纳米纤维结构的硅基复合材料的制备及电化学性能研究

发布时间：2018-05-30 01:22

本文选题：生物仿生合成 + 纤维素　；参考：《浙江大学》2017年博士论文

【摘要】：近年来,随着纳米科技的进步,锂离子电池的性能有了很大的提高,但是仍然存在许多瓶颈,尤其是在新型高性能电极材料的设计及构筑方面还面临挑战。作为未来锂离子电池理想的负极材料,硅基材料由于具有远高于传统商用石墨负极的理论比容量,受到极大的关注。但是如何解决硅基负极材料较低的导电性和在锂离子嵌入和脱出过程中巨大的体积变化是进一步提高其电化学性能的关键。基于自组装的仿生合成技术,它将客体基质的化学性能和生物物质优异的结构功能特点相结合,为设计和制备具有特定结构和性能的新的功能材料提供了一个有效途径。自然纤维素物质作为一种常见的天然高分子化合物,具有从宏观到分子层次的独特阶层结构及其在纳米层级上的多孔网状形貌,以其为模板或支架在构筑纳米结构电极材料中有很大的应用潜力。本论文的主要研究内容是以自然纤维素物质(实验室常用的定量滤纸)为模板和支架,实现了多种纳米纤维结构的硅基复合材料的制备,并较好的缓解了硅基负极材料在充放电过程中体积变化所导致的材料结构破坏,增强了其机械性能和导电性,改善了其电化学储能性能,为其在储能器件中的应用提供一定的指导。主要研究内容如下:1.以自然纤维素物质(实验室普通定量滤纸)为模板和碳支架,采用简单的溶胶-凝胶法将二氧化硅凝胶层组装到滤纸纤维素纤维的表面,经碳化得到纳米纤维结构的二氧化硅/碳复合材料。该复合材料中,纳米级厚度的二氧化硅层均匀的包覆在碳纤维的表面。由于该复合材料具有多孔网状结构、高比表面积以及碳纳米纤维基底,有利于电解液扩散和提高二氧化硅的导电性,有效缓冲了二氧化硅在充放电过程中巨大的体积变化,因此在用作锂离子电池负极材料时,表现出较高的比容量,较好的循环稳定性和倍率性能。进一步包覆无定形碳层或者沉积银纳米颗粒于二氧化硅/碳纳米纤维的表面,其电化学性能得到了进一步提高。2.为了进一步提高二氧化硅/碳复合材料的电化学性能,采用镁热还原法,将复合材料中二氧化硅还原为硅,得到了纳米纤维结构的硅/碳复合材料。该复合材料完整地保留了滤纸原有的多孔网状结构,纳米级厚度的硅层均匀的包覆在碳纤维的表面。当用作锂离子电池负极材料时,硅含量为25.7wt%,硅层厚度为40nm时,该复合材料的电化学性能最好。在100 mA g-1电流密度下,循环150次以后,放电比容量还可以保持在750.6 mAh g-1。将无定形碳层或者银纳米颗粒沉积在硅/碳纳米纤维表面,其电化学性能得到了进一步提高,在100mAg-1电流密度下,循环150次以后,放电比容量分别为775.3和1018.7 mAhg-1。3.以自然纤维素物质(滤纸)为模板,首先将二氧化硅凝胶膜包覆在滤纸纳米纤维表面,再用表面凝胶-溶胶法均匀的包覆一定厚度的二氧化钛凝胶层,经后续的煅烧和镁热还原处理,制备了纳米纤维结构的二氧化钛/硅复合材料。该复合材料中,锐钛矿型二氧化钛纳米颗粒层均匀完整的包覆在硅纳米纤维表面。当用作锂离子电池负极材料时,二氧化钛含量为54.3 wt%时,复合材料的电化学性能最好。在200 mA g-1电流密度下,循环200次以后,放电比容量为498.9 mAh g-1。由于复合材料的多孔网状结构以及二氧化钛层的均匀完整的包覆,有利于缓冲硅充放电过程中产生的巨大体积变化,增强电极材料的结构稳定性。因此制备的二氧化钛/硅纳米复合材料表现出增强的循环稳定性和倍率性能。4.以自然纤维素物质(滤纸)为模板和碳支架,结合自组装和低温镁热还原方法,制备了纳米纤维结构的TiOx/carbon/silicon复合材料。该材料在微观上具有独特的多孔网状结构及形貌特征,硅薄层(厚度～50 nm)夹在多孔碳纳米纤维和外面具有氧缺陷的二氧化钛薄层之间,并且二氧化钛层由大小约为5 nm的锐钛矿型二氧化钛纳米颗粒堆积而成,二氧化钛颗粒表面覆盖一层超薄无定形的碳层。将该复合材料应用于锂离子电池负极材料,由于多孔网状的碳支架、外层具有氧缺陷二氧化钛薄层、硅薄层的协同作用,该复合材料表现出较高的比容量和很好的倍率性能。当二氧化钛含量为24.1 wt%时,该复合材料在100mAg-1电流密度下,循环160次以后,放电比容量为792.6mAhg-1。相比没有钛氧化物层包裹的纳米纤维结构的硅/碳复合材料,该纳米复合材料的比容量和循环稳定性得到了很大的提高。5.以天然纤维素物质(滤纸)为模板,制备了纳米纤维结构的二氧化锡/硅复合材料。该复合材料中,二氧化锡纳米颗粒均匀的分散在硅纳米纤维表面。首先以滤纸为模板采用镁热还原法制备了硅纳米纤维材料,然后通过溶胶-凝胶方法在硅纳米纤维表面沉积二氧化锡凝胶层,再在惰性气中经过煅烧得到了二氧化锡/硅纳米复合材料。该复合材料具有原始滤纸模板的多孔网状结构,当用作锂离子电池负极材料时,比单一的二氧化锡纳米颗粒和硅纳米纤维材料表现出更好的电化学性能。当二氧化锡含量为58.8wt%时,该纳米复合材料在100mAg-1电流密度下,150次充放电循环以后,可逆比容量为54S.8mAhg-1。增强的电化学性能主要是因为复合材料的多孔网状结构,二氧化锡纳米颗粒的均匀分散以及二氧化锡和硅之间的协同作用。硅基材料,尤其是具有特殊结构的硅基复合材料,一直是锂离子电池负极材料研究的重点。本论文制备了一系列基于天然纤维素物质的纳米纤维结构的硅基复合材料,并对其电化学性能进行了研究。得益于自然纤维素物质高比表面和多孔网状结构,使得有关活性负极材料在充放电过程中体积变化所导致的材料结构破坏得到了缓解,增强了其机械性能和导电性,达到了更好的电池稳定性、较高的能量密度和较优的循环性能,从而提高了硅基负极材料的电化学性能。本论文工作表明基于自组装的仿生合成方法在设计制备高性能能源相关的电极材料方面具有很大的应用前景。
[Abstract]:In recent years, with the progress of nanotechnology, the performance of lithium ion batteries has been greatly improved, but there are still many bottlenecks, especially in the design and construction of new high performance electrode materials. As the ideal anode material for lithium ion batteries in the future, silicon based materials are far higher than the traditional commercial graphite negative. But how to solve the low conductivity of silicon based negative electrode and the huge volume change in the process of inserting and releasing lithium ion is the key to further improve its electrochemical performance. The combination of structural and functional characteristics provides an effective way for the design and preparation of new functional materials with specific structures and properties. As a common natural polymer compound, natural cellulose material has a unique hierarchical structure from macro to molecular levels and its porous network morphology at the nanoscale level. The main research content of this paper is the preparation of a variety of nanofiber structure silicon based composites by using natural cellulose material (the commonly