Preparation Of Hierarchical Carbon Based Composites And Their Application For Lithium-sulfur Batteries

Lithium-sulfur batteries have attracted extensive attention in recent years due to their high theoretical capacity(1672 m Ah g^-1)and energy density(2600 Wh kg^-1).However,there are still some problems in lithium-sulfur batteries,such as poor electronic and ionic conductivity of sulfur and its discharge products,huge volume change of positive and negative electrodes during the cycle,"shuttle effect"caused by soluble polysulfide intermediates,and dendrite growth of metal lithium anode.All these seriously hinder the development and commercial application of lithium-sulfur batteries.In this study,through reasonable structural design,one-dimensional carbon material was successfully introduced into the oxide with three-dimensional ordered macroporous（3DOM）structure,and two kinds of multistage carbon matrix composites were constructed.They are Co@TiO_xN_y/N-CNTs as sulfur host and CNTF/Co@Al₂O₃/CNFs as all-purpose flexible electrode.They have the advantages of good electrical conductivity,many active sites and adaptability to volume expansion.Their morphology,elemental properties and specific surface area were investigated and their electrochemical properties were tested.The main work content is divided into two parts as follows:（1）In this work,a Co-metal-decorated deficient titanium dioxide skeleton（Co@TiO_xN_y）featured with macroporous structure is fabricated by template method.Then,nitrogen doped carbon nanotubes with cobalt nanoparticles（Co@N-CNTs）were embedded in the oxide macropores by vacuum filtration.The resulting end product serves as a multifunctional sulfur host.The porous oxide skeleton is honeycomb and continuous.The unique three-dimensional ordered macroporous frame provides sufficient space for sulfur volume expansion and an effective path for electrolyte penetration.The stable titanium nitride framework also ensures good structural integrity of the cathode during repeated charge-discharge cycles.On the one hand,the introduction of oxygen defects not only improves the intrinsic conductivity of titanium dioxide skeleton,but also enhances its ability to capture polysulfides.On the other hand,Co@N-CNTs is embedded in the macroporous framework to form a highly conductive network that is interconnected and provides abundant micropores and physical constraints for the distribution of sulfur.In addition,highly dispersed cobalt nanoparticles were uniformly anchored to TiO_xN_yand N-CNTs as electrocatalysts to promote polysulfide conversion.Due to these properties,Co@TiO_xN_y/N-CNTs/S electrodes showed excellent rate performance and excellent cycling performance.A high area capacity of 5.05 m Ah cm^-2was obtained after 50 cycles,even in the condition of 6.34 mg cm^-2sulfur loading and poor electrolyte to sulfur(E/S=8μL mg^-1).This work offers a rational strategy for the design of advanced sulfur cathode in lithium-sulfur batteries.（2）In order to solve the"shuttle effect"caused by polysulfide intermediates in the positive sulfur electrode and the dendrite growth problem faced by lithium metal anode at the same time,The design idea of job one is applied to the flexible electrode and extended to lithium anode.In this work,CNTF/Co@Al₂O₃/CNFs was prepared as all-purpose flexible electrode for lithium-sulfur battery.It was prepared by loading a three-dimensional porous alumina framework on a carbon nanotube film（CNTF）by template method,and then growing carbon nanofibers in macroporous alumina framework by vapor deposition.As a flexible carrier of alumina,CNTF has certain electrical conductivity and mechanical properties.Polar macroporous alumina has strong interaction with polysulfide and provides high stability for the electrochemical reaction of sulfur species.The carbon network composed of carbon nanofibers is favorable for electron/ion transport and uniform distribution of sulfur.As the active center,cobalt nanoparticles in large pores can promote the rapid conversion of soluble lithium polysulfide into insoluble Li₂S.In addition,the alumina with large pore structure is well combined with lithium and provides enough free space to buffer the huge volume change in the lithium electroplating/stripping cycles.The uniform growth of lithium was successfully induced by carbon nanofibers and active cobalt particles in macroporous alumina framework,and dendrite-free lithium anode was achieved.A flexible pack cell assembled from CNTF/Co@Al₂O₃/CNFs/S electrode and CNTF/Co@Al₂O₃/CNFs/Li electrode shows a reversible specific capacity of 972 m Ah g^-1.These results suggest that CNTF/Co@Al₂O₃/CNFs has great application potential as an all-purpose flexible electrode for lithium-sulfur batteries.