New Insights into Li2S2/Li2S Adsorption on the Graphene Bearing Single Vacancy: A DFT Study

Abstract Graphene with single vacancy (SV) defect is investigated as the electrode substrate for Li-S battery and its reaction towards lithium polysulfide (LiPS) molecules are investigated by DFT calculations. It is found that the presence of SV defect changes the local electronic structure of graphene and enhances the adsorption energy to Li2Sx (x=1,2,4,8) molecule, especially for the insoluble final discharge product. When Li2S molecule approaches SV defected graphene, the S atom in Li2S directly connect to the C atom having unpaired electron in graphene. This unique S-C binding behavior is very seldom reported previously. Compared with other LiPSs, the adsorption height of Li2S on SV defected graphene is the lowest due to this strong S-C binding. At the same time, the interaction between SV defected graphene and Li2S2 molecule is also enhanced compared with pristine graphene due to the presence of unpaired electron, although the adsorption energy and adsorption height is smaller than that in the case of Li2S. Based on electronic structure analysis, covalent character bond is found between S and C atom because that the frontier orbitals of Li2S molecule and SV defected graphene have close energy and the symmetry also adapts. However, interaction between Li2S2 and SV defected graphene material is through a Li-C bond with ionic character, in which Li2S2 molecule act as an electron donor. The Li-C bond is also found when Li2S molecule is adsorbed on the pristine graphene. This work provides new insights to LiPS adsorption on carbon materials and gives inspiration to research on cathode of Li-S battery. Since it can work as the effective nucleation sites to enhance the affinity to Li2S, carbon materials with SV defect might be a good cathode substrate for Li-S battery to make Li2S particles deposit uniformly. Further theoretical and experimental research on the following growth of Li2S particle around the SV defect is still ongoing.