Structural Anomalies and Electronic Properties of an Ionic Liquid under Nanoscale Confinement

Abstract

Ionic liquids promise far greater electrochemical performance compared to aqueous systems, yet key physicochemical properties governing their assembly at interfaces within commonly used graphitic nanopores remain poorly understood. In this work, we combine synchrotron X-ray scattering with first-principles molecular dynamics simulations to unravel key structural characteristics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([TFSI] -) ionic liquids confined in carbon slit pores. X-ray scattering reveals selective pore filling due to size exclusion, while filled pores exhibit disruption in IL intermolecular structure, the extent of which increases for narrower slit pores. First-principles simulations corroborate this finding and quantitatively describe how perturbations in the local IL structure, particularly the hydrogenbond network, depend strongly on the degree of confinement. Despite significant deviations in structure under confinement, electrochemical stability remains intact, which is important for energy storage based on nanoporous carbon electrodes (e.g., supercapacitors).

Abnormal Enhanced Free Ions of Ionic Liquids Confined in Carbon Nanochannels

“…Such heterogeneous confined structures will also lead to the charge separation of ILs, resulting in the layered distribution of atomic charge as shown in Figure S2. The evolution trend of the confined structure of ILs is similar to that in the previous simulation and experimental works, ,, implying the rationality of the simulated procedure adopted here.…”

Machine learning predictions of diffusion in bulk and confined ionic liquids using simple descriptors

“…19,20 Predicting the behavior of ILs under confinement presents a unique challenge since the effect of confinement on ionion correlation is not well understood. 21 As reviewed recently, 10 experimental methods used to study IL structural properties in porous environments include NMR, 22,23 X-ray and neutron scattering, [24][25][26] X-ray absorption spectroscopy, 27,28 calorimetry, [29][30][31] microscopy, 32,33 and impedance spectroscopy. 34,35 Molecular dynamics (MD) simulation methods are well suited to investigate the properties of confined ILs since the properties of the liquid or individual ion components in porous environments can be unambiguously obtained.…”

“…21 Classical and ab initio MD simulations of ILs confined in graphite slit pores indicate that small slit pore sizes disrupt the bulk liquid structure, particularly the hydrogen bonding network. 26,39 Additionally, simulations of an IL dissolved in an organic solvent showed a large decrease in anion diffusion confined in a graphite slit pore compared to the bulk fluid, likely caused by anion adsorption onto the graphite surface. 23 Electrical conductivity of confined ILs has also been studied using MD simulation, showing that conductivity increases with slit pore size since the fraction of ions in a bulk environment increases.…”

Intercalation of Lithium into Graphite: Insights from First-Principles Simulations

“…Electrode–electrolyte interfaces are essential components in a variety of energy and environmental technologies, ranging from ion batteries − and supercapacitors − for energy storage to membranes and capacitive deionization for water desalination and purification. − Within these devices, a detailed understanding of ion transport and intercalation at the interface is crucial for predicting and optimizing performance. However, this remains a significant challenge since ion intercalation from electrolyte to electrode is a complex multiscale process that involves several steps, including ion desolvation and migration at the electrode–electrolyte interface that may involve multiple materials with complex chemical composition and chemistry.…”