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

Abstract

A new phase, Li_0.78V_0.75O₂, was fabricated for the first time on a large scale using a simple hydrothermal method. The structure of Li_0.78V_0.75O₂ 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. Li_0.78V_0.75O₂ showed structural evolution from cubic symmetry to thermodynamically stable rhombohedral symmetry Li_xV_yO₂ above 800 °C. Furthermore, the electrochemical performance of the as-prepared Li_0.78V_0.75O₂ as an anode for Li-ion batteries was studied for the first time. Galvanostatic battery testing showed that the Li_0.78V_0.75O₂ 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 Li_0.78V_0.75O₂ 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 Li_xV_yO₂ 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.