Nonaqueous electrolyte for sodium ion secondary battery and sodium ion secondary battery

Abstract

The present invention addresses the problem of providing a non-aqueous electrolyte solution for a sodium ion secondary battery, which can provide a sodium ion secondary battery with low resistance and little gas generation after a durability test, and a sodium ion secondary battery obtained by using the non-aqueous electrolyte solution for a sodium ion secondary battery. A nonaqueous electrolyte solution for a sodium ion secondary battery comprising a nonaqueous solvent, NaPF6, and a compound represented by the following formula (1), wherein the ratio of the content of the compound represented by formula (1) to the content of NaPF6, [compound represented by formula (1)] / [NaPF6] (molar ratio), is 0.001 or more and 1.5 or less. (In formula (1), R1 and R2 each independently represent a hydrogen atom or a hydrocarbon group having 1 to 6 carbon atoms, and n is 0 or 1.)

Description

[Technical Field]

[0001] The present invention relates to a nonaqueous electrolyte solution for a sodium ion secondary battery and a sodium ion secondary battery. More specifically, the present invention relates to a nonaqueous electrolyte solution for a sodium ion secondary battery that can provide a sodium ion secondary battery with low resistance and little gas generation after a durability test. [Background technology]

[0002] Lithium-ion secondary batteries are high-energy-density secondary batteries, and demand is increasing for them as power sources for transportation equipment such as electric vehicles and electric motorcycles, as well as large-scale power storage devices for home and commercial use. Lithium-ion secondary batteries use lithium ions as the charge carrier. However, lithium is a rare metal, and production is limited.

[0003] In recent years, research has been conducted on sodium-ion secondary batteries, which use sodium ions as a charge carrier. Sodium is more abundant and inexpensive than lithium, and thus has attracted attention as a potential low-cost, large-scale secondary battery. However, even if materials that are known to be usable as negative electrode active materials for lithium-ion secondary batteries are directly used as negative electrode active materials for sodium-ion secondary batteries, it is extremely difficult to realize a sodium-ion secondary battery with sufficient performance (Non-Patent Document 1). It has also been reported that the stability of the negative electrode film formed by the reductive decomposition of the solvent during the initial charge differs between sodium-ion secondary batteries and lithium-ion secondary batteries (Non-Patent Document 2). For these reasons, the development of electrode materials and electrolytes suitable for sodium-ion secondary batteries is needed. Patent Document 1 describes that high-capacity sodium-ion secondary batteries can be realized by using non-graphite carbon materials, such as heat-treated activated carbon, as the negative electrode material for sodium-ion secondary batteries.

[0004] Meanwhile, various additives have been investigated for improving the performance of electrolytes for lithium ion secondary batteries, including lithium salt compounds, nitrile compounds, isocyanate compounds, ether compounds, carbonate esters, carboxylate esters, sulfonate esters, and sulfate esters. Patent Documents 2 and 3 disclose that, among these, sulfate esters can suppress the capacity loss and battery voltage loss during high-temperature storage of nonaqueous electrolytes in lithium ion secondary batteries using LiCoO2 as the positive electrode and graphite as the negative electrode. Patent Document 4 describes the use of cyclic sulfate esters as additives for nonaqueous electrolytes in sodium ion secondary batteries. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Journal of The Electrochemical Society, 148(8) A803-A811 (2001) [Non-patent document 2] ACS Energy Lett. 2016, 1, 1173-1178 [Patent documents]

[0006] [Patent Document 1] WO2014 / 188722 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-319430 [Patent Document 3] WO2012 / 053644 [Patent Document 4] WO2017 / 111143 Summary of the Invention [Problem to be solved by the invention]

[0007] According to the study by the present inventors, it has become clear that when a non-graphite carbon material is used as the negative electrode material of a sodium ion secondary battery, the negative electrode resistance becomes high. On the other hand, Patent Documents 2 to 4 do not specifically study the effect of adding a cyclic sulfate ester to a sodium ion secondary battery.

[0008] The present invention has been made in view of the above background art, and an object of the present invention is to provide a nonaqueous electrolyte solution for a sodium ion secondary battery that can provide a sodium ion secondary battery that has low resistance and generates little gas after a durability test, i.e., that undergoes little volume change after a durability test, and a sodium ion secondary battery obtained by using this nonaqueous electrolyte solution for a sodium ion secondary battery. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that the addition of a cyclic sulfate ester reduces the negative electrode resistance in sodium-ion secondary batteries, an effect that was not observed in lithium-ion secondary batteries. Furthermore, they have found that the above-mentioned problems can be solved by using a sulfate ester compound and NaPF6 in a specific ratio in a nonaqueous electrolyte for sodium-ion secondary batteries. That is, the gist of the present invention is as follows: [1] to [6].

[0010] [1] A nonaqueous electrolyte solution for a sodium ion secondary battery, comprising a nonaqueous solvent, NaPF6, and a compound represented by the following formula (1), wherein the ratio of the content of the compound represented by formula (1) to the content of NaPF6, [compound represented by formula (1)] / [NaPF6] (molar ratio), is 0.001 or more and 1.5 or less.

[0011] [ka]

[0012] (In formula (1), R1 and R2 each independently represent a hydrogen atom or a hydrocarbon group having 1 to 6 carbon atoms, and n is 0 or 1.)

[0013] [2] The nonaqueous electrolyte solution for a sodium ion secondary battery according to [1], containing 0.01 parts by mass or more and 10 parts by mass or less of the compound represented by formula (1) relative to 100 parts by mass of the nonaqueous solvent.

[0014] [3] The nonaqueous electrolyte solution for a sodium ion secondary battery according to [1] or [2], wherein the nonaqueous solvent contains 0.001 mol / L or more and 5.0 mol / L or less of NaPF6.

[0015] [4] The nonaqueous electrolyte solution for a sodium ion secondary battery according to any one of [1] to [3], wherein the nonaqueous solvent contains a cyclic carbonate.

[0016] [5] A sodium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte solution, wherein the nonaqueous electrolyte solution for sodium ion secondary batteries according to any one of [1] to [4] is used as the electrolyte solution. [6] The sodium ion secondary battery according to claim 5, wherein the negative electrode comprises porous carbon. [Effects of the Invention]

[0017] According to the present invention, there are provided a nonaqueous electrolyte solution for a sodium ion secondary battery which can provide a sodium ion secondary battery having low resistance and little gas generation after a durability test, i.e., little volume change after a durability test, and this nonaqueous electrolyte solution for a sodium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below. The following description is an example (typical example) of the present invention, and the present invention is not limited thereto. Furthermore, the present invention can be implemented with any modifications within the scope of the gist thereof.

[0019] [Non-aqueous electrolyte for sodium ion secondary batteries] A nonaqueous electrolyte solution for a sodium ion secondary battery according to an embodiment of the present invention contains a nonaqueous solvent and a compound represented by the following formula (1): Hereinafter, the nonaqueous electrolyte solution for a sodium ion secondary battery according to this embodiment may be referred to as the "nonaqueous electrolyte solution according to this embodiment," and the compound represented by formula (1) may be simply referred to as "compound (1)." [ka] (In formula (1), R1 and R2 each independently represent a hydrogen atom or a hydrocarbon group having 1 to 6 carbon atoms, and n is 0 or 1.)

[0020] The nonaqueous electrolyte solution of this embodiment has the effect of reducing negative electrode resistance and gas generation. While the reason for this effect is unclear, it is presumed to be due to the following reasons. The formation of a negative electrode film is considered to be an important factor in increasing negative electrode resistance. In lithium-ion secondary batteries, an organic coating such as a lithium alkyl carbonate is formed due to the reduction of an organic solvent such as ethylene carbonate. In contrast, in sodium-ion secondary batteries, the reduction of the organic solvent does not stop, presumably making gas generation more likely. Furthermore, it is presumed that an inorganic coating made of a highly resistive component such as sodium carbonate is easily formed on the negative electrode due to reductive decomposition. It is presumed that the use of a nonaqueous electrolyte solution containing a cyclic sulfate ester in the process of forming the negative electrode film in such sodium-ion secondary batteries suppresses the reduction of the organic solvent and the formation of the inorganic coating, thereby reducing gas generation and lowering the negative electrode resistance.

