Preparation And Electrochemical Properties Of Lithium-Sulfur Battery Polymer/Sulfur Composite Cathode Materials

Elemental sulfur as the most potential cathode materials possesses an excellent theoretical specific capacity of 1675 mA h g-1, which is much higher than that of the traditional one applied in the lithium-ion battery. Generally, lithium sulfur battery is primarily made up of two parts, the active material of elemental sulfur acting as positive electrode and lithium as the negative electrode. With the advantages of high theoretical energy density (2600 W h kg-1), rich natural resources together with non-pollution, lithium-sulfur battery has gradually become a new generation of high energy density storage device. However, several limited factors has greatly confine the role of sulfur as cathode material among lithium-sulfur battery such as the insulating nature of sulfur (5 × 10-30 S cm-1), the high solubility of lithium polysulfides (Li2Sx,4? x? 8) during discharge processes and a large volumetric expansion during the formation of Li2S, which lead to low utilization of active material, poor cycling life and low coulombic efficiency. Such restrictions have awfully prevent the pace of progress of lithium-sulfur battery from moving forward.Focusing on the above problems, the paper mainly studies the modified properties of the lithium sulfur battery cathode material by preparing a series of polymer/sulfur composite materials. The main works and results are summarized as follows:(1) Polymer material of TSA-PPy has good conductivity (42.7 S cm-1) and porous structure. The TSA-PPy/S composite material is synthesized by combing TSA-PPy with elemental sulfur through traditional ball-milling in laboratory. We have investigated and analyzed the cycle performances of electrodes fabricated by TSA-PPy/S composite material with 76% sulfur and sulfur electrode at 0.5 C within 50 cycles, separately. The results show that the initial discharge specific capacity of the TSA-PPy/S-76% composite cathode is 1267 mA h g-1 at 0.5 C and reversible capacity maintains 557 mA h g-1 after 50 circles. In addition, the Coulombic efficiency can get up to 93%. Compared with sulfur electrode, lithium sulfur battery with TSA-PPy/S composite material shows better cycle performance due to improved conductivity and structure.(2) We have done a series of studies on both the synthesis and electrochemical properities of hypercrosslinked porous polymer/sulfur composite materials with high rate and long cycle life. On the basis of syntheses of porous polymer HCP by Friedd-Crafts, We combine it with sulfur via thermal treatment and obtain the HCP/S composite material with high rate and long cycle life. The results of the experiment indicate that the HCP/S composite material electrode with 80% sulfur content exhibits a initial discharge specific capacity of 1003 mA h g-1 at 1 C, discharge specific capacity of 605 mA h g-1 after 80 cycles. What's more, at a current rate of 0.5 C, it has a discharge specific capacity of 658 mA h g-1 after 120 cycles, maintaining 58% capacity retention and a high Coulombic efficiency of over 94% after 120 cycles. However, sulfur electrode only has 27% capacity retention, lower than that of the HCP/S-80% composite material electrode (80 circles), so it shows that the cycling stability of HCP/S composite material electrode has been improved by a wide margin. Additionally, after the comparison of sulfur electrode with HCP/S-80% electrode in the electrochemical impedance spectroscopy (EIS), it further confirms that HCP/S composite material electrode has more superior electrochemical performance.(3) The researches of the synthesis and electrochemical performance of new composite material PhMe@HCP/S is carried out. In details, PhMe@HCP/S-45% composite material is produced on the basis of the synthetic method of HCP/S-80% composite material. The initial discharge specific capacity of PhMe@HCP/S-45% composite material electrode is 885 mA h g-1 at 1 C. It can sustain 548 mA h g-1 discharge specific capacity after 100 cycles and maintain 61.9% capacity retention. Importantly, the Coulombic efficiency gets up to 97%, showing better recycle stability in the application of lithium sulfur battery.