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

Abstract

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.

A Novel and Highly Efficient Indolyl‐Based Electrolyte for Mg Batteries

“…This result is even more meaningful when compared with the available literature, which usually shows a cyclability of only 500 cycles, but nevertheless reported for a lower current density of about 0.5 mA cm −2 . [ 41–43 ] This long‐term stable cycling demonstrates a remarkable stability of the electrolyte. A similar performance was observed with Pt, Ni, SS, and Al substrates (cf., Figure S11 and S12, Supporting Information).…”

“…The cycling profile shows a slow diffusion kinetics as also reported by Ren et al . [ 42 ] Nevertheless, besides the similarly shape of the charge‐discharge curves for the de‐/intercalation of magnesium ions into the host material, the different states of charge are investigated by XRD (cf., Figure S14, Supporting Information) and in good agreement with the data reported in the literature. [ 46,47 ]…”

Toward practical issues: Identification and mitigation of the impurity effect in glyme solvents on the reversibility of Mg plating/stripping in Mg batteries

“…Thus, the instability of MgTFSI 2 itself at the Mg anode/electrolyte interface is another important factor that limits the electrochemical performance (to reach CE > 95%). Other relatively stable salts such as magnesium tetra (hexafluoroisopropyl)borate (Mg [B (hfip) 4 ] 2 and magnesium tetrakis (perfluoro-tert-butoxy)aluminate (Mg [TPFA] 2 ), or cosalt magnesium triflate [Mg (OTf) 2 ] with MgCl 2 in glymes, are now being developed to overcome this limitation (Deivanayagam et al, 2019;Lau et al, 2019;Dlugatch et al, 2021;Ren et al, 2021). Frontiers in Chemistry frontiersin.org…”

Boron–Based Electrolytes for Rechargeable Magnesium Batteries: Biography and Perspective

“…As shown in Figure 4(c), NuLi et al studied the electrostatic potential (ESP) iso-surface change as the deepening of fluorination degree, the positive and negative charge centers of B(OR F ) 3 molecular are no longer concentrated on a particular atom but tend to be conjugated. [51] When weaker fluorinated trifluoroethanol (tfe-OH) was used as the alcohol precursor, the diluted regional charge polarization is further manifested, and thus the weakened coordination effect brings an incomplete dissociation of B-centered anion and Mg 2 + . The synthesized [Mg • DME][B(Otfe) 4 ] 2 /THF (B(Otfe) 4 = tetra(trifluoroethyloxy) bo-rate) electrolyte also showed an oxidation decomposition potential reaching 3.3 V (vs. Mg, on SS) and 96 % CE of anodic Mg plating/stripping.…”