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.

Zero-strain Na4Fe7(PO4)6 as a novel cathode material for sodium-ion batteries.

We report for the first time a zero-strain cathode, Na4Fe7(PO4)6, for sodium-ion batteries (SIBs). This new iron-based polyanionic cathode delivers a reversible capacity of 66.5 mA h g-1 at 5 mA g-1 with almost 100% capacity retention over 1000 cycles under 200 mA g-1, and the outstanding performance benefits from single-phase-transition processes with a tiny volume change of only ∼0.24%.