyl, vinyl, or allyl, or may be a hydrogen atom, methyl, or ethyl, or may be a hydrogen atom or _methyl . A compound of formula (2) in which ^R3 is a hydrogen atom is represented by the chemical formula _FSO2NH2 . A compound of formula (2) in _which R3 is methyl is represented by _the chemical formula _FSO2NHCH3 . That is, the compound of formula (2) may _{be FSO2NH2} or _FSO2NHCH3 . The compound of formula (2) containing one or two hydrogen atoms as substituents on the nitrogen atom can suppress self-discharge during high-temperature storage to a greater extent than a compound that does not contain a hydrogen atom as a substituent on the nitrogen atom.

The content of the compound of formula (2) in the non-aqueous electrolyte may be, for example, 0.005% by mass or more and 15% by mass or less, 0.01% by mass or more and 10% by mass or less, 0.05% by mass or more and 5% by mass or less, 0.05% by mass or more and 3% by mass or less, or 0.1% by mass or more and 1% by mass or less. When the content of the compound of formula (2) is within the above range, self-discharge during high-temperature storage is less likely to occur.

The content of the compound of formula (2) in the non-aqueous electrolyte may be, for example, 1 molar part or more and 10,000 molar parts or less, 5 molar parts or more and 5,000 molar parts or less, 10 molar parts or more and 1,000 molar parts or less, 20 molar parts or more and 500 molar parts or less, or 50 molar parts or more and 200 molar parts or less, relative to 100 molar parts of the compound of formula (1).

<Non-aqueous solvent>
The non-aqueous solvent is an organic solvent that is not water. The non-aqueous solvent may be one that is commonly used by those skilled in the art as a solvent for electrolytes in secondary batteries. The non-aqueous solvent may be, for example, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, butyl propionate, isopropyl propionate, propyl propionate, ethyl acetate, methyl acetate, propyl acetate, or isopropyl acetate. The non-aqueous solvent may contain at least one solvent selected from the group consisting of 90% by volume or more of the total amount of non-aqueous solvent contained in the non-aqueous electrolyte. When these solvents are contained in a proportion of 90% by volume or more, storage stability at high temperatures is improved. From the viewpoint of further improving storage stability at high temperatures, these solvents may be contained in a proportion of 95% by volume or more, or even 100% by volume.

Furthermore, from the viewpoint of further improving storage stability at high temperatures, the nonaqueous solvent may contain at least one solvent selected from the group consisting of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, fluoroethylene carbonate, methyl propionate, and dimethyl carbonate in a proportion of 90% by volume or more, 95% by volume or more, or even 100% by volume of the total amount of nonaqueous solvent contained in the nonaqueous electrolyte.

The solvent contained in the non-aqueous electrolyte may also contain other organic solvents. Specific examples thereof include saturated cyclic carbonate (carbonate ester) solvents such as 2,3-dimethylethylene carbonate, 1,2-butylene carbonate, and erythritan carbonate; chain carbonate (carbonate ester) solvents such as diphenyl carbonate and methyl phenyl carbonate; 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; cyclic carbonate (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; fluorine-containing cyclic carbonate (carbonate ester) solvents such as fluoroethylene carbonate, 4,5-difluoroethylene carbonate, and trifluoropropylene carbonate; methyl benzoate, ethyl benzoate, and the like. aromatic carboxylic acid ester solvents such as butyl; lactone solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone; phosphate ester solvents such as trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, triethyl phosphate; nitrile solvents such as acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, isobutyronitrile; dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone Examples of suitable solvents include sulfur compound solvents such as benzene, sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane; aromatic nitrile solvents such as benzonitrile and tolunitrile; aromatic solvents such as toluene, amylbenzene, cyclohexylbenzene, fluorobenzene, anisole, 2,4-difluoroanisole, and trifluoromethoxybenzene; and nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, and 3-methyl-2-oxazolidinone. The solvent in the nonaqueous electrolyte does not need to contain water.

<Fluorine-containing lithium salt>
The non-aqueous electrolyte preferably further contains, in addition to the compound of formula (1), a fluorine-containing lithium salt other than the compound of formula (1).

