Synthesis And Electrochenmical Properties Of Conductive Polymers? Carbon/sulfur Composites As A Cathode Material For Lithium Sulfur Batteries

The emergence of the energy crisis prompted the advent of Lithium ion battery.Lithium ion battery is the most widely used electronic products in the market at present,and it is also the main research direction of high energy density storage applications.A lot of hard work has been made to improve the energy density and electrochemical performances of lithium ion batteries in recent 20 years,however,the theoretical specific capacity is limited by the cathode material,which is difficult to meet the needs of the development.Sulfur is one of the most plentiful elements on earth at present,lithium sulfur batteries have become one of the most widely studied lithium ion battery systems due to the high theoretical energy density(?2500 W h/kg)and the high theoretical specific capacity(?1673 mAh/g),as well as their safety and low cost.However,some serious problems have hindered the application and commercialization of lithium-sulfur batteries,these problems include:(1)The dissolution of lithium polysulfides(LPS)into electrolyte bring on fast decline of specific capacity and low coulombic efficiency.(2)Sulfur and its discharge products(Li2S/Li2S2)are typical ionic and electronic insulators.(3)In the process of discharge/charge,the sulfur-based cathode expands and contracts in succession,which causes the damage and collapse of structure.To solve these problems,we designed different conductive polymers@sulfur cathode composites and polymers@carbon/sulfur cathode composites.(1)The nanoscale S cathode was synthesized by the chemical reaction of sodium thiosulfate(Na2S2O3)and hydrochloric acid(HCl),polyvinyl pyrrolidone(PVP)was as surfactants and dispersants because of its rich functional groups.The S nanospheres are coated with the layer of polyvinyl pyrrolidone,and the coating can help tarp the lithium polysulfides to increase the coulombic efficiency and electrochemical performance.A relatively high initial discharge capacity of 1076.0 mAh/g is acquired for the PVP/S composite at 0.1 C rate.The PVP/S electrode maintains 401 mAh/g after 100 cycles while the coulombic efficiency remains 95%.SDS/PVP/sulfur nanocomposites were prepared by reducing the reaction temperature and using the crosslinking of SDS micelles and PVP surfactants.An initial discharge capacity of 645.3 mAh/g is acquired for the SDS/PVP/S composite at 0.1 C rate.The SDS/PVP/S electrode maintains 260.8 mAh/g after 200 cycles while the coulombic efficiency remains 98%.It is shown that the high dispersion state and nanometer scale enhance the electrochemical activity of sulfur monomer,and reduce the occurrence of the shuttle effect,thus optimize the electrochemical properties of lithium-sulfur batteries.(2)Polyaniline nanotubes(PANI-NT)/sulfur composites were prepared by static reaction and in-situ polymerization.On the basis of PVP/S composites,three kinds of conductive polymer coated nanocomposite sulfur was prepared by in-situ polymerization and the three conductive polymers are polyaniline(PANI),polypyrrole(PPY)and poly 3,4-ethylene dioxythiophene(PEDOT).The manganese dioxide(Mn02)hydrophilic layer on the surface of PEDOT/S was prepared by the reaction of PEDOT surface feature functional group and potassium permanganate(KMn04).A high initial discharge capacity of 1601.55 mAh/g which is close to theoretical capacity is acquired for the PANI-NT/S composite at 0.2 C rate.The PANI/S electrode maintains 598.8 mAh/g after 50 cycles.A high initial discharge capacity of 649.46 mAh/g is acquired for the PANI/S composite at 0.1 C rate.The PANI/S electrode maintains 365.69 mAh/g mAh/g after 50 cycles and the capacity retention rate is over 50%.A relatively high initial discharge capacity of 786.0 mAh/g is acquired for the PPY/S composite at 0.25 C rate.The PPY/S electrode maintains 680.13 mAh/g mAh/g after 50 cycles.A reversely high initial discharge capacity of 823.03 mAh/g is acquired for the PEDOT/S composite at 0.2 C rate.The PEDOT/S electrode maintains 701.22 mAh/g mAh/g after 50 cycles and the capacity retention rate is over 85.17%.A high initial discharge capacity of 1088.15 mAh/g is acquired for the Mn02@PEDOT/S composite at 0.1 C rate.The MnO2@PEDOT/S electrode maintains 1234.1 mAh/g mAh/g after 50 cycles and the capacity retention rate is over 100%.Thanks to high conductivity of conductive polymers,the first discharge capacities of electrodes are improved significantly,also because of conductive polymer coated layers of protection,the shuttle effect of lithium sulfur batteries is obviously inhibited.MnO2@PEDOT/S composite shows excellent electrochemical stability and cyclic performance.(3)The spherical ordered mesoporous carbon(S-OMC)and the rodlike ordered mesoporous carbon(R-OMC)were prepared by one-step soft template method.The carbon/sulfur(S-OMC/S)composite materials and the club-like ordered mesoporous carbon/sulfur composites(R-OMC/S)were prepared by using two kinds of mesoporous carbon as the carrier of sulfur monomers through thermal melting.A high initial discharge capacity of 1201.62 mAh/g is acquired for the R-OMC/S composite at 0.2 C rate.The R-OMC/S electrode maintains 640.29 mAh/g mAh/g after 100 cycles while the coulombic efficiency remains 90%.A high initial discharge capacity of 1333.4 mAh/g is acquired for the S-OMC/S composite at 0.2 C rate.The S-OMC/S electrode maintains 643.6 mAh/g mAh/g after 100 cycles while the coulombic efficiency remains 90%.The results showes that the electrochemical cycle performance and stability of the ordered mesoporous carbon/sulfur composite materials have been greatly improved thanks to the high specific surface area of the mesoporous channel and its considerable aperture ratio.(4)Polyaniline @ spherical ordered mesoporous carbon/sulfur(PANI @S-OMC/S)composite was prepared by the method of in-situ polymerization and one-step soft template.A high initial discharge capacity of 1647 mAh/g which is close to theoretical capacity is acquired for the PANI@S-OMC/S composite at 0.05 C rate.The PANI@S-OMC/S electrode maintains 1383 mAh/g mAh/g after 50 cycles.A high initial discharge capacity of 1663 mAh/g which is close to theoretical capacity is acquired for the PANI@S-OMC/S composite at 0.1 C rate.The electrode maintains 1302 mAh/g mAh/g after 100 cycles and the capacity retention rate is over 78.29%.This kind of special coating structure not only can adsorb lithium polysulfide and improve the activity of sulfur,but also can ameliorate the conductivity of Li-S batteries.The PANI@S-OMC/S demonstrates a promising electrochemical performance because of these advantages.