Templated and Catalytic Fabrication of N-Doped Hierarchical Porous

Sep 15, 2017 - ... conducting pathways and effective connections between the CNTs and HPC. Furthermore, CNTs grown on HPC can act as physical barriers...
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Templated and Catalytic Fabrication of N‑Doped Hierarchical Porous Carbon−Carbon Nanotube Hybrids as Host for Lithium−Sulfur Batteries Junjie Cai,*,†,‡ Chun Wu,‡ Shaoran Yang,‡ Ying Zhu,‡ Pei Kang Shen,*,§ and Kaili Zhang*,‡,∥ †

School of Materials and Energy, Center of Emerging Material and Technology, Guangdong University of Technology, Guangzhou 510006, China ‡ Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong § Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning 530004, China ∥ Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, China S Supporting Information *

ABSTRACT: Nitrogen-doped hierarchical porous carbon and carbon nanotube hybrids (N-HPC−CNTs) are fabricated by simple pyrolysis of the N-rich raw material melamine-formaldehyde (MF) resin in the presence of nano-CaCO3 and a bimetallic combination of Fe−Co catalyst. During carbonization, nano-CaCO3 acts as a template for creating a hierarchical porous carbon, and the N atoms originated from MF resin are in situ doped into the carbon matrix simultaneously. Meanwhile, volatile gases generated by the thermal decomposition of MF resin could serve as carbon and nitrogen sources to grow nitrogen-doped CNTs on HPC. The growth mechanism is the same as that for conventional chemical vapor deposition (CVD) growth of CNTs on the metal catalysts, but the technological requirements are obviously not as harsh as those for the CVD method. Low-cost raw materials and simple equipment are sufficient for the growth. Moreover, the density and length of the CNTs are tunable, which can be simply adjusted via applying different amounts of Fe−Co catalysts. Such an N-doped hybrid structured carbon with mesopores can not only effectively prompt the physical and chemical adsorption of polysulfides but also ensures a fast electron transfer because of the incorporation of CNTs, which provides sufficient conducting pathways and effective connections between the CNTs and HPC. Furthermore, CNTs grown on HPC can act as physical barriers to block the large pores on HPC, thereby reducing the polysulfide loss. Benefiting from the advantages, the N-HPC−CNT hybrids are a desirable host prospect for Li−S batteries. KEYWORDS: template, N doping, hierarchical porous carbon, CNTs, Li−S batteries

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INTRODUCTION

with an irreversible loss of sulfur, resulting in fast capacity decay and low Coulombic efficiency.7−9 To address these issues, carbon-based materials with high electronic conductivity and optimal porosity, such as porous carbons,10−12 graphene,13,14 hollow carbon,15 and carbon nanotubes (CNTs),16−18 have been widely investigated to accommodate sulfur and minimize the polysulfide shuttle via physical adsorption.19 Apart from physical adsorption, another strategy of using heteroatom (including N, P, S, and B) doped carbon chemical adsorption of polysulfides has successfully improved the cycling stability and Coulombic efficiency of Li−S batteries.20−24 Generally, this chemical adsorption ability is stronger than that of the physical interaction of nonpolar

Due to the high theoretical specific capacity of sulfur (1675 mAh g−1) and lithium (3860 mA h g−1), lithium−sulfur (Li−S) batteries have an unparalleled theoretical energy density of 2600 Wh kg−1, which is approximately five times higher than that of the commercialized Li-ion batteries (LiCoO2−graphite system).1,2 With additional advantages of low cost and high natural abundance of sulfur, Li−S batteries are considered as a promising alternative for next-generation high energy density rechargeable batteries.3,4 However, practical applications of the current sulfur cathode still seriously suffer from some inherent problems.5 Poor electronic conductivity of sulfur (5 × 10−30 S cm−1 at 25 °C) would inevitably lead to low sulfur utilization, low capacity, and limited rate performance of Li−S batteries.6 The lithium polysulfide intermediates (Li2Sn, 4 ≤ n ≤ 8) generated during cycling can be dissolved in the electrolytes and shuttle between the cathode and anode, which is associated © XXXX American Chemical Society

Received: July 6, 2017 Accepted: September 15, 2017 Published: September 15, 2017 A

DOI: 10.1021/acsami.7b09808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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as low-cost raw materials and simple equipment are enough for the procedure. Due to the ingenious combination of CNTs and HPC, and the simultaneous introduction of nitrogen, N-HPC− CNT hybrids have a suitable hierarchical porosity, high conductivity, and ideal ion transfer pathways; they can physically and chemically adsorb polysulfides. When the asprepared N-HPC−CNT hybrids are used as host to accommodate sulfur in Li−S batteries, significant enhancements in specific capacity retention, cycling performance, and rate capability are achieved.

