Redox catalysts for aprotic Li-O₂ batteries: Toward a redox flow system

Large-scale electrical energy storage with high energy density and round-trip efficiency is important to the resilience of power grids and the effective use of intermittent renewable energy such as solar and wind. Lithium-oxygen battery, due to its high energy density, is believed to be one of the most promising energy storage systems for the future. However, large overpotentials, poor cycling stability, and degradation of electrolytes and cathodes have been hindering the development of lithium-oxygen batteries. Numerous heterogeneous oxygen electrocatalysts have been investigated to lower the overpotentials and enhance the cycling stability of lithium-oxygen batteries. Unfortunately, the prevailing issues of electrode passivation and clogging remain. Over the past few years, redox mediators were explored as homogenous catalysts to address the issues, while only limited success has been achieved for these soluble catalysts. In conjunction with a flowing electrolyte system, a new redox flow lithium-oxygen battery (RFLOB) has been devised to tackle the aforementioned issues. The working mechanism and schematic processes will be elaborated in this review. In addition, the performance gap of RFLOB with respect to practical requirements will be analysed. With the above, we anticipate RFLOB would be a credible solution for the implementation of lithium-oxygen battery chemistry for the next generation energy storage.