Ultralow-freezing-point (−70 °C), organic solvent-free aqueous electrolyte with superior ionic conductivity and thermodynamics stability

Water-in-salt and organic solvent-additive strategies have significantly improved the electrochemical stability window of aqueous electrolyte, while it still faces poor low-temperature thermodynamics stability and unsatisfied transport kinetics, as well as flammability risks caused by organic solvent. Herein we report a novel “cation-water-urea ternary interaction strategy” via environmental-friendly and non-flammable urea (CO(NH2)2) to solve the above dilemma. The cation-water-urea ternary interaction strategy preferentially develops CO(NH2)2-cation solvation structures for weakened cation-anion associations, improved anti-salt-precipitation capability, and thus enhanced ionic conductivity, meanwhile it improves the low-temperature thermodynamics stability via the stronger CO(NH2)2-water hydrogen-bond interactions to suppress the freezing and electrolysis of water. The optimum ternary aqueous electrolyte exhibits an ultralow freezing point (−70 °C), wide electrochemical stability window (2.9 V) and outstanding ionic conductivity (135 mS cm−1@25 °C and 5.3 mS cm−1@−50 °C), demonstrating superior performance compared to traditional organic solvent-additive and water-in-salt electrolytes. Finally, to demonstrate its potential for practical applications, ternary aqueous electrolyte-based supercapacitors are fabricated using commercial activated carbon electrodes, which exhibit outstanding capacitance retention (71.4 %) and much lower ion transport resistance even at ultralow temperature of −40 °C. This work proposes a novel ternary aqueous electrolyte with improved thermodynamics stability and kinetics for safe, low-temperature energy storage applications.