Dynamic Solvent Participation Drives Sodium Storage in Oxygen Engineered Hard Carbon

Abstract The structural complexity of hard carbon (HC) has sparked persistent debates over its sodium storage mechanisms. To clarify the correlation between these mechanisms and the synthesis process for achieving enhanced electrochemical performance, citric acid was used to in situ engineer the surface oxygen architecture of cellulose-derived hard carbon. The optimized anode achieves a reversible capacity of 400 mAh·g–1 and an initial Coulombic efficiency of 91% in a diglyme-based electrolyte. Systematic in situ and quasi-in situ characterization reveals that charge-transfer resistance evolves cyclically with the sodium storage state, governed by the dynamic formation and attenuation of a solvation layer. These results provide evidence for the involvement of solvent molecules during (de)sodiation, validating the “adsorption–closed pore filling” pathway. This work demonstrates that the interfacial solvation structure is a key determinant of sodium storage kinetics and establishes surface oxygen tailoring as an effective strategy for hard carbon anodes with improved performance.