JP3451781B2 - Organic electrolyte secondary battery - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-energy-density and high-reliability lithium battery used as a power supply for driving electronic equipment or a power supply for holding a memory. 2. Description of the Related Art Along with rapid downsizing and weight reduction of electronic devices, secondary batteries capable of being repeatedly charged and discharged with a small size, light weight, high energy density, and a rechargeable battery. The demand for the development of is increasing. As a secondary battery satisfying these requirements, an organic electrolyte secondary battery is most promising. As the positive electrode active material of the organic electrolyte secondary battery, various materials such as titanium disulfide, lithium cobalt composite oxide, spinel type lithium manganese oxide, vanadium pentoxide and molybdenum trioxide have been studied. Have been. Among them, lithium cobalt composite oxide (L
Since iCoO ₂ ) and spinel-type lithium manganese oxide (LiMn ₂ O ₄ ) charge and discharge at a very noble potential of 4 V (Li / Li ⁺ ) or more, a battery having a high discharge voltage by using as a positive electrode Can be realized. As the negative electrode active material of the organic electrolyte secondary battery, various materials such as lithium metal, a Li-Al alloy capable of occluding and releasing lithium, and a carbon material have been studied. The material has an advantage that a battery having high safety and a long cycle life can be obtained. However, in this type of battery, while lithium having a low potential is used as a negative electrode active material, while a metal oxide having a noble potential is used for a positive electrode, the electrolyte is easily decomposed in each of the negative electrode and the positive electrode. It is in. Therefore, in selecting an electrolytic solution, it is indispensable to adopt a configuration in consideration of these points, and use of various electrolytic solutions has been proposed. Examples thereof include an electrolytic solution containing dimethyl sulfone (see JP-A-3-152879) and a mixed electrolytic solution of a cyclic carbonate and a chain carbonate (see JP-A-4-171774). On the other hand, as the solute, lithium perchlorate,
Lithium trifluoromethanesulfonate, lithium hexafluorophosphate and the like are generally used. Among them, lithium hexafluorophosphate has been widely used in recent years because of its high safety and high ionic conductivity of a dissolved electrolyte. [0007] However, although dimethyl sulfone itself has excellent thermal stability and oxidation resistance performance, good charge / discharge cycle characteristics can be obtained in a battery using a carbon material capable of occluding and releasing lithium as a negative electrode. Did not. On the other hand, the mixed electrolyte of the cyclic carbonate and the chain carbonate exhibited good cycle characteristics at room temperature, but deteriorated significantly at high temperatures. [0008] The present invention relates to a battery comprising a negative electrode made of a carbon material capable of inserting and extracting lithium, a positive electrode, and an organic electrolytic solution comprising a solvent and a solute.
The solvent is a cyclic carbonate and ethyl methyl sulfone
The above-mentioned problem is solved by containing . When a carbon material having a high degree of graphitization is used for the negative electrode, it is preferable to use ethylene carbonate as the cyclic carbonate . Also, ethyl methyl sulfone
To obtain a battery with excellent high-rate discharge performance
be able to. As described above, in this type of battery, a decomposition reaction of the electrolytic solution easily occurs, and this is a main cause of deterioration of battery performance. However, when the electrolyte contains a cyclic carbonate,
Using a mixture with ethyl methyl sulfone , it was found that a battery having excellent storage characteristics and good cycle characteristics was obtained, and the present invention was completed. The reason for this is not clear, but is speculated as follows. Since a non-cyclic sulfone molecule represented by dimethyl sulfone is smaller than a cyclic carbonate molecule such as ethylene carbonate or propylene carbonate, it is easily taken in between the layers of the negative electrode carbon together with lithium ions during charging. Thereby, the interlayer distance of the negative electrode carbon is increased. Therefore, it is considered that as the occlusion and release of lithium ions due to charge and discharge were repeated, the layer structure of the negative electrode carbon was destroyed and the battery capacity was reduced. However, as in the present invention, the cyclic carbonate and
When mixed with ethyl methyl sulfone , the cyclic carbonate forms a lithium ion conductive protective film on the surface of the negative electrode carbon, and suppresses the incorporation of ethyl methyl sulfone molecules between the negative electrode carbon layers, thus destroying the layer structure of the negative electrode carbon. It will be suppressed. Furthermore, since the electrolyte of the present invention does not contain a chain carbonate which is inferior in stability at high temperatures, it seems that the electrolyte exhibits excellent cycle performance even at high temperatures. Hereinafter, the present invention will be described with reference to preferred embodiments, but the present invention is not limited to the following embodiments without departing from the spirit of the present invention. The positive electrode is made of a lithium cobalt composite oxide (L
ICOO ₂ ), a carbon powder as a conductive agent, and a fluororesin powder as a binder are sufficiently mixed at a weight ratio of 90: 3: 7, and then molded under pressure. The negative electrode is
