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, ΔEdecomp, dissolved in the molten salt by cyclic voltammetry were not sufficiently precise because the oxidation wave of N3- 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 ΔEdecomp, and could be measured with higher accuracy. With this method, the activity of Li3N in molten salt was determined for anionic fractions of N3- ranging from xN3- = 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 (xN3- < 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 xN3- is around 10-3 at temperatures investigated in this study. ©2000 The Japan Institute of Metals.