Synthesis, Structure and Electrochemistry of Lithium Vanadium Phosphate Cathode Materials

The Li-ion battery positive electrode materials, α-LiVOPO4, β-LiVOPO4, and Li3V2(PO4)3 have been prepared as individual phases from a common precursor mixture by controlling the synthesis conditions. These phases result because of their distinct crystal symmetries and V oxidation states. The synthesis involves the decomposition of a precursor mixture prepared from NH4VO3, NH4H2PO4, LiF and hexanoic acid which when heat-treated under different conditions produced the three LiwVxOy(PO4)z products. The materials were characterized by means of elemental analysis, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and Li cell data. The Li cycling performance and the accompanying structural effects on α- and β-LiVOPO4 were also studied under normal and deep discharge conditions using XAS to probe the local vanadium environment. XANES and EXAFS confirm reversible changes in the two structures when intercalation does not exceed 1Li/VOPO4. Deeper discharge revealed more disruption to both structures, including an additional 0.25 Å increase to the V=O bond length. A greater range in the VO6 symmetry of α-LiVOPO4 resulted in a more flexible accommodating host, in agreement with its improved low voltage performance compared to β-LiVOPO4.