JP2001185210A - Electrolyte for lithium secondary battery and lithium secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel electrolyte and a lithium electrolyte which are excellent in safety while maintaining flame retardancy, having high electrical conductivity and electrochemical stability, as an optimal electrolyte for a lithium secondary battery. Provide the next battery. SOLUTION: The lithium salt is represented by the following general formula (I): (Wherein, R ¹ is alkyl having 1 to 4 carbons; R ² is 1 alkyl having 1 carbon)
Or a fluorine-substituted alkyl having from 4 to 4 carbon atoms; R ³ is a fluorine-substituted alkyl having 1 to 4 carbon atoms).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery using the same. In particular, an electrolyte for a lithium secondary battery in which a solute lithium salt is dissolved in an organic solvent containing a specific phosphate ester,
And a lithium secondary battery using the same.

[0002] The electrolyte of the present invention is flame-retardant (self-extinguishing).
And high electrical conductivity and electrochemical stability, and are excellent in safety.

[0003]

2. Description of the Related Art A carbon material such as graphite is used as a negative electrode active material.
LiCoO ₂ , LiNiO ₂ , LiMn as positive electrode active material
A lithium secondary battery using a lithium transition metal composite oxide such as ₂ O ₄ is rapidly growing as a new small secondary battery having a high voltage of 4 V and a high energy density. As an electrolyte for such a lithium secondary battery, generally, a high dielectric constant solvent such as ethylene carbonate and propylene carbonate, and an organic solvent obtained by mixing a low-viscosity solvent such as dimethyl carbonate and diethyl carbonate, A solution in which a lithium salt is dissolved is used.

However, since the conventional electrolyte is flammable, there is a danger of ignition or burning if the electrolyte leaks due to damage to the battery or an increase in pressure inside the battery due to any cause.

It is known to use phosphate esters as electrolytes for lithium batteries. For example, JP-A-58-
No. 206078, JP-A-60-23973,
JP-A-61-227377, JP-A-61-284
070 and JP-A-4-184870,
O = P such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, tris (2-chloroethyl) phosphate
The use of (OR) type ₃ phosphates is disclosed. Further, JP-A-8-88023 discloses a self-extinguishing electrolytic solution in which at least one of R is a halogen-substituted alkyl.

However, among the phosphoric esters used in these, trimethyl phosphate can provide an electrolytic solution having excellent flame retardancy, but has a disadvantage that it is easily decomposed by reduction depending on the material of the negative electrode. Therefore, when increasing the blending amount in the electrolyte or using natural graphite or artificial graphite as the negative electrode, the charge / discharge characteristics of the battery, for example, the charge / discharge efficiency and discharge capacity, do not satisfy the recently required characteristics. . On the other hand, an electrolytic solution containing triethyl phosphate or tributyl phosphate provides good charge / discharge characteristics without being restricted by the material of the negative electrode, but the flame retardancy of the electrolytic solution is not sufficient.

[0007] In particular, a phosphoric acid ester having a halogen atom such as chlorine or bromine in a molecule is inferior in oxidation-reduction resistance and is sufficiently charged when applied to a 4V class secondary battery or the like which generates a high voltage. A battery with discharge characteristics cannot be obtained. Furthermore,
A small amount of free halogen ions present as impurities corrode aluminum used as a positive electrode current collector, causing deterioration of battery characteristics.

Further, Japanese Patent Laid-Open No. Hei 4-18487 cited above.
No. 0 discloses the use of a cyclic phosphate as an electrolytic solution, and JP-A-11-67267 further discloses that 20 to 55% by volume of the cyclic phosphate is used in combination with a cyclic carbonate. An electrolyte for a battery is disclosed. However, in order to make the electrolytic solution of this system flame-retardant, it is necessary to incorporate 20% by volume or more of a cyclic phosphate, and there is a disadvantage that the conductivity decreases with an increase in the amount.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and provides flame retardancy (self-extinguishing property), high conductivity, and electrochemically. An object of the present invention is to provide a stable lithium secondary battery electrolyte and a lithium secondary battery using the same, which has excellent charge / discharge cycle characteristics and has both safety and reliability.

