Metal-Mediated Chlorine Transfer for Molten Salt-Driven Thermodynamic Change on Silicon Production

The development of silicon (Si) material poses a great challenge with profound technological advancements for semiconductors, photo/photoelectric systems, solar cells, and secondary batteries. Typically, Si production involves the thermochemical reduction of silicon oxides, where chloride salt additives help properly revamp the reaction mechanism. Herein, we unravel the chemical principles of molten AlCl₃ salt in metallothermic reduction. Above its melting temperature (T_m ≈ 192 °C), three AlCl₃ molecules coordinate with each metal (M) atom (e.g., conventional Al and Mg, or even thermodynamically unfeasible Zn) to form metal-AlCl₃ complexes, M(AlCl₃)₃. In the molten AlCl₃ salt media, all complexes directly lead to the universal formation of AlOCl byproduct and as-reduced Si spheres through internal Cl^* transfer during the reduction reaction. Intriguingly, highly oxophilic metal (i.e., Mg) establishes additional energetic shortcuts in reaction pathways, where AlCl₃ directly detaches an oxygen atom, accompanied by strong metal-oxygen interactions and Cl^* transfer within the same complex. Moreover, the thermodynamic stability of the metal-AlCl₃ complex residue (MAl₂Cl₈) and the microstructure of post-treated Si do change according to the metal choice, imparting disparate physicochemical properties for Si. This work offers insights into the scalable production of tailored Si materials for industrial applications, along with cost-effective operations at 250 °C.