Molecular engineering of pore structure/interfacial functional groups toward hard carbon anode in sodium-ion batteries

Hard carbon with abundant pore structure and suitable interface has become a promising anode for sodium-ion batteries. However, it is still a challenge to accurately regulate the hard carbon micropore structure and customize the appropriate interface. Herein, different heteroatoms are introduced into the precursor to regulate the pore structure of hard carbon through its pyrolytic components, and in-situ doping is also used to optimize the interface. The results show that the hard carbon cross-linked with oxy-hybrid (HCO) possesses affluent micropores (0.5∼0.9 nm) and groups of carbonyls (C = O). The micropores can accelerate the plateau capacity, while the C = O can induce the formation of inorganic rich solid electrolyte interface (SEI) to promote initial coulombic efficiency (ICE). Benefiting from the unique structure of HCO, the Na//HCO half-cell exhibits high reversible capacity of 352.9 mAh g^-1 and ICE of 88.0 %. In addition, the assembled HCO//Na₃V₂(PO₄)₂F₃@C full-cell reveals splendid rate performance and excellent cycling stability with capacity retention rate of 86.1 % after 300 cycles. The significance of different heteroatom cross-linked precursors on hard carbon modification is studied systematically, which provides new ideas and insights for designing hard carbon anodes of high-performance sodium-ion batteries.