Electricity: electrical systems and devices – Electrolytic systems or devices – Liquid electrolytic capacitor
Reexamination Certificate
2000-01-06
2001-10-09
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Liquid electrolytic capacitor
C361S512000, C361S523000, C361S502000, C361S516000, C429S199000, C429S218200
Reexamination Certificate
active
06301093
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrochemical capacitor.
2. Description of the Related Art
There has heretofore been known an electric double-layer capacitor having positive and negative electrode elements disposed in confronting relation to each other with a separator interposed therebetween. Each of the positive and negative electrode elements having a solid electrode made of activated carbon on the surface of a current collector such as of metal foil or the like. The positive and negative electrode elements are sealed together with an electrolytic solution in a casing which has terminals connected to the respective current collectors.
In the above conventional electric double-layer capacitor, the solid electrode is made of activated carbon as a substance having a large specific surface area. However, the electric double-layer capacitor having solid electrodes of activated carbon has an energy density lower than secondary cells that operate based on a chemical reaction. In view of such a drawback, there has been proposed an electrochemical capacitor having solid electrodes made of a material which is capable of producing pseudocapacitance owing to an electrochemical reaction. Various materials including ruthenium oxide capable of producing pseudocapacitance have been reviewed for use as solid electrode materials. However, these materials are disadvantageous in that they are expensive.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electrochemical capacitor which is relatively inexpensive to manufacture and has excellent discharging characteristics.
The inventors have employed a lithium vanadium oxide that is available inexpensively as a cell material capable of producing pseudocapacitance, and produced a solid electrode containing a lithium vanadium oxide. In the lithium vanadium oxide, a vanadium oxide can be either tetravalent or pentavalent. The lithium vanadium oxide exists stably as either a tetravalent or pentavalent oxide because of the presence of lithium. As a result, the lithium vanadium oxide can cause an electrochemical reaction and operates as a cell in the presence of an electrolytic solution, so that it can produce pseudocapacitance in a capacitor.
The inventors have studied various discharging characteristics of solid electrodes containing a lithium vanadium oxide. As a consequence, the inventors have found that excellent discharging characteristics are obtained by an electrochemical capacitor having a negative solid electrode containing a lithium vanadium oxide and a positive solid electrode made of activated carbon, and have completed by the invention based on the finding.
To achieve the above object, there is provided in accordance with the present invention an electrochemical capacitor comprising a casing and a pair of positive and negative electrode elements each having a solid electrode disposed on a surface of a current collector, the positive and negative electrode elements being disposed in confronting relation to each other with a separator interposed therebetween, the positive and negative electrode elements, together with an electrolytic solution, being housed in the casing, the solid electrode of the negative electrode element being made of a lithium vanadium oxide and an electrically conductive filler, the solid electrode of the positive electrode element being made of activated carbon.
In the electrochemical capacitor according to the present invention, the negative electrode element has a solid electrode made of a lithium vanadium oxide and an electrically conductive filler, and positive electrode element has a solid electrode made of activated carbon. These solid electrodes are effective to prevent a sharp voltage drop from occurring at the positive electrode element when the electrochemical capacitor starts being discharged, to increase the period of time for which the electrochemical capacitor can produce a high voltage, and to enable the electrochemical capacitor to discharge a large amount of electric energy.
The electrochemical capacitor according to the present invention can be manufactured inexpensively because the lithium vanadium oxide is used as a substance for producing pseudocapacitance. The electrically conductive filler, which together with the lithium vanadium oxide makes up the solid electrode of the negative electrode element, is required to increase the electric conductivity of the solid electrode.
Since the lithium vanadium oxide can cause an electrochemical reaction and operates as a cell in the presence of an electrolytic solution, the vanadium oxide should preferably be tetravalent or pentavalent. The lithium vanadium oxide may comprise at least one oxide selected from the group consisting of LiV
3
O
8
, LiV
2
O
5
, and Li
2
V
2
O
5
. The vanadium in LiV
3
O
8
is pentavalent, the vanadium in LiV
2
O
5
is a mixture of tetravalent and pentavalent vanadium, and the vanadium in Li
2
V
2
O
5
is tetravalent.
The electrically conductive filler may comprise carbon black or the like. The electrically conductive filler is used to adjust the electric conductivity of the solid electrode, and its amount differs depending on the application of the electrochemical capacitor. For example, the electrically conductive filler is added in an increased amount in order to reduce the resistance of the solid electrode for a high output level, and added in a reduced amount in order to reduce the output density for a high energy density.
The electrically conductive filler is in the range from 3 through 80 weight % of the total weight of the solid electrode. If the amount of the electrically conductive filler were less than 3 weight % of the total weight of the solid electrode, then the resistance of the electrode would be too high to discharge the electrochemical capacitor well. If the amount of the electrically conductive filler were in excess of 80 weight % of the total weight of the solid electrode, then the energy density of the electrode would be lowered.
The activated carbon of the solid electrode of the positive electrode element has a specific surface area in the range from 100 to 3000 m
2
/g. If the specific surface area of the activated carbon were less than 100 m
2
/g, then the electrostatic capacitance per volume would be excessively small. If the specific surface area of the activated carbon were greater than 3000 m
2
/g, then the bulk density would be reduced.
The electrolytic solution comprises a propylene carbonate solution of LiBF
4
or LiPF
6
. Positive ions of the electrolytic salt should preferably be Li
+
because the vanadium oxide capable of producing pseudocapacitance at the negative electrode element charges and discharges electric energy by reversibly doping and undoping Li
+
. Negative ions of the electrolytic salt should preferably be BF
4
−
or PF
6
−
because it has a high withstand voltage and a high electric conductivity, can easily be produced industrially, and has a low toxic level. The solvent of the electrolytic salt should preferably propylene carbonate since it has a high withstand voltage and a high electric conductivity and can be used in a wide temperature range.
The electrolytic solution has a concentration in the range from 0.5 to 1.5 mol/liter. If the concentration of the electrolytic solution were lower than 0.5 mol/liter, then the resistance would be increased. If the concentration of the electrolytic solution were greater than 1.5 mol/liter, then the electrolytic salt might separate out at low temperatures.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.
REFERENCES:
patent: 5219680 (1993-06-01), Fauteux
patent: 5428501 (1995-06-01), Bruder
patent: 5429893 (1995-07-01), Thomas
patent: 5549988 (1996-08-01), Reichert et al.
patent: 5549989 (1996-08-01), Anani
Higono Takashi
Matsumoto Kenji
Noguchi Minoru
Suzuki Tatsuya
Yamamoto Yoshio
Arent Fox Kintner & Plotkin & Kahn, PLLC
Ha Nguyen T
Honda Giken Kogyo Kabushiki Kaisha
Reichard Dean A.
LandOfFree
Electrochemical capacitor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrochemical capacitor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrochemical capacitor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2564297