Crystalline Evaluation of Size-Controlled Silicon and Silicon Oxide Nanoparticles Produced by Solution Plasma Discharge
2019 Volume 60 Issue 5 Pages 688-692
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2019 Volume 60 Issue 5 Pages 688-692
Li-ion batteries using Si particles as anode materials have attracted attention because of their extremely high theoretical capacities. However, they experience extensive volume expansion during lithiation in the process of charging, dramatically affecting the battery lifetime. To mitigate these expansion effects and improve the capacity, Si nanoparticles and occasionally SiOx particles have been effective. Therefore, the production of Si nanoparticles is necessary. For these purposes, the solution plasma discharge method was applied. We improved previous work on the plasma discharge method by adjusting the electrolyte medium conditions via controlling the pH, solution conductivity, electrolyte concentration, and voltage, successfully generating Si nanoparticles with diameters of <10 nm and SiOx particles of <50 nm in diameter. These crystalline Si and amorphous SiOx particles were characterized using microscopy and spectroscopy.
SiOx particles produced by plasma discharge method. (a) HAADF image, and (b) EELS data from boxed area in (a).
Energy storage is a global issue, particularly in the development of batteries with high energy capacities, which are needed for large-sized electronics including electric vehicles.1,2) Li-ion batteries using Si crystals, instead of commonly used carbon materials, as anode active materials have recently attracted attention. The energy capacity of carbon materials is limited to 372 mA·h/g, while Si has the theoretical capacity of 4200 mA·h/g.3)
However, the large volume change of Si anodes during the charge–discharge cycle can pulverize Si electrodes, shortening the battery cycle life. Electrodes using nano-sized Si particles (NSiP) can avoid such volume change4) and furthermore improve the performance of the Li-ion battery remarkably.5)
Decreases in NSiP size and the inclusion of SiOx nanoparticles may further effectively suppress electrode material volume changes;6,7) many investigations of the fabrication and structure of amorphous SiOx have been reported.8–13) For the commercial application of these nanoparticles in batteries, lower-cost mass production methods are needed. The solution plasma technique offers many advantages, including a simple experimental setup, the use of readily available precursors, avoidance of harmful chemicals, increased productivity relative to conventional solution processes, and suitability for mass production.14–17) Here, we produced desirable crystalline NSiP and SiOx nanoparticles using this technique, in which not only the electrolytic voltage and current but also PH of various electrolytes were controlled. The size distribution and crystallinity of the products were evaluated based on transmission electron microscopy (TEM), including lattice images, and energy-dispersive X-ray spectroscopy (EDS) analysis.
An electrolytic cell was used to produce the nanoparticles. The principle of the cell is shown in Fig. 1,4) in which the Pt anode and nanoparticle precursor of the 5-mm-diameter Si cathode are set in an electrolytic solution.
Schematic of electric cell to produce nano Si particles.
The main electrolysis conditions were applied as follows: the aqueous electrolyte solution comprised 0.1 mol/dm3 KCl or HCl, 0.2 mol/dm3 LiCl or HNO3, and 0.1 mol/dm3 potassium citrate or lithium citrate. The solution temperatures at plasma discharge were 353–363 K and the applied voltage and current were controlled within 100–185 V and 0.5–3.0 A, respectively. Furthermore, the electrolyte pH was adjusted because the production of crystalline NSiP and oxide NSiP (SiOx) is affected by acidic or alkaline conditions. The particles in the electrolytic solution were filtered and dispersed in deionized water. To observe these particles by TEM, a droplet of the water dispersion was placed on a carbon-coated micro-grid supported by a Cu mesh and dried in air at room temperature.
Thus, the NSiP or oxide NSiP particles on the mesh were observed by the high-resolution TEM (JEOL, JEM-2100) with EDS analysis, as well as electron energy-loss spectroscopy (EELS) analysis (FEI, Titan cubed TEM) at 300 kV.
