A mechanism for Li-S battery operation with a composite electrode and an adsorption additive obtained by using operando UV/Vis spectroscopy and X-ray absorption spectroscopy confirms the role of the adsorption additive and reflects the conversion mechanism of sulfur into Li 2 S. Operando UV/Vis spectroscopy shows a reversible appearance of the long-chain polysulfides in the separator in the fifth cycle, whereas the appearance of mid-and short-chain polysulfides suggests a polysulfide shuttle mechanism. By using a non-sulfur-containing electrolyte, a high-precision analysis of sulfur K-edge XANES and EXAFS spectra is possible. The XANES analysis shows that polysulfides reach the maximum concentration at the end of the high-voltage plateau, and the low-voltage plateau is characteristic of the polysulfides/Li 2 S equilibrium. The relative amount of Li 2 S increases linearly until the end of discharge and reaches a relative amount of 75 %. This is confirmed by sulfur K-edge EXAFS analysis. Additionally, a quantitative analysis of EXAFS spectra measured during discharge evidences a decrease of the average S-S coordination number. This can be interpreted as a decrease of the chain length of polysulfides. EXAFS analysis showed that there are no specific interactions of the polysulfide species with the matrix or with other species in the electrolyte.

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