Influence of residual inorganic components on the conversion behavior and sodium storage performance of coal-based hard carbons

As one of the most abundant carbonaceous resources, coal possesses a highly condensed aromatic framework and a low-cost advantage, making it a promising precursor for high-value carbon materials. However, high-performance coal-based carbons require ultralow-ash coal, whereas current deashing strategies still rely on corrosive acids and mainly evaluate precursors by total ash content, neglecting the role of residual inorganic components during pyrolysis and carbonization. Herein, a typical bituminous coal was treated with HCl, HNO3, and HF, followed by carbonization at 1300°C to prepare hard carbons. HCl and HF caused limited disturbance to the coal organic macromolecular framework and reduced Fe-containing components, resulting in enhanced reversible capacities compared with raw-coal-based hard carbon. Although HF further removed most Si- and Al-containing components, HC-F delivered 284.9 mAh g−1, only slightly higher than HC-H (280.1 mAh g−1), indicating that residual Si/Al-related components may mainly exist as electrochemically inert phases. In contrast, HNO3 achieved higher Fe-removal efficiency but induced overoxidation and excessive defects, decreasing reversible capacity and initial Coulombic efficiency. These results suggest that Fe-containing components are more detrimental than residual Si/Al components, providing a basis for inorganic-component-oriented precursor evaluation and targeted pretreatment strategies.