适用于电极材料的电化学扩散诱发应力松弛机制研究

Stress relaxation through electrochemical diffusion in electrodes

在近年建立的描述电极材料力-电-扩散耦合问题的连续介质理论框架下,提出一个简捷的有限变形本构理论,可直接描述发生在电极材料上的质量传输、变形和应力演化的强耦合过程,忽略了相变及塑性变形的影响;采用Bulter-Volmer动力学模型描述电解质/电极界面处电化学反应,可研究力、扩散对电化学性能的影响规律;同时构造了收敛、有效的有限元数值积分方案,并将其补充到商用有限元软件ABAQUS中,搭建了相应的计算模块.以具有橄榄石结构的LiFePO4电极材料为例,利用提出的理论和计算工具重点研究了锂离子在嵌入和脱嵌循环锂化过程中应力松弛机制、锂离子浓度的演化规律,以及力-电-扩散耦合机制对表征电池性能的充电状态变量SOC值的影响规律,并通过对平板和圆柱形两类常用电极的对比研究,揭示电极形状对电池性能的影响.此外,对于恒电流的情况,预测了电极界面处电化学量如交换电流密度、过电势及外电路电压随时间的变化规律,为进一步开展结合电化学实验来研究该类问题做铺垫.

In the framework of continuum constitutive theory recently developed that accounts for the coupling between mechanical behavior and electro-chemistry, a simple finite strain theory for electro-chemo-mechanics of electrode materials is proposed. Here, phase transformation and plasticity are neglected. The theory considers mass transport, deformation in electrodes, together with the electrochemical reactions at the electrolyte/electrode interface in terms of the Butler-Volmer kinetics, which can predict the effect of coupling mechanisms on electro-chemical response. A finite element algorithm is constructed which is stable and effective and is implemented into ABAQUS finite element platform. By using the proposed theory and numerical tool to LiFePO4 electrodes, we focus our investigations on stress relaxation mechanism and Li ion concentration during cyclic lithiation. A commonly used quantity in Li ion battery known as the state of charge （SOC） is investigated thoroughly. It is found that the geometry of electrodes has important effects on the stress level and capacity of electrodes through analyzing two types of electrodes which are plate and cylindrical electrodes. Besides, we predict exchange current, overpotential and cell voltage over time at the electrode interface under the galvanostatic condition. The present study paves the way for the further research considering electrochemical experiments.