JP7457620B2 - Non-aqueous electrolyte and non-aqueous electrolyte battery containing the non-aqueous electrolyte - Google Patents

Description

The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte battery containing the non-aqueous electrolyte.

BACKGROUND OF THE INVENTION Non-aqueous electrolyte batteries such as lithium secondary batteries are being put into practical use in a wide range of applications, from so-called consumer power supplies such as mobile phones and notebook computers to on-board power supplies for driving automobiles. However, in recent years, demands for higher performance for non-aqueous electrolyte batteries have been increasing, and in particular, various battery characteristics such as high capacity, low-temperature usage characteristics, high-temperature storage characteristics, cycle characteristics, and safety during overcharging are required. Improvements are requested.
To date, as a means to improve high-temperature storage tests and cycle tests for non-aqueous electrolyte secondary batteries, numerous studies have been conducted on various battery components, including positive and negative electrode active materials and non-aqueous electrolytes. technologies are being considered.
Patent Document 1 describes the formation of a film of a specific positive electrode active material, a lithium salt having an oxalate complex as an anion, and at least one kind selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, ethylene sulfite, and fluoroethylene carbonate. A technology has been disclosed that aims to prevent various characteristics of a non-aqueous electrolyte secondary battery from deteriorating when stored in a high-temperature environment by using a non-aqueous electrolyte to which a non-aqueous electrolyte is added. There is.
Non-Patent Document 1 describes a Li/graphite cell and a Li/petroleum coke cell using Li/graphite or Li/petroleum coke as the negative electrode and propylene carbonate (PC)/ethylene carbonate (EC) or PC as the non-aqueous solvent. Studies on electrochemical behavior have been reported. The electrolyte solution, which is a mixture of ethylene carbonate and propylene carbonate, reacts with the Li _x C negative electrode to form a film on the carbon surface of the negative electrode, suppressing the decomposition reaction between the electrode and electrolyte components, and improving the cycle characteristics of the cell. It is stated that it was done.
Furthermore, Non-Patent Document 2 reports a study on the structure of a film formed by lithium bisoxalate borate on the surface of a graphite negative electrode of a non-aqueous electrolyte secondary battery. According to Non-Patent Document 2, in lithium bisoxalatoborate, the bond between an atom with a formal negative charge and a ligand bonded to it (i.e., oxalate skeleton) is cleaved by electrochemical reduction, and further other It is conceivable that it may recombine with molecules (see Scheme 3 of Non-Patent Document 2).

Japanese Patent Application Publication No. 2006-196250

Fong et al., J. Electrochem. Soc. 137, 2009 (1990). Xu et al. , Electrochemical and Solid-State Lett. 6, A144 (2003).

However, when the present inventors investigated the method described in the above document, they found that there was a problem of an increase in initial output resistance in batteries involving a coating derived from a non-aqueous solvent or a coating-forming agent.
Therefore, an object of the present invention is to reduce the initial output resistance of a non-aqueous electrolyte secondary battery.

In view of the above-mentioned circumstances, the inventors of the present invention have made extensive studies and have found that the above-mentioned object can be achieved by incorporating a dihydroxy oxalate salt into a non-aqueous electrolytic solution, and have completed the present invention.
That is, the gist of the present invention is as follows.
[1] A non-aqueous electrolyte containing a dihydroxyoxalatoborate salt and a non-aqueous solvent.
[2] The non-aqueous electrolytic solution according to [1], wherein the non-aqueous electrolytic solution contains the dihydroxy oxalatoborate salt in an amount of 10 mass ppm or more and 0.1 mass % or less.
[3] The non-aqueous electrolyte according to [1] or [2], wherein the non-aqueous solvent contains a saturated cyclic carbonate.
[4] The nonaqueous electrolytic solution according to any one of [1] to [3], further containing an oxalate salt (excluding dihydroxyoxalate borate salts).
[5] The ratio of the content (mass %) of the dihydroxyoxalatoborate salt to the content (mass %) of the oxalate salt is 5 × 10 ^-4 or more and 2 × 10 ^-2 or less, [4] The non-aqueous electrolyte described in .
[6] A non-aqueous electrolyte battery containing the non-aqueous electrolyte according to any one of [1] to [5].

According to the present invention, it is possible to provide a nonaqueous electrolyte that can provide a battery with low initial output resistance, and a nonaqueous electrolyte battery using the same.

The non-aqueous electrolyte of the present invention contains a dihydroxyoxalatoborate salt and a non-aqueous solvent.
The mechanism of action of the present invention is speculated as follows. However, the present invention is not limited to this speculation.
A film formed particularly on the surface of the negative electrode by a film-forming agent can suppress deterioration of characteristics during battery storage. On the other hand, in a battery equipped with a non-aqueous electrolyte containing a film-forming agent, the film on the surface of the negative electrode continues to grow excessively, which is thought to increase the initial resistance of the battery.
For example, attempts have been made to use saturated cyclic carbonates such as ethylene carbonate to form a film on the surface of the negative electrode and suppress side reactions between the electrode and electrolyte components, thereby improving various characteristics of batteries. There is. However, since saturated cyclic carbonates are used as non-aqueous solvents, their concentration in the non-aqueous electrolyte is large, and excessive growth of the film is unavoidable.
In addition, in oxalate salts such as lithium bisoxalatoborate, the bond between an atom with a formal negative charge and a ligand bonded to it (i.e., oxalate skeleton) is cleaved by electrochemical reduction, and further It is thought that by repeatedly recombining with the molecules, a film is formed, especially on the surface of the negative electrode. In lithium bisoxalatoborate, since the above reaction occurs in the two oxalate skeletons, the film continues to grow excessively as if it were a polymerization reaction, which is thought to increase the resistance of the negative electrode in particular.
By introducing a dihydroxy oxalatoborate salt having only one oxalate skeleton, the hydroxyl groups that do not cause cleavage and recombination function as if they were terminal functional groups, thereby preventing excessive growth of the film. It is presumed that this suppresses the increase in resistance of the negative electrode.

[1. Non-aqueous electrolyte]
{1-1. Dihydroxyoxalatoborate salt}
The non-aqueous electrolyte according to this embodiment contains a dihydroxyoxalatoborate salt. Examples of the countercation of the dihydroxyoxalatoborate salt include alkali metals such as lithium, sodium, and potassium. Examples of dihydroxyoxalatoborate salts include lithium dihydroxyoxalatoborate, sodium dihydroxyoxalatoborate, and potassium dihydroxyoxalatoborate. Among these, lithium dihydroxyoxalatoborate and sodium dihydroxyoxalatoborate are preferred, and lithium dihydroxyoxalatoborate is particularly preferred from the viewpoint of solubility in non-aqueous solvents. The dihydroxyoxalatoborate salt can be synthesized by a known method (see, for example, Yang et al., Journal of power sources. 195, 1698 (2010)).
The content of the dihydroxyoxalatoborate salt in the nonaqueous electrolyte is preferably 10 mass ppm or more, more preferably 20 mass ppm or more, particularly preferably 50 mass ppm or more, but there is no particular restriction on the upper limit. However, it is preferably contained in an amount of 0.1% by mass or less, more preferably 0.05% by mass or less, particularly preferably 0.02% by mass or less. By containing the dihydroxyoxalatoborate salt in the above-mentioned range in the non-aqueous electrolytic solution, the dihydroxyoxalatoborate salt can satisfactorily exhibit its function as a terminal functional group. The dihydroxyoxalatoborate salt only needs to be contained in the non-aqueous electrolyte, and includes not only cases where it is added but also those generated within the non-aqueous electrolyte or within the non-aqueous electrolyte battery during battery operation. Identification and measurement of the content of the dihydroxyoxalatoborate salt are performed by nuclear magnetic resonance (NMR) spectroscopy.