The fluorine-containing lithium salt is a salt composed of an anion having a fluorine atom and a lithium ion. Specific examples of fluorine-containing lithium salts include at least one selected _{from the group consisting of LiPF6, LiBF4, LiPO2F2, FSO3Li , lithium difluorooxalatoborate ( LiBF2 ( C2O4 ) ,} LiDFOB), lithium difluorooxalatophosphate ( _LiPF2 ( _C2O4 ), _LiDFOP ), _{( SO2CF2CF2SO2} )NLi, _{( SO2CF2CF2CF2SO2} ) _NLi , _{LiFSO2 ( CH3SO2 ) N} , _LiFSO2 ( _C2F5SO2 ) _{N , LiFSO2 (C2H5SO2 ) N} , and _LiAsF6 . Note that (SO ₂ CF ₂ CF ₂ SO ₂ )NLi and (SO ₂ CF ₂ CF ₂ CF ₂ SO ₂ )NLi are salts formed from a cyclic sulfonimide anion and a lithium ion.

Among these, the fluorine-containing lithium salt may be at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , LiPO ₂ F ₂ , FSO ₃ Li, LiBF ₂ (C ₂ O ₄ ), and LiPF ₂ (C ₂ O ₄ ), or may be LiPF ₆ .

The content of the fluorine-containing lithium salt in the non-aqueous electrolyte may be, for example, 0.01 mol/L to 10.0 mol/L, 0.05 mol/L to 4.00 mol/L, 0.10 mol/L to 2.00 mol/L, 0.15 mol/L to 1.20 mol/L, 0.20 mol/L to 1.00 mol/L, or 0.40 mol/L to 0.80 mol/L. When the molar concentration of the fluorine-containing lithium salt is within the above range, self-discharge during high-temperature storage is less likely to occur.

<Other additives>
The non-aqueous electrolyte may further contain other additives in addition to the components described above. For example, the non-aqueous electrolyte may contain at least one additive selected from the group consisting of unsaturated cyclic carbonates, nitrile compounds, ester compounds, and fluorine-containing lithium salts.

The unsaturated cyclic carbonate may be, for example, at least one selected from the group consisting of vinylene carbonate (vinylene carbonate, VC), methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate. For example, the unsaturated cyclic carbonate may be vinylene carbonate.

A nitrile compound is a compound having a cyano group in its molecule. The nitrile compound of the present invention may be, for example, a compound having one cyano group in its molecule (mononitrile compound), a compound having two cyano groups in its molecule (dinitrile compound), or a compound having three or more cyano groups in its molecule. The nitrile compound of the present invention is preferably a compound having two or more cyano groups in its molecule. A compound having two or more cyano groups in its molecule may be a dinitrile compound or a compound having three or more cyano groups in its molecule.

Dinitrile compounds are compounds that have two cyano groups in the molecule. Examples of dinitrile compounds include succinonitrile, malononitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelanitrile, sebaconitrile, undecanedinitrile, dodecanedinitrile, methylmalononitrile, ethylmalononitrile, isopropylmalononitrile, tert-butylmalononitrile, methylsuccinonitrile, 2,2-dimethylsuccinonitrile, and 2,3-dimethylsuccinonitrile. , 2,3,3-trimethylsuccinonitrile, 2,2,3,3-tetramethylsuccinonitrile, 2,3-diethyl-2,3-dimethylsuccinonitrile, 2,2-diethyl-3,3-dimethylsuccinonitrile, bicyclohexyl-1,1-dicarbonitrile, bicyclohexyl-2,2-dicarbonitrile, bicyclohexyl-3,3-dicarbonitrile, 2,5-dimethyl-2,5-hexanedicarbonitrile, 2,3-diisobutyl-2,3-dimethylsuccinonitrile succinonitrile, 2,2-diisobutyl-3,3-dimethylsuccinonitrile, 2-methylglutaronitrile, 2,3-dimethylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,3,3-tetramethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 2,2,3,4-tetramethylglutaronitrile, 2,3,3,4-tetramethylglutaronitrile, maleonitrile, fumaronitrile, 1,4-dicyanopentane, 2,6 The dinitrile compound may be at least one selected from the group consisting of dicyanoheptane, 2,7-dicyanooctane, 2,8-dicyanononane, 1,6-dicyanodecane, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, 3,3'-(ethylenedioxy)dipropionitrile, 3,3'-(ethylenedithio)dipropionitrile, and 3,9-bis(2-cyanoethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane. The dinitrile compound may be at least one selected from the group consisting of succinonitrile, malononitrile, glutaronitrile, adiponitrile, pimelonitrile, and suberonitrile. For example, the dinitrile compound may be succinonitrile.

