Separated Solvated Ion Pair-Assisted Aqueous Electrolyte for Supercapacitor at Ultralow Temperature

Aqueous electrolytes have the advantages of low cost, environmental friendliness, and fast ion transport kinetics, which show great low-temperature potential. However, the high freezing point limit practical applications. In electrolytes, the design of ion pairs can effectively improve the charge transport properties, while its effect on low-temperature performance is neglected. Here, we acquired the lithium trifluoromethanesulfonate (LiOTf)-based aqueous electrolytes with different ion pair structures. Using advanced characterization and simulation, it was revealed that the 10 m LiOTf electrolyte-enriched separated solvated ion pairs (SSIPs) show excellent ultralow-temperature stability. The changes in energy storage behavior under different temperature were discussed, which revealed that SSIPs contribute to efficient adsorption at low temperatures. The supercapacitor with 10 m LiOTf reached high energy density of 34.67 Wh/kg at −40 °C and maintained long cycle stability at −70 °C. Our work suggests a strategy for the rational design of electrolytes that could enable next-generation ultralow-temperature electrochemical storage systems.