Enhanced electrochemical performance of Li3V2(PO4)3 structurally converted from LiVOPO4 by graphite nanofiber addition

In this work, the structural conversion of LiVOPO4 to Li3V2(PO4)3 due to the addition of graphene nanofiber (GNF) was investigated, and the resulting materials were found to exhibit enhanced capacity and cyclability. First, LiVOPO4 was synthesized using a solid-state method followed by annealing at 900 °C for 12 h under nitrogen atmosphere. Then, the conversion from the triclinic LiVOPO4 structure to the monoclinic Li3V2(PO4)3 structure due to the GNF addition was observed. No impurity peak was observed in the X-ray diffraction patterns of LiVOPO4 or Li3V2(PO4)3, and the structural conversion caused no defects to form in the resulting Li3V2(PO4)3 crystallite. Field emission-scanning electron microscope studies clearly demonstrate that larger corroded-structure-like particles formed which were mixed with GNF. This provided both a large active area and fast transport of lithium ions, which afforded enough active sites for simultaneous intercalation of many lithium ions, leading to improved electrochemical properties of the material. Compared with LiVOPO4, the Li3V2(PO4)3–GNF showed better properties, such as an improved lithium ion diffusion coefficient, improved cyclability, and smaller impedance. Furthermore, the optimized Li3V2(PO4)3–GNF (7%) battery showed the best discharge capacity of 181 mA h g−1 at 0.1 C and lithium ion diffusion coefficient of 6.01×10−9 cm2 s−1.