【作者】 胡爱平；

【导师】 陈小华；

【作者基本信息】 湖南大学 ， 材料科学与工程， 2013， 博士

【摘要】 锂离子二次电池具有比能量高、工作电压高、循环稳定性好、安全无污染等优点，是发展很快和最受重视的高能蓄电池之一。目前锂离子电池商用碳负极材料由于比容量低已不能满足锂离子电池在高功率器件中的应用需要，因此开发新型的高比能量的锂离子电池负极材料已成为电池研究领域的重要课题之一。Fe₃O₄具有理论比容量高、资源丰富、价格低廉、环境友好的优点，是一种非常具有应用前景的锂离子电池负极材料。但是，它存在循环稳定性和倍率性能差的缺点。本论文在探索制备Fe₃O₄和石墨烯的工艺条件基础上，原位合成了Fe₃O₄纳米棒/GNS复合材料，用水热法一步合成了Fe₃O₄/碳/还原的氧化石墨烯（简称Fe₃O₄/C/rGO）复合材料并对其电化学性能进行研究。以亚铁盐为铁源，水合肼为氧化剂，在pH>10的条件下，反应12h，可以形成单一物相的Fe₃O₄磁性纳米粒子。亚铁盐原料不同，反应速率不同。纳米Fe₃O₄在0.1C倍率下，放、充电比容量分别为1060和752mAh/g，首次库仑效率为71%，但是其循环性能和倍率性能差。石墨经浓硫酸插层反应1h，高锰酸钾氧化反应2.5h，高温反应0.25h后基本氧化完全，其中含有较多的羧基、羟基和环氧基。水合肼可以将氧化石墨烯中羧基、羟基及部分环氧基团还原，并在石墨烯中形成较小的共轭区域。pH值、反应时间、热处理温度等对氧化石墨烯的还原程度有一定影响。制备的石墨烯片无序排列，任意卷曲，片层与片层之间相互堆积。GNS中具有3-4nm的介孔结构，其比表面积为373.42m²/g，总孔体积为0.47mL/g。以硫酸亚铁铵和氧化石墨为反应原料，在pH大于11的水热条件下成功合成了Fe₃O₄纳米棒/石墨烯复合材料（FNGC）。FNGC中具有共轭结构，但有部分未还原的环氧基团。卷曲的石墨烯片层上覆盖着大量的Fe₃O₄棒状结构，分布均匀，棒之间基本无团聚现象。Fe₃O₄纳米棒直径11nm左右，长度大于100nm。Fe₃O₄为反尖晶石磁铁矿晶相。石墨烯片层距离大约为0.41nm。N2吸附脱附曲线表明，FNGC的比表面积为180m²/g，总孔体积0.26mL/g，具有介孔结构。通过对FNGC制备工艺条件如pH值、搅拌速度、反应温度、反应时间对产物形貌的影响，分析了FNGC的形成机理。FNGC负极首次放、充电比容量分别为：1538mAh/g和925mAh/g，库仑效率60.2%。在1C放、充电循环80次后，放、充电比容量保持率分别为80.5%和90%。在5C倍率下循环，依然具有66.78%和63.80%的放、充电比容量维持率。其电化学性能相对Fe₃O₄纳米颗粒/石墨烯复合材料，得到了有效地改善。通过四氧化三铁的形貌，GNS的含量、热处理温度对FNGC电化学性能的影响研究发现Fe₃O₄在石墨烯表面上的分布状况对复合物的倍率性能及循环稳定性有很大的关系。纳米Fe₃O₄均匀地覆盖在石墨烯表面的复合结构是得到优良电化学性能的有力保障。以FeCl3、葡萄糖、氧化石墨为原料，在15MPa，400℃下反应12h，一步合成了Fe₃O₄/C/rGO复合材料（FCRG）。Fe₃O₄主要附着在rGO片的边缘地带，并有一定的团聚现象。Fe₃O₄为晶态，rGO结构中存在有共轭区域，但有部分含氧基团未被还原。FCRG具有比Fe₃O₄/C更好的循环稳定性和倍率性能。本论文的研究为过渡金属氧化物锂离子电池负极材料的进一步开发奠定了良好的基础，也为合成一维金属氧化物/石墨烯复合材料提供了一条有效的途径。更多还原

