Metal-organic-framework-derived vanadium(iii) phosphate nanoaggregates for zinc-ion battery cathodes with long-term cycle stability

Aqueous zinc-ion batteries (ZIBs) are promising energy storage systems, owing to their low cost, environmental friendliness, and high safety. Many types of cathodic materials have been developed for practical application in ZIBs, particularly layered-structured vanadium-based oxides that exhibit high theoretical capacities. However, the intrinsic instability of these oxides, attributable to their loosely bound layered crystal structures, leads to significant capacity deterioration during Zn²⁺ access. Herein, we report upon the first-time application of VPO₄ as a cathodic material for ZIBs. In particular, the simultaneous carbonization/phosphidation processes in the vanadium-based metal-organic MIL-47 framework facilitate the in situ formation of highly crystalline VPO₄ (HVPO) nanoparticles, which are uniformly interconnected via an electrically conductive thin carbon network. In ZIBs, the HVPO cathode delivers a superior rate capability and long cycle life (almost no capacity fading at 10 A g⁻¹ for 20 000 cycles) with a pseudocapacitive charge-storage performance. Furthermore, the proton-insertion charge-storage mechanism and byproduct formation are identified using ex situ analyses. The results show that the polyanion-based compound HVPO is not only feasible as a new cathode material but also exhibits highly stable and rapid proton-insertion-based pseudocapacitive charge-storage kinetics, owing to its rigid open-channel-based polyanion structure.