Sodium layered oxides Na_xCoO₂ form one of the most fascinating low-dimensional and strongly correlated systems; in particular P2–Na_xCoO₂ exhibits various single-phase domains with different Na⁺/vacancy patterns depending on the sodium concentration. Here we used sodium batteries to clearly depict the P2–Na_xCoO₂ phase diagram for x≥0.50. By coupling the electrochemical process with an in situ X-ray diffraction experiment, we identified the succession of single-phase or two-phase domains appearing on sodium intercalation with a rather good accuracy compared with previous studies. We reported new single-phase domains and we underlined the thermal instability of some ordered phases from an electrochemical study at various temperatures. As each phase is characterized by the position of its Fermi level versus the Na⁺/Na couple, we showed that the synthesis of each material, even in large amounts, can be carried out electrochemically. The physical properties of the as-prepared Na_1/2CoO₂ and Na_2/3CoO₂ ordered phases were characterized and compared. Electrochemical processes are confirmed to be an accurate route to precisely investigate in a continuous way such a complex system and provide a new way to synthesize materials with a very narrow existence range.