In the realm of energy storage, a team of researchers from the University of Manchester, the Czech Academy of Sciences, and the University of Bologna have made a significant stride in understanding the behavior of water-in-salt electrolytes at graphitic interfaces. This research, published in the journal Nature Energy, could have practical implications for the design of next-generation aqueous energy storage systems.
The team, led by Dr. Hannah O. Wood and Prof. Robert A. W. Dryfe from the University of Manchester, combined classical and quantum-mechanical molecular dynamics simulations to study the electrical double layer (EDL) at carbon electrodes in aqueous LiCl electrolytes. The EDL, a structure formed at the interface between an electrode and an electrolyte, plays a crucial role in electrochemical performance. However, its behavior in super-concentrated aqueous electrolytes, known as water-in-salt (WiS) electrolytes, has remained poorly understood.
The researchers found that as the concentration of the electrolyte increases, the EDL undergoes a restructuring. Below a concentration of 6 mol/kg, solvated lithium ions (Li+) dominate the outer Helmholtz plane (OHP), a region within the EDL. However, at higher concentrations, chloride ions (Cl-) begin to co-adsorb onto the electrode through solvent-separated ion pairs, leading to a near 1:1 ratio of Li+ to Cl- at the interface. This transition expands the effective thickness of the EDL, redistributes the interfacial potential drop, and drives a decrease in the potential of zero charge (PZC), a key parameter in electrochemical systems.
The researchers also found that the capacitance, a measure of the electrode’s ability to store charge, varies strongly with concentration. However, the opposing trends of the EDL and quantum contributions to the capacitance make the total capacitance appear nearly constant for pristine few-layer graphite. For electrodes with smaller quantum capacitance, the concentration dependence of the EDL capacitance would be directly reflected in the total capacitance.
The key driver of this anomalous EDL behavior in LiCl WiS electrolytes is identified as solvent-separated ion pairing. This finding establishes important design considerations for tuning interfacial capacitance and stability in next-generation aqueous energy-storage systems. The research provides a deeper understanding of the behavior of WiS electrolytes, which could lead to the development of more efficient and stable energy storage devices.
Source: Wood, H.O., Zhou, F., Dočkal, J. et al. Anomalous double-layer restructuring in water-in-salt electrolytes at graphitic interfaces governs capacitance. Nat Energy (2023).
This article is based on research available at arXiv.