A bromo-nitro redox mediator of BrCH2NO2 for efficient lithium–oxygen batteries

The lithium−oxygen (Li–O2) battery is considered as a promising candidate for the next-generation energy storage systems owing to its high theoretic specific energy. However, the slow kinetics of Li–O2 batteries cause a high charging overpotential, which leads to poor cyclability. Redox mediators are introduced as a promising tool to reduce the overpotentials of Li–O2 batteries. Unfortunately, the oxidized redox mediators can diffuse to the lithium anode and induce undesirable reactions. This so-called “shuttle effect” leads to the consumption of the redox mediators and the decrease of battery round-trip efficiency. In this work, an organic bromine, bromonitromethane (BrCH2NO2), can act bifunctionally as a redox mediator and a lithium anode protective layer-forming agent for Li–O2 batteries. It generates bromine ion (Br−) and in situ forms a lithium anode protective layer via reducing on lithium anode. This layer suppresses the shuttle effect and allows efficient lithium ion (Li+) transfer. The superior cycling performance of batteries containing BrCH2NO2 demonstrates that the suitable bifunctional redox mediator is important for highly efficient and stable Li–O2 batteries.