carbons with polar polysulfides because it traps the polysulfides through stronger chemical bonding.25 Specifically, hierarchical porous carbon (HPC) materials with hierarchical porosity, high conductivity, ideal ion transfer pathways, and physical adsorption ability have been demonstrated as promising electrode materials for Li−S batteries.26 Recent research27,28 suggests that meso/macropores are beneficial for enhancing the capacity and rate performance because they can offer a large pore volume for high sulfur loading and ensure the electrolyte infiltration for fast Li ion transport. Micropores can confine sulfur and achieve physical adsorption of the polysulfides, thus improving the cycling performance of Li−S batteries.29−31 So far, strategies using diverse templates (such as silica, oxide/hydroxide/carbonate, polymer, etc.)32−35 are predominantly employed to obtain hierarchical porous carbon. The combining utilization of chemical vapor deposition (CVD) and versatile template strategies is explored to fabricate hierarchical porous carbon as host for high-performance Li−S batteries. For example, hierarchical porous CaO particles were adopted as catalytic templates for the CVD growth of graphene with tunable structural hierarchy for high-power Li−S battery.36 Zheng et al. reported the fabrication of hierarchical porous carbon rods by CVD growth on a Mg(OH)2 hierarchical microrod template for high sulfur loading cathodes.37 CNTs are also an attractive choice for potential applications in Li−S batteries because of their outstanding mechanical strength, excellent electronic conductivity, and large surface-tovolume ratio.38 As a sulfur host, sulfur cannot easily infiltrate into the tubes but rather affixes around the surface due to the small diameter of CNTs (10−20 nm). Therefore, attempts to increase the porosity or introduce heteroatoms in the CNTs are motivated to reinforce the mutual interaction between sulfur and CNTs. Besides CNT-supported sulfur composites, another approach intends to combine conducting CNTs with porous carbons/graphene to obtain a hybrid. In such hybrid structures, CNTs provide sufficient conducting pathways with lower interfacial contacting resistance, and porous carbons/graphene with large surface area and pore volume ensure high sulfur loading.39−42 For example, CNT-interpenetrated mesoporous nitrogen-doped carbon spheres were reported to act as a sulfur host.43 Zhang et al. used layered double hydroxide as substrate and employed one-step catalytic growth of graphene/singlewalled CNT hybrids for high-rate Li−S batteries.44 Despite these attempts, large-scale tailoring of the carbon host with high conductivity and optimal porosity by using a facile and reasonable low-cost strategy is still a significant challenge. In this work, we propose a facile method to prepare N-doped hierarchical porous carbon and CNT hybrids as host for highperformance Li−S batteries. The HPC and CNT hybrids are synthesized and underwent simultaneous in situ nitrogen doping by simple pyrolysis of the low-cost raw material melamine-formaldehyde resin with nano-CaCO3 as template and bimetallic combination of Fe−Co as catalyst. Notably, the CaCO3 template for porous carbons is more attractive due to its lower cost and easier post purification compared with the family of silica templates. The mechanism is the same as that for conventional CVD growth of CNTs on the metal catalysts, but the carbon and nitrogen sources for N-doped CNT growth are provided by pyrolysis of the MF resin which contains approximately 45 wt % of N and a substantial amount of triazine ring units.45 Obviously, the technological requirements of this strategy are not as harsh as those for the CVD method,

