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

Abstract

We use a multiconfigurational and correlated ab initio method to investigate the fundamental electronic properties of the peroxide MO 2 – (M = Li and Na) trimer to provide new insights into the rather complex chemistry of aprotic metal–O 2 batteries. These electrochemical systems are largely based on the electronic properties of superoxide and peroxide of alkali metals. The two compounds differ by stoichiometry: the superoxide is characterized by a M + O 2 – formula, while the peroxide is characterized by [M + ] 2 O 2 2– . We show here that both the peroxide and superoxide states necessarily coexist in the MO 2 – trimer and that they correspond to their different electronic states. The energetic prevalence of either one or the other and the range of their coexistence over a subset of the MO 2 – nuclear configurations is calculated and described via a high-level multiconfigurational approach.

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

“…While the detailed evaluation of overall geometrical rearrangements would require more demanding theoretical efforts (for example employing nudge-elastic band methods or molecular dynamic approaches), this first ab initio study on oxygen release from Li 2 O 2 surfaces via the EC(CASPT2-PC) approach aims for simplicity and explores the potential energy surfaces (PESs) in two fixed reaction coordinates, inspired by the similar approach adopted by Pierini et al to investigate the oxygen release from the LiO 2 − trimer and unveil the nature of the ground state as function of the O−O distance. 42 2 shows the resulting PESs for the oxygen release obtained for the periodic model at DFT-PBE level of theory considering two different spin multiplicity for the whole system, i.e., singlet and triplet (red and blue surfaces, respectively). The singlet/triplet state will be referred as S/T from this point forward, so that any point along the 41 and Pierini et al, 42 the oxygen−oxygen bond length is a useful coordinate to follow the subsequent oxidation up to the final oxygen release and thus can be used as a simple descriptor to predict the formation of oxidation products.…”

“…By monitoring the oxygen–oxygen bond lengths and the oxygen–lithium distance along the electron-transfer process, the triplet state is shown to form at distance d O–O,Li of ∼2.75 Å and associated to ∼2.5 eV, while the singlet oxygen can be released at ∼3.50 Å once provided an energy supply of ∼3.4 eV . In the work by Pierini et al , gas-phase CASPT2 calculations on the LiO 2 – anionic trimer show that the true nature of the electronic ground state is that of superoxide, which can only be unveiled by multiconfigurational methods . While a single-reference method would inevitably perceive the overall singlet multiplicity of the molecule as a closed shell arrangement of the electrons (thus yielding the M + O 2 2– peroxide configuration), the multiconfigurational nature of an open-shell singlet diradical ( i.e.…”

“…To the best of our knowledge, theoretical investigations on reaction mechanisms underlying singlet oxygen release have been reported only on small molecular systems in solution. , Nevertheless, reaction energetics and involved species, as well as mechanistic details, can be largely different if the process takes place at the interface with solid-state particles. For example, the exposed crystalline facets on Li 2 O 2 nanoparticles might act as reaction sites during the charging process, and the material surface can be key in the electron transfer occurring at the cathode boundary .…”

“…41 In the work by Pierini et al, gasphase CASPT2 calculations on the LiO 2 − anionic trimer show that the true nature of the electronic ground state is that of superoxide, which can only be unveiled by multiconfigurational methods. 42 superoxide configuration) can only be distinguished with multireference approaches. The occurrence of this low-lying M 0 superoxide channel in the possible products of superoxide disproportionation might have important consequences for the parasitic processes leading to cell death, for example, largely lowering the overpotentials needed to induce the 1 O 2 formation, which would explain why this reactive species has been observed despite the unfavorable energetics of superoxide oxidation.…”

“…The occurrence of this low-lying M 0 superoxide channel in the possible products of superoxide disproportionation might have important consequences for the parasitic processes leading to cell death, for example, largely lowering the overpotentials needed to induce the 1 O 2 formation, which would explain why this reactive species has been observed despite the unfavorable energetics of superoxide oxidation. 42 To the best of our knowledge, theoretical investigations on reaction mechanisms underlying singlet oxygen release have been reported only on small molecular systems in solution. 41,42 Nevertheless, reaction energetics and involved species, as well as mechanistic details, can be largely different if the process takes place at the interface with solid-state particles.…”