Inorganic Electrolyte for Low-Temperature Aqueous Sodium Ion Batteries

Aqueous sodium ion batteries have received widespread attention due to their great application potential and high safety. However, the serious capacity fading under low temperature dramatically restricts their practical application. Compared to flammable and toxic organic antifreezing additives, addition of common cheap inorganic inert additives to improve low-temperature performance is of interest scientifically. Herein, low-cost calcium chloride is served as antifreezing additive in 1 m NaClO₄ aqueous electrolyte due to its strong interaction with water molecules. The freezing point of the optimized electrolyte is significantly reduced to below -50 °C with an ultrahigh ionic conductivity (7.13 mS cm^-1 ) at -50 °C. All pure inorganic composition of the full battery delivers a high capacity of 74.5 mAh g^-1 under 1 C (1 C = 150 mA g^-1 ) at -30 °C. More importantly, when tested under 10 C at -30 °C, the battery can achieve an ultralong cycling stability of 6000 cycles with no obvious capacity decay, indicating fast Na⁺ transport under low temperature. Significantly, this work provides an easy-to-operate strategy by adding cheap inorganic salt to develop high-performance low-temperature aqueous batteries.