| Lithium-sulfur(Li-S) batteries have high specific capacity and energy density, meanwhile, the sulfur is cheap and environment-friendly, and the source of sulfur is extensive, so that Li-S battery receives much attention. However, about Li-S battery, there is the fade of specific capacity and cycle-life among the charge and discharge owing that the product of poly-sulphide during discharge is easier to precipitate, as well as the conductivity of sulfur is worse. Therefore, many researchers devoted to fabricating the sulfur composite and other materials to enhance the performance of Li-S batteries. Graphite nanosheets have high conductivity, less defects and high specific areas, and are hopeful to be used in Li-S batteries. In this dissertation, nano graphite sheets were prepared by combing liquid exfoliation and mechanical ball milling methods, and then sulfur, carbon nanofibers and silver nanowires were adopted to prepare a series of novel composite nanomaterials. And SEM/EDS, XRD, Raman and TG were used to characterize the properties of obtained nanomaterials. Finally, the electrochemical performance of the samples as the cathode materials for Li-S battery was investigated.First of all, this dissertation studied the preparation of graphite nanosheets by liquid exfoliation and mechanical ball milling, and the influence of some process parameters, mainly including the species and concentration of solvent, the ball milling revolutions and the weight ratio of milling ball on the thickness of graphite nanosheets. We prepared the graphite nanosheets with the thickness between 3 and 5 nm by optimize the process parameters. The comparatively ideal parameters are as follows: the initial graphite concentration is 0.5 mg/m L, and is dispersed into benzyl benzoate which contains 17.9 mg/m L phenyl salicylate, and is exfoliated with 10 hours by ultrasound to obtain graphite nanosheets with some certain thickness; and then the dissertation uses these factors which the weight ratio of milling ball and graphite nanosheets is 50:1, the weight ratio of big and small milling ball is 1:5, the volume of milling solvent is 50 ml, the revolutions are 500 rpm and the milling time is 5 hours to obtain nano-graphite sheets.Furthermore, the graphite nanosheets and sulfur were mixed to obtain sulfur/graphite nanosheets composite materials. After assembling Li-S batteries, it was found that the discharge capacity reached about 275 m Ah/g when the current density was 0.2C. However the capacity fade of battery is more serious among the charge-discharge, which indicates the nano graphite sheets are not ultra-high specific area and hole volume as same as graphene and play a limited role on sulfur.At last, in the light of the high conductivity of silver, we synthesized the composite samples containing carbon nanofiber-silver-sulfur and graphite nanosheet-sulfur-silver by dipping silver nanowire dispersive liquid into carbon/sulfur and graphite nanosheet/sulfur composite, respectively. In addition, we probed into the electrochemical performance of the composite materials by assembling Li-S batteries. It was shown that, when introducing silver nanowires, the conductivity and sulfur absorption of graphite nanosheets were improved significantly. After heat treatment, the carbon nanofiber-silver-sulfur composite materials exhibited unique morphology and structure, and when the current density was 0.2C, the discharge capacity was up to about 350 m Ah/g. Nevertheless, Ag2 S would form on the surface of carbon and graphite nanosheets, so that the carbon materials could not prevent the formation of shuttle and the serious capacity fading of Li-S battery. |
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