{1-2. Oxalate salts (excluding dihydroxyoxalatoborate salts)}
In the present invention, it is preferable that the non-aqueous electrolyte further contains an oxalate salt other than the dihydroxyoxalatoborate salt (hereinafter, may be simply referred to as "oxalate salt"). This is because the oxalate salt has an oxalate skeleton like the dihydroxyoxalatoborate salt, and the durability of the coating is improved by using it in combination with the dihydroxyoxalatoborate salt. Examples of the counter cation of the oxalate salt include alkali metals such as lithium, sodium, and potassium.
Examples of oxalate salts (excluding dihydroxyoxalatoborate salts) include lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium difluorooxalatophosphate, and lithium tetrafluorooxalatophosphate.
Among these, lithium bisoxalatoborate, lithium difluorooxalatoborate, and lithium difluorooxalatophosphate are preferred because they are highly effective in improving the initial output, high-rate charge/discharge characteristics, and output after high-temperature storage and after cycling.
The ratio of the content (mass%) of dihydroxyoxalatoborate salt to the content (mass%) of oxalate salt (excluding dihydroxyoxalatoborate salt) in the nonaqueous electrolyte (hereinafter also referred to as "ratio of dihydroxyoxalatoborate salt to oxalate salt (excluding dihydroxyoxalatoborate salt)" or "(dihydroxyoxalatoborate salt (mass%))/(oxalate salt (excluding dihydroxyoxalatoborate salt) (mass%))" is not particularly limited as long as the effects of the present invention can be obtained, but is preferably 5×10 ^-4 or more, more preferably 1×10 ^-3 or more, and particularly preferably 2×10 -3 or more. On the other hand, the upper limit is not particularly limited, but is preferably 2×10 ^{-2 or} less, more preferably 1.5×10 ^-2 or less, even more preferably 1×10 ^-2 or less, even more preferably 0.60×10 ^-2 or less, and particularly preferably 0.40×10 ^-2 or less. By setting the ratio of the dihydroxyoxalatoborate salt to the oxalate salt (excluding dihydroxyoxalatoborate salt) within the above range, the film formation of the oxalate salt (excluding dihydroxyoxalatoborate salt) can be adequately maintained while the function of the dihydroxyoxalatoborate salt as a terminal functional group can be satisfactorily exhibited. The oxalate salt may be contained in the non-aqueous electrolyte, and may be added or may be generated in the non-aqueous electrolyte or in the non-aqueous electrolyte battery during battery operation. The oxalate salt is identified and its content is measured by nuclear magnetic resonance (NMR) spectroscopy.

{1-3. Electrolytes}
The non-aqueous electrolytic solution of this embodiment usually contains an electrolyte as a component, like a general non-aqueous electrolytic solution. There are no particular limitations on the electrolyte used in the non-aqueous electrolyte of this embodiment, and any known electrolyte can be used.
The above dihydroxyoxalatoborate salts and oxalate salts (excluding dihydroxyoxalatoborate salts) can function as electrolytes, but typically the above dihydroxyoxalatoborate salts and oxalate salts (excluding dihydroxyoxalatoborate salts) ) Use electrolytes other than .
Lithium salt is usually used as the electrolyte in the non-aqueous electrolyte. As the lithium salt, there is no particular restriction as long as it is known to be used for this purpose, and any salt can be used, and specific examples include the following.
Examples of lithium salts include lithium fluoroborate salts, lithium fluoroborate salts, lithium fluorotantalate salts, lithium fluorophosphate salts, lithium tungstate salts, lithium carboxylate salts, lithium sulfonate salts, lithium imide salts, and lithium methide salts. and fluorine-containing organic lithium salts.
Among them, LiBF ₄ as lithium fluoroborate salts; LiSbF ₆ as lithium fluoroborate salts; LiTaF ₆ as lithium fluorotantalate salts; LiPF _6, Li ₂ PO ₃ F, LiPO ₂ F ₂ as lithium fluorophosphate salts; Lithium acid salts include LiFSO ₃ , CH ₃ SO ₃ Li; lithium imide salts include LiN(FSO ₂ ) ₂ , LiN(FSO ₂ )(CF ₃ SO ₂ ), LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , lithium cyclic 1,2-perfluoroethane disulfonylimide, lithium cyclic 1,3-perfluoropropanedisulfonylimide; as lithium methide salts, LiC(FSO ₂ ) ₃ , LiC(CF ₃ SO ₂ ) _3. LiC (C ₂ F ₅ SO ₂ ) ₃ ; As a fluorine-containing organic lithium salt, LiCF ₃ SO ₃ ; etc. improve low-temperature output characteristics, high-rate charge/discharge characteristics, impedance characteristics, high-temperature storage characteristics, cycle characteristics, etc. It is more preferable from the viewpoint of effectiveness. Particularly preferred are LiPF ₆ , LiN(FSO ₂ ) ₂ or LiFSO ₃ . Moreover, the above-mentioned electrolyte salts may be used alone or in combination of two or more kinds.
There are no particular restrictions on the combination of two or more electrolyte salts, but LiPF ₆ and LiFSO ₃ , LiPF ₆ and LiPO ₂ F ₂ , LiPF ₆ and LiN(FSO ₂ ) ₂ , or LiBF ₄ , LiPF ₆ and LiN( FSO ₂ ) ₂ combinations may be mentioned. Among these, combinations of LiPF ₆ and LiFSO ₃ and LiPF ₆ and LiPO ₂ F ₂ are preferred.

The total concentration of these electrolytes other than the dihydroxyoxalatoborate salt and the oxalate salt (excluding the dihydroxyoxalatoborate salt) in the non-aqueous electrolyte is not particularly limited, but is usually 8% by mass or more, preferably 8.5% by mass or more, more preferably 9% by mass or more, and is usually 18% by mass or less, preferably 17% by mass or less, more preferably 16% by mass or less, based on the total amount of the non-aqueous electrolyte. If the total concentration of the electrolyte is within the above range, the electrical conductivity becomes appropriate for the battery operation, and therefore sufficient output characteristics tend to be obtained.
The electrolytes such as the lithium salts mentioned above are identified and their contents are measured by nuclear magnetic resonance (NMR) spectroscopy.

