Structural phase transformation and Fe valence evolution in FeO _xF_2-x/C nanocomposite electrodes during lithiation and de-lithiation processes

In this study, the structural changes of FeO_xF_2-x/C during the first discharge and recharge cycles were studied by ex situ electron microscopy techniques including annular dark field scanning transmission electron microscopy (DF-STEM) imaging, selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) as well as by in situ X-ray absorption spectroscopy (XAS). The evolution of the valence state of Fe was determined by combined EELS using the Fe-L edge and XAS using the Fe-K edge. The results of this investigation show that the conversion reaction path during 1^st lithiation is very different from the re-conversion path during 1^st delithiation. During lithiation, intercalation is first observed followed by conversion into a lithiated rocksalt (Li-Fe-O-F) structure, and metallic Fe and LiF phases. During delithiation, the rocksalt phase does not disappear, but co-exists with an amorphous (rutile type) phase formed initially by the reaction of LiF and Fe. However, a de-intercalation stage is still observed at the end of reconversion similar to a single phase process despite the coexistence of these two (rocksalt and amorphous) phases.