Refined Sulfur Nanoparticles Immobilized in Metal–Organic

May 31, 2016 - Interfaces , 2016, 8 (23), pp 14328–14333 ... Importantly, the structure of the S@MOP hybrids was very stable, and they could last fo...
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Refined Sulfur Nanoparticles Immobilized in Metal−Organic Polyhedron as Stable Cathodes for Li−S Battery Linyi Bai,†,‡ Dongliang Chao,†,§ Pengyao Xing,‡ Li Juan Tou,‡ Zhen Chen,§ Avijit Jana,‡ Ze Xiang Shen,*,§,⊥ and Yanli Zhao*,‡,⊥ ‡

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, and §Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 ⊥ School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 S Supporting Information *

ABSTRACT: The lithium−sulfur (Li−S) battery presents a promising rechargeable energy storage technology for the increasing energy demand in a worldwide range. However, current main challenges in Li−S battery are structural degradation and instability of the solid-electrolyte interphase caused by the dissolution of polysulfides during cycling, resulting in the corrosion and loss of active materials. Herein, we developed novel hybrids by employing metal−organic polyhedron (MOP) encapsulated PVP-functionalized sulfur nanoparticles (S@MOP), where the active sulfur component was efficiently encapsulated within the core of MOP and PVP as a surfactant was helpful to stabilize the sulfur nanoparticles and control the size and shape of corresponding hybrids during their syntheses. The amount of sulfur embedded into MOP could be controlled according to requirements. By using the S@ MOP hybrids as cathodes, an obvious enhancement in the performance of Li−S battery was achieved, including high specific capacity with good cycling stability. The MOP encapsulation could enhance the utilization efficiency of sulfur. Importantly, the structure of the S@MOP hybrids was very stable, and they could last for almost 1000 cycles as cathodes in Li−S battery. Such high performance has rarely been obtained using metal−organic framework systems. The present approach opens up a promising route for further applications of MOP as host materials in electrochemical and energy storage fields. KEYWORDS: encapsulation, lithium−sulfur battery, metal−organic polyhedron, porous materials, sulfur nanoparticles

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ways to immobilize the active sulfur component and to enhance the dispersibility of polysulfide for decreasing undesirable effects on the performance of Li−S battery.10 To date, numerous porous materials have been tested for the construction of Li−S battery, which include porous carbon,11 conductive polymers,12,13 metal oxides,14,15 and metal−organic frameworks (MOFs),16,17 where sulfur is confined into these hollow or porous materials. Notably, MOFs show advantageous chemical and physical properties, in which small and naked sulfur particles are embedded inside the MOFs.18 However, the incorporation of sulfur nanoparticles into MOFs with a controllable amount of sulfur has not been well studied. Common methods of encapsulating sulfur into the pores of MOFs are vapor-phase infusion and melt diffusion,3,19 which cannot sufficiently solve the problems of immobilizing and separating the sulfur nanoparticles. Some sulfur nanoparticles unavoidably exist on the outer surface of the host materials as unprotected sulfur, contributing to the redox shuttle and

orldwide energy demand has been sharply increasing on account of the growing population and economic development. Electrical energy storage has therefore become a pressing need in these days. Lithium−sulfur (Li−S) battery has been increasingly attracting attention for its promising characteristics.1,2 Sulfur is best suited as a cathode material of the lithium battery, since it offers the highest theoretical capability of 1675 mAh g−1 over conventional transition metal oxide cathodes.3,4 In addition to exceptional high capacity, it brings a low operating voltage, which complies with the safety requirement of battery.5,6 Sulfur as one of the abundant elements on earth is cheap and environmentally friendly as compared to toxic transition metals. Thus, Li−S battery is believed to be a next-generation rechargeable battery system. However, its further development is still limited by several drawbacks. The main challenge in Li−S battery is structural degradation and instability of the solid-electrolyte interphase caused by the dissolution of polysulfide during cycling.7 Furthermore, the dissolved polysulfide in organic electrolyte might diffuse into the lithium anode, resulting in the corrosion and loss of active materials.8,9 Overall, these issues give rise to low utilization of active materials, low system efficiency, and poor cycle life. Hence, researchers have been seeking for new © XXXX American Chemical Society

Received: April 20, 2016 Accepted: May 31, 2016

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DOI: 10.1021/acsami.6b04697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of different MOP encapsulated sulfur: yellow represents sulfur nanoparticles, and orange represents PVP surfactant. The three hybrid MOPs used are 800 nm S@ZIF-8, 1 μm S@ZIF-67, and 500 nm S@HKUST-1.

