High-voltage ionic liquid electrolytes based on ether functionalized pyrrolidinium for electric double-layer capacitors

A series of ionic liquids containing imidazolium, pyrrolidinium, or ether-functionalized pyrrolidinium cations incorporated with the same counter anion of bis(fluorosulfonyl)imide (FSI⁻) were synthesized to investigate the effects of the cation structures on the physical and electrochemical properties of ionic liquids. The introduction of the polar ether group was able to compensate for the loss of the mobility of pyrrolidinium cation-based ionic liquids, while maintaining the high thermal and electrochemical stability. Extensive examinations of these ionic liquids in electric double-layer capacitor (EDLC) cells were performed to investigate their voltage stability under various operating conditions. Electrochemical evaluation revealed that the cell fabricated with ether-functionalized pyrrolidinium ionic liquid could deliver larger capacitance values and stable cycling performance compared with the cell containing imidazolium-based ionic liquid at high operating voltage; demonstrating its possible applications as high voltage electrolytes for EDLCs.