Connections Between the Speciation and Solubility of Ni(II) and Co(II) in Molten ZnCl2

Understanding the factors that control solubility and speciation of metal ions in molten salts is key for their successful use in molten salt reactors and electrorefining. Here we employ X-ray and optical absorption spectroscopies, and molecular dynamics simulations to investigate the coordination environment of Ni(II) in molten ZnCl2, where it is poorly soluble, and contrast it with highly soluble Co(II) over a wide temperature range. In solid NiCl2 the Ni ion is octahedrally coordinated, whereas the ZnCl2 host matrix favors tetrahedral coordination. Our experimental and computational results show that the coordination environment of Ni(II) in ZnCl2 is disordered among tetra- and penta-coordinate states. In contrast, the local structure of dissolved Co(II) is tetrahedral and commensurate with the ZnCl2 host’s structure. The heterogeneity and concomitant large bond length disorder in the Ni case is a plausible explanation for its lower solubility in molten ZnCl2.