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EXPERIMENTAL SECTION

Synthesis of CaCO3@MF Resin. In a typical synthesis, 4 g of melamine and 8 mL of formaldehyde solution (37 wt %) were mixed, and the pH value of the resulting milky suspension was adjusted to 9− 10 by adding 0.1 M NaOH solution. The polymerization reaction of melamine and formaldehyde was performed under stirring at 80 °C for 15 min. The soluble MF precursor was then adjusted to pH 5−6 by diluting with HCl. Amounts of 6.5 g of CaCO3 nanoparticles (50−100 nm, Nanjing XFNANO Materials Tech Co., Ltd.) and 10 mL of DI water were subsequently added into the solution and mixed thoroughly with vigorous stirring. The commercialized nano-CaCO3 was calcined at 400 °C to remove organic impurities before using. A gel-like mixture was prepared over a 2 h period, and then the composites were left to dry at 60 °C for 12 h. After complete solidification occurred, the monoliths were ground to fine powders. Synthesis of CaCO3@MF Supported Catalyst. CaCO3@MF supported bimetallic catalysts were prepared by an impregnation route using an aqueous mixture of CaCO3@MF and metal nitrates. Specifically, calculated amounts of Co(NO3)2·6H2O and Fe(NO3)3· 9H2O were dissolved in distilled water, and subsequently CaCO3@MF powder was suspended into the solution for catalyst loading. The total mass of Co−Fe catalysts was 60 mg, and the molar ration of Co:Fe is 4:1. Then the suspension was kept at 80 °C to evaporate all the water under stirring. The resulting product was further stabilized in air under 200 °C for another 1 h. Synthesis of N-HPC−CNTs. The precursor was annealed at 900 °C and kept for 2 h under argon atmosphere with ramping rate of 10 °C min−1. To remove the residues of calcium templates and metal catalyst, the products were washed thoroughly with 2 M HCl solution for 12 h under stirring. Finally, the N-CNT−HPC hybrids were filtered and rinsed with DI water until a neutral pH value was achieved and dried at 120 °C for 12 h. Moreover, N-NHPC and N-HPC−CNT hybrids were produced by carbonization in the same condition but using various amounts (0, 20, 60, and 100 mg) of metal catalyst. Synthesis of Sulfur Composites. N-HPC−CNT/S and N-HPC/ S composites were prepared via a facile melt-diffusion method. Typically, a mixture of the N-HPC−CNTs or N-HPC and sulfur in the desired mass ratio was heated at 155 °C for 12 h. The controlled experiments on various contents of sulfur were attained by adjusting the ratio of N-HPC−CNTs and sulfur with other conditions being fixed. Characterization Techniques. X-ray diffraction (XRD) measurements were performed on a D/Max-III (Rigaku Co. Ltd., Japan) using Cu Kα radiation and operated at 34 kV and 26 mA. The morphologies were observed by a field emission scanning electron microscope (FEI Quanta 450 FESEM) and transmission electron microscope (TEM, FEI Tecnai G2 F30). The elemental mapping by energy-dispersive Xray (EDX) analysis and the X-ray photoelectron spectroscopic (XPS, ESCALAB 250) measurements were carried out to analyze the constituent content and the composition of the specimens. Raman spectra were recorded on a micro-Raman spectrometer (Renishaw inVia, U.K.). A thermogravimetric analyzer (TA-DTG Q600) was used to study the synthesis mechanism and determine the sulfur content in the nanocomposite. The pore structures of the samples were characterized by analysis of the N2 adsorption−desorption isotherms, performed at 77 K with Micromeritics ASAP 2420 in a relative B

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Figure 1. (a) Common cross-linking sequence of MF resin; (b) schematic of N-HPC−CNT hybrids synthesized by the templated and catalytic growth of CNTs; and (c) schematic illustration of catalytic growth of CNTs on HPC.

Figure 2. N-doped hierarchical porous carbon synthesized with 6.5 g of nano-CaCO3: (a,b) typical SEM images and (c−e) typical TEM images of N-HPC. pressure (P/P0) range of 10−6 to 1 after degassing the samples at 150 °C for 5 h. Electrochemical Measurement. For the electrochemical measurements, sulfur composites with acetylene black and polyvinylidene fluoride were dispersed in methyl-2-pyrrolidene solution in a weight ratio of 8:1:1, and the mixed slurry was then coated on an aluminum foil. After vacuum-dried at 60 °C for 12 h, the electrodes were cut into 14 mm-diameter discs and assembled into 2032 coin cells in an argonfilled glovebox. The sulfur loading of each electrode is approximately 1.5 mg cm−2 if without specification. The lithium foil was used as the counter electrode and microporous membrane (Celgard 2400, USA) as the separator. The electrolyte is comprised of 1,3-dioxolane and 1,2dimethoxyethane (1:1 v/v) with 1.0 M bis(trifluoromethane) sulfonimide lithium and 1% LiNO3. The cells were galvanostatically

discharged and charged between 1.7 and 2.8 V versus lithium at room temperature on a program-controlled test system (Shenzhen Neware Battery Co., China). The cyclic voltammetric and electrochemical impedance spectroscopy measurements were performed on an electrochemical workstation (CHI660e, China). The EIS measurements were conducted in a frequency range of 105 to 10−2 Hz at the A.C. amplitude of 5 mV and at open-circuit voltage. The specific capacity of N-HPC−CNT/S and N-HPC/S composite was calculated based on the sulfur weight in the composite.

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RESULTS AND DISCUSSION The strategy for fabrication of N-HPC−CNT hybrids is illustrated in Figure 1. First, weak cross-linked polymer “glue” C

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implying that the decomposition of MF resin and the CNT growth occurred simultaneously. Finally, the N-CNT−HPC hybrids were obtained after removing the residues of calcium templates and metal catalysts. The N-HPC−CNT hybrids prepared by using 6.5 g of nanoCaCO3 and various amounts of catalysts are further studied by SEM in Figure 3 and Figure S6a,b. When the Fe−Co catalyst

