Interlayer Superstructure Stabilized Oxygen-Redox Ni- and Co-Free Cathodes

Oxygen redox is a mechanism that offers additional charge compensation, boosting the capacity of metal oxide cathodes in rechargeable batteries. However, the O^2- ions are susceptible to overoxidation and do not always recover during discharge, particularly during high-voltage charge operation, leading to structural irreversibility and fast electrochemical degradation. Herein, we unveil a critical factor influencing O-redox reversibility beyond its intrinsic thermodynamic theory: the role of interlayer superstructure. Leveraging comprehensive characterizations, we have discovered that P3-R-NMO that features a ribbon-ordered superstructure with O-□^Mn-O structural motifs exhibits highly reversible O-redox reactions for sodium-ion batteries. The remarkable enhancement in oxygen redox can be primarily attributed to the -A-B-C-interlayered superstructure. It introduces a substantial spatial separation between the interlayer O-□^Mn-O motifs that are essential for oxygen redox, consequently reinforcing the chemomechanical strength against interlayer planar gliding and effectively mitigating the two-phase transition and Mn migration during cycling. In a strict comparison, when the same ribbon superstructure adopts an -A-B- stacking sequence in P2-R-NMO, the {002} planar gliding is more readily activated, leading to the detrimental P2-to-O2 phase transition, severe Mn migration, and ultimately, the destruction of the O-□^Mn-O motifs and irreversible oxygen loss. Our findings suggest that achieving highly stable oxygen redox can be made feasible through the thoughtful design of an interlayer superstructure, where the elastic strength of the lattice against irreversible gliding can be significantly enhanced.