Structural and Rheological Signatures of the Salt-in-Solvent to Solvent-in-Salt Transition in a Fluorinated Ester Electrolyte

Abstract Experimental measurements and molecular dynamics (MD) simulations study the structural and rheological properties of a model fluorinated ester electrolyte, methyl 3,3,3-trifluoropropionate (MTFP) with LiFSI, over a wide concentration range (0.1–2.3 mol/kg). MD simulations using a calibrated Generalized AMBER Force Field report a transition from liquid-like solvation to anisotropic ion aggregation and intermediate-range structural ordering as the salt concentration increases. One-dimensional radial distribution functions and three-dimensional ion density maps link these structural changes to the experimentally observed evolution of macroscopic properties, including a sharp viscosity divergence near 1 mol/kg driven by percolation of ion aggregates. The results provide a mechanistic foundation for designing ester-based localized high-concentration electrolytes, a class of materials that remains significantly underexplored despite compelling practical advantages in safety, oxidative stability, and low-temperature performance. Using the aforementioned electrolyte as a model system, this study establishes a modeling approach which highlights the importance of the solvent molecular geometry in directing ion organization and electrolyte rheology.