Monoclinic ZIF-8 Nanosheet-Derived 2D Carbon Nanosheets as

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Monoclinic ZIF-8 Nanosheets Derived 2D Carbon Nanosheets as Sulfur Immobilizer for High Performance Lithium Sulfur Batteries Yi Jiang, Haiqiang Liu, Xinghua Tan, Limin Guo, Jiangtao Zhang, Shengnan Liu, Yanjun Guo, Juan Zhang, Hanfu Wang, and Weiguo Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04432 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017

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ACS Applied Materials & Interfaces

Monoclinic ZIF-8 Nanosheets Derived 2D Carbon Nanosheets as Sulfur Immobilizer for High Performance Lithium Sulfur Batteries

Yi Jiang†‡, Haiqiang Liu†‡, Xinghua Tan†‡, Limin Guo†‡, Jiangtao Zhang†, Shengnan Liu†‡, Yanjun Guo†, Juan Zhang *†, Hanfu Wang *†, and Weiguo Chu*†‡

† CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China. Fax: +86-10-62656765,

E-mail: [email protected], E-mail: [email protected],E-mail: [email protected],

‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China

1 ACS Paragon Plus Environment


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ABSTRACT

2D hierarchically porous carbon (2D-HPC) nanosheets with unique advantages are highly desired as host materials for lithium sulfur (Li-S) batteries and other energy storage devices. Herein, we propose a self-template and organic solvent-free approach to synthesize nanosheets of monoclinic ZIF-8 at room temperature from which 2D-HPC nanosheets (ZIF-8 Nanosheets Carbon denoted as ZIF-8-NS-C) are derived to be an efficient sulfur immobilizer for Li-S batteries for the first time. The anisotropic nanosheets are believed to relate to the symmetry of the monoclinic structure. The 2D ZIF-8-NS-C nanosheets with embedded hierarchical pores construct an effective conductive network through “plane-to-plane” modes to endow superior electron transfer and fast electrochemical kinetics. Moreover, the nitrogen rich feature of ZIF-8-NS-C can increase the affinity/interaction of carbon host with lithium polysulfides, favoring the cyclic performance. The sulfur/ZIF-8-NS-C (S/ZIF-8-NS-C) cathode shows a superior rate capability with high capacities of 1226 mA h g-1 at 0.2 C and 785 mA h g-1 at 2 C, and a sustainable cycling stability with a capacity attenuation of 0.12 % per cycle at 0.5 C for 300 cycles. The approach proposed here pioneers the controllable design of MOF-based structures to inspire the exploration of more variable MOF-derived porous materials for energy storage applications.

KEYWORDS

Green and template-free synthesis, monoclinic ZIF-8 nanosheets, two dimensional hierarchically carbon, superior electron conductivity, lithium-sulfur batteries 2 ACS Paragon Plus Environment

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1. Introduction High density energy storage devices are indispensable for powering ever increasing portable electronics and electric vehicles, and therefore receiving more and more attention.1 As promising next-generation electrochemical energy storage devices Lithium-sulfur (Li-S) batteries have been extensively explored because of high energy density (2600 W h kg-1), and natural abundance, low cost and non-toxicity of sulfur.2, 3 However, poor electrical conductivity of sulfur and Li2S, severe volume variation during cycling and the shuttle effect caused by the dissolution of reaction intermediate polysulfides (PS) are still obstacles to their practical applications.3, 4 Given that host materials for sulfur immobilization have a crucial role to play in addressing the above issues,5 tremendous efforts have been focused on their multi-scale/multi-functional designs.6 Among them, two-dimensional porous carbon (2D-C) materials are quite promising owing to their unique nanostructures: 1) 2D nanostructures construct a conductive network facilitating transport of electron;7 2) abundant pores can host a large amount of sulfur to guarantee a high sulfur loading and thus a high energy density of Li-S batteries.8, 9 3) porous 2D nanosheets can guarantee the availability of electrolyte and the close contact between carbon host and sulfur through a “plane-to-point” mode, favoring rapid transfer of charge; 4) N-doping is normally employed to not only enhance the electrical conductivity of carbon host, but also immobilize sulfur effectively to suppress the dissolution of PS due to the strong interaction between sulfur and nitrogen.10 It is known that two prominent and popular approaches such as plasma assisted chemical vapor deposition (PACVD) and 3 ACS Paragon Plus Environment


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template method (i.e., AAO, NaCl, MgO, biomass materials) are developed to prepare 2D-C or N-doped 2D-C.11-15

However, these processes are normally complex, quite

costly and time-consuming, which is not suitable for large-scale production. Furthermore, porous carbons thus obtained usually have narrow pore size distributions and irregular morphologies which are not favorable for rapid Li+ transport.11-13 Hence, it is imperative to develop novel and facile routes for the synthesis of highly ordered 2D carbon materials.

