Redox mediators for high-performance lithium–oxygen batteries

Aprotic lithium-oxygen (Li-O2) batteries are receiving intense research interest by virtue of their ultra-high theoretical specific energy. However, current Li-O2 batteries are suffering from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catalysts, i.e. redox mediators (RMs), have been explored to catalyse the oxygen electrochemistry in Li-O2 batteries and are regarded as an advanced solution. To fully unlock the capability of Li-O2 batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li-O2 batteries.