Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2000-12-19
2002-06-25
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S512000, C361S503000
Reexamination Certificate
active
06411497
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a separator for an electric double-layer capacitor.
BACKGROUND OF THE INVENTION
An electric double-layer capacitor has a relatively high capacity; moreover, it has a long life and a capability of being quickly charged and discharged. Therefore, it has been used not only for conventional applications such as leveling of power sources and absorption of noises but also as a memory backup power source for personal computers as well as a supplement or substitute for secondary batteries. In recent years, it has been expected to be used as a secondary battery for electric automobiles.
This electric double-layer capacitor has such a structure that a pair of electrodes is dipped in an ionic solution. When a voltage is applied to this electric double-layer capacitor, ions having an opposite sign to an electrode will be distributed near the electrode to form a layer of the ions while charges having an opposite sign to the ion will be accumulated within the electrode. Then, when a load is placed across the electrodes, the charges within the electrode will be discharged and, at the same time, the ions distributed near the electrode will leave therefrom whereby a neutral state will return.
When a pair of electrodes touch in such an electric double-layer capacitor, the formation of an ion layer near the electrode becomes difficult. Therefore, a separator is usually placed between each electrode pair. This separator is required to have not only a property of preventing the short circuit between the electrodes but also an excellent ionic permeability. For satisfying the first requirement, i.e., for being excellent in the property of preventing the short circuit between the electrodes, it is desired that the separator should have not only a high weight per unit area but also a better thickness. However, when the separator is of a high weight per unit area and thick as such, the capacity tends to reduce due to the disturbance of its ionic permeability and, therefore, such a one is practically unusable. When a thin separator of a low weight per unit area is used for increasing the ionic permeability, a short circuit between the electrodes is apt to take place; moreover, the strength of the separator tends to reduce whereby such a thing would be hard to handle. In order to improve the strength, there has been a proposal for a method in which a thermally fusible fiber is contained as a constituent fiber for the separator and the said fusible fiber it is subjected to a thermal fusion. However, when the separator is prepared by that method, a coat is formed therein by the thermal fusion and, therefore, the ionic permeation becomes bad in spite of making the weight per unit area and the thickness low with an object of improving the ionic permeability.
SUMMARY OF THE INVENTION
The separator of the present invention for an electric double-layer capacitor has an excellent property of preventing the short circuit, a good ionic permeability and a high strength. Since the separator is excellent in heart resistance as well, it is suitable for the manufacture of electric double-layer capacitors.
When the fibril-containing fiber is formed from a resin having a melting or carbonizing temperature of 300° C. or higher, its heat resistance is excellent. Therefore, the separator is suitable for the manufacture of electric double-layer capacitors.
Further, when none of the separator constituents for electric double-layer capacitors is thermally fused, the separator is excellent in ionic permeability.
Furthermore, when the fiber sheet consists of the wet-laid nonwoven fabric, the uniform dispersing property of the fiber is excellent whereby separator's reliability is high without resulting in a short circuit.
The present invention has been made for solving the above-mentioned problems. An object of the present invention is to provide a separator for an electric double-layer capacitor, said separator having an excellent property of preventing a short circuit between electrodes, a remarkable ionic permeability and a quite good strength.
The separator for electric double-layer capacitors according to the present invention (hereinafter, referred to as “separator”) is formed from a fiber sheet comprising a fibril-containing fiber (hereinafter, referred to as “fibril fiber”) and a fine polyester fiber having a fineness of not more than 0.45 dtex (decitex). The following has now been founds (1) since such a separator of the present invention is capable of having a densified structure relying on both the fibril fiber and the fine polyester fiber even if it is low in weight per unit area and thin, the separator has an excellent property of preventing a short circuit; (2) the separator is capable of forming fine micropores having an excellent ionic permeability due to the presence of the fine polyester fiber; and further (3) the separator has an excellent strength due to an entanglement of fibrils of the fibril fiber.
It has been noted that, since the above-mentioned fine polyester fiber has a softening temperature of about 240° C., thereby exerting an excellent heat resistance, it is possible to remove water at high temperature after assembling from the materials in the manufacture of the electric double-layer capacitor, thereby giving advantages in view of appropriate manufacturing processes.
When the above fibril-containing fiber is formed from a resin having a melting or carbonizing temperature of not lower than 300° C., its heat resistance is excellent. Therefore, water can be removed at high temperature after assembling from the materials in the manufacture of the electric double-layer capacitor, thereby giving an advantage in terms of appropriate manufacturing processes. Especially in the case of a fibril fiber formed from an aromatic polyamide, there are advantages that not only its affinity with electrolytes but also its ionic permeability is excellent.
When no separator constituents for electric double-layer capacitors are thermally fused, the separator is free from forming a coat, thereby leading to an excellent ionic permeability.
When the above-mentioned fiber sheet is composed of a wet-laid nonwoven fabric, the fiber has an excellent uniform dispersibility, thereby giving a high reliability free of short circuiting.
DETAILED DESCRIPTION OF THE INVENTION
The separators of the present invention are capable of having a densified structure by entangling the fibrils of the fibril fiber, thereby providing not only an excellent strength but also an excellent property of preventing the short circuit.
The fibril fiber is a fiber wherein numerous fine fibers are formed from one fiber. The fibril fiber may be composed of fine fibers alone or may contain portions forming a bundle of fine fibers, together with the fine fibers. When a fibril fiber containing portions forming a bundle of fine fibers in addition to the fine fibers is contained therein as in the latter case, it not only has an excellent strength but also shows an excellent ionic permeability due to a synergistic action with the fine polyester fiber as mentioned hereinbelow. Therefore, it can be appropriately used.
Although this fibril fiber may be formed from any resin, it is suitable for the manufacture of an electric double-layer capacitor when it is formed from a resin having a melting or carbonizing temperature of 300° C. or higher.
For example, when each material (such as a current collector electrode, an electrode and a separator) for an electric double-layer capacitor wherein an organic electrolyte liquid is employed contains water, it is difficult to manufacture an electric double-layer capacitor having a high withstand voltage or an electric double-layer capacitor having a high density. Therefore, it is necessary to dry each material well. However, both a prior art separator composed of conventionally used polypropylene fibers and a prior art separator composed of cellulose pulps are less heat resistant as compared to other materials. Accordingly, when they are dried at the temperature of 150°
Kimura Fuminori
Kimura Kouji
Kobayashi Takeshi
Japan Vilene Co. Ltd.
Oliff & Berridg,e PLC
Reichard Dean A.
Thomas Eric W.
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