Determining local structures around a metal ion dissolved in molten salts with high structural disorder is cumbersome. While neutron/X-ray diffraction experiments can provide direct insight on short- or intermediate-range structural ordering, they may face extreme difficulties to extract structural information for minority components typically found in corrosion products. Likewise, in the case of computational methods, force fields-based molecular dynamics fails to provide electronic-level accuracy required to treat the chemistry of multivalent ions. To this end, we have established a combined approach of ab initio MD (AIMD) and extended X-ray absorption fine structure (EXAFS) spectroscopy to accurately determine the local coordination structure of a metal ion dissolved in molten salts. In several case studies, including Ni2+ and Co2+ dissolved in pure ZnCl2 or in different mixtures of ZnCl2 and KCl, we find that our method can identify different stable/metastable coordination states, clearly distinguishing between disordered (e.g., multiple coordination number for Ni2+) and ordered (e.g., single coordination number for Co2+) solvent environments. While AIMD can predict the dominating coordination structures, their populations may be overestimated/underestimated due to intrinsic limitations of the current DFT methods and limited sampling. However, such inaccuracies can be detected and corrected with our new approach through a linear combination fit of the computed spectra of different coordination numbers to the experimental spectrum. The variation of temperature and solvent composition drastically affects the local coordination structure of Ni2+and its temporal changes. Specifically, our rate theory analysis reveals that Cl- exchange around Ni2+, which occurs via overcoordination over time, becomes faster with increasing KCl concentration in the solvent mixture with ZnCl2—this is due to KCl disrupting ordered ZnCl2 network and making Cl- more mobile and available for exchange. New structural and kinetic insights will lead to better understanding of the macroscopic behavior of structurally disordered molten salts.

Elucidating local structure, dynamics, and speciation in structurally disordered molten salts

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.