JP7770801B2 - Non-aqueous electrolyte solution and non-aqueous electrolyte secondary battery comprising the non-aqueous electrolyte solution - Google Patents

Description

The present invention relates to a nonaqueous electrolyte solution and a nonaqueous electrolyte secondary battery equipped with the nonaqueous electrolyte solution.

Lithium nonaqueous electrolyte secondary batteries, which use lithium-containing transition metal oxides for the positive electrode and nonaqueous solvents for the electrolyte, are capable of achieving high energy density and are therefore used in a wide range of applications, from small power sources for mobile phones and laptop computers to large power sources for automobiles, railways, and load leveling. However, in recent years, there has been an increasing demand for higher performance in nonaqueous electrolyte secondary batteries, and there is a strong demand for improvements in various characteristics.

For example, Patent Document 1 discloses an electrochemical element characterized by having a negative electrode containing a specific negative electrode active material, a non-aqueous electrolyte solution containing an organic lithium sulfonate salt having a carbon-carbon double bond, and a specific separator, and discloses that the element has excellent safety during overcharging, excellent high-temperature storage properties (i.e., the ability to suppress battery swelling), and excellent low-temperature charging characteristics.

Patent Document 2 discloses an electrolyte solution containing a monomer with a sulfonic acid ion group, and that secondary batteries using this electrolyte solution have an improved discharge capacity retention rate during charge-discharge cycles.

Patent Document 3 discloses a nonaqueous secondary battery equipped with a positive electrode having a positive electrode composite layer containing a specific positive electrode active material and a nonaqueous electrolyte containing an organic lithium sulfonate salt having a carbon-carbon double bond, and discloses that the battery has improved discharge capacity, charge-discharge cycle characteristics, and safety.

JP 2010-277723 A JP 2007-42387 A JP 2009-110943 A

In recent years, non-aqueous electrolyte secondary batteries have been required to meet increasingly higher performance requirements, particularly for battery-equipped automobiles, and there has been a strong demand for improved battery capacity after high-temperature storage. In particular, the addition of phosphates or sulfonates can improve battery performance such as capacity retention, and therefore, for example, lithium difluorophosphate is used as an additive in non-aqueous electrolyte secondary batteries.
However, in the nonaqueous electrolyte secondary battery described in Patent Document 1, it is described that battery swelling can be suppressed by combining a specific separator with a nonaqueous electrolyte containing an organic lithium sulfonate salt having a carbon-carbon unsaturated bond, but there is no study or description of battery capacity after high-temperature storage. Furthermore, in the nonaqueous electrolyte described in Patent Document 2, it is described that the discharge capacity retention rate during cycle testing can be improved, but there is no study or description of battery capacity after high-temperature storage. Furthermore, in the nonaqueous secondary battery described in Patent Document 3, it is described that cycle characteristics can be improved by combining a specific positive electrode mixture layer with an electrolyte having a carbon-carbon double bond, but there is no description of battery performance evaluation when a phosphate or sulfonate is co-added.

The objective of the present invention is to provide a nonaqueous electrolyte that can solve the above-mentioned problems in nonaqueous electrolyte secondary batteries and improve battery capacity after high-temperature storage.

As a result of extensive research into resolving the above-mentioned problems, the inventors discovered that the above-mentioned problems can be solved by adding a sulfonate salt having a specific carbon-carbon unsaturated bond and a specific phosphate salt or sulfonate salt to a non-aqueous electrolyte solution, thereby completing the present invention.

That is, the gist of the present invention lies in the following.
[1] A non-aqueous electrolyte solution comprising a compound (A) represented by formula (I) or formula (II), and one or more salts (B) selected from the group consisting of phosphates having a P—F bond and sulfonates having an S—F bond.


In formula (I), R ¹ , R ² and R ³ each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group, and R ¹ and R ² may be bonded to each other to form a cyclic structure; n represents an integer of 0 to 4; and M ¹ represents a monovalent cation.
In formula (II), ^R4 is a hydrogen atom, a halogen atom, or a hydrocarbon group; n is an integer of 0 to 4; and ^M2 is a monovalent cation.
[2] In the formula (I), R ¹ , R ² and R ³ each independently represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aralkyl group having 7 to 18 carbon atoms. [3] The nonaqueous electrolyte solution according to [1].
[3] The nonaqueous electrolyte solution according to claim [1] or [2], wherein in formula (II), R ⁴ is a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aralkyl group having 7 to 18 carbon atoms.
[4] The nonaqueous electrolyte solution according to any one of [1] to [3], containing 0.01% by mass or more and 10% by mass or less of the compound (A) represented by formula (I) or formula (II).
[5] The nonaqueous electrolyte solution according to any one of [1] to [4], containing 0.01% by mass or more and 10% by mass or less of the salt (B) selected from the group consisting of phosphates having a P—F bond and sulfonates having an S—F bond.
[6] A nonaqueous electrolyte secondary battery comprising: a positive electrode having a positive electrode active material capable of absorbing and releasing metal ions; a negative electrode having a negative electrode active material capable of absorbing and releasing metal ions; and the nonaqueous electrolyte solution according to any one of [1] to [5].

The present invention provides a nonaqueous electrolyte solution that can improve battery capacity after high-temperature storage. It also provides a nonaqueous electrolyte secondary battery equipped with this nonaqueous electrolyte solution.

The following describes in detail the embodiments of the present invention, but the following description is merely an example (typical example) of the present invention, and the present invention is not limited to these details as long as it does not depart from the gist of the claims.

[1. Non-aqueous electrolyte]
The nonaqueous electrolyte solution used in the nonaqueous electrolyte secondary battery according to the embodiment of the present invention, like a general nonaqueous electrolyte solution, contains an electrolyte and a nonaqueous solvent that dissolves the electrolyte, and contains at least one compound (A) represented by formula (I) or formula (II) (hereinafter, this may be referred to as a sulfonate salt having a carbon-carbon unsaturated bond), and one or more salts (B) (hereinafter, this may be referred to as a "phosphate or sulfonate salt having a fluorine atom") selected from the group consisting of phosphates having a P═O bond and a P—F bond and sulfonates having an S—F bond.


(In formula (I), R ¹ , R ² and R ³ each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group, and R ¹ and R ² may be bonded to each other to form a cyclic structure; n represents an integer of 0 to 4; and M ¹ represents a monovalent cation.
In formula (II), ^R4 is a hydrogen atom, a halogen atom, or a hydrocarbon group; n is an integer of 0 to 4; and ^M2 is a monovalent cation.
The non-aqueous electrolyte solution according to the present invention is preferable in that it can improve the battery capacity after high-temperature storage. The inventors speculate that the reason for such excellent effects is as follows.
It is believed that as the secondary battery is charged and discharged, the carbon-carbon unsaturated bonds of compound (A) represented by formula (I) or formula (II) polymerize on the electrode, and further react with the phosphate having a P-F bond or the sulfonate having an S-F bond (B), forming a hardly soluble and strong co-coating film on the electrode, thereby suppressing side reactions between the electrode active material and the solvent, etc.
That is, it is believed that the present invention makes it possible to efficiently improve the battery capacity after high-temperature storage to a higher level than before by including a sulfonate having a carbon-carbon unsaturated bond and a specific phosphate or specific sulfonate in a non-aqueous electrolyte solution.
Each component will be described below.

