Low-temperature hydrothermal synthesis of the three-layered sodium cobaltite P3-NaxCoO2 (x ∼ 0.60)

• We report direct synthesis of the high temperature stable phase, P3-Na0.6CoO2.
• The hydrothermal synthesis of P3-Na0.6CoO2 involves one step and low temperature.
• The yield diagram for Na–Co–H2O system has been builded up to 250 °C.
• We propose a formation mechanism of P3-Na0.6CoO2 phase using the unit cell theory.
• The thermal stability of P3-Na0.6CoO2 has been investigated by means of HT-XRD.

In order to obtain the layered sodium cobalt oxide materials by hydrothermal synthesis, the yield diagram for Na–Co–H2O system has been built and studied. In the same time, the well-known data of Co–H2O system have been extended at 250 °C in basic solution.We had first synthesized directly the high temperature stable phase, P3-Na0.6CoO2P3-Na0.6CoO2 by a one-step low-temperature hydrothermal method. The rhombohedral structure of P3-Na0.6CoO2P3-Na0.6CoO2 has been determined by X-ray diffraction (XRD) and the purity of phases has been confirmed by XPS. The thermal stability of P3-Na0.6CoO2P3-Na0.6CoO2 has been investigated by means of high temperature X-ray diffraction in 298–873 K range and when the temperature has reached 723 K, the completely transformation of P3-Na0.6CoO2P3-Na0.6CoO2 in the rhombohedral stable phase α-NaCoO2α-NaCoO2 (space group R-3m  ) was observed. Also, a formation mechanism of P3-Na0.6CoO2P3-Na0.6CoO2 phase using the unit cell theory in the hydrothermal process was proposed.

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