Application Of Porous Carbon-based Coated Composites In Cathode Of Lithium-sulfur Batteries

Lithium-sulfur batteries（LSBs）has recently drawn much attention,owing to its incredible energy density(2600 Wh kg^-1)and theoretical specific capacity(1675 m Ah g^-1).Despite its promising performance,the commercialization of LSBs has been beset by low electrical conductivity of sulfur,significant volume fluxes throughout the charge-discharge cycling,and capacity degradation resulting from either the dissolution and migration,or shuttle effect of lithium polysulfide（Li PS）intermediate.These represent the main obstacles in the development of practical LSBs.Therefore,it is imperative to seek alternative methods for fabricating sulfur host materials that mitigate the generation of Li PS,while simultaneously enhance the sulfur utility in LSBs.In view of the above problems,in this paper,different coating materials and preparation processes to coat nanomaterials was adopt that graphene-coated hollow-structured porous Mg ZIF-67/CNT@GO microspheres and covalent organic frameworks（COF）coated 3D ordered microporous（3DOM）structure 3DOM Tp Pa-1@Co/Ti O_xN_y were designed,using the coating layer to improve the cycle stability of the material while improving the ionic conductivity of the point electrode by loading the transition metal.The main contents and results of the paper are as follows:（1）A porous microsphere-based sulfur cathode was prepared by spray drying,consists of Mg²⁺etched ZIF-67 and carbon nanotubes encapsulated with graphene oxide（Mg ZIF-67/CNT@GO）,was fabricated.This composite has excellent electrical conductivity through CNTs and GO,and mitigate the loss of polysulfides through its porous structure.More importantly,the Mg²⁺etched ZIF-67 provides a large area of active interface,which promotes its trapping with polysulfides and catalytic performance towards polysulfides conversion.Accordingly,the Mg ZIF-67/CNT@GO composite cathode delivers good electrochemical performance.The battery exhibits highly capacity retention rate of 87.4%at 0.2 C and an advanced areal capacity of 3.35 m Ah cm^-2 under 3.0 mg cm^-2 sulfur loading at 0.2 C.In addition,the pouch cell cycled at 0.1 C retains its high capacity of 640.8 m Ah g^-l after 50cycles.The current work enables a promising design approach for the pragmatic implementation of LSBs.（2）The covalent organic framework（COF）-coated conductive porous metal oxide framework design strategy was demonstrated by coating a Tp Pa-1 COF layer on cobalt-decorated titanium oxynirtide（Ti O_xN_y）with a three-dimensional ordered microporous framework（3DOM）structure.In this strategy,the oxygen-deficient Ti O_xN_y framework ensures a good conductivity and structural stability of the cathode during the charge/discharge process.The 3DOM macrospores provide a high capacity for sulfur accommodation,whereas the coated Tp Pa-1 COF featured with ultrafine microspore offer an effective confinement of Li PS within the 3DOM framework,mitigating its shuttling effect.At the same time,the Co embedded in 3DOM Ti O_xN_yservers as efficient catalyst promoting the sulfur electrochemical reaction.The 3DOM structural design also endows the sulfur cathode with high specific surface area exposes mass active interfaces further facilitating the Li PS redox reactions.Attributed to these structural superiorities,the 3DOM Tp Pa-1@Co/Ti O_xN_y/S cathode exhibit excellent electrochemical performance,i.e.,stable cycling over 500 cycles with a low-capacity fading rate of 0.031%per cycle at 1 C.Even at high sulfur loading of 4.46 mg cm^-2and lean electrode condition(electrolyte/sulfur ratio of～2.8μL mg^-1),a high initial capacity of 3.38 m Ah cm^-2 is achieved at 0.2 C.In this paper,the two coated-composites based on highly conductive substrates show excellent electrochemical performance in the application of lithium-sulfur battery,effectively inhibit the shuttle effect,and significantly improve the cycle performance and rate performance of the battery.This design strategy offers an effective alternative strategy for the design of high-performance sulfur cathode in LSBs.