[1-1. Compound (A) represented by formula (I) or formula (II) (sulfonate salt having a carbon-carbon unsaturated bond)]
[1-1-1. Compounds represented by formula (I)]

In formula (I), R ¹ , R ² and R ³ each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group, with a hydrogen atom or a hydrocarbon group being preferred.
Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Among these, a fluorine atom is preferred in terms of stability in the electrolyte solution.
Examples of the hydrocarbon group include an alkyl group, an alkenyl group, an alkynyl group, an aryl group, and an aralkyl group. Among these, an alkyl group, an alkenyl group, an aryl group, and an aralkyl group are particularly preferred. The hydrocarbon group may have a substituent such as a fluorine atom. The aryl group also includes a heteroaryl group in which any ring atom is substituted with a heteroatom.
Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a tert-butyl group, an n-pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, and a decyl group. Of these, an alkyl group having 1 to 6 carbon atoms is preferred in terms of solubility in the electrolyte solution.
Examples of the alkenyl group include a vinyl group, an allyl group, a methallyl group, a 2-butenyl group, a 3-methyl-2-butenyl group, a 3-butenyl group, a 4-pentenyl group, a 5-hexenyl group, a 6-heptenyl group, and a 7-octenyl group. Of these, an alkenyl group having 2 to 6 carbon atoms is preferred in terms of solubility in the electrolyte solution.
Examples of the alkynyl group include an ethynyl group, a 2-propynyl group, a 2-butynyl group, a 3-butynyl group, a 4-pentynyl group, a 5-hexynyl group, a 6-heptynyl group, and a 7-octynyl group. Of these, an alkynyl group having 2 to 6 carbon atoms is preferred in terms of solubility in an electrolyte solution.
Examples of the aryl group include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, a 2-thienyl group, a 3-thienyl group, a 2-furyl group, a 3-furyl group, a 2-pyrrolyl group, a 3-pyrrolyl group, and a benzyl group. Of these, an aryl group having 6 to 12 carbon atoms is preferred, and a phenyl group, a 1-naphthyl group, and a 2-naphthyl group are particularly preferred in terms of solubility in an electrolyte solution.
Examples of the aralkyl group include a phenylmethyl group (benzyl group), a phenylethyl group (phenethyl group), a phenylpropyl group, a phenylbutyl group, and a phenylisopropyl group. Of these, a benzyl group and a phenethyl group are preferred, and a benzyl group is particularly preferred in terms of stability in an electrolyte solution.
^R1 and ^R2 in formula (I) may be bonded to each other to form a cyclic structure. Examples of the cyclic structure include a cyclohexene ring, a cyclopentene ring, a benzene ring, a naphthalene ring, a pyrrole ring, a thiophene ring, a furan ring, an imidazole ring, a thiazole ring, a carbazole ring, and a pyridine ring. Among these, a cyclohexene ring, a benzene ring, a naphthalene ring, a pyrrole ring, a thiophene ring, and a furan ring are preferred in terms of stability to the electrolyte, and a cyclohexene ring, a benzene ring, and a naphthalene ring are more preferred. In addition, the atoms forming the cyclic structure may have a substituent such as a fluorine atom.
^M1 in formula (I) is a monovalent cation, preferably a lithium ion, a sodium ion, a potassium ion, a rubidium ion, or a cesium ion. Among these, a lithium ion, a sodium ion, or a potassium ion is preferred in terms of solubility in an electrolyte solution, and a lithium ion or a sodium ion is more preferred.
n is an integer of 0 to 4, preferably an integer of 0 to 2, more preferably 0 or 1, in terms of solubility in the electrolyte.

Specific examples of the compound represented by formula (I) include the following compounds:

Among these, the following compounds are preferred:

More preferred are the following compounds:

More preferred compounds include the following:

Particularly preferred are the following compounds:

The content of the compound represented by formula (I) is not particularly limited, but the non-aqueous electrolyte solution contains the compound represented by formula (I) in an amount of preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 6% by mass or less, and particularly preferably 4% by mass or less. The compound represented by formula (I) may be used alone or in any combination and ratio of two or more.
When the compound represented by formula (I) and the compound represented by formula (II) described below are used in combination, the total amount of the compound represented by formula (I) and the compound represented by formula (II), i.e., the total amount of the sulfonate salt (A) having a carbon-carbon unsaturated bond, may be set within the above range, but it is preferable to use only the compound represented by formula (I).
The sulfonate salt having a carbon-carbon unsaturated bond is identified and its content is measured by nuclear magnetic resonance (NMR) spectroscopy.

[1-1-2. Compound represented by formula (II)]

^R4 in formula (II) is a hydrogen atom, a halogen atom, or a hydrocarbon group. Examples of the halogen atom or hydrocarbon group represented by ^R4 include those exemplified for ^R1 to ^R3 , and are preferably a hydrogen atom, a halogen atom, an alkenyl group having 2 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aralkyl group having 7 to 18 carbon atoms, more preferably a hydrogen atom or a phenyl group, and even more preferably a hydrogen atom.
^M2 in formula (II) is a monovalent cation, and specific examples thereof include those exemplified for ^M1 , with lithium ion, sodium ion, or potassium ion being preferred in terms of solubility in the electrolyte solution.
n is an integer of 0 to 4, preferably an integer of 0 to 2, more preferably 0 or 1, in terms of solubility in the electrolyte.

Specific examples of the compound represented by formula (II) include the following compounds:

Among these, the following compounds are preferred:

More preferred are the following compounds:

More preferred compounds include the following:

Particularly preferred are the following compounds:

The content of the compound represented by formula (II) in the non-aqueous electrolyte solution is not particularly limited, but the non-aqueous electrolyte solution contains the compound represented by formula (I) in an amount of preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 6% by mass or less, and particularly preferably 4% by mass or less. The compound represented by formula (II) may be used alone or in any combination and ratio of two or more.
As described above, when the compound represented by formula (I) and the compound represented by formula (II) are used in combination, the total amount of the compound represented by formula (I) and the compound represented by formula (II), i.e., the total amount of the sulfonate salt (A) having a carbon-carbon unsaturated bond, may be set to fall within the above range.

[1-2. Phosphates having fluorine atoms and sulfonates having S—F bonds]
[1-2-1. Phosphate having a P—F bond (phosphate having a fluorine atom)]
Specific examples of the phosphate salt having a P—F bond according to the present invention include lithium monofluorophosphate, sodium monofluorophosphate, potassium monofluorophosphate, rubidium monofluorophosphate, cesium monofluorophosphate, lithium difluorophosphate, sodium difluorophosphate, potassium difluorophosphate, rubidium difluorophosphate, cesium difluorophosphate, lithium monofluoromonomethylphosphate, sodium monofluoromonomethylphosphate, potassium monofluoromonomethylphosphate, rubidium monofluoromonomethylphosphate, and cesium monofluoromonomethylphosphate. Of these, lithium monofluorophosphate and lithium difluorophosphate are particularly preferred in terms of electrolyte stability.
The content of the phosphate having a P-F bond in the non-aqueous electrolyte is not particularly limited, but the non-aqueous electrolyte contains 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 of the phosphate having a P-F bond, and preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. The identification and content of the phosphate having a P-F bond are measured by nuclear magnetic resonance (NMR) spectroscopy.

[1-2-2. Sulfonate having an S—F bond (sulfonate having a fluorine atom)]
Specific examples of the sulfonate salt having an S—F bond of the present invention include lithium fluorosulfonate, sodium fluorosulfonate, potassium fluorosulfonate, rubidium fluorosulfonate, cesium fluorosulfonate, and lithium bisfluorosulfonylimide. Of these, lithium fluorosulfonate and lithium bisfluorosulfonylimide are particularly preferred in terms of electrolyte stability.
The content of the sulfonate salt having an S—F bond in the non-aqueous electrolytic solution is not particularly limited, but the non-aqueous electrolytic solution contains preferably 0.01 mass % or more, more preferably 0.05 mass % or more, and even more preferably 0.1 mass % or more of the sulfonate salt having an S—F bond, and preferably 10 mass % or less, more preferably 8 mass % or less, and even more preferably 6 mass % or less.

