JP2010176903A - Nonaqueous electrolytic solution for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic solution for a lithium secondary battery, which retains high ion conductivity even when containing a low dielectric constant solvent at high blending ratio, and shows less drop in ion conductivity under low temperature. <P>SOLUTION: The nonaqueous electrolytic solution (D) for the lithium secondary battery contains essential components of (A) a calixpyrrole class, (B) a lithium salt, and (C) a nonaqueous solvent. A mol ratio (A)/(B) is 0.1 to 1.0, and the content of lithium salt (B) is 0.1 to 3.0 mol/L. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte for a lithium secondary battery. More specifically, the present invention relates to an electrolytic solution for a lithium secondary battery that has high ionic conductivity at low temperatures and a small decrease in battery capacity.

In recent years, lithium secondary batteries having high energy density have been actively developed as power sources for portable electronic devices such as mobile phones and notebook computers. In addition, lithium secondary batteries have a high operating voltage and easily obtain high output, so that they are also becoming more important as power sources for electric vehicles and hybrid vehicles.
Lithium secondary batteries are required to operate reliably in a wide temperature range as their application fields expand. In particular, there is an urgent need to improve battery performance at low temperatures in preparation for use in cold regions.

A non-aqueous electrolyte composed of a non-aqueous solvent and a lithium salt is used for the lithium secondary battery. This non-aqueous solvent is typically a mixture of a high dielectric constant solvent such as a cyclic carbonate and a low viscosity solvent such as a chain carbonate.
Among these solvents, ethylene carbonate (EC) and dimethyl carbonate (DMC), which are widely used, have relatively high melting points of 37 ° C. and 3 ° C., respectively. Under viscosity is likely to increase and coagulate For this reason, when such a nonaqueous solvent is used at low temperature, there exists a problem that the ionic conductivity and battery capacity of electrolyte solution fall rapidly.

  As an improvement measure, ethyl methyl carbonate (EMC) (see Patent Document 1) or acetic acid ester (see Patent Document 2), which is a low-viscosity and low-melting solvent, is used in combination with the above cyclic carbonate or chain carbonate. It has been proposed to be used as

  However, when the mixing ratio of a low dielectric constant solvent such as EMC in the electrolytic solution is high, the solubility and dissociation degree of the lithium salt are lowered, so that sufficient ionic conductivity cannot be obtained in a wide temperature range. Have Further, when an acetate ester is blended in the electrolytic solution, the reduction resistance deteriorates, so that there is a problem that the battery capacity is greatly reduced during the cycle.

Japanese Patent Laid-Open No. 10-294016 Japanese Patent Laid-Open No. 9-245838

  Accordingly, it is an object to provide a non-aqueous electrolyte for a lithium secondary battery that has high ionic conductivity even when the blending ratio of the low dielectric constant solvent is high and has a small decrease in ionic conductivity at low temperatures.

The inventors of the present invention have reached the present invention as a result of studies to achieve the above object.
That is, the present invention comprises the calic spirols (A), the lithium salt (B), and the nonaqueous solvent (C) as essential components, and the molar ratio (A) / (B) is 0.1 to 1.0. There is an electrolyte for a lithium secondary battery.

  The electrolytic solution for a lithium secondary battery of the present invention can greatly improve the ion conductivity at low temperatures and can greatly improve the capacity reduction of the battery.

Hereinafter, the present invention will be described in more detail.
The electrolyte solution for a lithium secondary battery of the present invention contains calic spirols (A), a lithium salt (B) and a nonaqueous solvent (C) as essential components. By blending the calic spirols, even when the blending ratio of the low dielectric constant solvent is high, an effect of greatly increasing the dissociation degree of the lithium salt can be obtained, and an electrolytic solution having high ionic conductivity in a wide temperature range can be configured. That is, it is possible to provide an electrolytic solution that is superior in ionic conductivity at low temperatures.

  The calic spirols (A) in the present invention are cyclic oligomers in which pyrrole is connected by a tertiary carbon atom, and examples thereof include compounds represented by the following general formula (1).

In formula (1), R ^{1 to} R ⁴ are each independently an aliphatic hydrocarbon group or an aromatic hydrocarbon group.
The aliphatic hydrocarbon group may be either a saturated hydrocarbon group or an unsaturated hydrocarbon group, but is preferably a saturated hydrocarbon group from the viewpoint of the stability of the compound.
The saturated hydrocarbon group preferably has 1 to 4 carbon atoms from the viewpoint of solubility, and more preferably a methyl group from the viewpoint of ease of synthesis.
Examples of the aromatic hydrocarbon group include a phenyl group.

