The thermodynamic and electrochemical behavior of nickel species are investigated in Li + Na carbonate eutectic containing different amounts of calcium carbonate. NiO dissolution is predicted theoretically, and its solubility is determined experimentally by an electrochemical technique under P(CO2) = 1 atm. The addition of calcium increases the melt basicity and reduces the NiO solubility significantly. Chemical insertion and electrochemical intercalation of alkali ions (mostly Li+) in the NiO lattice are analyzed in Li + Na and Li + Na + Ca carbonate melts under P(CO2) = 0.1 atm + P(O-2) = 0.9 atm + P(CO2) = 0.3 atm + P(O-2) = 0.14 atm by potentiometry, voltammetry and SEM spectroscopy.