Highly Dispersed Catalytic Co3S4 among a Hierarchical Carbon

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Highly Dispersed Catalytic Co3S4 among a Hierarchical Carbon Nanostructure for High-Rate and Long-Life Lithium-Sulfur Batteries Hui Zhang, Mingchu Zou, Wenqi Zhao, Yunsong Wang, Yijun Chen, Yizeng Wu, Linxiu Dai, and Anyuan Cao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07843 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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Highly Dispersed Catalytic Co3S4 among a Hierarchical Carbon Nanostructure for High-Rate and Long-Life Lithium-Sulfur Batteries Hui Zhang,1 Mingchu Zou,1 Wenqi Zhao,1,2 Yunsong Wang,1 Yijun Chen,1 Yizeng Wu,1 Linxiu Dai1, Anyuan Cao1* 1Department 2Center

of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China

* Corresponding author: [email protected] Abstract Lithium-sulfur (Li-S) batteries are next-generation energy storage systems with high energy density, and the rate performance is a very important consideration for practical applications. Recent approaches such as introducing catalytic materials to facilitate polysulfide conversion have been explored, yet the results remain unsatisfactory. Here, we present an optimized Li-S electrode featured by a large amount of highly dispersed Co3S4 nanoparticles (~10 nm in size) throughout a hierarchical carbon nanostructure hybridized from ZIF-67 and carbon nanotube (CNT) sponge. This enables homogeneous distribution and close contact between infiltrated sulfur and Co3S4 nanoparticles within the ZIF-67-derived N-doped carbon nanocubes, leading to effective chemical interaction with polysulfides, maximum catalytic effect and enhanced lithium ion diffusion, while the built-in threedimensional CNT network ensures high electrical conductivity of the electrode. As a consequence, the Li-S battery exhibits both extraordinary rate performance by maintaining a capacity of ~850 mAh g-1 at very high charge/discharge rate (5 C), and long-term cycling stability with 85% retention after 1000 cycles at 5 C (an average capacity fading rate of only 0.0137% per cycle).

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Keywords: catalytic Co3S4, hierarchical carbon electrode, high-rate performance, Li-S battery, ZIF67 Lithium-sulfur (Li-S) battery represents a promising next-generation energy storage system with high theoretical energy density (2600 Wh kg-1); rational design and fabrication of sulfur electrodes are essential to fully explore its potential and achieve outstanding performance.1-3 To this end, a variety of approaches, for example introducing porous carbon materials as conducting scaffold and sulfur host, have been studied extensively, targeting major obstacles in Li-S batteries such as the low conductivity of sulfur and the shuttle effect caused by polysulfide dissolution into electrolyte.4-7 So far, considerable progresses have been made and many reports have shown promising properties in their carbon-sulfur hybrid electrodes such as enhanced specific capacity and cycling stability.8-10 However, such good performance is usually obtained at low current densities (e.g. 1C or lower), and there is significant degradation in the capacity as well as cycling behavior when the current density increases. This problem, how to improve the high-rate performance in Li-S batteries, remains a grand challenge which will severely limit future applications in circumstances that require fast charge/discharge abilities. One of the major reason for the inferior rate performance of Li-S batteries is the intrinsic complex and slow conversion kinetics between polysulfides and Li2S2/Li2S.11 Porous carbon matrix could spatially confine sulfur and improve the structural stability, but it is not beneficial to the sulfur conversion, rather, the carbon shell might further limit the reaction process.12 Recently, there is a growing interest in polar and catalytic materials that could potentially address this issue by promoting chemical interaction with polysulfides and catalyzing their conversion.13-19 Among a series of emerging materials including MoS2-x sheets,20 metal nanoparticles and TiN/TiO2 composite, 21-23 cobalt sulfides in different phases (e.g. CoS, CoS2, Co3S4, Co9S8) have gained particular interest owing to 2

