Electric double layer capacitor and method of forming the same

Details Electric double layer capacitor and method of forming the same Electric double layer capacitor and method of forming the same

2000-06-21

2002-01-15

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Double layer electrolytic capacitor

C361S503000, C361S508000, C361S509000, C361S516000

Reexamination Certificate

active

06339529

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electric double layer capacitor and a method of forming the same, and more particularly to an improvement in structure of a collector and a polarization electrode of an electric double layer capacitor which has a large capacity for various purposes, for example, of a semiconductor memory back-up.
FIG. 1
is a schematic cross sectional elevation view illustrative of a structure of a basic cell or a capacitor element of a conventional electric double layer capacitor. A pair of polarization electrodes
10
is provided. A separator
12
is also provided which separates the paired polarization electrodes
10
. A pair of collectors
11
is provided, so that the paired collectors
11
sandwich the paired polarization electrodes
10
. The sandwiched structure comprising the paired polarization electrodes
10
and the separator
12
has opposite ends which are spaced apart from each other in a direction parallel to interfaces between the separator
12
and the paired polarization electrodes
10
. The opposite ends of the sandwiched structure are sealed with a gasket
13
. Each of the paired polarization electrodes
10
is permeated with an electrolyte to form an electric double layer structure. Further, a pair of terminal plates
14
is provided, wherein the paired terminal plates
14
are in contact with the paired collectors.
11
.
It is necessary that the polarization electrodes
10
are electrically conductive and stable to the used electrolyte as well as have a large surface area. Powders or fibers of activated carbons such as coconut shell based activated carbon, or one of those activated carbons solidified by a binder such as polyetrafluoroethylene are available for material of the polarization electrodes
10
. The polarization electrodes
10
are permeated with an electrolyte therein. An available electrolyte of solution type is, for example, a sulfuric acid and potassium hydroxide. An available electrolyte of organic solvent type is, for example, quaternary ammonium salt. If the solution type electrolyte is used, then the collector
11
may be made of an organic material such as a rubber provided with a conductivity, for example, carbon powders. If the organic solvent type electrolyte is used, then the collector
11
may comprise a metal film. In order to reduce an internal resistance of the polarization electrodes
10
themselves and also reduce sa contact resistance between the polarization electrode
10
and the collector
11
, a thermal fusion splicing between the polarization electrode
10
and the collector
11
is carried out so that projections on a surface of the polarization electrode
10
partially enter into the surface of the collector
11
, if the collector
11
may be made of the organic material such as the rubber with the conductive powders. The separator
12
comprises a porous membrane with an ion-permeability but without electrical conductivity, for example, a polypropylene porous membrane. The gasket
13
may comprise an insulation rubber for maintaining a shape of the basic cell and preventing any leakage of the electrolyte as well as preventing a short circuit due to contact of the collectors
11
to a case. The terminal plates
14
are pressured toward the collectors
11
so as to reduce a contact resistance between the terminal plates
14
and the collectors
11
.
A withstand voltage of the basic cell depends upon the kind of the electrolyte. If, for example, the solution type electrolyte is used, then the withstand voltage is about 1.0V. If the organic solvent type electrolyte is used, then the withstand voltage is in the range of about 2.0 V to 3.0V. Therefore, in order to obtain the required withstand voltage, a plurality of the basic cells are laminated in series connection.
In the past, the electric double layer capacitor has been used for the purpose of applying a relatively small current, such as a back-up of a memory. In recent years, however, the electric double layer capacitor has been used for not only the above purpose but also another purpose of applying a large current in other fields of automobile, and electronic components which need relatively large currents. In order to apply a relatively large current, it is important to reduce the thicknesses of the polarization electrode and the collector so as to reduce an equivalent series resistance (ESR) of the basic cell. Upon the requirement for size-down of the electronic component, it is also required to reduce the thickness of the electric double layer capacitor.
The conventional electric double layer capacitor has the following problems. The surface roughness of the collector and the polarization electrode makes it difficult to obtain tight contacts between the collector and the polarization and between the collector and the terminal plate. Loose contacts between the collector and the polarization and between the collector and the terminal plate result in large contact resistances between them. In order to reduce the contact resistance, it is necessary that the basic cell is pressured from opposite sides to the center so as to obtain the required tight contacts between them. Time-passing makes the contacts between them loose whereby the contact resistances and the equivalent series resistance increase. Further, a long-time use of the electric double layer capacitor under conditions of a high temperature and a high voltage load, a gas is likely to be generated in the interior of the capacitor whereby the polarization electrodes are peeled to increase the internal resistance.
It has been known that in order to reduce the contact resistance between the polarization electrode and the collector, the polarization electrode and the collector are bonded with each other by a conductive adhesive. In Japanese laid-open patent publication No. 3-28318, it is disclosed that a conductive adhesive material is applied on the collector and a polarization electrode is made into tightly contact with the conductive adhesive material applied on the collector by a heat press. In Japanese laid-open patent publication No. 7-86098, it is disclosed that a solution dispersed with an activated carbon is put into a collector in the shape of a container to cause carbonization to form a polarization electrode, before the collector and the polarization electrode are bonded with each other by a conductive adhesive. In Japanese laid-open patent publication No. 9- 148202, it is disclosed that a polyvinylidene chloride resin melt with a solvent is applied on the collector before a polarization electrode is placed on the polyvinylidene chloride resin applied on the collector, and then the polarization electrode is pressured toward the collector to heat up the same but at a temperature lower than a carbonization temperature whereby the polarization electrode and the collector are bonded with each other.
The above conventional methods by using the conductive adhesive are effective to reduce the contact resistance between the polarization electrode and the collector. Since, however, the conductive adhesive is not so low in the resistance, then it is difficult to considerably reduce the internal resistance of the electric double layer capacitor.
In Japanese laid-open patent publication No. 5-326326, it is disclosed that activated carbon is used for the polarization electrode whilst graphite powders are used for the collector, but the polarization electrode and the collector are formed or by sintering process in a single member which comprises two compositions, for example, activated carbon and graphite but which is compositionally graded so that, from one side to opposite side, a first compositional ratio of activated carbon is decreased from 100% to 0% continuously or discontinuously, whilst a second compositional ratio of graphite is increased from 0% to 100% continuously or discontinuously.
The above later conventional method is effective to reduce the internal resistance of the electric double layer capacitor. The electrode is, however, poor in softness such as

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