The effect of anions, PF6- and ClO4-, on the oxidation decomposition mechanism of electrolyte solvent ethylene carbonate (EC) for lithium ion battery has been investigated by using density functional theory at B3LYP/6-311++G(d) level, in both gas phase (dielectric constant epsilon = 1) and solvent (dielectric constant epsilon = 20.5). It is found that the presence of PF6- and ClO4- anions significantly reduces EC oxidation stability and changes the order of the oxidation decomposition paths. The primary oxidation decomposition products of EC, EC-PF6- and EC-ClO4- are CO2 and acetaldehyde radical. The formation of CO is highly improbable in the decomposition reaction of EC with and without PF6- or ClO4-. (C) 2012 Elsevier B.V. All rights reserved.