PEG诱导下钼氧化物基复合材料的制备及其电化学性能研究

钼氧化物和钼氧基体杂化材料因其优异的光学、电化学、催化和机械性能，已被广泛应用于锂离子电池、催化剂、光致发光/变色和气敏传感器等许多领域。不同的应用领域对其微观结构和化学组成有特定的要求，但目前常采用的水热法、化学沉淀法、溶胶-凝胶法、火焰法等调控制备方法多存在反应条件苛刻、反应时间较长、使用毒性有机溶剂或原料价格昂贵等问题，不适合大规模生产。因此，本论文提出了聚乙二醇（PEG）诱导钼前驱体溶液调控制备多形态含钼杂化物的新方法，考察了反应条件对含钼杂化物物相、形貌和尺寸等的影响，揭示了PEG聚合物对钼氧化物微观结构的调控机理和PEG与钼前驱体的反应机理，通过后期热处理制备了三氧化钼（MoO3）纳米片和微米带以及多级多孔二氧化钼/碳（MoO2/C）和二氧化钼/碳化钼/碳（MoO2/Mo2C/C）微球，并测试了其用作锂离子电池负极材料的电化学性能。本论文主要在以下几个方面取得了创新性进展：（1）采用PEG聚合物诱导钼前驱体溶液调控制备了含钼杂化物，详细考察了反应温度、反应时间和PEG聚合物浓度等反应参数对含钼杂化物形貌、组成和尺寸等的影响，揭示了PEG聚合物对钼氧化物微观结构的调控机理和PEG与钼前驱体的反应机理。结果表明，PEG聚合物浓度对含钼杂化物的影响最为显著。PEG分子量为8000时，随着PEG浓度的增加，产物形貌由带状-针状-纤维状-球形-立方体转变，产物物相由过氧钼酸（H2MoO5）向钼酸（MoO3·H2O）再向无定形状态转变，产物颜色由黄色向绿色再向蓝色以及深蓝色转变，产物中钼的价态由六价向五价转变。PEG聚合物既是模板剂调节构筑不同形貌和尺寸的含钼杂化物，又是表面活性剂增强颗粒的分散性，而且是弱的还原剂将部分六价钼还原成五价钼。（2）在空气气氛下煅烧含钼杂化物调控制备了MoO3，系统研究了PEG浓度、煅烧温度、煅烧时间对MoO3产物晶型和形貌的影响，并测试了其作为锂离子电池负极材料的电化学性能。煅烧制备的MoO3纳米片具有独特的层状结构，能够加速锂离子和电子的传输以及有效的缓减体积效应，展现出了良好的电化学性能。电流密度为100 mA g-1时，首次放电比容量为1373 mAh g-1，循环100圈之后放电比容量仍可保持在540 mAh g-1。（3）在低温氮气气氛中煅烧含钼杂化物调控制备了多级多孔MoO2/C微球，考察了煅烧温度对MoO2/C微球结构和电化学性能的影响。通过微观结构观测，所合成的微球直径约1.5 μm，表面粗糙，具有良好的分散性，每个微球是由许多纳米颗粒所组成，颗粒和颗粒间有孔结构的存在。随着煅烧温度的增加纳米颗粒尺寸略有增加，石墨类碳含量则有减小趋势，孔结构由微孔向介孔转变。500oC煅烧温度下所制备的MoO2/C展现出高的放电比容量和长的循环寿命。电流密度为100 mA g-1时，MoO2/C首次放电比容量为768 mAh g-1，循环300圈之后放电比容量仍可保持在800 mAh g-1，结构能够很好的保持。 （4）在高温氮气气氛中煅烧含钼杂化物调控制备了多级多孔MoO2/Mo2C/C微球，研究了煅烧温度对生成产物的影响，并测试了其电化学性能。结果表明，煅烧温度为700和800oC时，产物为MoO2/Mo2C/C复合材料。PEG聚合物在氮气气氛下发生碳化，不仅将六价钼还原成四价钼，而且与部分二氧化钼反应生成碳化钼。700oC煅烧温度下所制备的MoO2/Mo2C/C微球呈现出高的比容量和良好的循环稳定性。在电流密度为100 mA g-1时，MoO2/Mo2C/C微球起始放电比容量为720 mAh g-1，循环100圈之后比容量保持在665 mAh g-1，且结构未发生破坏。;Molybdenum oxides and molybdenum-containing hybrid materials (MCHMs) have been widely used in many fields, such as lithiumion batteries (LIBs), catalysis, photo-chromism/luminescence and gas sensor due to their remarkable optical, electrochemical, catalytic and mechanical properties. Usually, different applications have specific requirements for the morphology, size and composition. At present, the hydrothermal method, chemical precipitation method, sol-gel method and flame method have been employed to synthesis the molybdenum oxides and MCHMs to meet the specific requirements. However, these methods exhibit various problems such as harsh reaction condition, long reaction time, use of toxic organic solvents, and high cost of raw materials. Most of them are unsuitable for large scaleproduction. Therefore, a new preparation method was proposed in this dissertation. The MCHMs with different morphologies were synthesized from molybdenum precursor solution induced by polyethylene glycol (PEG). The influence of various reaction conditions on the phase, morphology, and size of MCHMs was investigated in detail. Moreover, the regulation mechanism of molybdenum oxides in the PEG polymeric system and reaction mechanism of molybdenum precursor with PEG were revealed. MoO3 nanosheet and microribbon, hierarchical porous MoO2/C and MoO2/Mo2C/C microspheres were synthesized after heat treatment, and the related electrochemical properties as anode materials for LIBs were also tested.The main innovative progress is summarized as follows:(1) MCHMs were prepared from peroxomolybenum solution induced by PEG polymer. The effects of various reaction parameters including reaction temperature, reaction time and PEG concentration on the phase, morphology, and size of MCHMs were systemically investigated. The effect of PEG