Battery Relevant Electrochemistry of Ag₇Fe₃(P₂O₇)₄: Contrasting Contributions from the Redox Chemistries of Ag⁺ and Fe³⁺

Ag₇Fe₃(P₂O₇)₄ is an example of an electrochemical displacement material which contains two different electrochemically active metal cations, where one cation (Ag⁺) forms metallic silver nanoparticles external to the crystals of Ag₇Fe₃(P₂O₇)₄ via an electrochemical reduction displacement reaction, while the other cation (Fe³⁺) is electrochemically reduced with the retention of iron cations within the anion structural framework concomitant with lithium insertion. These contrasting redox chemistries within one pure cathode material enable high rate capability and reversibility when Ag₇Fe₃(P₂O₇)₄ is employed as cathode material in a lithium ion battery (LIB). Further, pyrophosphate materials are thermally and electrically stable, desirable attributes for cathode materials in LIBs. In this paper, a bimetallic pyrophosphate material Ag₇Fe₃(P₂O₇)₄ is synthesized and confirmed to be a single phase by Rietveld refinement. Electrochemistry of Ag₇Fe₃(P₂O₇)₄ is reported for the first time in the context of lithium based batteries using cyclic voltammetry and galvanostatic discharge-charge cycling. The reduction displacement reaction and the lithium (de)insertion processes are investigated using ex situ X-ray absorption spectroscopy and X-ray diffraction of electrochemically reduced and oxidized Ag₇Fe₃(P₂O₇)₄. Ag₇Fe₃(P₂O₇)₄ exhibits good reversibility at the iron centers indicated by ∼80% capacity retention over 100 cycles following the initial formation cycle and excellent rate capability exhibited by ∼70% capacity retention upon a 4-fold increase in current.