含缺陷锂电池硅负极材料断裂行为的原位观测与失效机理研究

Silicon anodes are promising materials for lithium-ion batteries due to their enhanced safety，low cost and high-energy capacity. However, the commercial use of silicon anodes has been hindered by their large volume change (up to 300%) during lithium insertion and extraction which often leads to pulverization of the active alloy particles and the unstable solid-electrolyte interphase (SEI) films. Therefore, it is very important to research the fracture mechanics of the silicon anodes under the electro-chemo-mechanical coupled effect. This project will (1) set up an experimental platform based on loading system, high resolution light microscopes, in-suit half-cell and charge-discharge system for observing the initiation and growth of cracks on silicon anodes with pre-cracks or voids. The challenges of this part are the productions of anodes contain pre-cracks and the design of in-suit half-cell which should meet the both the optic and electrochemical demands; (2) propose the different diffusion boundary conditions of lithium ion. Then, the nonlinear fracture model which consider the full diffusion-deformation coupling will be established using the experimental and simulation methods. The model will try to explain the competition mechanisms between the brittle to ductile transition and diffusion induced softening effect near the crack tip; (3) establish the Paris law of the silicon based on the observation of the fatigue crack propagation under different electrochemical conditions. The safe region of silicon anodes will be mapped for different factors including electrochemical conditions, geometrical features of electrodes and interplay between cracks and new SEI films. This project is expected to gain original achievements in electro-chemo-mechanical fracture theory and in-suit experimental results which will provide profitable guidance for the design, manufacture and life prediction for lithium-ion batteries with silicon anodes.

锂电池硅负极材料具有比容量高、放电平台低、资源丰富、环境友好等优点，应用前景广阔。然而，由于制备或首次充放电过程中不可避免产生微孔洞、微裂纹等缺陷，在循环充放电时，裂纹的起裂、扩展等断裂问题变得尤为突出，造成电池可逆容量急剧衰减。传统单一的电化学或力学理论方法不能很好解决脱嵌锂过程中缺陷/扩散/变形强耦合问题。本项目拟（1）解决电化学氛围力学加载、原位电化学池、高精度光学观测和精确预制脆性硅缺陷等技术难题，研制含缺陷硅负极材料裂纹起裂、扩展原位观测实验平台；（2）提出裂纹扩散边界条件，发展考虑离子扩散反应动力学的非线性断裂准则，揭示缺陷/扩散/变形耦合作用机理；（3）获得电化学环境下硅负极材料裂纹扩展Paris公式，提出基于裂纹扩展长度的容量衰减模型，揭示裂纹扩展对使用寿命影响机理。期望在原位实验和理论建模方面获得原创性成果，为锂电池硅负极材料的设计、制备和寿命预测提供指导。

锂离子电池由于其高比容量、高倍率性能等优势，广泛应用于消费电子、电动汽车等领域。而现有的高容量电极材料在脱嵌锂过程中有较大的本征体积膨胀，导致电极的变形与断裂问题突出，而且由于初始缺陷不可避免存在于电池生产与服役全寿命周期，进一步加剧了电池性能的劣化与失效。.本项目紧密围绕含缺陷锂电池负极材料力化耦合环境下的断裂与失效问题，率先研制1套力化耦合原位拉伸设备，实现定量化力学与电化学同时可控加载。通过离位俄歇电子能谱(AES)与光学原位平台监测结合的方法，首次实现对裂纹尖端锂浓度场集中现象的定量化表征。发展大变形弹塑性力化全耦合数值仿真方法，揭示裂尖应力集中驱动锂离子裂尖富集演化过程，与实验结果吻合较好。率先报道电极面内初始裂纹在二次嵌锂膨胀时的裂纹愈合、裂纹断面挤压导致界面脱粘失效的新现象，并定量化模拟了愈合、挤压、脱粘过程中的应力演化过程。基于石墨电极脱嵌锂变色原理，研究不同类型，不同几何构型缺陷对锂离子扩散过程的影响。进一步给出等效扩散系数定义，用于描述含缺陷电极的扩散演化规律。解决的电极多周循环裂纹在位观测表征难题，揭示含缺陷电极多周疲劳劣化机理。.本工作一方面发展了多场耦合实验力学加载与测试方法，丰富和拓展了多场耦合断裂力学的学科内涵，另一方面对电池生产过程中缺陷表征评价与缺陷阈值控制提供丰富的实验支撑。本项目在Script Materialia、Applie Physics Letter、Energy Storage Materials、Journal of Power Sources等高水平期刊发表相关论文10篇，并被Gerbrand Ceder、Lallit Anand、高华健、曲建民等知名教授引用，其中裂尖浓度集中等新现象被引用为论文理论或仿真预测工作的重要实验证据。