Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2001-01-31
2003-01-14
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C361S508000, C361S512000, C361S523000, C429S233000
Reexamination Certificate
active
06507479
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an electric double-layer capacitor (EDLC) having a laminated overcoat and, more particularly to such an EDLC having excellent electric characteristics and a longer lifetime.
(b) Description of the Related Art
The EDLC uses an electric double layer structure generated at the interfaces between polarized electrodes and an electrolytic solution for storing electric charge therein. The EDLC has the advantages of a lower thickness of the double-layer structure as low as several nanometers and capability of having a larger capacitance by using the polarized electrodes made of a material having a larger surface area, such as activated carbon.
EDLCs described in Patent Publications JP-2054380 and JP-A-57-60828 have polarized electrodes made of aluminum foils for achieving a larger capacitance and a lower internal resistance. These EDLCs are now being used for a variety of applications requiring a large electric power, such as for a hybrid car or electric car which enables an energy recovery due to an engine acting as a motor, a photovoltaic power generation or wind power generation for alleviation of fluctuation of generated power, a backup source to be used for a short-time power failure, a rash current source during startup of a motor, and alleviation of fluctuation of load of a fuel cell. The EDLC is generally requested to have a lower internal resistance and smaller dimensions for achieving discharge of large electric power in several seconds, for example.
The EDLCs are categorized into two types: one having an aqueous electrolytic solution such as including sulfuric acid and potassium hydroxide; and the other having an organic electrolytic solution using an organic solvent such as propylene carbonate and an electrolytic solution such as quaternary ammonium chloride. The EDLCs have different electric characteristics, constituent elements and structures depending on the types of the electric solution used therein.
The withstand voltage, for example, of the EDLC is limited by the electrolysis capability of the electrolytic solution used therein and assumes about one volt in the case of the aqueous electrolytic solution and about 2.3 to 3.3 volts in the case of organic electrolytic solution. More specifically, the EDLC using the organic electrolytic solution stores larger energy or can be made to have smaller dimensions.
On the other hand, the organic electrolytic solution has a specific resistance of about 100 &OHgr;-cm, whereas the aqueous electrolytic solution has a specific resistance of about 1 &OHgr;-cm. This means that the EDLC having the aqueous electrolytic solution has a desirable lower internal resistance due to the lower specific resistance of the electrolytic solution.
As for electrodes, a less expensive metal, such as aluminum, can be used in the EDLC having the organic electrolytic solution, whereas such a less expensive metal cannot be used in the EDLC having the aqueous electrolytic solution.
In view of the above facts, the EDLC having the organic electrolytic solution generally has either a wound structure wherein an aluminum foil is wound around the polarized electrodes or a coin cell structure using stainless steel. On the other hand, the EDLC having the aqueous electrolytic solution has a stacked structure including layers made of rubber or plastics.
Referring to
FIG. 1
, a conventional EDLC having the aqueous electrolytic solution has a plurality of basic cells
16
each including a separator
14
, a pair of polarized electrodes
12
sandwiching therebetween the separator
14
, and a pair of current collectors
13
sandwiching therebetween the pair of polarized electrodes
12
, and a gasket disposed between the pair of current collectors
13
for encircling the separator
14
and the pair of polarized electrodes
12
. The plurality of basic cells
16
are stacked one on another to form a stacked body
17
, with each of current collectors
13
sandwiched between adjacent basic cells being shared by the adjacent basic cells. Each separator
14
is impregnated with an aqueous electrolytic solution.
The stacked body
17
is sandwiched between a pair of terminal plates
18
, which are coupled together by bolts
19
and nuts
21
. An insulator bush
19
electrically isolates the bolt
19
from a corresponding nut
21
.
Each of the polarized electrodes
12
has a large surface area and a suitable electric conductivity, is made of material having an electrically-chemically stable property, and is impregnated with the aqueous electrolytic solution.
In general, the polarized electrode
12
is made of a material selected from the group consisting of activated carbon powder or activated carbon fibers having a specific surface area ranging between about 500m
2
/g and about 250m
2
/g, such activated carbon powder or fibers bonded by a fluorine-based binder, solid activated carbon which is bound by carbon as disclosed in Patent Publication JP-B-7-70448, and activated carbon/polyacene wherein activated carbon powder and/or activated carbon fibers are bonded by polyacene.
The current collector
13
electrically connects the polarized electrode
12
to an external circuit, prevents leakage of electrolytic solution, and is made of butyl rubber or elastomer added with carbon for achieving a suitable electric conductivity. In general, the current collector
13
has a thickness below about 500 &mgr;m and a specific resistance below about 10 &OHgr;-cm.
The separator
14
prevents a short-circuit failure between the pair of polarized electrodes
12
, allows electrolytic ions to pass therethrough and is made of unwoven cloth or porous film impregnated with the electrolytic solution. If the separator
14
is made of plastics such as polypropylene or polyethylene, the separator is added with a surfactant or silica for hydrophilic property.
The gasket
15
prevents a short-circuit failure between the pair of current collectors
12
and leakage of electrolytic solution, and is used as a structural material, which may be plastics, butyl rubber or elastomer.
If the gasket
15
is made of plastics, the gasket is bonded onto the current collector
12
by epoxy resin etc. If the gasket
15
is made of rubber or elastomer, the gasket
15
may be bonded onto the current collector
12
by vulcanization at a temperature of 100 to 130° C., as described in JP-A-60-216527.
The withstand voltage of the EDLC is restricted by the electrolysis of the electrolytic solution, as described above. Thus, if the EDLC is to be used at a higher working voltage, a plurality of basic cells
16
are connected in series.
Known structures for the EDLC include a simply-stacked structure wherein the basic cells
16
are simply stacked one on another and a bipolar structure, such as described in JP-A-6-005467 and described above with reference to FIG.
1
. The bipolar structure is obtained by modification of the simply-stacked structure wherein each current collector to be sandwiched between adjacent basic cells
16
in the simply-stacked structure is designed to be shared by the adjacent basic cells.
The basic cells
16
in the bipolar structure are subjected to an external pressure for reduction of the contact resistance by using rivets or bolts. In this case, especially in the case of using bolts, the EDLC has the drawback of a larger thickness.
For reduction of the thickness of the EDLC shown in
FIG. 1
, a laminated structure is recently used for the packing structure of the EDLC. Referring to
FIG. 2
, the EDLC having the laminated structure includes a stacked body (EDLC body)
17
, a pair of terminal plates
18
sandwiching therebetween the stacked body
17
to form a basic cell or a stack of basic cells, and a laminated overcoat
11
encapsulating the stacked body
17
. The laminated overcoat
11
is formed by folding a laminated film structure, and includes an innermost fused layer
11
c
having a fused edges, an intermediate foil
11
b
made of aluminum or aluminum alloy, and an outermost resin layer
11
a
. In the laminated
Kasahara Ryuichi
Saito Takashi
Ha Nguyen
Reichard Dean A.
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