Composition Dependent Phase Development of the High Capacity Conversion Cathode, FeO_xF_2-x

Conversion electrodes for lithium and sodium ion batteries are promising due to their high capacities compared to traditional intercalation electrodes, and because their mechanism of charge storage allows for greater compositional flexibility. The complete reduction of the metal upon lithium insertion yields this high capacity and flexibility, but it hinders the regeneration of the starting material. Improving the cyclability of these electrodes requires understanding the phase changes of these electrodes in operando. Iron oxyfluoride (FeO_xF_2-x) is an attractive candidate material because of its high theoretical capacity (885 mAh g-1) and because of iron’s natural abundance. We’ll discuss how the O:F ratio can also be tuned during synthesis to balance the high capacity of oxides and the high voltage windows of fluorides. Next, we’ll investigate how the composition of FeO_xF_2-x impacts the phase development of the material during battery cycling. To do so, we’ll use cyclic voltammetry and pair distribution function analysis of total scattering data to provide electrochemical behavior along with local structure information. Insights from this work will allow for modifications of FeO_xF_2-x and similar conversion electrodes to improve their cyclability without sacrificing their capacities.