Preparation Of Graphene-based Carbon Material Modified Separator And Its Application In Lithium-sulfur Batteries

Facing the problems of energy shortage and environmental pollution,there is an urgent need to find an advanced energy storage device with high energy density and low cost.In recent years,lithium-sulfur batteries have received much attention from researchers because of their high theoretical specific capacity(1675 m Ah·g^-1)and high energy density(2600Wh·kg^-1),as well as the advantages of high abundance of the active substance"sulfur",low cost,and environmental friendliness.However,the insulating properties of sulfur and its end product（Li₂S₂/Li₂S）,the"shuttle effect"caused by the intermediate product lithium polysulfide（Li₂S_x,4≤x≤8）,and the significant volume expansion（～80%）during cycling,all lead to rapid capacity degradation and poor cycle life of lithium-sulfur batteries.Therefore,it is important to develop a polar composite material with high conductivity and porous structure to promote the development of lithium-sulfur batteries.The separator is an important component of the battery to prevent short circuit and provide a channel for ion transport.However,commercial PP separators have large pores and cannot effectively inhibit the diffusion of soluble lithium polysulfides（Li PSs）into the lithium cathode,leading to a severe"shuttle effect".Graphene has been widely used in the study of modified separators due to its large specific surface area,high electrical conductivity,and ease of functionalization.However,the weak van der Waals forces between non-polar graphene and polar Li PSs have limited adsorption ability to Li PSs and cannot efficiently suppress the"shuttle effect".Based on this,in this thesis,N and P heteroatoms and Ni₂P nanoparticles were simultaneously introduced into the reduced graphene oxide（r GO）carbon matrix,and the N and P co-doped r GO homogeneously coated Ni₂P nanoparticles（Ni₂P@NPG）composites were prepared by heat treatment and used for separators modification.The studies are shown as follows:（1）Carbon materials of r GO and N,P co-doped r GO（NPG）were successfully prepared by ultrasonic dispersion,freeze-drying and high temperature thermal reduction using graphite oxide（GO）as the carbon source,melamine as the nitrogen source and phytic acid as the phosphorus source,and used for the study of separators modification.EDS and XRD demonstrated the successful preparation of NPG,and the isotherms of N₂adsorption/desorption and pore size distribution maps concluded that r GO and NPG is a graded porous structure consisting of microporous and mesoporous pores.Since phytic acid plays an activating role in the heat treatment to create pores,NPG has a larger specific surface area and looser morphology than r GO with a greater degree of defects.Therefore,NPG/PP modified separators（NPG/PP）can effectively inhibit the shuttle of Li PSs and improve the sulfur utilization.The test results show that NPG/PP has better rate performance(capacity remains at617.9 m Ah·g^-1at 2 C)and cycling performance(capacity remains at 658.6 m Ah·g^-1after 400cycles at 1 C,with a decay rate of 0.05%per cycle)than r GO/PP.（2）Ni₂P@NPG modified separators（Ni₂P@NPG/PP）composites were prepared by introducing N and P heteroatoms and Ni₂P nanoparticles into the r GO carbon matrix simultaneously using a one-step high-temperature heat treatment method.The successful preparation of Ni₂P@NPG nanocomposites was demonstrated by XRD,EDS,HRTEM and XPS.Adsorption experiments,symmetric cells,Tafel curves,Li⁺diffusion coefficients and the overpotential of charge/discharge curves proved that the Ni₂P@NPG nanocomposites not only have strong chemisorption ability to Li PSs,but also accelerate the rapid conversion of Li PSs,in addition,the graded porous structure can alleviate the volume expansion problem.Therefore,compared with NPG/PP and r GO/PP,the Ni₂P@NPG/PP modified separators exhibits high initial discharge capacity(1371.9 m Ah·g^-1at 0.1 C),good rate performance(750.3 m Ah·g^-1at 2 C)and cycling stability(735.2 m Ah·g^-1at 1.0 C after 400 cycles,with a decay rate of only 0.05%per cycle and a Coulomb efficiency of more than 98%).