Synthesis and characterization of ɛ-VOPO4 nanosheets for secondary lithium-ion battery cathode

Vanadium (III) phosphate monoclinic VPO4·H2O was synthesized hydrothermally. The ɛ-VOPO4 nanosheets, formed by the oxidative de-intercalation of protons from monoclinic VPO4·H2O, can reversibly react with more than 1 mol lithium atoms in two steps. Crystal XRD analysis revealed that the structure of the ɛ-VOPO4 nanosheets is monoclinic with lattice parameters of α=7.2588(4) Å, b=6.8633(2) Å and c=7.2667(4) Å. The results show that the ɛ-VOPO4 nanosheets have a thickness of 200 nm and uniform crystallinity. Electrochemical characterization of the ɛ-VOPO4 monoclinic nanosheets reveals that they have good electrochemical properties at high current density, and deliver high initial capacity of 230.3 mA·h/g at a current density of 0.09 mA/cm2. Following the first charge cycle, reversible electrochemical lithium extraction/insertion at current density of 0.6 mA/cm2 affords a capacity retention rate of 73.6% (2.0-4.3 V window) that is stable for at least 1000 cycles.