New Insights on Singlet Oxygen Release from Li-Air Battery Cathode: Periodic DFT Versus CASPT2 Embedded Cluster Calculations

[Image: see text] Li-air batteries are a promising energy storage technology for large-scale applications, but the release of highly reactive singlet oxygen ((1)O(2)) during battery operation represents a main concern that sensibly limits their effective deployment. An in-depth understanding of the reaction mechanisms underlying the (1)O(2) formation is crucial to prevent its detrimental reactions with the electrolyte species. However, describing the elusive chemistry of highly correlated species such as singlet oxygen represents a challenging task for state-of-the-art theoretical tools based on density functional theory. Thus, in this study, we apply an embedded cluster approach, based on CASPT2 and effective point charges, to address the evolution of (1)O(2) at the Li(2)O(2) surface during oxidation, i.e., the battery charging process. Based on recent hypothesis, we depict a feasible O(2)(2–)/O(2)(–)/O(2) mechanisms occurring from the (112̅0)–Li(2)O(2) surface termination. Our highly accurate calculations allow for the identification of a stable superoxide as local minimum along the potential energy surface (PES) for (1)O(2) release, which is not detected by periodic DFT. We find that (1)O(2) release proceeds via a superoxide intermediate in a two-step one-electron process or another still accessible pathway featuring a one-step two-electron mechanism. In both cases, it represents a feasible product of Li(2)O(2) oxidation upon battery charging. Thus, tuning the relative stability of the intermediate superoxide species can enable key strategies aiming at controlling the detrimental development of (1)O(2) for new and highly performing Li-air batteries.