Biomass hard carbon for sodium-ion batteries: Feedstock-process-performance relationships

The conversion of agricultural residues into high-value-added biomass-derived hard carbon anodes for sodium-ion batteries not only achieves the valorization of agricultural resources but also shows considerable potential for addressing the global energy crisis and mitigating environmental pollution. However, the practical application of biomass-derived hard carbons is hindered by persistent challenges such as low specific capacity, low initial Coulombic efficiency, and poor cycling stability, stemming from an unclear structure-performance relationship between modification strategies and electrochemical behavior. This review summarizes established models describing sodium storage mechanisms in hard carbons, with particular emphasis on active storage sites such as nanopores, graphitic layers, and defect sites. Addressing the practical issues associated with biomass-derived hard carbons, several sodium storage mechanism models are discussed in detail, including the “insertion-filling”, “adsorption-filling”, “adsorption-insertion”, and multistage mechanisms. Furthermore, relevant characterization analyses are integrated to elucidate the structure-performance relationship between hard carbon materials and sodium storage behavior. From a materials perspective, this review systematically outlines the preparation strategies, precursor selection, and the correlation between modification approaches and sodium storage mechanisms. Representative performance-enhancement strategies, including heteroatom doping, interfacial engineering, and morphology regulation, are explicitly summarized. Ultimately, in-depth investigation of these critical issues is expected to optimize the electrochemical performance of biomass-derived hard carbons and promote their practical application in sodium-ion batteries.