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

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. first synthesized directly the high temperature stable phase, P 3 - N a 0.6 C o O 2 by a one-step low-temperature hydrothermal method. The rhombohedral structure of P 3 - N a 0.6 C o O 2 has been determined by X-ray diffraction (XRD) and the purity of phases has been confirmed by XPS. The thermal stability of P 3 - N a 0.6 C o O 2 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 P 3 - N a 0.6 C o O 2 in the rhombohedral stable phase α - N a C o O 2 (space group R-3m) was observed. Also, a formation mechanism of P 3 - N a 0.6 C o O 2 phase using the unit cell theory in the hydrothermal process was proposed.