Weak-solvation dilution for interphase preservation and extreme low-temperature sodium-ion storage

Sodium-ion batteries (SIBs) show significant promise for their abundance and potential fast kinetics. However, ether-based electrolytes are limited by low anodic stability, and carbonate electrolytes typically suffer from reduced conductivity and sluggish desolvation at lower temperatures. Here, a weak-solvation dilution strategy that utilizes methyl difluoroacetate (MDFA) as a weakly solvated diluting agent to prepare an optimized carbonate/ester hybrid electrolyte is reported. MDFA reduces the bulk viscosity and takes part in the solvation structures to promote bulk ionic transportation and desolvation. Meanwhile, the reduced interaction between sodium and the solvent system limits salt dissolution to improve the interphase durability. The optimized electrolyte has an energy density of 60 Wh kg^-1 at -70°C with a full cell using Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) and hard carbon (HC), which exceeds the current limitation of carbonate electrolytes. It also enables Na(Ni_1/3Fe_1/3Mn_1/3)O₂||HC (NNFMO||HC) pouch cells to maintain 83% of room temperature capacity at -30°C and function with an appliance at -50°C. This strategy provides the possibility of operating high-power SIBs under low temperatures, which could also be extended to other batteries.