Hollow N-doped Carbon Polyhedrons with Hierarchically Porous Shell for Confinement of Polysulfides in Lithium–Sulfur Batteries

摘要

class="head no_bottom_margin" id="sec1title">IntroductionHigh energy density, low cost, and environmental benignity enable the Li–S battery to become a promising next-generation energy supplier (, , ). However, the performance of Li–S battery is strongly plagued and restrained by poor electrical conductivity and volume variation of sulfur, especially the dissolution of lithium polysulfides (, , ). Resolving these problems is thus essential to meet the practical requirements of commercial applications (). The key approach to figure out these obstructions is exploring appropriate host materials to serve as the conductive networks and immobilize sulfur effectively, such as the host materials with porous structures and chemical polar sites to suppress polysulfide shuttling by physical confinement and chemical adsorption (, , , , , , , ). N-doped porous carbon hosts have attracted great attention for sulfur cathodes because these hosts not only enhance the conductivity of sulfur cathodes but also increase the interaction of the polar N sites and polysulfides through the Li–N bond, which can alleviate polysulfide shuttling through physical and chemical confinement using the porous structure and chemisorption (href="#bib8" rid="bib8" class=" bibr popnode">Guo et al., 2017, href="#bib28" rid="bib28" class=" bibr popnode">Pang and Nazar, 2016, href="#bib32" rid="bib32" class=" bibr popnode">Su et al., 2017). Therefore, great efforts have been devoted to fabricate the N-doped carbon materials by using different precursors, such as urea, pyrrole, amines, carbohydrates, and porous organic polymers (href="#bib11" rid="bib11" class=" bibr popnode">Inagaki et al., 2018, href="#bib14" rid="bib14" class=" bibr popnode">Li et al., 2018b, href="#bib26" rid="bib26" class=" bibr popnode">Niu et al., 2015, href="#bib30" rid="bib30" class=" bibr popnode">Pei et al., 2016, href="#bib40" rid="bib40" class=" bibr popnode">Yang et al., 2014). Furthermore, the composites of the above-mentioned precursors with carbon nanotubes or graphene are also extensively designed to construct N-doped carbon hosts (href="#bib27" rid="bib27" class=" bibr popnode">Pan et al., 2017, href="#bib46" rid="bib46" class=" bibr popnode">Zhang et al., 2018b). However, the uncontrollability of pore structure or N-doping for the above-mentioned precursors brings some inconclusive or adverse effects on the practical electrochemical performances.Recently, it has been found that metal-organic frameworks (MOFs) are effective precursors to construct N-doped carbon hosts because of their great superiority in compositions and structures (href="#bib3" rid="bib3" class=" bibr popnode">Chen et al., 2014, href="#bib13" rid="bib13" class=" bibr popnode">Jiang et al., 2017, href="#bib17" rid="bib17" class=" bibr popnode">Li et al., 2016c, href="#bib23" rid="bib23" class=" bibr popnode">Liu et al., 2017, href="#bib34" rid="bib34" class=" bibr popnode">Tan et al., 2017a, href="#bib37" rid="bib37" class=" bibr popnode">Wu et al., 2013, href="#bib41" rid="bib41" class=" bibr popnode">Yang et al., 2017a). Compared with conventional precursors, MOFs possess regular porous structures, large specific surface areas, and controllable structure and composition, which have helped achieve the improved electrochemical performance in Li–S batteries by using MOFs as the carbon precursors (href="#bib1" rid="bib1" class=" bibr popnode">Chang et al., 2018, href="#bib2" rid="bib2" class=" bibr popnode">Chen et al., 2018, href="#bib21" rid="bib21" class=" bibr popnode">Li and Yin, 2015, href="#bib35" rid="bib35" class=" bibr popnode">Tan et al., 2017b, href="#bib44" rid="bib44" class=" bibr popnode">Zhang et al., 2018c, href="#bib45" rid="bib45" class=" bibr popnode">Zhang et al., 2018d). Nevertheless, the majority of MOF-derived N-doped carbon materials are almost microporous structures and have limited pore volumes, which are unfavorable for the large mass transport and the release of volume strain. In addition, the solid structure with micropores generally causes the slow adsorption kinetics for long-chain polysulfides owing to the physical barriers of diffusion pathways. Although the large pore volumes or hollow structures could be obtained through template strategies, the removal of template always uses highly corrosive agents (such as HF, NaOH), and the cavity size is largely dependent on the size of the template. Therefore, a straightforward method with mild condition is highly expected to create porous N-doped carbon hosts with large cavity.Herein, we fabricate a hollow N-doped carbon (HNC) material with hierarchical pores by simple etching and carbonizing of ZIF-8 particles. ZIF-8 can be mildly etched by tannic acid to easily get a shelled polyhedron precursor (href="#bib49" rid="bib49" class=" bibr popnode">Zhang et al., 2017). This polyhedron precursor-derived HNC particle efficiently combines the micropores, mesopores, and internal void space, which is first used as the sulfur host for Li–S battery. Different from the ZIF-8-derived solid microporous host, HNC polyhedron with hollow structure is favorable for sulfur immersion and polysulfide encapsulation via the void space and porous outer shell. With HNC serving as the sulfur host, the S@HNC cathode enables a 72 wt% sulfur loading. Additionally, this hybrid cathode combines the ingenious physical confinement with appropriate chemical adsorption for polysulfide anions, synergistically providing multilayered barriers for polysulfide diffusion by means of the interconnected carbon network and strong interaction between nitrogen and lithium polysulfides. These two functions work well together to not only increase the reversible capacity of hybrid cathode but also display fast adsorption for polysulfides compared with the ZIF-8-derived solid microporous carbon material.