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Is One of the Least Coordinating Anions Suitable to Serve as Electrolyte Salt for Magnesium Ion Batteries?

Von Wiley-VCH zur Verfügung gestellt

Weakly coordinating anions (WCAs) are promising as electrolyte ions in multivalent post-lithium batteries. The least coordinating WCA  [Al(ORF)4]− is a strong candidate as electrolyte salt for Mg batteries, i.e. [Mg(L)x][Al(ORF)4]2 ( L = MeCN (acetonitrile), DME (1,2-dimethoxyethane); x = 3,6; RF = C(CF3)3). A novel synthetic route, for which vibrational, NMR and XRD analyses show the formation of a clean, pristine electrolyte salt without the common contaminants found in published synthetic routes, is reported. The electrochemistry of this pristine and pure [Mg(DME)3][Al(ORF)4]2 electrolyte (0.2 M in DME) was investigated via cyclic voltammetry (CV). In contrast to previous work, the CV showed at all conditions tested no sign for Mg deposition from the pristine electrolyte. This also holds when including the additives LiCl, NaCl, MgCl2, Li[BH4] or a mixture of MgCl2/Mg[HMDS]2 ( HMDS = Hexamethyldisilazide) in THF. Quantum chemical calculations suggested anion decomposition at the negative electrode, leading to an electronically insulating MgF2 layer (Rutile structure) on all the electrode materials tested. This hypothesis was confirmed by an XPS investigation of electrodes exposed to the electrolytes containing the WCAs [Al(ORF)4]– and [Al(OC(H)(CF3)2)4]– at negative potentials, which only showed the presence of MgF2 for the [Al(ORF)4]– electrolyte, but not for the related [Al(OC(H)(CF3)2)4]– electrolyte (which deposited magnesium on the electrode).

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