Design And Construction Of Carbon-Based Cathode Materials For Lithium-Sulfur Batteries

With the development of global economics,environmental problems are becoming increasingly serious.Secondary battery systems are playing a significant role on solving the energy crisis and environmental problems.Comparing with other energy storage system,lithium metal batteries exhibit the highest theoretical capacity density,which are becoming the very important direction for the development of dynamic battery and large-scale energy storage battery systems.Owing to the ultrahigh theoretical capacity and energy density,lithium sulfur batteries have great advantages in the development competition of various electrode materials for the exploitation of the next generation of high-energy density batteries.Besides,the low price and environmental friendliness of cathode active materials sulfur are better fitting the future demand of low-cost,high-energy energy storage system.However,the insulating nature of active S/Li2S and the volume change(?80%)during charge and discharge,the sluggish redox kinetics of lithium polysulfides and the dissolution and diffusion of long-chain polysulfides(Li2Sx 4?x?8)and the consequent shuttling effect are still impeding the development of lithium sulfur batteries.The design of cathode structure to enhance the adsorption of polysulfide and accelerate the reaction kinetics in the conversion process so as to achieve long and stable cycle characteristics of the battery is an inevitable major scientific problem in the practical application of lithium sulfur battery system.Based on this comprehension,this thesis focuses on the adsorption and catalysis of polysulfide during the reaction process;the detailed results are as follows:1.Through simple treatment of widely available PVC with KOH in the presence of thiourea,the hierarchically porous N,S co-doped carbon materials were obtained,which possessed abundant porosity with large SSA of 808 m2 g-1 and rich heteriatom N and S doping.The NS-C was revealed to be capab le of efficiently trapping soluble lithium polysulifdes through the physical confinement of hierarchical pores and chemical absorption of N,S species,which can be of great help for the improvement of the reversible capacity,rate and cyclin g performance of Li-S batteries.2.The entangled carbon nanotubes array with N doping and encapsulated Co nanoparticles(Co-N-CNTA)was successfully constructed using the CoO nanowires array as both the template and pre-catalyst followed by chemical vapor deposition(CVD)process.Symmetric cells and in-situ UV-vis spectroscopy measurement were applied to confirm the catalytic effect of the hierarchical CNTs array on the polysulfides redox reactions.Moreover,DFT calculations were performed to further reveal the intrinsic interfacial confinement and catalysis of LiPSs at the atomic level.With unique orderly architecture,nitrogen doping and encapsulated Co nanoparticles,the Co-N-CNTA enables not only adsorption ability to LiPSs but also an efficient electrocatalysis promoting sulfur redox kinetics and thus efficiently improved the long cycle performance.3.The Cobalt mono-atom and nitrogen-carbon composites were synthesized by means of high temperature carbonization through solvent thermal synthesis,which were used as the ultrathin coating on the surface of cathode framework.The design not only effectively prevented the shuttling of polysulfide,but also accelerated the electrochemical process on the electrode surface.Besides the effect of physical isolation,the conversion of polysulfide in the form of chemical conversion effectively improves the long cycle capacity of lithium/sulfur batteries.This thesis focuses on the optimization and construction of cathode materials for lithium/sulfur batteries.The combination of physical adsorption and chemical conversion can significantly enhance the limited adsorption and conversion ability of polysulfide by the catalytic framework.In addition,the ultra-thin coating method is adopted to fully improve the long-cycle stability of lithium sulfur batteries.This work provides a new synthesis approach and strategy for the developing cathode with catalysis for lithium sulfur batteries.