The compound having three or more cyano groups in the molecule may be, for example, at least one selected from the group consisting of 1,2,3-propanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,2,3-tris(2-cyanoethoxy)propane, tris(2-cyanoethyl)amine, 1,3,5-cyclohexanetricarbonitrile, 1,3,5-cyclohexanetricyanobenzene, tris(2-cyanoethyl)amine, tris(2-cyanoethyl)phosphine, 7,7,8,8-tetracyanoquinodimethane, 2,5-dimethyl-7,7,7,8-tetracyanoquinodimethane, 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane, and 1,2,3,4-butanetetracarbonitrile.

The mononitrile compound may be at least one selected from the group consisting of, for example, acetonitrile, propionitrile, butyronitrile, pentanenitrile, hexanenitrile, heptanenitrile, octanenitrile, pelargononitrile, decanenitrile, undecanenitrile, dodecanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, acrylonitrile, methacrylonitrile, crotononitrile, 3-methylcrotononitrile, 2-methyl-2-butenenitrile, 2-pentenenitrile, 2-methyl-2-pentenenitrile, 3-methyl-2-pentenenitrile, and 2-hexenenitrile.

The ester compound is a compound having an ester bond in the molecule. The ester compound may be a carbonate ester compound. The carbonate ester compound is a compound having a divalent group represented by -O-C(=O)-O- in the molecule. The carbonate ester compound according to this embodiment may be a compound having two divalent groups represented by -O-C(=O)-O- in the molecule. For example, the ester compound may be a compound represented by the following formula (8):
[In formula (8), R ¹¹ and R ¹² each independently represent an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an alkynyl group having 2 to 6 carbon atoms which may have a substituent. R ¹³ represents an alkylene group having 1 to 6 carbon atoms which may have a substituent, an alkenylene group having 2 to 6 carbon atoms which may have a substituent, an alkynylene group having 2 to 6 carbon atoms which may have a substituent, or a bridged ring which may have a substituent. The substituent represents a halogen atom or an alkyl group.]
For example, the ester compound may be dimethyl 2,5-dioxahexanedioate.

As described above, the non-aqueous electrolyte may further contain at least one additive selected from the group consisting of vinylene carbonate, succinonitrile, and dimethyl 2,5-dioxahexanedioate. The non-aqueous electrolyte may also contain additives other than those described above. Examples of such 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, methyl methanesulfonate, busulfan, sulfolene, tetramethylthiuram monosulfide, and trimethylene glycol sulfate; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, and N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, and cycloheptane; sulfamic acid (amidosulfuric acid, H ₃ NSO ₃ sulfamates (e.g., 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, etc.); fluorosulfonic acid compounds such as sodium fluorosulfonate ( _NaFSO3 ), potassium fluorosulfonate ( _KFSO3 ), and magnesium fluorosulfonate (Mg( _FSO3 ) ₂ ).

When the non-aqueous electrolyte contains at least one additive selected from the group consisting of unsaturated cyclic carbonates, nitrile compounds, and ester compounds, the total content of these additives may be 0.1% by mass or more and 10% by mass or less, or 0.3% by mass or more and 5% by mass or less. When the content of these additives is 0.1% by mass or more, the effects derived from the additives tend to be more easily obtained, and when the content of these additives is 10% by mass or less, an increase in the viscosity of the non-aqueous electrolyte tends to be suppressed.

The non-aqueous electrolyte may have at least one dissolved therein selected from the group consisting of carbon dioxide (CO ₂ ), carbon monoxide (CO), bicarbonate ions (HCO ₃ ⁻ ) and carbonate ions (CO ₃ ²⁻ ).

