Low-Temperature Silicon Anodes from Biosilica via AlCl3-Assisted Magnesiothermic Reduction

Abstract

Silicon is a high-capacity anode material, yet its scalable production from sustainable precursors re- quires low-temperature and controllable synthesis routes. Diatom-derived SiO2 provides an abundant biogenic feedstock, but its conversion to silicon by magnesiothermic reduction (MgTR), typically con- ducted at 600900 ◦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 tem- perature and improve reduction eciency. By correlating heating ramp rate, isothermal hold time, and salt-to-silica ratio with phase evolution and silicon yield, we identify the parameters governing oxygen abstraction and Si formation. Time-resolved in situ synchrotron X-ray diraction provides direct evidence of the reduction mechanism, revealing the early formation of metallic Al as the eec- tive reducing species and establishing MgAl2Cl8 as a key intermediate controlling chlorine-mediated oxygen transfer. Silicon formation proceeds within a chloride-rich molten phase and is achieved at temperatures as low as 250300 ◦C. The silicon yield is primarily dictated by heating conditions and AlCl3 content, with optimized parameters maximizing Si fraction while suppressing inactive byprod- ucts. Electrochemical evaluation of graphiteSiOx electrode blends demonstrates enhanced reversible capacity relative to graphite together with stable cycling and high coulombic eciency after stabi- lization. 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.