JP2015069705A - Nonaqueous electrolytic solution for secondary batteries, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an incombustible nonaqueous electrolytic solution for secondary batteries which allows a lithium ion secondary battery superior in high-temperature shelf life to be obtained; and a safe lithium ion secondary battery superior in high-temperature shelf life.SOLUTION: A nonaqueous electrolytic solution for secondary batteries comprises: an electrolyte; and a liquid composition. The electrolyte includes at least one lithium salt. The liquid composition includes: at least one fluorine-containing solvent (α) selected from a group consisting of a fluorine-containing ether compound, a fluorine-containing chain carboxylic ester compound and a fluorine-containing chain carbonate compound; a cyclic carboxylic ester compound; and a compound (β) having an unsaturated carbon-nitrogen bond. A lithium ion secondary battery comprises the nonaqueous electrolytic solution for secondary batteries.

Description

  The present invention relates to a non-aqueous electrolyte for a secondary battery and a lithium ion secondary battery.

  In portable electronic devices such as mobile phones and notebook computers, lithium ion secondary batteries having a positive electrode, a negative electrode, and a non-aqueous electrolyte for secondary batteries are widely used. As a solvent for a non-aqueous electrolyte solution of a lithium ion secondary battery, a carbonate-based solvent (ethylene carbonate, dimethyl carbonate) is used because it can exhibit high ionic conductivity by dissolving lithium salt well and has a wide potential window. Etc.) are widely used. However, since the carbonate-based solvent is flammable, there is a risk of ignition due to heat generation of the battery.

Therefore, a nonaqueous electrolytic solution using a fluorine-containing solvent has been proposed as a nonaqueous electrolytic solution excellent in flame retardancy.
For example, non-aqueous electrolytes containing fluorinated solvents, non-fluorinated cyclic carbonates, non-fluorinated cyclic esters and lithium salts are known as flame retardant and having good battery characteristics (cycle characteristics, discharge capacity). (Patent Document 1).
However, as a result of studies by the present inventors, in the lithium ion secondary battery using the non-aqueous electrolyte of Patent Document 1, the discharge capacity tends to decrease and gas tends to be generated when stored at a high temperature. The high temperature storage characteristics were insufficient.

JP 2008-192504 A

  The present invention provides a non-aqueous electrolyte solution for a secondary battery that can obtain a lithium ion secondary battery excellent in high-temperature storage characteristics and is flame retardant. The present invention also provides a lithium ion secondary battery that is excellent in high-temperature storage characteristics and has safety.

In order to achieve the above object, the present invention adopts the following configuration.
[1] A nonaqueous electrolytic solution containing an electrolyte and a liquid composition,
At least one of the electrolytes is a lithium salt;
The liquid composition comprises at least one fluorine-containing solvent (α) selected from the group consisting of a fluorine-containing ether compound, a fluorine-containing chain carboxylic acid ester compound and a fluorine-containing chain carbonate compound, a cyclic carboxylic acid ester compound, And a non-aqueous electrolyte for a secondary battery comprising a compound (β) having a carbon-nitrogen unsaturated bond.
[2] The compound (β) is at least selected from the group consisting of a compound represented by the following formula (5), a compound represented by the following formula (6), and a compound represented by the following formula (7): The non-aqueous electrolyte for secondary batteries according to [1], which is one type.

(In the formula, each R ¹³ is independently an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, a fluorinated alkyl group having 1 to 10 carbon atoms, or a fluorinated group having 3 to 10 carbon atoms. A cycloalkyl group, an unsaturated hydrocarbon group having 2 to 10 carbon atoms having a carbon-carbon unsaturated bond, an alkyl group having 2 to 6 carbon atoms having one or more etheric oxygen atoms, or one or more ethers It is a C2-C6 fluorinated alkyl group having a reactive oxygen atom.
R ¹⁴ and R ¹⁵ are each independently an alkylene group having 1 to 10 carbon atoms, a cycloalkylene group having 3 to 10 carbon atoms, a fluorinated alkylene group having 1 to 10 carbon atoms, or a fluorinated cycloalkylene group having 3 to 10 carbon atoms. An alkylene group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, a fluorinated alkylene group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, or a combination of two or more thereof Or a divalent group having 6 to 10 carbon atoms having an aromatic ring. )
[3] The nonaqueous electrolyte for secondary batteries according to [1] or [2], wherein the ratio of the mass of the compound (β) to the total mass of the nonaqueous electrolyte is 0.01 to 10 mass%. .
[4] The non-secondary battery non-use according to any one of [1] to [3], wherein the ratio of the mass of the fluorinated solvent (α) to the total mass of the nonaqueous electrolytic solution is 30 to 80 mass%. Water electrolyte.
[5] for the total number of moles of lithium atoms from the lithium salt _{(N Li), N A /} N Li is the ratio of total moles _{(N A)} of the cyclic carboxylic acid ester compound is 1.5 to 7. The nonaqueous electrolytic solution for secondary batteries according to any one of [1] to [4], which is 0.
[6] The liquid composition is at least selected from the group consisting of a saturated cyclic carbonate compound, a saturated chain carbonate compound having no fluorine atom, a saturated cyclic sulfone compound (excluding an electrolyte), and a phosphate ester compound. The nonaqueous electrolytic solution for a secondary battery according to any one of [1] to [5], further including one kind of compound (γ).
[7] The total number of moles of the cyclic carboxylic acid ester compound (N _A ) and the total number of moles of the compound (γ) (N _B ) relative to the total number of moles of lithium atoms derived from the lithium salt (N _Li ) The nonaqueous electrolyte for secondary batteries according to [6] above, wherein (N _A + N _B ) / N _Li is a sum ratio of 3.0 to 7.0.
[8] The non-secondary battery non-use according to [6] or [7], wherein a ratio of a mass of the saturated chain carbonate compound having no fluorine atom to a total mass of the non-aqueous electrolyte is 30% by mass or less. Water electrolyte.
[9] The ratio of the total mass of the mass of the saturated cyclic carbonate compound and the mass of the saturated chain carbonate compound having no fluorine atom to the total mass of the non-aqueous electrolyte is 30% by mass or less. 6]-[8] A non-aqueous electrolyte for a secondary battery.
[10] The nonaqueous electrolytic solution for secondary battery according to any one of [1] to [9], wherein the fluorine-containing solvent (α) contains a fluorine-containing ether compound.
[11] The non-aqueous electrolyte for secondary battery according to [10], wherein the fluorine-containing ether compound is at least one selected from the group consisting of compounds represented by the following formula (1).
R ¹ —O—R ² (1)
(In the formula, R ¹ and R ² are each independently an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, a fluorinated alkyl group having 1 to 10 carbon atoms, or 3 to 10 carbon atoms. A fluorinated cycloalkyl group, an alkyl group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, or a fluorinated alkyl group having 2 to 10 carbon atoms having one or more etheric oxygen atoms. , One or both of R ¹ and R ² is a fluorinated alkyl group having 1 to 10 carbon atoms, a fluorinated cycloalkyl group having 3 to 10 carbon atoms, or 2 to 2 carbon atoms having one or more etheric oxygen atoms. 10 fluorinated alkyl groups.)
[12] The secondary of any one of [1] to [11], wherein the fluorine-containing chain carboxylic acid ester compound is at least one selected from the group consisting of compounds represented by the following formula (2): Non-aqueous electrolyte for batteries.

(However, R ³ and R ⁴ are each independently an alkyl group having 1 to 3 carbon atoms or a fluorinated alkyl group having 1 to 3 carbon atoms, and one or both of R ³ and R ⁴ are fluorinated alkyl groups. .)
[13] The non-secondary battery for a battery according to any one of [1] to [12], wherein the cyclic carboxylic acid ester compound is at least one selected from the group consisting of compounds represented by the following formula (4): Water electrolyte.

(However, R ^{7 to} R ¹² are each independently a hydrogen atom, a fluorine atom, a chlorine atom, an alkyl group having 1 to 2 carbon atoms, a fluorinated alkyl group having 1 to 2 carbon atoms, or a carbon having an etheric oxygen atom. (It is an alkyl group having a number of 2 to 3. a is an integer of 0 to 3.)
[14] The non-aqueous secondary battery according to [13], wherein the compound represented by the formula (4) is at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, and ε-caprolactone. Electrolytic solution.
[15] The lithium salt includes LiPF _6, wherein [1] to one of the non-aqueous electrolyte solution for a secondary battery [14].
[16] The non-aqueous electrolyte for secondary batteries according to any one of [1] to [15], wherein the content of the lithium salt in the non-aqueous electrolyte is 0.6 to 1.8 mol / L.
[17] The nonaqueous electrolysis for secondary battery according to any one of [1] to [16], wherein the ratio of the mass of the cyclic carboxylic acid ester compound to the total mass of the nonaqueous electrolyte is 4 to 50 mass%. liquid.
[18] A negative electrode using as an active material at least one selected from the group consisting of a positive electrode having a material capable of inserting and extracting lithium ions as an active material, and a lithium metal, a lithium alloy, and a carbon material capable of inserting and extracting lithium ions. And a non-aqueous electrolyte for a secondary battery according to any one of [1] to [17].

The non-aqueous electrolyte for a secondary battery of the present invention can provide a lithium ion secondary battery excellent in high-temperature storage characteristics and is flame retardant.
In addition, the lithium ion secondary battery of the present invention has excellent high-temperature storage characteristics and safety.

