【作者】 沈伟；

【导师】 刘海梅；

【作者基本信息】 北京化工大学 ， 化学， 2016， 博士

【摘要】 Na3V2(PO4)3具有三维开放的框架结构,充放电电压高,储能容量大,快速充放电能力和循环稳定性好等优点,是一类重要的钠离子电池正极材料。本论文将其作为主要研究对象,针对该材料电子导电率低的缺点,采用了不同的改性方法对其电化学性能进行改性研究。论文具体研究内容如下：(1)通过溶胶凝胶法结合瞬间冷冻干燥的处理手段制备出碳包覆均匀的,具有多孔结构的Na3V2(PO4)3通过液氮将Na3V2(PO4)3前驱体溶胶瞬间冷冻,然后转移至冷冻干燥机中低温进行干燥,在此过程中,材料的微观多孔结构得以良好保持。这种多孔结构的形貌能够显著的增加材料的比表面积,使电解液充分的与Na3V2(PO4)3接触,增加Na3V2(PO4)3电极中活性位点的数量,使其表现出了更为优异的电化学性能。当电压窗口为2.7-4.0 V,放电倍率从0.05 C依次逐渐增大到0.1,0.2,1,3,4和5 C时,放电比容量分别为118,108,105,99,95,92和90mAh g-1,当放电倍率再次回到O.1C时,放电比容量也能达到105mAhg-1,展现出了比较好的倍率性能和可逆性。(2)采用了p型硼掺杂碳包覆技术修饰Na3V2(PO4)3电极材料,并对其改性机理进行了详细的探究。通过简单的溶胶-凝胶法,首次实现了新型硼掺杂碳包覆技术对钠离子电池电极材料Na3V2(PO4)3的修饰,并显示出了极为有效的改性效果。在不同硼掺杂量的样品中,均出现四种掺杂类型：BC4,BC3,BC2O和BCO2。不同硼掺杂类型对于碳包覆的Na3V2(PO4)3的改性机理是各不相同的。与BC4和BC3相比,BC20和BC02中由于O原子的引入,能够明显的打破碳层的碳碳骨架结构,使碳层产生大量的缺陷,有利于提高Na+的扩散速度,进而改善Na3V2(PO4)3的电化学性能。因此,BC20和BC02含量最多的NVP-C-BO.38%样品表现出了最为优异的电化学性能,在小倍率0.2 C和0.5 C时放电比容量分别为95.8和95.2 mAhg-1,当倍率提高到1,2,3和5C时,放电比容量分别为93.1,93.0,93.0和90.3 mAhg-1,当倍率再次减小为3,0.5和0.2 C时,放电比容量依旧能够达到93.4,93.8和93.9 mAhg-1。(3)通过原位制备过程首次合成出了n型氮掺杂碳包覆修饰的Na3V2(PO4)3电极材料,并深入探究了其改性机理。分别使用柠檬酸和聚乙烯吡咯烷酮作为碳源和氮源,通过简便的溶胶凝-胶法,首次实现了氮掺杂碳包覆对Na3V2(PO4)3正极材料的修饰。相比于纯碳包覆的Na3V2(PO4)3电极材料,适量氮掺杂碳包覆表现出更为优异的电化学性能。这主要是因为,氮掺入碳包覆层后会产生三种掺杂构型：吡啶氮、吡咯氮和石墨氮。吡啶氮和吡咯氮不仅能够提高碳包覆层的电子电导率,而且还能够打破碳包覆层中原有的碳碳骨架结构,使之产生大量的缺陷,而石墨氮仅仅是取代部分的碳原子,不会对碳碳骨架结构产生破坏,因此不能产生缺陷。通过对其电化学性能测试发现,当石墨氮比例最少时,也就是吡啶氮和吡咯氮最多时,氮掺杂碳包覆的Na3V2(PO4)3 (NVP-C-N142)电化学性能最优,在0.2C,0.5 C,1C和2C时放电比容量都在100 mAhg-1左右,即使倍率提高到3 C和5 C时,放电比容量依旧高达93.8和84.3mAh g-1,当再次循环到3 C,1 C和0.2 C时,对应的放电比容也依旧高达90.1,96.4和98.2 mAh g-1。(4)双纳米碳(碳包覆和CNTs)协同修饰Na3V2(PO4)3正极材料及其电化学性能研究。通过简单的溶胶-凝胶法制备出了双纳米碳(碳包覆和CNTs)协同修饰Na3V2(PO4)3正极材料。通过对Raman光谱和XPS谱系统的分析发现,氮掺杂不仅能够提高碳包覆层的电子电导率,还能改善Na+通过碳包覆层的扩散速度；而碳纳米管能够显著改善电子在多个Na3V2(PO4)3颗粒间的传输速度。因此,双纳米碳协同修饰Na3V2(PO4)3正极材料表选出了极为优异的电化学性能,特别是倍率性能和循环稳定性。例如,当倍率从0.2 C增大到70 C时,放电比容量仅从94.5 mAh g-1下降到70 mAh g-1,容量保持率为74.5%;而且当倍率为30 C时,循环300周后的容量保持率高达87%。(5)体相Mn2+掺杂碳包覆修饰钠离子电池正极材料Na3V2(PO4)3的研究。通过简单的溶胶-凝胶法制备出不同Mn2+掺杂量(x=0,0.015,0.025and 0.035)的Na3V2-xMnx(PO4)3/C复合材料。通过对Na3V2-xMnx(PO4)3/C晶体结构的详细分析可知,Mn2+离子的掺杂有利于增大Na+的扩散系数,同时通过增大Na3V2(PO4)3晶胞中a轴和b轴增大晶胞的体积。此外,适量的Mn2+掺杂还有利于提高Na3V2(PO4)3的本征电子电导率。因此适量的Mn2+掺杂能够明显的改善Na3V2(PO4)3的电化学性能。例如,当倍率从0.2C逐渐增大到20 C时,Na3V2-xMnx(PO4)3/C的放电比容量只从96.8 mAh g-1下降到71.8 mAh g-1,容量保持率为74%；而且,即时在大倍率15C下循环时,首次放电比容量达到86.7 mAh g-1,100周循环后也高达80.4 mAhg-1,容量保持率为93%。更多还原

【Abstract】 As one of important sodium ion batteries cathode materials, Na3V2(PO4)3 exhibits a 3D framework, high charge-discharge potential, high theoretical specific capacity, good cycling performance and high rate capability. In this thesis, Na3V2(PO4)3 is used as a main research object. In order to improve its disadvantage of low electronic conductivity, several modification methods are adopted to enhance its electrochemical performance. The detailed research contents are as follows:(1) The porous Na3V2(PO4)3 was synthesized by the sol-gel method combined with a freeze-drying process. The