Deciphering the Role of Oxygen‐Containing Functional Groups in Hard Carbon Anodes for Sodium‐Ion Batteries

ABSTRACT Hard carbon (HC) is a promising anode material for sodium‐ion batteries (SIBs), yet the role of oxygen‐containing functional groups (OFGs) in shaping its structure and performance remains unclear. In this study, we systematically investigate the influence of OFGs on the microstructure and sodium storage behavior of HC. The combined X‐ray absorption near‐edge structure (XANES), neutron total scattering, and high‐resolution transmission electron microscopy (HRTEM) investigations reveal that carbonyl (C═O) and carboxyl (COOH) groups contribute to the curvature and dense stacking of graphitic‐like layers, promoting closed‐pore formation and enhancing plateau capacity. Meanwhile, the ether bond (C─O─C) is positively correlated with the lateral growth of graphitic domains ( L a ). Accordingly, an optimal OFG configuration consisting of low C─O─C content, moderate levels of C═O, and high COOH concentration enables a favorable balance between reversible capacity and initial Coulombic efficiency (ICE). This work provides new insights into the role of oxygen in HC and establishes a compositional and structural framework for the rational design of high‐performance HC anodes for next‐generation SIBs.