[0021] <1. Compound (1)> The non-aqueous electrolyte solution used in this embodiment contains a compound represented by the formula (1), in which R1 and R2 each independently represent a hydrogen atom or a hydrocarbon group having 1 to 6 carbon atoms, and n is 0 or 1.

[0022] Among the compounds represented by the formula (1), 1,3,2-dioxathiolane 2,2-dioxide, 4-methyl-1,3,2-dioxathiolane 2,2-dioxide, 4-vinyl-1,3,2-dioxathiolane 2,2-dioxide, 4-ethynyl-1,3,2-dioxathiolane 2,2-dioxide, 4,5-dimethyl-1,3,2-dioxathiolane 2,2-dioxide, 4-ethyl-1,3,2-dioxathiolane 2,2-dioxide, 4,4-dimethyl-1,3,2-dioxathiolane 2,2-dioxide, 4-propyl-1,3,2-dioxathiolane 2,2-dioxide, 4,5-dimethyl ... propyl-1,3,2-dioxathiolane 2,2-dioxide, 4-(1-methylethyl)-1,3,2-dioxathiolane 2,2-dioxide, 4,5-diethyl-1,3,2-dioxathiolane 2,2-dioxide, 4-phenyl-1,3,2-dioxathiolane 2,2-dioxide, 4-cyclohexyl-1,3,2-dioxathiolane 2,2-dioxide, 4-hexyl-1,3,2-dioxathiolane 2,2-dioxide, 4-(3-buten-1-yl)-1,3,2-dioxathiolane 2,2-dioxide, 4 -butyl-1,3,2-dioxathiolane 2,2-dioxide, 4-(1,1-dimethylethyl)-1,3,2-dioxathiolane 2,2-dioxide, 4-(1,1-dimethyl-2-propen-1-yl)-1,3,2-dioxathiolane 2,2-dioxide, 4-(3-methylbutyl)-1,3,2-dioxathiolane 2,2-dioxide, 4-cyclohexyl-5-methyl-1,3,2-dioxathiolane 2,2-dioxide, 4,5-dibutyl-1,3,2-dioxathiolane 2,2-dioxide, 4,5-dipe Preferred are 1,3,2-nyl-1,3,2-dioxathiolane 2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, 4-methyl-1,3,2-dioxathiane 2,2-dioxide, 5-methyl-1,3,2-dioxathiane 2,2-dioxide, 4,6-dimethyl-1,3,2-dioxathiane 2,2-dioxide, 4-ethyl-1,3,2-dioxathiane 2,2-dioxide, 4-propyl-1,3,2-dioxathiane 2,2-dioxide, 4-butyl-1,3,2-dioxathiane 2,2-dioxide, and the like.

[0023] Among these, from the viewpoint of small steric hindrance after reductive decomposition, 1,3,2-dioxathiolane 2,2-dioxide, 4-methyl-1,3,2-dioxathiolane 2,2-dioxide, 4-vinyl-1,3,2-dioxathiolane 2,2-dioxide, 4-ethynyl-1,3,2-dioxathiolane 2,2-dioxide, 4,5-dimethyl-1,3,2-dioxathiolane 2,2-dioxide, 4-ethyl-1,3,2-dioxathiolane 2,2-dioxide, 4,4-dimethyl-1,3,2-dioxathiolane 2,2-dioxide, and 4-propyl-1,3,2-dioxathiolane 2,2-dioxide, 4-(1-methylethyl)-1,3,2-dioxathiolane 2,2-dioxide, 4,5-diethyl-1,3,2-dioxathiolane 2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, 4-methyl-1,3,2-dioxathiane 2,2-dioxide, 5-methyl-1,3,2-dioxathiane 2,2-dioxide, 4,6-dimethyl-1,3,2-dioxathiane 2,2-dioxide, 4-ethyl-1,3,2-dioxathiane 2,2-dioxide, 4-propyl-1,3,2-dioxathiane 2,2-dioxide, and the like are particularly preferred.

[0024] The content of the compound represented by formula (1) in the non-aqueous electrolyte solution is not particularly limited as long as it does not significantly impair the effects of the present invention. Specifically, the lower limit of the content of compound (1) is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.1 parts by mass or more, relative to 100 parts by mass of the non-aqueous solvent. The upper limit is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, relative to 100 parts by mass of the non-aqueous solvent. When the concentration of this compound is within the above-mentioned preferred range, the effect of reducing the initial negative electrode resistance is more easily exhibited without impairing other battery performance.

[0025] <2. Non-aqueous solvent> The nonaqueous electrolyte solution of this embodiment, like a general nonaqueous electrolyte solution, usually contains, as its main component, a nonaqueous solvent that dissolves the electrolyte, which will be described later. There are no particular limitations on the nonaqueous solvent used here, and known organic solvents can be used. The organic solvent is preferably at least one selected from saturated cyclic carbonates, chain carbonates, chain carboxylic acid esters, cyclic carboxylic acid esters, ether-based compounds other than compound (1), and sulfone-based compounds, but is not particularly limited thereto. These can be used alone or in combination of two or more.

[0026] <2-1. Saturated cyclic carbonates> Examples of saturated cyclic carbonates include those having an alkylene group having 2 to 4 carbon atoms. Specific examples of saturated cyclic carbonates having 2 to 4 carbon atoms include ethylene carbonate, propylene carbonate, and butylene carbonate. Among these, ethylene carbonate and propylene carbonate are preferred from the viewpoint of improving battery characteristics due to an improved degree of dissociation of sodium ions. One type of saturated cyclic carbonate may be used alone, or two or more types may be used in any combination and ratio.

[0027] The content of the saturated cyclic carbonate is not particularly limited and may be any content as long as it does not significantly impair the effects of the present invention. However, when one type is used alone, the lower limit of the content is usually 3% by volume or more, preferably 5% by volume or more, based on 100% by volume of the non-aqueous solvent. By setting it within this range, a decrease in electrical conductivity due to a decrease in the dielectric constant of the non-aqueous electrolyte solution can be avoided, and the large current discharge characteristics, stability to the negative electrode, and cycle characteristics of the electricity storage device can be easily maintained in a good range. The upper limit is usually 90% by volume or less, preferably 85% by volume or less, and more preferably 80% by volume or less. Setting it within this range is preferable because it allows the viscosity of the non-aqueous electrolyte solution to be within an appropriate range, suppresses a decrease in ionic conductivity, and thereby further improves the input / output characteristics of the electricity storage device and further improves durability such as cycle characteristics and storage characteristics.

[0028] <2-2. Chain carbonate> The chain carbonate preferably has 3 to 7 carbon atoms. Specific examples of the chain carbonate having 3 to 7 carbon atoms include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, n-butyl methyl carbonate, isobutyl methyl carbonate, t-butyl methyl carbonate, ethyl-n-propyl carbonate, n-butyl ethyl carbonate, isobutyl ethyl carbonate, t-butyl ethyl carbonate, etc. Among these, dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, ethyl methyl carbonate, and methyl-n-propyl carbonate are preferred, and dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are particularly preferred.

[0029] Furthermore, chain carbonates having fluorine atoms (hereinafter sometimes abbreviated as "fluorinated chain carbonate") can also be suitably used. The number of fluorine atoms in the fluorinated chain carbonate is not particularly limited as long as it is 1 or more, but is usually 6 or less, and preferably 4 or less. When the fluorinated chain carbonate has multiple fluorine atoms, they may be bonded to the same carbon or different carbons. Examples of fluorinated chain carbonates include fluorinated dimethyl carbonate derivatives, fluorinated ethyl methyl carbonate derivatives, and fluorinated diethyl carbonate derivatives.

[0030] The chain carbonate may be used alone or in any combination of two or more kinds in any ratio.

[0031] The content of the chain carbonate is not particularly limited, but is usually 15% by volume or more, preferably 20% by volume or more, and more preferably 25% by volume or more, based on 100% by volume of the non-aqueous solvent. It is also usually 90% by volume or less, preferably 85% by volume or less, and more preferably 80% by volume or less. By setting the content of the chain carbonate within the above range, the viscosity of the non-aqueous electrolyte solution can be set to an appropriate range, a decrease in ionic conductivity can be suppressed, and the input / output characteristics and charge / discharge rate characteristics of the electricity storage device can be easily set to good ranges. Furthermore, a decrease in electrical conductivity due to a decrease in the dielectric constant of the non-aqueous electrolyte solution can be avoided, and the input / output characteristics and charge / discharge rate characteristics of the electricity storage device can be easily set to good ranges.

