Graphitic

Article Cite This: J. Phys. Chem. C 2019, 123, 15924−15934

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Three-Dimensional Functionalized Carbon Nanotubes/Graphitic Carbon Nitride Hybrid Composite as the Sulfur Host for HighPerformance Lithium−Sulfur Batteries Wenxiang He,†,§,∥ Xingchen He,†,∥ Meili Du,† Shiyu Bie,† Jianguo Liu,*,†,‡ Yiqing Wang,*,† Meng Liu,† Zhigang Zou,†,‡ Wuwei Yan,‡ and Haimin Zhao§

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Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, China ‡ Kunshan Innovation Institute of Nanjing University, Kunshan Sunlaite New Energy Co., Ltd., 1699# South Zuchongzhi Road, Kunshan, Suzhou 215347, China § R & D Department, Tianneng Battery Group Co., Ltd., Changxing, Zhejiang 313100, China S Supporting Information *

ABSTRACT: Improving the sulfur loading, conductivity, and polarity of the sulfur host is the main effective strategy used to solve the inherent problem of lithium−sulfur batteries for practical applications. Herein, a three-dimensional hybrid sulfur host composed of graphitic carbon nitride and functionalized carbon nanotubes is prepared via a selfassembly-assisted method assisted by calcination. The structural analysis results indicate that graphitic carbon nitride sheets are grown in situ on the surfaces of the carbon nanotubes and that the two components are connected by C− N bonds to form a larger π-conjugated system. This unique structural linkage between the two parts directs electron transfer from the carbon nanotubes to the carbon nitride, resulting in enhanced conductivity of the integrated hybrid composite and processing a sufficient number of nitrogen-containing functional groups. Thus, as a sulfur host, the hybrid composite can efficiently restrain the shuttle effect of intermediate polysulfides because of its high sulfur utilization and strong chemical interaction with polysulfides. As a result, a modified sulfur cathode with a high sulfur content of 80 wt % delivers a high specific capacity of 1351.2 mAh g−1 at a rate of 0.1 C. Even at a 5 mg cm−2 sulfur loading, the cathode exhibits an initial capacity of 758.9 mAh g−1 with a capacity retention of 77.1% after 200 cycles at 1 C.

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INTRODUCTION The development of new generation energy storage devices is always a popular topic worldwide today, especially in the field of battery engineering. Although poor electrical conductivity, low sulfur utilization, and the dissolution of intermediate lithium polysulfides (LiPSs) present significant challenges for commercial viability,1,2 lithium−sulfur batteries are still regarded as a very promising energy storage system for the next generation mainly due to their high theoretical specific energy density of 2600 Wh kg−1 and enormous sulfur resource. Virtually, great efforts have been implemented to solve these aforementioned issues with the cathode materials. Various carbon materials,3−6 metal oxides,7 metal sulfides,8 metal− organic frameworks,9,10 and MXene phases,11 etc.,12−15 have been investigated as sulfur host materials. Although these abovementioned works have achieved excellent performances, incorporating these materials into commercial lithium−sulfur batteries remains difficult and is impeded by many challenging © 2019 American Chemical Society

problems and Gordian techniques, including weak interfacial interactions between carbon materials and LiPSs and the poor conductivity of most metal compounds.16 Similarly, the practical feasibility of graphitic carbon nitride (g-C3N4), which has been studied and used in the application of lithium−sulfur batteries as a sulfur host,17−19 is also limited by its intrinsic low conductivity, resulting in a poor rate performance and low sulfur loading of the composite cathode. For example, Liu et al. designed a graphene-like oxygenated gC3N4 as the host material with 56 wt % sulfur that had a reversible capacity fading rate of 0.1% per cycle over 500 cycles at a 0.5 C rate.20 With a graphitic-phase structure and sp2hybridized C and N atoms, g-C3N4 has been evaluated in numerous studies in various fields due to its unique planar Received: March 13, 2019 Revised: June 10, 2019 Published: June 13, 2019 15924

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

Article

The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of the S/CN-CNT Synthesis Process

sulfur composite cathode can be prepared by further sulfur loading via the melt-impregnation method. The prepared sulfur composite cathode processes an integrative conductive network with high conductivity that is beneficial for the rate performance and sulfur utilization. On the other hand, of the many N-containing functional groups of the hybrid host material can facilitate the chemical adsorption of LiPSs, thus preventing the dissolution and migration of LiPSs in the electrolyte. More importantly, this scalable synthetic technique can be regarded as a low cost, environmentally friendly, and potentially large-scale route for preparation of new structures with complex properties that are intrinsically different from those of the isolated bulk materials or mechanical compounds.

