Deciphering the “Thermal Snap” on Ultra-High Ni Cathodes via Multi-Modal Operando Synchrotron X-ray and Mass Spectrometry

Deciphering the fundamental triggers of thermal instability in ultra-high nickel cathodes is vital for advancing safe, high-energy-density lithium-ion batteries. In this work, we present a reaction device that enables operando synchrotron XRD, XAFS, and online mass spectrometry (OMS) measurements simultaneously, individually, or in paired combinations under identical heating and gas conditions. Using delithiated NCM96 as a model cathode, the complementary measurements correlate structural degradation, transition-metal redox evolution, and gas release during thermal decomposition, providing comprehensive insights into the thermal instability of high-nickel cathodes. Our findings reveal that thermal failure is a synergistic process initiated by electronic instability and the onset of lattice oxygen loss at approximately 175 °C. Operando XRD identifies a distinct H3-to-H2 structural relaxation and subsequent amorphization, while XAFS captures rapid Ni reduction and local coordination expansion between 185–330 °C. These electronic and local fluctuations drive the bulk phase transition from layered to spinel, which is tightly coupled with a dominant oxygen evolution event (OT1 comprising 74.9% of total O2 release) and a vigorous exothermic response at ~210 °C. This study provides a refined mechanistic view of thermal instability of ultra-high Ni cathodes, offering a blueprint for the design of thermally safe high-nickel cathodes.