Structural Analysis of Magnesium Chloride Complexes in Dimethoxyethane Solutions in the Context of Mg Batteries Research

Abstract

Recently, MgTFSI2/MgCl2 electrolyte solutions in dimethoxyethane (DME) have been shown to function as viable electrolyte solutions for secondary Mg batteries that can facilitate reversible magnesium deposition/dissolution. MgCl2 is a crucial component in these solutions. On its own, however, it is practically insoluble in DME. Therefore, the fact that it is readily dissolved in MgTFSI2/DME solution is remarkable. Addition of MgCl2 greatly improves the electrochemical performance of MgTFSI2/DME electrolyte solutions. Thus, identifying the species formed in MgTFSI2/MgCl2 solutions is intriguing. In this study, we implemented a wide variety of analytical tools, including single crystal X-ray diffraction, multinuclear NMR, and Raman spectroscopy, to elucidate the structure of these solutions. Various solution species were determined, and a suitable reaction scheme is suggested.

Anode-Electrolyte Interfaces in Secondary Magnesium Batteries

“…Chlorides, in the current context, pertain to both complex anions and cations, serving as ligands, and solvating or surface-adsorbed anions. 4,25,28,39 As noted before, the first solutions that supported fully reversible magnesium electroplating were Grignard reagents in ethers. 26 These RMgX reagents included compounds where X = Cl, Br, and probably also I.…”

“…Interestingly, these cationic species are similar to those formed in MgTFSI 2 /MgCl 2 /DME solutions. 28 Borohydride-Based Electrolyte Solutions Mg(BH 4 ) 2 is a commercially available salt. Due to its strong reductive nature, it acts as scavenger for reducible contaminants in electrolyte solutions, similar to Grignard reagents.…”

“…38 A rigorous study of the structure of chlorides containing MgTFSI 2 solutions revealed the existence of Mg 2 Cl 2 2+ and Mg 3 Cl 4 2+ as solution species, at least in DME-based solutions. 28 It is clear that, in DME/MgTFSI 2 solutions, the strong complexation between the DME molecules and the Mg cations forms Mg(DME) 3 2+ moieties as major cationic species, the electrochemical reactivity of which is hindered by the strong solvation shell. The role of MgCl 2 in these solutions in discussed in the next section.…”

“…Better understanding was gained regarding the role of chlorides in the solution structures. 21,28,37 Apart from the process of electrochemical deposition/dissolution of magnesium, one of the greatest concerns associated with chlorides is their corrosive nature, both toward ''inert'' components in the cell and to cathode materials, in particular metal oxides. In fact, even gold electrodes were found to be heavily corroded under some circumstances.…”

“…Cations such as the solvate Mg 2+ 3DME, Mg 2 Cl 2 2+ , Mg 3 Cl 5 , and others have been identified. 12,21,28,37 The impact of chlorides on the magnesium deposition process in the simple salt-based solutions is critical. Solutions based on Mg(TFSI) 2 , Mg(PF 6 ) 2 in THF, DME, and glymes require a substantial proportion of chlorides to support magnesium deposition at high efficiency and reasonable overpotentials.…”