JPH09245829A - Organic electrolyte lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance a charge/discharge cycle characteristic and stabilize a constant voltage overcharge characteristic in an organic electrolyte lithium secondary battery. SOLUTION: By using an electrolyte prepared by dissolving LiN(C2 F5 SO2 )2 in an organic solvent, a charge/discharge characteristic is enhanced, and a constant voltage overcharge characteristic is stabilized. By containing ethylene carbonate(EC) as the solvent, the charge/discharge characteristic is further enhanced, and by containing γ-butyrolactone (γ-BL), the constant voltage overcharge characteristic is further stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile DC power supply,
The present invention relates to a chargeable / dischargeable organic electrolyte lithium secondary battery used as a memory backup power source or the like.

[0002]

2. Description of the Related Art As a rechargeable organic lithium secondary battery, there is a battery system using vanadium pentoxide for the positive electrode, metallic lithium or a lithium alloy for the negative electrode, and an organic solvent in which a lithium salt is dissolved in an electrolytic solution. It has a high voltage and an energy density of 1.5 to 2 times that of a NiCd battery, making it a useful battery. LiBF ₄ was used as the electrolyte of this battery system.
An organic solvent in which a lithium salt such as LiClO ₄ or LiCF ₃ SO ₃ is dissolved has been studied as an electrolytic solution.

[0003]

In the case of a combination using vanadium pentoxide for the positive electrode and a lithium aluminum alloy for the negative electrode, the discharge voltage is about 3V and the charging voltage is around 3.5V, but depending on the electrolyte used, charging / discharging may occur. The cycle life and constant voltage overcharge characteristics change. For example, propylene car
When LiClO ₄ is used as a lithium salt dissolved in a mixed solvent of carbon dioxide (hereinafter abbreviated as PC) and 1,2-dimethoxyethane (hereinafter abbreviated as DME), the charge / discharge cycle is somewhat good, The constant voltage overcharge characteristics deteriorate. For example, when continuously charged at a constant voltage of 3.5 V in a high temperature atmosphere of 60 ° C., the internal resistance starts to rise gradually,
Eventually, charging / discharging becomes impossible. When LiBF ₄ is used as the lithium salt in the same solvent, the constant voltage overcharge characteristics are good, but the charge / discharge cycle characteristics are LiCl 4.
It is much lower than O ₄ . In addition, LiCF ₃ SO
_{When 3} is used, both charge and discharge cycle characteristics and constant voltage overcharge characteristics deteriorate. The present invention is intended to improve both the charge / discharge cycle characteristics and the constant voltage overcharge characteristics by improving the electrolytic solution.

[0004]

As an electrolytic solution, an organic solvent in which a lithium salt represented by the chemical formula LiN (C ₂ F ₅ SO ₂ ) ₂ is dissolved is used. Further, it contains ethylene carbonate (hereinafter abbreviated as EC) or γ-butyrolactone (hereinafter abbreviated as γ-BL) as an organic solvent.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION LiN (C ₂ F ₅ SO ₂ ) ₂ is a kind of imide salt. LiN (CF ₃ S
O ₂ ) ₂ etc. are often considered as electrolytes,
When this is used, the charge-discharge cycle characteristics are further improved as compared with the case where LiClO ₄ is used, but unfortunately the overcharge characteristics due to the application of a constant voltage are not so good, and improvements have been desired. The reason for this is not clear, but it is considered that the methyl group (—CF ₃ ) decomposes at a high potential.
However, when LiN (C ₂ F ₅ SO ₂ ) ₂ of the present invention is used,
For example, when a combination of vanadium pentoxide for the positive electrode and a lithium aluminum alloy for the negative electrode is used, not only the charge / discharge cycle characteristics are improved, but also in a high temperature atmosphere of 60 ° C. 3.5
It was found that stability is exhibited even against constant voltage overcharge of V. It is considered that this is because the stability was improved at a high potential by changing from a methyl group to an ethyl group (—C ₂ F ₅ ). Further, as the organic solvent for dissolving the lithium salt of the present invention, a mixed solvent of PC and DME which is generally used for lithium secondary batteries can be used.
In addition to these, the addition of EC can further extend the charge / discharge cycle life, and the addition of γ-BL can improve the constant voltage overcharge characteristics. Of course, other well-used butylene carbonate (BC), diethoxy carbonate (DEC), dimethoxy carbonate (DMC) and the like can be mixed and used.

