Redox Mediators Degradation By Singlet Oxygen in Lithium Oxygen Batteries

Abstract

Non-aqueous lithium-oxygen batteries cycle by forming and decomposing the lithium peroxide to deliver a capacity. During charging, oxidizing the solid insulating lithium peroxide can greatly be facilitated by redox mediators which shuttle electron-holes between the porous substrate and lithium peroxide. Redox mediator stability is thus key for energy efficiency, reversibility, and cycle life. However, the gradual deactivation of redox mediators during repeated cycling has not conclusively been explained. Here, we show that organic redox mediators are predominantly decomposed by singlet oxygen that forms during cycling. Their reaction with superoxide, previously assumed to mainly trigger their degradation, peroxide, and dioxygen, is orders of magnitude slower in comparison. The reduced form of the mediator is markedly more reactive towards singlet oxygen than the oxidized form, from which we derive reaction mechanisms supported by density functional theory calculations. Thus, design of redox mediators considers the stability against singlet oxygen.