An Efficient Bulky Mg[B(Otfe)₄]₂ Electrolyte and Its Derivatively General Design Strategy for Rechargeable Magnesium Batteries

The requirement of practical high-performance electrolytes is a key bottleneck restricting the development of rechargeable magnesium batteries (RMBs). Electrolytes based on weakly coordinated and fluorinated bulky boron-center anions (B­(OR^F)₄^–) have attracted wide attention for their admirable oxidation stability, high ionic conductivity, and weak corrosion. However, the complex synthesis route and costly raw materials still hinder their wide application. Therefore, a magnesium tetra­(trifluoroethanoloxy)­borate (Mg­[B­(Otfe)₄]₂) is designed not only to inherit these merits above but also greatly cut down the synthetic costs. The as-prepared electrolyte is synthesized by two methods of microcrystal redissolution and in situ reaction, both of them cycle stably for reversible Mg plating–stripping with an average Coulombic efficiency of ∼99% and overpotentials as low as 0.2 V, as well as an oxidation stability of more than 3 V vs Mg at stainless steel. The design strategy of the electrolyte and its compatibility with insertion or replacement-type cathodes promote the realization of practical RMBs.