Focusing on the fact that cobalt oxide (COO) and nickel oxide (NiO) form a good solid solution, we examined the behavior of this material in molten carbonate in order to evaluate the feasibility of the NiO cathode coated with stable lithium cobaltite (LiCoO2). In the molten carbonate at 650 degreesC, the CoO-NiO, solid solution created LiCoO2 on its surface. The solubility measurement results indicated a suppression of the Ni dissolution which seems attributable to the LiCoO2 coating. However, during the long-range immersion treatment, the LiCoO2 layer on the solid solution surface grew and partially broke away from the solid solution matrix. In the powder-based immersion test, it was also verified that LiCoO2 particles grew to relatively large ones. For use as an alternative cathode material, it is necessary to suppress the crystal growth of the LiCoO2 coating layer.