JPH04363862A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve the discharge utilizing ratio of a positive electrode in a lithium secondary battery using a vanadium oxide as the main body of the positive electrode, lithium as the main body of a negative electrode, and an organic electrolyte. CONSTITUTION:Vanadium carbide is added to a positive electrode 5. Thus, the electric contact between vanadium oxide particles mainly forming the positive electrode is improved, the discharge utilizing ratio of the positive electrode of a lithium secondary battery is improved, and the charge/discharge cycle life of the battery is increased.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rechargeable and dischargeable lithium secondary battery used as a mobile DC power source, a memory backup power source, and the like.

0002

[Prior Art] Conventionally, there have been battery systems that use vanadium pentoxide as a positive electrode, metallic lithium or a lithium alloy as a negative electrode, and an organic solvent in which a lithium salt is dissolved in an electrolyte as a rechargeable lithium secondary battery. This battery has a high terminal voltage of 3 volts and an energy density of 1.5 to 2 times that of a nickel-cadmium battery, making it a promising battery.

0003

[Problems to be Solved by the Invention] In the conventional vanadium pentoxide-based batteries mentioned above, the positive electrode generally contains around 90% vanadium pentoxide, and is composed of carbon black and a binder each in several percentages. Vanadium pentoxide is
It is a one-electron reaction and has a capacitance of 1 faraday (26.8 ampere-hour) per 1 mole of V2O5 as shown in formula (1).

0004

[Chemical formula 1]

However, when a battery with the above-mentioned positive electrode is actually discharged, the reaction utilization rate is not high, and the discharge capacity drops to around 70% especially when discharged at a high load current. However, the problem was that the reaction efficiency of the positive electrode during discharge was low.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention uses vanadium oxide, such as vanadium pentoxide, vanadium octoxide, or a mixture of both, as the main ingredient for the positive electrode. , to which vanadium carbide was added. The amount of vanadium carbide added is effective within a wide range, but 5 to 20% by weight of the positive electrode mixture is particularly effective.

[0007]

[Function] In the lithium secondary battery of the present invention, by mixing vanadium carbide into the positive electrode, which is mainly made of vanadium oxide such as vanadium pentoxide or vanadium octoxide, the reaction efficiency of the positive electrode is greatly improved during discharge. There is.

Since vanadium carbide has semiconducting properties, it improves electrical contact between vanadium oxide powder particles that mainly constitute the positive electrode, and at the same time, vanadium oxide This is because the particles become well wetted by the electrolyte. Furthermore, vanadium carbide is thought to function as a battery reaction active material, with lithium ions moving in and out of the vanadium carbide.

In addition, vanadium oxide, such as vanadium pentoxide, has a high potential of 3.5 volts, but vanadium carbide is stable against such high potentials, and even if it decomposes, it will not decompose into vanadium and carbon. However, it is unlikely that it would have any adverse effect on vanadium oxide, which is the main component of the positive electrode.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, a case 1 also serves as a positive terminal, and is sealed by a sealing plate 2 that also serves as a negative terminal. 3 is a polypropylene gasket that insulates the case 1 and the sealing plate 2 and seals the case 1; 4 is a separator made of a polypropylene microporous membrane;
and is impregnated with an electrolyte. In order to clarify the effect of the amount of vanadium carbide added, the positive electrode 5 in this example was made using a constant amount of vanadium pentoxide of 204 mg (theoretical electric capacity of about 30 milliampere-hours), and an amount of vanadium carbide powder added thereto. are respectively 0 and 1
, 2, 5, 10, 20, 30, and 40% by weight, carbon black as a conductive material was mixed and kneaded at a weight ratio of 5% by weight, and fluororesin as a binder was 5% by weight. The pellets were molded into pellets with a diameter of 15 mm, and these were dehydrated by high-temperature vacuum drying to form batteries as positive electrodes. The negative electrode 6
is a lithium-aluminum alloy containing 5% by weight of lithium, and has a thickness of 0.3 mm and a diameter of 15 mm. A current collecting net 7 is included inside. The electrolytic solution used was one in which 1 mol/liter of lithium perchlorate was dissolved in a solvent containing a 1:1 mixture of propylene carbonate and 1,2-dimethoxyethane. The size of the battery was 20 mm in diameter and 2.5 mm in thickness.

Using the above-mentioned eight types of batteries in which the amount of vanadium carbide added to vanadium pentoxide, which mainly constitutes the positive electrode, is from 0 to 40% by weight, a constant current of 1 milliampere was used.
After charging and discharging for 0 hours 20 times, after fully charging for 10 hours with a constant voltage of 3.5 volts and a protective resistance of 100 ohms, 3
The battery was discharged at a constant current of 10 milliamperes and 10 milliamperes, and the electric capacity up to 2.5 volts was measured, and the discharge utilization rate with respect to the charging electric capacity was measured. The results are shown in FIGS. 2 and 3.

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

[0014] Particularly, in terms of the discharge utilization rate of the positive electrode for 10 milliampere discharge, the effect of adding vanadium carbide becomes great starting from an amount of about 5% by weight, 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 be the minimum volume and weight. Therefore, the optimum amount of vanadium carbide added is 5 to 20% by weight.

[0015] The effect of adding vanadium carbide is not limited to the case where the vanadium oxide that mainly constitutes the positive electrode is vanadium pentoxide (V2O5). Similar effects were also obtained using vanadium octoxide (V3O8) or a mixture of vanadium pentoxide and vanadium octoxide.

[0016]

[Effects of the Invention] As is clear from the description of the above embodiments, according to the present invention, the addition of vanadium carbide improves the discharge utilization rate, especially the high-load discharge characteristics, of a positive electrode mainly composed of vanadium oxide. This realizes a lithium secondary battery with a superior charge/discharge cycle life compared to conventional lithium secondary batteries.

[Brief explanation of the drawing]

[Fig. 1] A cross-sectional view of a coin-shaped lithium secondary battery in an embodiment of the present invention.

[Figure 2] Diagram showing changes in the 3 milliampere discharge utilization rate of the positive electrode depending on the content of vanadium carbide

[Figure 3] Diagram showing changes in the 10 mA discharge utilization rate of the positive electrode depending on the vanadium carbide content

[Explanation of symbols]

1 case 2 Sealing plate 4 Separator 5 Positive electrode 6 Negative electrode

Claims (2)

[Claims] 1. A rechargeable lithium secondary battery comprising a positive electrode mainly composed of vanadium oxide, a negative electrode mainly composed of lithium, and an electrolyte mainly composed of an organic solvent in which lithium salt is dissolved, The positive electrode is a lithium secondary battery containing 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.