Hindered aluminum plating and stripping in urea/NMA/Al(OTF)3 as a Cl-free electrolyte for Al batteries

Conventional electrolytes for aluminum metal batteries are highly corrosive because they must remove the Al2O3 layer to enable plating and stripping. However, such corrosiveness impacts the stability of all cell parts, thus hampering the real application of aluminum-metal batteries. The urea/NMA/Al(OTF)3 electrolyte is a non-corrosive alternative to the conventional [EMImCl]: AlCl3 ionic liquid electrolyte (ILE). Unfortunately, this electrolyte demonstrates poor Al plating/stripping, probably because (being not corrosive) it cannot remove the Al2O3 passivation layer. This work proves that no plating/stripping occurs on the Al electrode despite modifying the Al surface. We highlight how urea/NMA/Al(OTF)3 electrolyte and the state of the Al electrode surface impact the interphase layer formation and, consequently, the likelihood and reversibility of Al plating/stripping. We point up the requirement for carefully drying electrolyte mixture and components, as water results in hydrogen evolution reaction and creation of an insulating interphase layer containing Al(OH)3, AlF3, and re-passivated Al oxide, which finally blocks the path for the possible Al plating/stripping.