Macroscopic approach to studying the structure of concentrated aqueous LiTFSI solutions

Аннотация

This work systematically studies the physicochemical properties of aqueous LiTFSI solutions across a wide range of concentrations. The solutions’ density, viscosity, electrical conductivity, and water activity were measured, establishing quantitative relationships between macroscopic parameters and structural characteristics of the electrolyte. Analysis of electrical conductivity, incorporating viscosity corrections, demonstrated nearly complete ionic association at high concentrations (above 10 mol/kg), with water activity reaching exceptionally low values (~0.15) at maximum solubility. The study revealed nonlinear density changes with increasing concentration, indicating substantial structural reorganization in concentrated solutions. Hydration numbers, calculated using a novel thermodynamic approach combining water activity and conductivity data, showed unexpected solvation behavior: LiTFSI exhibited higher hydration numbers than traditional lithium salts in dilute solutions despite lower TFSI− charge density, attributed to spatial trapping of water molecules by its bulky structure. This pattern inverted at higher concentrations due to increased ionic association and charge screening effects. The calculated parameters aligned well with molecular dynamics simulations, validating our macroscopic approach. The study demonstrated that standard physicochemical measurements can accurately determine electrolyte structural parameters, offering practical advantages for optimizing electrolyte compositions, particularly in systems containing polymer additives and co-solvents where molecular dynamics modeling faces significant challenges.