In other words, the content of fluorine-containing phosphate or sulfonate (B) in the non-aqueous electrolyte solution 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, and preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. When two or more types of phosphate having a P-F bond and sulfonate having an S-F bond are contained, the total amount of phosphate having a P-F bond and sulfonate having an S-F bond, i.e., the total amount of salt (B), should be within the above-mentioned range.

[1-2. Electrolyte]
<Lithium salt>
As the electrolyte in the non-aqueous electrolytic solution, a lithium salt is usually used. The lithium salt is not particularly limited as long as it is known to be used for this purpose, and any lithium salt can be used, and specific examples thereof include the following:

Examples include lithium fluoroborates, lithium fluorophosphates, lithium tungstates, lithium carboxylates, lithium sulfonates, lithium imide salts, lithium methide salts, lithium oxalate salts, and fluorine-containing organic lithium salts.

Among them, lithium fluoroborates include LiBF ₄ ; lithium fluorophosphates include LiPF ₆ ; lithium sulfonates include CH ₃ SO ₃ Li; lithium imide salts include 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, and lithium cyclic 1,3-perfluoropropane disulfonylimide; and lithium methide salts include LiC(FSO ₂ ) ₃ , LiC(CF ₃ SO ₂ ) ₃ , and LiC(C ₂ F ₅ SO ₂ ) _{3 .} As lithium oxalate salts, lithium difluorooxalatoborate, lithium bis(oxalato)borate, lithium tetrafluorooxalatophosphate, lithium difluorobis(oxalato)phosphate, lithium tris(oxalato)phosphate, etc. are more preferred in terms of the effect of improving low-temperature output characteristics, high-rate charge/discharge characteristics, impedance characteristics, high-temperature storage characteristics, cycle characteristics, etc. More preferred are _LiPF6 and lithium bis(oxalato)borate, and particularly preferred is _LiPF6 . The above electrolytes may be used alone or in combination of two or more. However, when a lithium salt corresponding to "1-1. Compound (A) represented by formula (I) or formula (II) (sulfonate having a carbon-carbon unsaturated bond)" or "1-2. Phosphate having a fluorine atom and sulfonate having an S—F bond" is contained in a non-aqueous electrolyte solution, the non-aqueous electrolyte solution must contain an electrolyte other than the lithium salt corresponding to "1-1. Compound (A) represented by formula (I) or formula (II) (sulfonate having a carbon-carbon unsaturated bond)" or "1-2. Phosphate having a fluorine atom and sulfonate having an S—F bond".
The combination of two or more electrolytes is not particularly limited, but examples thereof include a combination of LiPF ₆ and LiBF ₄ and a combination of LiPF ₆ and LiN(CF ₃ SO ₂ ) _2. Of these, the combination of LiPF ₆ and LiBF ₄ is preferred.

There are no particular restrictions on the total concentration of electrolytes in the non-aqueous electrolyte solution, but it is typically 8% by mass or more, preferably 8.5% by mass or more, and more preferably 9% by mass or more, relative to the total amount of the non-aqueous electrolyte solution, and typically 18% by mass or less, preferably 17% by mass or less, and more preferably 16% by mass or less. When the total electrolyte concentration is within the above range, the electrical conductivity is appropriate for battery operation, and sufficient output characteristics tend to be obtained.

[1-3. Non-aqueous solvent]
Like common non-aqueous electrolytes, non-aqueous electrolytes usually contain a non-aqueous solvent that dissolves the above-mentioned electrolyte as its main component. The non-aqueous solvent used is not particularly limited as long as it dissolves the above-mentioned electrolyte, and known organic solvents can be used. Examples of organic solvents include saturated cyclic carbonates, chain carbonates, chain carboxylic acid esters, cyclic carboxylic acid esters, ether-based compounds, and sulfone-based compounds, but are not particularly limited thereto. The organic solvents can be used alone or in combination of two or more.
The combination of two or more organic solvents is not particularly limited, and examples thereof include a saturated cyclic carbonate and a chain carboxylic acid ester, a cyclic carboxylic acid ester and a chain carbonate, and a saturated cyclic carbonate, a chain carbonate and a chain carboxylic acid ester. Among these, a saturated cyclic carbonate and a chain carbonate, and a saturated cyclic carbonate, a chain carbonate and a chain carboxylic acid ester are preferred.

[1-3-1. Saturated cyclic carbonates]
Examples of saturated cyclic carbonates include those having an alkylene group having 2 to 4 carbon atoms, and saturated cyclic carbonates having 2 to 3 carbon atoms are preferably used from the viewpoint of improving the battery characteristics resulting from the improvement in the degree of lithium ion dissociation.

Specific examples of saturated cyclic carbonates include ethylene carbonate, propylene carbonate, and butylene carbonate. Among these, ethylene carbonate or propylene carbonate is preferred, and ethylene carbonate, which is less susceptible to oxidation and reduction, is more preferred. One saturated cyclic carbonate may be used alone, or two or more may be used in any combination and ratio.

The content of the saturated cyclic carbonate 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 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, relative to the total amount of the nonaqueous solvent in the nonaqueous electrolyte solution. By setting it in this range, a decrease in electrical conductivity due to a decrease in the dielectric constant of the nonaqueous electrolyte solution can be avoided, and the large current discharge characteristics, stability with respect to the negative electrode, and cycle characteristics of the nonaqueous electrolyte secondary battery tend to be in good ranges, and the oxidation/reduction resistance of the nonaqueous electrolyte solution is improved, and 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 atmosphere.

[1-3-2. Chain carbonate]
The chain carbonate usually has 3 to 7 carbon atoms, and in order to adjust the viscosity of the electrolyte solution within an appropriate range, a chain carbonate having 3 to 5 carbon atoms is preferably used.

Specific examples of chain carbonates include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, ethyl methyl carbonate, and methyl-n-propyl carbonate. Dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are particularly preferred.

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, the multiple fluorine atoms may be bonded to the same carbon or different carbons.
Examples of the fluorinated chain carbonate include fluorinated dimethyl carbonate derivatives such as fluoromethyl methyl carbonate; fluorinated ethyl methyl carbonate derivatives such as 2-fluoroethyl methyl carbonate; and fluorinated diethyl carbonate derivatives such as ethyl-(2-fluoroethyl) carbonate.

The chain carbonate may be used alone or in any combination and ratio of two or more types.

The amount of chain carbonate contained is not particularly limited, but is typically 15% by volume or more, preferably 20% by volume or more, and more preferably 25% by volume or more, relative to the total amount of nonaqueous solvent in the nonaqueous electrolyte solution, and is typically 90% by volume or less, preferably 85% by volume or less, and more preferably 80% by volume or less. By keeping the amount of chain carbonate contained within this range, the viscosity of the nonaqueous electrolyte solution is kept within an appropriate range, reducing the decrease in ionic conductivity, and ultimately making it easier to maintain the output characteristics of the nonaqueous electrolyte secondary battery within a favorable range.

Furthermore, by combining a specific chain carbonate with a specific amount of ethylene carbonate, battery performance can be significantly improved.

