Non-aqueous electrolyte secondary batteries

Abstract

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

Description

[Technical Field]

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

[0002] Various non-aqueous electrolytes and their materials used in secondary batteries have been investigated to improve the battery performance of secondary batteries such as lithium-ion secondary batteries. Through previous investigations, the applicant has found that non-aqueous electrolytes containing sulfonylimide compounds such as lithium bis(fluorosulfonyl)imide as the electrolyte salt improve the high-temperature durability and charge-discharge cycle performance of lithium-ion secondary batteries.

[0003] For example, in Patent Document 1, the applicant has proposed a non-aqueous electrolyte comprising LiN(FSO2)2 as the electrolyte and at least one of a silicon atom-containing compound, a boron atom-containing compound, a carbon atom-containing compound, a sulfur atom-containing compound, and a phosphorus atom-containing compound as an additive. In this non-aqueous electrolyte containing sulfonylimide compounds, by using specific compounds, it is possible to suppress the self-discharge of the battery and reduce the charge transfer resistance (impedance) and the DC resistance (DCR) of the battery, thereby improving battery performance. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2022 / 239807 [Overview of the project] [Problems that the invention aims to solve]

[0005] By the way, a non-aqueous electrolyte containing a sulfonylime compound and LiNi, which is commonly used as a positive electrode active material. 1 / 3 Co 1 / 3 Mn 1 / 3In a secondary battery including a positive electrode containing a lithium composite oxide containing Ni (nickel) such as O2 (NCM111), the use of an additive may conversely increase the resistance. That is, depending on the type of the positive electrode active material, there is a risk that the battery performance may deteriorate due to the additive in the non-aqueous electrolyte.

[0006] Also, in a conventional secondary battery using NCM111, it has been difficult to meet the requirements for the energy density of batteries for electric vehicles (EVs) in recent years. Therefore, the development of secondary batteries having a higher energy density than the prior art has been demanded. As a method for improving the energy density of a secondary battery, for example, a high-Ni-containing lithium composite oxide having a high Ni content ratio in transition metals can be considered as the positive electrode active material. However, in a secondary battery using a high-Ni-containing lithium composite oxide, since the increase in resistance associated with the use of the battery is large, a technique for reducing the resistance is required.

[0007] In a secondary battery, three types of resistances, namely, the initial resistance when the battery is completed by charging and discharging under predetermined conditioning conditions after manufacture, the resistance associated with the use of the battery, and the resistance after high-temperature storage, affect the battery performance. From the viewpoint of improving the battery performance, it is desirable that any of the resistance values is small.

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

Means for Solving the Problems

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

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

[0011] Further, the non-aqueous electrolyte secondary battery of the present disclosure · A non-aqueous electrolyte containing a sulfonylimide compound represented by the above general formula (1) and a nitrile compound having a linear alkyl group with 3 to 6 carbon atoms, · And a cathode including a cathode active material represented by the above general formula (2).

[0012] In the non-aqueous electrolyte secondary battery of this disclosure, the content of the nitrile compound relative to the sulfonyliimide compound may be 10 ppm by mass or more. The sulfonyliimide compound may also contain LiN(FSO2)2. [Effects of the Invention]

[0013] According to this disclosure, it is possible to reduce the resistance of a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing a sulfonylime compound and a high-Ni cathode containing a high-Ni lithium composite oxide. [Modes for carrying out the invention]

[0014] The following describes the embodiments in detail. The following description of preferred embodiments is essentially illustrative and is not intended to limit the invention, its applications, or its uses in any way.

[0015] <Nonaqueous electrolyte secondary battery> A non-aqueous electrolyte secondary battery refers to a secondary battery that includes a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery according to this embodiment includes a non-aqueous electrolyte, a positive electrode, and a negative electrode.

[0016] [Nonaqueous electrolyte] (Electrolyte salts) The non-aqueous electrolyte according to this embodiment is an electrolyte salt of general formula (1): [C1] LiN(RSO2)(FSO2)···(1) It contains a sulfonilimide compound represented by (hereinafter referred to as "sulfonilimide compound (1)," a fluorine-containing sulfonilimide salt). The non-aqueous electrolyte contains sulfonilimide compound (1) as an essential component.