When at least one selected from the group consisting of carbon dioxide (CO ₂ ), carbon monoxide (CO), bicarbonate ion (HCO ₃ ⁻ ) and carbonate ion (CO ₃ ²⁻ ) is dissolved in the non-aqueous electrolyte, the total dissolved amount of at least one selected from the group consisting of carbon dioxide (CO ₂ ), carbon monoxide (CO), bicarbonate ion (HCO ₃ ⁻ ) and carbonate ion (CO ₃ ²⁻ ) in the non-aqueous electrolyte may be 20 ppm by mass or more, 100 ppm by mass or more, or 250 ppm by mass or more, and may be equal to or less than the saturated dissolved amount at 25°C.

<Lithium-ion secondary battery>
Another aspect of the present invention is a lithium ion secondary battery including: a nonaqueous electrolyte solution according to one embodiment of the present invention; a positive electrode having a positive electrode composite layer and a positive electrode current collector; and a negative electrode having a negative electrode composite layer and a negative electrode current collector.

<Positive electrode>
The positive electrode of the lithium ion secondary battery according to this embodiment may be one in which a positive electrode mixture layer is formed on a positive electrode current collector.

The positive electrode of the lithium ion secondary battery according to one embodiment includes at least one positive electrode active material selected from the group consisting of a positive electrode active material represented by the following formula (A) and a positive electrode active material represented by the following formula (B):
Li _v Ni _x Co _{y Mnz} O _(2+w) …(A)
LiMPO ₄ ...(B)

In formula (A), 0.2≦v≦1.2, 0.6≦x≦0.9, 0<y≦0.3, 0<z<0.4, x+y+z=1, and -0.2≦w≦0.2.

In formula (A), v is preferably 0.5 or greater and 1.2 or less, more preferably 0.8 or greater and 1.1 or less, and even more preferably 1.

In formula (A), w is preferably -0.1 or greater and 0.1 or less, and more preferably 0.

_The positive _electrode active material represented _by formula (A _{) is preferably LiNi0.6Co0.2Mn0.2O2 , LiNi0.7Co0.2Mn0.1O2 , or LiNi0.8Co0.1Mn0.1O2 , and more preferably LiNi0.6Co0.2Mn0.2O2 or LiNi0.8Co0.1Mn0.1O2 .}

In formula (B), M represents Ni, Mn, Co, or Fe. In other words, in formula (B), M represents a transition metal, and the transition metal is selected from the group consisting of Ni (nickel), Mn (manganese), Co (cobalt), and Fe (iron).

The positive electrode active material represented by formula (B) may be LiFePO ₄ , LiNiPO ₄ , LiMnPO ₄ or LiCoPO ₄ .

From the viewpoint of improving the output characteristics and electrical characteristics of the lithium-ion secondary battery according to this embodiment, the content of the positive electrode active material in the positive electrode composite layer is preferably 75% by mass or more and 99% by mass or less, and more preferably 85% by mass or more and 95% by mass or less.

The positive electrode mixture layer may further contain a conductive additive. Examples of conductive additives include carbon black such as ketjen black and acetylene black, carbon fiber, and graphite, with acetylene black and graphite being preferred.

From the viewpoint of improving the output characteristics and electrical characteristics of the lithium-ion secondary battery according to this embodiment, the content of the conductive additive in the positive electrode mixture layer is preferably 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

The positive electrode mixture layer may further contain a binder. Examples of binders include fluororesins such as polyvinylidene fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide resins such as polyamideimide; polyolefin resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; and cellulose resins such as carboxymethyl cellulose; with polyvinylidene fluoride being preferred.

The content of the binder in the positive electrode mixture layer is preferably 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

The positive electrode mixture layer may further contain other components as necessary. Examples of other components include polymers such as non-fluorinated polymers such as (meth)acrylic polymers, nitrile polymers, and diene polymers, and fluorinated polymers such as polytetrafluoroethylene; emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymer dispersants such as styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salts), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers; preservatives, etc.

The content of the other components in the positive electrode may be 0% by mass or more and 15% by mass or less, or 0% by mass or more and 10% by mass or less.

