Decoding the Low‐Voltage Plateau in Hard Carbon Negative Electrodes: From Sodium‐Storage Models to Quantitative Active‐Site Design

Hard carbon (HC) is the leading anode candidate for sodium‐ion batteries, but its practical optimization remains constrained by an unresolved question: which structural motifs actually store Na + at low potential? Early voltage‐profile assignments separated sloping capacity from plateau capacity and attributed them to adsorption, intercalation, or pore filling. However, recent evidence indicates that these assignments are often sample‐dependent and may overinterpret individual characterization signatures. This Review re‐examines HC anodes from the perspective of site‐resolved Na storage. We first compare the evolution of sodium‐storage models and clarify what each model explains, where it fails, and why apparently contradictory conclusions have emerged. We then focus on the low‐voltage plateau, discussing monolayer Na accumulation, quasi‐metallic Na cluster formation, closed‐pore filling, and the remaining pore‐volume paradox. Particular attention is given to the strengths and limitations of WAXS/XRD, SAXS, XPS, Raman, NMR, titration, and theoretical simulations for assigning Na storage states. Finally, we connect mechanistic understanding to active‐site design, including defects, heteroatoms, pores, metal‐assisted motifs, and surface functional groups, emphasizing which design rules are established and which remain speculative. HC design must move beyond qualitative structure‐performance correlations toward quantitative active‐site identification, closed‐pore metrology, and full‐cell‐relevant optimization of plateau capacity, initial Coulombic efficiency, and scalability.