For example, if dimethyl carbonate and ethyl methyl carbonate are selected as the specific chain carbonates, the ethylene carbonate content is not particularly limited and can be any content as long as it does not significantly impair the effects of the present invention. However, it is typically 15% by volume or more, preferably 20% by volume or more, and typically 45% by volume or less, preferably 40% by volume or less, based on the total amount of nonaqueous solvent in the nonaqueous electrolyte solution. The dimethyl carbonate content is typically 20% by volume or more, preferably 30% by volume or more, and typically 50% by volume or less, preferably 45% by volume or less, based on the total amount of nonaqueous solvent in the nonaqueous electrolyte solution. The ethyl methyl carbonate content is typically 20% by volume or more, preferably 30% by volume or more, and typically 50% by volume or less, preferably 45% by volume or less, based on the total amount of nonaqueous solvent in the nonaqueous electrolyte solution. Keeping the contents within the above ranges tends to result in excellent high-temperature stability and suppress gas generation.

[1-3-3. Chain carboxylic acid ester]
Examples of chain carboxylic acid esters 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 these, methyl acetate, ethyl acetate, propyl acetate, and butyl acetate are preferred in terms of improving battery characteristics. Chain carboxylic acid esters in which some of the hydrogen atoms in the above-mentioned compounds are substituted with fluorine atoms (e.g., methyl trifluoroacetate, ethyl trifluoroacetate, etc.) can also be used suitably.
The amount of the chain carboxylic acid ester is typically 1% by volume or more, preferably 5% by volume or more, and more preferably 15% by volume or more, based on the total amount of the nonaqueous solvent. This range improves the electrical conductivity of the nonaqueous electrolyte and facilitates the enhancement of the large-current discharge characteristics of the nonaqueous electrolyte battery. Furthermore, the amount of the chain carboxylic acid ester is typically 70% by volume or less, preferably 50% by volume or less, and more preferably 40% by volume or less. By setting the upper limit in this manner, the viscosity of the nonaqueous electrolyte can be kept within an appropriate range, a decrease in electrical conductivity can be avoided, an increase in negative electrode resistance can be suppressed, and the large-current discharge characteristics of the nonaqueous electrolyte secondary battery can be easily maintained within a favorable range.

[1-3-4. Cyclic carboxylic acid esters]
Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Of these, γ-butyrolactone is more preferred. Cyclic carboxylic acid esters in which some of the hydrogen atoms in the above compounds have been substituted with fluorine atoms can also be used suitably.
The amount of the cyclic carboxylic acid ester is typically 1% by volume or more, preferably 5% by volume or more, and more preferably 15% by volume or more, based on the total amount of the nonaqueous solvent. This range improves the electrical conductivity of the nonaqueous electrolyte and facilitates the improvement of the large-current discharge characteristics of the nonaqueous electrolyte battery. Furthermore, the amount of the cyclic carboxylic acid ester is typically 70% by volume or less, preferably 50% by volume or less, and more preferably 40% by volume or less, based on the total amount of the nonaqueous solvent. By setting the upper limit in this manner, the viscosity of the nonaqueous electrolyte can be kept within an appropriate range, a decrease in electrical conductivity can be avoided, an increase in negative electrode resistance can be suppressed, and the large-current discharge characteristics of the nonaqueous electrolyte secondary battery can be easily maintained within a favorable range.

[1-3-5. Ether compounds]
Preferred 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 cyclic ethers having 3 to 6 carbon atoms, such as tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxane, 2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, and 1,4-dioxane. Some of the hydrogen atoms in the above-mentioned ether compounds may be substituted with fluorine.
Among these, as the chain ether having 3 to 10 carbon atoms, dimethoxymethane, diethoxymethane, or ethoxymethoxymethane is preferred because it has a high solvation ability for lithium ions, improves ionic dissociation, has low viscosity, and provides high ionic conductivity, and as the cyclic ether having 3 to 6 carbon atoms, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, or the like is preferred because it provides high ionic conductivity.

The content of the ether-based compound is not particularly limited and can be any amount as long as it does not significantly impair the effects of the present invention. However, it is typically 1% by volume or more, preferably 2% by volume or more, more preferably 3% by volume or more, and typically 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 non-aqueous solvent. When the content of the ether-based compound is within the preferred range, it is easy to ensure the improved ionic conductivity resulting from the improved degree of lithium ion dissociation of the ether and the reduced viscosity. Furthermore, when the negative electrode active material is a carbon-based material, the phenomenon of chain ethers being co-inserted with lithium ions can be suppressed, allowing the input/output characteristics and charge/discharge rate characteristics to be within appropriate ranges.

[1-3-6. Sulfone 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 standpoint of dielectric constant and viscosity, tetramethylene sulfones, tetramethylene disulfones, hexamethylene sulfones, and hexamethylene disulfones are more preferred, with tetramethylene sulfones (sulfolanes) being particularly preferred.

Preferred sulfolanes are sulfolane and sulfolane derivatives. Preferred sulfolane derivatives are 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.

Among these, 2-methylsulfolane, 3-methylsulfolane, 2-fluorosulfolane, 3-fluorosulfolane, 2,3-difluorosulfolane, 2-trifluoromethylsulfolane, 3-trifluoromethylsulfolane, etc. are preferred due to their high ionic conductivity and high input/output.

Examples of chain sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone, and pentafluoroethyl methyl sulfone. Of these, dimethyl sulfone, ethyl methyl sulfone, and monofluoromethyl methyl sulfone are preferred because they improve the high-temperature storage stability of the electrolyte.

The content of the sulfone-based compound is not particularly limited and can be any amount as long as it does not significantly impair the effects of the present invention. However, it is typically 0.3 vol.% or more, preferably 0.5 vol.% or more, and more preferably 1 vol.% or more, relative to the total amount of solvent in the nonaqueous electrolyte solution, and is typically 40 vol.% or less, preferably 35 vol.% or less, and more preferably 30 vol.% or less. If the content of the sulfone-based compound is within this range, an electrolyte solution with excellent high-temperature storage stability tends to be obtained.

[1-4. Auxiliaries]
The non-aqueous electrolyte solution of the present invention may contain various auxiliary agents within a range that does not significantly impair the effects of the present invention. As the auxiliary agent, any conventionally known agent can be used. Note that, one auxiliary agent may be used alone, or two or more auxiliary agents may be used in any combination and ratio.

Examples of auxiliary agents that may be contained in the non-aqueous electrolyte solution include cyclic carbonates having a carbon-carbon unsaturated bond, fluorine-containing cyclic carbonates, compounds having an isocyanate group, compounds having an isocyanuric acid skeleton, 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 an ether bond, carboxylic acid anhydrides, borates, oxalates, etc. Examples include compounds described in WO 2015/111676.
The content of the auxiliary agent is not particularly limited and may be any amount as long as it does not significantly impair the effects of the present invention. However, it is usually 0.001% by mass or more, preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and 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, relative to the total amount of the nonaqueous electrolyte solution.
Cyclic compounds having an ether bond can be used as auxiliary agents in non-aqueous electrolyte solutions, and as shown in 1-3, some of them can also be used as non-aqueous solvents. When used as auxiliary agents, cyclic compounds having an ether bond are used in an amount of less than 4% by mass. Borates and oxalates can be used as auxiliary agents in non-aqueous electrolyte solutions, and as shown in 1-2, some of them can also be used as electrolytes. When used as auxiliary agents, these compounds are used in an amount of less than 3% by mass.

