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 structural stability is required to devise alternative methods that overcome these challenges. In this study, the thermal evolution of three highly fluorinated phases, Li2TMO2F (TM = Mn, Co, and Ni), is investigated in an inert atmosphere. Diffraction and spectroscopic techniques are utilized to examine their electronic and chemical states up until conditions of decomposition. The analysis reveals that the materials phase-separate above 400 degrees C, at most. It is also observed that heat-treated DRS materials exhibit intricate changes in the local coordination of the metals, including their spin, and ordering compared to the pristine states. The changes upon annealing are accompanied by a modulation of the voltage profile, including reduced hysteresis, when used as electrodes. These results provide an in-depth understanding of the fundamental crystal chemistry of DRS oxyfluorides in view of their promising role as the next generation of Li-ion cathodes. Diffraction and spectroscopy probe the thermal stability of highly fluorinated Li-rich disordered rock salt oxyfluorides (DRS). Prior to decomposition below 500 degrees C, thermal treatments tune their local structure, which affects voltage hysteresis, providing a way to tune their overall performance.image