Sulfur speciation in Li–S batteries determined by operando laboratory X-ray emission spectroscopy

In this work, operando sulfur X-ray emission measurementson a Li−S battery cathode were performed using a laboratory setup as analternative to more common synchrotron radiation based absorptionstudies. Photoexcitation by an X-ray tube was used. Valence-to-core Kβ X-ray emission spectra were recorded with a wavelength dispersive crystalspectrometer in von Hamos geometry, providing excellent energyresolution and good detection efficiency. The setup was used to recordex situ S Kβ emission spectra from S cathodes from the Li−S battery andalso under operando conditions. Average S oxidation state within thebattery cathode during battery cycling was determined from the shape ofthe Kβ emission spectra. A more detailed S species characterization wasperformed by fitting a linear combination of previously measuredlaboratory synthesized standards to the measured spectra. Relative amounts of different S species in the cathode were determinedduring the cycling of the Li−S battery. The main advantage of X-ray emission spectroscopy is that it can be performed onconcentrated samples with S loading comparable to a real battery. The approach shows great promise for routine laboratory analysisof electrochemical processes in Li−S batteries and other sulfur-based systems under operando conditions.