Mechanisms and Product Options of Magnesiothermic Reduction of Silica to Silicon for Lithium-Ion Battery Applications

Abstract

Lithium-ion batteries (LIBs) have been one of the most predominant rechargeable power sources due to their high energy/power density and long cycle life. As one of the most promising candidates for the new generation negative electrode materials in LIBs, silicon has the advantages of high specific capacity, a lithiation potential range close to that of lithium deposition, and rich abundance in the earth’s crust. However, the commercial use of silicon in LIBs is still limited by the short cycle life and poor rate performance due to the severe volume change during Li++ insertion/extraction, as well as the unsatisfactory conduction of electron and Li+ through silicon matrix. Therefore, many efforts have been made to control and stabilize the structures of silicon. Magnesiothermic reduction has been extensively demonstrated as a promising process for making porous silicon with micro- or nanosized structures for better electrochemical performance in LIBs. This article provides a brief but critical overview of magnesiothermic reduction under various conditions in several aspects, including the thermodynamics and mechanism of the reaction, the influences of the precursor and reaction conditions on the dynamics of the reduction, and the interface control and its effect on the morphology as well as the final performance of the silicon. These outcomes will bring about a clearer vision and better understanding on the production of silicon by magnesiothermic reduction for LIBs application.

Endothermic Dehydrogenation-Driven Preventive Magnesiation of SiO for High-Performance Lithium Storage Materials

“…43,44 Note that the formation of the Mg 2 SiO 4 phase is thermodynamically more favorable compared to the formation of the MgO phase, which is a typical product in MTR of SiO 2 (Table S1). 25,31…”

“…21−23 Among the other elements, Mg was regarded as one of the most promising elements to improve the ICE of SiO x anode materials. 24,25,30 Magnesiothermic reduction (MTR) is the thermal reduction of SiO x with Mg, which reduces SiO x into Si with Mg-containing oxides, such as MgO and magnesium silicates, being etched out to obtain the Si phase. 25,31 Similar to MTR, the magnesiation process improves the ICE of SiO x by a preclusive reaction of a-SiO 2 in SiO x with Mg to form Mg-based oxides.…”

“…24,25,30 Magnesiothermic reduction (MTR) is the thermal reduction of SiO x with Mg, which reduces SiO x into Si with Mg-containing oxides, such as MgO and magnesium silicates, being etched out to obtain the Si phase. 25,31 Similar to MTR, the magnesiation process improves the ICE of SiO x by a preclusive reaction of a-SiO 2 in SiO x with Mg to form Mg-based oxides. 24,32 However, the magnesiation process of SiO x should be carefully designed to avoid the coarsening of the active Si phase, which is well known to cause poor cycle performance of Si-based anode materials.…”

“…Anode materials with higher capacity than the existing graphite anode materials have been intensively pursued to this end. − Silicon is among the most promising anode materials because of its theoretical capacity of 3580 mAh g –1 , about 10 times higher than graphite (375 mAh g –1 ). − However, pulverization and electrical contact loss of electrode materials and the delamination of the electrode from the current collector led to poor cycle performance of Si-based high-capacity anode materials, mainly due to a violent volume expansion of Si (∼300%) during cycling. − To improve the cycle performance of Si-based anode materials, various material concepts, including Si/C, SiO x, , and Si/Alloy, have been suggested as high-capacity anode materials for LIBs. SiO, an amorphous nanocomposite of Si and SiO 2 with the same molar ratio, has four times higher specific capacity than graphite and greater cycle performance than Si, thus being partly adopted in the commercial LIBs. − However, SiO x has a chronic problem of low initial Coulombic efficiency (ICE) because of its irreversible electrochemical reaction with lithium that forms lithium silicate phases in the initial cycle, which leads to the consumption of Li-ions coming from the cathode material and the decrease in the practical energy density of the full cell. , To resolve this issue of SiO x , numerous efforts have been devoted to preventing the irreversible electrochemical reaction of a-SiO 2 in SiO with lithium ions by preclusive reaction of the a-SiO 2 phase before cycling. − Based on the Ellingham diagram, several elements, including Li, C, Mg, and Al, were suggested as the effective elements that react with the a-SiO 2 phase before cycling, thereby increasing the ICE of SiO x . ,− Preventive formation of the Li–Si–O phase, which is also known as prelithiation, showed some promise in improving the ICE of SiO x anode materials. , However, the widely used materials for prelithiation are Li-metal and lithium-based organic materials, which are vulnerable to moisture and oxygen in the air, thus causing safety problems in their handling. − Among the other elements, Mg was regarded as one of the most promising elements to improve the ICE of SiO x anode materials. ,, Magnesiothermic reduction (MTR) is the thermal reduction of SiO x with Mg, which reduces SiO x into Si with M...…”

Formation of silicon layer through aluminothermic reduction of quartz substrates

“…Can be used as reductant metals to reduce SiO 2 and liberate elemental Si by removing O 2 (Ellingham, 1944). Several researchers have already reported the magnesiothermic reduction of SiO 2 at a temperature as low as around the melting temperature of magnesium (Entwistle et al, 2018;Tan et al, 2021). Mg is supposed to be a better reductant compared to Al, since Gibbs's free energy for the reaction with Mg and SiO 2 is more negative compared to that of the reaction between Al and SiO 2 (Ellingham, 1944).…”