(Digital Presentation) Electrodeposition of Si Metal in Molten CaCl2 with the Molar Ratio of CaO to SiO2=1.8, 2.2 Compounds

Abstract

An innovative production process of metallic Si is desired to reduce energy consumption and CO2 exhaustion. The authors carried out electrolysis in molten CaCl2 with CaSiO3 above 1300 K, and showed that Si metal could be electrodeposited. The influence of the molar ratio of CaO to SiO2 (R CaO/SiO2 , the same hereinafter) on Si metal deposition was also examined using various calcium silicates, and it was shown that R CaO/SiO2 seemed not to affect the deposition strongly. The suitable condition seemed R CaO/SiO2 =2.0, but it was suggested that the detailed study around R CaO/SiO2 =2.0 was necessary. In this study, the effect of Si electrodeposition in molten CaCl2 containing calcium silicates of R CaO/SiO2 =1.8, 2.2 was investigated, and the appropriate condition for Si metal deposition was investigated. The calcium silicates of R CaO/SiO2 =1.8 and 2.2 were prepared by sintering a molded mixture of CaO and SiO2. Molten CaCl2 containing the prepared calcium silicate was used in the baths at 1673 K. The concentration of solute in the bath was represented by the value calculated from SiO2 amount. A MoSi2 rod with a BN receiver to collect the electrodeposit efficiently was used as the working electrode. A graphite rod and a Mo wire were used as counter and quasi-reference electrodes, respectively. The potential of the quasi-reference electrode was calibrated against the Mo dissolution potential measured by cyclic voltammetry. The cathodic reaction was investigated by cyclic voltammetry, and potentio-static electrolysis was carried out at -1.85 ~ -2.1 V[vs. Mo diss., (the same hereinafter)]. The obtained electrodeposit was analyzed by XRD and SEM-EDX. Current peaks in cyclic voltammogram were not observed in all baths, which is thought due to high temperature. However, the cathode current got larger with increasing R CaO/SiO2 even in the bath of the same SiO2 concentration. This result showed that the reduction of Si ion in the bath occurred, and that the reduction depended on R CaO/SiO2 . In the baths of R CaO/SiO2 =1.8, metallic Si was deposited by the potentio-static electrolysis at -1.85 ~ -2.1 V. The current efficiency was about 28 % at -1.95 V, and slightly decreased at 2.1 V as shown in Fig.1. In the bath of R CaO/SiO2 =2.2, metallic Si was also obtained in the same potential region, but the current efficiency sharply decreased below -2.0 V. It was shown in our previous study that Ca co-deposited with metallic Si by electrolysis at -2.0 V in the bath of R CaO/SiO2 =2.0, and that the current efficiency became low in this condition. These results suggest that co-deposition of Ca with Si occurs more easily in the bath of R CaO/SiO2 2.0 and obstructs the efficient electrodeposition of metallic Si. It was indicated that the bath of R CaO/SiO2 < 2.0 was preferable for the electrodeposition of metallic Si. However, the current efficiency was still limited to around 30 %, so the detailed study to clarify the suitable condition for better electrolysis should be necessary. Figure 1