CAPACITOR DEVELOPMENT

Abstract: The present invention is directed to super or ultracapacitors. The ultracapacitor includes at least one bipolar electrode plate (12), an electrolyte (14) and a separator (16). The bipolar electrode plate (12) includes a current collector and an electrode material associated with the current collector. The electrolyte (14), which is positioned between adjacently oriented bipolar plates, includes an organic aprotic solvent and a salt. A stabilizing additive is associated with the electrolyte for stabilizing electrochemical activity between the solvent and at least one of the current collector and the electrode material. In addition, the electrolyte may further include a wetting agent for increasing wettability of the electrode.

Description: TITLE OF THE INVENTION CAPACITOR DEVELOPMENT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to capacitors, and, more particularly, super, or ultracapacitors of the type utilizing bipolar electrode plates. 2. Background Art Ultracapacitors, also known as super-capacitors, are well known in the art. These ultracapacitors store energy electrostatically by polarizing an electrolyte solution. Although the ultracapacitor is indeed an electrochemical device, there are no chemical reactions involved in its energy storage mechanism. The historical background and operational characteristics and conventional structures can be found in Ultracapacitors For Portable Electronics, Xavier Andrieu, The Big Little Book of Capacitors, Chapter 15, Pgs. 521-547. An ultracapacitor is typically constructed from two or more bipolar electrode plates separated from each other by electrolyte and a separator positioned within the electrolyte. The bipolar plates include a current collector comprised of an electronically conductive material, and, an electrode material associated with the current collector. The components of the capacitor are operatively secured and encased in a surrounding housing. The housing exposes electrical leads for connection with an item to receive the electrical benefit of the capacitor. In a conventional dielectric capacitor, energy is stored in the form of a separated electrical charge, wherein the greater the area for storing the charge, and the closer the separated charge, the greater the capacitance. In such a capacitor, the area for storing the charge is obtained from plates of a flat conductive material and the charged plates are separated with a dielectric material. In an ultracapacitor, the area for storing a charge is conventionally obtained from a porous carbon material The porous structure of the carbon allows the chargeable area to be much greater than the flat conductive plates of dielectric capacitors Furthermore, an ultracapacitor' s charge separation distance is typically determined by the size of the ions m the associated electrolyte, which are attracted to the charged electrode Such a charge separation is substantially smaller than that which can be obtained using conventional dielectric materials Accordingly, the combination of the relatively large surface area and the small charge separation results in superior capacitance relative to conventional capacitors SUMMARY OF THE INVENTION The present invention is directed to a capacitor, and, more particularly, a super capacitor comprising at least one bipolar electrode plate each comprising a current collector and an electrode material associated with the current collector, an electrolyte comprising an organic aprotic solvent, a salt and means for stabilizing electrochemical activity between the solvent and at least one of the current collector and/or the electrode material, and, a separator positionable in the electrolyte so as to prevent physical contact between adjacently positioned electrode plates In a preferred embodiment of the invention, the electrolyte further includes means lot increasing wettabihty of the electrode The wettabihty increasing means may comprise a surfactant In such a preferred embodiment, the surfactant is selected from the group comprising nomonic fluoro-alkanes In another preferred embodiment of the invention, the electiochemical activity stabilizing means compπses a chemical component which can be reduced in place of the solvent In such a preferred embodiment, the electrochemical activity stabilizing means is selected from the group compnsmg carbonates, such as vmylene carbonate, spirolactones, anhydrides and ohgomers In yet another preferred embodiment of the invention, the current collector is selected from the group of materials having low electπcal resistance, substantial impermeability to electrolyte penetration, chemical compatibility with at least one of the electrode mateπal and the electrolyte, and, chemical stability In one such preferred embodiment, the current collector includes carbon mateπal, while in another preferred embodiment the current collector compπses aluminum In still another prefeπed embodiment of the invention, the capacitor further includes a substantially inert frame structure In such a preferred embodiment, the frame structure includes end plates having a cuπent collector associated therewith Ultracapacitors fabricated in accordance with the above, and, more particularly, with carbon/ graphite fiber electioactive materials, a non-aqueous electrolyte, and polymer coated aluminum bipolar current collector plates have demonstrated capacitance of up to 0 95 F/cmf, energy of up to 1 mAh/cm 2 These capacitors have been assembled using commercially available components Materials and components have been selected in relation with known and anticipated manufacturing constraints and in order to enable manufacturing scale-up of the capacitor BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 of the drawings is a schematic cross-sectional view of the present invention, Fig 2 of the drawings is a perspective view of a bipolar plate of the present invention, Fig 3 of the drawings is a cross-sectional view of Fig 2, taken along lines 3-3, and showing the current collector mateπal and the electrode mateπal of the present invention, Fig 4 of the drawings is a graphical representation of the AC impedance of the present invention, Fig 5 of the drawings is a graphical representation of the normalized AC impedance of the present invention, Fig 6 of the drawings is a graphical representation of the initial charge and discharge voltage-time profile for the ultracapacitor of the present invention, Fig 7 of the drawings is a graphical representation of the discharge voltage-time profile for the ultracapacitor of the present invention, Fig 8 of the drawings is a graphical representation of the discharge capacity retention as a function of cycle number, Fig 9 of the drawings is a graphical representation of the initial power performance, as illustrated by the voltage-time profile, for the capacitor of the present invention, Fig 10 of the drawings is a graphical representation of the noπnalized voltage- time profile during power testing of the capacitor of the present invention, Fig 1 1 of the drawings is a graphical representation of the Total Delivered Energy as of function of time during discharge at different power rates, Fig 12 of the drawings is a graphical representation of comparative AC impedance, Fig 13 of the drawings is a graphical representation of an initial charge and discharge voltage-time profile, Fig 14 of the drawings is a graphical representation of comparative AC impedance, Fig 15 of the drawings is a graphical representation of comparative voltage-time profile, Fig 16 of the drawings is a graphical representation of comparative discharge voltage-time profile, Fig 17 of the drawings is a graphical representation of initial power performance, Fig 18 of the drawings is a graphical representation of normalized voltage-time profile during power testmg, Fig 19 of the drawings is a graphical representation of the Total Delivered Energy as function of time duπng discharge at different power rates, Fig 20 of the drawings is a graphical representation of the effect of the electrolyte additive on the charge behavior of an ultracapacitor; Fig 21 of the drawings is a graphical representation of the effect of the maximum charge voltage; Fig. 22 of the drawings is a graphical representation of the effect of co-solvent based electrolyte on the total cell resistance, Fig. 23 of the drawings is a graphical representation of the effect of co-solvent based electrolyte on the total cell resistance, and Fig. 24 of the drawings is a graphical representation of initial charge-discharge voltage-time profile. BEST MODE FOR PRACTICING THE INVENTION While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described detail, several preferred embodiments with the understanding that the present disclosuie is to be considered an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments so illustrated A schematic cross-sectional representation of ultracapacitor 10 is shown in Fig 1 as comprising bipolar electrode plates 12, electrolyte 14, separators 16, housing 18, housing end plates 19, 19' and exposed current collector leads 20. 22, positioned through corresponding end plates As can be seen, the separators are positioned in between adjacently positioned bipolar electrode plates, and, within the electrolyte, so as to prevent inadvertent contact between the bipolar plates Although an ultracapacitor having five bipolar electrode plates is shown in Fig 1, it will be readily understood to those having ordinary skill in the art that any number of desired plates are contemplated by the present invention A bipolar electrode plate 12, of the type shown m Fig 1 , is shown in Fig 2 as comprising current collector 24 (Fig 3) and electrode mateπal 26 (see also Fig 3) associated with the current collector The combination of the current collector 24 and electrode material 26 are secured to and surrounded by frame structure 28 Frame structure 28 is constructed from a substantially inert, non-electronically conducting material such as polyethylene and/or polypropylene Alternatively, glass fiber filled polyethylene having a low temperature induced deformation and a low melting temperature may also be utilized It is also contemplated that the frame structure be fabricated from other materials, such as low-density non-linear polyethylene and/or polypropylene The frame structures can be manufactured through conventional techniques such as die-cuttmg, injection molding and even extrusion The housing 18 (Fig 1) can also be made out of the same mateπal as the frame structure for the bipolar electrode plates The end plates 19, 19' can further be fabπcated from glass reinforced polyethylene sheets, and the associated end plate current collectors can be fabπcated from the same material as the current collector itself, or for example, expended copper mesh or aluminum Current collector 24 is fabricated from materials having low electrical resistance, impermeability to electrolyte penetration and permeation, chemical compatibility, and electrochemical stability A prefeπed material is polyethylene coated aluminum foil, commercially available under the trade name COER-Xal from a company called Rexam Graphics of South Hadley. Massachusetts It has been observed that the use of such mateπal provides several advantageous features Specifically - aluminum foil provides absolute separation of electrolyte in the bipolar electrode construction, - the PE coating