ИСТИНА

Nowadays Li-air batteries attract much attention worldwide due to their highest theoretical specific energy and opportunity to serve as a power sources for the electric vehicles. However, practically passivation of the positive electrode surface caused by the discharge products like lithium peroxide leads to a strong decrease in cycleability of the Li-air cells. Among different materials that can be used as the positive electrode carbons are the most promising ones due to its high catalytic activity towards oxygen reduction coupled with the high electronic conductivity, low specific weight and commercial availability. Using carbon cathodes, specific capacity higher then 10 Ah/g could be achieved at the first discharge cycle, however, it decreases significantly very rapidly. Near ambient pressure XPS and cyclic voltammetry studies recently revealed that during battery discharge oxygen is firstly reduced to superoxide radical that further reacts with both electrolyte and carbon electrode surface forming lithium carbonate and other byproducts. Such reactions lead to positive electrode degradation. Recent theoretical works discussed the influence of the carbon surface structure and defects both on the lithium peroxide and carbonate formation. Better understanding of these processes would sufficiently advance the design of an effective and stable cathodes for Li-air battery. In this work we compare oxygen reduction kinetics in presence of Li+ ions on different carbon disc electrodes: glassy carbon, highly oriented pyrolitic graphite (HOPG), basal plane and edge plane pyrolitic graphites. Amount of defects in graphite lattice for each type of carbon was estimated using Raman spectroscopy. Tafel slopes were utilized to determine the exchange current density for the oxygen reduction reaction in presence of Li+ ions on different carbon electrode surfaces. We demonstrated a correlation between the ORR exchange current density and amount of surface defects in carbon structure. The highest ORR exchange current density 2 μA/cm2 appeared on the edge plain graphite with the greatest amount of surface defects, while on the highly oriented pyrolitic graphite with almost no defects ORR exchange current density is 10 times smaller. At the same time detected amount of lithium carbonate formed due to a side reaction of superoxide radical with carbon electrode material is lowest for HOPG. It demonstrates that the defects and the edges of graphene layers can play a dual role increasing the electrocatalytic activity of carbon material on the one hand but decreasing its stability towards superoxide radical on the other. We also demonstrated that utilization of rather stable HOPG electrodes allows to observe superoxide-to-peroxide electrochemical reduction in case if the solvent with high donor number is employed. Thus the final discharge product Li2O2 can be generated via the second electron transfer to relatively stable Li+(DMSO)3--O2- ionic pairs instead of chemical disproportionation of Li⁺ – O2⁻ .