[0032] <2-3. Chain carboxylic acid esters> Examples of chain carboxylic acid esters include those having a total carbon number of 3 to 7 in their structural formula. Specific examples include methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, t-butyl propionate, methyl butyrate, ethyl butyrate, n-propyl butyrate, isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propyl isobutyrate, and isopropyl isobutyrate. Among these, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, methyl butyrate, and ethyl butyrate are preferred from the viewpoints of improving ionic conductivity by reducing viscosity and suppressing battery swelling during durability tests such as cycles and storage.

[0033] <2-4. Cyclic carboxylic acid esters> Examples of cyclic carboxylic acid esters include those having a total carbon atom number of 3 to 12 in their structural formula. Specific examples include gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, epsilon-caprolactone, etc. Among these, gamma-butyrolactone is particularly preferred from the viewpoint of improving battery characteristics due to an improved degree of dissociation of sodium ions.

[0034] <2-5. Ether compounds other than compound (1)> As the ether-based compound other than the compound (1), a chain ether having 3 to 10 carbon atoms and a cyclic ether having 3 to 6 carbon atoms other than the compound (1) are preferred.

[0035] Examples of the chain ethers having 3 to 10 carbon atoms include diethyl ether, di(2-fluoroethyl) ether, di(2,2-difluoroethyl) ether, di(2,2,2-trifluoroethyl) ether, ethyl(2-fluoroethyl) ether, ethyl(2,2,2-trifluoroethyl) ether, ethyl(1,1,2,2-tetrafluoroethyl) ether, (2-fluoroethyl)(2,2,2-trifluoroethyl) ether, (2-fluoroethyl)(1,1,2,2-tetrafluoroethyl) ether, and (2,2,2-trifluoroethyl). Ethyl)(1,1,2,2-tetrafluoroethyl) ether, ethyl-n-propyl ether, ethyl (3-fluoro-n-propyl) ether, ethyl (3,3,3-trifluoro-n-propyl) ether, ethyl (2,2,3,3-tetrafluoro-n-propyl) ether, ethyl (2,2,3,3,3-pentafluoro-n-propyl) ether, 2-fluoroethyl-n-propyl ether, (2-fluoroethyl)(3-fluoro-n-propyl) ether, (2-fluoroethyl)(3,3,3-trifluoro-n-propyl (2-fluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, 2,2,2-trifluoroethyl-n-propyl ether, (2,2,2-trifluoroethyl)(3-fluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether , (2,2,2-trifluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, 1,1,2,2-tetrafluoroethyl-n-propyl ether, (1,1,2,2-tetrafluoroethyl)(3-fluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3,3-Pentafluoro-n-propyl) ether, di-n-propyl ether, (n-propyl)(3-fluoro-n-propyl) ether, (n-propyl)(3,3,3-trifluoro-n-propyl) ether, (n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(3-fluoro-n-propyl) ether, (3-fluoro-n-propyl)(3,3,3-trifluoro-n-propyl) ether, (3-fluoro-n-propyl di(3,3,3-trifluoro-n-propyl)ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl)ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl)ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl)ether, di(2,2,3,3-tetrafluoro-n-propyl)ether, (2,2,3,3-tetrafluoro-n-propyl)ether Di(2,2,3,3,3-pentafluoro-n-propyl) ether, di(2,2,3,3,3-pentafluoro-n-propyl) ether, di-n-butyl ether, dimethoxymethane, methoxyethoxymethane, methoxy(2-fluoroethoxy)methane, methoxy(2,2,2-trifluoroethoxy)methane, methoxy(1,1,2,2-tetrafluoroethoxy)methane, diethoxymethane, ethoxy(2-fluoroethoxy)methane, ethoxy(2,2,2-trifluoroethoxy)methane, ethoxy(1,1,2 ,2-tetrafluoroethoxy)methane, di(2-fluoroethoxy)methane, (2-fluoroethoxy)(2,2,2-trifluoroethoxy)methane, (2-fluoroethoxy)(1,1,2,2-tetrafluoroethoxy)methane di(2,2,2-trifluoroethoxy)methane, (2,2,2-trifluoroethoxy)(1,1,2,2-tetrafluoroethoxy)methane, di(1,1,2,2-tetrafluoroethoxy)methane, dimethoxyethane, methoxyethoxyethane, methoxy(2-fluoroethoxy)ethane, methoxy(2,2,2-trifluoroethoxy)ethane, methoxy(1,1,2,2-tetrafluoroethoxy)ethane, diethoxyethane, ethoxy(2-fluoroethoxy)ethane, ethoxy(2,2,2-trifluoroethoxy)ethane, ethoxy(1,1,2,2-tetrafluoroethoxy)ethane, di(2-fluoroethoxy)ethane, (2-fluoroethoxy)(2,2,2-trifluoroethoxy)ethane, (2-fluoroethoxy) Examples include di(2,2,2-trifluoroethoxy)ethane, (2,2,2-trifluoroethoxy)(1,1,2,2-tetrafluoroethoxy)ethane, di(1,1,2,2-tetrafluoroethoxy)ethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether.

[0036] Examples of cyclic ethers having 3 to 6 carbon atoms other than compound (1) include tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxane, 2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and fluorinated compounds thereof. Among these, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether are preferred because they have a high solvation ability for sodium ions and improve sodium ion dissociation. Dimethoxymethane, diethoxymethane, and ethoxymethoxymethane are particularly preferred because they have low viscosity and provide high ionic conductivity.

[0037] <2-6. Sulfone compounds other than compound (1)> The sulfone-based compound other than compound (1) is preferably a cyclic sulfone having 3 to 6 carbon atoms other than compound (1) and a chain sulfone having 2 to 6 carbon atoms. The number of sulfonyl groups in one molecule is preferably 1 or 2.

[0038] Examples of cyclic sulfones other than compound (1) include monosulfone compounds such as trimethylene sulfones, tetramethylene sulfones, and hexamethylene sulfones; and disulfone compounds such as trimethylene disulfones, tetramethylene disulfones, and hexamethylene disulfones. Among these, from the viewpoints of dielectric constant and viscosity, tetramethylene sulfones, tetramethylene disulfones, hexamethylene sulfones, and hexamethylene disulfones are more preferred, and tetramethylene sulfones (sulfolanes) are particularly preferred.

[0039] The sulfolanes are preferably sulfolane and / or sulfolane derivatives (hereinafter, sulfolane may also be abbreviated as "sulfolanes"). The sulfolane derivatives are preferably those in which one or more hydrogen atoms bonded to the carbon atoms constituting the sulfolane ring are substituted with a fluorine atom or an alkyl group.

[0040] Among these, 2-methylsulfolane, 3-methylsulfolane, 2-fluorosulfolane, 3-fluorosulfolane, 2,2-difluorosulfolane, 2,3-difluorosulfolane, 2,4-difluorosulfolane, 2,5-difluorosulfolane, 3,4-difluorosulfolane, 2-fluoro-3-methylsulfolane, 2-fluoro-2-methylsulfolane, 3-fluoro-3-methylsulfolane, 3-fluoro-2-methylsulfolane, 4-fluoro-3-methylsulfolane, 4-fluoro-2-methylsulfolane, 5-fluoro-3-methylsulfolane Fluoro-2-methylsulfolane, 5-fluoro-2-methylsulfolane, 2-fluoromethylsulfolane, 3-fluoromethylsulfolane, 2-difluoromethylsulfolane, 3-difluoromethylsulfolane, 2-trifluoromethylsulfolane, 3-trifluoromethylsulfolane, 2-fluoro-3-(trifluoromethyl)sulfolane, 3-fluoro-3-(trifluoromethyl)sulfolane, 4-fluoro-3-(trifluoromethyl)sulfolane, 5-fluoro-3-(trifluoromethyl)sulfolane, and the like are preferred because of their high ionic conductivity and high input / output.

