Stepwise crosslinking-activation to create a closed pore structure of hard carbon for boosted sodium energy

Coal-derived hard carbons (HCs) hold significant promise as anode materials for sodium-ion batteries owing to their abundant availability, cost-effectiveness, and ease of processability. Nevertheless, it is a formidable challenge to precisely regulate the internal microstructure of HCs for advancing sodium storage. In this study, we demonstrate a stepwise crosslinking-activation design strategy for efficaciously constructing a pseudographitic and closed pore structure of HCs toward boosted sodium energy. Phosphoric acid could dually function as a steric linker to bridge the coal molecules and an activator to create open pores, which contributes to the formation of a carbon structure with rich closed pores and enlarged carbon interlayers during the subsequent high-temperature carbonization. Such a microstructurally favorable feature enables HCs to afford an exceptional reversible capacity of 373.9 mAh g⁻¹ along with a remarkable initial coulombic efficiency of 90.8% and an impressive cycling stability over 5000 cycles. The fundamental mechanism of the sodium storage reaction is further elucidated through kinetic analysis and in situ Raman spectroscopy. This work will pave a fresh structure-mediated doorway for the rational design and fabrication of high-capacity coal-based HCs desirable for battery technologies beyond sodium energy.