论文标题:锂离子电池的研制及凝胶聚合物电解质的研究
Preparation of Lithium Ion Battery and Research on Gel-type Polymer Electrolytes
论文作者
论文导师 吴浩青；吴宇平,论文学位 博士,论文专业 物理化学
论文单位 复旦大学,点击次数 300,论文页数 135页File Size11414K
2007-04-15论文网 http://www.lw23.com/lunwen_38229797/
Lithium ion batteries; cathode; anode; GPEs; stick-like morphology; crosslinking; blending; sandwiched structure; vesicant; PMMA; PVDF; PEGDA
锂离子电池自上世纪九十年代开始商品化以来，得到了飞速的发展。由于其电压高、比能量密度高、循环寿命长、自放电率低、环境友好等优点，在移动通讯、便携式电子设备、军事、医疗等领域获得了广泛的应用。因此，锂离子电池的研制及其循环衰竭机理的研究具有非常重要的意义。同时，为了进一步提高锂离子电池的性能，拓展其应用领域，有必要采用凝胶聚合物电解质。因此，本论文针对这两方面，开展了研究工作。 一、对于高性能锂离子电池而言，其性能与材料密切相关，同时也在很大的程度上受制于工艺。本论文通过工艺的设计，制备了锂离子电池的正极和负极，组成容量为1Ah的603450型铝壳锂离子电池。该电池具有优良的性能，例如循环1500次后，电池容量仍保持初始容量的60％以上。利用XRD、XPS、SEM、EIS、CV等手段研究不同循环次数下正负极的变化情况，对电池容量衰减机理有了一个较为深入的了解。在循环过程中，电池负极材料和正极材料的结构均未发生明显变化。随着循环次数的增加，由于正极表面与电解液有一定的作用，促使正极活性物质发生变化，从而使正极表面的粘结剂含量不断减少，导致部分容量衰减。负极材料与电解液不断反应，尽管该反应每次循环不明显，但是长期下来，导致SEI膜增加，交流阻抗增大，容量衰减明显。不同的负极材料，容量衰减的速率明显不同。这说明高性能锂离子电池容量衰减的主要原因是负极与电解液之间的反应，这为进一步提高锂离子电池的循环性能提供了一个良好的方向。 二、聚合物电解质的研究起源于1973年。30多年来，人们在固态聚合物电解质的理论研究及应用方面都取得了很大进展。但是目前全固态聚合物电解质的离子电导率还不能达到锂离子电池的实用要求。在这种背景下，作为液态电解质与全固态电解质的过渡产物，产生了凝胶聚合物电解质。它一方面具有聚合物的良好加工性能，另一方面又具有液体电解质的高离子电导率，安全性高，不仅可以作为电解质，还能代替隔膜，再加上聚合物良好的热塑性，聚合物锂离子电池可以做成多种形状，如平板形、方形、圆形等，应用前景广泛。 本文首先制备了以PMMA为基体的凝胶聚合物电解质，重点研究了PMMA／PVDF共混体系、PMMA／PEGDA／PVDF交联体系聚合物电解质的制备技术、导电机理、界面性质等，在PMMA／PVDF共混体系基础上制备了三种新型凝胶聚合物电解质：棒状排列、交联多孔和三明治结构，并首次探索了固相发泡成孔技术，制备多孔凝胶聚合物电解质。 棒状排列的凝胶聚合物电解质是首次制备，以热引发自由基聚合的PMMA(重均分子量约为4.7万)为基体，用冰水处理其热的溶液，使PMMA从溶剂中沉淀出来，形成一种表面形貌为棒状排列的聚合物。添加共混物PVDF后，棒变得细密。当PVDF用量超过20％时，则难以形成具有棒状排列形貌的聚合物。 棒状聚合物在沿着棒排列的方向，抗拉伸强度高，而在垂直棒状排列的方向，韧性比较好。在按照重量比1：1的比例加入电解液以后，所得凝胶聚合物具有较好的机械性能，离子电导率为0.32mS／cm。该聚合物基本上为非晶体，结晶度仅有4.45％左右，低的结晶度对于保证凝胶聚合物的良好的离子通道具有十分重要的作用。在共混物中PVDF和PMMA之间具有较强的相互作用，可以保证共混物混合十分均匀。该共混物具有良好的保液能力，其凝胶聚合物中电解液的挥发温度显著升高，有利于提高电池的安全性能和使用寿命。 电化学研究表明，该聚合物的电化学窗口大于4.7V，可以满足聚合物锂离子电池的需要。组装成聚合物锂离子电池以后，循环17次后，容量基本保持稳定，可望应用于实践中。 交联多孔的凝胶聚合物电解质是以PMMA／PVDF的共混物为基体。采用简单的共混方法，在PVDF含量占70wt％时还不能形成孔道结构；采用先挥发溶剂再交联的方法，也不能得到多孔的结构；采用先交联再挥发溶剂的方法，则可以得到交联型多孔聚合物。在加入电解液形成凝胶聚合物电解质以后，孔道结构获得保留。孔越多，电导率越高。采用交联的方法制得的多孔膜离子电导率可达5.5mS／cm。循环50次后，LiCoO_2正极材料的容量为初始容量的92％，具有良好的循环性能。 三明治结构的凝胶聚合物电解质是首次制备，它以PVDF和PMMA为基体，由PVDF、PMMA和PVDF三层组成，有效地解决了PMMA聚合物在电解液中的溶解问题，并且机械性能与离子电导率均有较大改善。外层PVDF为多孔结构，提供良好的离子通道；中间层PMMA为实心结构，具有良好的吸液性能。在聚合物与电解液重量比为25：75时，离子电导率达2.4mS／cm。DTA研究显示，该凝胶聚合物电解质对液体电解液具有良好的保持能力。在PMMA／PVDF共混体系中，电解液挥发温度比PP／PE复合膜提高了70℃，而采用PVDF／PMMA／PVDF体系作为凝胶聚合物电解质基体，则由共混的120℃提高到180℃，保液能力和热稳定性有了较大的提高。组成LiCoO_2／GPE／Li型电池的测试结果表明，110次循环后容量没有明显衰减，具有良好的循环性能。 首次利用固相发泡原理，制备了PVDF多孔凝胶聚合物电解质。发泡剂AC的用量、分解温度对孔隙结构具有明显的影响。所产生的孔和孔径大小分布均匀，孔径在亚微米级，与PP／PE复合膜相当，有效提高了PVDF凝胶聚合物电解质的离子电导率。该法与Bellcore公司的相转移法相比，不仅成本降低，而且易于工业化生产，对聚合物锂离子电池的生产将有可能产生重要的影响。
Since the birth of lithium ion batteries in 1990s, they have been developed very rapidly due to their predominate advantages, such as high voltage, high energy density, better cycling performance, and friendliness to environment. So far, they have been widely used in a lot of fields, for example mobile communication devices, portable electronics, military equipments and medical treatment. Conseeuqntly, the research on the preparation of lithium ion batteries and capacity fading is of great importance. In order to improve the performance of lithium ion batteries and broaden their applications, it is necessary to use gel polymer electrolytes. In this dissertation, work has been done on these two aspects. 