Vanadium Oxyfluoride/Few-Layer Graphene Composite as a High-Performance Cathode Material for Lithium Batteries
Metal oxyfluoride compounds are gathering significant interest as cathode materials for lithium ion batteries at the moment, due to their high theoretical capacity and resulting high energy density. In this regard, a new and direct approach is presented to synthesize vanadium oxyfluorides (VO 2 F), which is phase pure. The structure of VO 2 F was identified by Rietveld refinement of the powder XRD pattern. It crystallizes in perovskite type structure with a disorder of oxide and fluoride ions.
... he as synthesized VO 2 F was tested as a cathode material for lithium batteries after being surface coated with a few layer graphene. The VO 2 F delivered a first discharge capacity of 254 mAh g -1 and a reversible capacity of 208 mAh g -1 at a rate of C/20 for the first 20 cycles with an average discharge voltage of 2.84 V, yielding an energy density of 591 Wh kg -1 . Improved rate capability, which outperforms the previous report, has been achieved showing discharge capacity of 150 mAhg -1 for 1 C. The structural changes during lithium insertion and extraction were monitored by ex-situ XRD analysis of the electrodes discharged and charged to various stages. Lithium insertion results in irreversible structural change of the anion lattice from ¾ cubic close packing to a hexagonal close packing to accommodate the inserted lithium ions, but keeping the overall space-group symmetry. For the first time we revealed a structural change for the ReO 3 type structure of as-prepared VO 2 F to a RhF 3 structure after lithiation /delithiation, with structural changes that have not yet been observed in previous reports. Furthermore, the new synthetic approach described here would be a platform for the synthesis of new oxyfluoride compounds. 3