Kinetics of Magnesium Deposition and Stripping from Non-Aqueous Electrolytes

Abstract

Three previously reported non-aqueous electrolyte solutions were investigated electrochemically to determine the kinetics of the Mg deposition–dissolution process as well as the resulting morphology after Mg-ion reduction onto a bulk metal working electrode. Of the solutions examined, the 1.2 M ((CF3)2CH3)COMgCl and 0.2 M AlCl3 in THF solution (F6-t-butoxide) shows the lowest Tafel slope of 26.4 mV/dec, followed by 0.5 M RPhOMgCl and 0.25 M AlCl3 where R = 2,4,6-Me3 in THF (2,4,6-Me3 phenolate, 32.1 mV/dec) and 0.4 M PhMgCl and 0.2 M AlCl3 in THF (APC, 56.2 mV/dec). The fluorinated alkoxide results in complete magnesium coverage of the working electrode, with the metal growing along the [100] direction, orthogonal to the electrode surface. This behavior is contrary to the aromatic-based electrolyte solutions, which show less crystalline Mg deposits and incomplete surface coverage. Through a combination of electron microscopy, X-ray diffraction, and electrochemical methods, we show that this sporadic deposition, in addition to lower solution conductivities, drastically hinders the current density observed for phenolate and APC electrolytes. Electrochemical impedance spectroscopy demonstrates that the largest resistance originates at the platinum–magnesium interface and the solution resistance itself, rather than the actual charge transfer between the electrode and magnesium ions in solution. The facile kinetics of Mg-deposition, along with a strong dependence on surface crystallinity and coverage, suggests the importance of measuring solution conductivity in addition to fully characterizing the resulting magnesium deposits under chronopotentiometric conditions.