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

A comparative study was conducted on the electrodeposition of Si using molten salts of AF-ACl-A2SiF6 (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 > Cs systems with reference to Cl2/Cl-, and almost the same as with reference to F2/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 decompo-sition of SiF62-ions. The differences in the Si electrodeposition and thermal stability of SiF62-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 SiF62-. 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.