JP2819201B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, as a rechargeable lithium secondary battery, there has been a battery system using vanadium pentoxide for a positive electrode, metallic lithium or a lithium alloy for a negative electrode, and an organic solvent in which a lithium salt is dissolved in an electrolytic solution. It has a high terminal voltage of 3 volts, has an energy density 1.5 to 2 times that of nickel-cadmium batteries, and is known as a promising battery.

[0003]

The above-mentioned conventional vanadium pentoxide battery generally has a positive electrode containing about 90% of vanadium pentoxide, carbon black and a binder of several percent each. Vanadium pentoxide is
As shown in the chemical formula (1), a one-electron reaction has an electric capacity of 1 Faraday (26.8 ampere-hour) per mole of V ₂ O ₅ .

[0004]

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However, when a battery having the above-mentioned positive electrode is actually discharged, the reaction utilization rate is not high, and the discharge capacity particularly when discharged at a high load current is reduced to about 70%. Thus, there was a problem that the reaction efficiency of the positive electrode during this discharge was low.

[0006]

In order to solve the above-mentioned problems, the present invention mainly uses a vanadium oxide such as vanadium pentoxide or vanadium octoxide or a mixture of both as a mixture constituting a positive electrode. And vanadium carbide added thereto. The addition amount of vanadium carbide is effective in a wide range, but 5 to 20% by weight of the positive electrode mixture is particularly effective.

[0007]

According to the lithium secondary battery of the present invention, by mixing vanadium carbide into a cathode mainly composed of vanadium oxide such as vanadium pentoxide or vanadium octoxide, the reaction efficiency of the cathode during discharge is greatly improved. I have.

This is because vanadium carbide has semiconducting properties, so that electrical contact between powder particles of vanadium oxide mainly composed of a positive electrode is improved, and at the same time, vanadium oxide is improved. This is because the particles become well wetted by the electrolytic solution. Further, it is considered that vanadium carbide allows lithium ions to enter and exit, and acts as a battery reaction active material.

Further, vanadium oxide, for example, vanadium pentoxide has a high potential of 3.5 volts. Vanadium carbide is stable against such a high potential, and even if it is decomposed, it is decomposed into vanadium and carbon. It does not seem to adversely affect the vanadium oxide which is the main component of the positive electrode.

[0010]

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, reference numeral 1 denotes a case also serving as a positive electrode terminal, which is sealed by a sealing plate 2 also serving as a negative electrode terminal. Reference numeral 3 denotes a polypropylene gasket that insulates the case 1 from the sealing plate 2 and seals the case 1, and 4 denotes a separator made of a polypropylene microporous membrane.
And impregnated with an electrolytic solution. In order to clarify the effect of the added amount of vanadium carbide, the positive electrode 5 in this example was made to have a fixed amount of vanadium pentoxide of 204 mg (theoretical electric capacity of about 30 mA · h), and the added amount of vanadium carbide powder. To 0,
1, 2, 5, 10, 20, 30, and 40% by weight, carbon black as a conductive material is mixed at 5% by weight, and fluorocarbon resin as a binder is mixed at a weight ratio of 5% by weight and kneaded. These were formed into pellets having a size of 15 mm in diameter, and these were dehydrated by high-temperature vacuum drying to form respective batteries as positive electrodes. The negative electrode 6
Is a lithium aluminum alloy containing 5% by weight of lithium, having a thickness of 0.3 mm and a diameter of 15 mm. And a net 7 for current collection is inside. The electrolyte used was a solution in which lithium perchlorate was dissolved at 1 mol / L in a solvent in which propylene carbonate and 1,2 dimethoxyethane were mixed at a ratio of 1: 1. The size of the battery was 20 mm in diameter and 2.5 mm in thickness.

Using the above eight types of batteries in which the amount of vanadium carbide added to vanadium pentoxide mainly composed of the positive electrode is 0 to 40% by weight, the constant current is 1 mA and the current is 1 mA.
After 20 times of charging / discharging for 0 hours, the battery is fully charged for 10 hours with a constant voltage of 3.5 volts and a protection resistance of 100 ohms, and then charged for 3 hours.
Discharge was performed at a constant current of 10 mA and at a current of 10 mA, and the electric capacity up to 2.5 V was measured, and the discharge utilization rate with respect to the charged electric capacity was measured. The results are shown in FIGS.

As is apparent from FIGS. 2 and 3, when even a small amount of vanadium carbide is contained in the positive electrode mainly composed of vanadium pentoxide, the discharge utilization increases.

In particular, in the discharge utilization rate of the positive electrode of 10 mA discharge, the effect of adding vanadium carbide is increased from about 5% by weight of vanadium carbide, and becomes almost saturated at 20% by weight. Since the size of an actual battery is limited, it is desirable that the amount of vanadium carbide added is a minimum volume and weight. Therefore, the optimum addition amount of vanadium carbide is 5 to 20% by weight.

The effect of the addition of vanadium carbide is not limited to the case where the vanadium oxide mainly constituting the positive electrode is vanadium pentoxide (V ₂ O ₅ ). The same effect was obtained with vanadium pentoxide (V ₃ O ₈ ) or a mixture of vanadium pentoxide and vanadium pentoxide.

[0016]

As is apparent from the above description of the embodiments, according to the present invention, the addition of vanadium carbide improves the discharge utilization rate of a positive electrode mainly composed of vanadium oxide, particularly, a high-load discharge characteristic. Thus, a lithium secondary battery having a superior charge / discharge cycle life is realized.

[Brief description of the drawings]

FIG. 1 is a sectional view of a coin-shaped lithium secondary battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing a change in a 3 mA discharge utilization rate of a positive electrode depending on a content of vanadium carbide.

FIG. 3 is a diagram showing a change in the utilization rate of 10 mA discharge of a positive electrode depending on the content of vanadium carbide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Case 2 Sealing plate 4 Separator 5 Positive electrode 6 Negative electrode

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01M 4/62 H01M 4/48 H01M 4/58 H01M 4/02 H01M 10/40

Claims (2)

(57) [Claims] 1. A positive electrode mainly composed of vanadium oxide;
A rechargeable lithium secondary battery comprising a negative electrode mainly composed of lithium and an electrolyte mainly composed of an organic solvent in which a lithium salt is dissolved, wherein the positive electrode contains vanadium carbide. 2. The lithium secondary battery according to claim 1, wherein the content of vanadium carbide is 5 to 20% by weight based on the positive electrode.