石墨烯二氧化锡泡沫的制备及其电化学应用研究

论文总字数：69589字

摘 要

当今能源与环境问题日趋严重，开发新能源成为世界关注的焦点。其中锂电池作为能源转换设备，因具有比能量大、循环寿命长、生产污染小等优点引起了人们极大的重视。传统的负极材料有碳类材料和金属氧化物，但碳类材料的缺点是比容量较小、不可逆容量损失较大；金属氧化物虽然具有较高的比容量，但显著的缺点就是在充放电过程中存在巨大的体积膨胀，因而造成循环稳定性较差。随着石墨烯的兴起，科学家们采用复合材料的思路，将金属氧化物与石墨烯进行复合以期获得二者的协同互补。

本课题主要通过制备石墨烯/二氧化锡泡沫复合材料对其电化学性能进行研究。首先采用改进的Hummers法以石墨鳞片为原材料制备氧化石墨烯（GO）。然后通过溶液共混法制备复合材料。通过向GO中引入SnCl₄·5H₂O，再由Sn⁴ 自身水解后分解在GO上原位生成纳米级SnO₂粒子。此外，我们创新地引入PVA进行三元复合。PVA在溶液共混阶段起到交联剂的作用，对GO和SnO₂起到包覆作用并稳定二者之间的结合。接着我们再通过液氮速冻法及冷冻干燥法制备多孔的复合材料，之后采用高温热处理的方式得到还原后的三维泡沫复合材料。最后我们将还原后的样品作为负极材料进行锂电池的搭建、测试。我们在制备实验中设置了两大探究变量，一是不同的热处理方式，二是三元复合材料中GO/PVA的比例。我们通过研究这两个变量因素对样品性能的影响，分析其中的机理，以期优化制备工艺，实现更高效的石墨烯/二氧化锡锂离子电池负极材料的开发。

我们对制备的复合材料采用了多种手段技术进行表征。综合各项表征的结果来看，三元复合材料中确实含有r-GO，SnO₂纳米粒子以及碳化的PVA。碳化后的PVA包覆在r-GO和SnO₂纳米粒子上，SnO₂纳米粒子可以均匀的分散在r-GO 的片层间。复合材料整体具有三维空间网格结构，该结构十分有利于充放电过程中锂离子的嵌入和脱嵌，极大提高了载流子的传输效率。表征结果还证明，H₂ 600℃下的还原效果要优于Ar 250℃的还原效果，复合材料将具有更少的残余含氧官能团。当GO/PVA=1:3时，复合材料的三维孔洞结构最优，孔径大小约为20μm，排布规则。

我们通过搭建锂电池对不同的样品进行了循环伏安测试、恒流充放电测试以及倍率性能测试。测试的结果表明在我们所制备的三元复合材料中，对电池容量起主要贡献的碳材料包括r-GO和碳化PVA，SnO₂纳米粒子由于总量较少，对容量仅起到辅助作用。当GO/PVA=1:3时，样品的CV曲线表现出了较好的氧化还原峰，较高的比容量和较稳定的高倍率下循环性能。该样品的首次放电比容量可达948mAh/g，首次库伦效率为63.3%。稳定和的放电比容量约为380 mAh/g，容量保持率为40%。不同样品之间的电化学数据表明，Ar 250℃还原下的样品易发生严重的极化现象，因此说明其还原效果远不如H₂ 600℃，该结果与表征部分一致。结合微观结构与电学性能的关系我们可以得出结论：负极材料所具有的完整规则的三维空间网格结构及更小的孔洞直径将对其比容量和循环稳定性起到极为有利的作用。

关键词：石墨烯，二氧化锡，复合材料，聚乙烯醇（PVA），锂离子电池

Abstract

Nowadays, the problem of energy crisis and environmental pollution is more and more serious, thus developing new energy resource is becoming the focus of the whole world. As an energy transfer device, lithium ion battery arise people’s great attention due to its large specific energy, long cycling lifespan and less pollution during producing process. Traditional cathode materials of lithium ion battery include carbon materials and metal oxides. In general, the specific capacity of carbon materials is small and the irreversible capacity loss is large. For metal oxide cathode materials, although they have relatively large specific capacity, the biggest problem is caused by the large volume change during the charge-discharge cycles, which gives rise to the low cycling stability. With the researching popularity of graphene, the scientists are adopting such idea that combines graphene and metal oxide to produce composite materials with the expectation that they will have complementary cooperative effect.

