Substitution Index‐Prediction Rules for Low‐Potential Plateau of Hard Carbon Anodes in Sodium‐Ion Batteries

Abstract Establishing prediction rules for the low‐potential plateau (LPP) of hard carbon (HC) anodes is crucial for constructing high‐energy‐density sodium‐ion batteries (SIBs). While current studies suggest that the closed pores of HC can enhance the LPP performance, the rules for directly predicting the LPP from precursors have yet to be established. Here, prediction rules for the LPP of HC anodes in SIBs—the substitution index ( Δ ) of precursor are introduced. Three carbon models (disordered carbon, closed‐pore‐dominated carbon, and turbostratic carbon) are constructed to verify the accuracy of Δ and to explore the closed‐pore formation and LPP mechanism. In detail, as the Δ increases from 0.06 to 0.22, the LPP capacity rises from 25 to 278 mAh g⁻¹, revealing a strong linear correlation between Δ of precursor and LPP capacity. In situ XRD, Raman, and ex situ SAXS, EPR further confirm that sodium storage in HC can be categorized into adsorption (>0.4 V), interlayer storage (0.4 to 0.15 V), and pore‐filling (below 0.15 V). This work not only elucidates the sodium storage mechanisms, but also provides one efficient design guideline for advanced carbon anodes in SIBs.