Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
2002-08-23
2004-12-14
Nguyen, Ha Tran (Department: 2812)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C361S502000, C361S508000
Reexamination Certificate
active
06830594
ABSTRACT:
The present invention relates to a process for producing a non-aqueous electric double layer capacitor which has a low internal resistance, which has a high power output and a high energy density, and which is excellent in discharge properties, and a positive electrode for an electric double layer capacitor.
Heretofore, as a separator interposed between a positive electrode and a negative electrode of an electric double layer capacitor, a sheet obtained from e.g. fine fibers obtained by beating regenerated cellulose, craft pulp, sisal hemp or manila hemp, glass fibers or polyphenylenesulfide fibers by themselves or as mixed, a stretched microporous film made of polyolefin or PTFE, a polyethylene nonwoven fabric, a polypropylene nonwoven fabric and a polyester nonwoven fabric are known (JP-A-9-45586, JP-A-1-304719).
The role of a separator is to electrically insulate a positive electrode from a negative electrode on one hand and to facilitate transfer of ions in an electrolyte, which takes place accompanying charge and discharge, i.e. ion conduction on the other hand. Particularly for a high power output and large capacitance electric double layer capacitor for large current charge and discharge, decrease in the resistance and increase in the capacitance per unit volume i.e. the storage energy amount per unit volume (hereinafter referred to as energy density) have been desired, and it is accordingly required to make the separator as thin as possible. For such a high power output electric double layer capacitor, a separator having a thickness of from 50 to 150 &mgr;m has usually been used.
However, if the thickness of the separator is made to be less than 60 &mgr;m for the purpose of achieving a low resistance and an increase of the energy density, the insulating property between the positive and negative electrodes tends to be inadequate, thus leading to micro-short circuiting, which causes frequent self-discharge failure. Many of these problems are caused by the presence of pinholes generated on the separator by making the layer thin, and by electrically conductive coarse particles of e.g. activated carbon present on the surface of the electrodes. However, even in an electric double layer capacitor produced under conditions under which they are removed, it a thin layer separator of at most 60 &mgr;m is used, self-discharge failure still takes place in many cases. Accordingly, with a conventional electric double layer capacitor, the decrease in the resistance and the increase in the capacitance density by making the separator thin have been limited.
Accordingly, the present invention has been made to overcome the above problems, and it is an object of the present invention to provide a process for producing a high power output electric double layer capacitor, which has a low internal resistance, a high power output and a high energy density, and which has a low ratio of self-discharge failure, and a positive electrode for an electric double layer capacitor.
The present inventors have conducted extensive studies on the generation of the above-described self-discharge failure, and as a result, we have found such a phenomenon that metal impurities particularly copper present in the positive electrode which is polarized to a positive potential, electrochemically or partially chemically elute in the electrolyte in a voltage-applied state, and the copper ions are transferred to the negative electrode side and electrochemically reduced, and deposited on the surface of the negative electrode as metal copper. The present inventors have further found that when the metal copper grows and penetrates through the separator, micro-short circuiting takes place between the electrodes, which causes self-discharge failure.
Accordingly, the present invention is to provide a process for producing an electric double layer capacitor employing a thin layer separator, wherein the copper content in the positive electrode is decreased, to provide an electric double layer capacitor with which frequency of the self-discharge failure is low.
Specifically, the present invention provides a process for producing an electric double layer capacitor, which comprises a step of preparing a positive electrode and a negative electrode, each being a carbonaceous electrode containing as the main component a carbon material having a specific surface area of from 100 to 2,500 m
2
/g, a step of forming an element comprising the positive electrode and the negative electrode and a separator having a thickness of at most 60 &mgr;m interposed therebetween, and a step of impregnating the element with a non-aqueous electrolyte, wherein the copper content in the positive electrode before being impregnated with the non-aqueous electrolyte is at most 1 ppm.
Even when the thickness of the separator is at most 60 &mgr;m, more preferably at most 50 &mgr;m, by making the copper content in the positive electrode at most 1 ppm, micro-short circuiting due to deposition of metal copper can be suppressed, and at the same time, by making the separator thin, an electric double layer capacitor having a low internal resistance and a high energy density can be provided. It is more preferred that the thickness of the separator is at most 30 &mgr;m and the copper content is at most 0.5 ppm.
The present invention further provides a positive electrode for an electric double layer capacitor, which is a carbonaceous electrode containing as the main component a carbon material having a specific surface area of from 100 to 2,500 m
2
/g, wherein the copper content is at most 1 ppm.
In the electric double layer capacitors produced by the process of the present invention, both the positive electrode and the negative electrode are carbonaceous electrodes containing as the main component a carbon material having a specific surface area of from 100 to 2,500 m
2
/g. These capacitors work on the basis of a principle that electric charge is stored in an electric double layer formed at the interface between the electrodes and an electrolyte, or electric charge is stored in a pseudo electric double layer involving an electrochemical reaction. The carbon material preferably has a high specific surface area to increase the energy density per cell volume.
In the process of the present invention, first, a positive electrode and a negative electrode, each of which is a carbonaceous electrode containing as the main component a carbon material having a specific surface area of from 100 to 2,500 m
2
/g, are prepared. As the carbon material, activated carbon derived from natural plant tissue such as coconut shell, activated carbon derived from a synthetic resin such as a phenolic resin, activated carbon derived from fossile fuel such as coal, coke or pitch, carbon black, polyacene or a carbonaceous material in which graphite structure is developed, may, for example, be used.
The activated carbon is subjected to various activation treatment in order to increase the specific surface area and to increase adsorption properties. The activation method of the carbon material to obtain activated carbon varies depending upon the material to be used, but usually steam activation or alkali activation, particularly activation by KOH is applied. Activated carbon derived from natural plant tissue or fossile fuel contains a relatively large amount of metal impurities, and accordingly washing with e.g. an acid is usually required. Similarly, alkali activated carbon contains a large amount of alkali metals used for the activation or metal impurities brought from an activation apparatus due to the reaction with the alkali, and accordingly washing with e.g. an acid is required. Steam activated carbon made of a synthetic resin as a raw material, is most preferred since it contains a small amount of metal impurities, whereby the above-described washing is not required.
The carbonaceous electrode is prepared preferably in such a manner that a mixture comprising the carbon material having a high specific surface area which has a role of electric storage, a small amount of a binder and as the c
Hiratsuka Kazuya
Shinozaki Yasuo
Yoshida Naoki
Asahi Glass Company Limited
Nguyen Ha Tran
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