Understanding the Electrochemical Behavior of Vanadium Dioxyfluoride
Following our work on cryolites Li3MF6 (M=Fe and V) we report now part of our research on oxyfluorides presenting a very interesting example that fills the gap in the MO2F compounds of Group 5 elements: V, Nb and Ta. Up to now NbO2F and TaO2F were known, but no evidences of the existence of VO2F were found. Even although intercalation of lithium into NbO2F has been reported, vanadium is much more interesting owing to its lightness.
We have found a synthesis procedure to obtain pure samples of VO2F. Vanadium dioxyfluoride is isostructural with TiO2F exhibiting a hexagonal structure related to the ReO3 structure.
The investigation of chemical composition, structure and oxidation state of vanadium has been completed with a study of its electrochemical behavior. A long discharge from 4 to 1.5 volts develops ca. 450 mAh/g. The voltage profile during the whole first discharge – charge cycle and further cycling (reversible capacity of 350mAh/g) indicates that VO2F suffers an irreversible phase transformation. The investigation of the phase formed down to 1.5 V is now under progress. However, we are also focusing on the high voltage region where a kinetically limited reaction is likely occurring. Between 4 and 2.25 V a reversible cell capacity of 250 mAh/g is developed. This result is outstanding inasmuch as kinetics limitation arising from the highly insulating character of fluorides and many oxyfluorides tend to inhibit the intercalation of lithium, favoring instead convertible reactions at lower voltage.