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 M 2+ M 3+ 2 F 8 (H 2 O) 2 (M 2+ = Mn, Fe, Co, Ni, Cu; M 3+ = V, Fe) by microwave-assisted solvothermal synthesis. Besides the MnFe 2 F 8 (H 2 O) 2 and CuFe 2 F 8 (H 2 O) 2 phases, three new compounds, Fe 1.3 V 1.7 F 8 (H 2 O) 2, CoFe 2 F 8 (H 2 O) 2, and NiFe 2 F 8 (H 2 O) 2, which are isostructural with Fe 3 F 8 (H 2 O) 2, have been unraveled. The second step consists in the decomposition of M 2+ M 3+ 2 F 8 (H 2 O) 2 into amorphous oxyfluorides M 2+ M 3+ 2 F 8–2 x O x via suitable thermal treatments . . The amorphous materials show a greater electrochemical activity toward Li than their parent phases, among them being CuFe 2 F 6 O, 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.