Electrochemical investigation of the P2–NaxCoO2 phase diagram

Sodium layered oxides Na x CoO 2 form one of the most fascinating low-dimensional and strongly correlated systems; in particular P2–Na x CoO 2 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 x CoO 2 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 1/2 CoO 2 and Na 2/3 CoO 2 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. Sodium layered oxides are low-dimensional and strongly correlated systems that have been extensively studied because of their intriguing structural and physical properties. Electrochemical sodium intercalation is now used to investigate their different phase domains and thermal stability.