Plastic and nonmetallic article shaping or treating: processes – Forming electrical articles by shaping electroconductive... – Conductive carbon containing
Reexamination Certificate
1998-12-16
2002-05-07
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming electrical articles by shaping electroconductive...
Conductive carbon containing
C264S210200, C264S210600, C264S280000
Reexamination Certificate
active
06383427
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode for an electric double layer capacitor and a process for producing it.
2. Discussion of Background
The demand for electric double layer capacitors utilizing an electric double layer formed at the interface between a polarizable electrode and an electrolyte, particularly coin shaped ones, is rapidly increasing recently, as a memory backup power. On the other hand, the development of an electric double layer capacitor having a large capacitance per volume, small internal resistance, a high energy density and a high output density, is required, for a use wherein a large capacity is required, e.g. as a driving source for electric cars. Further, with regard to an electric double layer capacitor for memory backup, it is desired to reduce the internal resistance.
The electrode for an electric double layer capacitor is prepared, for example, by kneading an activated carbon powder with a solvent of an electrolytic solution such as sulfuric acid, and forming the mixture into a slurry, followed by press forming (U.S. Pat. No. 3,288,641). However, the electrode obtained by the process has a rigid porous structure and thus is likely to be cracked or broken, and it can not be used for a long period of time. On the other hand, a carbon-based electrode has been proposed which is made of a viscous material having a binder such as a polytetrafluoroethylene (hereinafter referred to as PTFE) added to a mixture comprising an activated carbon powder and an electrolytic solution, as the case requires (JP-B-53-7025, JP-B-55-41015). The electrode has crack resistance and break resistance. However, it is inadequate in the shape keeping property, and a cell having a special structure is required to supplement the strength to use the electrode.
In order to obtain an electrode having crack resistance and break resistance, and an excellent shape keeping property, a process has been proposed, which comprises preliminarily molding a kneaded material comprising a carbonaceous material, a binder such as PTFE and a liquid lubricant, followed by stretching or rolling to obtain a formed electrode of sheet shape (JP-A-63-107011, JP-A-2-235320). However, by this process, PTFE is randomly formed into fibers by kneading, and a part of PTFE is formed into fibers and the rest is not, and consequently the hardness of the two parts will be different. Therefore, when forming the sheet electrode into a thin film sheet having, for example, a thickness of at most 0.2 mm, the surface tends to be irregular, and holes are likely to be formed. Therefore, the capacitance per volume (hereinafter referred to as capacitance density) of the electric double layer capacitor can not be made large, and the internal resistance is large.
A process has also been proposed, which comprises mixing an activated carbon powder and PTFE to obtain a paste, coating the paste on a current collector, followed by drying, heating at a temperature higher than the melting point of PTFE, and press-forming the electrode to make it thin to increase the density (JP-A-9-36005). However, with this process, the production steps are complicated, and it is difficult to continuously conduct the process, and a part of PTFE melts so that the internal resistance will be high.
On the other hand, for extrusion of PTFE, a paste extrusion method has been known, which comprises using a polymer (fine powder) obtained by coagulating and drying an aqueous dispersion of PTFE made by emulsion polymerization of a tetrafluoroethylene, adding a processing aid such as naphtha or white lamp oil thereto, preliminarily molding the mixture to form it into a sleeve shape and filling it to a cylinder mold, followed by pressurizing by a ram to extrude it through a nozzle suitable for a shape of a rod or a sheet, and vaporizing the processing aid to obtain a formed product (U.S. Pat. No. 4,177,334, U.S. Pat. No. 4,250,138).
The process is generally applied to molding of PTFE alone or PTFE containing several wt % of a filler, and is not applied to molding wherein a filler is the main component and PTFE is used merely as a mechanical supplement. Namely, since the filler such as graphite, glass fiber or carbon fiber is not likely to undergo plastic deformation, in the case where it is mixed with PTFE and molded, the extrusion pressure tends to be high, and PTFE is highly deformed. Therefore, there is a problem that the obtained extruded product is fragile and has low strength.
SUMMARY OF THE INVENTION
The present invention has been made to overcome the problems of the prior art, and it is an object of the present invention to provide an electrode having a shape of thin film sheet and having high strength and low resistance, and a process for producing it, thereby to provide an electric double layer capacitor having a high capacitance density and small internal resistance, particularly an electric double layer capacitor suitable for use wherein a large current is required with a high capacitance.
The present invention provides a process for producing an electrode for an electric double layer capacitor, which comprises extruding a mixture comprising a carbonaceous material, PTFE and a processing aid by paste extrusion, and rolling the obtained extruded product by rolling rolls to form it into a sheet shape.
Further, the present invention provides an electrode for an electric double layer capacitor, which is a formed product of sheet shape prepared by forming a carbonaceous material by means of PTFE as a binder and which has a thickness of from 0.005 to 0.25 mm, a porosity of from 50 to 80% and the tensile strength in one direction is at least 1.5 kg/cm
2
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The electric double layer capacitor using the electrode of the present invention is based on a principle to form an electric double layer at the interface between an electrolytic solution and a carbonaceous material which is a material for an electrode, and to store electric charge in the electric double layer. As the carbonaceous material, a powder of e.g. activated carbon, polyacene or carbon black, which has a specific surface area of from 200 to 3500 m
2
/g, is preferred. Further, a fiber or a powder of e.g. carbon fiber, carbon whisker or graphite can also be preferably used, so long as its specific surface area is from 200 to 3500 m
2
/g. As the activated carbon, a phenol type, a rayon type, an acryl type, a pitch type or a coconut shell type may be used. The particle size of the activated carbon is preferably from 0.1 to 100 &mgr;m, particularly preferably from 1 to 20 &mgr;m, whereby it is easy to form the electrode into a thin film sheet, and the capacitance density can be made high.
It is also preferred to use carbon black in admixture with another carbonaceous material, as a conductive material. In the case of using it as a conductive material, the particle size of the carbon black is preferably from 0.001 to 1 &mgr;m, particularly preferably from 0.01 to 0.5 &mgr;m, thereby the resistance of the electrode can be made low, even when it is contained in the electrode in a small amount. Further, the specific surface area of the carbon black as a conductive material, is preferably from 200 to 1500 m
2
/g, particularly preferably from 500 to 1200 m
2
/g. An electrode comprising the carbon black as a conductive material, activated carbon having a specific surface area of from 200 to 3500 m
2
/g and a particle size of from 0.1 to 100 &mgr;m, and PTFE, is preferred since the internal resistance can be kept low, and the capacitance can be kept high.
PTFE of the present invention includes not only a homopolymer of tetrafluoroethylene but a copolymer obtained by adding at most 0.5 mol % of another monomer to tetrafluoroethylene, followed by copolymerization. When another monomer is at most 0.5 mol %, the melt fluidity is not given to PTFE, and it is possible to form the copolymer into fibers to prepare an electrode sheet having high strength and low resistance, just like the homopolymer of tetr
Ishikawa Takamichi
Kuroki Satoru
Suhara Manabu
Asahi Glass Company Ltd.
Eashoo Mark
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