摘要/Abstract

通过水热-热处理方法首次制备了K⁺离子掺杂的钒酸钠（Na₅V₁₂O₃₂）正极材料，对样品进行了TEM、XRD和XPS表征，详细研究了钾掺杂量对样品的结构和储钠性能的影响规律.TEM照片显示，合成的材料具有纳米片形貌.XRD/XPS谱图分析表明，K⁺离子掺杂在钒酸钠晶体的层间.恒流充放电测结果显示，当1 mol Na₅V₁₂O₃₂掺杂0.118 mol K⁺离子时，得到的Na₅K_0.118V₁₂O₃₂样品具有最佳的电化学性能：在1.5~4.0 V范围内，经过几次活化后，其于0.1C、0.2C、0.5C、1C、3C和10C倍率下的最大放电容量分别为169、160、148、132、98和69 mAh·g^-1；3C循环1000次后容量保持率为93.0%.研究结果表明，层间掺杂的K⁺离子不仅扩大了Na₅V₁₂O₃₂晶体的层间距，而且稳定了晶体的结构，从而显著改善了Na₅V₁₂O₃₂材料的倍率性能和循环性能.研究结果证明，适量K⁺离子掺杂的Na₅K_0.118V₁₂O₃₂纳米片有望发展为一种新型钠离子电池正极材料.
关键词: 钠离子电池, 钒酸钠, 阳离子掺杂, 正极材料, 电化学性能
Na-ion batteries with lower cost than Li-ion batteries would be developed to large-scale energy-storage device to store solar and wind energies. However, large radius renders Na⁺ ions to insert into/extract out the layered transition metal oxides (LTMOs) sluggishly. To improve the intercalation dynamics of Na⁺ ions, the interlayer spacing of crystals has to be expanded for those LTMOs that are capable of fast lithiation and delithiation. Herein, a LTMO based on vanadium is firstly doped with larger K⁺ ions to expand the interlayer spacing to yield K⁺-doped sodium vanadate (Na₅K_xV₁₂O₃₂) cathode material by a hydrothermal method at 200℃ for 24 h followed by calcination at 500℃ for 3 h. The samples were characterized by scanning electron microscope (SEM)/transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) technologies. Effect of the doping amount of K⁺ on the structure and sodium-storage performance of the sample was studied in detail. The synthesized materials display nanoplate morphology viewing from the TEM images. K⁺ ions are doped into the interlayer of the sodium vanadate crystallites, which is proved by analysis of the XRD patterns and XPS spectra. Expanded interlayer spacing favors Na⁺ ions' intercalation and deintercalation between the[V₃O₈]^- layers, which is testified by the chemical diffusion coefficient of cathodes, thus enhancing the rate capability. On the other hand, the chemically pre-intercalated K⁺ ions are pinned in the crystallites during insertion and extraction of Na⁺ ions and act as pillars to stabilize the layered structure, improving the cycliability of the cathode. However, excessive doping of K⁺ leads to a discounted rate capability of the cathode, suggesting an optimized amount of K⁺ doping into the crystal. The results from galvanostatic charge-discharge tests indicate that the obtained NVO(3K) sample, in which 0.118 mol of K⁺ ions are doped into per mol of Na₅V₁₂O₃₂, presents the best electrochemical performance among the various samples. It can deliver the maximum capacities of 169, 160, 148, 132, 98 and 69 mAh·g^-1 at the rates of 0.1C, 0.2C, 0.5C, 1C, 3C and 10C after activated for several times over the voltage window of 4.0~1.5 V (vs. Na⁺/Na), respectively. Even run at 3C rate, it can retain 93.0% of the maximum capacity after 1000 cycles, exhibiting excellent rate capability and stable cycliability. The results suggest that doping of K⁺ ions into the interlayer of crystallites can significantly improving the rate capability as well as cycling performance of the obtained Na₅V₁₂O₃₂. Our investigation demonstrates that design of K⁺-doped sodium vanadate cathode materials is beneficial for harvesting superior performance, of which the Na₅K_0.118V₁₂O₃₂ nanoplates can be developed into a novel cathode material for sodium-ion batteries in the future.
Key words: sodium-ion battery, Na₅V₁₂O₃₂, cation doping, cathode material, electrochemical performance


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