Partially graphitic structure-assisted hard carbon derived from lignin for sodium-ion battery anodes

The increasing demand for efficient energy storage has led to increased research on sodium-ion batteries (SIBs) as a promising alternative to lithium-ion batteries. However, the anode materials currently employed in lithium-ion batteries are not suitable for SIBs, highlighting the need for the development of appropriate anode materials. In this study, cellulose- and lignin-rich residues extracted from wood biomass were converted to hard carbon, and their performance as anode materials for SIBs was evaluated. Cellulose and lignin were separated from larch wood using a deep eutectic solvent, followed by carbonization to produce CF-1300C and LF-1300C, respectively. Lignin undergoes partial graphitization at elevated temperatures, enhancing its electrical conductivity and forming ion insertion and extraction pathways. LF-1300C demonstrated higher crystallinity than CF-1300C owing to this graphitization and featured an interlayer spacing of approximately 0.43 nm, which facilitates sodium-ion insertion. Consequently, LF-1300C achieved a higher initial discharge capacity and Coulombic efficiency (350 mAh g⁻¹ and 74%, respectively) than CF-1300C (331 mAh g⁻¹ and 71%, respectively). Furthermore, LF-1300C exhibited a 21% and 84% improvement in rate capability and cycle retention, as compared with CF-1300C. These results indicate that hard carbon with a partially graphitized structure exhibits significant potential for use as an anode material in SIBs, especially in cases where existing crystalline materials present challenges. This study highlights the advantages of lignin-derived hard carbon as a superior anode material for SIBs, providing an eco-friendly and scalable solution for energy storage.