In the present specification, a compound represented by the formula (1) is referred to as a compound (1). The same applies to compounds represented by other formulas.
The following definitions of terms apply throughout this specification and the claims.
The “non-aqueous electrolyte” is an electrolyte that does not substantially contain water, and even if water is included, the water content of the secondary battery using the non-aqueous electrolyte does not deteriorate in performance. It means an electrolyte solution in a range of amounts. The amount of water that can be contained in the non-aqueous electrolyte is preferably 500 ppm by mass or less, more preferably 100 ppm by mass or less, and particularly preferably 50 ppm by mass or less with respect to the total mass of the non-aqueous electrolyte. The lower limit of the moisture content is 0 mass ppm.
The “liquid composition” includes a fluorine-containing solvent (α), a cyclic carboxylic acid ester compound, and a compound (β). The liquid composition may contain a compound (γ). That is, the liquid composition in the present invention comprises only the fluorinated solvent (α), the cyclic carboxylic acid ester compound and the compound (β), or the fluorinated solvent (α), the cyclic carboxylic acid ester compound and the compound. (Β) and a compound (γ).
Lithium salt, fluorine-containing solvent (α), cyclic carboxylic acid ester compound, compound (β) and other compounds (other solvents, additives, etc.) other than compound (γ) are defined as “other components”, A distinction is made from lithium salts and liquid compositions.
The “fluorinated ether compound” means a chain or cyclic compound having an ether bond and having a fluorine atom.
“Fluorine-containing chain carboxylic acid ester compound” means a chain compound having an ester bond in a chain structure, no ring structure containing an ester bond, and having a fluorine atom.
The “fluorine-containing chain carbonate compound” is a fluorine atom having a carbonate bond represented by —O—C (═O) —O— in the chain structure, having no ring structure containing a carbonate bond. Means a chain-like compound having
The “fluorinated alkane compound” means a compound in which one or more hydrogen atoms of an alkane are substituted with fluorine atoms, and hydrogen atoms remain.
The “cyclic carboxylic acid ester compound” means a cyclic compound having an ester bond as a part of the ring skeleton.
“Saturated cyclic carbonate compound” is a ring skeleton composed of carbon atoms and oxygen atoms, and has a carbonate bond represented by —O—C (═O) —O— as a part of the ring skeleton, Means a cyclic compound having no carbon-carbon unsaturated bond.
An “unsaturated cyclic carbonate compound” is a molecule in which a ring skeleton is composed of a carbon atom and an oxygen atom, and has a carbonate bond represented by —O—C (═O) —O— as part of the ring skeleton. It means a cyclic compound having a carbon-carbon unsaturated bond inside.
The “saturated chain carbonate compound having no fluorine atom” has a carbonate bond represented by —O—C (═O) —O— in the chain structure and has a ring structure having a carbonate bond. It means a chain compound having no carbon-carbon unsaturated bond and no fluorine atom in the molecule.
“Fluorinated” and “fluorinated” mean that some or all of the hydrogen atoms bonded to the carbon atom are replaced with fluorine atoms.
The “fluorinated alkyl group” means a group in which part or all of the hydrogen atoms of the alkyl group are substituted with fluorine atoms. Among the partially fluorinated groups are hydrogen atoms and fluorine atoms.
“Carbon-carbon unsaturated bond” means a carbon-carbon double bond or a carbon-carbon triple bond.
“Unsaturated hydrocarbon group” means a hydrocarbon group having a carbon-carbon double bond or a carbon-carbon triple bond.

<Non-aqueous electrolyte for secondary battery>
The non-aqueous electrolyte for secondary batteries of the present invention (hereinafter also simply referred to as non-aqueous electrolyte) is composed of an electrolyte and a liquid composition, and contains other components as necessary.
The lower limit of the ionic conductivity at 25 ° C. of the nonaqueous electrolytic solution is preferably 0.4 S / m. A secondary battery using a non-aqueous electrolyte having an ionic conductivity at 25 ° C. of less than 0.4 S / m has poor output characteristics and poor practicality. If the non-aqueous electrolyte has an ionic conductivity at 25 ° C. of 0.4 S / m or more, the secondary battery has good output characteristics.

[Electrolytes]
At least one of the electrolytes is a lithium salt.
The electrolyte may be only a lithium salt, or a combination of a lithium salt and an electrolyte other than the lithium salt. Examples of electrolytes other than lithium salts include monofluorophosphoric acid, sodium and potassium salts of difluorophosphoric acid, NaPF _{6 and the} like. The lithium salt dissociates in the non-aqueous electrolyte and supplies lithium ions.
Examples of the lithium salt include Li ₂ PO ₃ F, LiPO ₂ F ₂ , LiPF ₆ , the following compound (A) (where k is an integer of 1 to 5), FSO ₂ N (Li) SO ₂ F, CF ₃ SO ₂ N (Li) SO ₂ CF ₃ , CF ₃ CF ₂ SO ₂ N (Li) SO ₂ CF ₂ CF ₃ , CF ₃ CFHSO ₂ N (Li) SO ₂ CFHCF ₃ , LiClO ₄ , the following compound (B) , The following compound (C), the following compound (D), the following compound (E), LiBF _{4 and the} like.
A lithium salt may be used individually by 1 type, and may use 2 or more types together.

The lithium salt contained in the nonaqueous electrolytic solution preferably contains LiPF ₆ .
LiPF ₆ can exhibit high ionic conductivity when dissolved in a solvent having a high solubility, but compared to other lithium salts such as CF ₃ CF ₂ SO ₂ N (Li) SO ₂ CF ₂ CF _3. It is difficult to dissolve in a fluorine-containing solvent. However, the combined use with a cyclic carboxylic acid ester compound improves the solubility of LiPF _{6 in} a fluorine-containing solvent. When LiPF ₆ is uniformly dissolved in the fluorine-containing solvent, it becomes easy to obtain a nonaqueous electrolytic solution having practically sufficient ionic conductivity. In addition, LiPF ₆ is likely to be thermally decomposed and easily reduce the thermal stability of the battery. However, the nonaqueous electrolyte of the present invention includes a cyclic carboxylic acid ester compound, so that even a lithium ion secondary battery using LiPF ₆ can be used. Thermal runaway is less likely to occur.

  Examples of the compound (A) include the following compounds (A-1) to (A-4). From the viewpoint of easily obtaining a nonaqueous electrolytic solution having high ionic conductivity, the compound (A) preferably includes a compound (A-2) in which k is 2, and only a compound (A-2) in which k is 2 More preferably, it consists of.

[Liquid composition]
The liquid composition contains a fluorine-containing solvent (α), a cyclic carboxylic acid ester compound, and a compound (β), and may contain a compound (γ) as necessary. That is, the liquid composition in the present invention comprises only the fluorinated solvent (α), the cyclic carboxylic acid ester compound and the compound (β), or the fluorinated solvent (α), the cyclic carboxylic acid ester compound and the compound. (Β) and a compound (γ).

(Fluorine-containing solvent (α))
The fluorine-containing solvent (α) includes at least one selected from the group consisting of fluorine-containing ether compounds, fluorine-containing chain carboxylic acid ester compounds, and fluorine-containing chain carbonate compounds, and other fluorine-containing solvents (if necessary) However, the fluorine-containing cyclic carbonate compound may be excluded). They are a fluorine-containing ether compound, a fluorine-containing chain carboxylic acid ester compound, and a fluorine-containing chain carbonate compound. These compounds have similar characteristics in terms of chemical stability due to introduction of fluorine atoms, compatibility with other compounds, and the like, and can be treated as equivalent compounds.
The fluorine-containing solvent (α) is a solvent having a fluorine atom in the molecule and is excellent in flame retardancy. A fluorine-containing solvent ((alpha)) may be used individually by 1 type, and may use 2 or more types together. When the number of fluorine-containing solvents (α) is two or more, the ratio can be arbitrarily determined.

Fluorine-containing ether compound:
The fluorine-containing solvent (α) preferably contains a fluorine-containing ether compound from the viewpoint of the solubility of the lithium salt, the flame retardancy, and the ionic conductivity of the nonaqueous electrolytic solution. As the fluorine-containing ether compound, the following compound (1) is preferable from the viewpoint of the solubility of the lithium salt, the flame retardancy, and the ionic conductivity of the nonaqueous electrolytic solution. A fluorine-containing ether compound may be used individually by 1 type, and may use 2 or more types together. When the compound (1) is included, the compound (1) may be used alone or in combination of two or more. When the number of fluorine-containing ether compounds is two or more, the ratio can be arbitrarily determined.

R ¹ —O—R ² (1)
However, R ¹ and R ² are each independently an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, a fluorinated alkyl group having 1 to 10 carbon atoms, or a fluorinated group having 3 to 10 carbon atoms. A cycloalkyl group, an alkyl group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, or a fluorinated alkyl group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, R ¹ And one or both of R ² are fluorinated alkyl groups having 1 to 10 carbon atoms, fluorinated cycloalkyl groups having 3 to 10 carbon atoms, or fluorine having 2 to 10 carbon atoms having one or more etheric oxygen atoms. Alkyl group.

Examples of the alkyl group and the alkyl group having an etheric oxygen atom in the compound (1) include a linear structure, a branched structure, or a group having a partial cyclic structure (for example, a cycloalkylalkyl group).
One or both of R ¹ and R ² are fluorinated alkyl groups having 1 to 10 carbon atoms, fluorinated cycloalkyl groups having 3 to 10 carbon atoms, or 2 to 10 carbon atoms having one or more etheric oxygen atoms. Of the fluorinated alkyl group. When one or both of R ¹ and R ² are these groups, the solubility of the lithium salt in the non-aqueous electrolyte and the flame retardancy are further improved. R ¹ and R ² may be the same or different.

As compound (1), R ¹ and R ² are both a fluorinated alkyl group having 1 to 10 carbon atoms and R ¹ and R ² because R ¹ is excellent in solubility in a liquid composition; ¹ is a fluorinated alkyl group having 2 to 10 carbon atoms having one or more ether oxygen atoms, ^{and R 2} is a fluorinated alkyl group having 1 to 10 carbon atoms and (1-B), ^{R 1} Is a fluorinated alkyl group having 1 to 10 carbon atoms, and R ² is an alkyl group having 1 to 10 carbon atoms. The compound (1-A) and the compound (1-C) are preferably More preferred is compound (1-A).

When the total carbon number of the compound (1) is too small, the boiling point is too low, and when it is too large, 4 to 10 is preferable, and 4 to 8 is more preferable.
When the molecular weight of the compound (1) is too small, the boiling point is too low, and when it is too large, the viscosity is preferably 150 to 800, more preferably 150 to 500, and particularly preferably 200 to 500.
When the compound (1) has an etheric oxygen atom, the number of etheric oxygen atoms in the compound (1) is preferably 1 to 4, more preferably 1 or 2, and even more preferably 1. The number of etheric oxygen atoms in the compound (1) affects flammability.
50 mass% or more is preferable and, as for the fluorine content in a compound (1), 60 mass% or more is more preferable. When the fluorine content in the compound (1) is high, the flame retardancy is excellent. The fluorine content is the ratio of the total mass of fluorine atoms to the molecular weight.