structure, optical and physicochemical properties, simple synthesis process, and cost-effective availability.21−23 As a host material, this unique two-dimensional (2D) planar graphite-phase structure with intrinsic nitrogen-containing functional groups can accommodate a large number of sulfur molecules and generate favorable chemical interactions with LiPSs. Therefore, a new g-C3N4-based host material with high conductivity, rich N-containing functional groups, and a large 2D loading platform is reasonably anticipated to be a perfect candidate for constructing a sulfur/g-C3N4 composite cathode with great performance. This ideal host substrate can offer dual functions to overcome the intrinsic drawbacks of pure g-C3N4: (i) the enhanced conductivity can support accelerated electron transport, leading to high sulfur utilization, and (ii) the intrinsic chemical adsorption ability can be inherited, thus suppressing the dissolution of intermediate LiPSs. Hydrogen bonding has been used frequently to synthesize supramolecular aggregates because of the strong directional component of noncovalent interactions (e.g., cyanuric acid molecules can form strong hydrogen bonds with melamine, yielding highly stable supramolecular structures).24 With this possibility in mind, we proposed a novel self-assembly-assisted method to construct a 3D conductive framework that integrated g-C3N4 and carbon nanotubes (CNTs). As schematically illustrated in Scheme 1, equimolar melamine and cyanuric acid molecules first deposit on the surfaces of oxidized CNTs and subsequently assemble into hydrogenbonded supramolecular structures. In this structure, the H atoms of the amino in melamine are attractive to the O atoms of the hydroxyl group in cyanuric acid and the carboxyl group in CNTs to form an intramolecular, six-membered, ring-like structure via the nonlinear hydrogen bond connections. Meanwhile, the hydrogen bonds of the N−H···O and N− H···N linkages between melamine and cyanuric acid result in formation of a triazine structure for g-C3N4 sheets. Then, this well-designed supramolecule can further evolve into a higher order 3D hybrid composite with excellent conductivity and a large-area sulfur loading platform through calcination. We can conclude that 2D g-C3N4 sheets derived from the supramolecule are grown in situ on the CNTs and completely cover them during the subsequent heating step. Finally, the final

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EXPERIMENTAL SECTION Preparation of g-C3N4-CNTs. First, the raw multiwall carbon nanotubes (MWCNTs) (95% purity, Jiaozuo) were soaked in a mixture of nitric acid and sulfuric acid (the volume ratio of nitric acid to sulfuric acid was 3:1) at 80 °C for 12 h and then were washed with distilled water until the pH value of the filtrate reached 7.0. The ratio of CNTs to mixed acid is 0.5 g of CNTs corresponding to 50 mL of mixed acid. Second, 0.2 g of the prepared oxidized CNTs was added to the aqueous solution with 1 g of cyanuric acid, and then the prepared suspension was poured into a melamine solution (the molar ratio of cyanuric acid to melamine = 1:1) at 60 °C and was mixed for 12 h. The precipitated mixed precursor was filtered and washed with deionized water thoroughly and then dried at 80 °C for 10 h. Finally, the precursor was calcined at 550 °C for 4 h under the protection of argon gas with a heating rate of 2 °C min−1. The obtained product containing approximately 50 wt % g-C3N4 was called g-C3N4-CNTs. For the comparative study, pure g-C3N4 was also synthesized using the same procedure without adding oxidized MWCNTs. Preparation of S/g-C3N4-CNT Composite Cathode Materials. The obtained g-C3N4-CNTs and sulfur (99.95%, Aladdin) were mixed and ground. The mass ratio of g-C3N4CNTs to sulfur was 2:8. Then, the powder was heated in a tube furnace at 155 °C for 12 h under argon gas protection (99.99%), and the final product was labeled S/CN-CNTs. For comparison, Super P, the prepared oxidized CNTs, and sulfur 15925

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

specially mentioned were measured at an ambient temperature of 25 ± 0.5 °C. Calculation Methods. The DMol3 code was used to calculate the structural optimization and transition states. The generalized gradient approximation (GGA) with the Perdew− Burke−Ernzerhof (PBE) functional and all-electron double numerical basis set with polarized function (DNP) was employed. The convergence tolerance of energy, maximum force, and maximum displacement were 1.0 × 10−5 Ha, 2.0 × 10−3 Ha/Å, and 5.0 × 10−3 Å (1 Ha = 27.21 eV), respectively, for geometric optimization. Li2S, Li2S2, and Li2S4 are absorbed on the surface of g-C3N4-CNTs. Each atom in the storage models is allowed to relax to the minimum enthalpy without any constraints.

