Deciphering Atomic Electronic Structure Dynamics and Site Occupancy Transitions in Dictating Sodium Storage in Hard Carbon

Deciphering the hard carbon (HC) sodiation mechanism is essential for HC's rational design, but is confused by sodium insertion and pore-filling in the plateau region due to HC's microstructural heterogeneity. This work leverages high-resolution ¹³C NMR and operando ²³Na NMR, which identify the carbon layer lining the micropore-wall as a contributor to HC's electronic structure beyond the quasi-metallic sodium clusters during the early plateau region. The stage coincides with a sodium occupancy transition of Na⁺ initially residing in graphitic interlayers into sodium clusters in micropores. The synergy of these changes consequently leads to the reduction of the Na⁺ diffusion coefficient. These results demonstrate how the carbon matrix itself, not just the pore architecture, controls the critical plateau electrochemistry in HC. Moreover, these findings provide conclusive evidence for an advanced theoretical framework of HC sodiation through three stages, and underscore the tailoring of electronic environments and Na occupancy for advanced HCs.