Electrochemical reduction of Ag_0.48vopo₄: A mechanistic study employing X-ray absorption spectroscopy and X-ray powder diffraction

The electrochemical reduction of Ag_0.48VOPO₄ · 1.9H₂O is accompanied by vanadium and silver oxidation state changes, characterized with X-ray absorption spectroscopy (XAS), and by structural changes, characterized with X-ray powder diffraction (XRD). The XAS data suggest that the initial reduction process, involving 0 to 0.5 electron equivalents, involved primarily the reduction of vanadium cations, while most of the silver cations are reduced between 0.5 to 1.0 electron equivalents. The XRD data display significant intensity decreases of absorbances associated with the 004 and 006 planes upon electrochemical reduction, consistent with a reduction-displacement of Ag⁺ with insertion of Li⁺. Retention of intensity of the absorbance associated with the 002 plane, with only minor decrease in interlayer spacing, indicates retention of the VOPO₄ sublattice structure. Uncovering the details of the discharge mechanism of bimetallic cathode materials such as Ag_0.48VOPO₄ should enable the design of future high current cathodes for secondary batteries displaying an enhanced current capacity based on a reduction-displacement strategy.