Reactions of Singly-Reduced Ethylene Carbonate in Lithium Battery Electrolytes: A Molecular Dynamics Simulation Study Using the ReaxFF

We have conducted quantum chemistry calculations and gas- and solution-phase reactive molecular dynamics simulation studies of reactions involving the ethylene carbonate (EC) radical anion EC– using the reactive force field ReaxFF. Our studies reveal that the substantial barrier for transition from the closed (cyclic) form, denoted c-EC–, of the radical anion to the linear (open) form, denoted o-EC–, results in a relatively long lifetime of the c-EC– allowing this compound to react with other singly reduced alkyl carbonates. Using ReaxFF, we systematically investigate the fate of both c-EC– and o-EC– in the gas phase and EC solution. In the gas phase and EC solutions with a relatively low concentration of Li+/x-EC– (where x = o or c), radical termination reactions between radical pairs to form either dilithium butylene dicarbonate (CH2CH2OCO2Li)2 (by reacting two Li+/o-EC–) or ester-carbonate compound (by reacting Li+/o-EC– with Li+/c-EC–) are observed. At higher concentrations of Li+/x-EC– in solution, we observe the formation of diradicals which subsequently lead to formation of longer alkyl carbonates oligomers through reaction with other radicals or, in some cases, formation of (CH2OCO2Li)2 through elimination of C2H4. We conclude that the local ionic concentration is important in determining the fate of x-EC– and that the reaction of c-EC– with o-EC– may compete with the formation of various alkyl carbonates from o-EC–/o-EC– reactions.