Ex Situ and Operando Characterization of Energy Storage Materials By Benchtop XAFS and XES

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Advanced x-ray spectroscopies permit the interrogation of a material’s local electronic structure in an element-specific manner. In particular, these techniques reveal the speciation and ligand environment of an electroactive element and can provide direct insight into the state-of-charge and state-of-health of a battery. Despite their utility, the scientific impact of advanced x-ray spectroscopies is necessarily constrained by the availability of instrumentation. For x-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES), studies are traditionally performed at synchrotron x-ray facilities. These facilities frequently serve to push the forefront of science and, as a result, necessarily operate under an access model which excludes projects which require routine analytical characterization, rapid feedback, or regular access. Over the last four years, our group at the University of Washington has developed several families of laboratory-based instruments to expand the accessibility of advanced x-ray spectroscopies. For the study of transition metal chemistry, we have constructed two Rowland circle spectrometers based upon related designs. Both spectrometers provide energy resolutions comparable to that observed at a synchrotron endstation. Their designs are also highly efficient, allowing studies to be completed with commercial x-ray tube sources in time scales relevant for materials research programs. For many such studies, the instrumentation can generate satisfactory spectra in a matter of minutes, permitting operando studies of battery materials at charge rates in excess of 1C. From the standpoint of instrumentation, a brief review of the present designs is presented with emphasis on recent advances in extended-XAFS capabilities. Differences between the spectrometer designs and the advantages of each will also be highlighted. Regarding materials inquiry, a survey of the above instrument’s application to energy storage research is provided. This will include XAFS analyses of two systems of nanoparticle compounds which were engineered to be catalysts or pseudocapacitors. These results will facilitate a review of the information attainable from the underlying technique. The rich electronic structure of vanadyl phosphate cathode materials, as well a suite of vanadium oxides, is probed via valence-to-core XES (VTC-XES). Finally, operando x-ray absorption near-edge structure (XANES) results obtained from a pouch cell containing a nickel-rich NMC cathode at a variety of charge rates are presented.