Due to the dissolution of the lithiated nickel oxide cathode, the life expectancy of a molten carbonate fuel cell is reduced. The use of a Li/Na carbonate electrolyte is expected to lead to a higher voltage and a longer life expectancy due to its higher ionic conductivity and its lower nickel oxide cathode solubility. Using the Li/Na electrolyte, single cells have been tested to evaluate their performance and their life expectancy. Empirical equations for these cells have been presented to determine the temperature, the CO2 partial pressure in the cathode gas, and the matrix thickness. The results prove that the life expectancy of Li/Na cells is reduced by nickel short-circuiting in comparison to Li/K cells, for which the life expectancy is many times longer. The dependence of the nickel-containing particle distribution in the matrix on the temperature has been evaluated using an image processing method. At 973 K, most of the particle distribution moves toward the anode more rapidly than at 873 K, because the rate of particle growth is lower at the higher temperature, and the particles move toward the anode due to the convection of the molten carbonate in the matrix. The initiation time for nickel short-circuiting was derived from the results of this study to explain the relationship between the shorting conductance and the volume of nickel-containing materials in the matrix porosity. Moreover, the results show that the predominant element contributing to short-circuiting is the nickel oxide, and not the metal.