Ion Pairing Restructures the Electric Double Layer in Water-in-Salt Electrolytes on Graphite and Alters Capacitance

The structure and thickness of the electrical double layer (EDL) at carbon electrodes strongly influence electrochemical performance, yet remain poorly understood in concentrated aqueous electrolytes. Here we combine classical and quantum-mechanical molecular dynamics simulations with experimental analyses to resolve the interfacial organization of aqueous LiCl from dilute to water-in-salt (WiS) (1–20 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 mol kg–1, solvated Li+ dominates the outer Helmholtz plane (OHP), but at higher concentrations coadsorption 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 a pristine graphite slab comprising four atomic layers; for other electrodes where quantum capacitance is not limiting and electrode-internal series contributions are small, the total capacitance would more directly reflect the concentration dependence of the EDL capacitance. Solvent-separated ion pairing is identified as the key driver of nonmonotonic EDL behavior in LiCl WiS electrolytes, establishing design considerations for tuning interfacial capacitance and stability in next-generation carbon-based aqueous energy storage systems.