JP3396990B2 - Organic electrolyte 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 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. Organic electrolyte secondary batteries are the most promising as secondary batteries that meet these requirements.

Various positive electrode active materials for organic electrolyte secondary batteries such as titanium disulfide, lithium cobalt composite oxide, spinel type lithium manganese oxide, vanadium pentoxide and molybdenum trioxide have been studied. Has been done. Among them, lithium cobalt composite oxide (L
iCoO ₂ ) and spinel-type lithium manganese oxide (LiMnO ₄ ) charge and discharge at an extremely noble potential of 4 V (Li / Li ⁺ ) or more, and therefore, by using these as a positive electrode, a battery having a high discharge voltage can be obtained. realizable.

Various negative electrode active materials for organic electrolyte secondary batteries, such as metallic lithium, Li-Al alloys and carbon materials capable of absorbing and desorbing lithium, have been investigated. Among them, carbon materials are especially preferred. Has the advantage that a battery with high safety and long cycle life can be obtained.

However, in this type of battery, while lithium having a base potential is used as a negative electrode active material, a metal oxide having a noble potential is used in the positive electrode, so that the electrolytic solution 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 have a configuration in consideration of these points, and it has been proposed to use various electrolytic solutions.

For example, an electrolytic solution containing a solvent containing an S--O bond (see JP-A-4-152879) or a mixed electrolytic solution of a cyclic carbonate and a chain carbonate (see JP-A-4-171674). And so on.

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 actively used in recent years because of its high safety and high ionic conductivity of the dissolved electrolytic solution.

However, when dimethylsulfite, dimethylsulfone, and dimethylsulfoxide, which are solvents containing S—O bond, were used in a battery using a carbon material capable of inserting and extracting lithium as a negative electrode, in all cases, No good charge / discharge cycle characteristics were obtained. Sulfolane and 3-methylsulfolane exhibited good charge / discharge cycle characteristics, but the discharge capacity was small. On the other hand, the mixed electrolytic solution of cyclic carbonate and non-cyclic carbonate shows good cycle characteristics at room temperature, but has a problem that the deterioration becomes large at high temperature.

[0009]

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 containing a solvent and a solute.
The above problem is solved by using a mixture of at least dioxide thiophene and acyclic sulfone as the solvent.

In order to obtain a battery excellent in high rate discharge performance, it is preferable to use sulfolane and 3-methyl-sulfolane as the dioxidothiophene compound and dimethyl sulfone, ethylmethyl sulfone and diethyl sulfone as the acyclic sulfone.

[0011]

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. However, it has been found that a battery having excellent storage characteristics and good cycle characteristics can be obtained by using a mixed solvent of a dioxidothiophene and an acyclic sulfone as an electrolytic solution, and has completed the present invention. The reason for this is not clear, but it is presumed as follows.

The molecules of dimethyl sulfite, dimethyl sulfone and dimethyl sulfoxide are sulfolane, 3-
Since it is smaller than the molecule of a cyclic compound such as methylsulfolane, it is easily taken in between the layers of the negative electrode carbon along with lithium ions during charging. As a result, the interlayer distance of the negative carbon is increased. Therefore, it is considered that the destruction of the layer structure of the negative electrode carbon progressed as the occlusion and release of lithium ions due to charge and discharge were repeated, and the battery capacity decreased. Therefore, these solvents could not be applied to the battery using the carbon material as the negative electrode. On the other hand, high viscosity solvents such as sulfolane and 3-methylsulfolane are difficult to use alone because the ionic conductivity of the electrolyte is low. However, when dioxidethiophene and an acyclic sulfone represented by dimethyl sulfone are mixed, the dioxidethiophene forms a lithium ion conductive protective film on the surface of the negative electrode carbon, and the acyclic sulfone molecule is incorporated into the negative electrode carbon layer. Therefore, even if the charge / discharge cycle is repeated, the layer structure of the negative electrode carbon is less likely to be destroyed. Further, compared with an electrolyte solution using a single solvent such as sulfolane or 3-methylsulfolane, the viscosity of the electrolyte solution is lowered and the ionic conductivity is improved, so that the polarization of the battery is reduced and the battery capacity is dramatically increased. I understood it.

However, the stabilizing effect of this protective film is specific only to non-cyclic sulfones such as dimethyl sulfone, ethyl methyl sulfone and diethyl sulfone, and dimethyl sulfite and dimethyl having the same S--O bond. No effect was seen with sulfoxide. The reason for this is not clear, but it is considered that dimethylsulfite and dimethylsulfoxide are molecules smaller than dimethylsulfone, so that they easily pass through the protective film and easily react with the negative electrode carbon material. Further, the electrolytic solution of the present invention has improved charge / discharge cycle characteristics at high temperatures, as compared with a mixed electrolytic solution of a cyclic carbonate and a chain carbonate which is a conventional electrolytic solution.
It is considered that this is because it does not contain a chain carbonate which is inferior in stability at high temperature.

[0014]

EXAMPLES The present invention will be described below with reference to preferred examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.

The positive electrode is a lithium cobalt composite oxide (L
iCoO ₂ ), carbon powder as a conductive agent and fluororesin powder as a binder were sufficiently mixed in a weight ratio of 90: 3: 7, and then pressure-molded. The negative electrode is
91: 9 graphite powder and fluororesin powder as a binder
After being sufficiently mixed at a weight ratio of 1, the mixture was pressure-molded.

