French laboratory highlights how lithium stays mobile in concentrated battery electrolytes

France’s Laboratoire Léon Brillouin highlighted a study on October 22 examining why water-in-salt electrolytes can conduct ions effectively even when the liquid becomes highly viscous. Célia Doublet and co-authors combined several experimental techniques to investigate concentrated aqueous lithium bis(trifluoromethanesulfonyl)imide, or LiTFSI. Their paper appeared in Journal of Physics: Energy on October 8; the later laboratory announcement describes that research rather than a newly manufactured battery.

The experiments examined solutions across a concentration range extending from 0.5 to 21 moles of salt per kilogram of water. As concentration increased across that range, water and lithium self-diffusion coefficients declined by approximately a factor of 18, while diffusion of TFSI anions fell by more than a factor of 35. These comparisons describe changes in molecular mobility; they are neither battery charging rates nor gains in energy capacity.

The contrast suggests that the different species do not all experience the liquid’s resistance in the same way. The researchers interpret their measurements as evidence of temporary regions rich in water, within a surrounding structure rich in anions. Water can move rapidly inside these small regions and exchange between them. The structure changes over time, providing a mechanism through which small solvated species retain mobility despite the bulk liquid’s increasing viscosity.

The team measured viscosity and conductivity alongside pulsed-field-gradient nuclear magnetic resonance and quasi-elastic neutron scattering. NMR tracked species over longer distances, while neutron measurements probed much shorter motions. Experiments used SHARPER, IN16B and IN13 at France’s Institut Laue–Langevin, as well as FOCUS at Switzerland’s Paul Scherrer Institute. The analysis distinguished water’s longer-range self-diffusion, localized translational movement and molecular rotation rather than assigning all motion to a single process.

Sample preparation was an important part of the method because LiTFSI readily absorbs moisture. The researchers dried the salt overnight under vacuum at 120°C, degassed water with argon and prepared solutions in an argon glovebox. The high-temperature NMR measurements covered 25–45°C. Viscosity measurements generally covered 5–50°C, with low-temperature limitations for the most concentrated solutions because salt could precipitate. Those temperatures are experimental conditions, not recommended commercial battery operating limits.

The study also compared neutron measurements with and without an applied polarization of 1 volt and found no detectable change in the water dynamics probed by that setup. The authors stress that the experiment predominantly measured bulk electrolyte: its very thin interfacial region contributed too little to resolve. The finding therefore does not establish that electric fields have no effect at electrode interfaces or under different test conditions.

The authors say the resulting dynamical parameters can help constrain molecular-dynamics simulations, including versions assisted by machine learning. The contribution is an experimentally supported account of transport in a particular electrolyte system. It supplies information for material design and modelling, while commercial performance still requires separate evidence on complete cells, cycling durability, production costs and manufacturing scale.