Sulfur valence-to-core X-ray emission spectroscopy study of lithium sulfur batteries

In this work, valence-to-core (VtC) K beta sulfur X-ray emission spectroscopy (XES) was used to perform quantitative analysis of different sulfur compounds produced in a lithium sulfur (Li-S) battery during discharge. The analysis is based on the theoretical sulfur K beta XES spectra obtained from ab initio quantum chemical calculations based on density functional theory. The emphasis is given to the K beta sulfur XES spectra of the polysulfide molecules (Li2Sx, x = 2,MIDLINE HORIZONTAL ELLIPSIS,8) produced electrochemically within the Li-S battery. Ab initio molecular dynamics calculations are used further to calculate also the K beta spectra of Li2Sx dissolved in a model solvent. Calculated spectra were directly compared with the experimental ones collected with a Johansson type tender XES spectrometer on laboratory synthesized Li2Sx reference standards and pre-cycled battery cathodes. These results demonstrate that sulfur VtC XES can be used effectively to quantitatively analyze electrochemical sulfur conversion, also in a smaller laboratory without the need for large scale synchrotron facilities.