Electrolyte composition for electric double layer capacitor,...

Details Electrolyte composition for electric double layer capacitor,... Electrolyte composition for electric double layer capacitor,...

2000-11-22

2002-08-13

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Double layer electrolytic capacitor

C361S508000, C361S509000, C361S523000, C361S528000, C029S025030

Reexamination Certificate

active

06433996

ABSTRACT:

This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/JP00/01732 which has an International filing date of Mar. 22, 2000 which designated the United States of America, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to electrolyte compositions and solid polymer electrolytes for electric double-layer capacitors that are highly suitable for use in such applications as backup power supplies for various types of electronic equipment. The invention also relates to polarizable electrode-forming compositions and polarizable electrodes, as well as to electric double-layer capacitors arrived at using the above compositions and components.
2. Background Art
Electric double-layer capacitors are currently employed as backup power supplies for memory in computers. These capacitors, which make use of the electric double layer that forms at the interface between an electrode and a liquid electrolyte, have a small size, a large capacitance and a long cycle life.
Recent rapid advances in portability and cordless features in consumer electronic devices such as mobile phones have led to a heightened demand for electric double-layer capacitors. Because electric double-layer capacitors which use nonaqueous electrolytes have a higher voltage and energy density than those made using aqueous electrolytes, they are viewed as especially promising and have been the object of accelerated research efforts.
Secondary cells have hitherto been used for such applications. However, electric double-layer capacitors have come into widespread use as lower power consumption by electronic equipment has reduced backup current requirements, and because of the longer cycle life and broader service temperature range of the capacitors themselves.
Such electric double-layer capacitors have a construction in which a positive and a negative electrode (polarizable electrodes) are provided on a left-right pair of current collectors.
The positive and negative electrodes (polarizable electrodes) have been built by adding a conductive material to a large surface area material such as activated carbon to improve the conductivity of the electrode, and using a binder to support the mixture on a current collector such as aluminum foil.
In this case, a separator lies between the positive and negative electrodes. The electrodes and the separator are generally impregnated with an electrolyte solution.
However, in such prior-art electric double-layer capacitors, adhesion and tackiness between the pair of electrodes and the electrolyte (separator) is low. This has an especially large impact in film-type electric double-layer capacitors.
That is, film-type electric double-layer capacitors, as noted above, have a positive electrode/electrolyte (separator)
egative electrode construction. Unlike cylindrical capacitors in which the positive electrode/electrolyte (separator)
egative electrode composite is coiled and placed in a case, the absence of a coiling pressure in film-type capacitors means that pressure is not applied between the positive electrode and the electrolyte and between the electrolyte and the negative electrode, allowing the electrolyte to separate readily from the positive and negative electrodes. Thus, the electrolyte (separator) placed between the positive electrode and the negative electrode, in addition to serving as an electrolyte, must also have the ability to strongly bond the positive and negative electrodes. In other words, it must have adhesion and tackiness.
Examples of binders used in electric double-layer capacitors to support a slurry of carbon material such as activated carbon on the metal current collector include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl pyrrolidone and carboxymethylcellulose. Of these, polyvinylidene fluoride has excellent film-formability.
However, none of these binders have the ability to dissolve ion-conductive salts to a high concentration. Nor does the binder itself have a high ionic conductivity. Moreover, these binders are unable to simultaneously satisfy the requirements for strong adhesion to the current collector, low dc resistance, and large electrostatic capacitance.
In addition, to lower the interfacial resistance between the electrodes and the electrolyte (separator), it is desirable for the same polymer used in the electrolyte to serve also as an electrode binder resin.
Such an electrode binder must retain the ability to bond large surface area materials such as activated carbon; that is, it must have adhesion and tackiness.
The binder resins and solid polymer electrolytes for electric double-layer capacitors that have hitherto been reported in the literature are all lacking in tackiness and adhesion, as well as other important properties. Hence, a need has been felt for further improvement.
Disclosure of the Invention The present invention was conceived in light of the above circumstances. One object of the invention is to provide polyurethane polymeric compounds (binder resins) in which a substituent having a large dipole moment has been introduced onto the polyurethane molecule to confer a high dielectric constant and the ability to dissolve ion conductive salts to a high concentration, which compounds also have a high tackiness that make them ideal as binders for firmly bonding large surface area materials and conductive materials, and are moreover capable of achieving an interfacial impedance comparable to that of an electrolyte solution. Further objects of the invention are to provide electrolyte compositions and solid polymer electrolytes for electric double-layer capacitors, which compositions and electrolytes are composed primarily of the above polymeric compound and an ion-conductive salt. Still further objects are to provide polarizable electrode-forming compositions and polarizable electrodes having a high adhesion and excellent dielectric properties, which compositions and electrodes are composed primarily of a polyurethane polymeric compound, a large surface area material and a conductive material. A yet further object of the invention is to provide electric double-layer capacitors composed of the foregoing compositions or components.
In the course of extensive and repeated investigations in order to achieve these aims, the inventors have made the following discoveries.
(1) Ion association readily arises in a low-dielectric-constant polymer matrix comprised of an ion-conductive solid polymer electrolyte containing a high concentration of ion-conductive salt, resulting in a decline in conductivity due to ion association.
(2) In such cases, introducing onto the polymer a substituent having a large dipole moment so as to increase the polarity of the matrix discourages ion association, thereby enhancing ionic conductivity.
(3) Introducing a substituent having a large dipole moment onto a polyurethane compound dramatically improves the compound's adhesion and tackiness.
Pursuing their investigations even further based on these findings, the inventors have found also that polyurethane polymeric compounds prepared by reacting an excess of an isocyanate compound with a polyol compound to form a polyurethane compound and reacting the hydroxyl group of an alcohol compound bearing a substituent having a large dipole moment with some or all of the remaining isocyanate groups on the polyurethane compound, wherein the substituent having a large dipole moment is coupled to the polyurethane compound through a NHCOO linkage, have a high dielectric constant, are able to dissolve ion-conductive salts to a high concentration, have excellent adhesive properties, enabling them to bond firmly with large surface area materials and conductive materials, and are capable of achieving an interfacial impedance comparable with that of electrolyte solutions. The inventors have additionally discovered that electrolyte compositions and solid polymeric electrolytes for electric double-layer capacitors which are composed primaril

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