Synthesis And Electrochemical Performance Of Cathode Material For Lithium-sulfur Batteries

Lithium-sulfur batteries are considered to be one of the most promising secondarybattery system at present due to their high theoretical specific capacity and theoreticalspecific energy. However, widespread practical applications for rechargeable lithium-sulfurbatteries are plagued with many problems, such as low utilization rate, poor cycle life andrate performance, poor safety. To solve the above problems, conductivity of elementalsulfur should be improved and shuttle effect should be inhibited. So, the research now isfocusing on suppression of the polysulfides dissolution, optimization of the compositematerials with high conductivity, modification of the electrolyte. In this paper, theutilization of Poly naphthalene (PPN) as cathode material was investigated; the synthesisprocesses of the graphene/S and porous carbon/S were optimized which enhance theelectrochemical performance of Li/S battery; the effect of electrolyte additive was studiedand its mechanism was explored. The main research contents are summarized as follows:(1) Synthesis and the electrochemical performance of PPN and PPN/S compositematerials. PPN is synthesized through3,4,9, two,10-perylenetetra carboxylicdianhy-dride under calcining in the tube furnace, its structure is characterized using IR、Raman,XRD and EDS measurements. PPN/S is synthesized using chemical reduction method, itselectrochemical performance is studied in detail. An initial discharge capacity of1145.9mAh·g1is delivered for S/PPN cathode at a current density of400mA·g1, with a capacityretention of59%after200cycles. The rate performance test shows that capacity of735mAh·g-1is obtained even at a current density of1600mAh·g1. When the currentdensity turn back to200mA·g1, the discharge capacity of850mAh·g1is remained. Goodcycle and rate performance is achieved by using the composite material of PPN/S.(2) Synthesis and the electrochemical performance of porous carbon/sulfurcomposites. The structure and electrochemical properties of porous carbon after activationare studied. The initial discharge capacity of AOPC/S cathode is1228.6mAh·g1,68.3%isremained after100cycles. The AOPC/S composite materials are prepared by loading different content of elemental sulfur,48.8%,64%and76%, respectively. The effect ofsulfur content on the electrochemical performance is investigated. The charge-discharge testshows that the composite material with sulfur content of48.8%has the best cycleperformance.(3) Study on the synthesis and properties of graphene/S composite materials.Accordion-like graphene is successfully prepared using MCMB as raw materials throughthe Hummers method. It is activated using KOH under high temperature to obtain the AG,then AG-S composite materials are prepared using chemical reduction method. Comparedto the G-S composite materials, AG-S displays much higher capacity and better cyclingstability. So, superior electrochemical properties are obtained for the composites afteractivation. At the current density of400mA g-1and1000mAg-1, the capacity retention ofAG-S are62.6%and54.7%after200cycles, respectively, with coulombic efficiency above95%. Increasing the current density from400mA·g1to5000mA·g1, the dischargecapacity decreases from1200mAh·g1to250mAh·g1. Then discharge capacity of1080mAh·g1can be returned after decrease the current density to400mA·g1. Theexperimental results indicate that the AG-S composite materials havegood rate performance.(4) The effect and mechanism of the additive on the performance of lithium sulfurbattery. Through adding5mg S powder in the PPN/S battery during cell assemblyprocess, its cycling stability is improved, capacity of948.2mAh·g1is remained after200cycles. SEM images of the200times cycles electrodes show that passivated layer can beeasily formed on lithium anode with the addition of sulfur powers, which can inhibit theformation of lithium dendrite. Meanwhile, the deposition of Li2S and Li2S2on cathodesurface is mitigated, which can alleviate the loss of active sulfur and enhance theperformance of Li/S battery. Different amount of LiNO3is added into electrolyte, its effecton the performance of AOGC/S cathode is studied. With the concentration of0.1mol L-1forLiNO3, an initial discharge capacity of1228.6mAh·g-1is delivered,85.8%of its capacitycan be remained after50cycles. Superior cycling stability of the AOGC-S/Li battery is achieved.（5） Synthesis and the electrochemical performance of TiO2-carbon-sulfurcomposites. The initial discharge capacity of TiO2-C-S cathode is1010.1mAh·g1, The100th discharge capacity of cathode is801.3mAh·g1,79.3%is remained after100cycles.Compared to the C-S composite materials, TiO2-C-S displays much higher capacity andbetter cycling stability...