Mechanisms of Si nanoparticle formation by molten salt magnesiothermic reduction of silica for lithium-ion battery anodes

Official URL: https://dx.doi.org/10.1002/celc.202100683

Molten salt methods enable the synthesis of Si nanostructures by moderating the thermal energy evolved in highly exothermic magnesiothermic reduction reactions (MRR) of silica. Due to their cost-effectiveness and scalability, these techniques are well suited for producing nanoscale Si for a number of applications, including energy storage. To control the microstructure morphology and particle size, it is necessary to understand the formation mechanism of the Si produced. By evaluating the time-resolved phase evolution, when NaCl moderates the thermal energy generated by MRR of SiO2, we elucidate 3 parallel interfacial reaction mechanisms yielding Si nanoparticles – via Mg vapor, Mg-rich eutectic liquid, and Mg ions dissolved in molten NaCl. These individual Si nanoparticles offer a striking contrast to the typical by-product of MRR of SiO2 with and without NaCl, which yields a 3-dimensional (3-D) porous network of sintered Si nanoparticles. Lithium-ion battery half-cells with electrodes composed of individual Si nanoparticles showed a greater first-cycle irreversible discharge capacity and faster capacity loss over the first 5 cycles at a current density of 200 mA g−1 compared to half-cells with electrodes of a porous 3-D Si network–indicative due to thicker solid electrolyte interphase (SEI) formation on individual particles. At a higher current rate of 400 mA g−1, once SEI formation and activation of Si are established, both cells exhibit a similar capacity retention rate over 100 cycles.