【作者】 赵娜红；

【导师】 吴宇平；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 博士

【摘要】 近年来,随着电子和信息产业的迅速发展,特别是便携式电子设备如笔记本电脑、移动电话、数码相机、数码摄像机等,以及航空航天和军用电子设备、混合电动汽车（HEV）的发展,对于其移动电源的能量密度,循环性能和可靠性等提出了更高的要求。为了满足世界范围内对能源转化和储备日益增长的需求,目前大量的研究工作直接与寻找新的材料概念以及多样化的合成方法相联系。在电极材料方面的突破是下一代锂离子电池成功开发的关键正逐步被广泛接受。纳米材料,纳米复合材料作为嵌锂材料,由于其特殊的纳米微观结构和形貌,可望更加有效的提高材料的可逆嵌锂容量和循环寿命。纳米材料具有大的比表面积和孔体积,锂离子嵌脱深度小,因而在大电流的充放电下表现出极化程度小、可逆性能高、循环稳定性好等优点。本论文主要采用湿化学方法或电沉积方法和氧化铝模板相结合的方式,合成各种一维纳米材料,包括碳纳米管、碳纳米管包覆的单晶和多晶SnO₂一维纳米阵列、碳纳米管包覆的多晶LiFePO₄纳米阵列、金属锡纳米管,以及TiO₂多晶纳米管。湿化学方法包括柠檬酸溶液法和以柠檬酸为络合剂的溶胶凝胶法。其中,溶胶凝胶法可以通过调节pH值、水量的大小以及温度等因素改变溶胶前驱体的交联度,提高阳极氧化铝模板的填充度。课题采用各种测试手段对合成材料进行物化表征,和电化学性能表证,研究其在锂离子电池中的应用。内容包括以下几个部分:首先,论文在第三章采用二次阳极氧化方法,在草酸电解液中制备出具有纳米级孔洞的高度有序的多孔阳极氧化铝膜。实验结果显示,氧化铝多孔膜的纳米孔纵横比大,孔道垂直有序、尺寸均一,孔密度可达10¹⁰/cm²,而且孔径大小可通过改变温度、后续扩孔处理等方式,在一定范围内进行调节。电化学测试结果显示,氧化铝模板是一种惰性基体,它对纳米电极阵列的电化学容量不产生较大影响。因此,氧化铝模板既可以在后续处理中加以去除,也可以作为惰性分散体,为微型电极阵列的基础和应用研究提供微型反应场所。碳纳米管（CNT）可以和金属一样有效地沿着其狭长的轴线导热和导电,同时,由于碳纳米管在管壁之间和管腔之中存在大量空间,因此它作为储氢材料、锂离子电池负极材料以及超级电容器材料得到了广泛的研究。论文第四章采用柠檬酸溶液-氧化铝模板法成功制备出了CNT材料和CNT/Al₂O₃复合材料。物化表征结果显示,制备的CNTs形貌均一,管壁厚度及长径比可调,而且采用这种方法制备碳纳米管产率高,重现性好,有望运用于场电子发射器材或储能器材领域。进一步的表征结果显示,氧化铝模板在450℃-600℃之间对碳纳米管的石墨化具有催化效果;而在热处理温度高于600℃时,两相界面对氧化铝基体的晶化更具催化效果,促使氧化铝在低于热力学的晶化温度即开始结晶,生成四方相的Al₂O₃晶体。电化学测试结果表明,CNT/Al₂O₃复合材料相比较CNT材料,具有更高的嵌脱锂容量,电化学循环稳定性较高,倍率放电性能优良,是一种较好的储锂材料。但是这种材料的电导性有待进一步提高。论文第五章首先研究了SnO₂粉体的电化学嵌脱锂性能,并进一步采用溶胶凝胶.氧化铝模板法成功制备出了碳纳米管包覆的SnO₂单晶纳米阵列和多晶纳米阵列,该合成方法对于合成其他碳纳米管包覆的单晶或多晶氧化物纳米阵列具有一定指导意义。研究结果证实,由于晶体SnO₂材料具有较小的电荷传递阻抗和较快的锂离子扩散速率,因而晶体SnO₂的电化学嵌脱锂容量和首次库仑效率高于无定形SnO₂;而另一方面,由于无定形SnO₂的物质形态有助于首次电化学还原反应生成弥散的金属锡,有效防止金属锡的团簇,因而具有较好的循环稳定性。通过对SnO₂一维合成材料的研究发现,SnO₂单晶纳米线阵列的填充度较好,纳米线的长度在1μm左右;进一步研究表明,单个纳米线具有不同的晶体生长方向,因而纳米线阵列不显示明显的晶体择优取向。通过降低溶胶前驱的pH值和增大溶胶的水量,可以成功合成出长度达数十微米的SnO₂多晶纳米线。电化学测试结果表明,CNT包覆的SnO₂一维纳米阵列综合两者优势,具有较高的锂离子扩散系数和径向电导,获得了较高的电化学嵌脱锂容量。另一方面,由于纳米阵列有限的自由空间和纳米线本身的尺寸效应,限制了SnO₂活性材料的体积膨胀,加上一维纳米材料充放电过程的一致性,都使得复合材料具有较好的循环稳定性。第六章首先采用恒流电沉积技术制备了金属锡薄膜,研究不同电流密度下电沉积锡薄膜的形貌、结构及其电化学嵌脱锂性能。并结合阳极氧化铝模板,采用恒压电沉积技术,在氧化铝模板的纳米孔洞中进行电化学沉积,制备金属锡的一维纳米结构材料。研究结果证实,恒电流沉积的电流密度越小,生成的锡膜越趋向于热力学稳定结构,即电极结构和晶界结构越致密,与基体结合越紧密,因而电化学循环性能较好,但是容量较低。分析认为,低电流密度下沉积得到的致密的电极结构和晶界结构,使得材料在嵌脱锂过程中能够保持结构的稳定,但是这种致密结构的较慢的嵌脱锂动力学过程,也限制了材料的电容量。相反,较高电流密度下获得的沉积膜为动力学稳定结构,电极结构和晶界结构较疏松,因而材料具有较高嵌脱锂容量,但是嵌脱锂过程中剧烈的体积膨胀,又使得材料粉化、脱落现象严重,循环性能变差。改性热处理可以在一定程度上结合两者的优势。在氧化铝模板中电沉积得到的金属锡的产物为管状的一维纳米材料,具有一定首创性。管状结构的生成与氧化铝模板及溅射的金膜的孔洞结构有关;另一方面,由于电镀液中的柠檬酸络合剂易吸附在孔道内壁,一定程度上也促进了管状电沉积产物的生成。相比较金属锡薄膜,锡纳米管的循环稳定性有一定程度的提高。以0.5mA/cm²沉积的锡膜为例,首次可逆容量为497mAh/g,循环至20周,容量衰减为88mAh/g;金属锡纳米管首次脱锂容量达到423mAh/g,循环20周后,容量保持在223mAh/g左右。这说明,纳米结构对电化学循环稳定性有一定改善作用,但是,由于合金化过程中固有的结构性膨胀没有得到根本性解决,因此,锡纳米管的容量衰减还比较大,循环稳定性还需进一步改进。本论文第七章采用超声、浸泡等不同手段,初步探索了溶胶凝胶-氧化铝模板法制备TiO₂纳米管的合成方法,并对合成的纳米管进行物化和电化表征。由于TiO₂和Al₂O₃的等电点在5～6之间和6～8之间,在酸度较大的溶液中,两种氧化物的表面均带有正电,具有静电排斥作用,因而TiO₂在氧化铝模板孔洞中的填充难度较大。实验证实,采用超声和浸泡等手段能够合成出具有一维纳米形貌的材料,但是填充率不高。物化表征显示,合成材料均为锐钛矿型TiO₂多晶纳米管。电化学测试显示,相对本体材料,TiO₂多晶纳米管的循环容量、循环稳定性及嵌脱锂动力学过程均有所促进。最后,论文第七章采用溶胶凝胶-氧化铝模板法初步合成出了碳纳米管包覆的磷酸亚铁锂多晶纳米线。进一步证实了溶胶凝胶-氧化铝模板法合成碳纳米管包覆的半导体一维纳米材料的有效性。更多还原

