Lithium-oxygen (Li-O₂) batteries employing a lightweight and gaseous oxygen cathode have been spotlighted because of their exceptional high-energy density (practically 2~3 times higher than lithium-ion cells).^[1-2] However, their poor efficiency and cyclability originating from the sluggish kinetics for the formation and decomposition of lithium oxide products (i.e., LiO₂, Li₂O₂) necessitate utilization of efficient catalysts. Although loading of the solid catalysts on the electrode effectively facilitates the Li-O₂ cell reactions, they are quickly deactivated upon cycling due to accumulating product residues.^[3-4]

Incorporation of soluble redox molecules into an electrolyte has been considered an effective alternative strategy to address the deactivation issues.^[5] Their redox properties enable rapid electron transfer from/to the electrode through their self-diffusion and subsequent redox reactions instead of a slow electron movement through the insulating discharge products. Development of potential redox catalysts for Li-O₂ cells should meet the following requirements: (i) high mobility in electrolyte; (ii) reversible redox properties for fast electron transfer; (iii) robust stability; and (iv) environmental friendliness.

In this work, we report a class of nature-inspired molecules as an efficient and stable redox catalyst for Li-O₂ batteries. Interestingly, the catalyst molecules exhibits catalytic activity for both oxygen evolution and reduction reactions under a certain condition when it forms a stable dispersion of molecular aggregates, which can be controlled by types of electrolyte solvents and exposure to light. As a result of the optimized catalytic activity, the Li-O₂ cells facilitated by redox reactions successfully achieve improved efficiency and a longer cycle life compared to reference cells. The reversibility of the Li-O₂ reactions in the presence of the molecular catalysts is confirmed by ex-situ characterizations.

[1] Ryu, W. H.; Yoon, T. H.; Song, S. H.; Jeon, S.; Park, Y. J.; Kim, I. D. Nano Lett 2013, 13, 4190-7.

[2] Gittleson, F. S.; Yao, K. P. C.; Kwabi, D. G.; Sayed; Ryu, W.-H.; Shao-Horn, Y.; Taylor, A. D., ChemElectroChem 2015, 2, 1446-1457.

[3] Ryu, W. H.; Gittleson, F. S.; Schwab, M.; Goh, T.; Taylor, A. D., Nano Lett 2015, 15, 434-441

[4] Ryu, W. H.; Gittleson, F. S.; Li, J.; Tong, X.; Taylor, A. D., Nano Lett 2016, 16, 4799-4806

[5] Ryu, W. H.; Gittleson, F. S.; Thomsen, J.; Li, J.; Schwab, M.; Brudvig, G.; Taylor, A. D., Nature Commun., 2016, 7, 12925.