Energy Dispersive X-Ray Diffraction (EDXRD): An Effective in-Situ Approach for Spatial Resolution of Electrode Reactions

Abstract

Energy dispersive x-ray diffraction (EDXRD) presents a special opportunity to achieve spatial resolution of an electrochemical process within the electrode inside a functioning electrochemical cell, providing the opportunity for tomographic-like data analysis. White beam radiation coupled with a synchrotron “wiggler” insertion device emits high energy radiation which can penetrate bulk engineering materials such as steel, enabling in-situ measurement of conventionally designed prototype and production level batteries. Unlike other in-situ methods, EDXRD does not require the use of a modified or special cell design. Rather, the method enables a direct probe of electrode reactions in cells fully encased in their steel housings. These studies used a family of AgxVOy(PO4)z materials as model systems to provide insights into the electrochemical reduction process for inherently poorly conducting phosphate and diphosphate based cathode materials. The systems proved to be particularly well suited for EDXRD in-situ studies as the Ag0metal product formed on reduction could be detected in small amounts. In-situ EDXRD was measured at several x-direction locations in several stainless steel coin cells with AgxVOy(PO4)z cathodes discharged to different depths of discharge under several rates. At each location spectra were collected in increments through the thickness of the electrode. The presence of Ag0 as a reduction product in silver vanadium phosphate cathodes had been verified ex situ using x-ray diffraction and scanning electron microscopy experiments. However, in situ measurements of the Ag0 provided the opportunity for further insight into the electrochemical reduction mechanism, regarding the location and distribution of the reduction products.