Investigation of Al(TfO)3-based deep eutectic solvent electrolytes for aluminium-ion batteries. Part I: understanding the positively charged Al complex formation

Abstract

Aluminium-ion batteries (AIB) are very attractive energy storage systems due to the high availability and theoretical energy density of metallic aluminium. However, the practical performance of AIBs in AlCl₃-based electrolytes is limited by the low reversible capacity of the positive graphite electrode for large AlCl₄⁻ anions. Moreover, the use of high energy oxide-based electrodes such as MnO₂ requires the presence of positively charged aluminium complexes in the electrolyte. In this work, the coordination of Al complexes in deep eutectic solvents (DESs) composed of Al triflate Al(TfO)₃ as the conducting salt, urea and/or acetamide as the hydrogen bond donor and formamide and N-methylacetamide as the solvent was investigated. Both solvents were able to dissolve and dissociate Al(TfO)₃ principally to a single positively charged Al complex [Al(TfO)₂–solvent_n]⁺ which was confirmed by Raman and NMR spectroscopy analyses. Furthermore, the addition of urea led to further dissociation to the [AlTfO–solvent_n–urea₂]²⁺ complex that can be assigned to the bidentate hydrogen bonding of urea with TfO at a specific ratio of urea/Al(TfO)₃ of 4 : 1. Because of their partial miscibility with urea, other conventional solvents such as propylene carbonate and acetonitrile dissociate Al(TfO)₃ only once to form [AlTfO₂–solvent_n]⁺. The as-prepared formamide and N-methylacetamide-based DES electrolytes show good conductivity values of 9.65 and 2.45 mS cm⁻¹, respectively.