In Situ Electrochemical XAFS Studies on an Iron Fluoride High-Capacity Cathode Material for Rechargeable Lithium Batteries

The reactions and structural evolution of FeF 3 during cell cycling are investigated in an in situ cell by using Fe K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The results of X-ray absorption near-edge structure spectroscopic analysis demonstrate that there are three stages in the reaction of FeF 3 with Li: (1) a two-phase intercalation reaction in the range of x = 0 to 0.46 Li, (2) a single-phase intercalation reaction in the range of x = 0.46 to 0.92 Li, and (3) a conversion reaction in the range of x = 0.92 to 2.78 Li. The coordination numbers ( CN s) and bond lengths of the Fe–F bonds or Fe–Fe bonds for the lithiated FeF 3 are obtained by performing XAFS fitting. The splitting trends of the Fe–F bond lengths and the Fe–F CN s in the range of x = 0 to 0.92 Li support the proposal that R -3 c -structured FeF 3 is transformed into R 3 c -structured Li 0.92 FeF 3 after the intercalation of 0.92 equiv. of Li, and that the intermediate Li 0.46 FeF 3 may be R 3-structured. The small Fe–Fe CN of Li 2.78 FeF 3 indicates that the average diameter of the Fe crystallites formed during discharge is <1 nm.