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片層纖維V2O5·1.6H2O干凝膠提升水系鋅離子電池循環性能

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

  • 摘要: 水系鋅離子電池憑借低成本和環境友好的特點具有極大的發展和應用前景. 具有高比表面、分層、或快速離子導體結構的釩基材料是鋅離子電池最具有前景的正極材料之一. 如何改善釩基材料的長循環性能是亟待解決的問題之一. 本文采用溶膠凝膠法并冷凍干燥成功制備了V2O5·1.6H2O干凝膠,利用X射線衍射儀、掃描電子顯微鏡對其物相和形貌進行了表征,發現制備的材料為V2O5·1.6H2O,結晶相良好,且成片狀纖維大孔結構. 電化學測試表明,在0.1 A?g–1電流密度下,首次放電比容量為388.4 mA·h?g–1,循環1000次后容量仍保持為129.7 mA·h?g–1,具有良好的長循環穩定性. 在0.1、0.2、0.5、1、2和3 A?g-1電流密度下,纖維狀V2O5干凝膠表現出良好的倍率性能,放電比容量分別為388.4、338.5、282.9、239.1、194.4和165.9 mA·h?g–1,遠高于商業化V2O5 (279.5、251.0、205.5、174.5、144.6和125.1 mA·h?g–1). 良好的電化學性能主要歸功于結合水的支撐作用增大了層間距,在循環過程中材料具有良好的結構穩定性,避免了放電容量衰減;同時纖維片狀結構縮短了鋅離子的遷移路徑. 對充放電機理研究發現,在鋅離子的嵌入脫出過程中伴隨有堿式硫酸鋅的生成與消失,且該過程可逆.

     

    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.

     

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