Electrochemical reduction of silver vanadium phosphorous oxide, Ag₂VO₂PO₄: Silver metal deposition and associated increase in electrical conductivity

This report details the chemical and associated electrical resistance changes of silver vanadium phosphorous oxide (Ag₂VO₂PO₄, SVPO) incurred during electrochemical reduction in a lithium based electrochemical cell over the range of 0-4 electrons per formula unit. Specifically the cathode electrical conductivities and associated cell DC resistance and cell AC impedance values vary with the level of reduction, due the changes of the SVPO cathode. Initially, Ag⁺ is reduced to Ag⁰ (2 electrons per formula unit or 50% of the calculated theoretical value of 4 electrons per formula unit) accompanied by significant decreases in the cathode electrical resistance, consistent with the formation of an electrically conductive silver metal matrix within the SVPO cathode. As Ag⁺ reduction progresses, V⁵⁺ reduction initiates; once the SVPO reduction process progresses to where the reduction of V⁵⁺ to V⁴⁺ is the dominant process, both the cell and the cathode electrical resistances then begin to increase. If the discharge then continues to where the dominant cathode reduction process is the reduction of V⁴⁺ to V³⁺, the cathode and cell electrical resistances then begin to decrease. The complex cathode electrical resistance pattern exhibited during full cell discharge is an important subject of this study.