Phosphate-engineered solid electrolyte interphase boosts Na+ transport and interfacial stability of hard carbon in fluoroethylene carbonate-rich ester electrolytes

Hard carbon (HC) is widely considered one of the most practical anodes for sodium-ion batteries because of its suitable Na-storage capacity and low cost. Ester electrolytes with high fluoroethylene carbonate (FEC) contents are often adopted to stabilize high-voltage cathodes by forming a robust cathode electrolyte interphase and mitigating parasitic reactions. For HC anodes, however, high FEC contents can aggravate interfacial instability and hinder Na + transport. Here, a bilayer coating consisting of an inner amorphous P x O y layer and an outer carbon layer is engineered on HC. Acting as a reactive interphase precursor rather than a passive coating, the bilayer promotes the in situ formation of a NaPO 3 /Na 3 PO 4 /NaPO x F y -rich inorganic solid electrolyte interphase (SEI) during the initial cycle. This phosphate-based SEI effectively improves Na + transport and interphase stability. Consequently, the modified anode delivers 270 mAh g −1 after 200 cycles at 100 mA g −1 and 71.4 mAh g −1 at 1000 mA g −1 . A full cell with a Na 3 V 2 (PO 4 ) 2 O 2 F (NVPOF) cathode delivers 119 mAh g −1 initially and retains 88.6% of its capacity after 100 cycles within 1.5-4.2 V. This work demonstrates that a reactive phosphate-forming surface layer can regulate SEI chemistry and improve the compatibility of HC with FEC-rich ester electrolytes.