Synergetic role of Li⁺ during Mg electrodeposition/dissolution in borohydride diglyme electrolyte solution: Voltammetric stripping behaviors on a pt microelectrode indicative of Mg-Li alloying and facilitated dissolution

We describe a voltammetric and spectroscopic study of Mg electrodeposition/dissolution (Mg_Dep/Dis) in borohydride diglyme electrolyte solution containing Li⁺ carried out on a Pt ultramicroelectrode (UME, r = 5 μm). The data reveal Li⁺ cation facilitation that has not been previously recognized in studies made using macroelectrodes. While a single broad, asymmetric stripping peak is expected following Mg_Dep on a Pt macroelectrode in 0.1 M Mg(BH₄)₂ + 1.5 M LiBH₄ diglyme solution on a Pt UME, the stripping reveals three resolved oxidation peaks, suggesting that Mg_Dep/Dis consists of not only a Mg/Mg²⁺ redox reaction but also contributions from Mg-Li alloying/dissolution reaction processes. Detailed XPS, SIMS, ICP, and XRD studies were performed that confirm the importance of Mg-Li alloy formation processes, the nature of which is dependent on the reduction potential used during the Mg_Dep step. Based on the electrochemical and surface analysis data, we propose an electrochemical mechanism for Mg_Dep/Dis in a borohydride diglyme electrolyte solution that, in the presence of 1.5 M Li⁺ ions, proceeds as follows: (1) Mg²⁺ + 2e^- ⇌ Mg; (2) (1 - x)Mg²⁺ + xLi⁺ + (2 - x)e^- ⇌ Mg_(1-x)Li_x, 0 < x ≤ ≤ and (3) (1 - y)Mg²⁺ + yLi⁺ + (2 - y)e^- ⇌ Mg_(1-y)Li_y, 0.02 < y ≤ 0.09. Most significantly, we find that the potential-dependent Mg_Dep/Dis kinetics are enhanced as the concentration of the LiBH₄ in the diglyme electrolyte is increased, a result reflecting the facilitating influences of reduced uncompensated resistance and the enhanced electro-reduction kinetics of Mg²⁺ due to Mg-Li alloy formation.