In my masters thesis, x-ray emission spectroscopy was used to characterise sulphur reduction in lithium-ion batteries during discharge. K\(\alpha\) and K\(\beta\) emission spectra were measured in a standard polysulfide sample and on a static cathode from a battery, stopped at different points during discharge. X-ray emission was induced by a proton beam with energies in a MeV range. Emission spectra were recorded with a Bragg crystal spectrometer with modified Johansson focusing geometry, designed for experiments in tender x-ray range (2-6keV). Experimental energy resolution was comparable to the natural lifetime broadening of the sulphur K emission line. The shape of K\(\alpha\) emission spectra does not depend on the chemical environment of the atom, but a small energy shift was observed for samples with different average oxidation numbers. Chemical sensitivity is more pronounced in the K\(\beta\) emission spectra corresponding to transitions from valence states and directly reflects valence electronic structure. For a qualitative description of the changes in the electron configuration, density functional theory (DFT) was used to calculate ground states of sulphur molecules. We confirmed a linear dependence of the energy shift of K\(\alpha\) emission spectra on the average oxidation state of the sulphur atoms. We applied the linear correlation to the in-operando measurements, to calculate the average oxidation state of sulphur in a lithium-sulphur battery during a discharge.
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