onto the aluminum foil provides tor additional mechanical and chemical stability to the aluminum foil, - the PE coating onto the aluminum foil provides a "hot-melt" adhesive characteristic to the current collector, thus enhancing the adhesion of the carbon electrode onto the current collector, and - the COER-Xal provides good electrical conductivity and is available in fairly thin configurations (75 um Al, 25 um PE on each side) As an alternative to the above, it is also contemplated that other materials, such as a polyethylene-carbon paper (25 um) and a polyethylene-carbon composite material can also be used With respect to electrode mateπal 26, such mateπal can be fabπcated from conventional types of electrode materials presently available for the fabπcation of super, ultracapacitors One example of acceptable electrode mateπal, and generally the most frequently described, is based on the utilization of a high surface area activated carbon powder The surface area of the carbon is generally greater than 1,500 m 2 /g These carbons generally have a total pore volume of 1 ml/g, and an average pore size of 20 A In order to fabricate an electrode material using these carbons, the selected carbon powder must be mixed with an appropriate "binder" and than applied onto a current collector to which the carbon powder must adhere A second acceptable type of carbon electrode material is based on activated carbon fibers, generally available in the foπn of woven and or non-wo\ en felts, fabrics foams, 01 ebs Carbon fibei s ha\ mg similar sui face area than the ones measured for the activated carbons are available commercially The use of carbon fibei webs simplifies the electrode assembly process bv eliminating the need of compounding the actu ated carbon powder with a binder and b\ eliminating the need to apply, generally via a solvent coating process, the compounded carbon powder onto a current collector It is believed that the mam difference between the carbon pow ders and the carbon fibers reside in the trade off between the energy storage capacity of the carbon electrode and its ease of processing Electrolyte 14 as shown in Fig 1 is comprised of an organic aprotic solvent, a salt, a stabilizer and a surfactant The solv ent can comprise conventionally known materials, such as the ones described by Dr Ue In T Electrochem Soc 141 , 1 1 ( 1994) 2989-2996 For example only, propylene carbonate ("PC") can be used Although PC has proven to be a good solvent, it has a high viscosity (n = 2 5 cp) and consequently has a less than optimum conductivity Accordingly, solvent mixtures have also been considered and tested Acetonitrile (n=0 3 cp) and y-buryrolacrone (n=l 7 cp) ha\ e been initially selected as a co-solvent to PC The salt used in the electrolyte formulation compπses tetra ethylene ammonium tetra fluoroborate However, other salts, such as fluonnated alkyl ammoniums, among several other conventional salts, are likewise contemplated for use Inasmuch as the electrochemical stability of organic aprotic electrolytes m contact with carbon powder/fiber anodes is of concern to the capacitor industry, the present invention addresses such a concern by utilizing an additive/chemical component that can preferentially be reduced in place of the solvent For example, in a preferred embodiment, the additive may compnse a carbonate (vmylene carbonate), a spπolactone an anhydride and/or an ohgomer In order to increase the wettabihty of the carbon electrode by the electrolyte, surfactants are added to the electrolyte solution These surfactants are generally selected to be no onic fluoro-alkanes (SaiNippon Ink F-142d, F177 and 3M FC-171 and FC 170C are examples) In support of the advantageous benefits from the ultracapacitors constmcted in accordance with the above, numerous analyses were performed and graphically depicted m Figs 4 - 24, wherein the additives (stabilizers and surfactants) were used in combination with conventional electrode materials (l e TSW2, TSW3 carbon fiber electrode mateπal, commercially available from a company called Toyobo in Japan, and, Satin based electrodes from a company called Calgon, located in the United Kingdom) Further information regarding the analyses can be found in corresponding U S Provisional Patent Application Serial No 60/165,865, filed November 16, 1999, from which the present application depends, the entirety of which is incorporated herein by reference In particular, and with respect to the graphs Fig 4 of the drawings is a graphical representation of the AC impedance of the present invention based on TSW2 carbon fiber electrodes, as measured Fig 5 of the drawings is a graphical representation of the normalized AC impedance of the present invention based on TSW2, illustrating the effect of adhesion between the cuπent collector and the electroactive mateπal Better adhesion provides for lower internal resistance Fig 6 of the drawings is a graphical representation of the initial charge and discharge voltage-time profile for the ultracapacitor assembled using the TSW2 based electrodes Although, some differences in charge time are observed, discharge time is almost constant for all systems This behavior is further illustrated in Figure 7 Fig 7 of the drawings is a graphical representation of the discharge voltage-time profile for the ultracapacitor assembled using the TSW2 based electrodes Fig 8 of the drawings is a graphical representation of the discharge capacity retention as a function of cycle number as illustrated for the initial 5 cycles for the capacitor assembled using the TSW2 based electrodes Fig 9 of the drawings is a graphical representation of the initial power performance, as illustrated by the voltage-time profile, for the