Paper making and fiber liberation – Processes and products – Electrical or magnetic product characteristic
Reexamination Certificate
1999-06-03
2001-02-20
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Electrical or magnetic product characteristic
C162S138000, C162S148000, C162S099000, C162S001000, C162S142000, C162S150000
Reexamination Certificate
active
06190501
ABSTRACT:
BACKGROUND OF THE INVENTION
2. Field of the Invention
The present invention relates to an electric double-layer capacitor. The present invention also relates to a separator for an electric double-layer capacitor and a method of making such a separator.
2. Description of the Related Art
Double-layer electrochemical capacitors are energy storage devices. Charge storage in double-layer electrochemical capacitors is a surface phenomenon that occurs at the interface between electrode and electrolyte. Because such double-layer capacitors have an extremely large capacitance on the order of several Farads or more, there has been considerable interest in improving their other properties to make a commercially viable energy storage device.
Double-layer capacitors typically utilize a pair of electrodes having a separator interposed therebetween. The separator absorbs and retains the electrolyte thereby maintaining close contact between the electrolyte and the electrodes.
The most common type of separator material for such capacitors is paper. Conventional paper separators have particle impurities and void defects that may lead to poor DC leakage characteristics or failure of the capacitor. Conventional paper separators also have poor mechanical properties and are prone to tearing during manufacture of the capacitor.
Polymers are common alternatives to paper separators. Polymer separators, however, generally have an unacceptably high electrical resistance. The separator is typically the largest contributor to the capacitors internal series resistance between the anode and cathode. Therefore, there is a need for a separator having low series resistance.
There is, a conventional electric double-layer capacitor including a separator made of a non-woven fabric formed from a polypropylene fiber that is sandwiched between a pair of porous electrodes formed essentially of activated carbon. In this conventional double-layer capacitor, the electrodes and the separator are impregnated with an electrolytic liquid comprised of an organic solvent (see Japanese Patent Application Laid-open No.63-187614, hereby incorporated by reference).
The reason why the organic solvent is used for the electrolytic liquid is that when the electric double-layer capacitor is used under a high voltage, electrolysis occurs especially when an aqueous electrolytic liquid such as sulfuric acid is used.
Another desirable property of separators is inhibiting self-discharge of the electric double-layer capacitor. More specifically, separators should obstruct the electrophoretic migration of charged carbon particles released from one of the electrodes toward the other electrode to reduce the electric neutralization. Furthermore, separators should be permeable to the electrolytic liquid to permit the migration of charged ions and have heat and chemical resistances.
However, known separators suffer from various problems and are susceptible to heat and chemicals especially when formed with polypropylene fibers.
Therefore, the present inventors have developed an electric double-layer capacitor including a separator which is formed from an ultra thin cellulose fiber, e.g., a bacterial cellulose aggregate (see Japanese Patent Application Laid-open No.9-129509, hereby incorporated by reference). This separator has heat and chemical resistances because it is formed from the cellulose fiber.
SUMMARY OF THE INVENTION
The present inventors have made further investigations concerning the electric double-layer capacitor. As a result, the inventors have found that the permeation of the electrolytic liquid into the separator is lower when an organic solvent is used as the electrolytic liquid as compared with other aqueous electrolytic liquids. This is because the organic solvent is hydrophobic, as compared with the aqueous electrolytic liquid, e.g., sulfuric acid, while a large number of OH groups on the surface of the cellulose fibers are hydrophilic.
Moreover, the degree of permeation of the electrolytic liquid into the separator is an important factor governing the resistance value of the separator because the electrolytic liquid is a medium for charged ions. Therefore, there is a need for increasing the permeation of the organic solvent into the separator when utilizing an organic solvent as the electrolytic liquid.
Accordingly, it is an object of the present invention to provide an electric double-layer capacitor, having an increased permeation of an organic solvent electrolyte into the separator as compared with the conventional art.
Another object of the present invention is to reduce the internal resistance of electric double-layer capacitors utilizing an organic solvent as the electrolyte.
Still another object of the invention is to improve the manufacturing methods for constructing a separator for use in a double-layer capacitor.
These objects are achieved by forming the separator with cellulose fibers having a modified surface.
These objects are further achieved by modifying the surface of a separator with an organic solvent.
To still further achieve the above objects, according to a first aspect and feature of the present invention, there is provided an electric double-layer capacitor including a casing, a pair of porous electrodes disposed within the casing, a permeable separator sandwiched between both the porous electrodes, and an electrolytic liquid which is formed of an organic solvent and which is provided in the casing and which permeates the porous electrodes and the separator, the separator being formed from an aggregate of cellulose fibers having a surface modified by an organic solvent.
With the above construction, the wettability of the cellulose fiber in the electrolytic liquid is improved, leading to excellent permeation of the electrolytic liquid into the separator, whereby a reduction in internal resistance of the electric double-layer capacitor is achieved. In addition, the separator is microporous and obstructs the electrophoretic migration of charged carbon particles as described above. Hence, the self-discharging of the electric double-layer capacitor is considerably inhibited. This is remarkable in a separator formed from bacterial cellulose.
The objects of the invention are even further achieved by providing a method of making a separator for an electric double-layer capacitor, including the steps of producing a cellulose fiber aggregate, forming a sheet of the cellulose fiber aggregate, modifying a surface of the cellulose fiber aggregate sheet by immersing the sheet in an organic solvent for a predetermined time period, drying the modified sheet, and cutting the dried sheets to form the separator of a desired size and shape.
The objects are still further achieved by pre-treating the cellulose fiber aggregate sheet by immersing the sheet in an amphipathic solvent.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
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Hara Ritsuo
Ishii Noriko
Okuma Jiro
Tanaka Yuichiro
Birch & Stewart Kolasch & Birch, LLP
Chin Peter
Halpern Mark
Honda Giken Kogyo Kabushiki Kaisha
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