Nonaqueous electrolyte and its storage method

Abstract

The present invention provides a nonaqueous electrolyte solution which contains a sulfonyl imide compound represented by general formula (1) and at least one hydroxyl group-containing compound that is selected from the group consisting of alcohols and compounds containing a phenolic hydroxyl group, wherein: the content of the sulfonyl imide compound represented by general formula (1) in the nonaqueous electrolyte solution is more than 1.5 mol / L; and the content of the hydroxyl group-containing compound relative to the sulfonyl imide compound represented by general formula (1) is 40 ppm by mass or more. (1): LiN(RSO2)(FSO2) (In the formula, R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms.)

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte and a method for storing the same. [Background technology]

[0002] In general, non-aqueous electrolytes used in secondary batteries such as lithium-ion secondary batteries deteriorate during storage, causing changes in their characteristics that significantly affect battery performance. Therefore, various non-aqueous electrolytes that can improve the safety, stability, storage characteristics, etc. of batteries have been studied.

[0003] For example, Patent Document 1 proposes a nonaqueous electrolyte for secondary batteries, which is obtained by mixing a fluorine-containing lithium salt with a nonaqueous solvent, containing a predetermined amount of hydrogen fluoride (HF), and further containing a predetermined proportion of lithium difluorophosphate as a specific compound. In this electrolyte, a nonaqueous solvent containing 3 ppm or more and 150 ppm or less (specifically, 10 ppm or more and 50 ppm or less) of alcohols is used as the nonaqueous solvent (for example, by adding alcohols to a purified nonaqueous solvent), thereby improving the output characteristics, high-temperature storage characteristics, and cycle characteristics of the battery.

[0004] Patent Document 2 proposes a non-aqueous electrolyte solution for lithium secondary batteries that contains a room-temperature molten salt, a lithium salt, and a compound having a hydroxyl group. In this electrolyte solution, an alcohol is used as the compound having a hydroxyl group, and it has been found that excellent current efficiency can be achieved by adjusting the content of alcohol to 30 to 10,000 ppm (specifically, 800 ppm) relative to the weight of the electrolyte solution.

[0005] Patent Document 3 proposes an electrolyte solution containing lithium salt, methyl acetate, and dimethyl sulfone as an electrolyte solution for a nonaqueous electrolyte secondary battery. This electrolyte solution provides a battery with high capacity and excellent high-temperature storage characteristics. Patent Document 3 also proposes an electrolyte solution that further contains alcohol as an additive, with the alcohol content in the electrolyte solution being 5 ppm to 500 ppm relative to the mass of the electrolyte solution.

[0006] Patent Document 4 proposes an electrolyte solution containing a lithium salt, an additive, and a solvent as the balance, with a p-benzoquinone derivative as the additive. Patent Document 5 proposes a non-aqueous electrolyte solution containing hydroquinone or a hydroquinone derivative.

[0007] Patent Document 6 proposes a non-aqueous electrolyte solution containing a specific anion and a lithium cation, and further containing 4 mass % or less of a radical scavenger and / or a redox stabilizer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-165294 [Patent Document 2] International Publication No. 2006 / 057447 [Patent Document 3] International Publication No. 2020 / 241438 [Patent Document 4] Japanese Patent Application Publication No. 2019-114346 [Patent Document 5] Japanese Patent Application Publication No. 08-203561 [Patent Document 6] Japanese Patent Application Laid-Open No. 2016-134283 Summary of the Invention [Problem to be solved by the invention]

[0009] The present applicants have found through their studies that a non-aqueous electrolyte solution containing a lithium salt such as lithium bis(fluorosulfonyl)imide as an electrolyte improves the high-temperature durability and battery performance of lithium ion secondary batteries, such as charge-discharge cycle performance. The present inventors have also proposed various non-aqueous electrolyte solutions containing the sulfonylimide compound that have improved storage stability (i.e., the property of suppressing decomposition reactions of the sulfonylimide compound even during long-term storage) and storage methods for the same.

[0010] However, although Patent Documents 1 to 3 discuss the safety, stability, storage characteristics, etc. of the battery, they do not discuss the storage stability or storage method of the electrolyte solution itself.

[0011] Furthermore, Patent Documents 1 and 2 do not discuss an electrolyte solution containing a sulfonylimide compound. Furthermore, Patent Document 3 does not specifically discuss an electrolyte solution further containing an alcohol as an additive, and it is unclear whether the above-mentioned effect can be obtained.

[0012] Patent Documents 4 and 5 discuss batteries containing hydroquinone or a hydroquinone derivative as an additive, but do not specifically discuss electrolytes containing sulfonylimide compounds.

[0013] Patent Document 6 discloses that a non-aqueous electrolyte containing a phenolic antioxidant suppresses the decomposition of a compound having a sulfonylimide anion at high temperatures. However, the non-aqueous electrolyte specifically studied contains lithium bis(fluorosulfonyl)imide as the compound at a concentration of 1.0 mol / L, and no studies have been conducted on higher concentrations.

[0014] The present disclosure has been made in view of the above points, and an object thereof is to improve the storage stability of a non-aqueous electrolyte solution itself that contains a relatively high concentration of a sulfonylimide compound. [Means for solving the problem]

[0015] In order to achieve the above object, in the disclosed technology, a compound containing a hydroxyl group (hereinafter also referred to as a "hydroxyl group-containing compound") is intentionally added to a non-aqueous electrolyte solution containing a sulfonylimide compound at a relatively high concentration (for example, more than 1.5 mol / L), thereby Imide The present invention has been found to suppress decomposition of a sulfonylimide compound by specifying the content of the compound and the hydroxyl group-containing compound.

