Atomic/nano-scale in-situ probing the shuttling effect of redox mediator in Na–O₂ batteries

Sodium-oxygen batteries (Na-O₂) have attracted extensive attention as promising energy storage systems due to their high energy density and low cost. Redox mediators are often employed to improve Na-O₂ battery performance, however, their effect on the formation mechanism of the oxygen reduction product (NaO₂) is still unclear. Here, we have investigated the formation mechanism of NaO₂ during the discharge process in the presence of a redox mediator with the help of atomic/nano-scale in-situ characterization tools used in concert (e.g. atomic force microscope, electrochemical quartz crystal microbalance (EQCM) and laser nano-particle analyzer). As a result, real-time observations on different time scales show that by shuttling electrons to the electrolyte, the redox mediator enables formation of NaO₂ in the solution-phase instead of within a finite region near the electrode surface. These findings provide new fundamental insights on the understanding of Na-O₂ batteries and new consequently perspectives on designing high performance metal-O₂ batteries and other related functions.