A Metal–Organic Framework with Open Metal Sites for Enhanced

Article pubs.acs.org/crystal

A Metal−Organic Framework with Open Metal Sites for Enhanced Confinement of Sulfur and Lithium−Sulfur Battery of Long Cycling Life Ziqi Wang,† Xiang Li,† Yuanjing Cui,† Yu Yang,† Hongge Pan,† Zhiyu Wang,† Chuande Wu,§ Banglin Chen,*,†,‡ and Guodong Qian*,† †

State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science, Zhejiang University, Hangzhou, 310027 China ‡ Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249 United States § Department of Chemistry, Zhejiang University, Hangzhou, 310027 China S Supporting Information *

ABSTRACT: The lithium−sulfur battery has a very high theoretical capacity and specific energy density, yet its applications have been obstructed by fast capacity fading and low Coulombic efficiency due to the dissolution of polysulfides. Herein we utilize HKUST-1 as the host material to trap sulfur and thus to diminish the dissolution problem. A large amount of sulfur (40 wt %) has been incorporated in HKUST-1 pore metrics to achieve HKUST-1⊃S composite whose structure has been established by both single and powder X-ray diffraction studies. The strong confinement of HKUST-1 for sulfur attributed to the suitable pore spaces and open Cu2+ sites has enabled the resulting Li−S⊂HKSUT-1 battery to show excellent performance with a capacity of about 500 mAh/g after 170 cycles.

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INTRODUCTION The rechargeable lithium−sulfur (Li−S) battery is very promising as the next-generation lithium battery because of its very high theoretical capacity (1675 mAh/g) and specific energy density (2500 Wh/kg).1 Although extensive research endeavors have been pursued on Li−S batteries, some scientific obstacles need to be cleared before they can be practically ulitized in our daily life.2 One of them is to reversibly and strongly trap polysulfides generated during the battery operation so that the release of polysulfides from the cathode to the electrolyte can be significantly slowed in order to retain battery capacity of long cycling life and higher Coulombic efficiency.3 Previous research efforts have been mainly focused on the implementation of porous materials such as mesoporous carbon,4 organic polymer,5 and porous silica6 to incorporate sulfur to control polysulfide release kinetics, though polymers instead of liquid-type electrolytes have been also tried to restrain the polysulfide solubility.7 Although porous metal−organic frameworks (MOFs) for the confinement of different substrates such as metal nanoparticles,8 small molecules,9 dyes,10 and drugs11 have been extensively explored for their diverse applications including heterogeneous catalysis,12 gas storage and separation,13 photonics,14 and drug delivery,15 their confinement of sulfur and thus their potential as cathode sulfur host materials for Li− S batteries were only recently pioneered by Tarascon6b who explored a mesoporous MIL-100 (Cr) for the enhanced cycling © 2013 American Chemical Society

life of the resulting Li−S battery. This result is really encouraging. Porous MOFs have much more variable pore structures than other types of porous materials,16 so the pore and cage sizes and window apertures can be systematically tuned for confinement of sulfur substrate. In addition, the pore surfaces can be straightforwardly functionalized for further stronger interactions with sulfur. These unique structural and functional characteristics have provided the bright promise of MOF materials as cathode additives for high-performing Li−S batteries. We speculate that those porous MOFs as the additives with stronger confinement effects for sulfur should further extend the cycling life of the Li−S batteries. Herein we report HKUST-117 with moderate pore spaces and open metal sites for the enhanced confinement of sulfur and thus for the Li−S battery of a very long cycling life over 170 times at a capacity of about 500 mAh/g.

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EXPERIMENTAL SECTION

Synthesis of HKUST-1. A mixture of 2.8 g of Cu(NO3)2·2.5H2O, 1.52 g of benzene-1,3,5-tricarboxylic acid, 150 mL of DMF, and 150 mL of EtOH was added into a 500 mL glass bottle and stirred for 30 min at room temperature. The bottle was tightly sealed and heated at 358 K for 20 h. After cooling naturally to room temperature, the Received: August 30, 2013 Revised: October 15, 2013 Published: October 16, 2013 5116

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Figure 1. Schematic illustration of the preparation of HKUST-1⊃S and the corresponding color changes.

