JP2002280060A - Nonaqueous electrolytic solution and lithium secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic solution and a lithium secondary battery having flame retardancy (self extinguishability) and excellent in charging and discharging characteristics. SOLUTION: This lithium secondary battery is furnished with the nonaqueous electrolytic solution which is a nonaqueous electrolytic solution for the lithium secondary battery made as lithium salt is dissolved in a nonaqueous solvent and contains phosphoric ester, five-membered ring thiol lactone and/or five- membered ring sultone and the nonaqueous electrolytic solution made by dissolving a positive electrode containing a compound free to occlude and discharge lithium, a negative electrode including at least one kind selected from carbide, lithium metal and lithium alloy free to occlude and discharge lithium and the lithium salt in the nonaqueous solvent, and the nonaqueous solvent contains phosphoric ester, five-membered ring thiol lactone and/or five-membered ring sultone.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte and a lithium secondary battery using the same. The non-aqueous electrolyte of the present invention has flame retardancy (self-extinguishing), and a secondary battery using the non-aqueous electrolyte exhibits excellent charge / discharge characteristics.

[0002]

2. Description of the Related Art A carbon material such as graphite is used as a negative electrode active material.
LiCoO ₂ , LiNiO ₂ , L
A lithium secondary battery using a lithium transition metal composite oxide such as iMn ₂ O ₄ is rapidly growing as a new small secondary battery having a high voltage of 4V class and a high energy density. Such lithium secondary batteries generally use a lithium salt in a non-aqueous solvent obtained by mixing a low-viscosity solvent such as dimethyl carbonate or diethyl carbonate with a high dielectric constant solvent such as ethylene carbonate or propylene carbonate as an electrolytic solution. The dissolved one is used.

[0003] In these lithium secondary batteries using an organic non-aqueous electrolyte, when the electrolyte leaks due to damage to the battery or an increase in pressure inside the battery due to some cause,
There is a risk of flammable combustion. Therefore, studies on adding a flame retardant to an organic non-aqueous electrolyte to impart flame retardancy have been energetically advanced. It is known to use phosphate esters as flame-retardant electrolytes for lithium batteries. For example, JP-A-58-206078 and JP-A-60-239
No. 73, JP-A-61-227377, JP-A-61-284070 and JP-A-4-184870.
The publication discloses the use of an O = P (OR) ₃ type phosphate such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, and tris (2-chloroethyl) phosphate. Further, Japanese Patent Application Laid-Open No. 8-88023 discloses a self-extinguishing electrolytic solution in which at least one of the above R is a halogen-substituted alkyl.

However, among the phosphoric esters used in these, electrolytic solutions containing trimethyl phosphate exhibit excellent flame retardancy, but have the disadvantage that they are liable to reductive decomposition depending on the material of the negative electrode. . Therefore, in order to enhance the manifestation of the flame retardant effect, it is necessary to increase the blending amount in the electrolyte solution.However, when natural graphite or artificial graphite is used as the negative electrode, the charge / discharge characteristics of the battery, for example, the charge / discharge efficiency and discharge The capacity cannot satisfy the characteristics generally required recently.

A phosphate ester having a halogen atom such as chlorine or bromine in a molecule is inferior in oxidation-reduction resistance, and is insufficient when applied to a 4V class secondary battery which generates a high voltage. A battery having charge / discharge characteristics cannot be obtained. Furthermore, a trace amount of free halogen ions present as an impurity in these phosphate esters corrode aluminum used as a positive electrode current collector and cause deterioration of battery characteristics.

Japanese Patent Application Laid-Open No. 4-184870 discloses that a cyclic phosphate ester is used as an electrolytic solution. Further, Japanese Patent Application Laid-Open No. 11-67267 discloses that cyclic phosphate esters 20 to 55 are used. Disclosed is an electrolyte for a lithium battery in which a volume% is used in combination with a cyclic carbonate. However, in order to make this type of electrolyte sufficiently flame-retardant, it is necessary to incorporate 20% by volume or more of a cyclic phosphate ester. There is a disadvantage that it decreases.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made to solve such problems, and is provided with flame retardancy (self-extinguishing properties), excellent charge / discharge characteristics, and safety. An object of the present invention is to provide a lithium secondary battery having both reliability and reliability.

