Accelerating Neodymium's First-Shell Dynamics toward Improved Metal Recovery

An increasing demand for rare earth elements (REE) such as neodymium has created a need to explore avenues for their reclamation from secondary sources. To this end, electrodeposition of Nd has proven promising with the use of phosphonium ionic liquids (IL) with dilute quantities of water. However, the detailed reasons for this performance remain unclear. Herein, force-field molecular dynamics simulations of a neodymium salt in two phosphonium ILs ([P_666,14][TFSI] and [P_1,444][TFSI]) were conducted in the presence of trace concentrations of water. These simulations reveal dominant coordination structures, supported by ab initio MD simulations. Addition of water leads to a range of Nd³⁺ structures with participation from water, a broadening of TFSI^- denticity, and accelerated first-shell anion dynamics. Overall, it is identified that trace quantities of water, along with a shorter alkyl-chained phosphonium, foster limited Nd ion hydration, while also significantly accelerating the dynamism of the Nd solvation sphere. The findings suggest that these factors could facilitate an easier deposition of Nd metal at the electrode interface. This work introduces a detailed molecular-scale understanding of the behavior of Nd³⁺ salts in phosphonium ILs and the effects of water on the coordination environment of Nd³⁺, which can guide future electrodeposition efforts for the recovery of REEs.