used quantitative filter paper in the laboratory) as the template and scaffold, and it is better to alleviate the charge and discharge of the silicon negative electrode materials. The material structure damage caused by the volume change in the process has enhanced its mechanical properties and electrical conductivity, improved its electrochemical energy storage performance, and provided some guidance for its application in energy storage devices. The main contents are as follows: 1. using natural cellulose material (laboratory fixed filter paper) as a template and carbon scaffold, simple sols are used. The silica gel layer is assembled on the surface of the filter paper cellulose fiber, and the silica / carbon composites of nanofiber structure are obtained by carbonization. In this composite, the nanometer thickness silica layer is coated evenly on the surface of carbon fiber. The composite has a porous network structure, high specific surface area and carbon. The nano fiber substrate is beneficial to the diffusion of electrolyte and the increase of the conductivity of silicon dioxide, which effectively cushions the huge volume change of silicon dioxide during the charge and discharge process. Therefore, when used as a anode material for lithium ion batteries, it shows higher specific capacity, better cycling stability and multiple performance. The electrochemical properties of the deposited silver nanoparticles on the surface of silica / carbon nanofibers have been further improved by.2. in order to further improve the electrochemical performance of silica / carbon composites. The reduction of silica in the composite materials to silicon by magnesium thermal reduction is used. The composite material of nanofiber structure is obtained. The material has retained the original porous network structure of the filter paper, and the nanoscale silicon layer is coated evenly on the surface of carbon fiber. When used as a lithium ion battery negative material, the silicon content is 25.7wt% and the thickness of the silicon layer is 40nm, the electrochemical performance of the composite is best. The discharge ratio is 150 times after the cycle of 100 mA g-1 density. The capacity can also be kept at 750.6 mAh g-1. by depositing amorphous carbon or silver nanoparticles on the surface of silicon / carbon nanofibers. The electrochemical performance is further improved. Under the current density of 100mAg-1, the discharge specific capacity is 775.3 and 1018.7 mAhg-1.3., respectively, after 150 cycles, first of the natural cellulose material (filter paper) as a template. The silica gel membrane was coated on the surface of the filter paper nanofibers, then the titanium dioxide gel layer with a certain thickness was coated evenly with the surface gel sol method. The nanofiber structure titanium dioxide / silicon composite was prepared by subsequent calcining and magnesium heat reduction. When used as a anode material for lithium ion batteries, when used as a anode material for lithium ion batteries, the electrochemical performance of the composite is the best when the content of titanium dioxide is 54.3 wt%. Under the 200 mA g-1 current density, the discharge specific capacity is 498.9 mAh g-1. due to the porous network structure of the composite and the titanium dioxide layer. A uniform coating is beneficial to the huge volume change produced during the charging and discharging of the buffer silicon, and the structural stability of the electrode material is enhanced. Therefore, the prepared titanium dioxide / silicon nanocomposites exhibit enhanced cyclic stability and multiplying