Unveiling the Reaction Mechanism of Aluminum and Its Alloy Anode in Aqueous Aluminum Cells

Aqueous aluminum-ion batteries (AAIBs) are attractive electrochemical cells for energy storage because of Earth's crust abundance, inexpensiveness, high theoretical capacity, and safety of aluminum. However, state-of-the-art AAIBs based on aluminum or its alloy anode show ambiguity in the detailed charge–discharge reactions, and the activation mechanisms still need to be explored. Herein, we investigate the effects of surface modification (treated aluminum in ionic liquids (T-Al)) or the alloying approach (Al–Cu alloy or Zn–Al alloy) in different anionic aqueous aluminum-based electrolytes (e.g., 1 M Al(OTF)3, AlCl3, and Al(NO3)3). Neither the surface modification nor the alloying approach can support the rechargeability of aluminum atoms from the employed aqueous electrolytes. During the charge–discharge cycles of the symmetric cells in the aqueous electrolytes, the dissolution of aluminum and its alloy, accompanied by severe gas evolution, was observed. These corrosion reactions resulted in corrosive activity of aluminum and its alloy in these selected aqueous electrolytes. The aluminum and T-Al anode activity in these aqueous electrolytes was mainly induced by the chloride anions present in the aqueous electrolytes or residual in the treated artificial solid–electrolyte interface layer. For the activity of the Al–Cu alloy, nanolamellar Al2Cu served as the catalyst and kept the hydrolysis by α-Al in aqueous electrolytes to continuously generate H2. The Zn–Al alloy anode exhibited a severe gas evolution reaction in these aqueous electrolytes and, therefore, showed the highest activity among these Al anodes. Thus, for the first time, the present work clarified the mechanisms of reactions occurring at the Al anode using the surface modification or alloying approach of aqueous electrolyte Al cells.