Boosting Sodium Storage in Hard Carbon: A Low-Temperature Ball Milling Approach for Superior Anode Performance

Hard carbon (HC) has emerged as a promising anode material for sodium-ion batteries (SIBs) owing to its superior sodium storage performance. However, the high cost of conventional HC precursors remains a critical challenge. To address this, coal─a low-cost, carbon-rich precursor─has been explored for HC synthesis. Nevertheless, the dense structure of coal tends to form highly graphitized microcrystalline domains during high-temperature carbonization, leading to a suboptimal reversible specific capacity and initial Coulombic efficiency (ICE). In this study, we propose a low-temperature assisted ball milling (LT-ABM) strategy to modify carbonized bituminous coal, employing dry ice (solid CO₂) as the grinding medium. This approach leverages a physicochemical synergistic effect to enhance defect concentration while mitigating particle agglomeration. The LT-ABM process facilitates surface etching of HC materials, suppresses cold agglomeration induced by van der Waals forces, and yields uniform nanoparticles with an elevated oxygen content. The synthesized hard carbon material demonstrates a reversible specific capacity of 308.14 mAh g^-1 under a current density of 30 mA g^-1, while maintaining 78% of its initial capacity after 500 cycles at an elevated current density of 300 mA g^-1. Kinetic analyses reveal that the slope capacity correlates strongly with surface defect concentration, aligning with the ″adsorption-insertion-filling″ sodium storage model. Our findings demonstrate that controlled defect engineering can significantly improve the reversible capacity of coal-derived HC. However, the limited plateau capacity restricts high-rate performance, suggesting a trade-off between defect density and Na⁺ diffusion kinetics. This work provides a viable pathway for the rational design of cost-effective, high-performance coal-based HC anodes.