Synthesis And Electrochemical Performances Of Sulfurized Polyacrylonitrile Composites

With the popularization of mobile electronic devices and the rapid development of electric vehicles,the demand for high-energy density,long-life lithium secondary battery is extremely urgent.Lithium-sulfur batteries are favored by researchers due to their high theoretical energy density(2600 Wh kg^-1),abundant of sulfur sources and low cost.In the conventional lithium-sulfur batteries,the"shuttle effect"is formed due to the insulation of sulfur and polysulfides dissolved into the ether-based electrolyte,resulting in low utilization of sulfur and poor cycle life.Researchers have found that small molecule sulfur and organic sulfide were matched with commercial carbonate-based electrolyte,showing excellent electrochemical performance.Among organic sulfide systems,sulfurized polyacrylonitrile composites have attracted attention due to its abundant resources and low cost.In this paper,we optimized the preparation process and discussed the effects of polyacrylonitrile with different molecular weight on the electrochemical performance of the composites.The effects of different current collectors on the electrochemical performance were investigated according to the optimized process.The research contents are as follows:（1）Based on different molecular weight commercial polyacrylonitrile（M_W=5W,15W）as raw materials,the effects of heat treatment process on the electrochemical performance of sulfurized polyacrylonitrile composites were studied,and the preparation processes were optimized.The results show that the composite synthesized at 350°C exhibits excellent electrochemical performance.The sulfurized polyacrylonitrile composites prepared with the polyacrylonitrile（15W）as precursors showed the better cycle stability compared to the polyacrylonitrile（5W）.The initial discharge specific capacity was 1089 mAh g^-1 at 0.5 C.After 300 cycles,the specific discharge capacity was 978 mAh g^-1,and the capacity retention rate was 89.8%.（2）Polyacrylonitrile synthesized via thermal polymerization using acrylonitrile monomer as precursor.The effects of thermal polymerization temperature on the physical and chemical properties of polyacrylonitrile（such as specific surface area,molecular weight,etc.）were studied.The electrochemical performance of as-prepared sulfurized polyacrylonitrile were tested.The results showed that the polyacrylonitrile synthesised at 65°C has a large specific surface area(147.6 m² g^-1)and a low dispersity index（1.46;narrow molecular weight distribution）.The initial discharge specific capacity is 1497 mAh g^-1 at 0.5 C.After 100 cycles,the specific discharge capacity is1403 mAh g^-1,the capacity retention rate was 93.7%.Even at 3 C,the specific discharge capacity is 987 mAh g^-1,it shows superior rate performance.（3）Based on the above optimized experimental results,the electrochemical properties of high sulfur loading(3.2 mg cm^-2)electrodes with different current collectors（carbon coated aluminum foil,graphene coated aluminum foil and microporous aluminum foil）were investigated.Compared with the graphene coated aluminum foil and the microporous aluminum foil,the surface of the electrode prepared by the carbon coated aluminum foil current collector shows no obvious crack.The initial discharge specific capacity of the electrode prepared by the carbon coated aluminum foil current collector is 1338 mAh g^-1 at 0.5 C,after 100 cycles,the specific discharge capacity was 1088 mAh g^-1,and the capacity retention rate was 81.3%,which was superior to the other two current collectors.Electrochemical impedance spectrum showed that the interface impedance of the electrode prepared by carbon coated aluminum foil was smallest and it exhibited the highest lithium ion migration rate,which was beneficial to improve the electrochemical stability of the electrode with high sulfur loading.