Revisiting metal fluorides as lithium-ion battery cathodes

Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed occur upon lithiation. We challenge this view by studying FeF3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, revisiting prior experimental studies of FeF2 CuF2. fluoride lithiation is instead dominated diffusion-controlled displacement mechanisms, a clear topological relationship between metal F? sublattices LiF established. Initial forms on particle’s surface, along cation-ordered stacking-disordered phase, A-LixFeyF3, which structurally related ?-/?-LiMn2+Fe3+F6 topotactically transforms B- then C-LixFeyF3, before forming Fe. Lithiation CuF2 results in buffer FeF2/CuF2 LiF. The resulting principles will aid future developments wider range isomorphic fluorides.