Non-aqueous metal-oxygen batteries depend critically on the reversible formation/decomposition of metal oxides on cycling. Irreversible parasitic reactions cause poor rechargeability, efficiency, and cycle life and have predominantly been ascribed to the reactivity of reduced oxygen species with cell components. These species, however, cannot fully explain the side reactions. Here we show that singlet oxygen forms at the cathode of a lithium-oxygen cell during discharge and from the onset of charge, and accounts for the majority of parasitic reaction products. The amount increases during discharge, early stages of charge, and charging at higher voltages, and is enhanced by the presence of trace water. Superoxide and peroxide appear to be involved in singlet oxygen generation. Singlet oxygen traps and quenchers can reduce parasitic reactions effectively. Awareness of the highly reactive singlet oxygen in non-aqueous metal-oxygen batteries gives a rationale for future research towards achieving highly reversible cell operation.

Aprotic alkali metal-oxygen batteries require reversible formation of metal superoxide or peroxide on cycling. Severe parasitic reactions cause poor rechargeability, efficiency, and cycle life and have been shown to be caused by singlet oxygen ( 1 O 2 ) that forms at all stages of cycling. However, its formation mechanism remains unclear. We show that disproportionation of superoxide, the product or intermediate on discharge and charge, to peroxide and oxygen is responsible for 1 O 2 formation. While the overall reaction is driven by the stability of peroxide and thus favored by stronger Lewis acidic cations such as Li + , the 1 O 2 fraction is enhanced by weak Lewis acids such as organic cations. Concurrently, the metal peroxide yield drops with increasing 1 O 2 . The results explain a major parasitic pathway during cell cycling and the growing severity in K-, Na-, and Li-O 2 cells based on the growing propensity for disproportionation. High capacities and rates with peroxides are now realized to require solution processes, which form peroxide or release O 2 via disproportionation. The results therefore establish the central dilemma that disproportionation is required for high capacity but also responsible for irreversible reactions. Highly reversible cell operation requires hence finding reaction routes that avoid disproportionation. Broader contextDecarbonizing the energy system requires energy storage with large capacity but equally low economic and ecological footprint. Alkali metal-O 2 batteries are considered outstanding candidates in this respect. However, they suffer from poor cycle life as a result of cathode degradation. Formation of the highly reactive singlet oxygen has been proposed to cause this degradation, but formation mechanisms have remained unclear. Here, we show that the singlet oxygen source is the disproportionation of thermodynamically unstable superoxide intermediates to the peroxides. The revealed mechanism conclusively explains the strongly growing degree of degradation when going from K-O 2 to Na-O 2 and Li-O 2 cells. A major consequence is that highly reversible cell operation of Li-O 2 and Na-O 2 cells requires them to form and decompose the peroxides without disproportionation. Achieving this requires finding new reaction routes. The work lays the mechanistic foundation to fight singlet oxygen as the predominant source of degradation in metal-O 2 cells.

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Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. The significant problem of oxidizing the solid insulating lithium peroxide can greatly be facilitated by incorporating redox mediators that 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. Redox mediators must thus be designed for stability against singlet oxygen.

Singlet oxygen ( 1 O 2 ) causes a major fraction of the parasitic chemistry during the cycling of non‐aqueous alkali metal‐O 2 batteries and also contributes to interfacial reactivity of transition‐metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4‐diazabicyclo[2.2.2]octane (DABCO), as an efficient 1 O 2 quencher with an unusually high oxidative stability of ca. 4.2 V vs. Li/Li + . Previous quenchers are strongly Lewis basic amines with too low oxidative stability. DABCOnium is an ionic liquid, non‐volatile, highly soluble in the electrolyte, stable against superoxide and peroxide, and compatible with lithium metal. The electrochemical stability covers the required range for metal–O 2 batteries and greatly reduces 1 O 2 related parasitic chemistry as demonstrated for the Li–O 2 cell.