Lithium metal anodes covered by a thin (6–10 Å) solid electrolyte interphase (SEI) film at its initial nucleation stage modeled as a single component (Li2O, LiF, Li2SO4, and Li3N) are examined with density functional theory and ab initio molecular dynamics simulations. The combined metal anode/SEI surfaces are exposed to an electrolyte containing salt, solvent, and lithium polysulfides. The polysulfide species are found to be easily reduced by the metal slab, even in the presence of the model SEI films. Although the chemical nature of the film and geometry of the exposed facet induces different decomposition kinetics, the reaction mechanisms are shown to be similar for the various SEI models, and end with the formation of Li2S on the surface of the anode or inside the SEI film. The density of states of the films at this nascent stage of SEI formation in contact with the Li metal surface are very different than those of the bulk crystals, usually showing new intermediate states between the metal and conduction bands, which suggests a change to an electronically conductive character that favors continuous reactivity.