The microstructure of electrodes significantly affects the performance of lithium-ion batteries (LiBs), and using bi-diameter active particles is a simple but effective way to regulate the microstructure of commercial LiB electrodes. Herein, to optimize the LiB cathode of bi-diameter active particles, a microstructure-resolved model is developed and validated. The results indicate that randomly packing of bi-diameter active particles is optimal when the electrolyte diffusion limitation is mild, as it provides the highest volume fraction of active materials. Under strong electrolyte diffusion limitations, layered packing with small particles near the separator is preferred. This is because particles near the current collector have a low lithiation state. Besides, optimizing the random packing can further improve the energy density. For energy-oriented LiBs, a low volume fraction of small particles (0.2) is preferred due to the higher volume fraction of active materials. For power-oriented LiBs, a high volume fraction of small particles (0.8) is better because it reduces diffusion limitations. This work should serve to guide the optimal design of electrode microstructure for achieving high-performance LiBs.