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their better electrocatalytic activity, electrical conductivity and suitable binding energy according to both theoretical and experimental studies.24-33 J. Pu, et al. fabricated hollow Co3S4 nanotubes by hydrothermal method which could serve for multifunctional purposes such as to host sulfur, adsorb polysulfide and catalyze their conversion.26 Their nanotube electrode showed a reasonable specific capacity of 517 mAh/g at 5 C and about 59% retention after 1000 cycles. In another work, T. Chen, et al. reported Co3S4 nanoboxes threaded by carbon nanotubes with a specific capacity of 702 mAh/g at 5 C and about 65% capacity retention after 500 cycles at 2 C.27 In both reports, Co3S4 is in the form of dense-shell nanotubes or nanoboxes possessing limited surface area and contact to polysulfides, which will decrease its catalytic efficiency, resulting in dramatic decline of specific capacity especially under a high current density. Besides, large sulfur pieces stored within the relatively big inner cavities of those nanotubes/boxes will increase the lithium-ion diffusion distance, further reducing sulfur utilization upon increasing the charge and discharge rates. Even catalytic materials have demonstrated some promising results as shown in the above studies, the sulfur electrode must be carefully configured in order to integrate the advantages of all components including catalyst, sulfur and the matrix they rest. Otherwise, the performance at high-rate would still be unsatisfactory. In particular, investigations on how to acquire maximum catalytic activity and its related mechanism remain in the infancy stage. Toward this goal, here, we directly synthesized uniformly dispersed Co3S4 nanoparticles with close contact to nanoscale sulfur species throughout a hierarchical porous carbon nanocubes built from metal-organic frameworks (MOFs) grafted on a threedimensional (3D) carbon nanotube (CNT) network. The catalytic effect of Co3S4 has been disclosed by detail mechanism analysis. Compared with previously reported Li-S electrodes, our optimized and highly catalytic electrode exhibits much superior high-rate and cycling performance including a 3

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capacity of ~850 mAh g-1 at 5 C and a fading rate of 0.0137% per cycle at 5 C for 1000 cycles.

Results & Discussion Our process of fabricating hierarchical catalytic sulfur electrodes involves mainly three steps: 1) grafting MOF particles onto CNTs, 2) carbonization and sulfurization, and 3) sulfur loading, as illustrated in Figure 1 (see Experimental for details). A bulk sponge consisting of 3D CNT networks (reported by our group previously)34 was used as the substrate to graft ZIF-67, a typical MOF possessing cobalt centers, microporous structure and ultrahigh surface area. Then the CNTs/ZIF-67 hybrid was subjected to thermal carbonization (to precipitate Co species within ZIF-67) and sulfurization (to convert Co to Co3S4), leading to in situ production of Co3S4 nanoparticles embedded within a N-doped carbon (NC) nanocube derived from ZIF-67; the resulting hierarchical structure is termed as CNTs/Co3S4@NC. Finally, sulfur was infiltrated into the micro- and meso-pores of carbonized ZIF-67, and maximum contact area is well established between these nanoscale sulfur species and those already embedded Co3S4 nanoparticles throughout the porous matrix. The resulting sulfur electrode, termed as S@CNTs/Co3S4@NC, represents a typical hierarchical hybrid structure with synergistic contribution from each component (In the symbol, “/” represents that CNTs network goes throughout the entire structure, “@” means Co3S4 particles are dispersed and encapsulated within NC formed after carbonization of ZIF-67). Specifically, our S@CNTs/Co3S4@NC structure integrates three important characteristics as a Li-S electrode: 1) the self-standing 3D CNT network acts as highly conductive paths for charge transport and also provides interconnected open pores for electrolyte infiltration throughout the bulk electrode, which is favourable for efficient sulfur loading and utilization, 2) the micro- and meso-pores inside carbonized ZIF-67 securely encapsulate sulfur as 4

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nanoscale pieces, while the Co3S4 and N-doped carbon layer enable chemical interaction with polysulfides and inhibit their dissolution, and 3) furthermore the highly dispersed Co3S4 nanoparticles (due to the presence of NC) with intimate contact to stored sulfur effectively catalyze the conversion reaction, a distinct feature compared with other reported electrodes. These uniformly distributed Co3S4 nanoparticles through the nanocube are not only highly catalytic, but also have a metallic behavior with relatively high electrical conductivity (3.3×105 S m-1),35 which is favorable for charge transport in our sulfur electrodes. In total, our optimized configuration simultaneously addresses key issues including electrical conductivity, polysulfide dissolution and conversion, with the potential to substantially improve Li-S battery performance especially at high rates. We have characterized the structural evolution during the fabrication process by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). One can see that many small cubes (~350 nm) are distributed uniformly throughout the whole CNT sponge, and this hybrid structure is well maintained over the entire process including CNTs/Co3S4@NC (after carbonization and sulfurization) and S@CNTs/Co3S4@NC (after sulfur loading) (Figure 2a and Supporting Information Figure S1). This is because the in situ growth of ZIF-67 nanocubes with strong adhesion to the CNT surfaces among the porous sponge, and each cube is traversed by multiple CNTs from different orientations, thus fixed tightly by the 3D CNT network (Figure 2b). This creates a free-standing 3D electrode with stable structure. In each nanocube, many nanoparticles have been produced and are distributed closely and evenly within the 3D space (Figure 2c). We also observe slightly concaved surfaces of the nanocubes indicating that the ZIF-67 framework has been well retained after annealing and sulfurization (Figure 2d). The regular chemical bonding between Co centers and organic ligands in ZIF-67 crystals ensures the formation of abundant tiny Co seeds during in situ carbonization, and 5

