Accumulation of Unreduced Molecular O ₂ Explains Abnormal Voltage Decay in P2‐Type Layered Oxide Cathode

Abstract The Na‐deficient P2‐type layered structure has been widely regarded as a more stable framework for facilitating anion redox reaction (ARR) relative to the Li‐rich O3‐type counterpart. Specifically, the notorious voltage decay typically associated with ARR is well‐documented to be intrinsically suppressed in P2 stacking. Herein, the presence of substantial voltage decay over cycling is paradoxically showcased in the exemplary high Na‐content P2‐type Na 0.8 Li 0.26 Mn 0.74 O 2 . Multimodal characterization reveals a stepwise ARR pathway during the initial cycle, involving sequential O 2− → O − (hole states on O) → O─O dimers → molecular O 2 evolution accompanied by varying degrees of Li displacement. Furthermore, electrochemical cycling induces dynamic intermediate evolution in desodiated Na 0.8 Li 0.26 Mn 0.74 O 2 , where oxidized oxygen species preferentially stabilize as molecular O 2 rather than O─O dimers. More importantly, further cycling irreversibly confines unreduced O 2 in nanovoids while triggering local phase segregation into Mn‐enriched domains and Li‐rich clusters in the discharged structure, synergistically driving the abnormal voltage decay. This study challenges the conventional wisdom of the structural stability of P2 phase during ARR and will establish a revised theoretical framework for ARR‐active layered cathode design.