Initial Evolution of Passivation Layers in Non-Aqueous Aluminium Batteries

Aluminium batteries (AlBs) have gathered considerable attention, primarily due to the high capacity, the low cost, the large abundance in the Earth’s crust, and the recyclability of the Al metal anode. However, several hurdles must be surpassed to make AlBs a feasible energy storage technology and two of them are interconnected; the presence of an ionic and electronically insulating native oxide layer on the Al metal anode that calls for special non-aqueous, most often ionic liquid based acidic electrolytes, to enable reversible plating and stripping of Al. We here find the passivation layer initially formed in contact with an ionic liquid electrolyte (ILE) to have a porous and very complex nature, i.e. an outer inorganic/organic layer and an inner oxide-rich layer. Furthermore, it grows under open circuit voltage conditions by simultaneous dissolution and re-deposition of dissolved products, while during galvanostatic cycling this is exacerbated by an electrochemical etching that causes pitting corrosion of the Al metal itself. All of this leads to unstable interfaces being formed and the co-existence of several species at the Al metal anode surface, of which a proper understanding and mitigation are crucial to make AlBs a reality.