Non-aqueous electrolyte secondary batteries

Abstract

This nonaqueous electrolyte secondary battery comprises: a nonaqueous electrolyte containing a sulfonylimide compound represented by general formula (1) as an electrolyte salt and a carbonate-based solvent as an electrolyte solvent; a negative electrode containing, as a negative electrode active material, a graphite in which the peak area ratio of a D band and a G band (D / G ratio) analyzed using Raman spectroscopy is 0.7 or less or a graphite in which the G band half width analyzed by Raman spectroscopy is 28 cm–1 or less; and a positive electrode. LiN(RSO2)(FSO2) (where R represents a fluorine atom, a C1-6 alkyl group, or a C1-6 fluoroalkyl group.) (1)

Description

[Technical Field]

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

[0002] In order to improve the battery performance of secondary batteries such as lithium ion secondary batteries, various non-aqueous electrolyte solutions and materials for use in secondary batteries have been investigated. Through previous investigations, the present inventors have found 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] As a result of further investigation, the present inventors have found that batteries using a non-aqueous electrolyte containing a sulfonylimide compound exhibit greater self-discharge from a fully charged state during storage than batteries using a non-aqueous electrolyte containing only a lithium compound other than a sulfonylimide compound (e.g., LiPF6, LiBF4, etc.) as the electrolyte salt, and that there is room for improvement in the storage characteristics of batteries. They have therefore proposed various techniques for improving this (e.g., Patent Document 1).

[0004] The present inventors have also proposed a non-aqueous electrolyte secondary battery, which is a secondary battery equipped with a non-aqueous electrolyte, and which comprises a non-aqueous electrolyte containing a sulfonylimide compound and a sulfone compound, and a negative electrode containing a specific carbon material (Patent Document 2).

[0005] As non-aqueous electrolytes for secondary batteries, for example, Patent Documents 3 to 6 propose non-aqueous electrolytes containing additives such as trimethylsilyl polyphosphate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2022 / 065198 [Patent Document 2] Patent No. 6646522 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-91785 [Patent Document 4] Japanese Patent Application Publication No. 2016-192401 [Patent Document 5] Korean Patent Publication No. 2017-0000903 [Patent Document 6] International Publication No. 2016 / 209840 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent Document 1 describes that dissolving CO2 or the like in a non-aqueous electrolyte solution containing a sulfonylimide compound can suppress self-discharge of a battery using the same, but does not consider or describe the crystallinity of graphite used as a negative electrode active material.

[0008] Patent Document 2 does not consider or describe self-discharge of a battery using a nonaqueous electrolyte containing a sulfonylimide compound. Furthermore, in the nonaqueous electrolyte secondary battery of Patent Document 2, the discharge capacity is improved by combining a negative electrode containing a specific carbon material as the negative electrode active material with a nonaqueous electrolyte that essentially contains a sulfone compound such as sulfolane as the electrolyte solvent. However, when a sulfone compound is used, low-temperature characteristics (for example, discharge capacity at 0°C (hereinafter also referred to as "low-temperature discharge capacity")) are insufficient, leaving room for improvement.

[0009] Patent Documents 3 to 6 do not provide detailed descriptions of the non-aqueous electrolyte solution containing a sulfonylimide compound and the negative electrode as components of the secondary battery, and do not discuss the crystallinity of graphite used as the negative electrode active material.

[0010] The present disclosure has been made in view of the above points, and an object of the present disclosure is to suppress self-discharge (improve storage characteristics) and increase low-temperature discharge capacity (improve low-temperature characteristics) in a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte containing a sulfonylimide compound, by combining a nonaqueous electrolyte containing a carbonate-based solvent with a negative electrode containing specific graphite as a negative electrode active material. [Means for solving the problem]

[0011] To achieve the above object, the disclosed technology aims to improve the low-temperature discharge capacity of a non-aqueous electrolyte secondary battery containing a sulfonylimide compound by using a carbonate-based solvent, and to suppress self-discharge by using a specific carbon material (graphite) without dissolving CO2 or the like in the non-aqueous electrolyte.

[0012] The nonaqueous electrolyte secondary battery of the present disclosure contains an electrolyte salt having the general formula (1): LiN(RSO2)(FSO2) (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 carbonate-based solvent as the electrolyte solvent; a negative electrode containing graphite as the negative electrode active material, the peak area ratio of the D band to the G band (D / G ratio) being 0.7 or less as analyzed by Raman spectroscopy; and a positive electrode.

[0013] The nonaqueous electrolyte secondary battery of the present disclosure also includes a negative electrode active material having a G-band half width of 28 cm as analyzed by Raman spectroscopy. -1 The battery is characterized by comprising a negative electrode containing graphite as described below, and a positive electrode.

[0014] In the non-aqueous electrolyte secondary battery of the present disclosure, the sulfonylimide compound represented by the general formula (1) may be contained in the non-aqueous electrolyte at 0.2 mol / L or more, and may contain LiN(FSO2)2. The carbonate-based solvent may be contained at 50 mass% or more based on the total amount of 100 mass% of the electrolyte solvent, and may contain at least one selected from the group consisting of a chain carbonate-based solvent and a saturated cyclic carbonate-based solvent. The non-aqueous electrolyte may contain, as an additive, a phosphorus atom-containing compound represented by the general formula (4): [-P(=O)(OR 1 )O-] n (In the formula (4), R 1 represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt or a hydrogen atom, and n represents 2 or more.) ··· (4), or may contain at least one selected from the group consisting of trimethylsilyl polyphosphate, ethyl polyphosphate, (triisopropylsilyl) polyphosphate and [(tert-butyl)dimethylsilyl] polyphosphate. The positive electrode may contain a positive electrode active material represented by the general formula (5): Li v Ni x Co y Mn z O 2+w 〔0.2 ≦ v ≦ 1.2, 0.5 ≦ x ≦ 0.9, 0 < y ≦ 0.3, 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).〕 ··· (5). The electrolyte salt is a compound represented by the general formula (2): LiPF a (C m F 2m+1 ) 6-a (a: 0 ≦ a ≦ 6, m: 1 ≦ m ≦ 4) ··· (2), and a compound represented by the general formula (3): LiBF b (C n F 2n+1 ) 4-bIt may contain at least one selected from the group consisting of a compound represented by (b: 0≦b≦4, n: 1≦n≦4) (3) and LiAsF 6 . [Effects of the Invention]

[0015] According to the present disclosure, in a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte containing a sulfonylimide compound, by combining a nonaqueous electrolyte containing a carbonate-based solvent with a negative electrode containing specific graphite as a negative electrode active material, it is possible to suppress self-discharge (improve storage characteristics) and increase low-temperature discharge capacity (improve low-temperature characteristics). [Brief explanation of the drawings]

[0016] [Figure 1] Figure 1 shows the D / G chart (Raman spectrum) of the graphite "MAGE" used in the manufacturing example. [Figure 2] Figure 2 is the D / G chart for the graphite "SFG15" used in the manufacturing example. [Figure 3] Figure 3 is the D / G chart for the graphite "SLP50" used in the manufacturing example. [Figure 4] Figure 4 is the D / G chart for the graphite "O-MAC" used in the manufacturing example. [Figure 5] Figure 5 is the D / G chart for the graphite "SMG" used in the manufacturing example. [Figure 6] FIG. 6 shows the 31P-NMR spectrum of the reagent trimethylsilyl polyphosphate (PPSE-1) used in Examples 4 and 5 series. [Figure 7] FIG. 7 shows the 31P-NMR spectrum of polytrimethylsilyl phosphate (PPSE-2) synthesized in Examples 4 and 5 series. [Figure 8] FIG. 8 shows the 31P-NMR spectrum of polytrimethylsilyl phosphate (PPSE-3) synthesized in Examples 4 and 5 series. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] <Nonaqueous electrolyte secondary battery> The nonaqueous electrolyte secondary battery according to this embodiment is a secondary battery including a nonaqueous electrolyte, a positive electrode, and a negative electrode.

[0019] [Nonaqueous electrolyte] The non-aqueous electrolyte contains an electrolyte salt and an electrolyte solvent.

[0020] (electrolyte salt) The electrolyte salt has the general formula (1): [C1] LiN(RSO2)(FSO2)···(1) (hereinafter referred to as "sulfonylimide compound (1)", a fluorine-containing sulfonylimide salt) represented by the following formula: That is, the nonaqueous electrolyte secondary battery according to this embodiment includes a nonaqueous electrolyte containing sulfonylimide compound (1) as an essential component as an electrolyte salt as one of its constituent materials.

[0021] 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.

[0022] Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Among the alkyl groups having 1 to 6 carbon atoms, a linear or branched alkyl group having 1 to 6 carbon atoms is preferred, and a linear alkyl group having 1 to 6 carbon atoms is more preferred.

[0023] 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.

[0024] 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.

[0025] Specific examples of the sulfonylimide compound (1) include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2, LiFSI), lithium (fluorosulfonyl)(methylsulfonyl)imide, lithium (fluorosulfonyl)(ethylsulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium (fluorosulfonyl)(pentafluoroethylsulfonyl)imide, and lithium (fluorosulfonyl)(heptafluoropropylsulfonyl)imide. The sulfonylimide compounds may be used alone or in combination of two or more. The sulfonylimide compound (1) may be a commercially available product or may be synthesized by a conventional method.

