JPH06223874A - Electrolyte for lithium secondary battery - Google Patents

Abstract

PURPOSE:To obtain an electrolyte for lithium secondary battery having the excellent lifetime characteristic, which can be charged to a high voltage without generating the electrolysis and which can repeat charge and discharge many times stably. CONSTITUTION:Lithium salt is solved in the mixture solvent, which consists of sulfolane at 20-80% by capacity and organic solvent having a low viscosity at 10-70% by capacity and organic solvent having a high dielectric constant at 10-30% by capacity. The rise of the pressure inside of a battery due to the electrolysis and the running short of electrolyte are thereby hard to be generated, and a lithium secondary battery using an electrode for high electromotive force can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic solution for a lithium secondary battery which is difficult to electrolyze and can be charged at a high voltage.

[0002]

2. Description of the Related Art In a lithium secondary battery using a lithium-cobalt-based electrode capable of generating high electromotive force as a positive electrode, an electrolytic solution that can utilize the high electromotive force is required. That is, conventionally, as an electrolytic solution for a lithium secondary battery, a solution in which a lithium salt is dissolved in a mixed solvent of propylene carbonate and a low-viscosity organic solvent such as 1,2-dimethoxyethane or tetrahydrofuran has been known. However, there is a problem in that such an electrolytic solution cannot be put into practical use because it is electrolyzed at a high voltage during charging, causing deformation or destruction of the battery or liquid depletion due to an increase in internal pressure.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides an electrolytic solution for a lithium secondary battery, which can be charged to a high voltage without electrolysis, and can be repeatedly charged and discharged stably a number of times and which is excellent in battery life. Development is an issue.

[0004]

The present invention provides sulfolane 2
Electrolysis for a lithium secondary battery, wherein a lithium salt is dissolved in a mixed solvent consisting of 0 to 80% by volume, an organic solvent having a low viscosity of 10 to 70% by volume, and an organic solvent having a high dielectric constant of 10 to 30% by volume. It provides a liquid.

[0005]

Function: Sulfolane widens the potential window for lithium to the positive side. The low-viscosity organic solvent dilutes sulfolane to reduce the viscosity of the electrolytic solution and improve the conductivity. The high-dielectric-constant organic solvent improves the dielectric constant of the electrolytic solution, prevents the dielectric constant from decreasing due to the addition of the low-viscosity organic solvent, and facilitates dissociation of the lithium salt. As a result, it is possible to form a lithium secondary battery that uses a lithium-cobalt-based electrode as the positive electrode, which can be charged to a high voltage without electrolysis of the electrolyte, has excellent conductivity and dielectric constant, and can generate high electromotive force. become.

[0006]

EXAMPLE The electrolytic solution of the present invention contains sulfolane, an organic solvent having a low viscosity, an organic solvent having a high dielectric constant and a lithium salt as components, and is used for forming a lithium secondary battery.

As the low-viscosity organic solvent, those having a viscosity at room temperature of 1 cP or less are preferably used, and examples thereof include methyl formate, methyl acetate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran,
Examples include 1,3-dioxolane and the like.

As the organic solvent having a high dielectric constant, for example, ethylene carbonate, dimethyl sulfoxide, γ-butyrolactone or the like is used.

Any appropriate lithium salt may be used, examples of which include Li ion, PF ₆ ion and ClO ₄
Ions, BF ₄ ions, CF ₃ SO ₃ ions, AlCl ₄ ions, AsF ₆ ions, and combinations of anions such as I ions are included.

The electrolytic solution can be prepared by dissolving a lithium salt in a mixed solvent of sulfolane, an organic solvent having a low viscosity and an organic solvent having a high dielectric constant. Upon mixing, one or two or more kinds of components may be used, and it is preferable to use an electrolyte solution having a water content as low as possible, especially 50 ppm or less. Water reacts with the positive and negative electrodes to cause deterioration of battery performance, and electrolysis of water occurs to prevent the original battery reaction.

The mixing ratio of each solvent component is sulfolane 20.
˜80% by volume, low viscosity organic solvent 10˜70% by volume, and high dielectric constant organic solvent 10˜30% by volume. If the mixing ratio is out of the above range, the balance between the electric potential window width and the electric conductivity and the dielectric constant may be poor, or the internal pressure of the battery may be greatly increased by the electrolysis of the electrolytic solution, or the performance may be significantly deteriorated due to repeated charging and discharging. The battery life is short. The content of the lithium salt in the electrolytic solution may be appropriately determined depending on the form and performance of the battery, and is generally about 0.1 to 3 mol / liter.

