One-step Hydrothermal Preparation And Electrochemical Properties Of Graphene/Sulfur Cathode For Lithium-Sulfur Battery

Lithium-sulfur battery is becoming a research hotspot on chemical power sources due to its abundant resources, environmental friendliness, low cost and high theoretical power density. Preparation of graphene/sulfur (RGO/S) composite materials were synthesized by hydrothermal method at high temperature. At the same time, modification was studied for the hydrothermal synthetic materials focusing on poor electrical conductivity, poor cycling stability and low utilizaiton of active material in lithium-sulfur battery. The main contents are shown as follows.Graphene/Sulfur (RGO/S) composites were synthesized by one-step hydrothermal method with mixture of sodium thiosulfate (Na2S2O3) and grapheme oxide (GO) solution reacting under acid condition. We explored the influence of the hydrothermal temperature, reaction time, the sulfur content on the materials, using X-ray diffraction (XRD), scanning electron microscope (SEM) and galvano static ally charge and discharge, the results showed that the RGO/S composite materials has excellent cycling performance, which synthesized by one-step hydrothermal method under180℃following the carbon and sulfur ratio of3:7for12h. The first discharge capacity is931mAh·g-1, its capacity remains in828.16mAh·g-1after50cycles. The coulomb efficiency of the RGO/S composite materials was above95%. At the same time, the rate capacity of the RGO/S composite was much better than that of sulfur. Sulfur molecular can evenly distributed on the surface of the graphene layer and sulfur molecules were fixed to the functional groups on graphene surface firmly by one-step hydrothermal method.Focused on stack phenomenon about graphene in the composite materials, two kinds of modification were studied on the graphene/sulfur composite cathode materials. Add a small amount of carbon nanotubes (MWCNTS) with oxidation treatment by concentrated HNO3. The MWCNTS and graphene sheets constitute a unique three-dimensional structure, which formed nanotube/graphene/sulfur (CNT-RGO/S) composite materials. Sulfur molecular can evenly distribute in the network structure of graphene and WMCNTS. Results show that GNT-RGO/S composite materials improved the sulfur content from50%to63%. The first discharge capacity at0.2C is1122mAh·g-1, its capacity remains in887mAh·g-1after50cycles. The electrochemical performance of CNT-RGO/S is better than RGO/S composites due to the addition of carbon nanotubes to improve the conductivity of the composite materials, which effectively have decreased the overlap between graphene layers. And also carbon nanotubes increase the sulfur content and maintain the stable structure of the cathode matrix.From the graphene layers structure itself, we synthetize N-RGO/S composite materials with urea [CO(NH2)2] as nitrogen source to bring in nitrogen atoms. The N-RGO/S composite materials has changed the electronic structure of graphene and improved the free carrier density. Results show that The first discharge capacity at0.2C is982mAh·g-1, its capacity remains in922mAh·g-1after50cycles. Its cycle stability is evidently very excellent. Good performances suggest that the introduction of the nitrogen functional groups have increased the activated adsorption on graphene surface to sulfur molecules and enhanced the coactions between sulfur molecules and graphene.