Nonaqueous electrolytes for electrochemical capacitors: Imidazolium cations and inorganic fluorides with organic carbonates

Electrolytes based on 1-ethyl-3-methylimidazolium cation (EMI+) and either the hexafluorophosphate (EMIPF6) or tetrafluorborate (EMIBF4) anion in organic alkyl carbonate solvents have been evaluated for use in electrochemical capacitors. The conductivity, capacitance, limiting oxidation and reduction potentials, and thermal stability were assessed. High conductivity and capacitance values were found regardless of whether cyclic (high viscosity/high dielectric constant) or acyclic (low viscosity/low dielectric constant) alkyl carbonates were used. The best correlation with conductivity for the EMIPF6 salt was found to be the 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 degrees C) make these electrolytes well suited for use in electrochemical double-layer capacitors.