Synthesis and electrochemical studies of layer-structured metastable αI-LiVOPO4

Abstract

The layer structured α_I-LiVOPO₄ was obtained via a two step chemical synthesis. In the first step, a hydrated phase, LiVOPO₄·2H₂O, was obtained by a simple hydrothermal route at 120 °C. Single crystal X-ray diffraction analysis revealed the structure of LiVOPO₄·2H₂O to be orthorhombic with lattice parameters: a = 8.9454(7) Å, b = 9.0406(7) Å and c = 12.7373(10) Å. Dehydration of the parent compound led to its structural transformation to tetragonal α_I-LiVOPO₄, which was only identified previously during the lithium insertion in VOPO₄. We have investigated the solid-state dehydration of LiVOPO₄·2H₂O and proposed a possible mechanism for the crystal structure transformation. Electrochemical characterization of this rarely studied tetragonal phase revealed its good lithium cycling at high operating voltage. Galvanostatic charge–discharge cycling of α_I-LiVOPO₄ was studied in a voltage window of 2.5–4.5 V, which shows a stable reversible capacity of 103(±3) mA h g⁻¹ at a current density of 16 mA g⁻¹ (0.1 C). At higher current rates, although it exhibited good cyclability, the capacity was found to decrease with increasing current rates. The long term cycling stability of the above material was demonstrated at a current rate of 0.5 C up to 200 cycles.