Two substituents may be connected to form a spiro ring. For example, R ¹ and R ² may be connected to form a 1,1-cyclohexane ring.

  In consideration of the solubility in the electrolytic solution and the degree of dissociation of the lithium salt, n needs to be 1 to 5. 1 to 3 are preferable.

  Specific examples of the calic spirols (A) include meso-octamethyl-calix [4] pyrrole (C4P) represented by the following general formula (2), 1,1,3 represented by the following general formula (3) 3,5,5-meso-hexamethyl-2,2,4,4,6,6-meso-hexaphenyl-calix [6] pyrrole (C6P).

The lithium secondary battery electrolytic solution of the present invention has a high ionic conductivity in a wide temperature range by improving the dissociation degree of the lithium salt by blending calic spirols in the electrolytic solution. .
The molar ratio (A) / (B) of the calic spirols to the lithium salt (B) is preferably 0.1 to 1.0, and more preferably 0.2 to 1.0. If it is less than 0.1, a sufficient degree of dissociation of the lithium salt cannot be achieved, so that sufficient ionic conductivity may not be obtained.
On the other hand, if it exceeds 1.0, sufficient ionic conductivity may not be obtained due to an increase in viscosity.

As lithium salt (B), what is used for the electrolyte solution for normal lithium secondary batteries can be used.
Specific examples include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCl, and LiI, and organic lithium salts such as LiCF ₃ SO ₃ and LiN (CF ₃ SO ₂ ) ₂ . Among these, LiPF ₆ and LiBF ₄ are preferable from the viewpoints of solubility in a non-aqueous solvent and ion conductivity.

  The concentration of these lithium salts (B) in the electrolyte is usually 0.1 to 3 mol / L, but is preferably 0.2 to 2 mol / L in view of ionic conductivity.

As the non-aqueous solvent (C) of the present invention, those used in ordinary electrolyte solutions for lithium secondary batteries can be used.
Specific examples include non-aqueous solvents such as cyclic or chain carbonates, cyclic or chain ethers, chain carboxylates, lactones, and nitriles, and these can be used alone or in combination.
Examples of the cyclic carbonate include ethylene carbonate (EC) and propylene carbonate (PC).
Examples of the chain carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).
Examples of the cyclic or chain ether include tetrahydrofuran, 1,2-dimethoxyethane and the like.
Examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, and methyl propionate.
Examples of the lactone include γ-butyrolactone, and examples of the nitrile include acetonitrile.
Of these non-aqueous solvents, cyclic or chain carbonates are preferred in view of electrochemical stability. Of these carbonic acid esters, DEC and EMC, which are low viscosity and low melting point solvents, are particularly suitable.

  The electrolytic solution for a lithium secondary battery of the present invention contains the above-described calic spirols (A), a lithium salt (B), and a nonaqueous solvent (C).

  The method for preparing the electrolyte solution for a lithium secondary battery of the present invention is not particularly limited, and can be prepared by dissolving calic spirols and a lithium salt in a non-aqueous solvent.

  Hereinafter, although an example and a comparative example explain the present invention further, the present invention is not limited to these. Hereinafter, unless otherwise specified, “%” represents “% by weight” and “parts” represents “parts by weight”.

Production Example 1: Synthesis of meso-octamethyl-calix [4] pyrrole (C4P) 33.5 parts of pyrrole and 29 parts of acetone were dissolved in 100 parts of methanol, 1.5 parts of methanesulfonic acid was added, and the mixture was stirred for 1 hour at room temperature. Reacted. After completion of the reaction, the precipitate was separated by filtration and washed with methanol until the filtrate became colorless. The obtained solid was reprecipitated in hexane to obtain 37 parts (yield 70%) of meso-octamethyl-calix [4] pyrrole [(C4P) represented by the general formula (2)]. .

