Probing the Water H-Bonding Network in Superconcentrated Aqueous Electrolytes with 2DIR Spectroscopy

Superconcentrated electrolytes have recently emerged as a promising material for various electrochemical applications, such as batteries and supercapacitors. These materials contain ions at concentrations comparable to, or greater than, the concentration of the solvent and can possess beneficial properties such as expanded electrochemical windows. Many fundamental questions about these materials remain, such as the details of the molecular-scale morphology and dynamics and how these relate to the ionic conductivity. Here we use ultrafast 2DIR spectroscopy to study the aqueous LiTFSI solutions as a function of concentration from the dilute to the superconcentrated regimes, focusing on the changes to the water H₂O stretch vibrational mode as well as the intra- and inter-molecular coupling to the H₂O bend and TFSI^- asymmetric SO2 stretch modes. We observe characteristic signatures of the disruption of the H₂O H-bonding network at increasing concentration of the salt, with no bulk-like water remaining at the superconcentrated limit, being replaced only by isolated water and linear H-bonding networks. This has important implications for the dominant mechanism of Li⁺ diffusion in these solutions, supporting the predominance of structural over vehicular transport.