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

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 electrochemical 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]− counteranions
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]­Cl3 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.