Tuning Interphase Composition in Sodium‐Ion Batteries via Co‐Solvent Selection and Anion‐Driven Solvation Shell Engineering

Abstract The formation of unstable interphases on the anode and cathode severely limits the long‐term cycling performance of sodium‐ion full cells. This study demonstrates that the use of low‐viscosity, weakly solvating linear carbonates‐dimethyl carbonate (DMC) and diethyl carbonate (DEC) ‐ promotes the involvement of the main salt anion (PF 6 − ) and additive anions (BF 4 − and TFSI − ) in the primary solvation shell. This leads to the formation of an anion‐rich, ion‐conducting interphase on the hard carbon (HC) anode surface, significantly enhancing its initial coulombic efficiency (ICE) and rate performance. Additionally, it promotes the formation of an inorganic‐rich cathode electrolyte interphase (CEI) on the titanium doped sodium nickel manganese oxide (NMTNO) cathode. The full‐cell utilizing the modified electrolyte demonstrates an excellent cycling stability, with a stable areal capacity of 1.25 mAh cm −2 at a current rate of 0.25 mA cm −2 after 200 cycles. Moreover, the sodium‐ion full‐cell exhibited impressive high‐rate performance, maintaining a stable capacity of 0.75 mAh cm −2 at a current rate of 1.5 mA cm −2 , with a capacity retention of over 90% after 300 cycles.