Interconnected Closed Pores Enable Dense and Facile Sodium Storage in Hard Carbon

ABSTRACT Achieving a high plateau capacity in hard carbon (HC) anodes is one of the most critical prerequisites for high‐energy‐density sodium–ion batteries (SIBs), yet it is fundamentally limited by inaccessible closed pores formed during conventional high‐temperature annealing. Here, we propose a potentially scalable oxidation‐reconfiguration strategy to unlock its latent capacity. By coupling controlled oxidative etching with subsequent thermal reconstruction, an interconnected closed‐pore network is constructed. Oxidative pretreatment opens blocked channels and interconnects isolated voids, while reconstruction promotes void fusion, generating accessible internal reservoirs for the nucleation and storage of quasi‐metallic sodium clusters. This structural evolution and storage mechanism are elucidated by total neutron scattering, SAXS, in situ techniques, and simulations. As a result, the optimized ICP‐HC anode delivers reversible capacity 437 mAh g −1 with an initial Coulombic efficiency of 91.5%. It achieves an initial discharge plateau capacity of 399 mAh g −1 with fast kinetics (300 mAh g −1 at 2C), overcoming the capacity‐rate trade‐off. Furthermore, an NVP//ICP‐HC full cell shows excellent rate capability (96 mAh g −1 at 5C) and stable cycling (95% retention over 200 cycles at 1C). This work provides a scalable strategy for advanced carbon anodes in high‐energy‐density SIBs.