Study On Preparation And Properties Of S-Based Cathode Materials

With a high theoretical specific capacity of 1674 m Ah·g-1、a theoretical specific energy of 2600 Wh·kg-1,volume energy density of 2800 Wh·L-1,and low cost,environmental friendliness, sulfur has been considered as one of the promising cathode materials for the next generation high energy density lithium secondary battery,therefore,lithium-sulfur batteries have become a research hotspot all over the world. Unfortunately,lithium-sufur batteries suffers from low sulfur utilization and poor cycle life performance due to the electrical insulating nature of sulfur,and the solubility of the reaction intermediates of polysulfides（Li₂Sx,4 ≤ x ≤ 8）, which can hindered the practical application of the lithium-sufur batteries. In this paper, the sulfur/polypyrrole composite（S/PPy）, sulfur/freeze-drying graphene composite（S@FD-rGO）, sulfur@porous carbon/graphene composite（S@PC/FDGO） are designed and prepared, these as-obtain samples were characterized by means of charging-discharging test、XRD、SEM、TEM、TGA and other characterization method to represent morphology,structure and electrochemical properties of the materials.The experimental results are as follows:（1） S/PPy composite with yolk-shell structure was prepared successfully by optimizing the preparation technology with Na₂S₂O₃·5H₂O 、 ethyl silicate and polypyrrole as raw material, S/PPy composite have a good electrochemical properties.The experimental results show that: the preparation of S@SiO₂ precursor with the cetyl trimethyl ammonium bromide as surfactant is core-shell structure. Then PPy was coated on the S@SiO₂ precursor by in situ hydrolysis, S/PPy composite with yolk-shell structure was prepared successfully after washing SiO₂. The morphology、structure and electrochemical of composite were characterized, it’s found that S/PPy composite with 25 wt.%SiO₂ and 20 wt.%PPy have the best electrochemical properties, it’s delivers an initial specific charge-discharge capacity of 869.6/953.6m Ah·g-1 at the current density of 100 m A·g-1, it maintaina the specific charge capacity of 284.2 m Ah·g-1 after 200 cycles. The cycling stability of S/PPy composite is improved compared with the pure S, dissolution of polysulfides was restrained, the performance improvement thanks to yolk-shell structure of S/PPy composite.（2） Graphene oxide（GO） was synthesized by chemical oxidation method, andthen was subjected to freeze-drying, the as-obtained freeze-drying graphene oxide and sulfur powder was heated treatment together, the final product（Freeze-drying graphene oxide/sulfur） was prepared by one-step solid phase method, it have a good electrochemical properties. The experimental results show that: freeze-drying graphene oxide/sulfur composite which the contents of S is 60% and the reaction temperature is 155℃×4 h+180℃×4 h have the best electrochemical properties: It delivers an initial specific charge-discharge capacity of 521.0/792.2 m Ah·g-1, it maintain the discharge capaticy of 384.4 m Ah·g-1, the capacity retention rate of 48.5%after 200 cycles, because sulfur can reduce graphene oxide to graphene in the heat treatment, which can improve the conductivity of the material, freeze-drying graphene oxide/sulfur have porous structure and high specific surface area, it can effectively restrain shuttle of polysulfide.（3） The porous carbon materials（PC） was obtained with the linear phenolic resin as raw material and hexamine as curing agent and foaming agent after solidifying and carbonizing, then PC、sulfur were composited together by fusion, and then coating freeze-drying graphene oxide to prepared sulfur@porous carbon/graphene composite（S@PC/FDGO） which have a good electrochemical properties. The experimental results show that: It delivers an initial specific charge-discharge capacity of946.7/1051.6 m Ah·g-1 at the current density of 100 m A·g-1, it maintaina the reversible capaticy of 401.6 m Ah·g-1after 200 cycles, coulombic efficiency（ratio of the charge capacity and discharge capacity） remained at around 103%. The reversible capacity stays 703.6 m Ah·g-1 at a current density of 400 m A·g-1, this illustrates sulfur@porous carbon/graphene composite has a good rate performance.