Evidence for the existence of Li2S2 clusters in lithium–sulfur batteries: ab initio Raman spectroscopy simulation

Abstract

Using density functional theory calculations and ab initio molecular dynamics simulations we have studied the structures and the Raman spectra of Li₂S₄ clusters, which are believed to be the last polysulfide intermediates before the formation of Li₂S₂/Li₂S during the discharge process in Li–S batteries. Raman spectra have been obtained using a new technique to estimate polarizabilities using Wannier functions. We have observed clear evidence of Li₂S₄ → Li₂S₂ transition by studying systematic changes in the simulated Raman spectra of (Li₂S₄)_n, n = 1, 4, and 8 towards that of (Li₂S₂)₈. Furthermore, we have shown that the dominant Raman peak of the Li₂S₂ cluster at ∼440 cm⁻¹ arises from sulfur–sulfur stretching mode. This peak has been experimentally observed in the discharged state of Li–S batteries and has also been attributed to the formation of Li₂S₂. We have also demonstrated that the transition is mainly due to the strong electrostatic interactions between Li₂S₄ monomers, which results in energy lowering by arranging the local Li^+δ–S^−δ dipole moments in an anti-parallel fashion.