Design And Preparation Of Polyimide COFs As Electroactive Sulfur Hosts In Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries are considered as a promising energy storage device due to their high theoretical energy density(2600 Wh kg^-1).However,due to the poor conductivity of sulfur,volume expansion,dissolution and shuttle effect of intermediate polysulfide,the cou-lomb efficiency of Li-S battery becomes low and the cycle performance becomes poor,which seriously restricts the practical application of Li-S battery.Covalent organic scaffolds（COFs）show good potential to effectively inhibit shuttle effect and improve the stability of Li-S bat-teries due to their strong design.However,most of the COFs reported so far do not have lithium storage function and have poor conductivity.As non-redox active sulfur carriers,they cannot contribute capacity.In view of the above problems,in this paper,starting from the molecular design,TATP、T4A、NDA、PMDA were selected to prepare polyimide COFs sulfur carriers with effective shuttle inhibition and redox activity.The relationship between the structure and properties of these cathode materials is systematically investigated.The main research contents are as fol-lows:（1）Design and synthesis of COFs active sulfur carriers with both shuttle inhibition and ca-pacity contribution.T4a-pmda-cof was prepared by solvent-thermal method using PMDA and4,4’4"-triamino-trianiline（T4A）.The results show that T4A-PMDA-COF has two pairs of re-dox peaks in the range of Li-S battery charging and discharging voltage,and has good redox performance.What’s more,the discharge capacity of T4A-PMDA-COF at 0.1 C can reach 95.8m Ah g^-1.The T4A-PMDA-COF/S composite positive sulfur electrode was prepared by melting and mixing sulfur as active sulfur carrier.The test results show that T4A-PMDA-COF/S has good cycling stability,cycling 150 times at 1 C,and the capacity retention rate is 80.6%.This work provides a new idea for the design of electroactive sulfur carrier for Li-S batteries with high energy density.（2）In order to improve the conductivity of COFs sulfur carrier,TATP-NDA COF@CNT active sulfur carrier was prepared by in situ condensation of 2,6,14-triamino-triophenene（TATP）and 1,4,5,8-naphthalene tetramethylene anhydride（NDA）to CNTs.Compared with TATP-NDA COF without CNTs,the addition of CNTs enhances the electronic transmission capacity of TATP-NDA COF@CNT.When the CNT content is 10%,the battery has good elec-trochemical performance.Lithium deposition experiments,visualization experiments and off-site infrared showed that TATP-NDA COF@CNT has a large number of C=O active sites,which can not only inhibit the dissolution of lithium polysulfide,but also form C-O-Li storage lithium contribution capacity in the process of charging and discharging,and promote the con-version of sulfur in electrode reaction.The battery with TATP-NDA COF@CNT-10/S as the positive electrode has good cycle stability and rate performance.At 0.1 C rate,the capacity retention rate is 82.22%after 100 charge-discharge cycles,and the initial discharge capacity can still be 75.03%after 500 charge-discharge cycles at 2 C rate.The attenuation is only 0.05%per cycle.（3）T4A-NDA-COF@CNT was synthesized in situ on CNTs using 1,4,5,8-naphthalene tet-ramethylene anhydride（NDA）and 4,4"4"-triamino-trianiline（T4A）,and in order to expose more active sites in COFs,E-T4A-NDA-COF@CNT was prepared by ball mill stripping T4A-NDA-COF@CNT.The results show that,compared with the unstripped COF,the stripped COF has a smaller diameter and a higher specific surface area,which is beneficial to the diffusion of Li⁺and the inhibition of shuttle effect.The specific discharge capacity of E-T4A-NDA-COF@CNT-20 can reach 113.4 m Ah g^-1at 0.1 C.Further using E-T4A-NDA-COF@CNT-20as the active sulfur carrier to load sulfur,the obtained battery has the best cycle performance.Specifically,at 1 C multiplier,the first cycle capacity of E-T4A-NDA-COF@CNT-20/S posi-tive electrode can reach 740.0 m Ah g^-1.After 200 charge-discharge cycles,the E-T4A-NDA-COF@CNT/S positive electrode still has a specific capacity of 615.5 m Ah g^-1,with a capacity retention rate of 83.18%.In this work,the design of redox active COFs sulfur host can effectively inhibit the shuttle effect while providing capacity contribution,and further improve the energy density of lithium sulfur batteries.This will provide a new idea for the design of the electroactive sulfur host of new high energy density Li-S batteries,and provide a new application for redox active COFs,which has potential application value.