Water as a catalyst for ion transport across the electrical double layer in ionic liquids

It was previously shown experimentally that the ionic liquid (IL) system based on 1-ethyl-3-methylimidazolium dicyanamide $\left(\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]\right)$ outperforms $N$-butyl-$N$-methylpyrrolidinium dicyanamide $\left(\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]\right)$ as an electrolyte in a rechargeable zinc battery. Specifically, $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ requires a lower overpotential to initiate electrodeposition of zinc, enables a higher zinc deposition peak current density, and supports a greater number of charge-discharge cycles. In the present work, a set of molecular dynamics simulations of two IL systems (i.e., IL + 9 mol % Zn + 3 wt % water) reveals that these differences could be related to the overscreening and crowding phenomena that exist in these IL systems. Furthermore, the work reveals the most likely mechanism by which water acts as an electrochemical catalyst in transferring metal cations to a negatively charged electrode. This transfer is a necessary step for subsequent metal deposition, as required for a rechargeable battery system, for example. In particular, the results of the simulations reveal three primary findings. First, it is shown that water activates the transfer of zinc onto the negative electrodes by temporarily disrupting zinc's [dca] coordination shell while zinc is located within the interfacial region; second, it is shown that the transfer of zinc is obviously greater in the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ system than in the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system, which is consistent with experimental measurements; and third, when the effects of crowding are largely eliminated in the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system (i.e., when the layering instability and the anion layer collapse in the anodic interfacial region are avoided by the use of a smaller electrode charge), the zinc transfer dynamics are completely changed and the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system starts providing a more favorable environment for zinc transfers. These findings present a significant advance in the understanding of the electrode-electrolyte interface in ILs.

Figure 1

2D structures of the studied IL cations and anions with atom labels as used in analyses.

Figure 2

3D overview and dimensions of each simulation cell (a snapshot taken at $t=10$ ns of the 700-K simulations at 5σ) for (a) $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ + 9% Zn + 3 wt % water and (b) $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ + 9% Zn + 3 wt % water systems. IL cations are colored in lime green, anions in marine blue, zinc in magenta, water's oxygen in red, water's hydrogen in white, negative electrode in marine blue, and positive electrode in dark red. Partial density profiles of water's oxygen (dotted black line), zinc (magenta line) and the central nitrogen atoms, N1 (red line) and N3 (blue line) on the IL cation and anion, respectively, are plotted for 1σ electrodes (c), (d), 3σ electrodes (e), (f), and 5σ electrodes (g), (h). The negative electrode is at $z=0\mathrm{nm}$ . For clarity, the zinc signal was increased by a factor of 3 and the inner IL density peaks were truncated. Small black arrows indicate the important peaks, and the direction of the arrows indicates the significant changes. $\left|\sigma \right|=0.38e\mathrm{n}{\mathrm{m}}^{\text{–}2}$.

Figure 3

Adsorbed layer orientations of the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]$ cation in the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ system [replicas 1 (a) and 2 (b)] at the -3σ electrodes and 1000 K.

Figure 4

3D snapshot of the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ + zinc + water system at the last frame of the 700-K simulation (step 2). IL ions are semitransparent so that zinc (magenta) and water (red/white) can be seen clearly.

Figure 5

3D Zn-RDF plots for the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ (left panels) and $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ (right panels). Center-of-mass (COM) plots (Zn-[dca] and Zn-water) are shown in the top panels and atomic plots ($\mathrm{Zn}-{\mathrm{N}}_{\mathrm{Z}}$ and Zn-O) are shown in the bottom panels. Full lines [offset on g(r) by +400] represent 300 K, dashed lines (offset by +200) represent 700 K, and dotted lines (no offset) 1000 K. Zn-[dca] coordination is represented with the blue color while Zn-water is represented by magenta.