【Abstract】 The protable batteries with higher energy density,better cycliability and reliability are most urged recently by the rapid development of the electronic and communication devices,such as notebook,mobile telephone,camera,aerial and space equipments,as well as Hybrid-Electric Vehicel（HEV）.More and more researchers focus on the discoveries of new material concept and the synthesis diversification to meet the increasingly demand for energy conversion and storage.Scientists have realized that breakthrough in the electrode materials is the key point for the next generation of the lithium ion batteries.Nano-materials and nano-composites are expectable for the higher capacity and better cycliability due to their special microstructure and micro morphology,which have higher specific surface area and bigger pore spaces.They can exhibit little polarization,better reversibility and cyliability when charged and discharged under large current density due to the short length of lithium ion diffusion.In this dissertaion,several one-dimensional nano-materials were synthesized by using the methods of combining the wet chemistry or electro-deposition with anodic aluminum oxide（AAO） template.These one-dimensional nano-materials include carbon nano-tubes（CNTs）,CNT-coated single-crystal and polycrystalline SnO₂ nanoarrays,CNT-coated polycrystalline LiFePO₄ nanoarrays,tin nanotubes and polycrystalline TiO₂ nanotubes.The wet chemistry methods primarily include the citric acid solution method and the sol-gel method which uses the citric acid as chelating agent.The pH value,water volume and temperature of the sol could affect the chelation degree and then enhance the loading rate of the AAO template.These one-dimensional nano-materials were physically characterized and electrochemically tested as electrode materials for lithium ion batteries.At first（3rd chapter）,nanometer-scale alumina template with highly ordered and closely packed hexagonal pore structure was prepared by two-step anodizing process in oxalic acid solution.The nanometer-scale pore holes have high aspect ratio with a pore density of 10¹⁰/cm²,and the pore size can be adjusted by tunning the temperature of the electrolytic cell and the following immersion in phosphoric acid.The electrochemical tests show that AAO template is an electrochemically inactive substrate,which has the least influence on the electrochemical capacities of the nanoarray materials.As a result,AAO template can not only be removed in the follow-up process but also be an inactive substrate to supply the absolute microtubes for the experiments of the microelectrode arrays.CNT has electronic and thermal conductivity in the axis direction like metal,and the large space in and between the nanotubes supplies the potential application in the fields of hydrogen storage,lithium ion storage and super capacitor.In the fourth chapter,large scale and uniform CNTs and CNT/Al₂O₃ composite were successfully synthesized by using citric acid solution-AAO template method.The wall