was prepared using the base-catalyzed reaction of melamine and formaldehyde at controlled PH. The common cross-linking sequence of MF resin is shown in Figure 1a. After uniformly dispersing nano-CaCO3 into the viscous gel-like MF resin, the mixture gradually turned into a solid with the increasing of polymer cross-linking degree, thereby leading to the embedding of CaCO3 nanoparticles that serve as a template to create a hierarchical porous structure during the carbonization. As shown in Figure 2a, the N-HPC prepared by adding 6.5 g of CaCO3 nanoparticles has a well-developed three-dimensional pore structure constructed by a disordered and interconnected porous network. The magnified SEM and typical TEM images (Figure 2b−d) of N-HPC exhibit roughly spherical macropores (50−100 nm) that are randomly located on HPC, which is in good agreement with the particle size distribution of nanoCaCO3 (Figure S1). Moreover, mesopores with diameters of 10−20 nm can also be observed in the HRTEM image (Figure 2e), and the pore wall has clear graphite lattice fringes, suggesting that the N-doped HPC is partially graphitized. NHPC samples prepared by various amounts of CaCO3 are also studied, and all samples show similar porosity morphology (Figure S2). Adding 6.5 g of CaCO3 in the precursor results in the N-HPC possessing the highest Brunauer−Emmett−Teller (BET) surface area of 687 m2 g−1 and Barrett−Joyner−Halenda (BJH) pore volume of 1.64 cm3 g−1, as determined by nitrogen adsorption−desorption measurements (Table S1). In this regard, this CaCO3 adding amount is fixed for the subsequent growth of CNTs. Second, the CaCO3@MF supported Fe−Co catalyst was prepared by an impregnation route for the growth of CNTs on HPC. After thorough impregnation, Fe−Co bimetallic nanoparticles were anchored on the surface of MF resin and the exposed nano-CaCO3. Nano-CaCO3 supported Fe−Co bimetallic is reported to be an excellent catalyst for the high-efficiency production of multiwalled CNTs, which gave a higher CNT yield than when the CaCO3-supported single metal was the catalyst.46 SEM images in Figure S3 revealed that CaCO3 in the precursor not only acted as a template for porous carbon but also played an important role in influencing the activity of the catalyst. When direct pyrolysis of MF resin supported Fe−Co catalyst, only sparse CNTs can be occasionally observed on the bulk carbon. This phenomenon could be caused by the metal−substrate reaction, stopping the catalytic behavior of the metal during pyrolysis. More CaCO3 is believed to be exposed on the composite, and the Fe−Co catalyst is favorable to high-yield CNT growth. During pyrolysis, the MF resin was first converted into carbon, and nitrogen was simultaneously in situ doped into the carbon. Meanwhile, MF resin underwent thermal decomposition and emitted volatile gases (such as NH3 and small cyano fragments) at 400−700 °C (Figure S4), which could serve as carbon and nitrogen sources to grow N-doped CNTs. In addition, metal nanoparticles encapsulated in the tip of the CNT can be observed in rare cases even after washing by acid (Figure S5), thereby demonstrating that the CNT growth is based on the vapor−liquid−solid mechanism. According to this model, the gaseous carbon fragments first diffuse into the metal droplet to form a metal−carbon interface, and the growth of CNTs is initiated at the liquid−solid interface.47 When supersaturation occurs, the precipitation of graphite carbon is induced from the metal surface, leading to the completion of the N-doped CNT assembly. TG curves in Figure S4 showed that the weight loss of the CaCO3@MF-supported metal catalyst was slower than that of the precursor without metal catalyst at 400−700 °C,

Figure 3. SEM images of N-HPC−CNTs synthesized with different amounts of Fe−Co catalyst: (a,b) 20 mg;(c,d) 60 mg; and (e,f) 100 mg.

was added at 20 mg, sparse CNTs of several micrometers grew on the surface of the hierarchical porous carbon (Figure 3a,b). As the catalyst loading increased from 20 mg to 100 mg, the density of CNTs significantly increased, and their lengths decreased. When the catalyst increased to 100 mg, the CNTs almost could not be distinguished, as their lengths are only a few hundred nanometers (Figure 3e,f). More catalysts anchoring on the precursor could offer more initial points for CNT growth. Thus, the density of CNTs in the as-obtained product is determined by the loading amount of metal nanoparticles on the precursor surface. Besides, the volatile carbon source derived from MF resin thermal decomposition was not unlimited but was determined by the amount of MF resin. These carbon sources allocated to each initial point are insufficient to grow micron meters of CNTs in the presence of a low amount of catalyst. The TEM images in Figure 4a−c show a typical morphology of multiwall CNTs with an outer diameter of approximately 14 nm and a wall thickness of ∼3 nm. Notably, the CNTs are constructed by wrinkled graphene layers, which could be commonly observed in CNTs prepared with nitrogen dopant. This wrinkled wall morphology of Ndoped CNTs is known to originate from the substitution of nitrogen atoms in the graphitic domain, which would increase the curvature of the graphene layer. The structures of the NHPC and N-HPC−CNT hybrid were studied by X-ray diffraction (XRD) as shown in Figure 4d. The characteristic peak of these two samples located at 26.2° corresponded to the (002) plane of graphite carbon. Noticeably, the characteristic D

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Figure 4. TEM (a) and HRTEM (b,c) images of N-HPC−CNTs; (d) XRD patterns of N-HPC and N-HPC−CNTs; and (e) Raman spectra of NHPC and N-HPC−CNTs.

