【作者】 王强；

【导师】 张卫新；

【作者基本信息】 合肥工业大学 ， 材料学， 2012， 博士

【摘要】 从1990年起,锂离子电池已经广泛使用于便携式电子设备、电动汽车以及不间断电源等领域中。纳米结构电极材料由于自身具有较小的尺寸可以显著的提升倍率性能,但是它在实际应用过程也存在着副反应和较低的振实密度等问题。微/纳多级结构电极材料作为一种特殊的纳米材料,一方面,其结构基元的纳米尺度有利于缩短电极反应过程中的锂离子的迁移路径,从而增加材料的倍率性能；另一方面,相对于单一纳米结构材料,其相对较小的比表面积和微米尺度聚集体的稳定结构能够缓解或弱化副反应的发生以及避免单一纳米结构材料由于较大比表面能所造成的团聚现象,从而增强材料的循环稳定性。在本论文中,作者分别采用共沉淀法、溶剂热法以及微乳液法制备出具有微/纳多级结构的LiMn2O4微米球、LiFePO4微米花和CuO齿轮状薄膜,并研究了其电化学性能与微结构之间的对应关系。此外,本论文还发展了基于水热法辅助后期改性的合成路线制备出Cu2+掺杂的LiFeP04/C正极材料,并进行性能研究。第一章,主要介绍了锂离子电池的发展简史和工作原理,归纳了各类锂离子电池电极材料的充放电机理、存在的缺陷以及相应的改性方法。此外,还着重介绍了纳米结构锂离子电池电极材料的发展方向。第二章,以蒸发结晶制备出的矩形状NH4FeP2O7·1.5H2O纳米片为前驱体,乙醇为溶剂,采用溶剂热合成方法制备出长为6-8μm,宽为1-2μm和厚约为50nm的矩形纳米片自组装而成的直径约为6-8μm的微/纳多级结构LiFePO4微米花。探讨了反应溶剂对材料形貌所产生的影响,实验结果表明,当采用水和乙醇的混合溶剂时,则制备出的是长约3-4μm,宽约为2μm的LiFePO4多面体微米粒子。LiFePO4微米花和LiFePO4微米多面体分别通过热处理进行碳包覆,包碳之后,它们的形貌都保持不变。与碳包覆的LiFePO4微米多面体相比,碳包覆的LiFePO4微米花显示出更高的放电比容量(0.1C下首次放电比容量为162mAhg-1)、更好的倍率性能(10C下的比容量为101mAh g-1)以及较好的循环性能。LiFePO4微米花电化学性能优异的原因可归因于其具有微/纳多级结构,这种特殊的结构能够在长期的充放电循环中保持材料的结构稳定性。而组成微米花的结构基元矩形纳米片则能有效地改善电化学反应动力学,最终提高其倍率性能。第三章,采用水热法合成纯相LiFePO4纳米粒子,后期通过高温煅烧形成Cu2+掺杂和碳包覆的粒径约为400-500nm的LiFePO4纳米粒子。实验结果表明,经过金属离子掺杂和包碳改性的LiFePO4材料在0.1C、1C、2C、10C和20C倍率下,其放电比容量分别为154mAh g-1,148mAh g-1,143mAh g-1,111mAh g-1和86mA g-1,材料以0.2C的放电倍率循环充放电50次后,容量的保持率可达到97.5%,表明所制备的LiFePO4正极材料具有很好的倍率性能和循环性能,满足动力电池的应用需要。此外,该材料还表现出优异的低温性能,在-30。C下,材料的放电容量还能保持在102mAhg-1(0.1C)。与传统的固相焙烧制备方法比较,该工艺方法具有生产成本低、材料性能好等诸多优点。第四章,通过热处理由共沉淀法获得的碳酸盐前驱物制备出由粒径为30-40nm的纳米粒子组装成的直径为600-900nm的微/纳多级结构LiMn2O4微米球。电化学实验结果显示煅烧温度750℃、煅烧时间9h时得到产物的电化学性能最好,当以0.1C的倍率放电时,产物的放电容量为124.71mAh g-1,而当采用1C高倍率放电,产物仍然具有122.99mAh g-1的放电容量,经50次循环以后,产物的放电容量仍保持在113.98mAh g-1。这种由纳米粒子组装而成的微米球,其结构基元能够缩短在充放电过程中锂离子的扩散路径,从而增加材料的放电容量和倍率性能,而相对于纳米粒子则具有相对较小的比表面积,在充放电过程中有效地缓解由于锰溶解所带来的容量损失。此外,这种LiMn2O4多孔微米球在2M Li2SO4水溶液中也保持着良好的倍率性能,在1Ag-1,1.5A g-1,2A g-11和2.5A g-1的电流密度下,其放电容量均在110mAh g-1左右,当电流密度增至5A g-1,其放电容量还能保持在71mAh g-1。第五章,分别采用水溶液法和微乳液法生长出直径为300-500nm的纳米棒薄膜以及由直径为20nm的纳米丝组装而成的直径约为4-6μm,轴向长度为3-5μm的齿轮状微/纳多级结构CuO薄膜,并将它们与CuO微米粒子的电化学性能进行对比。电化学实验结果表明,这种由极细纳米丝组装而成的CuO齿轮状多级结构薄膜在0.1C的倍率下首次放电容量和充电容量分别为1057mAh g-1和779mAh g-1。相比于第二次放电容量,循环50次后,材料的放电容量基本保持不变。此外,该材料还具有良好的倍率性能,即使在3C放电倍率下,材料的放电容量仍然能够保持在579mAh g-1。相比于其他两种CuO材料的电化学性能,这种CuO齿轮状多级结构薄膜表现出更小的首次不可逆容量、更好的循环性能和倍率性能,其主要原因是组成这种多级结构薄膜的结构基元能够有效地提升材料在充电过程中电化学反应的进行程度,减少锂离子的扩散路径以及很好地容纳材料在充放电过程中的体积变化。而具有相对较小的比表面积的聚集体则能够减小由于形成SEI膜所带来的首次不可逆容量损失。第六章,将微/纳多级结构LiFePO4微米花作为正极材料分别与石墨、Li4Ti5O12以及CuO齿轮状多级结构薄膜三种负极材料进行搭配,组装成全电池,研究它们的相关电化学性能。其中LiFePO4/CuO全电池具有最高的放电容量和最好的循环性能,以1C倍率进行充放电,电池中的LiFePO4微米花材料能够给出140mAh g-1的放电容量,经100次循环以后,产物的放电容量仍保持在134mAh g-1。第七章,总结了研究工作的创新性,对未来可能的研究方向进行展望。更多还原

【Abstract】 Since1990, lithium ion battery has been widely used in portable electronic devices, electric vehicles (EVs) and uninterruptible power system (UPS). Nanostructured electrode materials can remarkably promote charge/discharge rate due to their reduced dimensions, but it could encounter some problems in practical applications, such as side reactions, low tap density, etc. Micro/nano hierarchical electrode materials, as special nanomaterials, can take advantages of both nanomaterials and micromaterials. The ultraflne dimension of its building blocks can effectively shorten the lithium ion diffusion path and promote the rate performance during the charge-discharge process, while the microscale assemblies with relatively smaller specific surface area and better structure stability compared with unitary nanostructured materials could