Realizing reversible storage of trivalent aluminum ions using VOPO 4 ·2H 2 O nanosheets as cathode material in aqueous aluminum metal batteries

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, VOPO 4 ·2H 2 O nanosheets are successfully prepared using a refluxing method and an aqueous VOPO 4 ·2H 2 O/Al metal battery is constructed by employing the as-prepared VOPO 4 ·2H 2 O nanosheets as cathode, Al metal foil as anode, and Al(CF 3 SO 3 ) 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 VOPO 4 ·2H 2 O 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.