Preparation And Performance Of Novel Cathode Composite For Rechargeable Lithium-Sulfur Battery

Lithium sulfur battery is a most promising energy storage system due to its high theoretical energy density, low cost and environmental friendliness. However, there are still a number of problems in Li/S batteries, such as the low active material utilization and poor cycle life, which are due to the high electrical resistivity of sulfur and lithium sulfide, and the high solubility of the charge-discharge products in the electrolytes. According to the excellent electrical conductivity, high surface area and chemical stability of carbon materials, kinds of novel carbon-sulfur composites were designed and synthesized, that is, MWNTs-S composite, S/GNS composite and S/rGO composite. The structures and components of the synthesized composites are systematically measured by means of XRD、Raman、SEM、TEM、EDS and TGA, in order to testify the feasibility of the synthetic methods and the feasibility of application as cathode materials for rechargeable Li/S batteries.S/rGO and S/GNS composite materials were prepared by hydrothermal and thermal reduction methods. Sulfur coate on the GNS uniformly and stably after thermal reduction, the reduced graphene shows good conductivity. The unqiue staggered layer-like structure maintained after the freeze-drying method can improve the deeper ingress of the electrolyte, as well as the higher utilization of elemental sulfur; The S/rGO composite synthesized by hydrothermal shows seriously reunion structure. Element sulfur and S/GNS composite are tested by coin type cells to compare and analyze the differences of various electrochemical performances. The impedance measurements show that the contact resistance and the charge transfer resistance of S/GNS electrode are far less than the sulfur electrode; cyclic voltammetry and charge-discharge cycling tests show that the the reversibility of the electrochemical reaction, the sulfur utilization and cycling stability have greatly improved. The functional groups on the GO surface play the role of immobilizers that effectively confine any polysulfides from dissolving and reduce the occurrence of the shuttle; the S/GNS has an extremely high surface area and unique layer-like structure which can accommodates the volume change of the electrode during the charge and discharge process, enhance the stability of the electrode and weaken the detrimental effect of discharge products Li2S deposited on the cathode at certain extent. The S/GNS composite exhibits a high specific capacity up to1598mAh/g at the initial discharge and the sulfur utilization approaching to95.6%, the specific capacity remained above670mAh/g after80cycles.A series of MWNTs-S composite were synthesized at normal temperature by the method of vacuum impregnation. The results obtained show that the sulfur can be well coat on the outer surface homogeneously and filled into the inner of MWNTs, and the nanotubes form stable and uniform three-dimensional conductive network structure. The pores and defects on the surface of MWNTs are conducively to the transfer of electrolyte ions after pretreatment. Various sulfur content of MWNTs-S composites are tested by coin type cells. The composite materials exhibite that the increased utilization of sulfur and higher discharge capacity, which are due to the good electrical conductivity of MWNTs. The excellent cycling stability of Li/S cells are improved by the strong adsorption of nanotubes and the relatively closed nano-scale reactor structure of composites. The MW-S-70composite with70wt%sulfur presents the most excellent electrochemical performance of MWNTs-S composite materials with the average fading rate of about0.36%and0.71%per over100cycles at rate of0.1C and0.2C.