sol of Na3V2(PO4)3 precursor was momentarily frozen by liquid nitrogen; then the frozen sol was adopted by vacuum drying process in the vacuum freeze dryer, during which the porous structure of precursor is well-maintained. The special porous morphology could significantly increase the specific surface area of the material, which greatly enhances the contact area between the electrode and electrolyte, and supplies more active sites for sodium ions. Therefore, The porous Na3V2(PO4)3 exhibits excellent rate performance and cycling stability. It delivers an initial capacity as high as 118,108,105,99,95,92 and 90 mAh g-1 at 0.05,0.1,0.2, 1,3,4 and 5 C in the potential window of 2.7-4.0 V versus Na+/Na, respectively. When the rate returns to 0.1 C, this material can still deliver a discharge capacity of 105 mAh g-1.(2) The p-type B-doping carbon coated Na3V2(PO4)3 composite is synthesized and the modified mechanism is detailed investigated. The modification approach of B-doped carbon coating is initially applied on Na3V2(PO4)3 cathode materials for sodium ion batteries used a facile sol-gel process. It is found that there are four different B-doping species (B4C, BC3, BC2O and BCO2) in Na3V2(PO4)3 samples with different B doping contents; moreover, different B-doping species in the carbon coated layer have different influences on the improvement of the electrochemical properties of Na3V2(PO4)3. Compared to B4C and BC3, due to the introduction of the O atom in the carbon coated layer, BC2O and BCO2 can damage of the carbon skeletons and significantly increase numerous extrinsic defects and active sites, which could accelerate Na+ transport in the carbon layer. Therefore, it is unexpectedly demonstrated that Na3V2(PO4)3/C+B, which consists of the largest total amount of BC2O+BCO2, exhibits the best electrochemical properties. It delivers an initial capacity of 95.8 and 95.2 mAh g-1 at low rates of 0.2 and 0.5 C, respectively, which further demonstrates an excellent rate capability with the value of 93.1,93.0,93.0 and 90.3 mAh g-1 at 1,2,3 and 5 C. When recycled at 3,0.5 and 0.2 C, its can still deliver a discharge capacity of 93.4,93.8 and 93.9 mAhg-1, respectively.(3) The n-type N-doping carbon coated Na3V2(PO4)3 composite is synthesized via an in-situ preparation process and the modified mechanism is detailed investigated. Citric acid and PVP are used as carbon and nitrogen sources, respectively; the N-doping carbon coated Na3V2(PO4)3 is prepared by sol-gel method. Compared to the only carbon coated Na3V2(PO4)3, moderate nitrogen into the carbon coating layer could significantly improve the electrochemical properties of Na3V2(PO4)3. The reason is that there are three carbon-nitrogen species:pyridinic N, pyrrolic N, and quaternary N into the carbon coating layer. Pyridinic N and pyrrolic N could significantly increase the electronic conductivity and create numerous