Supercapacitor structure and method of making same

Details Supercapacitor structure and method of making same Supercapacitor structure and method of making same

1999-05-20

2001-02-13

Kincaid, Kristine (Department: 2831)

Metal working

Barrier layer or semiconductor device making

Barrier layer device making

C361S502000, C361S503000

Reexamination Certificate

active

06187061

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to capacitors which are capable of exhibiting high energy capacitance and high current density discharge over relatively extended time periods ranging from a few seconds to minutes. Such “supercapacitors” are particularly useful for delivering high levels of electrical current to utilization devices in a much shorter time than required by battery systems alone. The invention is directed in particular to a supercapacitor structure, and method of making the same, which incorporates the basic capacitor elements into a unitary, flexible structure which may be sized and shaped as desired to be most compatible with utilization devices while providing advantageously high energy capacities and current densities. The invention further takes advantage of available polymeric rechargeable battery technology in order to provide hybrid supercapacitor systems which are capable of retaining the power and dimensioning versatility of the novel supercapacitor structures.
In large measure, available supercapacitors are of the double layer type in which a pair of electrodes, typically comprising particulate activated carbon, are separated by a microporous, electron-insulating, ion-conducting sheet element comprising a uniformly-dispersed electrolyte component. The structure of the typical supercapacitor further comprises electrically-conductive current collector elements in intimate contact with the respective electrodes. Common among the structural variants of such prior supercapacitor devices are means, such as compressive arrangements, which maintain the essential close physical contact between elements in order to ensure low internal electrical resistance. An example of a capacitor of this type may be seen in U.S. Pat. No. 3,536,936 where the considerable compacting pressure required to reduce to usable levels the internal electrical resistance of the carbon particle electrode composition, as well as of the electrode/collector interface, creates severe difficulties in the fabrication of the capacitor cell.
Attempts have been made to reduce the internal electrical resistance in supercapacitor electrodes by means other than directly-applied physical pressure, notably through some manner of unifying the particulate carbon electrode composition and conductive collectors. A process of high-temperature sintering of the elements to achieve this end is described in U.S. Pat. No. 5,115,378, yet, as is apparent there, the extensive processing steps and high energy consumption lead to economic undesirability of this approach. Further limiting the general acceptance of the process is the intractability of the resulting solid and unyielding preformed capacitor body which cannot be readily shaped to conform to spacial requirements of varying utilization devices.
Other means for minimizing the internal resistance of supercapacitor structures have, for example, attempted to combine pyrolyzed aerogel, carbon foam electrodes with hightemperature soldering of conductive collector elements, as described in U.S. Pat. No. 5,260,855. Such approaches have realized limited success, however, due to the extensive processing and high energy and time consumption required, in addition to the lack of manipulability of the resulting devices.
Overcoming the limitations of prior supercapacitor structures and fabrication procedures, the present invention provides, in particular, means for readily preparing flexible, low resistance supercapacitor structures under economical ambient conditions. Such simple fabrication procedures enable the expanded use of these devices in a wide variety of configurations and applications, including combinations with integrated rechargeable battery energy sources of compatible composition and structure.
SUMMARY OF THE INVENTION
The supercapacitor structures and fabrication procedures of the present invention utilize in significant measure materials and techniques which have been successful in the preparation of polymeric rechargeable batteries, notably Li-ion batteries shown in such U.S. Pat. Nos. as 5,296,318, 5,418,091, 5,456,000, 5,571,634, and 5,587,253, the detailed descriptions of which are incorporated herein by reference.
Accordingly, supercapacitor devices having low internal resistance and being capable of yielding high energy and high current density over considerable time periods may be conveniently fabricated by lamination of electrode and separator films prepared from polymeric compositions comprising activated carbon and ion-conductive electrolyte.
Supercapacitor cell electrode and separator elements according to the present invention may utilize any of a wide variety of polymeric materials, e.g., poly(vinylidene fluoride-co-hexafluoropropylene) (VdF:HFP) and poly(vinylidene fluoride-co-chlorotrifluoroethylene) (VdF:CTFE) copolymers. Such elements preferably comprise a combination of a VdF:HFP copolymer matrix with 20 to 70% by weight of a compatible organic plasticizer which maintains a homogeneous composition in the form of a flexible, self-supporting film. The copolymer preferably comprises about 75 to 92% by weight vinylidene fluoride (VdF) and 2 to 25% hexafluoropropylene (HFP), a range in which the latter co-monomer limits the crystallinity of the final copolymer to a degree which ensures good film strength while enabling the retention of about 40 to 60% of preferred solvents for electrolyte salts. Within this range of solvent content, the 5 to 10% salt ultimately comprising a hybrid electrolyte membrane yields an effective room temperature ionic conductivity of about 10
−4
to 10
−3
S/cm, yet the membrane exhibits no evidence of solvent exudation which might lead to cell leakage or loss of conductivity.
Supercapacitor cells are constructed according to the invention by means of the lamination of electrode and separator cell elements which are individually prepared, by coating, extrusion, or otherwise, from compositions comprising a polymeric matrix of such a material as the noted polyvinylidene fluoride (PVdF) copolymers. For example, in the construction of such a supercapacitor cell an electrode film or membrane is prepared as a cast layer of a composition of activated carbon powder dispersed in a plasticized copolymer matrix solution which is dried to form the membrane. Sections of desired dimension are cut from the membrane and thermally laminated to respective electrically-conductive current collector foils, e.g., copper and aluminum reticulated grids, to form negative and positive capacitor electrodes. The copolymer matrix solution is similarly employed to prepare a separator membrane from which appropriately-sized sections are taken to interlay electrode pairs in an assembly which is then heated under pressure to thereby effect lamination of the cell elements into a unitary flexible supercapacitor structure.
At this stage the laminated structure comprises a significant measure of homogeneously distributed organic plasticizer which is then substantially removed by immersion of the cell laminate in a copolymer-inert solvent, such as diethyl ether or hexane, to selectively extract the plasticizer without significantly affecting the copolymer matrix of the cell element strata. The extracting solvent may then simply be evaporated to yield a dry, microporous supercapacitor cell which is thereafter activated by immersion or other contact with an electrolyte solution, e.g., 1M LiPF
6
in 2 parts ethylene carbonate and 1 part dimethyl carbonate, which fills the pores of the structure to ensure essential ionic conductivity.
The same fabrication procedure may be employed to form a supercapacitor structure having a pair of similar electrodes arranged on respective sides of a common electrode of opposite polarity. The result of this arrangement is a bicell supercapacitor having less weight than an equivalent parallel pair of single devices and thus providing greater specific energy density. A variant arrangement comprising a pair of opposite polarity single cell structures interfaced with a bimetallic current co

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