Highly Concentrated Mg(TFSA)2–MgCl2–Triglyme Electrolytes Toward High Potential Mg Rechargeable Batteries
Authors: Shimokawa Kohei , Matsumoto Hajime , Ichitsubo Tetsu 
Published: February 2020

There is a growing demand for large scale batteries applying to electric vehicles and stationary energy storage systems. Although lithium ion batteries are currently used for these purpose, further innovative batteries in terms of energy density and safety are required. Then, magnesium rechargeable batteries (MRBs) can be a promising candidate because Mg metal has several attractive features, such as its high specific capacity (2205 mAh g-
1), low electrode potential (-2.38 V vs. SHE), and dendrite-free electrodeposition. However, the sluggish Mg diffusion in cathode materials and the limited choice of electrolytes make it difficult to construct full-cell MRBs with high cell voltage. Recently, our group has reported that some spinel oxides (e.g., MgCo2O4) can work as 2–3 V class cathode materials, by utilizing the spinel-to-rocksalt transition with Mg insertion into spinel mother phase [1]. In the previous work, we used a CsTFSA-based ionic liquid as the electrolyte, and the operating temperature was set at 150°C to facilitate Mg diffusion in the spinel oxides. However, Mg metal anode is readily passivated in such electrolytes due to the decomposition of TFSA, which causes a large drop of the cell voltage. Recently, it was reported that the addition of MgCl2 is effective to suppress the passivation of deposited Mg in some ether based solutions of Mg(TFSA)2 [2–4], but the electrolytes that exhibit stable Mg dissolution behavior with a low overpotential as well as high thermal stability at elevated temperatures are not established yet.
In this work, we reveal that the Mg deposition/dissolution behavior in the Mg(TFSA)2–MgCl2/Triglyme (G3) solutions can be improved by restricting the amount of G3. Large amounts of MgCl2 salt can be dissolved in highly concentrated Mg(TFSA)2/G3 solutions at elevated temperatures around 150°C, by utilizing the chelating ability of G3 to Mg2+ ions and certain Shrenk-like reactions with Mg(TFSA)2. In particular, as for the mixture of Mg(TFSA)2:MgCl2:G3 = 1:1:2 (molar ratio), where the amounts of Mg and G3 are equimolar, the evaporation of the G3 solvent is efficiently suppressed even at 150°C. In addition, the mixture is crystallized by cooling down to room temperature without any phase separation or recrystallization of MgCl2. The behavior indicates that all G3 molecules coordinate to Mg2+ ions to form equimolar complexes as in the case of the so-called solvate ionic liquids [5], and it is also supported by Raman spectroscopy. More importantly, in the electrochemical measurements at elevated temperatures, the passivation of Mg is markedly suppressed in such highly concentrated solutions, in comparison to in the dilute solutions such as Mg(TFSA)2:MgCl2:G3 = 1:1:10 (molar ratio). In this presentation, we will discuss the solvation structure of Mg2+ ions in the highly concentrated solutions based on the results of Raman spectroscopy and powder-XRD measurements to grasp the origins of the improved Mg deposition/dissolution behavior. To assess the feasibility for MRBs operating at 150°C, battery tests are conducted using full-cells with MgCo2O4 cathodes and Mg metal anodes. We demonstrate that the cell with the electrolyte of Mg(TFSA)2:MgCl2:G3 = 1:1:2 delivers the highest cell voltage of about 2 V during a discharge process among the several electrolytes examined (Figure 1). We will mention how to design electrolytes with high potential cathodes to achieve high performance MRBs.
References:
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Figure 1