mixed together with the same weight ratio were prepared using the same method, and the obtained product was labeled S/SP/ CNTs. Similarly, mixtures of pure g-C3N4, CNTs, and sulfur were also prepared as S/CN/CNTs with the same amount of sulfur using the same method. Polysulfide Adsorption Sample Preparation. The 0.1 mol L−1 Li2S4 solution was prepared by mixing and dissolving stoichiometric amounts of Li2S and sulfur into dimethoxyethane (DME) and 1,3-dioxolane (DOL) mixed solvent with a volume ratio of 1:1. A total of 50 mg of the g-C3N4, CNT, and g-C3N4-CNT powders was added to three different vials with the same volume. All procedures were performed in an argonfilled glovebox with low H2O levels and O2 less than 0.1 ppm. Materials Analysis Techniques. X-ray powder diffraction (XRD) characterization was performed on a Philips APD 3520 diffractometer using Cu Kα radiation. The morphological analyses were carried out on a Helios 600i dual beam scanning electron microscope (SEM). The chemical composition and microstructure of the samples were characterized by a TECNAI F20 field-emission transmission electron microscope (TEM) equipped with a GIF 200 electron energy loss spectroscopy spectrometer (EELS). The specific surface areas of the samples were described by a TriStar 3000 analyzer using the Brunauer−Emmett−Teller (BET) method by N2 adsorption. The X-ray photoelectron spectroscopy (XPS) analysis was performed on an ULVAC-PHI PHI 5000 VersaProbe instrument using a monochromatic Al Kα source, and all binding energy spectra were calibrated using the C 1s peak at 284.6 eV. The thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449F3 thermal gravimetric analyzer in a nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR) was measured by a NEXUS 870 instrument. Sandwich electrodes consisting of stainless steel/thin disk/ stainless steel were assembled to test the conductivity of the materials using the I−V method at a voltage range from −0.2 to 2 V with a scan rate of 5 mV s−1 on a Princeton Applied Research analytical instrument (PARSTAT MC). These thin disks were prepared with 50 mg of the pure g-C3N4, hybrid gC3N4-CNT composite, and CNT powders under 10 MPa for 3 min using a tablet machine. Cell Fabrication and Characterization. A mixture of the prepared sulfur composite materials, Super P, and polyvinylidene fluoride binder with a weight ratio of 8:1:1 and an appropriate amount of N-methyl-2-pyrrolidone as the solvent were prepared at a high speed to form fresh slurry. Then, the slurry was coated on Al foil with a certain thickness and dried thoroughly. The sulfur mass loading of the cathode electrodes was 1−5 mg cm−2. Then, the working electrode plate was cut into circular flakes with the same size. Finally, the prepared working electrodes, microporous polypropylene membrane (Celgard 2400), an electrolyte composed of 1 M LiTFSI with 2% LiNO3 in 1:1 (by volume) DME and DOL, and Li metal foil were assembled into CR2025 coin cells in an argon-filled glovebox with low H2O and O2 below 0.1 ppm. The galvanostatic charge and discharge tests of the coin cells at various current densities in the voltage range of 1.8−2.8 V were measured by a NEWARE BTS (battery testing system). Cyclic voltammetry (CV) was carried out at a scan rate of 0.1 mV s−1, and electrochemical impedance spectroscopy (EIS) measurements were implemented in the frequency range between 100 kHz and 0.01 Hz with an amplitude of the input AC signal of 5 mV; both methods were supported by a CHI660E electrochemical workstation. All tests that were not

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RESULTS AND DISCUSSION Structural and Morphology Characterizations. Figure 1a presents typical diffraction peaks of the CNTs at

Figure 1. XRD patterns and inserted SEM micrographs of the (a) CNTs, (b) g-C3N4, (c) g-C3N4-CNTs, and (d) S/CN-CNTs.