The compound (1) is preferably a compound in which both R ¹ and R ² are alkyl groups in which some of the hydrogen atoms of the alkyl group are fluorinated from the viewpoint of excellent solubility in a lithium salt liquid composition, A compound in which one or both ends of R ¹ and R ² are —CF ₂ H is more preferable.

  Specific examples of the compound (1-A) and the compound (1-B), and specific examples of the fluorinated ether compound other than the compound (1-A) and the compound (1-B) include, for example, WO2009 / And the compounds described in No. 133899.

As the compound (1), CF ₃ CH ₂ OCF ₂ CHF ₂ , CF ₃ CH ₂ has excellent solubility in a liquid composition of lithium salt, excellent flame retardancy, low viscosity, and low boiling point. The group consisting of OCF ₂ CHFCF ₃ , CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , CH ₃ CH ₂ CH ₂ OCF ₂ CHF ₂ , CH ₃ CH ₂ OCF ₂ CHF ₂ , and CHF ₂ CF ₂ CH ₂ OCF ₂ CHFCF ₃ And at least one selected from the group consisting of CF ₃ CH ₂ OCF ₂ CHF ₂ , CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ and CHF ₂ CF ₂ CH ₂ OCF ₂ CHFCF ₃ is particularly preferred. preferable.

Fluorine-containing chain carboxylic acid ester compound:
The fluorine-containing chain carboxylic acid ester compound preferably contains the following compound (2), more preferably only the compound (2), from the viewpoints of viscosity and boiling point. A fluorine-containing chain carboxylic acid ester compound may be used individually by 1 type, and may use 2 or more types together. When the compound (2) is included, the compound (2) may be used alone or in combination of two or more. When the number of fluorine-containing chain carboxylic acid ester compounds is two or more, the ratio can be arbitrarily determined.

However, ^{R 3} and ^{R 4} are each independently an alkyl group having 1 to 3 carbon atoms or a fluorinated alkyl group having 1 to 3 carbon atoms, one or both of ^{R 3} and ^{R 4,} 1 carbon atoms 3 is a fluorinated alkyl group.

Examples of the alkyl group and the fluorinated alkyl group in the compound (2) include a linear structure and a branched structure, respectively.
One or both of R ³ and R ⁴ is a fluorinated alkyl group having 1 to 3 carbon atoms. By making one or both of R ³ and R ⁴ a fluorinated alkyl group having 1 to 3 carbon atoms, the oxidation resistance and flame retardancy of the compound (2) are improved. R ³ and R ⁴ may be the same or different.

R ³ is preferably a methyl group, an ethyl group, a difluoromethyl group, a trifluoromethyl group, a tetrafluoroethyl group, or a pentafluoroethyl group from the viewpoints of viscosity, boiling point, or availability of the compound. A fluoromethyl group is more preferred.
R ⁴ is methyl, ethyl, trifluoromethyl, 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trimethyl from the viewpoint of viscosity, boiling point, or availability of the compound. A fluoroethyl group is preferable, a methyl group, an ethyl group, and a 2,2,2-trifluoroethyl group are more preferable, and a methyl group and an ethyl group are more preferable.

When the total carbon number of the compound (2) is too small, the boiling point is too low, and when it is too large, the viscosity is increased, preferably 3 to 8, more preferably 3 to 6, and further preferably 3 to 5.
When the molecular weight of the compound (2) is too small, the boiling point is too low, and when it is too large, the viscosity is preferably 100 to 300, more preferably 100 to 250, and particularly preferably 100 to 200.
The fluorine content in the compound (2) is preferably 25% by mass or more and more preferably 30% by mass or more from the viewpoint of improving the flame retardancy.

  Specific examples of the compound (2) include acetic acid (2,2,2-trifluoroethyl), methyl difluoroacetate, ethyl difluoroacetate, ethyl trifluoroacetate and the like. From the viewpoint of availability and battery performance such as cycle characteristics, methyl difluoroacetate and ethyl trifluoroacetate are preferable.

Fluorine-containing chain carbonate compound:
The fluorine-containing chain carbonate compound preferably contains the following compound (3) from the viewpoint of viscosity, boiling point, etc., and more preferably consists only of the compound (3). A fluorine-containing chain carbonate compound may be used individually by 1 type, and may use 2 or more types together. When the compound (3) is included, the compound (3) may be used alone or in combination of two or more. When the number of fluorine-containing chain carbonate compounds is two or more, the ratio can be arbitrarily determined.

However, R < ⁵ > and R < ⁶ > is respectively independently a C1-C3 alkyl group or a C1-C3 fluorinated alkyl group, and one or both of R < ⁵ > and R < ⁶ > are C1-C1 3 is a fluorinated alkyl group.

Examples of the alkyl group and the fluorinated alkyl group in the compound (3) include a linear structure and a branched structure, respectively.
One or both of R ⁵ and R ⁶ is a fluorinated alkyl group having 1 to 3 carbon atoms. By making one or both of R ⁵ and R ⁶ a fluorinated alkyl group having 1 to 3 carbon atoms, the solubility and flame retardancy of the lithium salt are improved. R ⁵ and R ⁶ may be the same or different.

The compound (3) is preferably a compound in which both R ⁵ and R ⁶ are fluorinated alkyl groups having 1 to 3 carbon atoms from the viewpoint of viscosity, boiling point, or availability of the compound. R ⁵ and R ⁶ are preferably CF ₃ CH ₂ — or CHF ₂ CF ₂ CH ₂ —.

When the total number of carbon atoms of the compound (3) is too small, the boiling point is too low, and when it is too large, the viscosity is increased to 4 to 10, and 4 to 7 is more preferable.
When the molecular weight of the compound (3) is too small, the boiling point is too low, and when it is too large, the viscosity is preferably 180 to 400, more preferably 200 to 350, and particularly preferably 210 to 300.
The fluorine content in the compound (3) is preferably 25% by mass or more, and more preferably 30% by mass or more from the viewpoint of improving flame retardancy.

  Specific examples of the compound (3) include bis (2,2,2-trifluoroethyl) carbonate, bis (2,2,3,3-tetrafluoropropyl) carbonate and the like. Bis (2,2,2-trifluoroethyl) carbonate is preferred from the viewpoint of battery performance such as viscosity, availability, and output characteristics.

Other fluorine-containing solvents:
The fluorine-containing solvent (α) may contain a fluorine-containing alkane compound as another fluorine-containing solvent. When the fluorine-containing alkane compound is contained, the vapor pressure of the non-aqueous electrolyte is suppressed, and the flame retardancy is further improved.

As the fluorine-containing alkane compound, a fluorine-containing alkane compound having 4 to 12 carbon atoms is preferable. If the fluorine-containing alkane compound has 4 or more carbon atoms, the vapor pressure of the non-aqueous electrolyte is lowered. When the fluorine-containing alkane compound has 12 or less carbon atoms, the lithium salt has good solubility.
The fluorine content in the fluorine-containing alkane compound is preferably 50 to 80% by mass. If the fluorine content in the fluorine-containing alkane compound is 50% by mass or more, the flame retardancy is excellent. If the fluorine content in the fluorine-containing alkane compound is 80% by mass or less, the solubility of the lithium salt is easily maintained.

The fluorinated alkane compounds, compounds of the linear structure is preferable, for _{example, n-C 4 F 9 CH} 2 CH 3, n-C 6 F 13 CH 2 CH 3, n-C 6 F 13 H, n-C ₈ F ₁₇ H and the like. A fluorine-containing alkane compound may be used individually by 1 type, and may use 2 or more types together.

(Cyclic carboxylic acid ester compound)
The liquid composition contains a cyclic carboxylic acid ester compound. By including the cyclic carboxylic acid ester compound, the lithium salt is uniformly dissolved in the fluorine-containing solvent (α). In addition, the inclusion of the cyclic carboxylic acid ester compound makes it difficult for the non-aqueous electrolyte to react with the positive electrode and the negative electrode, thereby preventing thermal runaway in the secondary battery. The cyclic carboxylic acid ester compound may be one kind or two or more kinds.

As the cyclic carboxylic acid ester compound, a saturated cyclic carboxylic acid ester compound having no carbon-carbon unsaturated bond in the molecule is preferable from the viewpoint of stability to the redox reaction.
The cyclic structure in the cyclic carboxylic acid ester compound is preferably a 4- to 10-membered ring, more preferably a 4- to 7-membered ring from the viewpoint of structural stability and viscosity, and a 5- to 6-membered ring from the viewpoint of easy availability. Further preferred is a 5-membered ring. Moreover, 4-8 are preferable and, as for the total carbon number of a cyclic carboxylic acid ester compound, 4-6 are more preferable from an easy point. The cyclic carboxylic acid ester is preferably composed of only a carbon atom, a hydrogen atom and an oxygen atom, and the portion other than the ester bond represented by the —C (═O) —O— bond contained in the ring structure is carbon. More preferably, it consists only of atoms and hydrogen atoms.

The ring structure of the cyclic carboxylic acid ester compound is preferably a ring structure having one ester bond from the viewpoint of viscosity.
The cyclic carboxylic acid ester compound may be a compound in which one or more hydrogen atoms of the alkylene group are substituted with a substituent. Examples of the substituent include a fluorine atom, a chlorine atom, an alkyl group, and a fluorinated alkyl group. The alkyl group preferably has 1 to 2 carbon atoms, and the fluorinated alkyl group has preferably 1 to 2 carbon atoms.

  The cyclic carboxylic acid ester compound preferably contains the following compound (4), and more preferably consists only of the compound (4) from the viewpoints of stability to redox reaction, structural stability, and viscosity. A compound (4) may be used individually by 1 type, and may use 2 or more types together.

Provided that R ^{7 to} R ¹² are each independently a hydrogen atom, a fluorine atom, a chlorine atom, an alkyl group having 1 to 2 carbon atoms, a fluorinated alkyl group having 1 to 2 carbon atoms, or one or more etheric oxygen atoms And an alkyl group having 2 to 3 carbon atoms. a is an integer of 0-3. R ^{7 to} R ¹² may be the same or different.

