The discovery of effective catalysts is an important step towards achieving Li–O2 batteries with long cycle life and high round-trip efficiency. Soluble-type catalysts or redox mediators (RMs) possess great advantages over conventional solid catalysts, generally exhibiting much higher efficiency. Here, we select a series of organic RM candidates as a model system to identify the key descriptor in determining the catalytic activities and stabilities in Li–O2 cells. It is revealed that the level of ionization energies, readily available parameters from a database of the molecules, can serve such a role when comparing with the formation energy of Li2O2 and the highest occupied molecular orbital energy of the electrolyte. It is demonstrated that they are critical in reducing the overpotential and improving the stability of Li–O2 cells, respectively. Accordingly, we propose a general principle for designing feasible catalysts and report a RM, dimethylphenazine, with a remarkably low overpotential and high stability.

The rapid growth of portable energy demands has increased interest in next-generation batteries with high energy densities. Among several post-lithium ion battery candidates, Li–O2 batteries have attracted tremendous attention because of their exceptionally high theoretical energy density ( ∼3,500 Wh kg−1)1,2,3,4. The absence of a transition metal and the use of abundant resources of oxygen in the electrochemical reaction further make this chemistry appealing as a promising future battery system5,6,7. However, the present state of Li–O2 batteries involves the critical limitations of low cycle life and high charging overpotential (low energy efficiency) during cycling8,9,10. Although the cycle life and high overpotential are believed to be correlated to each other to some extent, the origin of the latter is often attributed to several factors in the decomposition process of Li2O2, such as the sluggish oxygen evolution from the Li2O2 crystal, low electrical conductivity, and formation/decomposition of unwanted products, including Li2CO3 (refs 11,12,13,14). Furthermore, the solid discharge product (Li2O2) in Li–O2 batteries is irregularly formed on the electrode; thus, the porous air cathode is easily clogged, hindering the efficient transport of oxygen and ions, causing further overpotential15,16. Addressing these issues and enhancing the energy efficiency by decreasing the overpotential are indispensable in taking advantage of the high energy density of the Li–O2 battery chemistry.