Insights Into the Effect of Humic Acids Extraction From Coal on Hard Carbon Microstructure and Sodium Storage Properties

ABSTRACT Coal is an attractive, cost‐effective raw material for producing hard carbon (HC) anodes for sodium‐ion batteries, offering a high carbon yield. However, direct carbonization of coal typically yields a graphitic structure, which limits its sodium storage capacity. To address this, this work uses a simple, scalable solid‐phase alkali‐oxygen oxidation method to extract valuable humic acids from coal. And then investigates the impact of these humic acids on the microstructure of the resulting HC. Since humic acids act as a natural soft template within coal, removing them helps form closed pores and exposes more sites for efficiently oxidizing the dense aromatic hydrocarbons in the coal. Notably, HC derived from coal treated with extracted brown humic acid exhibits superior electrochemical performance. This is attributed to its favorable molecular composition, resulting in a high specific capacity of 308 mAh g −1 and an impressive initial Coulombic efficiency of 82.15%. This enhanced performance is largely due to its significantly higher closed pore volume (0.151 cm 3 g −1 ) compared to untreated coal‐based HC (0.0092 cm 3 g −1 ) and its enlarged carbon interlayer spacing. The plateau capacity observed in this coal‐derived HC is primarily attributed to micropore filling, as confirmed by in situ X‐ray diffraction/Raman measurements.