[0006]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of a coin-shaped organic electrolyte lithium secondary battery according to the present invention. 1 is a case that also serves as a positive electrode terminal, 2 is a sealing plate that also serves as a negative electrode terminal, 3
Is a polypropylene gasket that insulates the case and the sealing plate, and 4 is a separator made of polypropylene nonwoven fabric. Reference numeral 5 is a positive electrode, and a certain amount of vanadium pentoxide is 2
It was made to be 04 mg (theoretical electric capacity of about 30 mAh), and kneaded so that the carbon black as the conductive material was 5 wt% and the fluororesin as the binder was 5 wt%, and the diameter was 15
Pellets were formed into a size of mm and dehydrated by high temperature vacuum drying. Reference numeral 6 denotes a negative electrode, which is a lithium-aluminum alloy containing 5 wt% of lithium, having a thickness of 0.3 mm and a diameter of 15 mm. The electrolyte is P
LiN (C ₂ F ₅ S
A solution of O ₂ ) ₂ dissolved at 1 M / l was used. The battery has a diameter of 20 mm and a thickness of 2.5 mm. This battery of the present invention is designated as A.

For comparison, LiClO under exactly the same conditions
₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ )
B, C, D and E are batteries produced by using ₂ respectively. Using these batteries, charging conditions 3.5V constant voltage,
The charging time was set to 24 hours with a protective resistance of 100Ω, the discharge condition was set to 2.5 V at 3 kΩ, charging and discharging were repeated, and the number of cycles at the time when the initial discharge capacity was reduced to 50% or less was compared. Table 1 shows the results. Battery C and D are about half of battery A, and battery B is 70%.
Slightly inferior in degree, and E has almost the same number of cycles.

[0008]

[Table 1]

Further, a constant voltage of 3.5 V was continuously applied in a high temperature atmosphere of 60 ° C., and after 30 days, the internal resistance of the battery was measured by an alternating current method of 1 kHz and the rate of change from the initial value was compared.
The results are shown in (Table 2). Batteries A and C have a low rate of change in internal resistance, and after that, there is a considerable upward trend. From this, it is understood that the battery A of the present invention is effective for charge / discharge cycle characteristics and constant voltage overcharge.

[0010]

[Table 2]

(Example 2) EC was added to the organic solvent of the electrolytic solution of the battery A of Example 1, and the volume ratio was PC: EC: DME.
= 1: 1: 1. This is referred to as a battery F. Similarly, γ-BL was added, and PC: γ-BL: DME = 1:
It was set to 1: 1. This is referred to as battery G.

These batteries were tested for charge / discharge cycles and constant voltage overcharge under the same conditions as in (Example 1).
The results are shown in (Table 3) and (Table 4).

[0013]

[Table 3]

[0014]

[Table 4]

As is clear from (Table 3), the number of charge / discharge cycles of F added with EC is extended. Also (Table 4)
As is clear from the above, the battery G containing γ-BL is more stable against constant voltage overcharge.

In the examples, crystalline vanadium pentoxide was used for the positive electrode, but amorphous type vanadium pentoxide, LiV ₂ O ₄ having a spinel structure containing lithium, and the like also had similar effects. Further, not only the lithium aluminum alloy but also a carbon material capable of inserting and extracting lithium can be used for the negative electrode.

[0017]

As described above, the use of the organic electrolytic solution of the present invention is effective in charge / discharge cycles and constant voltage overcharge in lithium secondary batteries.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a coin-shaped organic electrolyte lithium secondary battery of the present invention.

[Explanation of symbols]

 1. Positive electrode case 2. Seal plate 3. Gasket 4. Separator 5. Positive electrode 6. Negative electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuo Mori 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (3)

[Claims] 1. An organic electrolyte lithium secondary battery, wherein an organic solvent in which a lithium salt represented by the chemical formula LiN (C ₂ F ₅ SO ₂ ) ₂ is dissolved is used as an electrolyte. 2. The organic electrolyte lithium secondary battery according to claim 1, wherein the organic solvent contains ethylene carbonate. 3. The organic electrolyte lithium secondary battery according to claim 1, wherein the organic solvent contains γ-butyrolactone.