Exploring Species in Low-Molar-Ratio NaF-AlF ₃ Molten Salts Using Raman Spectroscopy and Quantum Chemical Calculations

Understanding the ionic structure of NaF-AlF₃ molten salts is essential for advancing aluminum electrolysis technologies. Raman spectroscopy and quantum chemical calculations have confirmed the presence of AlF₆^3-, AlF₅^2-, AlF₄^-, and Al₂F₇^- ions in the system. Notably, this study presents the first spectroscopic evidence of Al₂F₇^- in NaF-AlF₃ molten salts. Two structural isomers of Al₂F₇^-, exhibiting <i>D</i>_3<i>h</i> and <i>D</i>_3<i>d</i> point group symmetries, were characterized. Frontier orbital analysis revealed that the <i>D</i>_3<i>h</i> isomer possesses a narrower HOMO-LUMO gap, indicating enhanced ionic conductivity compared to the <i>D</i>_3<i>d</i> configuration. Wave function analysis further demonstrated that terminal fluorine atoms are preferentially oxidized at the anode, while aluminum centers act as the dominant electron acceptors at the cathode. The valence electron density maps and Mayer bond order analyses demonstrated the formation of highly polar covalent bonds between bridging fluorine atoms and terminal aluminum atoms, with significant electron localization centered on the bridging fluorine. Taken together, these insights offer a more nuanced mechanistic understanding of aluminum-fluoride coordination and redox dynamics in fluoride-rich environments, providing a solid theoretical basis for advancing low-carbon, energy-efficient aluminum electrolysis technologies.