Fingerprinting mean composition of lithium polysulfide standard solutions by applying high energy resolution fluorescence detected X-ray Absorption Spectroscopy

In a lithium/sulfur (Li/S) battery, the reduction of sulfur during discharge involves a particular mechanism, where the active material successively dissolves into the electrolyte to form lithium polysulfide intermediate species (Li2Sx), with x being a function of the state of charge. In this work, sulfur K-edge Resonant Inelastic X-ray Scattering measurements were performed for the characterization of different Li2Sx polysulfide standard solutions. High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy allowed clear separation the pre-edge absorption peak corresponding to terminal sulfur atoms from the main absorption peak due to internal atoms, and to evaluate quantitatively the evolution of the peak area ratio as a function of the polysulfide chain length. Results of this experimental work demonstrate that the normalized area of the pre-edge is a reliable fingerprint of Li2Sx mean chain length in agreement with recent theoretical predictions. As a perspective, this work confirms that operando HERFD XAS can be used to differentiate mean polysulfide composition, which is key issue in the characterization of Li/S cells.