Na3V2O2(PO4)2F(NVOPF) is a cathode material with potential application prospects in sodium-ion batteries. This is due to its suitably stable polyanion structure, high operating voltage, and high theoretical specific capacity. However, the intrinsic conductivity of the material is low, and it is prone to irregular agglomeration during the synthesis process, resulting in low actual specific capacity and unsatisfactory rate and cycling performances. Ion doping and micro/nanostructured materials have been found to benefit the intrinsic conductivity and stability of the material. This work, for the first time, reports the synthesis of Nb5+-doped NVOPF(NVNOPF, Na3V2-x Nb x O2(PO4)2F (0≤x≤0.15)) material with a hollow microspheric structure by the polyol-assisted hydrothermal method. The as-prepared NVOPF and NVNOPF materials were microspheres with sizes of 0.7-1.0 μm with hollow structures. The microspheres were found to be composed of nanoparticles of with sizes <100 nm. Nanoparticles shortened the diffusion distance between sodium ions, buffered the volume change caused by the intercalation/extraction of sodium ions, and improved the material cycling stability. Meanwhile, doping Nb5+ increased the lattice parameters of NVNOPF and enlarged the Na+ diffusion pathway. The solid-phase diffusion coefficient of Na+ in the material increased from 6.46×10-16 for Na3V2O2(PO4)2F to 3.52×10-15 cm2/s for Na3V1.90Nb0.10O2(PO4)2F. The discharge specific capacity of Na3V1.90Nb0.10O2(PO4)2F was 126.4 mAh/g (0.1 C rate) and 98.1 mAh/g (10 C rate). After 500 cycles of charge and discharge at the 10 C rate, the capacity retention was 95.2%, which is better than that of the undoped material (66.8%). The results showed that Nb-doped and hollow spherical micro-/nanostructures could effectively improve the electrochemical performance and cyclic stability of NVOPF.
Keywords：cathode material for sodium ion battery
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Na3V2O2(PO4)2F
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polyol assisted hydrothermal method
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hollow microspheres
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niobium doping