Metal organic frames (MOFs), a unique branch in the porous materials family, have recently gained particular attention as precursors to construct porous carbon nanostructures, inspired by their periodic and well-defined framework structures, high specific surface areas, controllable pore structures, and multiple heteroatom dopings (i.e., N and P).8, 16-20 As typical MOFs, zeolitic imidazolate frameworks (ZIFs-8) with multiple polymorphs have been intensively investigated because of their highly uniform porous structures and regular morphologies.21 However, to date, ZIFs-8 with both a monoclinic structure of low symmetry and a cubic SOD structure of high symmetry are generally synthesized with particle-like isotropic morphologies.21 Moreover, these primary particles are prone to aggregate into secondary particles with far larger sizes, reducing their specific surface areas significantly. Recently, graphene and layered double hydroxides (i.e., [Mg(OH)2]) has been reported as the structure oriented templates to fabricate 2D nanosheets-like ZIF-8 with large surface areas.22 However, further treatments are needed to remove the electrochemically inactive metal species (i.e., Co, Fe, Mg), which complicates the synthetic procedures and 4 ACS Paragon Plus Environment

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increases the cost. Additionally, these conventional synthetic approaches normally need environmentally hazardous and toxic solvents (i.e., methanol or DMF),21,

22

further limiting the scale-up and commercialization of ZIFs-8-derived 2D-C. Therefore, template-free, green, low cost, up-scalable and facile protocols for the synthesis of ZIFs-8-derived 2D-C host materials are quite significant from both the scientific and technological standpoints.

Herein, we propose a novel and facile method of preparing 2D hierarchical porous carbon (2D-HPC) nanosheets (labeled by ZIF-8-NS-C) by directly carbonizing monoclinic ZIF-8 nanosheets which are synthesized using a template free approach with water instead of organic solvents. 2D ZIF-8-NS-C networks serve as an efficient host to encapsulate sulfur owing to their large specific surface areas, hierarchical pore structures and sufficiently large pore volumes (intertwined micro-, meso-, and macro-pores). Besides this, nitrogen is also introduced into the carbon scaffold to enhance the electrical conductivity and immobilize sulfur, which helps suppress the shuttling effect of lithium polysulfide (LiPS). Distinct from the conventional approaches, we here demonstrate a template- and organic solvents-free protocol for the synthesis of ZIF-8 derived 2D-HPC, which is facile, environmentally benign, low-cost, and up scalable. S/ZIF-8-NS-C cathodes fabricated with this novel recipe exhibit high specific capacity, superior high-rate performance, and long cycle life.

2. Experimental Section 2.1 Materials preparation 5 ACS Paragon Plus Environment


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2.1.1 Synthesis of ZIF-8-Nanosheets and ZIF-8-Particles 0.33 g Zn(NO3)2•6H2O and 0.985 g 2-methylimidazole (Aladdin) were dissolved in two beakers of 90 mL deionized water to form solutions, which were then mixed and stirred for 24 h at room temperature (RT), accompanied by the formation of white deposits. The white deposits were washed, dried and collected, labeled as ZIF-8-Nanosheets. ZIF-8-Particles were prepared using the recipe reported with modified.22 In detail, 2.6064 g Zn (NO3)2•6H2O was first dissolved into 87 mL methanol to form a clear solution, which was subsequently poured into another 87 mL methanol solution with dissolved 2-methylimidazole of 5.7375 g under vigorous stirring for 24 h at RT. Likewise, the white precipitates were gradually produced under stirring, which were then washed, dried and collected, denoted as ZIF-8-Particles. 2.1.2. Carbonization and sulfur impregnation The ZIF-8-Nanosheets (ZIF-8-Particles) were transferred into a ceramic boat and then heated up to 920 °C at a rate of 2 °C min-1 and kept for 2 h under Ar flow in a tube furnace. After cooling down to RT, the black powders were mixed with KOH in a mass ratio of 1:4. The mixture was heated at 700 °C at a rate of 10 °C min-1 and kept for 2 h, and then cooled down under Ar atmosphere. Then, the black product was washed with water and methanol several times, and dried at 60 °C for 24 h to obtain ZIF-8-NS-C (ZIF-8-P-C). Sulfur impregnation was carried out by thoroughly mixing sulfur and ZIF-8-NS-C (ZIF-8-P-C) with a mass ratio of 7:3. The mixture was treated at 155 °C for 20 h and then cooled down under Ar flow. The resultant products were denoted as S/ZIF-8-NS-C (S/ZIF-8-P-C). 6 ACS Paragon Plus Environment