[0026] Among the sulfonylimide compounds (1), from the viewpoint of improving battery performance, LiN(FSO2)2, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, and lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide are preferred, with LiN(FSO2)2 being more preferred. In other words, among nonaqueous electrolytes, those containing LiN(FSO2)2 as the sulfonylimide compound (1) are preferred.

[0027] The concentration (content, total content when two or more types are used) of the sulfonylimide compound (1) in the nonaqueous electrolyte is preferably 0.01 mol / L or more, more preferably 0.05 mol / L or more, even more preferably 0.1 mol / L or more, still more preferably 0.2 mol / L or more, and even more preferably 0.5 mol / L or more from the viewpoint of improving battery performance. Moreover, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte and self-discharge of the battery, 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.

[0028] 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, and even more preferably 50 mol % or more, based on a total of 100 mol % of the electrolyte salts contained in the non-aqueous electrolyte solution.

[0029] 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 content 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, based on the total amount of the components contained in the non-aqueous electrolyte (100% by mass).

[0030] 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.

[0031] Examples of imide salts include fluorine-containing sulfonylimide salts other than sulfonylimide compound (1) (hereinafter referred to as "other sulfonylimide compounds"). Examples of other sulfonylimide compounds include non-lithium salts of the fluorine-containing sulfonylimides listed as sulfonylimide compound (1) (for example, salts in which the lithium (ion) in sulfonylimide compound (1) is substituted with a cation other than lithium ion). Examples of salts in which a cation other than lithium ion is substituted include alkali metal salts such as sodium salt, potassium salt, rubidium salt, and cesium salt; alkaline earth metal salts such as beryllium salt, magnesium salt, calcium salt, strontium salt, and barium salt; aluminum salt; ammonium salt; and phosphonium salt. The other sulfonylimide compounds may be used alone or in combination of two or more. In addition, commercially available products may be used as the other sulfonylimide compounds, or those synthesized by conventionally known methods may be used.

[0032] Examples of the non-imide salt include salts of non-imide anions and cations (lithium ions and the above-exemplified cations). [Case 2] LiPF a (C m F 2m+1 ) 6-a (a:0≦a≦6, m:1≦m≦4)···(2) (hereinafter referred to as "fluorophosphate compound (2)"), a compound represented by general formula (3): [C3] LiBF b (C n F 2n+1 ) 4-b (b:0≦b≦4, n:1≦n≦4)···(3) Examples of the non-imide salt include a compound represented by the formula (hereinafter referred to as "fluoroborate compound (3)"), lithium salts such as lithium hexafluoroarsenate (LiAsF), LiSbF, LiClO, LiSCN, LiAlF, CFSOLi, LiC[(CFSO)], LiN(NO), and LiN[(CN)]; and non-lithium salts (for example, salts in which the lithium (ion) in these lithium salts is substituted with one of the cations exemplified above (e.g., NaBF, NaPF, NaPF(CF)). The non-imide salts may be used alone or in combination of two or more. Furthermore, commercially available non-imide salts may be used, or those obtained by synthesis using a conventionally known method may be used.

[0033] Among the other electrolytes, non-imide salts are preferred from the viewpoints of ionic conductivity, cost, etc., and fluorophosphate compound (2), fluoroborate compound (3) and LiAsF6 are preferred, with fluorophosphate compound (2) being more preferred.

[0034] Examples of the fluorophosphate compound (2) include LiPF, LiPF(CF), LiPF(C,F), LiPF(C,F), LiPF(C,F), etc. Among the fluorophosphate compounds (2), LiPF and LiPF(C,F) are preferred, with LiPF being more preferred.

[0035] Examples of the fluoroboric acid compound (3) include LiBF, LiBF(CF), LiBF(C,F), LiBF(C,F) and LiBF(C,F), etc. Among the fluoroboric acid compounds (3), LiBF and LiBF(CF) are preferred, and LiBF is more preferred.

[0036] 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.

[0037] The electrolyte salt composition may be an electrolyte salt having a simple salt composition of sulfonylimide compound (1), or an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and another electrolyte. When an electrolyte salt having a mixed salt composition is used, an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and fluorophosphate compound (2) is preferred, and an electrolyte salt having a mixed salt composition containing LiN(FSO2)2 and LiPF6 is more preferred.

[0038] When using an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and other electrolytes, the concentration of the other electrolytes in the nonaqueous electrolyte (content, or the total content when two or more types are used in combination) is preferably 0.1 mol / L or more, more preferably 0.2 mol / L or more, even more preferably 0.5 mol / L or more, even more preferably 0.7 mol / L or more, and even more preferably 1 mol / L or more, from the viewpoint of improving battery performance. Furthermore, from the viewpoint of suppressing a decrease in battery performance due to an increase in electrolyte viscosity and self-discharge of the battery, 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.

[0039] The total concentration of the electrolyte salts in the nonaqueous electrolyte 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.

[0040] From the viewpoint of improving battery performance, it is preferable to increase the concentration of sulfonylimide compound (1). The molar ratio of sulfonylimide compound (1) to other electrolytes (the molar ratio of sulfonylimide compound (1) concentration to other electrolyte concentration) is preferably 1:25 or more, more preferably 1:10 or more, even more preferably 1:8 or more, even more preferably 1:5 or more, even more preferably 1:2 or more, particularly preferably 1:1 or more, and is 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.

[0041] (Electrolyte solvent) The electrolyte solvent contains a carbonate-based solvent. That is, the nonaqueous electrolyte secondary battery according to this embodiment uses a nonaqueous electrolyte containing a carbonate-based solvent as an essential component together with the sulfonylimide compound (1) as the electrolyte solvent as one of its constituent materials. In other words, the main component (the component contained in the largest amount) of the electrolyte solvent constituting the nonaqueous electrolyte is not a sulfone compound (a sulfur compound-based solvent described below), but a carbonate-based solvent. In this way, by using a carbonate-based solvent as the main component of the electrolyte solvent, the low-temperature discharge capacity is improved and the low-temperature characteristics are improved compared to when a sulfone compound is used.

[0042] The electrolyte solvent may contain only one type of carbonate-based solvent, may be a homogeneous mixed solvent containing two or more types of carbonate-based solvents, or may be a heterogeneous mixed solvent containing a carbonate-based solvent and another electrolyte solvent (an electrolyte solvent other than a carbonate-based solvent) in a predetermined content.

[0043] From the viewpoint of improving the low-temperature characteristics of the battery, the content of the carbonate solvent is preferably 50% by mass or more, more preferably more than 50% by mass, even more preferably 70% by mass or more, and even more preferably 80% by mass or more, relative to 100% by mass of the total amount of the electrolyte solvent (heterogeneous mixed solvent). The upper limit of the content may be 100% by mass (one type of carbonate solvent or a homogeneous mixed solvent containing two or more types of carbonate solvents).

[0044] When the electrolyte solvent is a heterogeneous mixed solvent containing other electrolyte solvents than a carbonate-based solvent, the content of the other electrolyte solvents is preferably less than 50 mass%, more preferably 30 mass% or less, and even more preferably 20 mass% or less, relative to 100 mass% of the total amount of the electrolyte solvent, from the viewpoint of improving battery performance.

[0045] Examples of carbonate solvents include chain carbonate (carbonate ester) solvents such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), diphenyl carbonate, and methyl phenyl carbonate; saturated cyclic carbonate solvents such as ethylene carbonate (EC), propylene carbonate (PC), 2,3-dimethylethylene carbonate, 1,2-butylene carbonate, and erythritol carbonate; cyclic carbonate solvents having an unsaturated bond such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate; and fluorine-containing cyclic carbonate solvents such as fluoroethylene carbonate (FEC), 4,5-difluoroethylene carbonate, and trifluoropropylene carbonate. The carbonate solvents may be used alone or in combination of two or more. Among carbonate solvents, chain carbonate solvents and saturated cyclic carbonate solvents are preferred, with DMC, EMC, DEC, EC, and PC being more preferred. Furthermore, the carbonate solvent preferably contains at least one selected from the group consisting of chain carbonate solvents and saturated cyclic carbonate solvents, more preferably a homogeneous mixed solvent containing two or more carbonate solvents including a chain carbonate solvent and a saturated cyclic carbonate solvent, and even more preferably a homogeneous mixed solvent containing EMC and EC.

[0046] As described above, the electrolyte solvent may contain a carbonate-based solvent, but may also contain other electrolyte solvents. The other electrolyte solvents are not particularly limited as long as they can dissolve and disperse the electrolyte salt, and examples thereof include non-aqueous solvents other than carbonate-based solvents, and any solvent generally used in batteries can be used.

[0047] Suitable non-aqueous solvents (excluding carbonate-based solvents) are those that have a high dielectric constant, high solubility for the electrolyte, a boiling point of 60°C or higher, and a wide electrochemical stability range. Organic solvents with a low water content are more preferred. Examples of such organic solvents include ether-based solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; aromatic carboxylic acid ester-based solvents such as methyl benzoate and ethyl benzoate; lactone-based solvents such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone; phosphate ester-based solvents such as trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, and triethyl phosphate; acetonitrile, ... Examples of suitable solvents include nitrile solvents such as tolyl, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, and isobutyronitrile; sulfur compound solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane; aromatic nitrile solvents such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, and 3-methyl-2-oxazolidinone; and chain ester solvents such as ethyl acetate, butyl acetate, and propyl propionate. These solvents may be used alone or in combination of two or more.

