Pre-oxidation of lignin precursors for hard carbon anode with boosted lithium-ion storage capacity

Lignin-derived hard carbon (HC) has great potential as energy storage materials. However, it is difficult to obtain desired electrochemical performances by direct carbonization of lignin. Herein, we demonstrate a pre-oxidation strategy to enhance the reversible capacity of hard carbon with lignin as precursor. The pre-oxidation mechanism and its influence on the microstructures of the resulted hard carbon are systematically studied. Based on in-situ FT-IR and 13C NMR spectrum, etc., it is confirmed that three dominant configurations of oxygen-containing functional groups are formed during the process, and the content of the desired carbonyl groups (CO) reaches a maximum value at a pre-oxidation temperature of 200 °C. Meanwhile, the alkyl groups are transformed into peroxides or alcohols, contributing to intermolecular cross-linkage within lignin. As a result, the obtained material with highly random orientation nanotexture gives a much larger d002 and abundant porous structure. Benefiting from these structural merits, the optimized lignin-derived hard carbon enables excellent Li-ion storage performance with a reversible capacity of 584 mA h g−1 at 50 mA g−1. This work provides insights into the rational design of high-performance hard carbon anodes for Li-ion batteries and beyond.