In this study, the diffusion induced stress of LiCoO2 half cell with a realistic 3D microstructure has been studied using finite element method. The electrochemical properties under various C rates were studied. The discharged curves under various C rates were simulated. Results show that the potential drops significantly with the increase of C rates. The lithium ion concentration distribution under high discharging rates shows strong inhomogeneity. At high C rates, the small LiCoO2 particles near the separator have higher lithium ion concentration because of the shorter lithium migration and diffusion paths. The diffusion induced stress inside LiCoO2 particles was calculated coupled with lithium diffusion. The results show that the stress near the concave and convex regions is the highest. The neck regions of the connected particles will break first and form several isolated particles. For isolated particles, cracks are more likely to form on the surface rather than inside the particle. Failure may occur in large grains ahead of small grains.