New Amorphous Iron-Based Oxyfluorides as Cathode Materials for High-Capacity Lithium-Ion Batteries

A novel two-step strategy to prepare amorphous oxyfluorides, containing both divalent and trivalent 3d-metals, as cathode materials for lithium batteries is presented. The first step involves the preparation of hydrated fluorides M2+M3+2F8(H2O)2 (M2+ = Mn, Fe, Co, Ni, Cu; M3+ = V, Fe) by microwave-assisted solvothermal synthesis. Besides the MnFe2F8(H2O)2 and CuFe2F8(H2O)2 phases, three new compounds, Fe1.3V1.7F8(H2O)2, CoFe2F8(H2O)2, and NiFe2F8(H2O)2, which are isostructural with Fe3F8(H2O)2, have been unraveled. The second step consists in the decomposition of M2+M3+2F8(H2O)2 into amorphous oxyfluorides M2+M3+2F8–2xOx via suitable thermal treatments.. The amorphous materials show a greater electrochemical activity toward Li than their parent phases, among them being CuFe2F6O, which displays the best performance as a cathode material with a first discharge capacity of 310 mAh·g–1. We show that such a large capacity results from cumulative insertion and displacement reactions.