Interfacial chemistry regulation by orbital hybridization for superior kinetics of hard carbon in an ester-based electrolyte.

Hard carbon is the most commercially viable anode material for sodium-ion batteries (SIBs), yet its application in ester-based electrolytes is hindered by sluggish interfacial ion diffusion and limited sodium nucleation kinetics. After comprehensive evaluation, an interfacial chemistry regulation strategy was proposed based on orbital hybridization between bismuth and electrolyte ions, which was realized through the introduction of ammonium bismuth citrate. The surface bismuth particles regulate the formation of a NaF-rich SEI through improved anion affinity. In collaboration with the generated highly ion-conductive NaN, a thin, compact and homogeneous SEI was constructed to enable fast and stable interfacial Na migration kinetics. Moreover, the Bi atoms can diffuse into the hard carbon structures, expanding the carbon interlayers to facilitate ion diffusion and intercalation as well as enhancing the sodiophilicity in closed pores to lower the nucleation barrier. Benefiting from these merits, the resulting T2-BiN exhibits superior sodium-storage kinetics with outstanding rate capability (185.6 mA h g at 0.5 A g) and long-term cycling stability (84.4% after 400 cycles at 0.5 A g) in the ester-based electrolyte. Even the practical full cell showed no capacity decay over 400 cycles at 2C. This work provides a simple and effective interfacial modification strategy, offering new insights into the advancement of hard carbon anodes for high-performance SIBs.