[0016] The nonaqueous electrolyte solution of the present disclosure has 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 at least one hydroxyl group-containing compound selected from the group consisting of alcohols and compounds containing a phenolic hydroxyl group, wherein the content of the sulfonylimide compound represented by general formula (1) in the nonaqueous electrolyte exceeds 1.5 mol / L, and the content of the hydroxyl group-containing compound relative to the sulfonylimide compound represented by general formula (1) is 40 ppm by mass or more. The acid concentration in the nonaqueous electrolyte may be 50 ppm by mass or less. The sulfonylimide compound represented by general formula (1) may contain LiN(FSO2)2. The alcohol may contain at least one selected from the group consisting of aliphatic monoalcohols having 1 to 4 carbon atoms and aliphatic dialcohols having 1 to 4 carbon atoms. The compound containing a phenolic hydroxyl group may contain at least one selected from the group consisting of hydroquinone and dibutylhydroxytoluene. The electrolyte solvent may further contain at least one selected from the group consisting of carbonate-based solvents, lactone-based solvents, ether-based solvents, and chain ester-based solvents.

[0017] The method for storing a non-aqueous electrolyte solution according to the present disclosure may include storing the non-aqueous electrolyte solution in a container, wherein the acid concentration in the non-aqueous electrolyte solution after storage at a temperature equal to or higher than room temperature for three months or longer is 50 ppm by mass or less.

[0018] The secondary battery of the present disclosure uses the nonaqueous electrolyte solution. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to improve the storage stability of a non-aqueous electrolyte solution itself that contains a relatively high concentration of a sulfonylimide compound. DETAILED DESCRIPTION OF THE INVENTION

[0020] 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.

[0021] <Nonaqueous electrolyte> The nonaqueous electrolyte solution according to this embodiment is a nonaqueous electrolyte solution (hereinafter also referred to as a "sulfonylimide electrolyte solution") that contains a lithium salt such as a specific sulfonylimide compound as an electrolyte and a hydroxyl group-containing compound as an additive.

[0022] (electrolyte) The electrolyte constituting the sulfonylimide electrolyte solution is represented by the general formula (1): [C1] LiN(RSO2)(FSO2) (1) The compound contains a sulfonylimide compound represented by the formula (hereinafter referred to as "sulfonylimide compound (1)" and a fluorine-containing sulfonylimide salt).

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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 compound (1) 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.

[0028] Among the sulfonylimide compounds (1), lithium bis(fluorosulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, and lithium (fluorosulfonyl)(pentafluoroethylsulfonyl)imide are preferred, with lithium bis(fluorosulfonyl)imide being more preferred, from the viewpoint of improving battery performance. In other words, the sulfonylimide compound (1) preferably contains LiN(FSO2)2.

[0029] The electrolyte (lithium salt) may contain the sulfonylimide compound (1), but may also contain other electrolytes (electrolytes other than the sulfonylimide compound (1)). Examples of other electrolytes include imide salts and non-imide salts.

[0030] 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.

[0031] Examples of the non-imide salt include salts of non-imide anions and cations (lithium ions and the above-exemplified cations). [C2] 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 (for example, 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] These electrolytes (sulfonylimide compound (1), other electrolytes, etc.) may be present (contained) in the form of ions in the non-aqueous electrolyte solution.

[0036] The content (concentration) of sulfonylimide compound (1) in the nonaqueous electrolyte (total content when two or more types are used in combination, the same applies hereinafter) is more than 1.5 mol / L, preferably more than 2 mol / L, more preferably 2.5 mol / L or more, and even more preferably 2.8 mol / L or more, from the viewpoint of improving the storage stability of the sulfonylimide electrolyte itself. Moreover, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte, the concentration is preferably 5 mol / L or less, more preferably 4 mol / L or less, and even more preferably 3.5 mol / L or less.

[0037] From the viewpoint of improving the storage stability of the sulfonylimide electrolyte solution itself, the content of the sulfonylimide compound (1) in the nonaqueous electrolyte solution is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and particularly preferably 50 mol % or more, based on a total of 100 mol % of the electrolytes contained in the nonaqueous electrolyte solution, with the upper limit being 100 mol %. In other words, the electrolyte may contain only the sulfonylimide compound (1).

[0038] The content of sulfonylimide compound (1) in the non-aqueous electrolyte is preferably 25% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more, based on the total amount of the non-aqueous electrolyte (based on 100% by mass of the total amount of components contained in the non-aqueous electrolyte), from the viewpoint of improving the storage stability of the sulfonylimide electrolyte itself. 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 60% by mass or less, and even more preferably 50% by mass or less. For example, the content of LiFSI in a solution (nonaqueous electrolyte) prepared by dissolving LiFSI as sulfonylimide compound (1) in dimethyl carbonate (DMC) as an electrolyte solvent (described later) to a concentration of 2 mol / L is [2 (mol / L) × 187.06 (molecular weight of LiFSI) / {1.27 (solution density) × 1000}] × 100 ≈ 30 mass%. In other words, the nonaqueous electrolyte is a 30 mass% LiFSI / 70 mass% DMC solution. Similarly, a DMC solution containing 2.9 mol / L LiFSI is a 40 mass% LiFSI / 60 mass% DMC solution. liquid The density is a value that depends on the type of electrolyte solvent and the concentration of the sulfonylimide compound (1), and can be determined by referring to literature such as "Superconcentrated electrolytes for a high-voltage lithium-ion battery" (NATURE COMMUNICATIONS | 7:12032 | DOI: 10.1038 / ncomms12032).

[0039] 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.

[0040] When an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and other electrolytes is used, the concentration of the other electrolytes in the nonaqueous electrolyte is preferably 0.1 mol / L or more, more preferably 0.2 mol / L or more, and even more preferably 0.5 mol / L or more, from the viewpoint of improving the storage stability of the sulfonylimide electrolyte itself. Furthermore, the concentration is preferably 1.5 mol / L or less, more preferably 1 mol / L or less, and even more preferably 0.8 mol / L or less, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte.