approximately 26 and 43°, which can be indexed as the (002) and (100) reflections of graphite, respectively.25,26 The corresponding SEM image in the top right corner shows the typical tube-like structure of CNTs intertwining with each other. The XRD pattern of pure g-C3N4 (Figure 1b) shows two diffraction peaks at approximately 13 and 27°, which can be indexed as (100) of the in-plane repeating packing and (002) of the interlayer stacking of g-C3N4, respectively.27,28 Pure gC3N4 displays a 2D sheet structure with an irregular shape according to the SEM characterization. Based on the examination of the XRD pattern (Figure 1c), two types of typical diffraction peaks corresponding to the two samples can be easily distinguished for the hybrid g-C3N4-CNT composite without any other secondary phase, indicating that a pure and stable hybrid g-C3N4-CNT composite can be synthesized using the self-assembly-assisted method. However, the SEM image of the g-C3N4-CNTs shows an integrated 3D conductive network, consisting of 2D g-C3N4 sheets and numerous interweaved 1D CNTs. This special structure, which differs from the morphology of the pure CNTs and g-C3N4, not only provides high conductivity but also processes a large sulfur loading platform. After the hybrid g-C3N4-CNT composite was loaded with sulfur using a melting method, the XRD pattern of the S/CN-CNTs (Figure 1d) represents the large quantities of 15926

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

Figure 2. XPS survey scan of the (a) CNTs, (b) g-C3N4, and (c) g-C3N4-CNTs. C 1s spectra of the (d) CNTs, (e) g-C3N4, and (f) g-C3N4-CNTs. N 1s spectra of the (g) g-C3N4 and (h) g-C3N4-CNTs. (i) Schematic structures of the oxidized CNTs, g-C3N4, and g-C3N4-CNTs. Gray, blue, red, and white balls represent carbon, nitrogen, oxygen, and hydrogen atoms, respectively.

orthorhombic sulfur (JCPDS no.08-0247) concentrated on the hybrid g-C3N4-CNT composite surface. Although distinguishing sulfur on the g-C3N4-CNT substrate surface in the SEM image is difficult, the S/CN-CNT composite surface is much smoother than that of the g-C3N4-CNTs, and no network pores can be observed, which confirms the even distributions of sulfur throughout the surface. Furthermore, the SEM analysis (shown in Figure S1) indicates that the mixtures of

pure g-C3N4 and CNTs, prepared by the physical mixing method are incompatible with each other and have two distinctly different morphologies. Chemical Analysis by XPS. XPS was employed to gain further insights into the chemical bonding of the hybrid gC3N4-CNT composite, as depicted in Figure 2. As shown in Figure 2a−c, two peaks (one sharp and one weak) at approximately 285.0 and 531.8 eV in the XPS survey spectra 15927

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

Figure 3. TEM images, corresponding EELS elemental mappings in the local domain, and schematic structures of the (a) g-C3N4, (b) g-C3N4CNTs, and (c) S/CN-CNTs.

(Figure 2g) can be fitted into three peaks at 398.5, 400.7, and 404.4 eV, which can be ascribed to the N characteristic of pyridinic-like N (CNC),32 graphitic-like N (C3−N)/ amino N (N−H), and oxidized N (N−O), respectively,33,34 as shown in Figure 2i. However, the N 1s spectra of the hybrid gC3N4-CNT composite can be fitted into four peaks. As shown in Figure 2h,i, the first three higher binding energy peaks are the same as those of pure g-C3N4, whereas the peak with the lowest binding energy located at 397.6 eV is assigned to the pyridinic-like N in the CNT structure. This result indicates that N atoms have been doped into the surface lattice of the CNTs, which has changed the electronic structure of the hybrid g-C3N4-CNT composite.35 As a result of N doping of the CNT structure, the extra valence electrons extracted from the N dopant can occupy the CNT conduction band and shift the Fermi level upwards from the conduction band, which can reduce the barrier height, resulting in overall improvement of the conductivity and the electron density of the hybrid g-C3N4CNT composite.36 The FTIR analysis and the core-level EELS spectra of the g-C3N4 and g-C3N4-CNTs also support these conclusions, as shown in Figures S2 and S3, respectively. In conclusion, these results suggest that the g-C3N4 sheets and CNTs in the g-C3N4-CNT composite are connected by C−N