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2.2 Structural characterization The morphology, structure and elemental distribution was characterized using field emission scanning electron microscope (FESEM, NOVA Nano SEM 430, FEI Company, USA) and transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN, FEI company, USA). Powder X-ray diffraction data (XRD) for crystal structure characterization were recorded in an angular range of 10-70° on a X-ray diffractometer (XRD, Rigaku - D/Max - 2500, Rigaku Corp. Japan) with a Cu-Kα radiation at 40 kV and 200 mA. Pore structure was analyzed using N2 adsorption and desorption isotherms performed on a Brunauer-Emmett-Teller surface area analyzer (BET, ASPS2020, Micromeritics Instrument Corporation, USA). Thermo gravimetric analyzer (TGA, Pyris Diamond Thermo gravimetric/Differential Thermal Analyzer， Perkin Elmer Instruments，USA) was used to determine the sulfur content in the S/C composites. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB250Xi (Thermo Fisher Scientific, UK) spectrometer. Infrared spectra were recorded on an FT-IR spectrometer (Spectrum One, Perkin Elmer Instruments Co. Ltd., USA). Raman spectroscopy was conducted on Renishaw inVia plus (Renishaw, UK) using laser excitation at 532 nm. The electronic conductivities of powder samples were derived using a Keithley 2400 digital multimeter based on the four probe method in which powder porous carbons were pressed into disk under 4 MPa. 2.3 Electrochemical measurements



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Electrochemical measurements were made using CR2016 coin cells assembled in an Ar-filled glove box. The cathodes were prepared by mixing S/C composites, acetylene black, and polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) with a weight ratio of 8:1:1. The slurry thus obtained was uniformly pasted onto aluminum foil, and dried at 60 °C for 12 h in vacuum. The areal sulfur loadings of electrode are around 2mg cm-2 with the thicknesses of about 35 µm. Celgard 2325 films were used as the separator between cathode and Lithium metal as anode. The electrolyte was 1.0 M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1, v/v) with 1 wt. % LiNO3 additive. The electrolyte/sulfur ratio (µL mg−1) is around 10: 1. Galvanostatic charge-discharge tests were performed in the range of 1.7–2.8 V at different rates by using a Neware battery test instrument (Neware Company, China). Both cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS) were recorded on an electrochemical workstation (CHI660D, Shanghai Chenghua Company, China). 3. Results and Discussion

Based on the design of cathodes for Li-S batteries shown in Scheme 1, a template- and organic solvents- free approach is proposed to synthesize ZIF-8 nanosheets then by carbonizing ZIF-8-Nanosheets (ZIF-8-NS) networks which serve as a host material for sulfur encapsulation. With this simple recipe, ZIF-8-NS networks

are

synthesized

simply

by

coordinating

zinc

ion

(Zn2+)

with

2-methylimidazole in an aqueous solution just at room temperature (RT). For comparison, particle-like ZIF-8-Particles (ZIF-8-P) is also obtained using the 8 ACS Paragon Plus Environment

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conventional recipe with methanol involved. To identify their crystal structures, XRD profiles are shown in Figure 1 to reveal that ZIF-8-P belongs to a cubic structure whereas ZIF-8-NS is indexed as a monoclinic structure.23 The monoclinic and cubic SOD structures are the polymorphs of ZIF-8.23 Based on their structural models shown in Figure 1, Rietveld refinements are performed on their XRD data, respectively. The experimental and simulated data for both samples are found to agree well, as shown in Figure 1. The lattice constants were derived to be a = 17.5065 Å, b = 7.7140 Å, c = 14.7689 Å, and β = 107.583° for ZIF-8-NS, and a = 17.0294 Å for ZIF-8-P, respectively, close to the values reported.23 Their infrared spectra are quite similar, as shown in Figure S1, which are consistent with those for monoclinic ZIF-8 and cubic SOD ZIF-8.23 The morphologies of ZIF-8-NS and ZIF-8-P are shown in Figure S2 in which ZIF-8-NS is characterized by typical nanosheets with thicknesses of 50~100 nm, and ZIF-8-P possesses a cuboid - like morphology with diameters of around 100 nm. Clearly, use of water and methanol as solvents has been found to result in different crystal structures of ZIF-8 with different morphologies, which could be ascribed to their different polarity and properties. To investigate the effects of solvents on the crystal structure and morphology of ZIF-8, we vary the volume ratio of methanol to water under the present experimental conditions. It is found that when the value of Vm/Vw is below 2, almost single phase cubic ZIF-8 nanoparticles are obtained, accompanied by an increase in size, as shown in Figures S3 and S4. Decrease of Vm/Vw (1:4) leads to the formation of an unidentified phase with an anisotropic 9 ACS Paragon Plus Environment


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morphology which is also observed in Ref. 22, as shown in Figures S3 and S4. When Vm/Vw reaches 1:9 the products with nanosheets crystallizes predominantly in a monoclinic structure with minute unidentified phase formed for 1:4. The unidentified phase shows an infrared spectrum very different from those of the cubic and monoclinic phases, as shown in Figure S5. It is therefore reasonable to conclude that the volume ratio of methanol to water plays a crucial role in controlling both the crystal structure and morphology of products. Monoclinic ZIF-8 is considered to be more stable than cubic SOD ZIF-8,23 and thus water as solvent leads to the formation of the stable monoclinic phase. The anisotropic nanosheets and isotropic cubes are believed to be related to the symmetries of the monoclinic and cubic structures, respectively. It is known above that solvent plays an important role not only in morphology but also chemistry of products, as revealed by the XRD data (Figures 1 and S4) and SEM observations (Figure S2). Solvents with different polarities and natures can change the morphologies from 0D to 2D.

