Functional Design And Electrochemical Study Of Carbon Materials For Lithium Sulfur Batteries

Lithium sulfur batteries possess the advantages of high theoretical energy density,environmental friendliness and low cost.However,there are some drawbacks to restrict its practical applications,such as low active material utilization,poor cyclic stability and serious safety problem.In this work,we usedthe carbonaceous material as the foundation and combined with the construction of the conductive network,confinement effect,pore structure control,heteroatom doping and surface chemical modification to prepare the novel lithium sulfur batteries with high energy density and long cycle life.In this work,a graphene/carbon nanotube(CNT)/sulfur(denoted GCS)hybrid with interconnected structure is prepared through a one-step reduction induced self-assembly approach by L-ascorbic acid reduction under a mild condition.In such a solution-based assembly process,the formation of an interconnected graphene/CNT conductive network is accompanied by the uniform loading of sulfur.This conductive network formed by interconnected graphene sheets and CNT can supply an unimpeded and continuous path for electron and Li ion transfer and accommodate the volume variation of sulfur during charge/discharge cycling.In addition,the residual functional groups on GCS can retain intimate contact of the conducting matrix with sulfur and effectively confine the diffusion of polysulfides.As a result,the as-prepared GCS hybrid shows an excellent cyclic performance with a capacity of 657 mAh g'1 after 450 cycles and the capacity decay is only 0.04%per cycle even at a high rate of 1.0 C.Chemical doping of carbon materials with heteroatoms,such as nitrogen,sulfur is an efficient way to tailor their chemical properties and enhance the chemical affinity between the carbon and sulfur.In this work,nitrogen and sulfur co-doped porous carbon spheres(NS-PCSs)were prepared using L-cysteine to control the structure and the functionalization during the hydrothermal reaction of glucose and the subsequent activation process.As the sulfur hosts in Li-S batteries,NS-PCS combine strong physical confinement and surface chemistry interaction to improve the affinity of polysulfide to carbon matrix.In addition,in order to further improve the electron and Li ion transport,a sheet-like carbon sandwich,which contains graphene sheets as the conductive filling with N-doped porous carbon layers uniformly coated on both sides,is designed for a novel sulfur reservoir for lithium-sulfur batteries and experimentally obtained by a hydrothermal process of a mixture of graphene oxide,glucose and pyrrole,followed by KOH activation.Both porous carbon layers act as a high capacity reservoir for sulfur and the polysulfide while the thin graphene sheets and the N-doped conducting matrix guarantee fast ion diffusion and electron transfer,ensuring a good rate performance of the carbon-sulfur hybrid.At a 2 C rate,the reversible capacity is up to 625 mAh g-1 and remains 461 mAh g-1 after 200 cycles with only 0.13%capacity fading per cycle.Starting from the integral structural design,a thin and multifunctional PEDOT:PSS@BP2000(PS@BP)coating tightly attached on the surface of the sulfur-carbon electrode was prepared by a simple and fast electrostatic spray deposition(ESD)technique.The as-prepared PS@BP coating composed of conductive polymer PEDOT:PSS and porous carbon BP2000 not only can restrain the dissolution of polysulfides through both physical adsorption of BP2000 and strong chemical binding between the PS and polysulfides,but also can serve as a upper current collector to reuse the trapped active materials and then improve the electrochemical performance of lithium sulfur batteries.