Figure 2. (a) X-ray crystal structure of HKUST-1⊃S, statistic sulfur molecules shown in yellow, and (b) PXRD patterns of the as synthesized HKUST-1 (green), simulated HKUST-1 (pink), HKUST-1⊃S (black), HKUST-1/S (blue), and sublimed sulfur (dark yellow) with an inset of magnification in the range of 22−24°. mixture was filtered and washed with EtOH (90 mL) three times and then dried in air, providing a sky blue powder as synthesized HKUST1. Solvent exchange was performed on the as-synthesized HKUST-1 by immersing it into MeOH (for 3 days, replacing fresh MeOH every 6 h) before activation. After that, HKUST-1 was heated at 423 K overnight under vacuum, resulting in purple activated HKUST-1. The activated HKUST-1 was transferred into an argon-glovebox (H2O, O2 < 1 ppm) immediately after cooling. Synthesis of HKUST-1/S and HKUST-1⊃S. Sublimated sulfur and activated HKUST-1 were loaded in a mortar in the weight ratio of 4:6. They were milled in the glovebox to a homogeneous purple powder labeled as HKUST-1/S. HKUST-1/S was loaded in a glass tube (filling 50% volume), which was tightly sealed and heated at 428 K for 24 h. Upon cooling, the military green HKUST-1⊃S powder was collected. Elemental analysis: S, 39.61 wt %. HKUST-1⊃S = Cu3(BTC)2S12.5. Synthesis of Cathode Materials. HKUST-1⊃S was coated with 20% Ketjenblack by ball milling (300 rpm, 1 h) and was hand milled with another 15% Ketjenblack and 25% PVDF (poly(vinylidene fluoride)). After the addition of NMP (N-methyl pyrrolidone), the mixture was stirred at room temperature into a homogeneous slurry. Then it was coated on an aluminum foil collector and dried at 343 K under vacuum for 24 h providing the HKUST-1⊃S cathode (sulfur content, ∼0.5 mg/cm2). The HKUST-1/S cathode was prepared by the same procedure. Battery Assembly. The 2025 coin cells were assembled in an argon glovebox with Celgard 2400 as separator, lithium foil as counter node, and 1 M LiTFSI with 2 wt % LiNO3 in DOL/DME (v:v, 1:1) as electrolyte.

X-ray Single-Crystal Studies. Crystallographic measurements for single crystal of HKUST-1⊃S were taken on an Oxford Xcalibur Gemini Ultra diffractometer with an Atlas detector using graphitemonochromatic Mo Kα radiation (λ = 0.71073 Å) at room temperature. Determinations of the unit cells and data collection for the crystal of HKUST-1⊃S were performed with CrysAlisPro.18 The data sets were corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structure of HKUST-1⊃S was determined by direct methods, and the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXL-9719 by the full-matrix least-squares method based on F2 values. The hydrogen positions were fixed geometrically at calculated distances and allowed to ride on the parent atoms. Highly disordered sulfur molecules were removed by the SQUEEZE routine in PLATON.20 Physical Measurements. Powder X-ray diffraction (PXRD) data were recorded by a PANalytical X’Pert PRO diffractometer at 40 kV, 25 mA for Cu Kα (λ = 1.541 Å). N2 sorption data were collected by a Quantachrome 20-E high speed gas sorption analyzer. The SEM morphology and EDS mapping were investigated using field-emission scanning electron microscopy (FE-SEM, Hitachi S4800) with a HORIBA EMAX energy dispersive spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG ESCALAB MARK II instrument with Mg Kα radiation (1253.6 eV) in a scan step of 0.2 eV. Thermogravimetric analysis (TGA) was carried out in a N2 atmosphere at a scan speed of 10 K/min on a Netzsch TG209 F3 system. Elemental analysis (EA) of sulfur was performed on a Kai Yuan SE-IRSII infrared sulfur analyzer. The cyclic voltammetry (CV) data were collected with an Arbin electrochemical workstation at a 5117

