【作者】 何敏；

【导师】 黄云辉； 袁利霞；

【作者基本信息】 华中科技大学 ， 材料学， 2013， 博士

【摘要】 锂离子电池目前已成为手机、摄像机、笔记本电脑等各种便携式电子设备的标准配套电源,同时也被公认为电动汽车的理想配套电源和潜在的风能太阳能等的储能电源。目前制约锂电池进入某些大容量大功率应用场合的主要问题之一是电池的能量密度仍需进一步提高。提高电池能量密度的关键在于开发高性能的新型电极材料。单质硫和锡基材料具有理论比能量高、价格便宜、与环境友好等突出优点,被认为是下一代锂二次电池正负极材料的理想选择。但是,这两种材料体系的发展还面临一系列问题：如活性物质利用率低、循环性差等。本文以硫正极、氧化锡和硫化锡负极为研究对象,以提高电导率和稳定电极结构为主线,以改善电化学性能为目标,以低成本和无污染为原则,系统探讨了材料结构、电极结构与锂二次电池性能的关系,并对其储锂性能的改善机理进行了详细的研究与分析。主要结果如下：为改善硫电极在充放电过程中的结构稳定性,采用具有表面活性的丁苯橡胶/羧甲基纤维素钠(SBR/CMC)复合粘接剂体系来制备高性能的单质硫电极。与传统的聚偏氟乙烯(PVDF)相比,SBR-CMC除良好的粘结功能外,还是一种良好的分散剂,其特有的负电性使硫和碳黑高度分散并形成良好的电接触网络,一方面可以提高硫的利用率,另一方面还可以有效抑制Li2S的团聚以保证充放电过程中电极结构的稳定性。另外,SBR/CMC粘结剂对电解液的吸收率远低于PVDF体系,电极溶胀程度大大降低,这也有助于电极结构稳定。在这些协同作用下,硫电极的活性物质利用率和循环性能均得到显著改善。针对单质硫正极材料存在的导电性差,中间产物易在电解液中溶解这两个关键问题,设计合成了硫/多孔碳纳米管复合材料(S-PCNTs)。通过氢氧化钾活化改性,多壁碳纳米管(MWCNTs)呈现多样孔道结构,比表面积和孔隙率显著增加。这种一维多孔结构既有助于充放电过程电子和锂离子的传输,又可有效抑制多硫化锂在电解液中的溶解,稳定充放电过程中的电极结构。与简单的碳纳米管复合电极相比,S-PCNTs电极显示出优良的电化学性能,首次充放电容量为1180mAh g-1,在100周充放电循环后可逆容量仍然维持在531mAh g-1;另外,在没有任何添加剂的情况下,库伦效率稳定在90%。针对二氧化锡材料在充放电过程中的体积效应这一主要问题,采用一锅水热法和碳化处理工艺合成出一种纳米簇复合材料,该纳米簇由相互连接的超细二氧化锡-碳核壳(SnO2@C)纳米球组成。作为锂离子电池负极材料,这种纳米结构可以有效缓解充放电过程中二氧化锡体积变化多电极结构的冲击,同时可以为锂离子和电子的快速传输提供三维通道。结合SBR-CMC粘结剂,SnO2@C纳米簇负极获得了优异的循环稳定性和倍率性能。在电流密度为100mAg-1时,200周循环后的放电容量高达1215mA h g-1。即使在电流密度为1600mA g-1时,容量仍然可保持在520mAh g-1,当电流密度又返回到100mA g-1时,容量可恢复到1232mAhg-1。其优异的电化学性能应主要归功于其独特的核壳结构：超细Sn02核具有很高的电化学活性和较小的体积变化,而外层的碳壳可以提高电导率、抑制颗粒团聚,为电化学反应提供连续的界面和缓冲重复嵌锂过程的机械压力。通过溶剂热法成功合成了乙炔黑修饰的多孔硫化锡(SnS2)纳米花。其结构特点是乙炔黑均匀环绕在由无数纳米片自组装而成的多孔SnS2二次微球周围,该复合材料的比表面积为129.9m2g1,电导率为0.345S cm-1。作为锂离子电池负极材料,该复合材料呈现良好的循环性能和倍率性能：在400mA g-1的电流密度下,70周的平均可逆容量高达525mAh g-1。电化学性能的提高应归功于乙炔黑和多孔SnS2的协同作用：乙炔黑不仅可作为导电剂加速电子在复合材料中的转移,而且可充当缓冲基抑制合金化／去合金化过程的体积变化和稳定电极结构。另外SnS2多孔的结构也有助于稳定电极结构和方便锂离子的高效传输。更多还原