polymeric system on the microstructure of molybdenum oxides and reaction mechanism of molybdenum precursor with PEG were revealed. It was found that PEG concentration has dramatic influence on the MCHMs. When the molecular weight of PEG is 8000, the change of the product with the increase of PEG concentration was as follows. The morphology changed in the sequence: ribbon-needle-fiber-sphere-cube. The phase altered from H2MoO5 to MoO3·H2O and then to amorphous state. The color shifted from yellow to green, blue and then to dark blue. The valence state of the molybdenum converted from +6 to +5. The PEG polymer acted not only as a structure-directing agent to construct MCHMs with different morphologies and sizes, but also as a surfactant to enhance the dispersion of particles. The mild reducibility of PEG polymer also converted a portion of Mo (VI) into Mo (V).(2) MoO3 was prepared by the calcination of MCHMs in air. The influence of PEG concentration, calcination temperature and calcination time on the phase and morphology of MoO3 was investigated in detail and the electrochemical properties of MoO3 as anode materials for LIBs were tested. The accelerated transport of lithium ions and electrons as well as the effective mitigation of the volume change due to the special layered structure contributed to the admirable electrochemical performance of MoO3 nanosheet. At a current density of 100 mA g-1, the MoO3 delivered the discharge specific capacity of 1373 mAh g-1 in the first cycle and 540 mAh g-1 after 100 cycles, respectively.(3) Hierarchical porous MoO2/C microsphere was synthesized through heat treatment of MCHMs in nitrogen at a relatively low temperature. The influence of calcination temperature on the structure and electrochemical performance of MoO2/C microsphere was investigated. The as-prepared microsphere with a diameter of 1.5 μm had rough surface and good dispersity. There are abundant pores in the microsphere consisting of many primary nanoparticles. With the increase of the calcination temperature, the nanoparticle size slightly grew, the content of carbon showed a reduction trend, and the pore structure changed from microporous to mesoporous. After calcination at 500oC, the MoO2/C microspheres exhibited the discharge specific capacity of 768 mAh g-1 in the first cycle and 800 mAh g-1 after 300 cycles at a current density of 100 mA g-1. Additionally, the microstructure can be preserved well.(4) Hierarchical mesoporous MoO2/Mo2C/C microsphere was prepared by heat treatment of MCHMs in nitrogen at a relatively high temperature. The effect of calcination temperature on the product was studied. In addition, MoO2/Mo2C/C composites obtained at different calcination temperatures were used as anode materials for LIBs and the electrochemical properties were tested. The results showed that MoO2/Mo2C/C composite was obtained at 700 and 800oC. The carbon from the carbonization of PEG polymer not only converted Mo (VI) into Mo (IV), but also reacted with a part of MoO2 resulting in the formation of Mo2C. The MoO2/Mo2C/C microspheres obtained from calcination at 700oC exhibited high capacity and good cycling stability. At a current density of 100 mA g-1, the MoO2/Mo2C/C microspheres exhibited the discharge specific capacity of 720 mAh g-1 in the first cycle and 665 mAh g-1 after 100 cycles without the destroy of microstructure.