In Situ Phosphoester Polymer Layer Locking Oxygen Migration in Ni‐Rich Cathodes Under Ultra‐High Voltage

ABSTRACT Raising the cutoff voltage of nickel‐rich layered cathodes is an effective strategy to increase the energy density of lithium batteries, yet it markedly aggravates structural degradation and gas evolution driven by lattice oxygen instability. Under diffusion‐limited conditions, the separator‐adjacent electrode region undergoes preferential over‐delithiation, serving as the primary initiation site for oxygen‐induced chemo‐mechanical failure. Here, we report an in situ formed phosphoester‐derived polymer interlayer on LiNi 0 . 8 Co 0 . 1 Mn 0 . 1 O 2 electrodes that operates via a synergistic chemical–physical oxygen migration locking mechanism. Chemically, the phosphorus‐containing polymer stabilizes lattice oxygen through robust metal–oxygen–phosphorus coordination, increasing the oxygen‐vacancy formation energy by 0.61 eV compared with pristine LiNi 0 . 8 Co 0 . 1 Mn 0 . 1 O 2 . Physically, the crosslinked polymer network regulates oxygen transport and captures evolved oxygen species, thereby mitigating parasitic reactions. This dual‐function interlayer suppresses gas evolution, reduces strain accumulation, and stabilizes bulk structural integrity under ultra‐high‐voltage operation. Consequently, the modified cathode delivers 81.5% capacity retention after 100 cycles at 1 C under 4.6 V, which is an improvement of 25.3% over the pristine counterpart, and enables stable cycling of a 3.2 Ah pouch full cell. This scalable in situ interfacial strategy provides an effective pathway to suppress oxygen‐related degradation in Ni‐rich cathodes, advancing safer and higher‐energy lithium batteries.