锂离子电池充放电过程中活性材料内应力的演变及其与性能的关系

During charge and discharge of the lithium ion battery, lithium ion intercalation and deintercalation in active material is usually accompanied by the expansion, contraction and phase transitions, resulting in the corresponding internal stress, which affects the safety and the electrochemical properties of the battery. However, the internal stress can not be directly perceived, and thus only theoretical results are available without quantitative test data. Base on in-situ and/or quasi-dynamic X-ray diffraction method, the dependence of the diffraction angle, the peak width and the diffraction intensity on the internal stress within the active material can be established, through which the macro-strain and micro-strain can be obtained using the methods for sin2ψ and separating multifold broadening effects. Further, by means of the Hooke's Law, the macro-stress and micro-stress can be quantitatively calculated. The object of this project is to study the evolution of the internal stress within the cathode and anode active materials under different charge and discharge conditions, and the effects of the internal stress on the materials bulk structure, thermal stability, electrode/electrolyte interphase characteristic, lithium ion intercalation/deintercalation dynamics, and to reveal the effect mechanisms of the internal stress within the active material on the battery performance (safety, capacity, power, high and low temperature performance , cycle life). It is expected to be helpful to improve the understanding of the underlying theory, and to guide the research, production and application of the lithium ion battery.

锂离子电池充放电过程中，由于锂离子的嵌入和脱出，导致活性材料颗粒体积的膨胀、收缩和相变等，随之产生相应的内应力，其大小直接影响到电池的安全性和电化学性能。由于内应力无法直接感知，因而以往多为理论模拟的结果，较少定量检测的数据。本项目采用原位和/或准动态X射线衍射准确测定衍射角度的漂移、峰宽和衍射强度的变化，并将其与活性材料中的内应变建立确定的关系，通过sin2ψ方法和分离多重宽化效应，分别测定宏观内应变和微观应变，再借助于虎克定律定量计算宏观内应力和微观应力，从而研究不同充放电条件下的电池正、负极活性材料内应力的变化及其对材料本体结构、热稳定性、电极/电解液界面特性、嵌/脱锂动力学特性的影响，揭示活性材料内应力对电池综合性能（安全性、容量、功率、高低温、循环寿命）的影响规律和机理，期望有助于提高人们对相关基础理论的认识，并对锂离子电池及其材料的研发、生产和使用具有一定的指导意义。

采用X射线衍射方法，测定了石墨/钴酸锂电池在不同化成阶段和循环过程中正、负极活性材料的内应力，得到化成和循环过程中活性材料内应力的演变规律。电池在循环过程中，随着循环次数的增加，LiCoO2电极的微应变逐渐增加，当累积的应变过大时，导致其颗粒表面出现裂纹，露出新鲜界面，与电解液发生副反应，一方面消耗了电池中有限的锂离子，导致电池容量降低；另一方面，电解液副反应产物覆盖在电极表面，增加了电池阻抗；而对于石墨电极，由于循环过程中，石墨负极体积的膨胀和收缩，使其表面SEI膜不断发生破裂-修复的过程，使石墨负极在循环后期压应力一直在增加；电池阻抗的增加，导致极化增大，电池的容量和放电电压逐渐降低。采用MOS法实时测量了一氧化硅电极在充放电过程中的应力演变过程，并从应力角度解释一氧化硅/铜/膨胀石墨复合电极循环性能提升的原因。