Reducing steric hindrance to enhance the oxidation reactivity of coal precursors for high-performance hard carbons in sodium-ion batteries

Abstract

The effective introduction of oxygen during pre-oxidation has been proved to be a prerequisite for producing coal-derived hard carbons in sodium-ion batteries. However, the influences of inherent oxygen functional groups within coal on its oxidation and pyrolysis behaviors are still underexplored due to the highly complex compositions of coal. Here, based on the molecular dipole moments, coal components with different oxygen-containing functional groups are separated to synthesize distinct hard carbons. Experimental and theoretical analyses demonstrate that small bond angle C=O groups endow carbonyl-rich coal components with high steric hindrance and thermal stability, limiting their oxidation reactivity. After removing these inert components, the oxidation reactivity of coal precursor is significantly enhanced, thereby introducing more C(O)–O groups during pre-oxidation. These C(O)–O groups elevate adjacent C–C bond energies, which not only mitigate the depolymerization of coal macromolecules but also prevent the thermal slip of carbon layers, facilitating the formation of closed micropores. Thus, the obtained coal-derived hard carbons achieve a high reversible capacity of 329 mAh/g and excellent kinetic performance. This study highlights the critical role of inherent oxygen functional groups of precursors in modulating oxidation reactivity, providing a rational precursor design guideline for high-performance coal-derived hard carbons.