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

In this work, valence-to-core (VtC) Kβ 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 Kβ XES spectra obtained from ab initio quantum chemical calculations based on density functional theory. The emphasis is given to the Kβ sulfur XES spectra of the polysulfide molecules (Li2S_x, x = 2,…,8) produced electrochemically within the Li–S battery. Ab initio molecular dynamics calculations are used further to calculate also the Kβ spectra of Li₂S_x 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 Li₂S_x 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.