The Synthesis And Electrochemical Performance Of Nanostruture Cathode Materials For Lithium-Sulfur Batteries

With the increasingly rigorous environmental problems,increasing energy demand and the rapid development of electronic equipment,it is high desire to design and develop efficient energy storage equipment.Lithium-sulfur(Li-S)batteries play a key role as a promising candidate of next-generation energy storage devices in terms of high theoretical specific capacity(1675 mAh g-1),high energy density(2600 W h kg-1)as well as the environmental friendliness,natural abundance and low cost of sulfur.However,the commercialization process of Li-S batteries faces issues,including the low sulfur utilization,poor rate capability,and short cycle life.All these problems are caused by the low electronic conductivity of sulfur and its discharge products(Li2S2/Li2S),the dramatic volume expansion upon lithiation,and the so-called shuttling effect of intermediate lithium polysulfides(LiPSs).In response to the above problems,in order to improve the electrochemical performance of Li-S batteries,this work has been based on the advantages of nano-structured materials,three novel nanostructured sulfur cathode materials were designed and prepared.Furthermore,the preparation conditions and microstructure as well as the electrochemical properties of those materials were studied,the feasibility of these materials as cathode materials for Li-S batteries is discussed,and the results are as follows:1.Iron carbide(Fe3C)embedded micro-mesoporous carbon nanospheres as the sulfur host material for Li-S batteries.An effective sulfur host material was prepared by implanting Fe3C nanocrystals homogeneously into micro-mesoporous carbon nanospheres.The high conductive specific surface area and ordered micro-mesoporous structure facilitate the homogeneous loading of sulfur before discharge and the uniform nucleation of Li2S after discharge.Moreover,compared with bulk Fe3C,the well-dispersed ultrafine Fe3C nanocrystals greatly enhance the adsorption and catalytic ability of LiPSs due to their increased number of polar active sites.Based on the above these advantages,the Fe3C/MMC-S NSs cathodes show impressive rate performance(656 mAh g-1 at 5.0 C)and outstanding cyclic stability with a high initial capacity of 1030 mAh g-1 and good capacity retention of 700 mAh g-1 after 1000 cycles at 1.0 C(a low capacity decay of 0.033%per cycle).2.Ultrafine NiS2-ZnS embedded micro-mesoporous carbon nanospheres as the sulfur host material for Li-S batteries.An effective sulfur host material was prepared by implanting ultrafine NiS2-ZnS(uNiS2-ZnS)nanocrystals homogeneously into micro-mesoporous carbon nanospheres.The uNiS2-ZnS/MMC-S cathodes make the full use of the respective advantages of uNiS2-ZnS heterostructures and micro-mesoporous structure,synergistic effect of physical confinement and chemical adsorption/catalysis of LiPSs is achieved.Furthermore,compared with bulk hetero structured materials,the uNiS2-ZnS heterostructures greatly enhance the adsorption and catalytic ability of LiPSs because catalysis interfaces effect and naturally formed in-plane interfaces can be magnified by the ultrafine dispersed nanoparticles.Benefit from the above advantages,the uNiS2-ZnS/MMC-S cathodes exhibit outstanding rate capacities(675.5 mAh g-1 at 5.0 C)and cyclic stability(710.5 mAh g-1 at 1.0 C after 1000 cycles with a low capacity decay of 0.033%per cycle).3.The fabrication and electrochemical performance of the pomegranate-like S@N-C@N-C NSs with a seed-pulp-peel nanostructure for Li-S batteries.Inspired by the structure of a pomegranate fruit,the pomegranate-like S@N-C@N-C NSs with a seed-pulp-peel nanostructure were obtained.The composite cathode material contains S nanocluster(seed)with an average diameter of 2.2 nm.These nanoclusters are perfectly encapsulated by N-C(pulp)to form S@N-C subunits with an average diameter of 5 nm and a certain cavity.Notably,these S@N-C subunits are re-coated with a layer of N-C(peel)to form the micro-mesoporous structure NSs with an average size of 150 nm.Based on the above advantages,the pomegranate-like S@N-C@N-C NSs with a seed-pulp-peel nanostructure not only can shorten the electron transfer distance but also provide abundant electron transport active sites.In addition,the nitrogen-doped micro-mesoporous carbon well limits the diffusion of Li2Sx,and at the same time,Li2S2 is obtained in the nanometer size range,and the rapid conversion of Li2S2 to Li2S is achieved.As a result,the S@N-C@N-C seed-pulp-peel NSs cathodes exhibit excellent sulfur utilization,superb rate performance(760 mAh g-1 at 10.0 C),and long cycle life(931 mAh g-1 and 800 mAh g-1 at 1.0 C and 4.0 C after 1000 cycles).