Unraveling the Mechanism of the Solution-Mediated Oxygen Reduction in Metal-O2 Batteries: The Importance of Ion Association

One of the bottlenecks in Li-O-2 batteries is the film like growth of Li2O2 on the electrode surface during discharge leading to early cell death. To tackle this problem 2,5-Di-tert-1,4-benzoquinone (DBBQ) was introduced as a soluble redox mediator. This redox mediator is avoiding the Li2O2 layer-by-layer growth on the electrode surface and thus leading to higher discharge capacities of the Li-O-2 cell. In this study, we investigate the ion pairing between the cation of the conducting salt and the DBBQ monoanion and the resulting impact on the ORR activity of the DBBQ monoanion. We investigate TBA(+), K+ and Li+ as cations and TEGDME and DMSO as solvents. We found out that there is a direct correlation between the ORR activity of DBBQ(-) and the ion pairing of DBBQ(-) with the cation of the supporting electrolyte: Only if DBBQ is strongly associated with the cations of the electrolyte it will reduce oxygen in the electrolyte. Increasing the Li+ concentration in the electrolyte shifts the ORR potential to more positive electrode potentials. In addition, we are describing a new experimental approach to investigate the kinetics of the homogenous ORR via time resolved mass spectrometry. With this approach we found out, that the reaction Li center dot center dot center dot DBBQ((sol))+O-2(sol)<-> k-1k1Li center dot center dot center dot DBBQ center dot center dot center dot O-2(sol) is 80 times faster in a TEGDME based electrolyte than in a DMSO-based electrolyte. We determined k1 with 5.1 center dot 10(2) s(-1) M(-1)and k-1 with 3.7 10(2) s(-1) M-1 in TEGDME whereas the constants in DMSO are k1=4.5 s(-1) M-1 and k-1=5.5 s(-1) M(-1)s.