富锂层状正极材料的多维度合成与局域结构调控研究

发布时间：2018-03-16 09:37

本文选题：锂离子电池　切入点：正极材料　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：富锂锰基固溶体正极材料由于其大于280 mAh g-1的超高比容量以及低成本等优势被认为是最有潜力的下一代锂离子电池正极材料,其超高比容量与特殊的充放电机制引起了科研工作者的浓厚兴趣。富锂材料的高容量与首次充电过程中4.5 V处氧的脱出有关,然而氧的脱出会造成材料的首次不可逆容量损失过大,以及氧空位的形成和局域离子迁移、重排,从而引起循环过程中的结构衍变,导致电压降与容量衰减,同时富锂材料的倍率性能较差,因此阻碍了其产业化道路。本论文以富锂锰基固溶体材料Li_(1.2)Ni_(0.13)Co_(0.13)Mn_(0.54)O_2为对象,以实现富锂材料的高能量密度和高功率密度,提高其比容量、倍率性能和循环结构稳定性为目标,就以下几个方面展开工作:采用高压静电纺丝法制备一维纳米纤维状的富锂材料Li_(1.2)Ni_(0.13)Co_(0.13)Mn_(0.54)O_2,同时采用共沉淀法合成纳米颗粒状材料作为对照,对纳米纤维状富锂材料与颗粒状富锂材料在形貌、比表面积、晶体结构、电化学性能与氧化还原机理之间的关系进行了全面的研究与报道,BET测试结果表明纳米纤维状富锂材料的比表面积为6.537 m2 g-1,大于纳米颗粒状材料(4.388 m2g-1),由于纳米纤维状材料具有更好的锂离子扩散动力学条件,在5 C倍率下能够释放126.6 mAh g-1的容量,大于纳米颗粒状材料的101.1 mAhg'1。此外,创新性地采用分段循环伏安法对首次氧化还原过程进行区别对比考察,利用电化学方法更好的解释了富锂层状正极材料在首次充放电过程中的O脱出与Mn活化的氧化还原机理。在静电纺丝法制备纳米纤维状富锂材料Li_(1.2)Ni_(0.13)Co_(0.13)Mn_(0.54)O_2的过程中,在对前驱体采取不同的热处条件时,其终产物会表现出不同的形貌,得出升温速度影响终产物的微观形貌这一结论,即当以1℃ min-1的速度升温时终产物表现出以为一维纳米纤维形貌,当升温速度为5 ℃ min-1时终产物会表现出花瓣状纳米片的微观形貌。样品在800℃锻烧8 h后表现出良好的倍率性能,这归因于其具有良好的动力学条件,5 C的放电容量达到127 mAh g-1。纳米线状与花瓣状纳米片材料具备纳米材料特有的尺寸效应小的特点,能缩短锂离子的迁移时间,另外,材料较大的比表面积能够使电极与电解液充分接触,减小电极在大电流密度下充放电时的极化现象。对富锂正极材料进行酸浸出处理会使材料发生锂离子与氧离子的化学脱出,即Li2O从晶格中脱出,这种方法可以有效降低富锂正极材料的首次不可逆容量损失,然而高浓度的酸浸出处理会对材料表面晶体结构造成较大破坏从而导致电极的循环性能恶化。本论文首次报道了采用稀释后的低浓度的酸溶液(0.01moL-lHNO3溶液)对富锂材料Li_(1.2)Ni_(0.13)Co_(0.13)Mn_(0.54)O_2进行温和的浸出处理,在将材料的首次库伦效率从82.4%提高到89.7%的同时,可以避免对材料表面晶格结构的破坏。为了探究稀酸处理对材料首次库伦效率提升的作用机理,创新性地设计并实施了工作电极在不同气氛下(O2或Ar)的原位循环伏安法测试,利用电化学手段第一次发现富锂材料中的氧可以被可逆还原,经过酸处理后的材料在充电时脱出的氧的还原催化活性得到了提高。本论文首次采用一种低温固相—电化学循环法对富锂材料Li_(1.2)Ni_(0.13)Co_(0.13)Mn_(0.54)O_2进行表面修饰,修饰物质在电化学循环过程中逐渐形成一层完整连续的保护层,同时聚阴离子BF4-逐渐介入到材料表面晶体结构中,抑制富锂材料表面处氧空位的形成与过渡金属离子的迁移,从而达到抑制晶体结构在循环过程中的衍变、抑制电化学循环中的电压降的目的。经过处理后的材料,放电比容量达到308.7mAhg-1,5C倍率下放电比容量为140 mAh g-1。经过55次循环后,处理后的材料电压降为65 mV,远小于原始材料的202mV,55次循环后容量为286.9mAhg-1。HRTEM测试说明表面处理能够有效阻止材料表面处晶格缺陷的形成。XPS测试表明,在首次放电至2.0 V时,经过处理的材料负二价氧离子明显增加,负一价氧离子显著减少,说明表面处理可以有效提高氧的可逆还原。
[Abstract]:Lithium rich manganese based solid solution cathode material due to its high specific capacity is more than 280 mAh g-1 and low cost advantages are considered to be the most promising cathode materials for the next generation of lithium ion battery, its high specific capacity and special charge discharge system has aroused great interest of researchers at 4.5 V high oxygen. The capacity of lithium rich materials and for the first time in the process of charging off, however, oxygen will cause the material from the initial irreversible capacity loss is too large, and the formation of oxygen vacancies and local ion migration, rearrangement, resulting in structural evolution during the cycle, resulting in voltage drop and capacity attenuation, poor rate capability and rich lithium the material, which has prevented the road of industrialization. The lithium rich manganese based solid solution materials Li_ (1.2) Ni_ (0.13) Co_ (0.13) Mn_ (0.54) O_2 as the object, in order to achieve the high energy density of lithium rich materials and high power density, improve The specific capacity and rate performance and cycle structure stability as the goal, to start work on the following aspects: the preparation of one-dimensional nanofibers by electrospinning method of lithium rich materials Li_ (1.2) Ni_ (0.13) Co_ (0.13) Mn_ (0.54) O_2 as control, co precipitation synthesis of nano granular materials used at the same time, the nano fibrous and granular material rich lithium lithium rich materials in morphology, surface area, crystal structure, electrochemical properties and redox mechanism between the studied and reported comprehensive, BET test results show that the lithium rich materials nano fibrous surface area of 6.537 M2 g-1, more than 4.388 (nano granular materials m2g-1), due to lithium ion nano fibrous material has better diffusion kinetic conditions, can release 126.6 mAh g-1 capacity at a rate of 5 C, more than 101.1 mAhg'1. nano granular material in addition, innovative. The difference of comparative