1. In the case of lithium ion batteries with high performance, their behaviors are closely related to materials and technologies. Through design of technologies, cathode (LiCoO_2) and graphitic anodes for lithium ion batteries were prepared and prismatic cells (type 603450, Al gasket) with 1 Ah capacity were assembled. The cells displayed excellent cycling behavior. After 1500, the capacity retention ratio is above 60%. Through measurements such as XRD, XPS, SEM, EIS and CV, the changes of the cathodes and anodes after different cycles were investigated and further understanding on capacity fading of lithium ion batteries were achieved. During cycling, the structures of the LiCoO_2 cathode and graphitic anode materials were not markedly changed. Due to reactions of the surface of LiCoO_2 with the liquid electrolyte, the surface structure of the cathode was changed and the binder content at the surface decreased, leading to partial capacity fading. The graphitic anode continuously reacted with the liquid electrolyte. Though the reactions were not very evident, after long term cycling the thickness of the SEI (solid-electrolyte-interface) film and the AC resistance increased, leading to evident capacity fading. Different graphitic anode materials resulted in different capacity fading rate. These results show clearly that the capacity fading of lithium ion batteries with high performance is mainly ascribed to the reactions of the anode with the liquid electrolytes, which provides a good direction to achieve lithium ion batteries with excellent cycling behavior. 2. Polymer electrolytes were invented in 1973. Since then, great progress on theretical research and practical applications of all solid-state polymer electrolytes have been achieved. However, the ionic conductivity of all solid-state polymer electrolytes cannot still meet the requirements of practical application of lithium ion batteries. As a result, compromise products, gel polymer electrolytes (GPEs), were given to birth in 1994. The GPEs have good mechanically processing performance of polymers and high ionic conductivity of liquid electrolytes. Consequently, it can not only be used as electrolytes but also substitute separators. Furthermore, since the polymers have a good thermoplasticity, polymer lithium ion batteries can be manufactured into different shapes such as platform, prismtatic and round with good applications. In this thesis, GPEs based on PMMA (poly methyl methacrylate) were at first synthesized, then the preparation, conducting mechanisms and interface properties of PMMA/PVDF (poly(vinyl difluoride)) blended system and PMMA/PEGDA/PVDF crosslinked system were investigated. Based on the blended PMMA/PVDF, three GPEs were prepared, i.e. stick-like, crosslinked porous and sandwiched structured ones. Finally, porous GPEs by adopting vesicant were first explored. Stick-like GPEs were first prepared based on PMMA from thermal polymerization, whose average molecular weight is about 47,000. By precipitating PMMA from its NMP solution via treatment with iced water, the precipitated PMMA presents a stick-like array. After adding PVDF, the sticks become thinner. However, when the amount of PVDF is above 20%, no array of sticks was obtained. The prepared stick-like polymer presents fine tractility along the parrallel direction of the array of the sticks and good tenacity along the vertical one. Adding liquid electrolyte in the weigh ratio of 1:1, the GPE exhibits good mechanical strength with ionic conductivity of 0.32 mS/cm. The polymer