Structural Design Of Heteroatom-doped Carbon Materials And Its Application In Lithium-Sulfur Batteries

After nearly three decades of rapid development,the lithium-ion battery（LIBs）has become the most widely used energy storage device in the market.It involves a wide range of fields,such as mobile communications,electric vehicles,electronic devices,etc.However,with the rapid development of science and technology,all kinds of equipment have higher and higher battery requirements.No matter now or in the future,all types of devices need batteries with large capacity,safe performance,and low price.The existing capacity of lithium-ion batteries can’t meet these stringent requirements.Therefore,the development of new batteries with high energy density and high specific capacity has become the focus of research in the energy field.Among many new types of batteries,lithium-sulfur battery（LSBs）has higher theoretical energy density(2600 Wh kg^-1)and higher theoretical specific capacity(1675 m Ah g^-1),so it has been widely concerned and developed in recent years.However,there is still a long way to go for the large-scale commercial application of lithium-sulfur battery,because some thorny technical problems need to be better solved.For example,sulfur has low conductivity and almost insulation;sulfur expands seriously in charge and discharge,which can easily lead to electrode rupture;lithium polysulfide is easy to dissolve into the organic electrolyte,resulting in"shuttle effect"and low utilization of active sulfur.In this paper,through reasonable assembly and design of carbon-based materials,composite carbon materials with high electrical conductivity and porous structure were prepared.By making use of the characteristics of large specific surface area,porosity,and polarity,the effective physical adsorption of lithium polysulfide（LiPSs）can be carried out to ensure the stability of the battery during the charge-discharge cycle.The introduction of heteroatoms and metal particles’addition can promote redox kinetics to a certain extent.The action mechanism and electrochemical properties of the prepared composites were explained through various characterization methods and testing methods.The specific research contents are as follows:（1）Effectively Confining lithium polysulfides（LiPSs）inside conductive cathodes material matrix represents a judicious way to extend the lifespan and rate performance of lithium-sulfur（Li-S）batteries.Herein,Nitrogen-Doped Hollow Carbon Polyhedrons with a thin CNTs Surface layer（CNTs/HNC）were prepared by directly pyrolyzing CNTs coated hollow ZIF-8 crystallites which were pre-treated with tannic acid,and served as sulfur hosts in Li-S batteries.The resulted product CNTs/HNC comprises capsule-like morphology,a high content of nitrogen（5.13 at.%）as well as excellent electrical conductivity,which helps to effectively adsorb/confine polysulfides and substantially improve the rate capacity of Li-S batteries.The S@CNTs/HNC based cathode shows a discharge capacity of 870.7 m Ah g^-1 at1.0 C,and can maintain 76.36%of its initial capacity after 500 charge-discharge cycles,corresponding to a capacity fade rate of only 0.047%per cycle.While with a higher sulfur loading(2.462 mg cm^-1),a discharge capacity of 649.7 m Ah g^-1 can be achieved at 0.5 C,and it shows a capacity fade rate of merely 0.044%per cycle during 200 cycles.This work enriches the ways to prepare complicate nanostructured sulfur hosts for long-life and high-rate Li-S batteries.（2）The effective inhibition of lithium polysulfides（LiPSs）and promotion of their conversion in the redox processes is indispensable to achieve the long cycle stability and excellent rate performance of lithium-sulfur（Li-S）batteries.Especially,the generally slow electrocatalytic sulfur redox kinetics and large interfacial Li₂S nucleation energy barrier have hindered the widespread of Li-S batteries.Herein,a robust three-dimensional sulfur carrier（denoted as Co-NCNT）is well-constructed by using the potassium citrate derived porous carbon sheets as substrate,and the catalytic growth nitrogen-doped porous carbon nanotubes as vertical scaffolds.Such a rationally designed structure guarantees efficient electrons transfer pathways and ions diffusion channels,and more importantly,it is conducive to the adsorption/catalytic conversion of intermediate lithium polysulfides and the nucleation of Li₂S.Due to these merits,the S@Co-NCNT cathode can achieve an initial discharge capacity up to 1072.7 m Ah g^-1 at 1.0 C,and it can retain a high capacity of 482.9 m Ah g^-1 with a capacity attenuation rate of only 0.045%per cycle after 1000 cycles.Upon a high sulfur loading of 5.87 mg cm^-2,it can still reach an initial capacity of 739.5 m Ah g^-1 at a charge/discharge rate of 0.3 C.Particular emphasis is that our work enriches the ways to prepare complicate carbon nano tubes structured sulfur hosts for long-life Li-S batteries.