5-6

Different capabilities of solvents

coordinating with metals may influence the configurations of functional groups significantly.

7

Solvents with different polarities and natures can change the

morphologies from 0D to 2D.24 Different capabilities of solvents coordinating with metals may influence the configurations of functional groups significantly. Water has far stronger affinity to coordinate with the metal centers compared to methanol.25 Furthermore, water has much more asymmetric molecular structure (noncollinear structure), which is in sharp contrast with methanol (nearly regular tetrahedron 10 ACS Paragon Plus Environment

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structure). From the FTIR spectra shown in Figure S1, almost the same surface functional groups were observed for the monoclinic ZIF nanosheets and cubic ZIF nanoparticles. Therefore, it is reasonable to think that the asymmetric structure of water molecule may more easily trigger the functional groups or ions to form the crystal structure with a lower symmetry such as monoclinic, whereas the much more symmetric structure of methanol molecule may more readily result in the formation of the more symmetric crystal structure such as cubic. Therefore, the polarity and the molecular structure of water and methanol may combine to play a critical role in the morphology and type of products. The exact underlying mechanism needs to be unveiled in future. ZIF-8-NS and ZIF-8-P synthesized above are chemically activated by KOH, followed by carbonizing to obtain ZIF-8-NS-C and ZIF-8-P-C, respectively. The pyrolysis process of ZIF-8-NS and ZIF-8-P is explored by thermogravimetric analysis (TGA). As shown in Figure S6, the weight loss during 350-550 ℃ is attributed to the evaporation of solvents remaining in the pores of ZIF-8-NS and ZIF-8-P. The weight loss at about 600 °C may result from the gradual decomposition and carbonization of the organic ligands. With the increased temperature the weight loss continued to increase which may arise from the evaporation and escape of the substances such as carbon, nitrogen and Zn atoms or clusters distributed in the samples. At 850°C there is ~18% Zn and 4.2% N remaining in ZIF-8-NS whereas the percentages of Zn and N become 0% and 3.5% at 950°C, respectively, as outlined in Table S1. Therefore, to remove all the Zn and keep a significant percentage of nitrogen in the sample and 11 ACS Paragon Plus Environment


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promote the graphitization of carbon as possible the temperature as high as much such as 920°C was chosen as the carbonization temperature to obtain ZIF-8-NS-C and ZIF-8-P-C. Sulfur is impregnated by using the facile melting diffusion method at 155 °C. SEM, TEM images and elemental mappings of sulfur impregnated S/ZIF-8-NS-C and S/ZIF-8-P-C composites are shown in Figures 2 and S7, respectively. Both samples show no sizable changes before and after impregnation, and no aggregated sulfur particles are observed, which reveal the effective impregnation of sulfur into the pores of the carbon hosts.26 Furthermore, the elemental mappings given by EDS shown in Figures 2 and S7 also demonstrate the uniform distribution of sulfur, carbon, and nitrogen in the composites. The N2 adsorption-desorption isotherms and pore size distributions of ZIF-8-NS-C, ZIF-8-P-C, S/ZIF-8-NS-C and S/ZIF-8-P-C are shown in Figure 3a-b. The isotherms of ZIF-8-NS-C are characterized by type I and IV with a typical mesopore hysteresis loop, revealing the coexistence of micro- and mesopores.14, 27 ZIF-8-P-C and ZIF-8-NS-C have the distributions of mesopores peaked at 2.4 and 2.8 nm, respectively (Figure 3b). Besides this, some stacked macropores around 100 nm only present in ZIF-8-NS-C samples originate from the siege of aggregated nanosheets. The unique pore structures from micro- to macropores facilitate the rapid transport of Li+, implying the fast electrochemical kinetics for Li-S batteries. ZIF-8-NS-C has a BET surface area of 3052 m2 g-1 and a pore volume of 2.35 cm3 g-1, far higher than the corresponding values of 2519 m2 g-1 and 1.90 cm3 g-1 for ZIF-8-P-C. The maximum theoretical percentages of impregnated sulfur are derived 12 ACS Paragon Plus Environment

