Isothermal, physics-based cell sandwich model has been validated with experimental galvanostatic discharge data at various current densities for advanced lithium-ion cells. The overpotential is higher at higher current densities and mostly increases with discharge at any given rate. The various contributions to cell overpotential during galvanostatic discharge at 5C and 3C constant-current densities are quantified in detail and analyzed. This work will aid in research and development efforts to mitigate these limitations for better cell performance in potential advanced electric vehicle applications.