Structures and bonding properties of lithium polysulfide clusters LiSn−/0 (n = 3–5) and Li2S4−/0: size-selected anion photoelectron spectroscopy and theoretical calculations

Abstract

The structures and bonding properties of several lithium polysulfide clusters LiS_n^−/0 (n = 3–5) and Li₂S₄^−/0 were investigated by size-selected anion photoelectron spectroscopy coupled with quantum chemistry calculations. The vertical detachment energies of LiS₃⁻, LiS₄⁻, and LiS₅⁻ were estimated to be 2.17 ± 0.08, 3.30 ± 0.08 and 3.66 ± 0.08 eV, respectively, and that of Li₂S₄⁻ was estimated to be 3.21 ± 0.08 eV. It is found that LiS₃⁻ and LiS₃ have planar quadrilateral structures, and LiS₄⁻ and LiS₄ have distorted five-membered ring structures. LiS₅⁻ has a distorted six-membered ring structure while neutral LiS₅ has a book-shaped structure. The lowest-lying structure of Li₂S₄⁻ can be viewed as a S₂ unit connecting to the Li–Li edge of a Li₂S₂ tetrahedron. The lowest-lying structure of neutral Li₂S₄ can be viewed as a S₂ unit connecting to the S atoms of a Li₂S₂ quadrilateral. The natural population analysis (NPA) and electron localization function (ELF) analyses show that the excess electron of LiS_n⁻ is mainly localized over the sulfur chains, especially on the S atoms interacting with Li, thus, the most stable structures of LiS_n⁻ can be regarded as a Li⁺ cation interacting with a S_n²⁻ dianion. The results may be useful for understanding the formation of lithium polysulfides in lithium sulfur batteries.