[0010]

Means for Solving the Problems The present inventors have conducted intensive studies in view of such circumstances, and as a result, by dissolving a lithium salt as a solute in a solvent containing a specific phosphoric acid ester as an organic solvent, flame retardancy has been achieved. Completed the present invention by finding that an electrolyte with excellent heat resistance (self-extinguishing properties), excellent electrical conductivity and electrochemical stability can be obtained, and that the charge / discharge cycle characteristics of a lithium secondary battery can be improved. I came to.

That is, the gist of the present invention is as follows. The lithium salt has the general formula (I):

[0012]

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(Wherein, R ¹ represents an alkyl group having 1 to 4 carbon atoms, R ² represents an alkyl group having 1 to 4 carbon atoms or a fluorine-substituted alkyl group, and R ³ represents a fluorine atom having 1 to 4 carbon atoms. 1. An electrolyte solution for a lithium secondary battery, which is dissolved in an organic solvent containing a phosphoric acid ester represented by a substituted alkyl group); A carbonaceous material or compound capable of occluding and releasing lithium;
A negative electrode containing a material selected from lithium metal or a lithium alloy; a positive electrode containing a compound capable of occluding and releasing lithium;
A lithium secondary battery comprising the electrolytic solution according to claim 1.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (Electrolyte Solution for Lithium Secondary Battery) The solvent of the electrolyte solution according to the present invention is represented by the following formula (I):

[0015]

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Wherein R ¹ represents an alkyl group having 1 to 4 carbon atoms, R ² represents an alkyl group having 1 to 4 carbon atoms or a fluorine-substituted alkyl group, and R ³ represents a fluorine atom having 1 to 4 carbon atoms. (Representing a substituted alkyl group). The organic solvent comprises (a) a phosphoric ester of formula (I) and (b) a formula (II):

[0017]

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Wherein R ⁴ , R ⁵ and R ⁶ each independently represent an alkyl group having 1 to 4 carbon atoms; and / or formula (III):

[0019]

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Wherein R ⁷ represents an alkylene group having 2 to ⁸ carbon atoms, and R ⁸ represents an alkyl group having 1 to 4 carbon atoms, and (c) a carbonate ester. preferable.

In the formula (I), when R ² is a linear or branched alkyl group having 1 to 4 carbon atoms, specific examples of the alkyl group include a methyl group, an ethyl group, a propyl group,
Isopropyl and butyl groups can be mentioned. A methyl group and an ethyl group are preferred from the viewpoint of exhibiting flame retardancy and high conductivity. When R ³ is a linear or branched fluorine-substituted alkyl group having 2 to 4 carbon atoms, specific examples of the fluorine-substituted alkyl group include a trifluoroethyl group, a pentafluoropropyl group, and a hexafluoroisopropyl group. And heptafluorobutyl groups. A trifluoroethyl group and a pentafluoropropyl group are preferred because the solvent has excellent conductivity and electrochemical stability.

Specific examples of the phosphoric ester of the formula (I) include, for example, trifluoroethyl dimethyl phosphate, pentafluoropropyl dimethyl phosphate, hexafluoroisopropyl dimethyl phosphate, heptafluorobutyl dimethyl phosphate, phosphate Trifluoroethylmethylethyl, pentafluoropropylmethylethyl phosphate,
Hexafluoroisopropylmethylethyl phosphate, heptafluorobutylmethylethyl phosphate, trifluoroethylmethylpropyl phosphate, pentafluoropropylmethylpropyl phosphate, hexafluoroisopropylmethylpropyl phosphate, heptafluorobutylmethylpropyl phosphate, phosphoric acid Trifluoroethylmethylbutyl, pentafluoropropylmethylbutylphosphate, hexafluoroisopropylmethylbutylphosphate, heptafluorobutylmethylbutylphosphate, trifluoroethyldiethylphosphate, pentafluoropropyldiethylphosphate, hexafluoroisopropyldiethylphosphate , Heptafluorobutyldiethyl phosphate, trifluoroethylethylpropyl phosphate, pentafluoropropylethylpropyl phosphate, phosphoric acid Hexafluorobutene isopropyl ethylpropyl, phosphoric acid heptafluorobutyl ethylpropyl,
Trifluoroethylethyl butyl phosphate, pentafluoropropylethylbutyl phosphate, hexafluoroisopropylethylbutyl phosphate, heptafluorobutylethylbutyl phosphate, trifluoroethyldipropyl phosphate, pentafluoropropyldipropyl phosphate, phosphoric acid Hexafluoroisopropyldipropyl, heptafluorobutyldipropyl phosphate, trifluoroethylpropylbutyl phosphate, pentafluoropropylpropylbutyl phosphate, hexafluoroisopropylpropylbutyl phosphate, heptafluorobutylpropylbutyl phosphate,
Examples thereof include trifluoroethyl dibutyl phosphate, pentafluoropropyl dibutyl phosphate, hexafluoroisopropyl dibutyl phosphate, heptafluorobutyl dibutyl phosphate and the like. These can be used alone or in combination of two or more. Among them, trifluoroethyl dimethyl phosphate, pentafluoropropyl dimethyl phosphate, trifluoroethyl methyl ethyl phosphate, and pentafluorophosphate can be obtained because of their high flame retardancy, high electrical conductivity and electrochemical stability. Propylmethylethyl,
Trifluoroethylmethylpropyl phosphate and pentafluoropropylmethylpropyl phosphate are preferred, and trifluoroethyldimethylphosphate and pentafluoropropyldimethylphosphate are particularly preferred.

