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

Abstract 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.