Preparation And Application Of Lithiated Covalent Organic Framework For Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries have become a promising secondary battery system because of their high energy density.In this dissertation,two kinds of Li⁺selective lithiated-covalent organic framework（Li-COF）modified separators were prepared.Due to the special 1D pore structure and rich lithiated-sites,the lithiated-nanochannel of Li-COF can promote lithium-ion transfer.Based on the construction of lithiated-sites in the ordered nanochannel,two different units were introduced into the two kind of Li-COF respectively for suppressing shuttle effect,and the corresponding mechanisms were adsorption conversion and electrostatic repulsion.The main research results are listed as follows:1.Covalent organic framework with triazole unit（（IISERP）-CON1）was selected to construct lithium-ion transfer sites in the nanochannel through chemical lithiation method,and the resulting Li-（IISERP）-CON1 nanosheet was assembled with graphene nanosheet to form separator modification layer.The one dimensional perforative nanochannel of Li-CON can shorten the lithium-ion transfer path in the interlayer of graphene,and reduce the diffusion barrier of lithium-ion,thus the lithiated sites in the nanochannel realize the rapid transfer of lithium-ion.Li-（IISERP）-CON1@GN/Celgard separator delivers a high lithium-ion conductivity(6.09×10^-4 S cm^-1),mobility（0.82）and a low diffusion energy barrier（0.53 e V）.2.Li-（IISERP）-CON1@GN/Celgard separator was used in Li-S battery systems,and the battery performance and mechanisms were explored.The triazole unit in Li-（IISERP）-CON1 can effectively adsorb polysulfides,and the fast conversion reaction of polysulfides occurs with the help of the electron conductive graphene and lithium-ion conductive Li-（IISERP）-CON1.Meanwhile,the high lithium-ion conductivity of Li-（IISERP）-CON1@GN/Celgard separator is beneficial to promote the cathode electrochemical reaction.The Li-S batteries using Li-（IISERP）-CON1@GN/Celgard separator exhibit excellent electrochemical performance.The reversible specific capacity is 981.5 m A h g^-1 at 1 C,and the specific capacity was maintained at 645 m A h g^-1 after 600 cycles.Even at a high C rate（4 C）,the specific capacity can still reach700 m A h g^-1.3.Interfacial polymerization was used to prepare covalent organic framework nanosheet（Tp Pa-SO₃H）,and the Tp Pa-SO₃Li nanosheet was synthesized from Tp Pa-SO₃H dispersion by a simple chemical lithiation method.Then,the Tp Pa-SO₃Li was covered on the commercial Celgard separator to form a continuous modification layer by vacuum filtration.On the one hand,Tp Pa-SO₃Li layer exhibits excellent lithium-ion selective conductivity.At the lean electrolyte consumption of 10μL,the lithium-ion conductivity of Tp Pa-SO₃Li/Celgard separator is 0.623 m S cm^-1,and the mobility is0.88.On the other hand,the negative framework of Tp Pa-SO₃Li can effectively suppress the shuttle effect based on electrostatic repulsion effect.In order to prevent the formation of an inert layer on the surface of cathode under high current condition,an electron-conductive carbon nanotube（CNT）layer was covered on the Tp Pa-SO₃Li/Celgard separator to prepare Tp Pa-SO₃Li/CNT/Celgard three-layer structure separator.The Li-S battery using Tp Pa-SO₃Li/CNT/Celgard separator has a specific capacity of 461 m A h g^-1 after 300 cycles at 2 C,and the average capacity decline is only 0.065%per cycle.