Oxygen Activation-Triggered Thermal Instability in Li(Ni0.8Co0.1Mn0.1)O₂ cathode

Ni-rich layered LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) is leading cathode candidate for next generation of lithium-ion batteries due to the distinguished energy density. Practically, NCM811 exhibits poor thermal stability and may well lead to thermal runaway, which is the bottleneck problem of this promising material. However, the fundamental factors underlying thermal failure remain ambiguous, and the correlation between surface and bulk degradation is still unclear. In this work, the evolution of atomic structure and electronic structure of high voltage delithiated NCM811 is fingerprinted by combining XRD, TEM and synchrotron-based soft X-ray absorption spectroscopy (sXAS). The oxygen hole states formed upon delithiation are thermodynamically unstable and release O 2 upon heating. This occurs prior to the phase transition and thus serves as the underlying cause of thermal failure in the NCM811 cathode. Although surface oxygen is inherently less stable, the presence of similar oxygen hole states leads to surface and bulk degradation occurring almost simultaneously. These findings present the detailed decay procedure of NCM811 during thermal runaway and offer rational guidelines for material design and optimization.