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