Spontaneous colloidal metal network formation driven by molten salt electrolysis

Abstract

The molten salt-based direct reduction process for reactive solid metal outperforms traditional pyrometallurgical methods in energy efficiency. However, the simplity and rapidity of this process require a deeper understanding of the interfacial morphology in the vicinity of liquid metal deposited at the cathode. For the first time, here we report the time change of electrode surface on the sub-millisecond/micrometre scale in molten LiCl-CaCl2 at 823 K. When the potential was applied, liquid Li-Ca alloy droplets grew on the electrode, and the black colloidal metal moved on the electrode surface to form a network structure. The unit cell size of the network and the number density of droplets were found to depend on the applied potential. These results will provide important information about the microscale mixing action near the electrode, and accelerate the development of metallothermic reduction of oxides.

Characterization of the Cathodic Thermal Behavior of Molten CaCl2 and Its Hygroscopic Chloride Mixture During Electrolysis

“…At E = E2, all currentvalues are almost constant, and the cathodic currents increase as the applied potential becomes more negative. This trend is similar to previously reported cyclic voltammograms[13,14,26,27]. The electrochemical reactions at each potential are…”

“…However, in LiCl-KCl, the behavior of Li droplets cannot be confirmed because a thick metal fog is generated in vicinity of the electrode. A previous report confirmed the formation of a metal fog with a hexagonal, network-like structure in LiCl-CaCl2[27]. In these mixtures, at t = 4.000 s, bubbles are generated while the liquid droplets dissolve rapidly (see Supplemental Movies 1 and 2 from the 14-s mark).…”

“…This is because the electrochemical dissolution rate of the electrodeposited metal is limited. According to previous reports[13,27], Ca-Li alloy is deposited in LiCl-CaCl2 and LiCl-KCl-CaCl2 and mainly Li in LiCl-KCl. This suggests that the dissolution rate of the electrodeposited Li is faster than that of Ca at E = E3.…”

“…Although a clear understanding of the cathodic behavior in molten salt is necessary to control and optimize the OS process, the transient behavior of the electrodeposition of liquid Me at the cathode is complicated because it depends on the morphology of liquid Me, including colloidal metal called "metal fog" [22][23][24][25]. In a previous study, we demonstrated that submillisecond high-speed microscopy synchronized with electrochemical measurement at submillimeter scale resolution is possible owing to advances in digital optical technology [26,27]. Such knowledge can be immediately applied to current molten salt electrolytic processes to achieve energy saving from the viewpoint of thermal efficiency in the material industry.…”

Fluid Structure of Molten LiCl–Li Solutions

“…found that a similar structure in the LiCl–KCl melt, a metallic Li “fog”, would be only observed on a “black film” that is formed on an electrode after applying a potential below −0.2 V (vs Li/Li + ) . Natsui et al reported the formation of a colloidal metal network near the electrode, and with a unit cell size and droplet density that depends on the applied potential . In the case of our current system, while the metallic Li is actively introduced into the system electrochemically, the dominant form of metallic Li in the system is suspended Li 8 nanoclusters.…”