performance.4. with natural cellulose material (filter paper) as a template and carbon scaffold, combined with self-assembly and low temperature. The TiOx/carbon/silicon composite of nanofiber structure has been prepared by the method of magnesium reduction. The material has unique porous network structure and morphologies on the microcosmic. The silicon thin layer (thickness to 50 nm) is sandwiched between the porous carbon nanofibers and the thin layer of oxygen deficient titanium dioxide, and the TiO2 layer is about 5 nm in size. Anatase titanium dioxide nanoparticles are stacked, and the surface of titanium dioxide particles is covered with a layer of ultra-thin amorphous carbon layer. The composite material is applied to the anode material of lithium ion battery. Due to the porous network of carbon scaffold, the outer layer has the thin layer of oxygen defect titanium dioxide and the synergistic action of silicon thin layer, the composite shows a higher ratio. When the content of titanium dioxide is 24.1 wt%, the composite material has no silicon / carbon composites with nanofiber structure wrapped in titanium oxide layer at 100mAg-1 current density and after 160 cycles at the current density, and the specific capacity and cyclic stability of the nanocomposite have been obtained. The two tin / silicon composite material with nanofiber structure was prepared by using the natural cellulose material (filter paper) as a template. In this composite, the two tin nanoparticles were evenly dispersed on the surface of silicon nanofibers. First, the silicon nanofibers were prepared by the magnesium heat reduction method with the filter paper as the template, and then the sol-gel was prepared by the sol-gel process. The gel method deposited two tin oxide gel layer on the surface of silicon nanofibers, and then calcined in the inert gas to get two tin / silicon nanocomposites. The composite has a porous network structure of the original filter paper template. When used as the anode material for lithium ion batteries, it is better than the single two tin oxide nanoparticles and silicon nanofiber material. When the content of two tin oxide is 58.8wt%, the electrochemical performance of the nanocomposite at the current density of 100mAg-1 and the 150 charge discharge cycle, the reversible specific capacity of 54S.8mAhg-1. is mainly due to the porous network structure of the composite, the uniform dispersion of two tin oxide nanoparticles and two oxidation. The synergistic effect between tin and silicon. Silicon-based materials, especially silicon based composites with special structures, have always been the focus of research on anode materials for lithium ion batteries. A series of silicon based composites based on nanofiber structures based on natural cellulose materials have been prepared in this paper, and their electrochemical properties are studied. The high specific surface and porous network structure of cellulose make the destruction of material structure caused by the volume change of the active anode material in the process of charge discharge and discharge, enhance its mechanical properties and electrical conductivity, achieve better battery stability, higher energy density and better cycling performance, thus increasing the negative silicon base. The electrochemical performance of polar materials shows that the bionic synthesis method based on self-assembly has great potential in the design and preparation of high performance energy related electrode materials.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB332;TM912

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