Anomalous double-layer restructuring in water-in-salt electrolytes at graphitic interfaces governs capacitance

TL;DR

Anomalous double-layer restructuring occurs in water-in-salt LiCl at graphitic interfaces, driven by solvent-separated ion pairing that promotes Cl$^-$ co-adsorption into the outer Helmholtz plane and Li$^+$ hydration-shell–assisted adsorption. The authors combine classical MD, quantum-mechanical MD, and experimental EIS to resolve interfacial ion distributions, EDL thickness, and capacitance components across 1–20 mol kg$^{-1}$, revealing a non-monotonic EDL thickness and concentration-dependent PZC shifts that align with measurements. Capacitance analysis shows $1/C_S=1/C_Q+1/C_{ ext{EDL}}$, with competing trends causing $C_S$ to appear concentration-invariant for pristine few-layer graphite, while thicker graphene enhances the role of $C_Q$ in the total response. These insights establish design rules for tuning interfacial capacitance and stability in high-concentration aqueous energy-storage systems by controlling SSIP formation and hydration structure.

Abstract

The structure and thickness of the electrical double layer (EDL) at carbon electrodes strongly influence electrochemical performance, yet remain poorly understood in super-concentrated aqueous electrolytes. Here we combine classical and quantum-mechanical molecular dynamics simulations to resolve the interfacial organisation of aqueous LiCl from dilute to water-in-salt (WiS) (1--$20~\mathrm{mol~kg^{-1}}$) concentrations at graphitic electrodes, and compare with electrochemical differential-capacitance measurements from which the potential of zero charge (PZC) is obtained. We uncover a concentration-driven restructuring of the EDL: below $6~\mathrm{mol~kg^{-1}}$, solvated Li$^+$ dominates the outer Helmholtz plane (OHP), but at higher concentrations co-adsorption of Cl$^-$ through solvent-separated ion pairs enforces a near 1:1 Li:Cl ratio at the interface. This transition expands the effective EDL thickness, redistributes the interfacial potential drop, and drives a decrease in the PZC, matching the trend inferred from differential-capacitance measurements on electrolyte-graphite interfaces. Capacitance calculations reveal that while both EDL and quantum contributions vary strongly with concentration, their opposing trends make the total capacitance appear nearly constant for pristine few-layer graphite; for electrodes with smaller quantum capacitance, however, the concentration dependence of the EDL capacitance would be directly reflected in the total capacitance. Solvent-separated ion pairing is identified as the key driver of anomalous EDL behaviour in LiCl WiS electrolytes, establishing design considerations for tuning interfacial capacitance and stability in next-generation aqueous energy-storage systems.

Figures (6)

• Figure 1: Schematic of the simulation cell showing a liquid electrolyte confined between two graphite electrodes in the $x$-$y$ plane. Each electrode consists of four stacked graphene sheets; the innermost sheet carries a surface charge density of $\sigma^0$= 0 $\mathrm{C~m}^{-2}$ for neutral systems, or $\sigma^+$ = +0.0571 $\mathrm{C~m}^{-2}$ (blue) and $\sigma^-$ = -0.0571 $\mathrm{C~m}^{-2}$ (red) in charged systems, while the three subsurface sheets remain neutral (cyan).
• Figure 2: Bulk-normalised number-density profiles, $\tilde{n}(z)$, for $\mathrm{Li}^+$ (red), $\mathrm{Cl}^-$ (orange), and water atoms (blue) as a function of distance $z$ from the electrode surface. Columns correspond to electrode surface charge densities $\sigma^0=0~\mathrm{C~m^{-2}}$ (left), $\sigma^+=+0.0571~\mathrm{C~m^{-2}}$ (middle), and $\sigma^-=-0.0571~\mathrm{C~m^{-2}}$ (right). Rows show concentrations: (a--c) $1~\mathrm{mol~kg^{-1}}$ LiCl; (d--f) $10~\mathrm{mol~kg^{-1}}$; (g--i) $20~\mathrm{mol~kg^{-1}}$. Vertical dotted lines indicate the inner Helmholtz plane (IHP, pink) and outer Helmholtz plane (OHP, cyan).
• Figure 3: Electric double layer thickness estimation. (a, b) Snapshots of the interfacial region for $1~\mathrm{mol~kg^{-1}}$ and $20~\mathrm{mol~kg^{-1}}$ LiCl in contact with neutral graphite. (c, d) Accumulated charge density profiles $Q(z)$ (black solid line) near the neutral surface, together with fitted damped oscillatory curves (blue dashed line), corresponding to $1~\mathrm{mol~kg^{-1}}$ and $20~\mathrm{mol~kg^{-1}}$ LiCl. (e) EDL thickness as a function of electrolyte concentration under three surface charge conditions: $\sigma^{0}$ (black), $\sigma^{+}$ (blue), and $\sigma^{-}$ (red).
• Figure 4: Hydration number, $N_{\mathrm{hyd}}$, calculated as the mean number of H$_2$O-ion intermolecular bonds per ion (solid lines), and solvent-separated ion pairs per ion, $N_{\mathrm{SSIP}}$ (dashed lines), as a function of distance from the electrode for $\mathrm{Li^{+}}$ (left column) and $\mathrm{Cl^{-}}$ (right column). Concentrations of 1, 10, and $20~\mathrm{mol~kg^{-1}}$ are shown in blue, red, and green, respectively. Vertical dotted lines mark the inner Helmholtz plane (IHP, pink) and the outer Helmholtz plane (OHP, cyan). Row labels indicate the electrode surface charge: $\sigma^{0}$ (top row, neutral), $\sigma^+=+0.0571~\mathrm{C~m^{-2}}$ (middle row), and $\sigma^-=-0.0571~\mathrm{C~m^{-2}}$ (bottom row). The images above depict representative hydration motifs for $\mathrm{Li^{+}}$ (left) and $\mathrm{Cl^{-}}$ (right).
• Figure 5: Charge density profiles, $\rho_q^{\alpha}$ (units $e~\mathrm{nm^{-3}}$), for species $\alpha\in\{\mathrm{H_2O},~\mathrm{Li}^+,~\mathrm{Cl}^-\}$ and the total ($\alpha=\mathrm{tot}$), as a function of $z$ distance from the electrode. Columns correspond to electrode surface charge densities $\sigma^0=0~\mathrm{C~m^{-2}}$ (left), $\sigma^+=+0.0571~\mathrm{C~m^{-2}}$ (middle), and $\sigma^-=-0.0571~\mathrm{C~m^{-2}}$ (right). Rows show concentrations: (a--c) $1~\mathrm{mol~kg^{-1}}$ LiCl; (d--f) $10~\mathrm{mol~kg^{-1}}$; (g--i) $20~\mathrm{mol~kg^{-1}}$. Colours: water (blue), $\mathrm{Li}^+$ (red), $\mathrm{Cl}^-$ (yellow), total (cyan).