Adsorption of insoluble polysulfides Li2Sx (x=1, 2) on Li2S surfaces

In lithium-sulfur batteries, the growth of insulating discharge product Li2S film affects the cathode microstructure and the related electron as well as lithium ion transport properties. In this study, chemical reactions of insoluble lithium polysulfides Li2Sx (x = 1, 2) on crystal Li2S substrate are investigated by a first-principles approach. First-principles atomistic thermodynamics predicts that the stoichiometric (111) and (110) surfaces are stable around the operating cell voltage. Li2Sx adsorption is an exothermic reaction with the (110) surface being more active to react with the polysulfides than the stoichiometric (111) surface. There is no obvious charge transfer between the adsorbed molecule and the crystal Li2S substrate. Analysis of the charge density difference suggests that the adsorbate interacts with the substrate via a strong covalent bond. The growth mechanism of thermodynamically stable surfaces is investigated in the present study. It is found that direct Li2S deposition is energetically favored over Li2S2 deposition and reduction process.