{1-4. Non-aqueous solvent}
The non-aqueous electrolyte of the present invention contains a non-aqueous solvent. As the non-aqueous solvent, as with a general non-aqueous electrolyte, it is not particularly limited as long as it dissolves the above-mentioned electrolyte or dihydroxyoxalatoborate salt as the main component, and a known organic solvent can be used. Examples of the organic solvent include saturated cyclic carbonates, chain carbonates, chain carboxylate esters, cyclic carboxylate esters, ether compounds, and sulfone compounds. Among them, saturated cyclic carbonates are preferred in that the effect of the dihydroxyoxalatoborate salt can be more effectively obtained. The organic solvent can be used alone or in combination of two or more kinds.
The combination of two or more organic solvents is not particularly limited, and may be a combination of saturated cyclic carbonates and chain carboxylate esters, cyclic carboxylate esters and chain carbonates, and saturated cyclic carbonates, chain carbonates and chain carboxylate esters. Among these, a combination of saturated cyclic carbonates and chain carbonates, and saturated cyclic carbonates, chain carbonates and chain carboxylate esters is preferred.

(1-4-1. Saturated cyclic carbonates)
Examples of saturated cyclic carbonates include those having an alkylene group having 2 to 4 carbon atoms, and from the viewpoint of improving battery characteristics resulting from an improvement in the degree of lithium ion dissociation, saturated cyclic carbonates having 2 to 3 carbon atoms are preferably used.
Specific examples of saturated cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, etc. Among them, ethylene carbonate or propylene carbonate is preferred, and ethylene carbonate, which is less susceptible to oxidation and reduction, is more preferred. The saturated cyclic carbonates may be used alone or in any combination and ratio of two or more.
The content of the saturated cyclic carbonates is not particularly limited and may be any content as long as it does not significantly impair the effects of the present invention, but the lower limit of the content when one type is used alone is usually 3% by volume or more, preferably 5% by volume or more, and is usually 90% by volume or less, preferably 85% by volume or less, more preferably 80% by volume or less, based on the total amount of the nonaqueous solvent in the nonaqueous electrolyte. By setting it in this range, the decrease in electrical conductivity due to the decrease in the dielectric constant of the nonaqueous electrolyte is avoided, and the large current discharge characteristics, stability to the negative electrode, and cycle characteristics of the nonaqueous electrolyte secondary battery are easily in a good range, and the oxidation/reduction resistance of the nonaqueous electrolyte is improved, and the stability during high-temperature storage tends to be improved.
In the present embodiment, the volume percentage refers to the volume at 25° C. and 1 atmospheric pressure.

(1-4-2. Chain carbonates)
As the chain carbonates, for example, those having 3 to 7 carbon atoms are used, and in order to adjust the viscosity of the electrolytic solution to an appropriate range, chain carbonates having 3 to 5 carbon atoms are preferably used.
Specific examples of chain carbonates include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propylisopropyl carbonate, ethylmethyl carbonate, and methyl-n-propyl carbonate. Particularly preferred are dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate.
Furthermore, chain carbonates having a fluorine atom (hereinafter sometimes abbreviated as "fluorinated chain carbonates") can also be suitably used. The number of fluorine atoms that the fluorinated chain carbonate has is not particularly limited as long as it is 1 or more, but it is usually 6 or less, preferably 4 or less. When the fluorinated chain carbonate has a plurality of fluorine atoms, the plurality of fluorine atoms may be bonded to the same carbon or different carbons. Examples of the fluorinated linear carbonate include fluorinated dimethyl carbonate derivatives such as fluoromethylmethyl carbonate, fluorinated ethylmethyl carbonate derivatives such as 2-fluoroethylmethyl carbonate; fluorinated diethyl carbonate such as ethyl-(2-fluoroethyl) carbonate; derivatives; etc.
One type of chain carbonate may be used alone, or two or more types may be used in combination in any combination and ratio.
The content of chain carbonates is not particularly limited, but is usually 15% by volume or more, preferably 20% by volume or more, more preferably 25% by volume or more, based on the total amount of the nonaqueous solvent in the nonaqueous electrolyte. Moreover, it is usually 90 volume% or less, preferably 85 volume% or less, and more preferably 80 volume% or less. By setting the content of chain carbonate within the above range, the viscosity of the non-aqueous electrolyte can be adjusted to an appropriate range, suppressing a decrease in ionic conductivity, and in turn keeping the output characteristics of the non-aqueous electrolyte secondary battery within a good range. It becomes easier to
Furthermore, by combining ethylene carbonate in a specific content with specific chain carbonates, battery performance can be significantly improved.
For example, when dimethyl carbonate and ethyl methyl carbonate are selected as specific chain carbonates, the content of ethylene carbonate is not particularly limited and is arbitrary as long as it does not significantly impair the effects of the present invention. The content of dimethyl carbonate is usually 15% by volume or more, preferably 20% by volume or more, and usually 45% by volume or less, preferably 40% by volume or less, based on the total amount of the nonaqueous electrolyte. The content of ethyl methyl carbonate is usually 20% by volume or more, preferably 30% by volume or more, and usually 50% by volume or less, preferably 45% by volume or less, based on the total amount of the nonaqueous solvent. The amount is usually 20% by volume or more, preferably 30% by volume or more, and usually 50% by volume or less, preferably 45% by volume or less, based on the total amount of the nonaqueous solvent. When the content is within the above range, high temperature stability is excellent and gas generation tends to be suppressed.

(1-4-3. Chain Carboxylic Acid Ester)
Examples of the chain carboxylate include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, methyl valerate, methyl isobutyrate, ethyl isobutyrate, and methyl pivalate. Among them, methyl acetate, ethyl acetate, propyl acetate, and butyl acetate are preferred from the viewpoint of improving battery characteristics. Chain carboxylates in which a part of the hydrogen of the above-mentioned compound is replaced with fluorine (e.g., methyl trifluoroacetate, ethyl trifluoroacetate, etc.) can also be used suitably. The content of the chain carboxylate compound is not particularly limited and is optional as long as it does not significantly impair the effect of the invention according to this embodiment, but is usually 1% by volume or more, preferably 2% by volume or more, more preferably 3% by volume or more, and usually 30% by volume or less, preferably 25% by volume or less, more preferably 20% by volume or less, based on the total amount of the nonaqueous solvent in the nonaqueous electrolyte.

(1-4-4. Cyclic carboxylic acid ester)
Examples of the cyclic carboxylic acid ester include γ-butyrolactone and γ-valerolactone. Among these, γ-butyrolactone is more preferred. Cyclic carboxylic acid esters in which part of the hydrogen atoms of the above-mentioned compounds are replaced with fluorine can also be suitably used. The content of the cyclic carboxylic acid ester compound is not particularly limited, and is arbitrary as long as it does not significantly impair the effects of the invention according to this embodiment, but it is usually 1% by volume based on the total amount of the nonaqueous solvent of the nonaqueous electrolyte. The content is preferably 2% by volume or more, more preferably 3% by volume or more, and usually 30% by volume or less, preferably 25% by volume or less, more preferably 20% by volume or less.