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

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

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

[0020] The substituent R is preferably a fluorine atom and 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, even more preferably a fluorine atom and a trifluoromethyl group, and still more preferably a fluorine atom.

[0021] Specific examples of sulfonylimide compounds (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, lithium (fluorosulfonyl)(heptafluoropropylsulfonyl)imide, etc. Sulfonylimide compounds may be used individually or in combination of two or more. Sulfonylimide compounds (1) may be commercially available products or those synthesized by conventionally known methods.

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

[0023] The concentration (content, total content if two or more types are used) of the sulfonylimide compound (1) in the non-aqueous electrolyte is preferably 0.2 mol / L or higher, more preferably 0.3 mol / L or higher, and even more preferably 0.5 mol / L or higher, from the viewpoint of improving battery performance (especially reducing resistance). Furthermore, from the viewpoint of suppressing the decrease in battery performance due to an increase in electrolyte viscosity, the concentration is preferably 5 mol / L or lower, more preferably 3 mol / L or lower, and even more preferably 2 mol / L or lower.

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

[0025] From the viewpoint of improving battery performance, 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, relative to the total amount of components contained in the non-aqueous electrolyte (100% by mass). Furthermore, from the viewpoint of suppressing the decrease in battery performance due to an increase in electrolyte viscosity, the concentration is preferably 70% by mass or less, more preferably 50% by mass or less, even more preferably 30% by mass or less, and even more preferably 20% by mass or less.

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

[0027] Examples of imide salts include other fluorine-containing sulfonylimide salts different from the sulfonylimide compound (1) (hereinafter referred to as "other sulfonylimide compounds"). Examples of other sulfonylimide compounds include non-lithium salts of fluorine-containing sulfonylimides listed as the sulfonylimide compound (1) (for example, in the sulfonylimide compound (1), salts in which lithium (ion) is replaced with a cation other than lithium ion). Examples of salts replaced with a cation other than lithium ion 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; phosphonium salt, etc. Other sulfonylimide compounds may be used alone or in combination of two or more. Also, other sulfonylimide compounds may be commercially available products or those obtained by synthesis by a conventionally known method.

[0028] Examples of non-imide salts include salts of non-imide anions and cations (lithium ion and the above-exemplified cations). Examples of non-imide salts include the compound represented by the general formula (3): [Chemical formula 2] LiPF a (C m F 2m+1 ) 6-a (a: 0 ≤ a ≤ 6, m: 1 ≤ m ≤ 4) ··· (3) hereinafter referred to as "fluorophosphoric acid compound (3)", and the general formula (4): [Chemical formula 3] LiBF b (C n F 2n+1 ) 4-b (b: 0 ≤ b ≤ 4, n: 1 ≤ n ≤ 4) ··· (4) Examples include compounds represented by (hereinafter referred to as "fluoroboric acid compounds (4)"), lithium salts of lithium hexafluoride (LiAsF6), LiSbF6, LiClO4, LiSCN, LiAlF4, CF3SO3Li, LiC[(CF3SO2)3], LiN(NO2), LiN[(CN)2], etc., and non-lithium salts. Examples of non-lithium salts include salts in which lithium (ions) are substituted with the cations exemplified above in these lithium salts (e.g., NaBF4, NaPF6, NaPF3(CF3)3, etc.). Non-imide salts may be used individually or in combination of two or more types. Furthermore, non-imide salts may be commercially available or synthesized by conventionally known methods.

[0029] Among other electrolytes, non-imide salts are preferred from the viewpoint of ionic conductivity and cost, with fluorophosphate compounds (3), fluoroboric acid compounds (4), and LiAsF6 being preferred, and fluorophosphate compounds (3) being more preferred.

[0030] Examples of fluorophosphate compounds (3) include LiPF6, LiPF3(CF3)3, LiPF3(C2F5)3, LiPF3(C3F7)3, and LiPF3(C4F9)3. Among the fluorophosphate compounds (3), LiPF6 and LiPF3(C2F5)3 are preferred, and LiPF6 is more preferred.

[0031] Examples of fluoroboric acid compounds (4) include LiBF4, LiBF(CF3)3, LiBF(C2F5)3, and LiBF(C3F7)3. Among the fluoroboric acid compounds (4), LiBF4 and LiBF(CF3)3 are preferred, with LiBF4 being more preferred.