[0041] The total electrolyte concentration in the nonaqueous electrolyte solution is preferably more than 1.5 mol / L, more preferably more than 2 mol / L, even more preferably 2.5 mol / L or more, and even more preferably 2.8 mol / L or more from the viewpoint of improving the storage stability of the sulfonylimide electrolyte solution itself. Also, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte solution, the concentration is preferably 5 mol / L or less, more preferably 4 mol / L or less, and even more preferably 3.5 mol / L or less.

[0042] From the viewpoint of improving the storage stability of the sulfonylimide electrolyte solution itself, it is preferable to increase the concentration of the sulfonylimide compound (1). The molar ratio of the sulfonylimide compound (1) to the other electrolyte (the molar ratio of the sulfonylimide compound concentration to the 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.

[0043] (Hydroxy group-containing compound) The hydroxyl group-containing compound is contained in the sulfonylimide electrolyte solution as an additive. In other words, the hydroxyl group-containing compound is an essential component intentionally added to the sulfonylimide electrolyte solution. The hydroxyl group-containing compound constituting the sulfonylimide electrolyte solution includes at least one selected from the group consisting of alcohols and compounds containing phenolic hydroxyl groups. In other words, the hydroxyl group-containing compound may include only an alcohol, may include only a compound containing a phenolic hydroxyl group, or may include both an alcohol and a compound containing a phenolic hydroxyl group.

[0044] 〔alcohol〕 Generally, when a sulfonylimide electrolyte solution contains an alcohol, the sulfonylimide decomposes due to solvolysis, and therefore, it is considered preferable to have a low alcohol content in the sulfonylimide electrolyte solution. In this regard, the present inventors have discovered that by specifying the content of sulfonylimide compound (1) within the above-mentioned range and specifying the content of alcohol relative to sulfonylimide compound (1) within the range described below, decomposition of sulfonylimide compound (1) can be suppressed. In this embodiment, by using sulfonylimide compound (1) and alcohol in combination at specific contents, the long-term (approximately 3 months) and high-temperature (approximately 40°C) storage stability of the sulfonylimide electrolyte solution itself is improved.

[0045] The alcohol is not particularly limited, and examples thereof include aliphatic alcohols. Examples of aliphatic alcohols include monoalcohols (aliphatic monoalcohols having 1 to 4 carbon atoms) such as methyl alcohol (methanol), ethyl alcohol (ethanol), n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, and t-butyl alcohol; diols (aliphatic dialcohols having 1 to 4 carbon atoms) such as ethylene glycol and propylene glycol; triols such as glycerin, and other polyols. Each alcohol may be used alone or in combination of two or more. Among alcohols, from the viewpoint of improving the storage stability of the sulfonylimide electrolyte solution itself, aliphatic monoalcohols having 1 to 4 carbon atoms and aliphatic dialcohols having 1 to 4 carbon atoms are preferred, with methanol, ethanol, and ethylene glycol being more preferred. In other words, the alcohol preferably includes at least one selected from the group consisting of the preferred alcohols described above. Commercially available alcohols can be used.

[0046] [Compounds containing a phenolic hydroxyl group] The compound containing a phenolic hydroxyl group refers to a phenol in which a hydrogen atom on an aromatic ring is substituted, and is distinguished from the alcohol. The inventors have also found that by specifying the content of the compound containing a phenolic hydroxyl group relative to the sulfonylimide compound (1) within the range described below, decomposition of the sulfonylimide compound (1) can be suppressed. In this embodiment, by using the sulfonylimide compound (1) and the compound containing a phenolic hydroxyl group in combination at a specific content, the storage stability of the sulfonylimide electrolyte solution itself over a long period (about 3 months) and at high temperatures (about 40°C) is improved.

[0047] The compound containing a phenolic hydroxyl group is not particularly limited, and examples thereof include hydroquinones, dibutylhydroxytoluene, alkoxyphenols, bisphenols, etc. Examples of hydroquinones include hydroquinone (hydroquinone), catechol, and resorcinol. Examples of alkoxyphenols include 4-methoxyphenol. Examples of bisphenols include bisphenol A and bisphenol B. The compounds containing a phenolic hydroxyl group may be used alone or in combination of two or more. Among the compounds containing a phenolic hydroxyl group, hydroquinone and dibutylhydroxytoluene are preferred from the viewpoint of improving the storage stability of the sulfonylimide electrolyte solution itself. Commercially available compounds containing a phenolic hydroxyl group can be used.

[0048] Examples of methods for incorporating a hydroxyl group-containing compound into a sulfonylimide electrolyte solution include adding the hydroxyl group-containing compound to the sulfonylimide electrolyte solution; when using an electrolyte solvent as described below, adding the hydroxyl group-containing compound to the electrolyte solvent in advance and using the resulting hydroxyl group-containing electrolyte solvent as a raw material to produce a sulfonylimide electrolyte solution. Another method involves utilizing alcohol contained in the components used as raw materials for the sulfonylimide electrolyte solution. For example, alcohol contained in sulfonylimide compound (1) and / or the electrolyte solvent can be used to produce a sulfonylimide electrolyte solution using alcohol-containing sulfonylimide compound (1) and / or the alcohol-containing electrolyte solvent as raw materials. The alcohol contained in sulfonylimide compound (1) refers to the production solvent used in the production of sulfonylimide compound (1) (residual solvent contained in sulfonylimide compound (1) obtained by the above-mentioned conventional production method). Residual solvent refers to the solvent used in the production reaction of sulfonylimide compound (1) or the solvent used in the purification process. The above methods may also be combined.

[0049] The content of the hydroxyl group-containing compound relative to the sulfonylimide compound (1) (the total content when multiple hydroxyl group-containing compounds are contained) is 40 mass ppm or more, preferably 50 mass ppm or more, more preferably 100 mass ppm or more, even more preferably 180 mass ppm or more, even more preferably 200 mass ppm or more, and even more preferably 220 mass ppm or more, from the viewpoint of improving the storage stability of the sulfonylimide electrolyte solution itself. The upper limit is preferably 3000 mass ppm or less, more preferably 1000 mass ppm or less, and even more preferably 500 mass ppm or less, from the viewpoint of suppressing deterioration of battery performance due to the hydroxyl group-containing compound. The content of the hydroxyl group-containing compound can be measured by the method described in the Examples below, for example, headspace gas chromatography or NMR.

