Investigating the Discharge Mechanism of Electrodes Using in-Situ Energy Dispersive X-Ray Diffraction
Authors: Kirshenbaum Kevin Charles , Marschilok Amy C , Takeuchi Kenneth J , Takeuchi Esther S 
Published: February 2020

Polyanionic electrode materials such as lithium iron phosphate (LiFePO4) have demonstrated significant success, but are hampered by low conductivity.  Conductive additives such as carbon are often used and are most frequently employed by coating or simply being mixed with the active material.  These additives have shown success, however they decrease the volumetric capacity of the electrode.  Bimetallic materials like silver vanadium oxide (Ag2V4O11) offer the ability to discharge multiple electrons per formula unit cell, generate a conductive network in-situ and thus provide high volumetric energy density.

The design of bimetallic materials that form conductive networks in-situ can be used to eliminate the need for conductive additives.  In these materials, a polyanionic metal center can be combined with a metal that reduces to form a conductive metal.  By utilizing active cathode materials that form conductive networks in-situ we can reduce or potentially eliminate the need for conductive additives that do not add to the capacity of the cell and require additional processing, thereby reducing processing complexity while providing a greater overall energy density.

In one such polyanionic material, silver vanadium diphosphate (Ag2VP2O8), both the vanadium and silver ions are reduced during discharge [1].  The silver metal formation provides a conductive matrix which allows for higher power output.  By determining the spatial location of silver as well as the discharge conditions under which the silver network is formed we can optimize the formation of this network, thereby improving performance of these electrodes [2].  In this study we use the combination of in-situ energy dispersive x-ray diffraction (EDXRD) and ex-situ x-ray absorption spectroscopy (XAS) measurements to spatially resolve the formation of the conductive silver network in Ag2VP2O8electrodes and to determine the effect of increased current on the discharge processes.

Li/ Ag2VP2O8 coin cells were discharged to various depths of discharge and left intact for in-situ EDXRD measurements performed at the National Synchrotron Light Source at Brookhaven National Laboratory.  The height of the incident beam was set to 20 μm, and the width of the slits on the detector was set so that the gauge volume (volume of cathode measured) was 0.02 x 2 x 2 mm3 (see Fig. 1A).  The coin cells were placed on a 3-axis stage so that diffraction patterns as a function of position could be obtained.  Fig. 1B shows an example of EDXRD spectra obtained within the cathode as a function of beam position.  By measuring the intensity of the Ag and Ag2VP2O8peaks we can determine the spatial distribution of silver metal within the cathode.

In addition to determining the location of silver, we can separate the Ag+ to Ag0 and V4+ to V3+reduction processes using XAS, the results of which are shown in Fig. 1C.  By measuring the fractions of reduced silver and vanadium in cells discharged at different rates, we find that silver is reduced preferentially at lower discharge rates, and conversely at higher rates more vanadium is reduced.

By combining the results from spatially-resolved EDXRD and XAS measurements, it is possible to determine the conditions under which a more isotropic silver network is formed.  In addition to materials where conductive networks are formed in-situ, these techniques have applications for a broader range of electrode materials.  Using these techniques it may be possible to determine the effect of discharge rate on the reduction processes in any material with multiple metal centers or structural changes as a function of discharge, and we will present our results in this broader context.

1.  E. S. Takeuchi, C. Y. Lee, P. J. Cheng, M. C. Menard, A. C. Marschilok, K. J. Takeuchi, Silver vanadium diphosphate Ag2VP2O8: Electrochemistry and characterization of reduced material providing mechanistic insights. J Solid State Chem
200, 232-240 (2013); published online EpubApr (DOI 10.1016/j.jssc.2013.01.020).

2.  K. C. Kirshenbaum, D. C. Bock, Z. Zhong, A. C. Marschilok, K. J. Takeuchi, E. S. Takeuchi, In situ profiling of lithium/Ag2VP2O8 primary batteries using energy dispersive X-ray diffraction. Physical chemistry chemical physics : PCCP
16, 9138-9147 (2014); published online EpubApr 16 (10.1039/c4cp01220h).

Figure 1