Preparation And Electrochemical Properties Of Cathode Materials For Lithium-Sulfur Batteries

With the rapid development of electric vehicles,portable energy storage devices,clean energy and other fields,lithium sulfur batteries have been deeply explored by researchers because of their high theoretical energy density and power density,as well as low cost.However,the inherent problems of lithium-sulfur batteries,such as the insulation of sulfur and discharge products?Li₂S₂/Li₂S?,the shuttle effect caused by the dissolution of intermediate discharge products,and the volumetric expansion of sulfur during battery reactions,severely reduce the active material utilization,and restricted the further commercial development and application of the battery.Reasonable design of sulfur cathode materials will help alleviate the above problems,to achieve high capacity,long cycle life performance.The performance of the cathode material depends on the choice of the anchoring material and its architectural design.In this thesis,a variety of nanostructured composites have been prepared by hydrothermal method,atomic layer deposition technique and freeze-drying method,and have been applied to the positive electrode of lithium-sulfur batteries,showing excellent electrochemical performance.The main research results of this thesis are as follows:?1?Watermelon juice was used as carbon source to synthesize BCSs with a simple one-step hydrothermal method.Then,BCSs@T-PPy composites were prepared through the in-situ polymerization of T-PPy.Finally,the BCS@T-PPy/S composites were prepared by compounding BCS@T-PPy with sulfur under the protection of inert gas,and applied to the electrochemical testing of the positive electrode of lithium-sulfur battery.Electrodes containing BCSs have more electrochemically active sites than pure T-PPy/S electrodes.At 0.5 C,the BCS@T-PPy/S electrode showed a high capacity of 1143.6mAh/g and remained at 685.8 mAh/g after 500 cycles of the battery.?2?Using potassium permanganate and glucose as raw materials,a cubic manganese carbonate precursor?MnCO₃?was prepared by simple hydrothermal reaction.The MnCO₃ to muffle furnace which,after 250 degrees for one hour,600 degrees two hours of calcination process,The formation of porous cubic manganese dioxide?Mn₂O₃?was produced by calcining the MnCO₃ precursor at 250°C for 1 h and 600°C for 2 h.By complexing with sulfur,a Mn₂O₃/S complex is formed.The preparation of the Mn₂O₃/S pole piece was completed by placing the pole piece in a R-200 Advanced ALD System and uniformly depositing a 2 nm thickness Al₂O₃ layer.The Al₂O₃ layer can increase the hydrophilicity of the electrode surface,reduce the interfacial transfer resistance,and adsorb the polysulfide,an intermediate discharge product,to reduce the shuttle effect.The MOS@Al₂O₃ electrode has a capacity up to 1012.1 mAh/g at 0.5 C.?3?Sodium chloride?NaCl?was used as a template,dopamine hydrochloride(C₈H₁₁NO₂·HCl)was used as carbon source and nitrogen source,copper nitrate was used as a copper source.After the procedure of freeze-drying and hydrogen reduction at high temperature,the Copper,Nitrogen co-doped three-dimensional porous graphitized carbon network?3D Cu@NC-F?was prepared.The 3D Cu_xS@NC/S-F composites were prepared by compounding with sulfur under inert gas at 155°C.Among them,the in situ formed copper sulfides?Cu_xS?can catalyze the conversion of polysulfides during battery reaction and provide extra capacity.The Cu_xS@NC/S-F composite electrode exhibits an ultra-high initial capacity of 1432 mAh/g at 0.1 C,after 120 cycles of the battery,the capacity is up to 1169 mAh/g,with the Coulombic efficiency nearly 100%.After 500cycles at 2 C,the capacity can still reach 619 mAh/g.