1D Nano-Fibers,Preparations And Their Applications In Lithium-sulfur Battery Cathodes

The traditional lithium ion batteries are difficult to meet the development needs of intelligent electronic products,high-performance electric vehicles and unmanned aircraft,etc.due to their relatively low energy density.Rechargeable lithium-sulfur?Li-S?batteries based on the multielectron redox conversion have recently been studied extensively as a promising candidate because of its terrific theoretical energy density(?2600 Wh kg^-1,based on overall reaction,S₈+16Li?8Li₂S)and the conspicuous advantages of sulfur such as extremely less expense,natural abundance and fewer safetyconcerns.Despitetheseappealinginherentcharacteristics,the commercialization of Li-S battery is still impeded by several obstacles,which include low electronic/ionic conductivity of both sulfur and its various discharge products?Li₂S_x,x=1–2?,the large volumetric expansion?80%?during lithiation,and the shuttle effect of intermediate polysulfides?Ps?,leading to low sulfur utilization,rapid capacity fading and low coulombic efficiency.To deal with the aforementioned problems,confining sulfur within various carbon substrates is believed as an effective approach to improve the electronic conductivity of sulfur and absorb the polysulfides preventing from the shuttle effect.Among them,the 1D porous carbon nanofiber prepared by electrospinning holds a great potential in improvement of the Li-S battery performance due to its good electrical conductivity,excellent flexibility and mechanical properties,and self-supporting characteristics.By now,although there are a few achievements have been made in the use of nanoporous carbon nanofibers,the mechanism responsible for such a dramatically improved performance has not been investigated in detail.In addition,the preparation process is complicated and inevitably involves the use of hazardous chemical reagents such as hydrofluoric acid.In this study,a simple electrospinning process was used to prepare1D carbon nanofibers,and the electrochemical behavior,and especially the enhancement mechanism with different diameter of carbon nanofibers and different surface modification conditions is investigated.This work offers practical approaches to fabricate high performance Li-S batteries while shedding a scientific light on the fundamental insights of the electrode kinetics.The specific works are as follows:?1?free-standing and flexible CNFs mats comprising carbon nanofibers with different diameters:results show that the nanofibers with the certain diameter?approximately 200 nm in this research?have the largest surface area,lowest charge transfer and the highest Li⁺diffusion coefficient?D?,which initiates the nucleation of sulfur species and the gradual formation of membrane at the fiber crossing-junctions.It is likely that the durable fluffy films formed in situ on the carbon fiber surface during cycling provide a self-sustaining architecture to support the polysulfides and avoid the precipitation of large agglomerates.What's more,the membrane also confines polysulfides transport and restricts the notorious shuttle effect.When directly using as a cathode in Li-S battery,it delivers a large capacity of 532.6 mAh g^-1 at 1 C after 1000cycles with a very low capacity decay rate of only 0.023%per cycle(from 100^th to1000^th cycles).In addition,the areal capacity of the foldable electrode can be further boosted to more than 9.5 mAh cm^-2 with the sulfur mass loading of 12.2 mg cm^-2.?2?1D carbon nanofibers decorated with Co₉S₈ nanoparticles as an efficient sulfur host in Li-S battery:the CNFs form a highly conductive network for rapid charge transfer,resulting in high sulfur utilization and good rate performance.Moreover,Co₉S₈ nanoparticles decorated on the surface of the carbon fibers can work synergistically with N heteroatoms to reserve the soluble polysulfides and promote the redox reaction kinetics of sulfur cathodes.As a result,the Co₉S₈?15%?+CNFs composite cathode exhibits high capacity and long cycle life with a low capacity decay rate of 0.026%per cycle over 600 cycles at 1 C.In brief,we have investigated the electrochemical performance and reaction kinetics of one-dimensional carbon nanofibers with different diameters and varied Co₉S₈ modification amount.The carbon fiber film with a diameter of 200 nm and the Co₉S₈?15%?+CNFs composite can effectively improve the specific capacity and cycle stability of the battery,providing new ideas for the development of high-capacity and long-cycle lithium sulfur batteries.The performance enhancement mechanism for the Li-S batteries are investigated.In the end,conclusions are made and perspectives of the battery development are offered.