[0048] The electrolyte solvent may be used as a medium such as a polymer or polymer gel used in place of the electrolyte solvent. When a polymer or polymer gel is used in place of the electrolyte solvent, the following methods may be employed. That is, a method in which a solution in which an electrolyte salt is dissolved in an electrolyte solvent is dropped onto a polymer film formed by a conventionally known method to impregnate and support the electrolyte salt and electrolyte solvent; a method in which a polymer and an electrolyte salt are melted and mixed at a temperature equal to or higher than the melting point of the polymer, and then a film is formed, and the film is impregnated with the electrolyte solvent (these are referred to as gel electrolyte); a method in which a nonaqueous electrolyte in which an electrolyte salt is dissolved in an electrolyte solvent in advance is mixed with a polymer, and then the mixture is formed into a film by a casting method or a coating method, and the electrolyte 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.

[0049] Examples of polymers that can be used in place of the electrolyte solvent include polyethylene oxide (PEO), which is a homopolymer or copolymer of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), polyether polymers such as polypropylene oxide, methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), fluorine-based polymers such as polyvinylidene fluoride (PVdF) and polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof. These polymers may be used alone or in combination of two or more.

[0050] (additives) The non-aqueous electrolyte may contain an additive for improving various properties of the lithium ion secondary battery. The additive may be added to the non-aqueous electrolyte or may be added during the preparation process of the non-aqueous electrolyte.Examples of additives include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, and trimethylene glycol sulfate; Yellow compounds; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, and cycloheptane; carbonate compounds such as vinylene carbonate, fluoroethylene carbonate (FEC), trifluoropropylene carbonate, phenylethylene carbonate, and erythrityl carbonate; sulfamic acid (amidosulfuric acid, H3NSO3); sulfamate salts ( Alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as calcium salts, strontium salts, and barium salts; other metal salts such as manganese salts, copper salts, zinc salts, iron salts, cobalt salts, and nickel salts; ammonium salts; guanidine salts; fluorosulfonic acid compounds such as lithium fluorosulfonate (LiFSO3), sodium fluorosulfonate (NaFSO3), potassium fluorosulfonate (KFSO3), and magnesium fluorosulfonate (Mg(FSO3)2); lithium monofluorophosphate (Li2PO3F Fluorophosphate compounds such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorooxalatophosphate (LIDFOP), lithium tetrafluorooxalatophosphate (LITFOP), lithium difluorobis(oxalato)phosphate (LiDFBOP), lithium tris(oxalato)phosphate, and other lithium salts having an oxalic acid skeleton; general formula (4): [C4] [-P(=O)(OR 1 )O-] n ···(4) (hereinafter referred to as "phosphorus atom-containing compound (4)") represented by the following formula: These additives may be used alone or in combination of two or more kinds.

[0051] 《Phosphorus atom-containing compound (4)》 The use of the phosphorus atom-containing compound (4) as an additive refers to the inclusion of a predetermined amount or more of the phosphorus atom-containing compound (4) in a nonaqueous electrolyte solution containing the sulfonylimide compound (1). In other words, the nonaqueous electrolyte solution according to this embodiment may contain the phosphorus atom-containing compound (4) as an additive. The phosphorus atom-containing compound (4) may be added to the nonaqueous electrolyte solution or may be added during the preparation process of the nonaqueous electrolyte solution. The nonaqueous electrolyte solution may contain the other additives listed above in addition to the phosphorus atom-containing compound (4).

[0052] General formula (4):[-P(=O)(OR 1 )O-] n In R 1 R represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt, or a hydrogen atom. 1Among these, preferred are linear alkyl groups having 1 to 6 carbon atoms (which may have a substituent), trifluoroalkyl groups having 1 to 6 carbon atoms (which may have a substituent), trialkylsilyl groups having 1 to 6 carbon atoms (which may have a substituent), and silyl groups formed by bonding an alkyl group having 1 to 6 carbon atoms (which may have a substituent) to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (linear, cyclic, etc.), etc.; linear alkyl groups having 1 to 3 carbon atoms (which may have a substituent), trifluoroalkyl groups having 1 to 3 carbon atoms (which may have a substituent), More preferred are a trialkylsilyl group having 1 to 4 carbon atoms (which may have a substituent), and a silyl group in which an alkyl group having 1 to 4 carbon atoms (which may have a substituent) is bonded to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (chain, cyclic, etc.); an ethyl group, a trifluoroethyl group, a trimethylsilyl group, a triethylsilyl group, a triisopropylsilyl group, a (tertiary (tert-)butyl)dimethylsilyl group, and a (tert-butyl)diphenylsilyl group are even more preferred; a trimethylsilyl group is particularly preferred. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 4 carbon atoms refers to a silyl group in which three alkyl groups having 1 to 6 or 1 to 4 carbon atoms are bonded. In addition, in general formula (4), R 1 may be a trialkoxysilyl group (-Si(-OR)3) in which three alkyl groups (R) having 1 to 6 carbon atoms or 1 to 4 carbon atoms are bonded to a silyl group via an oxygen atom. Examples of trialkoxysilyl groups include a trimethoxysilyl group, a triethoxysilyl group, a triisopropoxysilyl group, a (tert-butoxy)dimethoxysilyl group, and a (tert-butoxy)diphenoxysilyl group. The three alkyl or alkoxy groups may be the same or different. In general formula (4), R 1 are preferably the same groups.

[0053] In the general formula (4), n represents an integer of 2 or more (degree of polymerization), for example, n=2 to 200.

[0054] Specific examples of the phosphorus atom-containing compound (4) include ethyl polyphosphate (in general formula (4), R 1 represents an ethyl group), trimethylsilyl polyphosphate (in general formula (4), R 1 represents a trimethylsilyl group (TMS), triethylsilyl polyphosphate (in general formula (4), R 1 represents a triethylsilyl group (TES), poly(triisopropylsilyl)phosphate (in general formula (4), R 1 represents a triisopropylsilyl group (TIPS). ], polyphosphate [(tert-butyl)dimethylsilyl] [in general formula (4), R 1 represents a (tert-butyl)dimethylsilyl group (TBDMS). ], polyphosphate [(tert-butyl)diphenylsilyl] [in general formula (4), R 1 represents a (tert-butyl)diphenylsilyl group (TBDPS). ], trimethoxysilyl polyphosphate (in general formula (4), R 1 represents a trimethoxysilyl group), triethoxysilyl polyphosphate (in general formula (4), R 1 represents a triethoxysilyl group), polyphosphate (triisopropoxysilyl) [in general formula (4), R 1 represents a triisopropoxysilyl group.], polyphosphate [(tert-butoxy)dimethoxysilyl] [in general formula (4), R 1 represents a (tert-butoxy)dimethoxysilyl group.], polyphosphate [(tert-butoxy)diphenoxysilyl] [in general formula (4), R 1 represents a (tert-butoxy)diphenoxysilyl group. ]. The phosphorus atom-containing compounds (4) may be used alone or in combination of two or more. Among the phosphorus atom-containing compounds (4), trimethylsilyl polyphosphate is preferred.

[0055] Trimethylsilyl polyphosphate is, for example, 31By measuring the presence or absence of peaks indicating bonds between phosphorus atoms and surrounding groups and their abundance ratios (integral ratios of each peak) using P-NMR or the like, structures such as a chain structure represented by the following structural formula (4-1), a cyclic structure represented by the structural formula (4-2), and a branched structure represented by the structural formula (4-3) can be analyzed. 31 The P-NMR measurement conditions include those described in the Examples below.

[0056] [ka]

[0057] [ka]

[0058] [ka]

[0059] In structural formulas (4-1) to (4-3), "TMS" represents a trimethylsilyl group, n represents the degree of polymerization, and Pt, Pm, and Pb represent the peaks of phosphorus atoms. Pt represents the peak of a phosphorus atom at a terminal or side chain, specifically a phosphorus atom having one group in which the hydrogen atom of a hydroxyl group is substituted with another adjacent phosphorus atom (hereinafter also referred to as an "-OP group"). Pm represents the peak of a phosphorus atom with a linear structure, specifically a phosphorus atom having two "-OP groups." Pb represents the peak of a phosphorus atom with a branched structure, specifically a phosphorus atom having three "-OP groups." In this way, the presence or absence of branched structures, their abundance ratio, the degree of polymerization n, etc. can be estimated from the presence or absence of Pt, Pm, and Pb and their integral ratios. For example, if all trimethylsilyl polyphosphates have a chain structure (a structure without a Pb peak) represented by structural formula (4-1), and the integral of the Pt region is set to 1, the degree of polymerization (n) is expressed as 2 × {(integral value of Pt) + (integral value of Pm)} ≒ 2 × {1 + (integral value of Pm)}. Furthermore, if the integral of the Pt region is set to 2, the degree of polymerization (n) is expressed as (integral value of Pt) + (integral value of Pm) ≒ 2 + (integral value of Pm). For example, in the case of trimethylsilyl polyphosphate used in the examples described below, the average degree of polymerization (n) is calculated to be 4.87. It should be noted that if trimethylsilyl polyphosphate contains a cyclic structure represented by structural formula (4-2), the degree of polymerization (n) is estimated to be smaller than the above. Among trimethylsilyl polyphosphates, those with a Pb peak, i.e., those containing a branched structure represented by structural formula (4-3), are preferred from the viewpoints of suppressing battery self-discharge and further improving battery performance. In this case, the integral ratio of Pt, Pm and Pb is preferably Pt:Pm:Pb=1.0:3.0:0.2 to 1.0:9.0:3.0, and more preferably Pt:Pm:Pb=1.0:6.0:0.7 to 1.0:7.0:1.2.

