Silver vanadium phosphorous oxide, Ag₂VO₂PO₄, was used as a bimetallic cathode system to systematically study the impact of the constituents of a composite electrode, including polymeric and conductive additives, on the electrochemical polarization and the bimetallic discharge progression of the battery. Ag₂VO₂PO₄ has been developed as a cathode material intended for use in batteries that power implantable cardiac defibrillators (ICDs). This application demands primary battery systems that are reliable over long periods of implantation and are able to deliver high current pulses to charge the capacitors of the device. With these requirements in mind, primary Li/ Ag₂VO₂PO₄ batteries are particularly promising because of their high capacity (270 mAh/g), excellent rate capability with pulses >30 mA sq. 1/cm, and lower cathode dissolution compared to the commercially used Li/Ag₂V₄O₁₁system.

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.