Synthesis and crystallographic study of homeotypic LiVPO4F and LiVPO4O

LiVPO4F stands as an attractive positive electrode material for the next generation of Li-ion batteries to power electric and hybrid electric vehicles and is homeotypic to LiVIVPO4O with a Tavorite-type crystal structure. We report here on the full comparative structural determination of the crystal structures of LiVPO4F and LiVPO4O, thanks to X-ray and neutron diffraction data. Chains of distorted octahedra [VO4X2] (X = F, O) are connected through PO4 tetrahedra forming 3D frameworks with different types of tunnels. ***F***V***F*** sequences are encountered in LiVIIIPO4F with homogeneous V***F distances along the chains whereas successive short and long V***O distances characteristic of vanadyl bonds are observed in LiVIVPO4O. Combining chemical analyses, magnetic measurements, and X-ray and neutron diffraction, it was possible to propose optimized synthesis routes with a controlled high purity for the samples thus formed, as well as to fully describe their structure up to the localization of the lithium ions in five oxygen (fluorine) coordinated environments. LiVPO4F and LiVPO4O are electrochemically active versus lithium insertion or extraction, showing complex and, for one of them, uncommon redox processes. In particular, very distinct values are observed for the same V4+/V3+ redox couple in LiVIIIPO4F and LiVIVPO4O: 4.26 V vs Li0 upon Li+ extraction from LiVIIIPO4F versus on average 2.20 V vs Li0 upon Li+ insertion into LiVIVPO4O. These results highlight that in these Tavorite-type vanadium compounds the Vn+/V(n-1)+ redox couples can be tuned over a wide range of values vs Li0