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

[0033] The electrolyte salt composition may be that of a single salt of sulfonylimide compound (1), or it may be a mixed salt composition containing sulfonylimide compound (1) and other electrolytes. When using a mixed salt composition electrolyte salt, a mixed salt composition containing sulfonylimide compound (1) and fluorophosphate compound (3) is preferred, and a mixed salt composition containing LiN(FSO2)2 and LiPF6 is more preferred.

[0034] When using an electrolyte salt with a mixed salt composition containing a sulfonylime compound (1) and other electrolytes, the concentration (content, total content if two or more types are used) of the other electrolytes in the non-aqueous electrolyte 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, still more preferably 0.7 mol / L or more, and still 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, the concentration is preferably 5 mol / L or less, more preferably 3 mol / L or less, still more preferably 2 mol / L or less, and still more preferably 1.5 mol / L or less.

[0035] The total concentration of electrolyte salts in the non-aqueous electrolyte is preferably 0.8 mol / L or higher, more preferably 1 mol / L or higher, and even more preferably 1.2 mol / L or higher, 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, the concentration is preferably 5 mol / L or lower, more preferably 3 mol / L or lower, and even more preferably 2 mol / L or lower.

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

[0037] (Nitrile compounds having a chain-like alkyl group with 3 to 6 carbon atoms) Furthermore, the non-aqueous electrolyte according to this embodiment contains, as an essential component, a nitrile compound having a branched alkyl group with 3 to 6 carbon atoms (hereinafter also referred to as "branched alkyl nitrile compound") or a nitrile compound having a linear alkyl group with 3 to 6 carbon atoms (hereinafter also referred to as "linear alkyl nitrile compound"). A "linear alkyl nitrile compound" is distinguished from a "branched alkyl nitrile compound" in that the linear alkyl group with 3 to 6 carbon atoms does not have a branched structure. Branched alkyl nitrile compounds and linear alkyl nitrile compounds may be used individually or in combination of two or more types. Note that branched alkyl nitrile compounds and linear alkyl nitrile compounds are collectively referred to as "linear alkyl nitrile compounds."

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

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

[0040] The content of linear alkylnitrile compounds (total if two or more are used in combination) is preferably 10 ppm by mass or more, more preferably 20 ppm by mass or more, even more preferably 100 ppm by mass or more, even more preferably 500 ppm by mass or more, and even more preferably 1000 ppm by mass or more, relative to sulfonylimide compound (1) (total if two or more are used in combination), from the viewpoint of improving battery performance (especially resistance reduction). The upper limit of this content is preferably 6000 ppm by mass or less, and more preferably 5000 ppm by mass or less. The content of linear alkylnitrile compounds relative to sulfonylimide compound (1) is preferably 10 ppm by mass or more and 6000 ppm by mass or less. By specifying the ratio relationship between sulfonylimide compound (1) and linear alkylnitrile compounds within the above range, the resistance of the secondary battery equipped with this non-aqueous electrolyte can be reduced.

[0041] (Additives) The non-aqueous electrolyte may contain additives other than linear alkylnitrile compounds, for the purpose of improving various properties of the lithium-ion secondary battery. The additives may be added to the non-aqueous electrolyte or added during the preparation process of the non-aqueous electrolyte.Additives include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic acid anhydride, cyclopentanetetracarboxylic acid dianhydride, and phenylsuccinic acid anhydride; and sulfur-containing compounds such as ethylene sulfite, 1,3-propanesultone, 1,4-butanesultone, 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 erythritol 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, etc.); fluorosulfonic acid compounds such as lithium fluorosulfonate (LiFSO3), sodium fluorosulfonate (NaFSO3), potassium fluorosulfonate (KFSO3), and magnesium fluorosulfonate (Mg(FSO3)2); lithium monofluorophosphate (Li2PO3F Examples of additives include fluorophosphate compounds such as lithium difluorophosphate (LiPO2F2); fluorooxalate 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 lithium salts having an oxalic acid skeleton. These additives may be used individually or in combination of two or more types.

[0042] The additive is preferably used in an amount of 0.1% to 10% by mass, more preferably in an amount of 0.2% to 8% by mass, and even more preferably in an amount of 0.3% to 5% by mass, based on 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 effect derived from the additive, while if a large amount of additive is used, it may be difficult to obtain an effect commensurate with the amount added, and the viscosity of the non-aqueous electrolyte may increase, potentially reducing its conductivity.

