【作者】 陈建超；

【导师】 孙涛； Xiaodong Li；

【作者基本信息】 哈尔滨工业大学 ， 机械制造及其自动化， 2014， 博士

【摘要】 由于具有极佳的比容量，锂离子电池广泛地应用于便携式电子产品、绿色可替代能源储能及电动车等领域。尤其是电动车应用，它要求锂离子电池能够有更高的能量及功率密度、更长的循环及搁置寿命以及更具安全可靠性。为满足这些性能要求，学术及产业界针对锂离子电池中的电化学活性材料，如正负极活性材料及电解液，开展了大量的研究。相比，作为电池中电化学惰性材料的粘接剂和隔膜却受到极少的关注。实际上，它们对电池性能以及安全性的影响不容忽视：倘若电极颗粒间以及电极复合膜和集流体间的粘接强度不足以承受充放电循环，那么电池即会发生失效；对于绝缘隔离正负极片的多孔隔膜，一旦其发生变形、断裂或者击穿，将直接影响电池的性能及安全性。因此，本文通过从力学角度研究粘接剂和隔膜可能发生的力学行为，找到这两类材料导致锂离子电池发生失效及出现安全性问题的力学作用机理。本文首先分析了粘接剂在保持锂离子电池电极力学完整性方面所发挥的作用机制。基于对失效的锂离子电池负极片形态完整性的分析，将粘接剂粘接强度细分为微观的电极活性材料颗粒间的粘接强度以及宏观的电极复合膜与集流体间的结合强度，并针对上述宏-微观两类粘接强度提出了基于微刻划的综合评价方法与手段。应用该综合评价方法、原位拉伸/光学显微成像以及数字图像相关（DIC）分析技术对不同聚偏氟乙烯（PVDF）粘接剂含量的负电极片开展了实验研究。由粘接剂含量与微刻划中得到的摩擦系数及临界正压力之间的依赖关系揭示了粘接剂与其自身、颗粒（C）以及集流体（Cu）界面间的粘接强度大小顺序：Cu/PVDF<C/PVDF<PVDF/PVDF。此外，还应用上述实验手段，完成了电解液干涸处理过的粘接剂力学行为的实验研究，发现了电解液的干涸会增大颗粒间粘接强度，降低膜与集流体间结合强度的现象。其次分析了隔膜在外力作用下的变形行为对锂离子电池失效的影响。分析了隔膜在电池充放电以及搁置过程中的受力状态，并基于该分析推导并应用了Carroll-Holt模型计算压缩应力与隔膜微孔间的演化关系，设计完成了隔膜压缩实验，验证了上述模型。采用压缩后的隔膜制备了锂离子电池，完成了电池的电化学表征，表征结果揭示了隔膜的压缩所致闭合行为是导致锂离子电池容量衰减失效的一个重要影响机理。本文在研究了隔膜外力所致的力学行为之外，还研究了隔膜在高温热场作用下的热力学行为对锂离子电池失效的影响机理。通过原位热场原子力显微镜成像、功率谱密度和DIC分析技术，发现了聚丙烯-聚乙烯-聚丙烯三层复合隔膜（Celgard2325产品）的表面形貌在热场下的演化过程。实验结果表明：隔膜的聚丙烯表面在90°C温度下会在隔膜的机器方向发生收缩，同时在横向方向由于纳米纤丝的膨胀发生微孔闭合的行为。这种在低于隔膜的热闭合温度（120°C）以及其熔点（165°C）的温度下即发生的微孔闭合行为是锂离子电池在热场下快速失效的一个重要影响机理。在电池滥用条件下，隔膜的力学可靠性也直接影响着电池的安全性。因此，本文随后研究了隔膜的拉伸与断裂力学行为及形变机理。针对五种干湿法制得的商品化隔膜开展了常规拉伸以及原位拉伸/原子力显微镜成像实验。揭示了隔膜的微观结构与拉伸性能的演变机制：对于干法制得的隔膜，由于隔膜表面微观结构中的片晶组在不同方向有着显著不同的形变机理，因此隔膜的微观结构决定了隔膜的力学特性具有强烈的方向性。而且，为研究隔膜的断裂行为，本文采用了基本断裂功方法对隔膜完成了表征，明晰了隔膜的断裂性能与微观结构的依赖关系及演化机制。论文最后部分主要讨论了拉伸-压缩耦合应力对隔膜击穿强度的影响作用。在分析隔膜受力状态及传统击穿强度测试方法不足的基础上，发现了预拉伸应力可极大地降低隔膜的击穿强度的隐患点。为全面评价隔膜在多应力耦合作用下的可靠性，提出了一种拉伸-压缩多应力耦合作用下隔膜可靠性测试的新方法，实现了对已受到拉伸应力作用的隔膜击穿强度的测量。在大量的基于新方法的测试实验基础上，明确了隔膜在多应力耦合作用下的失效过程及机理。更多还原

