The magnesiothermic reduction of silica (SiO₂) has been widely explored as a sustainable synthetic route for Si-based anode materials toward advanced lithium-ion batteries. This method utilizes earth abundant SiO₂ as a compelling precursor for Si, which is readily attainable from, for example, sea sand, rice husk and diatom frustules. Also, the formation of 3D porous structure via simple removal of magnesium oxide (MgO) is beneficial to improving battery performance since it can help to accommodate 300% volume change during battery operation.

However, many studies have consistently reported incomplete conversion after the synthesis, which is typically in the form of undesirable remnants or side products: unreacted SiO₂ and intermediate magnesium silicide (Mg₂Si) that need to be chemically removed along with MgO, resulting in active material (Si) loss and uncontrollable porous structure. In addition, to enhance battery performance, it is widely believed that carbon protecting layer should be deposited on Si surface to obtain stable solid electrolyte interphase. The additional process for carbon coating, however, often requires costly and time/energy consuming steps and, therefore, hinders the practical deployment of Si anodes.

To obtain new insights into the incomplete conversion process, we carried out real-time phase evolution studies via in-situ high temperature X-ray diffraction during the magnesiothermic reduction of SiO₂ under various thermal treatment conditions. Based on these results, we proposed a novel processing route that can minimize the active material loss and produce in-situ carbon coating on macroporous Si by utilizing gaseous carbon dioxide (CO₂) as mild oxidant and precursor for carbon layer. The near-synchrotron X-ray nano-computed tomography was employed to obtain the processing-structure-property relationship. CO₂-derived, in-situ carbon coated macroporous Si showed good initial Coulombic efficiency (86%) and high reversible capacity of 2000 mAh/g at 0.2 C-rate with promising cycling performance.