Weakening H-bonds facilitate Li+ transport in concentrated aqueous LiTFSI solutions
Year: 2026
Type: preprint
Abstract: A comprehensive understanding of the microscopic processes governing ion transport in concentrated electrolyte solutions is essential for developing efficient, water-based batteries and energy storage devices. LiTFSI has emerged as a model system for Waterin-Salt (WiS) electrolytes because it can be investigated over a wide concentration range. Here, we combine Raman and THz spectroscopy with classical MD simulations to elucidate the reorganization of the water network across concentrations ranging from 1 to 21 mol·kg-1. Isotopic substitution of H2O by D2O allows us to disentangle the distinct spectroscopic contributions of ion rattling modes and H-bond motifs in the THz frequency range. The analysis reveals two solvent-dependent modes in the region of the translational and librational water network modes, rationalized as an overall strengthening of directional H-bonds and a decrease in orientational preference. This picture is consistent with the accompanying molecular dynamics simulations, which predict a continuous shift from solvent-separated to solvent-shared to contact ion pairs, alongside a softening of water-water H-bonds and a broadening in the angular distributions within the water-water H-bond network. A thermodynamic decomposition further reveals correlations among the number of H-bonds, the observed spectroscopic nonlinearities, and the concentration-dependent conductivity. Our findings reveal a gradual, continuous reorganization of the water network as LiTFSI concentration increases: the red-shifted librational mode indicates a lower barrier for the angular part of the intermolecular potential, consistent with the simulated broadening of the angular distributions within the H-bond network. The impact on water dynamics and charge transport is discussed.
Source: ChemRxiv
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7212154806