【作者】 王海燕；

【导师】 陈立泉； 黄可龙；

【作者基本信息】 中南大学 ， 应用化学， 2012， 博士

【摘要】 钒氧化合物和钒酸盐嵌锂材料由于成本相对低廉,合成方法简单,比容量高等特点成为了近年来研究的热点。我国钒资源丰富,但是综合利用程度不高,因此开发具有高容量的钒系化合物作为新型锂离子电池嵌锂材料,对优化我国钒资源利用和促进经济发展都具有重要意义。目前,钒系嵌锂材料存在的主要问题是循环性能差,倍率性能不高,限制了其商业化应用。本论文立足于结构相对稳定的三钒酸盐材料,通过新型制备方法的设计与优化,分别获得了高性能的钒酸锂和钒酸钠材料,在此基础上,提出用铵根离子取代层状钒酸锂中的Li+,设计合成了一种具有高比容量和长循环寿命的新型钒酸铵嵌锂材料,重点研究了相关材料的锂离子脱嵌机制。本论文的主要研究工作和结果如下：针对LiV308倍率性能差,容量衰减快的问题,设计了新型的水热—固相烧结两步法,首先通过简单的水热工艺可控合成分散均匀,厚度超薄的(NH4)0.5V205纳米片,然后与LiOH均匀混合低温烧结得到LiV308纳米薄片。纳米片分散均匀,厚度约20-50nm。该材料的倍率性能是目前相关文献结果中最好的。在5C和10C下,放电容量仍保持在148.7和105.8mAhg-1.在300和1000mA g-1前100次循环的容量保持率分别为84.1%和85.3%。材料优异的倍率性能可能归因于其独特的纳米薄片特征。研究了层状钒酸钠的循环稳定性能。采用简易的水热反应一步合成了钒酸钠纳米线,重点考察了结晶水的含量(通过热处理控制)对材料结构、形貌和电化学性能的影响。制备的纳米线尺寸60-100nm,部分长度达5μm。首次放电容量约272mAh g-1。热处理环节虽然会降低材料的容量,但是显著改善了循环性能。300℃热处理的材料在300mA g-1的首次放电容量为189.0mAhg-1。前80次保持率为94.4%。400℃后的材料100次循环没有出现容量衰减。研究表明层间的结晶水会增大层间距离,有利于容量的提高,但是会牺牲材料的循环性能。热处理后的钒酸钠可以作为一种新型具有高循环稳定性能的水溶液锂离子电池阳极材料.采用水热—固相法合成了高倍率的Na1.08V308纳米薄片。纳米片单层厚度小于10nm。中间产物的纳米薄片结构包含了纳米棒向纳米片融合转变的过程,在此基础上,提出了钒酸钠纳米薄片的形成机理。目标材料具有超高的倍率性能和优异的循环稳定性能。30mAg-1下材料的放电容量约220mAhg-1,在600和1000mAg-1下分别为164.1和154.6mAhg1,200次循环后保持在170.5和162.5mAhg-1。在30和50C倍率下容量仍能保持在95和75mAhg-1。该材料的倍率性能要高于迄今所有报道的钒酸盐材料,甚至要比很多经过碳改性处理后的钒酸盐更好,可能归因于其特殊的纳米薄片特征及本身的结构特点。在前面工作的基础上,提出NH4V308作为新型的锂离子电池正极材料,重点考察了合成因素如水热反应时间、溶液pH等对最终产物结构、形貌和电化学性能的影响,研究了NH4+在充放电过程中的行为及锂离子的脱嵌机理。结果表明NH4V308具有较好的锂离子可逆脱嵌能力,NH4+在充电过程中不会脱出。在pH值为4,水热反应24h的条件下制备的片状NH4V308电化学性能最好。该材料在30mA g-1放电容量最高达353.2mAhg-1,对应4个锂离子的插入。在300mAg-’下,100次循环容量保持稳定,600mA g-1下200次循环没有出现容量衰减,体现了优异的循环性能。研究表明材料在大电流下优异的循环稳定性能主要归因于晶体中分子内氢键的形成和分散均匀的片状形貌。不过该材料的倍率性能还有待提高,在300mAg-1下容量已经下降至202.5mAh g-1。针对倍率性能较差的问题,通过一步水热合成法得到NH4V308纳米片／碳纳米管(CNTs)的复合材料,考察了不同CNTs负载量的影响。研究发现,CNTs能在片状NH4V308上形成三维导电网络。CNTs的引入会导致(NH4)0.5V205相的产生。0.5wt%CNTs负载的复合材料体现了最好的电化学性能,在150mA g-1其放电容量为226.2mAh g-1,100次循环容量保持率约97%。而未负载的NH4V308放电容量只有181.5mAhg-1,容量保持率为95.2%。通过水热模板法制备了倍率性大幅提升的NH4V308纳米棒。纳米棒尺寸约30nm,长度小于1μm,其中大部分100nm左右。材料的BET比表面积为15.1m2g-1,是没有模板剂制备的纳米片的3倍多。相比于纳米片材料,制备的纳米棒具有明显改善的锂离子脱嵌平台和电化学性能。先后合成了(NH4)0.5V205纳米带和纳米薄片。纳米带宽50-200nm,长度约几个微米。该材料显示了良好的锂离子脱嵌能力。在15mA g-1下首次放电比容量为225.2mAh g-1。前11次循环,保持在197.5mAh g-1。在150mA g-1下,前100次循环的容量保持率为81.9%,相关电化学性能远超过了文献中报道的结果。通过FT-IR和XRD等手段证实,在首次的充电过程,有部分的NH4+会脱出。在锂离子的脱嵌过程中,材料物相会发生明显改变,但是过程是可逆的。制备的纳米薄片仅仅约单层晶胞厚度(1-1.5nm)。研究发现草酸的加入量对目标材料的电化学性能有较大影响。优化后的材料在0.5C下前200次循环并无容量衰减,体现了非常优异的循环稳定性能。本实验有效解决了文献中该材料循环性能差的问题。具体原因可能归因于其独特的超薄片状形貌和材料内部存在的分子氢键。更多还原

【Abstract】 There has been great interest in synthesizing vanadium oxides and their derivatives as electrode materials for Li-ion batteries because of their low cost, easy synthesis and high capacity. Vanadium-related resources in China are rich, however, the comprehensive utilization is still too low. Therefore, it is meaningful to develop the high capacity vanadium-related compounds as novel electrode materials for Li-ion batteries. It’s well known that poor cycling stability and rate capability are the main two problems for vanadium-related compounds, which limit their further application. This thesis focuses on the trivanadium compounds with relatively high structure stability. At first, lithium trivanadate and sodium trivanadate with high electrochemical performance were obtained by optimizing the preparation strategies. As follows, NH4+group was used to replace Li+in V3O8-layered structure to form NH4V3O8, a new cathode material candidate for LiV3O8. The as-prepared NH4V3O8exhibited a high discharge capacity and excellent long-term cycling life. The lithium ion insertion/extraction mechanisms of all three kinds of materials were emphasized. The main research work and results are shown as follows:A novel two-step method combining the hydrothermal process