Amide and Urea Based Solvents for Li–O2 Batteries. Part II: Evaluation of Decomposition Pathways Using Density Functional Theory

Potential decomposition mechanisms are investigated using density functional theory (DFT) for a set of amide and urea solvents. Reaction energies and barriers are reported for proton and hydrogen abstraction, and preferred abstraction sites are identified. The N–H bond of secondary amides and the α-hydrogen atoms are more susceptible to proton abstraction than other sites in the solvent molecules. Additionally, hydrogen abstraction is more favorable at the N-alkyl substituents as well as α-hydrogen atoms that result in the formation of secondary and tertiary radicals. All proton abstraction energies are sensitive to the presence of a coordinating Li⁺, but the hydrogen abstraction energies do not depend on the presence of a Li⁺. The stabilization due to the presence of Li⁺ is most pronounced for sites near the carbonyl group where the Li⁺ interacts with the lone pair of electrons formed by proton abstraction and the carbonyl O atom. The initial steps of a Baeyer–Villiger oxidation type mechanism are also examined. Barriers for these initial steps are significantly lower if HO₂– is the oxidant compared to LiO₂–. A comparison to our previously reported experimental results indicates that no single reaction could be identified as the rate-limiting step that would predict the performance of this set of solvents in Li–O₂ batteries.