Embedding Sulfur in MOF‐Derived Microporous Carbon Polyhedrons for Lithium–Sulfur Batteries

Abstract

As a promising rechargeable battery system, lithium– sulfur (Li–S) batteries can deliver an exceptionally high theoretical specific capacity of 1672 mAhg 1 and an energy density of 2500 Whkg 1 with the low-cost and environmentfriendly sulfur as the cathode material. Although the potential use of sulfur as a cathode material has long been discovered, several severe drawbacks have hindered the realization of Li–S batteries. One limitation is the insulating nature of sulfur with a very low conductivity of 5 10 30 Scm , which results in low utilization of sulfur. Another well-known problem is associated with the easy dissolution of polysulfides, the intermediate products formed during the electrochemical reaction, in organic electrolytes. The dissolved polysulfides “shuttle” between the electrodes, leading to the low Coulombic efficiency and deposition of a highly resistive layer on the surface of electrodes. These detrimental issues result in unsatisfactory electrochemical performance with rapid fading of capacity. Several approaches have been proposed to overcome the above-mentioned challenges in Li–S batteries, such as developing novel electrolytes and electrode materials. Among these efforts, using sulfur-containing composites instead of pure sulfur as the cathode materials has been demonstrated as an effective way towards high-performance Li–S batteries. Polymers and porous carbons are the common candidates to form composites with sulfur, which immobilize the loaded sulfur, and probably also the derived polysulfides via physical and/or chemical interactions. In addition, the electrical conductivity of composite materials is also better than that obtained with pristine sulfur. In particular, porous carbon materials have attracted intensive attention due to their good compatibility with sulfur, easy accessibility, and the abundance of candidates with diverse porosity and structures. Mesoporous carbon materials have been widely studied as the host materials to confine sulfur. For example, nanocomposites consisting of sulfur and ordered mesoporous carbon or mesoporous hollow carbon spheres have shown improved sulfur utilization and cycling stability. Nonetheless, continuous capacity fading upon prolonged cycling is still commonly observed, and the use of optimized ether-based electrolytes seems to be indispensable. Recent reports on carbon materials with rich micropores have revealed distinct characteristics. 25] Sulfur embedded in microporous carbon shows a pronounced discharge plateau at a lower potential of about 1.8 V versus Li/Li, which is different from the two plateaus of a typical sulfur cathode. More importantly, these microporous carbon/sulfur nanocomposites generally show outstanding capacity retention upon cycling and good compatibility with conventional carbonate-based electrolytes. However, the origins of the unusual characteristics of microporous carbon are not fully understood yet. In recently years, syntheses of porous carbon materials from metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have attracted growing attention due to the facile preparation procedures, high carbon yield, and unique porous structures. For example, carbonization of MOF-5 with furfuryl alcohol results in nanoporous carbon, which shows excellent supercapacitive performance. The carbon materials with fiber-like morphology prepared from Al-based PCPs exhibit remarkably high porosity. In particular, MOFs and PCPs are very attractive as both the template and the precursor for the fabrication of microporous carbon. Compared with many other highly porous carbon materials, such as those prepared by post-activation processes, the porous carbon derived from MOFs and PCPs exhibits highly uniform porosity, largely originating from the ordered crystalline structures of the MOFs and PCPs. However, the interesting application of these carbon materials derived from MOFs and PCPs for Li–S batteries needs to be further explored. Herein, we report the facile synthesis of microporous carbon polyhedrons (MPCPs) using unique MOF polyhedrons as both the template and precursor, and their use as carbon host to incorporate sulfur for Li–S batteries. The asprepared MPCPs with abundant and uniform micropores serve as an ideal model system for investigating the electrochemical behaviors of sulfur embedded in microporous [a] H. B. Wu, S. Wei, Dr. L. Zhang, Prof. R. Xu, Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459 (Singapore) E-mail : rxu@ntu.edu.sg xwlou@ntu.edu.sg Homepage: http://www.ntu.edu.sg/home/xwlou [b] H. B. Wu, Prof. H. H. Hng School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301689.