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Figure 3. (a) Galvanostatic discharge/charge profiles of HKUST-1⊃S. (b) Cycling performance of HKUST-1⊃S (cyan square) and HKUST-1/S (blue triangular) at a 0.1 C/0.05 C charge/discharge rate with Coulombic efficiency (yellow circle) of HKUST-1⊃S. (c) Cyclic voltammograms of HKUST-1⊃S at 0.1 mV/s. (d) XPS analysis S2p spectrum of (d) HKUST-1⊃S and (e) HKUST-1/S. scan rate of 0.1 mV/s between 1.0 and 3.0 V. The charge−discharge profiles, cyclability, and Coulombic efficiency were obtained with a LAND battery cycler in a discharge/charge rate of 0.05 C/0.1 C between 1.0 and 3.0 V.

conductivity and achieve uniform particles. The carbon coated materials were further mixed with another 15% Ketjenblack and 25% PVDF and dispersed in N-methyl pyrrolidone (NMP) as the cathode slurry. Aluminum foil was used as the current collector and 1 M LiTFSI with 2 wt % LiNO3 in DOL/DME (v:v, 1:1) was chosen as the electrolyte. Figure 3a shows the first three charge and discharge curves of Li−S⊂HKUST-1 battery at a charge rate of 0.1 C and discharge rate of 0.05 C. The first discharge curve shows two typical S8 discharge platforms at ∼2.35 and ∼2.05 V, corresponding to the cathode reactions of S8 → Li2Sx (x ≥ 4) and Li2Sx (x ≥ 4) → Li2Sx (x < 4),23 respectively. There exist two additional small shoulders at ∼1.4 and ∼1.7 V, which might be attributed to those strongly confined sulfur inside the pores of HKUST-1.6b During charging, the first charge curve exhibits two platforms at ∼2.7 and ∼2.45 V, and the first one disappears in the subsequent cycles. Such electrochemical behaviors have been exclusively confirmed by the cyclic voltammogram (CV) (Figure 3c), which was collected at a step of 0.1 mV/s. A single anodic peak occurs at ∼2.45 V as the potential scans to the charging voltage, which is attributed to the conversion of Li2S and polysulfides into elemental S. No peak corresponding to the charging platform at ∼2.7 V can be seen in first cycle, which we speculate is caused by the dynamic factors when conducting the CV test. In the subsequent cycles, the current density of the two reduction peaks at 2.35 and 2.05 V drop, indicating that the capacity of HKUST-1⊃S cathode irreversibly fades. The fact that the oxidation peak shifts to lower potential range after the first cycle demonstrates a discharge overpotential after recharging. As shown in Figure 3b, HKUST-1⊃S composite material displays much better cycling performance than the mechanically mixed HKUST-1/S. HKUST-1⊃S and HKUST-1/S reveal large initial discharge capacity of 1498 and 1507 mAh/

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RESULTS AND DISCUSSION Synthesis of HKUST-1⊃S composite material is illustrated in Figure 1. HKUST-1 was synthesized following the procedure reported,21 and its purity was confirmed by PXRD patterns, as shown in Figure 2b. The PXRD pattern of the HKUST-1⊃S (Figure 2b) shows that the sulfur has been completely incorporated into the pores of HKUST-1 with no trace of the crystallized sulfur. The crystal structure22 (Figure 2a) of HKUST-1⊃S has been established by both single and powder X-ray diffraction studies, indicating that HKUST-1 does confine a large amount of sulfur inside the pores attributed to the suitable pore spaces and open Cu2+ sites. HKUST-1⊃S is completely different from the mechanically mixed HKUST-1/S. HKUST-1/S is just mixture of the HKUST-1 and sulfur in which the strong (222) reflection of the crystalline sulfur at 22.8° has been clearly demonstrated in its PXRD pattern (Figure 2b). Furthermore, the surfaces of HKUST-1⊃S crystals are clean, and no residual sulfur particles can be observed on the crystal surfaces under the optical microscope (Figure S1, Supporting Information). As shown in Figure S2, Supporting Information, after the sulfur encapsulation, the BET surface area significantly drops from 1500 to 97 m2/g. The sulfur content in HKUST-1⊃S has been determined to be 39.61 wt % by EA, which matches with the added amount inside the sealed glass tube. The amount of the encapsulated sulfur can be controlled by adding different amounts of free sulfur with the highest one up to 55 wt %. Electrochemical studies have been carried out on HKUST1⊃S and HKUST-1/S. They were coated with 20% Ketjenblack by ball milling at 300 rpm for 1 h to improve electro5118

