JP2001160415A - Electrolyte for lithium secondary battery and lithium secondary battery using the electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new type of electrolyte with superior safety that has both high conductivity and electrochemical stability as well as maintaining excellent incombustibility, which is most suitable for electrolyte for lithium secondary battery and the lithium secondary battery incorporating same. SOLUTION: A lithium salt is dissolved in an organic solvent which contains 0.1-15% in volume of cyclic phosphoric ester as shown in formula (I) and/or phosphoric ester as shown in formula (II).

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. More specifically, the present invention relates to an electrolyte for a lithium secondary battery in which a solute lithium salt is dissolved in an organic solvent containing a specific phosphate, 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 excellent in safety.

[0003]

_2. Description of the Related Art A carbon material such as graphite is used as a negative electrode active material, and LiCoO ₂ , LiNiO ₂ , and LiMn are used as a 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 class and a high energy density. As an electrolyte for such a lithium secondary battery, lithium carbonate is generally used as a mixed carbonate obtained by mixing a high dielectric constant solvent such as ethylene carbonate and propylene carbonate with low-viscosity solvents such as dimethyl carbonate and diethyl carbonate. What dissolved the salt is used.

However, since such conventional electrolytes are flammable, if the electrolyte leaks due to damage to the battery or an increase in pressure inside the battery due to any cause,
There is a risk of ignition and burning.

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, an electrolytic solution having excellent flame retardancy can be obtained, but depending on the material of the negative electrode, trimethyl phosphate is easily decomposed and reduced. For this reason, when the amount of the compounded electrolyte is increased, or when natural graphite or artificial graphite is used as the negative electrode, the charge / discharge characteristics of the battery, such as charge / discharge efficiency and discharge capacity, are not satisfactory as compared with recent required characteristics. On the other hand, the electrolyte containing triethyl phosphate or tributyl phosphate
Although good charge / discharge characteristics can be obtained without being limited by the material of the negative electrode, the flame retardancy of the electrolyte 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 this type of electrolyte flame-retardant, it is necessary to incorporate 20% by volume or more of a cyclic phosphate ester, 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 in order to solve such problems, and is provided with flame retardancy (self-extinguishing properties), high conductivity, and high electrochemical performance. It is an object of the present invention to provide an electrolyte solution for a lithium secondary battery which is very stable, and a lithium secondary battery using the same, which is excellent in charge / discharge cycle characteristics and has both safety and reliability.

[0010]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and as a result, have dissolved a lithium salt of a solute in a solvent containing a specific phosphate as an organic solvent. By making it flame retardant (self-extinguishing)
The present inventors have found that an electrolytic solution having excellent electrical conductivity and electrochemical stability can be obtained, and that the charge / discharge cycle characteristics of a lithium secondary battery can be improved, thereby completing the present invention.

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

Embedded image (Wherein, R ¹ represents an alkylene group having 2 to 8 carbon atoms;
R ² represents an alkyl group having 1 to 4 carbon atoms), and / or a general formula (II):

Embedded image (Wherein, R ³ and R ⁴ are each independently a carbon number of 1 to
R ⁵ represents an allyl group, having 6 to 6 carbon atoms;
1. An electrolyte solution for a lithium secondary battery, which is dissolved in an organic solvent containing 0.1 to 15% by volume of a phosphoric acid ester represented by the formula (8). Further, a negative electrode including a material selected from the group consisting of a carbonaceous substance capable of inserting and extracting lithium, a lithium metal, and a lithium alloy; a positive electrode including a lithium transition metal composite; It is a secondary battery.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

(Electrolyte for Lithium Secondary Battery) The solvent of the electrolyte of the present invention contains (a) a phosphate compound of the formula (I) and / or a phosphate of the formula (II),
Preferably, the mixed solvent further comprises (a), (b) a lower phosphoric acid ester represented by the formula (III), and (c) a carbonate ester.

[0014]

Embedded image (Wherein, R ¹ represents an alkylene group having 2 to 8 carbon atoms;
R ² represents an alkyl group having 1 to 4 carbon atoms)

[0015]

Embedded image (Wherein, R ³ and R ⁴ are each independently a carbon number of 1 to
R ⁵ represents an allyl group, having 6 to 6 carbon atoms;
8 represents an aryl group or an aralkyl group)

[0016]

Embedded image (Wherein, R ⁶ , R ⁷ and R ⁸ each independently represent an unsubstituted or fluorine-substituted alkyl group having 1 to 4 carbon atoms)

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

R ² is a linear or branched 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. Of these, a methyl group and an ethyl group are preferred.

