Research On Modified Sulfur Cathode, Separator And Electrochemical Performance For Lithium-sulfur Battery

With the rapid growth of economy, human being’s demand for energy keeps increasing. Lithium-sulfur, as a novel form of green high-energy secondary battery, has attracted many researchers’ attention due to its high theoretical specific capacity（1675 mAh g-1） and energy density（2600 Wh kg-1）. However, the practical application of lithium-sulfur still faces some challenges, including the low conductivity of sulfur and lithium sulfide, the dissolution and diffusion of polysulfides in organic electrolyte, which cause the low active materials utilization and poor cycling performance, restricting the practical application of lithium-sulfur battery.In order to solve the above problems, such as the agglomeration of sulfur, the low active materials utilization, the diffusion of polysulfides, etc, this master thesis carried out some research on modified sulfur cathode and separator. The effects of one-dimensional metal oxide nanofibers on sulfur cathode’s structure and performance was studied. The scraper coating method was used to prepare metal oxide/conductive carbon composite modified separator. The metal Ni fiber, as an additive in sulfur cathode, was synthesized through improved coprecipitation-thermal decomposition method and its unique electrochemical properties and the mechanism of action was studied. The main research results are displayed as follows:Mg_0.6Ni_0.4O hollow nanofibers with a diameter from 300 to 500 nm and a length of tens of micron were prepared via electrospinning method. The shrinkage of precursor fibers appeared after sintered at different temperatures, and with the increase of temperature, the grain size grew constantly and the corresponding specific surface area declined gradually. When sintering temperature exceeded 700 ℃, small particles became obvious on the surface of nanofibers. Mg_0.6Ni_0.4O hollow nanofibers sintered at different temperatures as the substrate material for sulfur cathode were studied. Mg_0.6Ni_0.4O nanofibers prevented the agglomeration of sulfur and reduced the impedance of electrode; Metal oxygen bond was advantageous to the electrochemical reaction in lithium-sulfur battery, improving the reduction potential of electrode and reducing the polarization of electrode; Mg_0.6Ni_0.4O（700 ℃）/S composite electrode showed the best cycling performance.Mg_0.6Ni_0.4O nanoparticles with the particle size from 100 to 200 nm were synthesized by combustion method. Mg_0.6Ni_0.4O/ketjen black composite modified separator was firstly prepared through scraper coating method and the composite conductive layer’s thickness was about 7μm. The capacity of this cell can reach 530 mAh g-1 after 350 cycles at 0.5 C. Electrochemical analysis results illustrated the composite conductive layer on modified separator can effectively restrain the diffusion of polysulfides, and metal oxygen bond can promote the further reduction of Li2S4 to insoluble Li2 S, which can provide more capacity and reduce polarization. The close contact between the conductive and sulfur cathode can provide additional electronic transmission path, which not only reduced the resistance of electrode, but also reused the absorbed active materials.Ni fibers were prepared via improved coprecipitation-thermal decomposition method and the effects of different Ni doping amount on electrochemical performance of lithium-sulfur battery were studied. The results indicated the porous structure of Ni fibers can prevent the diffusion of polysulfides, and 3%-Ni/S composite cathode showed the best cycling performance. Its initial discharge specific capacity is 805 mAh g-1 at 0.5 C（0.766 mA cm-2） and the capacity can reach 440 m Ah g-1 after 50 cycles. CV and EIS analysis results illustrated that Ni can not only raise the electrochemical reaction in cathode region and reduce the polarization of electrode, but also provide better electronic transport channel for active materials and reduce the resistance of electrode, resulting in the improved performance of lithium-sulfur battery.