| Energy is the eternal power of social sustainable development.Rechargeable secondary battery is one of the most efficient energy storage devices among all the energy storage systems.Lithium ion batteries(LIBs)have been widely used in various fields in the past tens of years due to their high energy density.With the development of science,technology and social economy,people have put forward higher requirements for the development of secondary lithium ion batteries.Lithium-sulfur(Li-S)batteries have high energy density(2600 Wh kg-1)and theoretical specific capacity(1675 mAh g-1),which have attracted attention from scholars over the world,but several major issues have hindered their practical applications:the conductivity of sulfur and Li2S is extremely low,the dissolution of polysulfides,the volume expansion of sulfur during cycling and lithium ion deposition.These problems usually lead to severe capacity attenuation,low sulfur utilization and poor electrochemical performance of lithium-sulfur batteries.To solve these problems,this thesis focuses on the design of host materials for sulfur and the improvement of electrochemical performance.Two kinds of hosts,P-Fe4N@NPG and NiCo2Px,have been synthesized and their electrochemical performances have been studied for lithium-sulfur batteries.The main contents of this thesis are listed as follows:(1)Fe2O3@rGO precursor was synthesized through a solvothermalmethod,using iron(?)acetylacetonate [Fe(acac)3] and graphene oxide(GO)as raw materials and isopropanol(IPA)as solvent.After nitridation and phosphorization processes,PFe4N@NPG nanosheets were fabricated and used as sulfur host.The obtained material has good conductivity and stability,P-Fe4N nanoparticles decorate on N,P-codoped graphene nanosheets uniformly and add more reactive sites.In addition,P modification improves the electrocatalytic activity of iron and catalyzes the kinetics of chemical reactions effectively.The obtained P-Fe4N@NPG/S cathode showed good long-cycling stability.At a current density of 1 C after 1000 cycles,the cathodes retained a reversible capacity of 417.4 mAh g-1,and the coulomb efficiency was maintained above 98%.Even with a high areal sulfur loading of 3.52 mg cm-2,the cathodes still delivered an advanced performance of 1104.8 mAh g-1.(2)Ni-Co based precursor was synthesized using cobalt(?)acetate tetrahydrate[Co(CH3COO)2·4H2O],Nickel(?)acetate tetrahydrate [Ni(CH3COO)2·4H2O] and polyvinyl pyrrolidone(PVP)as reactants via a solvothermal method.After two subsequent calcination processes,we synthesized NiCo2Px with porous olive-like structure.After combining with rGO,the NiCo2Px/rGO composite was used as sulfur host for lithium-sulfur battery.The unique porous olive-like structure of the obtained material can reduce the distance of Li+transportation and wrap the polysulfides in a closed space,thereby limiting the volume expansion of sulfur during the cycling effectively.The polar nickel-cobalt-phosphide compound has strong affinity for the polysulfides,which could effectively reduce the shuttle effect and improve the utilization of sulfur.The obtained NiCO2Px/rGO/S cathode exhibited excellent electrochemical performance. |
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