The role of 1,3-propane sultone (PS) as an electrolyte additive for lithium ion batteries is explained by investigating the electroreductive decomposition of PS and (PS)Li⁺(PC)_n (n = 0,1) with the aid of density functional theory calculations. In the gas phase, the PS reductive decomposition is thermodynamically unfavorable as supported by the positive Gibbs free energy change and the negative gas phase vertical electron affinity values for the addition of an electron to give the radical anion intermediate. However, it is possible that PS can undergo one- as well as two-electron reduction processes in bulk solvent. The origin of this difference is explained by examining the frontier molecular orbitals of PS and its reduction intermediate both in solution and gas phase. A solvated PS is reduced prior to PC to give a stable intermediate which then undergo decomposition to yield a more stable primary radical. The products from the termination reactions of the primary radical (Li₂SO₃, (CH–CH₂–CH₂–OSO₂Li)₂, and Li–C carbides) and (PC–Li(O₂S)O(CH₂)₃)₂ from the reduction of (PC)–Li⁺(PS) would build up an effective solid electrolyte interphase.