Investigating the reversibility of in situ generated magnesium organohaloaluminates for magnesium deposition and dissolution

We compare the electrochemistry of Mg deposition and stripping from Mg(AlCl₂EtBu)₂ in THF, a common electrolyte used in Mg-ion battery prototypes, with EtMgBr, a simple Grignard reagent. Electrochemical quartz crystal microbalance measurements demonstrate that mass is gained and lost from the electrode with relatively high efficiency. However, the corresponding Coulombic efficiency is considerably less than the expected 100% for early stage cycles. SEM-EDS analysis shows accumulation of Mg and electrolyte constituents after stripping, highlighting the irreversibility of the Mg deposition and stripping process. GC-MS and NMR analysis of electrolytes reveal decomposition of the solvent-electrolyte system. These findings suggest that Mg organohaloaluminates are not ideal for use in robust Mg-ion batteries.