Synthesis and Crystallographic Study of Homeotypic LiVPO₄F and LiVPO₄O

LiVPO 4 F 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 LiV IV PO 4 O with a Tavorite-type crystal structure. We report here on the full comparative structural determination of the crystal structures of LiVPO 4 F and LiVPO 4 O, thanks to X-ray and neutron diffraction data. Chains of distorted octahedra [VO 4 X 2 ] (X = F, O) are connected through PO 4 tetrahedra forming 3D frameworks with different types of tunnels. ···F···V···F··· sequences are encountered in LiV III PO 4 F with homogeneous V···F distances along the chains whereas successive short and long V···O distances characteristic of vanadyl bonds are observed in LiV IV PO 4 O. 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. LiVPO 4 F and LiVPO 4 O 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 V 4+ /V 3+ redox couple in LiV III PO 4 F and LiV IV PO 4 O: 4.26 V vs Li 0 upon Li + extraction from LiV III PO 4 F versus on average 2.20 V vs Li 0 upon Li + insertion into LiV IV PO 4 O. These results highlight that in these Tavorite-type vanadium compounds the V n + /V ( n –1)+ redox couples can be tuned over a wide range of values vs Li 0 .