Examples of the positive electrode current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum, with aluminum being preferred.

The positive electrode is not particularly limited and can be manufactured by known methods. For example, it can be manufactured by dispersing the positive electrode active material, conductive additive, and binder in a solvent to form a slurry, applying it to a positive electrode current collector, drying it, and then performing roll pressing.

Examples of the solvent include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetonitrile, acetone, ethanol, ethyl acetate, and water, with N-methylpyrrolidone being preferred.

<Negative electrode>
The negative electrode of the lithium ion secondary battery according to this embodiment may be one in which a negative electrode mixture layer is formed on a negative electrode current collector.

The negative electrode composite layer may contain, as the negative electrode active material, graphite such as artificial graphite or natural graphite, mesophase sintered bodies made from coal or 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, and preferably contains graphite.

The content of the negative electrode active material in the negative electrode composite layer is preferably 80% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less.

The negative electrode composite layer may further contain a conductive additive. The conductive additive may be the same as that in the positive electrode composite layer, and is preferably carbon fiber. The content of the conductive additive in the negative electrode composite layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

The negative electrode mixture layer may further contain a binder. The binder may be the same as that used in the positive electrode mixture layer, and is preferably styrene-butadiene rubber or carboxymethyl cellulose. The content of the binder in the negative electrode mixture layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

The negative electrode composite layer may further contain other components as necessary. These other components may be the same as those in the positive electrode composite layer. The content of these other components in the negative electrode composite layer may be the same as that in the positive electrode composite layer.

The negative electrode current collector may be the same as the positive electrode current collector, and is preferably copper.

The negative electrode is not particularly limited and can be manufactured by known methods. For example, it may be manufactured in the same manner as the positive electrode. In this case, water is preferred as the solvent.

<Separator>
The lithium ion secondary battery according to this embodiment may include a separator. The separator is disposed to separate the positive electrode from the negative electrode. Examples of the separator include a porous sheet made of a polymer capable of absorbing and retaining a non-aqueous electrolyte (e.g., a polyolefin-based microporous separator, a cellulose-based separator, etc.), a nonwoven fabric separator, a porous metal body, etc. Examples of materials for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene/polyethylene/polypropylene. Examples of materials for the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, etc. A porous sheet made of polyethylene is preferred as the separator.

<Battery exterior materials>
The lithium ion secondary battery according to this embodiment may be housed in a battery exterior material. The material of the battery exterior material is not particularly limited, and any conventionally known exterior material may be used. If necessary, the battery exterior material may contain an overcurrent prevention element such as an expanded metal, a fuse, or a PTC element, a lead plate, or the like, to prevent pressure buildup within the battery and overcharging and discharging.

The shape of the lithium ion secondary battery according to this embodiment is not particularly limited and can be any known shape, such as cylindrical, rectangular, laminated, coin-shaped, large, etc.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples.

(Preparation of Positive Electrode)
A ternary positive _electrode active material _{, LiNi0.8Co0.1Mn0.1O2} (manufactured by Beijing Dangsheng), acetylene black (AB, manufactured by Denka Co., Ltd., product name: Denka Black (registered trademark)), graphite (manufactured by _Nippon Graphite Industries Co., Ltd., product number: SP270), and polyvinylidene fluoride (PVdF, manufactured by Kureha Corporation, product number: KF1120) were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry (positive electrode active material: AB: graphite: PVdF = 93:3:3:3 (mass ratio of solids)). Next, the obtained positive electrode mixture slurry was applied to one side of an aluminum foil (positive electrode current collector, manufactured by Nippon Foil Co., Ltd., thickness 15 μm) using an applicator so that the coating weight after drying would be 19.4 mg/ ^cm² , and the resultant was dried on a hot plate at 110°C for 10 minutes. The resultant was further dried in a vacuum drying furnace at 110°C for 12 hours. Thereafter, the resultant was pressure-molded using a roll press to a density of 3.1 g/ ^cm³ , thereby obtaining a sheet-like positive electrode.