Specific examples of the cyclic phosphate include ethylene methyl phosphate, ethylene ethyl phosphate,
Ethylene-n-propyl phosphate, ethylene-isopropyl phosphate, ethylene-n-butyl phosphate, ethylene-s-butyl phosphate, ethylene-t-butyl phosphate, propylene methyl phosphate, propylene ethyl phosphate, propylene phosphate -N-propyl, propylene isopropyl phosphate, propylene-n-butyl phosphate, propylene-s-butyl phosphate, propylene-t-butyl phosphate, trimethylene methyl phosphate, trimethylene ethyl phosphate, trimethylene phosphate- n-propyl, trimethylene isopropyl phosphate, trimethylene-n-butyl phosphate, trimethylene-s-butyl phosphate, trimethylene-t-phosphate
Butyl, butylene methyl phosphate, butylene ethyl phosphate, butylene-n-propyl, butylene isopropyl phosphate, butylene-n-butyl, butylene-s-butyl, butylene-t-butyl phosphate, phosphorus Isobutylene methyl phosphate, isobutylene ethyl phosphate, isobutylene phosphate-n-butyl, isobutylene phosphate
s-butyl, isobutylene-t-butyl phosphate, tetramethylenemethyl phosphate, tetramethyleneethyl phosphate,
Tetramethylene-n-propyl phosphate, tetramethyleneisopropyl phosphate, tetramethylene-n-butyl phosphate, tetramethylene-s-butyl phosphate, tetramethylene-t-butyl phosphate, pentamethylenemethyl phosphate,
Pentamethyleneethyl phosphate, pentamethylene phosphate
n-propyl, pentamethyleneisopropyl phosphate, pentamethylene-n-butyl phosphate, pentamethylene-s-butyl phosphate, pentamethylene-t-butyl phosphate,
Trimethylethylene methyl phosphate, trimethylethylene ethyl phosphate, trimethylethylene-n-propyl phosphate, trimethylethylene isopropyl phosphate, trimethylethylene-n-butyl phosphate, trimethylethylene-s-butyl phosphate, trimethylethylene phosphate t-butyl, hexamethylenemethyl phosphate, hexamethyleneethyl phosphate, hexamethylene-n-propyl, hexamethyleneisopropyl, hexamethylene-n-butyl, hexamethylene-s-butyl,
Hexamethylene-t-butyl phosphate, tetramethylethylene methyl phosphate, tetramethylethylene ethyl phosphate, tetramethylethylene-n-propyl phosphate, tetramethylethylene isopropyl phosphate, tetramethylethylene-n-butyl phosphate, Tetramethylethylene-s-butyl phosphate, tetramethylethylene-t-butyl phosphate, heptamethylenemethyl phosphate, heptamethyleneethyl phosphate, heptamethylene-n-propyl phosphate, heptamethyleneisopropyl phosphate, heptaphosphate Methylene-n-butyl, heptamethylene-s-butyl phosphate,
Heptamethylene-t-butyl phosphate, octamethylenemethyl phosphate, octamethyleneethyl phosphate, octamethylene-n-propyl phosphate, octamethyleneisopropyl phosphate, octamethylene-n-butyl phosphate, octamethylene phosphate- s-butyl, octamethylene phosphate
t-butyl and the like. These can be used alone or in combination of two or more. Of these, ethylene methyl phosphate and ethylene ethyl phosphate are preferred.

In the formula (II), R ³ and R ⁴ are each independently a linear or branched alkyl group having 1 to 4 carbon atoms, and specific examples thereof include a methyl group and an ethyl group. , A propyl group and a butyl group.

R ⁵ is an allyl group, an aryl group having 6 to 8 carbon atoms or an aralkyl group, and specific examples of the aryl group include a phenyl group and a tolyl group. Specific examples of the aralkyl group Includes a benzyl group and a 2-phenylethyl group.