(1-4-5. Ether compound)
Ether compounds include chain ethers having 3 to 10 carbon atoms such as dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether; and tetrahydrofuran. , 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxane, 2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, etc. cyclic having 3 to 6 carbon atoms Ether is preferred.
Among them, dimethoxymethane and diethoxymethane are used as chain ethers having 3 to 10 carbon atoms, which have a high ability to solvate lithium ions, improve ion dissociation, have low viscosity, and provide high ionic conductivity. , ethoxymethoxymethane is preferred, and tetrahydrofuran, 1,3-
Dioxane, 1,4-dioxane and the like are preferred.
The content of the ether compound is not particularly limited, and is arbitrary as long as it does not significantly impair the effects of the invention according to this embodiment, but it is usually 1% by volume or more, based on the total amount of the nonaqueous solvent in the nonaqueous electrolyte. The content is preferably 2% by volume or more, more preferably 3% by volume or more, and usually 30% by volume or less, preferably 25% by volume or less, more preferably 20% by volume or less. If the content of the ether compound is within the above range, it is easy to ensure the effect of improving the ionic conductivity resulting from the improvement in the degree of lithium ion dissociation by the ether compound and the decrease in the viscosity of the non-aqueous electrolyte. Furthermore, when the negative electrode active material is a carbonaceous material, the phenomenon in which chain ether is co-inserted with lithium ions can be suppressed, so that input/output characteristics and charge/discharge rate characteristics can be kept within appropriate ranges.

(1-4-6. Sulfone-based compounds)
The sulfone compound is not particularly limited, and may be a cyclic sulfone or a chain sulfone. In the case of a cyclic sulfone, the carbon number is usually 3 to 6, preferably 3 to 5, and in the case of a chain sulfone, the carbon number is usually 2 to 6, preferably 2 to 5. In addition, the number of sulfonyl groups in one molecule of the sulfone compound is not particularly limited, but is usually 1 or 2.
Examples of cyclic sulfones 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 viewpoint of dielectric constant and viscosity, tetramethylene sulfones, tetramethylene disulfones, hexamethylene sulfones, and hexamethylene disulfones are more preferred, and tetramethylene sulfones (sulfolanes) are particularly preferred.
As the sulfolanes, sulfolane and sulfolane derivatives are preferred. As the sulfolane derivatives, those in which one or more hydrogen atoms bonded to the carbon atoms constituting the sulfolane ring are substituted with a fluorine atom, an alkyl group, or a fluorine-substituted alkyl group are preferred.
Among these, 2-methylsulfolane, 3-methylsulfolane, 2-fluorosulfolane, 3-fluorosulfolane, 2,3-difluorosulfolane, 2-trifluoromethylsulfolane, 3-trifluoromethylsulfolane, and the like are preferred because they have high ionic conductivity and high input/output.
Examples of the chain sulfone include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone, pentafluoroethyl methyl sulfone, etc. Among these, dimethyl sulfone, ethyl methyl sulfone, and monofluoromethyl methyl sulfone are preferred in terms of improving the high-temperature storage stability of the electrolyte.
The content of the sulfone-based compound is not particularly limited and may be any as long as it does not significantly impair the effects of the present invention, but is usually 0.3 vol% or more, preferably 0.5 vol% or more, more preferably 1 vol% or more, and is usually 40 vol% or less, preferably 35 vol% or less, more preferably 30 vol% or less, based on the total amount of the non-aqueous solvent in the non-aqueous electrolyte. If the content of the sulfone-based compound is within the above range, an electrolyte having excellent high-temperature storage stability tends to be obtained.

{1-5. Additives}
The non-aqueous electrolyte may contain various additives within a range that does not significantly impair the effects of the present invention. Any additives known in the art may be used as the additives. The additives may be used alone or in any combination and ratio of two or more.
Examples of conventional additives that can be added to non-aqueous electrolyte solutions include cyclic carbonates having a carbon-carbon unsaturated bond, fluorine-containing cyclic carbonates, compounds having an isocyanato group (isocyanate group), sulfur-containing organic compounds, phosphorus-containing organic compounds, organic compounds having a cyano group, silicon-containing compounds, aromatic compounds, non-fluorine-containing carboxylic acid esters, cyclic compounds having a plurality of ether bonds, compounds having an isocyanuric acid skeleton, borates,
Examples of the oxalate salt include fluorosulfonate salts and the oxalate salts (excluding dihydroxyoxalate borate salts) described in the section {1-2. Oxalate salts (excluding dihydroxyoxalate borate salts)}. Specific examples include compounds described in WO 2015/111676.
Among them, cyclic carbonates having carbon-carbon unsaturated bonds, fluorine-containing cyclic carbonates, sulfur-containing organic compounds, and the above-mentioned oxalate salts (excluding dihydroxyoxalate borate salts) are preferred in that the effect of the dihydroxyoxalate borate salt as a film-forming agent is improved. The content of the additive is not particularly limited and is any as long as it does not significantly impair the effects of the present invention, but is usually 0.001% by mass or more, preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and is usually 10% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and particularly preferably less than 1% by mass, based on the total amount of the nonaqueous electrolyte. The cyclic compounds having multiple ether bonds also include those that can be used as nonaqueous solvents. When using a cyclic compound having multiple ether bonds as an additive, it is preferable to use it in an amount of less than 1% by mass. In addition, it also includes those that can be used as electrolytes, such as borates. When using these compounds as additives, it is preferable to use them in an amount of less than 1% by mass.

[2. Non-aqueous electrolyte secondary battery]
A non-aqueous electrolyte secondary battery, which is another embodiment of the present invention, includes a positive electrode having a positive electrode active material capable of occluding and releasing metal ions, and a negative electrode having a negative electrode active material capable of occluding and releasing metal ions. A non-aqueous electrolyte secondary battery comprising the above-mentioned non-aqueous electrolyte.

{2-1. Non-aqueous electrolyte}
As the non-aqueous electrolyte, the above-mentioned non-aqueous electrolyte is used. Note that it is also possible to mix and use other non-aqueous electrolytes with the above-mentioned non-aqueous electrolytes without departing from the spirit of the present invention.

{2-2. negative electrode}
The negative electrode has a negative electrode active material on at least a portion of the current collector surface.

(2-2-1. Negative Electrode Active Material)
The negative electrode active material used in the negative electrode is not particularly limited as long as it can electrochemically absorb and release metal ions. Specific examples include carbonaceous materials, particles containing a metal that can be alloyed with Li, lithium-containing metal composite oxide materials, and mixtures thereof. Among these, it is preferable to use carbonaceous materials, particles containing a metal that can be alloyed with Li, or a mixture of metal particles that can be alloyed with Li and graphite particles, in terms of good cycle characteristics and safety and excellent continuous charging characteristics. These may be used alone or in any combination of two or more.

