锂离子电池内耦合电化学反应的多物理过程的介观孔尺度传输机理研究

Lithium-ion batteries have been widely used as power sources for mobile and stationary consumer electronics, and are perceived to be promising alternatives to power sources of electric vehicles and other medium-to-large sized power or energy storage devices. However, fundamental transport mechanism of lithium-ion battery process still remains unknown or has not yet reached a widely-accepted cognition. Limited by the current experimental technique, many parameters are difficult to measure as well. How to explore the underlying mesoscopic pore-scale mechanism of multi-disciplinary transport coupled with electrochemical reaction gradually rises to be a major challenge to the further development and application of lithium-ion batteries. Firstly, by virtue of digital image processing technique we analyze the experimental (e.g., SEM/FIB) images of sliced porous electrodes and separator to obtain statistical information of mesoscopic pore-structure. Secondly, we propose to innovatively employ the simulated annealing method to reconstruct the porous electrodes and separator, and then to perform cluster analysis to examine the connectivity of the computer generated electrodes (and separator). Finally, SPH method will be deployed to establish mesoscopic pore-scale modeling of the multi-disciplinary transport coupled with electrochemical reaction in lithium-ion batteries. The results from pore-scale simulations will be compared with either our in-house experimental data or data in public literatures. Combining the simulated and experimental results we expect to unravel the mesoscopic transport mechanism of the involved process, to identify the key factors limiting the high C-rate charging/discharging performance of lithium-ion batteries, and to find out the reasons and controlling factors of thermal runway. Along with the accomplishing of this project, it is expected to provide theoretical basis and technical support for practical operation and management, design and optimization of lithium-ion batteries as well.

锂离子电池已经在小型消费电子元器件上得以广泛的应用，并且在电动汽车等大中型动力或储能设备中有着广泛的应用前景。但电池内部某些过程的机理至今还没有定论，许多参数在现有的实验条件下无法或难于测得。采用何种措施能够对锂离子电池过程的介观孔尺度传输机理进行深入的探索研究，已经成为锂离子电池进一步发展和应用亟待解决的关键问题。利用实验得到的锂离子电池多孔电极和隔膜的切片图，采用数字图像处理技术分别得到电极和隔膜的微结构统计信息，创新地提出采用模拟退火法分别对电极和隔膜进行重构，并对重构结构进行团簇分析。最后，利用SPH方法对锂离子电池进行模拟研究，重点探讨锂离子电池内耦合电化学反应的多物理过程的介观孔尺度传输机理，并将模拟结果分别与实验和文献结果进行对比，深入探讨和揭示限制锂离子电池充放电速度和引起热失控等现象的影响机制或关键控制因素，为电池的运行和管理、设计和优化提供理论依据和技术支持。

锂离子电池过程主要涉及固相材料中电子传导过程、固体活性物中Li扩散过程、电解液中Li+扩散与传导过程、电化学反应过程以及伴随上述四个过程的热量产生及传递过程。可见，除了电极材料性能之外，电池性能还取决于其内电子、Li、Li+和热量传输通量的大小、速度的快慢、路径的长短以及电化学反应所处的空间位置分布。因此，有必要深入研究电池内耦合电化学反应的多物理过程的传输机理。.首先，我们针对正、负极和隔膜内部微结构的特点，分别开发了具有最大影响距离的随机网格重建模型软件（软件I）、基于特定粒径分布的球形颗粒模拟退火重建模型软件（软件II）、基于特定粒径分布的椭球颗粒模拟退火重建模型软件（软件III）以及基于制备隔膜所用纤维形状和尺寸分布的随机泊松线重建模型软件（软件VI）。经研究后，发现软件I适用于重构细小颗粒构成的密实型块状或片状多孔电极；软件II适合重构颗粒堆积状电极；软件III特别适合重构具有鳞片状的石墨负极；而软件VI则适用于重建纤维状隔膜微结构。.然后，我们开发了微结构特征化分析软件，该软件集成了孔隙率、组分体积分数、比表面积、两点相关函数、连通性、扭曲率因子（形状因子）、孔径分布以及有效传输物性等重要特征参数的分析功能，其中预测有效传输物性有两个功能模块可供选择，即基于格子Boltzmann（ LB）和光滑粒子水力学（SPH）方法开发的有效传输物性虚拟测试功能模块，后者在求解时需遍历粒子，因而计算量相对较大。.最后，我们分别基于SPH和有限体积（ FV）方法开发了锂离子电池介观尺度电化学-热耦合模型软件，但前者在求解时需要遍历粒子，因而计算量偏大。为此，我们利用后者对电池充放电过程进行模拟，发现其在揭示电极/隔膜微结构对电池内耦合电化学反应的多物理过程传输机理方面具有巨大潜力，且计算量相对小。