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+O2− into Li2O2 and O2 leads to an increasing fraction of very reactive singlet oxygen (1O2) 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−O2 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 O2 evolution during oxidation at 3.1 V (vs. Li/Li+ in DMSO). The spurious solvent decomposition due to the very reactive 1O2 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−O2 battery with 1O2 quencher azide resulted in stable cycling, enhanced capacity and full charge recovery in a round trip.