Density Functional Theory Study of the Role of Anions on the Oxidative Decomposition Reaction of Propylene Carbonate

The oxidative decomposition mechanism of the lithium battery electrolyte solvent propylene carbonate (PC) with and without PF6- and ClO4- anions has been investigated using the density functional theory at the B3LYP/6-311++G(d) level. Calculations were performed in the gas phase (dielectric constant epsilon = 1) and employing the polarized continuum model with a dielectric constant epsilon = 20.5 to implicitly account for solvent effects. It has been found that the presence of PF6- and ClO4- anions significantly reduces PC oxidation stability, stabilizes the PC-anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. The primary oxidative decomposition products of PC-PF6- and PC-ClO4- were CO2 and acetone radical. Formation of HF and PF5 was observed upon the initial step of PC-PF6- oxidation while HClO4 formed during initial oxidation of PC-ClO4-. The products from the less likely reaction paths included propanal, a polymer with fluorine and fluoro-alkanols for PC-PF6- decomposition, while acetic acid, carboxylic acid anhydrides, and Cl- were found among the decomposition products of PC-ClO4-. The decomposition pathways with the lowest barrier for the oxidized PC-PF6- and PC-ClO4- complexes did not result in the incorporation of the fluorine from PF6- or ClO4- into the most probable reaction products despite anions and HF being involved in the decomposition mechanism; however, the pathway with the second lowest barrier for the PC-PF6- oxidative ring-opening resulted in a formation of fluoro-organic compounds, suggesting that these toxic compounds could form at elevated temperatures under oxidizing conditions.