Interfacial Effects of Electrochemical Lithiation of Epsilon-VOPO4 and Evolution of the Electronic Structure

The epsilon polymorph of vanadyl phosphate ε-VOPO₄ is a promising cathode material for high-capacity Li ion batteries, owing to its demonstrated ability to reversibly incorporate more than one lithium per redox center, with a total theoretical capacity of 331 mAh/g. As in the case of the olivines, the problem of the inherently low electronic conductivity of ε-VOPO₄ is overcome by the use of nano-sized particles within the cathode. At these dimensions the electrochemical reaction can be largely affected by the interfacial chemistry at the nanoparticle surface. An investigation of these surface reactions is critical for obtaining a complete understanding of the mechanism by which lithium intercalates and how it relates to the electrochemical performance.

         We performed x-ray photoelectron spectroscopy using both soft (XPS) and hard (HAXPES) x-rays to chemically distinguish and depth-resolve the interfacial phase transitions as a function of electrochemical discharge. Core level analysis supports a straightforward two-phase reaction as the first lithium is intercalated. In contrast, the insertion of the second lithium is more complicated with evidence of a significant radial lithium gradient, due to a possible to a disruption of the kinetics.  From inspection of the valence band region, we were able to monitor the reversible evolution of ε-VOPO₄ to Li₂VOPO₄ at the surface of our nanoparticles.  These assignments were supported by hybrid density functional theory of the three phases.  The origin of the radial gradient is likely associated with the presence of stable intermediate phases during the second reaction and warrants further attention.  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.