Design,Preparation And Optimization Of MOFs-Based Anode Materials For High Performance LIBs

Lithium-ion batteries have completely changed our daily life and the ever-increasing demands for high-energy-density devices promote the continuous development of lithium-ion battery technologies.The current market requires further improvement on the energy density and cycle life of LIBs,while commercial electrode materials are reaching their theoretical limits.Therefore,the exploration of electrode materials with better lithium storage performance and better cycle life,other than carbon anodes,is a research strategy to solve this urgent situation.Among many materials,metal-organic frameworks（MOFs）have attracted intensive attention in the electrochemical energy storage field,because of their large specific surface area and adjustable structure.Recently,a series of researches using MOFs as electrode materials for lithium-ion batteries have been published.In particular,much higher specific capacity than carbonaceous materials have been achieved.However,the industrial application of MOFs as electrodes still faces challenges such as unclear mechanism,insufficient capacity,and poor stability,so their industrial future is still pessimistic.This thesis aims to explore stable and efficient electrode materials based on MOFs,by analysising the structure-performance relationship,and optimizing the design strategy of MOFs-based electrode materials,so as to promote the commercialization process of MOFs-based electrode materials.First,the lithium storage mechanism of MOFs materials is studied,which clarifies the active sites and evolution of MOFs materials in the lithium storage process.Second,a high-performance composite material based on MOFs is designed and synthesized,which achieves high lithium storage capacity.Then,the crystal structure of a chosen MOF is adjusted to buffer the volume change,and therefore improve their cycle stability during charging and discharging.Finally,the influence of electrolyte on the composition and mechanical property of the solid electrolyte membrane（SEI）is studied,by adjusting the electrolyte composition,as a universal strategy to improve the cycle performance of MOFs.The specific contents are as follows:（1）The self-supported Cu-TCNQ/Cu electrode is prepared by a self-etching method,with arrayed Cu-TCNQ on the current collector.The characterization and mechanism research experiments confirm that Cu-TCNQ is composed of Cu⁺and TCNQ^-,which can be reduced by conversion reaction during the discharge process and contributes to the lithium storage capacity.The conjugated structure C₆can be inserted with lithium ions,as is clarified.This work proves that the transition metal ions,organic functional groups and conjugated structures in MOFs can all be used as active sites for lithium storage.（2）A series of rGO/Cu-BHT（1:10,1:3,1:1,3:1）composite materials are prepared and used as anodes for LIBs.The characterization results show that in r GO/Cu-BHT,Cu-BHT provides abundant redox active centers and high conductivity,and r GO improves the dispersion of Cu-BHT,making better lithium ion infiltration.The synergistic effect of r GO and Cu-BHT endows the composite material with good electrical conductivity,rich redox active centers and ion penetration efficiency,and results better performance.The r GO/Cu-BHT 1:1 composite delivers a stable specific capacity of 1249.5 m A g^-1,in 200cycles at a current density of 100 m A g^-1.（3）Cu-o BDC,Cu-m BDC,and Cu-p BDC,with similar structures are prepared.These materials are all composed of Cu²⁺and dicarboxylic acid in a 1:1 coordination ratio.The carboxyl groups in the ligands are distributed in the ortho,meta,and para positions respectively,so they have the same active site density.Different ligands makes diverse d-spacings of 13.02,6.79 and 5.21?,respectively.The performance test shows that the d-spacing has an important influence on the electrochemical performance.The SEI film of Cu-o BDC can be repaired in the long cycles,so its performance gradually is stabilized at674.9/683.6 m Ah g^-1,with a current density of 100 m A g^-1,in 250 cycles.（4）The performance of Cu-p BDC electrodes are studied in four different electrolytes（2%FEC,5%FEC,1%VC,and 3 M Li PF₆）.The internal structure,morphology and composition of SEI films are studies.The results show that the addition of 1%VC to the EC/EMC/DMC electrolyte with 1 mol L^-1Li PF₆cannot afford a stable SEI membrane,so the performance is worst.FEC can promote SEI membrane and improve the cycle stability of Cu-p BDC,to a certain extent,while the concentration of FEC has little effect on cycle performance.Concentrated Li PF₆electrolyte(3 mol L^-1)results the formation of SEI with a thickness of several micrometers and higher Li F content,and therefore the Cu-p BDC electrode shows the best cycling stability in 3 mol L^-1Li PF₆electrolyte,with a specific capacity of 378/387.2 m Ah g^-1after 100 cycles,at a current density of 100 mA g^-1.