Fluoride-Induced Coordination Restructuring and Chemical Behavior Modulation in Chloride Molten Salts

Fluoride incorporation is widely employed to regulate the physicochemical properties of chloride molten salts, yet the mechanistic link between electronic structure, coordination chemistry, and macroscopic behavior remains poorly understood. Here, the structural and chemical evolution of LiCl-KCl-LaCl₃ molten salts upon fluoride addition is elucidated by combining in situ optical basicity measurements, electrochemical analysis, and deep potential molecular dynamics simulations. Fluoride incorporation markedly lowers the optical basicity of the melt, indicating weakened electron-donating capability of the anionic framework. Atomistic simulations reveal that F^- preferentially coordinates with La³⁺ and progressively replaces Cl^- in the first coordination shell, stabilizing La-centered coordination complexes. This ligand substitution reorganizes the local coordination environment and suppresses ligand-exchange dynamics, leading to reduced ionic diffusivity and increased melt viscosity. Concomitantly, the La³⁺/La equilibrium potential shifts toward more negative values, and the corrosion current density of Ni decreases significantly. These findings uncover a cross-scale mechanism in which anion substitution drives coordination restructuring and electrochemical stabilization, providing a mechanistic basis for rational design of molten salt electrolytes in high-temperature energy systems.