Vanadium Oxyfluoride/Few-Layer Graphene Composite as a High-Performance Cathode Material for Lithium Batteries

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• 作者：Musa Ali Cambaz ; B. P. Vinayan ; Oliver Clemens ; Anji Reddy Munnangi ; Venkata Sai Kiran Chakravadhanula ; Christian Kübel ; Maximilian Fichtner
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：April 18, 2016
• 年：2016
• 卷：55
• 期：8
• 页码：3789-3796
• 全文大小：518K
• 年卷期：0
• ISSN：1520-510X

文摘

Metal oxyfluoride compounds are gathering significant interest as cathode materials for lithium ion batteries at the moment because of their high theoretical capacity and resulting high energy density. In this regard, a new and direct approach is presented to synthesize phase-pure vanadium oxyfluoride (VO₂F). The structure of VO₂F was identified by Rietveld refinement of the powder X-ray diffraction (XRD) pattern. It crystallizes in a perovskite-type structure with disorder of the oxide and fluoride ions. The as-synthesized VO₂F was tested as a cathode material for lithium ion batteries after being surface-coated with few-layer graphene. The VO₂F delivered a first discharge capacity of 254 mA h g^–1 and a reversible capacity of 208 mA h 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 W h kg^–1. Improved rate capability that outperforms the previous report has been achieved, showing a discharge capacity of 150 mA h g^–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 an irreversible structural change of the anion lattice from ³/₄ cubic close packing to hexagonal close packing to accommodate the inserted lithium ions while keeping the overall space-group symmetry. For the first time we have revealed a structural change for the ReO₃-type structure of as-prepared VO₂F to the RhF₃ structure after lithiation/delithiation, with structural changes that have not been observed in previous reports. Furthermore, the new synthetic approach described here would be a platform for the synthesis of new oxyfluoride compounds.