Revisiting Deep Delithiation of LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811) Cathode Materials

ABSTRACT Understanding the structural and electrochemical limits of Ni‐rich layered oxides under extreme delithiation is essential for advancing high‐energy‐density lithium‐ion batteries. Here, we show that subtle Li/Ni antisite disorder critically governs phase evolution and high‐voltage resilience in LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811). Nearly antisite‐free NMC811 undergoes a slow yet substantial O3→O1 transition during resting at the fully delithiated state. This O1 phase is largely reversible but becomes less accessible with cycling as structural heterogeneity accumulates. Pursuing deep delithiation at high voltages amplifies parasitic reactions, gas evolution, and transition‐metal dissolution, leading to accelerated capacity decay. In contrast, ∼2% antisite disorder plays a key role in preventing O1 formation in deep delithiated NMC811 and unexpectedly enhances cycling stability under constant‐voltage charging at 4.8 V. These results demonstrate crucial roles of modest antisite disordering in stabilizing deep‐delithiation cycling of NMC811 electrodes, offering valuable insights for designing Ni‐rich cathode materials.