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.