(2-2-2. Carbonaceous materials)
Examples of the carbonaceous material include natural graphite, artificial graphite, amorphous carbon, carbon-coated graphite, graphite-coated graphite, and resin-coated graphite. Of these, natural graphite is preferred. The carbonaceous material may be used alone or in any combination and ratio of two or more.
Examples of natural graphite include scaly graphite, scaly graphite, and/or graphite particles obtained by subjecting such graphite to treatment such as spheroidization or densification. Among these, from the viewpoint of particle packing property or charge/discharge rate characteristics, spherical or ellipsoidal graphite particles that have been subjected to spheroidization treatment are particularly preferred.
The average particle size (d50) of the graphite particles is usually 1 μm or more and 100 μm or less.

(2-2-3. Physical properties of carbonaceous material)
The carbonaceous material as the negative electrode active material preferably satisfies at least one of the characteristics such as physical properties and shape shown in (1) to (4) below, and it is preferable that it satisfies multiple items at the same time. Particularly preferred.

(1) X-ray diffraction parameters The d value (interlayer distance) of the lattice plane (002 plane) of a carbonaceous material determined by X-ray diffraction according to the Gakushin method is usually 0.335 nm or more and 0.360 nm or less. The crystallite size (Lc) of the carbonaceous material determined by X-ray diffraction according to the Gakushin method is 1.0 nm or more.

(2) Volume-based average particle size The volume-based average particle size of the carbonaceous material is the volume-based average particle size (median diameter) determined by a laser diffraction/scattering method, and is usually 1 μm or more and 100 μm or less.

(3) Raman R value, Raman half-value width The Raman R value of a carbonaceous material is a value measured using argon ion laser Raman spectroscopy, and is usually 0.01 or more and 1.5 or less.
Furthermore, the Raman half-width near 1580 cm ^-1 of the carbonaceous material is not particularly limited, but is usually 10 cm ^-1 or more and 100 cm ^-1 or less.

(4) BET specific surface area The BET specific surface area of a carbonaceous material is the value of the specific surface area measured using the BET method, and is usually 0.1 m ² ·g ⁻¹ or more and 100 m ² ·g ⁻¹ or less.
The negative electrode active material may contain two or more types of carbonaceous materials having different properties. The properties here refer to one or more properties selected from the group of X-ray diffraction parameters, volume-based average particle diameter, Raman R value, Raman half-width, and BET specific surface area.
Preferable examples include that the volume-based particle size distribution is not symmetrical about the median diameter, that it contains two or more types of carbonaceous materials with different Raman R values, and that the X-ray parameters are different. can be mentioned.

(2-2-4. Particles containing metals capable of being alloyed with Li)
The particles containing a metal capable of being alloyed with Li may be any of those known in the art, but from the viewpoint of capacity and cycle life, it is preferable that the particles are, for example, particles of a metal selected from the group consisting of Sb, Si, Sn, Al, As, and Zn or a compound thereof. In addition, when the particles containing a metal capable of being alloyed with Li contain two or more kinds of metals, the particles may be alloy particles made of an alloy of these metals.
Examples of the compound of a metal capable of forming an alloy with Li include metal oxides, metal nitrides, metal carbides, etc. The compound may contain two or more kinds of metals capable of forming an alloy with Li.
Among these, metallic Si (hereinafter, may be referred to as Si) or an inorganic compound containing Si is preferable from the viewpoint of increasing capacity.
In this specification, Si or Si-containing inorganic compounds are collectively referred to as Si compounds. Specific examples of Si compounds include _SiOx , _SiNx , _SiCx , and _SiZxOy (Z=C, N). As the Si compound, Si _oxide ( _SiOx ) is preferred because it has a larger theoretical capacity than graphite, and amorphous Si or nano-sized Si crystals are preferred because they allow easy ingress and egress of alkali ions such as lithium ions and allow high capacity to be obtained.
This general formula, SiO _x , can be obtained by using silicon dioxide (SiO ₂ ) and Si as raw materials, and the value of x is usually 0<x<2.
Specific examples of metal compounds alloyed with Li include Li _y Si (0<y≦4.4) and Li _2z SiO _2+z (0<z≦2).
The average particle size (d50) of the particles containing a metal capable of being alloyed with Li is usually 0.01 μm or more and 10 μm or less from the viewpoint of cycle life.

(2-2-5. Mixture of particles containing metal that can be alloyed with Li and graphite particles)
The mixture of particles containing a metal that can be alloyed with Li and graphite particles used as a negative electrode active material is a state in which the particles containing a metal that can be alloyed with Li and the graphite particles are independent of each other. It may be a mixture in which Li is mixed with Li, or it may be a composite in which particles containing a metal that can be alloyed with Li are present on the surface or inside of graphite particles.
The content ratio of particles containing a metal that can be alloyed with Li to the total of particles containing a metal that can be alloyed with Li and graphite particles is usually 1% by mass or more and 99% by mass or less.

(2-2-6. Lithium-containing metal composite oxide material)
The lithium-containing metal composite oxide material used as the negative electrode active material is not particularly limited as long as it can intercalate and release lithium, but from the viewpoint of high current density charge/discharge characteristics, lithium-containing composite metal oxide materials containing titanium may be used. is preferred, a composite oxide of lithium and titanium (hereinafter sometimes abbreviated as "lithium titanium composite oxide") is more preferred, and a lithium titanium composite oxide having a spinel structure is particularly preferred since it greatly reduces the output resistance. .
Furthermore, lithium and/or titanium in the lithium-titanium composite oxide may be substituted with another metal element, for example, at least one element selected from the group consisting of Al, Ga, Cu, and Zn.
As the lithium titanium composite oxide, Li _4/3 Ti _5/3 O ₄ , Li ₁ Ti ₂ O ₄ and Li _4/5 Ti _11/5 O ₄ are preferred. Moreover, as a lithium titanium composite oxide in which a part of lithium and/or titanium is replaced with another element, for example, _Li4/3 Ti _4/3 Al _1/3 O ₄ is preferable.

(2-2-7. Negative electrode configuration and manufacturing method)
Any known method can be used to manufacture the negative electrode as long as it does not significantly impair the effects of the present invention. For example, a binder, a solvent, and if necessary, a thickener, a conductive material, a filler, etc. are added to the negative electrode active material to form a slurry, and this is applied to a current collector, dried, and then pressed to form a negative electrode active material layer. It can be manufactured by forming a.

<2-2-7-1. Current collector>
Any known current collector can be used as the current collector for holding the negative electrode active material. Examples of the current collector of the negative electrode include metal materials such as aluminum, copper, nickel, stainless steel, and nickel-plated steel, and copper is particularly preferred from the viewpoint of ease of processing and cost. When the shape of the current collector of the negative electrode is plate-like, film-like, etc., the thickness of the current collector is arbitrary, but is usually 1 μm or more and 1 mm or less.