Figure 2. (a) TEM and (d) SEM images of 800 nm S@ZIF-8; (b) TEM and (e) SEM images of 1 μm S@ZIF-67; (c) TEM and (f) SEM images of 500 nm S@HKUST-1; Elemental mapping of (g) S@ZIF-8, (h) S@ZIF-67, and (i) S@HKUST-1 hybrids.

leading to a high loss of battery capacity over the first several cycles.20 Furthermore, there is also a poor control over the filling content of sulfur into the pores of the host materials, thus compromising the battery capacity.21,22 To address these issues, we herein present a unique hybrid material consisting of sulfur, polyvinylpyrrolidone (PVP) and metal−organic polyhedron (MOP), where MOP was employed to encapsulate the PVP-covered sulfur nanoparticles. We generalized this encapsulation methodology by using three representative MOPs, i.e., ZIF-8, ZIF-67, and HKUST-1. The

as-prepared S@MOP hybrids successfully solve the volume expansion issue, and effectively avoid the side reaction occurred to the electrodes and rapid fading of capacity during cycling. This study presents an ideal model system for further investigating lithium ion grafting-releasing behavior of sulfur embedded MOPs. The synthesis of S@MOP was based on a template method (Figure 1). This approach involves an amphiphilic and nonionic PVP surfactant that not only stabilizes the sulfur nanoparticles in polar solvents, but also controls physical properties of the B

DOI: 10.1021/acsami.6b04697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) FTIR spectra and (b) TGA diagrams of pure sulfur and the three S@MOP hybrids; (c) PXRD patterns and (d) N2 adsorption/ desorption isotherms at 77 K for three MOPs and corresponding S@MOP hybrids.

hybrids such as the shape and size.23 The sulfur nanoparticles were first synthesized by a liquid phase precipitation method with slight modifications,24 where sulfur powder was utilized as the starting reagent, formic acid as a precipitating agent, and polyethylene glycol-400 (PEG-400) as a dispersing agent. Sulfur powder was dissolved in alkali metal sulfide solution (Na2S) at room temperature to give polysulfide. Polysulfide is the source of S2−. When formic acid was added to polysulfide solution under acidic conditions, S2− was precipitated to afford sulfur nanoparticles with a uniform size (Figure S1). The obtained sulfur nanoparticles were then functionalized with PVP. Three well reported MOPs, i.e., ZIF-8, ZIF-67, and HKUST-1, were chosen as the host materials for the encapsulations of sulfur nanoparticles. The encapsulation procedure was initially demonstrated using ZIF-8, where ethanolic solutions of zinc nitrate (0.30 g, 1 mmol), 2methylimidazole (0.33 g, 8 mmol) and an appropriate amount of presynthesized sulfur nanoparticles were added to a beaker containing PVP (0.01 g). The mixed solution was stirred for 1 min and then kept at room temperature for 24 h without stirring. For the other two MOPs, the synthetic details of encapsulation can be found in the Table S1. A series of MOP polyhedra with different sizes from 70 nm to 2 μm were also synthesized. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images (Figures S2−S4) reveal that these MOPs have uniform polyhedral shape and good size distribution having smooth surface and fine crystallinity. FESEM and TEM images (Figure 2) of these S@MOP hybrids show that the uniformity of the morphology was well preserved after the encapsulation process, indicating that the

stability of the MOP framework was high. To verify the structure and composition of the S@MOP hybrids, their energy-dispersive spectroscopic (EDS) mapping/imaging and X-ray photoelectron spectroscopy (XPS, Figure S5) were performed. Elemental mapping clearly reveals the presence and uniform distribution of sulfur over a large area (Figure 2g−i), further confirming that sulfur nanoparticles were wellencapsulated by MOP. The S@MOP hybrids were then characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and N2 adsorption/desorption measurements. The FTIR spectra in Figure 3a present spectral nature of S@MOP hybrids as well as free sulfur nanoparticles. TGA was used to determine the percentage of sulfur content in the S@MOP hybrids (Figure 3b), where the degradation degree of sulfur was around 220 °C. For the three samples, i.e., 800 nm S@ZIF-8, 1 μm S@ZIF-67, and 500 nm S@HKUST-1 synthesized with 3 equiv base, the sulfur contents were found to be ∼24, ∼ 21, and ∼32%, respectively (Table S2 and Figure S6). In comparison with previous work using sulfur vapor,3 the sulfur percentage in this work was reduced mainly due to relatively higher molecular weight of MOP, indicating that more sulfur was accommodated within the MOP. Moreover, PXRD patterns also show the evidence of sulfur encapsulation (Figure 3c), and obvious differences before and after the encapsulation of sulfur nanoparticles could be easily observed at two positions of 15.2 and 42.7° that are characteristic peaks of sulfur nanoparticles.25 The N2 adsorption/desorption isotherms of the hybrids (Figure 3d and Table S3) exhibit type I isotherms with typical sharp uptake at low relative pressure, proving that C

