The role of ethylene sulfite (ES) as an electrolyte additive for Lithium ion batteries is explained by investigating the one and two-electron reductive decomposition of ES and (ES)Li+(PC)(n) (n = 0-2), both in vacuum and solvent, with the aid of high-level density functional. theory calculations. The open-chain radical, which is formed as a result of reduction of ES in solvent without first being coordinated with Li+, is further stabilized by a dissolved lithium ion. The resulting more stable intermediate releases a somewhat large amount of energy, which is utilized in the formation of a subsequent radical anion. On the basis of the study on the reductive decomposition of ES, (ES)Li+(PC), and (ES)Li+(PC)(2), the major products that are responsible for the formation of a protective solid electrolyte interphase film are Li2SO3, (CH2OSO2Li)(2), CH3CH(OSO2Li)CH2OCO2Li, and ROSO2Li.