R ^{7 to} R ¹² are preferably a hydrogen atom, a methyl group, an ethyl group, or a fluorine atom, more preferably a hydrogen atom, a methyl group, or an ethyl group from the viewpoints of stability to redox reaction, viscosity, and availability of the compound. preferable.
a is preferably 1 to 3, more preferably 1, from the viewpoint of viscosity and availability of the compound.

  Examples of the compound (4) include cyclic ester compounds such as γ-butyrolactone, γ-valerolactone, γ-hexanolactone, δ-valerolactone, ε-caprolactone, and carbon atoms forming the ring of the cyclic ester compound. 1 or more of the hydrogen atoms bonded to the carbon atom have a fluorine atom, a chlorine atom, an alkyl group having 1 to 2 carbon atoms, a fluorinated alkyl group having 1 to 2 carbon atoms, or one or more etheric oxygen atoms Examples thereof include compounds substituted with 2 to 3 alkyl groups. The compound (4) is preferably at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone and ε-caprolactone from the viewpoint of easy availability and high thermal runaway suppression effect, and γ-butyrolactone is preferable. Is more preferable.

(Compound β)
The compound (β) is a compound having a carbon-nitrogen unsaturated bond. When the nonaqueous electrolytic solution of the present invention contains the compound (β), a lithium ion secondary battery excellent in high temperature storage characteristics can be obtained.
As the compound (β), one type may be used alone, or two or more types may be used in combination. When there are two or more compounds (β), the ratio can be arbitrarily determined.

When the compound (β) has a carbon-nitrogen triple bond, the compound (β) is preferably a nitrile compound from the viewpoint that a lithium ion secondary battery excellent in high-temperature storage characteristics can be easily obtained. When the compound (β) is a nitrile compound, the number of nitrile groups is preferably 1 or 2.
When the compound (β) has a carbon-nitrogen double bond, the compound (β) is preferably an isocyanate compound. When the compound (β) is an isocyanate compound, the number of isocyanate groups is preferably 1 or 2, and more preferably 2.

  The compound (β) is at least one selected from the group consisting of the following compound (5), the following compound (6) and the following compound (7) from the viewpoint that a lithium ion secondary battery having excellent high-temperature storage characteristics can be easily obtained. Species are preferred.

In the formula, each R ¹³ is independently an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, a fluorinated alkyl group having 1 to 10 carbon atoms, or a fluorinated cyclohexane having 3 to 10 carbon atoms. An alkyl group, an unsaturated hydrocarbon group having 2 to 10 carbon atoms having a carbon-carbon unsaturated bond, an alkyl group having 2 to 6 carbon atoms having one or more etheric oxygen atoms, or one or more etheric groups It is a C2-C6 fluorinated alkyl group having an oxygen atom.
R ¹⁴ and R ¹⁵ are each independently an alkylene group having 1 to 10 carbon atoms, a cycloalkylene group having 3 to 10 carbon atoms, a fluorinated alkylene group having 1 to 10 carbon atoms, or a fluorinated cycloalkylene group having 3 to 10 carbon atoms. An alkylene group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, a fluorinated alkylene group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, or a combination of two or more thereof Or a divalent group having 6 to 10 carbon atoms having an aromatic ring.

Compound (5):
R ¹³ is preferably an alkyl group having 1 to 10 carbon atoms, and more preferably an ethyl group, a propyl group, or an isobutyl group because a lithium ion secondary battery having availability and excellent cycle characteristics is easily obtained.

Specific examples of the compound (5) include propionitrile, butyronitrile, isobutyronitrile and the like.
A compound (5) may be used individually by 1 type, and may use 2 or more types together.

Compound (6):
R ¹⁴ is preferably an alkylene group having 1 to 10 carbon atoms because it is easy to obtain a lithium ion secondary battery having excellent availability and cycle characteristics and has a higher gas generation suppressing effect.

Specific examples of the compound (6) include succinonitrile, glutaronitrile, adiponitrile, sepaconitrile and the like.
As the compound (6), one type may be used alone, or two or more types may be used in combination.

Compound (7):
As R ¹⁵ , a lithium ion secondary battery having availability and excellent cycle characteristics is easily obtained, and since the effect of suppressing gas generation is higher, an alkylene group having 1 to 10 carbon atoms, and 1 to 10 carbon atoms. And a divalent group having 6 to 10 carbon atoms having an aromatic ring.

Specific examples of the compound (7) include hexamethylene diisocyanate, 2,2,4-trimethyl-hexamethylene diisocyanate, 2,4,4-trimethyl-hexamethylene diisocyanate, isophorone diisocyanate, 1,3-bis (isocyanatomethyl). ) Cyclohexane, toluene diisocyanate and the like.
A compound (7) may be used individually by 1 type, and may use 2 or more types together.

(Compound (γ))
Since the liquid composition has excellent lithium salt solubility and ionic conductivity, a saturated cyclic carbonate compound, a saturated chain carbonate compound having no fluorine atom (hereinafter also referred to as a non-fluorine-based saturated chain carbonate compound), It is preferable to further contain at least one compound (γ) selected from the group consisting of a saturated cyclic sulfone compound (excluding the electrolyte) and a phosphate ester compound.

Examples of the saturated cyclic carbonate compound include propylene carbonate (PC), ethylene carbonate (EC), 4-fluoro-1,3-dioxolan-2-one (FEC), 4-trifluoromethyl-1,3-dioxolane- 2-one etc. are mentioned.
Examples of the non-fluorinated saturated chain carbonate compound include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and the like.
Examples of the saturated cyclic sulfone compound include sulfolane and 3-methylsulfolane.
Examples of the phosphoric acid ester compound include trimethyl phosphate, triethyl phosphate, and tris phosphate (2,2,2-trifluoroethyl).

  The liquid composition preferably contains a non-fluorinated saturated chain carbonate compound. When the non-fluorinated saturated chain carbonate compound is included, the viscosity of the non-aqueous electrolyte can be lowered, and the lithium ion diffusion coefficient in the non-aqueous electrolyte and the ionic conductivity of the non-aqueous electrolyte are easily increased.

[Other ingredients]
As long as the nonaqueous electrolyte solution does not impair the effects of the present invention, other than lithium salt, fluorine-containing solvent (α), cyclic carboxylic acid ester compound, compound (β), and compound (γ) as necessary A compound (other solvents, additives, etc.) may be contained.

(Other solvents)
The nonaqueous electrolytic solution may contain a solvent other than the fluorine-containing solvent (α), the cyclic carboxylic acid ester compound, the compound (β), and the compound (γ).

(Additive)
In order to improve the function of the non-aqueous electrolyte, the non-aqueous electrolyte may contain conventionally known additives as necessary. Examples of the additive include an overcharge inhibitor, a dehydrating agent, a deoxidizing agent, a property improving aid, and a surfactant.

Overcharge prevention agent:
Examples of the overcharge inhibitor include aromatic compounds (biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, dibenzofuran, etc.), aromatic Partially fluorinated compounds (2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene, etc.), fluorine-containing anisole compounds (2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole) All). An overcharge inhibitor may be used individually by 1 type, and may use 2 or more types together.

Dehydrating agent:
Examples of the dehydrating agent include molecular sieves, mirabilite, magnesium sulfate, calcium hydride, sodium hydride, potassium hydride, lithium aluminum hydride and the like. As the liquid composition or other solvent used for the non-aqueous electrolyte, those obtained by rectification after dehydration with a dehydrating agent are preferable. Moreover, what performed only the dehydration by the said dehydrating agent without performing rectification may be used.

Property improvement aids:
The characteristic improving aid is for improving capacity maintenance characteristics and cycle characteristics after high temperature storage. Examples of the property improving aid include unsaturated cyclic carbonate compounds (dimethyl vinylene carbonate, vinylene carbonate, vinyl ethylene carbonate, 4-acetylin-1,3-dioxolan-2-one, 3-methyl-4-vinyl ethylene carbonate, 4,5-divinylethylene carbonate, 4,5-bis (2-methylvinyl) ethylene carbonate, etc.), sulfur-containing compounds (ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, Busulfan, sulfolene, dimethylsulfone, diphenylsulfone, methylphenylsulfone, dibutyldisulfide, dicyclohexyldisulfide, tetramethylthiuram monosulfide, N, N-dimethylmethanesulfonamide, N, N-die Methane sulfonamide), hydrocarbon compounds (heptane, octane, cycloheptane and the like), a fluorine-containing aromatic compound (fluorobenzene, difluorobenzene, hexafluorobenzene and the like). A characteristic improvement adjuvant may be used individually by 1 type, and may use 2 or more types together.

Surfactant:
The surfactant assists the impregnation of the non-aqueous electrolyte into the electrode mixture or separator. As the surfactant, any of cationic surfactants, anionic surfactants, nonionic surfactants and amphoteric surfactants may be used. Anionic surfactants are easily available and have a high surfactant effect. Agents are preferred. As the surfactant, a fluorine-containing surfactant is preferable from the viewpoint of high oxidation resistance and good cycle characteristics and rate characteristics. Only one surfactant may be used, or two or more surfactants may be used.

[Ratio of each component]
(Ratio of lithium salt)
The upper limit of the content of the lithium salt in the nonaqueous electrolytic solution is not particularly limited, but is preferably 1.8 mol / L, more preferably 1.6 mol / L, and even more preferably 1.4 mol / L. Although the lower limit of content of lithium salt in a non-aqueous electrolyte is not specifically limited, 0.6 mol / L is preferable, 0.7 mol / L is preferable and 0.8 mol / L is more preferable.
In terms of mass standard, the ratio of the mass of the lithium salt to the total mass of the non-aqueous electrolyte is preferably 5 to 20% by mass, more preferably 7 to 17% by mass, and even more preferably 9 to 14% by mass.
If the ratio of lithium salt is more than the said lower limit, the ionic conductivity of a non-aqueous electrolyte will become high. If the proportion of the lithium salt is not more than the above upper limit value, the lithium salt is easily dissolved uniformly in the liquid composition, and the lithium salt does not precipitate even under low temperature conditions. Further, lithium ions are easily diffused into the non-aqueous electrolyte, and the lithium ion diffusion coefficient is increased.

