Thermodynamics, Kinetics and Structural Evolution of ε-LiVOPO4 over Multiple Lithium Intercalation

In this work, we demonstrate the stable cycling of more than one Li in solid-state-synthesized epsilon-LiVOPO4 over more than 20 cycles for the first time. Using a combination of density functional theory (DFT) calculations, X-ray pair distribution function (PDF) analysis and X-ray absorption near edge structure (XANES) measurements, we present a comprehensive analysis of the thermodynamics, kinetics, and structural evolution of epsilon-LixVOPO4 over the entire lithiation range. We identify two intermediate phases at x = 1.5 and 1.75 in the low-voltage regime using DFT calculations, and the computed and electrochemical voltage profiles are in excellent agreement. Operando PDF and EXAFS techniques show a reversible hysteretic change in the short (<2 angstrom) V-O bond lengths coupled with an irreversible extension of the long V-O bond (>2.4 angstrom) during low-voltage cycling. Hydrogen intercalation from electrolyte decomposition is a possible explanation for the similar to 2.4 angstrom V-O bond and its irreversible extension. Finally, we show that e epsilon-LixVOPO4 is likely a pseudo-1D ionic diffuser with low electronic conductivity using DFT calculations, which suggests that nanosizing and carbon coating is necessary to achieve good electrochemical performance in this material.