Potential dependent changes in the structural and dynamical properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on graphite electrodes revealed by molecular dynamics simulations

An understanding of the characteristics of ionic liquid/graphite interfaces is highly important for electrochemical devices such as batteries and capacitors. In this paper, we report microscopic studies of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on charged graphite electrodes using molecular dynamics simulations to reveal the two-dimensional arrangement of the ions and their dynamics at the interfaces. Analyses of surface distribution and mobility of ions revealed that the ion arrangement changes from a bilayer type to a checkerboard type with increasing applied potential. Whereas the bilayer type arrangement increases the ionic mobility parallel to the interfaces with the negative potential, the ions arranged in the checkerboard type tend to localize because of the increased lateral electrostatic interactions. Furthermore, we revealed that the inhomogeneity of ionic distribution at the positive potential propagates up to a few nanometers from the interface.