Sulfur/Carbon Composite Nanofibers As Cathode Materials For Lithium-Sulfur Batteries

With the rapid development of advanced portable electronics, zero-emission electric vehicles and renewable power stations, low cost and high capacity batteries are in great demand for widespread electric storage applications. In this technology development, lithium-sulfur (Li-S) batteries have attracted particular attention as a promising candidate for next-generation energy-storage devices because of their high theoretical energy density (2600 Wh Kg-1), wide availability, low cost and non-toxicity of sulfur resources, all of which offer competitive advantages for large scale applications. Despite such significant advantages, realizing a practical Li-S battery is difficult, primarily due to the intrinsic drawbacks of sulfur electrode. As is well known, sulfur and its discharge products (Li2S2 and Li2S) are electronically and ionically insulating, which leads to a poor electrochemical utilization of sulfur cathode. Besides, the dissolving and consequent shuttling of lithium polysulfides generated as intermediates during charge-discharge processes in the electrolyte can cause serious capacity fading and low coulombic efficiency. Moreovers the large mechanical stress induced by the volume expansfion from sulfur (d=2.03 g cm-3) to its discharge product, lithium sulfide (Li2S, d=1.66 g cm-3), may break the integrity of sulfur cathode, which accelerates, in turn, the capacity lading. These problems severely hindered the development of Li-S batteries. Considering one-dimensional electrode materials possessing large specific surface areas, high electronic conductivity, as well as their intrinsic advantages in constructing conductive network and accommodating the volume expansion, in this work, we designed and fabricated a series of sulfur/carbon (S/C) composite nanofibers and characterized the electrochemical performance of these nanofibers as cathode materials in Li-S batteries. The main results are as follows:1. Sulfur/carbon (S/C) nanocomposite-filled polyacrybnitrile (PAN) nanofibers (denoted as C/S/PAN) are synthesized by a single-nozzle electrospinning technique combined with a subsequent sulfurizing reaction. The preparation conditions including the mass ratios between carbon and sulfur, electrospinning conditions and cyclizing temperatures of PAN fibers, were optimized by comparing the electrochemical performance of thus-prepared C/S/PAN nanofibers in organic carbonate electrolytes. In the C/S/PAN nanofibers, the sulfurized PAN matrix acts not only as ionic and electronic channels to allow Li+ and electrons to arrive at and react with the S/C nanoparticles, but also as a protective barrier to prevent the S/C nanoparticles from contacting with electrolyte, thus avoiding the discharge intermediates of sulfur to dissolve in and react with the organic carbonate electrolyte. Since the redox reaction of sulfur in the nanofibers occurs mostly at the interior S/C interface through a solid state reaction mechanism, the micro structures and electrochemical interfaces in the nanofibers cathode remain stable during repeated cycles. As a consequence, the C/S/PAN cathode demonstrate a high reversible capacity of 1179 mA h g-1 at a current rate of 200 mA g-1, a high Coulombic efficiency of-100% after a few cycles, a good rate capability with 616 mA h g-1 at 4.0 A g-1 and a long cycling stability with 60% capacity retention over 400 cycles, showing a great prospect for Li-S battery applications. The novel structure and working mechanism of the C/S/PAN nanofibers may provide new insight into the design of long life and high capacity sulfur cathodes for the practical development of Li-S batteries.2. VGCF/S@PEDOT composite nanofibers were synthesized by first chemical depositing sulfur on the VGCF surface to obatain VGCF/S composite, and then coating the surface of VGCF/S composites with a conductive PEDOT polymer layer. The fabrication conditions were optimized by investigating the influence of sulfur loading, PVP surfactant and PEDOT coating layer on electrochemical behaviors of VGCF/S composites. The experimental results indicated that the amphiphilic PVP molecule as a surfactant can decrease the particle size of sulfur deposited on the surface of VGCF fibers and therefore improve the electrochemical utilization of sulfur in the composite nanofibers. The PEDOT polymer as a coating layer can significantly improve the conductivity of VGCF/S nanofibers and depress the dissolution loss of intermediates. As a result, the VGCF/S@PEDOT ranofiber cathode with a high sufur content of 70.8 wt.% demonstrate a high reversible capacity of 892 mAh g-1 after 50 cycles at a current density of 0.1 C. Even at a high rate of 1 C, the composite cathode still can deliver 54% capacity of that at 0.2 C, showing a great prospect for Li-S battery applications.3. Considering the high electronic conductivity of carbon fibers and the good inhibition capability of porous carbon matrix to the dissolution loss of intermediate polysulfide, an interconnected porous carbon fibers (IPCF) were proposed as sulfur matrix to fabricate S/C composite. The IPCF fibers were synthesized by electrospinning the mixed polymer solution of PANand PVP, followed by cyclization and carbonization of the thus-prepared polymer fibers. The experimental results demonstrated that the S/C composite using IPCF fibers as substrate can maintain a high reversible capacity of 891 mAh g-1 after 50 cycles at a current density of 0.1 C, corresponding to a capacity retention of 86%. Furthermore, the surface coating of PEDOT layer can further improve the electronic conductivity and the electrochemical performance of the IPCF/S composite.