[0008]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and as a result, obtained by dissolving a lithium salt in a nonaqueous solvent containing a specific component. The present inventors have found that a secondary battery in which the above-mentioned problems have been solved can be realized by using a liquid, and have completed the present invention.

That is, the gist of the present invention is to provide a non-aqueous electrolyte for a lithium secondary battery in which a lithium salt is dissolved in a non-aqueous solvent, wherein the non-aqueous solvent is a phosphate ester, a five-membered thiol lactone and / or Or a non-aqueous electrolyte solution containing a five-membered sultone. Further, another gist of the present invention is that a positive electrode including a compound capable of absorbing and releasing lithium, a carbonaceous material capable of absorbing and releasing lithium, a negative electrode including at least one selected from lithium metal and a lithium alloy, and a lithium salt are provided. A lithium secondary battery comprising a non-aqueous electrolyte dissolved in a non-aqueous solvent, wherein the non-aqueous solvent contains a phosphate ester and a five-membered thiol lactone and / or a five-membered sultone. Lithium secondary battery.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. (Non-Aqueous Electrolyte) The phosphate ester used in the non-aqueous electrolyte of the present invention is not particularly limited, but may be a compound represented by the following general formula (III) or a compound represented by the following general formula (IV). Preferably it is.

[0011]

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Wherein R ₇ , R ₈ and R ₉ each represent an alkyl group having 1 to 4 carbon atoms which may be substituted by a fluorine atom, provided that R ₇ , R ₈ and R _{9 have} the same number of carbon atoms. Is 3 to 12)

[0013]

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Wherein R ₁₀ represents an alkyl group having 1 to 4 carbon atoms which may be substituted by a fluorine atom, and R ₁₁ represents an alkylene group having 2 to 8 carbon atoms. , R _7, R ₈ and R ₉ are each independently of 1 to 4 carbon atoms, a straight-chain or branched, alkyl group substituted with an alkyl group or a fluorine atom. However, the total number of carbon atoms of R ₇ , R ₈ and R ₉ is 3
-12, preferably 3-7. And R ₇ , R ₈
And when R ₉ is an alkyl group, specific examples thereof include a methyl group, an ethyl group, a propyl group and a butyl group.

When R ₇ , R ₈ and R ₉ are a fluorine-substituted alkyl group, specific examples include a trifluoroethyl group, a pentafluoropropyl group, a hexafluoroisopropyl group and a heptafluorobutyl group. Can be mentioned. Specific examples of the phosphoric ester of the general formula (III) include, for example, trimethyl phosphate, triethyl phosphate, dimethylethyl phosphate, dimethylpropyl phosphate, dimethylbutyl phosphate, diethylmethyl phosphate, and dipropyl phosphate. Methyl, dibutyl methyl phosphate, methyl ethyl propyl phosphate, methyl ethyl butyl phosphate, methyl propyl butyl phosphate and the like.

Specific examples of the fluorine-substituted phosphate ester include, for example, trifluoroethyldimethyl phosphate,
Pentafluoropropyldimethyl phosphate, heptafluorobutyldimethyl phosphate, trifluoroethylmethylethyl phosphate, pentafluoropropylmethylethyl phosphate, heptafluorobutylmethylethyl phosphate, trifluoroethylmethylpropyl phosphate, pentafluorophosphate Propylmethylpropyl, heptafluorobutylmethylpropyl phosphate, trifluoroethylmethylbutyl phosphate, pentafluoropropylmethylbutyl phosphate,
Heptafluorobutyl methyl butyl phosphate, trifluoroethyl diethyl phosphate, pentafluoropropyl diethyl phosphate, heptafluorobutyl diethyl phosphate, trifluoroethyl ethyl propyl phosphate, pentafluoropropyl ethyl propyl phosphate, heptafluorobutyl phosphate Ethylpropyl, trifluoroethylethylbutyl phosphate, pentafluoropropylethylbutyl phosphate, heptafluorobutylethylbutyl phosphate, trifluoroethyldipropyl phosphate, pentafluoropropyldipropyl phosphate, heptafluorobutyldipropyl phosphate , Trifluoroethyl propyl butyl phosphate, pentafluoropropyl propyl butyl phosphate, heptafluorobutyl propyl butyl phosphate, trifluoroethyl dibutyl phosphate Pentafluoropropyl dibutyl phosphate, heptafluorobutyl dibutyl, and the like.

