Determination of Internal Cation Mobilities in the Molten System  ( Li ‐  K  ) Cl at 723 K

Abstract

Internal cation mobilities of Li + and K + in the system (Li-K)CI were determined experimentally using the counter current eleetromigration method over the temperature range 700-778 K, with initial equivalent fractions of KCI of 0.356, 0.416, and 0.451. The relative difference in internal cation mobilities ofLi + and K + was found to reach as much as 7% in this range.Lithium-aluminum/iron sulfide cells that contain multicomponent molten salt electrolytes such as (Li-K)C1 mixture are currently under development for energy-storage applications (1). During charge or discharge of such cells, composition gradients of ions, Li + and K + are induced within the molten salt by the electrode reactions. As a result, the salt compositions are shifted locally from the liquidus range, and precipitation of a solid phase may occur. In lithium/ sulfide cells, which are operated at a temperature range of 400°-430°C in (Li-K)C1 eutectic, Askew and Holland (2) noted that the lithium electrode was severely polarized when .discharged at high current densities. They conjectured that the polarization was caused by precipitation of LiC1 at the lithium electrode. Willars et al. (3) found that the cell capacity was inversely proportional to discharge rate, and the result was interpreted in terms of electrolyte phase separation at the iron sulfide electrode. Braunstein and Vallet (4) and Pollard and Newman (5) calculated current-induced composition profiles of LiC1 and KC1 in lithium/ sulfide cells containing (Li-K)C1 electrolyte. Furthermore, Vallet et al. (6) predicted a composition profile that indicates an increased KC1 concentration near the cathode. More recently, they presented direct evidence of KC1 precipitation (7). Furthermore, they have shown that the composition profile depends not only on the initial composition of the electrolyte, but also on the current density and distance between electrodes. The high KC1 concentration also favors the formation of J-phase (LiKsFe2~S26C1) in the iron sulfide electrode (8). Transition time required for the separation of the components, which was estimated by Braunstein and Vallet (5), is correlated with the transference number and diffusion coefficient. However, transference number is directly related to the emf because the concentration polarization is involved. Thus it is necessary to know the mobilities of the ions at the operating temperature for accurate determination of the transference numbers. At present, the experimental data reported on the mobilities of Li+ and K + in the system (Li-K)C1 are limited to the temperatures at 913 (9) and 1100 K (10), which are much higher than the normal operating temperature of lithium/iron sulfide cells, i.e., 723 K.In this study, the internal cation mobilities, ~.e., the cation mobilities relative to the common anion, were measured in the system (Li-K)CI at the cell operating temperature range by the counter current electromigration method developed by Klemm (11) and Okada (12).

A New Analysis of Internal Cation Mobilities in the Molten Binary System (Li, K)Cl

“…In the binary system (Li, K)Cl, the internal mobilities have so far been measured by several groups. The mole fractions and the temperatures where the data are obtained in the present study are given in Figure on the phase diagram, together with those in previous studies. − …”

“…Other data presented by two groups , were measured by the Klemm method; however, these data cannot be directly compared with the ones in the present study, as the temperature range is lower and the concentration range limited …”

“…Points on the phase diagram of (Li, K)Cl where the internal mobilities have been measured. ●, this work; ○, pure LiCl; ×, Moynihan and Laity; ∗, Smirnov et al; +, Takagi et al; ◆, Lundén and Okada . The mark ↓ indicates the concentrations where the equations of the conductivities, κ, are given as a function of temperature …”

A Molecular Dynamics Simulation of the Molten Ternary System (Li, K, Cs)Cl

“…Further investigations have shown that the difference in the mobilities of cations in such charge-symmetric binary systems may change sign on a change in temperature and/or concentration [2,3]. This Chemla effect in binary molten alkali binary chloride mixtures [4 -10] has been qualitatively interpreted in terms of quasi-complexes [2,3,[5][6][7][8][9] or polarized anions [8]. However, few papers dealt with ternary molten alkali chloride systems [9,10].…”

Internal Cation Mobilities in the Molten Binary Systems (Li, Na)Cl and (Na, K)Cl

“…In the molten alkali halide mixtures (Li, K)C1 [1,2,7,8], (Li, K)Br [9], (Li, Rb)Cl [10], (Li, Cs)Cl [2,11], (Na, K)C1 [2] and (Na, Cs)Cl [2] the internal cation mobilities have been measured. To our knowledge, the internal cation mobilities have not been measured in molten (Li, Na)Cl.…”