Figure 6

3D Zn-RDF plots for the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ and $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ systems on the same axes. Center-of-mass (COM) RDFs (Zn-[dca] and Zn-water in both systems and at all temperatures) are shown in (a) while atomic RDFs (Zn-${\mathrm{N}}_{\mathrm{Z}}$ and Zn-O for both systems at 1000 K) are shown in (b). In (a), full lines [offset on g(r) by +400] represent 300 K, dashed lines (offset by +200) represent 700 K, and dotted lines (no offset) 1000 K. Zn-[dca] coordination is represented with the blue color, while Zn-water is represented by magenta. Label $\mathrm{EMIM}=\left[{\mathrm{C}}_2\mathrm{mim}\right]$ and label $\mathrm{PYR}=\left[{\mathrm{C}}_4\mathrm{mpyr}\right]$. Light colors represent $\left[{\mathrm{C}}_2\mathrm{mim}\right]$ plots and dark colors $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]$. In (b), full lines represent the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]$ system and dashed lines the $\left[{\mathrm{C}}_2\mathrm{mim}\right]$. Water coordination is colored in magenta and [dca] coordination in blue, as before.

Figure 7

Time-resolved molecular coordination of the zinc atom (with unique identification number 12799) before, during, and after transfer onto the anode surface for the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ 5σ system with 9 mol % Zn and 3 wt % water. Coordination by [dca] anions is shown as a blue line and coordination by water as a red line (left axis), while zinc's $z$ coordinate is shown as the green line (right axis). The coordination sphere is taken as $r=0.4\mathrm{nm}$, and coordinating species were counted if any of their constituent atoms were found inside of this sphere at time $t$. Dark blue arrows indicate [dca] drops, while dark red arrows indicate water spikes. Horizontal black lines indicate layers that correspond to the partial density plots, while black arrows point to features of interest.

Figure 8

Time-resolved molecular coordination of the zinc atoms before, during, and after transfer onto the anode surface for the $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ 5σ system with 9 mol % Zn and 3 wt % water. Coordination by [dca] anions is shown as a blue line and coordination by water as a red line (both on the left axis), while zinc's $z$ coordinate is shown as the green line (right axis). The coordination sphere is taken as $r=0.4\mathrm{nm}$, and coordinating species were counted if any of their constituent atoms were found inside of this sphere at time $t$. The five-digit numbers in the chart titles are the unique identification numbers for zinc atoms, while the alphanumeric code in the brackets corresponds to the system replica number.

Figure 9

Time-resolved molecular coordination of the zinc atoms before, during, and after transfer onto the anode surface for the $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ 5σ system with 9 mol % Zn and 3 wt % water. Coordination by [dca] anions is shown as a blue line and coordination by water as a red line (left axis), while zinc's $z$ coordinate is shown as the green line (right axis). Coordination sphere is taken as $r=0.4\mathrm{nm}$, and coordinating species were counted if any of their constituent atoms were found inside of this sphere at time $t$. The five-digit numbers in the chart titles are the unique identification numbers for zinc atoms, while the alphanumeric code in the brackets corresponds to the system replica number.

Figure 10

$Z$ coordinates of the oxygen atom of water that forms part of the zinc's critical coordination shell before, during, and after four different zinc transfer events. The crossed transparent thick gray lines indicate the spatial and the temporal locations of the critical coordination shell. Water molecule (residue identification) numbers are used as line labels. Zinc atom five-digit unique identification numbers are used as zinc labels.

Figure 11

Graphical illustration of the proposed three-step zinc transfer mechanism across the electrical double layer in either $\left[{\mathrm{C}}_4\mathrm{mpyr}\right]\left[\mathrm{dca}\right]$ or $\left[{\mathrm{C}}_2\mathrm{mim}\right]\left[\mathrm{dca}\right]$ IL. CCS stands for the critical coordination shell (discussed in Sec. 3e). Magenta dashed arrows indicate the movement of zinc, while black dotted arrows indicate the movement of water and [dca]. The interfacial IL structure is shown as a transparent background for reference. [dca] is colored all in blue, zinc in magenta, graphene's carbon in gray, water's hydrogen in white, and water's oxygen in red.