Balancing the “disordered-ordered” microcrystalline structure to achieve high-performance of bituminous coal-based hard carbon anode for sodium-ion batteries

Bituminous coal, characterized by an optimal degree of metamorphism, volatile content, high aromaticity, and crosslinking potential, serves as an excellent precursor for the synthesis of hard carbon (HC). However, the ordered microcrystalline structure of derived HC increases the energy barrier for sodium-ion insertion, impairs diffusion kinetics, and restricts the localized storage of quasi-metallic sodium clusters, thereby significantly diminishing performance. Simple pyrolysis of resource-abundant while high aromatic bituminous coal induces highly graphitized carbon, exhibiting unsuitable microstructure for sodium storage. Herein, for the first time, the condensation reaction kinetics during the thermal treatment of bituminous coal were regulated, and the strong crosslinking network was optimized in-situ during the curing stage, establishing a “disordered-ordered” equilibrium state within the microcrystalline structure of the derived HC. The randomly arranged graphitic-like microcrystals facilitate sodium-ion adsorption, while the ordered graphitic domains provide efficient ion insertion sites and electronic conduction pathways. The coexistence of disordered and ordered domains generates structural mismatches at their interfaces, inducing carbon layer distortion that forms closed pores, which subsequently accommodate quasi-metallic sodium clusters. This balanced microstructural design enhances the sodium storage capacity of the bituminous coal-based HC anode from 225.6 mAh g −1 to 271.2 mAh g −1 , achieving an initial Coulombic efficiency (ICE) of 85.7%. The in-situ crosslinking control strategy effectively inhibits the excessive ordered stacking of graphitic microcrystals in bituminous coal-derived hard carbon at elevated temperatures, thereby establishing a “disordered-ordered” equilibrium state within the microcrystalline structure. When employed as an electrode material for sodium-ion batteries, it exhibits a high initial Coulombic efficiency (85.7%) and excellent rate performance (150.8 mAh g −1 at 1000 mA g −1 ). • The equilibrium state in the microcrystalline structure of hard carbon was constructed by in-situ crosslinking technology. • Hard carbon shows a high initial Coulombic efficiency of 85.7% and provides a capacity of 150.8 mAh g –1 at 1000 mA g –1 . • The disorderly arranged graphitic microcrystals facilitate the adsorption of sodium ion. • The formation of closed pores enhances the filling of quasi-metallic sodium clusters. • Multidimensional characterization techniques have been utilized to elucidate the sodium storage mechanisms in hard carbon.