No abstract

Silicon appears as a promising candidate to replace the graphite anode for lithium ion batteries (LIBs) due to its extremely high charge capacity. However, the large volume expansion during charging leads to serious stability problems. Silicon nanoparticles coated with protective coatings are reliable to solve this problem and facilitate the practical implementation of the silicon anode. In this study, induction thermal plasma was applied to synthesize silicon nanoparticles with amorphous hydrogenated carbon coating, and the effects of additional carbon sources were investigated. A novel but simple injection method of hydrocarbons was introduced to limit the unfavorable formation of byproducts. The thickness of carbon coating ranges from 2 to 8 nm with a higher hydrocarbon gas flow rate, while silicon particles show a constant mean diameter of around 70 nm. The properties of carbon coating, like sp 2 ratio and H-content, are tunable, and the thermal decomposition mechanism of carbon sources is believed as a key factor. Graphene flakes can also be obtained with abundant carbon and hydrogen radicals released. Based on the material characterizations, acetylene is regarded as a better candidate to prepare carbon coating. The above results show significance for the design of next generation of LIBs.

This study focus on the synthesis of amorphous silicon nanoparticles and understanding the formation mechanism. Counter-flow quenching gases with different flow rates were injected from downstream of the torch to understand the effect of quenching gas on the formation of silicon nanoparticles. Transmission electron microscopy show that nanoparticles with spherical shape and agglomerates consist of smaller particles were synthesized. X-ray diffraction analysis is used to calculate the amorphization degree, which is defined as fraction of amorphous silicon in the silicon nanoparticles including both crystal and amorphous. The obtained results show that higher quenching gas flow rate leads to smaller diameter with higher amorphization degree. Electron diffraction patterns reveal that nanoparticles with diameter less than 10 nm are amorphous and agglomerated together, while for the nanoparticles with diameter larger than 10 nm are crystal. The formation mechanism of amorphous silicon nanoparticles is explained by estimated nucleation temperature and experimental results. Consequently, silicon nucleates at about 2400 K and then silicon vapor condenses on the nucleus. Finally, smaller nanoparticles will keep amorphous phase, while nanoparticles with a larger diameter grow to form crystalline.

Induction thermal plasma is applied to prepare carbon coated silicon nanoparticles as the anode materials of a battery and the e ect of methane injection methods is investigated. Silicon nanoparticles are fabricated as main products and show spherical morphologies with an average diameter of around 50 nm. The unfavorable formation of SiC, which is a byproduct and limits the practical capacity of batteries, can be identi ed when the methane injection position is near to plasma torch. An amorphous hydrogenated carbon coating is synthesized successfully instead of pure carbon materials. The CH 4 injection position can determine the decomposition temperature of methane as well as the concentration of released H atoms. Consequently, the properties of prepared carbon coatings, including the sp 2 ratio and H content, are tunable with injection positions through the etching e ect of hydrogen atoms. These results are signi cant for the synthesis of silicon nanoparticles with carbon coating and the design of lithium ion batteries with higher energy density.