Preparation of hexagonal plate NaxCoOα・nH2O crystals via the thermal decomposition of sodium hexanitrocobaltate (III) and their properties

NaxCoO2 is a widely used cathode material for sodium-ion batteries, which are expected to replace lithium-ion batteries. In this study, NaxCoOα・nH2O crystals were prepared from Na3[Co(NO2)6] using a complex pyrolysis method. According to thermogravimetric (TG)?differential thermal analysis results, Na3[CoIII(NO2)6] was pyrolyzed into a mixture of NaNO3, Na2O, and CoII,III3O4 at 250-350 °C and Na2O and NaCoII,IIIO1.83 above 800 °C. It was found that Na3[Co(NO2)6] existed in the form of thin Co-rich hexagonal platelet crystals embedded into Na-rich or amorphous crystals after heat treatment at 700 °C for 2 h. After further washing with deionized water, only the thin hexagonal platelet crystals were observed, whereas the Na-rich or amorphous crystals disappeared. X-ray diffraction measurements revealed that the hexagonal platelet crystals consisted of a single γ-NaxCoO2 phase. Its chemical composition estimated via energy-dispersive X-ray spectroscopy and TG analysis was Na0.34CoO1.39・0.20H2O. The intragranular and intergranular resistivities of Na0.34CoO1.39・0.20H2O crystals compressed at 100 MPa were 200 and 80 Ω・cm, respectively, at room temperature. Furthermore, Na0.34CoO1.39・0.20H2O powder can be potentially used as a positive electrode in Na-ion batteries.