Cathode using the olivine particles was subjected to an open-circuit voltage measurement under the relaxation condition of 24 h at each SOC and DOD states. The electrochemical reaction in the LiFePO₄ cathode was composed of a large plateau around 3.45 V with sloped regions nearby for both the fully charged and discharged states. It was found that the potential profile in the sloped regions exhibited a hysteresis. Furthermore, both sloped regions became narrower when the operating temperature was raised from 30 to 60°C. Furthermore, it was found that the apparent diffusion coefficient of Li⁺ ions in the sloped regions was much smaller than that in the plateau region. These facts implied that the obtained profiles were not in an equilibrium state with a quasi-OCV profile than the real one, and that the potential relaxation in the sloped regions took an extremely long time.