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

Abstract Sodium-ion batteries (SIBs) show significant promises 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 de-solvation 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. MDFA reduces the bulk viscosity and takes part in the solvation structures to promote bulk ionic transportation and de-solvation. Meanwhile, the reduced interaction between sodium and solvent system limits salt dissolution to improve the interphases durability. The optimized electrolyte outputs an energy density of 60 Wh kg−1 at −70°C with full-cell using Na4Fe3(PO4)2P2O7 (NFPP) and hard carbon (HC), which exceed the current limitation of carbonate electrolytes. It also enables Na(Ni1/3Fe1/3Mn1/3)O2||HC (NNFMO||HC) pouch cell to maintain 83% of room temperature capacity at −30°C and function with 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.