DOI: 10.1021/acsami.6b04697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Electrochemical characterization of Li−S battery based on the S@MOP hybrids: (a-c) cyclic voltammetry curves (scan rate 0.3 mV/s) and (d-f) galvanostatic charge−discharge profiles of 800 nm S@ZIF-8, 1 μm S@ZIF-67, and 500 nm S@HKUST-1 used as electrodes at 0.2 C rate within a potential window of 1.4−3.0 V versus Li+/Li0. (g−i) Cycling stability and Coulombic efficiency of these electrodes at a 0.2 C rate for 1000 cycles.

Li2S, whereas the charge process of oxidation is relatively simple. The first two plateaus at 2.3 and 2.1 V comply with that of conventional carbon/sulfur hybrids, which could be attributed to the conversion of sulfur molecule (S8) to soluble polysulfide and the reduction of polysulfide to insoluble Li2S2/ Li2S, respectively. These two plateaus are most likely assigned to the presence of sulfur in large pores of the MOP, whereas the third plateau at ∼1.7 V is ascribed to the sulfur embedded in the micropores of the MOP.27 These characteristics also match well with the cycle voltammetry curves in Figure 4. Then, the same test was performed on other cells with different amounts of sulfur loading, i.e., S@ZIF-8 with 70 nm, 200 nm, and 2 μm size, S@ZIF-67 with 100 nm, 200 and 500 nm size, and S@HKUST-1 with 100 nm, 200 and 500 nm size. As expected, the S@MOP hybrids with different sulfur contents showed distinct charge−discharge voltage profiles (Figures S8− S10 in the Supporting Information). These distinct charge− discharge curves clearly indicate different electrochemical characteristics of sulfur embedded in the micropores of the MOP during charge/discharge processes when compared to that of bulk sulfur and typical carbon/sulfur hybrids.26 The high initial discharge capacity that is similar to the theoretical capacity, as well as the plateau with a reduced voltage could be attributed to strong affinity and close contact between the confined sulfur nanoparticles and microporous MOPs. In addition, the PVP coating layer on the sulfur nanoparticles plays a crucial factor in determining the cycling stability of the

the pores in the MOP were mostly micropores. The narrow pore size distribution calculated based on the nonlocal density functional theory method (Figure S7) verified the homogeneous size of the micropores in the S@MOP hybrids. MOP-based polyhedra as protecting materials exhibit several appealing features. The synthesized polyhedra show high porosity with uniform micropores. These uniform microporous polyhedra favor a high packing density, benefiting to the increased volumetric energy density as cathode materials. On the other hand, some defects such as poor conductivity and low sulfur uptake should be further improved. Driven by these promising properties, controllable sulfur loading into MOP was then investigated for measuring electrochemical performance of S@MOP based cathodes in Li−S battery. Coin cells were fabricated to evaluate the electrochemical performance of S@MOP-based cathodes using Li foil as the anode in an electrolyte of 1.0 M lithium bistrifluoromethanesulfonylimide in 1,3-dioxolane and 1,2-dimethoxyethane (volume ratio of 1:1). The charge−discharge capacity for three working electrodes with different S@MOP (800 nm S@ ZIF-8, 1 μm S@ZIF-67, and 500 nm S@HKUST) was measured (Figure 4), showing different charge and discharge curves at 0.2 C rate calculated by the amount of S@MOP hybrids. The C rate was dependent on theoretical specific capacity of sulfur, and 1 C could be equated to a current density of ∼1672 mAg−1.26 It could be seen from the discharge curves that several steps correspond to sequential reduction from S to D