FIG. 1 is a vertical sectional view of a battery of an embodiment of the present invention. In this figure, 1 is a case that also functions as a positive electrode terminal made by punching out an electrolytic solution resistant stainless steel plate, and 2 is 1
It is a sealing plate that also functions as a negative electrode terminal made by punching out a stainless steel plate similar to the above, and the negative electrode 3 is in contact with its inner wall. Reference numeral 5 is a separator made of polypropylene impregnated with an organic electrolytic solution, and 6 is a positive electrode. The battery is hermetically sealed by crimping the open end of the case 1 which also functions as a positive electrode terminal inward and tightening the outer periphery of the sealing plate 2 which also functions as a negative electrode terminal via the gasket 4.

The organic electrolytic solution used was one in which lithium hexafluorophosphate was dissolved at a concentration of 1 mol / liter in an organic solvent in which sulfolane and dimethyl sulfone were mixed at a volume ratio of 1: 1. About 150 μl of the above electrolytic solution was injected into the battery.

This battery has a diameter of 20 mm and a height of 2 mm.
Is. The battery of the present invention was designated as (A). Batteries (B) and (C) of the present invention having the same structure as in this example except that ethylmethyl sulfone and diethyl sulfone were used instead of dimethyl sulfone in the above-mentioned examples.

For comparison, a mixture of ethylene carbonate and diethyl carbonate (volume ratio 1: 1)
A comparative battery having the same structure as that of the battery of the present invention was used as (A), except that

Next, these batteries were charged at a constant current of 2.0 mA in a constant temperature bath at a temperature of 60 ° C. until the terminal voltage reached 4.2 V, and then the same constant current of 2.0 mA was applied. A charge / discharge cycle life test was conducted for 300 cycles, in which the terminal voltage was 2.7 V to discharge with a current. Figure 2 shows the change in discharge capacity as the charge and discharge cycle of each battery progresses.
Shown in.

As is clear from the results shown in FIG. 2, the batteries (A), (B) and (C) of the present invention using a mixed solvent of dioxidethiophene and acyclic sulfone are comparative batteries (A).
Compared with the above, the decrease in discharge capacity with the progress of charge / discharge cycles is small.

In the above examples, the case where sulfolane is used as the dioxide thiophene has been described.
Methylsulfolane or a mixed solvent of sulfolane and 3-methylsulfolane can be used.

In the above embodiment, the case where the dioxidothiophene and the acyclic sulfone were mixed at a volume ratio of 1: 1 was explained, but the mixture is not particularly limited.

The addition amount of these acyclic sulfones to the dioxide thiophene is 10 to 6 with respect to the total amount of both.
0% by volume is desirable. This is because when the content of the acyclic sulfone is less than 10% by volume, the electrolyte solution is likely to coagulate at low temperature when sulfolane having a high freezing point is used, while the content of the sulfone compound having a low dielectric constant is 60% by volume.
This is because the ionic conductivity of the electrolytic solution decreases when the value exceeds the range.

Further, as the sulfone compound used in the present invention, for example, at least one kind of dimethyl sulfone, ethylmethyl sulfone, diethyl sulfone and the like can be used. Among them, dimethyl sulfone is preferable in terms of ionic conductivity. Ethyl methyl sulfone is most desirable in terms of low temperature performance.

Further, in the above-mentioned embodiment, the case where the lithium cobalt composite oxide is used as the positive electrode active material has been described, but it is not particularly limited. Titanium disulfide, lithium nickel composite oxide (LiCoO ₂ ), and manganese dioxide, spinel type lithium manganese oxide, (Li
Various substances such as MnO4) vanadium pentoxide and molybdenum trioxide can be used.

In this embodiment, the case where a mixed system of a dioxide thiophene and an acyclic sulfone compound is used as the organic solvent has been described. However, the organic solvent conventionally used in lithium batteries is added as the third component and used. be able to. For example, γ-butyrolactone, ester solvents such as methyl formate, cyclic carbonate solvents such as ethylene carbonate, 1,2-dimethoxyethane, ether solvents such as tetrahydrofuran, dimethyl carbonate,
Examples thereof include chain carbonates such as ethylmethyl 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-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.

[0030]

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 containing a solvent and a solute, the solvent is a dioxidothiophene and an acyclic compound. By using a mixture with sulfone, 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 an organic electrolyte secondary battery.

FIG. 2 is a table showing changes in discharge capacity with progress of charge / discharge cycles of test batteries.

[Explanation of symbols]

1 battery case 2 Seal plate 3 Negative electrode 4 gasket 5 separator 6 Positive electrode

─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-3-214566 (JP, A) JP-A-3-152879 (JP, A) JP-A-8-78052 (JP, A) JP-A-7- 230824 (JP, A) JP 7-230825 (JP, A) JP 2-148663 (JP, A) JP 64-14879 (JP, A) JP 64-14877 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 10/40

Claims (3)

(57) [Claims] 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 containing a solvent and a solute, wherein the solvent is dioxide thiophene and acyclic sulfone. An organic electrolyte secondary battery comprising: 2. The dioxide thiophene is sulfolane, 3-
2. One or more selected from methylsulfolane.
The organic electrolyte secondary battery described. 3. A non-cyclic sulfone selected from dimethyl sulfone, ethylmethyl sulfone and diethyl sulfone.
The organic electrolyte secondary battery according to claim 1 or 2, which is more than one kind.