Study on Microscopic Properties of Molten NaF-AlF₃-CaF₂/LiF/KF Using First-Principles Molecular Dynamics

Area of Science:

• Materials Science
• Computational Chemistry
• Electrochemistry

Background:

• Understanding the microstructure and ion dynamics of molten salt electrolytes is crucial for efficient aluminum electrolysis.
• Existing models often lack detailed insights into the complex interactions within these multi-component systems.

Purpose of the Study:

• To investigate the microstructure, transport, electronic, and vibrational properties of NaF-AlF3-CaF2/LiF/KF molten salt systems.
• To elucidate the roles of cryolite ratio, additive type, and concentration on electrolyte behavior.
• To establish structure-property relationships for optimizing aluminum electrolyte performance.

Main Methods:

• First-principles molecular dynamics (FPMD) simulations were employed to model the molten salt systems.
• The Voronoi tessellation method was utilized to analyze the local atomic environments and bonding characteristics.
• Analysis included ion states, coordination numbers, bond types, diffusion coefficients, and Raman spectra.

Main Results:

• Free Na+, Ca2+, Li+, and K+ ions coexist with complex [AlF_x]n- species and free F- in the molten salts.
• Ion diffusion order was determined as Li+ > Na+ > F- > Al3+, with K+ exhibiting high mobility.
• Al-F bonds show mixed ionic and covalent character, with [AlF4]-, [AlF5]2-, and [AlF6]3- being the dominant complex ions.

Conclusions:

• The study successfully links microscopic properties to the composition of aluminum electrolytes.
• First-principles molecular dynamics combined with Voronoi tessellation is a suitable approach for probing aluminum electrolyte microstructures.
• Findings provide fundamental insights for the design and optimization of advanced molten salt electrolytes.