Identifying the Local Structures Formed during Lithiation of the Conversion Material, Iron Fluoride, in a Li Ion Battery: A Solid-State NMR, X-ray Diffraction, and Pair Distribution Function Analysis Study

The structural transformations that occur when FeF3 is cycled at room temperature in a Li cell were investigated using a combination of X-ray diffraction (XRD), pair distribution function (PDF) analysis, and magic-angle-spinning NMR spectroscopy. Two regions are seen on discharge. The first occurs between Li = 0 and 1.0 and involves an insertion reaction. This first region actually comprises two steps: First, a two-phase reaction between Li = 0 and 0.5 occurs, and the Li0.5FeF3 phase that is formed gives rise to a Li NMR resonance due to Li+ ions near both Fe3+ and Fe2+ ions. On the basis of the PDF data, the local structure of this phase is closer to the rutile structure than the original ReO3 structure. Second, a single-phase intercalation reaction occurs between Li = 0.5 and 1.0, for which the Li NMR data indicate a progressive increase in the concentration of Fe2+ ions. In the second region, the conversion reaction, superparamagnetic, nanosized (similar to 3 nm) Fe metal is formed, as indicated by the XRD and NMR data, along with some LiF and a third phase that is rich in Li and F. The charge process involves the formation of a series of intercalation phases with increasing Fe oxidation state, which, on the basis of the Li NMR and PDF data, have local structures that are similar to the intercalation phases seen during the first stage of the discharge process. The solid-state NMR and XRD results for the rutile phase FeF2 are presented for comparison, and the data indicate that an insertion reaction also occurs, which is accompanied by the formation of LiF. This is followed by the formation of Fe nanoparticles and LiF via a conversion reaction.