Solvation-Pore Coupling Governs Fast Na Storage in Hard Carbon Anodes

Hard carbon (HC) is widely regarded as the most promising anode for sodium (Na)-ion batteries (NIBs), yet its fast-charging capability is intrinsically limited by sluggish Na⁺ insertion and transport within disordered micropores. Here, we elucidate how Na⁺ solvation structures regulate pore-entry barriers and intra-pore transport, thereby dictating Na-storage chemistry and kinetics in HC. Using hard carbon spheres (HCSs) with well-defined micropore architectures, we show that ether-based electrolytes enable a micropore-mediated partial desolvation pathway that accelerates Na⁺ insertion and promotes the formation of quasi-metallic Na clusters/layers within HC featuring a broad pore-size distribution. In contrast, carbonate electrolytes induce deeper desolvation and sluggish Na⁺ diffusion, suppressing quasi-metallic Na formation and severely limiting high-rate performance. Comprehensive ²³Na MAS solid-state NMR (ssNMR) reveals a discontinuous, phase-transition-like transformation from ionic to quasi-metallic Na at deep sodiation, driven by enhanced Na-carbon electronic coupling. These results uncover a solvation-pore-coupled kinetic mechanism governing fast Na storage in HC, providing rational design principles for high-rate and long-life HC anodes via coordinated regulation of pore structure and interfacial desolvation chemistry.