of all samples are assigned to the C 1s and O 1s binding energies, respectively. Another peak at approximately 398.4 eV for g-C3N4 and g-C3N4-CNTs is attributed to the N 1s binding energy. From the fitted XPS analysis of the high-resolution C 1s XPS spectrum, the C 1s peak of the CNTs (Figure 2d) can be resolved into three individual peaks centered at 284.6, 285.8, and 288.6 eV corresponding to the sp2-, sp3-hybridized graphitic carbon atoms, and OCO bonds, respectively.29 This result indicates that trace amounts of carboxyl (−COOH) have been introduced into the CNT surfaces via acidizing treatment, as illustrated in Figure 2i. For the pure g-C3N4, Figure 2e shows that the C 1s peak of g-C3N4 can be divided into four individual peaks centered at 284.6, 285.8, 287.9, and 293.6 eV. The latter two peaks can be attributed to the C− (N)3 bonds and trace CO bonds.30 The fitted curve of the C 1s spectra of the hybrid g-C3N4-CNT composite (Figure 2f) differs from those of the pure g-C3N4 and CNTs in that the former can be divided into five peaks with binding energies of 284.6, 285.8, 286.8, 288.0, and 293.6 eV. A 286.8 eV binding energy of the g-C3N4-CNTs is related to the CO groups and originates from the CNTs after thermal annealing. 31 Compared to the N 1s spectra of the pure g-C3N4 and hybrid g-C3N4-CNT composite, the N 1s spectrum of pure g-C3N4 15928

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

Figure 4. (a) Calculated band structures of the hybrid g-C3N4-CNTs using fully relaxed structures. (b) Optimized HOMO − n and LUMO + n orbitals of the hybrid g-C3N4-CNTs. (c) Fully relaxed binding geometry of Li2S binding on the hybrid g-C3N4/CNTs. (d) Summary of the calculated adsorption energies of Li2Sx (x = 1, 2, and 4) on the g-C3N4-CNTs. Gray, blue, red, white, yellow, and purple balls represent carbon, nitrogen, oxygen, hydrogen, sulfur, and lithium atoms, respectively.

that 2D g-C3N4 sheets are grown in situ on the surface of onedimensional (1D) oxidized CNTs to construct a cross-linked 3D conductive network and that the g-C3N4 sheets and CNTs in the g-C3N4-CNT composite are chemically bonded. In this hybrid system, the CNTs can provide an efficient electron transport path; the latter acts as a loading platform by trapping sulfur particles on its nitrogen-containing functional groups. Therefore, the highly integrated g-C3N4-CNTs with nitrogencontaining functional groups are endowed with a strong LiPS confinement ability to achieve an excellent cycling ability. Density Function Calculations. To verify the abovementioned conclusion, the band structures of pure g-C3N4 and the hybrid g-C3N4-CNT composite were examined by firstprinciples calculations based on the density functional theory (DFT). We studied the energy bands and density of states of a 2 × 2 supercell layer of plain g-C3N4; the results are shown in Figure S4. The energy band structure diagram shows that pure g-C3N4 has a band gap of 2.73 eV, which is consistent with published data in the literature, indicating apparent semiconducting behavior and a satisfactory calculation outcome.37 Thus, the hybrid g-C3N4-CNT composite exhibits a very narrow band gap of 0.42 eV, which is indicative of a high conductivity feature, as shown in Figure 4a. The downward shift of the conduction band and upward shift of the valence band of the hybrid g-C3N4-CNTs relative to g-C3N4 indicate significant electron transfer between the 3D conductive networks, which can enhance electron mobility.38,39 Figure 4b shows the optimized HOMO − n and LUMO + n orbitals of the hybrid g-C3N4-CNTs; the increased dispersion of the contour distribution of the HOMO and LUMO caused by N

bonds. As shown in Figure 2i on the right, a larger delocalized π-conjugated system linkage between the two parts can direct electron transfer from the CNTs to g-C3N4, resulting in enhanced conductivity of the integrated g-C3N4-CNTs. This scenario provides effective electron transfer channels for charge and discharge under a large current. To further confirm that the g-C3N4 and CNTs are interconnected with each other via chemical bonds in the hybrid structure, TEM images and EELS mappings of the localized regions of the pure g-C3N4, hybrid g-C3N4-CNTs and S/CN-CNTs are shown in Figure 3. The TEM image of gC3N4 in Figure 3a shows that pure g-C3N4 has a uniform membrane structure with abundant wrinkles. In contrast, Figure 3b,c clearly demonstrates that CNTs interpenetrate through the sheet structure of g-C3N4 and interweave the gC3N4 into an integrated hybrid g-C3N4-CNT network. In comparison, the C, N, O, and color-mix mappings of the designated domain (marked with red boxes) clearly illustrate the uniform dispersion of C and N atoms throughout the whole hybrid g-C3N4-CNT structures. This result indicates that the g-C3N4 sheets are grown in situ on the surfaces of CNTs assisted by calcination. The inner structure of the hybrid g-C3N4-CNTs is depicted by the schematic structural images in Figure 3b. The EELS mapping of the local region of the S/CNCNTs (Figure 3c) clearly indicates the presence of sulfur in the hybrid g-C3N4-CNT composite. Additionally, the C, N, and S mappings are completely in accordance with the shapes of the samples according to the color-mix mapping, indicating that sulfur is uniformly deposited on the surfaces of the hybrid g-C3N4-CNTs. Based on the above results, we can conclude 15929

