True Reaction Sites on Discharge in Li-O-2 Batteries

In the pursuit of an advanced Li-O-2 battery, the true reaction sites in the cathode determined its cell performance and the catalyst design. When the first layer of insulating Li2O2 solid is deposited on the electrode substrate during discharging, the following O-2 reduction to Li2O2 could take place either at the electrode vertical bar Li2O2 interface or at the Li2O2 vertical bar electrolyte interface. The mechanism decides the strategies of catalyst design; however, it is still mysterious. Here, we used rotate ring-disk electrode to deposit a dense Li2O2 film and labeled the Li2O2 product with O-16/O-18 isotope. By identification of the distribution of the (Li2O2)-O-16 and (Li2O2)-O-18 in the Li2O2 film using new characteristic signals of (Li2O2)-O-16 and (Li2O2)-O-18, our results show that O-2 is reduced to Li2O2 at both interfaces. A sandwich structure of (Li2O2)-O-18 vertical bar(Li2O2)-O-16 vertical bar(Li2O2)-O-18 was identified at the electrode surface when the electrode was discharged under O-16(2) and then O-18(2). The electrode vertical bar Li2O2 interface is the major reaction site, and it contributes to 75% of the overall reaction. This new mechanism raises new challenges and new strategies for the catalyst design of Li-O-2 batteries.