Ag2VO2PO4 is especially well suited for studies of discharge progression in bimetallic cathode systems. Notably, it can be discharged as a pure electroactive material in the absence of a conductive additive as it generates an in situ conductive matrix via a reduction displacement reaction resulting in the formation of Ag0 nanoparticles. The electrochemical reduction of Ag+ and V5+ in Ag2VO2PO4 has previously been determined to take place sequentially at low rates, with reduction of Ag+ to Ag0 through a reduction-displacement reaction occurring first but with a small level of V5+ reduction occurring concurrently. The Ag0nanoparticles formed can be detected by diffraction based techniques and can be spatially resolved by in situ Energy Dispersive X-ray Diffraction (EDXRD) to assess the homogeneity of discharge.
EDXRD provides unique insight into the state of the electrode by enabling evaluation of discharged phases as a function of spatial location. Furthermore, the white beam energy of the synchrotron light source can penetrate the steel casing of the battery, providing in situ measurements of coin type cells without the need for specially designed X-ray windows.
Three different electrode compositions were investigated: Ag2VO2PO4 only, Ag2VO2PO4 with binder, and Ag2VO2PO4 with binder and carbon. Constant current discharge, pulse testing and impedance spectroscopy measurements were used to characterize the electrochemical properties of the electrodes as a function of depth of discharge. In situ EDXRD was used to spatially resolve the discharge progression within the cathode by following the formation of Ag0. Ex situ X-ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure (EXAFS) modeling were used to quantify the amount of Ag0 formed. Results indicated that the metal center reduced (V5+ or Ag+) were highly dependent on composite composition (presence of PTFE, carbon), depth of discharge (Ag0 nanoparticle formation), and spatial location within the cathode.