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.

TLDR: The structural transformations that occur when FeF(3) is cycled at room temperature in a Li cell were investigated using a combination of X-ray diffraction, pair distribution function (PDF) analysis, and magic-angle-spinning NMR spectroscopy, which indicated local structures that are similar to the intercalation phases seen during the first stage of the discharge process.

Abstract: 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 (∼3 nm) Fe metal is formed, as indicated by the XRD and N...