Preparation And Electrochemical Performance Research Of Graphene/Sulfur Nanocomposite Cathode

Lithium-sulfur batteries have the advantages of high theoretical specific capacity, low cost, environmental friendliness. So they have been studied as one of the most promising future systems for the new generation of high capacity lithium batteries. However, sulfur and the Li-S battery internal electrochemical reaction products of polysulfide lithium (Li2S2, Li2S) have high electrons and ions insulation. Besides, the high dissolution of Li-S battery intermediate polysulfide lithium in the electrolyte, prone to migration, formation the redox shuttle mechanism make Li-S batteries properties degradation. Herein, in this work, we used high electrical conductivity carbon materials of graphene oxide (GO), graphene and mesoporous carbon as supporter of sulfur. Graphene oxide, graphene and mesoporous carbon have high specific surface area. Simultaneously, oxygen-containing groups of GO and mesoporous carbon channels have the strong adsorption of polysulfide lithium and can largely improve the performance of Li-S batteries.The GO/S nanocomposite and GNs/S nanocomposite were prepared by one-step chemical reaction method. GO/S nanocomposite has higher conductivity after heated. The initial discharge capacity remain 1423mAh·g-1 at the rate of 0.2C, the reversible capacity maintained at 646mAh·g-1 after 50 cycles. GNs/S nanocomposite has been heated, the initial discharge capacity of GNs-S2 nanocomposite cathode remains at 1550mAh·g-1 up to 92.5% of the theory capacity (1675mAh·g-1) and delivers a capacity of 1033mAh·g-1 at 209mA·g-1. Which indicated that the sulfur in graphene oxide/sulfur nanocomposites and graphene/sulfur nanocomposite is smaller and distributed uniformly after heated. The special sheets structure of graphene and graphene oxide can effectively inhibit the dissolution of polysulfide lithium and accommodates volume expansion of the electrode in the process of charging and discharging, To improve the performance of the battery.The self-assembled and reducted graphene oxide/sulfur nano-composites (rGO/S) with porous and three-dimensional network structure were prepared by a one-pot hydrothermal method. The combination of the highly conductive interwoven fibrous graphene and the small size of the sulfur greatly improves the kinetics of charge and ion transfer in these rGO/S electrodes and has a strong absorption and interaction on polysulfide lithium. For the G-S3 nanocomposite, an initial reversible discharge capacity of 799mAh·g-1 is achieved at 1C and the reversible capacity remains at 724mAh·g-1 after 50 cycles, corresponding to about 93% capacity retention.We synthesized micrometer spherical ordered mesoporous carbon as the supporter of sulfur by one-step method. And we prepared GNs@s-OMC/S nanocomposite with graphene coated and s-OMC/S nanocomposite without graphene coated. The high conductivity and the special carbon channel of micrometer spherical ordered mesoporous carbon can reduce the dissolution of polysulfide lithium into the electrolyte and greatly improve the performance of Li-S batteries. After s-OMC/S was coated by graphene can further inhibit polysulfide lithium dissolved, and form the three dimensional conductive network, improve electrons and ions migration rate. The intial discharge capacity of s-OMC/S nanocomposite remain at 1589mAh·g-1 at 0.25C, up to 95% of the theory capacity. After initial activation processes at 0.25C for two cycles, the first discharge specific capacity of 904mAh·g-1 at high discharge current 2.5C and discharge specific capacity of 788mAh·g-1 after 45 cycles, corresponding to about 87% capacity retention. The charge and discharge efficiency is close to 100%. After coated by graphene, the discharge specific capacity of GNs@s-OMC/S nanocomposite is about 1150 mAh·g-1, the cycle curve nearly present a straight line basically, and the battery show the good cycle stability.