A Weak-Fluorine-Bond Molecule Stabilizes Hard Carbon Anodes for Practical Sodium-Ion Batteries

Solid-electrolyte interphase (SEI) is essential for improving the cycling stability in sodium-ion batteries (SIBs) by preventing direct contact between electrolytes and hard carbon (HC) anodes. Conventional C-F bond molecules like fluoroethylene carbonate (FEC) show poor SEI formation due to early sodium-ion adsorption on HC, delaying additive reduction. Herein, methyl 2, 2-difluoro-2-(fluorosulfonyl) acetate (MDFA), a weak-fluorine-bond molecule, is proposed to facilitate early SEI formation and suppress parasitic reactions. The strong electron-withdrawing O=S=O group destabilizes the S-F bond, enabling preferential reduction of MDFA and formation of inorganic SEI components that enhance ionic conductivity and accelerate interfacial charge transfer. As a result, the HC with MDFA shows over 5000 stable cycles and delivers a high capacity of 252 mAh g^-1 at 5 C, outperforming 108 mAh g^-1 of that with FEC. A 4.6 Ah pouch cell with MDFA enables 89.3% capacity retention after 1000 cycles. These findings provide valuable insights into fluorine-bond chemistry for the electrolyte additive design in long-life SIBs.