capacitor assembled using the YSW2 based electrodes Fig. 10 of the drawings is a graphical representation of the normalized voltage- time profile during power testing of the capacitor assembled using the TSW2 based electrodes. These results indicate a good capacity retention for these electrodes. This is further illustrated in Figure 11. Fig. 1 1 of the drawings is a graphical representation of the Total Delivered Energy as of function of time during discharge at different power rates. Fig. 12 of the drawings is a graphical representation of the comparative AC impedance between the TSW2 and TSW3 electrode material considered by the manufacturer as "equivalents." Fig. 13 of the drawings is a graphical representation of the initial charge and discharge voltage-time profile for the TSW2 and TSW3 electrode material. Fig. 14 of the drawings is a graphical representation of the comparative AC impedance between the TSW2 and TSW3 electrode material and the Satin electrode material. Fig. 15 of the drawings is a graphical representation of the comparative voltage- time profile between capacitor assembled using the TSW2 carbon paper and the Satin carbon fabric. Fig. 16 of the drawings is a graphical representation of the comparative discharge voltage-time profile between capacitor assembled using the TSW2 carbon paper and the Satin carbon fabric. Fig. 17 of the drawings is a graphical representation of the initial power performance, as illustrated by the voltage-time profile, for the ultracapacitor assembled using the Satin carbon fabric. Fig. 18 of the drawings is a graphical representation of the normalized voltage- time profile during power testing of the ultracapacitor assembled using the Satin based electrodes. These results indicate a good capacity retention for these electrodes. This is further illustrated in Figure 19. Fig. 19 of the drawings is a graphical representation of the Total Delivered Energy as function of time during discharge at different power rates. Fig 20 of the draw ings is a graphical lepresentation of the effect of the electiolvte additive on the charge beha\ ιor of an ultracapacitor Capacitor #1 10103 has no electrolyte additiv e and show s long self-decomposition behavioi during the initial charge w hile capacitoi #102902 sho s a normal charge behavior Fig 21 of the draw ings is a graphical representation of the effect of the maximum chaige voltage Fig 22 of the draw ings is a graphical representation of the effect of co-solvent based electrolyte on the total cell resistance, for the ultracapacitor assembled using TSW3 electrode material Fig 23 of the drawings is a graphical representation of the effect of co-solv ent based electiolvte on the total cell resistance, for the ultracapacitor assembled using Satin fabric Tests pei formed using a 1 1 mixture of propylene carbonate and acetionitπle indicate that although the electrolyte conductivity is increased, as expected, the electrochemical stability of the electrolyte is reduced This may be due in part to water contamination and to the presence of other residual impurities Fig 24 of the drawings is a graphical representation of the initial charge- discharge voltage-time profile for capacitor assembled using the Satin fabric and co solv ent based electrolyte (#101501 ) The foregoing description and drawings merely explain and illustrate the invention and the invention is not limited thereto except insofar as the appended claims are so limited as those skilled in the art who have the disclosure before them will be able to make modifications and v ariations therein without departing from the scope of the invention

Claims: CLAIMS What is Claimed is: 1. A capacitor comprising: - at least one bipolar electrode plate each comprising a current collector and an electrode material associated with the current collector; - an electrolyte comprising an organic aprotic solvent, a salt and means for stabilizing electrochemical activity between the solvent and at least one of the current collector and the electrode material; and - a separator. 2. The invention according to Claim 1 wherein the electrolyte further includes means for increasing wettabihty of the electrode. 3. The invention according to Claim 2 wherein the wettabihty increasing means comprises a surfactant. 4. The invention according to Claim 3 wherein the surfactant is selected from the group comprising nonionic fluoro-alkanes. 5. The invention according to Claim 1 wherein the electrochemical activity stabilizing means comprises a chemical component which can be reduced in place of the solvent. 6. The invention according to Claim 5 wherein the electrochemical activity stabilizing means is selected from the group comprising carbonates, spirolactones, anhydrides and oligomers. 7 The invention according to Claim 1 wherein the current collector is selected from the group of materials having low electrical resistance, substantial impermeability to electrolyte penetration, chemical compatibility with at least one of the electrode material and the electrolyte, and, chemical stability 8 The invention according to Claim 7 wherein the current collector includes carbon material 9 The invention according to Claim 7 wherein the current collector comprises aluminum 10 The invention according to Claim 1 wherein the capacitor further includes a substantially inert frame structure. 1 1. The invention according to Claim 10 wherein the frame structure include end plates having a current collector lead associated therewith

Inventors: FAUTEUX DENIS G, FAUTEUX, DENIS, G.
Applicants: FAUTEUX DENIS G, POWERCELL CORP, FAUTEUX, DENIS, G., POWERCELL CORPORATION
Application year: 2000
Main classification: H01G9
IPC codes: H01G9, H01M4, H01M6, H01M10, H01M14