Theoretical study on the utilization of N, O-functionalized graphene quantum dots in Li-S batteries

Lithium-sulfur (Li-S) batteries have high theoretical capacity and high energy density, and their practical application is difficult because of the “shuttle effect” induced by lithium polysulfides (LiPSs) and the difficulty of insoluble Li2S decomposition. Functionalized graphene quantum dots (GQDs) can anchor LiPSs and suppress the “shuttle effect” because of the various polar functional groups. This work investigates the effect of the size of GQDs and the N/O functional groups on the anchoring performance by first-principles calculations. In addition, the influence on the decomposition of Li2S, the diffusion of Li+, and the Gibbs free energy needed for the sulfur reduction reaction was also studied. The results show that the GQDs can effectively anchor LiPSs when the size is larger than 1.44 nm, and all the N/O-functionalized GQDs can also effectively anchor LiPSs. Among these N/O-functionalized GQDs, GQDs-COOH has the smallest Li+ diffusion energy barrier (0.52 eV) and low Li2S decomposition energy barrier (1.12 eV). It indicates that GQDs-COOH enables the rapid diffusion of Li+, ensures the smooth decomposition of Li2S, and catalyzes the effective conversion of LiPSs. Moreover, this work will promote the application of GQDs in Li-S batteries.