ZHANG Zinan. Preparation and property evaluation of Nb-doped Na3V2O2 （PO4 ） 2F hollow microspheres as cathode materials for sodium-ion batteries[J]. Energy Storage Science and Technology, 2023, 12(8): 2370-2381
本工作通过多元醇辅助水热法结合高温煅烧合成了由方形纳米颗粒组装而成的Na3V2-x Nb x O2(PO4)2F(0≤x≤0.15)(NVNOPF)空心微球。XRD精修结果证实了Nb在八面体V位的掺杂取代。Nb掺杂可以增加NVOPF的晶格参数，有利于提高钠离子的扩散速率。本工作还分别研究了NVOPF空心微球的形成对反应时间和多元醇的关联机制。与未掺杂的NVOPF相比，Nb掺杂的NVNOPF材料的比容量、倍率性能以及循环稳定性均明显优于未掺杂的对照样。
1 实验材料和方法
1.1 材料合成
采用水热法结合高温煅烧合成了掺杂不同含量Nb的Na3V2-x Nb x O2(PO4)2F(x = 0、0.05、0.10、0.15)材料，合成过程如图1所示。将(2-x) mmol NH4VO3、0.5x mmol Nb2O5和1 mL H2O2加入到10 mL去离子水中，搅拌30 min，得到混合溶液。然后，将2 mmol NaH2PO4和1 mmol NaF依次加入上述溶液中，在室温继续搅拌30 min至NaH2PO4和NaF完全溶解。再加入30 mL聚乙二醇(PEG200)，持续搅拌1 h。将上述溶液转移到50 mL内衬聚四氟乙烯的水热反应釜中，置于180 ℃烘箱中加热24 h。将获得的样品用去离子水和酒精洗涤、离心分离，共6次，然后在100 ℃下真空干燥12 h。最后，将干燥的样品在氩气气氛中，以5 ℃/min速率升温至400 ℃，煅烧3 h。所得样品为Na3V2O2(PO4)2F(NVOPF)、Na3V1.95Nb0.05O2(PO4)2F(NVNOPF-0.05)、Na3V1.90Nb0.10O2(PO4)2F(NVNOPF-0.10)和Na3V1.85Nb0.15O2(PO4)2F(NVNOPF-0.15)。
将所合成的Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)材料、乙炔黑和聚偏二氟乙烯以质量比70∶20∶10在N-甲基吡咯烷酮(NMP)溶液中充分研磨混合。将制好的浆料用刮刀均匀地涂覆在碳箔表面，于75 ℃鼓风烘箱中干燥3 h后，转入100 ℃真空干燥箱中过夜干燥。将干燥后的极片裁成直径10 mm的圆片备用，其中活性材料的负载量约为1.0～2.0 mg/cm2。
采用Versa STAT 4仪器，以0.10 mV/s的扫描速率在2.00～4.30 V(vs. Na+/Na)的范围内，进行循环伏安(CV)测试。
1.3.4 交流阻抗测试
采用Versa STAT 4仪器测试电池的电化学交流阻抗(EIS)。频率范围为0.01 Hz～100.00 kHz，振幅为5.00 mV。使用Zview软件对EIS谱图进行拟合分析。
2 结果与讨论
2.1 材料结构分析
图2(a)和(b)是Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)的XRD谱图。从图2(a)可以看出，Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)的XRD衍射谱图基本一致，所有强衍射峰都与具有NASICON结构的I4/mmm空间群[35](PDF#97-041-1950)相对应。表明本工作所选择的Nb5+掺杂量未明显改变NVOPF的晶体结构。从图2(b)中可以看到，随着材料中Nb5+含量的增加，(200)、(103)和(202)峰向低角度移动。这意味着Nb掺杂NVNOPF具有较未掺杂NVOPF更大的晶格常数[30]。
图2
(a) Na3V2-x Nb x O2(PO4)2F(x = 0、0.05、0.10、0.15)的XRD图；(b) 局部放大图
Fig. 2
(a) XRD pattern of Na3V2-x Nb x O2(PO4)2F(x = 0, 0.05, 0.10, 0.15); (b) partial enlarged image
图3(a)～(d)是利用Fullprof软件对Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)XRD谱图的精修图。可靠性因子Rp和Rwp的值均低于10%，表明精修是可以接受的。XRD精修计算得到相应的晶格参数列于表1。表1数据显示，随着NVNOPF材料中Nb掺杂量的增加，a值和c值以及晶胞体积逐渐增大。这主要是由于Nb5+的半径0.64 Å大于V4+(0.59 Å)，Nb掺杂后NVNOPF的晶胞参数增加[36]。表1中原子占位信息表明Nb占据了部分V位点。以上结果表明Nb5+成功掺杂到NVOPF的晶格中。图4(a)是NVOPF的晶体结构示意图。NVOPF由VO5F八面体和PO4四面体构成，它们沿c轴方向共享一个F原子[33]。通过这种框架连接，间隙通道沿a轴和b轴延伸，间隙空间中的Na+无序排列间隔很近。这种相对较大的隧道结构有利于Na+迁移[37]。图4(b)是Nb取代V位置的示意图。Nb掺杂的NVOPF晶体结构中形成了NbO5F八面体[38]。掺杂后晶胞参数明显增加，有利于Na+的扩散传输和NVOPF电化学性能的改善[25]。
