Hard carbon from wood and its constituents: Toward sustainable sodium and potassium-ion battery anodes

The growing demand for sustainable energy storage solutions necessitates the development of low-cost, environmentally friendly materials. Wood-derived hard carbons (HCs), including those from wood components such as cellulose, lignin and hemicellulose, have emerged as promising anode materials for sodium-ion (SIBs) and potassium-ion batteries (KIBs), offering a renewable and abundant carbon source with tuneable microstructure. The hierarchical structure of wood enables tuneable porosity and surface functionalities which improve ion diffusion and facilitate solid-electrolyte interphase (SEI) formation. This review focuses on the synthesis, structural characteristics, and electrochemical performance of wood-derived HCs in SIBs and KIBs. We discuss charge storage mechanisms, initial coulombic efficiency (ICE), SEI behaviour, and rate capability in the context of natural wood precursors. Although KIB research is still in early stages, wood-derived HCs have demonstrated specific capacity up to 650 mAh g-1, ICE values of up to 90 % and cycling stability exceeding 5000 cycles in SIBs, showcasing their advantages. Moreover, the use of forest-based resources supports circular economy principles and helps minimize the environmental impact. Despite their compelling performance and sustainability advantages, challenges remain in optimizing the structural design and synthesis scalability. Key research gaps include the need for a deeper mechanistic understanding of ion storage processes particularly the role of surface functionalities, porosity, and defect sites as well as the development of more sustainable, energy efficient and scalable synthesis route were identified. Addressing these challenges is essential for advancing SIBs and KIBs as viable, greener alternatives to lithium-ion batteries (LIBs). Insights from this review may further guide the rational design of wood-derived electrodes in next generation grid-scale and portable energy storage systems.