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 (CH ₂CH ₂OCO ₂Li) 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 (CH ₂OCO ₂Li) ₂ through elimination of C ₂H ₄. 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.