Non-symmetric capacitor

Electricity: electrical systems and devices – Electrolytic systems or devices – Liquid electrolytic capacitor

Reexamination Certificate

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C361S502000

Reexamination Certificate

active

06208502

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to non-symmetric electrolytic/electrochemical capacitors.
A typical symmetric aluminum electrolytic capacitor (
FIG. 1
) includes an aluminum anode foil, an aluminum cathode foil, and a conductive liquid electrolyte, such as ethylene glycol. Ethylene glycol is a substantially non-aqueous electrolyte, i.e. it contains less than 3% of water. The liquid electrolyte is retained by a porous paper separator which acts as a spacer between the anode and cathode foils. The anode and cathode foils are connected to external terminals via aluminum tabs.
The surfaces of the aluminum anode and cathode foils are coated with a layer of an insulating aluminum oxide, which is formed by an electro-chemical oxidation process called forming. For the forming process, a constant voltage is applied to the aluminum foils. The formation voltage is higher than a typical rated working voltage of the capacitor. The aluminum oxide thickness is proportional to the applied voltage. In one example, an aluminum electrolytic capacitor may have rated working voltages up to 600 C and forming voltages in the range of 850 to 900 V.
The insulating aluminum oxide is in contact with the conductive electrolyte. The aluminum anode and cathode foils, the corresponding aluminum oxides, and the electrolyte with the separator form two capacitors connected in series (FIG.
1
A). The thickness of the insulating aluminum oxide layer determines the breakdown voltage of the capacitor. By varying the aluminum oxide layer thickness, the specific capacitance (i.e., capacitance per surface area) of each capacitor is varied. Increasing the aluminum oxide layer thickness reduces the specific capacitance and increases the breakdown voltage of the capacitor. The specific capacitance may be increased by increasing the active surface area of the aluminum foil. The active surface area of the aluminum foil is increased by etching.
Another class of capacitors are the electrochemical capacitors. Electrochemical capacitors fall into two categories: Faradaic and double-layer. Double-layer capacitors rely solely on interfacial charge separation across a boundary between an electrolyte and a conducting surface or an insulating surface such as a metal oxide. The Faradaic capacitors are often referred to as pseudo-capacitors. Pseudo-capacitors have enhanced charge storage derived from charge transfer through a chemical reaction that takes place across the interface between an electrolyte and a conducting surface. The charge transfer can occur, for example by: (1) surface charge attachment to a metal hydride like ruthenium hydride, (2) volume charge diffusion into a metal like silver coated palladium, or (3) an oxidation/reduction reaction at the surface of an oxide like ruthenium oxide.
Non-symmetric electrolytic/electrochemical capacitors use a conventional electrolytic capacitor at the anode and an electrochemical capacitor at the cathode. Evans U.S. Pat. No. 5,737,181 describes a non-symmetric capacitor that has a pseudo-capacitor ruthenium oxide ceramic cathode, a tantalum anode and an aqueous electrolyte. Non-symmetric capacitors with modified metal cathode surfaces are disclosed in Libby U.S. Pat. No. 4,780,797 and Rogers U.S. Pat. No. 4,523,255, which describe very aggressive aqueous electrolytes (e.g., sulfuric acid) that have high conductivity and are compatible with tantalum and tantalum oxide anodes.
SUMMARY OF THE INVENTION
In one aspect, the invention features a capacitor having an electrochemical cathode, an electrolytic anode and a substantially non-aqueous electrolyte disposed between the cathode and anode. The cathode includes a metal and a finely divided material, the anode includes an oxide forming metal and a corresponding metal oxide, and the substantially non-aqueous electrolyte is in contact with the finely divided material and the metal oxide. The cathode structure results in high capacitance, permitting much higher energy density.
In preferred implementations of the invention, one or more of the following features may be incorporated. The cathode may be a metal selected from the group consisting of nickel, titanium, platinum, other noble metals, other non-oxidizing metals and metals forming a conducting or semiconducting oxide. The cathode may also be an expanded nickel mesh. The anode may be aluminum coated with aluminum oxide. The anode may also be a metal selected from the group consisting of tantalum, niobium, zirconium, titanium, and alloys thereof. The finely divided material may be selected from the group consisting of activated carbon powder, carbon fibers, and graphite. The substantially non-aqueous electrolyte may be an ethylene glycol solvent mixture with additives, or a butyrolactone solvent mixtures with additives.
In another aspect the invention features a capacitor having a plurality of electrochemical cathodes, a plurality of electrolytic anodes interleaved with the electrochemical cathodes, and the plurality of anodes and cathodes form a plurality of parallel connected capacitors. A plurality of separators separate the cathodes from the anodes, and an electrolyte may be disposed between the cathodes and anodes. A cathode lead electrically connects the cathodes to each other and to a cathode terminal. An anode lead electrically connects the anodes to each other and to an anode terminal.
In preferred implementations of the invention, the anode may be formed as a stack of individual metal sheets, and the individual metal sheets may be connected to each other by a collector strip.
In another aspect the invention features an AC start capacitor having a first electrolytic anode, a second electrolytic anode, a floating electrochemical cathode interleaved between the first and second electrolytic anodes, and a non aqueous electrolyte. The first and second electrolytic anodes include a metal and a corresponding metal oxide, and the electrochemical cathode includes a metal having top and bottom surfaces coated with a finely divided material. An AC voltage is connected to the first and second electrolytic anodes.
In another aspect the invention features forming a capacitor by fabricating a plurality of anodes, fabricating a plurality of cathodes, and then interleaving the anodes and cathodes while separating them with insulating separators. The anodes and cathodes are then connected in parallel to each other. The anodes are fabricated by transporting a continuous sheet of a first metal, cutting pieces of the first metal sheet at spaced intervals, stacking the pieces of the first metal, and welding the stacked pieces of the first metal. The cathodes are fabricated by transporting a continuous sheet of a second metal, and cutting pieces of the second metal.
Among the advantages of the invention are that the non-symmetric capacitors can be used in high voltage applications without a series construction. They have increased energy density over conventional electrolytic capacitors, improved service life, reduced time constant, and increased power density over serially connected chemical capacitors.
Other features and advantages of the invention will be apparent from the following description of preferred embodiments, and from the claims.


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