Surface and catalyst driven singlet oxygen formation in Li-O₂ cells

Large overpotentials upon discharge and charge of Li-O₂ cells have motivated extensive research into heterogeneous solid electrocatalysts or non-carbon electrodes with the aim to improve rate capability, round-trip efficiency, and cycle life. These features are equally governed by parasitic reactions, which are now recognized to be caused by formation of the highly reactive singlet oxygen (¹O₂). However, the link between the presence of electrocatalysts and ¹O₂ formation in metal-O₂ cells is unknown. Here, we show that, compared to pristine carbon black electrodes, a representative selection of electrocatalysts or non-carbon electrodes (noble metal, transition metal compounds) may both slightly reduce or severely increase the ¹O₂ formation. The individual reaction steps, where the surfaces impact the ¹O₂ yield are deciphered, showing that ¹O₂ yields from superoxide disproportionation as well as the decomposition of traces of H₂O₂ are sensitive to catalysts. Transition metal compounds in general are prone to increase ¹O₂. The results highlight the importance of ¹O₂ in metal-O₂ cells and to use a comprehensive set of metrics to judge the impact of catalysts on reversibility.