Electrosatic screening, dynamics and structure of [BMIM+][BF4-] and [BMIM+][PF6-] with confinement

Ionic liquids are spotlighted as electrolytes for batteries and supercapacitors. Such applications have nanoconfined systems, where deviation of electrostatic screening, dynamics, and structure of ionic liquids from bulk properties is caused. Although electrostatic screening is a fundamental interaction for any charged particles, electrostatic screening condition for ionic liquids is not discovered yet in confined systems. To fully utilize ionic liquids in electronic devices, it should be fully revealed how confinement alters physical properties of ionic liquids. Here, we conducted molecular dynamics simulations of [BMIM+][BF4-] and [BMIM+][PF6-] between two graphene sheets with gap of 1.5 to 4.5 nm. Confined ionic liquids show slower dynamics by 2 orders of magnitude compared to bulk dynamics and have nonmonotonic behavior in dynamics. In addition to dynamics, we observed that capacitance is independent of confinement length. To explain this confinement effect, we computed electrostatic screening condition derived from Stillinger-Lovett sum rules. Furthermore, we investigated polarization effect on electrolytes and electrodes in confined system.