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

Figure 5. (a) TGA curves of the S/CN-CNTs and S/SP/CNTs under a N2 atmosphere with a heating rate of 10 °C min−1. Charge−discharge profiles of the (b) S/CN-CNTs and (c) S/SP/CNTs from 0.1 to 2.0 C. (d) Rate capabilities of the S/CN-CNTs and S/SP/CNTs from 0.1 to 2.0 C. Cycling performances of the S/CN-CNTs and S/SP/CNTs at (e) 0.5 C and (f) 1 C. EIS spectra of the (g) S/CN-CNT and (h) S/SP/CNT cells at various cycling statuses. The sulfur mass loading of the cathode electrodes is 1 mg cm−2.

These results are consistent with the DFT calculation results, as shown in Figure S5. Moreover, the adsorptions of Li2Sx (x = 1, 2, and 4) onto the hybrid g-C3N4-CNT composites were also studied by DFT calculations to reveal the superiority of the g-C3N4-CNTs in restricting LiPS diffusion. The adsorption energy Ea of the

doping will enhance electron transfer and favorably affect the band gap of the hybrid g-C3N4-CNT composite.40 In addition, the electronic conductivities of the pure g-C3N4, hybrid gC3N4-CNT composite, and CNTs are fitted to be 8.62 × 10−10, 4.86 × 10−4, and 1.06 × 10−3 S cm−2, respectively. 15930

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

964.4, 911.5, and 846.6 mAh g−1 at rates of 0.1, 0.2, 0.5, 1, and 2 C, respectively. In contrast, lower discharge capacities of 1161.7, 882.9, 735.1, 629.7, and 541.0 mAh g−1 (based on the mass of sulfur) are achieved under the same rate. Both cells were tested under various rates to evaluate the rate capabilities; the results are shown in Figure 5d. The S/CN-CNT cells exhibit higher discharge capacities than the S/SP/CNT cells, especially under a high rate, indicating that the high sulfur utilization of the S/CN-CNT cathode material originates from the 3D conductive network of the hybrid g-C3N4-CNT host. The long-term cycling performance of the two samples was evaluated at 0.5 C (Figure 5e). The S/CN-CNTs deliver an initial capacity of 1068.2 mAh g−1, and the capacity retention is 52.4% after 500 cycles, indicating good conductivity and a strong adsorption capacity for the LiPSs offered by g-C3N4. In contrast, without the help of g-C3N4, the S/SP/CNTs show an initial capacity of 887.9 mAh g−1 with a capacity retention of 39.1%, indicating inadequate adsorption of LiPSs. However, at a higher rate of 1 C (Figure 5f), they exhibit more distinct differences. The initial reversible capacity of the S/CN-CNTs is 1023.6 mAh g−1 and retains a capacity of 583.7 mAh g−1 after 500 cycles. The corresponding capacity retention is 57%, representing a minimal capacity decay of 0.08% per cycle. The S/SP/CNTs only deliver an initial capacity of 744.8 mAh g−1 with a capacity retention of 16.1% after 500 cycles at 1 C. Notably, even when the sulfur mass loading of the S/CN-CNT electrode is up to 2 mg cm−2, as shown in Figure S7a, a reversible capacity of 679.7 mAh g−1 can be reached after 500 cycles. As the sulfur loading increases up to 5 mg cm−2, the cycling performance of the S/CN-CNTs at 1 C is displayed in Figure S7b where the cell shows a very stable cycling curve. The high sulfur loading electrode delivers an initial capacity of 758.9 mAh g−1 with a capacity retention of 77.1% after 200 cycles at 1 C. In contrast, the cycling curve of S/SP/CNTs with 5 mg cm−2 sulfur loading shows a sharp decline, indicating the poor cycle performance of S/SP/CNTs under high sulfur loading. Additionally, the corresponding physical mixture of pure g-C3N4 and CNTs was also used for comparison after loading with sulfur using a melting method. As shown in Figure S8, the S/CN/CNT mixture has better cycling stability than the S/SP/CNTs but lower stability than the hybrid S/CNCNTs. Therefore, the excellent electrochemical performance can be attributed to the unique structure of the hybrid g-C3N4CNTs, which possesses a high conductivity property, and the strong chemical interaction with polysulfides serves a dual function of electron transport and immobilization of polysulfides. Notably, the comparison of cycling performance with various g-C3N4- or nitrogen-doped carbon−sulfur composites reported in the literature indicates that the S/ CN-CNT composite cathode can deliver excellent electrochemical performance and cycling stability, as presented in Table S1. The Nyquist plots of the S/CN-CNT and S/SP/CNT electrodes at the zero cycle and after 10 and 500 charge− discharge cycles are shown in Figure 5g,h, respectively. Before the EIS measurements, all cells after 10 and 500 cycles were discharged to 2.1 V to reach an identical status. The equivalent electrical circuit in Figure 5g was proposed to analyze the impedance spectra, and the corresponding fitting data are given in Table 1. In this circuit, Re, Rf, Rct, and Zw indicate the electrolyte resistance, solid electrolyte interface (SEI) layer resistance, charge transfer resistance, and Warburg resistance, respectively. The constant phase element (CPE) is related to

