The Importance of Ion Packing on the Dynamics of Ionic Liquids during Micropore Charging

Abstract

There is an emerging concern that using room-temperature ionic liquids (RTILs) together with microporous electrodes may compromise supercapacitors’ power density in spite of their benefit for enhancing energy density due to possibly slow transport of ions inside narrow pores. Based on molecular simulations of the diffusion of EMIM+ and TFSI− ions in slit-shaped micropores (width < 2 nm,) under conditions similar to those during pore charging, we show that, in pores that accommodate only a single layer of ions, the ions diffuse increasingly faster as the pore becomes charged, even faster than Na^+ ions in bulk water. However, this trend can be reversed when the pore becomes highly charged. In pores wide enough to fit more than one layer of ions, the ion diffusion is typically slower than in the bulk, and only changes modestly as the pore becomes charged. Analysis of these results revealed that the fast (or slow) diffusion of ions inside a micropore is correlated most strongly with the dense (or loose) ion packing inside the pore during charging. The molecular details of ions and the precise width of pores modify these trends relatively weakly, except when the pore size is so narrow that the conformation of ions is strongly constrained by the pore walls. Insight from these results should be useful for establishing guidelines for the design of RTILs and porous electrode materials for supercapacitors.