【作者】 李慧；

【导师】 吴川；

【作者基本信息】 北京理工大学 ， 环境工程， 2016， 博士

【摘要】 金属钠资源丰富、价格低廉,因此钠离子电池非常适合用于大规模储能应用。NASICON型结构的Na₃V₂（PO₄）₃具有三维开放的网络结构,有利于在充放电过程中钠离子的脱嵌,且此材料具有较好的结构稳定性、热稳定性和高的充放电平台,是一种非常有潜力的钠离子电池正极材料。但是,Na₃V₂（PO₄）₃的自身电子电导率不高,这严重影响了其电化学性能。针对这一缺点,本文采用了几种不同的方法对其进行改性以提高其电化学性能。具体研究内容如下:（1）通过静电纺丝法成功制备发芽柳枝状Na₃V₂（PO₄）₃/C纳米纤维。所制备的Na₃V₂（PO₄）₃纳米纤维表面均匀粘附着一些柳牙状Na₃V₂（PO₄）₃/C纳米颗粒,观察高倍率透镜照片可以发现Na₃V₂（PO₄）₃纳米纤维外层由一层厚度均一的碳均匀包覆,碳包覆层内部为Na₃V₂（PO₄）₃活性材料。这种特殊的形貌结构提高了Na₃V₂（PO₄）₃的电化学性能:当在0.2 C的倍率下充放电时,首次放电容量为106.8 mAh g^-1,循环125周后,容量仍高达107.2 mAh g^-1。此外,发芽柳枝状Na₃V₂（PO₄）₃/C纳米纤维也展现了很好的倍率性能,当倍率从0.2 C增大到2 C,即增大十倍时,其容量保持率高达95.7%。好的循环性能和倍率性能主要是因为纳米纤维的形貌增大了电解液和电极的接触面积,同时外包碳层提高了材料的电子电导率。深入对Na₃V₂（PO₄）₃/C纳米纤维的暴露晶面研究发现,暴露的（113）晶面有利于充放电过程中的Na⁺的脱嵌。（2）探索Na₃V₂（PO₄）₃纳米形貌制备,利用水热法制备了三维分级结构Na₃V₂（PO₄）₃/C。此三维结构由Na₃V₂（PO₄）₃/C纳米片堆积而成,通过HRTEM发现Na₃V₂（PO₄）₃纳米片表面由一层厚度均匀的碳层包覆。作为钠离子电池正极材料时,三维分级结构Na₃V₂（PO₄）₃/C表现出了非常好的电化学性能:当在0.1 C的倍率下充放电时,其容量高达114.8 mAh g^-1,接近其理论比容量117 mAh g^-1。当倍率增大到0.2、0.5、1、2和5 C时,其放电容量分别为111.4、109.6、106.1、102.5和94.9 mAh g^-1。当倍率回到0.1 C时,其容量仍高达107.9 mAh g^-1,表现出很好的倍率性能。此外,三维分级结构Na₃V₂（PO₄）₃/C也具有很好的循环性能,1 C下首次放电容量为107.6mAh g^-1,600周后的容量保持率高达74.4%。如此好的循环性能和Na₃V₂（PO₄）₃/C的形貌具有很大的关系。（3）探索利用简单方法控制Na₃V₂（PO₄）₃形貌,并探索不同形貌对材料电化学性能的影响。通过控制草酸的量,利用简单的两步还原法制备了不同形貌的Na₃V₂（PO₄）₃/C--微米级无定形状、多孔泡沫状和片状Na₃V₂（PO₄）₃/C。草酸的量对Na₃V₂（PO₄）₃/C的形貌具有很大的影响,文章深入分析了草酸对Na₃V₂（PO₄）₃/C形貌的影响和不同形貌的形成过程。分别将三种形貌的Na₃V₂（PO₄）₃/C进行电化学性能测试,结果发现多孔泡沫状Na₃V₂（PO₄）₃/C具有最好的电化学性能。在30 C的高倍率下的首次放电容量高达101.77 mAh g^-1,循环700周后,容量仍高达89.28 mAh g^-1,仅有12%的容量损失。当电流密度增加到50 C,600周循环后仍有81%的初始容量,而放电倍率继续增大到70 C时,循环500周后,容量保持率为92.5%,多孔泡沫状Na₃V₂（PO₄）₃/C表现出非常好的倍率性能和长循环性能。（4）Ni²⁺体相掺杂Na₃V₂（PO₄）₃的研究。利用简单的溶胶-凝胶法设计和制备了一系列Ni²⁺掺杂富钠Na_3+xV_2-xNi_x（PO₄）₃/C（x=0、0.01、0.03、0.05和0.07）样品。由于Na、Ni和V的化学价不一样,为了保持电荷平衡,当Ni²⁺进入Na₃V₂（PO₄）₃不同的位置时,Na/V会表现出不一样的摩尔比变化趋势,因此,本章首先利用ICP探索了Na_3+xV_2-x-x Ni_x（PO₄）₃/C（x=0、0.01、0.03、0.05和0.07）的化学组成,通过计算Na/V摩尔比发现Ni²⁺进入到V位。此外,根据XRD和XRD精修结果可以发现,少量的Ni²⁺进入到Na₃V₂（PO₄）₃不会破坏其晶体结构。进一步对Ni²⁺掺杂Na_3+xV_2-xNi_x（PO₄）₃/C（x=0、0.01、0.03、0.05和0.07）的电化学性能测试发现,所有掺杂的材料的电化学性能都明显优于未掺杂的材料,尤其是Na_3.03V_1.97Ni_0.03（PO₄）₃/C。在1 C的倍率下充放电时,首次放电容量为107.1 mAh g^-1,100周后的容量保持率高达95.5%,表现出非常好的循环稳定性。Ni²⁺掺杂导致的材料形貌变化、晶体结构变化和电子电导率变化是导致掺杂材料性能提高的主要原因。（5）Mg²⁺体相掺杂Na₃V₂（PO₄）₃的研究。在Ni²⁺掺杂Na₃V₂（PO₄）₃研究的基础上,利用溶胶-凝胶方法合成了一系列富钠Na_3+xV_2-xMg_x（PO₄）₃/C（x=0、0.01、0.03、0.05、0.07和0.1）样品。利用第一性原理计算探索了不同掺杂位置所需活化能,计算结果发现Mg²⁺更倾向于掺杂到V位;同时,利用ICP探索了Na_3+xV_2-xMg_x（PO₄）₃/C（x=0、0.01、0.03、0.05、0.07和0.1）的具体分子式,同时通过对比Na/V摩尔比变化趋势,进一步发现Mg²⁺掺杂到V位,与第一性原理计算结果一致。实验又进一步对Mg²⁺分布进行了研究:通过对比精修所得晶胞参数和第一性原理的晶胞参数发现,绝大部分Mg²⁺掺杂在Na₃V₂（PO₄）₃表面,进一步通过EIS计算对比掺杂和不掺杂材料的离子扩散系数发现,所有材料的离子扩散系数相似,说明Na₃V₂（PO₄）₃颗粒内部只有很少量的Mg²⁺或没有Mg²⁺。将Na_3+xV_2-xMg_x（PO₄）₃/C（x=0、0.01、0.03、0.05、0.07和0.1）做钠离子电池正极材料充放电时,发现所有掺杂材料的电化学性能明显优于未掺杂材料,尤其是Na_3.05V_1.95Mg_0.05（PO₄）₃/C。在10 C的大倍率下循环时,首次放电容量为96.7 mAh g^-1,循环180周后,容量仍高达86 mAh g^-1,而未掺杂Na₃V₂（PO₄）₃首次放电容量为88.8 mAh g^-1,180周后容量仅剩63 mAh g^-1。更多还原