本工作通过多元醇辅助水热法成功制备了性能优异的Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)空心微球正极材料。研究发现，Nb掺杂Na3V2O2(PO4)2F材料的电化学性能显著提升。Nb的摩尔比为0.10的NVNOPF-0.10材料粒径为0.7～1.0 μm，壳厚度约为180 nm，空心微球均匀分散、具有优异的倍率性能和循环稳定性。NVNOPF-0.10材料以10 C倍率放电的初始比容量为98.1 mAh/g，循环500周后，容量保持率为95.2%。研究结果还显示，Nb掺杂后材料的电荷转移电阻明显降低，钠离子的固相扩散系数增大。合成的NVNOPF-0.05、NVNOPF-0.10和NVNOPF-0.15材料的倍率性能和循环稳定性均优于未掺杂NVOPF材料。
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... 图3(a)～(d)是利用Fullprof软件对Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)XRD谱图的精修图.可靠性因子Rp和Rwp的值均低于10%，表明精修是可以接受的.XRD精修计算得到相应的晶格参数列于表1.表1数据显示，随着NVNOPF材料中Nb掺杂量的增加，a值和c值以及晶胞体积逐渐增大.这主要是由于Nb5+的半径0.64 Å大于V4+(0.59 Å)，Nb掺杂后NVNOPF的晶胞参数增加[36].表1中原子占位信息表明Nb占据了部分V位点.以上结果表明Nb5+成功掺杂到NVOPF的晶格中.图4(a)是NVOPF的晶体结构示意图.NVOPF由VO5F八面体和PO4四面体构成，它们沿c轴方向共享一个F原子[33].通过这种框架连接，间隙通道沿a轴和b轴延伸，间隙空间中的Na+无序排列间隔很近.这种相对较大的隧道结构有利于Na+迁移[37].图4(b)是Nb取代V位置的示意图.Nb掺杂的NVOPF晶体结构中形成了NbO5F八面体[38].掺杂后晶胞参数明显增加，有利于Na+的扩散传输和NVOPF电化学性能的改善[25]. ...
... 图2(a)和(b)是Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)的XRD谱图.从图2(a)可以看出，Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)的XRD衍射谱图基本一致，所有强衍射峰都与具有NASICON结构的I4/mmm空间群[35](PDF#97-041-1950)相对应.表明本工作所选择的Nb5+掺杂量未明显改变NVOPF的晶体结构.从图2(b)中可以看到，随着材料中Nb5+含量的增加，(200)、(103)和(202)峰向低角度移动.这意味着Nb掺杂NVNOPF具有较未掺杂NVOPF更大的晶格常数[30]. ...
... 图3(a)～(d)是利用Fullprof软件对Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)XRD谱图的精修图.可靠性因子Rp和Rwp的值均低于10%，表明精修是可以接受的.XRD精修计算得到相应的晶格参数列于表1.表1数据显示，随着NVNOPF材料中Nb掺杂量的增加，a值和c值以及晶胞体积逐渐增大.这主要是由于Nb5+的半径0.64 Å大于V4+(0.59 Å)，Nb掺杂后NVNOPF的晶胞参数增加[36].表1中原子占位信息表明Nb占据了部分V位点.以上结果表明Nb5+成功掺杂到NVOPF的晶格中.图4(a)是NVOPF的晶体结构示意图.NVOPF由VO5F八面体和PO4四面体构成，它们沿c轴方向共享一个F原子[33].通过这种框架连接，间隙通道沿a轴和b轴延伸，间隙空间中的Na+无序排列间隔很近.这种相对较大的隧道结构有利于Na+迁移[37].图4(b)是Nb取代V位置的示意图.Nb掺杂的NVOPF晶体结构中形成了NbO5F八面体[38].掺杂后晶胞参数明显增加，有利于Na+的扩散传输和NVOPF电化学性能的改善[25]. ...
... 图2(a)和(b)是Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)的XRD谱图.从图2(a)可以看出，Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)的XRD衍射谱图基本一致，所有强衍射峰都与具有NASICON结构的I4/mmm空间群[35](PDF#97-041-1950)相对应.表明本工作所选择的Nb5+掺杂量未明显改变NVOPF的晶体结构.从图2(b)中可以看到，随着材料中Nb5+含量的增加，(200)、(103)和(202)峰向低角度移动.这意味着Nb掺杂NVNOPF具有较未掺杂NVOPF更大的晶格常数[30]. ...
