Realizing reversible storage of trivalent aluminum ions using VOPO4·2H2O nanosheets as cathode material in aqueous aluminum metal batteries

Highlights: • An aqueous VOPO₄·2H₂O/Al metal battery is constructed and optimized. • The battery shows a high specific capacity of 125.4 mAh g^–1 with a high operation voltage of ~0.9 V. • A reversible single-phase electrochemical reaction mechanism is demonstrated by ex-situ XRD, TEM and XPS techniques. -- Abstract: Aqueous Al metal batteries (AAMBs) are a very promising safe and cost-efficient energy storage system for large-scale applications because of the natural non-flammability of aqueous electrolyte and the high abundance of Al in the earth's crust. However, lack of proper cathode materials is the biggest stumbling block to the development of AAMBs. Here, VOPO₄·2H₂O nanosheets are successfully prepared using a refluxing method and an aqueous VOPO₄·2H₂O/Al metal battery is constructed by employing the as-prepared VOPO₄·2H₂O nanosheets as cathode, Al metal foil as anode, and Al(CF₃SO₃)₃ aqueous solution as electrolyte. The electrochemical performance of the batteries is tested and it is optimized by selecting appropriate electrolyte. The batteries exhibit a high discharge capacity of 125.4 mAh g⁻¹ at a current density of 20 mA g⁻¹, a high working voltage of ~0.9 V, and good capacity retention of 60% after 40 cycles. Moreover, a single-phase electrochemical reaction mechanism of the VOPO₄·2H₂O cathode is investigated by ex-situ XRD, TEM and XPS techniques. We believe that this work can promote the development of AAMBs and provide ideas for the design of cathode materials for other aqueous multivalent-ion batteries.