Engineering Alkali Lignin Structure Modification: Enhanced Hard Carbon Electrolyte Interface Toward Practical Sodium Ion Batteries

Hard carbon (HC) exhibits great potential as a promising candidate for sodium-ion batteries owing to its inherent advantages. However, the main challenges in utilizing HC stem from its low initial coulombic efficiency (ICE) and poor rate performance caused by its excessive surface defects. In this study, an effective strategy of employing alkali lignin (AL) is proposed, derived from pulp waste, as a binder for HC to create a uniform and inorganically enriched solid electrolyte interface. AL can modify the surface defects of HC through strong π–π interactions between the aromatic ring of AL and HC, while ingeniously grafting abundant active ─OH and ─COOH groups onto the electrode surface. The strong binder force between AL and electrolyte salts facilitates the formation of an ultra-thin NaF-rich solid electrolyte interface (SEI) layer (10 nm), thereby achieving an exceptional ICE of 91%. Furthermore, owing to its electrochemical activity, AL enables HC anode to exhibit an increasing slope capacity during cycling, compensating for capacity decay at high current densities. Consequently, when assembled into a full battery configuration, excellent rate performance is achieved with a reversible capacity of 282 mAh g⁻¹ even at a current density of 5A g⁻¹.