JPH10270077A - Electrolyte for lithium secondary battery - Google Patents

Abstract

(57) [Problem] To provide an electrolyte for a lithium secondary battery which can constitute a lithium secondary battery having excellent cycle characteristics and also excellent battery characteristics such as electric capacity and storage stability. I do. SOLUTION: This is achieved by a lithium secondary battery electrolyte solution having a non-aqueous solvent and a fluorine-containing electrolyte capable of releasing lithium ions, to which a lithium compound is added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery, which can form a lithium secondary battery having excellent cycle characteristics and also excellent battery characteristics such as electric capacity and storage stability.

[0002]

2. Description of the Related Art As an electrolyte for a lithium secondary battery, a non-aqueous electrolyte obtained by dissolving a fluorine-containing electrolyte such as LiPF _{6 in} a solvent such as a cyclic carbonate, a chain carbonate, or an ether has a high voltage and a high capacity. It is often used because it is suitable for obtaining batteries. However, HF in the electrolyte not only causes corrosion of the battery can,
It reduces battery capacity and adversely affects cycle characteristics. Therefore, a lithium secondary battery using such an electrolyte is not always satisfactory in battery characteristics such as cycle characteristics, electric capacity, storage stability, etc., and an electrolyte for a lithium secondary battery which does not cause deterioration in battery performance. Is desired.

[0003]

In view of the above-mentioned problems of the electrolyte for a lithium secondary battery, H in the electrolyte has been considered.
It is an object of the present invention to provide an electrolyte for a lithium secondary battery capable of forming a lithium secondary battery having a low F and excellent battery characteristics, particularly excellent cycle characteristics.

[0004]

SUMMARY OF THE INVENTION The present invention provides a non-aqueous solvent and
Lithium having a fluorine-containing electrolyte capable of releasing lithium ions
In an electrolyte for a lithium secondary battery, lithium is contained in the electrolyte.
Lithium secondary battery characterized by adding a lithium compound
It relates to an electrolyte for a pond. Lithium compound used in the present invention
The materials include metallic lithium and lithium hydride (Li
H) and further ROLi (wherein R is an alkyl having 1 to 6 carbon atoms)
Represents a kill group or an allyl group. ). Also before
Specific examples of the ROLi include lithium methoxide.
(CH_ThreeOLi), lithium ethoxide (C_TwoH_FiveO
Li), lithium iso-propoxide (iso-C)
_ThreeH₇OLi), lithium tert-butoxide (t
ert-C_FourH₉OLi) or lithium phenoxy
Do (C ₆H_FiveOLi) and the like. In the present invention
Therefore, the amount of the lithium compound to be added depends on the amount of HF in the electrolytic solution.
More usually, 0.05% by weight based on the electrolyte
Or less, preferably 0.03% by weight or less.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The non-aqueous solvent used in the present invention is preferably a solvent comprising a high dielectric constant solvent and a low viscosity solvent. Preferred examples of the high dielectric constant solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These high dielectric constant solvents may be used alone or in combination of two or more.

As the low-viscosity solvent, for example, dimethyl carbonate (DMC), methyl ethyl carbonate (M
EC), chain carbonates such as diethyl carbonate (DEC), tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane And lactones such as γ-butyrolactone, nitriles such as acetonitrile, esters such as methyl propionate, and amides such as dimethylformamide. These low-viscosity solvents may be used alone or in combination of two or more. The high dielectric constant solvent and the low viscosity solvent are each arbitrarily selected and used in combination. In addition,
The high dielectric constant solvent and the low viscosity solvent are used in a volume ratio (high dielectric constant solvent: low viscosity solvent) of usually 1: 9 to 4: 1, preferably 1: 4 to 7: 3.

The fluorine-containing electrolyte capable of releasing lithium ions used in the present invention includes, for example, fluorophosphates such as LiPF ₆ , fluoroborates such as LiBF ₄ , and fluoroarsenic acids such as LiAsF ₆ salt,
Or a triflate salt such as LiOSO ₂ CF ₃ . At least one of these electrolytes is selected and used. These fluorinated electrolytes are usually added to the non-aqueous solvent in an amount of 0.1 M to 3 M, preferably 0.5 to 1.5 M.
It is used after being dissolved at a concentration of M.

[0008] The electrolyte for a lithium secondary battery of the present invention is prepared by adding the above-mentioned lithium compound to the electrolyte in the electrolyte.
The F concentration can be less than 15 ppm. Therefore, the lithium secondary battery using the electrolyte of the present invention has good cycle characteristics.

The electrolyte for a lithium secondary battery according to the present invention can be suitably used as a component of a lithium secondary battery. The constituent members other than the electrolytic solution are not particularly limited, and various constituent members conventionally used can be used.
For example, as the positive electrode material (positive electrode active material), a composite metal oxide of lithium and at least one metal selected from the group consisting of chromium, vanadium, manganese, iron, cobalt, and nickel is used. Examples of such a composite metal oxide include LiCoO ₂ , LiMn ₂ O ₄ ,
LiNiO _{2 and the} like.

[0010] The positive electrode is made of a conductive material such as acetylene black or carbon black, polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (P).
VDF) and the like to form a positive electrode mixture, and this positive electrode mixture is rolled as a current collector into an aluminum or stainless steel foil or lath plate at a temperature of about 50 to 250 ° C. It is produced by heating under vacuum for about an hour.

