【作者】 周友元；

【导师】 李新海；

【作者基本信息】 中南大学 ， 冶金物理化学， 2008， 博士

【摘要】 锂离子电池以其工作电压高、能量密度大、循环寿命长、自放电率低、“绿色”环保等众多优点而备受人们关注。本文详细分析了锂离子电池及其炭负极材料的研究进展,提出进一步提高性能和降低成本是锂离子电池发展和改进的主要方向。在锂离子电池技术中,炭负极材料作为关键技术,由于研究时间短、种类繁多、性能各异、反应机理不明晰而增大了研究的复杂性,同时也给人们留下了很大的提高性能的空间。本文创新性地在石墨中添加钠盐及采用动态熔融炭化法在石墨及SnO₂-石墨复合材料表面包覆无定形炭并获得了很好的效果。本研究主要的研究内容如下:采用XRD、SEM、ICP、激光粒径分析及电化学性能测试等方法,对国内外多种典型石墨样品的结构与性能进行比较,研究了石墨材料的来源、结构、杂质含量、颗粒大小、比表面积等因素对其充放电性能的影响,确定了性能较好、价格低廉、来源广泛的普通天然石墨及天然石墨球作为掺杂改性以及复合结构炭材料研究的原料。在石墨中添加钠盐,通过钠盐的掺杂改性,普通天然石墨的可逆容量达到了364.8mAh·g^-1。在普通天然石墨中掺入不同的钠盐,并分别以NaCl、NaF、Na₂CO₃的形式存在于石墨材料中,石墨结构没有改变,但电极性能得以提高。经钠盐处理后,天然石墨得到了很好的修饰改性,Na⁺与阴离子共同作用,形成了很好的固体电解质膜,从而减小了锂离子电池在界面的极化,离子扩散系数增大了一个数量级。掺杂1%NaCl的石墨可逆容量为364.8mAh·g^-1,不可逆容量为47.4mAh·g^-1,首次充放电效率为88.5%,30个周期后容量保持率为91.97%。以掺杂改性石墨为负极的锂离子扣式电池（LiCO₂/C）的放电容量增加,放电中值电压升高,循环性能得到了明显提高。以沥青为前驱体,在650℃～950℃温度下热解得到无定形热解炭材料。随着热处理温度的升高和恒温时间的延长,所得炭材料的有序化程度增加,可逆嵌锂容量与不可逆容量均减小,首次充放电效率增大,电压滞后现象得到抑制。首创性地采用动态熔融炭化法对石墨进行无定形炭的包覆。采用固相混合-熔融炭化法或液相混合-熔融炭化法在天然石墨表面包覆无定形的沥青炭,并在一定温度下进行热处理制备了具有无定形炭/天然石墨结构的复合炭材料。XRD、SEM及粒径分析结果表明了石墨表面有无定形炭层的存在,随着包覆量的增加,复合炭材料的颗粒粒径增大。5%沥青经固相混合-熔融炭化法或液相混合-熔融炭化法在400℃动态炭化3h,再经850℃热处理2h的复合炭材料可逆容量分别为358.2 mAh·g^-1、362.0mAh·g^-1,首次充放电效率分别为89.3%、92.0%,30个周期后容量保持率分别为95.20%、96.55%。以不同方法包覆沥青炭的复合炭材料作负极的锂离子扣式电池（LiCO₂/C）性能测试结果表明,包覆适量的沥青所得的复合炭材料的充放电性能及循环性能明显得到提高。以煤焦油及天然石墨球为原料,经聚合、分离、炭化后得到中间相炭包覆石墨的新型复合炭材料。详细研究了天然石墨球的加入量及热处理温度、时间对复合炭材料的结构、比表面积、电化学性能的影响。天然石墨球添加量为70g/200mL时,充放电性能最佳,700℃2h处理的复合炭材料可逆容量为378mAh·g^-1,首次充放电效率为91.3%,同时复合炭材料的循环性能得到了充分改善,50个周期容量保持率96.0%。以复合炭材料作负极的锂离子扣式电池（LiCO₂/C）性能测试结果表明,中间相炭包覆石墨所得的复合炭材料的充放电性能及循环性能与天然石墨球相比明显得到提高。采用均相沉淀法制备了SnO₂-石墨复合材料,并首次采用液相-动态熔融炭化法在SnO₂-石墨复合材料表面进行沥青炭的包覆。研究了复合材料的结构、表面形貌和电化学性能。结果表明了SnO₂-石墨复合材料表面有无定形炭层的存在,对复合材料的电化学性能进行了研究,沥青量为10%经650℃2h热处理的C-SnO₂-石墨复合材料（SnO₂量为16%）首次可逆容量为485.4mAh·g^-1,不可逆容量为93.2mAh·g^-1,首次充放电效率为83.89%,30个周期容量保持率为85.49%。对C-SnO₂-石墨复合材料的交流阻抗数据使用Zsimpwin阻抗模拟软件进行模拟并提出等效电路图,无定形炭包覆后明显减小了SEI膜电阻,而且电化学反应电阻也是随着包覆量的增大而减小。同时对C-SnO₂-石墨复合材料的储锂机理进行了探讨。认为,无定形炭及石墨在复合材料中起到了骨架支撑的作用,在充放电过程中抑制了SnO₂的体积变化及锡颗粒的团聚,从而提高了复合材料的循环性能。采用线性极化及恒电位阶跃的方法研究了各种炭材料的嵌锂动力学行为。结果表明,随着嵌锂量的增加,炭电极的交换电流密度及锂在炭材料中的扩散系数增大:本实验研究的几类材料中,SnO₂的交换电流密度最大（53.22mA/g）,但扩散系数最小(9.48×10^-9cm²/s),天然石墨的交换电流密度（20.70 mA/g）及扩散系数均较小(2.38×10^-8cm²·s^-1),与天然石墨比较,改性石墨及复合炭材料的交换电流密度与锂的扩散系数增加了一个数量级,其中锂在沥青包覆的复合炭材料中具有最大的扩散系数（1.45×10⁷cm²/s）。将天然石墨、改性石墨、复合炭材料在不同倍率电流下放电,随着放电电流倍率的增加,各种炭材料的放电容量减小,其中以沥青炭-石墨的高倍率放电性能最好,其1C放电容量达到0.1C放电容量的99.14%。更多还原

【Abstract】 Advanced rechargeable lithium ion batteries are attractive for use in consumer electronic and electric vehicle（EV）application because of a favorable combination of voltage,energy density,cycling performance, self-discharge and environmental protection.The development of lithium ion batteries and carbon anode materials are reviewed in details,much attention is attached to the further improvement on performances of lithium ion batteries and reduction in the cost.Carbon anode materials are the key in the lithium ion batteries technologies.The short development, lots of species,different properties and indefinite reaction mechanism leave great promotion probability in performance.By creatively adding sodium salts and adopting dynamic melt-carbonization,the