Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2002-01-04
2002-12-10
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C361S508000
Reexamination Certificate
active
06493210
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrode metal material for electrical components such as capacitors and batteries which are used in contact with electrolyte, to a capacitor and a battery formed of the electrode metal material, and to a method of producing the electrode metal material and the capacitor and battery thereof.
PRIOR ART
At present, there are, for example, electric double-layer capacitors and electrolytic capacitors available as electrical components which are used in contact with electrolyte. Such electric double-layer capacitors have been applied to large-capacitance capacitors chargeable at up to about 3 V, and used for backup power sources of microcomputers, memory devices, timers, and the like.
Typically, an electric double-layer capacitor comprises a pair of polarizable electrodes or double-layer electrodes disposed face-to-face via an insulating separator therebetween and immersed in electrolyte. The electrode is produced by applying an activated carbon layer on the surface of an electrode metal material made of a valve metal and used as a mechanical supporter and, at the same time, electric collector.
Some types of electric double-layer capacitors use an organic-solvent based electrolytic solution as electrolyte, such as a tetraethyl ammonium salt which is added to an organic solvent, such as propylene carbonate. The examples of conventional electric double-layer capacitors using organic-solvent based electrolyte include a type in which a pair of electric double-layer electrodes is wound and enclosed in a container, and another type in which a pair of double-layer electrodes is laminated or stacked, both types having been disclosed in U.S. Pat. No. 5,150,283.
In the case of the winding type of capacitors, as shown in
FIG. 7
, an electrode metal material
1
is formed of etched aluminum foil having a thickness of 20 to 50 &mgr;m, and a paste obtained from a powder mixture of activated carbon particles, a desired binder and a desired conductive agent is applied to the above-mentioned metal foil to form a film. This film, that is, an activated carbon layer
30
(a polarizable electrode) mainly consisting of activated carbon particles, is used to form an electric double-layer electrode
3
.
A lead
6
is connected to each of the electrode metal materials
1
of the pair of electric double-layer electrodes
3
and
3
, respectively. These electrodes
3
and
3
are disposed face-to-face with a separator
5
therebetween and wound like a coil. The electric double-layer electrodes is immersed in non-aqueous electrolyte under vacuum to impregnate the activated carbon layers
30
and the separators
5
with the electrolyte, then placed in an aluminum case
70
, the opening
7
of the aluminum case
70
being sealed with a watertight packing
8
. The electrolyte in the electric double-layer capacitor has used polypropylene carbonate as an organic solvent, and a tetraethyl ammonium salt as an electrolyte, for example.
Furthermore, in a button-type electric double-layer capacitor, schematically shown in
FIGS. 9 and 10
, activated carbon layers
30
are joined to disc-like sheets
1
made of a valve metal material, respectively, to form a pair of double-layer electrodes
3
. The pair of double-layer electrodes
3
and
3
are disposed face-to-face via an insulating separator
5
therebetween, and accommodated in a metal container comprising two mating members. The valve metal material sheets of the two double-layer electrodes are joined to the inner surface sides of the bottom member
60
and the lid member
61
of the metal container. Both the bottom and lid members are joined to each other so as to be watertight by using an insulating ring packing
69
at the peripheral portion thereof. The interior of the capacitor is filled with non-aqueous electrolyte so that the double-layer electrodes and the activated carbon layers are immersed therein sufficiently. The non-aqueous electrolyte is a solution of tetraethyl ammonium perchlorate added in propylene carbonate in the same way as described above.
An electrolytic capacitor is known as a capacitor in which non-aqueous electrolyte is used. In the anode of the capacitor, a dielectric film is formed by chemically treating the valve metal foil. In the cathode, the valve metal foil is used as it is. Usually, both the electrodes are disposed face-to-face, wound into a coil, and hermetically enclosed in a container while being immersed in electrolyte.
In the case of the conventional electric double-layer capacitor, the valve metal sheet or foil, on which a polarizable electrode is formed as a film, has a naturally oxidized film specific to the valve metal constituting an electrode structure while the foil is handled. When this foil is used to form an electrode structure, a thin, insulating oxidized film
4
is frequently formed at the interface between the aluminum foil
1
used as a valve metal material and the polarizable electrode
3
, as schematically shown in FIG.
6
.
Furthermore, the above-mentioned non-aqueous electrolyte typically includes slight amounts of water and oxygen. For this reason, the valve metal material constituting the electrode structure reacts with the water content in the electrolyte during use of the capacitor, and the surface of the metal is oxidized. Therefore, when the electric double-layer capacitor formed of this kind of metal is used for extended periods of time, its equivalent series resistance (ESR), i.e., the internal resistance of the capacitor used as a power source, increases gradually, and, in some cases, its capacitance decreases.
This problem due to the oxidation of the metal portion of the electrode has also occurred in the case of the button-type electric double-layer capacitor in the same way.
Furthermore, the anode of the electrolytic capacitor using non-aqueous electrolyte is provided with a dielectric insulating layer formed by anodizing a valve metal such as aluminum. In addition, its cathode in direct contact with the electrolyte is also formed of the valve metal such as aluminum. In this case, an oxide film is formed on the surface of the metal used for the cathode because of oxidation with the water content in the electrolyte. This causes a problem of the capacitor increasing in internal resistance, just like the problem described above.
With respect to batteries using electrodes in contact to non-aqueous electrolyte, a lithium ion secondary battery is known which has high charge-discharge cycle performance with high energy density in a compact shape.
A lithium ion secondary battery, as shown in
FIG. 11
, comprises a positive electrode
35
, a negative electrode
37
, facing to the positive electrode, a film separator
5
for separating both electrodes
35
and
37
, and a non-aqueous electrolytic solution in which both the electrodes are placed and contained in a casing
71
. The positive electrode
35
is, as an example, formed of a mixture of positive active substance such as LiCoO
2
, conductive material such as acetylene black, and a binder including carboxylmethylcellulose and polyflorovinylidene which mixture is applied on both sides of aluminum foil as an electrode metal material
1
for an electric collector. On the other hand, the negative electrode
37
is formed of a mixture of negative active substance such as graphite and a binder such as carboxylmethylcellulose and styrene-butadiene rubber which mixture is applied on both sides of copper foil as an electric collector. The electrolytic solution is a non-aqueous solvent of a mixture of propylenecarbonate and 1,2-dimethoxyethane containing LiPF
6
as electrolyte. A porous polypropylene film is used as a separator.
In conventional lithium ion secondary batteries, aluminum foil is formed with natural oxide film on its surface during dealing with the foil so that thin isolating film have often been formed in the interface between the aluminum foil and the positive electrode on the aluminum foil.
Further, since the above non-aqueous electrolytic solution also contains slight amount of wat
Kojima Tamao
Nonaka Seiji
Shimada Mikinari
Tabata Munehiro
Tanahashi Masakazu
Dinkins Anthony
Matsushita Electric - Industrial Co., Ltd.
Thomas Eric
Wenderoth , Lind & Ponack, L.L.P.
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