Cationic ordering transition in oxygen-redox layered oxide cathodes

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-Na0.6Li0.2Mn0.8O2 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 Mn4+ to Jahn-Teller Mn3+ and the concomitant NaO6 stacking transformation from triangular prisms to octahedra, resulting in the loss of ribbon ordering and electrochemical decay. First-principles calculations reveal that Mn4+/Mn3+ charge ordering and alignment of the degenerate eg 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. The authors elaborate on the correlation between structural cationic ordering transition and electrochemical oxygen-redox (OR) degradation in ribbon-ordered P3-Na0.6Li0.2Mn0.8O2 via in situ structural analysis. The OR capacity can be improved approximately twofold when suppressing the in-plane cationic ordering transition, which originates from the Mn4+/Mn3+ charge ordering and alignment of the degenerate eg orbital-induced lattice-level collective Jahn-Teller distortion. image