Work

Surfactant self-assembly in water-in-salt electrolytes

Year: 2025

Type: article

Abstract: Hypothesis 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. Experiments 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 (LiNO 3 ), or sodium nitrate (NaNO 3 ) across various salt-to-surfactant ratios and temperatures (25–80 °C). Findings 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 NO 3 − counterions (LiNO 3 and NaNO 3 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.

Source: Journal of Colloid and Interface Science

DOI: 10.1016/j.jcis.2025.138791

Authors: Lucas N. Wong, Kathleen Wood, Jianan Wang, Gregory G. Warr, Rob Atkin

Institutions: The University of Western Australia, Australian Nuclear Science and Technology Organisation, Australian Centre for Neutron Scattering, The University of Sydney

Language: English

Topic: Surfactants and Colloidal Systems

Subfield: Organic Chemistry

Field: Chemistry

Domain: Physical Sciences

SDG: Affordable and clean energy

Open Access status: hybrid

Estimated APC paid: $4,470