Chemically lithiated layered VOPO4 by a microwave-assisted hydrothermal method and its electrochemical properties in rechargeable Li-ion batteries and supercapacitor applications

Lithium vanadyl phosphate (LiVOPO4) is a derivative of vanadyl phosphate (VOPO4) with different polymorphs that can be obtained based on employed synthesis approach. LiVOPO4 has recently gained attention due to multielectron electrochemical properties that result from varying vanadium redox activities as electrodes for lithium-ion batteries. In this work, we obtained single crystalline tetragonal layered LiVOPO4 (LVOP) by Li intercalation into solvothermal synthesized polycrystalline tetragonal layered VOPO4.2H2O (VOP) by a microwave-assisted hydrothermal method. We found that intercalation of Li+ into layered VOP resulted in a decrease in the interlayer spacing from 0.726 nm to 0.449 nm as well as a reduction of the vanadium oxidation state (+5 to +4), which resulted in a decrease of the band gap. Electrochemical properties of LVOP were evaluated as both a cathode and an anode in a half-cell lithium-ion battery system. The electrode used as a cathode, demonstrated a high capacity of 265 mAhg−1 at 0.1 C as well as 77 mAhg−1 at a high rate of 2 C. When used as an anode, the material exhibits an initial capacity of 865 mAhg−1 at 0.1 C. The electrode demonstrated good rate capability by maintaining a high capacity of 220 mAhg−1 at a high rate of 10 C. We also demonstrated for the first time that VOP can be utilized as a working electrode in an aqueous-based three-electrode supercapacitor and shows good capacitance properties.