In the formula (II), R ⁴ , R ⁵ and R ⁶ are
Each is independently a straight-chain or branched alkyl group having 1 to 4 carbon atoms. Specific examples of the alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group and a butyl group. Can be. A methyl group is preferable from the viewpoint of the effect of exhibiting flame retardancy, and an ethyl group, a propyl group, an isopropyl group, and a butyl group are preferable from the viewpoint of electrochemical stability.

Specific examples of the linear phosphate ester of the formula (II) include, for example, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, dimethylethyl phosphate, dimethylpropyl phosphate, phosphorus Dimethyl isopropyl phosphate, dimethyl butyl phosphate, methyl diethyl phosphate, diethyl propyl phosphate, diethyl butyl phosphate, dipropyl ethyl phosphate, dipropyl butyl phosphate, dibutyl ethyl phosphate, dibutyl propyl phosphate, ethyl propyl phosphate Butyl and the like. These can be used alone or in combination of two or more. Among these, trimethyl phosphate, triethyl phosphate, dimethylethyl phosphate, and dimethylpropyl phosphate are preferred.

In the formula (I), R ¹ is a linear or branched alkylene group having 2 to 8 carbon atoms, and specific examples thereof include a methylene group, an ethylene group, a trimethylene group, a propylene group and a tetraalkylene group. Methylene group, butylene group, 1,1-
Dimethylethylene group, pentamethylene group, 1,1,2-
Examples include a trimethylethylene group, a hexamethylene group, a tetramethylethylene group, a heptamethylene group, and an octamethylene group. Of these, an ethylene group is preferred.

R ⁸ is an alkyl group having 1 to 4 carbon atoms, and specific examples thereof include a methyl group, an ethyl group, a propyl group and a butyl group. Among these, a methyl group and an ethyl group are preferred because of excellent flame retardancy and electrical conductivity.

