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

Structural reversibility of multiple electron transfer cathodes is the key challenge for the consequent intercalation process, which can double the energy density of current commercialized lithium-ion batteries. VOPO₄ is a promising cathode that can possibly involve two redox transitions of V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺, while in fact the structure easily loses the reversibility with only ~1.2 lithium inserted¹. Here, we report ε-VOPO₄ from hydrothermal precursor with 5 % Mo substituted at V site with improved reversible initial capacity to as much as ~250 mAh/g (total ~1.6 Li) and ~80 % retention for up to 20 cycles at C/25. Faster kinetics and lower hysteresis of Mo substituted VOPO₄ can be attributed to the smaller and less ordered particles as shown by HRTEM images. X-ray absorption and X-ray photoelectron spectroscopy show the Mo⁶⁺ oxidation state with the charge compensated by the corresponding amount of V⁴⁺ confirmed by the magnetic analysis. The Mo⁶⁺ coordination and charge compensation mechanisms are also confirmed by hybrid density functional theory calculations. This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583.

1. Z. Chen, Q. Chen, L. Chen, R. Zhang, H. Zhou, N. A. Chernova and M. S. Whittingham, J. Electrochem. Soc., 2013, 160, A1777