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

ABSTRACT 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 ²³ Na 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.