Potential-Dependent Oxidation Mechanism of Li2O2 and Its Application in Li-O2 Batteries

Abstract

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. Figure 1