Correlating structural changes and gas evolution during the thermal decomposition of charged Li_xNi_0.8Co_0.15Al _0.05O₂ cathode materials

In this work, we present results from the application of a new in situ technique that combines time-resolved synchrotron X-ray diffraction and mass spectroscopy. We exploit this approach to provide direct correlation between structural changes and the evolution of gas that occurs during the thermal decomposition of (over)charged cathode materials used in lithium-ion batteries. Results from charged Li_xNi_0.8Co_0.15Al _0.05O₂ cathode materials indicate that the evolution of both O₂ and CO₂ gases are strongly related to phase transitions that occur during thermal decomposition, specifically from the layered structure (space group R3̄m) to the disordered spinel structure (Fd3̄m), and finally to the rock-salt structure (Fm3̄m). The state of charge also significantly affects both the structural changes and the evolution of oxygen as the temperature increases: the more extensive the charge, the lower the temperature of the phase transitions and the larger the oxygen release. Ex situ X-ray absorption spectroscopy (XAS) and in situ transmission electron microscopy (TEM) are also utilized to investigate the local structural and valence state changes in Ni and Co ions, and to characterize microscopic morphology changes. The combination of these advanced tools provides a unique approach to study fundamental aspects of the dynamic physical and chemical changes that occur during thermal decomposition of charged cathode materials in a systematic way.