[0060] Among the additives, from the viewpoint of suppressing self-discharge of the battery and Ni elution from the positive electrode, the phosphorus atom-containing compound (4) is preferred, trimethylsilyl polyphosphate, ethyl polyphosphate, (triisopropylsilyl) polyphosphate, and (tert-butyl)dimethylsilyl] polyphosphate are more preferred, and trimethylsilyl polyphosphate having a branched structure represented by structural formula (4-3) is even more preferred.

[0061] The additive is used in an amount of preferably 0.1% by mass to 10% by mass, more preferably 0.2% by mass to 8% by mass, even more preferably 0.3% by mass to 5% by mass, even more preferably 0.3% by mass to 3% by mass, and even more preferably 0.3% by mass to 1% by mass, relative to 100% by mass of the total amount of components contained in the non-aqueous electrolyte. If the amount of additive used is too small, it may be difficult to obtain the effects derived from the additive. On the other hand, even if a large amount of additive is used, it may be difficult to obtain effects commensurate with the amount added, and there is also a risk that the viscosity of the non-aqueous electrolyte will increase and the conductivity will decrease.

[0062] As described above, the nonaqueous electrolyte solution according to this embodiment is composed of components such as the sulfonylimide compound (1), a carbonate-based solvent, and, if necessary, other electrolyte salts, other electrolyte solvents, various additives, etc. The nonaqueous electrolyte solution can be prepared, for example, by mixing these components in a predetermined composition (mass) ratio.

[0063] [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.

[0064] 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.

[0065] 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.

[0066] In the secondary battery according to this embodiment, the positive electrode (positive electrode mixture) is preferably LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM111), LiNi 0.5 Co 0.2 Mn 0.3O2 (NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.8 Co 0.1 Mn 0.1 Ternary cathode active materials such as O2 (NCM811); LiFePO4, LiFe 0.995 Mn 0.005 Iron phosphate-based cathode active materials having an olivine structure such as PO4 can be preferably used. These cathode active materials may be used alone or in combination of two or more kinds.

[0067] Among the ternary cathode active materials, the general formula (5): [Chemical formula 8] Li v Ni x Co y Mn z O 2+w 〔0.2 ≤ v ≤ 1.2, 0.5 ≤ x ≤ 0.9, 0 < y ≤ 0.3, 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).〕···(5) The high-Ni content ternary cathode active material represented by (hereinafter referred to as "high-Ni content ternary cathode active material (5)") is preferred.

[0068] In the high-Ni content ternary cathode active material (5), the content ratio of Ni ( "x" in the general formula (5)) to the total amount 100% (100 mol%) on a molar basis of transition metals is 50% or more (0.5 ≤ x), preferably 55% or more (0.55 ≤ x), more preferably 70% or more (0.7 ≤ x). The upper limit of the content ratio is 90% or less (x ≤ 0.9), preferably less than 85% (x < 0.85), more preferably 80% or less (x ≤ 0.8). In addition, the respective content ratios of components other than Ni in the high-Ni content ternary cathode active material (5) ( "v", "y", "z", "w" (2 + w) in the general formula (5)) may be appropriately adjusted within the range of the above respective molar ratios.

[0069] The high Ni-containing ternary positive electrode active material (5) may be used alone or in combination of two or more. The high Ni-containing ternary positive electrode active material (5) 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 (5) include NCM523, NCM622, and NCM811.

[0070] The positive electrode preferably contains at least one of the above-mentioned ternary positive electrode active material and iron phosphate positive electrode active material, but may also contain other positive electrode active materials. The other positive electrode active materials may be any materials capable of absorbing and releasing lithium ions, and may be, for example, positive electrode active materials used in conventionally known secondary batteries (lithium ion secondary batteries).

[0071] Positive electrode active materials used in lithium ion secondary batteries include, for example, lithium cobalt oxide; lithium nickel oxide; lithium manganese oxide; LiNi 1-v-w Co v Al w O2 (0≦v≦1, 0≦w≦1) and other transition metal oxides other than the ternary oxides mentioned above; compounds with an olivine structure such as LiAPO4 (A=Mn, Ni, Co); solid solution materials incorporating multiple transition metals (solid solutions of electrochemically inactive layered Li2MnO3 and electrochemically active layered LiMO2 (M=Co, Ni, or other transition metals)); LiCo x Mn 1-x O2(0≦x≦1);LiNi x Mn 1-x O2 (0≦x≦1); compounds having a fluorinated olivine structure such as Li2APO4F (A=Fe, Mn, Ni, Co); sulfur, etc. can be used. Each of these may be used alone, or two or more types may be used in combination.

[0072] From the viewpoint of improving the output characteristics and electrical characteristics of the secondary battery, the content of the positive electrode active material (total content when multiple positive electrode active materials are included) is preferably 75% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the total amount of components included in the positive electrode composite, and is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 95% by mass or less.

[0073] 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, 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 matter of the positive electrode mixture is preferably 1 to 20 mass %, more preferably 1.5 to 10 mass %.

[0074] Examples of binders include fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber 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. Each binder may be used alone, or two or more types may be used in combination. Furthermore, the binder may be in a state of being dissolved in a solvent or dispersed in a solvent when used.

[0075] Examples of the solvent include N-methylpyrrolidone, 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.

[0076] The positive electrode mixture may contain other components as needed, such as non-fluorinated polymers such as (meth)acrylic polymers, nitrile polymers, and diene polymers; polymers such as 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 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.

[0077] 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.

[0078] The method for forming the positive electrode (coating method) is not particularly limited, and examples thereof include: (1) a method in which a positive electrode composite is applied to a positive electrode current collector by a conventional coating method (e.g., a doctor blade method, etc.) (and then dried); (2) a method in which a positive electrode current collector is immersed in the positive electrode composite (and then dried); (3) a method in which a sheet formed from the positive electrode composite is bonded to a positive electrode current collector (e.g., bonded via a conductive adhesive) and pressed (and then dried); (4) a method in which a positive electrode composite to which a liquid lubricant has been added is applied or cast onto a positive electrode current collector, formed into a desired shape, and then the liquid lubricant is removed (and then stretched in uniaxial or multiaxial directions); and (5) a method in which a positive electrode composite (or a solid content forming a positive electrode composite layer) is slurried with an electrolyte, transferred in a semi-solid state to a current collector (positive electrode current collector), and used as an electrode (positive electrode) without drying.

[0079] The positive electrode mixture layer may be dried or pressed after being formed or coated (applied), as needed.

[0080] [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.

[0081] 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.

[0082] 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.

[0083] In the nonaqueous electrolyte secondary battery according to this embodiment, the negative electrode active material has a peak area ratio of the D band and the G band (D / G ratio) of 0.7 or less when analyzed by Raman spectroscopy and / or a half width of the G band of 28 cm or less when analyzed by Raman spectroscopy. -1That is, the nonaqueous electrolyte secondary battery according to this embodiment is configured with a negative electrode containing graphite as a negative electrode active material having a D / G ratio of 0.7 or less and / or a G band half width of 28 cm or less. -1 The negative electrode contains the following graphite (hereinafter also referred to as "specific graphite") as an essential component as one of its constituent materials. By using a negative electrode active material containing specific graphite in this way, self-discharge of the battery caused by the sulfonylimide compound (1) (particularly LiN(FSO2)2) is suppressed and storage characteristics are improved, even without using a nonaqueous electrolyte solution in which CO2 or the like is intentionally dissolved.

[0084] The "peak area ratio of the D band and G band analyzed by Raman spectroscopy (D / G ratio)" refers to the peak area ratio of the D band and G band at 1580 cm due to the graphite structure (crystallinity) contained in the carbon material in the Raman spectrum measured using Raman light excited by a laser with a wavelength of 532 nm. -1 The area of ​​the peak intensity I near 1350 cm caused by defects in the graphite structure contained in the carbon material. -1 This refers to the ratio of the area of ​​the peak intensity ID near the center (ID / IG, D band / G band area ratio). Note that these peaks are within ±10 cm -1 It does not matter if the D / G ratio is slightly off. The above-mentioned effect is exhibited if the D / G ratio is 0.7 or less, but from the viewpoint of improving the effect, it is preferably 0.5 or less, more preferably 0.2 or less. The lower limit of the D / G ratio is not particularly limited, but is, for example, 0.05 or more. Graphite having a D / G ratio within the above range is graphite with high crystallinity and relatively few disturbances and defects in the graphite structure. Examples of methods for measuring Raman spectra include the methods described in the Examples below.

