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

Abstract

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.

Keywords

• zn-ion battery
• v2o5·1.6h2o
• cathode material
• cyclic performance
• mechanism exploration