Accumulation of Unreduced Molecular O₂ Explains Abnormal Voltage Decay in P2-Type Layered Oxide Cathode

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_0.8Li_0.26Mn_0.74O₂. Multimodal characterization reveals a stepwise ARR pathway during the initial cycle, involving sequential O²⁻ → O⁻ (hole states on O) → O─O dimers → molecular O₂ evolution accompanied by varying degrees of Li displacement. Furthermore, electrochemical cycling induces dynamic intermediate evolution in desodiated Na_0.8Li_0.26Mn_0.74O₂, where oxidized oxygen species preferentially stabilize as molecular O₂ rather than O─O dimers. More importantly, further cycling irreversibly confines unreduced O₂ 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.