Specific examples of the cyclic phosphate of the formula (III) include ethylene methyl phosphate, ethylene ethyl phosphate, ethylene-n-propyl phosphate, ethylene isopropyl phosphate, and ethylene-n-butyl phosphate. , Ethylene-s-butyl phosphate, ethylene-t-butyl phosphate, propylene methyl phosphate, propylene ethyl phosphate, propylene-n-propyl phosphate, propylene isopropyl phosphate, propylene-n-butyl phosphate, phosphoric acid Propylene-s-butyl, propylene-t-butyl phosphate, trimethylene methyl phosphate, trimethylene ethyl phosphate, trimethylene-n-propyl phosphate, trimethylene isopropyl phosphate, trimethylene-n-butyl phosphate, phosphoric acid Trimethylene-s-butyl, trimethylene-t-butyl phosphate, Phosphate butylene methyl, butylene ethyl phosphate, butylene phosphate -n- propyl, phosphoric acid butylene isopropyl, butylene phosphate -n- butyl,
Butylene-s-butyl phosphate, butylene-t-butyl phosphate, isobutylene methyl phosphate, isobutylene ethyl phosphate, isobutylene-n-butyl, isobutylene-s-butyl phosphate, isobutylene-t-butyl phosphate, Tetramethylene methyl phosphate, tetramethylene ethyl phosphate, tetramethylene-n-propyl phosphate, tetramethylene isopropyl phosphate, tetramethylene-n-butyl phosphate, tetramethylene-s-butyl phosphate, tetramethylene phosphate t-butyl, pentamethylenemethyl phosphate, pentamethyleneethyl phosphate, pentamethylene-n-propyl, pentamethyleneisopropyl, pentamethylene-n-butyl, pentamethylene-s-butyl, phosphorus Pentamethylene-t
-Butyl, trimethylethylene methyl phosphate, trimethylethylene ethyl phosphate, trimethylethylene phosphate-
n-propyl, trimethylethylene isopropyl phosphate, trimethylethylene-n-butyl phosphate, trimethylethylene-s-butyl phosphate, trimethylethylene-t-butyl phosphate, hexamethylenemethyl phosphate, hexamethyleneethyl phosphate, phosphorus Hexamethylene acid-n-
Propyl, hexamethylene isopropyl phosphate, hexamethylene-n-butyl, hexamethylene phosphate
s-butyl, hexamethylene-t-butyl phosphate, tetramethylethylene methyl phosphate, tetramethylethylene ethyl phosphate, tetramethylethylene-n-propyl phosphate, tetramethylethylene isopropyl phosphate, tetramethylethylene phosphate n-butyl, tetramethylethylene-s-butyl phosphate, tetramethylethylene-t-butyl phosphate, heptamethylenemethyl phosphate, heptamethyleneethyl phosphate, heptamethylene-n-propyl phosphate, heptamethyleneisopropyl phosphate , Heptamethylene-n-butyl phosphate, heptamethylene-s phosphate
-Butyl, heptamethylene-t-butyl phosphate, octamethylenemethyl phosphate, octamethyleneethyl phosphate,
Octamethylene-n-propyl phosphate, octamethylene-isopropyl phosphate, octamethylene-n-butyl phosphate, octamethylene-s-butyl phosphate, octamethylene-t-butyl phosphate, and the like. Among them, ethylene methyl phosphate and ethylene ethyl phosphate are preferable.

Examples of the carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, diisopropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate,
Ethyl isopropyl carbonate and the like can be mentioned. These can be used alone or in combination of two or more. Among them, ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate are preferred.

The ratio of each component in the organic solvent is as follows:
Component (a): 10 to 90% by volume, preferably 15 to 90% by volume based on the total amount of component (a), component (b) and component (c).
80% by volume, more preferably 20 to 70% by volume, (b)
Ingredient: 0 to 40% by volume (excluding 0), preferably 0
~ 35% by volume (not including 0), more preferably 0-3
0% by volume (not including 0), component (c): 10 to 90% by volume, preferably 20 to 85% by volume, more preferably 3%
0 to 80% by volume.

In the above-mentioned volume ratio, the value measured at 25 ° C. is used as the volume of each component. In the case of a solid such as ethylene carbonate at room temperature, a value measured in a molten state by heating to a melting point is used.

In addition to the above components (a), (b) and (c), other organic solvents conventionally known as electrolytes for lithium secondary batteries as long as the characteristics of the present invention are not impaired. Can also be used as a mixture.

The lithium salt of the solute according to the present invention includes lithium inorganic acid salts of LiPF ₆ and LiBF ₄ or the above-mentioned general formula (IV):

[0033]

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(Wherein m and n are each independently:
It is preferable to use an organic acid lithium salt represented by the following formula:

An organic acid lithium salt represented by the general formula (IV)
As LiN (SO_TwoCF_Three)_Two, LiN (SO_TwoC_Two
F_Five)_Two, LiN (SO_TwoC_ThreeF₇)_Two, LiN (SO_TwoC_FourF
₉)_Two, LiN (SO_TwoCF_Three) ・ (SO_TwoC_TwoF_Five), Li
N (SO_TwoCF_Three) ・ (SO_TwoC _ThreeF₇), LiN (SO_TwoC
F_Three) ・ (SO_TwoC_FourF₉), LiN (SO_TwoC_TwoF_Five) ・
(SO_TwoC_ThreeF₇), LiN (SO_TwoC_TwoF_Five) ・ (SO_TwoC_Four
F₉), LiN (SO_TwoC_ThreeF₇) ・ (SO_TwoC_FourF₉)Such
Is mentioned. Among them, LiN (SO_TwoCF_Three) _Two,
LiN (SO_TwoC_TwoF_Five)_Two, LiN (SO_TwoCF_Three) ・ (S
O_TwoC_FourF₉Is preferred.

