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

We have investigated the possibility of molybdenum substitution into {$\epsilon$}-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 {$^\circ$}C with further annealing at 550 {$^\circ$}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 {$\epsilon$}-VOPO4 and significantly improved electrochemical behavior. {$\epsilon$}-V0.95Mo0.05OPO4 shows the initial reversible capacity {$\sim$}250 mAh/g ({$\sim$}1.6 Li) and {$\sim$}80\% retention for up to 20 cycles at C/25. Sloping voltage profile, faster kinetics, and lower voltage hysteresis of Mo substituted VOPO4 are demonstrated by the galvanostatic intermittent titration technique. This enhanced electrochemical performance is attributed to the smaller particles and possible existence of partial LixMoyV1-yOPO4 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.