[2. Non-aqueous electrolyte secondary battery]
A nonaqueous electrolyte secondary battery according to one embodiment of the present invention is a nonaqueous electrolyte secondary battery including a positive electrode having a positive electrode active material capable of absorbing and releasing metal ions, and a negative electrode having a negative electrode active material capable of absorbing and releasing metal ions, and containing a nonaqueous electrolyte.

[2-1. Non-aqueous electrolyte]
The nonaqueous electrolyte solution used is the nonaqueous electrolyte solution described above. Note that, within the scope of the present invention, other nonaqueous electrolyte solutions may be mixed with the nonaqueous electrolyte solution described above.

[2-2. Negative electrode]
The negative electrode is composed of a negative electrode active material layer containing a negative electrode active material and a binder, and a current collector.
[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 carbon-based materials, materials containing metal elements and/or metalloid elements that can be alloyed with Li, lithium-containing metal composite oxide materials, and mixtures thereof. Among these, carbon-based materials are preferred because of their excellent cycle characteristics and safety, as well as excellent continuous charge characteristics. These materials may be used alone or in any combination of two or more.

[2-2-1-1. Carbon-based materials]
Examples of carbonaceous materials include natural graphite, artificial graphite, amorphous carbon, carbon-coated graphite, graphite-coated graphite, and resin-coated graphite. Of these, natural graphite is preferred. One type of carbonaceous material may be used alone, or two or more types may be used in any combination and ratio.
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, spherical or ellipsoidal graphite particles that have been subjected to spheroidization treatment are particularly preferred from the viewpoint of particle packing properties or charge/discharge rate characteristics.
The average particle size (d50) of the graphite particles is usually 1 μm or more and 100 μm or less.

[2-2-1-2. Physical properties of carbon-based materials]
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 the following items (1) to (4), and it is particularly preferable that the carbonaceous material simultaneously satisfies several of them.
(1) X-ray Diffraction Parameters The d value (interlayer distance) of the lattice plane (002 plane) of a carbon-based 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 carbon-based material determined by X-ray diffraction according to the Gakushin method is usually 1.0 nm or more.
(2) Volume-Based Average Particle Size The volume-based average particle size of a carbon-based 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 and Raman Half Width The Raman R value of a carbon-based material is a value measured using argon ion laser Raman spectroscopy, and is usually 0.01 or more and 1.5 or less.
The Raman half-width of the carbon-based material around 1580 cm ⁻¹ 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 carbon-based 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 carbonaceous materials with different properties, where the properties refer to X-ray diffraction parameters, volume-based average particle size, Raman R value, Raman half-width, and BET specific surface area.
Preferred examples include a case where the volume-based particle size distribution is not symmetrical about the median diameter, a case where two or more carbon-based materials having different Raman R values are contained, and a case where two or more carbon-based materials having different X-ray parameters are contained.

[2-2-1-3. Materials containing metal elements and/or metalloid elements that can be alloyed with Li]
Any conventionally known material containing a metal element and/or a metalloid element that can be alloyed with Li can be used, but from the viewpoint of capacity and cycle life, it is preferable that the material be, for example, a simple substance or a compound of a metal and/or a metalloid element selected from the group consisting of Sb, Si, Sn, Al, As, and Zn. Furthermore, when the material containing a metal element and/or a metalloid element that can be alloyed with Li contains two or more elements, the material may be an alloy material made of an alloy of these metals.
Examples of materials containing metal elements and/or metalloid elements that can be alloyed with Li include oxides, nitrides, carbides, etc. These may contain two or more metal elements and/or metalloid elements that can be alloyed with Li. Among these, metal Si (hereinafter sometimes referred to as Si) or Si-containing inorganic compounds are preferred in terms of increasing capacity.
Furthermore, the material of the metal element and/or metalloid element that can be alloyed with Li may already be alloyed with Li during the production of the negative electrode, which will be described later.

In this specification, Si or Si-containing inorganic compounds are collectively referred to as Si compounds. Specific examples of Si compounds include SiO _x (0≦x≦2). Specific examples of metal compounds alloyed with Li include Li _y Si (0<y≦4.4) and Li ₂ SiO _2+z (0<z≦2). As the Si compound, Si oxide (SiO _x1 , 0<x1≦2) is preferred because it has a larger theoretical capacity than graphite, and amorphous Si or nano-sized Si crystals are preferred because they allow alkali ions such as lithium ions to easily enter and exit the material, making it possible to obtain a high capacity.
When the material containing a metal element and/or a metalloid element that can be alloyed with Li is in the form of particles, the average particle size (d50) is usually 0.01 μm or more and 10 μm or less from the viewpoint of cycle life.

[2-2-1-4. Mixture of a material containing a metal element and/or a metalloid element capable of being alloyed with Li and a carbon-based material]
The mixture of a material containing a metal element and/or a metalloid element that can be alloyed with Li and a carbon-based material used as the negative electrode active material may be a mixture in which the material containing a metal element and/or a metalloid element that can be alloyed with Li and the carbon-based material are mixed in a state in which they are independent materials, or may be a composite in which the material containing a metal element and/or a metalloid element that can be alloyed with Li is present on the surface or inside of the carbon-based material.
The content of the material containing a metal element and/or a metalloid element that can be alloyed with Li relative to the total of the material containing a metal element and/or a metalloid element that can be alloyed with Li and the carbon-based material is usually 1 mass % or more and 99 mass % or less.

[2-2-1-5. 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 is capable of absorbing and releasing lithium ions. However, from the viewpoint of high current density charge/discharge characteristics, a lithium-containing metal composite oxide material containing titanium 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 because it significantly reduces output resistance.

Furthermore, the lithium and/or titanium of 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.
Preferred lithium titanium composite oxides are Li _4/3 Ti _5/3 O ₄ , Li ₁ Ti ₂ O ₄ and Li _4/5 Ti _11/5 O _4. Also preferred lithium titanium composite oxides in which part of the lithium and/or titanium has been substituted with other elements include Li _4/3 Ti _4/3 Al _1/3 O ₄ .

[2-2-1-6. Surface coating]
The negative electrode active material may have a substance (surface-attached substance) attached to its surface, the substance having a different composition from the negative electrode active material. 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 negative electrode active material, for example, by dissolving or suspending them in a solvent, adding them to the negative electrode active material by impregnation, and then drying.
The amount of the surface-adhering substance is preferably 1 μmol/g or more, more preferably 10 μmol/g or more, and is usually 1 mmol/g or less, relative to the positive electrode active material.
In this specification, the term "negative electrode active material" also refers to a negative electrode active material having the surface-adhering substance attached to its surface.

[2-2-2. Negative electrode configuration and manufacturing method]
The negative electrode can be produced by any known method as long as it does not significantly impair the effects of the present invention. For example, the negative electrode can be produced by adding a binder, a solvent such as an aqueous solvent or an organic solvent, and optionally a thickener, a conductive material, a filler, etc. to the negative electrode active material to form a slurry, which is then applied to a current collector, dried, and pressed to form a negative electrode active material layer. In this case, in order to increase the packing density of the negative electrode active material, it is preferable to compact it using a hand press, a roller press, etc.

[2-2-2-1. Active material content]
The content of the negative electrode active material in the negative electrode active material layer is usually 80% by mass or more and 99.5% by mass or less.

[2-2-2-2. Thickener]
A thickener is usually used to adjust the viscosity of the slurry. The thickener is not particularly limited, but specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, etc. These may be used alone or in any combination and ratio of two or more.
When a thickener is used, 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.

