X‑ray Absorption Spectra of Dissolved Polysulfides in Lithium–Sulfur Batteries from First-Principles

The X-ray absorption spectra (XAS)
of lithium polysulfides (Li2Sx) of various chain lengths (x) dissolved in a model
solvent are obtained from first-principles
calculations. The spectra exhibit two main absorption features near
the sulfur K-edge, which are unambiguously interpreted as a pre-edge
near 2471 eV due to the terminal sulfur atoms at either end of the
linear polysulfide dianions and a main-edge near 2473 eV due to the
(x – 2) internal atoms in the chain, except
in the case of Li2S2, which only has a low-energy
feature. We find an almost linear dependence between the ratio of
the peaks and chain length, although the linear dependence is modified
by the delocalized, molecular nature of the core-excited states that
can span up to six neighboring sulfur atoms. Thus, our results indicate
that the ratio of the peak area, and not the peak intensities, should
be used when attempting to differentiate the polysulfides from XAS.