study on the first redox process by using cyclic voltammetry, electrochemical method to better explain the oxidation Li rich layered cathode materials in the first charge discharge in the process of removal of the O and Mn activation reduction mechanism. The preparation of nano fibrous materials in lithium rich Li_ electrospinning (1.2) Ni_ (0.13) Co_ (0.13) Mn_ (0.54) O_2 in the process of taking heat at different conditions of the precursor, the final product will exhibit different morphology, the conclusion that the morphology effect of temperature rising speed of the end product, namely when 1 DEG min-1 speed of heating end products showed that one morphology, when the heating rate is 5 DEG min-1 when the final product will exhibit the morphology of petal shaped nano sheet. The sample at 800 C for 8 h after calcination showed good rate capability, which is attributed to its favorable dynamic conditions, the discharge of 5 C Capacity of 127 mAh g-1. nanowires and petal nanosheets possess the characteristics of unique small size effect of nanometer materials, can shorten the migration time, lithium ions in the material larger than the surface area of the electrode and the electrolyte can fully contact electrode polarization at high current density discharge to the rich. Lithium acid leaching treatment will make the material chemistry hernia lithium ion and oxygen ion, the removal of the Li2O from the crystal lattice, this method can effectively reduce the rich lithium cathode material the first irreversible capacity loss, but high concentration of acid leaching treatment will cause greater damage resulting in a deterioration of the cycle performance of electrode of crystal materials the surface structure. This is the first report of the acid solution of low concentration after dilution (0.01moL-lHNO3 solution) of lithium rich materials Li_ (1.2) Ni_ (0.13) Co_ (0.13) Mn_ (0.54) O_2 For leaching mild, the material for the first time in Kulun, the efficiency is increased from 82.4% to 89.7% at the same time, can avoid the damage on the surface of the material crystal structure. In order to explore the mechanism of dilute acid treatment on the material for the first time in Kulun to promote efficiency, innovative design and implementation of the working electrode in different atmospheres (O2 or Ar) the in situ cyclic voltammetry using electrochemical method first discovered lithium rich materials in oxygen can be reversible reduction, by reducing the catalytic activity from acid treated materials in charge of oxygen is improved. This paper firstly adopts a low temperature solid state electrochemical cycling method of lithium rich materials Li_ (1.2) Ni_ (0.13) Co_ (0.13) Mn_ (0.54) O_2 surface modification, modification of material gradually formed a layer of continuous protective layer on the electrochemical cycle, at the same time the polyanion BF4- gradually involved in crystal material surface In the structure, inhibit the formation of surface migration rich lithium oxygen vacancies and transition metal ions, thus inhibiting crystal structure evolution during the cycle, suppress the voltage drop in the electrochemical cycle. After processing the material discharge capacity reached 308.7mAhg-1,5C rate discharge capacity of 140 mAh after 55 g-1. cycles, material processing after the voltage is reduced to 65 mV, far less than the original material 202mV, after 55 cycles the capacity of 286.9mAhg-1.HRTEM test shows that surface treatment can effectively prevent the formation of.XPS test showed that the material surface defects, the first discharge at 2 V after treatment, the material price of two negative oxygen ions significantly increase of negative valence oxygen ion was significantly reduced, the surface treatment can effectively improve the reversible oxygen reduction.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM912

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