δ-VOPO₄ as a high-voltage cathode material for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) with excellent safety, low-cost and environmental friendliness have great application potential in large-scale energy storage systems and thus have received extensive research interest. Layered oxovanadium phosphate dihydrate (VOPO₄·2H₂O) is an appealing cathode for AZIBs due to the unique layered framework and desirable discharge plateau, but bottlenecked by low operation voltage and unstable cycling. Herein, we propose delta-oxovanadium phosphate (δ-VOPO₄) without conventional pre-embedding of metal elements or organics into the structure and paired it into AZIBs for the first time. Consequently, superior to the layered counterpart, δ-VOPO₄ exhibits better performance with a prominent discharge voltage of 1.46 V and a higher specific capacity of 122.6 mA h g^-1 at 1C (1C = 330 mA g^-1), as well as an impressive capacity retention of 90.88 mA h g^-1 after 1000 cycles under 10C. By investigation of structure resolution and theoretical calculation, this work well elucidates the structure-function relationship in vanadyl phosphates, offering more chances for exploration of new cathode materials to construct high performance AZIBs.