The nonaqueous electrolytic solution of the present invention preferably contains at least LiPF ₆ as a lithium salt. The lower limit of the molar ratio of LiPF ₆ with respect to the total number of moles of lithium salt contained in the nonaqueous electrolytic solution is preferably 40 mol%, more preferably 50 mol%, still more preferably 65 mol%, and particularly preferably 80 mol%. The upper limit of the molar ratio of LiPF ₆ with respect to the total number of moles of lithium salt contained in the nonaqueous electrolytic solution is 100 mol%. When the molar ratio of LiPF ₆ with respect to the total number of moles of lithium salt is equal to or more than the lower limit value, a nonaqueous electrolytic solution having excellent ion conductivity and high practicality is obtained.

(Ratio of fluorine-containing solvent (α))
The ratio of the mass of the fluorinated solvent (α) to the total mass of the nonaqueous electrolytic solution is not particularly limited, but the lower limit of the mass ratio of the fluorinated solvent (α) to the total mass of the nonaqueous electrolytic solution is 30% by mass. Is preferable, 35 mass% is more preferable, 40 mass% is further more preferable, and 45 mass% is especially preferable. The upper limit of the proportion of the fluorinated solvent (α) is preferably 80% by mass, more preferably 75% by mass, still more preferably 73% by mass, and particularly preferably 70% by mass.
If the ratio of the fluorine-containing solvent (α) is not less than the lower limit, the non-aqueous electrolyte has excellent flame retardancy, small positive electrode reactivity and negative electrode reactivity, hardly causes thermal runaway, and has high voltage resistance. Have. When the proportion of the fluorinated solvent (α) is not more than the above upper limit value, the lithium salt is uniformly dissolved and the lithium salt is unlikely to precipitate at a low temperature.

30-90 mass% is preferable, as for the ratio of the mass of the fluorine-containing solvent ((alpha)) with respect to the total mass of a liquid composition, 35-85 mass% is more preferable, 40-80 mass% is further more preferable, 45-75 mass%. Is particularly preferred.
If the ratio of the fluorine-containing solvent (α) is not less than the lower limit, the non-aqueous electrolyte has excellent flame retardancy, small positive electrode reactivity and negative electrode reactivity, hardly causes thermal runaway, and has high voltage resistance. Have. When the proportion of the fluorinated solvent (α) is not more than the above upper limit value, the lithium salt is uniformly dissolved and the lithium salt is unlikely to precipitate at a low temperature.

The fluorine-containing solvent (α) preferably contains a fluorine-containing ether compound from the viewpoints of the solubility of the lithium salt, the flame retardancy of the non-aqueous electrolyte, and the ionic conductivity.
The ratio of the mass of the fluorinated ether compound to the total mass of the fluorinated solvent (α) is preferably 25 to 100 mass%, more preferably 30 to 100 mass%, more preferably 50 to 100 mass%, and 60 to 100 mass%. % Is more preferable, and 70 to 100% by mass is particularly preferable. The fluorine-containing solvent (α) is most preferably composed of only a fluorine-containing ether compound.

  The ratio of the mass of the fluorinated ether compound to the total mass of the non-aqueous electrolyte is preferably 10 to 80% by mass. The lower limit of the proportion of the fluorine-containing ether compound is more preferably 20% by mass, further preferably 30% by mass, particularly preferably 40% by mass, and most preferably 45% by mass. Further, the upper limit of the proportion of the fluorine-containing ether compound is more preferably 75% by mass, further preferably 73% by mass, and particularly preferably 70% by mass.

  When the fluorine-containing solvent (α) contains a fluorine-containing chain carboxylic acid ester compound, the ratio of the mass of the fluorine-containing chain carboxylic acid ester compound to the total mass of the fluorine-containing solvent (α) is 0.01 to 50% by mass. Is preferable, 0.01-40 mass% is more preferable, 0.01-30 mass% is further more preferable, 0.01-20 mass% is especially preferable.

  When the fluorine-containing solvent (α) contains a fluorine-containing chain carbonate compound, the ratio of the mass of the fluorine-containing chain carbonate compound to the total mass of the fluorine-containing solvent (α) is preferably 0.01 to 50% by mass, 0 0.01-40 mass% is more preferable, 0.01-30 mass% is further more preferable, and 0.01-20 mass% is especially preferable.

When the fluorinated solvent (α) contains a fluorinated alkane compound, the ratio of the mass of the fluorinated alkane compound to the total mass of the nonaqueous electrolytic solution is preferably from 0.01 to 5 mass%.
If the ratio of a fluorine-containing alkane compound is 0.01 mass% or more, it will be excellent in the flame retardance of a non-aqueous electrolyte. When the proportion of the fluorinated alkane is 5% by mass or less, the solubility of the lithium salt is easily maintained.

  When the fluorine-containing solvent (α) is used in combination with one or more fluorine-containing ether compounds and one or more fluorine-containing chain carboxylic acid esters, fluorine-containing chain carbonate compounds, and fluorine-containing alkane compounds, the ratio thereof is arbitrary. I can decide.

(Ratio of cyclic carboxylic acid ester compound)
The ratio of the mass of the cyclic carboxylic acid ester compound to the total mass of the nonaqueous electrolytic solution is preferably 4 to 50 mass%, more preferably 7 to 45 mass%, still more preferably 10 to 40 mass%, and more preferably 15 to 35 mass%. Is particularly preferred.
If the ratio of the cyclic carboxylic acid ester compound is equal to or higher than the lower limit, the non-aqueous electrolyte uniformly dissolves the lithium salt, and the reactivity between the non-aqueous electrolyte and the positive and negative electrodes is small, causing thermal runaway. Hateful. If the ratio of the cyclic carboxylic acid ester compound is not more than the above upper limit value, the non-aqueous electrolyte is excellent in flame retardancy.

Contained in the nonaqueous electrolytic solution, the ratio N _{A /} N _Li of the total number of moles N _A cyclic carboxylic acid ester compound to the total number of moles N _Li of lithium atoms from the lithium salt is not particularly limited, 1.5 7.0 is preferred. The lower limit value of the N _A / N _Li is more preferably 2, more preferably 2.5, and particularly preferably 3. Moreover, 6.5 is preferable, as for the upper limit of said N _A / N _Li , 6 is more preferable, 5 is further more preferable, 4.5 is especially preferable, and 4.2 is the most preferable.
If N _A / N _Li is within the above range, the reactivity between the non-aqueous electrolyte, the positive electrode and the negative electrode can be reduced while the lithium salt is uniformly dissolved to obtain sufficient ionic conductivity for the following reasons. The thermal runaway of the secondary battery can be suppressed.

When a non-aqueous electrolyte is used for a secondary battery, the cyclic carboxylic acid ester compound forms a stable film on the electrode active material, particularly in the positive electrode, and the reaction between the electrode and the non-aqueous electrolyte is suppressed by the film. In connection with this, it is estimated that thermal runaway is suppressed. If N _A / N _Li is equal to or greater than the lower limit, the non-aqueous electrolyte contains a sufficient amount of the cyclic carboxylic acid ester compound, so that the coating is sufficiently formed and the reaction between the electrode and the non-aqueous electrolyte is suppressed. It is considered that a sufficient effect of suppressing thermal runaway can be obtained by sufficiently exhibiting the effect. In addition, the cyclic carboxylic acid ester compound has a high affinity with the lithium salt, and is considered to promote the dissolution of the lithium salt in a solvent. When N _A / N _Li is equal to or higher than the lower limit, the lithium salt is sufficiently dissolved in the solvent, and an electrolytic solution having practically sufficient ionic conductivity is easily obtained. Note that fluorine-containing compounds such as fluorine-containing ether compounds, fluorine-containing chain carboxylic acid ester compounds, and fluorine-containing chain carbonate compounds are considered to have low affinity with lithium salts, and promote dissolution of lithium salts in solvents. The effect of doing this tends to be very small.

The film formed on the electrode active material is considered to be easily dissolved in a highly polar solvent, and even if a film is formed in a solvent having a high polarity, the film formation is likely to be insufficient. Presumed. If N _A / N _Li is not more than the above upper limit, the content of the cyclic carboxylic acid ester compound in the non-aqueous electrolyte does not become excessive, and the polarity of the entire non-aqueous electrolyte is within an appropriate range, It is considered that dissolution of the film formed on the electrode active material hardly occurs. By maintaining a sufficient coating on the electrode active material, an exothermic reaction between the electrode and the non-aqueous electrolyte is unlikely to occur, and thermal runaway is unlikely to occur. Note that fluorine-containing compounds such as a fluorine-containing ether compound, a fluorine-containing chain carboxylic acid ester compound, and a fluorine-containing chain carbonate compound are low in polarity, and thus the effect of dissolving the coating is considered to be very low. Moreover, the flame retardance of a non-aqueous electrolyte improves also by reducing content of the highly flammable cyclic carboxylic acid ester compound.

(Ratio of compound (β))
The lower limit of the ratio of the mass of the compound (β) to the total mass of the nonaqueous electrolytic solution is preferably 0.01% by mass, more preferably 0.1% by mass, and further preferably 0.5% by mass. If the mass ratio of the compound (β) is at least the lower limit value, a lithium ion secondary battery excellent in high-temperature storage characteristics can be easily obtained.
10 mass% is preferable, as for the upper limit of the ratio of the mass of a compound ((beta)) with respect to the total mass of nonaqueous electrolyte solution, 7 mass% is more preferable, 5 mass% is further more preferable, and 3 mass% is especially preferable. When the proportion of the mass of the compound (β) is not more than the upper limit value, the reactivity of the positive electrode and the negative electrode with the non-aqueous electrolyte solution becomes low, and a non-aqueous electrolyte solution that hardly causes thermal runaway is easily obtained.

(Ratio of compound (γ))
When the liquid composition contains the compound (γ), the upper limit value of the mass ratio of the compound (γ) to the total mass of the non-aqueous electrolyte is preferably 30% by mass, more preferably 25% by mass, and 20% by mass. Further preferred. The lower limit of the ratio of the mass of the compound (γ) to the total mass of the non-aqueous electrolyte is 0% by mass.
If the ratio of a compound ((gamma)) is below the said upper limit, it will be easy to suppress reaction with a compound ((gamma)) and an electrode, and the nonaqueous electrolyte solution excellent in stability will be obtained. Moreover, since content of a fluorine-containing solvent ((alpha)) can be increased, the nonaqueous electrolyte solution excellent in the flame retardance is easy to be obtained.

