Molecular Dynamics Simulation for the Structural Properties of the KF-NaF-AlF3 System

近年来低温铝电解质的研究已成为铝电解工业上节能减排的热点问题，对其基本物理化学性质的探索是一个必然的过程。本文运用密度泛函理论与经典分子动力学方法相结合的方法系统研究了1100 K下xKF-2.2NaF-AlF3熔盐体系的结构性质和输运性质。研究结果表明在平衡的高温钾冰晶石熔盐体系中六配位[AlF6]3−、五配位[AlF5]2−、四配位[AlF4]−络合离子共同存在，且六配位铝氟离子集团占主导地位(70%以上)，并会发生部分的离解，生成五配位或者四配位离子团；当KF浓度增大时，电解质体系中游氟原子含量会增加，桥氟含量逐渐降低，体系的聚合程度也会随之下降。此外，KF的加入使得桥氟离子解离，Na、Al的自扩散系数会先增大后降低，体系显示出低粘度高电导率的特性，但当KF浓度达到2%后，电导率会逐渐降低。 The study of low temperature aluminium electrolytes has become a hot issue for energy saving and emission reduction in the aluminium electrolysis industry in recent years, and the exploration of their basic physicochemical properties is an inevitable process. In this paper, the structural and transport properties of the nKF-2.2NaF-AlF3 molten salt system at 1100 K have been systematically investigated using a combination of density flooding theory and classical molecular dynamics methods. The results show that in the equilibrium high-temperature potassium cryolite molten salt system, hexacoordinated [AlF6]3-, pentacoordinated [AlF5]2- and tetracoordinated [AlF4]- complex ions co-exist, and the hexacoordinated aluminium-fluorine ion group dominates (more than 70%) and undergoes partial dissociation to produce pentacoordinated or tetracoordinated ion groups; As the KF concentration increases, the amount of free fluorine atoms in the electrolyte system increases, the bridge fluorine content gradually decreases and the degree of polymerisation of the system decreases. In addition, the addition of KF causes the dissociation of the bridging fluorine ions, the self-diffusion coefficients of Na and Al will first increase and then decrease, and the system shows the characteristics of low viscosity and high conductivity, but when the KF concentration reaches 2%, the conductivity will gradually decrease.