Direct Evidence of Intermediate Oxygen Species Evolution in Cathode Oxides With Nominally Stable Superstructure

Anionic redox has fundamentally transformed the traditional approach to developing advanced cathode materials for secondary batteries. Nevertheless, how oxygen anions participate in the redox reaction and evolve during the dynamic intercalation process remains a topic of intense controversy, particularly in prototypical materials featuring a stable superstructure. Herein, we showcase that the oxidized oxygen species at the top of charge of ribbon-ordered P2-Na_0.6Li_0.2Mn_0.8O₂ continuously evolve during electrochemical cycling-shifting from a mixture of electron holes on O^2- and dimerized (O₂)^n- to a mixture of (O₂)^n- and molecular O₂. The intermediate oxygen species increasingly stabilize as molecular O₂ instead of (O₂)^n-, a process coupled with the expansion of Li-vacancy clusters and the growth of Mn-enriched domains. We further demonstrate the same trend in the P3 analogue, which features a more stable ribbon superstructure with a topological "-α-γ-" stacking pattern ("-α-β-" for P2-Na_0.6Li_0.2Mn_0.8O₂), further corroborating that molecular O₂ formation is inescapable even when Mn migration is restricted in the pristine structure. Collectively, our results provide direct evidence that electron holes on O^2- are unstable upon cycling and identify molecular O₂ as the ultimate oxygen-redox product for cathode oxides with nominally stable superstructure.