Lithium oxygen battery (LOB), as a promising next generation energy storage system, has attracted tremendous attention. However, for aprotic systems due to the deposition of insoluble and insulating discharging product Li₂O₂, surface passivation and pore clogging of the cathode occur. As a result, the charging overpotential of LOB is generally intolerably high. While people have tried to use homogeneous catalysts — redox mediators dissolved in the electrolyte to address the above issues, the passivation and clogging problems remain especially at deep discharge. Inspired by the redox targeting concept in redox flow lithium battery, we have recently proposed a new battery concept — redox flow lithium oxygen battery (RFLOB), which elegantly resolves the surface passivation and pore clogging issues.^1-3 As shown in Figure 1, with a pair of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) redox catalysts, the formation of Li₂O₂ during discharge and oxidation during charge could be performed in a separate gas diffusion tank rather than on the cathode in the cell. Therefore, the passivation and clogging problems of the electrode are feasibly obviated.⁴ Here, we will report a series of robust redox mediators as efficient ORR and OER catalysts in the RFLOB system. By optimizing the electrolyte compositions, extremely low charging overpotential (~0.20 V) has been achieved, which results of unprecedently high voltage efficiency. The underlying mechanism of the redox catalytic processes toward ORR and OER will be discussed in detail. We anticipate RFLOB provide a new vista to the development of Li-air battery with high round-trip efficiency for practical applications.

Figure 1. The structure of a redox flow lithium oxygen battery.

Reference:

                (1)           Wang, Q.; Zakeeruddin, S. M.; Wang, D.; Exnar, I.; Grätzel, M. Angewandte Chemie International Edition 2006, 45, 8197.

                (2)           Huang, Q.; Li, H.; Gratzel, M.; Wang, Q. Physical Chemistry Chemical Physics 2013, 15, 1793.

                (3)           Pan, F.; Yang, J.; Huang, Q.; Wang, X.; Huang, H.; Wang, Q. Advanced Energy Materials 2014, 4, n/a.

                (4)           Zhu, Y. G.; Jia, C.; Yang, J.; Pan, F.; Huang, Q.; Wang, Q. Chemical Communications 2015.