(Preparation of negative electrode)
Graphite (natural graphite (O-MAC)) as a negative electrode active material, carbon fiber (VGCF) as a conductive additive, and styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders were dispersed in ultrapure water to prepare a negative electrode composite slurry (negative electrode active material: conductive additive: SBR: CMC = 100: 2: 1: 1 (solid content mass ratio)). Subsequently, the obtained negative electrode composite slurry was applied to a copper foil (negative electrode current collector, manufactured by Fukuda Metal Foil and Powder Co., Ltd., thickness 15 μm) with an applicator to a single-sided coating weight of 10.8 mg / cm ² after drying, and dried on a hot plate at 80 ° C. for 10 minutes. Further, it was dried for 12 hours in a vacuum drying oven at 100 ° C. Then, it was pressure-molded using a roll press until the density became 1.2 g / cm ³ , thereby obtaining a sheet-shaped negative electrode (thickness 88 μm).

(Preparation of non-aqueous electrolyte)
A nonaqueous electrolyte solution (hereinafter simply referred to as "electrolyte solution") was prepared by dissolving an electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd., the same hereinafter) and LiPF ₆ (manufactured by Kishida Chemical Co., Ltd., the same hereinafter), or an electrolyte salt having a simple salt composition containing only LiPF ₆ , as well as a first additive and a second additive shown below, in a mixed solvent (manufactured by Kishida Chemical Co., Ltd., the same hereinafter) having an ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) composition as an electrolyte solution solvent, to the concentrations shown in Tables 1 to 6, respectively.

The first additives used were 1,3-propane sultone (PS) (Tokyo Chemical Industry Co., Ltd.), 1,3-butane sultone (1,3-BS) (Chemexpress), 2-methyl-1,3-propane sultone (AccelaChemBio), 2,4-butane sultone (2,4-BS) (Angene International), 1,4-butane sultone (1,4-BS) (BLD Pharmatech), 1-propene-1,3-sultone (PRS) (Tokyo Chemical Industry Co., Ltd.), and methylenemethane disulfonate (MMDS) (Tokyo Chemical Industry Co., Ltd.).

As the second additive, FSO ₂ NH ₂ (manufactured by Nippon Shokubai Co., Ltd.) or FSO ₂ NHCH ₃ (manufactured by Enamine Co., Ltd.) was used.

(Fabrication of Laminated Battery)
The prepared positive electrode was cut to an effective area of 12 ^cm2 , and a polarity lead was welded to the cut positive electrode using an ultrasonic welder. The prepared negative electrode was cut to an effective area of 13.44 ^cm2 , and a polarity lead was welded to the cut negative electrode using ultrasonic welding. These positive and negative electrodes were placed opposite each other with a 25 μm thick polyethylene separator in between, and the three sides were sealed with a laminate exterior to prepare an unfilled battery. Next, 700 μL of each electrolyte solution listed in Tables 1 to 6 was added to one of the unsealed sides of the unfilled battery. After the electrolyte was poured, the battery was vacuum sealed to prepare a 4.2 V, 30 mAh capacity laminate battery (cell). The obtained cell was charged at a constant current of 0.1 C (3 mA) for 3 hours at room temperature (25°C, hereinafter) using a charge/discharge tester (ASKA Electronics Co., Ltd., product number: ACD-01, hereinafter the same) and then left at room temperature for 36 hours. After leaving the cell, it was charged at a constant current/constant voltage (CCCV) of 4.2 V and 0.5 C (15 mA) for 5 hours at room temperature. Thereafter, it was discharged at a constant current of 0.2 C (6 mA) and a cutoff voltage of 2.75 V at room temperature, and the excess laminate was cleaved and vacuum sealed to degas the cell. After degassing, the cell was further charged at a constant current/constant voltage under the same conditions as above, and then discharged at a constant current of 1 C (30 mA) and a cutoff voltage of 2.75 V at room temperature. This was the aging process for the cell.

[Battery evaluation]
The self-discharge after high-temperature storage (capacity retention rate after high-temperature storage), the resistance increase rate after high-temperature storage, the low-temperature characteristics after high-temperature storage (low-temperature charge-discharge characteristics), and the charge-discharge cycle characteristics (resistance increase rate after cycle test) were evaluated by the methods described below. The results are shown in Tables 1 to 6.

