Study on the Structure and Transport Properties of Molten Salts of NaF–KF–AlF3 Electrolytes by First-Principles Molecular Dynamics Simulation

The NaF–KF–AlF3 is a promising low-temperature electrolyte for industrial application. Herein, we used a molecular dynamics and first-principles calculations to simulate NaF–KF–AlF3 molten salts with molar ratios(n(NaF + KF)/AlF3) of 1.3–1.6 and 30 mol % NaF at 1123 K. The results of the study reveal the law of influence of molar ratio on the structural properties and transport properties of the NaF–KF–AlF3 system. The results show that the average coordination number of the NaF–KF–AlF3 system was about 4.8, and the ionic structure was mainly [AlF4]– and [AlF5]2–, with strong covalent interactions between Al-F ions. As the molar ratio increased five-coordinated [AlF5]2– gradually became the dominant population in the molten salts, and the proportion of bridging fluoride ions increased, promoting ionic polymerization in the molten salts and the formation of complex ionic groups. The order of ion diffusion ability in the molten salts followed the order of K+ > Na+ > F– > Al3+, when the molar ratio was 1.3 and 1.4, changing to Na+ > K+ > F– > Al3+, when the molar ratio was 1.5 and 1.6. The viscosities and ionic conductivities of the molten salts were in the range of 1.1–1.65 mPa s, and 0.85–1.15 S/cm, respectively.