Densification of ionic liquid molecules within a hierarchical nanoporous carbon structure revealed by small-angle scattering and molecular dynamics simulation

The molecular-scale properties of the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C ₄mim⁺][Tf₂N^-], confined in nanometer-scale carbon pores have been investigated using small-angle X-ray and neutron scattering and fully atomistic molecular dynamics simulations. [C ₄mim⁺][Tf₂N^-] densities significantly higher than that of the bulk fluid at the same temperature and pressure result from the strong affinity of the RTIL cation with the carbon surface during the initial filling of slitlike, subnanometer micropores along the mesopore surfaces. Subsequent filling of cylindrical ∼8 nm mesopores in the mesoporous carbon matrix is accompanied by weak RTIL densification. The relative size of the micropores compared to the ion dimension, and the strong interaction between the RTIL and the slit-like micropore, disrupt the bulk RTIL structure. This results in a low-excluded volume, high-RTIL ion density configuration. The observed interfacial phenomena are simulated using a molecular dynamics model consisting of a linear combination of mesopore and micropore effects. These observations highlight the importance of including the effects of a porous substrate's internal surface morphology, especially roughness and microporosity, on the resulting electrolyte structural properties and performance in electrical energy storage applications.