Molybdenum Substituted Vanadyl Phosphate ε-VOPO₄ with Enhanced Two-Electron Transfer Reversibility and Kinetics for Lithium-Ion Batteries

Abstract

We have investigated the possibility of molybdenum substitution into ε-VOPO 4 structure and its effects on the electrochemical performance of this material as a cathode in Li-ion battery. We have found that up to 5% of Mo can substitute V upon hydrothermal synthesis at 180 °C with further annealing at 550 °C. The substitution is confirmed by the increase of the unit cell volume with Mo content. A combination of X-ray absorption and photoelectron spectroscopy, magnetic studies, and density functional theory calculations indicates an Mo 6+ oxidation state which is charge compensated by reduction of the same amount of V to 4+. Mo-substituted samples show much smaller particle size as compared to unsubstituted ε-VOPO 4 and significantly improved electrochemical behavior. ε-V 0.95 Mo 0.05 OPO 4 shows the initial reversible capacity ∼250 mAh/g (∼1.6 Li) and ∼80% retention for up to 20 cycles at C/25. Sloping voltage profile, faster kinetics, and lower voltage hysteresis of Mo substituted VOPO 4 are demonstrated by the galvanostatic intermittent titration technique. This enhanced electrochemical performance is attributed to the smaller particles and possible existence of partial Li x Mo y V 1– y OPO 4 solid solution supported by X-ray diffraction, which leads to less abrupt and completely reversible structure changes upon Li cycling evidenced by X-ray absorption spectroscopy.