Controllable Preparation Of Carbon-based Materials And Electrochemical Performance Study In Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)batteries have been considered as one of the most promising next-generation energy storage devices due to their high theoretical specific capacity,high energy density,and that elemental sulfur is high abundance,low cost,and environmentally-friendly.However,there are some problems for sulfur cathodes to be solved,including(1)the electric insulativity of elemental surfur and Li2S during charge/discharge processes,(2)the large volume change of active materials during cycling and(3)the inevitable "shuttle effect" caused by the dissolution and diffusion of the soluble lithium polysulfides(LiPSs)intermediates in electrolyte.These problems and challenges would cause the low utilization of active materials,the fast capacity fading,the poor cycling stability,and so on,and thus limit the commercialization of LiS batteries.To solve the above problems,the thesis devotes to design the sulfur host materials and the modified layer coated on the PP separator using the metal-based carbon composites with high conductivity,high chemisorption,and strong catalytic activity,which can improve the electrochemical performance of Li-S batteries.The main works are summarized as the following:(1)A hollow CoxSny alloy modified N-doped carbon(denoted as E-CoxSny@NC)composite as the sulfur host material of Li-S batteries is prepared through a facile stepwise coating-etching approach.The introduction of tetraethyl orthosilicate(TEOS)and the pyrogenation of polydopamine(PDA)can directly induce the formation of CoxSny alloy at high temperature,and the adding of ethylenediamine(EDA)can increase the N-doped content(5.69 wt%)of carbon framework,which can improve the polar adsorption of carbon surface and electronic conductivity.The novel metal-Ncarbon-based composite possesses strong chemisorption and electrocatalytic conversion activity for lithium polysulfides(LiPSs),which can accelerate the redox reaction kinetics and minimize the polysulfides loss.Therefore,the prepared ECoxSny@NC/S cathode shows good long-term cycling stability(1161 mAh g-1 at 0.2 C,a capacity retention of 81.2%after 500 cycles at 1.0 C and 0.0376%decay per cycle),superior rate capacity(778 mAh g-1 at 2.0 C),and low polarization.(2)A three-dimensional(3D)lather-like porous carbon framework containing Febased compounds(FeCFeOC)is designed by a freeze-drying method as the sulfur host and the interlayer on separator.The porous carbon framework of FeCFeOC composite can improve the electronic conductivity of active materials and accommodate their volume change during cycling.The polar Fe-based compounds nanoparticles have the strong chemisorption and catalytic activity.The soluble LiPSs are firstly adsorbed and anchored on the surface of cathode,and then are catalyzed to reduce the reaction barrier of LiPSs and accelerate their conversion reaction.In addition,the formed FeSx species with a larger size during cycling can not only be easy to block from the penetration of LiPSs cross the separator and suppress their shuttle,but also reversibly participate in the redox reaction between S8,and LiPSs and Li2S,and thus further improve the utilization of active materials.As a result,the assembled Li-S cells show excellent longterm stability(748 mAh g-1 over 500 cycles at 1.0 C,and～0.036%decay per cycle for 1000 cycles at 3.0 C),a superb rate capability with 659 mAh g-1 at 5.0 C,and lower electrochemical polarization.(3)A cobalt phosphide nanoparticle embedded in carbon spheres(CoP@C)as an interlayer on polypropylene(PP)separators are designed through a facile coprecipitation and phosphation method.Phosphation can increase the active sites on the surface of CoP nanoparticles.The designed spherical CoP@C composite can not only capture the LiPSs effectively via strong chemical anchoring ability,but also have good electrocatalytic effect to enhance Li+ion transport and reduce the conversion barrier,and ultimately accelerate the LiPSs conversion kinetics.In addition,the adsorped sulfur active materials on CoP@C interlayer can obtain electrons more easily and facilitate their re-participating into the redox reaction.As a result,the constructed Li-S batteries with CoP@C-PP separator show excellent electrochemical performance with a high initial capacity of 1213 mAh g-1 at 0.2 C,excellent cyclability with a low capacity decay of 0.058%per cycling upon long-term 800 cycles,and superior rate performance with a high reversible capacity of 654 mAh g-1 even at 5.0 C.