Mechanistic Analysis of Lithium-Air Battery with Organic Redox Mediator-Coated Air-Electrode

Redox mediators (RMs) suppress the charging overpotential to enhance the cycle performance of lithium-air batteries (LABs), but inappropriate RM incorporation can adversely shorten cycle life. In this study, three typical organic RMs; tetrathiafulvalene (TTF), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), and 10-methylphenothiazine (MPT), were incorporated into the air-electrode (AE) of the LAB (RM-on-AE), rather than dissolving them in the electrolyte (RM-in-EL), to maximize the RM effect throughout the cycle life. The discharge/charge cycle test confirmed that the cells with RM-on-AE prevented the reductive decomposition of RM with the lithium anode, deriving the RM effect for a longer cycle life than the cells with RM-in-EL. The measurement of AE deposits revealed that the TTF- and TEMPO-on-AE cells failed to generate a quantitative amount of Li 2 O 2 discharge product. In contrast, the MPT-on-AE provided a 96% yield of Li 2 O 2 after the first discharge because of the reductive tolerance of the MPT as organic RM. The quantitative analysis also revealed an accumulation of Li 2 CO 3 on the AEs, along with the generation of carboxylate, as the side products of irrelevant battery reactions. This study provides a practical methodology for selecting RMs and their incorporation for developing long-life LABs.