Solvent‐Dominated or Salt‐Controlled? Mechanistic Insights and Design Principles for Water‐in‐Salt and Deep Eutectic Solvent Electrolytes in Aqueous Energy Storage

Aqueous energy storage technologies are gaining renewed attention due to their safety, low cost, and environmental compatibility. Highly concentrated electrolytes, including water-in-salt (WIS) and deep eutectic solvent (DES) systems, significantly expand the electrochemical stability window (ESW) of water, enabling higher operating voltages. However, reported improvements often conflate thermodynamic, kinetic, and interfacial effects. This review presents a mechanistic analysis of ESW expansion by distinguishing bulk solvation behavior from interfacial chemistry, ion transport limitations, and solid-electrolyte interphase formation. WIS electrolytes are predominantly salt-controlled, whereas DES systems are solvent-dominated through hydrogen-bond networks. This framework clarifies their distinct transport and stability characteristics. The review further evaluates common experimental methodologies for ESW, conductivity, and interfacial analysis. By integrating literature trends, practical performance limits are defined, and standardized evaluation protocols are proposed. Finally, actionable design principles and an experimental checklist are provided to improve reproducibility and guide the rational development of advanced aqueous energy storage systems.