Interplay Between Phase Evolution, Fe Migration, and Oxygen Redox in O3‐Type Na x Fe y Mn 1−y O ₂ Cathodes

In the development of sustainable Na‐ion battery cathodes, the use of earth‐abundant elements, such as iron, is essential to reduce costs and mitigate reliance on scarce transition metals. However, Fe‐based layered oxides frequently exhibit complex redox behavior and structural instability, making it crucial to understand how Fe affects the redox properties and phase evolution during cycling. Here, we use Fe‐rich O3‐type layered Na x Fe y Mn 1−y O 2 ( y = 0.5–0.8) as a model system to elucidate the role of Fe by combining cyclic voltammetry, operando X‐ray absorption spectroscopy, and stacking‐fault‐sensitive operando powder X‐ray diffraction. Increasing Fe content shifts the high‐potential redox response to lower potentials and promotes earlier Fe 3+ migration into tetrahedral sites in O‐type NaO 2 layers during charge. This migration strongly influences the phase evolution: While moderate‐Fe compositions complete the O3→P3→OP2→O3 sequence in the first electrochemical cycle, Fe‐rich materials remain confined to stacking‐faulted O3‐like structures due to early structural “pinning” of the O‐type layers. The findings clarify the mechanistic role of Fe in the interplay between cationic and anionic redox processes and establish why Fe content exerts such a strong, composition‐dependent influence on the structural stability of O3‐type Na‐ion cathodes.