[0085] The "G-band half-width analyzed by Raman spectroscopy" refers to the G-band half-width at 1580 cm caused by the graphite structure contained in the carbon material in the Raman spectrum measured using Raman light excited by a laser with a wavelength of 532 nm. -1 The G-band half-width is related to the crystallinity or the amount of disorder and defects in the graphite structure. The G-band half-width is 28 cm -1 The above effect is achieved if the distance is less than 23cm.-1 Less than 20cm, preferably -1 The lower limit of the G-band half-width is not particularly limited, but is, for example, 10 cm -1 Graphite having a G-band half-width within the above range is highly crystalline and has relatively few disturbances and defects in the graphite structure.

[0086] The negative electrode (negative electrode active material) has a D / G ratio of 0.7 or less and / or a G band half width of 28 cm -1 It is sufficient if it contains specific graphite with a D / G ratio of 0.7 or less and / or a G-band half-width of 28 cm -1 The negative electrode may contain two or more types of graphite as long as the specific graphite is not more than 28 cm. -1 In this case, the graphite in the mixture (mixed graphite) has a D / G ratio of 0.7 or less and / or a G-band half width of 28 cm -1 The D / G ratio and G band half width of the mixed graphite can be calculated proportionally from the mixing ratio of the two, and the mixing ratio of the two may be adjusted so as to fall within the above ranges.

[0087] The content of the specific graphite is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the total amount of the negative electrode active material. The upper limit of the content may be 100% by mass.

[0088] From the viewpoint of suppressing self-discharge of the battery, the content of the other negative electrode active material is preferably 70% by mass or less, more preferably 50% by mass or less, even more preferably less than 50% by mass, still more preferably 20% by mass or less, and even more preferably 10% by mass or less, relative to 100% by mass of the total amount of the negative electrode active material.

[0089] D / G ratio is 0.7 or less and / or the G-band half width is 28 cm -1Examples of the specific graphite include MAGE manufactured by Hitachi Chemical Co., Ltd. and SFG15 and SLP50 manufactured by Imerys Co., Ltd. The specific graphites may be used alone or in combination of two or more.

[0090] As described above, the negative electrode active material may contain a specific graphite, but may also contain other negative electrode active materials. Examples of other negative electrode active materials include conventionally known negative electrode active materials used in various batteries (e.g., lithium secondary batteries) and the like, as long as they are capable of absorbing and releasing lithium ions. Examples of other negative electrode active materials include carbon materials such as mesophase sintered bodies made from coal or petroleum pitch, and 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 other negative electrode active materials may be used alone or in combination of two or more.

[0091] 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.

[0092] The negative electrode may be manufactured by the same method as the positive electrode.

[0093] (separator) The nonaqueous electrolyte secondary battery according to this embodiment may include a separator. The separator is disposed to separate the positive electrode from the negative electrode. There are no particular limitations on the separator, and any conventionally known separator can be used in the present disclosure. Specific examples of the separator include a porous sheet made of a polymer capable of absorbing and retaining an electrolyte (nonaqueous electrolyte) (e.g., a polyolefin-based microporous separator or a cellulose-based separator), a nonwoven fabric separator, a porous metal body, and the like.

[0094] Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

[0095] Examples of materials for the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, and glass. Depending on the required mechanical strength, the above-mentioned materials may be used alone or in combination of two or more.

[0096] (battery exterior materials) A battery element including a positive electrode, a negative electrode, and a non-aqueous electrolyte (and a separator) is usually housed in a battery exterior material to protect the battery element from external impacts during battery use, environmental degradation, etc. The material of the battery exterior material is not particularly limited, and any conventionally known exterior material can be used.

[0097] 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.

[0098] The shape of the battery (lithium ion secondary battery, etc.) is not particularly limited, and any of the conventionally known shapes of batteries (lithium ion secondary batteries, etc.) can be used, such as cylindrical, square, laminated, coin, large, etc. Furthermore, when used as a high-voltage power source (several tens to several hundreds of volts) to be mounted on electric vehicles, hybrid electric vehicles, etc., it can also be made into a battery module consisting of individual batteries connected in series.

[0099] The rated charging voltage of a secondary battery (lithium ion secondary battery, etc.) is not particularly limited, but when the secondary battery has a positive electrode containing the above-mentioned ternary positive electrode active material as a main component, it may be, for example, 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, for example, 4.6 V or lower (e.g., 4.5 V or lower).

[0100] <Manufacturing method of non-aqueous electrolyte secondary battery> The nonaqueous electrolyte secondary battery according to this embodiment can be easily manufactured, 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 nonaqueous electrolyte into the battery exterior material, and sealing the battery exterior material.

[0101] As described above, the nonaqueous electrolyte secondary battery according to this embodiment, which includes the sulfonylimide compound (1), comprises a specific electrolyte solvent containing a carbonate-based solvent (preferably 50% by mass or more relative to 100% by mass of the total amount of the electrolyte solvent), and a negative electrode active material having a D / G ratio of 0.7 or less and / or a G-band half width of 28 cm -1 The nonaqueous electrolyte secondary battery is used in combination with a specific negative electrode containing the following graphite (preferably 30% by mass or more relative to 100% by mass of the total amount of negative electrode active material): This provides a synergistic effect of improving self-discharge (storage characteristics) due to the specific negative electrode and improving low-temperature characteristics due to the specific electrolyte solvent. [Example]

[0102] 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.

[0103] <Raman spectroscopy of graphite> Using a JASCO NRS-3100 (manufactured by JASCO Corporation), Raman spectroscopy was carried out on various graphites shown in Table 1 under the following conditions. In the obtained Raman spectra (D / G chart, see Figures 1 to 5), -1 The area of ​​the peak intensity I G around 1350 cm is defined as the "G band peak area." -1 The area of ​​the peak intensity ID near 1580 cm was taken as the "D band peak area," and the ratio of these areas was calculated as the "D band and G band peak area ratio (D / G ratio)." -1The half-width of the peak intensity I G around this point was calculated as the "G-band half-width." The results are shown in Table 1. (Raman spectroscopy measurement conditions) Laser wavelength: 532nm Exposure time: 5 seconds x 4 times ·Center wave number: 2250cm -1 Slit: φ0.2mm -Dimmer: OD1 (laser output 0.7mW) Objective lens: 20x Baseline correction (400cm -1 ~2400cm -1 (linear correction between

[0104] [Table 1]

[0105] <Example 1 Series> (Manufacturing of negative electrodes) (Production Example 1) An aqueous slurry of MAGE (Hitachi Chemical Co., Ltd.): carbon fiber (VGCF, Showa Denko K.K.): carboxymethyl cellulose (CMC, commercially available): styrene butadiene rubber (SBR, commercially available) = 100:2:1:1 (mass ratio, same below) was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-1.

[0106] (Production Example 2) SFG15 (Imerys): A water-based slurry with a composition of VGCF:CMC:SBR=100:2:1:1 was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-2.

[0107] (Production Example 3) SLP50 (Imerys): A water-based slurry with a composition of VGCF:CMC:SBR = 100:2:1:1 was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2) and dried, followed by roll pressing to produce Negative Electrode 1-3.

[0108] (Production Example 4) A water-based slurry of mixed graphite (SLP50 and SFG15) in a mass ratio of 85:15, VGCF, CMC, and SBR (100:2:1:1) was prepared and applied to one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-4.

[0109] (Production Example 5) O-MAC (Osaka Gas Chemicals Co., Ltd.): A water-based slurry with a composition of VGCF:CMC:SBR = 100:2:1:1 was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-5.

[0110] (Production Example 6) A water-based slurry of SMG (manufactured by Hitachi Chemical Co., Ltd.) with a composition of VGCF:CMC:SBR=100:2:1:1 was prepared and applied to one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-6.

[0111] [Production of Laminated Batteries (Examples 1-1 to 1-12, Comparative Examples 1-1 to 1-12)] ·Ternary positive electrode (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM111, manufactured by Umicore), acetylene black (Denka, Denka Black), graphite (Nippon Graphite, SP270), and PVdF (Kureha, L#7208) were weighed in a composition (mass) ratio of 100:3:3:3 and dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The prepared slurry was coated on one side of aluminum foil (coating weight 19.8 mg / cm). 2 ), dried, and then roll-pressed to produce a positive electrode. The resulting positive electrode was cut into a shape with an effective area of ​​12 cm. 2 Cut to. The negative electrode obtained in each manufacturing example had an effective area of ​​13.44 cm 2The types of negative electrodes used are shown in Tables 2 and 3 below. 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.) and LiPF6 (manufactured by Stella Chemifa Corporation) in an electrolyte solvent of the type and composition (volume ratio) shown in Tables 2 and 3 to a concentration of 0.6 mol / L. A cell was fabricated by ultrasonically welding polarity leads to the cut positive and negative electrodes, placing them face-to-face with a 25 μm polyethylene (PE) separator, and sealing the three sides with a laminate exterior. 700 μL of the electrolyte solution shown in Tables 2 and 3 was injected into one of the unsealed sides of the resulting cell, producing a 30 mAh laminate battery (lithium ion battery) as a nonaqueous electrolyte secondary battery. After the electrolyte was injected, the battery was charged at a constant current of 6 mA for 3 hours, one piece was opened, and the battery was vacuum-sealed again to release the gas. After the release, the cell was stored at 25°C for 48 hours, and then charged and discharged under the following conditioning condition 1 to complete the evaluation battery. (Conditioning condition 1) 1st cycle: 3mA, constant current / constant voltage charge at 4.2V, terminated at 0.3mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 2nd cycle: 6mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 3rd cycle: 6mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 30mA, terminate at 2.75V.

