Influence of Simple Salts on Solvent Reduction Stability at Mg-Alloy Anodes Interface: A Potential-Dependent DFT Study

The significance of incorporating anion species into electrolyte solvation structures, particularly with doubly charged Mg²⁺ ions, is investigated using the grand canonical density functional theory (GC-DFT) approach. In an extension of previously established methodology, the work explores the thermodynamic stability in acetonitrile (AN) at the interface with Mg₃Bi₂ and Mg₂Sn. Two different anions, TFSI⁻ and ClO₄⁻, are strategically incorporated based on energy comparisons. Despite the known chemical compatibility of alloy anodes with the electrolyte solution, the research reveals a novel form of solvent degradation, which is also reported in the case of pure Mg anode with conventional electrolytes. Notably, the AN molecule adjacent to anion species exhibits reduced susceptibility to reduction (−0.8 – −0.4 V vs Mg²⁺/Mg) in the lower potential range in comparison with the solvation structure of full dissociation. Charge density difference and density of states analyses detail solvent molecules becoming electrophilic, with the LUMO overlapping with the Fermi level at lower potentials when the electrostatic interaction with anion species are considered. Experimental studies using nuclear magnetic resonance (NMR) spectroscopy and linear sweep voltammetry (LSV) validate the theoretical results, providing a comprehensive understanding. This methodology, augmenting prior approaches, provides valuable guidance for electrolyte composition based on predominant solvation structures in multivalent solutions.