Figure 5. N2 gas adsorption−desorption curves (a) and pore distribution (b) of N-HPC and N-HPC−CNTs. (c) XPS survey spectra of N-HPC and N-HPC−CNTs. (d) High-resolution XPS spectra of N1s for N-HPC and N-HPC−CNTs.

peak of N-HPC−CNTs became sharper than N-HPC, indicating that the graphitic degree increased, which can be ascribed to the catalytic graphitization function of transition

metal and the introduction of CNTs into the composite. The intensity ratio of D and G bands (ID/IG) obtained from the Raman spectra (Figure 4e) was further used to determine the E

DOI: 10.1021/acsami.7b09808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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moderate density of CNT sample, 345 mAh g−1 for low density of CNT sample, and 383 mAh g−1 for high density of CNT sample after 500 cycles. Theoretically, the sulfur content in this N-HPC−CNT hybrid can be as high as 71.1 wt %, calculated based on the sulfur density of 2.07 g/cm3. If further considering for the volume change of Li2S, the maximum sulfur loading of the NHPC−CNT hybrid should be 58 wt %, calculated based on the Li2S density of 1.66 g/cm3. A moderate amount of CNTs on HPC had little impact on the hierarchical porous structure. Most mesopores and macropores of HPC were retained after CNT growth, which could serve as buffering reservoirs for the sulfur/polysulfides and facilitate the fast ion transportation. Moreover, these N-doped CNTs grown on HPC play dual roles in the physical and chemical adsorption of polysulfides. On one hand, CNTs form physical barriers that block some large pores on HPC, thus reducing the polysulfides loss. On the other hand, N-doped CNTs could offer more active sites for chemical adsorption of polysulfides. To further confirm nitrogen incorporation into the asprepared carbon structure, X-ray photoelectron spectroscopy (XPS) was conducted, and typical XPS spectra for N-HPC and N-HPC−CNT hybrids were shown in Figure 5. Both XPS survey spectra show peaks for C 1s, N 1s, and O 1s, suggesting that nitrogen atoms have been successfully in situ doped into the carbon matrix by simple pyrolysis of MF resin. The surface elemental composition data obtained by XPS analysis (in Supporting Information Table S1) revealed that nitrogen content in the N-HPC significantly decreased as the template increased. The nitrogen contents were 7.4, 5.3, and 2.9 wt % in the carbon samples prepared by adding 4.5, 6.5, and 7.5 g of CaCO3 in the precursor, respectively. More CO2 gas would be generated during the thermal decomposition process when increasing the amount of CaCO3. Thus, the stronger activation treatment caused loss of nitrogen content. Comparing with NHPC, N-HPC−CNT hybrids retained lower nitrogen content, which was in good agreement with the intensity of N and O peaks for N-HPC−CNT hybrids that are slightly weaker than that of the N-HPC. In addition, the amount of catalyst in the precursor did not have a significant effect on the nitrogen content in N-HPC−CNTs. The nitrogen content of N-HPC− CNTs only decreased from 2.3 wt % to 1.2 wt % when the catalyst increased by adding 10−100 mg in the precursor. The high-resolution XPS N 1s spectra for N-HPC and N-HPC− CNT samples reveal the presence of four different bands, corresponding to pyridinic-N (∼398 eV), pyrrolic-N (∼399 eV), graphitic-N (∼401 eV), and oxidized pyridinic-N (∼404 eV), respectively. The curve fitting in Figure 5d and the corresponding normalized results (Supporting Information Table S2) suggest that the nitrogen atoms in N-HPC−CNT hybrids were mainly in the form of graphitic-N (53.1%). The pyridinic-N and pyrrolic-N in the hybrids decreased to 14.6% and 7.7% compared with N-HPC. The ratio change of nitrogen species indicated a conversion from pyridinic-N and pyrrolic-N to graphitic-N in the case of N-HPC−CNTs. Recent research demonstrated that the N heteroatoms in the carbon matrix with an extra pair of electrons can naturally act as Lewis base sites to interact with a strong Lewis acid of terminal Li atoms in lithium polysulfides via dipole−dipole electrostatic interaction, thereby effectively preventing the shuttle of polysulfides and allowing good cycling stability and high Coulombic efficiency. To investigate the electrochemical properties, the sulfur/NHPC−CNT composite was fabricated by a facile melt-diffusion method. In comparison, two sulfur loading composites were

