Long-Range Surface Forces in Salt-in-Ionic Liquids

Abstract

Ionic liquids (ILs) are a promising class of electrolytes with a unique combination of properties, such as extremely low vapor pressures and nonflammability. Doping ILs with alkali metal salts creates an electrolyte that is of interest for battery technology. These salt-in-ionic liquids (SiILs) are a class of superconcentrated, strongly correlated, and asymmetric electrolytes. Notably, the transference numbers of the alkali metal cations have been found to be negative. Here, we investigate Na-based SiILs with a surface force apparatus, X-ray scattering, and atomic force microscopy. We find evidence of confinement-induced structural changes, giving rise to long-range interactions. Force curves also reveal an electrolyte structure consistent with our predictions from theory and simulations. The long-range steric interactions in SiILs reflect the high aspect ratio of compressible aggregates at the interfaces rather than the purely electrostatic origin predicted by the classical electrolyte theory. This conclusion is supported by the reported anomalous negative transference numbers, which can be explained within the same aggregation framework. The interfacial nanostructure should impact the formation of the solid electrolyte interphase in SiILs.

Keywords: MD simulation; concentrated electrolytes; electric double layer; force measurements; salt-in-ionic liquid.

Figure 1

Wide-angle X-ray scattering of the SiILs. (A) Integrated intensity from 2D scans. The thin black lines show fits of Lorentzian functions to the integrated intensities. The fits of Lorentzian functions provide the d-spacing (d) and the half-width at half-maximum values (B) for reflections at (B) q₁, (C) q₂, and (D) q₃. The half-width at half-maximum values were corrected with the instrument divergence. The error bars are sometimes smaller than the marker size and, therefore, not visible.

Figure 2

Representative surface forces between mica surfaces for various mole fractions of NaTFSI. Force–distance curves for mole fractions (x_s) (A) 0.02, (B) 0.05, (C) 0.10, and (E) 0.15, after 12 h equilibration, 25 °C; (D) x_s = 0.05 after 24 h equilibration at 25 °C, (F) x_s = 0.15 at 40 °C after 12 h equilibration. The plots show FC1_ap (red circles), FC1_ret (black cross), FC2_ap (red triangles), and FC2_ret (gray x-marks). Panels (A) and (D) also show force–distance curves FC4. The black and blue lines show exponential fits with decay lengths of d₁ and d₂, respectively. Green arrow in (E) shows the shift of the hard wall during force measurements. Each force–distance curve displays the force normalized by the radius of the surface, F/R, vs the surface separation.

Figure 3

Relation between the decay length and long-range structure of SiILs. (A) Decay lengths of FC1_ap (d₁) and FC2_ap-FC5_ap (d₂) as a function of the mole fraction of NaTFSI, at 25 and 40 °C. The decay lengths influenced by the shift of the hard wall are shown with green markers. For x_s = 0.2 at 40 °C, d₂ is 18.9 nm (semitransparent red marker) before the shift of the hard wall, but d₂ decreases to 13.1 ± 2.6 nm when the hard wall shifts. The values for the saturated solution at 25 °C (for d₁ ∼ 3.3 nm and d₂ ∼ 14.1 0.6 nm, with significant shift of the hard wall) are not shown since the actual mole fraction of NaTFSI after filtering the precipitate is unknown. At least 3 values and at most 8 values were considered to determine average and standard deviation of d, except for d₁, x_s = 0.008, since here the forced separation only happened once. (B, C) Examples of steps in SFA force–distance curves; see also Figure S12. (D) Distribution of step size. The number of data points in the box and whisker plot is 28, 42, 23, 25, 16, and 7 for x_s = 0.008, 0.02, 0.05, 0.10, 0.15, and 0.20, respectively. (E) Conceptual picture of the confinement-induced gelation of this SiIL causing forced separation. The yellow circles represent Na⁺ partially screening the negative charge of the mica surface; IL ions and clusters are colored blue and red, respectively. We assume that the aggregate concentration is enhanced when the surfaces are approached so that c_conf > c_bulk. At a critical surface separation D_c, gelation is triggered, and the imbalance in the chemical potential inside and outside the pore justifies the mechanical work exerted by branched or cross-linked (compressible) aggregates against the mica surface, V_gelΔp. In subsequent force measurements, the osmotic pressure gradient justifies the enhanced repulsion.

Figure 4

AFM force–distance curves in dry SiILs. SiILs with x_s (A) 0, (B) 0.05, and (C) 0.10 NaTFSI in equilibrium with dry nitrogen. The maximum applied force was ∼12 nN. The mica surface was equilibrated for 2 h with the SiIL before measurements started. Each plot shows a heat map of superposed force–distance curves (34 in (a), 46 in (b), and 63 in (c)). Results were reproducible in other spots on the mica surface. Dimensions of IL cation 4 Å × 7.5 Å × 1.7 Å and IL anion 3 Å × 7.6 Å × 2.5 Å (see the molecular structure of the IL in Figure S14). An additional layer was resolved at varying applied force, with Δ = 2.4 ± 0.5 Å for the SiIL with x_s = 0.10; see Figure S15.

Figure 5

Theory and simulation of ionic aggregation in the bulk and at charged interfaces. (A) Probability distribution of the number of TFSI^– bound to Na⁺ for x_s = 0.05. (B) Structures and length scales of example clusters for a single Na⁺ and two Na⁺. (C, D) Concentration of TFSI-Na clusters (c_lm, in dimensionless units, number per lattice site) composed of m TFSI^– and l Na⁺, for x_s = 0.05 and 0.2, respectively. (E) Cluster bond density (CBD); the solid line denotes the Cayley-tree limit given by (l+m-1)/(l+m). (F) Volume fraction of EMIM⁺ (ϕ̅_⊕), Na⁺ (ϕ̅₊), and TFSI^– (ϕ̅_–), and (G) volume fraction of free Na⁺ (ϕ̅₁₀), free TFSI^– (ϕ̅₀₁), and clusters of ion pairs and larger (ϕ̅_11≥), both as a function of the applied potential Φ. (H) Screening length λ_s from the theory as a function of x_s.