Steric-hindrance engineering-induced formation of closed pores enhances sodium storage capacity in starch-derived hard carbon.

The simultaneous optimization of high reversible capacity and initial Coulombic efficiency (ICE) is critically important for hard carbon anodes. However, these two key parameters are typically limited by underdeveloped closed pores, which arises from the excessive growth of graphitic domains during pyrolysis. Herein, starch chain structures are engineered at the molecular level using terephthalic acid (PTA) as an external crosslinker. We demonstrate that the esterification between carboxyl groups (-COOH) in PTA and hydroxyl groups (-OH) on starch branched chains effectively enhances the thermal stability of starch. Furthermore, the benzene ring in PTA links and intertwines starch chains via ester bonds, generating a distinct steric hindrance effect. Consequently, the linear starch molecular structure is transformed into densely crosslinked 3D polymeric networks with abundant microscale cavities. Upon carbonization, these 3D cavities evolve into closed-pore cores, while the surrounding skeletons form robust closed-pore walls. Benefiting from these structural advantages, the optimized hard carbon delivers abundant closed pores with a uniform size of 2.62 nm and a pore volume of 0.69 cm3 g-1, achieving a reversible capacity of 338.7 mAh g-1 and a high ICE of 87.9%. This work provides fundamental guidance for the molecular-level design of closed-pore-rich hard carbon materials.