Local current density is an important parameter in battery modeling, which affects the performance of lithium-ion batteries. In this study, we take LiFePO4 cathode material as an example. A simplified mathematical model has been developed to study the internal mechanism of the electrode. According to the results of the model, the local current density distribution has a regular change at different time in the discharge process. The parameter “critical thickness” as an optimized variable has been presented for battery design. By qualitative analysis to estimate the critical thickness under different condition, we can optimize the design parameter of a battery according to the practical demand.