Synergistic Regulation of Na ⁺ Solvation Structure and Interfacial Chemistry Enabled by a Bifunctional Sultone Additive for High‐Voltage Sodium Metal Batteries

ABSTRACT Sodium metal batteries (SMBs) are regarded as promising candidates for next‐generation energy storage systems owing to their high theoretical energy density and low cost. However, their practical development is hindered by the instability of the electrode–electrolyte interphase (EEI), particularly under high‐voltage operation. Herein, we investigate the interfacial regulation mechanism of the bifunctional additive prop‐1‐ene‐1,3‐sultone (PES) in high‐voltage SMBs by employing Na metal || NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) full cells. Theoretical calculations and molecular dynamics simulations reveal that PES molecules preferentially undergo reduction and oxidation at the Na metal anode and NFM cathode interfaces, respectively, owing to their low LUMO and high HOMO energy levels. Furthermore, the highly polar sultone ring of PES can penetrate the primary Na + solvation sheath, enhance PF 6 − coordination in the inner solvation structure, and promote the formation of an inorganic‐rich EEI enriched in NaF and Na x SO y species. Consequently, batteries employing an electrolyte containing 1 wt% PES deliver a capacity retention of 82.5% after 300 cycles between 2.0 and 4.3 V, accompanied by an average Coulombic efficiency of 99.88%. Even at a high cut‐off voltage of 4.4 V, the capacity retention remains 80% after 150 cycles. This work provides a mechanistic understanding that may guide the rational design of electrolytes for high‐voltage SMBs based on synergistic solvation‐structure regulation and interfacial‐chemistry optimization.