부산대학교 도서관

Recently, sulfur nanoparticles (S-NPs) are widely used in different industrial applications, such as fungicides in agriculture field, pharmaceuticals, modification of carbon nanotubes, synthesis of sulfur nanowires with carbon for gas sensor and catalytic applications, nano-composites for lithium ion batteries, and so on. In this study, a surfactant assisted route was conducted to synthesize S-NPs by an acid catalyzed precipitation of sodium thiosulphate. Taguchi method is used for designing experiments to investigate how different parameters, such as acid concentration (1N, 5N, 10N), reaction temperature (15℃, 30℃, 40℃), PVP/S mole ratio (0.025, 0.05, 0.1) and reaction time (30min, 60min, 90min), affect the final obtained S-NPs. Through intuitive analysis, the optimal level of each parameter and the excellent combination of these four factors was determined as 10N, 40℃, 0.05 and 90min for acid concentration, reaction temperature, PVP/S mole ratio and reaction time, respectively, for synthesis of the most homogeneous S-NPs after drying. Additionally, the major-minor sequence was confirmed as that PVP/S ratio influences the final sulfur size most, then the reaction time and temperature, finally the acid concentration. In addition, another two sets of experiments under the preferred experimental conditions by only changing HCl to HNO3 or H2SO4, or changing PVP to CTAB or SDBS, were also done to further investigate the influence of acid and surfactant type on the synthesized S-NPs, respectively. In order to explain the formation mechanism of S-NPs in detail, the effect of acid type, PVP/S ratios, reaction temperature, acid concentration and surfactants on the final sulfur particles were investigated, respectively. And it was observed that the particle size and size distribution to a large extent depends on the concentration of the supersaturated solution (Q) generated, the larger the Q is, the faster the nucleation rate and the slower the growth rate, therefore the more homogeneous the sulfur suspension formed. Meanwhile, the average particle size, size distribution, and zeta potential, both as the most important indicators in evaluating the synthesis results, were measured by a Particle Size & Zeta Potential Analyzer at room temperature, while the morphology and structure of synthesized S-NPs were observed by FE-SEM and XRD, respectively. Furthermore, the effects of average particle size of synthesized sulfur particles on their electrochemical properties have been also investigated, by preparing several Li/S cells numbered from cell #1 to cell #5, with different mean size of 1.42 μm, 3.97 μm, 11.26 μm, 16.94 μm and 26.37 μm, respectively. When measured at 0.5C, cell #2 shows a highest initial discharge capacity of 621 mAh/g, then cell #4 with about 588 mAh/g, similar with that of cell #1 (about 564 mAh/g), finally, cell #3 and cell #5, separately with about 485 mAh/g and 272 mAh/g. And through comparison, sulfur particles with wide size distribution are beneficial to improve the initial discharge capacity, while sulfur with good homogeneity is conducive to the long cycle stability. Also, from the perspective of cycle life, S-NPs synthesized under the optimal experimental conditions are best, due to its most homogeneous particles formed after drying. Finally, S/Super-P and S/MWCNT composites were also prepared to further improve the electrochemical properties of Li/S cells, when measured at 0.5C, the former one shows a rather high initial discharge capacity of 892 mAh/g, then decayed to about 345 mAh/g, while the later decreased from the original 553 mAh/g to the final 293 mAh/g both after 50 cycles, mainly due to their large difference in average particle size, about 4.41 μm and 15.58 μm for S/Super-P and S/MWCNT, respectively.