and the following solid state reaction was designed to obtain LiV3O8nanosheets in this thesis. Firstly, well-dispersed (NH4)0.5V20s nanosheets were prepared by a simple hydrothermal approach. After the calcination of the mixture of the intermediate and LiOH, the LiV3O8nanosheets were yielded. The morphology of the as-prepared product was investigated by FE-SEM, TEM, AFM, etc. The nanosheets with the thickness of20-50nm showed the best rate capability for LiV3O8that have even been reported to date. The discharge capacities of148.7and105.8mAh g-1were retained at5C and10C, respectively. Meantime, LiV3O8nanosheets exhibited the excellent cycling stability with the capacity retention of84.1%and85.3%at300and1000mA g-1, respectively. The excellent rate performance should be due to the unique nanosheet morphology. A possible mechanism model was proposed to explain the reason why the electrode showed the better cycling stability at high rates in comparison with that at low current density.In order to address the poor cycling stability for vanadates, NaV3Og was investigated when Na+was used to substitute the Li+in LiV3O8. A simple hydrothermal method using V2O5and NaOH as raw materials was employed to obtain NaV3O8·xH2O nanowires. The effect of thermal treatment on the structure, morphology and electrochemical performance of as-prepared products was investigated. The nanowires showed a diameter of60-100nm and a length of up to5micrometers. Appropriate thermal treatment could effectively improve the cycling performance, although the discharge capacity was sacrificed to some extent. Na2V6O16·0.86H2O after heat treatment under300℃delivered an initial specific discharge capacity of235.2mAh g-1at30mA g-1, with a capacity retention of91.1%after30cycles. Long cycling test was demonstrated by the retention of90.4%and94.4%at150and300mA g"1after80cycles, respectively. Good rate capability was also achieved for this material. It is proposed that the improved cycling stability of the electrode after thermal treatment is mainly attributed to the removal of a part of crystal water, accompanied with certain structural arrangement.To further improve the rate performance, Na1.08V3O8nanosheets were prepared by the hydrothermal process combining the following solid state reaction. Ultra-thin and well-dispersed features with the mono-layer thickness of less than10nm were demonstrated for nanosheets. The BET specific surface area was9.5m2g-1. The formation mechanism of nanosheets involved the fusion and conversion of nanorods. When used as cathode material for Li-ion battery, the nanosheets showed superior rate capability, with the discharge capacities of ca.200.0,131.3,109.9,93.8and72.5mAh g"1at0.4,10,20,30and50C, respectively, which was the best value for all carbon-free coated vanadates and much better than those of most reported carbon-coated vanadates. Excellent cycling stability without considerable capacity loss over200cycles was observed at600and1000mA g-1. Cyclic voltammetry (CV) results revealed that the Li-ion diffusion coefficient in Na1.08V3O8nanosheets was-10-9cm2s-1. It is believed that the unique nanosheet morphology as well as its intrinsic structure feature greatly facilitates the kinetics of Li-ion diffusion and excellent structure stability, thus resulting in superior electrochemical performance.On the basis of the above-mentioned work, NH4V3O8was proposed as a novel cathode material for Li-ion batteries for the first time. Much work has been carried out. The effect of experimental conditions, including reaction time and pH value on the structure, morphology and electrochemical performance was