Oxidation Stability of Organic Redox Mediators as Mobile Catalysts in Lithium–Oxygen Batteries

Employing organic redox mediators (ORMs) for lithium–oxygen (Li–O2) batteries has emerged as an important strategy to suppress charging overpotentials. Judicious molecular designs of ORMs can also tailor their redox potential and electron-transfer rate to optimize the catalytic efficiency. However, the stability of ORMs in Li–O2 cells was scarcely studied. Here, the catalytic efficiency and stability of several important ORMs are assessed through in situ gas analysis and reactivity tests with singlet oxygen. Some well-known ORMs are detrimentally decomposed during the first cycle in Li–O2 cells, whereas nitroxyl-radical-based ORMs bear the most stable and efficient response. Analogous nitroxyl-radical derivatives further increase round-trip energy efficiency and electron-transfer kinetics. This study underlines chemical stability aspects of ORMs, which are mandatory for the long-term cyclability in Li–O2 cells. We emphasize that besides the importance of ORMs in these systems and their proper selection, an effective operation of Li–O2 cells depends also strongly on the stability of the carbonaceous cathodes and the electrolyte solutions. The stability of all the components in these systems is inter-related.