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diffusion process was alleviated by LiNO3,29 resulting an excess discharge capacity each cycle. After the 170 cycles, it was observed that many black corrosion spots emerged on the surface of the Li metal. To further study the sulfur encapsulation and cycling process, scanning electron microscopy (SEM) imaging along with electron energy dispersive spectroscopy (EDS) mapping were carried out on the HKUST-1⊃S cathode material before and after cycling. As shown in Figure 5a, HKUST-1⊃S crystals are

g, respectively, attributed to the excellent electroconductivity of Ketjenblack. Although the HKUST-1⊃S cathode suffers a continuously irreversible capacity fade during the first 50 cycles, which is typically observed in other types of cathode materials,24 its capacity stays nearly unchanged at ∼500 mAh/g even over 170th cycle. The capacity of HKUST-1/S declines faster to a lower one of about 350 mAh/g after 50 cycles. The cyclability of HKUST-1/S is also much worse than HKUST-1⊃S as a result of severe dissolution of polysulfides. Apparently, the strong confinement of HKUST-1 for the sulfur has played the most important role for the superior performance of HKUST-1⊃S cathode. As demonstrated in charge/discharge curves and CV and supported by the X-ray crystal structure, some of confined sulfur inside the pores exists in the state of S8, while a certain amount of sulfur might coexist as other types of smaller S molecules such as S2 and S4. It has been shown that smaller sulfur molecules (S2 and S4, etc.) can provide better battery performance by avoiding the production of polysufildes.25 Of course, the main role of HKUST-1 is to slow the release of sulfur into the electrolyte through such a strong confinement effect. The interactions of sulfur molecules with HKUST-1, particularly the open Cu2+ sites, have been also shown in the color of military green26 (Figure 1), which is not a simple mixed color of yellow (sulfur) and purple (activated HKUST-1). In fact, the HKUST-1/S mixture is still purple. This interaction between sulfur and open metal sites is reversible and not as strong as chemical bonds, which can facilitate battery operation and improve battery performance. The Cu(II)−S interactions within HKUST-1⊃S have been further proven by X-ray photoelectron spectroscopy (XPS) data. As shown in Figure 3d,e, S2p peaks in HKUST-1/S at 164.9 eV (S2p1/2) and 163.8 eV (S2p3/2) were shifted to lower energies at 164.0 and 162.9 eV in HKUST-1⊃S. Confinement of sulfur by HKUST-1 significantly delays the sulfur sublimation from 453 K (pure sulfur) to 493 K (HKUST1⊃S), as shown in the TGA (Figure 4).

Figure 5. (a) SEM morphology of HKUST-1⊃S cathode with elemental maps of sulfur (Kα1, b) and copper (Kα1, c) before cycling and after 70 cycles (d, e, f).

homogeneously embedded into the Ketjenblack layer in the range of micrometer size. The sulfur distribution (Figure 5b) is comparable to that of the copper (Figure 5c) in HKUST-1, indicating that sulfur was completely and homogeneously adsorbed into the HKUST-1 crystals as well. The SEM morphology of the 70 times cycled HKUST-1⊃S cathode (Figure 5d) does not change much, and the average particle size of HKUST-1 does not shrink, while XRD patterns (Figure S3, Supporting Information) of the 70-times cycled cathode still show sharp HKUST-1 peaks, demonstrating that HKUST1 is stable throughout the charge/discharge process. After cycling, although a small amount of sulfur (Figure 5e) spreads out of HKUST-1, which leads to the slightly different distribution of sulfur and copper (Figure 5f), overall, the majority of sulfur is still inside the pores of HKUST-1, enforcing the reversible capacity after the 50th cycle.

