Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Computational study of electron-transfers and singlet oxygen in aprotic metal-O2 batteries

The push for decreasing carbon emission is leading many societal sectors toward electrifi- cation and urging academic-industrial research toward the optimization of next-generation batteries with increasingly higher energy density. Aprotic metal-oxygen batteries (MOBs), based on the electroreduction of molecular oxygen at a porous cathode, have attracted a vast interest in research, owing to their potential upgrade in terms of energy density and costs over present lithium-ion batteries. Despite their highly promising features, aprotic MOBs based on alkali and alkaline-earth metals still suffer severe limitations in their practical applicability. One of the main unre- solved issues, especially with Li-O2 batteries, is represented by the high degree of parasitic reactivity. Singlet oxygen (1O2) is today held responsible for a major contribution to such re- activity, and the disproportionation of the superoxide anion is considered as one of the most likely source of 1O2 in the cell environment. Experimental evidences for electrolyte degra- dation and evolution of 1O2 have been reported, but the fundamental chemical mechanisms underlying these phenomena are still poorly understood. A valid strategy for contrasting the arise of side-reactions and materials degradation is to use redox mediators (RMs), which allow to recharge the battery with greatly reduced overpotentials. Understanding the con- nection of RM-assisted charging with the production 1O2 is likely to play a key role in the design of fully reversible and efficient practical MOBs in the future. In this thesis, quantum chemical computational methods were used to investigate reactive processes of electron-transfer involving reduced oxygen species in aprotic MOBs. The possibility of reactive pathways leading to the release of 1O2 was addressed in particular. The aim of the thesis was to apply theoretical methods to the modeling of reactive systems, in order to unravel part of the mechanisms which underpin the parasitic chemistry of MOBs. Despite their apparent simplicity, the reaction governing the chemistry of the cells involve a complex interplay of radical species and electronic excited states. For this reason, our approach was to use mainly ab-initio correlated multiconfigurational methods for a high-level description of potential energy surfaces and reaction energies. Owing to the computational costs of the methods, such an approach necessarily entails the resort to simplified models, including the exclusive use of implicit solvent and the neglect of solid phases and interfacial effects. Chapter One presents an introduction to MOBs and their dedicated literature. In the first section, the fundamental electrochemical reactions at the basis for discharge and charge cycling of alkali and alkaline-earth metals MOBs are analyzed, focusing in particular on the disproportionation of the superoxide radical. The second section discusses the implication of singlet oxygen in the parasitic chemistry of MOBs, from its production to current contrasting strategies. The third section covers the topic of redox mediators, in particular those based on iodine/iodide and bromine/bromide redox couples. Chapter Two offers a basic recap on the electronic structure theory and methods which were employed during the work for this thesis. The exposition is focused on the concepts, rather than on formulae, aiming at illustrating the main assumptions that underlie the application of different computational methods. The problem of electron correlation is briefly covered, and the most common strategies are presented to recover it via both single-reference and multi-reference methods. Chapter Three finally presents the results of the work, divided in two parts. The first section is focused on the formation of peroxide species upon disproportionation of the superoxide anion. Thanks to the detailed description of the electronic structure of the system, the proper disproportionation route is found to compete with a metal-reduction route which not only can affect the release of 1O2, but it also open the road for the production of additional reactive species for the onset of degradation reactions. The second section deals with the oxidation of peroxide clusters, as prototype of the discharge products of Li-O2 batteries, mediated by halogen/halide redox mediators. Different oxidation routes are compared, again trying to point out the most likely pathways leading to 1O2, also taking into account the occurrence of triplet-singlet spin transition triggered by spin-orbit coupling in presence of I and Br heavy atoms.