Preparation And Characterization Of Sulfur/Nanoporous Carbon Composite Cathode Materials

The ever-increasing demand for electric energy storage, ranging from portable electronics to electric vehicles and to renewable power stations, stimulates the development of improved rechargeable lithium batteries with substantially enhanced energy density and greatly reduced cost. In this technology development, lithium-sulfur batteries (Li-S) are revisited as a promising candidate for large scale electric storage, because of its high theoretical energy density of2600Wh Kg-1and particularly the natural abundance and low toxicity of sulfur. Despite a great progress has been achieved, commercial development of Li-S batteries is still hindered by the insufficient cyclability and low utilization of the electrode-active materials, which are given rise by the insulating nature of sulfur and the dissolution loss of the intermediates generated during discharge of sulfur. This PhD. work focuses on developing S/C composite cathode material with high capacity and good cycling performance. The main results are summarized as follows:1. Three kinds of mesoporous carbons, AMCMB, ACMS and CMK-3. were prepared and then used as conductive substrate for encapsulating sulfur to form S/C composite. The AMCMB and ACMS mesoporous carbon substrates were prepared by chemically activating graphitized mesophase carbon microbeads (MCMB) and non-graphitized mesophase carbon microbeads (CMS) using KOH as activation reagent, respectively. Highly ordered CMK-3mesoporous carbon was synthesized by a nanocasting method using silaceous SBA-15as hard template and sucrose as carbon source. The electrochemical performance of the S/C composites using these mesoporous carbon substrates were characterized by gavanostatically cycling them in organic ether-based electrolyte. The results showed that all three S/C composite electrods exhibit a superhigh coulombic efficiency of～99%, indicating that the mesoporous carbons as prepared in this work could effectively suppress the dissolution loss of the polysulfide intermediates. Benefiting from the highly-ordered and half-opened channel structure of CMK-3, the CMK-3/S compostie not only posseses high sulfur loading content, but also exhibits high capacity and good cycling performance. The initial discharge capacity of CMK-3/S composite with70wt.%sulfur content is1395mAh g-1. After50cycles, the composite still keeps a capacity of～848mAh g-1, corresponding to a capacity retention of61%.2. An alternative way to suppress the dissolution loss of the ploysulfide is proposed by dispersing or encapsulating sulfur into the microporous cabon spheres, which has rich inner micropores and small particle size, and simultaneously using the electrolytes with only poorer polysulfide solubility. To demonstrate the feasibility of this idea, several kinds of microporous carbon microspheres(MCS) were synthesized by hydrothermal method and heating reflux method with sucrose as carbon source, respectively, and then the S/MCS composites were prepared by vaporizing sulfur into the micropores of carbon spheres at elevated temperature. The effects of carbon sphere size and conductive additive on the electrochemical behaviors of the S/MCS composites were investigated by gavanostatic charge and discharge test. The experiment results showed that, in the carbonate electrolyte of1M LiPF6/PC-EC-DEC, the S/C composite obtained from a conductive matrix of nanofiber-wired carbon spheres, exhibits not only stable cycling performance with a reversible capacity of720mAh g-1after100cycles, but also superior high coulombic efficiency of～100%upon extended cycling (except the first three cycles), showing a good application prospect in Li-S batteries.3. The structural and electrochemical analysis indicates that the improved electrochemical behaviors of the S/MCS composite arise from a new reaction mechanism, in which Li+ions and electrons transport through the carbon matrix into the interior of the cathode and then react with the embedded sulfur in the S/C solid-solid interfaces, avoiding the dissolution of the intermediates into the bulk electrolyte. This working mechanism suggested a new idea for developing high perfrmance sulfur electrode in the future.4. To investigate the influence of conductive polymer coating on the electrochemical behaviors of S/C composite, we chose a high specific area carbon BP2000and a high electrical conductive carbon KS6as conductive supports to prerpare two kinds of PAN-coated S/C composites by ball milling the mixtures of conductive carbon with sulfur, and then spraying a suspension of S/C composite and PAN/DMF solution into a water bath, and their electrochemical performances were also evaluated by galvanostatic discharge—charge cycling in carbonate-based eletrolyte. The results showed that the PAN-BP2000/S composite can deliver a reversible capacity of568mAh g-1at the first cycle. After100cycles, the capacity still remains at557mAh g-1. The initial reversible capacity of the PAN-KS6/S composite with a mass ratio of5:1:4is1162mAh g-1.After50cycles, the reversible capacity still keeps at1000mAh g-1, indicating a highly stable cycling performance.