Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8749−8757
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Free-Standing Porous Carbon Nanofiber/Carbon Nanotube Film as Sulfur Immobilizer with High Areal Capacity for Lithium−Sulfur Battery Ye-Zheng Zhang,† Ze Zhang,† Sheng Liu, Guo-Ran Li,* and Xue-Ping Gao* Institute of New Energy Material Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China S Supporting Information *
ABSTRACT: Low sulfur utilization and poor cycle life of the sulfur cathode with high sulfur loadings remain a great challenge for lithium−sulfur (Li−S) battery. Herein, the free-standing carbon film consisting of porous carbon nanofibers (PCNFs) and carbon nanotubes (CNTs) is successfully fabricated by the electrospinning technology. The PCNF/CNT film with threedimensional and interconnected structure is promising for the uniformity of the high-loading sulfur, good penetration of the electrolyte, and reliable accommodation of volumetric expansion of the sulfur cathode. In addition, the abundant N/O-doped elements in PCNF/CNT film are helpful to chemically trap soluble polysulfides in the charge−discharge processes. Consequently, the obtained monolayer S/PCNF/CNT film as the cathode shows high specific capacity, excellent cycle stability, and rate stability with the sulfur loading of 3.9 mg cm−2. Moreover, the high areal capacity of 13.5 mA h cm−2 is obtained for the cathode by stacking three S/PCNF/CNT layers with the high sulfur loading of 12 mg cm−2. The stacking-layered cathode with high sulfur loading provides excellent cycle stability, which is beneficial to fabricate high-energy-density Li−S battery in future. KEYWORDS: lithium−sulfur battery, porous carbon nanofibers, carbon nanotubes, free-standing film, high areal capacity
1. INTRODUCTION The great demands for high-energy-density rechargeable batteries have aroused extensive interests in exploring new cathode materials with high specific capacity. Elemental sulfur is largely investigated as one of the promising cathode candidates owing to its high theoretical capacity of 1675 mA h g−1, as well as the cost-effectiveness and environmental friendliness.1,2 When combined with lithium-metal anode, lithium−sulfur (Li−S) battery owns the extremely high theoretical energy density of 2600 W h kg−1.3,4 Nevertheless, the commercialization of Li−S battery is plagued by several intrinsic problems, including low electronic/ionic conductivity of both sulfur and final-product Li2S, large volumetric expansion, high solubility, and shuttle effect of intermediate polysulfides. These issues bring about low sulfur utilization, rapid capacity fading, and low Coulombic efficiency.5−8 To deal with the aforementioned problems, significant approaches are explored via confining sulfur within various nanostructured carbon substrates, including meso/microporous carbons,9,10 hollow carbon spheres,11,12 and tube-in-tube carbon.13 These carbon materials with hierarchical pore structures are beneficial for building © 2018 American Chemical Society
conductive network to facilitate the electron transfer, buffer the volume expansion, and mitigate polysulfide dissolution. Apart from the physical adsorption of sulfur species in alternative pores, the chemical confinement of polysulfide offered by functionalized carbon (such as heteroatom-doped carbon14−16 and graphene oxide17,18) is more effective for improving the cycling stability of corresponding sulfur/carbon composites. Although the noteworthy progress is achieved in terms of the cycle stability, it should be particularly pointed out that the long-cycling performance is usually obtained via low sulfur areal loading in cathode (
