Variation in Crystalline Phases: Controlling the Selectivity between Silicon and Silicon Carbide via Magnesiothermic Reduction using Silica/Carbon Composites

Abstract

Magnesiothermic reduction of various types of silica/carbon (SiO 2 /C) composites has been frequently used to synthesize silicon/carbon (Si/C) composites and silicon carbide (SiC) materials, which are of great interest in the research areas of lithium-ion batteries (LIBs) and nonmetal oxide ceramics, respectively. Up to now, however, it has not been comprehensively understood how totally different crystal phases of Si or SiC can result from the compositionally identical parent materials (SiO 2 /C) via magnesiothermic reduction. In this article, we propose a formation mechanism of Si and SiC by magnesiothermic reduction of SiO 2 /C; SiC is formed at the interface between SiO 2 and carbon when silicon intermediates, mainly in situ-formed Mg 2 Si, encounter carbon through diffusion. Otherwise, Si is formed, which is supported by an ex situ reaction between Mg 2 Si and carbon nanosphere that results in SiC. In addition, the resultant crystalline phase ratio between Si and SiC can be controlled by manipulating the synthesis parameters such as the contact areas between silica and carbon of parent materials, reaction temperatures, heating rates, and amount of the reactant mixtures used. The reasons for the dependence on these synthesis parameters could be attributed to the modulated chance of an encounter between silicon intermediates and carbon, which determines the destination of silicon intermediates, namely, either thermodynamically preferred SiC or kinetic product of Si as a final product. Such a finding was applied to design and synthesize the hollow mesoporous shell (ca. 3−4 nm pore) SiC, which is particularly of interest as a catalyst support under harsh environments.

A Natural Transporter of Silicon and Carbon: Conversion of Rice Husks to Silicon Carbide or Carbon‐Silicon Hybrid for Lithium‐Ion Battery Anodes via a Molten Salt Electrolysis Approach

“…The formation of CaC 2 is helpful to induce the formation of SiC because the reaction of CaC 2 with SiO 2 can generate SiC and the reaction is spontaneous. As reported, the reaction of Si and C is governed by the contact surface area of Si and C. Since the CaO has a high solubility in molten CaCl 2 , the generated CaO will dissolve in the molten salt, allowing the sufficient contact of Si and C. Thus, SiC is the only electrolytic product by the electrolysis of C−RHs in molten CaCl 2 . However, MgC 2 forms at a potential more negative than Mg in molten NaCl−KCl−MgC l2 .…”

“…The compositions and morphologies of electrolytic products are different in molten CaCl 2 and NaCl−KCl−MgCl 2 . Since the reaction of Si and C is spontaneous, SiC is termed as the thermodynamically favored product . Thus, the component of electrolytic products can be solely adjusted by engineering the kinetic process.…”

“…Thus, the component of electrolytic products can be solely adjusted by engineering the kinetic process. Note that the contact of C and SiO 2 is crucial to promoting the generation of SiC . However, it is difficult to engineer the configuration of C and SiO 2 in C−RHs because they grow together.…”

Optimized Porous Si/SiC Composite Spheres as High‐Performance Anode Material for Lithium‐Ion Batteries

“…Even if NaCl is added to absorb heat generated from magnesiothermic reduction, the partial heat accumulation inside SiO 2 sphere is extremely severe due to the poor dispersion between SiO 2 and NaCl . By comparison, CTAB can be in‐situ carbonized and then reacted to Si forming SiC nanoparticles during Mg reduction process . The reactions in the whole process are presented as follows: …”

“…[29] By comparison, CTAB can be in-situ carbonized and then reacted to Si forming SiC nanoparticles during Mg reduction process. [35] The reactions in the whole process are presented as follows:…”