Lithium-sulfur (Li/S) batteries are attractive substitutes for current lithium-ion batteries (LIBs) because of the lower cost and higher theoretical capacity of the cathode materials compared to the current transition metal oxide cathode materials in LIBs. However, the polysulfide (PS) redox shuttle causes severe capacity fading of Li/S batteries during cycling, and a strong self-discharge behavior. Although there can be a lot of efforts to reduce or prevent the PS redox shuttle, the development of additives to cover the surface of the cathode is the most efficient way to resolve the PS redox shuttle in the Li/S battery. The passivation film generated by electrolyte additives can not only reduce the oxidation of electrolyte solvents on the surface of the sulfur cathode but also inhibit the dissolution of PS from the cathode into the electrolyte. Few studies have been carried out for this purpose in Li/S batteries even when there have been a variety of studies in LIBs for high-voltage cathode materials. In order to understand how surface films are generated, oxidation potentials and oxidative decomposed products should be studied first. In this study, the oxidation potentials of electrolyte solvents, Li salts, and additives are calculated using the density functional theory (DFT) with a continuum solvation model. Together with electrolyte solvents and Li salts in Li/S batteries, the mostly employed cathode passivation additives in Li-ion batteries and in Li/S batteries are investigated for additives. In addition, it is also investigated how oxidation potentials of these electrolyte systems are varied by complexations with Li⁺, anions of Li salts, and S₈ and pyrene as model systems to mimic the sulfur-carbon electrode materials in Li/S batteries.