Mixed anion chemistry as a way to tune the electrochemical properties of vanadium oxyfluoride phosphates in alkali-ion batteries

Abstract

Alkali vanadium fluoride phosphates such as Tavorite LiVPO4F, Na3V2(PO4)2F3 and KTP-type KVPO4F are attractive materials for positive electrodes in rechargeable alkali-ion batteries. In such compounds, the presence of fluoride ions further increases the electrode's electrochemical potential compared to their phosphate counterparts such as Na3V2(PO4)3. Furthermore, the fluoride anions in these structures can be fully or partially substituted by oxide anions, leading to the formation of V4+=O vanadyl centres and a great number of mixed valence and mixed anion compositions. The oxygen substitution for fluorine has a considerable impact on the structural, electronic, and electrochemical properties of the resulting compounds. In this review, the similarities and peculiarities in the crystal structures, electronic structures, and electrochemical properties of the three analogous LiVPO4F–LiVPO4O, Na3V2(PO4)2F3–Na3V2(PO4)2FO2, and KVPO4F−KVPO4O material families will be compared in detail. Such a comparison is expected to provide an insightful understanding on the factors governing the structure and properties of vanadium oxyfluoride phosphates.