Electrochemical reduction behavior of Si(IV) ions in LiF-SiO2 and KF-SiO2 melts at 1173 K

Metal electrodeposition from metal oxides is beneficial not only in the terrestrial metal smelting process but also in the process of recovering metals from minerals on the Moon and Mars. In the electrochemical process, a binary melt of metal oxide and metal fluoride can be a promising reaction media for constructing a practical electrolytic system that allows metal recovery directly from metal oxide at lower temperatures. In this study, the electrochemical reduction behavior of Si(IV) ions in LiF-SiO₂ and KF-SiO₂ melts at 1173 K, which included 10 mol% SiO₂, was studied toward controlling the electrodeposition process of crystalline Si in high-temperature melts with higher SiO₂ composition ratios. The dissolution behavior of SiO₂ was investigated by high-temperature Raman spectroscopy and density functional theory calculation, revealing that Si(IV) complex ions such as Si₃F₇–2 K are formed in the KF-SiO₂ system, whereas Si(IV) ion was not easily bonded to constituent ions of the LiF-SiO₂ melt. It was demonstrated that polycrystalline Si could be obtained by potentiostatic electrolysis in both melts, and the reduction rate of Si(IV) ions was faster in the LiF-SiO₂ system. The Raman spectroscopy and XRD analysis revealed that the electrodeposits were the polycrystalline Si and the alloys such as Li-Si and K-Si. The reported data on the interaction between fluoride ions and Si(IV) ions in the binary melt of metal fluoride and SiO₂ will contribute to realizing an in-situ resource utilization process that enables one-step Si recovery from SiO₂.