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

Abstract

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 indepth 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 (112̅ 0)−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.

Unveiling the Role of Surface Ir-Oxo Species in O2 Evolution at IrO2 Electrocatalysts via Embedded Cluster Multireference Calculations

“…Overall, these results highlight that a reliable description of oxygen evolution requires considering balanced active spaces for the A2 and A3 species, including more MOs of the released oxygen molecule, in a similar fashion as in a free oxygen molecule, namely, the π 2p * π 2p , σ p σ 2p * , and σ 2s σ 2s * MOs (see Figure S7b). The necessity of incorporating all the MOs of oxygen molecules for an accurate description of the release reaction is already emphasized in a prior study . If we exclude the aforementioned active spaces since they do not fulfill these conditions, the EEC­[NEVPT2] energy barrier for oxygen release converges to ∼0.8 eV at larger active spaces CAS­(14,11) and CAS­(18,13) (Figure S8).…”

“…The necessity of incorporating all the MOs of oxygen molecules for an accurate description of the release reaction is already emphasized in a prior study. 38 If we exclude the aforementioned active spaces since they do not fulfill these conditions, the EEC[NEVPT2] energy barrier for oxygen release converges to ∼0.8 eV at larger active spaces CAS (14,11) and CAS(18,13) (Figure S8). The fulfillment of a converged energy barrier reaffirms the appropriateness of the chosen active space.…”

Excited States by Coupling Piris Natural Orbital Functionals with the Extended Random-Phase Approximation

“…Excited states , are important for the description of photochemical and electrochemical processes, fluorescence and phosphorescence phenomena, spectroscopy, and chemical reaction mechanisms, , with a variety of cutting-edge chemical applications such as the development of new materials for organic solar cells and batteries. − The energy of these states can be calculated using the configuration interaction (CI) method; however, this becomes too expensive even for low levels of CI, although some variations have been developed to address this issue . There are other electronic structure tools that allow studying excited species at a more affordable cost, such as the equation-of-motion coupled-cluster (EOM-CC) − approach, the time-dependent density functional theory (TD-DFT), and the random-phase approximation, but the accuracy achieved is not as good as desired and the picture becomes more complex when it comes to static correlation, since multireference methods are required. , …”

A Computational Study on Halogen/Halide Redox Mediators and Their Role in 1O2 Release in Aprotic Li–O2 Batteries

“…In fact, the application of large voltages provides the chemical energy required for releasing 1 O 2 . A charging voltage of about 3.5–3.6 V vs Li represents a threshold for the release of significant amounts of singlet oxygen, which is formed upon direct, nonmediated oxidation of Li 2 O 2 on carbon-based electrodes. ,− Moreover, RMs can interact with already formed 1 O 2 either by chemical reaction, which progressively destroys the catalytic amount of the mediator, or by physical quenching. In the latter case, singlet oxygen gets deactivated to the triplet state without affecting the chemical nature of the RM.…”