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 high-density electrolytes with percolating conductive phases providing efficient ion flow.

Keywords: charge transport mechanism; high pressure; ionic liquids; liquid−liquid phase transition; self-assembly.

Figure 1

Differential scanning calorimetry (DSC) traces of [P_666,14]⁺-based ILs. Arrows indicate the onset of LLT (cyan), melting point (red), cold crystallization (green), and T_g (blue). The values of liquid–liquid transition temperature (T_LL), onset of cold crystallization (T_c), melting temperature (T_m), and enthalpy of these processes ΔH are collected in Table S1. The chemical structures of the [P_666,14]⁺ cation and [TCM]⁻, [DCA]⁻, and [SCN]⁻ anions are also presented.

Figure 2

X-ray scattering intensity of [P_666,14][DCA] at various temperatures recorded on cooling and subsequent heating (inset). Panel a presents SAXS results of [P666,14][DCA], while panel b presents WAXS data of the same IL.

Figure 3

Dielectric response of [P_666,14][DCA] under ambient pressure conditions. (a) Representative dielectric data of [P_666,14][DCA] in glass (blue scatters and lines), liquid 2 (violet scatters), and liquid 1 (solid lines) phases were obtained on cooling. (b) Representative dielectric data of [P_666,14][DCA] measured in the supercooled liquid 1 state (gray symbols) and self-assembled liquid 2 state (violet symbols) superimposed to each other.

Figure 4

(a) The isothermal-time-dependent dielectric measurements were performed in the crystallization range. Each scan was started after a quench from RT. (b) Time evolution of M″ at 0.1 MHz and various temperatures in liquid 1 and liquid 2 states of [P_666,14][DCA]. (c) Representative kinetic curve of [P_666,14][DCA] obtained from data presented in panel b and normalized using the M_norm^″(f) = (right axis). Avrami–Avramov plot constructed for [P_666,14][DCA] at 209 K (left axis). (d) Rate constant k of the crystallization process as a function of inverse temperature.

Figure 5

(a) Comparison between the temperature dependence of conductivity relaxation time for [P_666,14][SCN], [P_666,14][DCA], and [P_666,14][TCM] (from left to right) obtained cooling and heating scans. Heating has been performed after the quench cooling to the glassy state. Scatters indicate experimental data, and solid lines in liquid 1 state denote the fit of VFT function τ_σ = to experimental data. Dashed lines indicate T_g and the temperature of LLT (blue and red arrows). Zooms highlight different τ_σ(T_g) obtained on cooling (blue arrow) and heating (gray arrow) scans. (b) Apparent activation energy E_a = calculated from dielectric data obtained on cooling and heating for [P_666,14][SCN], [P_666,14][DCA], and [P_666,14][TCM].

Figure 6

Panel a presents the mechanical response of [P_666,14][DCA] recorded in supercooled liquid 2 and presented in the form of loss modulus peaks G″(f). In panel b, the mechanical data recorded at various temperatures in liquid 2 have been superimposed to each other and form the so-called masterplot. (c) Direct comparison between conductivity relaxation times (open circles) obtained on heating and structural relaxation times determined from rheology (blue stars) and TMDSC (green triangles) in supercooled liquids 1 and 2 and glass for [P_666,14][DCA]. The red point denotes T_g from standard DSC. The red star presents the predicted value of T_g for the liquid 1 state.

Figure 7

High-pressure data of [P_666,14][SCN], [P_666,14][DCA], and [P_666,14][TCM] (from left to right). Panel a presents the pressure dependence of conductivity relaxation time measured at various T. Solid lines denote fit of the Arrhenius equation to experimental data. Dashed lines separate liquid 1 from liquid 2 (black dashed line) and liquid 2 from glass (blue dashed line). (b) T_LL and T_g as a function of P is presented. The color areas on panels a and b denote the liquid 1 phase (gray) and glass region (blue) and crystalline state (pink). L1 denotes liquid 1, while L2 denotes liquid 2.

Figure 8

MD simulations snapshots. Panel a presents a single amphiphilic cation molecule with its counterion. Panel b illustrates IL morphologies under various T–P conditions. Columns present the molecular structure of the IL obtained at high (T = 5) and low (T = 2) reduced temperature and its variation with increased reduced pressure P (from low P = 2 through intermediate P = 5 to high P = 10). Rows display results for two different anion-to-cation size ratios, R_a/R_c (small anion with R_a/R_c = 1 and bulky anion with R_a/R_c = 2). Isosurface representation of ionic channels density consisting of head groups and anions is displayed in blue.