Specific examples of the phosphate ester of the formula (II) include, for example, allyl dimethyl phosphate, allyl diethyl phosphate, allyl dipropyl phosphate, allyl dibutyl phosphate, allyl methyl ethyl phosphate, allyl phosphate Methyl propyl, allyl methyl butyl phosphate, allyl ethyl propyl phosphate, allyl ethyl butyl phosphate, allyl propyl butyl phosphate, dimethyl phenyl phosphate, diethyl phenyl phosphate, dipropyl phenyl phosphate, dibutyl phenyl phosphate, phosphoric acid Methyl ethyl phenyl, methyl propyl phenyl phosphate, methyl butyl phenyl phosphate, ethyl propyl phenyl phosphate, ethyl butyl phenyl phosphate, propyl butyl phenyl phosphate, benzyl dimethyl phosphate, benzyl diethyl phosphate, benzyl dipropyl phosphate, Phosphate benzyl dibutyl, benzyl methyl ethyl phosphate, benzyl-methylpropyl, benzyl methyl butyl phosphate, benzyl ethyl propyl, benzyl ethyl butyl phosphate, benzyl propyl butyl, and the like. These can be used alone or
A mixture of more than one species can be used. Among these,
Allyl dimethyl phosphate, dimethyl phenyl phosphate, benzyl dimethyl phosphate and methyl propyl phenyl phosphate are preferred.

In the formula (III), R ⁶ , R ⁷ and R ⁸ are
Each independently is a linear or branched unsubstituted or fluorine-substituted lower alkyl group having 1 to 4 carbon atoms. Specific examples of the unsubstituted alkyl group include a methyl group, an ethyl group, a propyl group and a butyl group. Can be mentioned. Specific examples of the fluorine-substituted alkyl group include a trifluoroethyl group, a pentafluoropropyl group, and a heptafluorobutyl group.

Specific examples of the non-substituted lower alkyl phosphate of formula (III) include, for example, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, dimethylethyl phosphate, dimethylpropyl phosphate. , Dimethylbutyl phosphate, diethylmethyl phosphate, diethylpropyl phosphate, diethylbutyl phosphate,
Dipropylmethyl phosphate, dipropylethyl phosphate, dipropylbutyl phosphate, dibutylmethyl phosphate, dibutylethyl phosphate, dibutylpropyl phosphate, methylethylpropyl phosphate, methylethylbutylphosphate, ethylpropylbutylphosphate Specific examples of the fluorine-substituted lower alkyl ester include trifluoroethyl dimethyl phosphate, pentafluoropropyl dimethyl phosphate, heptafluorobutyl dimethyl phosphate, trifluoroethyl methyl ethyl phosphate, and phosphoric acid Pentafluoropropylmethylethyl, heptafluorobutylmethylethyl phosphate, trifluoroethylmethylpropyl phosphate, pentafluoropropylmethylpropyl phosphate, heptafluorobutylmethylpropyl phosphate,
Trifluoroethyl methyl butyl phosphate, pentafluoropropyl methyl butyl phosphate, heptafluorobutyl methyl butyl phosphate, trifluoroethyl diethyl phosphate, pentafluoropropyl diethyl phosphate, heptafluorobutyl diethyl phosphate, trifluoroethyl phosphate Ethylpropyl, pentafluoropropylethylpropyl phosphate, heptafluorobutylethylpropyl phosphate, trifluoroethylethylbutyl phosphate, pentafluoropropylethylbutyl phosphate, heptafluorobutylethylbutyl phosphate, trifluoroethyldipropyl phosphate , Pentafluoropropyldipropyl phosphate,
Heptafluorobutyl dipropyl phosphate, trifluoroethyl propyl butyl phosphate, pentafluoropropyl propyl butyl phosphate, heptafluorobutyl propyl butyl phosphate, trifluoroethyl dibutyl phosphate, pentafluoropropyl dibutyl phosphate, heptafluoro phosphate Butyldibutyl and the like. These can be used alone or in combination of two or more. Among these, trimethyl phosphate, dimethylethyl phosphate,
Dimethylpropyl phosphate, trifluoroethyldimethylphosphate, trifluoroethylmethylethylphosphate and trifluoroethylmethylpropylphosphate are preferred, and dimethylethylphosphate, dimethylpropylphosphate, trifluoroethyldimethylphosphate and trifluoroethyldimethylphosphate are preferred. Fluoroethylmethylethyl is particularly preferred.

Specific examples of the carbonate ester include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate,
Ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, diisopropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylisopropyl carbonate and the like. These can be used alone or in combination of two or more. Among these, ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate are preferred.

