Facile synthesis and electrochemistry of a new cubic rocksalt LixVyO2 (x=0.78, y=0.75) electrode material

A new phase, Li0.78V0.75O2, was fabricated for the first time on a large scale using a simple hydrothermal method. The structure of Li0.78V0.75O2 was identified by Rietveld refinement of the powder X-ray diffraction (XRD) data. XRD identified that the new Li-V-O compound had a cubic rocksalt structure, in which Li and V were evenly distributed on the octahedral sites in a cubic closely packed lattice of oxygen ions. Li0.78V0.75O2 showed structural evolution from cubic symmetry to thermodynamically stable rhombohedral symmetry LixVyO2 above 800 degrees C. Furthermore, the electrochemical performance of the as-prepared Li0.78V0.75O2 as an anode for Li-ion batteries was studied for the first time. Galvanostatic battery testing showed that the Li0.78V0.75O2 electrode exhibited a large reversible capacity, high rate performance and excellent cycling stability. A reversible capacity of 575 mA h g(-1) was maintained even after 500 cycles at a current density of 500 mA g(-1). Ex situ XRD and XPS studies on the lithiation mechanism of Li0.78V0.75O2 consistently suggested that two single-phase insertion-type lithiation processes proceed during the early stages of lithiation. Hence, a single-phase conversion reaction is followed, along with structural integration. The improved electrochemical performance can be ascribed to the unusual conversion-type lithiation process, which is not consistent with the intercalation process of LixVyO2 reported previously. More importantly, the lithiation and delithiation behavior suggests that the oxidation of V(II) to V(IV) upon cycling produces more suitable sites for Li+ insertion, which contributes to the enhanced specific capacity.