Singlet oxygen formation in Na–O2 battery cathodes catalyzed by ammonium Brönsted acid

Abstract

The presence of singlet oxygen in metal-O2 batteries is one of the major factors responsible for the degradation of the electrolyte. The formation of singlet oxygen, 1O2, at Na–O2 cathodes in sodium trifluoromethane sulfonate (NaOTf)-diglyme electrolyte has been detected in-operando by fluorescence quenching of 9,10-dimethylanthracene, DMA, a specific chemical monitor of singlet oxygen. Furthermore, ammonium Brönsted acid protonates superoxide and the further disproportionation of HO2 results in a catalytic EC' mechanism that recycles oxygen, a fraction of which has been found to be 1O2. Unlike, ammonium trifluoromethane sulfonate (NH4OTf), in tetrabutylammonium trifluoromethane sulfonate (TBAOTf), which lacks acidic protons, superoxide is stabilized by the large cation and no 1O2 could be detected. The addition of sodium azide to the electrolyte, an efficient specific physical quencher of singlet oxygen, results in negligible detection of this harmful oxygen reduction intermediate and proves the nature of 1O2.