Molybdenum Substituted Vanadyl Phosphate ε-VOPO4 for Lithium-Ion Batteries

Abstract

Maintaining the structural reversibility of multiple electron transfer cathodes is a challenge one has to address in order to increase the energy density of current lithium-ion batteries. ε-VOPO4 is a promising cathode material with two redox transitions of V3+/V4+ and V4+/V5+ at about 2.5 and 4 V, respectively. However, the reversibility and cycling stability is compromised with more than one lithium cycling. The goal of this work is to investigate if the substitution of molybdenum can enhance the electrochemical performance of ε-VOPO4. We have focused on molybdenum because it is predicted by the first-principle calculations to display multiple redox potentials in useful voltage window and to form relatively stable substituted ε-VOPO4 phases. We have refined the synthesis method, where by changing a solvent we can control morphology and obtain either loose uniform 100 nm particles, or make those particles form secondary micron-sized spheres. Combing the analysis of X-ray diffraction, and inductively coupled plasma mass spectrometry, we have determined the limits of molybdenum substitution in the structure. The effects of the substitution and morphology on the electrochemical performance will be discussed. This research is supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583. CS gratefully acknowledges the financial support from Graduate Assistance in Areas of National Need (GAANN) Fellowship and Binghamton University Provost's Doctoral Summer Fellowship.