Oxygen Reduction Reactions in Aprotic Ionic Liquids Based Mixed Electrolytes for High Performance of Li–O2 Batteries

The kinetics and mechanisms of oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) in the absence and presence of Li + have been systematically investigated in five ionic liquid (IL)/dimethyl sulfoxide (DMSO) mixed electrolytes for potential applications for high performance Li–O 2 batteries. The diffusion coefficient and solubility of oxygen in these mixed electrolytes are determined by nonlinear fitting of potential-step chronoamperometry obtained at a platinum microelectrode. The mixed electrolytes exhibit enhanced oxygen diffusion coefficient (6.2–11.3 × 10 –6 cm 2 s –1 ), compared to neat ILs, and solubility (2.9–6.8 mM), compared to DMSO. Cyclic voltammetry (CV) shows that all the IL-mixed electrolytes display good stability against superoxide (O 2 •– ) except for 1-butyl-3-methyl-pyridinium bis(trifluoromethylsulfonyl)imide [BMPy][NTf 2 ], where the generated superoxide is found to be unstable and reacts with the cations of the IL. Using digital simulation of cyclic voltammograms for ORR, the heterogeneous rate constant, k het, for reduction of O 2 to O 2 •– in these mixed electrolytes is obtained ( k het = 3.6–3.9 × 10 –3 cm s –1 ). The presence of Li + in the mixed electrolytes shows minor influence on the k het . The cations of ILs show strong interaction with O 2 •– thus stabilizing the ORR products. The role of anions of ILs on the stability of electrolytes and reversibility of ORR–OER processes is also investigated.