NONAQUEOUS ELECTROLYTES FOR ELECTROCHEMICAL CAPACITORS - IMIDAZOLIUM CATIONS AND INORGANIC FLUORIDES WITH ORGANIC CARBONATES

Electrolytes based on 1-ethyl-3-methylimidazolium cation (EMI+) and ei ther the hexafluorophosphate (EMIPF6) or tetrafluorborate (EMIBF4) ani on in organic alkyl carbonate solvents have been evaluated for use in electrochemical capacitors. The conductivity, capacitance, limiting ox idation and reduction potentials, and thermal stability were assessed. High conductivity and capacitance values were found regardless of whe ther cyclic (high viscosity/high dielectric constant) or acyclic (low viscosity/low dielectric constant) alkyl carbonates were used. The bes t correlation with conductivity for the EMIPF6 salt was found to be th e molecular weight (kappa proportional to 1/Mw) and to a lesser degree the viscosity (kappa proportional to 1/eta) Of the solvent. The high specific capacitance (130 Fig) and excellent stability (>3.5 V, >130 d egrees C) make these electrolytes well suited for use in electrochemic al double-layer capacitors.