<2-2-7-2. Binder>
The binder for binding the negative electrode active material is not particularly limited as long as it is a material that is stable to the non-aqueous electrolyte and the solvent used during electrode production.
Specific examples include rubbery polymers such as SBR (styrene-butadiene rubber), isoprene rubber, butadiene rubber, fluororubber, NBR (acrylonitrile-butadiene rubber), and ethylene/propylene rubber; polyvinylidene fluoride, polytetrafluoroethylene, Examples include fluorinated polymers such as fluorinated polyvinylidene fluoride and tetrafluoroethylene/ethylene copolymer. These may be used alone or in combination of two or more in any combination and ratio.
The ratio of the binder to the negative electrode active material is usually 0.1% by mass or more and 20% by mass or less.
In particular, when the binder contains a rubbery polymer represented by SBR as a main component, the ratio of the binder to the negative electrode active material is usually 0.1% by mass or more and 5% by mass or less.
Further, when the binder contains a fluorine-based polymer represented by polyvinylidene fluoride as a main component, the proportion of the binder to the negative electrode active material is usually 1% by mass or more and 15% by mass or less.

[2-1-7-3. solvent]
The solvent for forming the slurry may be of any type as long as it is capable of dissolving or dispersing the negative electrode active material, binder, thickener, conductive material, filler, etc. used as necessary. There is no particular restriction on the solvent, and either an aqueous solvent or an organic solvent may be used.

<2-2-7-4. Thickener>
Thickeners are commonly used to adjust the viscosity of slurries. The thickener is not particularly limited, but specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, and polyvinyl alcohol. These may be used alone or in combination of two or more in any combination and ratio.
Furthermore, when using a thickener, the ratio of the thickener to the negative electrode active material is usually 0.1% by mass or more and 5% by mass or less.

<2-2-8. Electrode density>
The electrode structure when the negative electrode active material is made into an electrode is not particularly limited, but the density of the negative electrode active material layer present on the current collector is usually 1 g cm ⁻³ or more and 2.2 g cm ⁻³ or less. It is.

<2-2-9. Thickness of negative electrode plate＞
The thickness of the negative electrode (also referred to as "negative electrode plate") is designed according to the positive electrode (also referred to as "positive electrode plate") used, and is not particularly limited, but depending on the thickness of the negative electrode plate, the thickness of the current collector The thickness of the negative electrode active material layer after subtracting the thickness is usually 15 μm or more and 300 μm or less.

<2-2-10. Surface coating of negative electrode plate>
Further, the above-mentioned negative electrode plate may have a material adhered to its surface having a composition different from that of the negative electrode active material (surface-adhered material). Examples of the surface-adhering substances include oxides such as aluminum oxide, sulfates such as lithium sulfate, and carbonates such as lithium carbonate.

{2-3. positive electrode}
The positive electrode has a positive electrode active material on at least a portion of the current collector surface. The lithium transition metal compound that is the positive electrode active material used in the positive electrode will be described below.

(2-3-1. Lithium transition metal compound)
A lithium-transition metal compound is a compound that has a structure that can remove and insert lithium ions, such as sulfide, phosphate compounds, silicate compounds, borate compounds, lithium-transition metal composite oxides, etc. Examples include things. Among these, lithium transition metal composite oxide is preferred.
Examples of lithium transition metal composite oxides include those belonging to a spinel structure that allows three-dimensional diffusion and a layered structure that allows two-dimensional diffusion of lithium ions. Those having a spinel structure are generally expressed as Li _x1 M ₂ O ₄ (M is at least one transition metal, x1 is usually 1 or more and 1.5 or less), and specifically, LiMn ₂ O ₄ , Examples include LiCoMnO ₄ , LiNi _0.5 Mn _1.5 O ₄ and LiCoVO ₄ . Those having a layered structure are generally expressed as Li _x2 MO ₂ (M is at least one transition metal, x2 is usually 1 or more and 1.5 or less). Specifically, LiCoO ₂ , LiNiO ₂ , LiNi _0.85 Co _0.10 Al _0.05 O ₂ , LiNi _0.80 Co _0.15 Al _0.05 O ₂ , LiNi _0.33 Co _0.33 Mn _{0 .33} O ₂ , Li _1.05 Ni _0.33 Mn _0.33 Co _0.33 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , Li _1.05 Ni _0.50 Mn _{0. 29} Co _0.21 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.8 C
o _0.1 Mn _0.1 O ₂ and the like. Among these, it is preferable to include a lithium transition metal compound represented by the following compositional formula (I).
Li _1+x M ³ O ₂ ...(I)
In compositional formula (I), M ³ is a plurality of elements containing at least Ni, and may also contain Co and/or Mn. The Ni/M ³ molar ratio (hereinafter also referred to as "Ni/M ³ molar ratio") is usually 0.2 or more and 1.0 or less. The lower limit of the Ni/M ³ molar ratio is preferably 0.30 or more, more preferably 0.45 or more, even more preferably 0.55 or more, particularly 0.65 or more. It is preferably 0.75 or more, and most preferably 0.75 or more. Further, the upper limit of the Ni/M ³ molar ratio may be 0.95 or less, preferably 0.85 or less, and more preferably 0.80 or less. If the Ni/M ³ molar ratio is within the above range, the ratio of Ni involved in charging and discharging will be sufficiently large, and the battery will have a high capacity, which is preferable.
When M ³ contains Co, the Co/M ³ molar ratio (hereinafter also referred to as "Co/M ³ molar ratio") is not particularly limited, but is preferably 0.05 or more, more preferably 0.08 or more. , more preferably 0.10 or more. Further, it is preferably 0.35 or less, more preferably 0.30 or less, even more preferably 0.25 or less, particularly preferably 0.20 or less, and most preferably 0.15 or less. It is preferable that the Co/M ³ molar ratio is within the above range because the charge/discharge capacity becomes large.
When M ³ contains Mn, the Mn/M ³ molar ratio is not particularly limited, but is larger than 0, preferably 0.05 or more, more preferably 0.08 or more, and even more preferably 0.10 or more. Further, it may be 0.35 or less, preferably 0.30 or less, more preferably 0.25 or less, even more preferably 0.20 or less, and particularly preferably 0.15 or less. If the Mn/M ³ molar ratio is within the above range, the ratio of Mn that does not participate in charging and discharging will be sufficiently small, and the battery will have a high capacity, which is preferable.

(2-3-2. Introduction of different elements)
In the lithium transition metal compound, an element (different element) other than Ni, Co, and Mn may be introduced in the above-mentioned compositional formula (I). Furthermore, a different element may be introduced into the specifically exemplified compounds.

(2-3-3. Surface coating)
The positive electrode active material may have a surface to which a substance having a different composition from the positive electrode active material is attached (a surface-attached substance). Examples of the surface-attached substance include oxides such as aluminum oxide, sulfates such as lithium sulfate, and carbonates such as lithium carbonate.
These surface-adhering substances can be attached to the surface of the positive electrode active material, for example, by dissolving or suspending them in a solvent, adding them to the positive electrode active material by immersion, and drying.
The amount of the surface-attaching substance is preferably 1 μmol/g or more and usually 1 mmol/g or less relative to the positive electrode active material.
In this specification, a positive electrode active material having the above-mentioned surface-adhering substance attached to the surface thereof is also referred to as a "positive electrode active material".

(2-3-4. Blend)
One type of positive electrode active material may be used alone, or two or more types may be used in combination in any combination and ratio.

