Stacking‐Fault Enhanced Oxygen Redox in Li2MnO3

Abstract Lattice oxygen redox yields anomalous capacity and can significantly increase the energy density of layered Li‐rich transition metal oxide cathodes, garnering tremendous interest. However, the mechanism behind O redox in these cathode materials is still under debate, in part due to the challenges in directly observing O and following associated changes upon electrochemical cycling. Here, with 17 O NMR as a direct probe of O activities, it is demonstrated that stacking faults enhance O redox participation compared with Li 2 MnO 3 domains without stacking faults. This work is concluded by combining both ex situ and in situ 17 O NMR to investigate the evolution of O at 4i, 8j sites from monoclinic C2/m and 6c(1), 6c(2), 6c(3) sites from the stacking faults (P3 1 12). These measurements are further corroborated and explained by first‐principles calculations finding a stabilization effect of stacking faults in delithiated Li 2 MnO 3 . In situ 17 O NMR tracks O activities with temporal resolution and provides a quantitative determination of reversible O redox versus irreversible processes that form short covalent OO bonds. This work provides valuable insights into the O redox reactions in Li‐excess layered cathodes, which may inspire new material design for cathodes with high specific capacity.