Insights into the Dynamical Sodium Occupancy Evolution and Rate-Limiting Steps in Hard Carbon

Accurate understanding of the sodium-storage mechanisms and behaviors is essential for advancing hard carbon (HC) anodes, yet significant controversies persist regarding the sloping and low-voltage-plateau sodiation processes. This work leverages quantitative in situ NMR with Raman spectroscopy and electrochemical analysis to achieve a critical quantified understanding. The approach definitely identifies a transition of Na⁺ from intercalation/adsorption sites to quasi-metallic sodium clusters within closed pores in the early stage of the plateau and subsequently cluster-grow alongside adsorption/intercalation-reoccupy during the late plateau. Notably, our results demonstrate that adsorbed Na⁺ maintains a significantly higher mobility than intercalated Na⁺ during the transition. This transition exhibits strong correlation with decreasing diffusion coefficient during the process, critically governing the rate performance of HC. This understanding clearly explains the enhanced plateau kinetics of HC by introducing abundant defects and closed pores and enlarging carbon layers, which provide a fast transition pathway into quasi-metallic sodium. As a result, our strategically designed HC material achieves a high reversible capacity of 413.2 mAh g^-1 at 30 mA g^-1 and an exceptional rate capability of 253.0 mAh g^-1 at 1500 mA g^-1. These fundamental insights into Na⁺ release and the transition-storage mechanism provide a critical foundation for the rational design of high-performance HC materials.