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 Li 2 O 2 solid is deposited on the electrode substrate during discharging, the following O 2 reduction to Li 2 O 2 could take place either at the electrode|Li 2 O 2 interface or at the Li 2 O 2 |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 Li 2 O 2 film and labeled the Li 2 O 2 product with 16 O/ 18 O isotope. By identification of the distribution of the Li 2 16 O 2 and Li 2 18 O 2 in the Li 2 O 2 film using new characteristic signals of Li 2 16 O 2 and Li 2 18 O 2 , our results show that O 2 is reduced to Li 2 O 2 at both interfaces. A sandwich structure of Li 2 18 O 2 |Li 2 16 O 2 |Li 2 18 O 2 was identified at the electrode surface when the electrode was discharged under 16 O 2 and then 18 O 2 . The electrode|Li 2 O 2 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.