Synergistic Engineering of Hollow Nanospherical Structure and Closed‐Pore in Hard Carbons for Ultra‐Stable and High‐Utilization Sodium Storage

ABSTRACT Hard carbons are among the most promising anode materials for sodium‐ion batteries, yet synergistic architectural control across multiple scales to improve comprehensive Na + storage performances remains elusive. Herein, we propose a salt‐mediated micelle interfacial polymerization to fine nanospherical swelling/shrinkage and closed‐pore within nanoshells for excellent holistic Na + storage. Metal salts screening reveals that Mn 2+ promotes hollow sphere swelling via hydrolysis‐driven micelle expansion, while Cu 2+ oxidatively polymerizes monomer within micelles, causing shrinkage to solid spheres. The hollow architecture enables bidirectional Na + diffusion with >95% storage‐site utilization, outperforming unidirectional‐limited solid spheres with size‐dependent utilization. Meanwhile, this architecture, combined with 0D isotropy mitigates sodiation‐induced stress upon ultralong cycling. In situ salt‐induced expanded closed pores within the nanoscale shell enhance low‐potential capacity with fast kinetics. Such synergistic multi‐scale design leads to simultaneous exceptional high capacity of 324 mA g −1 , outstanding rate capability retention of 90.5% at 1 A g −1 , and unprecedented cyclic stability up to 20000 cycles with decay rate as low as 0.0014% per cycle. Negligible volume expansion (