thickness of the nanotube and the aspect ratio can be adjusted.These CNT arrays have potential application in Field Emission Display（FED） devices.Further investigation shows that the nanopores of AAO have catalyze the graphization of the CNTs during 450℃-600℃,while the interfaces prefer to catalyze the crystallization of the alumina substrate for the tetrahedral Al₂O₃ above 600℃,which is lower than the thermal crystallization temperature of the AAO.The electrochemical tests show that CNT/Al₂O₃ composite exhibits higher lithium storage capacity compared with CNT material,with excellent cycling property and relatively high rate capacities.However, the electronic conductivity of the composite should be improved.In the fifth chapter,the lithiation and delithiation properties of two kinds of SnO₂ powders were investigated at first.Then sol gel-AAO template method was applied to synthesize CNT-coated single-crystal and polycrystalline SnO₂ nanoarrays.This synthesis process is instructive for preparation of the other CNT-coated single-crystal or polycrystalline oxides nano-arrays.According to the results,the crystal SnO₂ exhibits higher capacity and higher coulombic efficiency in the 1st cycle than the amorphous SnO₂ due to the less charge transferring impedance and the faster lithium ion diffusion.In the other hand,the amorphous SnO₂ exhibits better cycliability than crystal SnO₂ because the amorphous structure conduces to forming the highly dispersed tin atoms during the first reduction process.The tin atoms aggregation is avoided,and then good cycliability is obtained.According to the one-dimensional SnO₂ materials,the loading rate of the single-crystal SnO₂ nanoarrays is high,with a length of 1μm or so.Further investigations show that the single-crystal nanowires have random crystallization directions.As a result,the XRD pattern of the nanoarrays shows no strongly preferential orientation.Furthermore,the CNT-coated polycrystalline SnO₂ nanoarrays were successfully synthesized by decreasing the pH value and increasing the water volume of the sole precursor,of which the nanowire length is about several micrometers.Electrochemical tests show that CNT-coated SnO₂ nanoarrays obtained relatively high capacities and greatly improved cycliability due to the combined superiority of both the fast lithium ion diffusion and the good axial conductivity.It is thought that the limited free space in the nanoarrays and the dimensional confinement of the nanowires helped a lot to avoid large volume expansion when lithiation and delithiation processes performed,which promoted the cycling performance.In the sixth chapter,several tin films were electrodeposited with different current densities using constant current technique.The morphology,structure and the electrochemical properties of these tin films were investigated.Then we used the AAO template and constant voltage technique to electrodeposit the one-dimensional tin material.Thermaldynamically steady tin