JPH10189042A - Lithium secondary battery electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery electrolyte having a small capacity reduction effected together with a progress of a charge and discharge cycle progresses. SOLUTION: This electrolyte contains lithium salt as a solute, a cyclic sulfuric ester represented by the formula (R1 and R2 represent independent alkyl groups having 1 to 2 hydrogen or carbon; (n) is 0 or 1) and an organic solvent for dissolving the lithium salt. In this case, the cyclic sulfuric ester is contained in an electrolyte at a concentration of 0.1 to 50wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrolyte for a lithium secondary battery. In particular, the present invention relates to improvement of cycle characteristics of an organic solvent electrolyte for a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, as electronic devices have become smaller and more portable, there has been a demand for the development of batteries with higher energy density. As a promising candidate, carbon materials such as coke and graphite are replaced with lithium secondary batteries using metal lithium anodes that have been proposed previously because there is no danger of internal short-circuiting due to the growth of dendritic electrodeposited lithium. It is receiving attention as a negative electrode material.

[0003]

However, even when such a carbon material is used for the negative electrode, there is a problem that the organic solvent electrolyte is decomposed on the carbon negative electrode as the charge / discharge cycle progresses, and the battery capacity gradually decreases. was there. When the degree of graphitization of the carbon material increases, the capacity increases, but the organic solvent electrolyte tends to decompose and the cycle characteristics tend to deteriorate. An object of the present invention is to provide an electrolyte for a lithium secondary battery in which decomposition on a carbon negative electrode is small with progress of a charge / discharge cycle.

[0004]

The present invention provides a lithium salt as a solute, a cyclic sulfate represented by the formula (I),

Embedded image [Wherein, R ₁ and R ₂ each independently represent hydrogen or an alkyl group having 1 to 2 carbon atoms. Further, n in the formula is 0 or 1. ] And an electrolyte for a lithium secondary battery containing an organic solvent dissolving the lithium salt, wherein the cyclic sulfate is contained in the electrolyte at a concentration of 0.1 to 50% by weight. A liquid is provided.

[0005]

In the present invention, the electrolyte for a lithium secondary battery contains a cyclic sulfate, which reacts with the carbon electrode to form a film having high lithium ion permeability on the surface of the carbon electrode. This film suppresses decomposition of the electrolytic solution.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION

Lithium salt: As the lithium salt as a solute, those conventionally used as a solute in an electrolyte for a lithium secondary battery can be used. For example, LiPF ₆ , LiClO ₄ , L
iBF ₄ , CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NL
i, LiAsF ₆ and the like. The solute is dissolved in the organic solvent. The concentration of the solute in the electrolyte is 0.5 to 1.5M
(Mol / liter).

Organic solvents: Examples of organic solvents for dissolving solutes include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ- One selected from butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like is used as a single solvent or as a mixture of a plurality of solvents. The amount of the organic solvent used is 18 to 96% by weight in the electrolytic solution.

Cyclic sulfate: Examples of the cyclic sulfate represented by the formula (I) include ethylene glycol sulfate, 1,2-propanediol sulfate,
1,2-butanediol sulfate, 2,3-butanediol sulfate, 2,3-pentanediol sulfate, 3,4-hexanediol sulfate,
Examples thereof include 1,3-propanediol sulfate, 1,3-butanediol sulfate, 2,4-pentanediol sulfate, and 3,5-heptanediol sulfate.

[0009] The content of the cyclic sulfate is 0.1 to 50% by weight, preferably 0.3 to 10% by weight in the electrolyte for a lithium secondary battery. If the concentration is less than 0.1% by weight, a sufficiently thick film is not formed on the surface of the carbon electrode, so that decomposition of the electrolytic solution on the surface of the carbon electrode cannot be suppressed, and the cycle characteristics cannot be sufficiently improved. It is. On the other hand, if the concentration exceeds 50% by weight, the film becomes too thick and the lithium ion permeability deteriorates, so that the reaction resistance of the electrode plate increases and the cycle characteristics deteriorate. The sum of the use of the cyclic sulfate and the above-mentioned organic solvent is preferably 68 to 96% by weight in the electrolytic solution.

Lithium secondary battery: FIG. 1 shows a lithium secondary battery (coin type; diameter: 20 mm, thickness: 16) using a carbon electrode prepared in Examples and Comparative Examples described later as a positive electrode.
mm) is a sectional view. This coin-shaped cell includes a stainless steel case 1, a stainless steel sealing plate 2, a positive electrode 3 covered with natural graphite on the same sheet, a negative electrode 4 of a metal lithium sheet, and a separator 5 of a porous polypropylene film immersed in an organic solvent electrolyte. , And an insulating gasket 6.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. Example 1 A coin-type cell (lithium secondary battery) shown in FIG. 1 was produced. Here, the electrolyte was 22.9 g of ethylene carbonate (EC).
5.6 g of LiPF ₆ was dissolved as an electrolyte in a mixed solvent obtained by mixing 21.0 g of propylene carbonate (PC) with propylene carbonate (PC) at a volume ratio of 5: 5, and 0.5 g of ethylene glycol sulfate was further contained. The concentration of LiPF ₆ in the electrolyte was 1.0 M (mol / liter), and the concentration of ethylene glycol sulfate was 1% by weight.

