Crosslinking modification of starch improves the structural stability of hard carbon anodes for high-capacity sodium storage.

Compared with the complex components of raw biomass, biomass derivatives with defined structures are more conducive to the controllable synthesis of hard carbon (HC) materials. Starch-based HC has garnered significant attention because of its cost-effectiveness; however, its practical applicability is limited by poor thermal stability. Herein, we propose a strategy for improving the stability of starch through self-assembly crosslinking modification, yielding high-performance HC. Starch and citric acid form a dense crosslinked structure through esterification between hydroxyl and carboxyl groups, effectively overcoming the poor thermal stability. The resulting HC exhibits a low specific surface area (SSA) and abundant closed pore structures, thereby enabling substantial sodium-ion storage. The optimized HC exhibits an improved reversible capacity of 378 mAh g^-1 and an initial Coulombic efficiency (ICE) of 90.9 %. After 100 cycles at 0.5 C, it retains 98 % initial capacity. The assembled full-cell shows a high energy density of 248 Wh kg^-1. Furthermore, the structure-performance relationship analysis reveals that the slope capacity is primarily affected by the defect concentration, while the plateau capacity is mainly determined by the closed pore structure. Galvanostatic intermittent titration technique (GITT) tests and in-situ Raman spectroscopy reveal that the sodium-ion storage mechanism in starch-based HC is "adsorption-intercalation/filling."