Understanding the Electrochemical Behavior of Vanadium Dioxyfluoride

After intensive investigation of light transition metal oxides as electrodes for lithium ion rechargeable batteries oxides new chemistries based on different ligands are needed to fulfill performance requirements in highly energy demanding devices.  In this context transition metal (TM) fluorides have been investigated as both convertible and intercalation electrodes. In the latter case the main drawback is the high insulating behavior arising from the high ionic character of M-F bond. However, the increment of intercalation potential with respect to similar oxides can be advantageous. In some cases the intercalation potential has been in fact reported to exceed the electrochemical stability window of the commonly used liquid electrolytes. The partial substitution of oxygen by fluorine may have a more controlled effect on potential rising. Proper doping with fluorine may produce an increase of the intercalation potential high enough to get a significant increase of specific energy without exceeding the high voltage stability limit of the electrolyte. Nonetheless, stoichiometric control is not easily achieved. On the other hand, crystal chemistry differences between oxygen and fluorine may provide new crystalline structure with relevant topotactical features for intercalation. Note that among TM fluorides a large variety of structures are found, ranging from crystal structure with isolated M-F₆ octahedra to true tridimensional structures.

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.