Preparation And Electrochemical Properties Of MOF Modified Separator For Lithium Metal Anodes

The wide application of electronic products provides a great application field for the development of batteries,but also put forward higher requirements for batteries.Thin,light,high energy density,good safety,low cost and environmental friendliness are the goals that everyone pursues.Lithium-ion batteries are widely used in a variety of energy storage electronic components,but the limited capacity is difficult to meet people’s demand for energy.In this context,high specific energy lithium-sulfur battery system has become a research hotspot.However,in the negative electrode,due to the high activity of lithium metal itself,lithium dendrite growth,interface instability,volume expansion,resulting in low coulombic efficiency,poor cycling performance,high safety risks;On the sulfur anode side,there are many problems,such as the shuttle of polysulfide and the non-conduction of active sulfur,which limit the development of lithium sulfur battery system.This thesis sets about from the diaphragm modification,and conducts the following research:1.Based on the preparation of hydrophobic and thermal stable metal polyazolium frame（MAF）material MAF-4/5/6 by liquid phase diffusion method,MAF was used as coating material to modify the membrane surface of commercial polypropylene（PP）to obtain MAF/PP.Test its lithium ion transference number,lithium deposition overpotential,and further explore different MAF were calculated based on first principles of lithium ion adsorption configuration,based on the molecular dynamics simulation reveals the MAF crystal for ion transport in the channel running,Fourier infrared spectrum analysis MAF-6and double trifluoro methane sulfonyl imide anion（TFSI^-）interaction.The results show that the chelating coordination between lithium and nitrogen atoms in the MAF structure may induce the uniform transport of lithium ions and lead to the deposition/stripping of lithium without dendrite.Among the three MAFs,MAF-6 has the widest pore structure and the largest mean orientation shift（MSD）of lithium.There is host-guest interaction between MAF-6 and TFSI^-,which can effectively fix TFSI^-.MAF-6 modified membrane lithium ion migration number is the largest,up to 0.78,lithium deposition overpotential is the minimum of 32.3 m V.2.Based on MAF-6/PP,the effects of single-sided coating and incomplete coating on the migration number of lithium ions were further investigated,and it was concluded that double-sided coating was the best process condition.Based on MAF-6/PP,half cell,symmetric cell and whole cell were assembled,and the modification mechanism of MAF-6on lithium metal anode was studied by electrochemical performance test,scanning electron microscope and non-in-situ XPS.The experimental results show that MAF-6/PP can not only increase the ionic conductivity,broaden the electrochemical window,improve the coulomb efficiency,reduce the polarization,and achieve good cycling performance,but also further regulate the deposition morphology of lithium metal,inhibit the growth of lithium dendrite,and make the SEI film more stable which on the surface of lithium metal.Based on MAF-6/PP of Li|Cu half cell in the current density of 0.5 m A cm^-2,capacity of 5 m Ah cm^-2,lithium sedimentary thickness of 25.33μm,dense without dendrite deposit morphology.Based on MAF-6/PP of Li|LiFePO₄（LFP）(LFP loads to 10 mg cm^-2)under 1C loop 500times,capacity remain at a rate of 71.4%.3.MAF-6/PP was used to assemble lithium-sulfur batteries,and the interaction between MAF-6/PP and sulfur cathode was studied based on Fourier transform infrared spectroscopy and non-in-situ XPS.The results showed that MAF-6/PP could inhibit the shuttle of polysulfide.The MAF-6/PP lithium-sulfur battery can cycle 500 cycles at 0.5C,with a capacity loss rate of 0.088%per cycle,while the PP-based lithium-sulfur battery has a capacity loss rate of 0.165%per cycle.The high energy density lithium-metal battery system is deeply disturbed by the growth of lithium dendrite.MAF-6/PP formed by special hydrophobic MAF-6 can achieve uniform lithium ion flux in the lithium-metal battery system,and MAF-6 can inhibit the movement of polysulfide molecules.