Among these, trimethyl phosphate, triethyl phosphate, dimethylethyl phosphate, dimethylpropyl phosphate, methyl diethyl phosphate, trifluoroethyl dimethyl phosphate, pentafluoropropyl dimethyl phosphate, trifluoroethyl methyl ethyl phosphate , Pentafluoropropylmethylethyl phosphate, trifluoroethylmethylpropyl phosphate, pentafluoropropylmethylpropyl phosphate is preferred, especially trimethyl phosphate,
Dimethylethyl phosphate, dimethylpropyl phosphate, methyldiethyl phosphate, and trifluoroethyldimethylphosphate are preferred.

These may be used alone or in combination of two or more. In the general formula (IV), R ₁₀ represents a linear or branched, preferably linear, alkyl group or fluorine-substituted alkyl group having 1 to 4 carbon atoms, preferably 1 to 2 carbon atoms, preferably an alkyl group. Represent. Specific examples thereof include a methyl group, an ethyl group, a propyl group, a butyl group, a trifluoroethyl group, a pentafluoropropyl group, a hexafluoroisopropyl group, and a heptafluorobutyl group. Of these, a methyl group and an ethyl group are preferred.

R ₁₁ has 2 to 8 carbon atoms, preferably 2 carbon atoms.
Represents a straight-chain or branched, more preferably a straight-chain alkylene group having 2 carbon atoms. Specific examples thereof include an ethylene group, a propylene group, a trimethylene group, a butylene group, a tetramethylene group, a 1,1-dimethylethylene group, a pentamethylene group, a 1,1,2-trimethylethylene group, a hexamethylene group, and a tetramethylene group. Examples include a methylethylene group, a heptamethylene group, and an octamethylene group. Of these, an ethylene group is preferred.

In the compound of the general formula (IV), R ₁₁
There an ethylene group, R ₁₀ is preferably a group selected from methyl group, ethyl group and trifluoroethyl group. Specific examples of the cyclic phosphate of the general formula (IV) in which R ₁₀ is an alkyl group include ethylene methyl phosphate, ethylene ethyl phosphate, ethylene-n-propyl phosphate, ethylene isopropyl phosphate, and phosphoric acid. Ethylene-n-butyl, ethylene-sec-butyl phosphate, ethylene-t-butyl phosphate, propylene methyl phosphate, propylene ethyl phosphate, propylene-n-propyl phosphate, propylene isopropyl phosphate, propylene-n phosphate -Butyl, propylene phosphate-sec-butyl, propylene-t-butyl phosphate, trimethylene methyl phosphate, trimethylene ethyl phosphate, trimethylene phosphate-
n-propyl, trimethylene isopropyl phosphate, trimethylene-n-butyl, trimethylene-s
ec-butyl, trimethylene-t-butyl, butylene methyl phosphate, butylene ethyl phosphate, butylene-n-propyl, butylene isopropyl phosphate, butylene-n-butyl, butylene phosphate-sec-
Butyl, butylene-t-butyl phosphate, isobutylene methyl phosphate, isobutylene ethyl phosphate, isobutylene-n-butyl phosphate, isobutylene-sec-butyl, isobutylene-t-butyl phosphate, tetramethylene methyl phosphate, Tetramethyleneethyl phosphate, tetramethylene-n-propyl phosphate, tetramethyleneisopropyl phosphate, tetramethylene-n-butyl phosphate, tetramethylene-sec-butyl, tetramethylene-t-butyl, phosphoric acid Pentamethylene methyl, pentamethyleneethyl phosphate, pentamethylene-n
-Propyl, pentamethyleneisopropyl phosphate, pentamethylene-n-butyl phosphate, pentamethylene-sec-butyl, pentamethylene-t-butyl phosphate, trimethylethylenemethyl phosphate, trimethylethyleneethyl phosphate, phosphoric acid Trimethylethylene-n-propyl, trimethylethyleneisopropyl phosphate, trimethylethylene-n-butyl phosphate, trimethylethylene-sec-butyl phosphate, trimethylethylene-t-butyl phosphate, hexamethylenemethyl phosphate, hexamethylene phosphate Ethyl, hexamethylene-n-propyl, hexamethyleneisopropyl, hexamethylene-n-butyl, hexamethylene-se
c-butyl, hexamethylene-t-butyl phosphate, tetramethylethylene methyl phosphate, tetramethylethylene ethyl phosphate, tetramethylethylene-n-propyl phosphate, tetramethylethylene isopropyl phosphate, tetramethylethylene phosphate n-butyl, tetramethylethylene phosphate-sec-butyl, tetramethylethylene-t-butyl phosphate, heptamethylenemethyl phosphate, heptamethyleneethyl phosphate, heptamethylene-n
-Propyl, heptamethylene-isopropyl phosphate, heptamethylene-n-butyl phosphate, heptamethylene-sec-butyl phosphate, heptamethylene-t-butyl phosphate, octamethylenemethyl phosphate, octamethyleneethyl phosphate, phosphoric acid Octamethylene-n-propyl, octamethyleneisopropyl phosphate, octamethylene-n-butyl phosphate, octamethylene-sec-butyl phosphate, octamethylene-t-butyl phosphate and the like can be mentioned. Among them, ethylene methyl phosphate and ethylene ethyl phosphate are preferable.