is almost amorphous with very small crystallization degree of 4.45%, which is very important to ensure good channels for the movement of ions in the GPEs. In the blend of PVDF and PMMA, there is an interaction between PVDF and PMMA, which ensures homogeneous blending. This blend has a good retention ability of liquid since the evaporation temperature of liquid electrolyte in the GPE increases dramatically, which is favorable for the improvement of safety and cycling life of lithium ion batteries. Results from electrochemical evaluation show that the electrochemical window of the GPE is above 4.7 V, which can meet the requirements of polymer lithium ion batteries. The fabricated polymer lithium ion battery adopting the GPE exhibits good capacity retention, indicating promising application of the blending GPEs in lithium ion batteries. The crosslinked porous GPEs are based on the blend of PVDF and PMMA. There are no pores in the simple blend membrane when PVDF is up to 70 wt%. In the case of vaporizing the solvent followed by crosslinking there are also no pores in the blend of PVDF and PMMA. While in the case of crosslinking followed by vaporizing the solvent, a crosslinked porous PVDF-PMMA-PEGDA was achieved. After gelled by adding liquid electrolytes, these pores were retained. When there are more pores in the polymer membrane, the ionic conductivity can be higher. The ionic conductivity of the crosslinked porous membrane can reach to 5.5 mS/cm. After 50 cycles, the capacity retention of the Li/GPE/LiCoO2 is 92%, which indicates good cycling behavior. The sandwiched structured PGEs were first prepared, which are based on PVDF and PMMA and consist of PVDF layer, PMMA layer and PVDF layer. In this novel sandwiched structure, the solubility of PMMA into liquid electrolyte is greatly alleviated, and the mechanical processing ability is improved and the ionic conductivity is increased. The outer PVDF layers present porous structure, which provides pathways for ions into and from the GPEs, and the middle PMMA layer is solid and has good capacity to absorb liquid electrolytes. The ionic conductivity is 2.4 mS/cm when the weight ratio between the polymer and the liquid electrolyte is 25:75. DTA results demonstrate that this sandwiched GPE exhibits good retention ability of liquid electrolyte. The evaporation temperature of liquid electrolyte in the blend of PMMA/PVDF is increased for 70°C compared with the PP/PE composite film. In the case of the sandwiched GPEs, it is 180℃, much higher than that of the blending GPE (120℃), which shows marked improvement of the retention ability of the liquid electrolytes and thermal stability. The fabricated LiCoO_2/sandwiched GPE/Li battery exhibits good cycling performance since there is no obvious capacity fading after 110 cycles. Porous PVDF GPEs were prepared first by adopting vesicant. The amount of the vesicant AC and the decomposition temperature present evident influence on the pore structure. The dispersion and the diameters of the pores are homogenenous, and the pore size is at the range of sub-micrometer, as equal as that of PP/PE film. The ionic conductivity is also effectively increased. Compared with the phase inverstion method of Bellcore Company, this method presents low cost and is easily for industrialization, which will produce great influence on the production of polymer lithium ion batteries.
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