Oxygen activation-triggered thermal instability in Li(Ni_0.8Co_0.1Mn_0.1)O₂ cathode

Ni-rich layered LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) is a leading cathode candidate for the next generation of lithium-ion batteries because of its high energy density. In practice, NCM811 exhibits poor thermal stability that can lead to thermal runaway, which is a critical bottleneck for the practical application of this promising material. The fundamental factors underlying thermal failure and the relationship between surface and bulk degradation, however, remain unclear. In this work, we track the evolution of the atomic and electronic structures of high-voltage delithiated NCM811 using x-ray diffraction (XRD), transmission electron microscopy (TEM), and synchrotron-based soft x-ray absorption spectroscopy (sXAS). Oxygen hole states formed upon delithiation are thermodynamically unstable and lead to O₂ release upon heating. This O₂ release occurs prior to phase transitions and therefore constitutes the primary cause of thermal failure in NCM811 cathodes. Although surface oxygen is inherently less stable, the presence of similar oxygen hole states at the surface and in the bulk causes surface and bulk degradation to proceed almost simultaneously. These findings delineate the degradation pathway of NCM811 during thermal runaway and provide rational guidelines for material design and optimization.