Although the operating mechanism of sodium-ion battery (SIB) resembles that of lithium-ion battery, common film-forming additive for lithium-ion battery does not play its role in SIB. Therefore, it is essential to tailor new additives for SIB. Hard carbon (HC), as the most used anode material of SIB, has the disadvantage of interphasial instability, especially under the condition of long-term cycling. The incessant accumulation of electrolyte decomposition products leads to a significant increase in interphasial impedance and a sharp decline in discharge capacity. In this work, N-phenyl-bis(trifluoromethanesulfonimide) (PTFSI) was proposed as a novel film-forming electrolyte additive, which effectively enhances the long-term cycling performance for HC anode in SIB. The passivation film generated from the preferential reduction of PTFSI improves the capacity retention of HC/Na half-cell from 0% to 68% after 500 cycles. Profoundly, the enhanced interphasial stability of HC anode results in a 52% increase in capacity retention of HC/Na₃V₂(PO₄)₃ full-cells after 100 cycles.

The non-flammability and high oxidation stability of sulfolane (SL) make it an excellent electrolyte candidate for lithium-ion batteries (LIBs). However, its incompatibility with graphitic anode prevents the realization of these advantages. To understand how this incompatibility arises on molecular level so that it can be suppressed, we combined theoretical calculation and experimental characterization and reveal that the primary Li⁺ solvation sheath in SL is depleted of fluorine source. Upon reduction, SL in such fluorine-poor solvation sheath generates insoluble dimer with poor electronic insulation, hence leading to slow but sustained parasitic reactions. When fluorine content in Li⁺-SL solvation sheath is increased via salt concentration, a high stability LiF-rich interphase on graphite can be formed. This new understanding of the failure mechanism of graphite in SL-based electrolyte is of great significance in unlocking many possible electrolyte solvent candidates for the high-voltage cathode materials for next-generation LIBs.