Elucidating the Requirements for Reversible Magnesium Electrodeposition and Dissolution

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We compare the electrochemistry of Mg deposition and stripping from three Mg-battery electrolytes: EtMgBr, Mg(AlCl2EtBu)2, and MgCl2-AlCl3. These electrolytes represent three generations of progress in Mg-battery development, and include a simple Grignard reagent, an organohaloaluminate, and an all inorganic system, respectively. Our electrochemical measurements coupled with surface and bulk analytical techniques elucidate the requirements for reversible Mg deposition and definitively establish that voltammetric cycling of the electrolyte significantly alters their composition and performance. In the cases of EtMgBr and Mg(AlCl2EtBu)2, electrochemical quartz crystal microbalance measurements demonstrate that mass is gained and lost from the electrode with relatively high efficiency. However, the charge passed during stripping is consistently less than that passed during deposition, resulting in chronoamperometric charge efficiencies between 80-100% for EtMgBr and 50-70% for Mg(AlCl2EtBu)2. SEM-EDS analysis of electrode surfaces demonstrates irreversible Mg deposition and stripping. GC-MS and NMR analysis of electrolytes reveal decomposition of the solvent-electrolyte system. For the all inorganic system containing MgCl2-AlCl3 in THF, elemental analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (SEM-EDS) results demonstrate that irreversible Mg and Al deposits form during early cycles. Electrospray ionization-mass spectrometry (ESI-MS) data show that inhibitory oligomers develop in THF-based solutions. These oligomers form via the well-established mechanism of a cationic ring-opening polymerization of THF during the initial synthesis of the MgCl2-AlCl3 electrolyte and under resting conditions. In contrast, solutions containing MgCl2-AlCl3 in 1,2-dimethoxyethane (DME), an acyclic solvent, do not evolve as dramatically at open circuit potential. From these results, we propose a mechanism describing how the conditioning process of MgCl2-AlCl3 in THF improves its performance by both tuning the Mg:Al stoichiometry and eliminating oligomers. We also explore the role that other solvents and metal chlorides play in the properties of inorganic Mg-battery electrolytes.