A Key Concept of Utilization of Both Magnesium Chloride and Imide Salts for High Temperature Rechargebale Mg Battery Electrolytes
Authors: Mandai Toshihiko , Kanamura Kiyoshi 
Published: February 2020

Magnesium rechargeable batteries are an emerging class of attractive technologies for next generation large scale energy storage devices much owing to the inherent favorable properties of magnesium metal compared to those of lithium metal, i.e. large volumetric capacity, large natural abundance, low cost, and high safety. Since the pioneering work done by Aurbach in the realization of magnesium batteries,1 the development especially in electrode and electrolyte materials has indeed been accelerated in past few decades. Despite such significant efforts, the materialization of any practical batteries has not yet been achieved but still in the early stage mainly due to the lack of a suitable electrolyte that is compatible with both cathode and anode materials. A recent theoretical study of high voltage cathode materials for multivalent-metal based batteries suggested that high operation temperature is required for cycling such batteries due to a large barrier of charge transfer at the electrode/electrolyte interface and limited migration of multivalent ions.2 Although the magnesium aluminum chloride complexes (MACC) and their families in THF or dimethoxythane exhibited excellent electrochemical properties for the magnesium metal anode,3,4 poor thermal stability of these electrolytes has hampered their adoption for practical battery applications. On the other hand, the high temperature stability is achievable by using higher glymes. Most magnesium chloride salts however alone are hardly soluble in e.g. triglyme and tetraglyme. In addition, the electrolyte solutions of Mg(TFSI)2/glyme are modestly active electrochemically compared to the MACC-based electrolytes.5 Here we propose a concept of the electrolyte design for high temperature magnesium rechargeable batteries.6 Appropriate combination of modified magnesium chloride and magnesium imide salts dissolved in triglyme at certain molar ratio exhibited remarkable electrochemical properties at 100 ºC, in spite of the absence of activation by strong Lewis acid, e.g. AlCl3. Raman spectra of the selected electrolyte solutions aided by the crystal structure and electrochemical characterization revealed that the formation of the structurally “unstable” μ-complex would impart a favorable activity while the thermodynamically stable Mg–Cl complex is inactive electrochemically. This strategy and finding will inspire the rational designing of electrolytes applicable to high temperature rechargeable magnesium batteries.
Reference
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