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

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 Mn²⁺ promotes hollow sphere swelling via hydrolysis-driven micelle expansion, while Cu²⁺ 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 (<0.0002 % per cycle) is observed for hollow spheres, nearly 15 times less than that of solid spheres (0.0026%). This work establishes fundamental design principle of multi-scale architectural engineering for advanced hard carbon anodes.