graphite and SnO_2^-graphite composites are covered amorphous carbon to gain a better performance.The main contents in this research as follows:The structures and characteristics of several graphite samples are measured by means of power X-ray diffraction（XRD）,scanning electron microscope（SEM）,Brunauer-Emmer-Teller（BET）surface area measurement,inductively coupled plasma（ICP）spectroscopy,particle size analysis and electrochemical measurements.The effects of origin, structures,impurity,particle size,specific surface area of carbon materials on the electrochemical characteristics are studied.The common natural graphite and the spherical natural graphite with abundant resources,low cost and favorable performance are determined as the raw materials for modification of graphite.The reversible capacity of natural graphite reaches 364.8mAh.g^-1by sodium salts modification.Three kinds of sodium salts are doped into graphite and exist in the form of NaCl,NaF,Na₂CO₃ respectively.The structure of the graphite is not changed and the electrochemical characteristics of graphite anode doped with sodium salts are enhanced. The performances of natural graphite are improved by the modification of sodium salts.It is believed that the improvement is attributed to the merged effects of the sodium/negative ions on the SEI（Solid Electrolyte Inter-face）.The polarization of lithium diffusion at the interface is reduced and an order of magnitude of diffusion coefficient is gained.The sample modified with 1%NaCl had the best electrochemical performances with a reversible capacity of 364.8mAh·g^-1,an irreversible capacity of 47.4mAh·g^-1,and an initial coulombic efficiency of 88.5%.The cycling stability of the Li/C cell with modified graphite as anode was improved.The capacity retention ratio at the 30th cycle was up to 91.97%.Compared with the button lithium ion batteries using untreated graphite as anode material,those using modified graphite as anode materials have larger capacity,higher discharge voltage and better cycling capability.For the first time,the dynamic melt-carbonization is adopted to cover amorphous carbon on graphite.Disordered carbon materials are obtained by pyrolysis of pitch at high temperature.As the temperature of pyrolysis and the soak time increasing,the carbon materials become more stacked,the specific surface area reduces,both reversible capacity and irreversible capacity decrease,the initial coulombic efficiency increases, and the hysteresis in the voltage profile between charge and discharge is cut down.The graphite is covered with a thin film of disordered carbon according to the measurements of XRD,SEM and particle size analysis, pitch is used as the precursor of the shell-carbon materials by dynamic melt-carbonization,also the composite carbon materials are heat-treated at high temperature.Structure and performance of the composite structure carbon are studied in detail.The samples of solid-mixed and liquid-mixed for 5%pitch by dynamic melt-carbonization at 400℃for 3 h and heat-treated at 850℃for 2h have the best electrochemical performances with a reversible