A coupled electrochemical-mechanical model is developed and applied to predict transient three-dimensional stress fields within reconstructed Li x CoO2 cathode particles from commercial Li-ion batteries. The reconstructed particle geometries are derived from focused-ion-beam–scanning-electron-microscopy (FIB-SEM) experiments. The study uses three individual particles, representing typical sizes and shapes. The mechanical model incorporates measured anisotropic strain within the Li x CoO2 lattice and includes strains due to phase transformations. The stresses are generally found to be compressive in the particle interiors and tensile near the surfaces. Small-scale surface morphology, high Li concentration gradients, and phase transformations are found to have a major influence on the stresses, with particularly high tensile stresses near small protuberances and concave notch-like features on the electrode surfaces. The study considers 1C and 5C discharge rates. The qualitative behaviors are similar at different discharge rates, but the stress magnitudes are higher at higher discharge rates.