【Abstract】 The lithium ion secondary battery with the advantages of high specific energy,high operating voltage, good cycling stability, safety and low pollution etc, has beenone of the most important high-energy batteries and developed rapidly. Carbon，as thecommercial lithium-ion battery（LIB） anode material， cannot have met the need oflithium-ion battery applications in high-power devices due to its low specificcapacities. Therefore, it needs urgently to develop new anode materials for LIB. Ironoxide has been a promising anode material for the LIBs owing to its high theoreticalspecific capacity， abundance， low cost and friendly environment. However，thestudies show that Fe₃O₄has the disadvantage of poor cycle stability and rate property.In this paper, in the basis of exploring the technique conditions of synthesizing Fe₃O₄and graphene in the aqueous solution， respectively， Fe₃O₄nanorods/graphenenanocomposites were prepared via in situ self-assembly, Fe₃O₄/C/r-GO compositeswere also synthesized by one-step strategy in hydrothermal conditions and theirelectrochemical properties were analyzed.Pure granular Fe₃O₄nanoparticles could be synthesized with ferrous salts as reactantsand hydrazine as oxidant at pH>10for12h in aqueous solution. The reaction rate wasdifferent with various ferrous salts. The discharge/charge specific capacities were1060and752mAh/g， respectively. The first Columbic efficiency was71%. But it’scycling performance and rate capacity properties were poor.Most of graphite was oxidized and some hydroxyl groups, carboxyl groups andepoxy groups formed when concentrated sulfuric acid intercalation for1h，oxidationreaction with potassium permanganate for2.5h and high temperature reaction for0.25h. The hydrazine could reduce the oxide graphite to graphene. It could be foundthat oxygen-containing functional groups except some epoxide groups of oxidegraphite were reduced and conjugated region were formed at pH>10. PH value,reaction time and heat-treatment temperature had important role in the reducingdegree of oxide graphite. However, epoxy groups could not be reduced completel ywith hydrazine as reducer with changing the reaction conditions. Graphene sheetswere arranged disorderly and curled randomly. Some accumulations occurred amonggraphene sheets. Graphene has mesoporous structure of3-4nm，specific surface areaof373.42m²/g, the total pore volume of0.47mL/g. Fe₃O₄nanorods graphene composites （FNGC） have been successfully preparedwith graphite oxide and （NH4）2Fe（SO4）2in water as reactants at pH>11for12h.Fe₃O₄was inverse spinel magnetite crystal phase. Some conjugated region and someepoxy groups existed in FNGC. The as-formed Fe₃O₄nanorods, about11nm indiameter and more than100nm in length， were uniformly anchored on curledgraphene nanosheets. The distance between graphene nanosheets was about0.41nm.The specific surface area of180m²/g, a total pore volume of0.26mL/g andmesoporous structure existed in FNGC. Through studying the effect of pH, agitationspeed, temperature and reaction time on the morphologies of the products, theformation mechanism of FNGC was supposed.The first discharge/charge specific capacities were1538and925mAh/g，respectively. The Columbic efficiency was60.2%. After80cycles at1C, thedischarge/charge specific capacities sustainities was80.5%and90%, respectively. Atcurrent densities of5C, the sustainities of discharge and charge capacity remain at66.78%and63.80%. The FNGC exhibited improved electrochemical properties incomparison with Fe₃O₄nanoparticles/graphene composites. Through studying the ironoxide morphology, GNS content and the heat treatment temperature on theelectrochemical properties of FNGC, it was found that the surface distribut ion ofFe₃O₄and graphene have an important role in the rate performance and cyclestability of the composites. Fe₃O₄nanostructures evenly covering in the surface ofGNS by bonding force is strong guarantee for excellent electrochemical properties ofiron oxide composite materials.Fe₃O₄/C/r-GO composites were synthesized with ferric chloride, glucose andgraphite oxide by hydrothermal method at15MPa,400℃for12h. Fe₃O₄nanoparticles with some aggregation were mainly anchored on the edge of graphenesheets. Fe₃O₄was crystalline. Many r-GO sheets were stacked together. Someconjugated regions existed in Fe₃O₄/C/r-GO composites but some oxygenated groupwere not reduced.Fe₃O₄/C/r-GO composites have improved electrochemical propertiesin comparison with Fe₃O₄/C.The studies in the paper have laid a good foundation for the further developmentof the transition metal oxide anode material for lithium-ion batteries and also providean effective way for the synthesis of one-dimensional metal oxide/graphenecomposites.更多还原