[0050] The content of the hydroxyl group-containing compound in the non-aqueous electrolyte (relative to the mass of the non-aqueous electrolyte) is preferably 10 mass ppm or more, more preferably 20 mass ppm or more, and even more preferably 50 mass ppm or more from the viewpoint of improving the storage stability of the sulfonylimide electrolyte itself. The upper limit is preferably 1500 mass ppm or less, more preferably 500 mass ppm or less, even more preferably less than 200 mass ppm, and even more preferably 150 mass ppm or less from the viewpoint of suppressing deterioration of battery performance due to the hydroxyl group-containing compound.

[0051] When a carbonate-based solvent, which will be described later, is used as the electrolyte solvent, a trace amount of alcohol may be generated by hydrolysis of the carbonate compound. Thus, the alcohol generated during the production process of the sulfonylimide electrolyte solution is also included in the content of the hydroxyl group-containing compound.

[0052] (additives) The non-aqueous electrolyte solution according to this embodiment may contain, in addition to the hydroxyl group-containing compound, additives for improving various properties of the lithium ion secondary battery.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 difluorooxalatophosphanite (LIDFOP), lithium tetrafluorooxalatophosphate (LITFOP), lithium difluorobis(oxalato)phosphate (LiDFOP), lithium tris(oxalato)phosphate, and the like, and fluorooxalato compounds such as lithium salts having an oxalic acid skeleton. These additives may be used alone or in combination of two or more.

[0053] The additive is preferably used in an amount of 0.1% by mass to 10% by mass, more preferably 0.2% by mass to 8% by mass, and even more preferably 0.3% by mass to 5% by mass, relative to 100% by mass of the total amount of components contained in the non-aqueous electrolyte. If the amount of additive used is too small, it may be difficult to obtain the effects derived from the additive. On the other hand, even if a large amount of additive is used, it may be difficult to obtain effects commensurate with the amount added, and the viscosity of the non-aqueous electrolyte may increase, resulting in a decrease in conductivity.

[0054] (Electrolyte solvent) The non-aqueous electrolyte according to this embodiment may contain an electrolyte solvent. In other words, the non-aqueous electrolyte may be a sulfonylimide electrolyte containing three essential components: a sulfonylimide compound (1), a hydroxyl group-containing compound, and an electrolyte solvent. In this case, the electrolyte solvent is a component different from the hydroxyl group-containing compound. The electrolyte solvent is not particularly limited as long as it can dissolve and disperse the electrolyte. Examples of the electrolyte solvent include non-aqueous solvents, polymers used in place of electrolyte solvents, polymer gels, and other media, and any solvent commonly used in batteries can be used.

[0055] The non-aqueous solvent is preferably a solvent that has a high dielectric constant, high solubility for the electrolyte, a boiling point of 60° C. or higher, and a wide electrochemical stability range, and more preferably an organic solvent with a low water content.Examples of such organic solvents include ether solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; chain carbonate ester (carbonate) solvents such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), diphenyl carbonate, and methyl phenyl carbonate; saturated cyclic carbonate ester solvents such as ethylene carbonate (EC), propylene carbonate (PC), 2,3-dimethyl ethylene carbonate, 1,2-butylene carbonate, and erythrityl carbonate; cyclic carbonate ester solvents having an unsaturated bond such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate; fluoroethylene carbonate, 4,5-difluoroethylene carbonate, trifluoropropylene carbonate, and propylene carbonate; fluorine-containing cyclic carbonate solvents such as benzoate, ethyl benzoate, and other aromatic carboxylic acid ester solvents; γ-butyrolactone, γ-valerolactone, δ-valerolactone, and other lactone solvents; trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, triethyl phosphate, and other phosphate ester solvents; acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, isobutyronnitrile Examples of suitable solvents include nitrile solvents such as tolyl; sulfur compound solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane; aromatic nitrile solvents such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, and 3-methyl-2-oxazolidinone; and chain ester solvents such as ethyl acetate, butyl acetate, and propyl propionate. These solvents may be used alone or in combination of two or more.

[0056] Among the electrolyte solvents, carbonate solvents such as chain carbonate ester solvents and cyclic carbonate ester solvents, lactone solvents, ether solvents, and chain ester solvents are preferred, with dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, γ-butyrolactone, and γ-valerolactone being more preferred, carbonate solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate being even more preferred, and chain carbonate solvents such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate being even more preferred. In other words, the electrolyte solvent preferably contains at least one solvent selected from the group consisting of the above-mentioned preferred solvents.

[0057] When a polymer or polymer gel is used instead of the electrolyte solvent, the following methods may be employed: a method in which a solution of an electrolyte salt dissolved in a solvent is dropped onto a polymer film formed by a conventionally known method to impregnate and support the electrolyte salt and non-aqueous solvent; a method in which a polymer and an electrolyte salt are melted and mixed at a temperature equal to or higher than the melting point of the polymer, and then a film is formed and the film is impregnated with a solvent (these are referred to as gel electrolytes); a method in which a non-aqueous electrolyte in which an electrolyte salt has been dissolved in an organic solvent is mixed with a polymer, and then the mixture is formed into a film by a casting method or a coating method, and the organic solvent is volatilized; a method in which a polymer and an electrolyte salt are melted at a temperature equal to or higher than the melting point of the polymer, mixed, and molded (true polymer electrolyte); and the like.

[0058] 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.

