A Self-Promotion Effect during ORR Process in Lithium Oxygen Battery

Abstract

Rechargeable lithium oxygen batteries have attracted enormous attention due to its high theoretical energy density which could fulfill the ever increased energy requirement for electrification of road transport. There are numerous significant problems hinder it moving from theory to practice, especially the fundamental understanding of ORR and OER processes. Great efforts have been paid on the fundamental mechanism of O2 reduction in nonaqueous electrolyte. In this study, by combining cyclic voltammetry (CV) with in-situ surface enhanced Raman spectroscopy (SERS), we show a self-promotion effect during ORR process in aprotic lithium oxygen battery, that is the residual lithium peroxide after charge can act as an effective nucleation site for subsequent discharge process, promoting the lithium peroxide precipitation. The promotion effect becomes more obvious as cycles continue, then recedes after dozens of cycles since the electrode surface is blocked by side reaction products. This self-promotion effect is strongly affected by the solvent, as reported previously, the pathway of O2 reduction to form Li2O2 is related to the solvent donor number (DN). We proved here the self-promotion effect of lithium peroxide is obviously observed in intermediate DN solvents in which lithium peroxide precipitation undergoes electrode surface and solution pathways simultaneously. As a result of the self-promotion effect, the discharge capacity increases as cycles continue during first dozens of cycles. In summary, this work provide more insights into the ORR process for further fundamental understanding of lithium oxygen battery.