# Ultrafast Kerr Spectroscopy Reveals Bulk‐Like Solvent Dynamics in Concentrated LiTFSI–Acetonitrile Electrolytes

## TL;DR

This study uses ultrafast spectroscopy to show that acetonitrile-based electrolytes maintain high ion conductivity at high salt concentrations due to dynamic solvent molecules.

## Contribution

The paper provides time-domain evidence of bulk-like solvent dynamics in concentrated LiTFSI–acetonitrile electrolytes using ultrafast Kerr spectroscopy.

## Key findings

- Highly dynamic, bulk-like acetonitrile molecules persist even at high salt concentrations.
- The presence of free solvent molecules reduces viscosity and enhances ionic conductivity.
- Slower components from coordinated solvent and anion relaxation are also observed.

## Abstract

Electrolyte solutions are vital to energy storage devices, significantly influencing their capacity, safety, and cost efficiency. Lithium salts based on multidentate anions have shown remarkable potential in energy storage, particularly when dissolved in acetonitrile. These solutions exhibit exceptionally high ionic conductivities, even for concentrations above the standard 1 mol L−1 solutions. To directly probe bulk solvent and solvation shell dynamics in lithium salt solutions, the ultrafast optical Kerr effect (OKE) method is utilized. We investigate the microscopic dynamics of LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) solutions at various concentrations in acetonitrile. The measured data, combined with a global analysis method, reveal that the solvent remains highly dynamic and nearly bulk‐like, even at high concentrations where a significantly reduced number of solvent molecules are available to solvate the cations in solution. These findings support recent explanations as to why acetonitrile‐based electrolyte solutions exhibit higher conductivity compared to, for instance, other nonaqueous electrolyte solutions. In electrolytes based on acetonitrile, a greater proportion of free solvent molecules results in lower overall viscosity. An abundance of uncoordinated solvent molecules facilitates higher ion conduction, compared with the more limited ion mobility observed in other LiTFSI electrolyte systems.

Ultrafast OHD‐OKE spectroscopy is used to directly probe the intermolecular and solvation‐shell dynamics of LiTFSI–acetonitrile electrolytes. Despite strong ion–solvent interactions at high salt concentrations, the measurements reveal a persistent population of highly dynamic, bulk‐like ACN molecules, together with distinct slower components arising from coordinated solvent and intrinsic anion polarizability relaxation. These results provide microscopic, time‐domain evidence explaining why ACN‐based electrolytes maintain low viscosity and high ionic conductivity even at elevated LiTFSI loadings.© 2026 WILEY‐VCH GmbH