Novel Transmetalation Reaction for Electrolyte Synthesis for Rechargeable Magnesium Batteries
Authors: Zhao-Karger Zhirong , Mueller Jonathan E. , Fuhr Olaf , Fichtner Maximilian 
Published: February 2020

Emerging applications of rechargeable batteries in vehicles raise concerns about long cycle life, low cost, and safety. Due to the limited lithium resources and safety issues, new battery chemistries are appealing. Magnesium batteries have been recognized as promising alternative. Magnesium is abundant element in the earth’s crust and Mg metal can be safely used as anode. Owing to the divalent nature of Mg2+, magnesium batteries could potentially provide a volumetric capacity of up to 3832 mA h cm-3, which is significantly higher than those of lithium (2062 mAh cm-3) and sodium (1136 mAh cm-3).1,2 One of the major challenges in the commercialization of Mg batteries has been the development of an electrolyte which is stable in contact with the electrode materials, does not form a blocking layer and has a wide electrochemical window. Gregory et al. have initially demonstrated the magnesium ion conductivity of the solutions containing Mg organo-borates or organo-aluminates.1 Aurbach et al. improved the oxidative stability of the electrolytes by reacting AlCl3-xRx Lewis acids with R2Mg Lewis bases in ethers at various ratios, and demonstrated the first prototype of a rechargeable Mg battery.2 Subsequently, the properties of the electrolytes were further optimized by combining different organomagnesium compounds and Lewis acids using the same concept.3,4

Interestingly, it is found in the literature that crystallographic studies of the electrolytes generated from various combinations of organomagnesium compounds and Lewis acids reveal very similar structures, in which a cation, consisting of two magnesium atoms bridged by three chlorines, pairs with a counter anion such as an organo-aluminate or borate.5-8These results imply that such binuclear magnesium compounds are thermodynamically favored in the reaction between an organomagnesium and a Lewis acid. Moreover, these binuclear magnesium complexes are capable of reversible Mg deposition and show enhanced oxidative stability compared to the reaction mixtures.
Herein we present a novel transmetalation reaction between MgCl2and organoaluminum compounds which leads exclusively to the electrochemical active aluminate complex, as expressed in the general chemical equation 1 (x = 1, 2, 3; neglecting the solvent ligands).
2MgCl2 + RxAlCl3-x → [Mg2Cl3]+[RxAlCl4-x]- 1
Through this reaction, we are also able to obtain a chemical bond between aluminum and the ligand in the complexes which is more stable than the Al-C bonds and are thus able to optimize the properties of the electrolyte. Owing to the higher electronegativity of nitrogen and oxygen, the polar Al-N or Al-O bonds in the aluminate anion should benefit the voltage stability of the electrolyte. We have developed a straightforward synthetic route for the Al-O bond containing electrolyte by employing the transmetalation reaction. The as-prepared electrolyte shows an anodic stability of up to 3.4 V, good air-stability and ionic conductivity.
The one-step synthesis without organomagnesium compounds can serve as a useful tool to design the electrolyte composition and improve the performance of the electrolytes. The feasibility of the transmetalation reaction with various aluminium Lewis acids opens the door for the accessibility of air-stable and non-nucleophilic electrolyte for high energy magnesium batteries such as magnesium air and magnesium sulphur batteries.
References
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