defect density and graphitic degree of the materials. The ID/IG value for the N-HPC−CNTs (0.36) was smaller than that of NHPC (0.58). This can be attributed to the increase of ordered graphitic structure originating from CNTs, which is consistent with the XRD result. To further investigate the porosity characteristics of NHPC−CNT hybrids, nitrogen adsorption−desorption measurements were conducted and shown in Figure 5a,b. The N2 adsorption−desorption isotherm of N-HPC−CNTs exhibited a typical characteristic of type VI in the IUPAC classification with a mesopore hysteresis loop at a relative pressure range of 0.45− 0.85. This can be classified into type H4, indicating the presence of mesopores. The pore size distribution derived from the N2 desorption branches revealed that the N-HPC−CNTs with a hierarchical pore structure are mainly composed of small mesopores (∼7 nm and ∼11 nm), large mesopores (∼33 nm), and marcopores (50−150 nm). According to the TG analysis (Figure S4), CaCO3 decomposes into CaO and CO2 between 700 and 900 °C, and such a considerable amount of gas inside the carbon would cause explosive expansion and finally break through the surface surrounding the template. The control experiment with the sample prepared at 700 °C only can obtain dense monolithic carbon (Figure S6c,d), confirming that the interconnected pores could be produced by such a CO2induced pore generating mechanism. Additionally, such large quantities of CO2 gas can chemically activate the carbon and generate small mesopores. For the large mesopores, they were replicated from the hard templates. During decomposition from CaCO3 to CaO, the size of the template shrunk and became smaller than their original size. Accordingly, large mesopores were left inside the carbon matrix after acid washing that removed the embedded templates. The pore distribution of the samples with or without CNTs was similar, except for a significant decrease of macropore distribution in N-HPC− CNTs compared with the N-HPC sample. This change was due to the growth of intertwined CNTs on the N-HPC surface, thereby blocking some of the macropores. In addition, surface area and pore volume for N-HPC−CNT samples were smaller than those for N-HPC, which may be attributed to the aggregation of the higher graphitic structure, as demonstrated by XRD and Raman characterization. Specifically, this is true for the sample with sparse CNTs, whose increased surface area contributed by these CNTs is not sufficient to offset the decreased surface area caused by graphitization. However, too many CNTs on the HPC surface also led to surface area decrease because the pores on HPC were completely covered with the dense intertwined short CNTs. In this regard, it is reasonable that the N-HPC−CNTs with a moderate amount of CNTs (sample prepared by loading 60 mg of metal catalyst) have the highest BET surface area (651 m2 g−1) and pore volume (1.19 cm3 g−1). Such a large pore volume suggests that the N-HPC−CNTs as host favor high sulfur loading. The density of the CNT has direct effect on the surface area and pore volume of the N-HPC−CNTs (Table S1). Generally, large surface area and pore volume are beneficial for better sulfur distribution and higher sulfur loading, thereby leading to better electrochemical performance. In the view of these, the NHPC−CNT samples with a moderate density of CNTs which have the highest BET surface area and pore volume were mainly studied in the following discussion, and the cycling performance of N-HPC−CNT/S (1:2) samples with different density of CNTs (Figure 8a and Figure S9) was also tested for comparison. The discharge capacities are 438 mAh g−1 for F

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Figure 6. XRD patterns of N-HPC−CNT/S (a); TGA curves of N-HPC−CNT/S cathode with various sulfur loadings (b); TEM (c); and the corresponding STEM (c1) and elemental mapping (c2−c4) images of N-HPC−CNT/S composite with 62.5 wt % sulfur content.

peaks located at 2.3 and 2.05 V (vs Li/Li+) in the anodic scan and two overlapping oxidation peaks appear at 2.3 and 2.35 V in the cathodic scan. These peaks correspond to the reversible reaction of elemental from sulfur to polysulfide (Li2Sn, 4 ≤ n ≤ 8) and further to Li2S. The current density slightly increased during the anodic scan because the sulfur in the electrode was more efficiently utilized with the soaking and penetrating of electrolyte. The current density decrease during the anodic scan is probably originated from the shuttle effect that results in sulfur loss, and the Li2S/Li2S2 cannot completely convert into sulfur in the electrochemical oxidation reaction. Both reduction/oxidation peaks show such slight changes during the first five cycles of the CV profiles, indicating that the cathode processes high electrochemical stability. The galvanostatic charge/discharge curves of the N-HPC−CNT/S at current density from 0.1 to 2 C were presented in Figure 6c. All charge/discharge profiles exhibited a typical plateau of the sulfur cathode, which is in good agreement with the results confirmed by CV measurement. Both charge/discharge capacity decrease and voltage hysteresis can be observed as the current density increased due to the polarization phenomenon. It is worth noting that the plateaus are maintained well even at a high current rate of 2 C, suggesting