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

[0044] As a non-aqueous solvent, a solvent with a high dielectric constant, high solubility of the electrolyte, a boiling point of 60°C or higher, and a wide electrochemical stability range is preferred. More preferably, an organic solvent with a low water content is used. Such organic solvents include ether-based solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; linear carbonate ester (carbonate) solvents such as dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), diphenyl carbonate, and methylphenyl carbonate; saturated cyclic carbonate ester solvents such as ethylene carbonate (EC), propylene carbonate (PC), 2,3-dimethylethylene carbonate, 1,2-butylene carbonate, and erythritol carbonate; cyclic carbonate ester solvents with unsaturated bonds such as vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, 2-vinylethylene carbonate, and phenylethylene carbonate; and fluoroethylene carbonate and 4,5-difluoroethylene carbonate. Fluorine-containing cyclic carbonate ester solvents such as trifluoropropylene carbonate; aromatic carboxylic acid ester solvents such as methyl benzoate and ethyl benzoate; lactone solvents such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone; phosphate ester solvents such as trimethyl phosphate, ethyldimethyl phosphate, diethylmethyl phosphate, and triethyl phosphate; nitrile solvents such as acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, and 2-methylglutaronitrile. Examples include tolyl solvents; sulfur compound solvents such as dimethyl sulfone, ethylmethyl sulfone, diethyl sulfone, sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane; 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-like ester solvents such as ethyl acetate, butyl acetate, and propyl propionate.These solvents may be used individually or in combination of two or more types.

[0045] Among the electrolyte solvents, carbonate-based solvents such as linear carbonate ester solvents and cyclic carbonate ester solvents, lactone-based solvents, ether-based solvents, and linear ester-based solvents are preferred, with dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, γ-butyrolactone, and γ-valerolactone being more preferred, and carbonate-based solvents such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate being even more preferred.

[0046] When using polymers or polymer gels as electrolyte solvents, the following methods can be employed: Specifically, a method in which a solution of an electrolyte salt dissolved in a solvent is dropped onto a polymer film formed by a conventionally known method to impregnate and support the electrolyte salt and a non-aqueous solvent; a method in which the polymer and electrolyte salt are melted and mixed at a temperature above the polymer's melting point, a film is formed, and the solvent is impregnated therein (the above are gel electrolytes); a method in which a non-aqueous electrolyte solution, in which the electrolyte salt is dissolved in an organic solvent beforehand, is mixed with the polymer, a film is formed using a casting or coating method, and the organic solvent is evaporated; a method in which the polymer and electrolyte salt are melted and mixed at a temperature above the polymer's melting point to form a mold (intrinsic polymer electrolyte), etc.

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

[0048] Based on the above, the non-aqueous electrolyte according to this embodiment consists of a sulfonylimide compound (1) and a branched alkylnitrile compound or a linear alkylnitrile compound (chain alkylnitrile compound) as essential components, and is composed of other electrolyte salts, electrolyte solvents, various additives (other than chain alkylnitrile compounds) as needed. The non-aqueous electrolyte can be prepared, for example, by mixing these components in a predetermined composition ratio.

[0049] [Positive electrode] The positive electrode includes a positive electrode current collector and a positive electrode composite layer, the positive electrode composite layer being formed on the positive electrode current collector and usually in sheet form.

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

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

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

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

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

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

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

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

[0058] Conductive additives are used to improve the output of lithium-ion secondary batteries. Conductive additives mainly consist of conductive carbon. Examples of conductive carbon include carbon black, fibrous carbon (carbon fiber), and graphite. Conductive additives may be used individually or in combination of two or more types. Among the 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 conductive additives in the non-volatile components is preferably 1 to 20% by mass, more preferably 1.5 to 10% by mass.

[0059] Examples of binders include fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber (SBR) and nitrile butadiene rubber; polyamide resins such as polyamide-imide; polyolefin resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; and cellulose resins such as carboxymethylcellulose (CMC). Binders may be used individually or in combination of two or more types. Furthermore, binders may be used either dissolved in a solvent or dispersed in a solvent.

