Nano-composite silicon powders have been produced at a maximum process throughput of 6 g/min by plasma spraying with metallurgical grade silicon powder as raw material. The obtained powders are found to be fundamentally composed of crystalline silicon particles of 20-40 nm in diameter, and are coated with an $5-nm-thick amorphous carbonous layer when methane gas is additionally introduced during plasma spraying. The performance of half-cell batteries containing the powders as negative electrodes has shown that the capacity decay observed for the raw Si coarse particles is significantly improved by plasma treatment. The carbonous coating potentially contributes to an improvement in capacity retention, although coexisting SiC particles that inevitably form during high-temperature processing reduce the overall capacity. V C 2014 AIP Publishing LLC.
Nanocomposite Si/SiOx powders were produced by plasma spray physical vapor deposition (PS-PVD) at a material throughput of 480 g h−1. The powders are fundamentally an aggregate of primary ∼20 nm particles, which are composed of a crystalline Si core and SiOx shell structure. This is made possible by complete evaporation of raw SiO powders and subsequent rapid condensation of high temperature SiOx vapors, followed by disproportionation reaction of nucleated SiOx nanoparticles. When CH4 was additionally introduced to the PS-PVD, the volume of the core Si increases while reducing potentially the SiOx shell thickness as a result of the enhanced SiO reduction, although an unfavorable SiC phase emerges when the C/Si molar ratio is greater than 1. As a result of the increased amount of Si active material and reduced source for irreversible capacity, half-cell batteries made of PS-PVD powders with C/Si = 0.25 have exhibited improved initial efficiency and maintenance of capacity as high as 1000 mAh g−1 after 100 cycles at the same time.
Core-shell SiO x nanocomposite powders have been produced in a single continuous plasma spray process. The addition of CH 4 at appropriate amounts during plasma spraying of SiO was found to be quite effective in promoting the reduction of SiO and thus increasing the crystalline Si amount after the disproportionation reaction. The half-coin cell assembled using these powders for the negative electrode has exhibited a stable capacity higher than 1000 mAh/g with the coulombic efficiency of around 99.3%, both of which are higher values than those of the cell with raw SiO. Electrochemical analysis has revealed that the resistance at the SiO x particle surface decreases potentially with Li 2 O formation from the beginning of the first lithiation. The decrease in the resistance is further enhanced by the addition of CH 4 , although more volume change is expected because of the increased crystalline Si phase content. As a result, the core-shell SiO nanocomposite produced by plasma spraying with CH 4 becomes advantageous in attaining high capacity and high retention efficiency simultaneously.
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