Screening electrolytes designed for high voltage electrochemical capacitors

Standard acetonitrile/tetraalkylammonium-tetrafluoroborate-based electrochemical capacitor electrolytes were modified with both fluorinated additives (including fluorinated analogs of ethylmethyl carbonate and diethyl carbonate, lithium difluoro(oxalato)borate, and tris(2,2,2-trifluoroethyl) phosphite) and spiro(1,1′)-bipyrrolidinium tetrafluoroborate salt, to improve stability at higher voltages (>2.7 V). Compounds investigated included (2,2,2-trifluoroethyl) methyl carbonate (TFEMC), ethyl(2,2,2-trifluoroethyl) carbonate (ETFEC), bis(2,2,2-trifluoroethyl) carbonate (bTFEC), lithium difluoro(oxalato)borate (LiDFOB), and tris(2,2,2-trifluoroethyl) phosphite (TTFEPi). Screening of modified electrolytes in prototype coin cells using constant current cycling and voltage hold test protocols revealed good retention of capacitance and a reduced cell internal resistance growth when operating at maximum voltages between 3.5 and 4.0 V. These results suggest new strategies for increasing the operating limits of electric double-layer type supercapacitors, to increase stored energy and improve their compatibility with lithium-ion battery cells for use in hybrid configurations.