capacity of 358.2mAh·g^-1,362.0mAh·g^-1,and an initial coulombic efficiency of 89.3%,92.0%.The capacity retention ratios at the 30th cycle are up to 95.20%and 96.55%.Electrochemical measurements of button lithium ion batteries show that composite carbon coated with appropriate amount of pitch have improved charge/discharge and cycling performance.A mesocarbon-graphite composite is prepared by depositing mesocarbon from the coal tar pitch on the surface of global natural graphite though polymerization,separation,carbonization.The effects of the weight of spherical natural graphite,the temperature and time of heat-treatment on the structure,specific surface area and electrochemistry properties of composite carbon are studied in detail.The sample heat-treated at 700℃for 2h has the best electrochemical performances with a reversible capacity of 378mAh·g^-1and an initial coulombic efficiency of 91.3%.The cycling stability of the Li/C cells with carbon composite as anodes is improved,its capacity retention ratio at the 50th cycle is 96%.Electrochemical measurements of button lithium ion batteries show that composite carbon coated with appropriate amount of mesocarbon have improved charge/discharge and cycling performance more than spherical natural graphite.SnO₂-graphite composites are prepared by homogeneous precipitation method and for the first time it is modified by dynamic melt-carbonization of the pitch.The structure,morphology and electrochemical properties are studied.The results suggest that SnO₂ is uniformly distributed on the surface of graphite,and the SnO₂-graphite is covered with a thin film of disordered carbon.The composites of C-SnO₂-graphite（16%SnO₂）heat-treated at 650℃for 2h and modified by pitch（10%）have a reversible capacity of 465.4mAh·g^-1,an irreversible capacity of 103.2mAh·g^-1,and an initial coulombic efficiency of 81.85%.A mechanism of Li storage in C-SnO₂-graphite composite was proposed.Since the carbon and graphite acts as the framwork of the composite,restraining the volume change of SnO₂ and the reunition of the Sn particles in charge/discharge,the cycling performance is improved obviously.The kinetics behaviors of carbon anodes are studied by means of linear sweep voltammetry and chronoamperometry measurements.It is found that the exchange current（i₀）and diffusion coefficient of lithium (D_Li)increase with Li intercalation into carbon anodes,SnO₂ has larger i₀ （53.22mA/g）and smaller D_Li(9.48×10^-9cm²/s)than other materials.The natural graphite has smaller i₀（20.70 mA/g）and D_Li(2.38×10^-8cm²·s^-1). The i₀ and D_Lifor modified graphite and composite carbon materials improve an order of magnitude compared with natural graphite with the best value reaching 1.45×10^-7cm²/s for pitch covered carbon composites. The discharge capacities of carbon electrodes decrease with the increase of current rate,the composite carbon of modified by pitch has the best fast discharge capability among the untreated graphite,modified graphite by sodium salts,other composite carbon materials,its discharge capacity at 1C is up to 99.14%of that at 0.1C.更多还原