Specific examples of the fluorine-substituted cyclic phosphate of the general formula (IV) wherein R ₁₀ is a fluorine-substituted alkyl group include ethylene trifluoroethyl phosphate,
Ethylene pentafluoropropyl phosphate, ethylene hexafluoroisopropyl phosphate, ethylene heptafluorobutyl phosphate, propylene trifluoroethyl phosphate, propylene pentafluoropropyl phosphate, propylene hexafluoroisopropyl phosphate, propylene heptafluorobutyl phosphate, phosphorus Trimethylene trifluoroethyl phosphate, trimethylene pentafluoropropyl phosphate, trimethylene hexafluoroisopropyl phosphate, trimethylene heptafluorobutyl phosphate, butylene trifluoroethyl phosphate, butylene pentafluoropropyl phosphate, butylene hexafluorophosphate Isopropyl, butylene heptafluorobutyl phosphate, tetramethylene trifluoroethyl phosphate, tetramethylene pentafluorophosphate Pills, phosphate tetramethylene hexafluoroisopropyl, tetramethylene heptafluorobutyl phosphate, dimethyl ethylene trifluoroethyl phosphate, dimethyl ethylene pentafluoropropyl, phosphate dimethylethylene hexafluoroisopropyl, dimethylethylene heptafluorobutyl phosphoric acid,
Pentamethylene trifluoroethyl phosphate, pentamethylene pentafluoropropyl phosphate, pentamethylene hexafluoroisopropyl phosphate, pentamethylene heptafluorobutyl phosphate, trimethylethylene trifluoroethyl phosphate, trimethylethylene pentafluoropropyl phosphate, phosphoric acid Trimethylethylene hexafluoroisopropyl, trimethylethylene heptafluorobutyl phosphate, hexamethylene trifluoroethyl phosphate, hexamethylene pentafluoropropyl phosphate, hexamethylene hexafluoroisopropyl phosphate, hexamethylene heptafluorobutyl phosphate, tetramethyl phosphate Ethylene trifluoroethyl, tetramethylethylene pentafluoropropyl phosphate, tetramethylethylene hexaphosphate Fluoroisopropyl, tetramethylethylene heptafluorobutyl phosphate, heptamethylene trifluoroethyl phosphate, heptamethylene pentafluoropropyl phosphate, heptamethylene hexafluoroisopropyl phosphate, heptamethylene heptafluorobutyl phosphate, octamethylene triphosphate Fluoroethyl,
Octamethylenepentafluoropropyl phosphate, octamethylenehexafluoroisopropyl phosphate, octamethyleneheptafluorobutyl phosphate and the like can be mentioned. Among them, ethylene trifluoroethyl phosphate is preferred.