In the electrolyte of the present invention, LiPF ₆ , L
The inorganic acid lithium salt of iBF ₄ ; or the organic acid lithium salt represented by the formula (IV) is dissolved in a mixed solvent containing the phosphoric acid ester to be used, thereby maintaining flame retardancy and achieving high electrical conductivity. Along with obtaining an electrochemically superior electrolyte,
A battery having excellent charge / discharge capacity and charge / discharge cycle characteristics can be obtained.

In order to obtain a high conductivity of the electrolyte, the concentration of the solute in the electrolyte is usually 0.5%.
２2 mol / dm ³ , preferably 0.5 to 1.5 mol / dm ³ .

(Lithium Secondary Battery) The lithium secondary battery of the present invention comprises the above-mentioned electrolytic solution, a negative electrode and a positive electrode.

As the negative electrode material constituting the battery, a carbonaceous substance capable of occluding and releasing lithium; lithium metal; and an alloy of lithium with a metal such as aluminum, tin, zinc, silver, and lead can be used. As the carbonaceous material,
Examples thereof include graphite, non-graphitizable carbon, and amorphous carbon; and those having a mesophase exhibiting an intermediate layer structure. These negative electrode materials can be used alone or in combination of two or more.

The negative electrode can be formed in any shape such as a sheet electrode and a bellet electrode. The sheet electrode is formed by, for example, mixing with a binder and a conductive agent as necessary, and then coating the current collector. The pellet electrode is formed by, for example, press molding.

As the positive electrode material constituting the battery, a lithium transition metal composite oxide material such as lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide or the like which can insert and extract lithium can be used. These can be used alone or in combination of two or more.

The shape of the positive electrode is not particularly limited, and for example, a sheet electrode and a pellet electrode can be used. These are similar to those shown for the negative electrode,
It is formed.

The lithium secondary battery of the present invention includes, in addition to the above-described electrolyte solution, negative electrode and positive electrode,
A current collector for a negative electrode, a current collector for a positive electrode, a separator, and the like can be provided.

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 preferred from the viewpoint of easy processing into a thin film and cost.

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 an alloy thereof is particularly preferable in terms of light weight and energy density.

As the separator, a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used. The separator is impregnated with the electrolytic solution of the present invention.

The battery has a known shape such as a cylinder type in which the sheet electrode and the separator are formed in a spiral shape, a cylinder type having an inside-out structure in which the pellet electrode and the separator are combined, and a coin type in which the pellet electrode and the separator are laminated. It is possible.

[0048]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these embodiments unless it exceeds the gist.

The performance of the electrolyte and the battery performance were evaluated by the following methods.

1. Self-extinguishing property evaluation of electrolyte solution: width 15 mm,
A strip of glass fiber filter paper having a length of 300 mm and a thickness of 0.19 mm was immersed in a beaker containing an electrolyte for 10 minutes or more to sufficiently impregnate the glass fiber filter with the electrolyte. 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 a lighter for about 3 seconds, and the presence or absence of self-extinguishing property with the fire source removed and the time to extinguish the fire were measured.

2. Measurement of conductivity of electrolyte: conductivity meter CM-30S and conductivity cell CG- manufactured by Toa Denpa Kogyo KK
The conductivity at 25 ° C. was measured using 511B.

3. Measurement of charge / discharge cycle characteristics: 1) Test cell A: In order to evaluate the electrolytic solution and the negative electrode, a test cell A as a coin-type battery was prepared as described below. That is, a carbon material described below, which is an active material, and a fluororesin, which is a binder, are mixed at a weight ratio of 90:10, and this is dispersed in N-methylpyrrolidone to form a slurry. A working electrode having a diameter of 12.5 mm was prepared by applying the coating to a copper foil used as a body and drying the coating. As a counter electrode, a lithium metal sheet having a diameter of 12.5 mm and a thickness of 1.0 mm was used. Next, the working electrode and the counter electrode are accommodated in a stainless steel case also serving as the positive electrode terminal, with the working electrode and the counter electrode stacked one on top of the other through a porous polypropylene film separator impregnated with an electrolytic solution, and the stainless steel serving as the negative electrode terminal is also interposed via a polypropylene gasket. Seal with a sealing plate, diameter 2
A coin-type battery having a thickness of 0 mm and a thickness of 1.6 mm was manufactured.