[2-2-2-3. 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 solution and the solvent used in producing the electrode.
Specific examples include rubbery polymers such as SBR (styrene-butadiene rubber), isoprene rubber, butadiene rubber, fluororubber, NBR (acrylonitrile-butadiene rubber), and ethylene-propylene rubber; and fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, and tetrafluoroethylene-ethylene copolymer. These may be used alone or in any combination and ratio of two or more.
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 rubber-like polymer such as SBR as a main component, the ratio of the binder to the negative electrode active material is preferably 0.1% by mass or more and 5% by mass or less. Also, when the binder contains a fluorine-based polymer such as polyvinylidene fluoride as a main component, the ratio of the binder to the negative electrode active material is preferably 1% by mass or more and 15% by mass or less.

[2-2-2-4. Solvent]
The solvent for forming the slurry is not particularly limited as long as it is capable of dissolving or dispersing the negative electrode active material, the binder, and the thickener, conductive material, filler, and the like that are used as needed, and either an aqueous solvent or an organic solvent may be used.

[2-2-2-5. Current collector]
Any known current collector can be used to support the negative electrode active material. Examples of the negative electrode current collector include metal materials such as aluminum, copper, nickel, stainless steel, and nickel-plated steel, but copper is particularly preferred from the standpoints of ease of processing and cost.
The current collector may be in the form of a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punched metal, a foamed metal, or the like. Of these, a metal foil or a metal thin film is preferred. The metal foil or the metal thin film may be formed into a mesh shape as appropriate.
When the negative electrode current collector is in the form of a plate or film, the thickness of the current collector is not limited, but is usually 1 μm or more and 1 mm or less.

[2-2-2-6. Thickness and density of negative electrode active material layer]
The thickness of the negative electrode active material layer is the thickness obtained by subtracting the thickness of the current collector from the thickness of the entire negative electrode, and is not particularly limited, but is usually 15 μm to 300 μm from the viewpoint of high capacity and high output. The density of the negative electrode active material layer is usually 0.8 g cm to ^{1.7 g} cm.
The density of the negative electrode active material layer is measured by measuring the thickness and weight of the negative electrode active material layer.

[2-2-2-7. Surface coating of negative electrode plate]
The negative electrode plate may have a surface to which a substance having a different composition from the negative electrode active material (surface-attached substance) is attached, such as an oxide such as aluminum oxide, a sulfate such as lithium sulfate, or a carbonate such as lithium carbonate.

[2-3. Positive electrode]
The positive electrode is composed of a positive electrode active material layer containing a positive electrode active material and a binder, and a current collector.
[2-3-1. Cathode active material]
The positive electrode active material used in the positive electrode is not particularly limited as long as it can electrochemically absorb and release metal ions. Specific examples include lithium transition metal compounds. These may be used alone or in any combination of two or more.

[2-3-1-1. Lithium transition metal compounds]
Examples of lithium transition metal compounds include sulfides, phosphate compounds, silicate compounds, borate compounds, lithium transition metal composite oxides, etc. Among these, phosphate compounds and lithium transition metal composite oxides are preferred, and lithium transition metal composite oxides are more preferred.
Examples of lithium transition metal composite oxides include those having a spinel structure that allows three-dimensional diffusion and those having a layered structure that allows two-dimensional diffusion of lithium ions. Those having a spinel structure are generally represented by the following composition formula (1):
Li _x' M' ₂ O ₄ ...(1)
(In formula (1), x' is 1≦x'≦1.5, and M' represents at least one transition metal element.)
Specific examples include LiMn ₂ O ₄ , LiCoMnO ₄ , LiNi _0.5 Mn _1.5 O ₄ , and LiCoVO ₄ .
Those having a layered structure are generally represented by the following composition formula (2).
Li _1+x MO ₂ ...(2)
(In formula (2), x is −0.1≦x≦0.5, and M represents at least one transition metal element.)
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 Co _0.33 Mn _0.33 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , Li _1.05 Ni _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _{0.8Co0.1Mn0.1O2 , etc. ​}

Among these, from the viewpoint of improving the battery capacity, a lithium transition metal composite oxide having a layered structure is preferred, and a transition metal composite oxide represented by the following composition formula (3) is more preferred.
Li _a1 Ni _b1 M _c1 O ₂ ...(3)
(In formula (3), a1, b1, and c1 are numerical values that satisfy 0.90≦a1≦1.10, 0.30≦b1≦0.98, and 0.01≦c1≦0.5, respectively, and satisfy 0.50≦b1+c1 and b1+c1=1. M represents at least one element selected from the group consisting of Mn, Al, Mg, Zr, Fe, Ti, and Er.)
In the composition formula (3), it is preferable that d1 satisfies the numerical value 0.01≦d1≦0.50.

In particular, from the viewpoint of the structural stability of the lithium transition metal composite oxide, a transition metal oxide represented by the following composition formula (4) is preferred.
Li _a2 Ni _b2 Co _c2 M _d2 O ₂ ...(4)
(In formula (4), a2, b2, and c2 are numerical values that satisfy 0.90≦a2≦1.10, 0.50≦b2≦0.98, and 0.01≦c2<0.50, respectively, and b2+c2=1 is satisfied. M represents at least one element selected from the group consisting of Mn, Al, Mg, Zr, Fe, Ti, and Er.)
_Specific examples _of suitable _lithium transition _metal oxides _{represented by composition formula ( 4 ) include LiNi0.85Co0.10Al0.05O2 , LiNi0.80Co0.15Al0.05O2 , LiNi0.5Co0.2Mn0.3O2 , Li1.05Ni0.50Co0.20Mn0.30O2 , LiNi0.6Co0.2Mn0.2O2 , and LiNi0.8Co0.1Mn0.1O2 .}
In each composition formula, M preferably contains Mn or Al, more preferably contains Mn, and further preferably is Mn or Al, because this increases the structural stability of the lithium transition metal oxide and suppresses structural deterioration during repeated charge and discharge.

[2-3-1-2. Introduction of different elements]
Furthermore, elements (foreign elements) other than the elements included in the above composition formula may be incorporated into the lithium transition metal composite oxide.

[2-3-1-3. Surface coating]
The positive electrode active material may have a substance (surface-attached substance) attached to its surface, the substance having a different composition from the positive electrode active material. 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 by, for example, dissolving or suspending them in a solvent, adding them to the positive electrode active material by impregnation, and drying them.
The amount of the surface-adhering substance is preferably 1 μmol/g or more, more preferably 10 μmol/g or more, and is usually 1 mmol/g or less, relative to the positive electrode active material.
In this specification, a positive electrode active material having the surface-adhering substance attached to its surface is also referred to as a "positive electrode active material."
[2-3-1-4. Blend]
These positive electrode active materials may be used alone or in any combination of two or more in any ratio.

[2-3-2. Positive electrode configuration and manufacturing method]
The configuration and manufacturing method of the positive electrode are described below. In this embodiment, a positive electrode using a positive electrode active material can be manufactured by a conventional method. That is, a positive electrode can be obtained by a coating method in which a positive electrode active material, a binder, and, if necessary, a conductive material and a thickener are mixed in a dry state to form a sheet and then pressed onto a positive electrode current collector, or by dissolving or dispersing these materials in a solvent such as an aqueous solvent or an organic solvent to form a slurry, which is then applied to a positive electrode current collector and dried to form a positive electrode active material layer on the current collector. Furthermore, for example, the above-mentioned positive electrode active material may be roll-formed into a sheet electrode, or may be compression-molded into a pellet electrode. In this case, in order to increase the packing density of the positive electrode active material, it is preferable to compact it using a hand press, roller press, or the like.
Hereinafter, the case where the slurry is sequentially applied to the positive electrode current collector and then dried will be described.

