Prevention of redox shuttle using electropolymerized polypyrrole film in a lithium-oxygen battery

Among the recent advancements in lithium-oxygen (Li-O₂) chemistries, redox mediators (RMs) have been revealed to play a significant role in decreasing overpotential on charging and in improving cycling performance. However, an intrinsic problem is redox shuttle of RMs, which leads to degraded RM utilization and induces the accumulation of discharge products on the cathode surface; this remains a significant issue in the current battery cell configuration (Li anode/separator/cathode). To address this detrimental problem, herein we propose a novel Li-O₂ cell incorporating a freestanding electropolymerized polypyrrole (PPy) film for the restriction of the redox-shuttle phenomenon of lithium iodide (Li anode/separator/PPy film/cathode). In this study, a PPy film, which is prepared through oxidative electropolymerization using an ionic liquid of 1-methyl-1-butylpyrrolidinium mixed with pyrrole and lithium bis(trifluoromethanesulfonyl)imide, is introduced between the cathode and the separator. From the charge-discharge voltage profile, it is confirmed that the PPy film suppresses the diffusion of the oxidized I₃^- to the Li anode, while allowing Li ion transport. Secondary scanning electron microscope measurements confirm that the chemical reactions between I₃^- and Li₂O₂ are facilitated by the presence of the PPy film because I₃^- remains near the cathode surface during the charging process. As a result, the cycling performance in the Li-O₂ cells with PPy film exhibits a cycling life four times as long as that of the Li-O₂ cells without PPy film.