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

[0061] The positive electrode mixture may contain, as necessary, other components such as polymers including non-fluorinated polymers such as (meth)acrylic polymers, nitrile polymers, and diene polymers, fluorinated polymers such as polytetrafluoroethylene, emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymeric dispersants such as styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethylcellulose (CMC), hydroxyethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers; and preservatives. The content of other components in the non-volatile content of the positive electrode mixture is preferably 0 to 15% by mass, more preferably 0 to 10% by mass.

[0062] A positive electrode composite material can be prepared, for example, by mixing a positive electrode active material, a conductive additive, a binder, a solvent, and other components as needed, and then dispersing them using a bead mill, ball mill, agitator, or the like.

[0063] The method for forming the positive electrode (coating method) is not particularly limited and includes, for example, (1) a method of applying (and drying) the positive electrode composite material to the positive electrode current collector using a conventional coating method (e.g., the doctor blade method), (2) a method of immersing (and drying) the positive electrode current collector in the positive electrode composite material, (3) a method of joining (e.g., joining via a conductive adhesive) a sheet formed from the positive electrode composite material to the positive electrode current collector and pressing (and drying), (4) a method of applying or casting a positive electrode composite material with added liquid lubricant onto the positive electrode current collector to form a desired shape, and then removing the liquid lubricant (and then stretching in a uniaxial or multiaxial direction), and (5) a method of slurrying the positive electrode composite material (or the solid component forming the positive electrode composite layer) with an electrolyte, transferring it to the current collector (positive electrode current collector) in a semi-solid state, and using it as an electrode (positive electrode) without drying.

[0064] Furthermore, the positive electrode composite layer may be dried or pressed after formation or coating, if necessary.

[0065] [Negative electrode] The negative electrode includes a negative electrode current collector and a negative electrode composite layer, the negative electrode composite layer being formed on the negative electrode current collector and usually molded into a sheet.

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

[0067] The negative electrode composite layer is formed from a negative electrode composite (negative electrode composition). The negative electrode composite contains a negative electrode active material, a conductive additive, a binder, a solvent for dispersing these components, etc.

[0068] As the negative electrode active material, conventionally known negative electrode active materials used in various batteries (e.g., lithium secondary batteries) can be used, as long as they are capable of intercalating and releasing various ions (e.g., lithium ions). Specific negative electrode active materials include graphite materials such as artificial graphite and natural graphite, carbon materials such as mesophase calcined bodies made from coal and petroleum pitch, 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. Each negative electrode active material may be used individually, or two or more may be used in combination.

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

[0070] The same method used for manufacturing the positive electrode may be employed for manufacturing the negative electrode.

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

[0072] Examples of materials for porous sheets include polyethylene, polypropylene, and laminates having a three-layer structure of polypropylene / polyethylene / polypropylene.

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

[0074] (Battery casing material) A battery element comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte (and even a separator) is typically housed in a battery casing to protect it from external shocks, environmental degradation, and other factors during battery use. The material of the battery casing is not particularly limited, and any conventionally known casing material can be used.

[0075] The battery casing may include, if necessary, expanded metal, fuses, overcurrent protection elements such as PTC elements, lead plates, etc., to prevent pressure buildup inside the battery and overcharging / discharging.

[0076] The shape of the battery (lithium-ion secondary battery, etc.) is not particularly limited; any conventionally known shape for batteries (lithium-ion secondary batteries, etc.), such as cylindrical, prismatic, laminated, coin-type, or large, can be used. Furthermore, when used as a high-voltage power supply (several tens to several hundred volts) for electric vehicles, hybrid electric vehicles, etc., it can also be configured as a battery module by connecting individual batteries in series.

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

[0078] Non-aqueous electrolyte secondary batteries can be easily manufactured, for example, by stacking a positive electrode and a negative electrode (with a separator if necessary), placing the resulting laminate into a battery casing, and then pouring a non-aqueous electrolyte into the battery casing and sealing it.

