A new model of lithium-ion transport processes in the LiFePO4 electrode is proposed. This model takes into account the phase transition LiFePO4 ↔FePO4 accompanying reversible lithium intercalation into the electrode during potential or current steps. The diffusion coefficient of Li+ ion and its dependence on the LiFePO4/FePO4 phase ratio have been determined by means of processing of experimental potential and current transients in accordance with the model's equations. The results of galvanostatic and potentiostatic intermittent titration techniques are in good agreement. The value of diffusion coefficient varies within 10−10–10−16 cm2 s−1 depending on the lithium content in solid solution Li X FePO4 and Li1−X FePO4 (X <0.02) or the LiFePO4/FePO4 phase ratio.