JPH07296849A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To efficiently restrain a discharge capacity from decreasing as a cycle of charge and discharge proceeds by providing a negative electrode composed chiefly of a carbonaceous material storing and releasing Li, a positive electrode, and a nonaqueous electrolyte, and containing fluorosulfuric acid Li in the electrolyte. CONSTITUTION:A positive electrode 6 is formed when a complex oxide of Li and Co (LiCoO2), carbon powder serving as a conducting agent, and fluororesin powder serving as a binding agent are mixed sufficiently in a weight ratio of 90:3:7 and pressure molded. A negative electrode 3 is formed when graphite and fluororesin powder serving as a binding agent are mixed sufficiently in a weight ratio of 91:9 and pressure molded. The negative electrode 3 is made to abut the sealing plate 2 of a case 1 made of a stainless steel plate. A separator 5 is made from a polypropylene impregnated with an organic electrolyte. The positive electrode 6 is sealed by inwardly calking the open end of the case 1 serving also as a positive element, and clamping the outer periphery of the sealing plate 2 via a gasket 4. An organic solvent formed by the mixing of sulfolane and dimethyl carbonate in a volume ratio of 1:1 with fluorosulfuric acid Li dissolved therein at a concentration of 1mol/l is used as the organic solvent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery having high energy density and high reliability as a power source for driving electronic equipment or a memory holding power source.

[0002]

2. Description of the Related Art With the rapid miniaturization and weight reduction of electronic equipment, the development of a secondary battery that is smaller, lighter in weight and high in energy density, and that can be repeatedly charged and discharged with respect to the power source battery The demand is increasing. Non-aqueous electrolyte secondary batteries are the most promising secondary batteries that meet these requirements.

Various positive electrode active materials for non-aqueous electrolyte secondary batteries such as titanium disulfide, lithium cobalt composite oxide, spinel type lithium manganese oxide, vanadium pentoxide and molybdenum trioxide have been investigated. ing. Among them, the lithium cobalt composite oxide and the spinel type lithium manganese oxide are 4 V (Li / Li ⁺ )
Since charging / discharging is performed at the above extremely noble potential, a battery having a high discharge voltage can be realized by using it as a positive electrode.

As a negative electrode active material for a non-aqueous electrolyte secondary battery, various materials such as metallic lithium, Li-Al alloys capable of inserting and extracting lithium, and carbon materials have been studied. Among them, carbon materials are particularly preferable. Has the advantage that a battery with high safety and long cycle life can be obtained.

However, the electrolyte is liable to be electrochemically decomposed in the charge / discharge reaction in this type of secondary battery. Therefore, in selecting an electrolytic solution, it is indispensable to have a configuration in consideration of these points, and it has been proposed to use various electrolytic solutions. Most of them are
As a solvent, a mixture of propylene carbonate, ethylene carbonate, γ-butyrolactone, a high dielectric constant solvent such as sulfolane and a low viscosity solvent such as 1,2-dimethoxyethane, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl methyl sulfone is used. is there.

On the other hand, as the solute, lithium perchlorate,
Lithium trifluoromethanesulfonate, lithium tetrafluoroborate, lithium hexafluorophosphate and the like are generally used. Among them, lithium hexafluorophosphate has been actively used in recent years because of its high safety and electrochemical stability and the high ionic conductivity of the dissolved electrolyte.

However, even when the above-mentioned electrolytic solution is used, there is a problem that the discharge capacity of the battery decreases as the charging / discharging cycle progresses.

[0008]

The present invention relates to a secondary battery comprising a negative electrode containing a carbon material which absorbs and releases lithium as a main component, a positive electrode and a non-aqueous electrolyte, wherein the electrolyte contains lithium fluorosulfate. By doing so, the above problem is solved.

[0009]

As described above, in this type of battery, decomposition reaction of the electrolytic solution is likely to occur, which is the main cause of deterioration of battery performance. The inventor has obtained the following findings when the above decomposition reaction was examined in detail. When lithium perchlorate is used as the electrolyte, lithium perchlorate is decomposed and active oxygen is generated due to the noble potential of the positive electrode active material. It was found that this active oxygen attacks the solvent and accelerates the decomposition reaction of the solvent. Lithium trifluoromethanesulfonate as electrolyte
When lithium tetrafluoroborate and lithium hexafluorophosphate are used, decomposition of the electrolyte proceeds due to the noble potential of the positive electrode active material and fluorine is produced. It was found that this fluorine attacks the solvent and accelerates the decomposition reaction of the solvent. However, they have found that the use of lithium fluorosulfate as the electrolyte provides a battery having excellent storage characteristics and excellent cycle characteristics, and has completed the present invention. That is, when lithium fluorosulfate is used as the electrolyte, it is considered that the decomposition reaction of the electrolytic solution is difficult to occur because it is chemically and electrochemically stable in itself.

[0010]

EXAMPLES The present invention will be described below with reference to preferred examples.