Preferably, the organic solvent comprises (a), (b)
And (c) component. The proportion of each component in the organic solvent is 0.1 to 15% by volume, preferably 0.5 to 15% by volume, based on the total amount of component (a), component (b) and component (c). 10% by volume, more preferably 1%
Component (b): 0 to 40% by volume, preferably 0 to 35% by volume, more preferably 0 to 30% by volume.
Component (c): 10 to 90% by volume, preferably 2%
0 to 85% by volume, more preferably 30 to 80% by volume. If the concentration of the component (a) is less than 0.1% by volume, a battery having sufficient charge / discharge characteristics cannot be obtained depending on the material of the negative electrode. If the concentration exceeds 15% by volume, the conductivity of the electrolytic solution decreases, An electrolyte having a practical conductivity cannot be obtained.

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 used as a solute in the present invention includes a lithium salt of an inorganic acid such as LiPF ₆ or LiBF ₄ ; or the above-mentioned general formula (IV):

Embedded image (Wherein, m and n each independently represent an integer of 1 to 4).

Examples of the organolithium compound represented by the general formula (IV) include LiN (SO ₂ CF ₃ ) ₂ and LiN (SO ₂ C _{2
F 5) 2, LiN (SO} 2 C 3 F 7) 2, LiN (SO 2 C 4 F 9)
₂ , LiN (SO ₂ CF ₃ ) · (SO ₂ C ₂ F ₅ ), LiN
(SO ₂ CF ₃ ) · (SO ₂ C ₃ F ₇ ), LiN (SO ₂ CF
₃ ) · (SO ₂ C ₄ F ₉ ), LiN (SO ₂ C ₂ F ₅ ) · (S
O ₂ C ₃ F ₇ ), LiN (SO ₂ C ₂ F ₅ ) · (SO ₂ C
₄ F _9), such as _{LiN (SO 2 C 3 F 7} ) · (SO 2 C 4 F 9) and the like. Among them, LiN (SO ₂ CF ₃ ) ₂ , Li
N (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) · (SO ₂
C ₄ F ₉ ) is preferred.

In the electrolyte of the present invention, LiPF ₆ , L
By dissolving an inorganic lithium salt such as iBF ₄ or the like or an organolithium compound represented by the above formula (IV) in the above mixed solvent containing the phosphoric ester, flame retardancy is maintained and high conductivity is maintained. A battery excellent in charge / discharge capacity and charge / discharge cycle characteristics can be obtained while obtaining an electrolytic solution excellent in rate and electrochemical properties.

In order to obtain a high conductivity of the electrolyte, the concentration of the solute in the electrolyte is usually 0.5 to 2 mol.
l / 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 material capable of occluding and releasing lithium; lithium metal; and an alloy of lithium and a metal such as aluminum, tin zinc, silver, lead, and the like can be used. As the carbonaceous material,
Graphite, non-graphitizable carbon and amorphous carbon; and those having a mesophase exhibiting an intermediate layer structure among them are exemplified. These 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 a 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.

[0038] In addition to the above-mentioned electrolyte solution, negative electrode and positive electrode, the lithium secondary battery of the present invention comprises:
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 non-woven 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.

[0043]

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 glass fiber filter in the form of a strip having a length of 300 mm and a thickness of 0.19 mm is immersed in a beaker containing an electrolyte for at least 10 minutes.
The electrolyte was sufficiently impregnated into the glass fiber filter paper. Next, after removing excess electrolyte attached to the glass fiber filter paper at the edge of the beaker, clip one end of the glass fiber filter paper,
Hanged vertically. The lower end was heated by a small gas flame of lighters for about 3 seconds, and the self-extinguishing property in a state where the fire source was 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
The conductivity at 25 ° C. was measured using 511B.

3. Preparation of test battery and 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 electrolyte 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 7 and Comparative Examples 1 to 6 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, self-extinguishing properties (flame retardancy) and electrical conductivity were measured.
Further, the charge / discharge capacity of a coin-type battery (test cell A) using natural graphite for the working electrode was measured. Table 1 shows the results. 1 and 2 show the results of the initial charge / discharge curve and the cycle characteristics of the coin-type battery prepared using the electrolytic solution of Example 1.

Examples 8 to 12 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 . A coin-type battery (test cell A) was prepared in the same manner as in Example 1 except that artificial graphite was used for the working electrode, and the self-extinguishing property (flame retardancy) of the electrolyte, the conductivity, and the charge / discharge capacity of the battery were used. Was measured. Table 1 shows the results.

[0053]

[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 Comparative Example 1 containing no phosphoric acid ester, and contains 20% by volume of trimethyl phosphate. The blended electrolyte solution provided a superior discharge capacity compared to the graphite electrode that did not have a discharge capacity, and had a higher conductivity than the electrolyte solution blended with 30% by volume of cyclic phosphate. showed that.

