Preparation Of Cobalt Compounds And Electrochemical Performance Of Lithium Sulfur Batteries

Lithium-sulfur batteries are expected to be the next generation of high energy density battery candidates due to their high theoretical specific capacity and high energy density.However,the development and application of lithium-sulfur batteries are seriously hampered by some inherent problems,such as low sulfur utilization and increased polarization due to the low conductivity of sulfur and final discharge products（Li₂S）,rapid decay of battery capacity due to the shuttle effect of lithium polysulfide during charging and discharging,and pulverization and shedding of electrode materials caused by the volume change of sulfur species during the chemical reaction,all of which limit the commercialization of lithium-sulfur batteries.Improving the conductivity of the cathode material,inhibiting the shuttling of lithium polysulfide,and improving the polysulfide conversion kinetics play a crucial role in enhancing the performance of lithium-sulfur batteries.In this thesis,we analyzed the effect of cobalt-based metal chalcogenides compounds（Co₃O₄,Co S,Co₃Se₄）on the performance of lithium-sulfur batteries in terms of physical and chemical properties,and investigated the intrinsic factors that affect the performance of batteries as the anion changes from O to Se.Based on this,further optimization of the separator material is carried out.ZIF-67-derived nitrogen-doped porous carbon embedded with Co₃Se₄nanoparticles（Co₃Se₄@N-C）was compounded with MXene（Ti₃C₂T_x）to modify the conventional commercial polypropylene separators（PP）.Using the synergistic effect of the two to further optimize the role of cobalt-based metal compounds in enhancing the performance of lithium-sulfur batteries.Graphene oxide（GO）was prepared by a modified Hummers’method,and Co₃O₄nanoparticles were uniformly grown on GO by using a hydrothermal method,followed by high-temperature sulfidation and selenization to obtain reduced graphene oxide（rGO）uniformly loaded with cobalt-based metal nanoparticles as sulfur host material.Combined with the experimental results and theoretical calculations,the analysis shows that the structure of the formed compounds gradually changes as the non-metallic elements change from O to Se.Along with the gradual elongation of the chemical bonds,the band gap width gradually decreases making the conductivity of the compounds gradually increase,making the catalytic effect of the compounds on lithium polysulfide gradually enhanced as well as the decrease of the polarization voltage;with the increase of the radius of the non-metallic elements,leading to the polarity of the compounds are decreased.At the same time,the upward shift of the metal p-band center and the decrease ofΔd-p due to the different non-metallic elements also play a certain influence on the performance of lithium-sulfur batteries.The combined experimental results and theoretical calculations show that Co₃Se₄shows the best performance as the sulfur host catalyst,which is related to its high catalytic activity and excellent physical properties.Nitrogen-doped porous carbon（Co₃Se₄@N-C）embedded with Co₃Se₄nanoparticles was obtained by one-step high-temperature selenization using ZIF-67 as the substrate material.The few-layer MXene（Ti₃C₂T_x）nanosheets were obtained by mild chemical etching and ultrasonic exfoliation.The composite material comprised of MXene and Co₃Se₄@N-C was obtained by electrostatic adsorption.Then,it was used to modify the PP separator which was prepared by vacuum filtration method.Combining the synergistic effect of all the component,the electrochemical performance of lithium-sulfur batteries is significantly enhanced.And the reasons behind the enhancement were systermatically investigated.Due to the excellent catalytic activity of Co₃Se₄and the adsorption by abundant functional groups on the surface of Ti₃C₂T_x,sulfur utilization together with rate performance and coulombic efficiency was silmutaneously enhanced.The lithium-sulfur battery with Co₃Se₄@N-C/Ti₃C₂T_x-PP as the battery separator material obtained a discharge specific capacity of 1479 m Ah g^-1at 0.2C and still had a reversible capacity of 1077m Ah g^-1after 50 cycles.Meanwhile,it showed a superior rate capability of 778 m Ah g^-1at 4C.Even at a sulfur loading of 2.24 mg cm^-2,a area capacity of 2.37 m Ah cm^-2was realized.