Using the coin-type battery manufactured as described above, a voltage of 0.2 mA / cm ² in a voltage range of 0 to 1.5 V.
The charge / discharge cycle characteristics were measured by charging / discharging using the current density of the sample. As the battery capacity, the discharge capacity (dedoping capacity) after 5 cycles was determined.

2) Test cell B: In order to evaluate the electrolytic solution and the positive electrode, a test cell B as a coin-type battery was prepared as follows. That is, a lithium manganese composite oxide (LiMn ₂ O ₄ ) was used as a positive electrode active material, and the oxide, acetylene black as a conductive agent, and fluororesin as a binder were mixed in a weight ratio of 90: 5: 5 and dispersed in N-methylpyrrolidone to form a slurry, coated and dried on an aluminum foil as a positive electrode current collector, and then punched into a 12.5 mm diameter disk to form a positive electrode. Produced. The negative electrode has a diameter of 12.5 mm and a thickness of 1.
A 0 mm lithium metal sheet was used. Next, in a stainless steel case also serving as a positive electrode terminal, the positive electrode and the negative electrode are stacked and accommodated via a separator made of a porous polypropylene film impregnated with an electrolytic solution, and a stainless steel seal serving also as a negative electrode terminal is provided via a polypropylene gasket. It sealed with the board and produced the coin-shaped battery of diameter 20mm and thickness 1.6mm.

Using the coin-type battery manufactured as described above, the charge / discharge cycle characteristics were measured by a charge / discharge cycle at a current density of 0.6 mA / cm ² in a voltage range of 4.3 to 3.5 V. went. As the battery capacity, an average value (discharge capacity) in the initial 20 cycles was obtained.

Examples 1 to 12 and Comparative Examples 1 and 2 The solutes shown in Table 1 were dissolved in the mixed solvents shown in Table 1 to prepare an electrolyte having a solute concentration of 1 mol / dm ³ . Using this electrolytic solution, the self-extinguishing property (flame retardancy), the electrical conductivity, and the charge / discharge capacity of a coin-type battery (test cell A) using amorphous carbon for the working electrode were measured. Table 1 shows the results. Example 1
1 and 2 show the results of the initial charge / discharge curve and cycle characteristics of the coin-type battery manufactured using the electrolyte solution of FIG.

Examples 13 to 15 and Comparative Example 3 A solute shown in Table 1 was dissolved in a mixed solvent shown in Table 1 to prepare an electrolyte having a solute concentration of 1 mol / dm ³ , and natural graphite was used as a working electrode. Except for this, a coin-type battery (test cell A) was produced in the same manner as in Example 1, and the self-extinguishing property (flame retardancy) of the electrolytic solution, the conductivity, and the charge / discharge capacity of the battery were measured. Table 1 shows the results.
Shown in 3 and 4 show the results of the initial charge / discharge curve and cycle characteristics of the coin-type battery manufactured using the electrolyte solution of Example 13.

[0058]

[Table 1]

As is clear from Table 1, the electrolytic solution of the present invention has excellent flame retardancy as compared with the electrolytic solution of the comparative example not containing the phosphate ester of the present invention, and was mixed with trimethyl phosphate. In the case of the electrolytic solution, a discharge capacity was not obtained in the graphite electrode, but an excellent discharge capacity was obtained.

Examples 16 to 17 LiPF ₆ as a solute was dissolved in a mixed solvent shown in Table 2 to prepare an electrolyte having a solute concentration of 1 mol / dm ³ . Using this electrolytic solution, the self-extinguishing property (flame retardancy), the electrical conductivity, and the charge / discharge capacity of the coin-type battery (test cell B) were measured. Table 2 shows the results. 5 and 6 show the results of the initial charge / discharge curve and the cycle characteristics of the coin-type battery manufactured using the electrolyte solution of Example 14.

[0061]

[Table 2]

The abbreviations in the table are as follows. LiBETI: LiN (SO ₂ C ₂ F ₅ ) ₂ TFEDMP: trifluoroethyl dimethyl phosphate PFPDMP: pentafluoropropyl dimethyl phosphate EDMP: dimethyl ethyl phosphate PrDMP: dimethyl propyl phosphate MEP: ethylene methyl phosphate EEP: phosphoric acid Ethylene ethyl TMP: Trimethyl phosphate EC: Ethylene carbonate PC: Propylene carbonate DMC: Dimethyl carbonate DEC: Diethyl carbonate

[0063]

The electrolytic solution for a lithium secondary battery of the present invention comprises:
The present invention has excellent unique effects such as good charge / discharge cycle characteristics, flame retardancy (self-extinguishing properties), no toxicity, high safety and high reliability.

