Explicit study on the oxidation mechanism of nickel in molten Li₂CO₃-K₂CO₃

The oxidation behavior of nickel in Li + K carbonate melt is followed by measuring the open-circuit potential and by electrochemical impedance spectroscopy under an O₂ + CO₂ gas mixture in the ratio 90/10 at a total pressure of 1 atm at 650 °C. X-ray diffraction (XRD) and energy-dispersive spectroscopy are employed for qualitative and quantitative analyses of the different compounds involved during the oxidation of nickel. Atomic force microscopy is used for both imaging the evolution of the oxide layer and determining its surface roughness. The in situ oxidation process of nickel demonstrates three stages: rapid formation of a compact surface oxide (first stage), thicker oxide layer (second stage), and a porous oxide structure (third stage). The lithiation reaction has been identified to occur during the second stage. Formation of an intermediate and unstable compound, namely NiCO₃, has been confirmed by XRD.