Cross-linking engineering of starch molecular topology tailoring hard carbon microstructure and sodium storage kinetics

Owing to the abundance and low cost of their precursors, along with their superior sodium storage capability, biomass-derived hard carbons hold great promise as anodes for sodium-ion batteries. However, how precursor molecular structure governs hard carbon properties remains unclear. Herein, corn starch is employed as a model system to compare linear amylose and highly branched amylopectin on crosslinking and hard carbon anodes. The abundant branch points and amorphous regions of amylopectin facilitate more thorough crosslinking with (NH₄)₂HPO₄, forming a denser three-dimensional network. In contrast, dense crystalline domains of amylose restrict crosslinker penetration. These differences dictate the microstructure of carbonized products: high-amylopectin derived hard carbon (A-HC) exhibits smaller interlayer spacing and higher graphitization, whereas high-amylose derived hard carbon (S-HC) shows larger specific surface area (78.19 m² g^-1) and open porosity (0.0187 cm³ g^-1). Electrochemically, A-HC exhibits a reversible capacity of 346.01 mAh g^-1 alongside an initial Coulombic efficiency of 88.51%, outperforming S-HC (275.67 mAh g^-1, 79.97%). Combined with multidimensional in situ characterizations, the sodium storage mechanism follows an adsorption-intercalation/pore filling model. This work establishes a clear structure property relationship linking precursor molecular topology to hard carbon performance, offering new insights for the rational design of biomass-derived hard carbon anodes.