|Electrochemical cells based on aluminum (Al) are of long-standing interest because Al is earth abundant, low cost, and chemically inert. The trivalent Al3+ ions also offer among the highest volume-specific charge storage capacities (8040 mAh cm−3), approximately four times larger than achievable for Li metal anodes. Rapid and irreversible formation of a high-electrical bandgap passivating Al2O3 oxide film on Al have, to date, frustrated all efforts to create aqueous Al-based electrochemical cells with high reversibility. Here, we investigate the interphases formed on metallic Al in contact with ionic liquid (IL)–eutectic electrolytes and find that artificial solid electrolyte interphases (ASEIs) formed spontaneously on the metal permanently transform its interfacial chemistry. The resultant IL-ASEIs are further shown to enable aqueous Al electrochemical cells with unprecedented reversibility. As an illustration of the potential benefits of these interphases, we create simple Al
|MnO2 aqueous cells and report that they provide high specific energy (approximately 500 Wh/kg, based on MnO2 mass in the cathode) and intrinsic safety features required for applications.