[0059] (summary) As described above, the nonaqueous electrolyte according to this embodiment is composed of the sulfonylimide compound (1), the hydroxyl group-containing compound, and, if necessary, other electrolytes, electrolyte solvents, various additives, etc. The nonaqueous electrolyte can be prepared, for example, by mixing these components in a predetermined composition ratio.

[0060] The nonaqueous electrolyte according to this embodiment contains a sulfonylimide compound (1) in an amount exceeding 1.5 mol / L. The sulfonylimide compound (1) contains a specific hydroxyl-containing compound at a specific content (e.g., 40 mass ppm or more) relative to the sulfonylimide compound (1), thereby suppressing decomposition of the sulfonylimide compound (1). As a result, the generation of acids such as HF due to the decomposition of the sulfonylimide compound (1) is suppressed, and an increase in the acid concentration in the sulfonylimide electrolyte is suppressed. Specifically, the acid concentration (HF equivalent) in the sulfonylimide electrolyte is preferably 50 mass ppm or less, more preferably 30 mass ppm or less, even more preferably 20 mass ppm or less, even more preferably less than 10 mass ppm, and even more preferably 8 mass ppm or less. The acid may not be substantially present (may be approximately 0 mass ppm). The acid concentration can be measured, for example, by the method described in the Examples below. A sulfonylimide electrolyte solution in which the acid concentration is reduced and maintained within the above range has excellent storage stability even at high temperatures, and as described below, its component composition is maintained stably before and after storage at room temperature or higher for a certain period of time. Because the sulfonylimide electrolyte solution is resistant to deterioration during storage, there is also the advantageous effect that differences in battery performance are unlikely to occur even when sulfonylimide electrolyte solutions stored for different periods of time are used.

[0061] <Storage method for non-aqueous electrolyte> The method for storing a non-aqueous electrolyte according to this embodiment is a method for storing a sulfonylimide electrolyte (hereinafter also referred to as "stored electrolyte") in a container.

[0062] (container) The container is preferably a sealed container. The sealed container is preferably made of a material and structure that makes it difficult for moisture to get mixed in, more preferably one that is highly airtight so that the internal pressure of the container can be maintained, and even more preferably one that can be sealed (closed system). An example of a means for sealing the container is a form in which a valve is provided in part of the container.

[0063] The material of the sealed container (the material of the part that comes into contact with the contents (the electrolyte to be stored)) is not particularly limited, and examples include metals such as stainless steel (e.g., SUS316), aluminum, aluminum alloys, and Hastelloy (registered trademark); fluorine-based resins such as tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (perfluoroalkoxyalkane, PFA) and polytetrafluoroethylene (PTFE); olefin-based resins such as polyethylene (PE) and polypropylene (PP); and glass. Of these, stainless steel and PFA are preferred.

[0064] The inner surface of the sealed container made of the metal material may be coated with a resin. The resin used for the coating is not particularly limited, and examples thereof include fluorine-based resins (PTFE, PFA, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), etc.) and olefin-based resins (PP, etc.).

[0065] Examples of the structure of the sealed container include a canister, a plastic container, a container made of a fluorine-based resin, a pouch-type container, etc. The sealed container may also have a structure consisting of a resin inner bag and a metal outer body.

[0066] The shape of the sealed container is not particularly limited, and examples thereof include a bottle shape, a cylindrical shape, an aluminum-lined paper pack shape, and an aluminum pouch shape.

[0067] The volume of the sealed container is not particularly limited, and is about 100L to 20,000L.

[0068] In a sealed container, the ratio (space volume ratio) of the volume of the gas phase (void space (headspace) other than the liquid phase) to the volume of the liquid phase (portion where the contents (the electrolyte to be stored) are present) is preferably 50% or less, more preferably 30% or less, even more preferably 15% or less, and most preferably 10% or less, from the viewpoint of the filling efficiency of the electrolyte to be stored. The lower limit of the space volume ratio is preferably 5% or more, because the presence of the gas phase allows the addition of a gas, as described below.

[0069] After the electrolyte to be stored is filled (introduced) into a sealed container, the gas phase (void) present inside the sealed container may be filled with a gas, if necessary. Examples of the gas include active gases such as air, oxygen (O), and carbon dioxide (CO); inert gases such as nitrogen (N), helium (He), and argon (Ar); dry air (e.g., with a dew point of −60°C or less), and combinations thereof. Commercially available active gases, inert gases, and dry air can be used.

[0070] The temperature of the stored electrolyte (internal temperature inside the sealed container during storage) is not particularly limited, but may be, for example, 60°C or lower (50°C or lower, 40°C or lower) or -40°C or higher (-30°C or higher, -20°C or higher, -10°C or higher, 0°C or higher). By appropriately adjusting the temperature, solidification and decomposition of the stored electrolyte can be suppressed.

[0071] The acid concentration (HF equivalent) in the non-aqueous electrolyte (sulfonylimide electrolyte) after storage for 3 months or more at room temperature or higher (for example, 25°C or higher, or a high temperature (for example, about 40°C); the same applies below) is preferably 50 mass ppm or less, more preferably 30 mass ppm or less, even more preferably 20 mass ppm or less, even more preferably less than 10 mass ppm, and even more preferably 8 mass ppm or less. Substantially no acid may be present (it may be about 0 mass ppm). The acid concentration can be measured, for example, by the method described in the Examples below.

[0072] The water concentration in the non-aqueous electrolyte after storage at room temperature or higher for 3 months or longer is preferably 50 mass ppm or less, more preferably 30 mass ppm or less. It is not necessary for the water content to be substantially zero (it may be approximately 0 mass ppm). The water concentration can be measured by the method described in the Examples below, for example, using a Karl Fischer water content analyzer.

