Hydrothermal Synthesis, Structural Transformations and Electrochemistry of Lithium Vanadyl Phosphates

Abstract

The polymorph of lithium vanadyl phosphate ε-LiVOPO4 is a promising multi-electron intercalation cathode material for Li ion batteries with theoretical capacity of 330 mAh/g. It has been shown to reversibly intercalate and deintercalate Li ions over two redox potentials corresponding to V3+/4+ and V4+/5+ oxidation states. Here we report a low-temperature (LT) hydrothermal synthesis method that produces pure LiVOPO4·2H2O between 120 and 160 oC and pure ε-LiVOPO4 between 180 and 220 oC. We also investigate the structure, electrochemical and thermal properties of the products focusing on ε-LiVOPO4. X-ray diffraction indicates that the lattice parameters of LT ε-LiVOPO4 change linearly with synthesis temperature increase, approaching those reported for high-temperature (HT) synthesized ε-LiVOPO4. Thermogravimetric analysis with mass-spectroscopy (TGA-MS) indicates water loss at 350 °C decreasing from 1.5 to 0.5 wt.% as the synthesis temperature increases from 180 to 220 °C. We have interpreted these observations as an evidence of proton intercalation into the LT ε-LiVOPO4 structure. We have further investigated the process of water loss by in-situ XRD and x-ray absorption spectroscopy (XAS) upon heating, combined with pair-distribution function analysis (PDF) and magnetic measurements of LT ε-LiVOPO4 heated to 350 and 750 °C. In-situ XRD shows abrupt change in the lattice parameters dependences upon temperature at 350 °C. Ex-situ studies indicate that the structure and properties of LT samples heated to 750 °C resemble those synthesized by the solid state method at 750 °C. Interesting reversible change of lattice symmetry upon heating to 550 °C was observed and will be further discussed. LT ε-LiVOPO4 has large particle size, over 10 μm, therefore high-energy ball-milling with carbon is required to bring the particle size to below 1 μm to achieve good electrochemistry. Ball-milled product shows high initial capacity around 300 mAh/g, which holds for about 10 cycles and then gradually decays. The voltage profile of LT ε-LiVOPO4 is sloping upon delithiation, in contrast with the step-like profile of HT ε-LiVOPO4. The detailed distinctions between LT and HT ε-LiVOPO4 leading to variations in the electrochemical performance will be discussed. This work was supported as part of the North East Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583.