JPH0973918A - Nonaqueous electrolyte for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the high temperature storage characteristics of a battery by using an electrolyte containing the specified amount of diphenyl picrylhydrazyl serving as a polymerization inhibitor based on the weight of the nonaqueous electrolyte. SOLUTION: A nonaqueous electrolyte for a battery has a base electrolyte prepared by dissolving 1M of LiPF6 as a lithium salt in a nonaqueous solvent of a mixture of ethylene carbonate and propylene carbonate of 1:2, and diphenyl picrylhydrazyl serving as a polymerization inhibitor is added to the base electrolyte. In order to decide the optimum adding amount, zero and various weight percent of diphenyl picrylhydrazyl is added to the base electrolyte to prepare the electrolyte, and charge/discharge test of the batteries using these electrolytes was conducted. From the test results, the adding amount of diphenyl picrylhydrazyl is set to 1wt.% or less. The battery using this electrolyte shows no increase in internal resistance and no deterioration in charge/discharge characteristics even after high temperature storage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte solution for batteries used in lithium secondary batteries and the like.

[0002]

2. Description of the Related Art For example, in a lithium secondary battery, a lithium metal composite oxide is used as a positive electrode active material, a carbon material or metallic lithium is used as a negative electrode active material, and an electrolyte solution in which a lithium salt is appropriately dissolved in a non-aqueous solvent is used. I'm using it. As the non-aqueous solvent, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DE
C) and the like are used alone or in a suitable mixture of two or more kinds. A lithium salt such as LiPF ₆ is used as the solute. A coin type or a cylindrical type is generally used as the form of the battery.

[0003]

In the lithium secondary battery described above, when stored in a high temperature environment of about 80 ° C.,
As compared with storage at room temperature, there are problems that the internal resistance increases significantly, the capacity decreases, and the charge-discharge cycle characteristics deteriorate significantly.

This is because the molecules of the non-aqueous solvent are decomposed or polymerized by the heat during storage at high temperature to deteriorate the electrolytic solution, which is considered to be due to the following mechanism. That is, molecules of the non-aqueous solvent are decomposed by heat, and radicals, that is, free radicals are generated by this thermal decomposition. The generated free radicals cause free radical polymerization in a chained manner, and decomposed molecules are polymerized one after another to generate a polymer. As a result of this polymer hindering the electrode reaction of the battery and the movement of ions, the internal resistance increases and the capacity deteriorates.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is excellent in high-temperature storage characteristics in which internal resistance and charge / discharge cycle characteristics are not deteriorated even when used in a battery and stored at high temperatures. Another object is to provide a non-aqueous electrolyte solution for a battery.

[0006]

The nonaqueous electrolytic solution for a battery of the present invention contains diphenylpicrylhydrazyl as a polymerization inhibitor in an amount of 1% by weight or less based on the nonaqueous electrolytic solution.

[0007] Preferably, the solvent of the non-aqueous electrolyte contains ethylene carbonate, propylene carbonate and diethyl carbonate, and the solute is lithium salt Li.
It is made by dissolving PF ₆ .

In the above-mentioned constitution of the present invention, by containing diphenylpicrylhydrazyl as a polymerization inhibitor in an amount of 1% by weight or less with respect to the non-aqueous electrolytic solution, this electrolytic solution can be used in a battery at a high temperature. It is possible to prevent deterioration of internal resistance and charge / discharge cycle characteristics even after storage.

The mechanism for preventing the deterioration of the characteristics is considered as follows. That is, diphenylpicrylhydrazyl has stable free radicals, and has the property of absorbing and stabilizing free radicals from other sources. Therefore, the polymerization of the non-aqueous solvent by the free radicals does not occur, and the electrolytic solution does not deteriorate.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The nonaqueous electrolytic solution for a battery according to the embodiment of the present invention is a non-aqueous electrolyte prepared by mixing ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) in a ratio of 1: 1: 2. A solution obtained by dissolving 1M LiPF ₆ as a lithium salt in a water solvent is used as a basic electrolytic solution, and diphenylpicryl as shown in the structural formula of FIG. 1 is used as a polymerization inhibitor which is a characteristic component of the present invention with respect to the basic electrolytic solution. Hydrazil (DPPH) was added.

