Redox Chemistry and Reversible Structural Changes in Rhombohedral VO₂F Cathode during Li Intercalation

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Metal oxyfluorides are promising for high-energy-density rechargeable batteries.
• Rhombohedral VO2F enables two-electron transfer for high theoretical capacity.
• The operational mechanism and structural changes of VO2F during cycling remain unclear.

Purpose of the Study:

• To elucidate the crystal structure and electrochemical mechanism of VO2F during lithium insertion.
• To investigate the role of anions in the charge compensation during battery operation.
• To challenge the assumption of fluoride acting as a spectator ion.

Main Methods:

• Synchrotron X-ray and neutron diffraction for crystal structure analysis.
• In operando high angular synchrotron diffraction for real-time monitoring.
• Rietveld refinements of neutron powder diffraction data.
• Analysis of electronic state changes for redox evaluation.

Main Results:

• Lithium insertion up to one ion follows a solid-solution reaction.
• Lithiated states adopt the noncentrosymmetric R3c framework with octahedral Li-(O/F)6 coordination.
• Fluoride anions actively participate in charge compensation via covalent interactions with V 3d states.

Conclusions:

• The study clarifies the structural and electronic changes of VO2F during lithium cycling.
• It provides evidence for the active role of fluoride in redox reactions, contrary to prior beliefs.
• This work deepens the understanding of metal oxyfluorides for advanced battery applications.