[2-3-2-1. 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 99.5% by mass or less.

[2-3-2-2. Conductive material]
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 carbon-based materials such as amorphous carbon such as needle coke. One type of conductive material may be used alone, or two or more types may be used in any combination and ratio. The conductive material is typically used so that it is contained in the positive electrode active material layer in an amount of 0.01% by mass to 50% by mass.

[2-3-2-3. Binder]
When the positive electrode active material layer is formed by, for example, a coating method, the type of binder used in producing the positive electrode active material layer is not particularly limited as long as it is a material that can be dissolved or dispersed in a solvent for the slurry. However, in view of weather resistance, chemical resistance, heat resistance, flame retardancy, and the like, preferred binders include fluorine-based resins such as polyvinyl fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; and CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide.
It is also possible to use mixtures, modified products, derivatives, random copolymers, alternating copolymers, graft copolymers, block copolymers, etc. The binder may be used alone or in any combination and ratio of two or more types.
Furthermore, when a resin is used as a binder, the weight-average molecular weight of the resin is not particularly limited 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 suitably formed.
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.

[2-3-2-4. Solvent]
The solvent for forming the slurry is not particularly limited in type as long as it is capable of dissolving or dispersing the positive electrode active material, conductive material, binder, and thickener used as needed, and either an aqueous solvent or an organic solvent may be used.

[2-3-2-5. 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 these, aluminum is preferred.
The current collector may be in the form of a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punched metal, a foamed metal, or the like. Of these, a metal foil or a metal thin film is preferred. The metal foil or the metal thin film may be formed into a mesh shape as appropriate.
When the current collector of the positive electrode is in the form of a plate or film, the thickness of the current collector is not limited, but is usually 1 μm or more and 1 mm or less.

[2-3-2-6. Thickness and density of positive electrode active material layer]
The thickness of the positive electrode active material layer is the thickness obtained by subtracting the thickness of the current collector from the thickness of the entire positive electrode, and is not particularly limited, but is usually 10 μm or more and 500 μm or less on one side of the current collector from the viewpoint of high capacity and high output. In addition, the density of the positive electrode active material layer is usually 1.5 g cm or ^more and 4.5 g cm or ^less .
The density of the positive electrode active material layer is measured by measuring the thickness and weight of the positive electrode active material layer.

[2-3-2-7. Surface coating of positive electrode plate]
In addition, the positive electrode plate may have a substance of a different composition attached to its surface, and the substance may be the same as the surface-attached substance that may be attached to the surface of the positive electrode active material.

[2-4. Separator]
A separator is usually interposed between the positive electrode and the negative electrode to prevent short circuits, and in this case, the non-aqueous electrolyte is usually impregnated into the separator before use.

[2-4-1. material]
The separator material is not particularly limited as long as it is stable against the non-aqueous electrolyte solution, but preferred examples include oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, sulfates such as barium sulfate and calcium sulfate, inorganic materials such as glass filters made of glass fibers, and resins such as polyolefins, more preferably polyolefins, and particularly preferably polyethylene or polypropylene. These materials may be used alone or in any combination and ratio of two or more. The above materials may also be used in a laminated state.

[2-4-2. form]
The form is not particularly limited, but preferably, a thin film such as a nonwoven fabric, woven fabric, or microporous film is used. Thin film separators with a pore size of 0.01 to 1 μm and a thickness of 1 to 50 μm are preferably used. In addition to independent thin film separators, separators may be used in which a composite porous layer containing inorganic particles is formed on the surface of the positive electrode and/or negative electrode using a resin binder. The separator is preferably a microporous film or nonwoven fabric because of its excellent liquid retention properties.

[2-4-3. Porosity]
When a porous material such as a porous sheet or nonwoven fabric is used as the separator, the porosity of the separator is not limited, but is usually 20% or more and 90% or less.

[2-4-4. Air permeability]
The air permeability of a separator in a nonaqueous electrolyte secondary battery can be determined by its 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. The Gurley value of a separator is arbitrary, but is typically 10 to 1,000 seconds/100 mL.

[2-5. Battery design]
[2-5-1. Electrode group]
The electrode group may have either a laminated structure in which the positive electrode plate and the negative electrode plate are sandwiched between the separator, or a structure in which the positive electrode plate and the negative electrode plate are spirally wound with the separator sandwiched between them. 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 usually 40% to 90%.

[2-5-2. Current collection structure]
When the electrode group has the aforementioned laminated structure, a structure in which the metal core portions of each electrode layer are bundled and welded to a terminal is preferably used. A structure in which multiple terminals are provided within the electrode to reduce resistance is also preferably used. 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 electrode and negative electrode and bundling them to a terminal.

[2-5-3. Protection elements]
Examples of protective elements that can be used include a PTC (Positive Temperature Coefficient) element whose resistance increases with heat generation due to excessive current, a thermal fuse, a thermistor, a valve (current cutoff valve) that cuts off the current flowing in the circuit due to a sudden increase in the internal pressure or temperature of the battery during abnormal heat generation, etc. It is preferable to select the above protective element so that it will not operate under normal use at high current, and it is more preferable to design it so that abnormal heat generation or thermal runaway does not occur even without a protective element.

[2-5-4. Exterior body]
A non-aqueous electrolyte secondary battery is usually constructed by housing the above-mentioned non-aqueous electrolyte, negative electrode, positive electrode, separator, etc. in an exterior body (exterior case). There are no limitations on 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 exterior case is not particularly limited as long as it is stable against the non-aqueous electrolyte solution used, but from the viewpoint of weight reduction, metals such as aluminum or aluminum alloys, and laminate films are preferably used.
Examples of exterior cases using the above metals include those in which metals are welded together by laser welding, resistance welding, or ultrasonic welding to form a sealed, airtight structure, and those in which the above metals are used via a resin gasket to form a crimped structure.

[2-5-5. shape]
The shape of the exterior case may also be arbitrary, and may be, for example, cylindrical, rectangular, laminated, coin-shaped, large, or the like.

Next, specific embodiments of the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
The compounds used in the present examples and comparative examples are shown below.

[Examples 1-1 to 1-2, 2-1 to 2-2, Comparative Examples 1-1 to 1-6, 2-1 to 2-6]
[Fabrication of non-aqueous electrolyte secondary battery]
<Preparation of non-aqueous electrolyte>
Under a dry argon atmosphere, a non-aqueous electrolyte solution in which thoroughly dried LiPF ₆ was dissolved at 1.0 mol / L (12.3 mass%, as the concentration in the non-aqueous electrolyte solution) in a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (volume ratio 3:4:3) was used as a reference electrolyte solution, and additives 1 and 2 were dissolved as shown in Tables 1 and 2. The non-aqueous electrolyte solutions of Examples 1-1 to 1-2, 2-1 to 2-2 and Comparative Examples 1-2 to 1-6, and 2-2 to 2-6 were prepared. The non-aqueous electrolyte solutions of Comparative Examples 1-1 and 2-1 were standard electrolyte solutions.

<Preparation of positive electrode>
85 parts by mass _{of Li1.05Ni0.34Mn0.33Co0.33O2} as the positive _electrode active material, 10 parts by mass of acetylene black as the conductive material, and 5 parts by _mass of polyvinylidene fluoride (PVdF) as the binder were mixed in _N -methyl-2-pyrrolidone to form a slurry. This was uniformly applied to a 15 μm thick aluminum foil, dried, and then roll-pressed to form a positive electrode. The plate density of the positive electrode was 2.6 g/ ^cm3 .

