Improved Electrolytes For Electrochemical Capacitors

Currently, the best performing ultra-capacitors, an important technology for hybrid electric vehicles, utilize a non-aqueous electrolyte based on acetonitrile, which is highly flammable and considered toxic. Alternative electrolytes that do not contain acetonitrile are based primarily on propylene carbonate and have about 35% of the conductivity of those with acetonitrile, which significantly degrades the performance of the capacitor. This project will develop and synthesize a number of new quaternary ammonium tetrafluoroborates and determine their solubility and maximum conductivity in non-acetonitrile electrolytes. Phase I successfully found and developed a group of non-acetonitrile electrolytes with conductivity as high as 60% of the acetonitrile-containing electrolytes. In addition, the flash point of these electrolytes was increased from 2oC to 120oC (increased safety), and the allowable capacitor voltage was increased from about 2.8 volts to over 4.2 volts (increased performance). The increased performance and safety were accomplished with relatively non-toxic, low-cost solvents and newer versions of quaternary ammonium tetrafluoroborate salts. Phase II will complete the optimization and final selection of the preferred electrolyte compositions. Processes for manufacturing and purifying the preferred electrolytes will be developed on a small pilot scale. Commercial Applications and other Benefits as described by the awardee: The new ultra-capacitor electrolytes should overcome a major safety and hazard barrier in the wide acceptance of the use of ultra-capacitors in hybrid electric vehicles (HEV) and electric vehicles (EV). The replacement of the currently used acetonitrile-based electrolytes with these essentially non-flammable, non-toxic, higher-voltage electrolytes could eliminate the safety issues that have been raised for ultra capacitors. Further, the use of ultra-capacitors with these new safe, high performance electrolytes should aid the rapid commercialization of HEVs and EVs