Synthesis And Application Of N-rich Nanomaterials For Lithium Sulfur Batteries

Li-S battery is regarded as a very promising energy storage system because of the advantages of high specific capacity(1675 mAh g-1),high energy density(2500 wh kg-1),low price and environmental friendliness.However,up to now,the commercial application of Li-S batteries are greatly hindered by three key problems,including the insulting nature of active material S,the volume expansion of sulfur electrode during charge and discharge and the diffusion of intermediate polysulfide between the anode and cathode.In this work,the modification of cathode materials is studied in view of the problems existing in lithium-sulfur batteries.The aim is to prepare multi-functional nanomaterials,which can be used as a carrier of sulfur to increase the electrical conductivity of the cathode electrode material,relieve the volume expansion in the charge-discharge process,and have strong adsorption to lithium polysulfide.The main research content is as follows:1.The nitrogen-doped carbon nanotubes@mesoporous carbon composites were prepared using biomass corn cobs as raw materials.After calcining at 800? for 2 hours,the corn cob provided not only a carbon source but also a small amount of iron as a catalyst,while melamine was used as a nitrogen source.In this process,a large number of carbon nanotubes grow vertically on the mesoporous carbon framework,forming a composite with novel structure.Nanotubes have a diameter of about 50 nm and their length varies from 0.1 to 20 mm and can be controlled by adjusting the ratio of the pretreated corncob to melamine.As a lithium-sulfur battery cathode material,the assembled battery exhibits excellent rate performance and cycle stability.2.An ultra-thin carbon-doped C3N4(C-CNN)nanosheet was prepared by a liquid exfoliating method,and then sulfur was uniformly distributed on the surface via an in-situ deposition method.Through electrochemical impedance spectroscopy and conductivity measurement,the role of carbon doping in improving the conductivity of C-CNN was revealed.The change of electrode before and after the battery reaction was analyzed by X-ray photoelectron spectroscopy,which proved the strong chemical interactions between C-CNN and polysulfide.As a cathode material for lithium-sulfur batteries,it exhibits good rate performance and excellent cycle stability with only 0.07%capacity decay per cycle at 1C.3.A three-dimensional porous network of C3N4/reduced graphite oxide(PCN@rGO)was prepared by a simple hydrothermal reaction.The three-dimensional porous PCN@rGO not only chemically adsorbs polysulfides but also provides a fast electron transfer path which can enhance the redox kinetics.The material has a large specific surface area and can be loaded with more sulfur while providing more adsorption sites.As a lithium-sulfur battery cathode material,it exhibits excellent performance.After 800 cycles,it still maintained 680 mAh g-1 and and the capacity per cycle was 0.048%,demonstrating excellent cycle stability.