Haloaluminate-Free Cationic Aluminum Complexes: Structural Characterization and Physicochemical Properties

Journal article, 2016

The large electrochemical activities of haloaluminate anions [AlnX3n+1](-), anionic complexes derived from AlX3 and Lewis basic fertilizers, have significantly contributed to the development of industrial coatings and more recently also to eledrochernical energy storage. In contrast, cationic metal complexes have just emerged as a class of species interesting as multivalent main charge carriers for Mg, Ca, and especially here Al batteries. Despite the potential of such complexes to efficiently deliver Al3+ cations at the electrode! electrolyte interfaces, very few cationic aluminum complexes that do not contain moisture sensitive [AlnX3n+1](-) counter anions have been reported due to the few, and difficult to synthesize, commercially available parent aluminum salts. Here a range of cationic aluminum complexes with different ligands and, anionic structures were successfully synthesized by complexation of AlCl3 with certain ligands to create fully solvated [Al(L)(6)]Cl-3 complexes and subsequent application of anion metathesis reactions. X-ray crystallography aided by vibrational spectroscopy corroborates the formation of discrete complexes with hexacoordinated octahedral Al3+ cations balanced by three isolated anions. The resulting physicochemical properties are strongly dependent on the constituent ions, and one special choice of ligand and anion results in a, novel design of a room temperature quasi-ionic liquid having high ionic conductivity. Although the high-melting complexes with DMSO ligands are inactive, the:molten complex exhibits both cathodic and anodic currents. This is the first electrolyte that allows quasi-reversible electrochemical plating/stripping of Al metal without any fragile anion being present.