[Brief description of the drawings]

FIG. 1 is a diagram showing an initial charge / discharge curve of a coin-type battery (test cell A) produced using the electrolytic solution prepared in Example 1.

FIG. 2 is a diagram showing the cycle characteristics of a coin-type battery (test cell A) manufactured using the electrolytic solution prepared in Example 1.

FIG. 3 is a diagram showing an initial charge / discharge curve of a coin-type battery (test cell A) manufactured using the electrolytic solution prepared in Example 13.

FIG. 4 is a diagram showing the cycle characteristics of a coin-type battery (test cell A) manufactured using the electrolytic solution prepared in Example 13.

FIG. 5 is a diagram showing an initial charge / discharge curve of a coin-type battery (test cell B) produced using the electrolytic solution prepared in Example 14.

FIG. 6 is a diagram showing the cycle characteristics of a coin-type battery (test cell B) manufactured using the electrolytic solution prepared in Example 14.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Asato Kominato 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture Within the Tsukuba Research Laboratory, Mitsubishi Chemical Corporation (72) Inventor Kenichi Ishigaki Chuo-Hachi, Ami-cho, Inashiki-gun, Ibaraki Prefecture No.3-1, Mitsubishi Tsukuba Research Laboratory (72) Inventor Shigeaki Kasuya 8-3-1, Chuo, Ami-cho, Inashiki-gun, Ibaraki Pref. AL07 AL08 AL12 AL18 AM02 AM03 AM05 AM07 CJ08 HJ02 HJ07

Claims (7)

[Claims] The lithium salt is represented by the general formula (I): (Wherein, R ¹ represents an alkyl group having 1 to 4 carbon atoms, R ² represents an alkyl group having 1 to 4 carbon atoms or a fluorine-substituted alkyl group, and R ³ represents a fluorine-substituted alkyl group having 1 to 4 carbon atoms. Which is dissolved in an organic solvent containing a phosphoric acid ester represented by the following formula: 2. The organic solvent is used in an amount of 10 to 90 volumes of (a) the phosphoric ester represented by the general formula (I) based on the total amount of the components (a), (b) and (c). %When,
(B) General formula (II): (Wherein R ⁴ , R ⁵ and R ⁶ each independently represent an alkyl group having 1 to 4 carbon atoms); and / or a general formula (III): 3] (Wherein, R ⁷ represents an alkylene group having 2 to 8 carbon atoms, R ⁸
Represents an alkyl group having 1 to 4 carbon atoms), and (c) carbonate ester 1
The electrolyte solution for a lithium secondary battery according to claim 1, which is a mixed solvent containing 0 to 90% by volume. 3. The component (b) is at least one selected from the group consisting of trimethyl phosphate, triethyl phosphate, dimethylethyl phosphate, dimethylpropyl phosphate, ethylene methyl phosphate and ethylene ethyl phosphate. The electrolytic solution for a lithium secondary battery according to claim 2, wherein 4. The method according to claim 1, wherein the phosphate represented by the general formula (I) is selected from the group consisting of trifluoroethyl dimethyl phosphate and pentafluoropropyl dimethyl phosphate. The electrolytic solution for a lithium secondary battery according to claim 1. 5. The carbonate ester is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, di-n-carbonate.
Propyl, diisopropyl carbonate, methyl ethyl carbonate, methyl-n-propyl carbonate, methyl isopropyl carbonate, ethyl-n-propyl carbonate, ethyl isopropyl carbonate and at least one selected from the group consisting of -n-propyl isopropyl carbonate, The electrolyte for a lithium secondary battery according to claim 2. 6. The lithium salt is LiPF ₆ , LiBF ₄ or the general formula (IV): (Wherein m and n each independently represent an integer of from 1 to 4).
The electrolyte solution for a lithium secondary battery according to any one of claims 1 to 5. 7. A carbonaceous substance or compound capable of occluding and releasing lithium; a negative electrode including a material selected from lithium metal or a lithium alloy; a positive electrode including a compound capable of occluding and releasing lithium; A lithium secondary battery comprising the electrolytic solution according to claim 1.