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

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. ... mehrFor 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.