Remarkable improvement of cyclic stability in Li–O2 batteries using ruthenocene as a redox mediator

Abstract

Nonaqueous Li–O2 batteries attract attention for their theoretical specific energy density. However, due to the difficulty of decomposition of Li2O2, Li–O2 batteries have high charge overpotential and poor cycling life. So all kinds of catalysts have been studied on the cathode. Compared to heterogeneous solid catalysts, soluble catalysts achieve faster and more effective transport of electrons by reversible redox pairs. Here, we first report ruthenocene (Ruc) as a mobile redox mediator in a Li–O2 battery. 0.01 mol/L Ruc in the electrolyte effectively reduces the charging voltage by 610 mV. Additionally, Ruc greatly increases the cycling life by four-fold (up to 83 cycles) with a simple ketjen black (KB) cathode. The results of SEM, XPS and XRD confirm that less discharge product residue accumulated after recharge. To verify the reaction mechanisms of the mediator, free energy profiles of the possible reaction pathways based on DFT are provided.