锌离子电池正极材料V2O5的储能机理和容量衰减原因

水系锌离子电池凭借低成本和环境友好的特点具有极大的发展和应用前景. 具有高比表面、分层、或快速离子导体结构的钒基材料是锌离子电池最具有前景的正极材料之一. 如何改善钒基材料的长循环性能是亟待解决的问题之一. 本文采用溶胶凝胶法并冷冻干燥成功制备了V₂O₅·1.6H₂O干凝胶，利用X射线衍射仪、扫描电子显微镜对其物相和形貌进行了表征，发现制备的材料为V₂O₅·1.6H₂O，结晶相良好，且成片状纤维大孔结构. 电化学测试表明，在0.1 A·g^–1电流密度下，首次放电比容量为388.4 mA·h·g^–1，循环1000次后容量仍保持为129.7 mA·h·g^–1，具有良好的长循环稳定性. 在0.1、0.2、0.5、1、2和3 A·g^-1电流密度下，纤维状V₂O₅干凝胶表现出良好的倍率性能，放电比容量分别为388.4、338.5、282.9、239.1、194.4和165.9 mA·h·g^–1，远高于商业化V₂O₅ (279.5、251.0、205.5、174.5、144.6和125.1 mA·h·g^–1). 良好的电化学性能主要归功于结合水的支撑作用增大了层间距，在循环过程中材料具有良好的结构稳定性，避免了放电容量衰减；同时纤维片状结构缩短了锌离子的迁移路径. 对充放电机理研究发现，在锌离子的嵌入脱出过程中伴随有碱式硫酸锌的生成与消失，且该过程可逆.

Fundamental understanding and effect of anionic chemistry in zinc 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.