Operando Synchrotron XRD Investigation of Silver Metal Formation upon Electrochemical Reduction of Silver Iron Pyrophosphate (Ag₇Fe₃(P₂O₇)₄)

The formation of conductive metallic silver upon electrochemical reduction and lithiation of Ag₇Fe₃(P₂O₇)₄ is investigated. Alternating current impedance spectroscopy measurements show a 34% decrease in charge transfer resistance upon one electron equivalent (ee) of reduction, which is coincident with the formation of a Ag metal conductive network evidenced by both ex situ and operando X-ray diffraction. Quantitative assessment of Ag metal formation derived from operando XRD shows that only Ag⁺ ions are reduced during the first 3ee, followed by simultaneous reduction of Ag⁺ and Fe³⁺ reduction for the next 5ee (3ee to 8ee), culminating in reduction of the remaining Ag⁺. Scanning electron microscopy images show smaller Ag metal crystallite size and shorter nearest neighbor distance between and among Ag particles with higher depth of discharge. A high rate intermittent pulsatile discharge test is conducted where the cell delivers 12 total pulses during full discharge to probe the effect of Ag metal formation on the Li/Ag₇Fe₃(P₂O₇)₄ cell electrochemistry. The Ohmic resistance is derived from the voltage drop of each pulse. The resistance is 65 Ω initially, reaches its minimum of 26 Ω at 4.5 ee discharge, and levels off at 35 Ω after 7.0 ee reduction. The initial Ag reduction is more significant for the conductive network formation indicated by the decrease of both R_ct and Ohmic resistance, which facilitates the high power output of the cell.