weaken the side reactions and avoid the active materials aggregation, which can enhance the cycling performance. In this dissertation, micro/nano hierarchical LiMn2O4microspheres, LiFePO4microflowers and CuO microcog films were successfully prepared and the electrochemical performances related to their microstructures were investigated. Besides, a novel route based on hydrothermal method was developed for preparing Cu2+doped LiFePO4/C cathode materials.In chapter1, a general introduction was given to the history and working principles of lithium ion batteries, including three important cathode materials (LiCoO2, LiMn2O4and LiFePO4) and four kinds of anode materials (carbon, Li-alloy, Li4Ti5O12and transitional metal oxides). Particularly, the developments of nanostructured electrode materials for lithium ion batteries were reviewed.In chapter2, hierarchical LiFePO4microflowers have been successfully synthesized via a solvothermal reaction in ethanol solvent with the self-prepared ammonium iron phosphate rectangular nanoplates as precursors, which were obtained by a simple water evaporation method beforehand. The hierarchical LiFePO4microflowers are self-assemblies of a number of stacked rectangular nanoplates with length of6-8μm, width of1-2μm and thickness of around50nm. When ethanol was replaced with the water-ethanol mixed solvent in the solvothermal reaction, LiFePO4micro-octahedrons instead of hierarchical microflowers were prepared. Then both of them were modified respectively with carbon coating through a post-heat treatment and their morphologies were retained. As cathode materials for rechargeable lithium ion batteries, the carbon-coated hierarchical LiFePO4microflower sample delivers high initial discharge capacity (162mAh g-1at0.1C), excellent high-rate discharge capability (101mAh·g-1at10C), and cycling stability. It exhibits better electrochemical performances than the carbon-coated LiFePO4micro-octahedron sample. The enhanced electrochemical properties can be attributed to the micro/nano hierarchical structure of the electrode material. It can take advantage of structure stability of micromaterials for long-term cycling. Furthermore, the rectangular nanoplates as the building blocks can improve the electrochemical reaction kinetics and finally promote the rate performance.In chapter3, Cu2+-doped LiFePO4/C nanoparticles with the particle size of400-500nm were successfully prepared by Cu2+doping and carbon coating pure phase LiFePO4nanoparticles from hydrothermal synthesis. Compared with undoped LiFePO4/C sample, the Cu2+-doped LiFePO4/C sample exhibit remarkably-improved electrochemical performance at high charge/discharge rate. It can deliver discharge capacities up to154mAh g-1,148mAh g-1,143mAh g-1,111mAh g-1and86mAh g-1at0.1C,1C,2C,10C and20C. Meanwhile, the Cu2+-doped LiFePO4/C cathode material also gives an excellent cycling performance, with no obvious capacity loss over50cycles. In addition, this Cu2+-doped LiFePO4/C composites present an outstanding low-temperature electrochemical performance, when the environmental temperature decreases to-30℃, it can still deliver a discharge capacity of102mAh g-1at0.1C rate. The electrochemical performance can meet the need of power batteries. In comparison with the solid state reaction method, this preparation process can fabricate LiFePO4cathode materials with higher electrochemical performance