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

Aprotic alkali metal-O₂ 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 (¹O₂). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect ¹O₂ 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 ¹O₂ formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li⁺ drive ¹O₂ evolution from superoxide oxidation, while disproportionation always generates some ¹O₂. We find that ¹O₂ suppression requires oxidation to be faster than the generation of ¹O₂ from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.