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CONCLUSION In summary, we have successfully targeted a microporous metal−organic framework (HKUST-1) for the confinement of sulfur. The suitable pore sizes and open Cu2+ sites within HKUST-1 have enabled it to strongly trap sulfur and thus to significantly slow the release of sulfur into the electrolyte, leading to the superior HKUST-1⊃S cathode material to those explored via the usage of oxygenated porous architectures SBA15, MIL-100 (Cr), and mesoporous carbon,6b as demonstrated in the long life cycling of 170 at the capacity of about 500 mAh/g. The rich MOF chemistry in which the pore structures and sizes within porous MOFs can be systematically tuned and functional sites can be rationally immobilized for their recognition of sulfur might provide very bright promise and alternative strategies to realize novel cathode materials for Li−S batteries of high performance. It is expected that new MOF⊃S cathode materials for even longer life cycling and larger capacity will emerge in the near future for their potential applications.

Figure 4. TGA curves of HKUST-1 (blue), HKUST-1⊃S (black), and sulfur (red) under N2 atmosphere at a scan speed of 10 K/min.

An abnormal Coulombic efficiency of about 105% has been observed in the HKUST-1⊃S cathode after the 25th cycle (yellow circles in Figure 3b). We speculate that the overdischarge phenomenon27 is caused by the slow but continuous dissolution and migration of the polysulfides, while the “shuttle” problem has been passivated by LiNO3 in the electrolyte.28 During discharging, a small amount of polysulfides migrated and deposited on the Li anode, but the back5119

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(17) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (18) CrysAlisPro, version 1.171.33.56; Oxford Diffraction Ltd.: Oxfordshire, U.K., 2010. (19) Sheldrick, G. M. SHELXL-97: Program for the Refinement of Crystal Structures; University of Gottingen: Gottingen, Germany, 1997. (20) Platon Program: Spek, A. L. Acta Crystallogr. 1990, A46, 194. (21) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304. (22) Crystal data for HKUST-1⊃S (C18H6Cu3O12S12.5): cubic, Fm3̅m, a = b = c = 26.3657(9) Å, α = β = γ = 90°, V = 18328.1(11) Å3, Z = 16, ρcalcd = 1.458 g m−3, μ(Mo Kα) = 1.990 mm−1; 2θmax = 52.66°; reflections collected 990; R1 = 0.0756 (I > 2σ(I)), wR2 = 0.2165; GOF = 1.224. (23) Su, Y. S.; Manthiram, A. Electrochim. Acta 2012, 77, 272. (24) (a) Qiu, L.; Zhang, S.; Zhang, L.; Sun, M.; Wang, W. Electrochim. Acta 2010, 55, 4632. (b) Guo, J.; Xu, Y.; Wang, C. Nano Lett. 2011, 11, 4288. (c) Lin, T.; Tang, Y.; Wang, Y.; Bi, H.; Liu, Z.; Huang, F.; Xie, X.; Jiang, M. Energy Environ. Sci. 2013, 6, 1283. (25) Xin, S.; Gu, L.; Zhao, N. H.; Yin, Y. X.; Zhou, L. J.; Guo, Y. G.; Wan, L. J. J. Am. Chem. Soc. 2012, 134, 18510. (26) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745. (27) (a) Trinidad, F.; Montemayor, M. C.; Fatás, E. J. Electrochem. Soc. 1991, 138, 3186. (b) Wang, Y. X.; Huang, L.; Sun, L. C.; Xie, S. Y.; Xu, G. L.; Chen, S. R.; Xu, Y. F.; Li, J. T.; Chou, S. L.; Dou, S. X.; Sun, S. G. J. Mater. Chem. 2012, 22, 4744. (28) Zhang, S. S.; Tran, D. T. J. Power Sources 2012, 211, 169. (29) Zhang, S. S.; Read, J. A. J. Power Sources 2012, 200, 77.

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data of HKUST-1⊃S, photographs of HKUST-1 and HKUST-1⊃S, and N2 sorption data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Numbers 51272231, 51229201, and 51010002) and the Fundamental Research Funds for the Central Universities, and partially supported by the Grant AX1730 (B.C.) from the Welch Foundation.

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