Abstract
Silicon is a high-capacity anode material, yet its scalable production fromsustainable precursors requires low
temperature and controllable synthesis routes. Diatom-derived SiO2 provides an abundant biogenic
feedstock, but its conversion to silicon by magnesiothermic reduction (MgTR), typically conducted at 600
900 °C, is limited by the highly exothermic nature of the reaction, which induces local overheating,
promotes side-phase formation, and often results in incomplete SiO2 reduction. Here, we elucidate the
reaction pathway of AlCl3-assisted MgTR as a strategy to decrease synthesis temperature and improve
reduction efficiency. By correlating the heating ramp rate, isothermal hold time, and salt-to-silica ratio with
phase evolution and the crystalline silicon fraction, we identify the parameters governing oxygen
abstraction and Si formation. Time-resolved in situ synchrotron X-ray diffraction provides direct insight into
the reaction mechanism, revealing the early formation of metallic Al, the transient formation of MgAl2Cl8 as
an intermediate, and the subsequent crystallization of Si concurrent with the consumption of metallic Al,
thereby suggesting that Al acts as an effective reducing agent. Silicon formation proceeds within
a chloride-rich molten phase and is achieved at temperatures as low as 250–300 °C. The crystalline silicon
fraction is primarily dictated by heating conditions and AlCl3 content, with optimized parameters
maximizing the Si fraction while suppressing inactive byproducts. Electrochemical evaluation of the
graphite-SiOx electrode blends demonstrates enhanced reversible capacity relative to graphite together
with moderate cycling stability, confirming the electrochemical activity of the synthesized material. Overall,
this work unveils the mechanistic framework of AlCl3-assisted MgTR and provides synthesis guidelines for
the low-temperature conversion of diatom biosilica into silicon-based anode materials.