【Abstract】 Due to the abundance and low cost of sodium,it is very important to probe sodium ion batteries to meet the development of large scale energy storage.Na₃V₂（PO₄）_3,which owns open NASICON framework,high specific energy density（⁴00 Wh/kg）and high thermal stability,is considered to be a prospective cathode of sodium ion batteries.However,similar to other lithium metal phosphates,the electronic conductivity of Na₃V₂（PO₄）₃ is very poor,which greatly affects its electrochemical properties.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）Budding willow branches shaped Na₃V₂（PO₄）₃/C nanofibers were successfully synthesized by a simple electrospinning technique with Poly（vinyl pyrrilidone）（PVP）.The Na₃V₂（PO₄）₃/C nanoparticles that anchored on the nanofibers surface seemed like the willow buds;the inner core of the nanofibers,which composed Na₃V₂（PO₄）₃,looked like willow twig and the uniform carbon layer was same with willow bark.Such special morphology played a vital role in improving cycle stability and rate capability of the electrode due to the conductive network built up by nanofibers.The Na₃V₂（PO₄）₃/C nanofibers cathode exhibited an initial specific capacity of 106.8 mAh g^-11 at a current density of 0.2 C,still stabling at 107.2 mAh g^-11 after 125 cycles with excellent cycle stability.Moreover,a capacity retention of 95.7%was obtained when Na₃V₂（PO₄）₃/C nanofibers cycled stepwise from 0.2 to 2 C.Good electrochemical performance should be ascribed to both the special morphology and preferential growth of the（113）plane.The simple synthesis technique and good electrochemical performance suggests that this material with the special shape of budding willow branches is a promising cathode for sodium ion batteries.（2）Na-ion batteries have been regarded as promising alternatives for Li-ion batteries due to the extensive sodium reserves in the world.Na₃V₂（PO₄）₃ has been proved to be a good candidate of the cathode materials in Na-ion batteries,but the intrinsic low electrical conductivity and sluggish kinetics handicapped its application.Here,3D hierarchical Na₃V₂（PO₄）₃ particles were synthesized by a facile hydrothermal method,constructed by carbon-coated 2D Na Na₃V₂（PO₄）₃ nanowalls.Superior cell performance of high rate capability and cycle stability were observed in the well-defined structure.As the cathode in Na-ion batteries,it delivered a high capacity almost reaching the theoretical one and exhibited high capacity retention.The enhanced rate capability and cycle performance can be attributed to the improved electrical conductivity from the interconnected carbon layer and the shortened ion diffusion length and high specific surface area from the nanowalls.（3）Structural and morphological control is an effective approach for improvement of electrochemical performance in rechargeable batteries.In this paper,three different morphological Na₃V₂（PO₄）₃（irregular shaped,the porous sponge-like and plate like）were successfully prepared through controlling the amount of oxalic acid by a simple two-step reduction method.It is found that the amount of oxalic acid had vital impacts on the morphology of Na₃V₂（PO₄）₃;moreover,the morphological evolution and formation mechanism were proposed based on the reactions of different amount of oxalic acid occurring in the two-step reduction process.The excellent electrochemical performances of the porous sponge-like Na₃V₂（PO₄）₃ were attributed to the unique morphology.The initial capacity of the porous sponge-like Na₃V₂（PO₄）₃ was 101.77 mAh g^-11 at 30 C;after 700cycles,it remains as high as 89.28 mAh g^-11 with only 12%capacity loss.When the current density increases to 50 C and 70 C,the capacity retentions of 81%after 600 cycles,and92.5%after 500 cycles are achieved,respectively.