High-voltage, low-temperature supercapacitors enabled by localized “water-in-pyrrolidinium chloride” electrolyte

Aqueous electrolytes offer superior prospects for advanced energy storage. “Water-in-salt” (WIS) electrolytes exhibit a wide electrochemical stability window (ESW), but their low conductivity, high viscosity, and precipitation at low temperatures restrict their application. Herein, we report a novel localized “water-in-pyrrolidinium chloride” electrolyte (LWIP; 1 ​mol/L, N-propyl-N-methylpyrrolidinium chloride/(water and N,N-dimethylformamide, 1:4 by molality)) enabling high-voltage, low-temperature supercapacitors (SCs). The greatly improved ESW (3.451 ​V) is mainly attributed to the strong solvation between Cl− and water molecules, which broadens the negative stability. This water-binding mechanism is very different from that of a WIS electrolyte based on alkali metal salt. SCs using LWIP electrolytes not only yield a high operating voltage of 2.4 ​V and excellent capacity retention (82.8% after 15,000 cycles at 5 ​A ​g−1) but also operate stably at −20 ​°C. This work provides new approaches for the design and preparation of novel electrolytes.