optimized. And the behavior of NH4+and Li ion insertion/extraction mechanism were also studied. It was found that the NH4V3O8possessed good lithium ion insertion/extraction ability and NH4+could not be extracted in the charge process. NH4V3O8nanosheets prepared by the hydrothermal reaction with the pH of4and reaction time of24h showed the best electrochemical performance. The nanosheets showed the high reversible discharge capacity and superior cycling stability, probably owning to their unique nanosheet morphology and formed molecular H-bond in crystal. A maximum discharge capacity of353.2mAh g-1was exhibited at30mA g"1and202.1mAh g-1discharge capacity was maintained well over100cycles at300mA g-1. Even at600mA g-1, no capacity fading was observed over200cycles. A slight structure arrangement should occur during the prolonged cycling since the lithium ion intercalation/deintercalation plateaus were meliorated, which was further confirmed by FT-IR and CV results. Note that the capacity decreased to202.5mAh g-1at300mA g-1, indicating the inferior rate performance.NH4O3carbon nanotubes (CNTs) composites were synthesized by one-step hydrothermal method and the influence of the coated amount of CNTs was studied. The CNTs were clearly observed on the surface of modified NH4V3O8. It was found that incorporation of CNTs could result in the impurity of (NH4)0.5V205.0.5wt%CNTs coated composite showed the best electrochemical performance. It delivered a discharge capacity of226.2mAh g-1with excellent capacity retention of97%after100cycles at150mA g-1,45mAh g-1larger than that of the pristine one. The greatly improved electrochemical performance of NH4V3O8should be attributed to incorporation of CNTs, which facilitated the interface charge transfer and Li+diffusion. NH4V3O8nanorods with further improved rate capability were hydrothermally prepared in the presence of sodium dodecyl benzene sulfonate (SDBS). The diameter of nanorods is about30nm and the length is less than1μm. The BET surface area is15.1m2g-1. In comparison with the flakes prepared without surfactant, the nanorods are better suited as lithium inserting electrode material, with superior lithium ion insertion/deinsertion plateaus, higher discharge capacity and better cycling stability.(NH4)o.5V205nanobelts and nanosheets were synthesized, respectively. The diameter of the nanobelts was50-200nm and length was about several micrometers. The reversible lithium intercalation behavior was evaluated. It delivered an initial specific discharge capacity of225.2mAh g-1at15mA g-1and still maintained a high discharge capacity of197.5mAh g-1after11cycles. Cycling stability with the capacity retention of81.9%after100cycles at150mA g-1was much better than that in literature. Interestingly, the excess120mAh g-1capacity in the first charge process was observed, most of which could be attributed to the extraction of NH4+group, verified by Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) results. Meanwhile, XRD results showed that the structure of lithium inserted electrode changed during lithium ion insertion and extraction, but the structure change was reversible. It is interesting to note that the thickness of the as-prepared nanosheets was less than1.5nm, which was equivalent to the c axis value of single unit-cell. It was found that the amount of the added oxalic acid influenced the electrochemical performance of the as-prepared materials. The optimized electrode exhibited the superior cycling stability, without capacity fading after200cycles at0.5C, which was probably due to the unique nanosheet morphology and molecular H bond in the crystal.更多还原