To achieve a high energy density for Li-ion batteries, it is important to optimize the electrode thickness and electrode density. It is common to design the electrodes to be thick and dense to achieve a high energy density. However, highly tortuous transport paths in thick and dense electrodes can lead to severe transport losses, which negatively affect the cell performance. In this work, we investigated the effects of varying the electrode thickness and density on lithium ion transport in the electrolytes by means of both experiments and simulations. Both results indicated that an additional capacity loss occurs from the electrode with low porosity because the effective diffusivity decreased in the electrolyte phase. Optimal ranges of the electrode thickness and density (porosity) that can be used to help design high-power LiFePO4/graphite batteries were also suggested in this work.