Surfactant self-assembly in water-in-salt electrolytes

Water-in-salt electrolytes (WiSEs) are safer alternatives to organic electrolytes in lithium-ion batteries. While surfactants have been proposed as performance enhancing additives, their self-assembly behaviour in these concentrated systems is completely unknown. We hypothesise that ionic surfactants can form micelles in WiSEs with their structure dependent on salt type, salt-to-surfactant ratio, and temperature.

The self-assembly of dodecyltrimethylammonium bromide (DTAB) in various WiSEs was investigated using small-angle neutron scattering. We examined systems containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium nitrate (LiNO₃), or sodium nitrate (NaNO₃) across various salt-to-surfactant ratios and temperatures (25-80 °C).

The surfactant DTAB forms micelles in all WiSEs studied, persisting at salt concentrations far beyond those in conventional aqueous solutions. In LiTFSI WiSEs, salt concentration determines micelle structure. Hydrophobic TFSI^- counterions screen the micelle surface at 5 mol/kg, forming elongated micelles, while at ≥8 mol/kg they intercalate between headgroups, producing globular micelles. At certain high salt/low surfactant compositions, traditional micelles do not form. Instead, small, disordered clusters are formed that are further enhanced at elevated temperatures. In contrast, hydrophilic NO₃^- counterions (LiNO₃ and NaNO₃ systems) maintain near identical morphologies regardless of salt concentration, though co-ion type strongly affects morphology. Na⁺ produces well-defined wormlike micelles while Li⁺ causes instability, resulting in highly polydisperse assemblies. These findings provide a platform for enhancing WiSEs through protective electrode interfaces that further reduce water activity, inhibit lithium dendrite formation, and control ion transport. This work also advances fundamental understanding of surfactant self-assembly in concentrated electrolytes and in nanostructured fluids.