Induction thermal plasma (ITP) attracts great attention due to its excellent
properties like extremely high temperature (>1e4 K), high chemical reactivity,
rapid quenching effect (1e3~1e6 K/s) and electrodeless discharge. As the
development of lithium ion batteries (LIBs), interests in new anode materials
with higher energy density increased exponentially. Silicon (Si) is proposed
as the most appealing candidate of anode material, because Si can provide
a highest charge storage capacity of 4200 mAh/g (Li22Si5), which is approximately
10 times higher than graphite as anode. However, the irreversible volume
expansion of up to 400% during charge and discharge for silicon and intrinsic
low electrical conductivity frustrate the replacement of graphite anode
severely. Some special strategies were applied to overcome those problems,
such as silicon nanoparticles (SiNPs), amorphous silicon (a-Si) materials
and applying carbon coating on SiNPs. In this dissertation, silicon-based
materials with these complicated structure are synthesized with ITP method,
and the formation mechanisms are also investigated.
In chapter 1, the method of ITP is reviewed and the objective of this dissertation
is presented.
In chapter 2, a-SiNPs are successfully synthesized by induction thermal
plasma with additional counter-flow quenching gas, since a-Si shows smaller
volume variation compared with crystalline silicon (c-Si). The obtained
a-SiNPs show totally different morphology with c-Si, that amorphous phase
is characterized with random shapes and smaller diameters (˂5 nm), while
crystal phase can be identified with spherical shape and much larger size.
The amount ratio of a-SiNPs is improved from 33% to 64% with quenching
gas flow rate, and the quenching rate increases from 3.2e4 to 8.9e5 K/s
which is demonstrated by numerical studies. The increased quenching rate
promotes nucleation and the number of nuclei increases subsequently. Therefore,
larger amount of small particles (˂5 nm) are synthesized, while amorphous
is thermodynamically preferable in such small size for silicon.
In chapter 3, SiNPs with carbon coating are successfully fabricated in
one-step by ITP method and ethylene is applied as additional carbon source.
The obtained coating is amorphous hydrogenated carbon (a-C:H) with a thickness
ranges from 2 to 8 nm. Since the concentration of H radical increases with
more ethylene injection, etching effect for sp2 bonds is enhanced. Then
more sp2 hybrid orbitals will be destroyed and the ratio of sp2 in final
products will be lower. For the same reason, hydrogen content in the carbon
coating will be lower with more ethylene injection.
In chapter 4, the effect of hydrocarbon sources (methane, ethylene, and
acetylene) on the formation of SiNPs as well as a-C:H coating are investigated.
The formation of unfavorable SiC can be identified in all cases and the
amount increases from 2% to 22% with initial C/Si ratios. Graphene flakes
are fabricated in the cases of methane and acetylene due to the abundant
formation of free C and H atoms. The obtained results of coating characterization
reveal that acetylene is a better choice for the preparation of carbon
coating at SiNPs in this research due to the highest sp2 ratio and hardness
in comparison with other two carbon sources.
In chapter 5, the effect of sheath gas composition on plasma parameters
and the formed silicon-carbon composite are investigated. The ratio of
tangential and radial flow rates (T/R), which compose plasma sheath gas,
varies from 0.5 to 1. Results shows the plasma shape as well as diameter
will be compressed in the case of higher T/R values, which will lead to
an enhanced quenching rate on SiNPs while the temperature of methane injection
positions will be limited. The improved amount of a-SiNPs and limited H
etching effect with higher T/R values further proves this conclusion. For
the same reason, formation of unfavorable SiC can be ignored in the case
of higher tangential gas flow rate, which can improve the energy density
of synthesized particles in batteries.
In chapter 6, the obtained conclusions are marked and some ideas for the
future research are presented.
In summary, the obtained results in this research can improve the understanding
of formation mechanisms for silicon material in ITP method, which is significant
for the design of LIBs with higher energy density and better cycling performance.
Outstanding Paper Award of Journal of Chemical Engineering of Japan (2022年9月) 「Effect of Methane Injection Method on Preparation of Silicon Nanoparticles with Carbon Coating in Induction Thermal Plasma」 |
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