Research On Preparation And Lithium-ion Conductivity Of Polyethylene Glycol Functionalized Covalent Organic Frameworks

With the continuous improvement of the society’s requirements for energy density and safety of lithium-ion batteries,traditional liquid electrolytes have been difficult to meet the demand.The development of solid electrolytes with high safety and high specific capacity negative electrodes is an effective solution.Inorganic solid electrolytes have some problems such as poor interface and large brittleness,which hinder its developement.Polymer electrolytes have been widely used in research due to their low cost and high flexibility.However,polymer electrolytes still suffer from poor thermal stability,low free volume,and difficulty in understanding the relationship between conformation and performance.Covalent organic framework is a kind of porous crystalline polymer formed by organic unit structure through covalent bonds.Because of high thermogravimetric stability,ordered pore structure and high structural designability,it has huge application potential in electrochemical energy.However,the lithium ion conductivity of COF materials is still very low.Designing of COF materials with high conductivity has a wide range of application prospects in the energy field.Because of the unique segmental motion,polyethylene glycol（PEG）is a medium that can accelerate lithium ion transport.Based on this concept,we design a series of PEG functional covalent organic framework materials,and systematically studies the lithium ion conduction of COF and full battery performance at different temperatures.These contents mainly include the following two parts:1:Preparation of polyethylene glycol functionalized covalent organic framework powder and its lithium ion conductivity application researchA series of branched PEG functionalized covalent organic frameworks（COFs）were constructed by solvothermal method.These COFs have good crystallinity,high chemical and thermal stability,and good lithium ion conductivity after doping with lithium salts.The COF material with longest chain branch（COF-PEG-B6-Li）can reach a conductivity of 1.5×10^-3 S cm^-1 at a temperature of 200℃.What is more,the COF-PEG-B6-Li solid electrolyte is also assembled with the LiFePO₄cathode to form a button battery,after 60 cycles of stable cycling,it can reach an efficiency of more 90%.In addition,the crowded pores can limit the movement of anions.The lithium ion migration number of COF-PEG-B6-Li is 0.3,while the lithium ion migration number of COF-PEG-B3-Li is only 0.18.2:Synthesis of polyethylene glycol functionalized covalent organic framework film and study on its lithium ion conductivityA series of PEG functionalized covalent organic framework membranes were prepared by interfacial polymerization.The interfacial polymerization can complete the morphology transformation of COF from powder to membrane.At the same time,these materials have good crystallinity and thermal stability.The electrical conductivity of the longest linear COF membrane（COF-PEG-2）was tested after electrolyte exchange（LITFSI）.Because of the better processability of the membrane,its lithium ion conductivity can reach 1.8×10^-5 cm^-1at 160°,more than 3 times of the same series of COF powders.