at lower cost.In chapter4, micro/nano hierarchical LiMn2O4microspheres have been successfully prepared through calcinating the carbonate precursor from co-precipitation. The results indicate that the samples are pure-phase spinel LiMn2O4and exist in microspheres with the diameter of600-900nm, which are assembled by nanoparticles of about30-40nm. The electrochemical performance of the as-prepared LiMn2O4microspheres has been investigated by galvanostatic charge-discharge test. The results show that the sample prepared at750℃for9h possesses excellent electrochemical performance. Its discharge capacities are124.71mAh g-1at0.1C and122.99mAh g-1at1C, and after50times cycling, the specific discharge capacity is still as high as113.98mAh g-1. Such excellent rate and cycling performance can be attributed on one hand to its micro/nano hierarchical structure with ultrafine nanoparticles as the building blocks, which can shorten the lithium ion diffusion path, and on the other hand to its micro-sized assembly with relatively smaller specific surface area compared to nanoparticles, which can ease the dissolution of Mn3+in the electrolyte. In addition, these micro/nano hiearchical LiMn2O4microspheres present excellent rate performance in2M Li2SO4aqueous solution. When the current density increases to1A g-1,1.5A g-1,2A g-1and2.5A g-1, the specific discharge capacity are all about110mAh g-1. Even at5A g-1, the specific discharge capacity can still retain71mAh g-1.In chapter5, CuO nanorod film with diameter of300-500nm and CuO hierarchical microcog film with diameter of4-6μm which are assembled by ultrafine nanofibers with the diameter of20nm were respectively prepared by aqueous solution method and micro-emulsion method. These two kinds of films and CuO microparticles were characterized by electrochemical tests. The electrochemical performance shows that the first discharge and charge capacity of this hierarchical CuO microcog film are1057mAh g-1and779mAh g-1, respectively. Moreover, it shows excellent rate performance and cycling performance. The discharge capacity of the microcog film hardly decays during the50times cycling process and has a discharge capacity of579mAh g-1at3C rate. Compared with CuO microparticles and CuO nanorod film, this microcog film shows smaller irreversible capacity, better cycling and rate performance. Such better performance can be ascribed to the small dimension of its building blocks which can improve the electrochemical reaction extent during the charge process, shorten the lithium ion diffusion path and accommodate the volume expansion during the charge-discharge process. Meanwhile, the assemblies with relatively smaller specific surface area could confine the excessive SEI formation, thus reduce its initial capacity loss.In chapter6, we assembled full batteries by using the LiFePO4microflowers as the cathode materials to combine with three kinds of anode materials (graphite, Li4Ti5O12and CuO microcog film) respectively. The LiFePO4/CuO full battery presents the best electrochemical performance compared with other kinds of full batteries. The specific discharge capacity is140mAh g-1at1C rate (based on LiFePO4) and after100cycles, the specific discharge capacity is still as high as134mAhg-1.Finally, in chapter7, an overview on the research achievements is summarized and some prospects of the future research are pointed out.更多还原