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to be 73% and 69% for ZIF-8-NS-C and ZIF-8-P-C, respectively (SI). The real impregnated sulfur percentages for ZIF-8-NS-C and ZIF-8-P-C are determined to be about 70 wt. % by a carbon sulfur analyzer, which is shown in Figure 3c. Therefore, there are still about 4% pores remained in S/ZIF-8-NS-C composites, which is expected to facilitate the infiltration of electrolyte and accommodate the volume expansion during cycling. However, such is not the case for S-ZIF-8-P-C composites, and no pores remained. To explore the crucial role of KOH activation, we also prepared the carbon hosts without KOH activation, which are denoted as ZIF-8-NS-C (no KOH) and ZIF-8-P-C (no KOH). As shown in Figure S8a，both ZIF-8-NS-C (no KOH) and ZIF-8-P-C (no KOH) show the typical type IV adsorportion-desorption isotherms. The pore volume distributions of ZIF-8-P-C (no KOH) in Figure S8b reveal the presence of fewer mesopores and macropores compared to ZIF-8-NS-C (no KOH). The BET surface areas of ZIF-8-NS-C (no KOH) and ZIF-8-P-C (no KOH) are 548 and 128 m2 g−1, respectively, and the corresponding total pore volumes are 0.22 and 0.08 cm3 g-1 (Table S2), which are much lower than 2.37 and 1.90 cm3 g-1 after KOH activation. Therefore, two hosts without KOH activation can’t provide enough active sites for electrochemical reactions and sufficient space for sulfur species accommodation, and thus the worse electrochemical performances would be expected (as shown in the following). Figure 3d gives XRD profiles of crystalline sulfur, ZIF-8-NS-C, S/ZIF-8-NS-C, ZIF-8-P-C and S/ZIF-8-P-C. Two broad diffraction peaks located at around 25.0 ° and 44.0 ° belong to the (002) and (100) planes of graphite for ZIF-8-NS-C and ZIF-8-P-C, respectively, suggesting the 13 ACS Paragon Plus Environment


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enhanced graphitization of carbon. After sulfur impregnation, no diffraction peaks of crystalline sulfur are observed, revealing well dispersed amorphous sulfur.28 Raman spectroscopy (Figure S9) is used to further investigate the structural features of ZIF-8-NS-C and ZIF-8-P-C. Two peaks are observed at around 1590 and 1350 cm-1. The peak at 1590 cm-1 (G band) is assigned to the E2g mode from graphitic carbon, and the peak at 1350 cm-1 (D band) corresponds to the A1g mode from disordered carbon.29 The intensity ratio of the D to G band (ID/IG) is normally used to estimate the graphitization degree of carbon materials. The ID/IG value for ZIF-8-NS-C is a little higher than that for ZIF-8-P-C, implying a slightly higher degree of disorder. This could be the result of the formation of oxygen-contained functional groups upon KOH activation for ZIF-8-NS-C, which is believed to show stronger chemical adsorption to polysulfides, and hence benefits the cyclic stability.28,

30

Moreover,

according to the XPS results as shown in Table 1 (the percentages of nitrogen in ZIF-8-NS-C and ZIF-8-P-C are 3.417 and 1.251 at. %, respectively) and BET results (the BET surface areas: 3052 m2 g-1 for ZIF-8-NS-C and 2519 m2 g-1 for ZIF-8-P-C; pore volume: 2.34cm3 g-1for ZIF-8-NS-C and 1.90 m3 g-1 for ZIF-8-P-C), the higher percentage nitrogen and more pores in the ZIF-8-NS-C resulted in its little higher value of ID/IG.31 XPS is utilized to investigate the chemical states of C, N and O involved in ZIF-8-NS-C and ZIF-8-P-C composites, as shown in Figure 4. The C 1s band (Figure 4a) for ZIF-8-NS-C could be deconvoluted into four components arising from C–C (284.4 eV), C–OH (285.0 eV), C-N-C (286.2 eV), C=O (288.5 eV), and COOH 14 ACS Paragon Plus Environment

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(290.1 eV), respectively.28 The presence of oxygen and nitrogen contained functional groups can potentially provide a higher concentration of active sites for LiPS adsorption.28 For ZIF-8-P-C, the fine C 1s band (Figure 4d) is also composed of almost the identical components with a smaller binding energy (286.0 eV) of the C-N-C component except for the absence of the COOH component. The total nitrogen content of ZIF-8-NS-C (3.417 at. %) is much higher than its counterpart (1.251 at. % for ZIF-8-P-C), in accord with the CHN elemental analysis results (3.5 at. % for ZIF-8-NS-C and 1.1 at. % for ZIF-8-P-C). Furthermore, the N 1s bands for both ZIF-8-NS-C and ZIF-8-P-C are successfully deconvoluted into three different types of piridinic N (397.9 eV), pyrrolic N (399.3 eV), and quaternary N (400.7 eV). The spectra are fitted with Gaussian-Lorentzian functions and a Shirley-type background using the CasaXPS software from which the percentages of three different types of nitrogen involved in two samples were derived to be presented in Table 1 and Figure S10. The notable difference lies in trace pyridinic (0.025 at. %) and quaternary N (0.057 at. %) for ZIF-8-P-C. The pyridinic N and the carbon neighboring to quaternary N would strongly bond with LiPS, which helps alleviate the dissolution of LiPS whereas the pyrrolic N would interact with LiPS more weakly. Therefore, ZIF-8-P-C is believed to have the poorer interaction with LiPS.32, 33 To test the cycling stability of S/ZIF-8-NS-C and S/ZIF-8-P-C cathodes, the galvanostatic discharge–charge experiments are carried out at a C-rate of 0.2 C, which are shown in Figure 5. S/ZIF-8-NS-C delivers an initial discharge capacity of around 1226 mA h g−1, and still exhibits a high capacity of about 800 mA h g-1 after 100 15 ACS Paragon Plus Environment


