Solid electrolyte interphases for high-energy aqueous aluminum electrochemical cells

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.