The positive electrode is a lithium cobalt composite oxide (L
iCoO ₂ ), a carb powder as a conductive agent and a fluororesin powder as a binder were sufficiently mixed in a weight ratio of 90: 3: 7, and then pressure-molded. The negative electrode was obtained by sufficiently mixing graphite and fluororesin powder as a binder in a weight ratio of 91: 9, and then pressure-molding the mixture.

FIG. 1 is a vertical sectional view of a battery. In this figure, 1 is a case that also serves as a positive electrode terminal made by punching a stainless steel (SUS316) steel plate, and 2 is a sealing plate that also serves as a negative electrode terminal made by punching a stainless steel (SUS316) steel plate,
The negative electrode 3 is in contact with the inner wall thereof. Reference numeral 5 is a separator made of polypropylene impregnated with an organic electrolyte, 6 is a positive electrode, and the opening end of the case 1 also serving as a positive electrode terminal is caulked inward, and the outer periphery of the sealing plate 2 also serving as a negative electrode terminal is tightened through a gasket 4. It is hermetically sealed.

The organic electrolyte used was an organic solvent prepared by mixing sulfolane and dimethyl carbonate in a volume ratio of 1: 1 and dissolving lithium fluorosulfate at a concentration of 1 mol / liter. Approximately 150 μl of the above electrolyte is used in the battery
It was injected.

This battery has a diameter of 20 mm and a height of 2 mm.
Is. The battery thus prepared was used as the battery (A) of the present invention.

As the organic solvent, a mixture of ethylene carbonate and ethyl methyl sulfone (volume ratio 1: 1) and a mixture of sulfolane and ethyl methyl sulfone (volume ratio 1:
Batteries of the present invention having the same configuration as that of this example except that (1) was used were designated as (B) and (C), respectively.

Further, for comparison, the same constitution as that of the battery of the present invention is used except that lithium perchlorate, lithium trifluoromethanesulfonate, lithium tetrafluoroborate and lithium hexafluorophosphate are used as electrolytes, respectively. The prepared comparative batteries are referred to as (A), (B), (C) and (D), respectively.

Next, these batteries are charged with a constant current of 2.0 mA until the terminal voltage reaches 4.2 V, and are also charged with a constant current of 2.0 mA until the terminal voltage reaches 3 V. A discharge cycle life test was conducted at a temperature of 60 ° C. FIG. 2 shows a change in discharge capacity with the progress of charge / discharge cycles of each battery.

As is clear from the results shown in FIG. 2, the batteries (A), (B) and (C) of the present invention are comparative batteries (A),
Compared to (a), (c) and (d), the decrease in discharge capacity with the progress of the charge and discharge cycle is small.

In the above embodiment, the case where the lithium cobalt composite oxide is used as the positive electrode active material has been described.
Various materials such as lithium nickel composite oxide (LiNiO 2), titanium disulfide, manganese dioxide, spinel type lithium manganese oxide (LiMn 2 O 4), vanadium pentoxide and molybdenum trioxide can be used. The carbon material for the negative electrode is also not particularly limited. In addition to natural graphite and artificial graphite, spherical or fibrous graphite or a low crystalline carbon material can be used. Further, the solvent is basically not limited, and the solvent used in the conventional non-aqueous electrolyte secondary battery can be used. Examples of the organic solvent include cyclic esters such as ethylene carbonate, which is an aprotic solvent, and ethers such as tetrahydrofuran, dioxolane, and the like. These can be used alone or in a mixture of two or more thereof. For example, polyethylene oxide or the like can be used. In addition, although the case where lithium fluorosulfate is used alone has been described in the above examples, it is mixed with at least one of lithium perchlorate, lithium trifluoromethanesulfonate, lithium tetrafluoroborate, and lithium hexafluorophosphate. Can be used.

Although the batteries according to the above-mentioned embodiments are all coin type batteries, the same effect can be obtained by applying the present invention to cylindrical, prismatic or paper type batteries.

[0021]

As described above, in a secondary battery including a negative electrode containing a carbon material that absorbs and releases lithium as a main material, a positive electrode, and a non-aqueous electrolyte, the electrolyte contains lithium fluorosulfate, It is possible to effectively suppress the decrease in discharge capacity that accompanies the progress of charge and discharge cycles, which is a problem of this type of battery, and its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing an internal structure of a button battery which is an example of a non-aqueous electrolyte secondary battery.

FIG. 2 is a diagram showing a change in discharge capacity as a charge / discharge cycle of a test battery progresses.

[Explanation of symbols]

 1 Battery Case 2 Sealing Plate 3 Negative Electrode 4 Gasket 5 Separator 6 Positive Electrode

Claims (1)

[Claims] 1. A negative electrode comprising a carbon material which absorbs and releases lithium as a main material, a positive electrode, and a non-aqueous electrolyte, wherein the electrolyte contains lithium fluorosulfate. Water electrolyte secondary battery.