Long cycling aqueous sodium-ion batteries at - 30 °C enabled by solvation structure reorganization

Aqueous sodium-ion batteries (ASIBs) have surfaced as viable solutions for grid-scale applications characterized by exceptional safety, cost efficiency and eco-friendliness. However, the high freezing point severely restricts low-temperature viability. To overcome this issue, the dimethylacetamide (DMAC) is employed as a co-solvent for the inorganic and cheap 2 m NaCl (m: mol kg^-1) electrolyte, achieving a freezing point below -45 °C with remarkable ionic conductivity (2.93 mS cm^-1 under -30 °C). Theoretical calculations and experimental measurements reveal that carbonyl group in DMAC engages hydroxyl group from H₂O molecules to primarily form 1H₂O-DMAC conformation, disrupting the intrinsic hydrogen bonds interaction of H₂O molecules, effectively lowering freezing point of hybrid system. Using optimized electrolyte, the assembled Na₂CoFe(CN)₆//activated carbon (AC) batteries deliver 70.7 mAh g^-1 at 1C (1 C = 150 mA g^-1)，with 95 % capacity retention over 10,000 cycles at 10C at -30 °C. Notably, ASIBs successfully power light emitting diodes (1.8 V) at -40 °C. The electrolyte engineering strategy not only significantly enhances the performance of aqueous batteries in cold environments but also underscores their substantial potential for energy storage.