Anionic Bottom-Up Flux Orchestrated via Hard Carbon Surface Chemistry for Stable Sodium-Ion Batteries

Hard carbon anodes for sodium-ion batteries suffer from unstable solid-electrolyte interphase formation, leading to low initial Coulombic efficiency and poor cycle stability. Herein, we demonstrate a molecular-level surface design that actively orchestrates an anionic bottom-up flux from the electrolyte bulk to the hard carbon interface. By engineering a synergistic pair of pyridinic-N and carbonyl groups on the carbon surface, we create a functional-differentiation microenvironment where pyridinic-N selectively anchors PF₆^- anions, while the cooperative carbonyl repels solvent molecules. This dual functionality establishes a sustained concentration gradient that drives a continuous reverse flux of anions toward the interface, fundamentally redirecting the electrolyte decomposition pathway from solvent-dominated to anion-preferential. The tailored surface lowers the PF₆^- decomposition barrier by over 70%, yielding a thin, inorganic-rich solid-electrolyte interphase dominated by NaF and Na₂O. The optimized anode achieves 91.9% Coulombic efficiency with high reversible capacity of 368.2 mAh g^-1, and 96.5% capacity retention after 5,000 cycles. A pouch cell assembled with NFPP cathode achieves an energy density of 239.1 Wh kg^-1 and stable operation over 500 cycles, demonstrating strong practical potential. This work establishes active surface-guided anionic transport as a powerful strategy for interphase engineering in advanced sodium-ion batteries.