Water-in-salt electrolytes (WISE) defy classical transport models by maintaining high ionic conductivity despite extreme viscosities.
Here, we uncover the multiscale mechanisms behind this anomaly in superconcentrated aqueous Lithium bis(trifluoromethane)sulfonimide (LiTFSI) solutions. Using viscosity and conductivity measurements alongside pulsed-field gradient nuclear magnetic resonance and Quasi Elastic Neutron Scattering (QENS), we reveal a pronounced decoupling between species mobility and bulk viscosity. Lithium species and protons retain high mobility, while TFSI- anions are increasingly hindered. QENS identifies transient, water-rich nanodomains that enable rapid local diffusion and dynamic interdomain exchange. This work provides direct experimental evidence for hierarchical, decoupled transport in WISE, offering a new framework for designing high-performance electrolytes for energy storage.
Doublet Celia, Villevieille Claire, Lecarme Laureline, Judeinstein Patrick, Belime Agathe, Embs Jan Peter, Appel Markus, Lyonnard Sandrine, Alloin Fannie, Berrod Quentin