Unveiling the Formation of Solid Electrolyte Interphase and its Temperature Dependence in "Water-in-Salt" Supercapacitors.

"Water-in-salt" (WIS) electrolytes have emerged as an excellent superconcentrated ionic medium for high-power energy storage systems such as supercapacitors due to their extended working potential compared to the conventional dilute aqueous electrolyte. In this work, we have investigated the performance of WIS supercapacitors using hollow carbon nanoplates as electrodes and compared it to that based on the conventional "salt-in-water" electrolytes. Moreover, the potentiostatic electrochemical impedance spectroscopy has been employed to provide an insightful look into the charge transport properties, which also, for the first time, reveals the formation of a solid-electrolyte interphase (SEI) and their temperature-dependent impedance for charge transfer and adsorption. Furthermore, the effect of temperature on the electrochemical performance of the WIS supercapacitors in the temperature range from 15 to 60 °C has been studied, which presents a gravimetric capacitance of 128 F g-1 and a volumetric capacitance of 197.12 F cm-3 at 55 °C compared to 87.5 F g-1 and 134.75 F cm-3 at 15 °C. The in-depth understanding about the formation of SEI layer and the electrochemical performance at different temperatures for WIS supercapacitors will assist the efforts toward designing better aqueous electrolytes for supercapacitors.