Critical Issues of Fluorinated Alkoxyborate-Based Electrolytes in Magnesium Battery Applications

The development of noncorrosive but highly efficient electrolytes has been a long-standing challenge in magnesium rechargeable battery (MRB) research fields. As fluorinated alkoxyborate-based electrolytes have overcome serious problems associated with conventional electrolytes, they are regarded as promising for practical MRB applications. An electrolyte containing representative magnesium fluorinated alkoxyborate Mg[B(HFIP)₄]₂ ([B(HFIP)₄]: tetrakis(hexafluoroisopropoxy) borate) was prepared through general synthetic routes using Mg(BH₄)₂; however, it shows poor electrochemical magnesium deposition/dissolution behavior. Herein, we report an alternative synthetic route of highly reactive Mg[B(HFIP)₄]₂ and several critical issues associated with the use of Mg[B(HFIP)₄]₂/glyme electrolytes in MRBs. The cycling performance of the electrolytes as well as the synthetic reproducibility of the salt was significantly improved upon adopting a transmetalation reaction between certain magnesium and boron compounds for the salt preparation. Despite the outstanding electrochemical activity of Mg[B(HFIP)₄]₂/glyme, the electrolytes were unstable with the magnesium metal. The remarkably high dissociativity of Mg[B(HFIP)₄]₂ in glyme solutions and the resulting enhanced induction interaction of Mg²⁺ with coordinated glymes make the solutions reductively unstable. Surface passivation by [TFSA]-based electrolytes (TFSA: bis(trifluoromethanesulfonyl)amide) effectively suppressed the decomposition of Mg[B(HFIP)₄]₂/glyme electrolytes. This passivation simultaneously caused a large overpotential for electrochemical cycling. The short-circuiting of the cells upon repeated deposition/dissolution cycling is rather problematic. Here, the findings disclose the issues of fluorinated alkoxyborate-based electrolyte solutions that should be resolved for practical MRB materialization. We also emphasize the importance of systematic strategies in manipulating the electrolytes and interfaces as well as base magnesium metal based on each appropriate approach.