“Water in Ionic Liquid” Electrolyte Toward Supercapacitors With High Operation Voltage, Long Lifespan, and Wide Temperature Compatibility

ABSTRACT Aqueous electrolytes, with their inherent safety, low cost, and eco‐friendliness, provide a promising alternative for energy storage devices, but their application is limited due to the narrow electrochemical stability window of water. Using super‐concentrated electrolytes has been demonstrated effectives in expanding the electrochemical window of aqueous electrolytes. However, this approach also brings in several challenges, including decreased ionic conductivity, poor wettability, and increased temperature sensitivity due to the near‐saturated salt concentrations. In this study, we employed a water‐miscible ionic liquid (i.e., 1‐butyl‐3‐methylimidazolium trifluoromethanesulfonate) to break the solubility limitations faced in super‐concentrated electrolytes and created a new “water in ionic liquid” electrolyte that simultaneously featured with broad electrochemical window, decent ionic conductivity, and wide temperature compatibility. Moreover, a prototype of electrochemical double‐layer supercapacitor utilizing the “water in ionic liquid” electrolyte demonstrates outstanding performance characteristics, including a high operating voltage (2.6 V), excellent rate capability with 81% capacitance retention from 0.5 to 30 A g –1 , remarkable cyclic stability with 75% capacitance retention after 120,000 cycles, along with broad temperature compatibility from –20°C to 60°C. These findings not only provide new insights into electrolyte engineering but also offer a pathway for designing innovative aqueous electrolytes for energy storage devices with balanced electrochemical performance.