Al conductive haloaluminate-free non-aqueous room-temperature electrolytes

Abstract

Al batteries are yet rather unexplored as a promising technology to respond to the growing electrochemical energy storage demands. Despite the outstanding electrochemical activity of haloaluminate-based electrolytes, no prospect of practical re-chargeable Al batteries has yet materialized, partly due to these electrolytes' extremely sensitive nature. Hence alternative aluminum conducting electrolytes with sufficient stability are strongly needed. Here a series of room-temperature ternary electrolytes consisting of aluminum trifluoromethanesulfonate (Al[TfO]₃), N-methylacetamide (NMA), and urea are presented, which provide excellent ionic conductivities by selecting appropriate ratios. Compared to conventional organic electrolytes, unprecedented solvation ability for Al-salts and remarkable ion transport properties were observed. For the optimized composition, Al[TfO]₃/NMA/urea = 0.05/0.76/0.19, physicochemical properties and vibrational spectroscopy data imply a decoupling of the Al conduction mechanism from viscosity limitations and furthermore that the dissociation state of the Al[TfO]₃ salt drastically changes. These phenomena are likely due to the unique coordination environment of the Al³⁺ ions and the multiple functions of urea in these ternary mixtures. The electrochemical properties of the optimized ternary electrolyte were studied with respect to the electrochemical stability window and using cyclic voltammetry.