[0041] Examples of chain sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, n-propyl methyl sulfone, n-propyl ethyl sulfone, di-n-propyl sulfone, isopropyl methyl sulfone, isopropyl ethyl sulfone, diisopropyl sulfone, n-butyl methyl sulfone, n-butyl ethyl sulfone, t-butyl methyl sulfone, t-butyl ethyl sulfone, monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone, monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone, trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethyl monofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyl trifluoromethyl sulfone, and perfluoroethyl methyl sulfone. Examples thereof include fluoromethyl-n-propyl sulfone, ethyl trifluoroethyl sulfone, ethyl pentafluoroethyl sulfone, di(trifluoroethyl) sulfone, perfluorodiethyl sulfone, fluoromethyl-n-propyl sulfone, difluoromethyl-n-propyl sulfone, trifluoromethyl-n-propyl sulfone, fluoromethyl isopropyl sulfone, difluoromethyl isopropyl sulfone, trifluoromethyl isopropyl sulfone, trifluoroethyl-n-propyl sulfone, trifluoroethyl isopropyl sulfone, pentafluoroethyl-n-propyl sulfone, pentafluoroethyl isopropyl sulfone, trifluoroethyl-n-butyl sulfone, trifluoroethyl-t-butyl sulfone, pentafluoroethyl-n-butyl sulfone, and pentafluoroethyl-t-butyl sulfone.

[0042] Among these, dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, n-propyl methyl sulfone, isopropyl methyl sulfone, n-butyl methyl sulfone, t-butyl methyl sulfone, monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone, monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone, trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethyl monofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyl trifluoromethyl sulfone, ethyl trifluoroethyl sulfone, ethyl pentafluoroethyl sulfone, trifluoromethyl-n-propyl sulfone, trifluoromethyl isopropyl sulfone, trifluoroethyl-n-butyl sulfone, trifluoroethyl-t-butyl sulfone, trifluoromethyl-n-butyl sulfone, trifluoromethyl-t-butyl sulfone, and the like are preferred in terms of high ionic conductivity and high input / output.

[0043] <3. Electrolytes> The non-aqueous electrolyte solution of this embodiment contains NaPF6, which is necessary to ensure the conductivity of sodium ions in the non-aqueous solvent.

[0044] <3-1.NaPF6> In this embodiment, NaPF6 is used in such a manner that the ratio of the content of the compound represented by formula (1) to the content of NaPF6, [compound represented by formula (1)] / [NaPF6] (molar ratio), is in the range of 0.001 to 1.5. If this value is too low or too high, the resistance of the battery increases, and the amount of gas generated after a durability test also increases. From the viewpoint of improving these battery characteristics, [compound represented by formula (1)] / [NaPF6] (molar ratio) is preferably 0.003 or more, more preferably 0.005 or more, and even more preferably 0.007 or more, and is preferably 1.4 or less, more preferably 1.3 or less.

[0045] The content of NaPF6 in the non-aqueous solvent is preferably 0.001 mol / L or more, more preferably 0.01 mol / L or more, even more preferably 0.1 mol / L or more, and particularly preferably 0.5 mol / L or more, and is preferably 5.0 mol / L or less, more preferably 4.0 mol / L or less, even more preferably 3.0 mol / L or less, and particularly preferably 2.0 mol / L or less.

[0046] <3-2. Other sodium salts> In the nonaqueous electrolyte solution of this embodiment, in addition to NaPF6, other sodium salts listed below can be used to improve battery performance, such as improving capacity retention, suppressing gas generation, and reducing battery internal resistance. Examples of other sodium salts include NaClO4, NaBF4, NaAlF4, NaSbF6, NaTaF6, NaWOF5, NaWF7, HCO2Na, CH3CO2Na, CH2FCO2Na, CHF2CO2Na, CF3CO2Na, CF3CH2CO2Na, CF3CF2CO2Na, CF3CF2CF2CO2Na, CF3CF2CF2CO2Na, CH3SO3Na, FSO3Na, sodium methyl sulfate, sodium ethyl sulfate, sodium 2-propynyl sulfate, sodium 1-methyl-2-propynyl sulfate, sodium 1,1-dimethyl-2-propynyl sulfate, sodium 2,2,2-trifluoroethyl sulfate, and disodium ethoxylate. Examples of suitable amines include ethylene disulfate, NaC(SO2F)3, NaC(CF3SO2)3, NaC(C2F5SO2)3, Na(FSO2)2N, Na(CF3SO2)2N, Na(C2F5SO2)2N, NaBF3CF3, NaBF3C2F5, NaBF3C3F7, NaBF2(CF3)2, NaBF2(C2F5)2, NaBF2(CF3SO2)2, NaBF2(C2F5SO2)2, NaPO2F2, NaPO3F, NaPF4(C2O4), NaPF2(C2O4)2, NaP(C2O4)3, NaBF2(C2O4), and NaB(C2O4)2, but are not particularly limited to these. These amines may be used singly or in combination of two or more.

[0047] Among these, NaPO2F2, NaPO3F, NaPF2(C2O4)2, NaB(C2O4)2, FSO3Na, and Na(FSO2)2N are preferred from the viewpoint of improving battery performance, such as improving capacity retention, suppressing gas generation, and reducing internal battery resistance.

[0048] When other sodium salts are used, their content is not particularly limited as long as it does not significantly impair the effects of the present invention. Specifically, the lower limit of the content of other salts is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more, based on the total amount of the non-aqueous electrolyte. The upper limit is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 2% by mass or less, based on the total amount of the non-aqueous electrolyte. The total concentration of all the above-mentioned other sodium salts in the non-aqueous electrolyte is not particularly limited, but being within the above range is preferable because it provides a good balance of battery performance such as improved capacity retention, suppressed gas generation, and reduced internal resistance of the battery.

[0049] <4. Other additives> In addition to the various compounds listed above, the nonaqueous electrolyte solution of this embodiment may contain, as a storage property improver, a compound having a cyano group such as malononitrile, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelanitrile, sebaconitrile, undecanedinitrile, or dodecanedinitrile; and as a negative electrode protectant, a diisocyanatomethyl cyclohexane, 1,4-bis(isocyanatomethyl)cyclohexane, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 1,2-bis(isocyanatomethyl)benzene, 1,3-bis(isocyanatomethyl)benzene, or 1,4-bis(isocyanatomethyl)benzene. Compounds: durability improvers such as carboxylic acid anhydride compounds such as acrylic anhydride, 2-methylacrylic anhydride, 3-methylacrylic anhydride, benzoic anhydride, 2-methylbenzoic anhydride, 4-methylbenzoic anhydride, 4-tert-butylbenzoic anhydride, 4-fluorobenzoic anhydride, 2,3,4,5,6-pentafluorobenzoic anhydride, methoxyformic anhydride, and ethoxyformic anhydride; overcharge inhibitors such as cyclohexylbenzene, t-butylbenzene, t-amylbenzene, biphenyl, alkylbiphenyls, terphenyls, partially hydrogenated terphenyls, diphenyl ether, and dibenzofuran; and various other additives may be blended within a range that does not significantly impair the effects of the present invention. These compounds may also be used in appropriate combinations.

[0050] [Sodium-ion secondary battery] The above-mentioned nonaqueous electrolyte solution, positive electrode, and negative electrode can be used to prepare an electrolyte solution for a sodium ion secondary battery (hereinafter, sometimes referred to as "the sodium ion secondary battery of this embodiment.") The sodium ion secondary battery of this embodiment usually comprises the above-mentioned nonaqueous electrolyte solution, a positive electrode having a current collector and a positive electrode active material layer provided on the current collector and capable of absorbing and releasing sodium ions, and a negative electrode having a current collector and a negative electrode active material layer provided on the current collector and capable of absorbing and releasing sodium ions.

[0051] <1. Positive electrode> The positive electrode used in the sodium ion secondary battery of this embodiment typically contains a composite oxide, a polyanion compound, a fluoride, etc. The positive electrode typically has a positive electrode active material layer on a current collector, and the positive electrode active material layer contains a positive electrode active material. The positive electrode active material will be described below.

[0052] An example of the composite oxide is one represented by the following formula (2). Na x M 1 1-y M 2 y O2(2)

[0053] In formula (2), 0 <x<1.2、0<y<1である。

[0054] M in Equation (2) 1 is a transition metal, preferably Mn, Fe, Co, or Ni. 1 The composite oxide may contain only one type of element, or may contain multiple different types.

