Singlet Oxygen in Electrochemical Cells: A Critical Review of Literature and Theory.

Rechargeable metal/O batteries have long been considered a promising future battery technology in automobile and stationary applications. However, they suffer from poor cyclability and rapid degradation. A recent hypothesis is the formation of singlet oxygen (O) as the root cause of these issues. Validation, evaluation, and understanding of the formation of O are therefore essential for improving metal/O batteries. We review literature and use Marcus theory to discuss the possibility of singlet oxygen formation in metal/O batteries as a product from (electro)chemical reactions. We conclude that experimental evidence is yet not fully conclusive, and side reactions can play a major role in verifying the existence of singlet oxygen. Following an in-depth analysis based on Marcus theory, we conclude that O can only originate from a chemical step. A direct electrochemical generation, as proposed by others, can be excluded on the basis of theoretical arguments.