1
... 图3(a)～(d)是利用Fullprof软件对Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)XRD谱图的精修图.可靠性因子Rp和Rwp的值均低于10%，表明精修是可以接受的.XRD精修计算得到相应的晶格参数列于表1.表1数据显示，随着NVNOPF材料中Nb掺杂量的增加，a值和c值以及晶胞体积逐渐增大.这主要是由于Nb5+的半径0.64 Å大于V4+(0.59 Å)，Nb掺杂后NVNOPF的晶胞参数增加[36].表1中原子占位信息表明Nb占据了部分V位点.以上结果表明Nb5+成功掺杂到NVOPF的晶格中.图4(a)是NVOPF的晶体结构示意图.NVOPF由VO5F八面体和PO4四面体构成，它们沿c轴方向共享一个F原子[33].通过这种框架连接，间隙通道沿a轴和b轴延伸，间隙空间中的Na+无序排列间隔很近.这种相对较大的隧道结构有利于Na+迁移[37].图4(b)是Nb取代V位置的示意图.Nb掺杂的NVOPF晶体结构中形成了NbO5F八面体[38].掺杂后晶胞参数明显增加，有利于Na+的扩散传输和NVOPF电化学性能的改善[25]. ...
1
... 图3(a)～(d)是利用Fullprof软件对Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)XRD谱图的精修图.可靠性因子Rp和Rwp的值均低于10%，表明精修是可以接受的.XRD精修计算得到相应的晶格参数列于表1.表1数据显示，随着NVNOPF材料中Nb掺杂量的增加，a值和c值以及晶胞体积逐渐增大.这主要是由于Nb5+的半径0.64 Å大于V4+(0.59 Å)，Nb掺杂后NVNOPF的晶胞参数增加[36].表1中原子占位信息表明Nb占据了部分V位点.以上结果表明Nb5+成功掺杂到NVOPF的晶格中.图4(a)是NVOPF的晶体结构示意图.NVOPF由VO5F八面体和PO4四面体构成，它们沿c轴方向共享一个F原子[33].通过这种框架连接，间隙通道沿a轴和b轴延伸，间隙空间中的Na+无序排列间隔很近.这种相对较大的隧道结构有利于Na+迁移[37].图4(b)是Nb取代V位置的示意图.Nb掺杂的NVOPF晶体结构中形成了NbO5F八面体[38].掺杂后晶胞参数明显增加，有利于Na+的扩散传输和NVOPF电化学性能的改善[25]. ...
1
... 图3(a)～(d)是利用Fullprof软件对Na3V2-x Nb x O2(PO4)2F(x=0、0.05、0.10、0.15)XRD谱图的精修图.可靠性因子Rp和Rwp的值均低于10%，表明精修是可以接受的.XRD精修计算得到相应的晶格参数列于表1.表1数据显示，随着NVNOPF材料中Nb掺杂量的增加，a值和c值以及晶胞体积逐渐增大.这主要是由于Nb5+的半径0.64 Å大于V4+(0.59 Å)，Nb掺杂后NVNOPF的晶胞参数增加[36].表1中原子占位信息表明Nb占据了部分V位点.以上结果表明Nb5+成功掺杂到NVOPF的晶格中.图4(a)是NVOPF的晶体结构示意图.NVOPF由VO5F八面体和PO4四面体构成，它们沿c轴方向共享一个F原子[33].通过这种框架连接，间隙通道沿a轴和b轴延伸，间隙空间中的Na+无序排列间隔很近.这种相对较大的隧道结构有利于Na+迁移[37].图4(b)是Nb取代V位置的示意图.Nb掺杂的NVOPF晶体结构中形成了NbO5F八面体[38].掺杂后晶胞参数明显增加，有利于Na+的扩散传输和NVOPF电化学性能的改善[25]. ...
