Ultrathin carbon layer-engineered spherical hard carbon anodes with high initial Coulombic efficiency and kinetic-enhanced sodium storage

In recent years, hard carbon (HC) has become a promising choice of the anode materials for sodium-ion batteries (SIBs). Nevertheless, its practical application remains constrained by inadequate rate capability coupled with insufficient initial Coulombic efficiency (ICE). This work presents a solution by applying an ultrathin carbon coating onto sucrose-derived hard carbon to address these issues. The composite of spherical hard carbon with an ultrathin carbon coating (HC@PC-1) possesses a core–shell structure, which is prepared by using polyaniline as the carbon precursor of the outer layer. As an anode for SIBs, HC@PC-1 demonstrated a notable reversible capacity reaching 373.3 mAh g⁻¹ under 0.03 A g⁻¹, accompanied by an exceptional high ICE of 90.2 %. Additionally, HC@PC-1 also demonstrates exceptional rate performance and cycling durability, achieving a capacity of 215.1 mAh g⁻¹ at 3 A g⁻¹, and maintaining a capacity of 338.5 mAh g⁻¹ after 100 cycles at 0.3 A g⁻¹. Furthermore, an “adsorption-intercalation-pore filling” mechanism was promoted to reveal the sodium storage mechanism during discharging/charging process. Besides, a stable SEI layer with a thickness of 20 nm was formed on the surface of HC@PC-1 after the first discharge at 0.03 A g⁻¹. The ultrathin carbon coating suppresses the decomposition of the electrolyte salt throughout electrochemical cycling, and promotes the formation of an SEI layer with balanced organic and inorganic components. This work proposes a novel surface modulation strategy for constructing anode materials of SIBs, achieving high ICE and kinetic-enhanced sodium storage.