Production Example 2: Synthesis of 1,1,3,3,5,5-meso-hexamethyl-2,2,4,4,6,6-meso-hexaphenyl-calix [6] pyrrole (C6P) (1) Synthesis of diphenyldi- (2-pyrrolyl) methane:
After dissolving 18.2 parts of benzophenone and 13.4 parts of pyrrole in 300 parts of ethanol, 10 parts of boron trifluoride-diethyl ether complex was added and reacted at room temperature for 5 days. After completion of the reaction, the precipitate was separated by filtration and washed with methanol until the filtrate became colorless. The obtained solid was reprecipitated in hexane to obtain 11.9 parts (yield 40%) of diphenyldi- (2-pyrrolyl) methane.

(2) Synthesis of C6P:
After dissolving 11.9 parts of the above diphenyldi- (2-pyrrolyl) methane and 11.9 parts of boron trifluoride-diethyl ether complex in a mixed solution of 200 parts of acetone and 200 parts of ethanol, 10 parts of trifluoroacetic acid. And allowed to react at room temperature for 5 days.
After completion of the reaction, the precipitate was separated by filtration and washed with methanol until the filtrate became colorless. The obtained solid was reprecipitated in hexane to give 4.1 parts (yield 30%) of 1,1,3,3,5,5-meso-hexamethyl-2,2,4,4,4. 6,6-Meso-hexaphenyl-calix [6] pyrrole [(C6P) represented by the general formula (3)] was obtained.

Example 1
In a non-aqueous solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed so that EC: DEC = 0.3: 0.7 (volume ratio), LiPF ₆ is 1 mol / L, Production Example 1 (C4P) obtained in (1) was dissolved to 0.5 mol / L to prepare a non-aqueous electrolyte.

Example 2
In Example 1, instead of LiPF _6, the same operation as in Example 1 was performed except that LiBF ₄ was changed to 1 mol / L to prepare a nonaqueous electrolytic solution.

Example 3
In Example 1, instead of LiPF _6, the same operation as in Example 1 was performed except that LiCl was changed to 1 mol / L to prepare a nonaqueous electrolytic solution.

Example 4
In Example 1, except that (C4P) was changed to (C6P), the same operation as in Example 1 was performed to prepare a nonaqueous electrolytic solution.

Example 5
In Example 2, the same operation as in Example 2 was performed except that (C4P) was changed to (C6P) to prepare a nonaqueous electrolytic solution.

Comparative Example 1
In Example 1, except that (C4P) was not blended, the same operation as in Example 1 was performed to prepare a nonaqueous electrolytic solution.

Comparative Example 2
In Example 2, the same operation as in Example 2 was performed except that (C4P) was not blended to prepare a nonaqueous electrolytic solution.

Comparative Example 3
In Example 1, the same operation as in Example 1 was performed except that the concentration of (C4P) was changed to 0.05 mol / L to prepare a nonaqueous electrolytic solution.

  The ion conductivity at 25 ° C., 0 ° C. and −20 ° C. was measured for the non-aqueous electrolyte of the present invention prepared in Examples 1 to 5 and the non-aqueous electrolyte for comparison prepared in Comparative Examples 1 to 3. did. The ionic conductivity was measured by the AC impedance method. The results are shown in Table 1.

  As shown in Table 1, each of the nonaqueous electrolyte solutions prepared in Examples 1 to 5 has excellent ionic conductivity, and the ionic conductivity at low temperature was particularly prepared in Comparative Examples 1 to 3. It can be seen that there is a significant improvement over the liquid.

  According to the present invention, it is possible to provide a non-aqueous electrolyte for a lithium secondary battery that exhibits high ionic conductivity in a wide temperature range, and that is particularly excellent in ionic conductivity at low temperatures. Moreover, by using the non-aqueous electrolyte of the present invention, it is possible to provide a lithium secondary battery in which battery performance at a low temperature is greatly improved.

Claims (4)

Carrick spirols (A), lithium salt (B), and non-aqueous solvent (C) are essential components, and the molar ratio of (A) / (B) is 0.1 to 1.0. Electrolytic solution (D) for lithium secondary battery. The electrolyte solution (D) for a lithium secondary battery according to claim 1, wherein the calic spirols (A) are represented by the following general formula (1).

[Wherein, R ^{1 to} R ⁴ are each independently an aliphatic hydrocarbon group or an aromatic hydrocarbon group, and two substituents may be linked to form a ring. n represents an integer of 1 to 5. ]   The electrolyte solution (D) for a lithium secondary battery according to claim 1 or 2, wherein the content of the lithium salt (B) is 0.1 to 3.0 mol / L. The lithium secondary battery characterized by including the electrolyte solution (D) in any one of Claims 1-3.