91: 9 graphite powder and fluororesin powder as a binder
And then press-molded. FIG. 1 shows a battery according to one embodiment of the present invention. In this figure, reference numeral 1 denotes a case also serving as a positive electrode terminal punched out of an electrolytic solution-resistant stainless steel plate, and 2 denotes a sealing plate serving also as a negative electrode terminal punched out of the same stainless steel plate as in 1, and a negative electrode 3 is provided on the inner wall thereof. Is abutted. 5
Is a separator made of polypropylene impregnated with an organic electrolyte, and 6 is a positive electrode. The battery is hermetically sealed by caulking an opening end of the case 1 also serving as a positive electrode terminal inward and tightening an outer periphery of a sealing plate 2 also serving as a negative electrode terminal via a gasket 4. The organic electrolyte includes ethylene carbonate and
A solution prepared by dissolving lithium hexafluorophosphate at a concentration of 1 mol / liter in an organic solvent mixed with ethyl methyl sulfone at a volume ratio of 1: 1 was used. About 150 μl of the above electrolyte was injected into the battery. The dimensions of this battery are 20 mm in diameter and 2 mm in height. The battery of the present invention was designated as (A). For comparison, a comparative battery having the same structure as the battery of the present invention except that a mixture of ethylene carbonate and diethyl carbonate (volume ratio 1: 1) was used ( A) Next, these batteries were charged at a constant current of 2.0 mA in a thermostat at a temperature of 60 ° C. until the terminal voltage reached 4.2 V, followed by a constant current of 2.0 mA. so,
A charge / discharge cycle life test for discharging until the terminal voltage reached 2.7 V was performed 300 times. FIG. 2 shows a change in the discharge capacity as the charge / discharge cycle of each battery progresses. [0020] As apparent from the results of FIG. 2, ethylene
The battery of the present invention (A) using the mixed solvent of carbonate and ethyl methyl sulfone has a smaller decrease in the discharge capacity with the progress of the charge / discharge cycle than the comparative battery (A). In the above embodiment, the case where ethylene carbonate is used as the cyclic carbonate is described. However, if a carbon material having a high degree of graphitization is not used for the negative electrode, propylene carbonate or ethylene carbonate is used instead of ethylene carbonate. A mixed solvent with propylene carbonate can be used. In the above embodiment, cyclic carbonate and ethyl
Although the case where 1: 1 by volume ratio with dimethyl sulfone was mixed was described, it is not particularly limited. The amount of ethyl methyl sulfone added to the cyclic carbonate is preferably 20 to 80% by volume based on the total amount of both. Because, ethyl
When the content of tyl sulfone is less than 20% by volume, the use of ethylene carbonate having a high freezing point makes it easier for the electrolyte to coagulate at a low temperature, while ethyl methyl sulfonate having a low dielectric constant is used.
This is because if the content of rufone exceeds 80%, the ionic conductivity of the electrolytic solution decreases. The ethyl methyls used in the present invention
Lefon is desirable in terms of low temperature performance. Further, in the above embodiment, the case where the lithium-cobalt composite oxide is used as the positive electrode active material has been described, but the present invention is not particularly limited to this. Various materials such as titanium disulfide, lithium nickel composite oxide (LiNiO ₂ ), manganese dioxide, spinel lithium manganese oxide (LiMn ₂ O ₄ ), vanadium pentoxide, and molybdenum trioxide can be used. . In this embodiment, the case where a mixed system of cyclic carbonate and acyclic sulfone is used as the organic solvent has been described.
It can be used by being added as a component. For example, γ-
Ester solvents such as butyrolactone and methyl formate; cyclic sulfur compounds such as sulfolane; ether solvents such as 1,2-dimethoxyethane and tetrahydrofuran; and linear carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. . As the electrolyte, one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium fluorosulfate and the like can be used. Although the batteries according to the above embodiments are all coin-shaped batteries, the same effects can be obtained by applying the present invention to cylindrical, square or paper batteries. As described above, in a battery including a negative electrode made of a carbon material capable of inserting and extracting lithium, a positive electrode, and an organic electrolytic solution comprising a solvent and a solute, a cyclic carbonate is used as the solvent. By using a mixture with ethyl methyl sulfone , it is possible to effectively suppress a decrease in discharge capacity due to progress of a charge / discharge cycle, which is a problem of this type of battery, and its industrial value is extremely large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an internal structure of a button battery as an example of an organic electrolyte secondary battery. FIG. 2 is a diagram showing a change in discharge capacity as a charge / discharge cycle of a test battery progresses. [Description of Signs] 1 Battery case 2 Sealing plate 3 Negative electrode 4 Gasket 5 Separator 6 Positive electrode

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 10/40

Claims (1)

(57) [Claim 1] A battery comprising a negative electrode made of a carbon material capable of inserting and extracting lithium, a positive electrode, and an organic electrolytic solution comprising a solvent and a solute, wherein the battery comprises: Contains a cyclic carbonate and ethyl methyl sulfone .