Effects of Magnesiothermic Reduction Process on SiO_x Anode Materials for Lithium-Ion Batteries

As regulations on internal combustion engine vehicle have been tightened due to the recent climate crisis, the development of lithium-ion batteries (LIBs) and other electric vehicle batteries is actively undergoing. A LIB consists of an anode, a cathode, an electrolyte, and a separator. Among them, high capacity and energy density in the anode materials are crucial for development. Graphite has been used as a representative anode material, allowing Li insertion and extraction during charging and discharging. However, its energy density improvement is limited by its low theoretical capacity (372 mAh/g). Thus, there is an ongoing need to develop high performance anode materials.<br/>Silicon has a theoretical capacity about 4,200 mAh/g much higher than that of graphite. However, a critical issue with Si anodes is capacity reduction due to significant volume expansion and contraction (300% or more) during the alloying and dealloying processes of Li. To address this challenge, extensive studies has been dedicated to silicon-based anode materials. Among these, the synthesis of SiO_x (0 &lt; x &lt; 2) particles have attracted much attention due to their ease of synthesis, high chemical and structural stability, and reasonable cost. Additionally, the expansion issue in anode materials is mitigated by the formation of lithium oxides during the initial charging/discharging cycles.<br/>We utilized a magnesiothermic reduction reaction (MRR) process, a widely used method for synthesizing silicon oxide particles. The reaction, SiO₂ + 2Mg -&gt; Si + 2MgO, involves the simple mixing and reduction of SiO₂ particles with Mg powder, producing SiO_x (0 &lt; x &lt; 2) particles. Since the reaction is exothermic, SiO₂ can be converted to SiO_x (0 &lt; x &lt; 2) at a lower temperature compared to conventional methods. Additionally, the reduction process results in smaller SiO2 particles, which can alleviate volume expansion during cycling and potentially extend the lifespan of LIBs by reducing structural degradation in anode electrodes.<br/>We synthesized SiO_x particles through the MRR process and employed them as an anode material for high-capacity LIBs. Sodium silicate solution and tetraethyl orthosilicate were employed as precursors for SiO₂. Spherical SiO₂ particles, approximately 100-500 nm size, were synthesized using a sol-gel method. After mixing the synthesized SiO₂ with Mg power, SiO_x (0 &lt; x &lt; 2) particles were obtained through the MRR process under an argon and hydrogen gas mixture. Standard characterization methods, such as SEM, XRD, Raman spectroscopy, and XPS, were used to analyze the particle’s shape, crystallinity, and chemical composition.<br/>After optimizing the anode composition of graphite and SiO_x particles, we fabricated a half-cell for electrochemical characterization. We observed a significant increase in initial discharge capacity with the increase of reduction temperature. Finally, we will present a comprehensive analysis of the relationship between silicon content in the particles, charge capacity, and capacity retention rate to elucidate the effects of the MRR process.