Correlating Electrochemical Behavior and Speciation in Neodymium Ionic Liquid Electrolyte Mixtures in the Presence of Water

Electrodeposition of rare earth metals at moderate temperature as a means to recover these critical materials from spent magnets or electric motors would be a major breakthrough to close the loop using a sustainable approach. This work highlights that neodymium electrodeposition in a phosphonium-based ionic liquid requires the presence of water in order to achieve a continuous layer of metal deposition. Herein, we describe that the nature of the Nd3+ solvation sphere being composed of both bis(trifluoromethylsulfonyl)imide (TFSI) and water determines neodymium deposition and that this process is not entirely dependent on the physicochemical properties of the electrolyte. The importance of the concentration of neodymium salt for the electrodeposition process has also been investigated, showing that 0.1 mol/kg Nd(TFSI)3/[P66614][TFSI] with 0.4 wt % water is the most promising electrolyte investigated, giving a favorable Nd3+ reduction process and better quality deposits.