We investigate the grain boundary effect on Li-ion diffusivity and intercalation-induced stress in a single-particle Li-ion cell. The measured activation energy for self-diffusion at the grain boundary is a fraction of that in the lattice due to the loosely packed structure, and this results in a diffusivity that is 3–16 orders of magnitude higher in the grain boundary than in the lattice. To study how grain boundaries affect Li-ion battery performance, grain boundaries are modeled inside ellipsoidal cathode (LiMn2O4) particles and placed under potentiodynamic and galvanostatic control simulations. A Voronoi grain distribution is employed in modeling grain boundaries. The grain boundary effect on Li-ion diffusivity is evaluated by computing an apparent diffusion coefficient from the cathode particles containing different grain boundary densities. It is shown that the apparent diffusion coefficient increases with increasing grain boundary densities. With enhanced Li-ion diffusivity, particles are found to have higher capacity utilizations, especially under high discharge C-rates. The inclusion of grain boundaries also lowered intercalation-induced stress by reducing the overall Li+ concentration gradients developed within particles during cycling. However, as local Li+ concentration distribution depends on grain boundary network geometries, intercalation-induced stress varied appreciably within the different grain boundary network geometries.