Electrolyte-dependent deposition morphology on magnesium metal utilizing MeMgCl, Mg[B(hfip)4]2 and Mg(HMDS)2–2AlCl3 electrolytes

Abstract

The deposition behavior of two state-of-the-art electrolytes, magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)₄]₂) in dimethoxyethane (DME) and magnesium bis(hexamethyldisilazide) with two equivalents of aluminum chloride (Mg(HMDS)₂–2AlCl₃) in tetrahydrofuran (THF) was investigated. Using symmetric flooded magnesium–magnesium cells with different electrolyte concentrations and current densities the deposition process was monitored optically in situ by a video microscope. The deposits were also investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy and compared to deposition from methylmagnesium chloride (MeMgCl) in THF, known for its dendritic growth. Furthermore, the chemical composition of the surfaces after deposition was tested by X-ray photoelectron spectroscopy (XPS). In this work, MeMgCl showed unidirectional growth and for the harshest applied conditions, mossy deposition, but no branching dendrites as previously reported in the literature. Mg[B(hfip)₄]₂ and Mg(HMDS)₂–2AlCl₃ did not show the formation of dendrites or a dendrite preform but also did not result in the desired smooth layer but in spherical deposits. For the Mg[B(hfip)₄]₂ electrolyte, the influence of magnesium borohydride (Mg(BH₄)₂) as an additive was additionally tested, resulting in a more planar growth.