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

We prepared LiF/Fe(V2O5) nanocomposites with varying (0-20 wt.%) V2O5 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.% V2O5. 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 similar to 35% FeF2, and during the second discharge, the initial V3.9+ oxidation state was reduced to V3.5+. Using principal component analysis, we decomposed the series of Fe K-edge spectra into three components consisting of Fe, FeF2 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 Fe-57 Mossbauer spectroscopy to be the inverse spinel V[FeV]O-4. From the calculations, we also identified the lithium vanadate LixVO2-xFx. Both LixVO2-xFx 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(V2O5) nanocomposites is discussed. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.