Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2003-02-24
2004-08-31
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C029S025030, C361S503000
Reexamination Certificate
active
06785122
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for preparing an electrolytic solution, an electrolytic solution and an electric double-layer capacitor. It relates particularly to a method for preparing an electrolytic solution capable of preventing overcharge of an electric double-layer capacitor, an electrolytic solution and an electric double-layer capacitor.
BACKGROUND OF THE INVENTION
Electric double-layer capacitors, which have high capacitance of farad class, excellent characteristics of charge/discharge cycle and the capability of undergoing rapid charge, have been used for a backup power supply of electronic component, an onboard battery for vehicle (an energy buffer) and the like.
An electric double-layer capacitor will be briefly described referring to FIG.
1
.
FIG. 1
is a sectional view showing the main structure of an electric double-layer capacitor.
As shown in
FIG. 1
, an electric double-layer capacitor
101
includes a casing
102
housing a pair of carbon electrodes (polarizing electrodes)
104
which interposes a separator
103
, and a pair of collectors (elements)
105
. And the casing
102
is filled with an ion conductive electrolytic solution. The electric double-layer capacitor
101
employs electric charges (shown by + and − in
FIG. 1
) as dielectrics in a normal capacitor, which are generated at an interface between the solid carbon electrodes
104
and the liquid electrolytic solution and spaced at a distance of molecule.
Electrolytic solutions used for electric double-layer capacitors are roughly categorized into an aqueous electrolytic solution made of dilute sulfuric acid added with an electrolyte and an organic electrolytic solution made of an organic solvent added with an electrolyte. An appropriate type of electrolytic solution is selectably applied to an electric double-layer capacitor taking into account the usage thereof. An electric double-layer capacitor using an aqueous electrolytic solution is advantageous in terms of lower internal resistance and higher power density, which also enables flexibility for setting of voltage requirements. On the other hand, an organic electrolytic solution, which allows a higher withstand voltage per cell, is advantageous in terms of energy density. It also allows selection of inexpensive and light metals such as an aluminum alloy for a casing.
An activated carbon or activated carbon fiber with a large specific surface area is generally used for an electrode of electric double-layer capacitor so that the capacitor can attain high electrostatic capacitance. Since the more the specific surface area of an activated carbon increases, the more the number of pores therein will increase, the amount of adsorption of electrolytic ions grows, thereby resulting in higher electrostatic capacitance. Generally speaking, an activated carbon having a specific surface area of some thousands square meters per gram (m
2
/g) is used for an electrode. It is reported that application of an activated carbon with a large specific surface area can provide an electric double-layer capacitor with high capacitance such as some hundreds to some thousands farads (F).
Volume and specific surface area per weight for an activated carbon are substantially linearly proportional each other. However, volume per electrode reaches maximum when the specific surface area of an activated carbon falls into the range of 2000 to 2500 m
2
/g, and decreases if the area exceeds the range, which is reported in the document (DENKI KAGAKU, 59, P.607). The reason for it is that the density of an electrode seemingly decreases due to an increase in volume of pores according as a specific surface area increases. Improvement of the specific surface area of an activated carbon is believed to relate closely to higher electrostatic capacitance. However, as the bulk density of an activated carbon decreases with the increase of pores in connection with improvement of specific surface area, electrostatic capacitance per volume of an electrode will accordingly fall.
In an effort for increasing the electrostatic capacitance per volume of an electrode, the development of an electrode using a graphitized carbon has been started.
Especially, an activated carbon (mesophase carbon fiber or mesophase microsphere), which is made of alkali activated mesophase pitch, has been recently used for a polarizing electrode of electric double-layer capacitor.
Mesophase carbon fiber is a kind of carbon fiber which is produced from pitch and the like as an ingredient (graphitized carbon). Pitch is optically isotropic, but when it is heated, pitch molecules start regular orientation, thereby a portion of optical anisotropy (optically anisotropic microsphere) is generated. Eventually, the portion is transformed into coke completely, which is optically anisotropic and shows a flow pattern. Such a portion of optical anisotropy is called mesophase. Mesophase carbon fiber is a type of carbon fiber produced from pitch, which is transformed into mesophase to some extent, by spinning such as melt-blow method.
Mesophase carbon fiber has optical anisotropy and relatively high degree of orientation. The basic orientation is immune to oxidization (infusible treatment) by air, and what is more it is remarkably improved by carbonization and high temperature treatments. Also mesophase carbon fiber has high graphitization. In this way mesophase carbon fiber can be applied to a polarizing electrode having high electrostatic capacitance per volume.
Japanese Patent Application Publication 05-258996 discloses an electrode employing mesophase carbon made of carbonaceous fiber, which is activated by an aqueous solution of alkaline metal hydroxide and crushed. The carbonaceous fiber is produced from pitch with melt spinning and subsequent heat treatment. Also Japanese Patent Application Publication 09-275042 discloses a polarizing electrode with high electrostatic capacitance using an activated carbon, which is produced from vinyl chloride resin with baking and subsequent alkali activation.
An electric double-layer capacitor arranged as described above is adaptive for rapid charge when the capacitor is operated under cyclic charge and discharge. Generally, the capacitor is operated while electrically connected to a charge and discharge control circuit or to an overcharge protection circuit in order to prevent overcharging. When the voltage of an electric double-layer capacitor exceeds a predetermined value, namely a predetermined charge control voltage, the circuit cuts off power supply to the capacitor.
However, when there is not provided a circuit of this type or the function of overcharge protection of the circuit does not work properly, the performance of charge and discharge of the electric double-layer capacitor may deteriorate, which is overcharged with a higher voltage than the predetermined value.
Furthermore as described before, when electrostatic capacitance per volume is increased, the volume of an electrode generally tends to expand. It accordingly requires a reduction in filling factor of electrode. If the filling factor is thus set smaller, which leads to a reduction in energy density, an electric double-layer capacitor cannot fully demonstrate its features. Typically, this holds true of an electric double-layer capacitor which employs a material with high electrostatic capacitance for an electrode, such as an activated carbon produced from mesophase pitch with alkali activation.
Instead of reducing the filling factor, it may be possible to increase the thickness of casing for an electric double-layer capacitor so that the casing can withstand the load generated by expansion of the electrode.
However, the increase in casing thickness results in an undesirable increase in the gross weight of the capacitor.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for preparing an electrolytic solution and the electrolytic solution which can prevent overcharging of an electric double-layer capacitor.
The other object of the present invention i
Fujino Takeshi
Noguchi Minoru
Oyama Shigeki
Arent & Fox PLLC
Dinkins Anthony
Honda Giken Kogyo Kabushiki Kaisha
LandOfFree
Method for preparing electrolytic solution, electrolytic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for preparing electrolytic solution, electrolytic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for preparing electrolytic solution, electrolytic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3316535