Thermal Decomposition Mechanisms and Inherent Stability Differences Between O2‐ and O3‐Lithium‐Rich Manganese‐Based Oxide Cathodes

Abstract O2‐type lithium‐rich manganese‐based oxides (O2‐LRMOs) promise to exhibit highly reversible electrochemical behavior and suppressed voltage decay for use in high‐energy‐density lithium‐ion batteries. However, how oxygen layer stacking governs their phase evolution and thermal stability remains elusive. This study investigates the thermal decomposition mechanism of O2‐ and O3‐Li x Li 0.17 Ni 0.133 Co 0.133 Mn 0.564 O 2 (x = 0.39, 0.78) at elevated temperatures. Combined with in situ time‐resolved synchrotron X‐ray diffraction and thermal analysis, it reveals that O2‐LRMOs, owing to its ABAC oxygen layer arrangement, follow a two‐step thermal failure pathway. Specifically, a phase transformation occurs from metastable O2 to an intermediate O3‐type layered phase during the initial heating process, followed by the formation of the spinel phase. Notably, the formation of the intermediate O3 phase delays the formation of the spinel phase compared to that of pure O3‐type lithium‐rich manganese‐based oxides (O3‐LRMOs). O2‐ and O3‐LRMOs exhibit comparable thermal release behavior upon incorporation of the electrolyte, despite significant inherent stability differences. This indicates surface reactions rather than oxygen act as the dominant factor in the initial stage of thermal runaway. Overall, these findings provide an important theoretical basis for optimizing the thermal stability and electrochemical performance of O2‐type lithium‐rich manganese‐based cathode materials in the future.