Electrochemical capacitor

Details Electrochemical capacitor Electrochemical capacitor

2000-08-28

2002-12-31

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S503000, C361S508000, C361S523000, C361S528000, C429S321000, C429S321000

Reexamination Certificate

active

06501640

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electrochemical capacitor that exhibits a high electrostatic capacity density based on pseudo-capacity by using a carbon material having fine crystals of graphite-like carbon as a polarized electrode material, and by using a reaction product of an electrochemical reaction as an active material.
An electrochemical capacitor (hereafter called “capacitor”) is used in a backup power source for electronic devices and in batteries for various transport machines such as motor vehicles because of its large farad-class capacity and excellent charge-discharge cycle characteristics. The use in the storage of electric power at nighttime has also been examined from the viewpoint of effective use of energy.
A single electrode cell
10
, one of the basic structures of such a capacitor, generally comprises as
FIG. 1
shows, a positive-side polarized electrode
24
and a negative-side polarized electrode
26
formed on current collectors
20
and
22
consisting of a metal material, respectively. The polarized electrodes
24
and
26
are separated by a separator
28
, and are impregnated with an electrolyte solution consisting of a solvent and an electrolyte.
FIG. 2
shows the structure of a single capacitor cell
12
, in which a plurality of single electrode cells
10
are electrically connected in parallel by electrode extensions
30
and
32
formed on current collectors
20
and
22
. Such a single capacitor cell
12
is suitably used in a capacitor of a relatively large capacity used for motor vehicles and the like. These single electrode cells
10
and single capacitor cell
12
are both flat plate types, and are characterized in the ease of increasing packing densities and surface areas.
In addition to such a flat-plate type capacitor, there is a winding-type capacitor
70
as
FIG. 3
shows. In a winding-type capacitor
70
, a wound body
76
fabricated by cylindrically winding a positive electrode sheet
72
comprising a positive-side polarized electrode
24
formed on a current collector
20
and a negative electrode sheet
74
comprising a negative-side polarized electrode
26
formed on a current collector
22
via a separator
28
is used as an electrode body. For example, this wound body
76
is accommodated in a case
78
filled with an electrolyte solution, and the open end of the case
78
is sealed with a seal plate
82
on which electrode terminals
80
are formed, while securing electrical conduction between the electrode sheets
72
,
74
and the electrode terminals
80
.
As a material of the polarized electrodes for such capacitors, an activated-charcoal-based material that has a specific surface area of 1000 m
2
/g or more has been used. The major electrostatic capacity produced in such capacitors using activated charcoal is electric double layer capacity. In this case, the electrostatic capacity density is limited to 20F/cc, and the withstand voltage is limited to about 3 V.
In order to further improve the performance of capacitors, a new material for polarized electrodes that has a high electrostatic capacity is demanded, and a capacitor utilizing pseudo-capacity by a redox reaction of a metal oxide such as ruthenium oxide or a conductive high polymers such as polypyrrole is attracting attention.
However, since capacitors that use metal oxides require expensive materials such as ruthenium, and can use only aqueous electrolyte solutions, the operating voltage cannot be elevated. When a conductive high polymer is used, although the withstand voltage can be elevated because an organic electrolyte solution can be used, chemical synthesis is complicated and expensive, and thick films are difficult to form, making an increase in the capacity difficult.
Considering the above problems, the object of the present invention is to provide a capacitor that exhibits a high electrostatic capacity based on pseudo-capacity, and uses an active substance that can be synthesized at low costs.
SUMMARY OF THE INVENTION
According to the present invention, an electrochemical capacitor is provided, comprising an organic electrolyte solution, and polarized electrodes immersed in the organic electrolyte solution, wherein a reaction product formed by an electrochemical reaction of a carbon material that has partially oxidized fine crystals of graphite-like carbon in the organic electrolyte solution as an active substance of the polarized electrodes.
In the electrochemical capacitor of the present invention, the carbon material is preferably an easily graphitizable carbon material that has been heat-treated in an inert gas atmosphere at a temperature between 500° C. and 1000° C. One of the methods of partial oxidation comprises a heat treatment of the carbon material In the presence of at least one of an alkali metal compound that contains an alkali metal and oxygen in an inert gas atmosphere above the temperature at which a vapor of the alkali metal is generated. Another method of partial oxidation comprises the immersion of the carbon material in an oxidant that forms graphite acid when graphite is immersed therein. Furthermore, heat treatment of the carbon material may be carried out in an oxidizing atmosphere that contains an oxidizing gas at a temperature below the carbonization temperature of the easily graphitizable carbon material.
The electrochemical reaction for generating electrostatic capacity is preferably carried out by immersing an assembly of a sheet-like electrode fabricated using the partially oxidized carbon material, a separator, and a current collector in an organic electrolyte solution to construct a capacitor, and then passing an electric current through the capacitor. In this case, the electrolyte solution used for carrying out the electrochemical reaction may be different from the electrolyte solution that acts as a capacitor.


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patent: 5907472 (1999-05-01), Farahmandi et al.
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U.S. patent application Ser. No. 09/473,416, Niiori et al., filed Dec. 29, 1999.
U.S. patent application Ser. No. 09/387,995, Niiori et al., filed Sep. 1, 1999.
U.S. patent application Ser. No. 10/068,484, Niiori et al., filed Feb. 5, 2002.

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