Topology-Derived Closed Pores in Hard Carbon Boost Sodium-Ion Storage

The closed-pore architecture of hard carbon governs reversible capacity, initial Coulombic efficiency (ICE), and Na-storage kinetics, but its precise regulation and the underlying mechanism remain elusive. To address such issues, a molecular-cavity engineering strategy is developed using cyclic molecular precursors, whose inherent cavities are preserved by low-temperature cross-linking, to construct topology-derived intrinsic closed pores in hard carbon during carbonization. Using β-cyclodextrin as a model precursor, the resulting hard carbon (β-ICHC) delivers a reversible capacity of 410.2 mAh g^-1, an ICE of 94.7% and 99.7% capacity retention over 300 cycles. Combined theoretical and in situ/ex situ analyses reveal that intrinsic closed pores simultaneously enable high reversible capacity, high ICE, and fast Na-storage kinetics: the intrinsic closed pores with sub-0.35 nm electrolyte-sieving mouths can not only serve as reversible Na-storage spaces to increase capacity but also suppress irreversible Na⁺ consumption to enhance ICE; the pseudo-graphitic walls with enlarged interlayer spacing enable fast Na⁺ transport and provide additional capacity. Furthermore, a three-stage Na-storage mechanism of surface/defect adsorption, closed-pore-wall adsorption, and closed-pore filling is supported by the combined experimental and theoretical results. This work provides a paradigm for hard-carbon structural design and offers new insight into sodium-storage mechanisms.