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

Area of Science:

• Materials Science
• Electrochemistry
• Inorganic Chemistry

Background:

• Metal oxyfluorides are promising cathode materials for lithium-ion batteries due to their high theoretical capacity and energy density.
• Vanadium oxyfluoride (VO2F) is of particular interest, but efficient synthesis methods are needed.

Purpose of the Study:

• To present a new, direct synthesis method for phase-pure vanadium oxyfluoride (VO2F).
• To evaluate the electrochemical performance of VO2F as a cathode material in lithium-ion batteries.
• To investigate the structural changes during lithium ion insertion and extraction.

Main Methods:

• Phase-pure VO2F synthesized via a new direct approach.
• Structure identified using Rietveld refinement of powder X-ray diffraction (XRD) data.
• Electrochemical testing of graphene-coated VO2F as a cathode material.
• Ex-situ XRD analysis to monitor structural evolution during cycling.

Main Results:

• VO2F crystallizes in a perovskite-type structure with disordered oxide and fluoride ions.
• Graphene-coated VO2F delivered a high initial discharge capacity (254 mA h g⁻¹) and reversible capacity (208 mA h g⁻¹ at C/20).
• Achieved an energy density of 591 W h kg⁻¹ and improved rate capability (150 mA h g⁻¹ at 1 C).
• Observed irreversible structural transformation from ReO3-type to RhF3-type upon lithiation/delithiation.

Conclusions:

• The novel synthesis provides a scalable route to phase-pure VO2F.
• Surface-coated VO2F demonstrates excellent electrochemical performance as a lithium-ion battery cathode.
• The revealed structural changes offer fundamental insights into the cycling mechanism of oxyfluoride cathodes.
• This work establishes a platform for developing new metal oxyfluoride materials.