Higher power Li ion rechargeable batteries are important in many practical applications. Higher power output requires faster charge transfer reactions in the charge/discharge process. Because lower activation energy directly correlates to faster Li ion diffusion, the activation energy for ionic diffusion throughout the electrode materials is of primary importance. In this study, we demonstrate a simple, versatile electrochemical method to determine the activation energy for ionic diffusion in electrode materials via temperature dependent capacitometry. A generalized form of the temperature dependence of the discharge capacity was derived from the diffusion equation. This method yielded activation energy values for Li ion diffusion in LiCoO2 comparable to those obtained from ab initio calculations.