Structural and Chemical Evolution of Highly Fluorinated Li-rich Disordered Rocksalt Oxyfluorides as a Function of Temperature

Li-rich disordered rocksalt (DRS) oxyfluorides have emerged as promising high-energy cathode materials for lithium-ion batteries. While a high level of fluorination in DRS materials offers performance advantages, it can only be achieved via mechanochemical synthesis, which poses challenges of reproducibility and scalability. The definition of relationships between fluorination and thermal stability is required to devise alternative methods that overcome these challenges. In this study, we investigated the thermal stability of three highly fluorinated Li2MO2F (M=Mn, Co, and Ni) in inert atmosphere. We utilized diffraction and spectroscopic techniques to examine changes in their electronic and chemical states up until conditions of decomposition. The analysis revealed that the materials phase-separate above 400C, at most. We also observed that heat-treated DRS materials exhibited intricate changes in local coordination and ordering compared to the pristine states. Furthermore, the heat-treated materials displayed a reduced voltage hysteresis on the electrochemical voltage profiles compared to pristine states. These results provide an in-depth understanding of the fundamental crystal chemistry of DRS materials in view of their promising role as next generation of Li-ion cathodes.