Design Of Low-dimensional Carbon Composite Structure Modified By Co-Ni Nanoparticles And Study On Li-S Battery

Lithium-sulfur(Li-S)batteries are believed to be next generation commercial secondary battery system benefiting from their ultra-high energy density and high theoretical capacity.Meanwhile,sulfur as the cathode material in Li-S batteries,is not only abundant in the earth's crust,but also environmental friendly.Despite these advantages,the commercialization of Li-S batteries is limited by the shortcomings of research,such as low conductivity,severe volume expansion and inevitable shuttle effect.The positive discharge process of Li-S battery is mainly divided into two parts.The specific discharge capacity can account for about 70%of the whole discharge process within the discharge range of 2.1 V.In view of the current research focus,it has been changed from preventing the dissolution and diffusion of lithium sulfide to promoting and inducing the conversion of lithium sulfide to discharge product Li2S,which is considered to be the fundamental way to completely suppress the "shuttle effect" and maximize the energy output of Li-S batteries.Therefore,in this study,N-doped graphitic ladder-structured carbon nanotubes and bimetal CoNi active sites on mesoporous carbon nanosheets were designed to dynamically promote and induce the conversion of LiPs to discharge product Li2S.The main contents are as follows:(1)Herein,a new confinement strategy originates a distinctive sulfur scaffold,namely nitrogen doped graphitic ladder-structured carbon nanotubes(NGLCNTs)loaded with appropriate amount of metallic cobalt nanoparticles.Due to the unique graphite ladder structure of carbon nanotubes and the presence of metal cobalt nanoparticles in the nitrogen-containing carbon nanotubes,these one-dimensional nanotubes with high aspect ratio can easily form a three-dimensional interconnected conductive network,which is conducive to the transport of electrons and ions in the cycling process.And more importantly,new ladder structure composed of carbon nanotube inside not only can be used as a physical isolation to create enough internal space,to adapt to the high content of sulfur,alleviate the volume expansion,adsorption soluble sulfur compounds,can also effectively prevent the repeat of polysulfide in the process of charging and discharging lithium dissolved and weaving effect.The positive electrodes of NGLCNTs not only promote ion transfer and electron transfer in the redox reaction of high-order polysulfides to Li2S2/Li2S,but also effectively limit the diffusion and dissolution of polysulfides,which is conducive to the nucleation and growth of Li2S.As a result,the NGLCNTs/S cathode has successfully overcame the crucial issues based on sulfur electrodes,and it presents greatly enhanced electrochemical performances with excellent rate capabilities up to 8 C and longed life over 500 cycles.(2)The doping of polar materials in the material can effectively improve the electrochemical performance of Li-S batteries.The synergistic effect of binary transition metal nanoparticles shows strong catalytic performance and has been widely used in catalytic reactions.Based on this,we designed mesoporous carbon supported Co-Ni bimetallic composite dynamically-promoted Li-S battery.In order to solve the problem that soluble polysulfide intermediates diffuse between cathode and anode during charging and discharging,which leads to rapid attenuation of battery cycle life,the separator modification materials come into people's sight.Herein,a mesoporous carbon-supported Co-Ni bimetal composite(CoNi@MPC)has been synthesized and directly coated on the original separator to serve as a secondary current collection for Li-S batteries.CoNi@MPC exhibits multiple Co-Ni active sites,able to catalyze the reactions of soluble polysulfides,specifically accelerating the generation and decomposition of insoluble Li2S in lithiation and delithiation process testified by the electrochemical results and density functional theory calculation.Due to the bifucntional electrocatalyst,the shuttle effect of LiPs can be effectively alleviated.Moreover,porous carbon as the conductive scaffold favors the improvement of electronic conductivity.Benefiting from the above advantages,the capacity of the battery can be maintained at 724.7 mAh g-1 after 500 cycles at 1 C current density,showing excellent electrochemical performance(the capacity decay rate is as low as 0.090%).