Electrolyte Based on Easily Synthesized, Low Cost Triphenolate–Borohydride Salt for High Performance Mg(TFSI)₂-Glyme Rechargeable Magnesium Batteries

Abstract

A new class of electrolyte based on TFSI and triphenolate-borohydride anions was designed and produced which fulfill all requirements of easy synthesis, high ionic conductivity, wide potential window, and noncorrosion of Al current collector. The electrolyte composed of magnesium triphenolate borohydride and Mg(TFSI) in glyme simultaneously displays a high conductivity of 5.5 mS cm at 25 °C and a reversible Mg plating/stripping with high current density and Coulombic efficiency at room temperature. By addition of a slight amount of MgCl to this electrolyte, a Coulombic efficiency of 90% in an SS/Mg cell, stable cycling performance, and a wide anodic potential of 3.4 V vs Mg/Mg on Al current collector can be reached. Reversible and efficient Mg insertion/deinsertion with a high capacity of 94 mAh g and 96% Coulombic efficiency was obtained in a MoS Chevrel cathode phase.

Mg Cathode Materials and Electrolytes for Rechargeable Mg Batteries: A Review

“…Also, the charge‐delocalized framework makes this conjugated [NTf 2 ] − anion highly resistant to positive potential polarization, and thus high anodic stability can be expected in such electrolytes. There have been a number of studies using Mg[NTf 2 ] 2 in different solvents for reversible Mg electrochemistry, particularly in polyether(s) ,. Electrolytes using 0.5 M Mg[NTf 2 ] 2 in a glyme/diglyme mixture showed a high conductivity of 5.2 mS cm −1 , and the anodic stability on stainless steel (SS) substrate was up to 4.2 V vs Mg …”

“…There have been a number of studies using Mg[NTf 2 ] 2 in different solvents for reversible Mg electrochemistry, particularly in polyether(s). [43,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] Electrolytes using 0.5 M Mg[NTf 2 ] 2 in a glyme/diglyme mixture showed a high conductivity of 5.2 mS cm À1 , and the anodic stability on stainless steel (SS) substrate was up to 4.2 V vs Mg. [46] Glymes with higher oligomer degree exhibit stronger ability to solvate Mg[NTf 2 ] 2 . [43,51] For example, a higher solubility of Mg has been reported in tetraglyme (G4) than in diglyme (G2).…”

Beyond Typical Electrolytes for Energy Dense Batteries

“…Note that in the latter case, reaction of the reductive borohydride with the acidic additive was indicated thereby partially transforming the borohydride (further investigation is needed to identify the new species). Another approach that was applied to modify Mg(TFSI)2 solutions with BH4 − was using a BH4 − anion with hydride groups that were partially substituted with phenol [91]. Both 0.5 M Mg(TFSI)2 and 0.15 M Mg(B(OPh)3H)2 were combined in diglyme and exhibited low deposition overpotentials (ca −0.64 V); however, the coulombic efficiency remained at low as 64% even after extended cycling [91].…”

“…− was using a BH 4 − anion with hydride groups that were partially substituted with phenol [91]. Both 0.5 M Mg(TFSI) 2 and 0.15 M Mg(B(OPh) 3 H) 2 were combined in diglyme and exhibited low deposition overpotentials (ca −0.64 V); however, the coulombic efficiency remained at low as 64% even after extended cycling [91].…”

Rechargeable Magnesium Batteries using Conversion‐Type Cathodes: A Perspective and Minireview

“…Also, Watanabe and co‐workers demonstrated that using molecular sieves to remove water impurities in tetraglyme solvents allows reversible Mg striping/plating . In light of the difficult access to very pure and dry Mg(TFSI) 2 and ether solvents, the introduction of an “impurity scavenger” (such as MgCl 2 , Mg(BH 4 ) 2 , Bu 2 Mg, or triphenolate‐borohydride) has been demonstrated to be an effective strategy to achieve highly reversible Mg stripping/plating processes (Table ) . By adding these highly reactive or reductive reagents, the extremely reduced overpotential for Mg deposition and stripping processes could be achieved as expected due to the consumption of water impurities.…”