[0055] M in Equation (2) 2 is at least one selected from V, Fe, Cu, Nb, Mo, Ta, W, Zn, Ti, Zr, Al, B, Mg, Li, Na, and K. Among these, from the viewpoint of improving output, at least one selected from V, Fe, Cu, Nb, Mo, Ta, and W is preferred, and among these, at least one selected from Nb, Mo, Ta, and W is more preferred. On the other hand, from the viewpoint of capacity retention rate after a durability test, at least one selected from Zn, Ti, Zr, Al, B, Mg, Li, Na, and K is preferred, and among these, at least one selected from Zr, Al, Mg, and Li is more preferred.

[0056] Preferred composite oxides include, for example, Na 2 / 3 Fe 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Ni 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Ni 1 / 3 Mn2 / 3 O2, Na 4 / 5 Ni 1 / 3 Mn 2 / 3 O2, NaCoO2, NaCrO2, NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 Examples include O2.

[0057] The polyanion compound is, for example, one represented by the following formula (3). Na x` M' y` (QO4) z (3)

[0058] In formula (3), 1 <x`<2、1<y`<3、1<z<3である。

[0059] M' in formula (3) is a transition metal, preferably Mn, Fe, Co, or Ni. The polyanion compound may contain only one type of M' in formula (3), or may contain multiple different types.

[0060] In formula (3), Q is at least one selected from P, As, Sb, Bi, S, Se, Te, Po, Si, Ge, Sn, and Pb. Among these, from the viewpoint of the stability of the compound, at least one selected from P, S, and Si is preferred, and among these, at least one selected from P and S is more preferred.

[0061] Preferred examples of polyanion compounds include NaFePO4, Na2Fe3(PO4)3, Na2Fe2(SO4)3, Na2Fe2(SiO4)3, NaMnPO4, NaMnFe2(PO4)3, Na2Mn2(SO4)3, and Na2Mn2(SiO4)3.

[0062] The fluoride is, for example, NaM"F3 or Na2M"PO4F (wherein M" is a transition metal, preferably Mn, Fe, Co, or Ni, and may contain only one type or a plurality of different types). Examples include NaFeF3, Na2FePO4F, NaFMnF3, Na2MnPO4F, NaFNiF3, and Na2NiPO4F.

[0063] Among the above-mentioned positive electrodes, those containing a composite oxide, a polyaniline compound, and a fluoride are preferred. However, other positive electrode active materials may also be included as long as they do not impair the effects of the present invention. The other positive electrode active materials are not particularly limited as long as they are not classified as any of the composite oxides, polyanion compounds, or fluorides and are capable of electrochemically absorbing and desorbing s-block metal ions. For example, materials containing an alkali metal and at least one transition metal are preferred. Specific examples include sodium-transition metal composite oxides, sodium-containing transition metal phosphate compounds, and sodium-containing transition metal silicate compounds. Examples include Na2FeP2O7 and Na4Fe3(PO4)2(P2O7). The above other positive electrode active materials may be used alone or in combination of two or more.

[0064] <2. Negative electrode> The negative electrode usually has a negative electrode active material layer on a current collector. The negative electrode active material will be described below.

[0065] There are no particular limitations on the negative electrode active material, so long as it is capable of electrochemically absorbing and releasing s-block metal ions such as lithium ions, sodium ions, potassium ions, and magnesium ions. Specific examples include carbonaceous materials, metal alloy materials, and s-block metal-containing metal composite oxide materials. These materials may be used alone or in any combination of two or more.

[0066] Carbonaceous materials used as negative electrode active materials include natural graphite, non-graphitizable carbon, and artificial carbonaceous materials, but are not particularly limited. Generally, they should have a porous structure that allows the insertion and desorption of sodium ions. Specifically, the porous carbon materials described in WO2014 / 188722 (Patent Document 1) are preferred from the viewpoint of high capacity. The porous carbon material is a carbon material having a plurality of open pores that are connected to the surface, a plurality of closed pores that are not connected to the surface, and a solid portion made of a carbon material. The distance between the (002) planes of carbon in at least a portion of the solid portion may be 0.36 nm or more and 0.41 nm or less. Furthermore, the volume ratio of the closed pores to the total volume of the open pores, the closed pores, and the solid portion may be 30% or more and 90% or less.

[0067] <3. Separator> A separator is usually interposed between the positive electrode and the negative electrode to prevent short circuits, and in this case, the nonaqueous electrolyte solution of the present invention is usually impregnated into the separator before use.

[0068] The separator is not particularly limited in material or shape, and any known material may be used as long as it does not significantly impair the effects of the present invention. Among these, it is preferable to use a material such as a resin, glass fiber, or inorganic material that is stable with respect to the nonaqueous electrolyte solution of this embodiment, and to use a porous sheet or nonwoven fabric that has excellent liquid retention properties.

[0069] Examples of materials that can be used for the resin and glass fiber separator include polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene, polyethersulfone, and glass filters. Among these, glass filters and polyolefins are preferred, and polyolefins are more preferred. These materials may be used alone or in any combination and ratio of two or more.

[0070] The thickness of the separator is optional, but is usually 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and is usually 50 μm or less, preferably 40 μm or less, more preferably 30 μm or less. If the separator is thinner than the above range, the insulating properties and mechanical strength may be reduced. If the separator is thicker than the above range, not only may the battery performance such as rate characteristics be reduced, but also the energy density of the entire power storage device may be reduced.

[0071] Furthermore, when a porous material such as a porous sheet or nonwoven fabric is used as the separator, the porosity of the separator is optional, but is usually 20% or more, preferably 35% or more, more preferably 45% or more, and is usually 90% or less, preferably 85% or less, more preferably 75% or less. If the porosity is too low below the above range, the membrane resistance tends to increase, resulting in poor rate performance. If the porosity is too high above the above range, the mechanical strength of the separator tends to decrease, and its insulating properties tend to deteriorate.

[0072] The average pore size of the separator can also be any value, but is usually 0.5 μm or less, preferably 0.2 μm or less, and usually 0.05 μm or more. If the average pore size exceeds the above range, short circuits are likely to occur. If the average pore size is below the above range, the membrane resistance increases, which may result in a decrease in rate characteristics.

[0073] On the other hand, inorganic materials include, for example, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate, and these are used in particulate or fibrous form.

[0074] As for the form, a thin film such as a nonwoven fabric, a woven fabric, or a microporous film is used. A thin film with a pore size of 0.01 to 1 μm and a thickness of 5 to 50 μm is preferably used. In addition to the independent thin film, a separator can be used in which a composite porous layer containing the inorganic particles is formed on the surface layer of the positive electrode and / or negative electrode using a resin binder. For example, a porous layer can be formed on both sides of the positive electrode using alumina particles with a 90% particle size of less than 1 μm and a fluororesin as a binder.

[0075] <4. Conductive materials> The positive electrode and negative electrode may contain a conductive material to improve conductivity. Any known conductive material can be used as the conductive material. Specific examples include metal materials such as copper and nickel; graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; and carbonaceous materials such as amorphous carbon such as needle coke. These materials may be used alone or in any combination and ratio of two or more.

[0076] The conductive material is used in an amount of usually 0.01 parts by mass or more, preferably 0.1 parts by mass or more, more preferably 1 part by mass or more, and usually 50 parts by mass or less, preferably 30 parts by mass or less, more preferably 15 parts by mass or less, per 100 parts by mass of the positive electrode material or negative electrode material. If the content is below the above range, the conductivity may be insufficient. On the other hand, if the content is above the above range, the battery capacity may decrease.

[0077] <5. Binder> The positive electrode and the negative electrode may contain a binder to improve binding properties. The binder is not particularly limited as long as it is a material that is stable to the non-aqueous electrolyte solution and the solvent used in producing the electrodes.

[0078] In the case of the coating method, any material that can be dissolved or dispersed in the liquid medium used in manufacturing the electrode may be used. Specific examples include resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, and nitrocellulose; rubber polymers such as SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluororubber, isoprene rubber, butadiene rubber, and ethylene-propylene rubber; styrene-butadiene-styrene block copolymers or their hydrogenated products, EPDM (ethylene-propylene-diene terpolymer), and styrene-butadiene-styrene block copolymers. Examples of suitable materials include thermoplastic elastomeric polymers such as ethylene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers, and hydrogenated products thereof; soft resinous polymers such as syndiotactic 1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorinated polymers such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; and polymer compositions with ionic conductivity for alkali metal ions (especially sodium ions). These materials may be used singly or in any combination and ratio of two or more.