(2-3-5. Positive electrode configuration and manufacturing method)
The configuration and manufacturing method of the positive electrode are described below. In this embodiment, the positive electrode using the positive electrode active material can be manufactured by a conventional method. That is, the positive electrode active material, the binder, the conductive material, and if necessary, the thickener, etc. are mixed in a dry state to form a sheet, which is then pressed onto the positive electrode current collector, or these materials are dissolved or dispersed in a liquid medium to form a slurry, which is then applied to the positive electrode current collector and dried to form a positive electrode active material layer on the current collector, thereby obtaining a positive electrode by a coating method. In addition, for example, the above-mentioned positive electrode active material may be roll-formed into a sheet electrode, or may be compression-formed into a pellet electrode.
Hereinafter, the case where the slurry is successively applied to the positive electrode current collector and then dried will be described.

<2-3-5-1. Active material content>
The content of the positive electrode active material in the positive electrode active material layer is usually 80% by mass or more and 98% by mass or less.

<2-3-5-2. Packing density of positive electrode active material>
The positive electrode active material layer obtained by coating and drying is preferably compacted by hand pressing, roller pressing, etc. in order to increase the packing density of the positive electrode active material. The density of the positive electrode active material layer is usually 1.5 g/cm ³ or more, preferably 2.5 g/cm ³ or more, more preferably 3.0 g/cm ³ or more, and usually 3. It is 8g/ ^cm3 or less.

<2-3-5-3. Conductive material>
As the conductive material, any known conductive material can be used. 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. In addition, these may be used individually by 1 type, and may use 2 or more types together in arbitrary combinations and ratios. The conductive material is used so as to be contained in the positive electrode active material layer, usually in an amount of 0.01% by mass or more and 50% by mass or less.

<2-3-5-4. Binder>
The binder used for producing the positive electrode active material layer is not particularly limited, and in the case of a coating method, the type is not particularly limited as long as it is a material that can be dissolved or dispersed in the liquid medium used during electrode production. Fluororesins such as polyvinyl fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; and CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide are preferred from the viewpoint of weather resistance, chemical resistance, heat resistance, flame retardance, and the like.
Further, mixtures, modified products, derivatives, random copolymers, alternating copolymers, graft copolymers, block copolymers, etc. of the above-mentioned polymers can also be used. In addition, one type of binder may be used alone, or two or more types may be used in combination in any combination and ratio.
Further, when a resin is used as a binder, the weight average molecular weight of the resin is arbitrary as long as it does not significantly impair the effects of the present invention, but is usually from 10,000 to 3,000,000. When the molecular weight is within this range, the strength of the electrode is improved and the electrode can be formed suitably.
The proportion of the binder in the positive electrode active material layer is usually 0.1% by mass or more and 80% by mass or less.

<2-3-5-5. Solvent>
There are no particular restrictions on the type of solvent used to form the slurry, as long as it can dissolve or disperse the positive electrode active material, conductive material, binder, and thickener used as necessary. Either an aqueous solvent or an organic solvent may be used.

<2-3-5-6. Current collector>
The material of the positive electrode 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, and tantalum. Among them, aluminum is preferred.
Examples of the shape of the current collector include metal foil, metal cylinder, metal coil, metal plate, metal thin film, expanded metal, punched metal, and foamed metal. Among these, metal foil or metal thin film is preferred. Note that the metal foil or thin film may be formed into a mesh shape as appropriate. When the shape of the current collector of the positive electrode is plate-like, film-like, etc., the thickness of the current collector is arbitrary, but is usually 1 μm or more and 1 mm or less.

<2-3-5-7. Thickness of positive electrode plate＞
The thickness of the positive electrode plate is not particularly limited, but from the viewpoint of high capacity and high output, the thickness of the positive electrode active material layer, which is the thickness of the positive electrode plate minus the thickness of the current collector, is the thickness of the positive electrode active material layer on one side of the current collector. On the other hand, it is usually 10 μm or more and 500 μm or less.

<2-3-5-8. Surface coating of positive electrode plate>
Further, the above-mentioned positive electrode plate may have a substance attached to its surface having a composition different from that of the positive electrode plate, and the substance may be a surface-attached substance that may be attached to the surface of the positive electrode active material. The same substance is used.

{2-4. separator}
A separator is usually interposed between the positive electrode and the negative electrode to prevent short circuits. In this case, the separator is usually impregnated with the non-aqueous electrolyte.
The material and shape of the separator are not particularly limited, and any known materials can be used as long as they do not significantly impair the effects of the present invention.

(2-4-1. Materials)
The material for the separator is not particularly limited as long as it is stable against non-aqueous electrolytes, but preferable examples include oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfuric acid. Examples include sulfates such as barium and calcium sulfate, inorganic substances such as glass filters made of glass fibers, and resins such as polyolefins, more preferably polyolefins, and particularly preferably polyethylene or polypropylene. One type of these materials may be used alone, or two or more types may be used in combination in any combination and ratio. Further, the above materials may be used in a stacked manner.

(2-4-2. Form)
Although the form is not particularly limited, it is preferably in the form of a thin film such as a nonwoven fabric, a woven fabric, or a microporous film. In the case of a thin film, one having a pore diameter of 0.01 μm to 1 μm and a thickness of 1 μm to 50 μm is preferably used. In addition to the independent thin film shape, a separator may be used in which a composite porous layer containing inorganic particles is formed on the surface layer of the positive electrode and/or negative electrode using a resin binder. The form of the separator is preferably a microporous film or a nonwoven fabric because it has excellent liquid retention properties.

(2-4-3. Porosity)
When using a porous material such as a porous sheet or a nonwoven fabric as a separator, the porosity of the separator is arbitrary, but is usually 20% or more and 90% or less.

(2-4-4. Air permeability)
The air permeability of a separator in a non-aqueous electrolyte secondary battery can be determined by the Gurley value. The Gurley value indicates the difficulty of air passing through the film in the thickness direction, and is expressed as the number of seconds required for 100 mL of air to pass through the film. Although the Gurley value of the separator is arbitrary, it is usually 10 to 1000 seconds/100 mL.

{2-5. Battery design}
(2-5-1. Electrode group)
The electrode group may have a laminated structure consisting of the above positive electrode plate and negative electrode plate with the above separator in between, or a structure in which the above positive electrode plate and negative electrode plate are spirally wound with the above separator interposed therebetween. Either is fine. The ratio of the volume of the electrode group to the internal volume of the battery (hereinafter referred to as electrode group occupancy) is usually 40% or more and 90% or less.

(2-5-2. Current collection structure)
In the case where the electrode group has the above-mentioned laminated structure, a structure formed by bundling the metal core parts of each electrode layer and welding them to a terminal is preferably used. It is also preferably used to reduce resistance by providing multiple terminals in the electrode. In the case where the electrode group has a wound structure, the internal resistance can be reduced by providing multiple lead structures on each of the positive electrode and the negative electrode and bundling them to a terminal.