【Abstract】 Because of their superior gravimetric and volumetric capacities, lithium-ion secondary batteries (Li-ion batteries) are increasingly employed in systems such as mobile electronics, alternative energy storage, and plug-in hybrid electric vehicles (PHEV)/all-electric vehicles (EV). Especially for PHEV and EV applications, much higher energy/power density, longer shelf/cycle life, and greater reliability are essential. To achieve such performance, intense research has been focused on the electrochemically active cell materials: positive and negative electrode active materials (AM) and electrolytes. In contrast, less attention has been paid to binders and separators, the electrochemically inactive materials, although for the former the electrochemical performance of batteries such as specific capacity and cycle life cannot be achieved if the adhesion strengths between electrode particles and between electrodes film and current collectors are insufficient to endure charge-discharge cycling, while for the latter any pore closure or rupture/penetration could lead to cell performance degradation or catastrophic consequences such as explosion and thermal runway. Therefore, in this thesis the mechanical behaviors of binders and separators were characterized to unveil the mechanical mechanisms that contribute to the ageing and reliability in Li-ion batteries.First, the role of binders in the mechanical integrity of electrodes for Li-ion batteries was studied. Based on the morphological analysis of the anode electrodes from aged Li-ion batteries, the strength of the binder was redefined as the micro-scale carbon particle/particle cohesion strength and macro-scale the electrode-film/copper-current-collector adhesion strength. Accordingly, a comprehensive evaluation method of the above micro-macro strength based on microscratch was proposed. This method coupled with microindentation and digital image correlation (DIC) techniques were used to study the mechanical properties of anode electrodes with different polyvinylidene fluoride (PVDF) binder loading. The dependences of microscratch coefficient of friction and the critical delamination load on the polyvinylidene fluoride (PVDF) binder content suggest that the strength of different interfaces is ranked as follows: Cu/PVDF<carbon-particle/PVDF<PVDF/PVDF. The electrolyte soaking-and-drying process leads to an increase in particle/particle cohesion but a decrease in electrode-film/Cu current-collector adhesion.The role of separator deformation in response to external mechanical stimuli in electrochemical performance of batteries was then studied. Based on the analysis of stress generated in separator due to lithium insertion/deinsertion induced electrode expansion and storage conditions, the process of pore closure in separators resulted from external stress was determined by Carroll-Holt pore-collapse relation model, which was then varificated by compression testing. Electrochemical characterization of mechanically stressed separators was also performed. We find the pore closure in the electrochemically inactive separator to be a cause of battery capacity fade.In addition to the external mechanical stress induced deformation, the mechanical behavior of polymeric separators in Li-ion batteries at elevated temperatures was also characterized by in-situ high temperature surface imaging using an atomic force microscope (AFM) coupled with power spectral density (PSD) analysis and DIC technique. The temperature dependent micro-scale morphology change of PP (polypropylene)-PE (polyethylene)-PP sandwiched separators (Celgard2325) was found. Both PSD and DIC analysis results show that the PP phase significantly closes its pores by means of dilation of the nanofibrils surrounding the pores in the transverse direction and shrinkage in the machine direction, when cycled at90°C, even below the separator’s shutdown temperature (120°C) and its own melting temperature (165°C).Besides the important role of separator in the battery ageing, the reliability of the separator is also crucial to the abuse tolerance of a battery. Therefore, deformation and fracture behaviors of five commercially available wet and dry processed polymer separators were investigated by conventional tensile testing coupled with in-situ tensile testing under an AFM. The evolution mechanism between the separator tensile properties and its microstructures was explained. For separators made by the dry process, material direction dictates the significant diversity of overall mechanical integrity of the separator, which is a result of the distinct deformation mechanism of the stacked lamellea in the separator. Moreover, in order to evaluate the fracture properties of these separators, the essential work of fracture (EWF) approach was adopted. The EWF results show that the fracture properties for the dry processed separators also present orientation dependence, which is then found to be results of different toughening mechanisms.The remainder of this dissertation focuses on the effects of tensile-penetration coupled stress on the penetration strength of lithium-ion battery separators. Because the pre-tensile stress has the potential to dramatically decrease the penetration strength of the separator, we thus proposed a new method that is able to measure the penetration strength of a battery separator under the pre-stressed condition. Moreover, the process and mechanism of the penetration into the separator under coupled stress were also elucidated.更多还原