[0079] The proportion of the binder is typically 0.1 parts by mass or more, preferably 1 part by mass or more, more preferably 3 parts by mass or more, relative to 100 parts by mass of the positive electrode material or negative electrode material, and typically 50 parts by mass or less, preferably 30 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 8 parts by mass or less. If the proportion of the binder is within the above range, the binding properties of the electrode can be sufficiently maintained and the mechanical strength of the electrode can be maintained, which is preferable in terms of cycle characteristics, battery capacity, and conductivity.

[0080] <6.Liquid medium> The liquid medium for forming the slurry is not particularly limited in type, and either an aqueous solvent or an organic solvent may be used, as long as it is a solvent capable of dissolving or dispersing the active material, conductive material, binder, and thickener used as needed.

[0081] Examples of aqueous media include water and mixed media of alcohol and water. Examples of organic media include aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphalamide and dimethylsulfoxide. These solvents may be used alone or in any combination and ratio of two or more.

[0082] <7. Thickener> When an aqueous medium is used as the liquid medium for forming the slurry, it is preferable to form the slurry using a thickener and a latex such as styrene butadiene rubber (SBR). The thickener is usually used to adjust the viscosity of the slurry.

[0083] There are no limitations on the thickener as long as it does not significantly restrict the effects of the present invention, and specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein, and salts thereof. These may be used alone or in any combination and ratio of two or more.

[0084] Furthermore, when a thickener is used, it is usually 0.1 part by mass or more, preferably 0.5 part by mass or more, more preferably 0.6 part by mass or more, and usually 5 parts by mass or less, preferably 3 parts by mass or less, more preferably 2 parts by mass or less, per 100 parts by mass of the positive electrode material or negative electrode material. If the amount is less than the above range, the coating properties may be significantly reduced, and if the amount is more than the above range, the proportion of active material in the active material layer may decrease, resulting in problems such as a decrease in battery capacity and an increase in resistance between active materials.

[0085] <8. Current Collector> The material of the current collector is not particularly limited, and any known material can be used. Specific examples include metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and copper; and carbonaceous materials such as carbon cloth and carbon paper. Among these, metal materials, especially aluminum, are preferred.

[0086] The shape of the current collector may be, for example, a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punched metal, or a foamed metal, for a metal material, or a carbon plate, a carbon thin film, or a carbon cylinder, for a carbonaceous material. Of these, a metal thin film is preferred. The thin film may be formed into a mesh as appropriate.

[0087] The thickness of the current collector is optional, but is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and is usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thin film is within the above range, the strength required as a current collector is maintained and it is also preferable from the viewpoint of handleability.

[0088] <9.Battery design> [Electrode group] The electrode group may have either a laminated structure consisting of the positive and negative electrode plates sandwiched between the separator, or a spirally wound structure consisting of the positive and negative electrode plates sandwiched between the separator. The ratio of the volume of the electrode group to the internal volume of the battery (hereinafter referred to as the electrode group occupancy rate) is typically 40% or more, preferably 50% or more, and typically 90% or less, preferably 80% or less. If the electrode group occupancy rate is below the above range, the battery capacity decreases. On the other hand, if the electrode group occupancy rate exceeds the above range, the void space is small, and the battery becomes hot, causing components to expand and the vapor pressure of the electrolyte liquid components to increase, resulting in an increase in internal pressure. This can degrade various battery characteristics such as repeated charge / discharge performance and high-temperature storage, and may even activate the gas release valve that releases internal pressure.

[0089] [Current collection structure] Although the current collecting structure is not particularly limited, in order to more effectively realize the improvement of discharge characteristics by the nonaqueous electrolyte solution of the present invention, it is preferable to use a structure that reduces the resistance of wiring parts and joint parts. When the internal resistance is reduced in this way, the effect of using the nonaqueous electrolyte solution of the present invention is particularly well exhibited.

[0090] When the electrode group has the aforementioned laminated structure, a structure formed by bundling the metal core portions of each electrode layer and welding them to a terminal is preferably used. When the area of ​​a single electrode is large, the internal resistance increases, so it is also preferable to provide multiple terminals within the electrode to reduce the resistance. When the electrode group has the aforementioned wound structure, the internal resistance can be reduced by providing multiple lead structures on each of the positive and negative electrodes and bundling them to a terminal.

[0091] [Outer case] The material of the outer case is not particularly limited as long as it is stable against the non-aqueous electrolyte solution used. Specifically, metals such as nickel-plated steel sheet, stainless steel, aluminum or aluminum alloy, magnesium alloy, or a laminate film of resin and aluminum foil (laminate film) can be used. From the viewpoint of weight reduction, metals such as aluminum or aluminum alloy and laminate film are preferably used.

[0092] Examples of exterior cases using metals include those in which metals are welded together to form a sealed, airtight structure by laser welding, resistance welding, or ultrasonic welding, or those in which the metals are used via a resin gasket to form a crimped structure. Examples of exterior cases using laminate films include those in which resin layers are heat-sealed to form a sealed, airtight structure. In order to improve sealing properties, a resin different from the resin used in the laminate film may be interposed between the resin layers. In particular, when a sealed structure is formed by heat-sealing the resin layers via a current collecting terminal, a resin having a polar group or a modified resin into which a polar group has been introduced is preferably used as the interposed resin, since the metal and the resin are bonded together.

[0093] [Protection element] Examples of the protective element mentioned above include a PTC (Positive Temperature Coefficient), whose resistance increases when abnormal heat is generated or an excessive current flows, a thermal fuse, a thermistor, a valve (current cutoff valve) that cuts off the current flowing in the circuit due to a sudden rise in the internal pressure or temperature of the battery when abnormal heat is generated, etc. It is preferable to select a protective element that will not operate under normal use at high current, and from the perspective of high output, it is even more preferable to design it so that abnormal heat generation or thermal runaway does not occur even without a protective element.

[0094] [Exterior body] The electricity storage device of the present invention is usually configured by housing the above-mentioned nonaqueous electrolyte solution, negative electrode, positive electrode, separator, etc. There are no limitations on this housing, and any known housing can be used as long as it does not significantly impair the effects of the present invention.

[0095] Specifically, the material of the exterior body is not limited, but typically, for example, nickel-plated iron, stainless steel, aluminum or an alloy thereof, nickel, titanium, or the like is used.

[0096] The shape of the exterior body may also be arbitrary, and may be, for example, cylindrical, rectangular, laminated, coin-shaped, large, or the like. [Example]

[0097] The present invention will be explained in more detail below by way of examples and reference examples, but the present invention is not limited to these examples as long as it does not depart from the gist of the invention.

[0098] (Example 1-1) [Preparation of positive electrode] The complex metal compound is NaNi 0.3 Mn 0.3 Fe 0.4 Na2CO3, Ni2CO3, Mn3O4, and Fe2O3 were weighed out to form O2, dispersed in an ethanol solvent, and then wet-pulverized using a Picograin Mill (Asada Iron Works Co., Ltd., PCM-L) until the median diameter d50 was 0.4 μm or less, yielding a mixture of metal-containing compounds. When weighing, an additional 5 mol% of Na2CO3 was added to account for losses during production. The resulting mixture was evaporated and dried, then filled into an alumina crucible and fired in an electric furnace in an air atmosphere at 900°C for 24 hours to produce a composite metal oxide, O3-type NaNi 0.3 Mn 0.3 Fe 0.4 Got O2.

[0099] The composite metal oxide as the positive electrode active material, acetylene black (HS-100 manufactured by Denka Co., Ltd.) as the conductive material, and polyvinylidene fluoride (#7500 manufactured by Kureha Corporation) as the binder were weighed in a mass ratio of 95:5:5 and dispersed in NMP solvent to obtain a slurry. The obtained slurry was coated onto aluminum foil using a coating machine. The coated electrode plate was rolled using a rolling mill, punched into a 30 mm x 40 mm rectangle, and processed into an electrode state to obtain a test electrode.

