Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation

Abstract

Aprotic alkali metal-O-2 batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (O-1(2)). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect O-1(2) formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress O-1(2) formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above similar to 3.5 V versus Li/Li+ drive O-1(2) evolution from superoxide oxidation, while disproportionation always generates some O-1(2). We find that O-1(2) suppression requires oxidation to be faster than the generation of O-1(2) from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.