Preparation Of Grapehen-modificatied Cathode Materials For Lithium Sulfur Battery And Their Electrochemical Properties

Lithium-sulfur(Li-S) batteries are widely considered as one of the most promising next generation of secondary batteries, due to its various advantages, such as high theoretical specific capacity, low cost, environmental friendliness. As an important part of Li-S batteries, the sulfur-based cathode materials play the crucial role on the performance of the battery. In this paper, sulfur-based cathode materials with good electrochemical performance were obtained by the structure design, modification, synthesis of cathode materials. It will provide an important reference for the practical application of Li-S batteries.The reduced graphene oxide-sulfur(RGO-S) cathode composites were synthesized using ultrasonic-assisted chemical co-deposition and vacuum filtration methods. The effect of the sulfur content on the electrochemical properties of RGO-S cathode materials was studied. Morphology, crystal structure, specific surface area, electric conductivity and electrochemical performance of the cathode materials were characterized and analyzed by scanning electron microscope(SEM), transmission electron microscope(TEM), X-ray diffraction(XRD), battery testing system and electrochemical work station, in order to study the relationship between the structure and electrochemical properties of the cathode. As a result, the as-prepared RGO-S cathode materials show a core-shell structure by graphene wrapping over sulfur particles, as well as high specific surface area and conductivity. When the sulfur content was26wt%, the RGO-S cathode could deliver a high initial capacity of1250mAh·g-1sulfur, and a reversible capacity of598mAh·g-1sulfur after50charge-discharge cycles at0.1C. The RGO wraping over the sulfur could shorten the transmission path of both electron and ion, leading the enhancement of the reaction activity and the rate of chemical reaction kinetics. And also, it could trap the diffused intermediates and remit the shuttle effect. As a result, the electrode materials showed a high specific capacity, good cycle performance and rate properties.The RGO-S cathode materials were modified by functional carbon nanofibers (FCNFs). Firstly, FCNFs-S composites were synthesized with a uniform sulfur layer on the outside of FCNFs using chemical deposition method. And it was used as the precursor in the preparation of RGO-S-FCNFs multilayered coaxial cathode composites. In order to study the modification mechanism of the FCNFs for RGO-S cathode materials, micro-morphology, crystal structure, conductivity and electrochemical performance of RGO-S materials before and after the modification were studied. As a result, the cathode with33%sulfur could deliver an initial specific capacitance of745mAh·g-1sulfur, and maintained273mAh·g-1sulfur after1500charge-discharge cycles at a rate of1C, showing excellently long cycle stability at high rate. The conductivity of RGO-S cathode materials were improved by synergistic effect of RGO and FCNFs. Because of the coaxial-coated structure, sulfur and intermediates during charge-discharge process could be trapped between graphene and FCNFs, the reunion and growth of big sulfur particles was suppressed, the volume change was accommodated, the shuttle effect, the internal stress of the cathode materials and the resistance of charge-transfer was decreased. As a result, the specific capacity, cycle performance and rate capability could be further improved.FCNFs modified three-dimensional graphene-sulfur cathode materials with large sulfur content and areal mass loading were prepared by hydrothermal method and thermal treatment. In order to investigate the modification mechanism of the FCNFs for3D-CGOS cathode materials, micro-morphology, crystal structure, conductivity and electrochemical performance of RGO-S materials before and after the FCNFs modification were studied. The results showed that the reversible discharge-specific capacity of the3D-CGOS composites were increased from0.5mAh·cm-2cathode to3.1mAh·cm-2cathode at2C, when the sulfur content was80%and the areal mass loading of sulfur was11.9mg cm-2. The modification of FCNFs can be enhanced and the conductivity of3D-GOS materials was improved. Both of the electron and ion transportation length were shorted, the reaction activity and the rate of chemical reaction kinetics were accelerated, as well as the shuttle effect was slowed. Meanwhile, the mechanical property of3D graphene can be enhanced, which is benefit for the accommodation of volume changes during the charge-discharge process. As a result, the specific capacity, cycle performance and rate capability were significantly improved.