【Abstract】 Now the lithium-ion batteries (LIBs) have been the dominant power source for advanced portable electronic devices and are also considered as the most competitive system to supply power for future electric vehicles (EVs). For LIBs, in order to meet the large-scale power requirements, searching for new electrode materials with higher energy density is still a challenging task. Sulfur cathode and tin-based anode, both show some outstanding advantages such as high theoretical specific capacity, low cost and environmental friendliness, which has been considered as one of the most promising cathode candidates for next-generation batteries. However, the practical application of these two kinds of electrodes suffers from low sulfur/tin utilization and poor cycle life. The problems can be ascribed to the poor electrical conductivity of sulfur and some tin-based composites, and to the large volume changes during the repeated charge-discharge cycling. This Ph.D work was aimed at developing some sulfur/tin based composites applicable for rechargeable lithium batteries. The main results are summarized as follows:The mixture of styrene butadiene rubber (SBR) and sodium carboxyl methylcellulose (CMC), a water-soluble binder system, have been successfully introduced to the sulfur cathode system. Compared with conventional poly (vinylidene fluoride)(PVDF) binder, the SBR-CMC binder significantly improves cycling performance of the sulfur cathode. The SBR-CMC mixture is not only a high adhesion agent but also a strong dispersion medium, which favors the uniform distribution between insulating sulfur and conductive carbon black (CB) and ensures a good electrical contact, leading to a high sulfur utilization. Furthermore, the improvement in cyclability is ascribed to structural stability of the sulfur cathode promoted by the SBR-CMC binder during charge/discharge cycles due to the combined effects of homogeneous distribution of the S and CB particles in the composite cathode, the low electrolyte uptake, and the suppressed agglomeration of L12S.A sulfur composite based on a porous multi-walled carbon nanotubes (PCNTs) with multiple mesopores structure was synthesized and studied as a cathode material for Li-S battery. The PCNTs was prepared via a simple activation of multi-walled carbon nanotubes (MWCNTs) by potassium hydroxide. The potassium hydroxide activation process results in a significantly enhanced specific surface area with numerous small pores. The as-obtained PCNTs are employed as the conductive matrix for sulfur in the sulfur cathode, SBR-CMC was used as binder. Compared with the composite sulfur cathode based on the original MWCNTs, the sulfur-PCNTs cathode shows a significantly improved cycle performance and columbic efficiency. The improvement in the electrochemical performance for S-PCNT is mainly attributed to the enlarged surface area and the porous structure of the unique mesopores carbon nanotube host, which cannot only facilitate transport of electrons and Li+ions, but also trap polysulfides, stabilize the electrode structure during charge/discharge process.A nanocluster composite assembled by interconnected ultrafine SnO2-C core-shell (SnO2@C) nanospheres is successfully synthesized via a simple one-pot hydrothermal method and subsequent carbonization. As an anode material for lithium-ion batteries, the thus-obtained nano-construction can provide a three-dimensional transport access for fast transfer of electrons and lithium ions. With the mixture of styrene butadiene rubber and sodium carboxyl methyl cellulose as a binder, the SnO2@C nanocluster anode exhibits superior cycling stability and rate capability due to a stable electrode structure. Discharge capacity reaches as high as1215mA h g-1after200cycles at a current density of100mA g-1. Even at1600mA g-1, the capacity is still520mA h g-1and can be recovered up to1232mA h g-1if the current density is turned back to100mA g-1. The superior performance can be ascribed to the unique core-shell structure. The ultrafine SnO2core gives a high reactive activity and accommodates volume change during cycling; while the thin carbon shell improves electronic conductivity, suppresses particle aggregation, supplies a continuous interface for electrochemical reaction and alleviates mechanical stress from repeated lithiation of SnO2.Acetylene black incorporated porous3-dimensional (3D) SnS2nanoflowers have been successfully synthesized via a simple solvothermal route. The composites are composed of acetylene black adorned SnS2secondary microspheres which are assembled from a number of nanosheets. The nanocomposites possess a large specific surface area of129.9m2g-1and a high conductivity of0.345S cm-1. As anode materials for lithium ion batteries, the nanocomposites show high cyclability and rate capability and deliver an average reversible capacity as high as525mAh g-1at a current density of400mA g-1over70cycles. The high electrochemical performance can be attributed to the synergistic effect of acetylene black and the unique microstructure of SnS2. The acetylene black serves as not only a conductive agent to accelerate the transfer of electrons in the composites, but also as a buffer matrix to restrain the volume change and stabilize the electrode structure during the alloying/dealloying process. The porous structure of SnS2also helps to stabilize the electrode structure and facilitates the transport for lithium ions.更多还原