Example 2 A coin cell was prepared in the same manner as in Example 1 except that 1,3-butanediol sulfate was used as the cyclic sulfate.

Comparative Example 1 As an electrolyte containing no cyclic sulfate, 23.1 g of ethylene carbonate (EC) and propylene carbonate (PC) 2 were used.
LiP was added to a mixed solvent obtained by mixing 1.2 g at a volume ratio of 5: 5.
A coin-shaped cell was produced in the same manner as in Example 1 except that 5.7 g of F ₆ was dissolved.

(Cycle Characteristics) The coin cells obtained in Examples 1 and 2 and Comparative Example 1 were discharged at 0.613 mA to a discharge end voltage of 0.0 V, and then at 0.613 mA to a charge end voltage of 1.0 V. After charging, the cycle characteristics of the coin-shaped cell using each of the electrolytes were examined. Figure 2 shows the result.
Shown in FIG. 2 shows the cycle characteristics of each coin cell.
A graph in which the vertical axis represents the carbon electrode capacity (mAh / g) which is the capacity per 1 g of the carbon material, and the horizontal axis represents the number of cycles (times). As shown in the figure, the carbon electrode capacity of the coin-shaped cell using the electrolyte of the present invention is larger from the initial cycle than in the case of using the comparative electrolyte.

As shown in FIG. 3, the capacity retention rate (Example 1: 93%, Example 2: 92%) of the coin-type cell using the electrolytic solution of the present invention in 20 cycles was determined by cyclic sulfuric acid. It is larger than the capacity retention rate in the same cycle when an electrolyte solution containing no ester is used (Comparative Example 1: 85%). From this, it is understood that the cyclic sulfate contained in the electrolytic solution generates a film having high lithium ion permeability on the surface of the carbon electrode, and suppresses the capacity reduction due to the decomposition of the electrolytic solution during charge and discharge. .

In the above embodiment, the case where ethylene glycol sulfate, 1,3-propanediol sulfate and 1,3-butanediol sulfate were used as the cyclic sulfate was described as an example. Diol sulfate, 1,2-butanediol sulfate, 2,3-butanediol sulfate, 2,3
-Pentanediol sulfate, 3,4-hexanediol sulfate, 1,3-propanediol sulfate, 2,4-pentanediol sulfate, 3,
Even when another cyclic sulfate such as 5-heptanediol sulfate is used, an electrolytic solution exhibiting similar excellent cycle characteristics can be obtained.

[0017]

The cyclic sulfate contained in the electrolyte for lithium secondary batteries reacts on the surface of the carbon electrode to form a film (protective film) having high lithium ion permeability, and the decomposition of the electrolyte on the electrode surface Deterioration is suppressed. Therefore, the coin-type cell using the electrolytic solution of the present invention exhibits excellent unique effects such as a small capacity deterioration that occurs with the progress of the charge / discharge cycle.

[Brief description of the drawings]

FIG. 1 is a sectional view of a coin cell.

FIG. 2 is a graph showing cycle characteristics of a coin cell.

FIG. 3 is a graph showing a capacity retention rate of a coin cell.

Claims (4)

[Claims] 1. A lithium salt as a solute, a cyclic sulfate represented by the formula (I): [Wherein, R ₁ and R ₂ each independently represent hydrogen or an alkyl group having 1 to 2 carbon atoms. Further, n in the formula is 0 or 1. ] And an electrolyte for a lithium secondary battery containing an organic solvent that dissolves the lithium salt, wherein the cyclic sulfate is contained in the electrolyte at a concentration of 0.1 to 50% by weight. Lithium secondary electrolyte. 2. The electrolytic solution for a lithium secondary battery according to claim 1, wherein the cyclic sulfate is ethylene glycol sulfate or 1,3-butanediol sulfate. 3. The lithium salt is LiPF ₆ , LiClO.
_{4, LiBF 4, CF 3 SO} 3 Li, (CF 3 SO 2)
₂ NLi and LiAsF ₆ is a more selected compounds according to claim 1 for a lithium secondary battery electrolyte according. 4. An organic solvent comprising ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2- The electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte is selected from dimethoxyethane and 1,2-diethoxyethane.