This project is about the preparation of Graphene/Tin Dioxide foam composite material and the study of its electro-chemical properties. Firstly, the modified Hummers method id adopted to prepare for the oxide graphene (GO) with the natural graphite flake as raw materials. Then mixing-solution method is hired for the preparation of composite materials. SnCl₄·5H₂O is adding to GO hydrosol and Sn⁴ hydrolyzes forming Sn(OH)₄. Sn(OH)₄ is not stable in the system thus decomposing to in-situ SnO₂ nanoparticles on GO layers. Besides, PVA is introduced innovatively to form ternary composite materials. As a crossing-link agent, PVA coats GO as well as SnO₂ and stabilize their combination. Afterwards, liquid nitrogen is employed for quick-freezing of the composite hydrosol and then they will be put into the vacuum freeze drier for 48 hours to form a porous structure. Finally, high-temperature treatment is applied to the samples to have them reduced. The coin-type lithium ion batteries are constructed with the reduced samples as the cathode materials. By means of electro-chemical working station, their electro-chemical properties can be studied. During the whole sample preparation process, two kinds of variance are set, one is the way of heat treatment and the other is the ration of GO/PVA. According to study these to variance by comparison of different samples, the mechanism can be analyzed and the preparation process can be optimized. A more effective Graphene/ Tin Dioxide cathode materials of lithium ion battery is in the hope to be developed.

Advanced analysis technology is employed to characterize the samples. Integrating the outcomes, it can be confirmed that r-GO, SnO₂ nanoparticles and carbonized PVA indeed exist in the ternary composite materials. SnO₂ coated by carbonized PVA distribute uniformly between r-GO layers. The ternary composite material has a 3D porous structure which is favorable to the embedding and de-embedding of Li as well as the transfer of charge carriers during charge-discharge process. The outcomes also prove that the reduced effect under the condition of H₂ 600 is better that that under Ar 250 because there is less residual oxygen-containing function groups. Furthermore, it’s found that the ternary composite material obtains a best the 3D porous structure when GO/PVA=3:1, in which the holes have diameter of 20μm and arrange periodically.

The lithium batteries used the different reduced sample experience tests including Cyclic voltammetry, galvanostatic charge-discharge and capacity at high charge-discharge rate. The test results show that in our reduced sample, it’s the carbon including r-GO and carbonized PVA that plays leading role in specific capacity while SnO₂ nanoparticles only have auxiliary contribution. The lithium ion battery used r-GO/PVA/SnO₂ (1:3, H₂) shows good oxidation and reduction peaks on CV curve, relatively high specific capacity and stable cycling properties at high charge-discharge rate. Its discharge specific capacity is up to 948mAh/g and the coulomb efficiency in the initial cycle. After stabilization, the discharge specific capacity maintains 380mAh/g which is 40% of the initial value. By comparison of different samples, it’s found that severe polarization problems occur in lithium ion batteries that used samples reduced in Ar 250. This also means the reduced effect of Ar 250 is far worse than H₂ 600, which is corresponding to the outcome in characterization part. Last but not least, combing the microstructure and electrochemical properties the conclusion can be reached that for the ternary composite material, the intact and ordered 3D porous structure with smaller-diameter holes has great benefit to the specific capacity and cycling stability of a lithium ion battery when it works as the cathode material.

Key words: Graphene, Tin dioxide, Composite materials, PVA, Lithium ion battery

目 录

摘 要 I

Abstract II

第一章 绪 论 1

1.1 引言 1

1.2 石墨烯简介 2

1.2.1 石墨烯的发展和特性^[11] 2

1.2.2 石墨烯的制备方法 3

1.2.4 石墨烯在锂电池中的应用 4

1.3 锂离子电池简介 5

1.3.1 锂离子电池的发展 5

1.3.2 锂离子电池的结构及原理 6

1.3.3 锂离子电池的负极材料 10

1.4 石墨烯/二氧化锡复合材料在锂电池中的应用 12

1.4.1 石墨烯/SnO₂复合材料的处理机理 12

1.4.2石墨烯/SnO₂复合材料的制备方法 12

1.4.3石墨烯/SnO₂复合材料现存的问题及解决手段 13

1.5 本课题研究背景及内容 14

第二章 石墨烯/二氧化锡泡沫材料的制备 16

2.1 引言 16

2.2 实验试剂与设备 17

2.2.1 药品和试剂 17

2.2.2 仪器和设备 17

2.3 复合材料的制备 17

2.3.1 GO的制备 17

2.3.2 GO/PVA的制备 19

2.3.3 GO /PVA/SnO₂三元泡沫复合材料的的制备 21

2.3.4 样品热还原 22

2.4 本章小结 24

第三章 石墨烯泡沫复合材料的表征 26

3.1 表征原理及设备 26

3.1.1 X射线衍射仪（X-ray Diffraction Spectroscopy, XRD） 26

3.1.2 扫描电子显微镜（Scanning Electron Microscope, SEM） 26

3.1.3 透射电子显微镜（Transmission Electron Microscopy, TEM） 27

3.1.4 傅里叶变换红外光谱仪（Fourier Transform Infrared Spectroscopy, FTIR） 27

3.1.5 激光拉曼光谱仪（Raman Spectroscopy） 28

3.1.6 热重分析仪（Thermogravimetric Analysis, TGA）和差示扫描热法（Differential Scanning Calorimetry, DSC） 28

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