Degradation of an Ethylene Carbonate/Diethyl Carbonate Mixture by Using Ionizing Radiation

The reactivity of ethylene carbonate (EC) and of a EC/diethyl carbonate (DEC) mixture was studied under ionizing radiation to mimic the aging phenomena that occur in lithium-ion batteries. Picosecond-pulse radiolysis experiments showed that the attachment of the electron to the EC molecule is ultrafast (k(e^-_EC +EC)=1.3×10⁹  L mol^-1  s^-1 at 46 °C). This reaction rate is accelerated by a factor of 5.7 compared with the electron attachment to propylene carbonate, which implies that the presence of the methyl group significantly slows the reaction. In a 50:50 EC/DEC mixture, just after the electron pulse the electron is solvated by a mixture of EC and DEC molecules, but its fast decay is attributed exclusively to electron attachment to the EC molecule. Stable products detected after steady-state irradiation were mainly H₂ , CH₄ , CO, and CO₂ . The evolution of the radiolytic yields with the EC fraction shows that H₂ and CH₄ did not exhibit linear behavior, whereas CO and CO₂ did. Indeed, H₂ and CH₄ mainly arise from the excited state of DEC, the formation of which is significantly affected by the evolution of the dielectric constant of the mixture and by the electron attachment to EC. CO formation is mainly due to the reactivity of the EC molecule, which is not affected in the mixture, as proven by pulse-radiolysis experiments.