DOI: 10.1021/acsami.6b04697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces sulfur nanoparticles in the S@MOP hybrids. The PVP coating may provide a cushioning effect to withstand the changes in volume and stress. Furthermore, PVP acts in a similar way to that of PEG for trapping polysulfide.24 In whole, all of these factors led to excellent performance of the S@MOP-based cathodes. Next, cycling performance of the three Li−S cells based on 800 nm S@ZIF-8, 1 μm S@ZIF-67, and 500 nm S@HKUST-1 was studied (Figure 4). At a rate of 0.2 C, an initial capacity of 250 mAhg−1 was measured for S@ZIF-8, followed by a decrease to relatively stable capacity of 210 mAhg−1 after 80 continuous cycles. Within the next 420 cycles, the capacity only dropped by 14%, indicating that the S@ZIF-8 cathode had a high cycling stability. For the other two hybrids as working electrodes, slightly higher decay of battery capacity was observed. For S@ZIF-67, about 28% decrease in the capacity from the first to the 500th cycle at the same rate of 0.2 C was noted. At the end of the cycling test, the discharge capacity remained at about 150 mAhg−1 based on the mass of the S@ ZIF-67 hybrid, corresponding to about 750 mAhg−1 based on the mass of sulfur content. As for the S@HKUST-1 hybrid, better cycling performance (25% of decay from the first to the 1000th cycle) was achieved (Figure 4). High capacity and good cycling stability of these hybrid materials are consistent with different amounts of sulfur encapsulated in the MOP (Figure S8−S10), i.e., 8 and 12% capacity decay over 500 cycles for 200 nm and 2 μm S@ZIF-8 hybrids, 27 and 31% capacity decay over 500 cycles for 200 and 500 nm S@ZIF-67 hybrids, and 17 and 16% capacity decay over 500 cycles for 100 and 200 nm S@HKUST-1 hybrids. The relatively poor performance of S@ ZIF-67 hybrid is mainly caused by lower loading amount of sulfur and the side product formation between sulfur and cobalt site in ZIF-67.28 The stability of the S@MOP hybrids was also tested for 200 cycles (Figures S11−S13), indicating that the MOP host can be well-retained during the cycling experiments. It was observed that the S@MOP hybrids were well-dispersed without obvious changes in the shape and size after the cycling experiments. To reveal the resistance and rate performance of the S@ MOP-based cathodes, the charge transfer resistance (Rct) and specific discharge capacities at different current densities ranging from 200 mAg−1 to 1000 mAg−1 were tested (Figure 5). It should be noted that the S@HKUST-1 hybrid presents the smallest charge transfer resistance value (13 Ohm) as compared with those of S@ZIF-8 (27 Ohm) and S@ZIF-67 (25 Ohm) samples. For the rate tolerance performance shown in Figure 5b, the S@HKUST-1 electrode delivered superior rate capability of more than 200 mAhg−1 at the current density of 1000 mAg−1, which is in correspondence with obtained charge transfer resistance results. In addition, the S@MOPbased electrodes at different rates exhibited good recovery (from 500 mAg−1 to 1000 mAg−1 and then back to 500 mAg−1), indicating their high reversibility. The high specific capacity and rate performance are comparable to or even higher than those of previously reported MOF-based cathodes.3,17,29 In summary, we have developed novel S@MOP hybrids by encapsulating PVP-functionalized sulfur nanoparticles with MOP. The as-synthesized S@MOP hybrids as the cathode materials for Li−S battery could effectively enhance the utilization of sulfur component, showing high specific capacity with good cycling stability. Moreover, the amount of sulfur embedded into the MOP could be controlled according to the preference and requirements. In comparison with conventional

Figure 5. (a) Nyquist plots of three different hybrid-based electrodes: S@ZIF-8, S@ZIF-67 and S@HUKST-1. Inset shows impedance plots at the full-charged state after the first cycle. The resistance was simulated using an equivalent circuit of RS(Q1Rb)(Q2(RctZW)), where RS is the ohmic resistance of solution, Rct is the charge transfer resistance, Rb is the resistance in the Li2S (or Li2S2) film, Q is the constant phase elements, and ZW is the Warburg impedance due to the diffusion of polysulfide. (b) Rate tolerance performance of the three hybrid-based electrodes.

carbon/sulfur and metal oxide/sulfur hybrids, the MOP encapsulation can well confine the sulfur nanoparticles, and obtained S@MOP has higher stability to endure longer period of treatments in the following fabrication processes. In addition, the porosity of MOP does not influence the Li ion to interact with the active sulfur component. Importantly, the S@MOPbased cathodes present the cycle time of 1000 cycles. The long lifetime of the hybrids makes the decay rate per cycle to be only 0.028%. These hybrids with desirable performance show high applicability as novel cathode materials for Li−S battery.

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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.6b04697. Additional synthesis details, SEM and TEM images, XPS and pore size distribution data for three kinds of S@ MOP hybrids, and the reproducibility of coin-cell devices for S@MOP hybrids (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

L.B. and D.C. contributed equally to this work.

Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acsami.6b04697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) Programme−Singapore Peking University Research Centre for a Sustainable Low-Carbon Future, as well as the NTU-A*Star Centre of Excellence for Silicon Technologies under Grant 11235100003.

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DOI: 10.1021/acsami.6b04697 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX