Flexible Precursor Modulation toward Selective Heteroatom Doping in a Hard-Carbon Anode for Sodium-Ion Batteries

Hard carbon (HC) doped with heteroatoms is considered an ideal anode for sodium-ion batteries (SIBs) due to its abundance and stable physicochemical properties. However, it is still necessary to break through the constraints of batch consistency, low Coulombic efficiency, and limited cyclability in practical applications. Herein, a flexible molecular design of precursors toward selective heteroatoms doping strategy is proposed, and uniform nitrogen/sulfur mono- or codoped hard carbon with batch consistency is prepared in situ from benzoxazine resin in one step as an HC anode of SIBs. The HC prepared by this efficient batch-consistent and controllable synthesis strategy forms a multiactive site-wide interlayer spacing-stabilized skeleton coupling structure, which facilitates electron/ion transport, improves electrolyte wettability, and comprehensively improves sodium storage performance. The nitrogen-sulfur codoped hard carbon (N/S-HC) shows excellent rate performance (280 mAh g^-1 at 30 mA g^-1 and 166 mAh g^-1 at 600 mA g^-1) and long cycle life with capacity retention of 88% at 600 mA g^-1 after 2000 cycles. Kinetic investigation indicates that N/S codoping enhanced the adsorption and diffusion of Na⁺, and ex situ Raman test revealed the Na⁺ storage mechanism of N/S-HC. This work provides an important view of optimizing Na⁺ storage performances of HC anodes by molecular design engineering, which can be broadened into other electrode materials.