Increased energy density is required for rechargeable batteries to keep up with the consumer demand for electronic devices, electric vehicles, and smart grids. Current lithium ion battery technology utilizes cathodes with high voltage and capacity against a carbon anode, shuttling Li ions back and forth on charge and discharge in a non-aqueous electrolyte. No new commercially viable cathodes with greater capacity than those identified by Goodenough and co-workers¹ have been yet found, with no real increase in capacity over the original LiCoO₂ composition since 1980. Options to increase energy density in Li-ion batteries can be summarized by increasing the voltage or lowering the molar mass of the active material in the cathode. Families of compounds that have yet to be investigated are scarce, however one such family are those containing fluorides. By introducing a more electronegative anion, higher oxidation states of transition metals can be stabilized which opens up the opportunity of higher redox potentials as well as the possibility of more than one Li⁺ ion deintercalation per transition metal; a phenomenon yet to be observed in Li-ion batteries. Lithium-transition metal-fluorides have potential as candidates for high voltage, low molar mass cathode materials, resulting in high energy density.
Thus far, it has only been shown that lithium intercalation into Li₃MF₆ (M^III to M^II) structures has been possible^2,3, while the elusive reversible deintercalation (M^III to M^IV) has not been achieved. Our preliminary studies show that in certain conditions, reversible deintercalation of these family of compounds is possible providing the synthetic methods are carefully controlled. Previously reported synthetic routes for these class of materials utilized aqueous precipitation methods and sol-gel processing methods involving water. It is already a well-established principle that at high voltages, run-away reactions between the electrolyte and water can occur. By synthesizing these materials in a completely air and water free environment, we have observed evidence of reversible oxidation and changes in crystals structure of Li₃VF₆ through electrochemical analysis at 4.4V vs Li. Changes in the oxidation state of the central transition metal were confirmed by hard x-ray absorption spectroscopy and changes in the crystal structure were shown using high resolution x-ray diffraction. We are currently investigating this family of materials with other transition metals and anticipate these compounds will show similar results.
These promising results have given hope to redox chemistry at much higher operating voltages than has been previously observed for fluoride compounds.

1. K. Mizushima, P. C. Jones, P. J. Wiseman, and J. B. Goodenough, Mater. Res. Bull., 15, 783–789 (1980).
2. A. Basa, E. Gonzalo, A. Kuhn, and F. García-Alvarado, J. Power Sources, 207, 160–165 (2012).
3. G. Lieser et al., J. Power Sources, 294, 444–451 (2015).