[0112] [Evaluation of laminated batteries] [Measurement of low-temperature discharge capacity (evaluation of low-temperature characteristics)] The evaluation battery was charged at a constant current and constant voltage of 30 mA at 25°C with a 0.6 mA cutoff at 4.2 V, and then soaked at 0°C for 2 hours. The discharge capacity was then measured at 30 mA at 0°C with a 2.75 V cutoff. The results are shown in Tables 2 and 3. Note that a larger low-temperature discharge capacity indicates better low-temperature characteristics.

[0113] [Measurement of self-discharge capacity (OCV) (evaluation of storage characteristics)] The evaluation battery was fully charged by constant current and constant voltage charging at 30 mA at 4.2 V with a cutoff of 0.6 mA. The open circuit voltage (OCV) of the battery after full charge was measured (initial OCV). After measurement, the battery was stored at 60°C for 28 days, and then at 25°C for 4 hours, after which the OCV was measured (OCV after 28 days (storage) at 60°C). Using these measurements, the difference (ΔV) between the initial OCV and the OCV after storage was calculated as self-discharge. The results are shown in Tables 2 and 3. Note that the higher the OCV after storage, i.e., the smaller the self-discharge (ΔV), the more suppressed the battery's self-discharge (excellent storage characteristics).

[0114] [Table 2]

[0115] [Table 3]

[0116] [Discussion of Example 1 Series] A comparison of batteries (e.g., Examples 1-1 and 1-7) that differed only in the LiFSI concentration in the non-aqueous electrolyte revealed that the higher the LiFSI concentration, the lower the OCV after storage at 60°C for 28 days, and the greater the self-discharge (ΔV), confirming that the self-discharge from a fully charged state was large. In this regard, a comparison of batteries in which only the type of graphite used in the negative electrode was different (for example, Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-2; Examples 1-7, 1-10 to 1-12 and Comparative Examples 1-7 to 1-8, etc.) showed that the D / G ratio (D / G band area ratio) was 0.7 or less and / or the G band half width was 28 cm -1It was confirmed that batteries using specific graphite (single graphite of MAGE, SFG15 or SLP50, or mixed graphite containing SLP50 and SFG15) with a D / G ratio and / or G-band half width outside the above ranges had higher OCV and smaller self-discharge after storage than batteries using graphite (O-MAC or SMG). Therefore, it was confirmed that batteries using graphite with a D / G ratio of 0.7 or less and / or a G-band half width of 28 cm -1 It has been confirmed that by using the following graphite as the negative electrode active material, it is possible to suppress self-discharge, which was an issue with batteries that use electrolytes containing LiFSI. In addition, Comparative Examples 1-3, 1-4, 1-9, and 1-10, which contained only sulfolane (a sulfone compound) without a carbonate-based solvent as the electrolyte solvent, exhibited significantly lower low-temperature discharge capacity at 0°C than Examples or Comparative Examples containing a carbonate-based solvent. This is thought to be because sulfolane has a relatively high melting point of 27.5°C, which causes it to solidify at low temperatures, resulting in an increase in viscosity. Furthermore, in the heterogeneous mixed solvents containing carbonate-based solvents and sulfolane, those containing a high sulfolane content, for example, Comparative Examples 1-5 and 1-11 containing 60 mass% or more of sulfolane relative to 100 mass% of the total amount of electrolyte solvent, showed a significant decrease in low-temperature discharge capacity due to sulfolane, as described above. On the other hand, in Examples 1-5, 1-6, 1-8, and 1-9 or Comparative Examples 1-6 and 1-12, in which the heterogeneous mixed solvent contained 50 mass % or more of a carbonate-based solvent relative to 100 mass % of the total amount of the electrolyte solvent, the low-temperature discharge capacity was significantly improved. Furthermore, a comparison of batteries differing only in the sulfolane content in the electrolyte solvent (Comparative Examples 1-4 to 1-6 and Comparative Example 1-1, or Comparative Examples 1-10 to 1-12 and Comparative Example 1-7) confirmed that a lower sulfolane content not only improves low-temperature discharge capacity but also suppresses self-discharge. Therefore, it was confirmed that increasing the content of carbonate-based solvent in the electrolyte solvent (to 50% by mass or more relative to 100% by mass of the total amount of electrolyte solvent) improves both the low-temperature characteristics and storage characteristics of the battery.

[0117] <Example 2 Series> (Manufacturing of negative electrodes) The composition is the same as that of negative electrodes 1-1 to 1-6, but the coating weight is 10.8 mg / cm 2 Negative electrodes 2-1 to 2-6 were produced in the same manner as in Production Examples 1 to 6, except for the above change.

[0118] [Production of Laminate Batteries (Examples 2-1 to 2-6, Comparative Examples 2-1 to 2-6)] The positive electrode active material is LiNi manufactured by Beijing Dongben Co., Ltd. 0.8 Co 0.1 Mn 0.1 A positive electrode was produced in the same manner as in Example 1 series, except that O2 (NCM811) was used. A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 series, except that the type and composition (volume ratio) of the electrolyte solvent was changed to those shown in Table 4. Using the obtained positive and negative electrodes, a 30 mAh laminate battery was manufactured in the same manner as in Example 1 series, and was charged and discharged under the above-mentioned Conditioning Condition 1 to complete a battery for evaluation. The types of negative electrode and electrolyte used are shown in Table 4 below.

[0119] [Evaluation of laminated batteries] Using the evaluation battery, the low-temperature discharge capacity and OCV were measured in the same manner as in Example 1. The results are shown in Table 4.

[0120] [Table 4]

[0121] [Discussion of Example 2 Series] Positive electrode: LiNi 0.8 Co 0.1 Mn 0.1 Even if we change to O2 (NCM811), the D / G ratio is less than 0.7 and / or the G-band half width is 28 cm -1 It has been confirmed that batteries using negative electrodes containing the specific graphite described below have high OCV after storage and can suppress self-discharge. Furthermore, with regard to the electrolyte solvent, as in Example 1 series, a significant decrease in low-temperature discharge capacity due to sulfolane was observed, and it was confirmed that by increasing the content of carbonate-based solvent (to 50 mass% or more), the low-temperature discharge capacity was improved and self-discharge was suppressed, i.e., both the low-temperature characteristics and storage characteristics of the battery were improved.

[0122] <Example 3 Series> (Manufacturing of negative electrodes) The composition is the same as that of negative electrodes 1-1 to 1-6, but the coating weight is 8.8 mg / cm 2 Negative electrodes 3-1 to 3-6 were produced in the same manner as in Production Examples 1 to 6, except for changing the above.

[0123] [Production of Laminate Batteries (Examples 3-1 to 3-6, Comparative Examples 3-1 to 3-6)] The positive electrode active material was changed to commercially available LiFePO4, and LiFePO4, acetylene black (HS-100), and PVdF (Kureha Corporation, L#7208) were weighed in a composition (mass) ratio of 100:9:6 and dispersed in NMP to prepare a slurry. The prepared slurry was coated on one side of aluminum foil (coating weight 20.20 mg / cm). 2 ) and dried, followed by roll pressing to produce a positive electrode. A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 series, except that the type and composition (volume ratio) of the electrolyte solvent was changed to those shown in Table 5. Using the obtained positive and negative electrodes, a 25 mAh laminate battery was manufactured in the same manner as in Example 1. The types of negative electrode and electrolyte used are shown in Table 5 below. After the electrolyte was injected, the battery was charged at a constant current of 5 mA for 3 hours, one piece was opened, and the battery was vacuum-sealed again to release the gas. After the release, the battery was stored at 25°C for 48 hours, and then charged and discharged under the following conditioning condition 2 to complete the evaluation battery. (Conditioning condition 2) 1st cycle: Charging: 2.5mA, constant current constant voltage charging at 3.6V, termination at 0.25mA ⇒ Discharge: Discharge at 5mA, terminate at 2.0V. 2nd cycle: Charging: 2.5mA, constant current constant voltage charging at 3.6V, termination at 0.5mA ⇒ Discharge: Discharge at 5mA, terminate at 2.0V. 3rd cycle: Charging: 2.5mA, constant current constant voltage charging at 3.6V, termination at 0.5mA ⇒ Discharge: Discharge at 25mA, terminate at 2.0V.

[0124] [Evaluation of laminated batteries] The evaluation battery was charged and discharged under the following conditions (25° C.), and the initial capacity was confirmed. (Conditions) Charging: 3.6V, 25mA constant current constant voltage charging, 0.5mA termination ⇒ Discharge: 2.5mA terminates at 2.0V.

[0125] [Measurement of low-temperature discharge capacity (evaluation of low-temperature characteristics)] The batteries whose initial capacity had been measured were charged at a constant current and constant voltage of 25 mA at 3.6 V with a 0.5 mA cutoff, and then soaked at 0°C for 2 hours. The discharge capacity was then measured at 25 mA at 0°C with a 2.0 V cutoff. The results are shown in Table 5.

