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Crystalline Si core–amorphous SiOx shell nanoparticles were synthesized from SiCl4 using an atmospheric microwave plasma process and their microstructures were investigated. It was found that the amorphous SiOx shell thickness was determined by varying the applied microwave power. The increase of applied power directly increased the length of the flame at the end of the plasma. Through observation of the microstructures with varying lengths of the flame, it was found that a highly crystalline Si phase was formed in the plasma and amorphous SiOx phase was formed in the flame. The longer flame resulted in the thicker SiOx shell around the Si core. The electrochemical properties of the varying SiOx shell thicknesses were also investigated in that the synthesized Si-SiOx nanoparticles were used as anode materials in a lithium-ion battery (LIB). Thin shells such as native oxide did not influence the electrochemical behaviour of the Si core, whilst some shells were too thick to allow Li-ions to react with the Si core. Using Si-SiOx nanoparticles with the optimum SiOx:Si ratio of 18:82, the following electrochemical properties were obtained: a first reversible capacity of 932 mAh/g, an initial columbic efficiency (ICE) of 52.5%, and a capacity retention of 83.7% at the 100th cycle.

Nanoparticles having four different ratios of crystalline silicon (Si) cores and amorphous SiOx shells were synthesized by using a microwave-generated plasma. Their microstructures were analyzed to find quantitative relationships with electrochemical properties. By measuring XRD, SEM, HR-TEM, and Raman spectra of four samples with different core-shell microstructures, the quantitative fractions of crystalline Si core and amorphous SiOx shell in the nanoparticles were calculated. Also, electrochemical properties of the nanoparticles were measured and compared with the calculated fractions. The electrochemical properties such as the initial reversible capacity, the initial coulombic efficiency, and the capacity retention exhibited tendencies remarkably similar to those implied by the calculated fractions. Analysis of each sample's dQ/dV profile also gave us good evidence of understandable relationships between the fractions and electrochemical properties. Details of calculating the fractions from microstructural data are given, with suitable consideration having been given to the fact that several assumptions could lead to errors during the analysis. The approach introduced in this research offers a good means of analysis with which to further the optimal design of nanoparticle microstructure for Li-ion battery anodes.

We synthesized silicon (Si) nanoparticles using an atmospheric microwave plasma process, and investigated the effects of hydrogen (H2) injection on their microstructure during the synthesis. Two nozzles were applied to inject H2 (swirling and rectilinear H2). Our microstructural analysis indicated that the amount and method of H2 injection were critical for completion of the reaction from silicon tetrachloride (SiCl4) to Si, as well as to obtain highly crystalline Si nanoparticles. The swirling H2 was especially critical due to its formation of vortex flow, which allowed relatively long residence time of the H-ions in plasma. The Si nanoparticles synthesized by the atmospheric plasma process had core-shell structures that consisted of crystalline Si cores with amorphous SiOx shells of 5–15 nm thickness. We also investigated the feasibility of the synthesized Si nanoparticles as anode materials in a lithium-ion battery (LIB). For the core-shell structured Si nanoparticles, we obtained the first reversible capacity of 1204 mAhg−1, and a capacity retention of 82.2% at the 50th cycle.

Silicon-carbon (Si-C) composites with various weight ratios were prepared through heat treatment of water-soluble carboxymethyl cellulose (CMC) with Si nanoparticles synthesized by using an inductively-coupled plasma. Microstructures of the Si-C composites were thoroughly investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The results indicate that we obtained a micro-sized Si-C composite with homogeneously-distributed crystalline Si nanoparticles in an amorphous C-matrix. Pores, which were due to the volatilization of CO2 from CMC during heat treatment, was detected when the concentration of carbon was increased. The electrochemical properties of those Si-C composites for use as anode materials in lithium-ion batteries (LIBs) were also investigated. The C-matrix enhanced the capacity retention, as well as the rate capability of Si nanoparticles, due to the dense and homogeneous microstructures of the composite. The Si-C composites (7:3 weight ratio) retained a reversible capacity of > 1,000 mAh/g with a capacity retention of 88.9% even after 100 cycles. The reversible capacity ratio at a 1.5 C-rate was about 80% as compared with that at a 0.1 C-rate.

Si nanoparticles were synthesized by inductively coupled plasma and a specially designed double tube reactor. By injection of large amount of PH3 during the synthesis, the effects of phosphorous incorporation on their microstructures and chemical binding environments were investigated. Injection of PH3 gas during the synthesis resulted in a change from crystalline to amorphous phase, a reduction of particle size as well as a process yield. All of the above results were attributed to a lower plasma density when higher amount of PH3 was injected. From energy-dispersive X-ray spectroscopy, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy analysis, it was revealed that P was doped in Si nanoparticles. However, secondary phases such as P4 and P2O5 were formed as amorphous ones in nano-scale when a relatively large amount of PH3 was injected. In addition, those nanoparticles were applied as an active material in the lithium-ion battery's anode. Unexpectedly, amorphous Si nanoparticles with secondary phases showed improved electrochemical properties. P-doping in Si nanoparticles could not directly advance cycling performance by improvement of electrical conductivity of Si nanoparticles. It was rather assumed that a secondary phase influenced and enhanced electrochemical properties by additional capacity due to a formation of Li3P and forming an effective buffer against large volumetric change of Si nanoparticles during the charge/discharge. The initial reversible capacity of amorphous Si nanoparticles synthesized with 100 sccm of PH3 flow rate was 2113 mAh g− 1, and that at the 100th cycle was still about 1000 mAh g− 1, which was twice as high as that of Si nanoparticles synthesized without PH3 injection.

Phosphorous (P) incorporated silicon nanoparticles (Si NPs) were synthesized by using inductive coupled plasma (ICP) and a specially designed double tube reactor.Their microstructures were investigated by injecting various amounts of PH3 gas during the synthesis. Injection of PH3 gas during the synthesis resulted in a change from crystalline to amorphous phase, a reduction of particle size as well as process yield. These results were attributed to a lower plasma density when higher amount of PH3 was injected.From EDS, SIMS and XPS analysis, it was revealed that P was successively incorporated in Si NPs.However, secondary phases such as P4 (red P) and P2O5were formed as amorphous ones in nanoscale when a relatively large amount of PH3 was injected.