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