Study On Preparation And Electrochemical Performance Of Sulfur Cathodes Based Carbon Materials And Metal Complex For Lithium-Sulfur Batteries

With the rising expectations for the performances of energy density and power density of batteries,lithium-sulfur battery has been expected to be the next generation of storage system with high-energy density.In theory,it has a specific energy of up to2567 m Ah g^-1,which is almost an order of magnitude higher than the current commercialized Li Fe PO₄ and Li Co O₂ lithium ion batteries.As an active material of lithium sulfur battery cathode material,sublimated sulfur has the advantages of abundant natural reserves,low price,no pollution and it can be produced in large scale easily,therefore,it has been considered to be the most promising cathode material at present.However,sulfur and lithium sulfide,the final product of discharge,are both low-conductivity substances,which leads to low utilization rate and poor multiplier performance of active substances of anode materials.In addition,polysulfide lithium,the intermediate product generated in the reaction process,is easily dissolved in the electrolyte,resulting in shuttle effect that causes the loss of active substances and the volume expansion of sulfur in the lithium process.It seriously affects the marketization process of lithium-sulfur battery.In order to solve the above problems,this dissertation focuses on the preparation of positive electrode materials to improve the overall electrochemical performance of lithium-sulfur battery,to reveal the mechanism of electrochemical behavior of lithium-sulfur battery,and to explore the application of carbon materials and metal compounds in positive electrode materials.The main research contents are as follows:The deserted clothes microporous carbon（DCMC）with specific surface area and pore structure enhancement was prepared by combining high temperature carbonization and KOH activation with cotton cloth as carbon source,and the structure of DCMC was optimized by regulating the ratio of carbon base to alkali.When the ratio of carbon to alkali was 1:3,the specific surface area and pore volume reached the maximum value.Microstructure analysis showed that sulfur and microporous carbon materials in DCMC/S composites were well combined.Some of them were attached to the surface of DCMC and more of them penetrated into the pore structure of DCMC.DCMC3/S composites were prepared by melt leaching method.The electrical performance test showed that the initial discharge capacity was maintained at 1082.6m Ah g^-1 at the current of 0.2 C.After 100 cycles,the capacity of DCMC3/S was maintained at 655m Ah g^-1.At a higher current density（1 C）,the initial discharge capacity of DCMC3/S electrode was 939.6 m Ah g^-1.After 500 cycles,the capacity of422.8 m Ah g^-1 was still maintained.The coulombic efficiency was always above 98%,and the average attenuation was only 0.11%.Taking CNT as carbon source,the carboxy-modified CNT-COOH on the surface was prepared by mixed acid acidification,and the CNT-COOH/S and CNT/S composites were prepared by melting leaching method.The combination of sulfur and CNT-COOH could effectively improve the conductivity of sulfur,provide transmission channels for electrons,and improve the utilization rate of positive active substances.The first discharge specific capacity of CNT/S electrode was 863.5m Ah g^-1,while the first discharge specific capacity of CNT-COOH/S electrode was 1076.2m Ah g^-1.After acidification,some ports of carbon nanotubes were opened,and folds showing on the surface could provide more active sites for electrochemical reactions,which was conducive to improving the electrochemical activity of the sulfur anode,thus slowing down the shuttle effect,improving the coulomb efficiency,and improving the cycling stability of the battery.The electrical performance test showed that the first discharge specific capacity of CNT-COOH/S was 1076.2 m Ah g^-1 at the1C ratio.After 200 cycles,the high specific capacity of 438.8 m Ah g^-1 was still maintained,and the coulombic efficiency was always above 98%.Compared with the initial capacity,the capacity retention rate was 40.78%,and the average attenuation was only 0.28%each time.Mn O₂@Fe₃O₄ nanoparticles were prepared by solvent thermal method and liquid deposition method.The silver-ear ternary composite material Mn O₂@Fe₃O₄@C was prepared by dopamine-coated and carbonized method.The composite material Mn O₂@Fe₃O₄@C/S-3 and Mn O₂@Fe₃O₄@C/S-24 were prepared by melting leaching method after Fe₃O₄ was partially etched by strong acid.The surface and internal morphology of ternary composites The physical adsorption of the outer carbon shell of Mn O₂@Fe₃O₄@C-3 ternary composite material and the chemical bond of the inner core of Mn O₂@Fe₃O₄ cooperated to form a synergistic effect to bind the polysulfide generated by the electrochemical reaction,which could effectively inhibit the dissolution and shuttling of polysulfide,so that the electrochemical reaction achieved a certain degree of balance and showed excellent performance.The electrical performance test showed that Mn O₂@Fe₃O₄@C/S-3 maintained a capacity of 983m Ah g^-1 and a capacity retention rate of 80.6%after the initial discharge ratio of 1219m Ah g^-1,100 cycles at the current of 0.2 C.At a higher current density（1 C）,the initial discharge specific capacity of Mn O₂@Fe₃O₄@C/S-3 electrode was 924.3 m Ah g^-1.After 200 cycles,the high specific capacity of 608.9 m Ah g^-1was still maintained,and the coulombic efficiency was always above 97%,with an average attenuation of0.17%each time.Under different charge and discharge current densities,the cathode material of lithium-sulfur battery exhibited excellent electrical properties,indicating that the microstructure of the material could have an important impact on the electrochemical behavior of lithium-sulfur battery.ZIF-67 nanoparticles were prepared by solvent thermal method.Through high-temperature carbonization,ZIF-67 was converted into a Co-C complex,which was oxidized by strong acid.The three-component composites of Co-C-CNTCOOH-1and Co-C-CNTCOOH-2 were prepared by electrostatic self-assembly with carboxylic carbon nanotubes.Co-C-CNTCOOH/S-1 and Co-C-CNTCOOH/S-2 composites were prepared by melting leaching method.The results showed that after high temperature carbonization,cobalt ions were reduced to cobalt metal,and the Co-C composite still maintained the dodecahedral structure of ZIF-67,because the organic matter was carbonized and the material was folded.Subsequently,strong acid oxidation was adopted to make cobalt ions positively charged and carboxylic carbon tubes negatively charged.The two self-assembled by electrostatic gravity formed a three-dimensional conductive network structure,which was conducive to improving the utilization rate of sublimated sulfur in electrochemical reactions and alleviating the shuttle of lithium sulfide.When sulfur was compounded with Co-C,Co-C-CNTCOOH-1 and Co-C-CNTCOOH-2 respectively,it could effectively improve the conductivity of sulfur,provide transmission channels for electrons,improve the utilization rate of anode active substances,and alleviate the shuttle of polysulfide.The Co-C chemical bond cooperation in the Co-C-CNTCOOH-2 ternary composite and the excellent electron conduction of carbon nanotubes were conducive to the activation of sublimated sulfur,which could more effectively inhibit the dissolution and shuttling of polysulfide,thus improving the circularity of electrode materials.The electrical performance test showed that Co-C-CNTCOOH/S-2maintained a capacity of 967.2 m Ah g^-1 and a capacity retention rate of 77%after the initial discharge ratio of 1256 m Ah g^-1,100 cycles at the current of 0.2 C.At a higher current density（0.5 C）,the first discharge specific capacity of Co-C-CNTCOOH/S-2electrode was 759.7 m Ah g^-1.After 450 cycles,the high specific capacity of 356 m Ah g^-1 was still maintained,and the coulombic efficiency was always maintained at more than 97%,with an average attenuation of 0.12%each time.