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by controlling the process parameters we obtained highly dispersed Co3S4 nanoparticles with diameters up to 10 nm. As the sulfurization process using a low concentration thiourea solution as sulfur source at a reaction temperature of 200 C is relatively mild, both CNTs and the ZIF-67-derized carbon nanocubes were not sulfur-doped under this condition (Figure S2). In contrast, direct solvothermal sulfurization of CNTs/ZIF-67 followed by thermal annealing results in hollow nano-boxes where Co3S4 nanoparticles are distributed only on the box surface as a dense shell, as described in literature.27 The crystal structure and lattice constants of Co3S4 nanoparticles have been confirmed by highresolution TEM and selected area electron diffraction (SAED) (Figure 2e,S3), while energy-dispersive X-ray (EDX) spectroscopy reveals a cobalt-to-sulfur atom ratio of ~0.73, approaching the stoichiometric composition of Co3S4, and a high Co3S4 weight ratio of ~83.3 wt% (by adding the weight ratios of Co and S) in the hybrid CNTs/Co3S4@NC (Figure S4). In the final structure of S@CNTs/Co3S4@NC, one can see that each nanocube has been infiltrated by sulfur, leading to a homogeneous grey background throughout the cube (Figure 2f). Sulfur is present only inside the porous nanocubes rather than the space between CNTs, making a neat hybrid sulfur electrode. There are ~5000 Co3S4 nanoparticles embedded inside each 350 nm-side-length nanocube, estimated based on an average inter-particle distance of 10 nm. Uniform mixing of such a large number of dispersed Co3S4 nanoparticles with sulfur is the key advantage in our S@CNTs/Co3S4@NC which greatly enhances the catalytic effect. Correspondingly, EDX mapping reveals an even distribution of both cobalt and sulfur elements throughout the N-doped nanocubes, and weak sulfur signal out of nanocubes further demonstrate that sulfur tends to infiltrate into the nanocubes (Figure 2g). In our method, a sulfur solution was infiltrated into the hybrid CNTs/Co3S4@NC for loading sulfur. The ZIF-67-derived nanocubes with a hierarchical pore structure and high specific surface area play a significant role in 6

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loading sulfur safely, compared to the CNTs with a relatively smooth and functionalized surface. In addition, we carried out an annealing process (at 300 C) to remove/sublime those sulfur adhered on the outer surface of nanocubes and CNTs, whereas the sulfur encapsulated within the carbon nanocubes are well protected. X-ray diffraction (XRD) has been carried out to monitor the structural transformation during the fabrication process, which reveals distinct patterns in the original highly crystalline CNTs/ZIF-67, Co peaks in the intermediate CNTs/Co@NC, strong peaks of Co3S4 (JCPDS No. 42−1448) in the CNTs/Co3S4@NC, and appearance of both sulfur and Co3S4 peaks in the final S@CNTs/Co3S4@NC (Figure 3a). It indicates successful loading of a large amount of sulfur into the Co3S4-embedded nanocubes. In Figure 3b, we use TGA to characterize our hybrid structures and determine the loading of ZIF-67 and sulfur, since ZIF-67 decomposes (but only partially) at temperatures of 500-600 C, sulfur sublimes at 200-300 C, and CNTs do not decompose until 800 C, when heated in N2 atmosphere. First, from the TGA curve of pure ZIF-67, the weight loss ratio is 47 wt% (some light elements such as hydrogen were burned while metal species and most of carbon remained). For the hybrid of CNTs/ZIF-67, the weight loss is 26 wt% which is decreased compared with pure ZIF-67, because of the CNTs presence. Therefore, the weight ratio of ZIF-67 in CNTs/ZIF-67 can be directly calculated by 26 wt%/47wt%, which is 55 wt%. Finally, for the S@CNTs/Co3S4@NC, the weight loss (74 wt%) between 200-300 C is attributed to sulfur sublimation, corresponding to the sulfur loading in the S@CNTs/Co3S4@NC electrode. Thus, a high sulfur loading of ~74 wt% (corresponding to an area loading of 7.4 mg cm-2) has been obtained in the resulting freestanding S@CNTs/Co3S4@NC cathode (with an absolute sulfur content of ~7.4 mg in the whole cathode of 10 mg weight), which is beneficial for high energy density. We have studied the porous structure by nitrogen adsorption7

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desorption isotherms at 77 K, which show that the specific surface area has decreased from 803 m2 g1

in CNTs/Co3S4@NC to 112 m2 g-1 in S@CNTs/Co3S4@NC (Figure 3c). Meanwhile, the isotherms

have changed from the combination of type I (micropore) and IV (mesopore) to only the presence of type IV after infiltration, indicating that sulfur mainly occupies micropores. Accordingly, the pore size distribution shows a major volume decrease of