An anti-freezing pure inorganic electrolyte for long cycle life aqueous sodium-ion batteries at -40 °C

Aqueous electrolytes have the great application potential for sodium-ion batteries owing to eco-friendliness, high-safety, and low cost. However, the high freezing point of common aqueous electrolytes greatly limits the normal operation of aqueous sodium-ion batteries (ASIBs) at low temperatures. Herein, MnCl2·4H2O is introduced into NaCl solution to form the hybrid NaCl/MnCl2·4H2O electrolyte to expand the application scope of ASIBs. Due to the strong interaction between Mn2+ and water molecules, the hydrogen bond network in water is damaged on a large scale. Thus, the hybrid electrolyte maintains a liquid state and has a high ionic conductivity (2.44 mS cm−1) at -50 °C. When used the optimized hybrid inorganic electrolyte, the pure inorganic compositions full battery assembled with Na2CoFe(CN)6 cathode and active carbon anode delivers a high specific capacity of 54.0 mAh g−1 at -40 °C under 1 C (1 C = 150 mA g−1). Excitingly, when tested at -40 °C under 10 C, the battery can achieve an ultra-long cycle stability of 10,000 cycles with a capacity retention of ∼ 99 %. Significantly, this work opens a new path to explore the ASIBs with superior electrochemical performance at low temperatures.