prepared by adjusting the mass ratio of carbon host and sulfur (1:2 and 1:4). After melting S with the as-prepared N-HPC− CNT hybrids at 155 °C for 12 h, the XRD patterns of composites (Figure 6a) are a perfect match of the wellcrystallized orthorhombic type of sulfur (JCPDS no. 08-0247). The TG curves in Figure 6b showed that a sharp mass loss occurred between 200 and 300 °C, corresponding to the evaporation of sulfur in the composite. The actual sulfur contents in these two samples are 62.5 and 76.4 wt %, which are very close to the theoretical value. Figure 6c shows a typical morphology of N-HPC−CNTs with clear hierarchical porous texture and interlinked CNTs even after sulfur loading. More importantly, no discernible sulfur particles can be observed in the composite. Scanning TEM (STEM) and elemental mapping analyzed by EDS present a uniform distribution of the carbon, sulfur, and nitrogen elements in the nanocomposite, further confirming that heteroatoms of nitrogen have been successfully doped into N-HPC−CNTs and sulfur is homogeneously dispersed in the carbon matrix. Cyclic voltammetry (CV) was first implemented to investigate the electrochemical performance of the N-HPC− CNT/sulfur cathodes at a scan rate of 0.1 mV s−1 in the potential range from 1.7 to 2.8 V. Figure 7a shows that two G

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Figure 7. Electrochemical performance of the as-prepared N-HPC−CNT/S cathode with 62.5 wt % sulfur content: (a) cyclic voltammogram curves; (b) rate performance; (c) corresponding galvanostatic charge/discharge curves at various current rates; and (d) cycling performance of the N-HPC/ S and N-HPC−CNT/S prepared by the same mass ratio of carbon to sulfur (1:2) at a current rate of 0.5 C.

Figure 8. (a) Long-term cycling performance of the N-HPC−CNT/S with 62.8 and 76.4 wt % sulfur content at a current density of 1 C; (b) galvanostatic charge/discharge curves of N-HPC−CNT/S with 62.8 wt %; and (c) Nyquist plots from EIS of fresh cells.

H

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plateau voltage stabilized at approximately 2 V during longterm cycling, further demonstrating excellent cycling stability of the cathode even at high charge/discharge rates. Compared with the cathode with 62.5 wt % sulfur, the N-HPC−CNT/S76.4 wt % electrode exhibits lower discharge plateau voltage (Figure S7) and inferior capacity, delivering an initial discharge capacity of 470 mAh g−1 and retaining 296 mAh g−1 (∼63% capacity retention) after 600 cycles. According to the BET result, the pore volume of N-HPC−CNT can accommodate a maximum sulfur loading of 71.1 wt %. In this case, the redundant sulfur of the N-HPC−CNT/S-76.4 wt % sample would deposit on the carbon surface, thus increasing the resistance of the electrode and resulting in low sulfur utilization. It is worth noting that such capacities of the N-HPC−CNT/S cathode with 62.5 and 76.4 wt % sulfur are comparable to those of the current cathode materials (such as LiCoO2, LiFePO4, and LiMn2O4) even after 600 cycles at high current density. Electrochemical impedance spectroscopy (EIS) measurements were conducted to further demonstrate the conductivity of the sulfur cathode, shown as Figure 8c. Under the condition of the same sulfur loading, the charge transfer resistance (Rct, corresponding to the diameter of the semicircle at high frequencies) of the N-HPC−CNT/S electrode is lower than that when using N-HPC as a host for the sulfur cathode. The better conductivity of the N-HPC−CNT/S electrode can be attributed to the CNTs in the hybrid materials, which provide sufficient conducting pathways and effective connections between the CNTs and HPC, ensuring a fast electron transfer. In addition, the HPC−CNT/S-76.4% electrode exhibited a much larger charge-transfer resistance than the sample with 62.5 wt % sulfur. It is well explained by the phenomenon that the HPC−CNT/S cathode delivers higher capacity compared with that of a high sulfur loading sample. The comparison of Nyquist plots of the fresh electrode and the electrode after 600 cycles is presented in Figure S12. Apparently, the two semicircles that appear in the high to medium frequency in the Nyquist plots for the cycled electrode are significantly smaller than compared to the fresh samples, indicating their charge-transfer resistant decreased after cycling. This phenomenon could be ascribed to the sufficient infiltration of electrolyte and the redistribution of sulfur.

