Role of Superoxide and Singlet Oxygen on the Oxygen Reduction Pathways in Li-O-2 Cathodes at Different Li+ Ion Concentration

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+O2- into Li2O2 and O-2 leads to an increasing fraction of very reactive singlet oxygen (O-1(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 Li2O2 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 O-1(2) from superoxide disproportionation is revealed by gravimetric EQCM of insoluble by-products. Furthermore, DEMS provides evidence of CO2 gas evolution from decomposition of Li2CO3 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 O-1(2) quencher azide resulted in stable cycling, enhanced capacity and full charge recovery in a round trip.