Signatures of Ion Pairing and Aggregation in the Vibrational Spectroscopy of Super-Concentrated Aqueous Lithium Bistriflimide Solutions

Aqueous electrolytes in the super-concentrated regime, where water and ions are present in a similar concentration, have recently drawn considerable interest for their potential application in electrochemical systems, such as Li+ batteries. In this regime, the classical theories for the structure and properties of electrolyte solutions break down, and even such fundamental properties as the solution morphology on the molecular length scale can be unclear, with specific ion effects often driving the properties. Here, we study the role of ion–ion interactions in aqueous lithium bistriflimide (LiTFSI) solutions as a function of concentration, ranging from the dilute to the super-concentrated regimes. We use vibrational spectroscopy of the asymmetric −SO2 stretch modes of the TFSI– anion, specifically the linear absorption, two-dimensional (2D) IR, and polarization-dependent 2D IR and IR transient absorption spectroscopies, as a method for probing for both the cation–anion interactions and anion–anion interactions in these solutions and demonstrate specific signatures of the formation of contact ion pairs and higher-order ion aggregates. These results are validated by comparison with molecular dynamics simulations of several concentrations of LiTFSI in water, and the spectral observables are interpreted with the analysis of the normal modes of small clusters with geometries consistent with solution-phase environments derived from these simulations. Using these techniques, we find evidence that contact ion pairs, both cation–anion and anion–anion pairs, begin to have substantial population at concentrations around 2 M (∼3 m) and that an extended, highly interconnected ion-rich network forms in the super-concentrated regime.