Novel Guidelines of Redox Mediators for Practical Lithium–Oxygen Batteries: Characterization Mechanisms, Design Principle, and Engineering Strategies

In recent years, aprotic lithium–oxygen (Li–O₂) batteries have received extensive academic attention for their ultrahigh capacity. However, their practical development faces the problems of low capacity, low rate, and short lifetime. Soluble catalysis with efficient redox mediators (RMs) is considered a feasible strategy owing to its good interfacial contact and flexible action. However, the mutual constraints of RMs charging/discharging catalysis, the erosion of anode by RMs shuttle effect leading to deactivation, and the decomposition of RMs or the initiation of side reactions have greatly limited the effectiveness of RMs in Li–O₂ batteries. Therefore, it is necessary to optimize RMs and find traceable principles and directions. Based on this, this work systematically reviews the mechanism, effectiveness, and characterization of RMs in Li–O₂ batteries. The design principles of novel RMs constructed by two research tendencies of kinetics and thermodynamics are pioneered, and the key roles of ionization energy and site-resistive groups are especially pointed out. In addition, the current optimization design strategies for RMs are summarized. Specifically, the introduction of functional groups such as adsorption, conductivity, active sites, and the use of intermolecular forces for efficient RMs are highlighted, designed to provide direction for optimization and development of RMs.