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 a-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 a-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-O-2 batteries.

Automated summary

In this study, potential decomposition mechanisms of amide and urea solvents were investigated using density functional theory (DFT). Proton and hydrogen abstraction reactions were examined, and the preferred abstraction sites were identified. It was found that the N-H bond of secondary amides and the α-hydrogen atoms were more susceptible to proton abstraction. Additionally, hydrogen abstraction was more favorable at the N-alkyl substituents and α-hydrogen atoms, resulting in the formation of secondary and tertiary radicals. The presence of a coordinating Li+ ion affected the proton abstraction energies, but not the hydrogen abstraction energies. The study also examined the initial steps of a Baeyer-Villiger oxidation type mechanism and found that barriers for these steps were lower with HO2- as the oxidant compared to LiO2-. A comparison with previous experimental results indicated that no single reaction could be identified as the rate-limiting step for predicting the performance of these solvents in Li-O-2 batteries.