ABSTRACT Prussian blue analogues (PBAs) continue to attract significant interest as low‐cost cathode materials for sodium‐ion batteries. Of these, sodium manganese hexacyanomanganate (NaMnHCMn) offers the highest capacity. However, its redox mechanism, which involves three redox plateaus, remains elusive due to the complexity of mastering the materials’ stoichiometry, vacancies, and hydration levels. Here, we combine in situ X‐ray diffraction, electrochemical quartz‐crystal‐microbalance, and operando infrared fibre‐optic‐evanescent‐wave‐spectroscopy (IR‐FOEWS) to investigate the redox processes in NaMnHCMn. Strikingly, we found that low‐spin Mn–C environments alone drive all three redox events, which corresponds to the three distinct voltage plateaus observed. This contrasts with other Na‐based PBAs (Na x FeFe(CN) 6 and Na x MnFe(CN) 6 ), in which multiple metal centers are redox‐active. Density‐functional‐theory (DFT) calculations of the vibrational modes and projected‐density‐of‐states (PDOS) of NaMnHCMn explain this difference and show that charge compensation occurs exclusively at low‐spin Mn. The stepwise redox of Mn C centers involves an intermediate Mn 1+ state, the detection of its IR signature at 1900–2000 cm −1 by operando IR‐FOEWS, corroborated by DFT calculations of C≡N vibrational modes with PDOS, represents a new finding. Overall, Mn C centers’ unique involvement in the redox process is believed to account for the limited cycling stability of this material when cycled over the 3e − range.
Li et al. (2026) studied this question.