[0073] Fluoride ions (F) in the non-aqueous electrolyte after storage for more than three months at room temperature or higher - ) concentration is preferably 20 mass ppm or less, more preferably 15 mass ppm or less, and further Preferably 12 mass ppm or less. In addition, the sulfate ions (SO4 2- ) concentration is preferably 25 mass ppm or less, more preferably 10 mass ppm or less, and further Preferably The concentration is 5 ppm by mass or less. Fluoride ions and sulfate ions may be substantially absent (may be approximately 0 ppm by mass). The concentrations of fluoride ions and sulfate ions can be measured by the method described in the Examples below, for example, ion chromatography.

[0074] The alcohol concentration relative to the sulfonylimide compound (1) in the nonaqueous electrolyte solution after storage at room temperature or higher for 3 months or longer is in the same numerical range as the alcohol content relative to the sulfonylimide compound (1) because decomposition of the stored electrolyte solution is suppressed and the component composition is stably maintained during the storage period.

[0075] <Application> The nonaqueous electrolyte solution configured as described above and the nonaqueous electrolyte solution after storage by the storage method are used, for example, in batteries (batteries having a charge / discharge mechanism), electricity storage (electrochemical) devices (or ion conductor materials constituting these), etc. Specifically, the electrolyte solution can be used as an electrolyte constituting, for example, primary batteries, secondary batteries (e.g., lithium (ion) secondary batteries), fuel cells, electrolytic capacitors, electric double layer capacitors, solar cells, electrochromic display elements, etc. Hereinafter, a description will be given taking batteries (particularly secondary batteries) as an example.

[0076] <Secondary battery> The secondary battery according to the present embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The nonaqueous electrolyte according to the present embodiment, i.e., a nonaqueous electrolyte containing a sulfonylimide compound (1) and a hydroxyl group-containing compound as essential components in specific amounts, is used in the secondary battery. Use of the nonaqueous electrolyte according to the present disclosure can improve the self-discharge capacity of the battery.

[0077] (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.

[0078] 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.

[0079] 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.

[0080] 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, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 Ternary positive electrode active materials such as O2; LiFePO4, LiFe 0.995 Mn 0.005 Suitable positive electrode active materials include iron phosphate-based positive electrode active materials having an olivine structure such as PO4, etc. These positive electrode active materials may be used alone or in combination of two or more.

[0081] 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).

[0082] 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.

[0083] 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.

[0084] 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 %.

[0085] 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.

[0086] 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.

[0087] The positive electrode mixture may contain other components as needed, such as polymers such as non-fluorinated polymers such as (meth)acrylic polymers, nitrile polymers, and diene polymers, and fluorinated polymers such as polytetrafluoroethylene; emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymer dispersants such as styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salts), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers; preservatives, etc. The content of 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.

[0088] 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.

[0089] 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.

[0090] The positive electrode mixture layer may be dried or pressed after being formed or coated (applied), as needed.

[0091] (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.

[0092] 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.

[0093] 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.

[0094] The negative electrode active material may be any conventionally known negative electrode active material used in various batteries (e.g., lithium secondary batteries), as long as it is capable of absorbing and releasing lithium ions. Specific negative electrode active materials that can be used include graphite materials such as artificial graphite and natural graphite, mesophase sintered bodies made from coal and petroleum pitch, carbon materials such as non-graphitizable carbon, Si-based negative electrode materials such as Si, Si alloys, and SiO, Sn-based negative electrode materials such as Sn alloys, lithium metal, and lithium alloys such as lithium-aluminum alloys. The negative electrode active materials may be used alone or in combination of two or more.

[0095] 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.

[0096] The negative electrode may be manufactured by the same method as the positive electrode.

[0097] (separator) The secondary battery may include a separator. The separator is disposed to separate the positive electrode from the negative electrode. There are no particular limitations on the separator, and any conventionally known separator can be used in the present disclosure. Specific examples of the separator include porous sheets made of polymers capable of absorbing and retaining an electrolyte solution (non-aqueous electrolyte solution) (e.g., polyolefin-based microporous separators, cellulose-based separators, etc.), nonwoven fabric separators, porous metal bodies, etc.

[0098] Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

[0099] 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.

[0100] (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.

[0101] 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.

[0102] 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.

[0103] 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-described ternary positive electrode active material as a main component, it may be 3.6 V or higher, preferably 4.0 V or higher, more preferably 4.1 V or higher, and even more preferably 4.2 V or higher. The higher the rated charging voltage, the higher the energy density can be, but from the viewpoint of safety, etc., the rated charging voltage may be 4.6 V or lower (e.g., 4.5 V or lower).

[0104] <Secondary battery manufacturing method> The secondary battery according to this embodiment can be easily manufactured by, for example, stacking a positive electrode and a negative electrode (with a separator interposed therebetween as necessary), placing the resulting laminate in a battery casing, injecting a nonaqueous electrolyte into the battery casing, and sealing the battery casing. [Example]

[0105] 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.

[0106] <Preparation of non-aqueous electrolyte> Example 1 Lithium bis(fluorosulfonyl)imide (LiN(FSO2)2, hereinafter referred to as "LiFSI", manufactured by Nippon Shokubai Co., Ltd.) was used as the electrolyte. 50 ppm by weight of methanol (Fujifilm Wako Pure Chemical Industries, Ltd., ultra-dehydrated grade) was added as an additive (hydroxyl group-containing compound), and the resulting solution was dissolved in dimethyl carbonate (hereinafter referred to as "DMC", manufactured by Kishida Chemical Co., Ltd., LIB grade) to prepare a non-aqueous electrolyte solution with a LiFSI concentration of 2.91 mol / L (approximately 40% by weight). The acid concentration in the non-aqueous electrolyte solution (acid concentration before storage (HF equivalent), hereinafter the same) measured according to the method described below was 1.5 ppm by weight.

[0107] Examples 2 and 3 Non-aqueous electrolyte solutions were prepared in the same manner as in Example 1, except that the amount of methanol added was changed to the content shown in Table 1. The acid concentrations in the non-aqueous electrolyte solutions were 2.9 ppm by mass in Example 2 and 2.5 ppm by mass in Example 3.

