Removal of hemicellulose from alkaline lignin improved electrochemical performance of hard carbon for sodium-ion battery application

Lignin, a natural aromatic biopolymer, is often recovered as low-value by-product during the delignification process of lignocellulosic biomass for producing cellulosic pulps. In this study, crude alkaline lignin (CAL) derived from the NaOH-pretreated sugarcane bagasse was used to produce hard carbon (HC) for sodium-ion battery anode applications. The results showed that the direct use of CAL led to HC with a maximum initial Coulombic efficiency (ICE) and reversible capacity of only 60.2% and 198.5 mAh g^−1, respectively. To improve the electrochemical performance, solvent extraction was applied to purify CAL. The use of purified alkaline lignin (PAL) led to HC with a maximum ICE of 76.1% and reversible capacity of 277.5 mAh g⁻¹, which were significantly higher than CAL-derived HC. The effects of carbonization temperature (1200 °C–1400 °C) and heating rate (1 °C–8 °C min⁻¹) were also examined. Structural analyses revealed that hemicellulose removal resulted in HC with favorable microstructural structures, including short-range graphitic layers, closed pores, and reduced defects, facilitating the storage of Na ions. Additionally, the overall yield of PAL-HC was comparable to CAL-HC. These results demonstrated that the removal of hemicellulose is a critical initial step toward improving the electrochemical performance of HC before the application of other strategies.