Sulfur Speciation in Li-S Batteries Determined by Operando Laboratory X-ray Emission Spectroscopy

In this work, operando sulfur X-ray emission measurements on a Li-S battery cathode were performed using a laboratory setup as an alternative to more common synchrotron radiation based absorption studies. Photoexcitation by an X-ray tube was used. Valence-to-core Kβ X-ray emission spectra were recorded with a wavelength dispersive crystal spectrometer in von Hamos geometry, providing excellent energy resolution and good detection efficiency. The setup was used to record ex situ S Kβ emission spectra from S cathodes from the Li-S battery and also under operando conditions. Average S oxidation state within the battery cathode during battery cycling was determined from the shape of the Kβ emission spectra. A more detailed S species characterization was performed by fitting a linear combination of previously measured laboratory synthesized standards to the measured spectra. Relative amounts of different S species in the cathode were determined during the cycling of the Li-S battery. The main advantage of X-ray emission spectroscopy is that it can be performed on concentrated samples with S loading comparable to a real battery. The approach shows great promise for routine laboratory analysis of electrochemical processes in Li-S batteries and other sulfur-based systems under operando conditions.