Effects of Fluorinated Additives in Molten Salt Electrolytes for Calcium Batteries

Abstract

Fluorinated additives offer a promising route to tailor the structure and transport properties of electrolytes in general, yet their role in molten salt electrolytes (MSEs) remains poorly understood. Here, the influence of three fluorinated additives, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 1,2- bis(2,2,2-trifluoroethoxy) ethane (BTFE), and PhF, on the structure and dynamics of an MSE composed of [Li, Na, K, Ca]FSI, is investigated using molecular dynamics simulations—with the end-goal of improved calcium battery (CaB) electrolytes. The differences in additive chemical structure affect cation coordination, ionic cage persistence, and ligand exchange kinetics; while TTE and BTFE directly participate in cation coordination, PhF acts as a non-coordinating diluent, weakening the ionic network through spatial disruption. These additive-specific effects result in cation-dependent trends in coordination and mobility. Overall, the findings provide a proof-of-concept for rational additive selection in MSEs, shifting the design paradigm from optimizing bulk conductivity toward engineering coordination and interfacial behavior for CaB electrolytes.