Aqueous aluminum ion batteries (AAIBs) have received growing attention because of their low cost, safe operation, eco-friendliness, and high theoretical capacity. However, one of the biggest challenges for AAIBs is the poor reversibility due to the presence of an oxide layer and the accompanying hydrogen evolution reaction. Herein, we develop a strongly hydrolyzed/polymerized aluminum-iron hybrid electrolyte to improve the electrochemical behavior of AAIBs. On the one hand, the designed electrolyte enables aluminum ion intercalation/deintercalation on the cathode while stable deposition/stripping of aluminium occurs on the anode. On the other hand, the electrolyte contributes to the electrochemical energy storage through an iron redox reaction. These two reactions are parallel and coupled through an Fe-Al alloy on the anode, thus enhancing the reversibility and energy density of AAIBs. As a result, this hybrid-ion battery delivers a specific volumetric capacity of 35 A h L-1 at the current density of 1.0 mA cm-2, and remarkable stability with a capacity retention of 90% over 500 cycles. Furthermore, the hybrid-ion battery achieves a high energy density of approximately 42 W h L-1 with an average operating voltage of 1.1 V. This green electrolyte for high-energy AAIBs holds promises for large-scale energy storage applications.
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