UC Berkeley

In cation-disordered rocksalt Li-ion cathode materials, an excess of Li with respect to the transition metal content is necessary for the creation of percolating pathways for Li transport. Because of the lower amount of redox-active transition metal, a substantial part of the charge transfer must occur via less reversible oxygen redox. Fluorination can be used to minimize this dependence on oxygen redox by increasing the amount of low-valent transition metal in the compound, but it adds complexity to materials design. Here, we investigate the feasibility of using computationally constructed phase diagrams to facilitate the search for optimal oxyfluorides. We use the phase diagram of LiF-Li3NbO4-NiO to identify Li1.13Ni0.57Nb0.3O1.75F0.25 and Li1.19Ni0.59Nb0.22O1.46F0.54 as two promising compositions and demonstrate that they can be successfully synthesized. These compounds exhibit significantly reduced hysteresis and higher energy density than the previously reported Li1.3Ni0.27Nb0.43O2 compound in this space. Although we generally attribute the improved performance to the increased Ni content enabled by fluorination, a more nuanced relation between fluorination and the cycling behavior is revealed through electrochemical tests, X-ray absorption spectroscopy, solid-state nuclear magnetic resonance spectroscopy, and density functional theory. We find that fluorination increases the voltage, improves cycle life, but reduces the accessibility of Ni redox. Consideration of these effects will facilitate the future design of optimized disordered-rocksalt oxyfluoride cathodes.