Electrodeposition of Crystalline Si in Molten Alkali Metal Fluoride–Chloride Mixtures: Comparative Study of Li, Na, K, and Cs Systems

• The potential for Si deposition was negative for Na, K, and Cs, in that order with reference to Cl 2 /Cl − and almost the same as with reference to F 2 /F − . • The decomposition of SiF 6 2− ions was observed in the Na and Li systems, but not in K and Cs systems. • Film-like and whisker-like Si were electrodeposited in the K and Cs systems and Na system, respectively. A comparative study was conducted on the electrodeposition of Si using molten salts of AF–ACl–A 2 SiF 6 (A = Li, Na, K, Cs, AF–ACl = 50:50 mol%) at 1073 K. Cyclic voltammetry revealed that the electrodeposition potential of Si was negative in the order of Na, K, and Cs systems with reference to Cl 2 /Cl − , and almost the same as with reference to F 2 /F − . In the Li system, no cathodic current peak was observed, owing to the formation of the Li–Si alloy. In the Na and Li systems, the cathodic peak current density decreased with time owing to the decomposition of SiF 6 2− ions. The differences in the Si electrodeposition and thermal stability of SiF 6 2− ions can be explained by the interactions of alkaline cations and F − ions; smaller cations interact more strongly with F − , resulting in the reduced thermal stability of SiF 6 2− . Electrodeposited Si was confirmed to be crystalline by XRD, and its morphology was film-like in the K and Cs systems, and whisker-like in the Na system. A difference in nucleation and growth was observed between the K and Na systems, which explains the difference in the morphology of the Si deposits.