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One- or Two-Electron Transfer? The Ambiguous Nature of the Discharge Products in Sodium-Oxygen Batteries.

Rechargeable lithium-oxygen and sodium-oxygen cells have been considered as challenging concepts for next-generation batteries, both scientifically and technologically. Whereas in the case of non-aqueous Li/O2 batteries, the occurring cell reaction has been unequivocally determined (Li2O2 formation), the situation is much less clear in the case of non-aqueous Na/O2 cells. Two discharge products, with almost equal free enthalpies of formation but different numbers of transferred electrons and completely different kinetics, appear to compete, namely NaO2 and Na2O2. Cells forming either the superoxide or the peroxide have been reported, but it is unclear how the cell reaction can be influenced for selective one- or two-electron transfer to occur. In this Minireview, we summarize available data, discuss important control parameters, and offer perspectives for further research. Water and proton sources appear to play major roles.