As the negative electrode material (negative electrode active material), lithium metal, lithium alloy, carbon material (pyrolyzed carbons, cokes, graphites, glassy carbons, organic polymer compound burner, carbon fiber, activated carbon) Materials that can occlude and release lithium, such as tin and tin composite oxides, are used. A powder material such as a carbon material is kneaded with a binder such as ethylene propylene diene monomer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) to be used as a negative electrode mixture.

The structure of the lithium secondary battery is not particularly limited, and may be a coin-type battery having a positive electrode, a negative electrode, and a single-layer or multiple-layer separator, a cylindrical battery having a positive electrode, a negative electrode, and a roll-shaped separator. A prismatic battery is an example. In addition, as a separator,
Known microporous polyolefin membranes, woven fabrics, nonwoven fabrics and the like are used.

[0013]

EXAMPLES Next, the present invention will be described specifically with reference to examples and comparative examples, but these do not limit the present invention in any way. Example 1 [Preparation of electrolytic solution] Commercially available EC and DEC were
After preparing C: DEC (volume ratio) = 1: 2, the solution was passed through Molecular Sieves 4A (LHSV; 3 / hour). LiPF ₆ was dissolved therein to a concentration of 1M. Thereafter, 3 × 10 ⁻² wt% of lithium metal powder as a lithium compound was added to the electrolyte solution, and room temperature (25%) was added.
C) for 1 day to treat the electrolyte. Unreacted metallic lithium powder was used after filtration. The content of HF in this electrolytic solution was 14 ppm.

[Preparation of Lithium Secondary Battery and Measurement of Battery Characteristics] LiCoO ₂ (cathode active material) was 70% by weight, acetylene black (conductive agent) was 20% by weight, and polytetrafluoroethylene (binder) was 10% by weight. The mixture was mixed at a ratio of weight%, and this was compression molded to prepare a positive electrode. 95% by weight of natural graphite (negative electrode active material) and 5% by weight of ethylene propylene diene monomer (binder) were mixed and compression molded to prepare a negative electrode. Then, using a separator made of polypropylene microporous film, impregnated with the above-mentioned electrolytic solution, a coin-type battery (diameter: 20 mm, thickness: 3.2 m)
m) was prepared. Room temperature (2
0 ° C.) at a constant current of 0.8 mA and a charge end voltage of 4.2
V, discharge capacity retention rate of 100% after 100 cycles when charging and discharging are repeated as a potential regulation of a discharge end voltage of 2.7 V
Met.

Example 2 An electrolytic solution was prepared in the same manner as in Example 1 except that lithium hydride was used as a lithium compound and 3 × 10 ^-2 wt% was added to the electrolytic solution. At this time, the content of HF in the electrolyte was 13 ppm, and 100
The discharge capacity retention after the cycle was 85%.

Example 3 Using lithium ethoxide as a lithium compound,
An electrolyte was prepared in the same manner as in Example 1, ^except that 1 × 10 ^-2 wt% was added to the electrolyte. At this time, H
The content of F was 3 ppm, and 1
The discharge capacity retention rate after the 00 cycle was 89%.

Example 4 An electrolytic solution was prepared in the same manner as in Example 3 except that lithium iso-propoxide was used as a lithium compound, and 1 × 10 ^-2 wt% was added to the electrolytic solution. At this time, the content of HF in the electrolyte was 2 ppm, and the discharge capacity retention rate after 100 cycles using a coin-type battery was 89%.

Example 5 An electrolytic solution was prepared in the same manner as in Example 3 except that lithium tert-butoxide was used as a lithium compound, and 2 × 10 ^-2 wt% was added to the electrolytic solution. At this time, the content of HF in the electrolyte was 2 ppm, and the discharge capacity retention rate after 100 cycles using a coin-type battery was 89%.

Example 6 Lithium phenoxide was used as a lithium compound, and 2 × 10 ⁻² wt% was added to the electrolyte.
An electrolytic solution was prepared in the same manner as in Example 3. At this time, the content of HF in the electrolytic solution was 4 ppm, and the discharge capacity retention rate after 100 cycles using a coin battery was 87%.

Comparative Example 1 An electrolyte was prepared in the same manner as in Example 1 except that no lithium compound was added. At this time, the HF amount of the electrolyte was 4
It was 0 ppm, and the discharge capacity retention ratio after 100 cycles using the coin battery was 71%.

[0021]

According to the present invention, it is possible to provide an electrolyte for a lithium secondary battery capable of forming a lithium secondary battery having excellent cycle characteristics and also excellent battery characteristics such as electric capacity and storage stability. .

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yukio Nakata 5 Ube Kogushi 1978 Kobe, Ube City, Yamaguchi Prefecture

Claims (4)

[Claims] 1. An electrolyte for a lithium secondary battery having a non-aqueous solvent and a fluorine-containing electrolyte capable of releasing lithium ions, comprising a lithium compound added to the electrolyte. 2. The electrolyte according to claim 1, wherein the lithium compound is metallic lithium. 3. The electrolyte for a lithium secondary battery according to claim 1, wherein the lithium compound is lithium hydride (LiH). 4. The electrolyte for a lithium secondary battery according to claim 1, wherein the lithium compound is ROLi (where R represents an alkyl group or an allyl group having 1 to 6 carbon atoms). .