Enhanced Ionic Conductivity of confined Ionic-Liquid in Angstrom-scale 2D channels

This study demonstrates that confining the ionic liquid [EMIM]+[TFSI]- within angstrom-scale 2D channels induces structural rearrangements that maximize ionic conductivity at specific heights, achieving over 30 times the bulk value, while further enhancement to ~145 S/m is achieved by introducing co-solvents with high dielectric constants and low viscosity.

The Gist

Imagine you have a very thick, sticky honey made of tiny, charged particles (ions) instead of sugar molecules. Normally, this honey flows slowly because the particles get stuck to each other, forming tight little clumps. Scientists call this an "ionic liquid."

This paper is about what happens when you squeeze this sticky honey into a hallway so narrow that it's only a few atoms wide. You might think squeezing it would make it move even slower, like trying to run through a crowded hallway. But the researchers discovered something surprising: if you squeeze it just the right amount, the honey suddenly starts flowing incredibly fast—30 times faster than usual.

Here is a breakdown of their discovery using simple analogies:

1. The "Goldilocks" Hallway

The researchers built tiny, flat tunnels (channels) using layers of special materials like graphene and boron nitride. They could adjust the height of these tunnels with extreme precision, down to the size of a single atom.

• Too Narrow (The Traffic Jam): When the tunnel was extremely tight (about 6.8 Ångströms high), the ions were crushed together. They couldn't move because they were too crowded. It was like trying to dance in a closet; the walls were too close, and everyone was stuck.
• Too Wide (The Normal Flow): When the tunnel was wide (like a normal room), the ions behaved like they did in a jar of honey. They moved at a normal, slow pace.
• Just Right (The Superhighway): When they made the tunnel a specific "just right" size (about 10.2 Ångströms high), something magical happened. The ions rearranged themselves into neat, organized layers. Instead of being a messy, sticky clump, they lined up like soldiers or cars in a well-ordered traffic lane. This structure broke up the sticky clumps, allowing the ions to zip through the tunnel at record speeds.

2. The "Lubricant" Effect

The researchers then tried adding different liquids (solvents) to their sticky honey to see if they could make it flow even better. Think of these solvents as different types of oil or water you mix into the honey.

• Acetonitrile (The Magic Lubricant): They added a liquid called Acetonitrile (ACN). This liquid is like a super-powerful lubricant. It has a special ability to pull the sticky ions apart, breaking the clumps so they can move freely. When they mixed this into the "Goldilocks" tunnel, the flow speed skyrocketed to 145 S.m-1. This is a massive jump, making the liquid conduct electricity almost 150 times faster than the original thick honey.
• Other Liquids: They tried other liquids (DMC and DEC) that were less effective. These were like thinner oils that didn't separate the ions as well, so the speed boost wasn't as dramatic.

3. Why This Matters (According to the Paper)

The paper explains that this isn't just about making things faster; it's about understanding how matter behaves when it is squeezed into tiny spaces.

• Structure is Key: The speed boost happens because the narrow space forces the ions to organize. In the "Goldilocks" zone, the ions stop hugging each other (which slows them down) and start sliding past one another easily.
• The Balance: If you squeeze too hard, you get a traffic jam. If you don't squeeze enough, the ions stay in their slow, clumpy state. You need that perfect, atomic-scale squeeze to unlock the super-speed.

Summary

The scientists took a thick, slow-moving liquid, squeezed it into a hallway that was only a few atoms high, and found that at a specific width, the liquid suddenly became a super-fast conductor. By adding a special "lubricant" liquid, they made it even faster. They proved that by controlling the size of the hallway and the type of liquid inside, you can manipulate how fast electricity moves through it, turning a slow, sticky substance into a high-speed flow.

Technical Summary

Technical Summary: Enhanced Ionic Conductivity of Confined Ionic-Liquid in Ångstrom-scale 2D Channels

Problem Statement
Understanding ion transport under molecular confinement is critical for advancing next-generation energy technologies, particularly where ionic motion occurs within nanoscale or Ångstrom-scale channels. While bulk properties of room-temperature ionic liquids (ILs), such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM]+[TFSI]-), are well-characterized, their behavior under extreme confinement remains poorly understood. Classical continuum models often fail in these regimes, where ion-wall interactions, strong Coulomb correlations, and steric effects dominate. Previous studies on nanoporous materials have shown improved energy density, but a systematic investigation into how Å-scale confinement specifically modulates ionic conductivity ($\sigma$) and the underlying structural mechanisms was lacking.

