Sulfolane Co‐Solvent‐Tailored Interfacial Stability for High‐Temperature Sodium‐Ion Batteries

Designing and optimizing electrolyte compositions beyond carbonate/NaPF ₆ is vital to advancing commercial sodium‐ion batteries, as electrode–electrolyte interface instability gravely affects the battery cycling stability and rate performance. Sulfones are known as high‐voltage solvents/additives in commercial lithium‐ion batteries. Here, sulfolane has been used as a cosolvent with a baseline electrolyte containing 1 M NaPF ₆ in EC:PC (v/v, 1:1) in SIBs to improve performance and safety metrics. The electrolyte’s oxidative stability increases with the addition of SL, which has two S═O bonds, thereby forming a robust sulfur‐rich cathode‐electrolyte interface layer. The battery metrics of the commercial oxide cathode, O3‐NaNi _0.33 Fe _0.33 Mn _0.33 O ₂ , were optimized with 15 vol% sulfolane co‐solvent at high temperature (60 °C) and voltage (4.2 V) to improve rate capability and capacity retention. The optimized electrolyte retains ∼90% of its specific capacity compared to the baseline electrolyte (78%). To understand interfacial chemistry, the influence of the cosolvent on electrolyte solvation structure on the cathode interface layer was mapped. The thermostability and non‐flammability of sulfolane further enhance the safety of commercial SIBs.