锂离子电池高容量电极材料的破坏预测

发布时间：2019-01-03 07:00

【摘要】：锂离子电池因其高电压平台、高能量密度、低自放电率、绿色环保等诸多优良性能得到了广泛应用。电极材料是锂离子电池的重要组成部分,对锂离子电池电化学性能的好坏起决定性作用。然而当前商业化的电极材料理论比容量较低,已远远不能满足锂离子电池在电动汽车等高比容量需求领域的应用,因此,高容量电极材料的发展应用成为研发新型高性能锂离子电池的关键。但高容量电极材料在充放电过程中面临着严重的体积变形问题,大体积变形伴随着大锂化应力的产生,会造成电极活性材料的破坏甚至粉化,直接导致锂离子电池容量的衰减和循环性能的降低。为了解决锂离子电池高容量电极材料的锂化变形破坏问题,本论文通过理论分析和有限元数值模拟,建立了不同结构高容量电极材料的相变锂化模型及应力破坏预测模型,旨在明确电极材料锂化过程中的应力演化规律,得到电极材料临界破坏状态与其结构尺寸、基本力学性能之间的锂化破坏失效机制图,并进一步通过实验加以验证。本论文的主要研究工作包括以下几个方面:(1)建立了基于理想弹-塑性变形的相变锂化模型,并通过ABAQUS有限元软件模拟了薄膜结构和球结构(包括实心球、空心球和空心-核壳结构)电极材料的相变锂化进程,分析了锂化过程中浓度场、应力场的动态演化。发现锂化应力在相界处发生了突变,且电极材料表面和界面处演化增大的拉应力是材料破坏的主要原因。空心结构不仅可以加快锂化反应速率,还可以在一定程度上缓解向外的锂化膨胀,空心核-壳结构有效控制了电极材料向外的锂化变形。(2)在相变锂化模型的基础上,基于能量破坏准则,建立了薄膜结构和空心核-壳结构电极材料的断裂、剥离破坏模型。根据量纲分析和ABAQUS模拟,通过函数拟合,确定了电极材料临界破坏SOC与其结构尺寸、基本力学性能之间的无量纲函数关系,得到了相应的锂化破坏失效机制图,从而为判断高容量电极材料在锂化过程中的安全性提供了理论依据。(3)通过电镀法制备了不同厚度的锡薄膜,并用其装配成扣式电池进行了充放电实验,通过扫描电镜观察得到了不同厚度的锡薄膜在锂化过程中的临界破坏SOC。实验表明对于厚度分别为10μm、18μm、26μm和35μm的锡薄膜,其临界断裂状态分别为SOC=1、SOC=0.72、SOC=0.5和SOC=0.36,实验结果与理论预测基本吻合,证实了锂化破坏失效机制图的准确性。
[Abstract]:Lithium ion battery has been widely used for its high voltage platform, high energy density, low self discharge rate, green environment and so on. Electrode material is an important component of lithium ion battery, which plays a decisive role in the electrochemical performance of lithium ion battery. However, the current commercial theoretical specific capacity of electrode materials is relatively low, which is far from being able to meet the needs of lithium ion batteries in the field of high specific capacity, such as electric vehicles. The development and application of high-capacity electrode materials have become the key to the development of new high-performance lithium ion batteries. However, the high capacity electrode material is facing the serious volume deformation problem in the process of charge and discharge. The large volume deformation accompanied by the large lithium stress will lead to the destruction and even pulverization of the electrode active material. It directly leads to the attenuation of lithium-ion battery capacity and the decrease of cycling performance. In order to solve the problem of lithium deformation and failure of high capacity electrode materials for lithium ion batteries, the phase change lithium transition model and stress failure prediction model of high capacity electrode materials with different structures are established by theoretical analysis and finite element numerical simulation in this paper. In order to clarify the stress evolution law of electrode material during lithiation, the failure mechanism diagram of lithium damage between the critical failure state of electrode material and its structure size and basic mechanical properties is obtained, and further verified by experiments. The main work of this thesis includes the following aspects: (1) A phase transition lithiation model based on ideal elastic-plastic deformation is established, and the thin film structure and spherical structure (including solid sphere) are simulated by ABAQUS finite element software. The process of phase transition lithiation of hollow sphere and hollow core-shell structure) electrode materials was studied. The dynamic evolution of concentration field and stress field in the process of lithiation was analyzed. It is found that the Lithium stress changes at the phase boundary and the tensile stress at the electrode surface and interface is the main reason for the material failure. The hollow structure can not only accelerate the lithiation reaction rate, but also alleviate the outward lithiation expansion to a certain extent. The hollow core-shell structure can effectively control the lithiation deformation of the electrode material. (2) based on the model of phase transition lithiation, the hollow core-shell structure can effectively control the lithiation deformation of the electrode material. Based on the energy failure criterion, the fracture and delamination failure models of thin film structure and hollow core-shell structure electrode materials were established. According to dimensionality analysis and ABAQUS simulation, the dimensionless function relationship between critical failure SOC of electrode material and its structure size and basic mechanical properties was determined by function fitting, and the corresponding failure mechanism diagram of lithium failure was obtained. This provides a theoretical basis for judging the safety of high capacity electrode materials in the process of lithiation. (3) Tin thin films with different thickness have been prepared by electroplating, and the charging and discharging experiments have been carried out by assembling them into button batteries. The critical failure of SOC. of tin thin films with different thickness during lithiation was observed by scanning electron microscope (SEM). The experimental results show that the critical fracture states of tin films with thickness of 10 渭 m ~ 18 渭 m ~ (26 渭 m) and 35 渭 m are SOC=1,SOC=0.72,SOC=0.5 and SOC=0.36, respectively, which are in good agreement with the theoretical prediction. The accuracy of the failure mechanism diagram is verified.
【学位授予单位】：湘潭大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TM912

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