The Preparation And Application Of ZIF-9 Metal Organic Framework Derived-Porous Carbon Materials Of Lithium Sulfur Batteries

Lithium-sulfur battery theoretical energy density of up to 2500Wh/Kg,specific capacity of up to 1675Ah/Kg,and the positive electrode active material of lithium-sulfur battery sulfur is non-toxic,cheap,environmentally friendly advantages.This makes the lithium-sulfur batteries has become one of the most promising future secondary battery.Although lithium-sulfur batteries have many advantages,they have not been commercialized yet.This is because there are still some problems in lithium-sulfur batteries,Such as:active material sulfur and discharge products?Li₂S and Li₂S₂?insulation;discharge intermediate products easily soluble in electrolyte;volume expansion.These problems lead to low sulfur utilization and poor cycling performance of the active material of lithium-sulfur batteries.Therefore,improving the utilization ratio and cycle performance of the cathode active material has become a research hotspot for lithium-sulfur batteries.This article reviews the research progress of cathode materials for lithium-sulfur batteries in recent years,and summarizes the cathode materials.We have found that the microstructure and stability of the cathode conductive materials are important factors affecting the performance of lithium-sulfur batteries,and that the performance of batteries and cathode The catalytic effect of electrochemical reactions is related.We solve the problem of poor stability of the positive electrode material in the discharge process by preparing three-dimensional integrated conductive materials,and prepare positive electrode conductive materials with different microstructures,change the material composition of the positive electrode material,and thus In order to develop a lithium-sulfur battery cathode material research.The specific research content and conclusions are as follows:?1?MOF-5 Preparation of porous carbon material as precursor carbonization temperature performance of the lithium-sulfur batteryIn this dissertation,the hydrothermal method was used to synthesize MOF-5 crystals.The porous carbon materials were prepared by one-step pyrolysis method.HPCN-n?n=800,900,950,1000?conductive materials were prepared by changing the pyrolysis temperature.Using the fusion method and sulfur composite,after stirring to paddle,coating film,slicing to prepare a lithium-sulfur battery positive pole piece.The positive pole piece and the assembled battery were prepared under the optimum process conditions that the ratio of graphene was 1,the total amount of electrolyte added was 60?L,and the sealing pressure was unipolar plate assembly pressure of 7 MPa.Electrochemical test results show that the battery performance is best when the carbonization temperature is 1000°C,the first cycle specific discharge capacity is 1144.9mAh/g,and the discharge specific capacity is maintained at 857mAh/g after 50 cycles.Compared with other carbonization temperature materials,the S/HPCN-1000 positive electrode material has a high specific discharge capacity and good cycle stability.The physical characterization of XRD,BET,and TG shows that when the carbonization temperature is 1000°C,the central metal element zinc does not exist,the specific surface area is large,and the pore volume and pore size are the smallest.It is because of the higher specific surface area and the appropriate pore volume that the HPCN-1000 conductive material exhibits better electrochemical performance.?2?MOF-5 Preparation of porous carbon material as precursor carbonization time performance of lithium-sulfur batterySynthesize MOF-5 using the same method as described above.Carbonized at 1000°C,and HPCN-1000-n?n=1,2,3,6?conductive material was prepared by changing the time of carbonization.Using the fusion method and sulfur composite,after stirring to paddle,coating film,slicing to prepare a lithium-sulfur battery positive pole piece.The results of electrochemical tests show that the performance of the cathode material prepared at the carbonization time of 3 h is the best,in which the discharge capacity of the first cycle is 1144.9mAh/g,and the discharge specific capacity is maintained at 857 mAh/g after 50cycles.Other positive electrode materials prepared carbonization time average polarization occurs.The BET characterization results showed that the specific surface area,pore volume,and pore size all reached a minimum at the carbonization time of 3 h,and the AC impedance test results showed that the SEI film appeared at the latest on the electrode surface at a carbonization time of3 h.It can be seen that the material with large pore volume and pore size is not suitable for the most conductive material of the lithium-sulfur battery cathode.?3?Preparation of ZIF-9-based metal oxide porous carbon/carbon fiber composite material and its application in lithium-sulfur battery cathodeIn this paper,high-density arrays of ZIF-9 are grown on carbon fiber carbon fibers,followed by one-step pyrolysis to prepare the CN-Co/CP composite material,and then the CN-Co/CP composite material was oxidized in the atmosphere for several hours.CN-Co₃O_4?n?/CP?n=1.5,2,4,6?composites.Then,Preparation of S/C-N-Co/CP and S/C-N-Co₃O_4?n?/CP?n=1.5,2,4,6?Cathode Materials by Sulfur Adsorption with Solvent Method,and finally a battery was assembled.The SEM characterization results show that the ZIF-9crystals grow densely on the carbon paper,and the gap between the carbon fiber and the carbon fiber is also filled.After the carbonization and oxidation,the ZIF-9 crystal skeleton morphology does not change.The BET characterization results show that the pores around 9 nm are opened when the oxidation time is short.With continued oxidation,the carbon pore walls are oxidized and etched so that the average pore size becomes smaller and the most accessible pore size increases.EDS and XPS characterization results show the same percentage of atomic content.The galvanostatic charge-discharge test results show that the assembled battery of S/CN-Co₃O_4?2?/CP positive electrode material has the best performance,the first ring discharge capacity is 1154.4mAh/g,and it remains at708.2mAh/g after 100 cycles.And the average Coulomb efficiency remains above 98%.The cyclic voltammetry?CV?test results show that the reduction peak current of the S/C-N-Co/CP cathode material is the smallest,and the reduction peak current of the S/C-N-Co₃O_4?2?/CP cathode material is the largest.The impedance of the EIS test shows that the S/C-N-Co/CP positive electrode material has the highest impedance,and the prepared S/C-N-Co₃O_4?2?/CP has the smallest impedance.This is because Co₃O₄ has a biphasic catalytic effect on the electrochemical reaction of a lithium-sulfur battery cathode,and an appropriate increase in the pore size of about 9 nm can improve the electrochemical performance of the lithium-sulfur battery.