Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
2000-03-02
2004-08-10
Nguyen, Ha Tran (Department: 2812)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C361S502000
Reexamination Certificate
active
06773466
ABSTRACT:
The present invention relates to a method for producing an electric double layer capacitor which is excellent in reliability of the performance for a long period of time and which has a high withstand voltage and a high energy density.
An electric double layer capacitor has excellent properties with respect to the output density and the long term reliability for charge and discharge cycles, but it has a problem such that its energy density is low as compared with e.g. a lithium ion secondary cell. The amount of energy to be stored in a capacitor is proportional to the capacitance of the capacitor and to the square of the applied voltage. Accordingly, the energy density can effectively be increased by increasing the withstand voltage of the capacitor.
The withstand voltage of an electric double layer capacitor is restricted basically by the electrochemical decomposition voltage of the electrolyte. When an organic electrolyte having a high withstand voltage as compared with an aqueous electrolyte, is used as the electrolyte, electrolysis is likely to take place due to impurities, particularly water, contained in the electrolyte. Accordingly, for the organic electrolyte, both the solvent and the electrolytic salt will be used as highly purified and dehydrated.
On the other hand, for the electrodes for an electric double layer capacitor, an electrode material having a high surface area is used, and when it is to be combined with the above organic electrolyte, activated carbon is usually employed as the electrode material having a high surface area. Activated carbon is a porous material having fine pores of a few nm, but it is likely to adsorb moisture in the environment, as it has a high adsorbing ability. Accordingly, activated carbon is required to be highly dehydrated during the process for producing an electric double layer capacitor.
In order to completely remove moisture from fine pores of activated carbon, it is usually required to carry out dehydration treatment at a high temperature of at least 300° C. in vacuum or in an inert gas atmosphere. However, activated carbon particles are usually molded by means of a binder such as an organic polymer on a current collector to form an electrode, and the binder undergoes thermal decomposition by high temperature treatment of at least 300° C. Therefore, heat treatment may usually be possible only at a temperature of not higher than 200° C., whereby it is difficult to completely remove moisture in the activated carbon electrode.
For the purpose of increasing the energy density, an electric double layer capacitor employing an organic electrolyte is operated at a voltage of at least 2 V which is higher than the theoretical decomposition voltage (1.23 V) of water. Therefore, when a capacitor cell is assembled and then a voltage is applied for operation, moisture remaining in the pores will be electrolyzed to generate a gas. It has been found that the generated gas will gradually accumulate in pores of activated carbon and will remain in the interior of the element comprising electrodes facing each other with a separator interposed between them and impregnated with the electrolyte, without being discharged from the element.
If the capacitor is used for a long period of time under such a condition, the electrolyte in pores will be expelled by the generated gas, whereby the expected capacitance will not be obtained. Further, electrically conductive paths by ion transfer within pores will be cut off, thus leading to deterioration of the performance such as a decrease in the capacitance or an increase of the internal resistance.
Under these circumstances, it is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a method for producing an electric double layer capacitor which is excellent in reliability of the performance for a long period of time and which has a high operation voltage and a high energy density.
The present invention provides a method for producing an electric double layer capacitor, which comprises impregnating an element comprising positive and negative electrodes facing each other with a separator interposed between them, with an organic electrolyte capable of forming an electric double layer on the surface of the electrodes to store electric charge, and then applying a voltage to the element, wherein said positive and negative electrodes are made of electrodes containing a carbonaceous material having a specific surface area of from 100 to 3,000 m
2
/g, and said organic electrolyte contains benzene or its chlorine derivative having at least one hydrogen atom of benzene substituted by a chlorine atom.
Now, the present invention will be described in detail with reference to the preferred embodiments.
In the present invention, benzene or its chlorine derivative having at least one hydrogen atom of benzene substituted by a chlorine atom (hereinafter referred to as “a chlorine derivative of benzene”) is represented by a compound of the formula 1. In the formula 1, n is an integer of from 1 to 6.
In the present invention, the benzene or its chlorine derivative is preferably at least one member selected from the group consisting of benzene, monochlorobenzene, dichlorobenzene and trichlorobenzene. The structure of dichlorobenzene is not particularly limited, and any one of o-dichlorobenzene, m-dichlorobenzene p-dichlorobenzene may be preferably employed. Likewise, the structure of trichlorobenzene is not particularly limited.
In the present invention, the amount of the benzene or its chlorine derivative in the electrolyte may suitably be adjusted depending upon the amount of the remaining moisture or the pore characteristics of the carbonaceous material contained in the electrodes. It is preferred, however, that the benzene or its chlorine derivative is maintained to be completely dissolved in the organic electrolyte. Further, as the benzene or its chlorine derivative is added, the dielectric constant of the organic electrolyte decreases. Accordingly, the amount of the benzene or its chlorine derivative is preferably controlled within a range where a decrease in the ion conductivity due to a decrease in the dielectric constant of the organic electrolyte, is little. Thus, the amount is preferably from 0.1 to 30 wt %, particularly preferably from 1 to 10 wt %, in the electrolyte.
The operation mechanism in the present invention is not clearly understood, but it is considered that the benzene or its chlorine derivative contained in the electrolyte has high affinity to a pseudo-graphite surface present on the inner walls of pores of the carbonaceous material and is likely to be adsorbed in substitution with the water remaining in fine pores without being removed by the heat treatment. Thus, the remaining water can readily be expelled from the fine pores and discharged in the electrolyte outside the fine pores. When a voltage is applied to the element impregnated with the electrolyte, such water will readily be electrolyzed and gasified.
The storage of electric charge by formation of an electric double layer takes place mainly in the interior of pores. With a conventional electric double layer capacitor containing no benzene or chlorine derivative in the electrolyte, if a voltage is applied to the element impregnated with the electrolyte, a gas generated by the electrolysis of water remaining in the fine pores will remain in the fine pores, whereby the performance of the electric double layer capacitor tends to deteriorate. Whereas, in the present invention, as mentioned above, it is considered that the gas generated by the electrolysis by the remaining water by the application of a voltage, will be present outside the fine pores i.e. not in the fine pores. Namely, it is present in macro pores within the activated carbon particles or in spaces among the particles, or it will be discharged as bobbles out of the element. Thus, it is considered that deterioration of the performance of the electric double layer capacitor can be controlled to be little.
However, the decomposition gas
Hiratsuka Kazuya
Kawasato Takeshi
Yoshida Naoki
Asahi Glass Company Limited
Nguyen Ha Tran
LandOfFree
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