Effect of Ionic Association in the Catalytic Mechanism of Redox Mediators for Li-air Batteries: a Theoretical Framework

We present a theoretical framework to explain how interactions between redox mediators and electrolyte components impact electron transfer kinetics, thermodynamics, and catalytic efficiency. Specifically focusing on ionic association, we use DBBQ as a case study to demonstrate these effects. Our analytical equations reveal how observed potential and electron transfer rate constants evolve with Li+ concentration, evidencing different redox activity mechanisms. Experimental validation shows that DBBQ bounds to 3 Li+ ions in its reduced state and 1 Li+ ion in its neutral form, leading to a maximum in the electron transfer kinetic constant at around 0.25M. We also explore parallel equilibria, emphasizing the pivotal role of free Li+ concentration influenced by ionic association with the supporting salt counteranion or interactions with solvent molecules. Furthermore, we investigate the impact of Li+ concentration on oxygen reduction reaction (ORR) catalysis using DBBQ, considering effects on electron transfer and catalytic kinetics. Cyclic voltammograms are presented as illustrative examples. The proposed theoretical framework's strength lies in its adaptability to various redox mediators and their interactions. By understanding these effects, we open up a new avenue to tune electron transfer and catalytic kinetics and thus improve the energy efficiency and rate capability of Li-O2 batteries.