Biomass-derived hard carbon anodes for sodium-ion batteries: structure–property engineering, mechanistic insights, and data-driven design strategies

Sodium-ion batteries (SIBs) have emerged as promising alternatives to lithium-ion systems for large-scale energy storage owing to the abundance, low cost, and widespread availability of sodium. Among anode materials, biomass-derived hard carbon (HC) has gained considerable attention because of its low operating potential, structural tunability, and sustainable origin. This review critically examines recent advances in biomass-derived HC anodes, highlighting the relationships between precursor chemistry, carbonization strategies, and microstructural evolution. Particular emphasis is placed on expanded interlayer spacing (>0.37 nm), closed nanopores, and ultramicroporosity, which collectively govern sodium storage and plateau capacity. Recent studies report reversible capacities of 278–335 mAh g−1, initial Coulombic efficiencies up to 93%, and ~96% capacity retention after 5000 cycles. The review further discusses heteroatom engineering and emerging machine learning (ML) approaches for precursor selection and process optimization. Finally, it identifies key challenges including initial efficiency, tap density, scalability, and full-cell validation to accelerate commercial deployment of sustainable SIB technologies.