Investigation of the usage of carbon-based two dimensional materials in lithium sulfide (Li-S) batteries

After the synthesis of graphene (Gr), the other two-dimensional (2D) allotropes of carbon based materials have been predicted theoretically or synthesized by experimentally. Two of these nanomaterials are graphyne (Gy) and graphdiyne (Gdy) which are close relatives of Gr and can be synthesized in bulk. While carbon atoms in Gr bonds with sp2 bonds, Gy and Gdy comprise sp and sp2 hybridized carbon atoms with high degrees of π conjugation that features uniformly distributed pores. So, these intrinsic nanopores add distinction to Gy and Gdy with respect to the Gr especially for energy storage applications. Nowadays, the researchers are studying to improve the storage capacity of Lithium-ion batteries (LIBs). However, Lithium–Sulfur (Li–S) batteries can store approximately five times more energy in terms of energy density compared to LIBs. In this study, we have systematically investigated the lithium polysulfides (LixSy; 1≤x≤2,1≤ y ≤ 8) adsorbed on Gr, Gy and Gdy monolayers by means of density functional theory (DFT) calculations. Our results reveal that, the presence of sp bonds and the size of nanopores in the structure extremely affect the adsorption energy of small sized LixSy clusters. However, the adsorption energies obtained with increasing the size of LixSy clusters are almost similar in Gr, Gy and Gdy monolayers. The diffusion barrier energy values obtained for LixSy clusters indicate that the migration of clusters on Gr may be easy but it is hard on Gdy and especially on Gy due to the pore networks in the structures acting as traps to anchor the LixSy clusters. In addition, we have investigated the double adsorption of LixSy clusters on Gr, Gy and Gdy structures to search the shuttle effect. It can be reported that LixSy clusters do not bind each others and do not create long polysulfide chains, which is not requested.