A molecular dynamics simulation study on ion-conducting polymer sPBI-PS(Li+)

To understand the mechanism of ionic migration in the amorphous matrixes of polymer electrolytes is crucial for their applications in modern technologies. Here, molecular dynamics (MD) simulation was carried out to investigate the ionic conduction mechanism of a particular conjugated rigid-rod polymer, sPBI-PS(Li+). The backbone of this polymer is poly[(1,7-dihydrobenzo[1,2-d:4,5-d']diimidazole-2,6-diyl)-2-(2-sulfo)-p-phenylene]. The polymer has pendants of propane sulfonate Li+ ionomer. The MD simulations showed that the main chains of sPBI-PS(Li+) are in a layer-like structure. The further detailed structure analysis suggested that the pi-electron of this polymer is not delocalized among aromatic rings. This agrees with the experimental result that sPBI-PS(Li+) shows no electronic conductivity, and the conductivity of this polymer is mainly ionic. The calculated migration channels of lithium ions and electrostatic potential distributions indicated clearly that the polymer matrix is anisotropic for the migrations of ions. The migration of lithium ions along the longitudinal direction is more preferable than that along the transverse direction. The relaxations of the polymer host were found to play important roles in the transfer process of lithium ions. The hopping of lithium ions from one -SO3-1 group to another correlated strongly with characteristic motions of -SO3-1 group on a time scale of about 10(-13) s.