Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [Cnmim][TFSI] Ionic Liquids at Graphite Electrodes

The dependence on electrode potential of the interfacial structure and differential capacitance (DC) for 1-alkyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)mim][TFSI], n = 2, 4, 6, and 8) ionic liquids (IL) near basal (flat) and prismatic edge face (rough) graphite electrodes was investigated here with atomistic simulations. Overall camel-shaped DCs were observed for both surfaces. The prismatic graphite generated systematically larger capacitances than the atomically flat basal face. Although on the flat electrodes the DC is almost constant at electrode potential bellow saturation (i.e., roughly within +/- 2 V), on the prismatic edge face the DC showed large amplitude changes between minima and maxima. This trend in DC was explained from the dependence versus potential of the structure and composition of the interfacial electrolyte layer; specifically, faster counterions accumulation and ion segregation in the interfacial layer are observed for atomically corrugated electrode surfaces as compared to the flat ones. Surprisingly, the increase of the charge-neutral alkyl tail length of the cation resulted only in a small reduction in DC, indicating ions ability to rearrange/reorient charge-caring groups such that it maximizes the counterions charge near the surface. This finding shows a promising route for optimization of ions structure to achieve the desired/optimal properties of electrolyte (e.g., low melting point and viscosity) without significant reduction of energy density storage capabilities.