The proportion of the phosphate ester in the non-aqueous solvent used in the present invention is usually 9 to 50% by volume, preferably 15 to 40% by volume. In the present invention, the non-aqueous solvent containing a phosphate ester preferably contains at least one organic solvent selected from carbonates, lactones, ethers, sulfolanes and dioxolanes. Carbonates, lactones, ethers,
In both sulfolane and dioxolane,
The compounds are not limited, and known compounds can be used.

For example, carbonates include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethylene glycol dimethyl carbonate, propylene glycol dimethyl carbonate, ethylene glycol diethyl carbonate, vinylene carbonate and the like. Among these, ethylene carbonate, propylene carbonate, dimethyl carbonate,
Diethyl carbonate and methyl ethyl carbonate are preferred.

Examples of the lactones include γ-butyrolactone, γ-valerolactone, γ-caprolactone, γ
-Octanolactone, β-butyrolactone, δ-valerolactone, ε-caprolactone and the like. Among these, γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone are preferred.

Examples of the ethers include tetrahydrofuran, 2-methyltetrahydrofuran and 1,4-dioxane. Examples of sulfolane include sulfolane, and examples of dioxolane include 1,3-dioxolan. The non-aqueous solvent used in the present invention contains a 5-membered thiol lactone and / or a 5-membered sultone.

The five-membered thiol lactone is a cyclic thiol ester having a thiol ester group (—CO—S—) in the five-membered ring, and its type is not particularly limited. Preferably, the compound is represented.

[0027]

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(In the formula, the bond not indicated by a carbon atom having a deficient number of bonds is a part of a double bond or is bonded to a hydrogen atom, and R ₁ , R ₂ and R ₃ each represent A hydrogen atom, an aliphatic hydrocarbon group or an aromatic hydrocarbon group, which may be bonded to each other to form a ring, or the ring may be an aromatic ring. In the carbon atom in the five-membered ring does not indicate all of the bonds corresponding to the number of bonds, but the bonds not shown are part of a double bond or are bonded to a hydrogen atom, The five-membered ring is a saturated ring or an unsaturated ring.

In the general formula (I), R ₁ , R ₂ and R
₃ is an aliphatic hydrocarbon group or an aromatic hydrocarbon group,
They may combine with each other to form a ring, and the ring may be an aromatic ring. Specific examples of the five-membered thiol lactone represented by the general formula (I) include γ-thiobutyrolactone, naphtho [1,8-B
C] thiophen-2-one and the like.

The five-membered ring sultone is a cyclic sulfonic acid ester having a sulfonic acid ester group (—SO ₂ —O—) in a five-membered ring, and its type is not particularly limited. Compounds represented by II) are preferred.

[0031]

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(In the formula, the carbon atom having a deficient number of carbon atoms without a bond is a part of a double bond or is bonded to a hydrogen atom, and R ₄ , R ₅ and R ₆ each represent A hydrogen atom, an aliphatic hydrocarbon group or an aromatic hydrocarbon group, which may be bonded to each other to form a ring, or the ring may be an aromatic ring). The notation of the bond of the carbon atom in the five-membered ring in is the same as that described for the general formula (I).

In the general formula (II), R ₄ , R ₅ and R
₆ is an aliphatic hydrocarbon group or an aromatic hydrocarbon group,
They may combine with each other to form a ring, and the ring may be an aromatic ring. Such a general formula (I
Specific examples of the five-membered sultone represented by I) include 1,
Examples thereof include 3-propane sultone, α-hydroxy-o-toluenesulfonic acid-γ-sultone, and 1,8-naphthosultone.

These may be used in combination of two or more. The addition amount of the five-membered thiol lactone and the five-membered sultone used in the present invention is preferably 0.1 to 15% by weight in total with respect to the electrolytic solution, and particularly preferably 1 to 10% by weight. . The addition of the five-membered thiol lactone or the five-membered sultone to the nonaqueous solvent has the effect of improving the charge / discharge characteristics (charge / discharge efficiency, charge / discharge capacity) of the battery generated when the nonaqueous solvent is used. .

