Unlocking the potential of lignin for sodium-ion battery anodes: From biorefining to hard carbon engineering

• Lignin is a renewable, abundant, low-cost, and carbon-rich precursor for hard carbon. • Lignin-based hard carbon can be integrated in a multiproduct biorefinery process. • Challenges and opportunities of lignin-derived hard carbon are discussed. The rising demand for sustainable and cost-effective energy storage solutions has intensified interest in sodium-ion batteries (SIBs) as a promising alternative to lithium-ion batteries (LIBs). Among potential anode materials, hard carbon (HC) offers advantageous sodium storage characteristics, with lignin emerging as a highly sustainable and economical precursor due to its abundance, high carbon yield, aromatic structure and tuneable physicochemical properties. This review comprehensively addresses the latest advancements in lignin-derived HC for SIBs. Initially, the overview of SIBs and different types of anode materials is outlined and subsequently, the structural properties of HC and mechanisms of sodium storage are systematically presented. Following the introduction of various precursors for HC, the significance of lignin structure and extraction methods is discussed, emphasizing the benefits of utilizing lignin for HC production and various advanced synthesis approaches. Challenges associated with lignin-based HC are then discussed alongside opportunities for material engineering, including morphological design, microstructural property regulation, heteroatom doping and functional group modification, are comprehensively elaborated. Finally, the review outlines future perspectives and identifies pathways to unlock the full potential of lignin-derived HC as high-performance and sustainable anodes for next-generation SIB technology.