 Increasing electrode thickness, thus increasing the volume ratio of active materials, is one effective method to enable the development of high energy density Li-ion batteries. In this study, an energy density versus power density optimization of LiNi0.8Co0.15Al0.05O2 (NCA)/graphite cell stack was conducted via mathematical modeling. The energy density was found to have a maximum point versus electrode thickness (critical thickness) at given discharging C rates. The physics-based factors that limit the energy/power density of thick electrodes were found to be increased cell polarization and underutilization of active materials. The latter is affected by Li-ion diffusion in active materials and Li-ion depletion in the electrolyte phase. Based on those findings, possible approaches were derived to surmount the limiting factors. The improvement of the energy–power relationship in an 18,650 cell was used to demonstrate how to optimize the thick electrode parameters in cell engineering.