Precision-Engineered hard carbon from bituminous coal via small-molecule extraction for high-rate sodium-ion batteries

Abstract

The challenge in decoupling the order and disorder phase evolution impedes the precise microstructure control of hard carbon (HC) anodes, fundamentally limiting the reconciliation of high capacity and fast kinetics for sodium storage. Here, we realize a precise modulation of order and disorder structures in HCs using bituminous coal precursors containing inherent aromatic units and aliphatic side chains. By removing the volatile small aromatic species (molecular weight <500 amu) with methanol extraction, more active edges of coal macromolecules (molecular weight >500 amu) and C–O/CO/O–CO functional groups are exposed, which enhances the cross-linking of macromolecules aromatic units, yielding long, curved microcrystalline domains with enlarged interlayer spacing and abundant closed nanopores. The extended carbon microcrystals boost electronic conductivity, while expanded interlayers and closed pores facilitate Na⁺ diffusion and storage. As a result, the obtained HC has a high capacity of 256 mAh g⁻¹ and long cycling stability (capacity retention of 80.8 % after 1500 cycles) at a very high current density of 2000 mA g⁻¹. This work demonstrates a scalable route for precise structural modulation of coal-derived HC to reconcile capacity and rate performance in sodium-ion batteries.