a considerable reaction kinetics of the N-HPC−CNT/S electrode. The rate performance of the N-HPC−CNT/S composite tested stepwise increased the current density from 0.1 to 2 C (1 C = 1675 mA g−1) for every 10 successive cycles. Figure 7b showed that the N-HPC−CNT/S cathode delivered an initial discharge capacity of 1181 mAh g−1 at 0.1 C, 1016 mAh g−1 at 0.2 C, 809 mAh g−1 at 0.5 C, 624 mAh g −1 at 1 C, and 400 mAh g−1 at 2 C, respectively. After the charge/ discharge process at the high current rate of 2 C, a discharge capacity of 757 mAh g−1 recovered at 0.5 C and still remained at 699 mAh g−1 after another 50 cycles. The excellent rate capability of N-HPC−CNT/S can be ascribed to the fast electron and ion transportation due to nitrogen doping and the hierarchical porous structure, as well as the distribution of CNTs. Except for a few current rate switched cycles, the Coulombic efficiency in most cycles achieved over 99%, thereby implying an excellent electrochemical reversibility. For comparison, the cycling performance of N-HPC/S and N-HPC−CNT/S prepared by the same mass ratio of carbon to sulfur (1:2) is tested at a current density of 0.5C. Figure 7d shows that two electrodes deliver similar initial capacities (∼800 mAh g−1) and good Coulombic efficiencies (Figure S11a), but the capacity of the N-HPC/S cathode decreased rapidly in the subsequent cycles. Although N-HPC has larger surface area and pore volume, it theoretically can allow better sulfur impregnation into the carbon matrix, enhancing the utilization of sulfur and guaranteeing a high capacity. However, N-HPC is mainly composed of macropores and large mesopores. Such large pores have been proved to be inefficient for blocking the polysulfides loss, whereas CNTs grown on NHPC PC acted as a physical barrier and blocked some large pores on HPC and thus slowed down the polysulfide loss. In addition, nitrogen atoms incorporated into the CNTs could create more active sites, which can afford strong chemical adsorption capability for polysulfides. As a result, the N-HPC− CNT/S cathode exhibited significantly higher capacity retention than that of the N-HPC/S. The long-term cycling performance of N-HPC−CNT/S samples with different sulfur content was investigated by evaluating the evolution of 600 charge/discharge cycles (Figure 8a and Figure S10). Except for the first two cycles performed at 0.1 C, the subsequent 600 cycles are performed at 1 C. The discharge capacity of N-HPC−CNT/S-62.5 wt % decreased to 622 mAh g−1 from 986 mAh g−1 when the current density switched to 1 C from 0.1 C. Then its discharge capacity slowly increased and reached to the highest capacity of 625 mAh g−1 at the 120th cycle. The N-HPC−CNT/S-62.5 wt % cathode finally delivered a discharge capacity of 407 mAh g−1 after 600 cycles, corresponding to capacity retention of 65.4% and only 0.066% capacity decay per cycle for over 600 cycles. Figure 8b presents several typical galvanostatic charge/discharge curves in long-term cycling. More information about capacity retention was obtained from these cycles, whose capacity retention is 98.7% after 150 cycles, 82.3% after 300 cycles, and 73.2% after 450 cycles, respectively. Despite that CNTs form physical barriers that block some large pores on HPC and can alleviate the polysulfide loss, the presence of large macropores still retained in the N-HPC−CNT is mainly responsible for continuous capacity fading with cycling which is because such macropores cannot effectively trap the polysulfides.48 The Coulombic efficiency of N-HPC−CNT/S-62.5 wt % (Figure S11b) is not be lower than 97% during the 600 cycles, and charge/discharge profiles maintained well, whose discharge

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CONCLUSIONS In summary, the N-doped hierarchical porous carbon−carbon nanotube hybrids have been successfully achieved via a facile pyrolysis strategy using melamine-formaldehyde resin as carbon and nitrogen source and nano-CaCO3 as template for hierarchical porosity structure and the thermal decomposition components of MF resin incorporated with bimetallic Fe−Co catalyst for CNT growth on HPC. We demonstrate that lowcost raw materials and simple equipment are enough for tunable fabrication of N-doped HPC and CNTs. The porosity of the HPC and the density and length of the CNTs are tunable, which can be simply adjusted via applying different amounts of CaCO3 template and Fe−Co catalyst. The asprepared N-HPC−CNT hybrids are composed of meso/ macropores. They can not only provide enough pore volume for sulfur impregnation but also effectively adsorb the soluble polysulfide intermediates. In the hybrid structure, CNTs play a dual role in enhancing the electrical conductivity and acting as physical barriers that block some large pores on HPC, thus reducing the shuttle effect. Moreover, simultaneously introduced nitrogen into the carbon matrix induces stronger I

DOI: 10.1021/acsami.7b09808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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chemical adsorption ability for trapping the polysulfides. As a result, the obtained N-HPC−CNT/S cathode exhibits considerable reversible capacity and good long-term cycling stability, which makes it evident that N-HPC−CNT hybrids can be a promising host for Li−S batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09808. SEM, TEM, and additional electrochemical performance figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Junjie Cai: 0000-0002-8670-6578 Chun Wu: 0000-0002-9783-1307 Ying Zhu: 0000-0003-2460-6544 Pei Kang Shen: 0000-0001-6244-5978 Kaili Zhang: 0000-0002-5926-2019 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Shenzhen Science and Technology Innovation Council (Project number JCYJ20160428154522334) and Hong Kong Research Grants Council (Project numbers CityU 11216815 and CityU 11338016) and supported by Hong Kong Innovation and Technology Commission via the Hong Kong Branch of National Precious Metals Material Engineering Research Center (Chinese National Engineering Research Center (CNERC)).

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DOI: 10.1021/acsami.7b09808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX