Ab Initio Study of the Gas- and Liquid-Phase Hydrogen Abstraction From Dimethyl-, Diethyl-, and Ethyl–Methyl Carbonates by H ̇ ${\dot{\mathrm{H}}} $ and C ̇ ${\dot{\mathrm{C}}} $ H3 and Subsequent Reactions

Abstract

Dimethyl-, diethyl, and ethyl–methyl carbonate are important components of lithium batteries. They are used as solvents and comprise the medium through which the lithium ions move between the anode and the cathode during charge and discharge. However, these species are susceptible to decomposition if thermal runaway occurs, forming flammable gases inside the battery, and eventually leading to mechanical failure and ignition with the surrounding air. These events have been reported and are extremely hazardous. To avoid these incidents, it is important to understand the reactivity of carbonates by building chemical kinetic mechanisms based on experimental testing and theoretical calculations. These models are also important when using these species in combustion as additives or replacements to fossil fuels. Because of their high oxygen content, researchers believe that including carbonates in combustion processes would decrease soot and particulate matter emissions. Existing models typically use estimated reaction rate parameters; thus, more accurate rate parameters would benefit existing and new models. In this study, the rate coefficients of H-atom abstraction reactions by $\dot{H}$ and $\dot{C}$ H₃, β-scission, isomerization, and internal radical migration reactions are computed from CCSD(T)/aug-cc-pV(D+T)Z//B3LYP-D3BJ/def2-TZVP calculations. Additionally, solvation effects have been investigated to allow for comparison between liquid and gas phase kinetics. Consistent with the literature, H-atom abstraction by $\dot{H}$ is found to be faster than that by $\dot{C}$ H₃. At the low-temperature end of the investigated range (300 K), available literature rate coefficients and the present rate coefficients are deviating up to three orders of magnitude. Notably, uncertainties in the imaginary frequency computation are found to contribute most to deviations between the present calculations and combined theoretical and experimental literature data for DMC + $\dot{H}$. For the unimolecular reactions of the fuel radicals, β-scission is found to dominate radical consumption and differs from previous analogy-based rates by up to about one order of magnitude. Solvation effects are found to be pronounced for diethyl and ethyl–methyl carbonate, with up to two orders of magnitude faster isomerization in the liquid phase compared to the gas phase. The presented rate coefficients will aid future detailed chemical kinetic modeling.