<Preparation of negative electrode>
To 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 1 part by mass of an aqueous dispersion of styrene-butadiene rubber (concentration of styrene-butadiene rubber: 49% by mass) as a binder were added, and the mixture was mixed in a disperser to form a slurry. The resulting slurry was uniformly applied to a 10 μm-thick copper foil, dried, and roll-pressed to form a negative electrode.

<Production of non-aqueous electrolyte secondary battery>
The positive electrode, negative electrode, and polyolefin separator were stacked in this order, and the resulting battery element was wrapped in an aluminum laminate film, and the nonaqueous electrolyte solution was poured into the battery, followed by vacuum sealing to prepare a sheet-shaped nonaqueous electrolyte secondary battery.
The remaining capacity and recovered capacity of the obtained non-aqueous electrolyte secondary battery were evaluated as described above. The results are shown in Tables 1 and 2.

[Evaluation of non-aqueous electrolyte secondary batteries]
The non-aqueous electrolyte secondary batteries prepared in the examples were evaluated as follows.
Evaluation of remaining capacity and recovered capacity after storage In a thermostatic bath at 25°C, a sheet-shaped non-aqueous electrolyte secondary battery was charged at a constant current of 0.025C (the current value at which the rated capacity based on the discharge capacity at a 1-hour rate is discharged in 1 hour is defined as 1C; the same applies hereinafter) to 3.6V, then charged at a constant current and constant voltage of 0.167C to a voltage of 4.2V, and then discharged at a constant current of 0.167C to 2.5V.
Furthermore, the nonaqueous electrolyte secondary battery was stabilized by constant current-constant voltage charging at 0.167 C to 4.1 V and then storing at 60 °C for 12 hours. Thereafter, a constant current discharge was performed at 25 °C to 2.5 V, followed by a constant current-constant voltage charge at 0.167 C to a voltage of 4.2 V. A constant current discharge was then performed at 0.167 C to 2.5 V. Next, a constant current-constant voltage charge was performed at 0.2 C to a voltage of 4.2 V, completing the initial charge and discharge. The nonaqueous electrolyte secondary battery after the initial charge and discharge was left standing at 60 °C for 14 days. After leaving for 14 days, a constant current discharge was performed at 25 °C to 2.5 V, and the discharge capacity at this time was recorded as the remaining capacity. A constant current-constant voltage charge was then performed at 0.2 C to a voltage of 4.2 V. A constant current discharge was then performed at 0.2 C to 2.5 V, and the discharge capacity at this time was recorded as the recovered capacity. The results are shown in Tables 1 and 2. The remaining capacity and recovery capacity in Tables 1 and 2 are values normalized by setting the values of Comparative Example 1-1 and Comparative Example 2-1 to 100. It can be said that the larger the remaining capacity and recovery capacity, the more preferable it is.

From Table 1, comparing Examples 1-1 and 1-2 with Comparative Examples 1-1 to 1-3 and 1-6, it can be seen that the co-addition of a sulfonate having an unsaturated bond and a phosphate having a P-F bond to a non-aqueous electrolyte solution further improves the remaining capacity and recovered capacity of a non-aqueous electrolyte secondary battery. Furthermore, comparing Comparative Example 1-4 with Comparative Example 1-1, it can be seen that the use of a non-aqueous electrolyte solution containing Compound 2, which is a sulfonate having no unsaturated bonds, results in almost no change in the remaining capacity and recovered capacity of a non-aqueous secondary battery. Furthermore, Comparative Examples 1-3 to 1-5 show that when a non-aqueous electrolyte solution containing a sulfonate having no unsaturated bonds and a phosphate having a P-F bond is used, both the remaining capacity and recovered capacity of the non-aqueous secondary battery are lower than when a non-aqueous electrolyte solution containing only a phosphate having a P-F bond is used. Therefore, it can be seen that the non-aqueous electrolyte solution of the present invention can specifically improve battery performance by co-adding a sulfonate having an unsaturated bond and a phosphate having a P-F bond.

Comparing Example 2-1 and Example 2-2 with Comparative Examples 2-1 to 2-3 and Comparative Example 2-6 in Table 2, it can be seen that the remaining capacity and recovered capacity of the nonaqueous electrolyte secondary battery are further improved by co-adding a sulfonate having an unsaturated bond and a sulfonate having an S—F bond to the nonaqueous electrolyte.
Furthermore, a comparison between Comparative Example 2-4 and Comparative Example 2-1 shows that the use of a sulfonate salt having no unsaturated bonds results in almost no change in either the remaining capacity or the recovered capacity. On the other hand, Comparative Examples 2-3 to 2-5 show that even when a non-aqueous electrolyte solution containing Compound 2, which is a sulfonate salt having no unsaturated bonds and a sulfonate salt having an S—F bond, is used, only the effect of adding a sulfonate salt having an S—F bond is exhibited. Furthermore, when a non-aqueous electrolyte solution containing a sulfonate salt having no unsaturated bonds and a sulfonate salt having an S—F bond is used, the remaining capacity of the non-aqueous electrolyte secondary battery is found to be lower than when a non-aqueous electrolyte solution containing only a sulfonate salt having an S—F bond is used.
These results demonstrate that the nonaqueous electrolyte solution of the present invention can specifically improve battery performance by adding a sulfonate having an unsaturated bond and a sulfonate having an S—F bond together.

Claims (8)

The composition is characterized by comprising a compound (A) represented by formula (I) or formula (II), and one or more salts (B) selected from the group consisting of phosphates having a P—F bond and sulfonates having an S—F bond ,
The nonaqueous electrolyte solution , wherein the phosphate having a P—F bond is at least one selected from the group consisting of alkali metal monofluorophosphates and alkali metal difluorophosphates .

In formula (I), R ¹ , R ² and R ³ each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group, and R ¹ and R ² may be bonded to each other to form a cyclic structure; n represents an integer of 0 to 4; and M ¹ represents a monovalent cation.
In formula (II), ^R4 is a hydrogen atom, a halogen atom, or a hydrocarbon group; n is an integer of 0 to 4; and ^M2 is a monovalent cation. 2. The nonaqueous electrolyte solution according to claim 1, wherein the phosphate having a P—F bond is at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate. 2. The nonaqueous electrolyte solution according to claim 1, wherein the salt (B) selected from the group consisting of a phosphate having a P—F bond and a sulfonate having an S—F bond is a sulfonate having an S—F bond. 4. The nonaqueous electrolyte solution according to claim 1, wherein in formula (I), R ¹ , R ² and R ³ each independently represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aralkyl group having 7 to 18 carbon atoms. The nonaqueous electrolyte solution according to any one of claims 1 to 4, wherein in formula (II), ^R4 is a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aralkyl group having 7 to 18 carbon atoms. The nonaqueous electrolyte solution according to any one of claims 1 to 5 , containing 0.01% by mass or more and 10% by mass or less of the compound (A) represented by formula (I) or formula (II). The nonaqueous electrolyte solution according to any one of claims 1 to 6 , comprising 0.01% by mass or more and 10% by mass or less of the salt (B) selected from the group consisting of phosphates having a P-F bond and sulfonates having an S-F bond. A non-aqueous electrolyte secondary battery comprising: a positive electrode having a positive electrode active material capable of absorbing and releasing metal ions; a negative electrode having a negative electrode active material capable of absorbing and releasing metal ions; and the non-aqueous electrolyte solution according to any one of claims 1 to 7 .