Residual silica removal and nanopore generation on industrial waste silicon using ammonium fluoride and its application to lithium-ion battery anodes

• A new process using ammonium fluoride (NH 4 F) to remove residual SiO 2 was developed. • Purity of Si was improved to a level similar to that of treatment with HF solution. • Nanopores were also generated, resulting in an increase in the surface area. • A reaction mechanism was proposed and the safety of the process was demonstrated. • The modified Si showed improved performance as an anode of Li-ion batteries. Silicon is considered an important industrial element, primarily owing to its use in semiconductors, which are key components of various electronic devices; thus, the development of low-cost and efficient purification silicon processes has become a research focus. In this study, we developed a new process using ammonium fluoride (NH 4 F) to remove residual silica (SiO 2 ) and generate nanopores on raw silicon (Si) materials without the use of highly toxic reagents and expensive procedures. We conducted low-temperature heat treatment of raw Si with NH 4 F under an inert atmosphere and found that the purity of raw Si was improved to a level similar to that of raw Si treated with hydrofluoric acid solution. Moreover, through this process, the surface area of raw Si was increased by surface tearing and the formation of nanopores. We also proposed a reaction mechanism for removing residual SiO 2 from raw Si through X-ray diffraction analysis and demonstrated the safety of the process by analyzing the by-products generated during the heat treatment. When the modified Si material was applied to Li-ion battery anodes, they showed improved capacity, initial coulombic efficiency, and cycle performance as compared with those using raw Si material. We expect that this new NH 4 F-based method will not only be used to modify various Si materials for their efficient application, but will also be used to obtain nanostructured materials that require the removal of SiO 2 during synthesis.