(2-5-3. Protection element)
Protective elements include PTC (Positive Temperature Coefficient) elements whose resistance increases as heat is generated due to excessive current, thermal fuses, thermistors, and valves that cut off current flowing through the circuit due to a sudden rise in battery internal pressure or temperature when abnormal heat is generated. (current cutoff valve) etc. can be used. It is preferable that the above-mentioned protective element is selected so that it does not operate under normal use of high current, and it is more preferable that the design is such that abnormal heat generation or thermal runaway does not occur even without the protective element.

(2-5-4. Exterior body)
A non-aqueous electrolyte secondary battery is usually constructed by housing the above-mentioned non-aqueous electrolyte, a negative electrode, a positive electrode, a separator, etc. in an exterior body (exterior case). There is no limit to this exterior body, and any known exterior body can be used as long as it does not significantly impair the effects of the present invention.
The material of the outer case is not particularly limited as long as it is stable with respect to the nonaqueous electrolyte used, but from the viewpoint of weight reduction, metals such as aluminum, aluminum alloy, or laminate films are preferably used. .
For exterior cases using the above metals, the metals are welded together using laser welding, resistance welding, or ultrasonic welding to create a hermetically sealed structure, or the metals are caulked using a resin gasket. There are things that do.

(2-5-5. Shape)
Further, the shape of the outer case is arbitrary, and may be, for example, cylindrical, square, laminated, coin-shaped, large-sized, etc.

Next, specific embodiments of the present invention will be explained in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Preparation of non-aqueous electrolyte secondary battery]
<Preparation of non-aqueous electrolyte>
In a dry argon atmosphere, sufficiently dried LiPF ₆ was added to a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (volume ratio 3:4:3) at a concentration of 1.0 mol/L (12 A non-aqueous electrolytic solution was prepared by dissolving 2.0% by mass of lithium bisoxalatoborate (based on the entire non-aqueous electrolytic solution) and 2.0% by mass of lithium dihydroxyoxalate at the concentration shown in Table 1. . Lithium dihydroxyoxalate was described by Yang et al. , Journal of power sources. 195, 1698 (2010) p. It was synthesized and used based on the method described in lines 5-6 of the right column of 1702. Table 1 also shows the ratio of the content (mass %) of lithium dihydroxyoxalatoborate to the content (mass %) of lithium bisoxalatoborate (LiBOB).

<Preparation of positive electrode>
85 parts by mass of Li _1.05 Ni _0.34 Mn _0.33 Co _0.33 O ₂ as a positive electrode active material, 10 parts by mass of acetylene black as a conductive material, and 5 parts by mass of polyvinylidene fluoride (PVdF) as a binder. , N-methyl-2-pyrrolidone to form a slurry, this was uniformly coated on a 15 μm thick aluminum foil, dried, and then roll pressed to form a positive electrode. Note that the density of the positive electrode active material layer was 2.6 g/cm ³ .

<Preparation of negative electrode>
49 parts by mass of graphite powder, 50 parts by mass of an aqueous dispersion of sodium carboxymethylcellulose (concentration of sodium carboxymethylcellulose 1% by mass) as a thickener, and an aqueous dispersion of styrene-butadiene rubber (concentration of styrene-butadiene rubber) as a binder. 1 part by mass (50% by mass) was added and mixed with a disperser to form a slurry. The obtained slurry was uniformly applied to a copper foil having a thickness of 10 μm, dried, and roll pressed to obtain a negative electrode.

The above positive electrode, negative electrode, and polyolefin separator were laminated in the order of negative electrode, separator, positive electrode. The battery element thus obtained was wrapped in an aluminum laminate film, and the above-mentioned nonaqueous electrolyte was injected and then vacuum sealed to produce a sheet-shaped nonaqueous electrolyte secondary battery.

[Evaluation of non-aqueous electrolyte secondary battery]
The produced non-aqueous electrolyte secondary battery was evaluated as follows. Note that it can be said that the smaller the initial output resistance is, the more preferable it is, and the larger the capacity retention rate is, the more preferable it is.

・Initial output resistance A sheet-shaped non-aqueous electrolyte secondary battery is heated at 0.05C in a constant temperature bath at 25°C (the current value that discharges the rated capacity in 1 hour based on the discharge capacity at a 1-hour rate is 1C. The same applies hereinafter) ), then constant current-constant voltage charging was performed at 0.2C to a voltage of 4.2V, and then constant current discharge was performed at 0.2C to 2.5V.
Further, after constant current-constant voltage charging at 0.2C to 4.1V, initial charging and discharging was performed by storing at 60°C for 24 hours to stabilize the non-aqueous electrolyte secondary battery.
Thereafter, constant current discharge was performed at 25° C. to 2.5 V, and then constant current-constant voltage charging was performed at 0.2 C to a voltage of 3.7 V. This was discharged at 0.25C, 0.5C, 0.75C, and 1C at 25°C, and the voltage was measured at 10 seconds from the start of each discharge process, and the initial output was determined from the slope of the obtained current-voltage line. The resistance (Ω) was determined. Table 2 shows the initial output resistance (relative value with the initial output resistance value of Comparative Example 1 as 100) of each non-aqueous electrolyte secondary battery.

- Capacity Retention Rate The non-aqueous electrolyte secondary battery that had undergone the initial charging and discharging described above was placed in a constant temperature bath at 45°C, and constant current discharge was performed at 0.2C to 2.5V. Next, after performing constant current-constant voltage charging at 0.2C to a voltage of 4.2V, the discharge capacity when performing constant current discharge to 2.5V at 0.2C was defined as the capacity (A). . Thereafter, constant current-constant voltage charging at 1C to a voltage of 4.2V and constant current discharging to a voltage of 2.5V were repeated 99 times. Thereafter, constant current-constant voltage charging was performed at 0.2C to a voltage of 4.2V, and then constant current discharge was performed to 2.5V at 0.2C, and the discharge capacity was defined as the capacity (B). The ratio of capacity (B) to capacity (A) (B/A×100) was defined as the “capacity retention rate”. Table 3 shows the capacity retention rate.

As is clear from Table 2, by containing lithium dihydroxyoxalate borate, the initial output resistance is reduced and the performance as a battery is improved.
Moreover, as is clear from Table 3, by containing lithium dihydroxyoxalatoborate, the capacity retention rate can also be maintained favorably.

Claims (6)

A non-aqueous electrolytic solution containing a dihydroxyoxalatoborate salt and a non-aqueous solvent. The non-aqueous electrolytic solution according to claim 1, wherein the non-aqueous electrolytic solution contains the dihydroxyoxalatoborate salt in an amount of 10 mass ppm or more and 0.1 mass % or less. The non-aqueous electrolytic solution according to claim 1 or 2, wherein the non-aqueous solvent contains a saturated cyclic carbonate. The non-aqueous electrolytic solution according to any one of claims 1 to 3, further containing an oxalate salt (excluding a dihydroxyoxalate borate salt). 5. The method according to claim 4, wherein the ratio of the content (mass%) of the dihydroxyoxalate borate salt to the content (mass%) of the oxalate salt is 5×10 ⁻⁴ or more and 2×10 ⁻² or less. Non-aqueous electrolyte. A non-aqueous electrolyte battery containing the non-aqueous electrolyte according to any one of claims 1 to 5.