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

TL;DR

This study combines simulations and experiments to reveal how water-in-salt electrolytes induce a concentration-dependent restructuring of the electrical double layer at graphitic interfaces, affecting capacitance and electrochemical performance.

Contribution

It uncovers the concentration-driven transition in interfacial ion organization and its impact on capacitance, providing new insights into electrolyte-electrode interactions in super-concentrated aqueous systems.

Findings

Solvated Li+ dominates at low concentration, while Cl- co-adsorption occurs at higher concentrations.

The EDL thickness increases with salt concentration, affecting the potential of zero charge.

Total capacitance remains nearly constant in pristine graphite due to opposing EDL and quantum effects.

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.