[0079] Based on the above, the non-aqueous electrolyte secondary battery according to this embodiment has the following constituent materials: A non-aqueous electrolyte containing a sulfonyliimide compound (1) and a branched alkylnitrile compound or a linear alkylnitrile compound (chain alkylnitrile compound), A high-Ni positive electrode containing a high-Ni ternary positive electrode active material (2) in which the Ni content in the transition metal is 50 mol% or more (0.5 ≤ x) is used in combination. In this non-aqueous electrolyte secondary battery, the above configuration not only reduces all three types of resistance—initial resistance, resistance associated with battery use, and resistance after high-temperature storage—but also provides a high energy density that can meet the performance requirements of EV batteries. [Examples]

[0080] The present disclosure will be described below based on examples. However, the present disclosure is not limited to the following examples, and the following examples can be modified or changed in accordance with the spirit of the present disclosure, and such modifications do not exclude them from the scope of the present disclosure.

[0081] (1) Preparation of evaluation batteries (Non-aqueous electrolyte) Standard electrolytes (Comparative Examples 1-3) were prepared by dissolving LiPF6 (commercial product) and LiFSI (manufactured by Nippon Shokubai, a sulfonyliimide compound) as electrolyte salts at concentrations of 0.6 mol / L each in a mixed solvent (EC / EMC = 30 / 70 (vol%)) containing ethylene carbonate (EC, commercially available) and ethyl methyl carbonate (EMC, commercially available). To the standard electrolytes, isobutyronitrile (IBN, commercially available) shown in Table 1 as a branched alkyl nitrile compound having 3-6 carbon atoms (branched alkyl nitrile compound) or butyronitrile (BN, commercially available) shown in Table 1 as a linear alkyl nitrile compound having 3-6 prime atoms (linear alkyl nitrile compound) was added and dissolved in the standard electrolytes at the concentrations shown in Table 1 (content of linear alkyl nitrile compound relative to LiFSI) to prepare "nitrile compound-containing electrolytes". In the table, branched alkyl nitrile compounds and linear alkyl nitrile compounds are collectively referred to as "linear alkyl nitrile compounds".

[0082] (positive electrode) As a ternary positive electrode active material, see "NCM111" (LiNi) shown in Table 1. 1 / 3 Co 1 / 3 Mn 1 / 3 O2, manufactured by UmiCore, "NCM523" (LiNi 0.5 Co 0.2 Mn 0.3 O2 (manufactured by Beijing Dangben Co., Ltd.) or "NCM811" (LiNi 0.8 Co 0.1 Mn 0.1O2 (manufactured by Beijing Dangben Co.), acetylene black (manufactured by Denka, Denka Black), graphite (manufactured by Nippon Graphite, SP270), and polyvinylidene fluoride (PVdF, #1120, commercially available) were weighed in a mass ratio of 100:3:3:3 and dispersed in N-methyl-2-pyrrolidone (NMP, commercially available) to prepare a slurry. The prepared slurry was coated onto one side of aluminum foil (NCM111 had a coating weight of 19.7 mg / cm²). 2 NCM523 has a coating weight of 19.5 mg / cm². 2 NCM811 has a coating weight of 15.7 mg / cm². 2 After drying, the positive electrode was fabricated by roll pressing.

[0083] (Negative electrode) A water-based slurry was prepared using graphite (O-MAC, manufactured by Osaka Gas Chemical Co., Ltd.), carbon fiber (VGCF, manufactured by Showa Denko Corporation), styrene-butadiene rubber (SBR, commercially available), and carboxymethylcellulose (CMC, commercially available) in a mass ratio of 100:2:1:1. This slurry was then applied to one side of copper foil (coating weight 9.8 mg / cm³). 2 After drying, the negative electrode was fabricated by roll pressing.

[0084] (Evaluation battery) The obtained positive and negative electrodes were cut, the polarity lead connections were ultrasonically welded, and the electrodes were placed opposite each other with a 25 μm polyethylene (PE) separator. Three sides were then sealed with a laminate casing. The electrolyte was poured through the unsealed end, and the battery was vacuum-sealed and charged at 25°C at 3 mA for 3 hours. After that, it was left at room temperature for 2 days, one side of the laminate casing was slit, and degassing was performed by vacuum-sealing again. After degassing, the battery was charged and discharged under the following conditioning conditions to complete the evaluation battery. (Conditioning conditions) Cycle 1: Charging: Constant current and voltage charging at 3mA, 4.2V, terminates at 0.3mA. ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. Second cycle charging: 15mA, constant current / voltage charging at 4.2V, terminated at 0.6mA. ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 3rd cycle charging: Constant current and constant voltage charging at 15 mA and 4.2 V, termination at 0.6 mA ⇒ Discharge: Discharge at 30 mA, termination at 2.75 V. 4th cycle charging: Constant current and constant voltage charging at 15 mA and 4.2 V, termination at 0.6 mA ⇒ Discharge: Discharge at 60 mA, termination at 2.75 V.