The properties of crystalline Si particles were previously successfully controlled by manipulating the electrolytic solution composition and cell voltage.14) Based on these results, we attempted to produce desirably sized NSiP and oxide NSiP of <10 nm and <50 nm, respectively, by controlling the electrolytic solution compositions and pH and adjusting the cell voltage.
3.1 Effects of electrolytic solutions on NSiP productionFigure 2 shows the NSiP distributions produced in different electrolytic solutions. The NSiP sizes are changed depending on the electrolytic solution; relatively larger crystalline NSiP are produced in acidic solutions with pH < 7 of 0.1 mol/dm3 HNO3 and 0.1 mol/dm3 HCl.
Si or SiOx particle distributions produced in various electrolytes: (a) 0.1 mol/dm3 HNO3 at 135 V, (b) 0.1 mol/dm3 HCl at 157.5 V, (c) 0.1 mol/dm3 KCl at 170 V, (d) 0.1 mol/dm3 potassium citrate at 160 V, (e) 0.1 mol/dm3 lithium citrate at 175 V, and (f) 0.2 mol/dm3 LiCl at 177.5 V.
However, in the 0.1 mol/dm3 KCl and 0.2 mol/dm3 LiCl solutions with pH < 5, most NSiP sizes were definitely <5 nm. These results suggest that not only the electrolytic solution but also the solution pH affect the produced particle sizes. However, when the potassium citrate and lithium citrate solutions of 0.1 mol/dm3 and higher initial pH (>7) were used, the pH was increased to >7.5 during plasma electrolysis. In an alkaline electrolytic solution, the produced Si particles were not crystalline but amorphous in phase.
This clarified that acidic and alkaline solutions significantly affect the resultant NSiP phases, with crystalline Si particles formed in acidic solutions and amorphous oxide particles formed in alkaline solutions.
3.2 H3BO3 addition effect on Si particle sizeFigure 3 shows the TEM observation of Si particles obtained when plasma discharge is performed at 185 V in an electrolytic solution containing 0.125 mol/dm3 H3BO3 as well as 0.1 mol/dm3 HCl, 0.1 mol/dm3 KCl, and 0.2 mol/dm3 LiCl. The Si particle sizes are clearly decreased by the addition of 0.125 mol/dm3 H3BO3 to the acidic electrolytic solutions. The effect is particularly remarkable in the HCl solution compared to that in other electrolytes. However, the addition of H3BO3 to the KCl solution did not decrease the particle size; the effect of H3BO3 addition depended on the electrolyte. Further investigation is required to explain this difference.
Si crystalline particle size distribution change with 0.125 mol/dm3 H3BO3 addition to electrolytes of (a) 0.1 mol/dm3 HCl, (b) 0.1 mol/dm3 KCl, (c) 0.1 mol/dm3 LiCl, (d) 0.1 mol/dm3 HCl + 0.125 mol/dm3 H3BO3, (e) 0.2 mol/dm3 KCl + 0.125 mol/dm3 H3BO3, and (f) 0.2 mol/dm3 LiCl + 0.125 mol/dm3 H3BO3.
Furthermore, NSiP production was performed using 0.2 mol/dm3 LiCl and 0.125 mol/dm3 H3BO3 as the electrolyte with cell voltages of 165 to 185 V to determine the effect of voltage on NSiP size. The results are shown in Fig. 4; most of the obtained NSiP are very fine, with little influence by cell voltage.
Nano-Si crystalline size distribution depending on cell voltage in 0.2 mol/dm3 LiCl + 0.125 mol/dm3 H3BO3 electrolyte: (a) 185 V, (b) 180 V, (c) 170 V, and (d) 165 V.
The crystallites of NSiP produced in 0.2 mol/dm3 LiCl + 0.125 mol/dm3 H3BO3 electrolytic solution with 5 pH was analyzed. Figure 5(a) and (b) show the bright-field image and dark-field image taken from a part of SAD pattern. In addition, Fig. 5(c) shows the lattice images observed by high-resolution TEM, and any oxide films were not observed in these particles. From these results it was identified that these small NSiP are crystalline ones with small nano size.
