Preparation Of Melamine Polymers And Their Electrochemical Performance As Lithium Sulfur Battery Interlayer

Lithium sulfur batteries（LSBs）as a new type of energy storage device,because of high theoretical energy density(～2600 Wh kg^-1)and high theoretical specific capacity(1672 mAh g^-1),as well as the environmentally friendly,natural resources Rich and other advantages have attracted widespread attention.LSBs faces the main obstacle of commercialization:Poor conductivity of sulfur,and lithium polysulfide（Li PSs）is easily soluble in the organic electrolyte to produce shuttle effects.With the development of porous carbon materials in recent years,sulfur composite porous carbon materials can effectively solve the problem of poor conductivity of sulfur cathode.However,how to solve the shuttle effect of Li PSs is still a difficult problem for the commercialization of LSBs.Research has found that designing and introducing LSBs multifunctional interlayers could become an effective way to prevent Li PSs shuttle effect.Polymer interlayers are widely used in LSBs because of their multi-active functional groups,rich microporous structure,simple preparation,and low cost.Cost-effective melamine（MA）with three amino groups can form macromolecules with microporous structures with various monomers.This thesis mainly uses melamine as the main monomer,and by changing the type of monomer reacted with melamine,the microstructure and functional group composition of melamine-type polymers can be controlled.Different polymers with developed microporous structures and multi-active functional groups can be obtained.Using this polymer as an interlayer material in LSBs can effectively reduce the shuttle effect of Li PSs.The research content of this thesis is as follows:1.Polyimide is rich in active carbonyl groups,which can adsorb and catalyze Li PSs.Porous rod polyimide（PI-1）and porous block polyimide（PI-2）were designed and coated on commercial separators to prepare two kinds of lithium-sulfur battery interlayers.PI-1is rich in microporous structure,and assembling it as interlayer in LSBs can effectively improve the cycle stability of batteries.The evolution process and mechanism from polyamide acid to porous polyimide were explored,and the adsorption capacity of polyimide to Li PSs was quantified by density functional theory（DFT）calculation.PI-1has narrower band gap and lower LUMO orbital than PI-2,and the binding energy between PI-1 and Li PSs is higher than PI-2,so PI-1 has better adsorption and catalytic performance for Li PSs.The lithium-ion diffusion coefficients of LSBs sandwich made of two kinds of polymers were investigated.The results show that the lithium-ion diffusion coefficients of PI-1 sandwich are higher than those of PI-2 sandwich and both are higher than commercial separators.The LSBs prepared with PI-1 as the interlayer can still achieve a considerable specific capacity(495.2 mAh g^-1)after 2000 cycles of charge and discharge at 1 C current density.2.On the basis of the first chapter,the reaction solvent was replaced by a mixed solvent of glacial acetic acid and dimethyl sulfoxide,and a two-dimensional polyimide（PI）nanosheet was prepared.With the assistance of Super-P,PI The commercial polypropylene（PP）separator is modified by nanosheets to obtain a unique sandwich structure separator（PI/PP/PI-Super P）,which can uniformly transport lithium ions,thereby reducing lithium dendrites,and more importantly,the side of the modified separator close to the cathode can adsorb Li PSs and catalyze its conversion,which can effectively alleviate the shuttle effect.In this chapter,DFT calculations are used to quantify the interaction between PI and Li PSs,and it is found that PI exhibits a higher adsorption binding energy for Li PSs,and PI has a lower LUMO orbital and a narrow band gap,which is beneficial to catalyze the conversion of Li PSs.Due to the large proportion of carbonyl groups in PI molecules,and its two-dimensional sheet structure can expose more carbonyl active sites,which is convenient for fully and effectively adsorbing and catalyzing the conversion of Li PSs,LSBs using this modified separator can achieve high specific capacity.And long cycle life(after 500 cycles at a current density of 4 C,the battery capacity is still 617.3 mAh g^-1);in addition,PI also has good high temperature resistance and flame retardancy,and the sandwich structure obtained by using PI modification can be Effectively protect the PP diaphragm,the PI/PP/PI-Super P diaphragm can still maintain dimensional stability at 175 ^oC,and will not burn violently after encountering an open flame,and the LSBs using the PI/PP/PI-Super P diaphragm can have Higher service temperature（at 80 ^oC,after 100 cycles at 1 C current density,the average capacity decay rate is 0.469%）.3.Use melamine to react with phenyl formaldehyde and phenyl chloride,respectively,and synthesize two polymers（MSb-1 and MSb-2）.The appearance of the MSb-1 is one-dimensional nano-navigation,and the morphology of MSb-2 is two-dimensional nanosheets.Both polymers had high nitrogen content（MSb-1:30.60%,MSb-2:28.62%）,can effectively carry out chemisorption on Li PSs.However,by further analyzing the influence of the introduction of oxygen in polymer on its Li PSs adsorption performance through DFT calculation,it was found that MSb-2 showed better Li PSs adsorption capacity due to its rich oxygen（24.37%）.LSBs equipped with MSb-2sandwich shows better energy storage performance.When the current density is 0.1C,it can reach a high specific capacity of 1396.5mAh g^-1.Even after 500 cycles of charging and discharging at the current density of 2 C,it still has a specific capacity of 459.2mAh g^-1.Much higher than the 291.9mAh g^-1 battery with the MSb-1 sandwich.In addition,the electrochemical performance of MSb-2 sandwich under high sulfur load was studied.After 60 cycles of charging and discharging under a high sulfur load of 4.5 mg cm^-2,LSBs with MSb-2 sandwich could still provide an excellent area specific capacity of 5.21 mAh cm^-2.