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

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

[0087] (DCR after 200 cycle test) After the initial DCR measurement, the batteries underwent a 45°C cycle test for 200 cycles. The cycle conditions were as follows: Charging: 4.2V, 30mA (1C), 0.6mA (0.05C) termination, 10-minute rest ⇒ Discharging: 30mA (1C), 2.75V termination, 10-minute rest. After the cycle test, the "DCR after 200 cycles" was measured and calculated at 25°C in the same manner as above. The decrease in DCR after 200 cycles was calculated in the same manner as in formula (1) above, except that "initial DCR" was changed to "DCR after 200 cycles". A smaller decrease in DCR after 200 cycles indicates a greater decrease in DCR due to battery use.

[0088] (DCR after 28 days at 60°C (after high-temperature storage)) After the initial DCR measurement, the batteries were fully charged using the same method as described above, stored at 60°C for 28 days, left to stand at 25°C for 4 hours, and then the "DCR after 28 days at 60°C" was measured and calculated at 25°C using the same method as described above. The decrease in DCR after 28 days at 60°C was calculated using the same method as in formula (1) above, except that "initial DCR" was replaced with "DCR after 28 days at 60°C". A smaller decrease in DCR after 28 days at 60°C indicates a greater decrease in the battery's DCR after high-temperature storage.

[0089] <Percentage of impedance reduction> (Initial impedance) The evaluation battery was fully charged using the same method as described above. Next, the impedance of this evaluation battery was measured using an impedance analyzer (Bio Logic, part number: VSP-300) at 25°C from 1 GHz to 1 mHz. The actual axial resistance (interface resistance) was determined from the frequency at which the arc of the obtained measurement diverged. The frequency at which the arc diverges is the frequency between 1 kHz and 0.001 Hz at which the imaginary axis value reaches a minimum. Specifically, the actual axial resistance at which the imaginary axis resistance becomes 0 was defined as the bulk resistance, and the value obtained by subtracting the bulk resistance from the actual axial resistance at which the imaginary axis resistance is maximum in the low-frequency region below 1 kHz was defined as the "initial impedance." The initial impedance reduction rate was calculated in the same way as in formula (1) above, except that "initial DCR" was changed to "initial impedance". A smaller initial impedance reduction rate means that the initial impedance of the battery decreases.

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

[0091] [Table 1]

[0092] (Summary of examples) Table 1 shows that in a non-aqueous electrolyte secondary battery equipped with a high-Ni cathode containing a "high-Ni-containing ternary cathode active material" (NCM523, NCM811) in which the Ni content in the transition metal is 50 mol% or more, each example equipped with a "nitrile compound-containing electrolyte" containing a branched alkylnitrile compound or a linear alkylnitrile compound together with a sulfonylimide compound (1) showed a reduction in all three types of resistance—initial resistance, resistance associated with battery use, and resistance after high-temperature storage—compared to each comparative example (with the same cathode) equipped with a "reference electrolyte" that does not contain a linear alkylnitrile compound. On the other hand, in a non-aqueous electrolyte secondary battery equipped with a low-Ni cathode containing a low-Ni ternary cathode active material (NCM111) in which the Ni content in the transition metal is less than 50 mol%, Comparative Examples 4 to 13, equipped with a "nitrile compound-containing electrolyte," were found to have all three types of resistances equivalent (no decrease) or increased compared to Comparative Example 3 (same cathode) equipped with a "reference electrolyte." The cause of this is thought to be the chain-like alkylnitrile compound added to the non-aqueous electrolyte. In other words, in a non-aqueous electrolyte secondary battery equipped with a low-Ni cathode, it is thought that the chain-like alkylnitrile compound added to the non-aqueous electrolyte does not exhibit its additive effect or even degrades the battery performance.

Claims

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

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

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that the content of the nitrile compound relative to the sulfonylimide compound is 10 ppm by mass or more.

4. The above sulfonylimide compound is LiN(FSO) ２ ) ２ A non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized by including the following:

Images