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

Li-air batteries are a promising energy storage technology for large-scale applications, but the release of highly reactive singlet oxygen (¹O₂) during battery operation represents a main concern that sensibly limits their effective deployment. An in-depth understanding of the reaction mechanisms underlying the ¹O₂ 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 ¹O₂ at the Li₂O₂ surface during oxidation, i.e., the battery charging process. Based on recent hypothesis, we depict a feasible O₂^2-/O₂^-/O₂ mechanisms occurring from the (112̅0)-Li₂O₂ surface termination. Our highly accurate calculations allow for the identification of a stable superoxide as local minimum along the potential energy surface (PES) for ¹O₂ release, which is not detected by periodic DFT. We find that ¹O₂ 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 Li₂O₂ 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 ¹O₂ for new and highly performing Li-air batteries.