(1) Field of the Invention
The present invention relates to an electric double layer capacitor, particularly to an electric double layer capacitor having high voltage retention and excellent reliability.
As a shape of a conventional electric double layer capacitor, there may be a coin type wherein an element having a separator sandwiched between a pair of polarized electrodes composed mainly of activated carbon formed on current collectors, is accommodated together with an electrolytic solution in a metal casing, which is then sealed by a metal cover via a gasket, or a cylindrical type wherein an element having a pair of polarized sheet electrodes wound with a separator interposed therebetween, is accommodated together with an electrolytic solution in a metal casing, which is then sealed so that the electrolytic solution will not evaporate from an opening of the casing.
Further, as one for a large current and large capacitance, a lamination type electric double layer capacitor has also been proposed wherein an element having many polarized sheet electrodes laminated via a separator disposed therebetween, is incorporated (JP-A-4-154106, JP-A-3-203311, JP-A-4-286108). Namely, rectangular polarized sheet electrodes are used as a positive electrode and a negative electrode, and they are alternately laminated with a separator interposed therebetween, to form an element, which is then accommodated in a casing in such a state that a positive electrode lead member and a negative electrode lead member are connected by caulking to the terminals of the positive and negative electrodes, respectively, then the element is impregnated with an electrolytic solution, and the casing is closed with a cover.
(2) Description of Related Art
As an electrolytic solution for a conventional electric double layer capacitor, not only an aqueous electrolytic solution containing a mineral acid such as sulfuric acid, an alkali metal salt or an alkali, but also various organic electrolytic solutions have been used. As the solvent for such organic electrolytic solutions, propylene carbonate, γ-butyrolactone, acetonitrile, dimethyl formamide (JP-A-49-068254) or a sulfolane derivative (JP-A-62-237715), has been known. When the withstand voltages are compared, the aqueous electrolytic solution has a withstand voltage of 0.8 V, while the organic electrolytic solution has a withstand voltage of from 2.5 to 3.3 V. The electrostatic energy of a capacitor corresponds to the square of the withstand voltage. Accordingly, from the viewpoint of the electrostatic energy, the organic electrolytic solution is more advantageous.
The withstand voltage of an electric double layer capacitor is basically restricted by the electrochemical decomposition voltage of the electrolytic solution. In a case where an organic electrolytic solution which has a high withstand voltage as compared with an aqueous electrolytic solution, is used as an electrolytic solution, it is used by an application of a voltage higher than the decomposition voltage of water, whereby electrolysis takes place due to an impurity, particularly water, contained in the electrolytic solution. Accordingly, it is common to use an organic electrolytic solution as dehydrated and having both the solvent and electrolyte highly purified.
On the other hand, for the electrodes for an electric double layer capacitor, an electrode material having a high specific surface area, is used, but when it is combined with the above organic electrolytic solution, it is common to use activated carbon as the electrode material. Activated carbon is a porous material having fine pores of a few nm, but has high adsorbing ability and thus is likely to adsorb moisture in an environment. Accordingly, activated carbon is required to be highly dehydrated in the process for producing an electric double layer capacitor. 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 in order to completely remove moisture from fine pores of activated carbon. However, activated carbon particles are usually formed on a current collector by means of a binder such as an organic polymer to constitute an electrode, and the binder undergoes thermal decomposition by treatment at a high temperature of at least 300° C. Accordingly, heat treatment is usually carried out at a temperature of at most 200° C., whereby it is difficult to completely remove moisture in the activated carbon electrodes.
Further, there has been a problem that due to the presence of a portion where the activated carbon surface and the electrolytic solution are not in contact with each other, i.e. the activated carbon surface is not wetted by the electrolytic solution due to inadequate impregnation of the electrolytic solution, or a portion where a gas generated by electrolysis is retained within fine pores of activated carbon, the area to store electric charge tends to be small, the capacitance developing ratio tends to decrease, and the resistance tends to increase.
An electric double layer capacitor employing an organic electrolytic solution, is operated at a voltage of at least 2 V which is higher than the theoretical decomposition voltage (1.23 V) of water for the purpose of increasing the energy density. Accordingly, in a state where the voltage is applied after assembling a capacitor cell, water remaining in the above-mentioned fine pores will be electrolyzed to generate a gas. It has been found that the generated gas will be gradually accumulated in the fine pores of activated carbon and will remain in the interior of the element without being discharged out of the element formed by impregnating the electrolytic solution to a pair of electrodes facing each other via a separator. If the electric double layer capacitor is used for a long period of time in such a state, the electrolytic solution present in the pores of activated carbon is likely to be driven out by the generated gas, whereby the capacitance normally expected to be obtainable, tends to be hardly obtainable, and further, an electroconductive path created by movement of ions in the pores, is likely to be blocked off. Consequently, there will be a decrease in the capacitance of the electric double layer capacitor or a deterioration in performance such as an increase of the internal resistance. Further, water remaining in the pores can not completely be removed, electrolysis due to adsorbed electric charge will take place continuously. Accordingly, there has been a problem that the voltage retention is poor after applying a voltage and opening the circuit.