The type of the solute lithium salt dissolved in the non-aqueous solvent is not particularly limited, and a known lithium salt used for a non-aqueous lithium secondary battery can be used. Preferably, LiPF ₆ and LiBF are used. Inorganic lithium salts such as ₄ or organic acid lithium salts represented by the following general formula (V) can be used.

[0036]

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(Where m and n are integers of 1 to 4, respectively)
The organic acid lithium salt of the general formula (V) includes LiN (S
O_TwoCF_Three)_Two, LiN (SO_TwoC_TwoF_Five)_Two, LiN
(SO_TwoC_ThreeF₇)_Two, LiN (SO_TwoC_FourF ₉)_Two,
LiN (SO_TwoCF_Three) ・ (SO_TwoC_TwoF_Five), LiN
(SO_TwoCF_Three) ・ (SO_TwoC_ThreeF₇), LiN (SO
_TwoCF_Three) ・ (SO_TwoC_FourF₉), LiN (SO_TwoC_Two
F_Five) ・ (SO_TwoC_ThreeF₇), LiN (SO_TwoC
_TwoF_Five) ・ (SO_TwoC_FourF₉), LiN (SO_TwoC_ThreeF
₇) ・ (SO_TwoC_FourF₉). these
Medium, LiN (SO_TwoCF_Three)_Two, LiN (SO_TwoC_TwoF
_Five)_Two, LiN (SO_TwoCF_Three) ・ (SO_TwoC_FourF₉)
Is preferred.

The lithium salt has a solute concentration in the electrolytic solution of usually 0.5 to 2 mol / dm ³ , preferably 0.5 to 2 mol / dm ³ .
１．５1.5 mol / dm ³ is used. 0.
If it is less than 5 mol / dm ³ or more than 2 mol / dm ³ , the conductivity of the electrolyte tends to decrease. (Lithium Secondary Battery) The lithium secondary battery of the present invention comprises the above-mentioned non-aqueous electrolyte, a negative electrode and a positive electrode.

The negative electrode material constituting the battery is not particularly limited as long as it is a material capable of inserting and extracting lithium, but a known carbonaceous material can be preferably used. Specific examples include coke, glassy carbon, artificial graphite, natural graphite, non-graphitizable carbon, pyrolytic carbon, and carbon fiber. Alternatively, as the negative electrode material, lithium metal or an alloy of lithium with a metal such as aluminum, tin, zinc, silver, lead, or the like can be preferably used. A known negative electrode material such as a conductive polymer can be used as the negative electrode material. These negative electrode materials may be used as a mixture of two or more kinds.

As the shape of the negative electrode, a sheet electrode applied to a current collector after mixing with a binder and a conductive agent, if necessary, and a pellet electrode subjected to press molding can be used. As the material of the current collector for the negative electrode, metals such as copper, nickel, and stainless steel are used, and among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost. As the positive electrode material constituting the battery, various materials capable of inserting and extracting lithium, such as a lithium transition metal composite oxide material such as lithium manganese oxide, lithium cobalt oxide, and lithium nickel oxide, can be used. The above-mentioned lithium transition metal composite oxide is preferable.

The shape of the positive electrode is not particularly limited, and examples thereof include a sheet electrode applied to a current collector after mixing with a binder and a conductive agent, if necessary, and a pellet electrode subjected to press molding. Can be used. As the material of the positive electrode current collector, a metal such as aluminum, titanium, and tantalum or an alloy thereof is used. Among these, aluminum or its alloy is desirable in terms of energy density because it is lightweight.

As the shape of the battery, known types such as a cylinder type in which a sheet electrode and a separator are formed in a spiral shape, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are laminated can be used. It is. As the separator constituting the battery, a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used.

[0043]

EXAMPLES Hereinafter, the embodiments of the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples unless it exceeds the gist. In addition,
The performance of the electrolyte and the battery performance were evaluated by the following methods. 1. Evaluation of self-extinguishing property of electrolyte: width 15 mm, length 300
mm, a glass fiber filter paper with a thickness of 0.19 mm,
The glass fiber filter paper was sufficiently impregnated with the electrolytic solution by immersing it in a beaker containing the electrolytic solution for 10 minutes or more. Next, after removing excess electrolyte adhering to the glass fiber filter paper at the edge of the beaker, one end of the glass fiber filter paper was hung vertically with a clip. The lower end was heated with a small gas flame of lighters for about 3 seconds, and the self-extinguishing property with the fire source removed and the time until the fire was extinguished were measured.

