Secondary Lithium Battery Research, Preparation And Electrochemical Properties Of Sulfur-containing Cathode Material

Jiulin Wang (Materials Physics and Chemistry) Directed by Naixin Xu, Jun Yang and Jingying XieDue to its abundant resources, low toxicity and the highest charge capacity in all known cathode materials, elemental sulfur is a promising cathode material for the next generation of high-performance lithium batteries. However, previously reported Li/S batteries encounter many serious problems, including very low active material utilization and poor rechargeablity, which inhibit the commercialization of rechargeable Li/S batteries.In this study, two types of nano particle sulfur composite materials were designed and prepared, that is, conductive-polymer/sulfur composites and active-carbon/sulfur composites, which can be used as advanced cathode materials for rechargeable lithium batteries.Sulfur/conductive-polymer composites were prepared simply by thermal treatment of a mixture of elemental sulfur and polymer. The polymer precursors include PVC, PE, PAN, PP and so on. The structures and components of the resulting composites were characterized by XPS, XRD, FTIR, Raman, NMR, SEM, BSE, TEM, BET and element analysis. The mechanisms of dehydrogenization and sulfuration reaction were deduced on the basis of the characterization results. The charge capacities, reversibilities and self-discharge properties of the above-mentioned composites were systematically studied in lithium batteries based on gel electrolytes.The structure of the polymer precursor affects the mechanism of the dehydrogenization and sulfuration reaction. Especially, the side chain of the precursor almost has a decisive effect on the structure of the resulting composite and their electrochemical properties. By optimization of side functional group and selection of polymer precursor, the resulting composite materials shows high reversible specific capacity about 850 mAh g"1 in first discharge which remained above 600 mAh g"1 after 50 cycles at the current density of 0.3 mA cm"2 at room temperature. The utilization of-II-sulfur in the composites approaches 90% and galvanic efficiency is about 100%. Moreover, the Li/S batteries with above-mentioned composite cathode materials have very low self-discharge rate at room temperature.Another type of composites, sulfur/active carbon composites, possess the structure of sulfur incorporated in nano and micro pores of active carbon. Active carbon could effectively inhibit the reaction products ?polysulfides dissolving into electrolyte for its strong absorbability arising from very large surface area. So Li/S batteries show good cyclability with high utilization of the active materials.All above results indicate that many problems in traditional Li/S batteries are dramatically improved, even overcomed. The above-mentioned composites show excellent electrochemical performance.