posted on 2020-08-21, 19:03authored byToshihiko Mandai
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.