Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2000-02-02
2001-11-20
Bowers, Charles (Department: 2813)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C429S231800, C429S232000, C429S217000, C029S025030
Reexamination Certificate
active
06320740
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a polarized electrode for an electric double-layer capacitor having an electrolytic solution.
DESCRIPTION OF A RELATED ART
The electric double-layer capacitor uses an electric double-layer generated in the interface between a polarized electrode and the electrolytic solution for storing electric charge in the interface. The electric double-layer has a thickness on the order of as low as several nanometers (nm), and the polarized electrode thereof can be made from a material, such as activated carbon, having a large specific surface area for achieving a large electric capacity. Since the materials for the electric double-layer capacitor do not include any harmful stuff such as heavy metals, it has the advantage of less environmental burdens. In addition, it has a large lifetime against the iterative charge and discharge thereof due to no chemical reaction accompanied thereby differently from the secondary cells. Thus, the electric double-layer capacitor has been expected to have variety of potential applications such as a backup electric source for a microcomputer, a memory device etc. in place of the secondary cell.
The current electric double capacitor has a larger electric capacity and a lower internal resistance due to the recent invention of an activated carbon/polyacene composite material (Patent Publication JP-B-7-91449) and the development of the process for forming an activated carbon layer on an aluminum foil by using a binder such as described in JP-A-57-60828. Thus, electric double-layer capacitor now has wide actual applications for power uses such as for an energy recovery in a hybrid motor car or an electric vehicle, alleviation of fluctuation in the power source generated by photovoltaic generation or wind power generation, backup source against instantaneous service interruption, rash current source during start of a motor, and alleviation of the load fluctuation in the fuel power system.
It is desired that the electric double-layer capacitor have a lower internal resistance because these applications require a high-speed charge/discharge function wherein a power of several hundreds of kilowatts to several tens of kilowatts, for example, is charged or discharged in a very short time, e.g., several seconds.
The electric double-layer capacitors are generally classified depending on the electrolytic solution thereof into two types: an aqueous solution system such as including sulfuric acid or potassium hydroxides; and an organic solution system including organic solvents and quaternary ammonium salts as the electrolyte thereof. The two types have different electric characteristics, different constituent elements and different structures. The aqueous solution system has the disadvantage of lower breakdown voltage as low as 1 volt compared to 3 volts of the organic solution system, and yet has the advantage of lower internal resistance due to the low resistivity of the electrolytic solution. However, low-cost metals such as aluminum cannot be used in the aqueous solution system and can be used in the organic system. This allows the organic solution system to have a winding structure, wherein a collector made of an aluminum foil is used for winding the polarized electrode, or a coin cell structure having a stainless steel foil. On the other hand, the aqueous solution system has a laminated structure wherein rubber or plastics is used as a basic material.
FIG. 1
shows a sectional view of a conventional electric double-layer capacitor including a single basic cell having an aqueous solution as an electrolyte. A pair of polarized electrodes
11
are made from a material having a large specific surface area, an electric conductivity, and a chemical stability. The material for the polarized electrode
11
may be selected from materials having a specific surface area of 500 to 2500 m
2
/gram, such as activated carbon powder or fibers, activated carbon powder or fibers combined (bound) by a binder such as fluorine-based material, a solid active carbon obtained by binding activated carbon powder or fibers with carbon as described in Patent Publication JP-B-7-70448, and an activated carbon/polyacene composite material prepared by binding activated carbon powder or fibers by using polyacene.
A collector
12
electrically connects the polarized electrode
11
to the external circuit while preventing leakage of the electrolytic solution. The collector
12
is made from a butylene-isoprene rubber (butyl rubber) or elastomer by adding carbon thereto for electric conductivity. The collector
12
generally has a thickness equal to or less than 500 &mgr;m and a specific resistivity equal to or less than 10 &OHgr;-cm.
A separator
13
separates the pair of polarized electrodes
11
for prevention of a short-circuit failure or contact therebetween, while passing therethrough electrolytic ions. The separator
13
is generally made from nonwoven fabric or porous film. Plastic material including polypropylene or polyethylene, if used for the separator
13
, may be added with a surface active agent or silica to have a hydrophilic property.
A gasket
14
is used as a structural material for preventing a short-circuit failure between the pair of collectors
12
and leakage of the electrolytic solution. The gasket
14
may be made from plastics, butyl rubber or elastomer. The plastics, if used for the gasket
14
, may be bonded to the collectors
12
by using a epoxy resin adhesive. The butyl rubber or elastomer, if used for the gasket
14
, may be cured at a temperature of 100 to 130° C., as described in JP-A-60-216527, to be bonded to the collectors
12
.
The breakdown voltage or operating voltage of the electric double-layer capacitor is limited by the electrolysis of water and is generally around 1 volt. To obtain a desired breakdown voltage, a specified number of electric double-layer capacitors are connected in series. In this case, either a stacked structure wherein the basic cells are stacked one on another or a bipolar structure wherein a single collector is used in common for a pair of basic cells disposed on both surfaces of the collector, as described in JP-A-6-5467, may be used.
The polarized electrode
11
, if made from activated carbon powder or fibers or made as a solid body by binding activated carbon powder or fibers with a binder such as fluorine-based material, should be subjected to a fastening pressure of 10 kg/cm
2
or more to reduce the contact resistance between the powder or fibers and the contact resistance between the polarized electrode
11
and the collector
12
. The electric double-layer capacitor
10
has a smaller internal resistance along with a larger opposing area between the pair of polarized electrodes
11
. However, an electric double-layer capacitor having a lower internal resistance is difficult to handle because of large dimensions thereof which are required for maintaining the pressure. On the other hand, an activated carbon/carbon composite material wherein activated carbon powder or fibers are bound with carbon, or an activated carbon/polyacene composite material wherein activated carbon powder or fibers are bound with polyacene can provide a suitable specific resistivity. for the electric double-layer capacitor as low as several tens of &OHgr;-cm or less, whereby the electric double-layer capacitor thus fabricated has a higher electric capacity and a lower internal resistance.
The activated carbon/carbon composite material or the activated carbon/polyacene composite material has a high rigidity, and thus a lower transformation capability. As a result, the polarized electrode
11
made from these materials has a smaller contact area with respect to the collector
12
in the case of a low surface flatness of the polarized electrode
11
, whereby the contact resistance between the polarized electrode and the collector is higher compared to the contact resistance between the collector and the activated carbon powder or fibers. In view of the above, the electric conta
Kasahara Ryuichi
Kibi Yukari
Saito Takashi
Bowers Charles
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
Kielin Erik
NEC Corporation
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