When the liquid composition contains a saturated cyclic carbonate compound, the ratio of the mass of the saturated cyclic carbonate compound to the total mass of the non-aqueous electrolyte is preferably 0.01 to 30% by mass, more preferably 0.01 to 20% by mass. 0.01 to 15 mass% is more preferable, 0.1 to 15 mass% is particularly preferable, and 1 to 15 mass% is most preferable.
If the ratio of a saturated cyclic carbonate compound is below the said upper limit, a saturated cyclic carbonate compound and an electrode will be hard to react, a nonaqueous electrolyte solution is excellent in stability, and is excellent in a flame retardance.

When the liquid composition contains a non-fluorinated saturated chain carbonate compound, the ratio of the mass of the non-fluorinated saturated chain carbonate compound to the total mass of the non-aqueous electrolyte is preferably 0.01 to 30% by mass, and 01-20 mass% is more preferable, 0.01-15 mass% is further more preferable, 0.1-15 mass% is especially preferable, and 1-15 mass% is the most preferable.
If the proportion of the non-fluorinated saturated chain carbonate compound is less than or equal to the above upper limit, the non-fluorinated saturated chain carbonate compound and the electrode are less likely to react, and the non-aqueous electrolyte is excellent in stability and flame retardancy. .

When the liquid composition contains a saturated cyclic carbonate compound and a non-fluorinated saturated chain carbonate compound, the ratio of the total mass of the saturated cyclic carbonate compound and the non-fluorinated saturated chain carbonate compound to the total mass of the non-aqueous electrolyte is: 0.01-30 mass% is preferable, 0.01-20 mass% is more preferable, 0.01-15 mass% is further more preferable, and 1-15 mass% is especially preferable.
If the ratio of the total mass is equal to or less than the upper limit, even when a saturated cyclic carbonate compound and a non-fluorinated saturated chain carbonate compound are used, The dissolution of the coating can be suppressed, the reactivity between them and the electrode can be kept low, and it is easy to obtain a non-aqueous electrolyte with excellent stability. Moreover, it is easy to set it as the non-aqueous electrolyte which has the outstanding flame retardance by restraining content of a combustible compound low.

When the liquid composition contains a phosphate ester compound, the mass ratio of the phosphate ester compound to the total mass of the non-aqueous electrolyte is preferably 0.01 to 5 mass%.
When the proportion of the phosphate ester compound is equal to or less than the upper limit, the phosphate ester compound and the electrode are difficult to react, and the non-aqueous electrolyte is excellent in stability and flame retardancy.

The ratio of the mass of the cyclic carboxylic acid ester compound to the total mass of the cyclic carboxylic acid ester compound and the compound (γ) in the liquid composition is preferably 30 to 100% by mass, more preferably 35 to 100% by mass, and 40 to 100. % By mass is more preferred, 45 to 100% by mass is more preferred, and 50 to 100% by mass is particularly preferred.
When the ratio of the cyclic carboxylic acid ester compound is within the above range, the reactivity between the non-aqueous electrolyte, the positive electrode, and the negative electrode can be reduced, and thermal runaway of the secondary battery can be suppressed.

If the liquid composition comprises compound (gamma), to the total mole number N _Li of lithium atoms from the lithium salt, the total number of moles N _A and the compound of the cyclic carboxylic acid ester compound with total number of moles N _B of (gamma) The sum ratio (N _A + N _B ) / N _Li is preferably 3.0 to 7.0. The lower limit of the _{(N A + N B) /} N Li is more preferably 3.2, 3.5 is more preferable. Furthermore, the _{(N A} + N B) the upper limit of the _{/ N Li} is more preferably 6.5, 6 more preferably, particularly preferably 5.5, 4.5 is most preferred.

The compound (γ) has a high affinity with the lithium salt similarly to the cyclic carboxylic acid ester compound, and is considered to have an effect of promoting dissolution of the lithium salt in a solvent. (N _A + N _B ) / N _Li is not less than the above lower limit, that is, the total amount of the cyclic carboxylic acid ester compound and the compound (γ) considered to have a high lithium salt dissolution promoting effect is a certain amount or more with respect to the amount of the lithium salt. If the lithium salt solubility in the fluorine-containing solvent (α) is improved, the ionic conductivity of the nonaqueous electrolytic solution is improved, and in particular, when a lithium salt such as LiPF ₆ that is difficult to dissolve in the fluorine-based solvent is used. Can be dissolved in a fluorine-based solvent, and practically sufficient ionic conductivity is easily obtained.

When the polarity of the solvent is high, the film of the cyclic carboxylic acid ester compound formed on the electrode active material is dissolved, and the film formation tends to be insufficient. Since the compound (γ) is also highly polar, it is considered that it exhibits an action of dissolving the film. _{(N A + N B) /} N Li is less than the upper limit, that is, if constant less is the lithium salt total amount of the compound cyclic carboxylic acid ester compound showing the effect of dissolving the coating (gamma), the coating It is considered that the solubility is low, and the film formation is difficult to be insufficient. As a result, it is presumed that the reactivity between the non-aqueous electrolyte and the positive electrode and the negative electrode becomes smaller, and thermal runaway of the secondary battery is less likely to occur. Moreover, the flame retardance of a non-aqueous electrolyte is also improved by reducing the content of a highly flammable cyclic carboxylic acid ester compound or compound (γ) in the non-aqueous electrolyte.

In particular, by using a lithium salt containing LiPF ₆ and controlling N _A / N _Li and (N _A + N _B ) / N _Li within the above ranges, practically sufficient ion conductivity and thermal runaway can be achieved. It is easy to obtain a non-aqueous electrolyte that has excellent stability that does not easily occur.

(Ratio of other ingredients)
When the nonaqueous electrolytic solution contains another solvent, the ratio of the total mass of the other solvent and the compound (γ) to the total mass of the nonaqueous electrolytic solution is preferably 0.01 to 30 mass, and preferably 0.1 to 20 The mass% is more preferable. If the ratio of the total mass of the other solvent and the compound (γ) is less than or equal to the above upper limit value, the non-aqueous electrolyte solution that easily suppresses the reaction between the other solvent and the compound (γ) and the electrode and has excellent stability. Is obtained. Moreover, since content of a fluorine-containing solvent ((alpha)) can be increased, the nonaqueous electrolyte solution excellent in the flame retardance is easy to be obtained.

When the non-aqueous electrolyte contains an overcharge inhibitor, the mass ratio of the overcharge inhibitor to the total mass of the non-aqueous electrolyte is preferably 0.01 to 5% by mass.
If the ratio of the overcharge inhibitor is within the above range, it becomes easier to suppress the secondary battery from bursting and firing due to overcharge, and the secondary battery can be used more stably.

  When the non-aqueous electrolyte contains a property improving aid, the ratio of the mass of the property improving aid to the total mass of the non-aqueous electrolyte is preferably 0.01 to 5% by mass.

  When the non-aqueous electrolyte contains a surfactant, the ratio of the mass of the surfactant to the total mass of the non-aqueous electrolyte is preferably 0.05 to 5% by mass, more preferably 0.05 to 3% by mass, 0.05-2 mass% is further more preferable.

[Function and effect]
Since the non-aqueous electrolyte of the present invention described above contains the compound (β), a lithium ion secondary battery having excellent high-temperature storage characteristics can be obtained. The mechanism by which this effect is obtained is not necessarily clear, but it is because the reaction between the positive electrode and the non-aqueous electrolyte is suppressed by the compound (β) adsorbing to the positive electrode and thereby protecting the positive electrode. Conceivable.
Moreover, since the non-aqueous electrolyte of this invention contains a fluorine-containing solvent ((alpha)), it is a flame retardance.
In addition, since the non-aqueous electrolyte of the present invention contains a fluorine-containing solvent (α) and a cyclic carboxylic acid ester compound, the reactivity of the positive and negative electrodes with the non-aqueous electrolyte is small, and the amount of heat generated by these reactions is reduced. Therefore, it is possible to obtain a lithium ion secondary battery that hardly causes thermal runaway.

<Lithium ion secondary battery>
The lithium ion secondary battery of this invention has a positive electrode, a negative electrode, and the non-aqueous electrolyte of this invention.

[Positive electrode]
Examples of the positive electrode include an electrode in which a positive electrode layer including a positive electrode active material, a conductivity-imparting agent, and a binder is formed on a current collector.

(Positive electrode active material)
The positive electrode active material may be any material that can occlude and release lithium ions. As the positive electrode active material, a known positive electrode active material for a lithium ion secondary battery can be employed. For example, a lithium-containing transition metal oxide, a lithium-containing transition metal composite oxide using one or more transition metals, a transition metal Examples thereof include oxides, transition metal sulfides, metal oxides, and olivine type metal lithium salts. A positive electrode active material may be used individually by 1 type, and may use 2 or more types together.

Examples of the lithium-containing transition metal oxide include lithium cobalt oxide (LiCoO ₂ etc.), lithium nickel oxide (LiNiO ₂ etc.), lithium manganese oxide etc. (LiMnO ₂ , LiMn ₂ O ₄ , Li ₂ MnO ₃ etc.). Can be mentioned.

As the metal contained in the lithium-containing transition metal composite oxide, Al, V, Ti, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Yb, and the like are preferable. Examples of the lithium-containing transition metal composite oxide include a lithium ternary composite oxide (Li (Ni _p Co _q Mn _r ) O ₂ (where p, q, r ≧ 0, p + q + r = 1), and the like). Some of the transition metal atoms that are the main components of these lithium transition metal composite oxides are Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, and Yb. And the like substituted with other metals.
Specific examples of the lithium-containing transition metal composite oxide include the following.
LiMn _0.5 Ni _0.5 O ₂ ,
LiMn _1.8 Al _0.2 O ₄ ,
LiNi _0.85 Co _0.10 Al _0.05 O ₂ ,
LiMn _1.5 Ni _0.5 O ₄ ,
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ,
LiMn _1.8 Al _0.2 O ₄ etc.

