Synergistic engineering of closed pores and oxygenic groups via vapor-phase carbon deposition for hard carbon anodes with high initial Coulombic efficiency in sodium-ion batteries

Hard carbon anodes suffer from low initial Coulombic efficiency (ICE), mainly due to surface defects and unfavorable functional groups that trap sodium ions from the electrolyte. Conventional optimization strategies—such as presodiation, passivation layers, and high-temperature treatment—often improve ICE at the expense of sodium-ion storage capacity. Herein, a vapor-phase deposition (CVD)-assisted carbonization process is proposed. This method synergistically engineers closed pores and oxygen-containing groups through an in-situ repair mechanism and deposition behavior, thereby achieving a simultaneous enhancement in ICE and plateau capacity. The deposition behavior effectively converts micropores into supermicropores/closed pores, resulting in a ninefold increase in closed pore volume over the control sample. Meanwhile, the repair mechanism preferentially eliminates surface defects, thereby preserving the C=O groups beneficial to ICE against reduction. This mechanism fosters a more robust solid electrolyte interphase and enhances ion diffusion kinetics. Consequently, the optimized hard carbon anode achieves an ultrahigh ICE of 94 %, a boosted plateau capacity of 211 mAh g⁻¹, and a reversible capacity of 335 mAh g⁻¹. Furthermore, an adsorption/intercalation-pore filling mechanism for sodium-ion storage is elucidated. This work provides valuable insights for developing high-performance hard carbon anodes for sodium-ion batteries.