2. Measurement of conductivity of electrolyte: conductivity meter CM-30S and conductivity cell CG- manufactured by Toa Denpa Kogyo KK
Using 511B, the conductivity at -20 ° C and 25 ° C was measured. 3. Measurement of charge / discharge cycle characteristics: 1) Test cell A: Measurement of charge / discharge cycle characteristics was performed using a diameter of 2
0mm, 1.6mm thick coin type battery
Charge / discharge was performed at a current density of 0.2 mA / cm ² within a voltage range of 1.5 V. As for the battery capacity, the discharge capacity (dedoping capacity) after three cycles was obtained.

In a battery, a positive electrode (working electrode) and a negative electrode (counter electrode) are accommodated in a stainless steel case also serving as a positive electrode terminal via a porous polypropylene film separator impregnated with an electrolytic solution, and are then inserted through a polypropylene gasket. It was manufactured by sealing with a stainless steel sealing plate also serving as the negative electrode terminal. As a working electrode, a carbon material as an active material, a fluororesin as a binder were mixed at a weight ratio of 90:10, and this was dispersed in a solvent (N-methylpyrrolidone) to form a slurry. After coating and drying on copper foil as a current collector,
A working electrode having a diameter of 12.5 mm was produced. As the counter electrode, a lithium metal sheet having a diameter of 12.5 mm and a thickness of 1.0 mm was used.

2) Test cell B: For measurement of charge / discharge cycle characteristics, a coin-type battery having a diameter of 20 mm and a thickness of 1.6 mm was manufactured, and 0.2 mA / c in a voltage range of 4.2 to 2.5 V.
Charge and discharge were performed at a current density of m ² . For the battery capacity, the charge capacity (dope capacity) after three cycles was determined. The battery contains a positive electrode and a negative electrode in a stainless steel case that also serves as the positive electrode terminal, with a porous polypropylene film separator impregnated with the electrolyte interposed therebetween, and is sealed with a stainless steel sealing plate that also serves as the negative electrode terminal via a polypropylene gasket. And made it. For the positive electrode, lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material, acetylene black as a conductive agent and a fluororesin as a binder were mixed at a weight ratio of 90: 5: 5, and this was mixed with N 2. -A slurry prepared by dispersing in methylpyrrolidone was applied to an aluminum foil as a positive electrode current collector, dried, and then dried to a diameter of 12.5 mm.
It was used after cutting into a disk of mm. As the negative electrode, a lithium metal sheet having a diameter of 12.5 mm and a thickness of 1.0 mm was used.

Examples 1 to 5 and Comparative Examples 1 and 2 The solutes and additives shown in Table 1 were dissolved in the nonaqueous mixed solvents shown in Table 1 to prepare an electrolyte having a solute concentration of 1 mol / dm ^3. did. Using this electrolytic solution, self-extinguishing properties (flame retardancy) and electrical conductivity were measured. Further, the undoping capacity of a coin-type battery (test cell A) using natural graphite for the working electrode and the doping capacity of a coin-type battery (test cell B) using lithium cobalt oxide were measured. Table 1 shows the results. FIG. 1 shows the results of the charge / discharge efficiency characteristics of the coin-type batteries (test cells A) produced using the electrolytes of Examples 1, 3, 4 and Comparative Example 2.

The abbreviations in the table indicate the following. EC: ethylene carbonate PC: propylene carbonate GBL: γ-butyrolactone TMP: trimethyl phosphate EDMP: dimethylethyl phosphate SBL: γ-thiobutyrolactone PS: 1,3-propane sultone NS: 1,8-naphthosultone

[0049]

[Table 1]

As shown in Table 1, the electrolytic solution of Comparative Example 1 provided high capacity in test cells A and B, but did not have self-extinguishing properties. In Comparative Example 2, the addition of the phosphoric acid ester exhibited high self-extinguishing properties and improved conductivity, but the charge / discharge capacity was very low. On the other hand, in the electrolyte solutions to which the additives of Examples 1 to 5 were added, self-extinguishing properties were exhibited, high conductivity was maintained, and charge / discharge capacity was greatly improved.