Examples of the transition metal oxide include TiO ₂ , MnO ₂ , MoO ₃ , V ₂ O ₅ , V ₆ O _{13 and the} like.
Examples of the transition metal sulfide include TiS ₂ , FeS, and MoS ₂ .
Examples of the metal oxide include SnO ₂ and SiO ₂ .

Olivine type metal lithium _{salt, Li L X x Y y O} z F s ( although, X is Fe (II), Co (II ), Mn (II), Ni (II), V (II) or Cu (II And Y is P or Si, and 0 ≦ L ≦ 3, 1 ≦ x ≦ 2, 1 ≦ y ≦ 3, 4 ≦ z ≦ 12, and 0 ≦ s ≦ 1) Or these composites.
Specific examples of the olivine-type metal lithium salt include the following.
LiFePO ₄ ,
Li ₃ Fe ₂ (PO ₄ ) ₃ ,
LiFeP ₂ O ₇ ,
LiMnPO ₄ ,
LiNiPO ₄ ,
LiCoPO ₄ ,
Li ₂ FePO ₄ F,
Li ₂ MnPO ₄ F,
Li ₂ NiPO ₄ F,
Li ₂ CoPO ₄ F,
Li ₂ FeSiO ₄ ,
Li ₂ MnSiO ₄ ,
Li ₂ NiSiO ₄ ,
Li ₂ CoSiO ₄ etc.

A material in which a substance having a composition different from that of the main constituent of the positive electrode active material is attached to the surface of the positive electrode active material can also be used. Surface adhesion substances include oxides (aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, bismuth oxide), sulfate (lithium sulfate, sodium sulfate, potassium sulfate, sulfuric acid) Magnesium, calcium sulfate, aluminum sulfate, etc.), carbonates (lithium carbonate, calcium carbonate, magnesium carbonate, etc.) and the like.
The amount of the surface adhesion substance with respect to the positive electrode active material is preferably 0.1 mass ppm or more and 20 mass% or less, more preferably 1 mass ppm or more and 10 mass% or less, and particularly preferably 10 massppm or more and 5 mass% or less. The surface adhering substance can suppress the oxidation reaction of the nonaqueous electrolytic solution on the surface of the positive electrode active material, and can improve the battery life.

As the positive electrode active material, a lithium-containing composite oxide (LiCoO ₂ , LiNiO ₂ , LiMnO _2, etc.) based on an α-NaCrO ₂ structure, a spinel type, because it has a high discharge voltage and high electrochemical stability. A lithium-containing composite oxide (LiMn ₂ O ₄ or the like) having a structure as a base is preferable.

(Conductivity imparting agent)
Examples of the conductivity-imparting agent include carbon materials, metal substances (such as Al), and conductive oxide powders.

(Binder)
Examples of the binder include a resin binder (such as polyvinylidene fluoride) and a rubber-based binder (such as hydrocarbon rubber and fluororubber).

(Current collector)
Examples of the current collector include a metal thin film mainly composed of Al or the like.

[Negative electrode]
Examples of the negative electrode include an electrode in which a negative electrode layer containing a powdered negative electrode active material, a conductivity-imparting agent, and a binder is formed on a current collector. In addition, when a negative electrode active material can maintain a shape by itself (for example, when it is a lithium metal thin film), a negative electrode can be formed only with a negative electrode active material.

(Negative electrode active material)
Examples of the negative electrode active material include at least one selected from the group consisting of a lithium metal, a lithium alloy, and a carbon material capable of inserting and extracting lithium ions.
Examples of the lithium alloy include a Li—Al alloy, a Li—Pb alloy, and a Li—Sn alloy.
Examples of the carbon material include graphite, coke, and hard carbon.

(Conductivity imparting agent, binder)
As the binder for the negative electrode and the conductivity-imparting agent, the same as those for the positive electrode can be used.

(Current collector)
Examples of the current collector include a metal thin film mainly composed of Cu or the like.

[Separator]
A separator is interposed between the positive electrode and the negative electrode to prevent a short circuit. An example of the separator is a porous film. A non-aqueous electrolyte is used by impregnating the porous membrane. Moreover, you may use as a gel electrolyte what impregnated the porous film with the nonaqueous electrolyte solution, and was made to gelatinize.

As the porous film, one that is stable with respect to the non-aqueous electrolyte and excellent in liquid retention can be used. As a porous film, a porous sheet or a nonwoven fabric is preferable.
Examples of porous membrane materials include fluororesins (polyvinylidene fluoride, polytetrafluoroethylene, copolymers of ethylene and tetrafluoroethylene, etc.), polyimides, polyolefins (polyethylene, polypropylene, etc.), oxidation resistance, air permeability From the viewpoint of availability, polyolefin is preferred.

[Battery exterior]
Examples of the material for the battery outer package include nickel-plated iron, stainless steel, aluminum or an alloy thereof, nickel, titanium, a resin material, and a film material.

[shape]
The shape of the lithium ion secondary battery may be selected according to the application, and may be any shape such as a coin shape, a cylindrical shape, a square shape, and a laminate shape. Moreover, the shape of a positive electrode and a negative electrode can be suitably selected according to the shape of a secondary battery.

[Charging voltage]
The charging voltage of the lithium ion secondary battery of the present invention is preferably 3.4 V or higher, more preferably 4.0 V or higher, and further preferably 4.2 V or higher. When the positive electrode active material is a lithium-containing transition metal oxide, a lithium-containing transition metal composite oxide, a transition metal oxide, a transition metal sulfide, or a metal oxide, the charge voltage is preferably 4.0 V or more, and 4.2 V The above is more preferable. When the positive electrode active material is an olivine type lithium metal salt, the charging voltage is preferably 3.2 V or more, and more preferably 3.4 V or more.

[Function and effect]
The lithium ion secondary battery of the present invention described above is excellent in high temperature storage characteristics because it uses the non-aqueous electrolyte of the present invention. Further, since the non-aqueous electrolyte of the present invention is used, the battery characteristics (cycle characteristics, discharge capacity) are excellent, thermal runaway is unlikely to occur, flame retardancy, and safety are excellent.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by the following description. Examples 1 to 13 are Examples, and Examples 14 to 16 are Comparative Examples.
[Compound]
The compounds and abbreviations used in the examples are as follows.
(Lithium salt)
LPF: LiPF ₆ .

(Fluorine-containing solvent (α))
AE3000: CF ₃ CH ₂ OCF ₂ CHF ₂ (trade name: AE-3000, manufactured by Asahi Glass Co., Ltd.),
_{HFE5510: CHF 2 CF 2 CH 2} OCF 2 CHFCF 3,
_{HFE458: CHF 2 CF 2 CH 2} OCF 2 CHF 2,
MFA: methyl difluoroacetate.

(Cyclic carboxylic acid ester compound)
GBL: γ-butyrolactone.

(Compound (β))
GN: glutaronitrile,
AN: adiponitrile,
CEE: 2-cyanoethyl ether,
BN: butyronitrile,
HDI: hexamethylene diisocyanate.

(Compound (γ))
DMC: dimethyl carbonate.

(Other ingredients)
VC: vinylene carbonate.

[Evaluation method]
(Production of electrode for evaluation)
1. Preparation of electrode for evaluation (negative electrode):
Artificial graphite (4.25 g) and a conductive material, carbon black (manufactured by Denki Kagaku Kogyo Co., Ltd., 0.15 g) are mixed, and a rotating and rotating stirrer (manufactured by Shinky Co., Ltd., Awatori Kentaro AR-E310) The step of stirring for 1 minute at a rotational speed of 2000 rpm was performed 3 times. Subsequently, 1 mass% carboxymethylcellulose aqueous solution (4.25g) was added, and also the process stirred for 5 minutes at the rotation speed of 2000 rpm using the said stirrer was performed twice. Furthermore, the same aqueous carboxymethyl cellulose solution (4.25 g) was added, and the mixture was stirred for 10 minutes at a rotational speed of 2000 rpm using the stirrer. Thereafter, a styrene-butadiene rubber aqueous dispersion latex binder (0.13 g) adjusted to a solid content concentration of 40% by mass was added, and the mixture was stirred for 5 minutes at a rotational speed of 2000 rpm using the stirrer to prepare a slurry for electrode coating. Obtained.
The slurry was applied onto a 20 μm thick copper foil, dried, and then cut into a 5.5 cm × 4.5 cm square to obtain an evaluation electrode (negative electrode).

2. Preparation of electrode for evaluation (positive electrode):
LiCoO ₂ (manufactured by AGC Seimi Chemical Co., Ltd., trade name “Selion C”, 32.0 g) and carbon black (manufactured by Denki Kagaku Kogyo Co., Ltd., trade name “Denka Black”, 0.80 g) are mixed, and the agitator is mixed. The step of stirring for 1 minute at a rotational speed of 2000 rpm was performed 3 times. Next, a step of adding N-methyl-2-pyrrolidone (7.50 g) and stirring for 3 minutes at a rotational speed of 2000 rpm using the stirrer was performed three times. Next, the step of adding N-methyl-2-pyrrolidone (1.0 g) and stirring for 3 minutes at a rotational speed of 2000 rpm using the stirrer was performed three times. Further, an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride (11% by mass, 7.45 g) was added and stirred for 1 minute at a rotational speed of 2000 rpm using the stirrer to obtain a slurry. The slurry was applied to an aluminum foil having a thickness of 20 μm, dried, and then cut into a square of 5.2 cm × 4.2 cm to obtain an evaluation electrode (positive electrode).

(Production of evaluation battery)
The positive electrode and the negative electrode were laminated so as to alternately face each other, and a polyolefin microporous film was present as an electrode separator for evaluation between each electrode to prepare a battery element. After inserting the obtained battery element into a bag made of a laminate film that seems to place importance on both sides of the aluminum foil so that the terminals protrude, the non-aqueous electrolyte shown in Table 1 as a non-aqueous electrolyte, Furthermore, the electrolyte solution (1.0 mL) added so that it might become 2.0 mass% of vinylene carbonate was added, and the battery for evaluation which consists of a LiCoO ₂ electrode-graphite electrode was created by sealing under reduced pressure. The capacity of the created cell was 200 mAh at 4.3 V charge-3.0 discharge.

