Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Impact of Cell Formulation on Degradation Processes in Aprotic Li-O2 Batteries and Novel Cell Designs

The redox chemistry of lithium-oxygen batteries offers the potential to surpass the energy density achievable by intercalation-based batteries. This advantage is owed to their outstanding theoretical gravimetric capacity combined with the ∼3 V voltage difference between the positive and negative electrodes during cycling. A crucial issue that hinders the feasibility of this technology is the charge process, which is the oxidation of Li2O2 to molecular O2, affected by high overpotentials able to trigger parasitic reactions. Simoultaneously, the discharge process, i.e. the reduction of O2 to solid Li2O2, needs to be optimized to prevent the progressive loss of gaseous O2 during the cycling of the battery. The use of soluble catalysts called redox mediators in the electrolyte formulation is currently considered the most promising strategy to optimize the oxygen redox within Li-O2 cells. Charge redox mediators are able to reduce overpotentials by facilitating Li2O2 oxidation, and discharge redox mediators are key to maximize the cell capacity by promoting an optimal morphology of the discharge product Li2O2. In this thesis work, multiple facets of the functioning of aprotic Li-O2 batteries containing redox mediators were addressed. Electrochemical and chemical-physical characterization techniques were coupled to unveil fundamental insights of the redox mediation of lithium halides, LiI and LiBr, during the charge process. The ongoing of Li2O2 oxidation by halide species was correlated with the production of 1O2, which is one of the most reactive species formed during the operation of a Li-O2 battery, that can lead to the deterioration of cell components. Basing on the identification of the most critical aspects of LiI and LiBr redox mediation, cell optimization strategies employing a halogen-complexing polymer, polyvinylpyrrolidone, have been designed and subjected to the preliminary test phase. A parallel research line was focused on an emerging class of discharge redox mediators: quinones. The reactivity of three representative quinones, pBQ, DBBQ and AQ, in their mono-reduced and doubly-reduced forms towards molecular oxygen was explored in both solid and solution phase to investigate their fundamental behaviour in the frame of the O2 -related redox reactions.