Purifying Metallurgical-Grade Silicon to 4N with 3D Porous Structure by Integrated Metallurgy-Materials Phase Separation

• A phase separation method consisting of alloying, nitriding, and etching is described. • Metallic impurities such as Ca, Fe, and Al and non-metallic impurities such as B and P are removed with high efficiency. • This simple, economical, and environmentally green process is conducted at a low temperature. • The process achieves purification and nanostructure formation simultaneously. Silicon (Si) is widely used in photovoltaics, semiconductors, and lithium-ion batteries but high purity is required in most applications. Conversion of metallurgical-grade Si (MG-Si) to Si with 4N purity and desired structure by an economical and environmentally friendly technique is still challenging, albeit desirable. Herein, an integrated metallurgy-materials technique is described to produce high-purity Si (99.99%) with a three-dimensional (3D) porous structure from metallurgical Si. This green and cost-effective strategy involves controllable phase separation of impurities from the Si matrix via Mg alloying, nitriding/dealloying (< 800 °C), and acid etching, which can remove metallic impurities (Fe, Al, Ca, et al.) and non-metallic impurities (B, P, et al) with the efficiency of above 90% and 80% respectively. The intermediate product of Mg 3 N 2 serves as both the pore-forming medium and impurity carrier /remover to separate impurities, improve the exposed area, and enhance dissolution of impurities. Different from conventional metallurgical processes, B and P are converted into MgB 2 and Mg 3 P 2 resulting in easy removal during phase separation. The as-obtained 3D porous Si shows outstanding Li-ion storage performance because of high-purity and unique architecture. This green process is effective in purifying Si and forming a porous structure on a large scale as demonstrated by the conversion of metallurgical Si to high-purity Si. Moreover, the integrated metallurgy-materials phase separation technique can be extended to other industrial applications.