Cationic ordering transition in oxygen‐redox layered oxide cathodes

Abstract Understanding the structural origin of the competition between oxygen 2p and transition‐metal 3d orbitals in oxygen‐redox (OR) layered oxides is eminently desirable for exploring reversible and high‐energy‐density Li/Na‐ion cathodes. Here, we reveal the correlation between cationic ordering transition and OR degradation in ribbon‐ordered P3‐Na 0.6 Li 0.2 Mn 0.8 O 2 via in situ structural analysis. Comparing two different voltage windows, the OR capacity can be improved approximately twofold when suppressing the in‐plane cationic ordering transition. We find that the intralayer cationic migration is promoted by electrochemical reduction from Mn 4+ to Jahn–Teller Mn 3+ and the concomitant NaO 6 stacking transformation from triangular prisms to octahedra, resulting in the loss of ribbon ordering and electrochemical decay. First‐principles calculations reveal that Mn 4+ /Mn 3+ charge ordering and alignment of the degenerate e g orbital induce lattice‐level collective Jahn–Teller distortion, which favors intralayer Mn‐ion migration and thereby accelerates OR degradation. These findings unravel the relationship between in‐plane cationic ordering and OR reversibility and highlight the importance of superstructure protection for the rational design of reversible OR‐active layered oxide cathodes.