[0108] Comparative Example 1 A non-aqueous electrolyte solution was prepared in the same manner as in Example 1, except that methanol was not added. The acid concentration in the non-aqueous electrolyte solution was 0.9 ppm by mass.

[0109] <Evaluation of non-aqueous electrolyte> The LiFSI concentration, type of hydroxyl group-containing compound added, amount (content) of hydroxyl group-containing compound added relative to LiFSI, and type of electrolyte solvent for each nonaqueous electrolyte obtained in the above Examples and Comparative Examples are shown in Table 1. Furthermore, the acid content (HF equivalent), water content, and fluoride ion (F ) content in each electrolyte after storage at 40°C for 3 months in a sealed container made of PFA (fluororesin) were measured. - ), sulfate ions (SO4 2- The concentrations of the hydroxyl group-containing compounds were measured by the following method, and the results are shown in Table 1.

[0110] [Acid content measurement] Each nonaqueous electrolyte was diluted 14.3 times with ultrapure water (over 18.2 Ω cm) to prepare a measurement solution. Using a COM-1700A (manufactured by Hiranuma Sangyo Co., Ltd.), the acid content of each nonaqueous electrolyte was measured by titration with a 0.01N sodium hydroxide solution.

[0111] [Moisture measurement] A Karl Fischer moisture content analyzer AQ-2000 (manufactured by Hiranuma Sangyo Co., Ltd.) was used. The moisture content of each nonaqueous electrolyte was measured using Aqualite RS-A (manufactured by Hiranuma Sangyo Co., Ltd.) as the generating solution and Aqualite CN (manufactured by Hiranuma Sangyo Co., Ltd.) as the counter electrode solution.

[0112] [Ion chromatography measurement] Each nonaqueous electrolyte was diluted 100 times with ultrapure water (over 18.2 Ω cm) to prepare a measurement solution. The concentrations of fluoride ions and sulfate ions in each nonaqueous electrolyte were measured using an ion chromatography system ICS-3000 (manufactured by Nippon Dionex Co., Ltd.). The measurement conditions were as follows: (Ion chromatography measurement conditions) Separation mode: Ion exchange ·Eluent: 4.5mM Na2CO3 / 0.5mM NaHCO3 aqueous solution Detector: Electrical conductivity detector Column: Ion PAC AS-23 column for anion analysis (manufactured by Nippon Dionex Co., Ltd.).

[0113] [Measurement of hydroxyl group-containing compound content] The amount of hydroxyl group-containing compounds contained in each non-aqueous electrolyte was measured using a headspace gas chromatograph SHIMADZU HS-GC20 / GC-2010Plus (manufactured by Shimadzu Corporation). (Headspace gas chromatography measurement conditions) Detector: Hydrogen flame ionization detector Separation column: Rtx-200 (Restek) The content of the hydroxyl group-containing compound is 1 It can also be measured using H-NMR. 1 H-NMR measurements were carried out using a Varian "Unity Plus-400" (internal standard: trifluorotoluene, number of accumulations: 64).

[0114] [Table 1]

[0115] The results in Table 1 show that in Examples 1 to 3, in which a predetermined amount of methanol was added as a hydroxyl group-containing compound to a nonaqueous electrolyte containing 2.91 mol / L of LiFSI, the amounts of LiFSI decomposition products, such as acid, fluoride ions, and sulfate ions, were all lower after 3 months of storage at 40°C compared to Comparative Example 1, in which no methanol was added. Therefore, the nonaqueous electrolytes of Examples 1 to 3, in which the LiFSI (sulfonylimide compound) content exceeded 1.5 mol / L and the methanol (hydroxyl group-containing compound) content relative to LiFSI was 40 ppm by mass or more, suppressed LiFSI decomposition and exhibited better storage stability than the nonaqueous electrolyte of Comparative Example 1, in which the LiFSI content exceeded 1.5 mol / L but the hydroxyl group-containing compound content was less than 40 ppm by mass.

[0116] Examples 4 to 10 and Comparative Example 1 Non-aqueous electrolyte solutions were prepared in the same manner as in Example 1, except that the LiFSI concentration, the type and amount (content) of the hydroxyl group-containing compound, and the type of electrolyte solvent were changed to the amounts shown in Table 2. Comparative Example 1 had the same composition as the above-mentioned Comparative Example 1. The acid concentrations in the non-aqueous electrolyte solutions were 1.5 ppm by mass in Example 4, 2.9 ppm by mass in Example 5, 2.5 ppm by mass in Example 6, 4.2 ppm by mass in Example 7, 4.2 ppm by mass in Example 8, 6.7 ppm by mass in Example 9, 4.2 ppm by mass in Example 10, and 0.9 ppm by mass in Comparative Example 1, respectively.

[0117] In addition, in the same manner as in Example 1, the acid content (HF equivalent), moisture content, and fluoride ion (F ) content in each electrolyte solution were measured after storing the solution in a sealed PFA container at 40°C for one month. - ), sulfate ions (SO4 2- The concentrations of the hydroxyl group-containing compounds were measured, and the results are shown in Table 2. In Table 2, "na" indicates that the concentration was below the lower limit of detection by the measuring device.

[0118] [Table 2]

[0119] From the results in Table 2, it can be seen that the examples in which a predetermined amount of ethanol or ethylene glycol (alcohol), or hydroquinone or dibutylhydroxytoluene (a compound containing a phenolic hydroxyl group) was added instead of methanol as a hydroxyl group-containing compound were used. 7 In all of Examples 4 to 10, the amount of decomposition products of LiFSI, such as acid and sulfate ions, after storage for one month at 40°C was smaller than that of Comparative Example 1, which did not contain a hydroxyl group-containing compound. From this result, as with the results in Table 1, it is expected that the amount of decomposition products after storage for three months at 40°C will also be smaller in the nonaqueous electrolytes of Examples 4 to 10 than in the nonaqueous electrolyte of Comparative Example 1.

