Preparation And Properties Of Carbon Composite Anode Material For Lithium-sulfur Battery

With the rapid development of science and technology,the industrial and portable electronic equipment field puts forward a higher demand for energy storage system.Compared with traditional lithium-ion battery?LIB?,due to higher theoretical energy density?about five times that of commercial LiCoO₂ battery?and lower cost,lithium-sulfur battery?Li-S?is highly expected by researchers in the next generation of secondary battery energy storage systems.Due to the poor cyclic stability,low sulfur utilization rate and the shuttle effect caused by the dissolution of polysulfide in electrolyte during charging and discharging,commercial application of Li-S has been seriously hindered.Study of carbon/sulfur composite anode materials is the key to solve the above problems.In this thesis,three different carbon matrix materials were used to compound with sulfur.The main research content is as follows:?1?An in-situ nitrogen-doped porous carbon material with ultrahigh specific surface area?2163.776 m²/g?and suitable total pore volume?1.048 cm³/g?was obtained by using hickory-shell as carbon source and through the chemical activation process of potassium hydroxide?KOH?.Many micro-pores provide a large number of active sites for sulfur adsorption and effectively improve the problem of volume expansion during the cycling process.At the same time,the nitrogen doping promotes polysulfide adsorption and improves the conductivity of the carbon.In addition,the effects of different sulfur loadings on the performance of Li-S batteries were further studied.The composite with 55%sulfur loading reveals best cyclic stability and sulfur utilization.At the current density of 0.1 C and 1 C rate,the initial discharge capacity is up to 1446.1mAh/g and 991.7 mAh/g,respectively.Meanwhile,after 1000 cycles at the current density of 1 C,the electrode material still has a good capacity retention rate.Considering that the nitrogen-doped porous carbon has the advantages of low cost,being environmentally friendly and excellent electrochemical performance,it has a great potential to be used in high performance lithium-sulfur batteries.?2?Bimetal–organic framework?MOF?-derived carbon material was synthesized by utilizing the template agent 1,2,3,4-butanetetracarboxylic acid?BTCA?and cobalt?II?and nickel?II?acetylacetonate.The unique sheet structure of MOF porous carbon and the synergistic effect of cobalt and nickel double doping effectively inhibit the dissolution of polysulfide from physical and chemical levels,thus improving the electrochemical performance of Li-S batteries.On this basis,it is proved that bimetal doping has better electrochemical performance than single metal doping.At a current rate of 0.2 C,the initial discharge capacity of bimetal MOF-derived carbon sulfur composite is 1395.4 mAh/g.Even at a current rate of 1C,it still delivers a discharge capacity of 673 mAh/g after 50 cycles and the coulombic efficiency is up to 99.3%.?3?Nanofiber membranes were synthesized from polyacrylonitrile?PAN?,ammonium tetrathiomolybdate?ATTM?and polymethyl methacrylate?PMMA?by electrospinning technology and then reduced to molybdenum disulfide?MoS₂?carbon nanofibers by calcining at high temperature.The good nanostructure of the carbon-based material and the catalytic effect of MoS₂ in some extent effectively solved the problem that hindered the commercial application of Li-S batteries.Due to the poor conductivity of MoS₂,the influence of different MoS₂ contents on the performance of Li-S battery were further explored.It was proved that the electrochemical performance was best when the ratio of ATTM to PAN was 60%:100%.The initial discharge capacity is 1166.4 mAh/g at 0.5 C rate.After 500 cycles,the discharge capacity is still 516.8mAh/g,showing excellent electrochemical stability.These experimental results provide some ideas for the future research on cathode materials of Li-S batteries.