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

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 (PF₆ ^-) and additive anions (BF₄ ^- 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.