Preparation Of ZIF-derived Carbon Nanomaterials And Their Application In Sulfur-based Batteries

Environmental pollution and the shortage of fossil resources have promoted the green recycling and utilization of waste and the development of new clean energy.Lithium-ion battery?LIB?,a popular chemical power source,is no longer sufficient to meet the rapid development of portable electronic devices and electric vehicles due to its low energy density.The theoretical mass capacity of the new lithium-sulfur battery is much higher than that of traditional lithium batteries,and it has become one of the most promising new-generation energy storage systems.However,its commercial application has been hindered by poor sulfur conductivity and"shuttle effect".In order to solve the above problems,the composition of the sulfur-based positive electrode active material and the modification of the positive electrode material have become a hot topic of research.In this thesis,zeolite-like imidazolate framework?ZIFs?materials are used as precursors,and through the synthesis of heteroatom doping and the construction of hollow or multi-level pore carbon materials,the effective control of the composition and microstructure of the cathode-supported sulfur?selenium sulfide?/carbon composite material is achieved.Finally,a sulfur-based battery cathode material with good electrochemical performance was prepared.The work of this thesis mainly includes the following aspects:1.Firstly,the two-dimensional ZnO nanosheets prepared by hydrothermal method were used as template and zinc source,and 2-methylimidazole was used as organic ligand.By controlling the amount of Co metal source,we prepared a series of core-shell ZnO@ZIF precursors.Then,the precursor was carbonized at a high temperature?600?700?800?900°C?in a nitrogen atmosphere to obtain a series of two-dimensional porous carbon nanosheet materials;Finally,sulfur/carbon?S/C?nanosheet composites are prepared by melt-diffusion method.The experimental results showed that the sample S/Co/NCP-2-900 has the best electrochemical performance when the cobalt-doped source is 20 mg and the carbonization temperature is 900°C.At 0.5 C rate,the specific capacity of the first cycle discharge reached1170 mAh g^-1?the utilization rate of active material was 69.9%?,and after 200 cycles,the specific capacity was still 535 mAh g^-1.It has great potential in the application of rechargeable energy storage systems.2.On the basis of the previous chapter,we prepared a two-dimensional selenium sulfide/carbon?SeS₂/C?nanosheet composite with selenium sulfide instead of elemental S.The results showed that the SeS₂/Co/NCP-2-900 has the best electrochemical performance.At a current density of 1 A g^-1 the initial discharge specific capacity reached 1003 mAh g^-1,and the specific capacity was 383 mAh g^-1 after 500 cycles;at the same time,at a lower current density of 0.2 A g^-1,The first cycle discharge capacity of SeS₂/Co/NCP-2-900 is 1019 mAh g^-1,and there is still 425 mAh g^-1 after 100 cycles,showing excellent specific capacity and cycle performance.This was because SeS₂ combines the common advantages of sulfur and selenium.It not only greatly improved the electrical conductivity,but also solved the problem of expensive selenium,which is a potential development object in the future energy storage system.3.About 800,000 tons of waste cigarette butts are produced every year in the world,which caused serious environmental pollution problems.The main component of these cigarette butts is non-degradable cellulose acetate,which is a good precursor for the preparation of carbon materials.In order to build a new connection between waste cigarette butts and energy storage equipment,we prepared three kinds of nitrogen-doped carbon nanofiber materials?NCF,Co/NCF-1 and Co/NCF-2?by using acetate fiber in waste cigarette butts as the main carbon source,and introducing different Co-based ZIF materials as auxiliary carbon sources.The carbon nanofiber materials were combined with elemental sulfur to prepare sulfur/carbon composite materials?S/CNF,S/Co/NCF-1 and S/Co/NCF-2?and used for lithium-sulfur battery cathode material.The test results showed that,the carbon material obtained when the two metals of Zn and Co are doped simultaneously?the Zn/Co molar ratio is 1:1?is the best.At 0.5 C,the specific discharge capacity of the first cycle was 1247 mAh g^-1.After 100 cycles,it can still reached 528 mAh g^-1,and the coulombic efficiency was 99.91%,which was much higher than the carbon material NCF derived only from waste cigarette butts fiber.It fully demonstrated that the introduction of ZIF material effectively regulated the composition and pore structure of the derivatized carbon material,and achieved the improvement of active material utilization rate and cycle stability.At the same time,the use of waste cigarette butts as a main raw material is an extremely economical and environmentally friendly strategy,which not only can alleviate the environmental pollution caused by excessive waste cigarette butts,but also provide good support for the development of lithium-sulfur battery cathode materials.