Li2Sx groups on the substrate surface is calculated using the equation Ea = E*Li2Sx − (E* + E*Li2Sx) (x = 1, 2, and 4), where *Li2Sx and * denote the adsorption of Li2Sx groups onto the substrate and the bare substrate, respectively, and E*Li2Sx denotes the energy of the Li2Sx groups. The calculated Ea of the different adsorption systems is shown in Figure 4d. The end-product Li 2S was used to simulate LiPSs as the representative polysulfide, and the fully relaxed binding geometry of the LiPSs in the g-C3N4-CNTs is shown in Figure 4c. The theoretical calculation results show that significant Li−N bonding exist between Li+ and pyridinic-like N, resulting in strong interactions for Li2S with an adsorption energy of −3.75 eV. The calculated Ea values of the Li2Sx (x = 2 and 4) on the g-C3N4-CNT surface are −3.67 and −2.55 eV, respectively. The more negative Ea indicates a stronger adsorption capability.41,42 The computed result demonstrates that, as a sulfur host substrate, the hybrid g-C3N4-CNT composite can inhibit the dissolution of LiPSs more effectively. The chemical absorption test of LiPSs and the BET measurements support the DFT calculation results, as shown in Figure S6. The same amounts of g-C3N4, CNT and g-C3N4CNT powders were immersed in Li2S4 solution with the DOL/ DME solvent for comparison; the visual discrimination is shown in Figure S6c. After adding the g-C3N4 powders, the yellow Li2S4 solution completely changed to colorless after approximately 1 h. The color of the Li2S4 solution with the gC3N4-CNTs becomes nearly colorless but not completely transparent, indicating the existence of leftover Li2S4 species in the solution. However, the color of the CNTs only lightened slightly, indicating a poor interaction between Li2S4 and the CNTs. The abovementioned results further confirm the advanced intrinsic adsorption capability of g-C3N4 for LiPSs although the CNTs have a higher specific surface area than that of g-C3N4. Electrochemical Properties. A series of contrast experiments, such as rate capabilities, cyclic stabilities, and kinetic processes of the S/CN-CNT and S/SP/CNT electrodes, are presented in Figure 5. These experiments are used to evaluate the effect of the hybrid g-C3N4-CNT host on improvement of the electrochemical performance. As shown in Figure 5a, the weight ratio of sulfur in the S/CN-CNTs and S/SP/CNTs are both 80 wt % based on the TGA results, which represent two weight loss peaks with different weight loss rates in the DSC curve. The initial small amount of weight loss that occurs from 100 to 130 °C can be attributed to evaporation of the moisture adsorbed on the surface. The second larger weight loss peak process from 240 to 380 °C is caused by evaporation of sulfur. Furthermore, a unique weight loss for S/CN-CNTs in the temperature range of 620−680 °C is related to the thermal decomposition of g-C3N4. By comparing the electrochemical measurement results, we found that the S/CN-CNTs exhibited a superior electrochemical performance compared to that of the S/SP/CNTs. Figure 5b,c shows the charge−discharge profiles of the S/CNCNT and S/SP/CNT cells at variable rates, respectively. These profiles exhibit typical charge−discharge curves with two discharge plateaus at approximately 2.4 and 2.1 V, which are ascribed to the formation of soluble long-chain polysulfides with a series of S8 → S82− → S62− (Li2Sx, 6 ≤ x ≤ 8) reduction reactions and the generation of insoluble short-chain polysulfides with a series of S62− → S42− → S22− (Li2Sx, 2 ≤ x ≤ 4) reduction reactions, respectively. The S/CN-CNT cathode delivers high discharge capacities of 1351.2, 1064.8, 15931

DOI: 10.1021/acs.jpcc.9b02356 J. Phys. Chem. C 2019, 123, 15924−15934

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The Journal of Physical Chemistry C

shuttle effect of intermediate polysulfides, poor conductivity, and low sulfur loading of lithium−sulfur batteries.

