Valorizing low cost and renewable lignin as hard carbon for Na-ion batteries: Impact of lignin grade

Lignin, the second most abundant biopolymer and rich in aromatic entities, is a by-product of paper industry and mainly burned to produce energy. Its commercialization for high-value added products remains largely unexplored today. Herein, we explored the preparation of hard carbon anodes for sodium-ion batteries using the two most common lignin grades, i.e, lignin kraft and lignin sulphonate. The lignin kraft hard carbon (LK-HC) shows small specific surface area (1.8 m² g⁻¹) and a dense random-like morphology while the latter lignin sulphonate hard carbon (LS-HC) presents a high surface area (180 m² g⁻¹) and spherical particles containing macropores. Their electrochemical performances vs. Na⁺ revealed a steady capacity of 181 mAh g⁻¹ over cycling for LK-HC, as compared to a capacity of 205 mAh g⁻¹ for LS-HC which fades after 30 cycles due to an impurity-driven growth of a blocking solid electrolyte interphase (SEI). To circumvent this issue, an efficient washing procedure was successfully implemented enabling to obtain (LSW-HC) carbons which deliver a high and stable capacity (284 mAh g⁻¹) along efficiency (78.1%).