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基于力-化-电耦合理论的电极结构有限元模拟及结构优化
英文题名The Finite Element Simulation of Electrode Structure Based on Electrochemical-Mechanical Coupling and its Structure Optimization
温济慈
导师郑仰泽 ; 魏宇杰
2018-12-04
学位授予单位中国科学院大学
学位授予地点北京
培养单位中国科学院力学研究所
学位专业固体力学
关键词锂离子电池 高能量密度电极材料 力-化-电耦合理论 有限单元法 结构优化设计
摘要

    可充电的锂离子电池作为新型绿色能源,是便携式电子设备电池的首选,并且广泛应用于动力能源领域。随着对高能量密度存储设备需求的增长,对锂离子电池的电容密度和循环寿命提出了更高的要求。硅基、锡基等电极材料由于其高的电容密度而成为锂离子电池理想的负极材料。然而,电极材料的高能量密度意味着充放电过程中,单位质量(或者体积)的电极材料需要接纳更多的锂离子,导致结构产生大的体积变形和高的应力状态,并引发电极结构的断裂、疲劳等破坏问题,严重影响到锂离子电池的工作性能。因此,从力学的角度分析电极材料充放电过程的应力演化,并通过合理的结构设计,规避可能出现的结构破坏,提高电池的循环稳定性,对于研发兼具高能量密度和高循环寿命的电能存储设备具有重要的工程和科学意义。

    电极材料的充放电过程是一个与力学、化学和电学相关的多物理场耦合过程,其复杂的非线性耦合关系对结构变形和应力演化过程的求解造成了极大的困难。针对纳米线电极结构充放电过程中的体积变形和应力演化问题,我们建立了基于位错沿半径方向的插入(或抽出)引起的变形和应力的求解框架。在弹性小变形假设下,该方法可以方便地直接给出圆柱结构任意体积变形状态下的应力解。

    区别于传统的电极材料,高能量密度电极材料的充放电过程表现出的明显的不可回复的大变形。因此,需要借助高性能数值计算方法用来刻画电极材料在充放电过程中多物理场的演化过程。基于弹塑性大变形的力--电耦合理论,该研究工作通过发展相应的有限元计算程序,用以准确刻画电极材料在充放电过程中的锂离子扩散,弹塑性大变形及其应力演化的过程。针对薄膜电极结构,我们通过基于力--电耦合的有限元程序进行有限元模拟,阐述了由电极薄膜的弹塑性大变形引起的原位曲率测量试验过程中通过Stoney公式预测薄膜应力所导致的误差,并讨论了电极薄膜的大变形、弹塑性本构关系和界面材料性质对应力-充放电状态曲线的影响,以及电极材料参数与应力-充放电状态曲线特征之间的对应关系。该工作可以提供了一种可用于准确提取电极材料的弹塑性材料性质及界面材料性质的方法,为研究电极材料在充放电过程中的力--电耦合本构关系提供了帮助。

    既然高能量密度电极材料的工作性能受制于高应力状态引起的严重的结构破坏问题,于是,基于合理假设的力--电耦合的本构关系及可靠的实验数据,我们采用有限元计算程序,模拟不同电极结构在充放电过程中的多物理场演化过程,并通过梯度设计的多孔电极结构,研究了不同孔隙分布对结构电化学和力学的宏观性能的影响,为大尺度的电极结构的优化设计提供有效的指导。

    本项工作的意义在于,从理论和结构设计两方面,对不同电极结构的力学表现做了深入研究,为解决高能量密度电极材料在充放电过程中由结构断裂和疲劳问题引起的严重的容量衰减和寿命低等问题提供了重要的指导作用。

英文摘要

    Lithium ion secondary batteries have become ubiquitous power sources for mobile applications, and have been used widely for power energy industries. It is the need for high density energy storage devices that prompts to the lithium ions battery with high capacity and long life. Thus, high charge capacity anode materials, like Si and Sn, have attracted great interests in recent years. However, lithiation and de-lithiation in anode materials with high capacity inevitably result in huge volume expansion and contraction, lead to high internal stresses and, thus, poor cycle life of high capacity anode batteries. Therefore the analysis of electrode material’s stress evolution during charging and discharging, and the improvement of electrode structure’s mechanical properties by electrode structure’s optimization have important engineering and scientific significance.

    There are complex strongly coupled interaction between electrochemical and mechanical process, the analytical solutions of the deformation and stress are difficult to obtain for the complex nonlinear coupling relation. Here by taking the equivalence of volume expansion (or shrinkage) as continuous insertion (or distraction) of infinitesimal dislocations, we supply a framework to solve the stress field of a cylinder with arbitrary insertion (distraction) profile of materials along the radial direction. Under the assumptions that the deformation is small and elastic, we supply analytical solutions of stress fields to several typical volume expansion or shrinkage profiles.

    Different from the traditional electrode materials, there are large irreversible deformation for electrode materials with high capacity during charging and discharging, so it is necessary to develop high performance numerical calculation method for simulating the changes of electrodes during charging and discharging. Here we developed a robust electrochemical-mechanical coupled numerical procedure to simulate electrode structures’ diffusive process, large elastic-plastic deformation and stress evolution. With the numerical results of thin film electrode structure, we analyze on examining the validity of the Stoney equation for in-situ stress measurements, identifying how the constitutive behavior of electrode materials and film-substrate interfacial properties affect the measured stress-capacity curves of electrodes, and establishing the relationship of electrode material parameters with the characteristics of stress-capacity curves.

    In fact, the working performance of lithium ion batteries has been restricted by serious structural damage which is caused by high stress status, hence, we simulated the stress evolution for several kinds of electrode structures during charging and discharging, based on reasonable assumption on the electrochemical-mechanical coupling constitutive relationship and the reliable experimental data. We designed porous electrode media with gradient distributed pores and validated its improvement on electrode structure’s stress status.

    This thesis is focused on the mechanical behavior of electrode materials in the view of electrochemical-mechanical coupling theory and structure design, and aims at providing direction to improve the electrochemical behavior influenced by fracture and fatigue problem on mechanical properties during charging and discharging.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/78076
专题非线性力学国家重点实验室
推荐引用方式
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温济慈. 基于力-化-电耦合理论的电极结构有限元模拟及结构优化[D]. 北京. 中国科学院大学,2018.
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