Physicochemical studies of lithium nitride in molten LiCl-KCl

Abstract

Thermodynamic properties of lithium nitride in the molten LiCl-KCl eutectic salt were explored using electrochemical methods at temperatures between 723 and 900 K. Attempts to measure the decomposition voltage of Li₃N, Δ _Edecomp, dissolved in the molten salt by cyclic voltammetry were not sufficiently precise because the oxidation wave of N^3- ion could not be located with high accuracy. As an alternative method, lithium activity of Pb-Li alloy in equilibrium with the molten salt containing dissolved Li₃N under nitrogen atmosphere was measured electrochemically. Under the conditions used in this study, the electrochemical potential of the Li-Pb electrode with reference to Li/Li⁺ electrode is equal to the Δ _Edecomp, and could be measured with higher accuracy. With this method, the activity of Li₃N in molten salt was determined for anionic fractions of N^3- ranging from x_N^3-=3× 10^-4 to 0.015. The nitride ion concentration in this salt was determined by chemical analysis, and compared with nominal values. The activity coefficient of Li₃N at high dilution (x_N^3-<10^-3) was found to be very low, around 10^-2 to 10^-4 at 723 K. The activity coefficient increases sharply with composition and has a value in the range 10^-1 to 1 when x_N^3- is around 10^-3 at temperatures investigated in this study.