A direct molecular orbital-molecular dynamics study on the diffusion of the Li ion on a fluorinated graphene surface

A direct molecular orbital-molecular dynamics (MO-MD) method has been applied to diffusion processes of the Li+ ion on a fluorinated graphene surface. A graphene sheet composed Of C96F24 (denoted by F-graphene) was used as a model of the fluorinated graphene surface. The total energy and energy gradient on the full dimensional potential energy surface of the Li+C96F24 system were calculated at each time step in the trajectory calculation. The calculations were carried out at the AM I level. Simulation temperatures were chosen in the range 200-1000 K. At low temperatures, below 200 K, the diffusion of lithium ion did not occur, and the ion vibrates around an equilibrium point. At around room temperature (similar to 300 K), the lithium ion diffused freely on the surface, but the ion did not approach to the edge region of the surface. This is due to the repulsive interaction with positively charged carbon atom connecting to the fluorine atom where the C-F bond is polarized as C delta+-F delta-. The repulsive interaction strongly dominates the diffusion path of the Li+ ion on the F-graphene. However, the order of magnitude of diffusion coefficient for the Li+ ion moving on the F-graphene surface was close to that of the normal graphene surface (H-graphene). At higher temperatures, the Li+ ion moves freely on the F-graphene, and it fell in the edge region. On the basis of theoretical results, we designed a molecular device composed of F-graphite sheets.