[0100] [Preparation of negative electrode] A test electrode was fabricated using a porous carbon material (AT Electrode Co., Ltd., LN0010) as the negative electrode active material and copper foil as the current collector. The porous carbon material was used as the negative electrode active material, carbon black (TIMCAL, Super P) as the conductive material, and polyvinylidene fluoride (Kureha Corporation, #1120) as the binder in a mass ratio of 95:2:3. The mixture was dispersed in NMP solvent to obtain a slurry. The resulting slurry was coated onto copper foil using a coating machine. The coated electrode plate was rolled using a rolling mill, punched into a 32 mm x 42 mm rectangle, and processed into an electrode to obtain a test electrode.

[0101] [Preparation of electrolyte] An electrolyte solution (manufactured by Kishida Chemical Co., Ltd.) prepared by dissolving NaPF6 as an electrolyte in a mixed solvent (volume ratio 30:70) of ethylene carbonate (EC) and diethyl carbonate (DEC) at a ratio of 1 mol / L was used as a base electrolyte solution. 1.43 parts by mass of ethylene sulfate (compound (1-1)) was mixed with 100 parts by mass of the mixed solvent to prepare the electrolyte solution of Example 1.

[0102] [Battery manufacturing] The above positive electrode, negative electrode, and polypropylene separator were stacked in this order to prepare a battery element. This battery element was inserted into a bag made of a laminate film of aluminum (40 μm thick) coated on both sides with a resin layer, with the positive and negative terminals protruding, and then the above electrolyte was injected into the bag, which was then vacuum-sealed and fully charged at 4.0 V. A sheet-type battery of Example 1-1 was fabricated.

[0103] [Battery evaluation] A sodium-ion secondary battery was charged to 4.0 V at 25°C with a constant current equivalent to 0.1 C and then discharged to 1.5 V at a constant current of 0.1 C. This cycle was repeated twice to stabilize the battery. Next, the battery was charged to 4.0 V at a constant current equivalent to 0.1 C and then subjected to impedance measurements at a temperature of -20°C, a voltage amplitude of 10 mV, and a frequency range of 100,000 Hz to 0.001 Hz. The analysis program ZView (Ver. 3.2b) was used to separate the positive and negative electrode resistances from the impedance measurements. A durability test was performed by charging to 4.0 V at a constant current equivalent to 1 C at 60°C and then discharging to 1.5 V at a constant current of 1 C. This cycle was repeated 100 times, and the volume change of the battery was measured using Archimedes' principle.

[0104] (Comparative Example 1-1) A sheet-form battery was produced in the same manner as in Example 1-1 except that the compound (1-1) was not mixed, and was evaluated under the same conditions as in Example 1-1.

[0105] (Comparative Example 1-2) LiNi as the positive electrode active material 1 / 3 Mn 1 / 3 Co 1 / 3 A sheet-shaped battery was fabricated in the same manner as in Example 1-1, except that a positive electrode containing O2, carbon black as a conductive material, and polyvinylidene fluoride as a binder in a mass ratio of 90:7:3, and LiPF6 was used as the electrolyte instead of NaPF6, was used. In addition, evaluation was performed under the same conditions as in Example 1-1, except that the discharge voltage was set to 3.0 V.

[0106] (Comparative Examples 1-3) A sheet-form battery was produced in the same manner as in Comparative Example 1-2, except that the compound (1-1) was not mixed, and was evaluated under the same conditions as in Example 1-1.

[0107] (Comparative Examples 1-4) A sheet-type battery was produced in the same manner as in Comparative Example 1-2, except that a negative electrode was used in which graphite was used as the negative electrode active material and carboxymethyl cellulose sodium (aqueous dispersion with a concentration of 1% by mass) and styrene-butadiene rubber (aqueous dispersion with a concentration of 50% by mass) were used as the thickener and binder in a mass ratio of 97.5:1.5:1, and evaluation was carried out under the same conditions as in Comparative Example 1-2.

[0108] (Comparative Examples 1-5) A sheet-form battery was produced in the same manner as in Comparative Example 1-4, except that the compound (1-1) was not mixed in. Evaluation was also carried out under the same conditions as in Comparative Example 1-2.

[0109] (Example 1-2) [Preparation of positive electrode] Complex metal compounds are Na 2 / 3 Ni 1 / 3 Mn 2 / 3 Na2CO3, Ni2CO3, and Mn3O4 were weighed and dispersed in an ethanol solvent to obtain a composition of P2-type Na2CO3. The mixture was then wet-pulverized using a Picograin Mill (PCM-L, manufactured by Asada Iron Works Co., Ltd.) to a median diameter d50 of 0.4 μm or less, yielding a mixture of metal-containing compounds. When weighing, 10 mol% of Na2CO3 was added to account for losses during production. The resulting mixture was evaporated and dried, then filled into an alumina crucible and fired in an electric furnace at 1000°C for 24 hours in an air atmosphere to obtain a composite metal oxide P2-type Na2CO3. 2 / 3 Ni 1 / 3 Mn 2 / 3 O2 was obtained. The composite metal oxide was used as the positive electrode active material, acetylene black (HS-100 manufactured by Denka Co., Ltd.) as the conductive material, and polyvinylidene fluoride (#7500 manufactured by Kureha Corporation) as the binder in a mass ratio of 95:5:5, and dispersed in an NMP solvent to obtain a slurry. The obtained slurry was coated onto aluminum foil using a coating machine. The coated electrode plate was rolled using a rolling mill, punched into a 30 mm x 40 mm rectangle, and processed into an electrode state to obtain a test electrode.

[0110] [Preparation of negative electrode] Activated carbon material (specific surface area 1700m 2 The activated carbon material (carbon dioxide, SiO2 ...

[0111] A sheet-type battery was fabricated in the same manner as in Example 1-1, except that the positive and negative electrodes fabricated by the above method were used as test electrodes, and evaluation was carried out under the same conditions as in Example 1-1.

[0112] (Examples 1-3) A sheet-type battery was produced in the same manner as in Example 1-2, except that the compound (1-1) was not mixed and an electrolyte solution containing 1.60 parts by mass of 1,2-propylene sulfate (compound (1-2)) was used, and evaluation was performed under the same conditions as in Example 1-2.

[0113] (Comparative Examples 1-6) A sheet-form battery was produced in the same manner as in Example 1-2 except that the compound (1-1) was not mixed, and was evaluated under the same conditions as in Example 1-2.

[0114] [Table 1]

[0115] The types of active materials and additives used in the evaluated batteries, as well as the resistance ratio and volume change ratio, are shown in Table 1. The initial resistance ratio represents the ratio of the negative electrode resistance when no additives are used to the negative electrode resistance when additives are used in a battery system using the same positive and negative electrode active materials, and was calculated using the following formula. [Initial resistance ratio] = [[Negative electrode resistance with additive] / [Negative electrode resistance without additive]]

[0116] The volume change ratio represents the ratio of the volume change in a battery system using the same positive and negative electrode active materials when no additive is used to the volume change in a battery system using an additive, and was calculated using the following formula. [Volume change ratio] = [[Volume change of battery with additive] / [Volume change of battery without additive]]

[0117] Although a porous carbon material was used as the negative electrode active material in Example 1-1 and Comparative Examples 1-1 to 1-3, the results shown in Table 1 indicate that the addition of compound (1-1) did not have an effect of reducing the negative electrode resistance in lithium ion secondary batteries. However, it can be seen that the addition of compound (1-1) has an excellent effect of reducing the negative electrode resistance in sodium ion secondary batteries.

[0118] Furthermore, although Comparative Examples 1-4 and 1-5 do not contain NaPF6 and do not exhibit the effects of the present invention, the results shown in Table 1 show that in lithium ion secondary batteries using graphite as the negative electrode active material, the addition of compound (1-1) tends to increase the negative electrode resistance.

[0119] These results indicate that the same compound, when used as an additive, can have different effects depending on the battery system. Regarding the difference in the effect on anode resistance between sodium-ion and lithium-ion secondary batteries, even with the same compound, the reaction mechanism is unclear, but it is thought to be due to the lithium-ion secondary battery system during the initial anode formation process. Compound (1) is presumed to reduce anode resistance by suppressing the formation of an inorganic coating during the anode coating formation process in sodium-ion secondary batteries.

[0120] A comparison between Example 1-2 and Comparative Example 1-6, which use positive and negative electrode active materials for sodium secondary batteries different from those in Example 1-1, reveals that the addition of compound (1-1) significantly reduces the negative electrode resistance. A comparison between Example 1-3 and Comparative Example 1-6 reveals that a similar effect can be obtained with compound (1-2) having a chemical structure similar to that of compound (1-1).

