Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
1998-09-29
2001-03-06
Kincaid, Kristine (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C029S025030
Reexamination Certificate
active
06198620
ABSTRACT:
BACKGROUND OF THE INVENTION
Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per weight than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Each electrode is in intimate contact with a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a layer at the solid liquid interface region. This is accomplished by absorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The absorbed charge is held in this region by opposite charges in the solid electrode to generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
A major advantage of an ultracapacitor is its ability to deliver electrical energy at high power rating. A high power operation is predicated on a low internal resistance. Hence, ultracapacitor separators are made of highly porous materials that provide minimal resistance to electrolyte ion movement and that at the same time, provide electronic insulator properties between opposing electrodes. Further, the separators must be cost effective to provide a commercial ultracapacitor.
Various materials have been used as separators in ultracapacitors, including (1) aquagel and resorcinol formaldehyde polymer, (2) polyolefin film, (3) nonwoven polystyrene cloth (4) acrylic resin fibers and (5) nonwoven polyester film. Other materials such as porous polyvinyl chloride, porous polycarbonate membrane and fiberglass paper are suitable as separators for ultracapacitors. Some separator materials such as polyesters, show high ionic resistance in nonaqueous electrolyte because of poor wettability by organic solvents such as propylene carbonates. On the other hand, some of the separator materials demonstrate good features as separators in nonaqueous electrolyte but are too expensive for commercialization. The present invention relates to a separator material that is reasonable in cost and that exhibits good performance for a nonaqueous ultracapacitors. The separator material of the present invention has excellent mechanical strength that is improved by wetting with electrolyte. These properties are important for ultracapacitors because a separator must possess good mechanical integrity for handling during fabrication and wetability to electrolyte to achieve a low resistance in completed cells. According to the present invention, stacks of bipolar configured ultracapacitor cells can be easily fabricated. The stacks are characterized by an increased energy density.
SUMMARY OF THE INVENTION
The invention relates to an ultracapacitor and to a method of making an ultracapacitor. The ultracapacitor of the invention comprises at least one cell comprising two solid, nonporous current collectors, two porous electrodes separating the collectors, a porous separator between the electrodes and an electrolyte occupying the pores in the electrodes and separator. The electrolyte comprises a polar aprotic organic solvent and a salt. The porous separator comprises a cellulosic tissue material. The invention also relates to a stack of ultracapacitor cells having at least one separator layer comprising a cellulosic tissue material.
The method of the invention comprises providing a multilayer structure comprising two solid, nonporous current collectors, two porous electrodes separating the current collectors, a porous separator between the electrodes and an electrolyte occupying the pores in the electrodes and separator. The electrolyte comprises a polar aprotic organic solvent and a salt. The porous separator layer comprises a cellulosic tissue material. The multilayer structure is sealed to form the ultracapacitor.
In another aspect, the invention relates to a method of making a stack of ultracapacitor cells. In this method, a plurality of bipolar double layer ultracapacitor cells is provided in stacked relationship. At least one cell comprises porous, oppositely charged electrodes with a cellulosic tissue material separator disposed between the electrodes. A non-porous current collector is provided between each cell with each current collector having adjoining polarized electrodes of different cells bonded thereto. The electrodes and separators are saturated with electrolyte and the cells, current collectors and separators are sealed to form the stack of ultracapacitor cells.
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pate
Jerabek Elihu Calvin
LeBlanc, Jr. Oliver Harris
Wei Chang
General Electric Company
Johnson Noreen C.
Kincaid Kristine
Stoner Douglas E.
Thomas Eric
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