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.