Water-based synthesis of spiro-(1,1′)-bipyrrolidinium bis(fluorosulfonyl)imide electrolyte for high-voltage and low-temperature supercapacitor

A novel electrolyte salt spiro-(1,1')-bipyrrolidinium bis(fluorosulfonyl)imide (SBP-FSI) is successfully synthesized via one-step method in aqueous solvent. We systematically investigate, for the first time, physic-chemical properties and electrochemical stability of the new electrolyte. Compared with commercial electrolyte, the new electrolyte system possesses higher conductivity and lower viscosity, especially at ultra-low temperature. Moreover, SBP-FSI incorporates with propylene carbonate (PC) for supercapacitors which exhibits excellent electrochemical capacitance, rate performance and cyclic stability under 2.7 V at -40 degrees C. Interestingly, commercial electrolytes appear 'electrolyte starvation' at high voltages due to a large amount of precipitation at -40 degrees C, while SBP-FSI/PC is not affected. Surprisingly, SBP-FSI/PC obtains a working voltage of 3.2 V to effectively compensate for the loss of ion mobility in low temperature, greatly improving the energy density and power density of supercapacitors. This work emphasizes that SBP-FSI is a promising high-voltage electrolyte for ultra-low temperature applications of supercapacitors.