Effect of Electrode Shape on Lithium Metal Fog in LiCl-KCl

Abstract

INTRODUCTION Molten salt electrolysis is a common production process of reactive metal. The so-called metal fog formation causes the decrease in current efficiency, whereas it can be used as reductant because of its high reactivity. In our previous research, two types of metal fog were observed in LiCl-KCl; one was dark blue, and considered the dissolution of Li metal. The other had gray color and seemed to start forming at the edge of the electrode. Since this gray fog was seen at high current density, it was suggested that the current concentration at the edge caused its formation. In this paper, the effect of the surface shape of electrode on the gray metal fog formation was investigated in LiCl-KCl. EXPERIMENTAL LiCl-KCl eutectic mixture was melted at 700K. Cyclic voltammetry and potentiostatic electrolysis were performed. Mo rods of various diameters with/without screw cutting were used as working electrode. The surface area of the screw-cut electrode was estimated assuming the series of conic shapes. A carbon rod was used as counter electrode. A reference electrode was an Ag/Ag+couple in a mullite membrane. The appearance around the working electrode was directly observed to confirm the fog formation. RESULT AND DISSCUTION Cyclic voltammograms at the working electrodes of f2.0mm with/without screw cutting surfaces are shown in Fig. 1. The dark blue metal fog was formed at both electrodes, while the gray metal fog was generated only at the screw-cut electrode. The gray metal fog was observed with current decrease as reported in our previous study. It was suggested that edges affected the gray metal fog formation. In the case of f4.9mm electrode, the gray metal fog formation didn’t occur regardless of the electrode shape. The gray metal fog appeared at the screw-cut electrode of f1.5m, and the formation was intenser than the electrode of f2.0mm. To compare the voltammograms at the screw-cut electrode of different diameter, the results were re-plotted with the linear current density (Ia /Acm-1) as shown in Fig. 2, where the linear current density was defined as the division of electrolytic current by the length of the ridge. The linear current density of f2mm and f1.5mm were approximately the same, while that of f4.9mm was very small. In the formers case, the gray metal fog appeared, but its formation didn’t occur in the later. These results suggest that the gray fog formation strongly depends on the linear current density and that the current concentration should cause its formation. The changes in current density during constant potential electrolysis at the screw-cut electrodes of different diameters are shown in Fig. 3. In the electrode of f2.0mm, the gray metal fog was seen for 10 sec from the beginning, and then its formation stopped. Lithium metal deposit was seen after the stop of the formation, and the sphere-like deposit fully covered the surface about 40 sec. The behavior of the electrode of f1.5mm was almost the same, but the time length of the gray fog formation was longer than at the f2.0mm-electrode. In the electrode of f4.9mm, the gray metal fog didn’t appear from the beginning. The gray fog formation occurred at a fine electrode intenser than at a thick electrode, and screw cutting seemed to accelerate the formation. However, lithium metal deposition occurred simultaneously, and the effect by screw cutting was lost by its covering the surface. CONCLUSIONS The effect of the surface shape of electrode on metal fog formation was investigated in LiCl-KCl. Screw cutting accelerated the gray metal fog formation and the formation also depended on the diameters of the electrodes. It is concluded that the shape of electrode strongly affects the gray metal fog formation and the current concentration at the edge causes the formation. Figure 1