（4）In order to get element substituted Na₃V₂（PO₄）₃/C in appointed V site,the simple sol-gel method is used to design and prepare a series of Na-rich Na_3+xV_2-xNi_x（PO₄）₃/C（x=0,0.01,0.03,0.05 and 0.07）compounds.To get a charge balance,the ratio of Na,V and Ni would be changed differently if Ni goes into different site.Hence,ICP is applied to probe the real stoichiometry of the as-prepared Na_3+xV_2-xNi_x（PO₄）₃/C（x=0,0.01,0.03,0.05 and 0.07）.According to Na/V ratio from ICP result,it indicates that Ni²⁺goes to V site,and more Na⁺will be introduced into the crystal to keep the charge balance.In addition,the crystal structure changes are explored by XRD and Rietveld refinement,it can be indicated from the results that Ni²⁺doping does not destroy the lattice structure of Na₃V₂（PO₄）₃.When applied as Na-storage material,the electrochemical property of all Ni²⁺doped Na3+xV2-xNix（PO4）3/C composites have been significantly improved,especially for the Na_3.03V_1.97Ni_0.03（PO₄）₃/C sample.For example,107.1 mAh g^-11 can be obtained at the first cycle,after 100 cycles,the capacity retention is as high as 95.5%.Moreover,when charging/discharging at higher rate of 5 C,the capacity still remains 88.9 mAh g^-1,displaying good rate performance.The good electrochemical performance is ascribed to the optimized morphology,stabled crystal structure and improved ionic conductivity.（5）Based on the research of Ni²⁺doped Na₃V₂（PO₄）₃,Mg²⁺is also chosen to dope into Na₃V₂（PO₄）₃.A series of Na_3+xV_2-xMg_x（PO₄）₃/C（x=0,0.01,0.03,0.05,0.07 and 0.1）samples were prepared by simple sol-gel method.First principles calculation was applied to calculate the form energy for doping at different site,the result indicates that Mg²⁺prefers to go to V site.ICP is applied to probe the real stoichiometry of the as-prepared Na_3+xV_2-x-x Mg_x（PO₄）₃/C（x=0,0.01,0.03,0.05,0.07 and 0.1）.According to Na/V ratio from ICP result,it indicates that Mg²⁺goes to V site,and more Na⁺will be introduced into the crystal to keep the charge balance.The experimental result is similar with first principle calculation result.In addition,the comparison of crystal parameters based on XRD refinement and model from first principle demonstrated that Mg is not in the bulk of Na₃V₂（PO₄）₃.According to the sodium ion diffusion coefficient results,it can indicate again that most of Mg²⁺exist in the surface of Na₃V₂（PO₄）₃.When used as cathode of sodium ion batteries,all the doped samples show much better electrochemical performance than the undoped one,especially for Na_3.05V_1.95Mg_0.05（PO₄）₃/C.The initial capacity is 96.7mAh g^-11 when cycled at 10 C,after 180 cycles,the capacity is still as high as 86 mAh g^-1.However,the undoped sample only display 88.8 mAh g^-11 at the first cycle at 10 C,only 63mAh g^-1left after 180 cycles.更多还原