[0126] [Measurement of self-discharge capacity (OCV) (evaluation of storage characteristics)] The OCV was measured in the same manner as in Example 1 series, except that the batteries for which the initial capacity had been measured were charged at a constant current of 25 mA and a constant voltage of 3.6 V with a cut-off of 0.5 mA to reach a fully charged state. Furthermore, after 28 days (storage) at 60°C, the batteries for which the OCV had been measured were discharged at a constant current of 2.5 mA with a cut-off of 2.0 V, and the remaining capacity after 28 days of storage at 60°C was measured. The remaining capacity thus obtained and the initial capacity were used to calculate the formula (1): [Number 1] Remaining rate (%) = remaining capacity / initial capacity × 100 (1) The survival rate was calculated by the above method, and the results are shown in Table 5. A high survival rate means a low self-discharge capacity, which indicates that self-discharge is suppressed.

[0127] [Table 5]

[0128] [Discussion of Example 3 Series] Even if the positive electrode is changed to LiFePO4, the D / G ratio is less than 0.7 and / or the G-band half width is 28 cm -1 It has been confirmed that batteries using negative electrodes containing the specific graphite described below have high OCV after storage and can suppress self-discharge. Because the discharge voltage of the LiFePO4 cathode is flat, the relationship between OCV and self-discharge (ΔV) may not be clear. Therefore, when the residual rate during storage was checked, the D / G ratio was 0.7 or less and / or the G-band half width was 28 cm. -1 It has been confirmed that batteries using negative electrodes containing the specific graphite listed below have a high survival rate and are able to suppress self-discharge. Furthermore, with regard to the electrolyte solvent, as in Example 1 series, a significant decrease in low-temperature discharge capacity due to sulfolane was observed, and it was confirmed that by increasing the content of carbonate-based solvent (to 50 mass% or more), the low-temperature discharge capacity was improved and self-discharge was suppressed, i.e., both the low-temperature characteristics and storage characteristics of the battery were improved.

[0129] <Example 4 Series> [Synthesis and Analysis of Additives] ( 31 P-NMR analysis) Trimethylsilyl polyphosphate (hereinafter referred to as "PPSE-1"), a reagent manufactured by Sigma-Aldrich, was used. 31 When analyzed by P-NMR, two peaks were confirmed, one appearing at a chemical shift of -28 ppm to -33 ppm (Pt) and the other appearing at a chemical shift of -35 ppm to -41 ppm (Pm), as shown in Figure 6. The integral ratio of the two peaks was Pt:Pm = 1.00:1.43. On the other hand, the peak appearing at -41 ppm to -45 ppm (Pb) was not confirmed. 31 P-NMR measurements were performed using a JNM-ECA500 manufactured by JEOL (Japan Electronics Corporation) with a double sample tube as the sample tube, and the chemical shift was determined with the phosphorus peak of H3PO4 added to one of the tubes as 0 ppm.

[0130] (Synthesis of trimethylsilyl polyphosphate 1) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of methylene chloride solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. The mixture was then stirred at room temperature for about a day. The solvent was then distilled off to synthesize polytrimethylsilyl phosphate (hereinafter also referred to as "PPSE-2"). ( 31 P-NMR analysis) The synthesized trimethylsilyl polyphosphate (PPSE-2) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm, a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm, and a peak (Pb) appearing at chemical shifts of -41 ppm to -45 ppm, as shown in Figure 7. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.73:1.00. From these analysis results, PPSE-2 is presumed to be polytrimethylsilyl phosphate containing a large amount of branched structures (branched structures represented by the above structural formula (4-3)).

[0131] (Synthesis of trimethylsilyl polyphosphate 2) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of toluene as a solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. The mixture was then stirred at room temperature for about a day. The solvent was then distilled off to synthesize polytrimethylsilyl phosphate (hereinafter also referred to as "PPSE-3"). ( 31 P-NMR analysis) The synthesized trimethylsilyl polyphosphate (PPSE-3) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm, a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm, and a peak (Pb) appearing at chemical shifts of -41 ppm to -45 ppm, as shown in Figure 8. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.51:0.81. From these analysis results, PPSE-3 is presumed to be polytrimethylsilyl phosphate containing a large amount of branched structures (branched structures represented by the above structural formula (4-3)).

[0132] (Synthesis of ethyl polyphosphate) 10 g of diphosphorus pentoxide, 10 g of chloroform as a solvent, and 20 g of diethyl ether were placed in a test tube and stirred at 900 rpm at 35°C for 3 days. The solvent was then removed using an evaporator, and the residue was vacuum dried for 24 hours to synthesize ethyl polyphosphate (hereinafter also referred to as "PPE"). ( 31 P-NMR analysis) The synthesized polyethyl phosphate (PPE) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -12 ppm to -15 ppm, a peak (Pm) appearing at chemical shifts of -25 ppm to -31 ppm, and a peak (Pb) appearing at chemical shifts of -39 ppm to -46 ppm. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:3.59:0.42. From these analysis results, it is estimated that PPE is polyethyl phosphate containing a large amount of branched structures (branched structures represented by the above structural formula (4-3) in which TMS is an ethyl group).

[0133] (Synthesis of triisopropylsilyl polyphosphate) 0.53 g of indium(III) bromide was dissolved in 30 mL of tetrahydrofuran, followed by the addition of 4.75 g of triisopropylsilane. The reaction was allowed to proceed with stirring at room temperature for one day. 30 mL of hexane was added to the reaction solution, which was then allowed to stand, resulting in separation into two layers. The separated upper layer was concentrated under reduced pressure to yield 4.01 g of hexaisopropyldisiloxane, a colorless liquid. Next, 1.55 g of phosphorus pentoxide and 1.70 g of the hexaisopropyldisiloxane obtained above were added to 10 mL of dichloromethane, and the reaction was allowed to proceed with stirring at 35°C for three days. The reaction solution was filtered and concentrated under reduced pressure to synthesize 2.10 g of poly(triisopropylsilyl)phosphate (hereinafter referred to as "PPSE(TIPS)") as a viscous liquid.

[0134] (Synthesis of (tert-butyl)dimethylsilyl polyphosphate) Poly(tert-butyl)dimethylsilyl]phosphate (hereinafter also referred to as "PPSE(TBDMS)") was synthesized by the same procedure as in the synthesis of PPSE(TIPS), except that the triisopropylsilane used in the synthesis of PPSE(TIPS) was changed to tert-butyldimethylsilane.

[0135] [Production of Laminated Batteries (Examples 4-1 to 4-15, Comparative Examples 4-1 to 4-8)] The positive electrode active material is LiNi manufactured by Beijing Dongben Co., Ltd. 0.5 Co 0.2 Mn 0.3 Changed to O2 (NCM523), coating weight 19.5 mg / cm 2 A positive electrode was produced in the same manner as in Example 1 series, except that the above was changed. The negative electrodes 1-1, 1-2, 1-3, 1-5, and 1-6 prepared in Example 1 series were used. A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 series, except that the type and composition (volume ratio) of the electrolyte solvent was changed to those shown in Table 6. If necessary, an additive shown in Table 6 was added to the non-aqueous electrolyte solution obtained above to give the content shown in Table 6, and the mixture was stirred for one day to prepare a non-aqueous electrolyte solution containing the additive. Using the obtained positive and negative electrodes, a 30 mAh laminate battery was manufactured in the same manner as in Example 1. The types of negative electrode and electrolyte used are shown in Table 6 below. After the electrolyte was injected, the battery was vacuum sealed and charged at a constant current of 3 mA at 25°C for 3 hours. It was then left at room temperature for 2 days, and one piece of the laminate exterior was cleaved and vacuum sealed again to allow for degassing. After degassing, the battery was charged and discharged under the following conditioning condition 3 to complete the evaluation battery. (Conditioning condition 3) 1st cycle: 3mA, constant current / constant voltage charge at 4.2V, terminated at 0.3mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 2nd cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 3rd cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 30mA, terminate at 2.75V. 4th cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 60mA, terminate at 2.75V.

[0136] [Evaluation of laminated batteries] [Measurement of self-discharge capacity (OCV) (evaluation of storage characteristics)] The conditioned batteries were charged to a full charge state using a charge / discharge tester at a constant current of 30 mA (1C), 4.2 V, and a 0.6 mA cutoff. The fully charged batteries were stored at 80°C for 14 days, and then left at 25°C for at least 6 hours. The OCV was measured in the same manner as in Example 1. The results are shown in Table 6.

[0137] [Detected amount of transition metals] After the OCV measurement, the battery was discharged and then disassembled. The removed PE separator and negative electrode were washed with 50 mL of EMC and dried. The negative electrode composite (negative electrode active material) was then peeled from the current collector (copper foil) and immersed in 1 g of nitric acid for 24 hours to dissolve. The PE separator was then also immersed in the same nitric acid for 24 hours. The resulting nitric acid solution was filtered and diluted with ultrapure water to prepare a measurement sample. The measurement sample was analyzed using an ICP atomic emission spectrometer (Shimadzu Corporation) to determine the amounts of nickel (Ni), cobalt (Co), and manganese (Mn) (per battery (cell)) in the measurement sample. The results are shown in Table 6.

