Structural Evolution and Thermal Behavior of High-Ni Cathodes: Coupled Operando XAS, XRD, and Mass Spectrometry Analysis

Understanding the intrinsic thermal instability of Ni-rich cathodes is critical for elucidating the thermal runaway mechanisms in lithium-ion batteries. Here, we investigate the thermally induced decomposition of Ni-rich cathodes under electrolyte-free conditions using time-resolved operando approaches, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and online mass spectrometry (OMS). The results reveal a coupled evolution of oxygen release, Ni redox, and structural transformations during heating. Oxygen release is initiated prior to detectable bulk phase transition and correlates with early structural disorder, as tracked by the evolution of the (003)L and (104)L reflections. Deep delithiation at 4.8 V (∼95% SOC) accelerates Ni reduction and amplifies structural instability, leading to approximately 3-fold higher early stage oxygen release compared with the 4.3 V (∼85% SOC) condition. These findings establish a coupling-based framework for understanding cathode-driven thermal instability and provide guidance for improving battery safety.