ИСТИНА

Oxygen reduction reaction (ORR) plays a key role in lithium-air batteries (LABs) that attract great attention thanks to their high theoretical specific energy several times exceeding that of lithium-ion batteries. Because of their high surface area, high electric conductivity, and low specific weight, various carbons are often materials of choice for applications as the LAB cathode. Unfortunately, the possibility of practical application of such batteries is still under question as the sustainable operation of LABs with carbon cathodes is not demonstrated yet and the cyclability is quite poor, which is usually associated Li2CO3 byproduct formation. It is generated at the cathode surface due to lithium superoxide (intermediate product) or peroxide (final discharge product) reactions with carbon. However, the mechanisms of carbon reactivity toward these species are still unclear. Here, we report an in situ X-ray photoelectron spectroscopy study of oxygen reduction reaction on graphene electrode in Li-air battery. Although lithium peroxide (Li2O2) and lithium oxide (Li2O) reactions with carbon are thermodynamically favorable, neither of them was found to react even at elevated temperatures. In contrast, superoxide species were demonstrated to be strongly reactive towards carbon. Using model chemical systems, we study reactivity of graphene with different concentration of vacancies and impurities towards lithium and potassium superoxides. As a result, we suggest detailed mechanism of carbon electrode degradation in LAB.