The electrolytic solution of the present invention is for forming a lithium secondary battery as described above, and can be particularly preferably used for forming an electrode which can generate a high electromotive force as a positive electrode. Incidentally, as the positive electrode according to the present invention, for example, a lithium-cobalt type electrode having an active material represented by the general formula: Li _w Co _1-xy M _x P _y O _{2 + z} can be used. M in the above formula may contain two or more kinds of transition metals such as Ni. Also, w is 0 <w ≦ 2,
x is 0 ≦ x <1. Further, y is 0 ≦ y <1, and
z is -1≤z≤4.

In addition to the method of using the electrolytic solution of the present invention as it is in the formation of a lithium secondary battery, for example, a method of holding it by impregnating it with an appropriate polymer, a porous support, or a gel-like substance. Can be used in any method.

Example 1 Sulfolane, 1,2-dimethoxyethane and ethylene carbonate having a water content of 50 ppm or less by dehydration and refining treatment were mixed in a volume ratio of 50:40:10, and LiClO ₄ was mixed in the mixed solvent. An electrolytic solution was obtained by dissolving the solution to a concentration of 1 mol / liter. The LiClO ₄ used
Is a product that has been vacuum dried at 200 ° C. for 24 hours.

Example 2 An electrolytic solution was obtained in the same manner as in Example 1 except that sulfolane, 1,2-dimethoxyethane and ethylene carbonate were used in a volume ratio of 20:70:10.

Example 3 An electrolytic solution was obtained in the same manner as in Example 1 except that sulfolane, 1,2-dimethoxyethane and ethylene carbonate were used in a volume ratio of 20:50:30.

Example 4 An electrolytic solution was obtained in the same manner as in Example 1 except that sulfolane, 1,2-dimethoxyethane and ethylene carbonate were used at a volume ratio of 80:10:10.

Comparative Example 1 An electrolytic solution was obtained in the same manner as in Example 1 except that a mixed solvent prepared by mixing propylene carbonate and 1,2-dimethoxyethane at a volume ratio of 50:50 was used.

Comparative Example 2 An electrolytic solution was obtained in the same manner as in Example 1 except that sulfolane, 1,2-dimethoxyethane and ethylene carbonate were used in a volume ratio of 5: 90: 5.

Comparative Example 3 An electrolytic solution was obtained in the same manner as in Example 1 except that sulfolane, 1,2-dimethoxyethane and ethylene carbonate were used in a volume ratio of 5:45:50.

Comparative Example 4 An electrolytic solution was obtained in the same manner as in Example 1 except that sulfolane, 1,2-dimethoxyethane and ethylene carbonate were used at a volume ratio of 90: 5: 5.

Evaluation Test Internal Pressure of Battery Using the electrolytic solutions obtained in the above-mentioned Examples and Comparative Examples, Li
A closed container type lithium secondary battery having Co _0.5 P _0.5 O ₂ as a positive electrode and Li as a negative electrode was prepared, and a pressure sensor and a pressure gauge were attached to the cap type negative electrode to repeat charging and discharging, and at the initial stage,
The internal pressure of the battery was examined at the 50th time and the 100th time.

Charge / Discharge Efficiency A sealed container type lithium secondary battery was prepared according to the above, and the charge / discharge was repeated to examine the ratio of the charge / discharge to the initial value at the 50th or 100th cycle.

The results are shown in Table 1.

[Table 1]

[0025]

INDUSTRIAL APPLICABILITY The electrolytic solution of the present invention is unlikely to cause an increase in battery internal pressure or liquid depletion due to electrolysis, can be charged at a high voltage, and can form a lithium secondary battery using a high electromotive force electrode. The obtained battery can be repeatedly charged and discharged a number of times stably and has an excellent life.

Claims (1)

[Claims] 1. Sulfolane 20 to 80% by volume, low viscosity organic solvent 10 to 70% by volume, and high dielectric constant organic solvent 10
An electrolytic solution for a lithium secondary battery, characterized in that a lithium salt is dissolved in a mixed solvent of ˜30% by volume.