Engineering Small-Sized Cations with Strong Solvation for High-Voltage Supercapacitors

Supercapacitors (SCs) have attracted much attention in the field of energy storage due to their high power density and long cycle life. However, their relatively low energy density limits their application in a wider range of fields. Methods to increase energy density include enhancing specific capacitance and extending operating voltage window. Herein, we report a novel electrolyte salt, tetramethylammonium bis(fluorosulfonyl)imide (TMA-FSI), the resulting electrolyte formed with propylene carbonate (PC), designated as TMF, exhibited a wide electrochemical stability window (ESW) of 5.09 V. On the one hand, we found that the small size of TMA+ enables it to enter the pores more fully during charging and discharging, thereby increasing the specific capacitance. On the other hand, the strong solvation of TMA+ and PC inhibited the decomposition of PC, thereby broadening the ESW. After 10,000 cycles at a current density of 2 A·g–1 and a voltage of 3.0 V using the TMF electrolyte, the capacity retention rate of the SCs was still 96.66%, verifying the efficient kinetic characteristics of the strong solvation system.