Methodology
The study employed a combined experimental and computational approach using a model system of [EMIM]+[TFSI]- confined within precisely tunable, slit-shaped 2D channels.

• Fabrication: The 2D channels were constructed via van der Waals (vdW) assembly of hexagonal boron nitride (hBN) top and bottom layers with graphene strips acting as spacers. This allowed for precise control of channel height ($h$) ranging from ~6.8 Å to 660 Å, determined by the number of graphene layers ($h = n \times 3.4$ Å).
• Electrochemical Measurements: Ionic conductivity was measured using a custom electrochemical cell with Ag/AgCl electrodes. Current-voltage (I-V) characteristics were recorded for pure IL and IL mixtures with co-solvents (acetonitrile/ACN, dimethyl carbonate/DMC, and diethyl carbonate/DEC) at various concentrations.
• Molecular Dynamics (MD) Simulations: All-atom MD simulations were performed using GROMACS to model the IL within the slit channels. The Green-Kubo (GK) formalism was utilized to calculate conductivity, accounting for all interactions in dense ionic systems. Simulations analyzed ion number-density distributions, rotational degrees of freedom, and free-ion populations to correlate structural changes with transport properties.

Key Contributions and Results

1. Non-Monotonic Conductivity Dependence on Confinement:
   The study revealed a non-monotonic relationship between channel height ($h$) and ionic conductivity ($\sigma$).

   • Bulk Regime: For $h \ge 92$ Å, $\sigma$ matched bulk values (~0.87 S.m$^{-1}$).
   • Enhanced Regime: As confinement increased, $\sigma$ rose significantly, reaching a maximum of ~26.7 S.m$^{-1}$ at $h \approx 10.2$ Å. This represents an enhancement of over 30 times the bulk value.
   • Steric Hindrance Regime: Further reducing $h$ to 6.8 Å caused $\sigma$ to drop below bulk values due to steric hindrance limiting ion mobility.

2. Structural Mechanism of Enhancement:
   MD simulations elucidated that the conductivity peak at $h \approx 10.2$ Å arises from a specific structural reorganization of the ionic liquid:

   • At the narrowest limit ($h \approx 3.3$ Å), ions form a single mixed layer with crystal-like ordering and strong cation-anion interlocking, which restricts mobility.
   • At $h \approx 10.2$ Å, the ions reorganize into a distinct three-layer structure (cation-anion-cation). In this configuration, cations near the walls and in the center adopt a parallel orientation relative to the channel walls, while anions occupy the intermediate layer.
   • This arrangement disrupts crystal-like ordering, weakens electrostatic caging, and increases the population of "free ions" (peaking at ~19% at 10 Å), thereby enhancing in-plane diffusivity and conductivity.

3. Solvent Modulation:
   The introduction of co-solvents further modulated conductivity by altering ion dissociation and mobility:

   • Acetonitrile (ACN): Due to its high dielectric constant ($\varepsilon \approx 37.5$) and low viscosity ($\eta$), ACN promoted significant ion pair dissociation. In a 10.2 Å channel, diluting the IL with ACN increased $\sigma$ to a maximum of ~145 S.m$^{-1}$.
   • DMC and DEC: Solvents with lower dielectric constants (DMC: $\varepsilon \approx 3.1$, DEC: $\varepsilon \approx 2.8$) showed lower peak conductivities compared to ACN, highlighting the critical role of dielectric screening in promoting free ion populations under confinement.
   • The optimal conductivity in all solvent systems was consistently observed at $h \approx 10.2$ Å, confirming that geometric confinement and solvent properties act synergistically.

Significance
The paper establishes confined [EMIM]+[TFSI]- as a representative model system for probing mechanisms of nano- and Å-scale ion transport. The primary significance lies in demonstrating that ionic conductivity can be systematically manipulated at the molecular level by tuning two parameters:

1. Geometric Confinement: Precisely controlling channel height to induce specific ion layering and orientation that maximizes free-ion fractions and mobility.
2. Solvent Environment: Selecting co-solvents with optimal dielectric constants and viscosities to further enhance ion dissociation and transport.

The work provides a foundational understanding of how extreme confinement alters the fundamental physics of ionic liquids, moving beyond bulk properties to reveal transport regimes dominated by molecular packing and surface interactions. This knowledge is presented as essential for the development of next-generation iontronic technologies, including energy storage devices and molecular sensing, where ion transport occurs within Ångstrom-scale channels.