A new comparative investigation of lithium sulfur cells employing a tetraethylene glycol dimethyl ether–lithium trifluoromethanesulfonate (TEGDME-LiCF₃SO₃) electrolyte charged by various polysulfide species (Li₂S₂, Li₂S₄, Li₂S₆, and Li₂S₈) is here reported. We carefully detect the effects of lithium polysulfide addition by originally combining X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The measurements clearly reveal how the polysulfide addition affects the nature and composition of the solid electrolyte interphase (SEI) in terms of precipitated S-based species determined by XPS. The study demonstrates that the SEI layer formed on the Li anode decreases in impedance and stabilizes by the presence of polysulfide. This, together with a buffer effect strongly mitigating the sulfur-cathode dissolution and the shuttle reaction, significantly improves the stability of the lithium–sulfur cell. The data here reported clearly suggest the polysulfide as an effective additive to enhance the performance of the lithium–sulfur battery.