Oxidation state and local structure of a high-capacity LiF/Fe(V2O5) conversion cathode for Li-ion batteries

Abstract We prepared LiF/Fe(V 2 O 5 ) nanocomposites with varying (0–20 wt.%) V 2 O 5 by high-energy ball milling and found a stable specific capacity of 450 mA h g −1 for a period of 20 cycles without a noticeable reduction in capacity for the composite with 15 wt.% V 2 O 5 . X-ray diffraction was unable to identify new phases present in the nanocomposite. To identify the phases formed during ball milling and cycling, we collected in situ X-ray absorption spectra at the Fe K- and V K-edges. During the first charge, LiF/Fe was converted to ∼35% FeF 2 , and during the second discharge, the initial V 3.9+ oxidation state was reduced to V 3.5+ . Using principal component analysis, we decomposed the series of Fe K-edge spectra into three components consisting of Fe, FeF 2 and a new phase, which was identified by comparison with theoretical X-ray absorption near edge structure spectra of model compounds with tetrahedral and octahedral V coordination and 57 Fe Mossbauer spectroscopy to be the inverse spinel V[FeV]O 4 . From the calculations, we also identified the lithium vanadate Li x VO 2− x F x . Both Li x VO 2− x F x and V[FeV]O 4 have open crystal structures with the ability to reversibly store lithium in interstitial lattice sites, and the effect of these compounds on capacity and cyclic stability of the LiF/Fe(V 2 O 5 ) nanocomposites is discussed.