Table 1. Electrode Impedance Parameters Obtained from Equivalent Circuit Fitting of Experimental Data from the S/ CN-CNT and S/SP/CNT Cells electrodes

condition

Re (Ω)

Rf (Ω)

Rct (Ω)

S/CN-CNTs

original after 10 cycles after 500 cycles original after 10 cycles after 500 cycles

5.15 6.20 7.86 5.07 6.66 3.21

79.83 60.83 48.99 89.02 48.84 83.47

22.50 22.09 49.13 2.22 131.70 182.60

S/SP/CNTs

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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/acs.jpcc.9b02356. SEM measurements, FTIR spectrum, core-level EELS spectra, energy band structure diagrams, density of states, I−V measurements, N2 adsorption/desorption isotherms, pore size distributions, results of long-term cycle performance, photos of cycled separators, and CV profiles (PDF) (PDF)

the constant phase that is adopted to compensate for the capacitance effect on the elements.43 Notably, both Rf and Rct of the S/CN-CNTs are lower than those of the S/SP/CNTs, which suggests easier exchange of electrons/charges between the g-C3N4-CNTs and LiPSs. According to the fitted results, the Re of both cells maintains a stable value, whereas the Rf decreases to different degrees, indicating that an immature SEI layer forms on the electrode solid−liquid interface before activation cycling. Notably, the high Rf value of the S/SP/CNT electrode returns after 500 cycles with formation of the dense outer SEI layer on the interface, which is not favorable for lithium-ion transport during charging/discharging.44 Additionally, the charge transfer resistance of the S/CN-CNT electrode becomes more than 2.7 times higher than that of the S/SP/ CNT electrode after 500 cycles, demonstrating higher utilization of the active material, enhanced charge/ion transfer, and a lower kinetic barrier in the electrochemical redox of the LiPSs. These results are definitely ascribed to the high conductivity offered by the hybrid g-C3N4-CNT host.45 Disassembling and observing the yellow LiPS areas on the separator of these cycled cells can reflect the dissolution of the LiPSs in the cathode; these results are shown in Figure S9. The yellow area on the separator of the S/SP/CNT cell is more distinct and much larger than that of the S/CN-CNT cell, indicating that a significant amount of LiPSs has been dissolved from the S/SP/CNT cathodes during cycling while maintaining good effective restriction of the LiPSs in the S/ CN-CNTs. The above conclusions are consistent with the results interpreted from the CV measurements shown in Figure S10.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (Y.W.). ORCID

Jianguo Liu: 0000-0002-9229-4936 Yiqing Wang: 0000-0002-5626-8589 Author Contributions ∥

W.H. and X.H. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Graduate Innovation Foundation of Nanjing University (2017ZDL05), the Key Science and Technology project for Zhejiang Province (2017C01035). The authors also thank the support of the Jiangsu Province Natural Science Foundation (BK20171247 and BK20171245), the National Scientific Instrument Develop Major Project of National Natural Science Foundation of China (51627810), and the Fundamental Research Funds for the Central Universities, China.

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CONCLUSIONS In summary, a novel 3D hybrid g-C3N4-CNT composite was synthesized using a self-assembly-assisted method as the sulfur loading host material. The S/CN-CNT cathode material with a high sulfur loading of 80 wt % can be prepared via a subsequent melt-diffusion method. The S/CN-CNT cathode can deliver a high discharge capacity, excellent rate performance, and cycling stability that is much superior compared to that of the control samples consisting of simple sulfur filled without g-C3N4 or CNTs. The outstanding electrochemical performance can be attributed to the unique structure of the hybrid g-C3N4-CNT host. This 3D hybrid structure can effectively improve the conductivity, reduce the interface resistance, and facilitate electron transport all over the electrode with high sulfur loading. Meanwhile, the inherent N-containing groups on the surface of the hybrid host composite can suppress the shuttle effect by strong absorption with polysulfides. Moreover, the method proposed in this paper is a simple process with a low cost and easy large-scale production that offers an effective technique to solve the

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