Surfactant self-assembly in water-in-salt electrolytes.

Area of Science:

• Materials Science
• Electrochemistry
• Physical Chemistry

Background:

• Water-in-salt electrolytes (WiSEs) offer enhanced safety as alternatives to organic electrolytes in lithium-ion batteries.
• The self-assembly behavior of surfactants, potential performance-enhancing additives, in these highly concentrated WiSEs remains largely unexplored.
• Understanding surfactant self-assembly is crucial for designing stable electrode interfaces and improving battery performance.

Purpose of the Study:

• To investigate the self-assembly of ionic surfactants in various WiSEs.
• To determine how salt type, salt-to-surfactant ratio, and temperature influence micelle formation and structure in WiSEs.
• To explore the potential of surfactant-modified WiSEs for advanced lithium-ion battery applications.

Main Methods:

• Small-angle neutron scattering (SANS) was employed to study the self-assembly of dodecyltrimethylammonium bromide (DTAB).
• Systems investigated included WiSEs with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium nitrate (LiNO3), and sodium nitrate (NaNO3).
• Experiments were conducted across a range of salt-to-surfactant ratios and temperatures (25–80 °C).

Main Results:

• Dodecyltrimethylammonium bromide (DTAB) forms micelles in all studied WiSEs, even at salt concentrations exceeding those in conventional aqueous solutions.
• In LiTFSI-based WiSEs, micelle structure is salt-dependent: elongated micelles form at 5 mol/kg, while globular micelles appear at ≥8 mol/kg due to counterion intercalation.
• Hydrophilic nitrate counterions (in LiNO3 and NaNO3 systems) lead to stable micelle morphologies, with sodium nitrate favoring well-defined wormlike micelles and lithium nitrate causing instability.

Conclusions:

• Ionic surfactants can form stable micelles in WiSEs, offering a pathway to engineer protective electrode interfaces.
• Tailoring micelle structure through salt selection and concentration control can reduce water activity and inhibit lithium dendrite growth.
• This fundamental understanding of surfactant self-assembly in concentrated electrolytes advances the development of safer and more efficient lithium-ion batteries.