(Evaluation method)
The volume V1 before storage was measured by immersing the evaluation battery in an ethanol bath.
Thereafter, the battery for evaluation was charged with being sandwiched between resin plates and charged at 25 ° C. with a constant current corresponding to 0.05 C to 3.4 V, and further with a current corresponding to 0.2 C. The battery was charged to .3 V, and further charged until the current value reached 0.02 C at the charge lower limit voltage. Then, it discharged to 3.0V with the electric current corresponding to 0.2C.
From the second cycle to the fourth cycle, the battery was charged to 4.3 V with a current corresponding to 0.2 C, and further charged until the current value reached a current corresponding to 0.02 C at the charge lower limit voltage. Then, it discharged to 3.0V with the electric current corresponding to 0.2C.
In the fifth cycle, the battery was charged to 4.3 V with a current corresponding to 1.0 C, and further charged until the current value reached a current corresponding to 0.02 C at the charge lower limit voltage. Then, after storing for 7 days at 60 ° C., the battery was discharged to 3.0 V at a current corresponding to 0.2 C at 25 ° C., and the discharge capacity was measured to obtain the discharge capacity after storage.
The capacity retention rate was calculated according to the following formula.
(Capacity maintenance rate) = (Discharge capacity after storage) / (Charge capacity at the fifth cycle) × 100
Moreover, the volume V2 after a preservation | save was measured by immersing the battery for evaluation after discharge in an ethanol bath, and the gas generation amount at the time of high temperature preservation was calculated from the volume change (V2-V1) before and after the preservation.

[Example 1]
After diffusing LPF (0.15 g), which is a lithium salt, into AE3000 (1.02 g), which is a fluorine-containing solvent (α), GBL (0.35 g), which is a cyclic carboxylic acid ester compound, and a compound (β ) (0.03 g) was mixed to obtain a uniform solution. Then, VC (0.03g) was added with respect to this solution, and it was set as the non-aqueous electrolyte.

[Examples 2 to 16]
A nonaqueous electrolytic solution was obtained in the same manner as in Example 1 except that the composition of each compound such as lithium salt was changed as shown in Tables 1 and 2.
Tables 1 and 2 show the results of capacity retention rate and gas generation amount in each example.

As shown in Table 1 and Table 2, in Examples 1 to 13 using the nonaqueous electrolytic solution containing the fluorine-containing solvent (α), the compound (β) and the cyclic carboxylic acid ester compound, the capacity retention rate after storage at 60 ° C. In addition, the amount of gas generated was small and the high temperature storage characteristics were excellent.
On the other hand, in Examples 14 to 16 using a non-aqueous electrolyte containing a fluorine-containing solvent (α) and a cyclic carboxylic acid ester compound but not containing the compound (β), compared with Examples 1 to 13 after storage at 60 ° C. The capacity retention rate was low, the amount of gas generated was large, and the high temperature storage characteristics were inferior.

The nonaqueous electrolytic solution of the present invention is useful as a nonaqueous electrolytic solution for a lithium ion secondary battery.
The lithium ion secondary battery of the present invention includes a mobile phone, a portable game machine, a digital camera, a digital video camera, an electric tool, a notebook computer, a portable information terminal, a portable music player, an electric vehicle, a hybrid vehicle, a train, an aircraft, an artificial It can be applied to various uses such as satellites, submarines, ships, uninterruptible power supplies, robots, and power storage systems. The lithium ion secondary battery of the present invention is particularly useful as a large secondary battery for electric vehicles, hybrid vehicles, trains, aircraft, satellites, submarines, ships, uninterruptible power supplies, robots, power storage systems, and the like. is there.

Claims (18)

A non-aqueous electrolyte containing an electrolyte and a liquid composition,
At least one of the electrolytes is a lithium salt;
The liquid composition comprises at least one fluorine-containing solvent (α) selected from the group consisting of a fluorine-containing ether compound, a fluorine-containing chain carboxylic acid ester compound and a fluorine-containing chain carbonate compound, a cyclic carboxylic acid ester compound, And a non-aqueous electrolyte for a secondary battery comprising a compound (β) having a carbon-nitrogen unsaturated bond. The compound (β) is at least one selected from the group consisting of a compound represented by the following formula (5), a compound represented by the following formula (6), and a compound represented by the following formula (7): The nonaqueous electrolyte solution for secondary batteries according to claim 1.

(In the formula, each R ¹³ is independently an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, a fluorinated alkyl group having 1 to 10 carbon atoms, or a fluorinated group having 3 to 10 carbon atoms. A cycloalkyl group, an unsaturated hydrocarbon group having 2 to 10 carbon atoms having a carbon-carbon unsaturated bond, an alkyl group having 2 to 6 carbon atoms having one or more etheric oxygen atoms, or one or more ethers It is a C2-C6 fluorinated alkyl group having a reactive oxygen atom.
R ¹⁴ and R ¹⁵ are each independently an alkylene group having 1 to 10 carbon atoms, a cycloalkylene group having 3 to 10 carbon atoms, a fluorinated alkylene group having 1 to 10 carbon atoms, or a fluorinated cycloalkylene group having 3 to 10 carbon atoms. An alkylene group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, a fluorinated alkylene group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, or a combination of two or more thereof Or a divalent group having 6 to 10 carbon atoms having an aromatic ring. )   The non-aqueous electrolyte for secondary batteries according to claim 1 or 2, wherein a ratio of the mass of the compound (β) to the total mass of the non-aqueous electrolyte is 0.01 to 10% by mass.   The nonaqueous electrolysis for a secondary battery according to any one of claims 1 to 3, wherein the ratio of the mass of the fluorinated solvent (α) to the total mass of the nonaqueous electrolytic solution is 30 to 80 mass%. liquid. N _A / N _Li which is a ratio of the total number of moles (N _A ) of the cyclic carboxylic acid ester compound to the total number of moles (N _Li ) of lithium atoms derived from the lithium salt is 1.5 to 7.0. The non-aqueous electrolyte for secondary batteries as described in any one of Claims 1-4.   The liquid composition is at least one selected from the group consisting of a saturated cyclic carbonate compound, a saturated chain carbonate compound having no fluorine atom, a saturated cyclic sulfone compound (excluding an electrolyte), and a phosphate ester compound. The nonaqueous electrolytic solution for a secondary battery according to any one of claims 1 to 5, further comprising a compound (γ). The sum of the total number of moles of the cyclic carboxylic acid ester compound (N _A ) and the total number of moles of the compound (γ) (N _B ) relative to the total number of moles of lithium atoms derived from the lithium salt (N _Li ). The nonaqueous electrolytic solution for a secondary battery according to claim 6, wherein the ratio (N _A + N _B ) / N _Li is 3.0 to 7.0.   The nonaqueous electrolyte for secondary batteries according to claim 6 or 7, wherein a ratio of a mass of the saturated chain carbonate compound having no fluorine atom to a total mass of the nonaqueous electrolyte is 30% by mass or less.   The ratio of the total mass of the mass of the saturated cyclic carbonate compound and the mass of the saturated chain carbonate compound having no fluorine atom with respect to the total mass of the nonaqueous electrolytic solution is 30% by mass or less. The nonaqueous electrolyte for secondary batteries as described in any one of these.   The nonaqueous electrolytic solution for a secondary battery according to any one of claims 1 to 9, wherein the fluorine-containing solvent (α) contains a fluorine-containing ether compound. The non-aqueous electrolyte for secondary batteries according to claim 10, wherein the fluorine-containing ether compound is at least one selected from the group consisting of compounds represented by the following formula (1).
R ¹ —O—R ² (1)
(In the formula, R ¹ and R ² are each independently an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, a fluorinated alkyl group having 1 to 10 carbon atoms, or 3 to 10 carbon atoms. A fluorinated cycloalkyl group, an alkyl group having 2 to 10 carbon atoms having one or more etheric oxygen atoms, or a fluorinated alkyl group having 2 to 10 carbon atoms having one or more etheric oxygen atoms. , One or both of R ¹ and R ² is a fluorinated alkyl group having 1 to 10 carbon atoms, a fluorinated cycloalkyl group having 3 to 10 carbon atoms, or 2 to 2 carbon atoms having one or more etheric oxygen atoms. 10 fluorinated alkyl groups.) The secondary battery according to any one of claims 1 to 11, wherein the fluorine-containing chain carboxylic acid ester compound is at least one selected from the group consisting of compounds represented by the following formula (2). Non-aqueous electrolyte.

(However, R ³ and R ⁴ are each independently an alkyl group having 1 to 3 carbon atoms or a fluorinated alkyl group having 1 to 3 carbon atoms, and one or both of R ³ and R ⁴ are fluorinated alkyl groups. .) The non-aqueous electrolysis for a secondary battery according to any one of claims 1 to 12, wherein the cyclic carboxylic acid ester compound is at least one selected from the group consisting of compounds represented by the following formula (4). liquid.

(However, R ^{7 to} R ¹² are each independently a hydrogen atom, a fluorine atom, a chlorine atom, an alkyl group having 1 to 2 carbon atoms, a fluorinated alkyl group having 1 to 2 carbon atoms, or a carbon having an etheric oxygen atom. (It is an alkyl group having a number of 2 to 3. a is an integer of 0 to 3.)   The nonaqueous electrolytic solution for a secondary battery according to claim 13, wherein the compound represented by the formula (4) is at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, and ε-caprolactone. . The lithium salt includes LiPF _6, a nonaqueous electrolyte solution for a secondary battery according to any one of claims 1 to 14.   The nonaqueous electrolyte for secondary batteries according to any one of claims 1 to 15, wherein the content of the lithium salt in the nonaqueous electrolyte is 0.6 to 1.8 mol / L.   The nonaqueous electrolyte for secondary batteries as described in any one of Claims 1-16 whose ratio of the mass of the said cyclic carboxylic acid ester compound with respect to the total mass of the said nonaqueous electrolyte is 4-50 mass%.   A positive electrode having a material capable of inserting and extracting lithium ions as an active material, a negative electrode having at least one selected from the group consisting of lithium metal, a lithium alloy, and a carbon material capable of inserting and extracting lithium ions as an active material; Item 18. A lithium ion secondary battery comprising: the non-aqueous electrolyte for secondary battery according to any one of items 1 to 17.