Revisiting irreversible capacity in lignin-derived hard carbons for sodium-ion batteries: The dominant role of surface functional groups over surface area

Hard carbons are promising materials as anode for sodium-ion batteries due to their good reversible capacities and the huge diversity of bio-based precursors available for their synthesis. Nevertheless, their low initial coulombic efficiency (ICE), often below 80% for bio-based hard carbons, is one of the main challenges for their commercial applications. In this work, the surface of lignin-based hard carbons is modified with a thin layer of soft carbon obtained from petroleum pitch. Soft carbon is known to have rich sp 2 -hybridized domains, with very few heteroatoms, leading to a surface with less defects. After pyrolysis at different temperatures (900 °C to 1400 °C), hard carbon materials were coated using an impregnation coating method. Structural, textural and surface analyses were carried out to understand the impact of soft carbon coating on the electrochemical performance. The composite carbon initially pyrolyzed at 1200 °C exhibited the best performance, achieving a reversible capacity of 310 mAh.g −1 , and an excellent initial coulombic efficiency of 89%. Notably, this ICE is among the highest reported for lignin-based hard carbons, which highlights the effectiveness of surface engineering in enhancing their electrochemical properties for sodium-ion batteries. • Hard carbons derived from lignin promote sustainable Na-ion battery design. • Pitch-derived soft carbon coating reduces surface defects of hard carbon. • Surface engineering boosts initial coulombic efficiency of bio-based hard carbons. • The optimized soft carbon-hard carbon composite achieves 310 mAh g −1 with 89% ICE.