First, in order to investigate the optimum amount of diphenylpicrylhydrazyl to be added, diphenylpicrylhydrazyl is not added to the basic electrolyte or the amount of diphenylpicrylhydrazyl is 0.1 to 5.0% by weight. Nine kinds of electrolytic solutions represented by electrolytic solutions A to I added in various amounts were prepared. Then, these electrolytes A to I were used for an AA type lithium secondary battery having a spiral electrode structure described later, and a charge / discharge test was performed on each battery at room temperature. The conditions of this charge / discharge test are
Current density is 1 mA / cm ² and voltage is 3.0 from start to finish.
The charging / discharging cycle was set to 100 cycles at 100 to 4.2V. As a result, as shown in FIG. 2, for the batteries (using the electrolyte solutions B to D) in which the amount of diphenylpicrylhydrazyl added was up to 1.0% by weight, the additive-free battery (using the electrolyte solution A was used). Battery with almost 1.1% by weight of addition (electrolyte E).
(The battery using the electrolyte solutions F to I) does not have good characteristics and is not suitable for practical use. Therefore, the optimum addition amount of diphenylpicrylhydrazyl to the electrolytic solution is 1% by weight or less.

Among the above-mentioned batteries, the batteries using the electrolyte solutions A (comparative example), B (present invention 1) and D (present invention 2), respectively, were tested for internal resistance during high temperature storage at 80 ° C. When the change was examined, the results shown in FIG. 3 were obtained. As shown in the figure, in the comparative example, the internal resistance remarkably increased with the lapse of the storage period, and reached 1500 milliohms after 3 months. On the other hand, in Inventions 1 and 2 of the present application, the internal resistance does not increase so much even after 3 months and stays at a small value of 200 to 300 milliohms, which means that the high temperature storage characteristics are extremely excellent. understood.

Furthermore, the above-mentioned comparative example and the invention 1 of the present application,
About 2, 20 ℃ after high temperature storage at 80 ℃ for 1 month
When the cycle characteristics in Example 1 were examined under the same conditions as in the charge / discharge test described above, the results shown in FIG. 4 were obtained. As shown in the figure, the comparative example could not be charged / discharged for one cycle, but the inventions 1 and 2 of the present application showed good cycle characteristics.

Here, the structure of the AA-type lithium secondary battery having the spiral electrode structure, which was used for investigating the change in the internal resistance and the cycle characteristics described above, will be briefly described.
As shown in FIG. 5, a sheet-shaped positive electrode 1 and a sheet-shaped negative electrode 2 are spirally wound with a sheet-shaped separator 3 interposed therebetween, and the sheet-shaped positive electrode 1 and the sheet-shaped negative electrode 2 are loaded into a metal case 4 together with the above-described electrolytic solution, and a metal lid member 5 is provided. It is sealed by a sealing gasket 6. The positive electrode 1 is internally connected to the metal lid member 5, and the negative electrode 2
Is internally connected to the case 4. The positive electrode 1 is an active material Li
A sheet formed by mixing CoO ₂ , carbon powder as a conductive material, and an aqueous dispersion of PTFE as a binder, kneading in a paste form with water, applying thinly on both sides of an aluminum foil, drying and rolling. Is. The negative electrode 2 is formed by thinly applying a carbon material as an active material to a metal foil, drying and rolling it into a sheet shape.

[0015]

According to the present invention, diphenylpicrylhydrazyl as a polymerization inhibitor is contained in the non-aqueous electrolyte in an amount of 1% by weight or less, so that this electrolyte can be used in a battery and stored at high temperature. The internal resistance and charge / discharge cycle characteristics are not deteriorated and the high temperature storage characteristics are excellent.

[Brief description of drawings]

FIG. 1 is a diagram showing a structural formula of diphenylpicrylhydrazyl of the present invention.

FIG. 2 is a graph showing charge / discharge cycle characteristics with respect to the addition amount of diphenylpicrylhydrazyl of the present invention.

FIG. 3 is a graph showing changes in internal resistance during high temperature storage of the present invention and comparative examples.

FIG. 4 is a graph showing charge / discharge cycle characteristics of the present invention and a comparative example after high temperature storage.

FIG. 5 is a vertical cross-sectional view showing a typical structure of a lithium secondary battery.

[Explanation of symbols]

 1 sheet-like positive electrode 2 sheet-like negative electrode 3 sheet-like separator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nozomu Narita 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd.

Claims (2)

[Claims] 1. A nonaqueous electrolytic solution for a battery, which contains diphenylpicrylhydrazyl as a polymerization inhibitor in an amount of 1% by weight or less based on the nonaqueous electrolytic solution. 2. The nonaqueous electrolytic solution for a battery according to claim 1, wherein ethylene carbonate, propylene carbonate and diethyl carbonate are contained as a solvent of the nonaqueous electrolytic solution, and LiPF ₆ which is a lithium salt is dissolved as a solute.