Room Temperature Magnesium Electrodeposition from Glyme-Coordinated Ammonium Amide Electrolytes

Abstract

We prepared less volatile and halide-free electrolytes for room temperature non-dendritic magnesium (Mg) electrodeposition by mixing a Mg 2+ -amide-containing ionic liquid (IL) with equimolar glyme (Mg 2+ +IL : glyme = 1:1). Raman spectroscopy suggested that in the equimolar mixture most glyme molecules are coordinated to Mg 2+ cations and/or IL cations, which is also supported by a single crystal X-ray diffraction study. The glyme-coordinated IL electrolytes showed sizable redox currents (order of mA cm -2 ), while aging deterioration of electrochemical properties was observed for the triglyme mixture due to partial bath decomposition. The tetraglyme-coordinated IL electrolyte enabled flat electrodeposition of Mg with a metallic luster and showed with very high anodic stability (ca. +4 V vs. Mg) because of decrease in uncoordinated glymes, which can be used for high-voltage Mg ion batteries. In the last few decades elemental magnesium (Mg) has come to be viewed as one of the most interesting negative electrode materials for post lithium ion secondary batteries because of its high-theoretical capacity (3839 mAh cm -3 ), low electrode potential (-2.356 V vs. SHE), and natural abundance. The well-known electrolytes for electrodepositing Mg metal at room temperature are Grignard electrolytes comprised of an ether solvent tetrahydrofuran (THF) and alkylmagnesium halides RMgX (R = alkyl or aryl groups; X = Cl, Br). Addition of AlCl 3 makes more electroactive species of organo-halo-aluminates, giving larger current density and/or higher anodic stability.1-10 However, the highly volatile THF and the highly moisture sensitive RMgX and AlCl 3 are difficult to use practically. Safer alternatives to both solvents and solutes for Mg-ion battery electrolytes are still being sought.Several groups have reported deposition of elemental Mg without using THF or RMgX. Ionic liquids (ILs) are one of the least volatile solvents and efforts have been made to electrodeposit Mg from ILs. For example, Mg redox behavior has been observed in an IL solution of Mg(ClO 4 ) 2 or MgCl 2 , although their current densities were significantly lower than those for RMgX-dissolved ILs.11 Some organic solvents that enable redox of Mg are less volatile than THF; these include 2-methyl-tetrahydrofuran, 12 and ethyleneglycol dimethylethers (glymes), [13][14][15] where Mg halides and hydrides were dissolved. Notably, glymes have boiling points above 150• C and are relatively safe at room temperature. However, in halide or hydride solutions hazardous halogen gas or hydrogen gas may evolve through anodic oxidation. From this viewpoint amide electrolytes are quite desirable as they hardly yield dissociated halogen ions.Glyme solutions of magnesium amides such as bis [(trifluoromethyl) 16 Therefore, it is of special interest to investigate glyme-based electrolytes where all glymes coordinate to any cations such as metal cations and/or ammnonium cations of ILs. Notably, the reversible deposition/dissolution cycle of Mg was reported using Mg 2+ -amide-containi...

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).…”

In-situ Multimodal Imaging and Spectroscopy of Mg Electrodeposition at Electrode-Electrolyte Interfaces

“…A uniform smooth thin film of Mg was achieved and no dendritic growth was observed. Although, other ex situ studies indicated non dendrite growth 10 30 31 , this work provides the first in situ study to support the theoretical prediction that Mg batteries should not have safety related issues imposed by dendritic growth 20 .…”

Ionic liquid hybrids: Progress toward non-corrosive electrolytes with high-voltage oxidation stability for magnesium-ion based batteries

“…Ionic liquids (ILs), or room temperature molten salts, are possible additives for Mg-ion batteries because of their dual role as electrolyte and solvent, large windows of voltage stability, low volatility and inability to ignite. [12][13][14] Some of the inherent limitations of ILs are high viscosities, low transference numbers and low conductivities which can be mitigated with cosolvents like THF [15][16][17][18][19] and glycol ethers [20,21]. Cosolvents with ILs have also been suggested to promote Mg 2+ dissociation [22] because Mg electrodeposition and dissolution from pure ILs with magnesium salts has been difficult [15,[23][24][25] or poorly reproducible [26][27][28].…”

“…[15] Kitada et al studied the oxidative stability of electrolytes with IL, glyme and magnesium bis[(trifluoromethyl)sulfonyl]amide salt and determined that the IL/glyme mixtures had a higher oxidative stability than IL-free electrolytes with only glyme solvent. [20] They attributed the improved stability to the lack of free glymes in solution when an IL is also present.…”