Recent Progress of Halide Redox Mediators in Lithium–Oxygen Batteries: Functions, Challenges, and Perspectives

Lithium–oxygen batteries (LOBs) have received much research interest owing to their ultra-high energy density, but their further development is restricted by the erosion of the Li anode, the degradation of the electrolyte, and especially the sluggish oxygen-involving reactions on the cathode. To facilitate the oxidation of discharge products, halide redox mediators (HRMs), a subclass of soluble additives, have been explored to promote their decomposition. Meanwhile, some other intriguing functions were discovered, like protecting the Li anode and redirecting the discharge pathway to form LiOH. In this Review, after a brief introduction of LOBs and HRMs, the various functions of HRMs, not limited to promoting the oxidation of discharge products, are discussed and summarized. In addition, the challenges and controversies confronted by HRMs in LOBs are highlighted and the future opportunities of HRMs for achieving better LOBs are proposed.