As shown in FIG. 1, the charge / discharge efficiency is low in Comparative Example 2, but the charge / discharge efficiency is maintained due to the effect of the additive, and the cycle characteristics are excellent.

[0052]

The electrolytic solution for a lithium secondary battery of the present invention comprises:
It has flame retardancy (self-extinguishing), good charge / discharge characteristics, and high safety and reliability.

[Brief description of the drawings]

FIG. 1 is a diagram showing the cycle characteristics of charge and discharge efficiency of a coin-type battery (test cell A) produced using the electrolyte of Example 1 and the electrolyte of Comparative Example.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Asato Kominato 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture Mitsubishi Chemical Corporation Tsukuba Research Laboratory F-term (reference) AL18 AM02 AM03 AM04 AM05 AM06 AM07 BJ03 HJ01 HJ02

Claims (9)

[Claims] 1. A non-aqueous electrolyte for a lithium secondary battery comprising a lithium salt dissolved in a non-aqueous solvent, wherein the non-aqueous solvent is a phosphate ester and a five-membered thiol lactone and / or
Alternatively, a non-aqueous electrolyte solution containing a five-membered ring sultone. 2. A 5-membered thiol lactone having the general formula (I): (In the formula, the bond without the indicated carbon atom having a shortage of the number of bonds is a part of a double bond or is bonded to a hydrogen atom, and R ₁ , R ₂ and R ₃ are each a hydrogen atom,
An aliphatic hydrocarbon group or an aromatic hydrocarbon group, which may be bonded to each other to form a ring, or the ring may be an aromatic ring). Item 2. The non-aqueous electrolyte according to Item 1. 3. The five-membered ring sultone has the general formula (II): (In the formula, the bond not indicated by a carbon atom having a deficient number of bonds is a part of a double bond or is bonded to a hydrogen atom, and R ₄ , R ₅ and R ₆ are each a hydrogen atom,
An aliphatic hydrocarbon group or an aromatic hydrocarbon group, which may be bonded to each other to form a ring, or the ring may be an aromatic ring). Item 3. The non-aqueous electrolyte according to Item 1 or 2. 4. The phosphoric ester of the general formula (III): (In the formula, R ₇ , R ₈ and R ₉ each represent an alkyl group having 1 to 4 carbon atoms which may be substituted by a fluorine atom.
However, the total number of carbon atoms of R ₇ , R ₈ and R ₉ is 3 to 12
Or a compound represented by the general formula (IV): (Wherein, R ₁₀ represents an alkyl group having 1 to 4 carbon atoms which may be substituted by a fluorine atom, and R ₁₁ represents an alkylene group having 2 to 8 carbon atoms). Item 1
4. The non-aqueous electrolyte according to any one of claims 1 to 3. 5. The non-aqueous solvent according to claim 1, wherein the non-aqueous solvent contains at least one organic solvent selected from the group consisting of carbonates, lactones, ethers, sulfolane and dioxolane. Non-aqueous electrolyte. 6. The lithium salt is LiPF ₆ , LiBF ₄
Or an organic acid lithium salt represented by the general formula (V):
The non-aqueous electrolyte according to claim 1. Embedded image (Wherein, m and n each represent an integer of 1 to 4) 7. The non-aqueous electrolyte according to claim 1, wherein the content of the phosphate in the non-aqueous solvent is 9 to 50% by volume. 8. The non-aqueous electrolyte according to claim 1, wherein the total content of the five-membered thiol lactone and the five-membered sultone in the non-aqueous electrolyte is 0.1 to 15% by weight. Electrolyte. 9. A positive electrode containing a compound capable of occluding and releasing lithium, a negative electrode containing at least one selected from a carbonaceous material capable of occluding and releasing lithium, lithium metal and a lithium alloy, and a lithium salt containing a non-aqueous solvent. A non-aqueous electrolytic solution dissolved in a lithium secondary battery, wherein the non-aqueous electrolytic solution is the non-aqueous electrolytic solution according to any one of claims 1 to 8. battery.