Silver vanadium diphosphate Ag₂VP₂O₈: Electrochemistry and characterization of reduced material providing mechanistic insights

Silver vanadium phosphorous oxides (Ag_wV_xP _yO_z) are notable battery cathode materials due to their high energy density and demonstrated ability to form in-situ Ag metal nanostructured electrically conductive networks within the cathode. While analogous silver vanadium diphosphate materials have been prepared, electrochemical evaluations of these diphosphate based materials have been limited. We report here the first electrochemical study of a silver vanadium diphosphate, Ag₂VP₂O₈, where the structural differences associated with phosphorous oxides versus diphosphates profoundly affect the associated electrochemistry. Reminiscent of Ag₂VO ₂PO₄ reduction, in-situ formation of silver metal nanoparticles was observed with reduction of Ag₂VP₂O ₈. However, counter to Ag₂VO₂PO₄ reduction, Ag₂VP₂O₈ demonstrates a significant decrease in conductivity upon continued electrochemical reduction. Structural analysis contrasting the crystallography of the parent Ag₂VP ₂O₈ with that of the proposed Li₂VP ₂O₈ reduction product is employed to gain insight into the observed electrochemical reduction behavior, where the structural rigidity associated with the diphosphate anion may be associated with the observed particle fracturing upon deep electrochemical reduction. Further, the diphosphate anion structure may be associated with the high thermal stability of the partially reduced Ag₂VP₂O₈ materials, which bodes well for enhanced safety of batteries incorporating this material.