Preparation Of Graphene-base Carbon Material And Research On Modified Separator In Lithium Sulfur Batteries

Lithium sulfur battery attracts people’s attention because the cathode material,sulfur,is rich in source,non-toxic and environmentally friendly,meanwhile,lithium sulfur battery has high energy density and high theoretical specific capacity(about 5 times of conventional lithium-ion batteries).However,there are still some problems for lithium sulfur batteries,which greatly limits the commercialization process of lithium sulfur batteries.For example,sulfur and its final discharge products Li2S and Li2S2 are of poor conductivity.What’s more,the batteries capacity always decreases rapidly due to the volume expansion of sulfur during the lithiation process and the dissolution of discharge intermediate lithium polysulfide in the electrolyte(shuttle effect).In this paper,transition metal(compounds)/non-metal heteroatom doped carbon materials as multifunction coating for lithium sulfur batteries separators were prepared in order to adsorb polysulfide in the discharge process of lithium sulfur batteries through physical and chemical method.The as-prepared materials are employed to suppress the shuttle effect and enhance the electrochemical performance of lithium sulfur batteries.The main contents of this article are as follows:1.Graphene oxide was prepared by the modified Hummers method and used as carbon matrix material.Melamine is used as nitrogen source and cobalt acetate tetrahydrate is used as cobalt source.After a high temperature pyrolysis process,reduced graphene oxide loading Co nanorods composite material is obtained.The diameter of the Co nanorods is about 20 nm and the length is about 200～500 nm.The prepared Co-N-rGO material then is used as a separator modified material of lithium sulfur batteries and the results show that Co-N-rGO exhibits excellent electrochemical and high rate performance in the application of lithium sulfur battery separator because transition metal Co can promote the conversion of lithium polysulfide and doped nitrogen atoms can enhance the conductivity of the carbon material and form the chemical adsorption to fix lithium polysulfide to suppress the shuttle effect effectively.2.Fe3C and N co-doped carbon reduced graphene oxide composite material are prepared using graphene oxide as carbon source,melamine as nitrogen source and ammonium ferric citrate as iron source.The material has a high specific surface area of 250.68 m2 g-1 because the exist of iron source can catalyze the formation of nitrogen-doped carbon nanotubes.The diameter of bamboo-like NCNTs is 30～40 nm and its length 300 nm.The Fe3C nanoparticles are found to be encapsulated in carbon nanotubes,which can not only change the charge distribution of the carbon layer nearby with enhanced absorbability for lithium polysulfide,but also exhibit a catalytic effect on the transform of polysulfide during the charge and discharge process.3.Herein,the effect of non-metallic heteroatom doped carbon materials modified separator on the performance of lithium-sulfur batteries was studied.The preparation of N,P co-doped reduced graphene oxide is a metal free process in a liquid system under normal temperature and normal pressure conditions followed by one-step pyrolysis.The N,P double doped carbon material can offer more active sites than the single doped carbon material.Moreover,the synergistic reaction of the dual activation of carbon atoms induced a stronger chemical adsorption to polysulfide than single heteroatom doped.Electrochemical experiments showed that compared with single nitrogen doping carbon material,N,P co-doped reduced graphene oxide for separator modification is more helpful to improve the performance of lithium-sulfur batteries.