Redox mediators for oxygen reduction reactions in lithium-oxygen batteries: governing kinetics and its implications

Despite the high theoretical energy density, the realization of lithium-oxygen batteries with practically high energy density is limited by the nature of oxygen reduction reactions generating insulating lithium peroxide and inevitable electrode passivation. Redox mediators have been recently introduced as a promising soluble catalyst that can effectively manage the complex multi-phase reactions in lithium-oxygen batteries, facilitating the oxygen reduction reaction with a significant performance enhancement. Nevertheless, little is known regarding the factors governing kinetics of redox mediators and its implication on cell performance. Herein, we conducted a comparative study employing benzoquinone derivatives to elucidate the kinetic mechanism of redox-mediated oxygen reduction reactions in lithium-oxygen batteries. It is revealed that the oxygen reduction by the redox mediator occurs via inner-sphere electron transfer, and its kinetics is significantly affected by the steric hindrance effects. More importantly, we show that electrochemical performance (e.g., the discharge capacity) is concurrently governed by both the kinetics of redox mediators and their steric hindrance, contrary to the conventional belief that RMs with fast kinetics would be simply beneficial. These findings suggest a new direction in the development of redox mediators not only for lithium-oxygen batteries but also for other areas where the redox mediators have been conventionally employed. Intrinsic properties of quinones such as steric hindrance and heterogeneous electron transfer kinetics that follows Marcus theory concurrently govern their performance as redox mediators for oxygen reduction reactions in lithium-oxygen batteries.