A XANES study of lithium polysulfide solids: a first-principles study

Abstract

X-ray absorption spectroscopy (XAS) is used for capturing the reaction mechanisms at the molecular level via the determination of local electronic configurations inside Li–S batteries. In this paper, a comprehensive investigation of the S K-edge X-ray absorption near-edge structure (XANES) of a series of stoichiometric lithium polysulfide solids was performed, assuming that these polysulfide solids are formed during the battery cycling process. The results based on density functional theory show that the pre-edge peak in the S K-edge XANES spectra is not a common feature in the solid phase in contrast to the case of pristine polysulfide molecules where the pre-edge peak constitutes a unique signature when dissolved in liquid electrolytes. A general redshift in the S K-edge XANES peak is found when a Li vacancy is introduced in these lithium polysulfide solids. Furthermore, the first peak positions of the S K-edge XANES spectra of the lithium polysulfide solids appear within a small range (2471–2473 eV) overlapping with the white line of the α-S₈ bulk (∼2472 eV). Therefore, the results suggest that the use of additional characterization methods might be necessary for accurately identifying the elusive solid phase lithium polysulfides in Li–S batteries instead of relying only on S K-edge XANES spectroscopy.