Lamellar fiber V₂O₅·1.6H₂O for improving the cyclic performance of aqueous Zn-ion batteries

Aqueous zinc-ion batteries have great development and application prospects due to the low cost and environmental friendliness. Vanadium-based materials with high specific surface area and layered or fast ionic conductor structures are among the most promising cathode materials for zinc-ion batteries. Layered vanadium pentoxide cathodes have higher capacity and adjustable interlayer spacing, which have been extensively examined. As a layered vanadium pentoxide, V₂O₅·nH₂O is widely evaluated because of its high theoretical capacity, simple synthesis process, etc. However, the practical application of layered V₂O₅·nH₂O is still hindered by structural collapse during cycling and slow Zn²⁺ diffusion in the V₂O₅·nH₂O cathode. How to improve the long-cycle performance of V₂O₅·nH₂O remains to be solved. In this study, V₂O₅·1.6H₂O xerogel was successfully prepared by the sol-gel method combined with the freeze-drying technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the phase composition and morphology. The results showed that the prepared material was primarily V₂O₅·1.6H₂O with good crystallinity, and little V₂O₅ still existed. V₂O₅·1.6H₂O grew like macroporous lamellar fibers of approximately 100 nm thick. Compared with commercialized V₂O₅, V₂O₅·1.6H₂O has larger interlayer space, which benefits the diffusion of Zn²⁺, and the crystal H₂O may help stabilize the structure. Electrochemical performance results revealed that the fibrous V₂O₅·1.6H₂O cathode material showed an initial discharge capacity of 388.4 mA·h·g^–1 at a constant current of 0.1 A·g^–1 and it still maintained at 129.7 mA·h·g⁻¹ after 1000 cycles, with nearly no capacity decay. At 0.1, 0.2, 0.5, 1, 2, and 3 A·g⁻¹, the fibrous V₂O₅·1.6H₂O xerogel show capacities of 388.4, 338.5, 282.9, 239.1, 194.4, and 165.9 mA·h·g⁻¹, respectively. The capacity was much higher than that of commercialized V₂O₅, which only showed 279.5, 251.0, 205.5, 174.5, 144.6, and 125.1 mA·h·g⁻¹, respectively, at the same discharge current density. The good electrochemical performance was mainly attributed to the large layer spacing, combined with the supporting effect of H₂O, which contributed to the good structural stability of the material during the cycle and avoided the degradation of material properties. In addition, the fibrous structure shortened the Zn²⁺ diffusion path and increased the electronic conductivity also contributed to the enhanced electrochemical performance. The mechanism of the charge and discharge process was examined by ex-situ X-ray photoelectron spectroscopy (XPS) and XRD. The results showed that the formation and disappearance of basic zinc sulfate are accompanied by the embedding and removal of zinc ions, and the process is reversible.