Si nanocrystalline particles distribution from 0.2 mol/dm3 LiCl + 0.125 mol/dm3 H3BO3 solution. (a) Bright-field TEM image, (b) its high-resolution lattice image, (c) dark-field image, and (d) particle size distribution.
In the previous section, SiOx particles produced in alkaline electrolytic solution were shown to be amorphous in phase.
To study the more detailed structures of the oxide particles, the particles were analyzed by high-angle annular dark-field (HAADF) imaging and EELS. The results are shown in Fig. 6. The SiOx particles are produced from the potassium citrate electrolytic solution. The average size of the oxide NSiPs is ∼50 nm, and the selected area diffraction (SAD) pattern shows a ring indicating an amorphous phase. The HAADF image depicts nanoscale (∼4 nm in diameter) domains with faint white contrast, as observed in Fig. 6(a). Furthermore, Fig. 6(c) is the EELS profile taken from a white-contrast domain indicated with a box in Fig. 6(b), in which the profiles of standard Si and SiO2 are indicated for comparison.8)
SiOx particles produced by plasma discharge method. (a) TEM image of SiOx particles, (b) HAADF image, and (c) EELS data from boxed area in (b).
The obtained spectrum from the white domain region is not coincident with both Si and SiO2, but seems to be a mixture. A similar spectrum for SiO was reported.9,18) The spectrum analyzed in the present study may reveal the existence of Si-rich domains such as SiO.
To confirm the Si-rich domain formation in the oxide NSiPs, the structural change under electron irradiation was observed during in situ observation at 200 kV because Si oxides such as SiO2 and SiO experience phase changes under electron irradiation.19,20)
Figure 7 shows an example of structural change during TEM observation under focused ion beam irradiation. Figure 7(a) shows one SiOx particle of ∼50 nm in diameter before electron exposure (irradiation), while Fig. 7(b) shows the same particle after about 5 min electron exposure corresponding to 5 × 1010 e/cm2.
Structural change due to electron irradiation: SiOx particle before irradiation (a), (b) after 5 min irradiation at 200 kV, (c) lattice image of the enlarged area (circled) of (b), (d) and (e) FFT from lattice image area and inverse FFT, respectively, and (f) intensity profile of inverse FFT.
The circumference of the oxide particle becomes irregular; simultaneously, a dot-like domain of ∼3 nm in diameter appears with darker contrast. With continued electron irradiation, nano-sized particles are often observed with lattice fringes around the circumference of the SiOx particle, as shown in Fig. 7(c). The SAD pattern and its fast Fourier transform (FFT) image taken from this region (marked with a circle) identified the lattice spacing of 0.31 nm, which corresponds well to the (111) plane of Si crystals. These results indicate that the oxide NSiP produced in the present experiment showing amorphous phase may comprise crystalline NSiP with Si-rich phases and SiO2.13,21)
In addition, EDS analysis was performed on the oxide NSiP. The Si and O concentrations before electron irradiation were ∼37 at% Si and ∼62 at% O. However, the Si and O concentrations of the particle shown in Fig. 6(b) are 55 at% Si and 45 at% O after electron irradiation. This indicates that electron irradiation preferentially removes O atoms from silicon oxide phases such as SiO2,20) thus increasing the Si concentrations measured after irradiation because of the Si-rich domains remaining in the particles. Based on the analysis of the oxide NSiP, nanosized Si crystalline particles produced in the process of plasma emission seem to aggregate, forming clusters; these clusters are oxidized in alkaline solutions, but are rapidly cooled in electrolytic solutions. Thus, the Si-rich NSiP would include clusters of oxide NSiPs.
The main results obtained in the present experiments on NSiP production via plasma discharge are as follows:
A part of this work was supported by the “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This paper is based on results obtained from a project commissioned by the NEW ENERGY and Industrial Technology Development Organization (NEDO), and also was financially supported partially by NAKAYAMAGUMI Co., Ltd.
