Design Of Triazine-based Porous Organic Framework Materialand Its Application In Lithium-sulfur Batteries

With the increasing environmental pollution and energy crisis in the world,recyclable secondary batteries have attracted much attention.Lithium sulfur（Li-S）batteries are considered as an ideal new generation of secondary batteries because of their high energy density,environmental friendliness and low cost.However,there are some problems that seriously affect the cycle performance and actual specific capacity of lithium sulfur battery.Firstly,the conductivity of sulfur and the final product lithium sulfide is low,resulting in low utilization of active substances and slow conversion kinetics;The intermediate polysulfide will dissolve in the electrolyte and diffuse,causing"shuttle effect",and self-discharge in the lithium negative electrode,resulting in irreversible loss of active substances and affecting the cycle performance and coulomb efficiency of the battery;The volume of sulfur positive electrode will change during charging and discharging,and the active material will fall off with the collector.In order to solve the above scientific problems,based on porous organic polymer materials,three new functional membrane modified layer materials for lithium sulfur battery were designed and synthesized.The effects of functional membrane on inhibiting shuttle effect,promoting polysulfide conversion and lithium ion conduction were studied.The main research contents are as follows:（1）A triazine based porous organic polymer（POP）was designed and synthesized by in-situ polymerization with reduced graphene oxide（r GO）to obtain POP@r GO compound material.It was prepared on commercial polypropylene membrane by vacuum suction filtration POP@r GO layer.The physical limitation of r GO and the increase of electronegativity of nitrogen atoms on the triazine ring strongly adsorb polysulfides can effectively inhibit the shuttle effect.At the same time,the captured polysulfides can be reactivated by the conductive skeleton r GO and promote the transformation,so as to improve the utilization rate of active substances.In addition,the porous characteristic of POP improves the wettability of electrolyte and the conduction rate of Li⁺.The Li-S battery with POP@r GO-PP functional separator can maintain the specific capacity of 700 m Ah g^-1after 500 cycles at1 C rate,showing excellent electrochemical performance.（2）The SAuaric acid based porous organic framework（POF）was synthesized.The polypropylene membrane was modified by scraper coating.The SAuaric acid amphoteric ion has two characteristics of lithium affinity and sulfur affinity,which can not only repel the diffusion of polysulfides,but also promote the migration of lithium ions.The Li-S battery with POF-PP functional separator has an initial specific capacity of 1295 m Ah g^-1at the rate of 0.2 C,and can maintain 968 m Ah g^-1after 100 cycles,with good cycle stability.（3）Covalent organic framework（Tp Pa-SO₃H）with sulfonic acid anion was prepared in situ on the surface of commercial polypropylene membrane by interfacial polymerization.The ultra-thin modified layer ensures the original high energy density of Li-S battery.The Li⁺mobility and ionic conductivity of the functional membrane are as high as 0.90 and 0.18 m S cm^-1.Anions effectively promote lithium ion migration,repel polysulfides and effectively inhibit the shuttle effect.The Li-S battery with Tp Pa-SO₃H@PP functional separator can maintain a specific capacity of 605 m Ah g^-1at a high rate of 4 C,and maintain a complete charge discharge curve at a high rate.