Temperature-dependent structure and performance evolution of “water-in-salt” electrolyte for supercapacitor

“Water-in-salt” (WIS) electrolytes exhibit enlarged electrochemical stability windows compared to conventional dilute aqueous electrolytes, which helps to achieve high-voltage aqueous electrochemical energy storage devices. However, the ultra-high concentration of WIS electrolytes raises a serious concern about their temperature availability. Herein, we demonstrate the correlation of the temperature-dependent electrochemical performance of different-concentration electrolytes with their solvation structures and intermolecular interactions as elaborated via Raman spectroscopy and molecular dynamics simulations. The 21 m (mol kg−1) WIS electrolyte is advantageous in the high-temperature range of >25 °C, whereas the 5 m WIS electrolyte shows superior low-temperature performance, remaining a stable gel state even at −30 °C ascribed to the muscular hydrogen bond network. The electrochemical performance evaluation of carbon supercapacitors assembled with different-concentration WIS electrolytes further verify our findings. This study provides guidance to choose suitable WIS electrolytes for electrochemical energy storage devices working at different temperatures.