(Self-discharge: Capacity retention rate after high-temperature storage)
The aged cell was charged at 4.2 V, 0.5 C (15 mA) by constant current constant voltage (CCCV) at room temperature. Then, a constant current discharge of 0.1 C (3 mA) at room temperature with a cut-off voltage of 2.75 V was performed. The discharge capacity at this time was designated as "discharge capacity before high-temperature storage." The discharged cell was again charged at 4.2 V, 0.5 C (15 mA) by constant current constant voltage (CCCV) at room temperature to a fully charged state. After storage at 80°C for 15 days, a constant current discharge of 0.1 C (3 mA) at room temperature with a cut-off voltage of 2.75 V was performed. The discharge capacity at this time was designated as "discharge capacity after high-temperature storage." The capacity retention rate after high-temperature storage was calculated using the following formula (2):
Capacity retention rate after high-temperature storage (%)=(discharge capacity after high-temperature storage/discharge capacity before high-temperature storage)×100 (2)
The larger the capacity retention rate after high-temperature storage, the more the self-discharge of the battery is suppressed.

(Resistance increase rate after high-temperature storage)
The aged cell was subjected to constant current/constant voltage charging at room temperature at 1C (30mA) and 4.2V with a termination of 0.02C (0.6mA) to obtain a fully charged state (SOC 100%). Subsequently, after leaving the cell for 30 minutes from the fully charged state (SOC 100%), it was discharged at 6mA for 10 seconds, then left for 30 minutes, and then discharged at 30mA for 10 seconds. After leaving the cell for another 30 minutes, it was discharged at 90mA for 10 seconds. Each discharge current was plotted on the horizontal axis, and the difference (ΔV) in closed-circuit voltage at the start of discharge and after 10 seconds at each discharge current was plotted on the vertical axis. The slope of the IV line was taken as the "initial DCR" of the cell. After the self-discharge evaluation described above, the same operation was performed again to obtain the DCR after high-temperature storage. The resistance increase rate after high-temperature storage was calculated using the following formula (3):
Resistance increase rate after high-temperature storage (%)=(DCR after high-temperature storage/initial DCR)×100 (3)
The smaller the DCR after high-temperature storage, the more the increase in the resistance of the battery is suppressed.

(Low-temperature discharge capacity after high-temperature storage: low-temperature charge-discharge characteristics)
After high-temperature storage and DCR measurement, the cell was discharged at 0.2 C (6 mA) to 2.75 V at 25 ° C., and then subjected to constant current/constant voltage charging at 1 C (30 mA) with a 0.02 C (0.6 mA) cutoff at 25 ° C. to 4.2 V. The charged cell was then left at -20 ° C. for 3 hours, and the constant current discharge capacity was measured at 1 C (30 mA) and 2.75 V cutoff at -20 ° C. Subsequently, the cell was left at room temperature for 3 hours, and then subjected to constant current discharge at 0.2 C (6 mA) and 2.75 V cutoff at 25 ° C. The discharged cell was then left at -20 ° C. for 3 hours, and the constant current charge capacity was measured at 1 C (30 mA) and 4.2 V cutoff at -20 ° C.

(Charge-discharge cycle characteristics: resistance increase rate after cycle test)
The aged cell was charged at room temperature at 1 C (30 mA) and a constant current/constant voltage of 0.02 C (0.6 mA) at 4.2 V, and fully charged (SOC 100%). Subsequently, the cell was discharged at 6 mA for 10 seconds after leaving it for 30 minutes from the fully charged state (SOC 100%), then discharged at 30 mA for 10 seconds after leaving it for 30 minutes, and then discharged at 90 mA for 10 seconds after leaving it for another 30 minutes. Each discharge current was plotted on the horizontal axis, and the difference in closed-circuit voltage (ΔV) between the start of discharge and 10 seconds after each discharge current was plotted on the vertical axis. The slope of the IV line was taken as the "initial DCR" of the cell. A total of 500 cycles were tested at 45 ° C under the following charge/discharge conditions (cycle conditions).
Charge: constant current/constant voltage charge at 4.2 V, 1 C (30 mA), terminated at 0.02 C (0.6 mA), rested for 10 minutes. Discharge: constant current (CC) discharge at 1 C (30 mA), terminated at 2.75 V, rested for 10 minutes. After the cycle test, DCR was measured in the same manner as above, and the "post-cycle DCR" was calculated. The resistance increase rate after cycling was calculated using the following formula (4):
Resistance increase rate after cycling (%) = (DCR after cycling/initial DCR) × 100 (4)
The smaller the DCR after cycling, the more the increase in the resistance of the battery is suppressed.

