Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
1999-12-29
2002-08-27
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C361S512000
Reexamination Certificate
active
06442015
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to electrochemical capacitors wherein reliability and durability are improved by reducing the pressure generated by change in volume accompanying charging and discharging, when high-performance carbon materials with large electrostatic capacity are used as polarizing electrodes. The present invention also relates to electrostatic capacitors having structures relaxing the stress generated by expansion of the polarizing electrodes on charging.
Besides being used as back-up power sources and batteries for transportation vehicles from automobiles down, as they have large capacity of the Farad class and are excellent in charging and discharging cycle characteristics, electric double layer capacitors, a type of capacitors, are investigated for use in off-peak power storage from the viewpoint of effective utilization of energy.
A single electrode cell
10
, one of the basic structures of such electric double layer capacitors, has the structure as. shown in
FIG. 15
in which a positive side polarizing electrode
24
and a negative side polarizing electrode
26
are respectively formed on collectors
20
and
22
generally comprised of metallic materials, said polarizing electrodes
24
and
26
being separated by a separator
28
, and are impregnated with electrolytic solution comprising the solvent and electrolytes.
FIG. 16
shows the structure of a single capacitor cell
12
, wherein plurality of single electrode cells
10
are electrically connected parallel with each other to electrode outlet parts
30
and
32
formed on collectors
20
and
22
. Such a single capacitor cell
12
is suitably used as an electric double layer capacitor with relatively large capacity used for automobiles, etc. Both single electrode cells
10
or single capacitor cells
12
are planar and characterized by ease of tight packing and expansion.
In contrast to such planar electric double layer capacitors, there are also coil type electric double layer capacitors
70
having structures suitable for capacity enlargement similar to a single capacitor cell
12
as shown in
FIG. 17. A
coil type electric double layer capacitor
70
is prepared using a coil
76
prepared by cylindrically coiling a positive electrode sheet
72
, in which a positive electrode side polarizing electrode
24
is formed on a collector
20
, and a negative electrode sheet
74
, in which a negative pole side polarizing electrode
26
is formed on a collector
22
, with a separator
28
between them, and for example by putting said coil
76
in a case
78
and filling the case with electrolytic solution, and sealing the opening end surface of the case
78
with a sealing plate
82
having an electrode terminal
80
formed on it, while securing continuity between electrode sheets
72
and
74
and each electrode terminal
80
.
As polarizing electrode materials for such electric double layer capacitors, those having activated carbon with relative surface area of 1000 m
2
or more as a main ingredient have traditionally been used. The construction of a pair of electrodes of an electric double layer capacitor using such activated carbons will be explained referring to FIG.
18
.
As shown in
FIG. 18
, this pair of electrodes
1
is constructed to have a positive pole
2
and a negative pole
3
stuck to one another with a separator
6
between them.
To explain more precisely about the positive pole
2
and the negative pole
3
, they have layers of polarizing electrodes
5
containing activated carbon as an essential ingredient and optionally binders such as polytetrafluoroethylene and conduction aids such as carbon black, on the surface of a collector
4
comprising aluminum foil, etc.
Further, the layer of polarizing electrode
5
is formed for example by blending a mixture of fixed ratio of activated carbon, a binder and a conduction aid to give a sheet of fixed shape and area, which is then adhered onto a collector
4
. It may also be formed by coating the surface of a collector
4
with a paste of a mixture of activated carbon, a binder, a conduction aid, and a solvent and drying it to remove the solvent by evaporation. Known coating methods such as spray coating and brush coating may be adopted.
Here the separator
6
is impregnated with an organic electrolytic solution (not shown) such as ethylene carbonate, propylene carbonate, &ggr;-butyrolactone, and sulfolane, containing a fixed concentration, for example, of 1 mol/l of an electrolyte such as Et
4
NBF
4
(Et
4
N; tetraethylammonium), Et
4
NPF
6
, Bu
4
NBF
4
(Bu
4
N: tetrabutylammonium) or Bu
4
NPF
6
.
When such an electric double layer capacitor having a pair of electrodes
1
and a separator
6
is charged, the ions of the electrolyte are adsorbed inside micropores of the activated carbon which is the essential ingredient of the polarizing electrode, and this adsorption causes capacitance to occur in the electric double layer capacitor.
As the separator
6
, materials may be used that electrically insulate the positive pole
2
and the negative pole
3
to prevent them to short, and may allow ions of the electrolytes to penetrate so that ions of the electrolytes can migrate toward the positive pole and the negative pole
3
during charging and discharging. Cellulose blend paper is particularly suitable. Cellulose blend paper also has advantages that it is inexpensive, enabling cost reduction, and may be formed thinner, enabling reduction of internal resistance of the electric double layer capacitor.
Further, cellulose blend paper will not melt even when the positive and negative electrodes short, as it has a high melting point. Therefore, it is seldom used for electric cell separators. This is because migration of electrolyte ions is not prevented, as ion penetrating pores of the separator are not blocked even when the internal temperature of an electric cell rises due to excessive reaction of the cell when electrodes short, resulting in further increase of the internal temperature.
Internal temperature of an electric double layer capacitor, however, scarcely rises even when the separator does not melt in a short circuit. In other words, there is no problem in using high melting point separators, since internal temperature would not rise significantly in a short circuit. These and the advantages described above are the reason why cellulose blend paper is widely used.
Capacitance and upper limited voltage of conventional electric double layer capacitors described above depend on the activation method of activated carbon. For example, capacitance is 15 F/cc and upper limited voltage is 3 V for an electrochemical capacitor using steam activated carbons, and capacitance is 20 F/cc and upper limited voltage is 2.5 V for an electrochemical capacitor using alkali-activated carbons.
Capacitance and upper limited voltage of various capacitors including electrical double layer capacitors are expected recently to improve further, those with these values above 30 F/cc and 3.5 V being desired.
Thus, alkali-activated new polarizing electrode materials are reported in Japanese patent provisional publications No. 275042/1997 and 320906/1997 in order to increase the capacity of polarizing electrode materials.
In the case of electrochemical capacitors using alkali-activated polarizing electrode materials as disclosed in Japanese patent provisional publications No. 275042/1997 and 320906/1997, however, there is a problem of volume expansion of the polarizing electrode materials due to charging. There was no such problems of volume expansion in electric double layer capacitors using conventional activated carbons.
This volume expansion tends to increase as the amount of electricity contributing to charging and discharging increases, and is especially a big problem in uses wherein it is preferable that capacitance is big, charging and discharging capacity is big, and further capacitance density (density of capacitance for unit volume) or capacitance weight-density (density of capacitance for unit weight) is big,
Katsukawa Hiroyuki
Niiori Yusuke
Yoshida Hitoshi
Burr & Brown
NGK Insulators Ltd.
Reichard Dean A.
Thomas Eric W.
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