Unraveling the Positive Roles of Point Defects on Carbon Surfaces in Nonaqueous Lithium-Oxygen Batteries

Carbon has been widely used to form cathodes for nonaqueous lithium-oxygen (Li-O-2) batteries due to its high specific surface area, high electrical conductivity, and cost-effectiveness. The mechanistic understanding of carbon materials, particularly the effect of carbon surface properties on the battery's performance, however, is limited. In this work, we perform first-principles calculations to study the roles of point defects on carbon surfaces. Five representative defective structures, including SV (single vacancy), DV5-8-5 (two pentagons and one octagon), DVSSS-777 (three pentagons and three heptagons), DVSSSS-6-7777 (four pentagons, one hexagon, and four heptagons), and SW (Stone-Wales) defects, are considered. On the basis of the adsorption energies of O-2 and Li, the different Li4O4 growing pathways on these structures are identified, and free energy diagrams are then obtained. It is found that the presence of DVS555-6-7777 and SW defects is beneficial to nonaqueous Li-O-2 batteries because (i) DVS555-6-7777 and SW defects exhibit zero-band-gap semiconductor behaviors, ensuring excellent electrical conductivity; (ii) DVSSSS-6-7777 and SW defects lead to a high discharge voltage but low charge voltage; DV5555-6-7777 and SW defects are stable during battery cycling and do not promote the formation of side product Li2CO3; and (iv) by using DMSO as a sample, DVS555-6-7777 and SW defects are not supposed to decompose electrolytes. Hence, carbon materials containing DVSSSS-6-7777 and SW defects are desired for nonaqueous Li-O-2 batteries.