Rechargeable battery such as Li-ion battery will be the main focus worldwide for decades to come. However, energy storage beyond the capabilities of Li-ion technologies are becoming increasingly important to satisfy society’s future energy storage needs. One such alternative is aprotic Li-air (O₂) battery with its theoretical specific energy several times higher than that of Li-ion. However, many challenges including early cell death, high overpotential and poor cycle life need to be addressed before its commercialisation. Fundamental studies of the processes at the positive electrode have shown that this is a result of passivating Li₂O₂ film at the electrode surface.

It is generally accepted that solution growth of discharge product Li₂O₂ is required to achieve high rates and capacities. One way to achieve solution mechanism is use of high donor or acceptor number electrolytes, however, such electrolytes are less stable towards the reactive oxygen species in aprotic Li-O₂ cell. Here, we discuss our new strategies to use redox mediators as soluble catalysts, to reduce O₂ to Li₂O₂ by halving the overpotential on discharge, without forming passivating films and with less side reactions in ether solvents, while on charge Li₂O₂ can be oxidized rapidly to O₂ even when the former is not in contact with the electrode surface, Fig. 1. As a result, cycling of an aprotic Li-O₂ cell has been demonstrated with rates and capacities of several mA and mAh cm^-2 respectively. The advantages and disadvantages of redox mediators during cycling and decomposition of carbon electrode will be discussed.

Figure 1: Schematics of positive electrode reactions on discharge and charge in the presence of dual mediators.