Optimization of Electrolytes for High-Performance Aqueous Aluminum-Ion Batteries

Aqueous rechargeable batteries based on aluminum chemistry have become the focus of immense research interest owing to their earth abundance, low cost, and the higher theoretical volumetric energy density of this element compared to lithium-ion batteries. Efforts to harness this huge potential have been hindered by the narrow potential window of water and by passivating effects of the high-electrical band-gap aluminum oxide film. Herein, we report a high-performing aqueous aluminum-ion battery (AIB), which is constructed using a Zn-supported Al alloy, an aluminum bis­(trifluoromethanesulfonyl)­imide (Al­[TFSI]₃) electrolyte, and a MnO₂ cathode. The use of Al­[TFSI]₃ significantly extends the voltage window of the electrolyte and enables the cell to access Al³⁺/Al electrochemistry, while the use of Zn–Al alloy mitigates the issue of surface passivation. The Zn–Al alloy, which is produced by in situ electrochemical deposition, obtained from Al­[TFSI]₃ showed excellent long-term reversibility for Al electrochemistry and displays the highest performance in AIB when compared to the response obtained in Al₂(SO₄)₃ or aluminum trifluoromethanesulfonate electrolyte. AIB cells constructed using the Zn–Al|Al­[TFSI]₃|MnO₂ combination achieved a record discharge voltage plateau of 1.75 V and a specific capacity of 450 mAh g^–1 without significant capacity fade after 400 cycles. These findings will promote the development of energy-dense aqueous AIBs.