Role of Superoxide and Singlet Oxygen on the Oxygen Reduction Pathways in Li−O2 Cathodes at Different Li+ Ion Concentration**

Abstract The oxygen reduction reaction (ORR) on Au electrodes has been studied in DMSO at different Li + concentrations. In‐operando fluorescence decay of 9,10‐dimethyl anthracene (DMA) has shown that disproportionation of lithium superoxide Li + O 2 − into Li 2 O 2 and O 2 leads to an increasing fraction of very reactive singlet oxygen ( 1 O 2 ) at high lithium concentration. Singlet oxygen has been identified as the major cause of parasitic reactions leading to capacity fading and high charge overpotential of Li−O 2 batteries. Rotating ring‐disk electrode shows quantitative formation of soluble superoxide at low Li + concentration, a decrease in superoxide yield at high Li + concentrations is consistent with electrochemical quartz crystal microbalance (EQCM) evidence of Li 2 O 2 deposits. Differential electro chemical mass spectrometry (DEMS) confirms oxygen depletion at the electrode surface during ORR, and O 2 evolution during oxidation at 3.1 V (vs. Li/Li + in DMSO). The spurious solvent decomposition due to the very reactive 1 O 2 from superoxide disproportionation is revealed by gravimetric EQCM of insoluble by‐products. Furthermore, DEMS provides evidence of CO 2 gas evolution from decomposition of Li 2 CO 3 by‐product at 3.7 V (vs. Li/Li + in DMSO). Preliminary in‐operando full discharge‐charge tests of a Li−O 2 battery with 1 O 2 quencher azide resulted in stable cycling, enhanced capacity and full charge recovery in a round trip.