Examples 13 and 14 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, self-extinguishing properties (flame retardancy), electrical conductivity, and charge / discharge capacity of a coin-type battery (test cell B) were measured. Table 2 shows the results. FIGS. 3 and 4 show the results of the initial charge / discharge curve and the cycle characteristics of the coin battery prepared using the electrolyte solution of Example 13.

[0056]

[Table 2]

The abbreviations in the table are as follows. LiBETI: LiN (SO ₂ C ₂ F ₅ ) ₂ MEP: ethylene methyl phosphate EEP: ethylene ethyl phosphate ADMP: allyl dimethyl phosphate BzDMP: benzyl dimethyl phosphate DMPP: dimethyl phenyl phosphate TMP: trimethyl phosphate EDMP: phosphorus Dimethyl ethyl phosphate PrDMP: Dimethyl propyl phosphate BuDMP: Dimethyl butyl phosphate TFEDMP: Trifluoroethyl dimethyl phosphate EC: Ethylene carbonate PC: Propylene carbonate DMC: Dimethyl carbonate DEC: Diethyl carbonate

[0058]

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.

In particular, an excellent discharge capacity can be obtained by combining with a negative electrode made of a wide range of materials, as compared with an electrolytic solution in which trimethyl phosphate in an amount necessary for obtaining flame retardancy alone is incorporated into a carbonate ester. Similarly, a higher conductivity can be obtained as compared to an electrolyte solution in which the amount of cyclic phosphate necessary for obtaining flame retardancy is used alone or in a high concentration in a carbonate ester. Therefore, the electrolytic solution of the present invention is extremely useful as an electrolytic solution for a 4V-class small lithium secondary battery.

[Brief description of the drawings]

FIG. 1 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 1.

FIG. 2 is a diagram showing 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 B) manufactured using the electrolytic solution prepared in Example 13.

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

 ──────────────────────────────────────────────────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 3-1 Citizen, Mitsubishi Chemical Co., Ltd. Tsukuba Research Laboratory (72) Inventor Shigeaki Kasuya 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Pref. Mitsubishi Chemical Corporation Tsukuba Research Laboratory F-term (reference) 5H029 AJ05 AJ12 AK03 AL06 AL12 AM02 AM03 AM04 AM05 AM07 BJ02 BJ03 BJ04 BJ12 BJ14 HJ02 HJ07

Claims (5)

[Claims] The lithium salt is represented by the general formula (I): (Wherein, R ¹ represents an alkylene group having 2 to 8 carbon atoms;
R ² represents an alkyl group having 1 to 4 carbon atoms); and / or a general formula (II): (Wherein, R ³ and R ⁴ are each independently a carbon number of 1 to
R ⁵ represents an allyl group, having 6 to 6 carbon atoms;
8 represents an aryl group or an aralkyl group), which is dissolved in an organic solvent containing 0.1 to 15% by volume of a phosphoric acid ester represented by the formula (8): 2. The method according to claim 1, wherein the organic solvent is selected from the group consisting of:
And / or a phosphate ester represented by the general formula (II):
(B) general formula (III): (Wherein R ⁶ , R ⁷ and R ⁸ each independently represent an unsubstituted or fluorine-substituted lower alkyl group having 1 to 4 carbon atoms) 10 to 90% by volume of a phosphoric acid lower ester represented by the formula:
And (c) a mixed solvent containing 10 to 90% by volume of a carbonic acid ester. 3. The carbonate ester is ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, methyl ethyl carbonate, methyl-n-propyl carbonate, methyl isopropyl carbonate, carbonic acid The electrolytic solution according to claim 1 or 2, wherein the electrolytic solution is at least one selected from the group consisting of ethyl-n-propyl, ethyl isopropyl carbonate, and -n-propyl isopropyl carbonate. 4. The lithium salt according to claim 1, wherein the lithium salt is LiPF ₆ , LiBF ₄ or the general formula (IV): (Wherein m and n each independently represent an integer of 1 to 4).
4. The electrolytic solution according to any one of 3. 5. A carbonaceous substance capable of inserting and extracting lithium,
A negative electrode comprising a material selected from the group consisting of lithium metal and a lithium alloy; a positive electrode comprising a lithium transition metal composite oxide; and a lithium secondary battery comprising the electrolytic solution according to claim 1. Next battery.