Sulfonium Bis(trifluorosulfonimide) Plastic Crystal Ionic Liquid as an Electrolyte at Elevated Temperature for High-Energy Supercapacitors

This study describes the use of the ionic liquid trimethylsulfonium bis(trifluorosulfonimide) [Me 3 S][TFSI] as an electrolyte for carbon-based supercapacitors at temperatures up to 80 °C. [Me 3 S][TFSI] is synthesized by a metathesis reaction, possesses a low melting point ( T m = 45.5 °C), and presents crystal plastic behavior indicated by a strong organizational structure with many possible conformations corresponding to several solid–solid phase transitions T S–S . Furthermore, [Me 3 S][TFSI] is thermally stable up to T = 280 °C and presents a high conductivity up to 20.42 mS cm –1 at 80 °C and a low viscosity, 3 mPa s, at the same temperature. The combination of a good cycling ability with high capacities up to 150 F g –1 at elevated voltage and temperature, i.e., Δ E = 3 V, T = 80 °C, enables the realization of supercapacitors with high specific energies at high temperatures.