[0121] From the above comparison, it is clear that by using the nonaqueous electrolyte for sodium ion secondary batteries of the present invention, a sodium ion secondary battery with low resistance can be provided.

[0122] Example 2-1 [Preparation of positive electrode] Complex metal compounds are Na 2 / 3 Ni 1 / 3 Mn 2 / 3 Na2CO3, Ni2CO3, and Mn3O4 were weighed and dispersed in an ethanol solvent to obtain a composition of P2-type Na2CO3. The mixture was then wet-pulverized using a Picograin Mill (PCM-L, manufactured by Asada Iron Works Co., Ltd.) to a median diameter d50 of 0.4 μm or less, yielding a mixture of metal-containing compounds. When weighing, 10 mol% of Na2CO3 was added to account for losses during production. The resulting mixture was evaporated and dried, then filled into an alumina crucible and fired in an electric furnace at 1000°C for 24 hours in an air atmosphere to obtain a composite metal oxide P2-type Na2CO3. 2 / 3 Ni 1 / 3 Mn 2 / 3 O2 was obtained. The composite metal oxide was used as the positive electrode active material, acetylene black (HS-100 manufactured by Denka Co., Ltd.) as the conductive material, and polyvinylidene fluoride (#7500 manufactured by Kureha Corporation) as the binder in a mass ratio of 95:5:5, and dispersed in an NMP solvent to obtain a slurry. The obtained slurry was coated onto aluminum foil using a coating machine. The coated electrode plate was rolled using a rolling mill, punched into a 30 mm x 40 mm rectangle, and processed into an electrode state to obtain a test electrode.

[0123] [Preparation of negative electrode] Activated carbon material (specific surface area 1700m 2The activated carbon material (carbon dioxide, SiO2 ...

[0124] [Preparation of electrolyte] Ethylene sulfate (compound (1-1)) and NaPF6 were dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 30:70) according to Example 2-1 in Table 2 to prepare an electrolyte solution.

[0125] [Battery manufacturing] The positive electrode, negative electrode, and polypropylene separator were stacked in this order to prepare a battery element. This battery element was inserted into a bag made of a laminate film of aluminum (40 μm thick) coated on both sides with a resin layer, with the positive and negative electrode terminals protruding, and the electrolyte solution was then injected into the bag, which was then vacuum-sealed to prepare a sheet-like battery of Example 2-1, which was fully charged at 4.0 V.

[0126] [Battery evaluation] A sodium-ion secondary battery was charged to 4.0 V at 25°C with a constant current equivalent to 0.1 C, and then discharged to 1.5 V at a constant current of 0.1 C. This cycle was repeated twice to stabilize the battery. Next, the battery was charged to 4.0 V with a constant current equivalent to 0.1 C, and impedance measurements were performed at a temperature of 25°C, a voltage amplitude of 10 mV, and a frequency range of 100,000 Hz to 0.001 Hz to confirm the battery resistance. For the durability test, the battery was charged to 4.0 V and stored at 60°C for one week, and the volume change of the battery was measured using Archimedes' principle.

[0127] (Example 2-2) Ethylene sulfate (compound (1-1)) and NaPF6 were dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 30:70) according to Example 2-2 in Table 2 to prepare an electrolyte solution. A sheet-type battery was fabricated in the same manner as in Example 2-1, except that the battery was evaluated under the same conditions as in Example 2-1.

[0128] (Example 2-3) Ethylene sulfate (compound (1-1)) and NaPF6 were dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 30:70) as in Example 2-3 of Table 2 to prepare an electrolyte solution. A sheet-type battery was fabricated in the same manner as in Example 2-1, except that the battery was evaluated under the same conditions as in Example 2-1.

[0129] (Examples 2-4) Ethylene sulfate (compound (1-1)) and NaPF6 were dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 30:70) as in Example 2-4 of Table 2 to prepare an electrolyte solution. A sheet-type battery was fabricated in the same manner as in Example 2-1, except that the battery was evaluated under the same conditions as in Example 2-1.

[0130] (Comparative Example 2-1) Ethylene sulfate (compound (1-1)) and NaPF6 were dissolved in a mixed solvent (volume ratio 30:70) of ethylene carbonate (EC) and diethyl carbonate (DEC) according to Comparative Example 2-1 in Table 2 to prepare an electrolyte solution. A sheet-type battery was fabricated in the same manner as in Example 2-1, and evaluated under the same conditions as in Example 2-1.

[0131] (Comparative Example 2-2) Ethylene sulfate (compound (1-1)) and NaPF6 were dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 30:70) as shown in Table 2 to prepare an electrolyte solution. A sheet-type battery was fabricated in the same manner as in Example 2-1, and evaluated under the same conditions as in Example 2-1.

[0132] [Table 2]

[0133] Table 2 shows the moles of ethylene sulfate (compound (1-1)) and NaPF6, the ethylene sulfate / NaPF6 molar ratio, the initial resistance ratio, and the volume change ratio of the evaluated batteries. The initial resistance ratio represents the ratio of the negative electrode resistance when no additives are used to the negative electrode resistance when additives are used in a battery system using the same positive and negative electrode active materials, and was calculated using the following formula. [Initial resistance ratio] = [[Negative electrode resistance with additive] / [Negative electrode resistance without additive]]

[0134] The volume change ratio represents the ratio of the volume change in a battery system using the same positive and negative electrode active materials when no additive is used to the volume change in a battery system using an additive, and was calculated using the following formula. [Volume change ratio] = [[Volume change of battery with additive] / [Volume change of battery without additive]]

[0135] The results of Examples 2-1 to 2-4 clearly show that when the ethylene sulfate / NaPF6 (molar ratio) is within a specific range, the battery resistance and the change in battery volume after the durability test are smaller than those of Comparative Example 2-1. Furthermore, as shown in Comparative Example 2-2, when the ethylene sulfate / NaPF6 (molar ratio) is greater than the specific range, the initial resistance is higher and the change in battery volume after the durability test is larger than those of Comparative Example 2-1, in which ethylene sulfate is not added. In other words, when the molar ratio of ethylene sulfate / NaPF6 is within an appropriate range, a sodium ion secondary battery with low initial resistance and small change in battery volume after the durability test can be provided. [Industrial Applicability]

[0136] The nonaqueous electrolyte solution for sodium ion secondary batteries and the sodium ion secondary batteries according to the embodiments of the present invention can be used in a variety of known applications, including, for example, power sources for portable electronic devices such as power tools and smartphones, emergency power storage systems for homes, etc., power sources for transportation devices such as electric vehicles, power sources for load leveling, and natural energy storage power sources.

Claims

1. A sodium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte solution, The electrolyte is a non-aqueous solvent, NaPF ６ and a compound represented by the following formula (1), and NaPF ６ The ratio of the content of the compound represented by formula (1) to the content of the compound represented by formula (1) [compound represented by formula (1)] / [NaPF ６ ] (molar ratio) is 0.001 or more and 1.5 or less, The negative electrode comprises porous carbon. A sodium ion secondary battery (excluding the case where the electrolyte solution contains a sultone-based compound represented by the following formula II): 【Chemistry 1】 (In formula (1), R １ and R ２ is a hydrogen atom, and n is 0 or 1. 【Chemistry 2】 (In formula II, R 2 and R 3 are each independently a hydrogen atom or a hydrocarbon group having 1 to 3 carbon atoms, and n 2 is 1, 2, or 3.)

2. The sodium ion secondary battery according to claim 1 , wherein the compound represented by formula (1) is contained in an amount of 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the non-aqueous solvent.

3. The NaPF ６ The sodium ion secondary battery according to claim 1 or 2, containing 0.001 mol / L or more and 5.0 mol / L or less.

4. The sodium ion secondary battery according to claim 1 , wherein the non-aqueous solvent contains a cyclic carbonate.

5. The sodium ion secondary battery according to claim 1 , wherein n in the formula (1) is 0.

6. 6. The sodium ion secondary battery according to claim 1, wherein the porous carbon has a plurality of openings that communicate with the surface, a plurality of openings that do not communicate with the surface, and a solid portion made of a carbon material.

7. The sodium ion secondary battery according to claim 6, wherein the distance between (002) planes of carbon in at least a part of the solid portion is 0.36 nm or more and 0.41 nm or less.

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