Effect of the Substrate and the Morphology on Electrochemical Separation of Lithium Isotopes: Insights from DFT

Abstract

The electrochemical separation of lithium isotopes has been revisited during the last years due to their applications in different aspects of nuclear technology, from the use of 7 Li in the chemical treatment of pressurized water reactors to the production of tritium from 6 Li for fusion nuclear plants. However, little is still known about the role played by each of the physicochemical variables in the separation factor, such as the applied overpotential, the electrolytes, or the materials used as electrodes. Particularly, a large isotope fractionation factor has been previously reported using planar nickel electrodes at very low overpotentials. In this work, we analyze how the electrode material (nickel or gold) and the morphology of the deposited lithium could affect the isotopic separation by means of Density Functional Theory (DFT) simulations. We consider the isotopic equilibrium exchange reaction between two phases: the strongly solvated lithium in solution and the adsorbed lithium structure onto the metallic substrates. The results suggest that there is a strong correlation between the strength of the deposited lithium bonds and the obtained fractionation. For the considered phases in equilibrium, the morphology with fewer and weaker lithium bonds could greatly improve the electrochemical isotope separation. Furthermore, we find that the bonding between lithium and the nickel surface is detrimental toward the enrichment. The calculations of lithium deposited onto gold indicate weaker bonds and, consequently, on average a larger fractionation factor R. These findings could guide the development of better electrochemical processes for lithium isotope separation, by clarifying not only the role of the lithium morphology, but also its relationship with the cathode material. DFT simulations have proven to be a useful tool to search for new materials that promote electrochemical isotopic separation.