Revealing Reaction Pathways of Collective Substituted Iron Fluoride Electrode for Lithium Ion Batteries.

Metal fluorides present a high redox potential among the conversion-type compounds, which make them specially work as cathode materials of lithium ion batteries. To mitigate the notorious cycling instability of conversion-type materials, substitutions of anion and cation have been proposed but the role of foreign elements in reaction pathway is not fully assessed. In this work, we explored the lithiation pathway of a rutile-FeCoOF cathode with multimodal analysis, including and transmission electron microscopy and synchrotron X-ray techniques. Our work revealed a prolonged intercalation-extrusion-cation disordering process during phase transformations from the rutile phase to rocksalt phase, which microscopically corresponds to topotactic rearrangement of Fe/Co-O/F octahedra. During this process, the diffusion channels of lithium transformed from 3D to 2D while the corner-sharing octahedron changed to edge-sharing octahedron. DFT calculations indicate that the Co and O cosubstitution of the FeCoOF cathode can improve its structural stability by stabilizing the thermodynamic semistable phases and reducing the thermodynamic potentials. We anticipate that our study will inspire further explorations on untraditional intercalation systems for secondary battery applications.