True Reaction Sites on Discharge in Li-O₂ Batteries

In the pursuit of an advanced Li-O₂ 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 O₂ reduction to Li₂O₂ could take place either at the electrode|Li₂O₂ interface or at the Li₂O₂|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 ¹⁶O/¹⁸O isotope. By identification of the distribution of the Li₂¹⁶O₂ and Li₂¹⁸O₂ in the Li₂O₂ film using new characteristic signals of Li₂¹⁶O₂ and Li₂¹⁸O₂, our results show that O₂ is reduced to Li₂O₂ at both interfaces. A sandwich structure of Li₂¹⁸O₂|Li₂¹⁶O₂|Li₂¹⁸O₂ was identified at the electrode surface when the electrode was discharged under ¹⁶O₂ and then ¹⁸O₂. The electrode|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-O₂ batteries.