Understanding electrochemical behavior requires inquiry of the material fundamental properties as well as the behavior of the material when configured into a fully functioning device.  Thus, the ability to use local electrochemical probes and integrate the results with system level behavior is needed.  In this work, quantitative electrochemistry was conducted on individual particles of vanadium phosphorous oxides, (specifically SVPO, Ag_wV_xP_yO_z) cathode materials. Silver ions incorporated into the layered vanadium phosphorous oxide structure facilitated monitoring of the reaction progress through the formation of silver metal.  The results obtained from the single particle were related to the systems level through in-situ examination of the reduction progress in fully assembled cells probed through energy dispersive x-ray diffraction, EDXRD.

  At the particle level, individual particles of active material, Ag₂VO₂PO₄, were electrochemically reduced to various depths of discharge.  After the material was reduced, the conductivity of the individual particles was determined using a nanoprobe integrated with a scanning electron microscope, Fig. 1.  The individual particles showed a drastic reduction in local resistance when discharged.

At the system level, in situ energy dispersive X-ray spectroscopy (EDXRD) was used to probe Li/ Ag₂VP₂O₈ cells at several stages of reduction, allowing depth profile analysis of the discharge process. This technique enables micron scale spatial resolution of reduced Ag₂VP₂O₈ and Ag⁰ discharge products in the bulk electrode.  Results suggest the formation of a 3-dimensional Ag⁰ percolation network that increases conductivity throughout the entire electrode, occurs concurrently with a substantial decrease in the charge transfer resistance. The resistance of the reduced cathode is dependent upon discharge rate even in the presence of conductive additives.

Development of the next generation of battery systems with improved energy utilization requires accessing all of the active material in an electrode.  The multiscale inquiry methodology described provides a template for future investigations of both particle level and system level properties of electrochemically discharged materials highlighting the importance of advanced instrumental techniques to gain insight into complex discharge processes.