Spectroscopic Insight\non Neodymium Solvation in Lithium\nBorohydride-Supported Electrolyte

Borohydride-based\nelectrolytes have recently emerged as promising\nmedia for the electrodeposition of electropositive metals, including\nrare earth (RE) elements. While the presence of supporting alkali\nmetal cations and RE counteranions provides essential electrochemical\nconductivity for achieving fast metal electrodeposition, interactions\nbetween the host ligand and solvated neodymium (Nd) complexes remain\nunclear. This study provides insights into the coordination structure\nof a concentrated and directly solvated Nd salt in a lithium borohydride-supported\nelectrolyte. Our spectroscopic results indicate that the RE coordination\nenvironment is significantly influenced by the solvation mechanism,\nwhich can vary between metathesis and complexation pathways, primarily\ndictated by stoichiometric factors. Under dilute conditions, nearly\ncomplete metathesis of anions leads to a high coordination number\nfor the host ligand (borohydride), consistent with the previously\nreported solvated Nd speciation in chlorine-free electrolytes. In\ncontrast, concentrated dissolution of the Nd salt in the supported\nelectrolyte is dominated by a complexation pathway featuring a Li-ion-paired\ncomplex with a low coordination number of the host ligand. Density\nfunctional theory (DFT) calculations indicated that the observed blue\nshift in the borohydride vibration was the result of an increase in\nelectron density drawn into the terminal B–H interbond region\nfrom the hydride as the coordination changed from Li to Nd. In conjunction\nwith DFT results, vibrational analyses allowed correlation of the\nexperimental shifts associated with changes in Nd ligation and coordination\nspheres, further consolidating the prevalence of highly chloride-coordinated\nspecies under concentrated conditions. The outcomes of this work illuminate\nthe distinctive and heterogeneous coordination structures that the\nelectroactive RE species can adopt at high concentrations in lithium\nborohydride-supported electrolytes, as a key step to comprehend the\nreported metal electrodeposition performance in these media.