Pathways to Triplet or Singlet Oxygen during the Dissociation of Alkali Metal Superoxides: Insights by Multireference Calculations of Molecular Model Systems

Abstract

Recent experimental investigations demonstrated the generation of singlet oxygen during charging at high potentials in lithium/oxygen batteries. To contribute to the understanding of the underlying chemical reactions a key step in the mechanism of the charging process, namely, the dissociation of the intermediate lithium superoxide to oxygen and lithium, was investigated. Therefore, the corresponding dissociation paths of the molecular model system lithium superoxide (LiO2) were studied by CASSCF/CASPT2 calculations. The obtained results indicate the presence of different dissociation paths over crossing points of different electronic states, which lead either to the energetically preferred generation of triplet oxygen or the energetically higher lying formation of singlet oxygen. The dissociation to the corresponding superoxide anion is energetically less preferred. The understanding of the detailed reaction mechanism allows the design of strategies to avoid the formation of singlet oxygen and thus to potentially minimize the degradation of materials in alkali metal/oxygen batteries. The calculations demonstrate a qualitatively similar but energetically shifted behavior for the homologous alkali metals sodium and potassium and their superoxide species. Fundamental differences were found for the covalently bound hydroperoxyl radical.

Study of the Electronic Structure of Alkali Peroxides and Their Role in the Chemistry of Metal–Oxygen Batteries

“…The tendency to favor the superoxide state when increasing the size of the alkali metal in the anionic MO 2 – systems shows a clear analogy with the general, well-known trend of increased stability of neutral alkali superoxide vs peroxide phases. 9 , 26 , 37 − 39 Interestingly, a somehow similar trend is observed in ref ( 27 ), where the dissociation of neutral MO 2 superoxide (M = Li, Na, K) is investigated through multireference correlated ab initio methods. Here, along the OO–M dissociation coordinate, the energy curves relative to the formation of neutral M 0 atoms are remarkably closer to the equilibrium ground state energy for Na than for Li.…”

“…Very recently, Zaichenko et al have applied multiconfigurational methods to the study of the dissociation pathways of alkali superoxides. 27 In a previous work, we have explored the superoxide disproportionation reaction when catalyzed by protons or Li ions using multiconfigurational methods. 14 From an oversimplified standpoint, the superoxide disproportionation reaction takes place between two O 2 − anions…”

“…Density functional theory (DFT) calculations have been used to characterize the bulk phase of sodium superoxide , and have been accompanied with coupled-cluster methods in refs , to explore various possible intermediates in the disproportionation reaction of Li and Na superoxides. Very recently, Zaichenko et al have applied multiconfigurational methods to the study of the dissociation pathways of alkali superoxides . In a previous work, we have explored the superoxide disproportionation reaction when catalyzed by protons or Li ions using multiconfigurational methods…”

Singlet Oxygen in Electrochemical Cells: A Critical Review of Literature and Theory

“…A possible reaction path for the formation of singlet oxygen from the dissociation of the lithium superoxide as simplest model system was investigated by Zaichenko et al on the basis of multireference CASSCF/CASPT2 calculations in the gas phase . The reaction path for singlet oxygen generation is about 0.9 eV higher in energy than the reaction path to triplet oxygen considering the dissociation curves, which show no additional barrier.…”

Superoxide Anion Disproportionation Induced by Li+ and H+: Pathways to 1O2 Release in Li−O2 Batteries

“…Such level of theory goes beyond all previous reports (e. g. see Bryantsev and co-workers [16] and Mourad and co-workers [20] ). CASSCF has been very recently exploited by Zaichenko [24] and co-workers to study the dissociation sequence XO 2 !X + + O 2 À !X + O 2 (with X=H, Li, Na, K) on both the triplet and singlet PES, to model a unimolecular superoxides homolytic bond cleavage. Zaichenko and co-workers demonstrate that CASSCF effectively allows to correctly model the electronic structure of O 2 both in triplet and singlet spin states, whereas the most common computational approaches, being based on a single reference paradigm, e. g. DFT or MP2, fail.…”