Self-Assembled Nanostructures in Aprotic Ionic Liquids Facilitate Charge Transport at Elevated Pressure

Abstract

Ionic liquids (ILs), revealing a tendency to form self-assembled nanostructures, have emerged as promising materials in various applications, especially in energy storage and conversion. Despite multiple reports discussing the effect of structural factors and external thermodynamic variables on ion organization in a liquid state, little is known about the charge-transport mechanism through the self-assembled nanostructures and how it changes at elevated pressure. To address these issues, we chose three amphiphilic ionic liquids containing the same tetra(alkyl)phosphonium cation and anions differing in size and shape, i.e., thiocyanate [SCN] − , dicyanamide [DCA] − , and tricyanomethanide [TCM] − . From ambient pressure dielectric and mechanical experiments, we found that charge transport of all three examined ILs is viscosity-controlled at high temperatures. On the other hand, ion diffusion is much faster than structural dynamics in a nanostructured supercooled liquid (at T < 210 ± 3 K), which constitutes the first example of conductivity independent from viscosity in neat aprotic ILs. High-pressure measurements and MD simulations reveal that the created nanostructures depend on the anion size and can be modified by compression. For small anions, increasing pressure shapes immobile alkyl chains into lamellar-type phases, leading to increased anisotropic diffusivity of anions through channels. Bulky anions drive the formation of interconnected phases with continuous 3D curvature, which render ion transport independent of pressure. This work offers insight into the design of highdensity electrolytes with percolating conductive phases providing efficient ion flow.

Inflection Point in Pressure Dependence of Ionic Conductivity as a Fingerprint of Local Structure Formation

“…In this context, an alternative explanation that takes into account the changes in IL self-organization needs to be considered. In this context, a significant deviation from a single VFT law recently observed for several phosphonium ILs at the temperature of liquid–liquid transition, T LL , , needs to be recalled. Namely, it has been shown that tetra­(alkyl)­phosphonium ILs with a 14-carbon alkyl chain exist in two supercooled phases differing from each other in a tendency toward self-assembly and temperature sensitivity of ion dynamics.…”

“…A slow, sub-α relaxation in the dielectric spectra attributed to polarization at the interfaces of polar and nonpolar domains of the aggregates has been found in a series of alkyl-imidazolium 24 and phosphonium 25 ILs. Furthermore, the temperature dependence of conductivity relaxation times (τ σ ), describing the time scale of ion motions, demonstrated an increased slope in the vicinity of the liquid− liquid transition, originating from the self-organization of trihexyl(tetradecyl)phosphonium cations, 26,27 as evidenced by SAXS measurements. The same behavior has also been observed for the temperature evolution of the viscosity (η) of [P 666,14 ][DCA] and the ion dynamics of other ILs with evidence of liquid−liquid transition.…”

Non-Isochronal Behavior of Charge Transport at Liquid–Liquid and Liquid–Glass Transition in Aprotic Ionic Liquids

“…On the other hand, below the LLT in a nanostructured supercooled phase, their ion diffusion becomes much faster than structural dynamics. 18 It has been shown that long alkyl chains of cations form a skeleton that contributes substantially to structural dynamics, while anions travel easily through created channels and thus govern the charge transport. Such a decoupling between the time scale of charge transport (τ σ ) and structural relaxation (τ α ) is maintained down to the liquid–glass transition ( T g ).…”

“…As a result, the decoupling between the time scales of conductivity and structural relaxation revealed a maximum at P g = 170 MPa for [P 666,14 ][SCN]. 18 On the other hand, as the anion size increases, the formation of channels for charge diffusion was found to be more challenging. Consequently, the effect of pressure on molecular packing of [P 666,14 ][TCM] was weak, leading to the constant value of τ σ ( T g , P g ) under various temperature and pressure conditions.…”

“…The self-assembly of phosphate cations originating LLT was found when the temperature drops below 203 K. Above this temperature limit, [P 666,14 ]-based ILs (e.g., trihexyl­(tetradecyl)­phosphonium dicyanamide [P 666,14 ]­[DCA], trihexyl­(tetradecyl)­phosphonium tricyanomethanide [P 666,14 ]­[TCM], or trihexyl­(tetradecyl)­phosphonium thiocyanate [P 666,14 ]­[SCN]) exist in a supercooled liquid state with the charge transport being viscosity-controlled, as for typical aprotic ionic liquids. On the other hand, below the LLT in a nanostructured supercooled phase, their ion diffusion becomes much faster than structural dynamics . It has been shown that long alkyl chains of cations form a skeleton that contributes substantially to structural dynamics, while anions travel easily through created channels and thus govern the charge transport.…”