Study Of Sulfur-Based Composite Cathode Materials With High Conductivity

With the rapid development of clean energy industry, rechargeable and high energy density batteries received great attention. Sulfur-based materials are regarded as one of promising rechargeable batteries with higher specific capacity/energy. In this work, we focused on the study of Polyacrylonitrile (PAN)/Sulfur composite cathode material for Lithium-Sulfur batteries, improved the electrochemical performance by using block PAN and optimizing structure of the composite. Test results were compared and discussed.PAN generally exists in the form of block copolymer in chemical industry. In this study, two block PAN were used as polymer precursor instead of Co-PAN. Also the reaction conditions including time and mass ratio of raw materials were investigated. The SEM indicates that the block PAN are porous and spherical, in semi-closed state with diameters exceeding 10μm. Cyclic voltammetry (CV) indicates a serious polarization of the cell. Ball milling treatment can break the semi-closed particle into numerous small fragments, which is expected to raise sulfur content of composite and improve the electrochemical performance. The charge-discharge test shows the potential plateau of composite became relatively flattened, average discharge voltage rise and specific capacity was close to theory value.Based on ball milling treatment, the performance was further enhanced by adding metallic vanadium precursor, conductive carbon and carbon nanotubes. SEM shows that the composite surface was coated with hexagonal cylindrical vanadium sulfide crystal layer, which may enhance the conductivity of composite and be available as part of the active material. However, the crystal layer was so unstable that the capacity retention rate was only 57% after 30 cycles; Conductive carbon can go into the internal space and contact with primary particle of block PAN closely, test results show that the sulfur utilization cycle performance of composite improved significantly; MWCNTs were evenly dispersed in the composites and form an electrically conductive framework, test results show that the composite presents the capacity retention rate of 96.5 % after 100 cycles, delivered close to 71% of its initial reversible capacity at 7C. Cycle life and rate capability of composite are improved remarkably.To overcome the disadvantages of agglomeration and large size of PAN, film electrode was prepared based on the self-assemble property of block polymer. Dissolved block PAN into solvent then coated on aluminum foil, the film electrode with nano-fiber structure can be obtained by controlling external conditions. Moreover, the film electrode process specific reaction structure. Setting the sulfur in the interlayer will promise sulfur fully reaction process with upper PAN network. Test results show that the capacity of the composite improved greatly, while the nano-fiber structure is helpful for ion and electron diffusion. The performance is superior to conventional particle materials.