Non-aqueous lithium-oxygen (Li-O₂) batteries have attracted intensive research attentions owing to their potential to provide gravimetric energy density 3–5 times that of conventional Li-ion batteries. In-depth understandings of the reaction mechanisms during discharge and charge are the prerequisites for further advancement of the Li-O₂ technology. The ORR has been widely established to generate Li₂O₂ by surface (electrochemical) or solution (chemical) pathway via manipulating the fate of superoxide intermediate (LiO₂). However, the reaction mechanism of the charging reaction in Li-O₂ batteries (OER, or Li₂O₂ oxidation) is still under debates.

In this study, we report a potential-dependent oxidation mechanism of Li₂O₂ (charging mechanism) by examining the reaction intermediate species at a wide range of charging overpotential via thin-film rotating–ring disk electrode (RRDE) and synchrotron-based X-ray absorption near edge structure (XANES). To investigate the oxidation behavior of Li₂O₂ at lower overpotential, we used ruthenium (Ru) as the model catalyst to control the charging potential. To make the amount of reactant comparable to that in regular Li-O₂ cells, we here employ thin-film RRDE by drop casting Valcun Carbon (VC) or VC supported Ru (Ru/VC) particles onto the glass carbon electrode (disk) to increase the total surface area, as shown in Figure 1. XANES further provides the chemical information of reaction intermediates and parasitic product that caused by the intermediates/discharge product. We will discuss potential-dependent charging reaction pathways and provide insights in the design strategy to achieve efficient Li-O₂ batteries.

Acknowledgement

This work is supported by two grants from Research Grants Council (RGC) of the Hong Kong Special Administrative Region, China, under No. C7051-17G and CUHK 14207517.