Water-in-salt electrolytes: An interfacial perspective

Liquid electrolytes with high ionic conductivity, high transference number for the target ions, and excellent electrochemical, chemical, and thermal stability are essential for electrochemical energy storage devices. Water-in-salt (WIS) electrolytes, in which the salt-water ratio is larger than one, are gaining intensive attention in the electrochemical community. Here, we review the recent work on WIS electrolytes and the closely related water-in-ionic liquid electrolytes. We highlight the fact that many properties of these electrolytes, in bulk and at electrolyte-electrode interfaces, are underpinned by the physics and chemistry of the interfaces formed between water and ions (or aggregated water/ion clusters). Manipulating these interfaces by tailoring the selection of ions and water-ion ratio opens up new dimensions in the optimization of liquid electrolytes but also poses new challenges. We conclude the review by highlighting several directions for research on WIS electrolytes, in particular, the study of WIS electrolyte-electrode interfaces using surface force measurements.