Redox Mediators for Li-O₂ Batteries: Status and Perspectives

Li-O₂ batteries have received much attention due to their extremely large theoretical energy density. However, the high overpotentials required for charging Li-O₂ batteries lower their energy efficiency and degrade the electrolytes and carbon electrodes. This problem is one of the main obstacles in developing practical Li-O₂ batteries. To solve this problem, it is important to facilitate the oxidation of Li₂ O₂ upon charging by using effective electrocatalysis. Using solid catalysts is not too effective for oxidizing the electronically isolating Li-peroxide layers. In turn, for soluble catalysts, red-ox mediators (RMs) are homogeneously dissolved in the electrolyte solutions and can effectively oxidize all of the Li₂ O₂ precipitated during discharge. RMs can decompose solid Li₂ O₂ species no matter their size, morphology, or thickness and thus dramatically increase energy efficiency. However, some negative side effects, such as the shuttle reactions of RMs and deterioration of the Li-metal occur. Therefore, it is necessary to study the activity and stability of RMs in Li-O₂ batteries in detail. Herein, recent studies related to redox mediators are reviewed and the mechanisms of redox reactions are illustrated. The development opportunities of RMs for this important battery technology are discussed and future directions are suggested.