extrinsic defects and active sites. While quaternary N only increases the electronic conductivity without creating extrinsic defects. In consequence, it is unexpectedly demonstrated that the Na3V2(PO4)3/C+N (NVP-C-N142), in which with minimize content of quaternary N or exist most extrinsic defects, exhibits the best electrochemical properties. The NVP-C-N142 electrode could deliver high discharge capacities of about 100 mAh g-1 at 0.2,0.5,1 and 2C, which further demonstrates an excellent rate performance with a value of 93.8 at 3C, 84.3 mAh g-1 at 5 C. When recycled at 3 C,1 C and 0.2 C, its discharge capacity can still reach 90.1,96.4 and 98.2 mAh g-1, respectively.(4) Nitrogen-doped carbon-coated Na3V2(PO4)3 hybriding with multi-walled carbon nanotubes (CNTs) composite, namely double nano-carbon synergistically modified Na3V2(PO4)3 of sodium ion battery, was synthesized by a facile sol-gel method. Based on the systemical analysis of Raman spectra, X-ray photoemission spectroscopy results about this composite structure, it is found that suitable N-doping not only increases the electric conductivity of carbon layer, but also increases its Na-ion migration velocity across the carbon layer. Moreover, due to the intimate contacts between active materials and CNTs, the CNTs 3D conducting network could significantly accelerate the electron transport between multiple-particles of Na3V2(PO4)3. Therefore, the electrochemical properties of this double nano-carbon modified Na3V2(PO4)3 is significantly improved, especially the rate performance and long lifetime. For instance, when the discharging rate increased from 0.2 C to 70 C, its capacity of 94.5 mAh g-1 decreases to 70 mAh g-1 and an unexpective capacity retention of 74% is obtained. Moreover, even at a higher current density of 30 C, an excellent capacity retention of 87% is obtained after 300 cycles.(5) Mn2+-doping carbon-coating was used to modified electrochemical performance of Na3V2(PO4)3 cathode material for sodium ion batteries. Na3V2-xMnx(PO4)3/C composites with different Mn2+ doping contents (x= 0, 0.015,0.025 and 0.035) were prepared by a simple sol-gel method. Based on the precise analysis of crystal structure of Na3V2-xMnx(PO4)3/C, it is found that Mn2+ plays an important role in enlarging the Na-ion migration velocity and in increasing the lattice volume by elongating the a- and b-axis; moreover, suitable Mn2+ doping also could increase the electric conductivity of Na3V2-xMnx(PO4)3/C, thereby improving the electrochemical performance. For example, when the C-rates increased from 0.5 C to 20 C, the discharge specific capacity only decreased from 96.8 mAh g-1 to 71.8 mAh g-1 and an unexpective capacity retention of 74% is obtained. Furthermore, even cycling at a high rate of 15 C, an excellent capacity retention of 93% is maintained from the initial value of 86.7 mAh g-1 to 80.4 mAh g"1 after 100 cycles.更多还原