Competitive Solvation Structures via Hetero-Salt Additive Strategy Enables Temperature-Adaptive Sodium-Ion Batteries

Abstract

Sodium metal batteries (SMBs) are the ultimate choice for high-energy density sodium-based batteries. However, their practicality is still constrained by the uncontrolled side reactions and unstable solid electrolyte interface (SEI) between the electrolyte and Na-metal anode. Herein, a hetero-salt additive (Lithium difluorophosphate, LiDFP) strategy is proposed to optimize the solvation structure of pseudo gel-polymer electrolyte (pGPE) and construct robust SEI on Na metal anode, thereby enhancing Na⁺ deposition/stripping kinetics. Detailed characterization reveals that Li⁺ with lower binding energy to solvent molecules can steal solvent molecules from Na⁺ coordination environment, and then push anions to further coordinate with Na⁺. The formed anion-enriched solvation structure can assist to restrain side reactions and form a robust inorganics-dominated SEI on Na metal anode. Consequently, such optimization effectively realizes outstanding electrochemical performance of the cells under wide operation temperature (−20 to 70 °C) and prolongs cycling life (1900 h) of Na plating/stripping. Furthermore, paired with commercial NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathode and hard carbon (HC) anode, the pouch cell using pGPE+0.5%LiDFP delivers a high-capacity retention of 87.5% at 45 °C after 250 cycles. It is believed that this work affords emerging chances for the rational electrolyte design of stable wide-temperate SIBs.