Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2001-04-02
2003-07-01
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S502000, C423S414000, C423S44500R, C429S231800, C361S502000, C361S503000, C264S029100
Reexamination Certificate
active
06585915
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a carbon material useful for an electrode for e.g. an electric double layer capacitor, a process for producing an electric double layer capacitor electrode and a process for producing an electric double layer capacitor. The electric double layer capacitor of the present invention can be widely used for power sources for portable apparatus, standby power sources for domestic electrical equipment, UPS for optical communication, power sources for electric automobiles and the like.
2. Discussion of Background
As an electric double layer capacitor, a coin type obtained in such a manner that an element having a pair of electrodes consisting mainly of activated carbon and formed on a current collector and a separator sandwiched therebetween, together with an electrolytic solution, is sealed in a metal casing by means of a metal lid and a gasket insulating the casing from the lid, and a wound type obtained in such a manner that a pair of sheet electrodes is wound by means of a separator interposed therebetween to obtain a wound element, which is accommodated in a metal casing together with an electrolytic solution, and sealed in the casing so that the electrolytic solution does not evaporate from an opening of the casing, have been known.
Further, for an application which requires a large current and a large capacitance, a stack type electric double layer capacitor having an element obtained by stacking a large number of sheet electrodes by means of a separator interposed therebetween, incorporated therein, has been proposed (JP-A-4-154106, JP-A-3-203311, JP-A-4-286108). Namely, a plurality of sheet electrodes formed into a rectangle as positive electrodes and negative electrodes, are alternately stacked one on another by means of a separator interposed therebetween to obtain a stacked element, a positive electrode lead material and a negative electrode lead material are connected with the respective terminals of the positive electrodes and the negative electrodes by caulking, and the element in such a state is accommodated in a casing, impregnated with an electrolytic solution and sealed with a lid.
Conventionally, an electrode constituting an electric double layer capacitor has been one made mainly of activated carbon having a large specific surface area, and as the electrolytic solution, a solvent having a high dielectric constant such as water or propylene carbonate has been used so as to dissolve an electrolyte at a high concentration.
In such an electrode made mainly of activated carbon, the charge of an electric double layer formed on the surface of the activated carbon itself contributes to the capacitance of the electric double layer capacitor, and thus activated carbon having a large specific surface area has been employed.
Activated carbon is produced usually by carbonizing and activating a carbon source derived from a plant such as sawdust or coconut shell, a carbon source derived from a coal/petroleum material such as coke or pitch, or a synthetic high polymer carbon source such as a phenolic resin, a furfuryl alcohol resin or a vinyl chloride resin.
The carbonization is carried out usually by heating a carbon source in a non-oxidizing atmosphere at a temperature of from 300° C. to 2,000° C. The activation is carried out by heating the carbonized product thus obtained in a weak oxidizing gas containing carbon dioxide or water vapor to a temperature of from 500° C. to 1,100° C., so as to oxidize and exhaust the carbonized product to being it to have a porous structure and to increase its surface area. Otherwise, the carbonized product is mixed with an alkali metal hydroxide (such as KOH) in an amount of several times the mass of the carbonized product, and then the mixture is heated at a temperature of from the melting point of said metal hydroxide to 1,000° C. in an inert atmosphere for from several tens minutes to 5 hours so as to increase the surface area. Since the alkali metal hydroxide may cause deterioration in durability of an electric double layer capacitor, it is removed by adequate washing after the activation. Here, in the present specification, “from A to B (A and B are numerical values)” means at least A and at most B.
As important performances required for an electric double layer capacitor, a) a large capacitance, b) a high energy density, c) a high durability when charging and discharging cycles are repeated, and d) a low internal resistance, may, for example, be mentioned. Among these properties required, as an electrode material presenting a large capacitance, activated carbon obtained by activating a carbon material derived from pitch by heating in the coexistence of an alkali metal hydroxide (alkali activation) or activated carbon obtained by activating a carbon material derived from coke by an alkali has been reported (JP-A-5-258996, JP-A-10-199767, JP-A-63-78513, JP-A-10-199767, JP-A-11-31637). Particularly, it is reported that activated carbon obtained by alkali activation of a carbon material having a relatively developed crystallinity, such as pitch showing optical anisotropy i.e. so-called mesophase pitch as a carbon source, has a large capacitance per mass, and has a relatively high bulk density, and accordingly, when the activated carbon is formed into an electrode, said electrode has a high density, whereby an electric double layer capacitor having a large capacitance per unit volume can be obtained (JP-A-2-185008, JP-A-10-121336).
Further, activated carbon obtained by water vapor activation or alkali activation of a carbon material having a relatively low crystallinity, such as a thermosetting resin such as a phenol resin or pitch showing optical isotropy, as a carbon source, to bring the specific surface area to be high, has a large capacitance per unit mass, and has a high durability when charging and discharging are repeated for a long period of time. However, as the bulk density of the activated carbon is low, the capacitance per unit volume tends to be small.
In general, it tends to be difficult to increase the specific surface area of a carbon material having a developed crystallinity by activation by an oxidizing gas such as water vapor (gas activation), and accordingly activation by using a chemical (chemical activation) is employed to increase the surface area. Particularly, an alkali activation employing an alkali metal hydroxide as a chemical is frequently employed. Although the mechanism of the activation by an alkali metal hydroxide is not clearly understood in detail for the most part, it is considered that, in a case of an activation by KOH for example, KOH infiltrates into between carbon layers at a relatively low temperature of from 400 to 500° C., and during this step, carbonation of carbon and gasification of carbon with generated water or carbonic acid gas takes place, whereby carbon is consumed to increase the specific surface area, and metal potassium generated by reduction of KOH is intercalated into between carbon layers to weaken the bonding force between carbon layers (New Edition Activated Carbon, Yuzo Sanada, Kodansha Ltd. Publishers, Scientific). It is considered that in such an activated carbon subjected to alkali activation, bonding force between carbon layers becomes weak as mentioned above, and accordingly when it is used as an electrode for an electric double layer capacitor, not only ions are adsorbed into pores in the activated carbon but a part of ions is adsorbed into pores to widen the space between carbon layers at the time of charging, to present the capacitance.
When the above-mentioned known activated carbon is used for an electrode for an electric double layer capacitor, the capacitance per unit mass of the electrode will be relatively large, but the bulk density tends to be low since the pore volume of the activated carbon is large, whereby when an electrode is prepared by using this activated carbon, it tends to be difficult to fill an adequate mass of activated carbon per unit volume of
Hiratsuka Kazuya
Ikeda Katsuji
Shinozaki Yasuo
Asahi Glass Company Limited
Kopec Mark
Vijayakumar Kallambella
LandOfFree
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