Thermodynamics and Kinetics of the Multi-Electron ε-VOPO4 cathode Across Two Lithium Insertion

LiVOPO₄ is one of the few polyanion cathode materials that can reversibly insert/extract more than one lithium ion per transition metal and has a very high theoretical capacity of 318 mAh/g. Here, we present a detailed first principles and experimental investigation on ε-Li_xVOPO₄ (0 ≤ x ≤ 2) to understand its structural evolution, thermodynamics and kinetics during lithium intercalation. We show that while no intermediate phases are predicted to be stable in the range of 0 ≤ x ≤ 1, first principles calculations predict two intermediate phases in the low voltage regime (1 ≤ x ≤ 2), in agreement with recent experiments.¹ These intermediate phases are confirmed by V L-edge soft XAS and XRD data. The calculated voltage profile over 0 ≤ x ≤ 2 is also in excellent agreement with experiments. We predict decreasing phase stability with the incorporation of two Li into ε-VOPO₄, suggesting that there are inherent challenges to achieving stable two-electron cycling without structural modification. We will also present a comprehensive analysis of the kinetics (ionic and electronic) of ε-Li_xVOPO₄over the entire lithiation range. This work was supported as part of 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-SC0001294.

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