The aim of this study is to investigate the correlation of mechanical stresses and electrochemical battery degradation. Specifically, mechanical stress and strain caused by lithium intercalation and diffusion in both cathode and anode were calculated and crack initiation in particles was predicted. Furthermore, a modified Butler-Volmer equation, which accounts for the influence of mechanical stress on electrochemical reactions, is considered. In this study, we adopted real 3D microstructures of both anode (graphite) and cathode (NMC) based on over 100 images from a synchrotron radiation X-ray tomographic microscopy (SRXTM). The 3D finite element model (FEM) was developed by COMOSL Multiphysics 5.2a and followed by coupled electrochemical and mechanical analyses of a whole cell, including cathode, anode, and electrolyte.
As the first step, the electrochemical model based on the microstructure was compared with results from the previous study with a simple geometry (e.g. spherical particles). Specifically, the real 3D microstructure model, concentration, polarization, over potential, and von-Mises stress distribution across both electrodes (e.g. graphite and NMC) were computed under different C-rates. We observed that effects of phase transformation of NMC on mechanical stress was significant during discharging processes. Moreover, the model revealed that at particle connections where exhibit complicated geometries and the bulged regimes of smaller curvature showed higher stress and were vulnerable to particle cracking. Finally, it is confirmed that mechanical stress plays important role in electrochemical performance.
This computational model can make a solid foundation to understand the relationship between mechanical stress and electrochemical performance in NMC/graphite battery chemistry. It might be helpful to understand key factors in the deterioration of Lithium-ion batteries with higher C-rates and longer cycle life.