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cycles, with a capacity retention of 65.2%. In contrast, S/ZIF-8-P-C shows an initial capacity of 1104 mA h g-1 and a capacity of 561 mA h g-1 after 100 cycles. The cycling performance of S/ZIF-8-NS-C at 0.5 C is also displayed in Figure 5b-c, respectively. An obvious activation process is observed upon cycling initially. After 15 cycles, the cathode delivers a reversible discharge capacity of 887 mA h g-1 with a coulumbic efficiency of 99%. After 300 cycles, a capacity of 587 mA h g-1 could still be achieved with a low decay rate of 0.12% per cycle. Figure 5b presents the discharge / charge profiles of S/ZIF-8-NS-C for different cycles. It can be seen that the capacity dropped slightly from the 200th to 300th cycle, implying the excellent cycling stability.

The C-rate performances of both S/ZIF-8-NS-C and S/ZIF-8-P-C electrodes from 0.2 C to 2 C are illustrated in Figure 6. S/ZIF-8-NS-C delivers the capacities of 1162, 1023, 969 and 785 mA h g-1 at 0.2, 0.5, 1 and 2 C, respectively. In contrast, the S/ZIF-8-P-C cathode undergoes a drastic drop of capacity from 1014 to 399 mA h g-1 as the rate increases from 0.1 C to 2 C, indicating the poor kinetics. The excellent rate capability of S/ZIF-8-NS-C can be ascribed to the fast reaction kinetics arising from its unique hierarchically porous structures of carbon host, the effective conducting nanosheet 3D networks, and the N-doping .27, 32 In contrast, both S/ZIF-8-NS-C (no KOH) and (S/ZIF-8-P-C (no KOH)) exhibit the far worse electrochemical performances, as shown in Figure S11.


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In order to compare the Li+/e- transport rates in S/ZIF-8-NS-C and S/ZIF-8-P-C, two important parameters of U1 and Q2/Q1 defined in Figure 6e are introduced to assess the interfacial reaction kinetics and the transport kintetics of Li+/e-, respectively (Supporting information).29, 34 The values of U1 and Q2/Q1 are derived to be shown in Figure 6f. The values of U1 for both electrodes are higher than 2.2 V, revealing the superior interfacial reaction kinetics owing to their high specific areas. The values of Q2/Q1 for both electrodes decrease with C-rate. As the C-rates from 0.2 to 1 C, the values of Q2/Q1 for both cathodes are roughly equal. However, for 2 C, the value for S/ZIF-8-NS-C, Q2/Q1=1.54, is far higher than 1.08 for S/ZIF-8-P-C, implying its faster transport kinetics of Li+/e-. Cyclic voltammograms (CVs) of S/ZIF-8-NS-C and S/ZIF-8-P-C cathodes are acquired in Figure 7a-b to further reveal their electrochemical processes during discharge and charge. The CVs of sulfur cathodes here are quite typical, including two redox peaks.35, 36 Two reduction peaks correspond to the formation of polysulfides (Li2Sn, 4~8) and Li2S2 and Li2S, respectively. The reduction peak at around 2.25 V (vs Li/Li+) is attributed to the conversion of sulfur (S8) to long-chain lithium polysulfides. The reduction peak at about 1.95 V (vs Li/Li+) corresponds to the further reduction to insoluble Li2S2/Li2S of long-chain soluble polysulfides.37-40 Two oxidation peaks arise from the delithiation of different polysulfides.36 Close inspection shows that the reduction peak of S/ZIF-8-NS-C located at 2.35 V is slightly higher than that of S/ZIF-8-P-C, which suggests its lower dissociation energy of S - Li bonds, and smaller polarization. 17 ACS Paragon Plus Environment