[0138] [Table 6]

[0139] [Discussion of Example 4 Series] LiNi as the positive electrode 0.5 Co 0.2 Mn 0.3 When using O2 (NCM523), the D / G ratio must be less than 0.7 (or the G-band half-width must be less than 28 cm -1When compared with batteries using anodes containing graphite with a D / G ratio (or G-band half-width) outside the above range (batteries with a different anode only), all batteries using anodes containing specific graphite (see below) had a relatively high OCV after storage at 80°C for 14 days, and it was confirmed that self-discharge (ΔV) during storage was suppressed. Furthermore, batteries using negative electrodes containing the specific graphite exhibited lower amounts of Ni, Co, and Mn than batteries using negative electrodes containing other graphites, confirming that the elution of Ni, Co, and Mn from the positive electrode is suppressed. This result suggests that self-discharge is suppressed by reducing the side reaction of transition metal elution from the positive electrode. It was confirmed that the addition of polytrimethylsilyl phosphate (PPSE-1) to the electrolyte significantly reduced the amount of Ni, Co, and Mn eluted, and as a result, self-discharge during storage at 80°C for 14 days was further suppressed. Furthermore, it was confirmed that the above reduction and suppression effect of adding PPSE-1 was greater in batteries using anodes containing the specific graphite mentioned above than in batteries using anodes containing other graphites (i.e., the amount of Ni, Co, and Mn eluted was more effectively reduced, and self-discharge was more effectively suppressed). The use of polytrimethylsilyl phosphate (PPSE-2, PPSE-3), which contains a high proportion of branched structures, also showed a similar trend to that of the commercially available polytrimethylsilyl phosphate (PPSE-1). In other words, the addition of PPSE-2 or PPSE-3 to the electrolyte reduced the amount of Ni elution, and as a result, self-discharge during storage at 80°C for 14 days was also confirmed to be suppressed. While the reasons for this are unclear, PPSE-2 and PPSE-3, which contain a peak (Pb) indicating a branched structure, detected less Ni than PPSE-1, which does not exhibit a Pb peak, confirming that Ni elution from the positive electrode was further suppressed. This is thought to be the reason for the improvement in self-discharge. The use of polyethyl phosphate (PPE), which contains a large amount of branched structures, showed a similar trend to that of polytrimethylsilyl phosphate (PPSE-1). In other words, it was confirmed that adding PPE to the electrolyte reduced the amount of Ni elution, and therefore also suppressed self-discharge during storage at 80°C for 14 days. The use of poly(triisopropylsilyl)phosphate [PPSE(TIPS)] or poly(tert-butyl)dimethylsilyl]phosphate (PPSE(TBDMS)) showed a similar trend to that of poly(trimethylsilyl)phosphate (PPSE-1). In other words, the addition of PPSE(TIPS) or PPSE(TBDMS) to the electrolyte reduced the amount of Ni elution, and as a result, self-discharge during storage at 80°C for 14 days was also suppressed.

[0140] <Example 5 Series> [Production of Laminated Batteries (Examples 5-1 to 5-15, Comparative Examples 5-1 to 5-8)] The positive electrode active material is LiNi manufactured by Beijing Dongben Co., Ltd. 0.8 Co 0.1 Mn 0.1 Changed to O2 (NCM811), coating weight 15.7 mg / cm 2 A positive electrode was produced in the same manner as in Example 1 series, except that the above was changed. The negative electrodes 1-1, 1-2, 1-3, 1-5, and 1-6 prepared in Example 1 series were used. A non-aqueous electrolyte solution was prepared in the same manner as in Example 4 series, except that the type and composition (volume ratio) of the electrolyte solvent was changed to those shown in Table 7. Using the obtained positive and negative electrodes, evaluation batteries were completed in the same manner as in Example 4. The types of negative electrodes and electrolytes used are shown in Table 7 below.

[0141] [Evaluation of laminated batteries] The test batteries were evaluated in the same manner as in Example 4. The results are shown in Table 7 below.

[0142] [Table 7]

[0143] [Discussion of Example 5 Series] Positive electrode: LiNi 0.8 Co 0.1 Mn 0.1Even when changed to O2 (NCM811), the same tendency as in Example 4 series was observed. In other words, when NCM811 is used as the positive electrode, the D / G ratio is 0.7 or less (or the G band half width is 28 cm -1 Batteries using negative electrodes containing specific graphite (see below) had relatively high OCVs after 14 days of storage at 80°C compared to batteries (batteries with only a different negative electrode) containing graphite with a D / G ratio (or G-band half-width) outside the above range, confirming that self-discharge (ΔV) during storage was suppressed. However, the suppression effect was smaller than the self-discharge observed during 28 days of storage at 60°C in Example 2 series using the same NCM811. This is thought to be due to the accelerated elution of transition metals (Ni, Co, Mn) from the positive electrode as a side reaction when the battery was stored in a high-temperature environment with a higher load. Furthermore, as in the fourth example series, the batteries using the negative electrodes containing the specific graphite had a lower amount of Ni detected than the batteries using negative electrodes containing other graphites, confirming that Ni elution from the positive electrode was suppressed. This result suggests that the reduction in side reactions suppresses self-discharge. Even when the positive electrode was changed to NCM811, as in the Example 4 series, the amount of Ni elution was reduced by adding to the electrolyte trimethylsilyl polyphosphate (PPSE-1), highly branched trimethylsilyl polyphosphate (PPSE-2, PPSE-3), highly branched ethyl polyphosphate (PPE), triisopropylsilyl polyphosphate [PPSE(TIPS)], or [(tert-butyl)dimethylsilyl] polyphosphate (PPSE(TBDMS)), and this was confirmed to also suppress self-discharge during storage at 80°C for 14 days. Furthermore, as in the Example 4 series, PPSE-2 and PPSE-3, which have a peak (Pb) indicating a branched structure, had a lower amount of Ni detected than PPSE-1, which does not have a Pb peak, confirming that Ni elution from the positive electrode was further suppressed. This is thought to be the reason for the improvement in self-discharge.

Claims

1. A lithium ion secondary battery comprising a positive electrode, a nonaqueous electrolyte solution containing an electrolyte salt and an electrolyte solvent, and a negative electrode containing a negative electrode active material, the electrolyte salt contains a sulfonylimide compound represented by general formula (1), the electrolyte solvent contains a carbonate-based solvent, the negative electrode active material includes a first graphite having a peak area ratio (D / G ratio) of a D band to a G band as analyzed by Raman spectroscopy of 0.7 or less; When the negative electrode active material contains second graphite having a peak area ratio (D / G ratio) of a D band to a G band analyzed by Raman spectroscopy of greater than 0.7, the peak area ratio (D / G ratio) of a mixed graphite of the first graphite and the second graphite is 0.7 or less, A lithium ion secondary battery, characterized in that the content of the carbonate-based solvent is 80 mass % or more with respect to a total amount of 100 mass % of the electrolyte solvent. 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)

2. A lithium ion secondary battery comprising a positive electrode, a nonaqueous electrolyte solution containing an electrolyte salt and an electrolyte solvent, and a negative electrode containing a negative electrode active material, the electrolyte salt contains a sulfonylimide compound represented by general formula (1), the electrolyte solvent contains a carbonate-based solvent, The negative electrode active material has a G band half width of 28 cm as analyzed by Raman spectroscopy. －１ a first graphite, When the negative electrode active material contains second graphite having a G-band half width greater than 28 cm −1 as analyzed by Raman spectroscopy, the G-band half width of a mixed graphite of the first graphite and the second graphite is 28 cm −1 or less; A lithium ion secondary battery, characterized in that the content of the carbonate-based solvent is 80 mass % or more with respect to a total amount of 100 mass % of the electrolyte solvent. 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)

3. 3. The lithium ion secondary battery according to claim 1, wherein the sulfonylimide compound represented by the general formula (1) is contained in the non-aqueous electrolyte solution in an amount of 0.2 mol / L or more.

4. The sulfonylimide compound represented by the general formula (1) is LiN(FSO ２ ) ２ 3. The lithium ion secondary battery according to claim 1, further comprising:

5. 3. The lithium ion secondary battery according to claim 1, wherein the carbonate solvent includes at least one selected from the group consisting of chain carbonate solvents and saturated cyclic carbonate solvents.

6. 3. The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte contains a phosphorus atom-containing compound represented by general formula (4) as an additive. [-P(=O)(OR １ )O-] ｎ ・・・(4) (In formula (4), R １ represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt or a hydrogen atom, and n represents 2 or more.

7. 3. The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte solution contains at least one additive selected from the group consisting of trimethylsilyl polyphosphate, ethyl polyphosphate, (triisopropylsilyl) polyphosphate, and (tert-butyl)dimethylsilyl polyphosphate.

8. 3. The lithium ion secondary battery according to claim 1, wherein the positive electrode contains a positive electrode active material represented by general formula (5). Li ｖ Ni ｘ Co ｙ Mn ｚ O ２＋ｗ [0.2≦v≦1.2, 0.5≦x≦0.9, 0<y≦0.3, 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)] (5)

9. The electrolyte salt is a compound represented by general formula (2), a compound represented by general formula (3), and LiAsF ６ 3. The lithium ion secondary battery according to claim 1, further comprising at least one selected from the group consisting of: LiPF ａ (C ｍ F ２ｍ＋１ ) ６－ａ (a:0≦a≦6、m:1≦m≦4)・・・ (2) LiBF ｂ (C ｎ F ２ｎ+１ ) ４－ｂ (b:0≦b≦4、n:1≦n≦4)・・・(3)

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