Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
1999-01-28
2001-07-24
Graybill, David E. (Department: 2814)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C428S208000, C264S108000
Reexamination Certificate
active
06264707
ABSTRACT:
The present invention relates to a process for producing an electrode for an electric double layer capacitor.
The demand for an electric double layer capacitor utilizing an electric double layer formed at the interface between a polarizable electrode and an electrolyte, particularly for a coin shaped one, is rapidly increasing recently, as a memory backup power source. On the other hand, it is desired to develop an electric double layer capacitor having a large capacitance per volume, small internal resistance, a high energy density and a high output density, also for a use which requires a large capacity such 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 for 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 stiff 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 made of 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.
Further, by the above processes, it is difficult to prepare an elongate electrode sheet, and the electrode can not be obtained by continuous operation, whereby production efficiency is poor.
On the other hand, as a method for extruding 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 (JP-B-61-54578).
This method 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 a 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. Further, in the paste extrusion method, the amount extruded at a time depends on the shape of sleeve, and thus it is difficult to continuously obtain an elongate extruded product.
The present invention has been made to overcome the above-mentioned 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 a process for continuously producing the electrode, thereby to provide an electrode for an electric double layer capacitor having a high capacitance density and small internal resistance, particularly an electrode for 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, a polytetrafluoroethylene and a processing aid by screw extrusion, and rolling the obtained extruded product by rolling rolls to form it into a sheet shape, and an electrode obtainable by the process.
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 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. 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, and the specific surface area of the carbon black is preferably from 200 to 1500 m
2
/g, particularly preferably from 500 to 1300 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.
In the present invention, PTFE 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 and to prepare an electrode sheet having high strength and low resistance, just like the homopolymer of tetrafluoroethylene. As another monomer, hexafluoropropylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), trifluoroethylene or (perfluoroalkyl)ethylene may, for example, be mentioned.
PTFE is a liquid or a gel when it is of low molecular weight, and in such a state, it can hardly be formed into fibers. Therefore, it is preferred that PTFE contains at least 50 wt % of a polymer having a molecular weight of at least 1×10 as calculated from the standard specific gravity. Further, PTFE obtained by emulsion polymerization is preferred since it is easily formed into fi
Ishikawa Takamichi
Kanetoku Sadao
Kuroki Satoru
Suhara Manabu
Asahi Glass Company Ltd.
Graybill David E.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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