Claims (11)

A non-aqueous electrolyte solution for a lithium ion secondary battery, comprising: a compound represented by the following formula (1), a cyclic sulfonic acid ester, a compound represented by the following formula (2), and a non-aqueous solvent:
LiN(R ¹ SO ₂ ) (R ² SO ₂ )...(1)
[In formula (1), R ¹ and R ² each independently represent a fluorine atom or an alkyl group having 1 to 6 carbon atoms which may be substituted with a fluorine atom.]
FSO ₂ NHR ³ …(2)
[In formula (2), ^R3 represents a hydrogen atom or an optionally substituted hydrocarbon group having 1 to 18 carbon atoms.] 2. The nonaqueous electrolyte solution according to claim 1, wherein the compound represented by formula (1) comprises at least one selected from _the group consisting of LiN( _FSO2 ) ₂ and LiN( _CF3SO2 ) ₂ . The nonaqueous electrolyte according to claim 1, wherein the compound represented by formula (1) includes LiN(FSO ₂ ) ₂ . The nonaqueous electrolyte solution according to claim 1, wherein the cyclic sulfonate ester comprises at least one selected from the group consisting of a compound represented by the following formula (3) and a compound represented by the following formula (4):
[In formulas (3) and (4), R ⁴ to R ⁶ each independently represent a divalent saturated hydrocarbon group having 1 to 10 carbon atoms or a divalent unsaturated hydrocarbon group having 2 to 10 carbon atoms.] The nonaqueous electrolyte solution according to claim 4, wherein the cyclic sulfonate ester includes a compound represented by formula (3). The nonaqueous electrolyte solution according to claim 5, wherein the cyclic sulfonate ester comprises a compound represented by the following formula (3-1):
[In formula (3-1), R ¹¹ to R ¹⁶ each independently represent a hydrogen atom or a methyl group.] 7. The nonaqueous electrolyte solution according to claim 6, wherein in formula (3-1), at least one of R ¹¹ to R ¹⁶ is a methyl group. The nonaqueous electrolyte solution according to claim 5, wherein the cyclic sulfonate ester comprises a compound represented by the following formula (3-2):
[In formula (3-2), R ¹⁷ to R ²⁴ each independently represent a hydrogen atom or a methyl group. The total number of carbon atoms contained in R ¹⁷ to R ²⁴ is 0 to 6.] The nonaqueous electrolyte solution according to claim 1, wherein the content of the cyclic sulfonic acid ester in the total amount of the nonaqueous electrolyte solution is 0.05% by mass or more and 5% by mass or less. The nonaqueous electrolyte solution according to claim 1, wherein the content of the compound represented by formula (2) in the total amount of the nonaqueous electrolyte solution is 0.005% by mass or more and 15% by mass or less. The nonaqueous electrolyte solution according to any one of claims 1 to 10,
a positive electrode having a positive electrode mixture layer and a positive electrode current collector;
a negative electrode having a negative electrode mixture layer and a negative electrode current collector,
The positive electrode comprises at least one positive electrode active material selected from the group consisting of a positive electrode active material represented by the following formula (A) and a positive electrode active material represented by the following formula (B):
Li _v Ni _x Co _{y Mnz} O _(2+w) …(A)
[In formula (A), 0.2≦v≦1.2, 0.6≦x≦0.9, 0<y≦0.3, 0<z<0.4, x+y+z=1, and −0.2≦w≦0.2.]
LiMPO ₄ ...(B)
[In formula (B), M represents Ni, Mn, Co, or Fe.]