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In addition, EIS for both S/ZIF-8-NS-C and S/ZIF-8-P-C, and their corresponding real impedances versus ω-1/2 at RT are shown in Figure 7c-d, respectively. The EIS spectra consist of a short line in the low frequency and a semicircle in the high frequency, quite typical for a sulfur cathode.36 An equivalent circuit is given in the inset of Figure 7c, where Rs, Rct, CPE and WO represent the ohmic resistance, charge–transfer resistance, double-layer capacitance and the warburg impedance, respectively. The impedances derived from the equivalent circuit model are outlined in Table 2.36 Rct for S/ZIF-8-NS-C, 66.10 Ω cm-2, is smaller than 76.71 Ω cm-2 for S/ZIF-8-P-C, suggesting its faster interfacial kinetics. From the linear relationship between the real impedance and ω-1/2 the diffusion coefficients of Li+ (D) for S/ZIF-8-NS-C and S/ZIF-8-P-C are derived to be 3.53 × 10-10 cm2 s-1, and 2.45 × 10-10 cm2 s-1, respectively, which reveals a faster transport kinetics of Li+ for S/ZIF-8-NS-C as a result of the shorter path and the faster kinetics of Li+ diffusion.41 To estimate the diffusion activation barrier, EIS of both samples at different temperatures are given in Figure S12. From these profiles the diffusion coefficients at different temperatures are derived to be plotted against 1/T in Figure S13. According to the Arrhenius equation, the diffusion activation energies (Ea) are derived to be 0.295 and 0.818 eV for S/ZIF-8-NS-C and S/ZIF-8-P-C, respectively, as outlined in Table 2. The far lower diffusion activation energy for S/ZIF-8-NS-C suggests the more active electrochemical reaction, which is consistent with the results of Q1/Q2. The electrical conductivity of ZIF-8-NS-C (1.24 S cm-1) is higher than that of ZIF-8-P-C (0.19 S cm-1). The increase in electronic conductivity favors the 18 ACS Paragon Plus Environment

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electrochemical activity of sulfur.42 The electronic conductivities of two samples derived from the four probe method are in agreement with the results obtained from the Nyquist plots (Figure 7). The LiPS adsorptivity of pyridinic N and quaternary N in ZIF-8-NS-C carbon matrix was investigated using XPS and SEM by discharging the S/ZIF-8-NS-C electrode to 1.7 V, which is shown in Figure 8. The N 1s XPS spectra before/after discharge can be deconvoluted into three subpeaks, including pyridinic N (397.89/398.21 eV), pyrrolic N (399.39/399.29 eV), and quaternary N (400.66/399.91 eV). The binding energies of pyridinic N (397.89 eV) in the S/ZIF-8-NS-C composite were shifted progressively up to 398.21 eV after discharging, while that of the quaternary N was shifted from 400.66 eV to 399.91 eV (Figure 8a). The higher binding energy of pyridinic N after discharging shows a stronger interaction with PS in that the electron sufficient pyridinic N with a lone pair of electrons located in edges of graphene plane transfers charge from N to Li+ more easily.30 Such a charge transfer from pyridinic N to PS is also supported by the asymmetry of the Li dominant peak at 55.6 eV accompanied by an extra peak with a higher binding energy after discharge (Figure 8b), as reported in References.32, 33 One should bear in mind that the pair of electrons for pyrrolic N form large pi bonds (π bonds), and therefore pyrrolic N is thought to have no electron donating nature, resulting in the weak enough interaction with PS acceptors.32, 33 For quaternary N, the substitutional central N atom serves as an electron donor to the delocalized π-system, and thus N has a tendency to transfer charge to the carbon framework, as substantiated by a 0.7 eV shift of quaternary N 19 ACS Paragon Plus Environment


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peak toward the lower binding energy (Figure 8b).30 The strong interaction between PS and the ZIF-8-NS-C host is further evidenced by the shift of the C 1s peak at 286.1 eV for the C–N bond to the lower binding energy of 285.7 eV after discharge, which arises from the C–S interaction (Figure 8c). In addition, the S/ZIF-8NS-C cathode upon full discharge after 100 cycles still maintained the relatively smooth surface and the pristine morphology of nanosheets with no significant damage, as shown in the SEM image (Figure S14a). The homogeneous distribution of sulfur into the pores of the ZIF-8-NS-C host was evidenced by the high resolution STEM-EDS elemental mappings (Figure S15). In contrast, the discharged S/ZIF-8-P-C electrode after 100 cycles exhibits a blurry surface on which PS deposits diffused out from the pores of ZIF-8-P-C were observed (Figure S14b).43 These observations imply that the shuttling effect can be effectively suppressed by the nitrogen-doped host through the stronger interaction of the pyridinic N and quaternary N with the discharge products. Therefore, the S/ZIF-8-NS-C shows the superior cycling stability. 4. Conclusion A novel self-template and organic solvent free approach at RT is proposed to successfully synthesize 2D nanosheets of monoclinic ZIF-8 from which 2D hierarchically porous carbon nanosheets are derived to be host materials for advanced Li-S batteries by direct carbonization. The obtained S/ZIF-8-NS-C electrode exhibits high specific capacity, excellent rate performance, and long cycle life, which can be ascribed to its unique structures as designed. 1) The 2D nanosheets act as the “plane 20 ACS Paragon Plus Environment

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to plane” conducting networks, facilitating the transport of electron and ion. 2) Large specific surface area and hierarchical pores (intertwined micro-, meso-, and macro-pores) enable to impregnate a high percentage of sulfur (70 wt. %) to increase the energy density, and help accommodate the volume expansion during cycling. 3) The N doping not only enhances the electrical conductivity but also immobilizes polysulfides effectively. The novel approach proposed here is facile, low-cost, energy-saving, and environmentally benign, which is well suited for mass production. This work demonstrates the potential of using monoclinic ZIF-8 derived carbon nanosheet networks for high energy/power density Li-S batteries, and would undoubtedly promote the wide applications of MOF-derived porous materials with 2D orientations to other energy storage systems. ASSOCIATED CONTENT

Supporting Information Supporting Information Available: The infrared spectra, SEM images, XRD profiles, TEM images, The relationships between lnDLi+ and 1/T, Derivation of theoretical sulfur loading percentages, Assessment of interfacial and diffusion kinetics, Calculation of diffusion coefficients of Li+.

