钾掺杂对钒酸钠纳米片储钠性能的影响

Effect of K-Doping on the Sodium-storage Performance of Sodium Vanadate Nanoplates

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Na5K0.118V12O32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Na5K0.118V12O32纳米片有望发展为一种新型钠离子电池正极材料.

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(Na5KxV12O32)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 Na5K0.118V12O32 nanoplates can be developed into a novel cathode material for sodium-ion batteries in the future.