Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
1999-04-22
2001-05-22
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C029S025030
Reexamination Certificate
active
06236560
ABSTRACT:
The present invention relates to an electrode for an electric double layer capacitor, particularly an electrode for an electric double layer capacitor having excellent mechanical strength and a large capacitance.
The electric double layer capacitor is based on a principle to store electric charge in an electric double layer formed at the interface between an electrode and an electrolyte. In order to obtain an improved capacitance density of the electric double layer capacitor, it is common to employ as a material for the electrode, a material having a high specific surface area, including a carbonaceous material such as activated carbon and carbon black, and fine particles of a metal and conductive metal oxide. For efficient charging and discharging, the electrode is bonded to a current collector made of a layer or a foil having low resistance, such as a metal or graphite. As the current collector, it is common to employ a valve metal such as aluminum or a stainless steel such as SUS304 or SUS316L, which are electrochemically highly corrosion resistant.
As an electrolyte for the electric double layer capacitor, an organic electrolyte and an aqueous electrolyte are available. However, an attention has been drawn to an electric double layer capacitor employing an organic electrolyte, as the operating voltage is high, and the energy density in the charged state can be made high. In the case of using an organic electrolyte, if water is present in the interior of an electric double layer capacitor cell, the water may be electrolyzed and deteriorate the performance. Accordingly, the electrode is required to have water completely removed, and usually the electrode is subjected to drying treatment by heating under reduced pressure.
In the case of using an organic electrolyte, activated carbon is mainly used as the main component for the electrode. However, activated carbon is usually in a powder form. Accordingly, activated carbon is mixed with a binder such as polytetrafluoroethylene, molded into an electrode sheet, which is then electrically connected with a current collector by means of an electroconductive adhesive layer, to form an electrode assembly.
Since polytetrafluoroethylene is excellent in heat resistance and chemical stability, it has been known to use an electrode which is made by using polytetrafluoroethylene as a binder, mixing activated carbon therewith followed by kneading and stretching, thereby forming polytetrafluoroethylene into fibers to form a continuous fine porous structure (JP-B-7-105316). However, in such a case, if polytetrafluoroethylene is not adequately formed into fibers, it can not support activated carbon adequately, thus lowering mechanical strength of the electrode. In order to supplement mechanical strength, the amount of polytetrafluoroethylene may be increased. However, in such a case, the amount of activated carbon will be relatively small. Accordingly, the capacitance of an electric double layer capacitor per unit volume (hereinafter referred to as capacitance density) decreases.
The present invention has been made to overcome above-mentioned problems, and it is an object of the present invention to provide an electrode suitable for an electric double layer capacitor having excellent mechanical properties and high capacitance, and an electric double layer capacitor employing the electrode.
The present invention provides an electrode for an electric double layer capacitor comprising a carbonaceous material and a binder, wherein the binder comprises a polytetrafluoroethylene which is made from a fine powder of polytetrafluoroethylene having a specific gravity of at most 2.18 as defined in JIS-K6891, and an electric double layer capacitor employing the electrode.
The present invention further provides an electrode for an electric double layer capacitor comprising a carbonaceous material and a binder, wherein the binder comprises a polytetrafluoroethylene which is made from a fine powder of polytetrafluoroethylene made of aggregate of particles having an average primary particle size of from 0.26 to 0.40 &mgr;m, and an electric double layer capacitor employing the electrode.
The present inventors have conducted extensive studies particularly on polytetrafluoroethylene as a binder to overcome above-mentioned problems, and as a result, they have found that it is effective to use a fine powder of polytetrafluoroethylene having a high molecular weight, or a fine powder of polytetrafluoroethylene made of aggregate of particles having a specific primary particle size.
The electrode of the present invention is preferably a porous sheet wherein the polytetrafluoroethylene is formed into fibers to form a continuous fine porous structure, and a carbonaceous material is supported by said structure. The fine powder of polytetrafluoroethylene is three-dimensionally formed into fibers by extraneous stress such as sheer, rolling or stretching, to form a continuous fine porous structure. As the carbonaceous material can be contained densely in the continuous fine porous structure, with an electrode made of the structure, high capacitance can be obtained. Further, as fine fibers are present, suitable flexibility and high mechanical strength can be obtained.
The above electrode is produced preferably in such a manner, for example, that a mixture comprising a carbonaceous material, a fine powder of polytetrafluoroethylene and a liquid lubricant is kneaded and molded into a sheet, and the molded product is subjected to rolling treatment and/or stretching treatment. In such a case, the rolling treatment may be conducted either monoaxially or multiaxially, and the liquid lubricant is removed before or after the rolling treatment and/or the stretching treatment. It is particularly preferred to conduct extrusion molding of the mixture by e.g. paste extrusion or screw extrusion, followed by the rolling treatment and/or the stretching treatment, as the fine powder of polytetrafluoroethylene is formed into fibers in both length and breadth direction to constitute a three-dimensional network structure, and a thin electrode sheet can be continuously produced.
The fine powder of polytetrafluoroethylene of the present invention is a powder obtained through an aggregation step from a dispersion, which is obtainable by a dispersion polymerization conducted in an aqueous medium, which is one of common polymerization methods of polytetrafluoroethylene. The primary particle size of polytetrafluoroethylene obtained by the polymerization method, i.e. the dispersed particle size in water during the polymerization, is usually from 0.15 to 0.40 &mgr;m.
Particularly, the particle size of a fine powder to be aggregate after the polymerization is from 0.20 to 0.25 &mgr;m.
In the present invention, a fine powder of polytetrafluoroethylene made of aggregate of particles having an average primary particle size of from 0.26 to 0.40 &mgr;m, is used as a binder for the electrode. By using a fine powder of polytetrafluoroethylene having a particle size of such a range, an electrode having excellent mechanical strength and high capacitance, can be obtained. Further, above-mentioned porous structure can be effectively produced.
Usually, primary particles of a fine powder of polytetrafluoroethylene are not spherical, but in a form of straw bags having the aspect ratio not being 1. Accordingly, the primary particle size in the present invention is represented by average of the minor axis and the major axis of the primary particles observed by an electron micrograph. As a specific measuring method, the primary particle size of from 100 to 10,000 particles of the fine powder of polytetrafluoroethylene is measured at random by observation by penetration electron microscope, and the average is taken as an average primary particle size.
If the average primary particle size of the fine powder of polytetrafluoroethylene is less than 0.26 &mgr;m, the polytetrafluoroethylene is not adequately formed into fibers by shear, rolling or stretching, thus lowering mechanical strength of the obtained electrode sheet.
Ikeda Katsuji
Kawasato Takeshi
Suhara Manabu
Asahi Glass Company Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Reichard Dean A.
Thomas Eric
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