AUTHOR INFORMATION Corresponding Author * Fax: +86-10-62656765, E-mail: [email protected], E-mail: [email protected], E-mail: [email protected] 21 ACS Paragon Plus Environment


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, grant No. XDA09040101.

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FIGURES CAPTIONS Scheme 1. Schematic illustration of the S/ZIF-8-NS-C composites preparation. Figure 1. (a) Experimental, simulated and different XRD profiles of ZIF-8-NS and ZIF-8-P. Open framework structures of (b) cubic ZIF-8, (c) monoclinic ZIF-8. Figure 2. (a) ~ (c) SEM images, (d) ~ (f) TEM images of the S/ZIF-8-NS-C at different magnifications. (g) STEM image of the S/ZIF-8-NS-C and corresponding mappings of (h) carbon, (i) nitrogen and (j) sulfur. Figure 3. (a) Nitrogen adsorption/desorption isotherms and (b) Pore size distribution curves of ZIF-8-NS-C, S/ZIF-8-NS-C, ZIF-8-P-C and S/ZIF-8-P-C composites. (c) TG curves of S/ZIF-8-NS-C and S/ZIF-8-P-C composites. (d) XRD profiles of orthorhombic sulfur, ZIF-8-NS-C, S/ZIF-8-NS-C, ZIF-8-P-C and S/ZIF-8-P-C composites. Figure 4. Fine XPS of C 1s, N 1s and O 1s for ZIF-8-NS-C (a, b, c) and ZIF-8-P-C (d, e, f).

Figure 5. (a) Cycling performance of S/ZIF-8-NS-C and S/ZIF-8-P-C cells at 0.2 C. (b) Charge/discharge curves of cell S/ZIF-8-NS-C at 0.5 C (the data were collected from (c) ). (c) Long-term cycling performance of cell S/ZIF-8-NS-C at 0.5 C.

Figure 6. (a) Rate capability of S/ZIF-8-NS-C and S/ZIF-8-P-C cells. (b) The comparison of their rate performances. Charge/discharge profiles of (c) cell S/ZIF-8-P-C, (d) cell S/ZIF-8-P-C at different rates. (e) Schematic illustration of a 26 ACS Paragon Plus Environment

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typical discharge potential profile. U1 is the onset potential of the upper plateau. Q1 and Q2 are the discharge capacities in stages 1 and 2, respectively. (f) The dependence of Q2/Q1 and U1 on the current rate. (data are collected from c,d).

Figure 7. (a) CV curves of the 6th cycle for S/ZIF-8-NS-C and S/ZIF-8-P-C cells. (b) CV curves of 6 cycles for cell S/ZIF-8-NS-C. (c) EIS of S/ZIF-8-NS-C and S/ZIF-8-P-C cells and the equivalent circuit model. (d) relationship between Z′and square root of frequency (ω-1/2 ) in the low-frequency region. Figure 8. High-resolution XPS spectra of the S/ZIF-8-NS-C electrode after full discharge, (a) N 1s, (b) Li 1s, and (c) C 1s. Table 1. The total percentages of N, and the percentages of pyrrolic N, pyridinic N and quanternary N in ZIF-8-NS-C and ZIF-8-P-C. Table 2. The impedances and Li+ diffusion coefficients of ZIF-8-NS-C and ZIF-8-P-C derived from the experimental EIS data based on the proposed equivalent circuit model, and their diffusion activation energies.



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Scheme 1.


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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Figure 7.



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Figure 8.


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Table 1 Total nitrogen

Quaternary

Pyrrolic N

Pyridinic N

content (at. %)

N (at. %)

(at. %)

(at. %)

ZIF-8-P-C

1.251

0.057

1.169

0.025

ZIF-8-NS-C

3.417

0.621

2.371

0.425

Samples

Table 2 D (cm2 s-1)

Activation

Samples

Rs (Ω cm-2)

Rct (Ω cm-2)

S/ZIF-8-P-C

2.92

75.71

2.45× 10-10

0.818

S/ZIF-8-NS-C

3.10

66.10

3.53 × 10-10

0.295

barriers (eV)



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Table of Contents


Monoclinic ZIF-8 Nanosheet-Derived 2D Carbon Nanosheets as

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