[0120] Examples 11 and 12 and Comparative Example 2 The present inventors have found through previous studies that batteries using a non-aqueous electrolyte containing a sulfonylimide compound such as LiFSI as the electrolyte exhibit greater self-discharge from a fully charged state 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, and that there is room for improvement in the storage characteristics of the battery. They have therefore investigated various techniques to improve this. In the following examples, it was confirmed whether the self-discharge of a battery during storage can be suppressed by adding a hydroxyl group-containing compound to a non-aqueous electrolyte containing a sulfonylimide compound.

[0121] (1) Preparation of evaluation battery Commercially available LiFePO4 was used as the positive electrode active material. Acetylene black (HS-100) and PVdF (Kureha #L7208) were weighed in a composition (mass) ratio of 100:9:6, 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 20.20 mg / cm). 2 ) and dried, followed by roll pressing to prepare a positive electrode. Graphite (O-MAC (Osaka Gas Chemicals Co., Ltd.)) was used as the negative electrode active material, carbon nanotubes (VGCF, Showa Denko K.K.) as the conductive additive, and SBR (styrene-butadiene rubber) and CMC (carboxymethyl cellulose) as the binders. An aqueous slurry with a composition (mass ratio) of graphite:VGCF:SBR:CMC = 100:2:1.5:1.5 was prepared. The prepared slurry was applied to copper foil at a coating weight of 8.8 mg / cm. 2 After drying, the mixture was roll-pressed to prepare a negative electrode. The resulting positive and negative electrodes were cut into 3cm x 4cm pieces, and the polarity leads were ultrasonically welded. Then, a 20μm polyethylene (PE) separator was placed between the electrodes, and the three sides were sealed with a laminate exterior to form a cell. 700μL of the electrolyte solution shown in Table 3 below was added to one of the unsealed sides of the resulting cell. The electrolyte used in Comparative Example 2 was prepared by dissolving LiFSI at 1.51 mol / L in a mixed solvent of EC / DMC=3 / 7 (wt / wt).The electrolytes used in Examples 11 and 12 were prepared by adding methanol to the electrolyte used in Comparative Example 2 in amounts of 250 ppm by mass and 1000 ppm by mass relative to LiFSI, respectively. After the electrolyte was injected, the cell was charged at a constant current of 5 mA for 3 hours, one piece was split open, and the cell 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 conditions to complete the evaluation battery. [Conditioning conditions] 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.

[0122] (2) Characterization of the test battery Using the evaluation battery obtained in (1) above, the self-discharge capacity rate after high-temperature storage was measured by the following method. The results are shown in Table 3. <Self-discharge capacity rate measurement after high-temperature storage> The self-discharge amount of the battery was calculated as the self-discharge capacity rate after storage at 80°C for 7 days (high-temperature storage) using the following formula (1). Note that the smaller the self-discharge capacity rate, the more suppressed the self-discharge of the battery is.

[0123] [Number 1] Self-discharge capacity rate = 100 × {(discharge capacity before storage) - (discharge capacity after storage)} / (discharge capacity before storage) (1) (Discharge capacity before storage) The evaluation battery was charged and discharged under the following conditions (25° C.), and the discharge capacity before storage was confirmed. Charge: 3.6V, 25mA constant current, constant voltage charge, 0.5mA termination ⇒ Constant current discharge: 2.5mA, 2.0V termination. (Discharge capacity after storage) After checking the discharge capacity before storage, the evaluation battery was charged at room temperature at a constant current of 1 C (25 mA) with a termination of 0.02 C (0.5 mA) at 3.6 V, and then fully charged and stored at 80°C for 7 days. The discharge capacity after high-temperature storage was determined by discharging the stored battery at a constant current of 2.5 mA at 25°C with a termination of 2.0 V.

[0124] [Table 3]

[0125] The results in Table 3 show that in Examples 11 and 12, which used non-aqueous electrolyte solutions containing more than 1.5 mol / L of LiFSI (sulfonylimide compound) and containing 40 mass ppm or more of methanol as a hydroxyl group-containing compound relative to LiFSI, the self-discharge capacity rate after storage at a high temperature of 80°C for 7 days was lower than in Comparative Example 2, which used a non-aqueous electrolyte solution without added methanol (the content was less than 40 mass ppm), and that self-discharge of the battery was suppressed.

Claims

1. A non-aqueous electrolyte solution for a lithium ion secondary battery, comprising a sulfonylimide compound represented by general formula (1) and at least one hydroxyl group-containing compound selected from the group consisting of alcohols and compounds containing a phenolic hydroxyl group, the content of the sulfonylimide compound represented by the general formula (1) in the nonaqueous electrolyte solution is more than 1.5 mol / L, the content of the hydroxyl group-containing compound relative to the sulfonylimide compound represented by general formula (1) is 40 ppm by mass or more and 3000 ppm by mass or less, The non-aqueous electrolyte solution includes at least one alcohol selected from the group consisting of methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, t-butyl alcohol, ethylene glycol, propylene glycol, and glycerin. 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. The non-aqueous electrolyte according to claim 1 , wherein the acid concentration in the non-aqueous electrolyte is 50 ppm by mass or less.

3. The sulfonylimide compound represented by the general formula (1) is LiN(FSO ２ ) ２ The non-aqueous electrolyte solution according to claim 1 , comprising:

4. 2. The nonaqueous electrolyte solution according to claim 1, wherein the compound containing a phenolic hydroxyl group comprises at least one selected from the group consisting of hydroquinone and dibutylhydroxytoluene.

5. 2. The nonaqueous electrolyte solution according to claim 1, further comprising, as an electrolyte solvent, at least one solvent selected from the group consisting of carbonate-based solvents, lactone-based solvents, ether-based solvents, and chain ester-based solvents.

6. A lithium ion secondary battery using the nonaqueous electrolyte solution according to any one of claims 1 to 5.