films were obtained at low depositing current,which have compacter electrode structure and firmly packed crystalline interface.The tin film and copper substrate also conected to each other firmly.As a result,these tin films exhibited the excellent cycliability while the capacity is low.It is considered that the compact electrode structure and the firmly packed crystalline interface help a lot to the integrity of the electrode during lithiation and delithiation processes,which greatly improved the cycliability,although the capacity was limited by the low lithiation and delithiation processes due to the compact electrode structure. Inversely,the higher capacities and worse cycliability are got for the tin films electrodeposited at high current densities,due to the less compact electrode structures and crystalline structures.According to the one-dimensional tin deposition,it is tubelike shape.The formation of the tin nanotubes is related to the porous structure of the AAO template and the sputtered porous Au film as well.Besides,the inclination of the adsorption of the citric acid chelating agent to the alumina pore wall is accountable.Compared to the tin films,tin nanotube array exhibits further improved cyliability.For example,tin film deposited at 0.5mA/cm² exhibited the delithiation capacities of 497 mAh/g and 88 mAh/g in the 1st and 20th cycle,while tin nanotube arrays exhibited the delithiation capacities of 423 mAh/g and 223 mAh/g in the 1st and 20th cycle,which has the better capacity retention ability.As a result,the nanotube morphology improved the electrochemical cyliability due to the dimensional confinement of nanosize effect to a certain degree although the essential structural changes during the alloying process are still unresolved.The cycliability of the tin nanotube arrays still needs further improvement.In the seventh chapter,ultrasonic and long-time immersion measurement were used to initiate the primary synthesis of TiO₂ nanotubes through sol-gel-AAO route,then the physical chemistry and the electrochemical properties of the TiO₂ nanotubes were characterized.The isoelectric points of TiO₂（5～6） and Al₂O₃（6～8） are close to each other.That means the surfaces of these two oxides are both positively charged in the high acidic solution,and they will be repulsed by each other.Accordingly,the loading of the TiO₂ into the AAO pore holes will be relatively difficult.Ultrasonic and long-time immersion measurements are effective methods to load TiO₂ sol into the AAO pore holes although the loading rate is low.The end products are polycrystalline TiO₂ nanotubes with anatase phase,which exhibited the promoted electrochemical cycling property and the promoted lithiation dynamics campared to the TiO₂ powder.In the last,we primarily synthesized CNT-coated polycrystalline LiFePO₄ nanowire arrays,which further proved the validity of the sol-gel-AAO method for the preparation of CNT-coated one-dimensional semiconductor materials.更多还原