Metal–Organic Framework with Aromatic Rings Tentacles: High Sulfur

DOI: 10.1021/acsami.6b10946. Publication Date (Web): November 9, 2016. Copyright © 2016 American Chemical Society. *E-mail: [email protected]., ...
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Metal–Organic Framework with Aromatic Rings Tentacles: High Sulfur Storage in Li–S Batteries and Efficient Benzene Homologs Distinction Meng-Ting Li, Yu Sun, Kai-Sen Zhao, Zhao Wang, Xin-Long Wang, Zhong-Min Su, and Hai-Ming Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10946 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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Metal–Organic Framework with Aromatic Rings Tentacles: High Sulfur Storage in Li–S Batteries and Efficient Benzene Homologs Distinction Meng-Ting Li, Yu Sun, Kai-Sen Zhao, Zhao Wang, Xin-Long Wang,* Zhong-Min Su and HaiMing Xie* Institute of Functional Material Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun, 130024 Jilin, People’s Republic of China. KEYWORDS. Metal-organic framework (MOF); Aromatic rings tentacles; Li–S batteries; chemical sensor; Benzene homologs ABSTRACT: We designed and fabricated a fluorophore-containing tetradentate carboxylate ligand based metal–organic framework (MOF) material with open and semi-open channels, which acted as the host for sulfur trapped in Li–S batteries and sensor of benzene homologs. These channels efficiently provide a π-π* conjugated matrix for the charge transfer and guest molecule trapping. The open channel ensured a much higher loading quantitative of sulfur (S content-active material: 72 wt%, electrode: 50.4 wt%) than most of the MOF/sulfur composites while the semi-open channel possessing aromatic rings tentacles tentacles guaranteed an outstanding specific discharge capacity (1092 mA h g-1 at 0.1 C) accompanied by good cycling stability. To our surprise, benefiting from special the π-π* conjugated conditions, compound 1 could be a chemical sensor for benzene homologs, especially for 1,2,4-Trimethylbenzene (1,2,4-TMB). This is the first example of MOFs materials serving as sensor of 1,2,4-TMB among benzene homologs. Our works may be worthy of use for references in other porous materials systems to manufacture more long-acting Li–S batteries and sensitive chemical sensor.

1. INTRODUCTION As conversion cathode, sulfur is rapidly evolved into a big attraction for the second-generation rechargeable batteries in recent years by taking advantage of incomparable theoretical capacity (~1675 mAh g-1) and specific energy density (2600 W h kg-1), which provides nearly quintuple capacity higher than lithium-ion batteries (LIBs).1,2 Besides, the reserves of sulfur are extremely abundant, such as formed by volcanic eruptions. In spite of the numerous advantages of sulfur, some hurdles of sulfur restrict Li–S batteries from academic study to industrial applications, specifically polysulfides “shuttle phenomenon”.3,4 Additionally, undissolving polymeric dendrite precipitate inevitably cover on electrode surface during electrochemical reactions of cycling discharge and charge procedure. Hence, a few pioneers hypothesized metal–organic frameworks (MOFs) materials may resolve the impediments.5-8 By tuning the selection of both metal center and multitopic ligands, which act as “bridge abutment” and “bridge span” respectively, MOFs possess unparalleled synthetic flexibility and tunable pore diameter.9-12 As “ideal host materials”, the original photoelectricity natures of

MOFs can be induced by guest species as a matter of course. In other words, no matter how insulative overwhelming majority MOFs are, the channels or cages of MOFs can trap and immobilize sulfur species to fabricate cathode composites through the weak mutual effect of host-guest.13,14 Tarascon and Férey et al. initially applied the mesoporous MIL-100 (Cr) for sulfur storage, which performed inspiring capacity retention.5 After that, Wang’s group demonstrated that particle size of MOFs has a significant connection with specific capacity and cycling stability.13 However, finding an appropriate MOF host both as charge pathway and trap for mobile polysulphide centers are still great challenges. Herein, we report a novel MOF with aromatic rings tentacles as the host material, which based on cadmium and fluorophore-containing tetradentate carboxylate ligand, namely 1,4-bis(3,5-isophthalic acid) naphthalene (H4L), formulated as [(CH3)2NH2]2[Cd(L)]·5DMF. By anchoring the fluorescent ligand into the framework, compound 1 reveals two kinds of channels, namely semi-open channel, and open channel. The channels within aromatic rings tentacles can effectively immobilize sulfur and mobile polysulphide species in Li–S batteries. As a result, the open channel guaranteed a much higher loading quanti-

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tative of sulfur (S content-active material: 72 wt%, electrode: 50.4 wt%) than most of the MOF/sulfur composites, and the semi-open channel generated an outstanding specific discharge capacity (1092 mAh g-1 at 0.1 C and 571 mAh g-1 at 1 C) and good cycling stability. To help understand the relative importance of channels within aromatic rings tentacles to the guest molecule, we also discussed the luminescence feature of compound 1.

2. EXPERIMENTAL 2.1. Synthesis of [(CH3)2NH2]2[Cd(L)]·5DMF (1) Cd(NO3)2∙4H2O (30 mg, 0.8 mmol), H4L (10 mg, 0.2mmol) were dispersed in 8 mL acidic mixed solvent of tetrahydrofuran (THF) /1,4-dioxane (DOX) /deionized water (v/v/v=2/1/1) in a 10mL sealed vial and heated at 100°C for three days. Then we collected the colorless octahedral blocks at ambient temperature without purification. Yield: 53 % based on Cd. Anal. calc for C45H63CdN7O13: C, 52.86; H, 6.21; O, 20.34; N, 9.59. Found: C, 52.89; H, 6.19; O, 20.31; N, 9.61. IR (KBr, cm-1): 1669m, 1558s, 1423m, 1371s, 1018w, 918w, 847w, 775m. Crystal data of compound 1 for details see ESI†. 2.2. Synthesis of S/Compound 1 (Abbreviate to S/1) Composite According to the description reported by Wang et al,13 we preparation the S/1 composite. Compound 1 were soaked in acetone for a week and renewal the solvent per 12h, and then the filtered crystals were evacuated. The mixture of degassed compound 1 and sulfur (m/m=1:3) was ground in the glove box and heated to 155°C under argon for maximum 12 h. 2.3. Materials and Measurements 1,4dibromonaphthalene reacted with diethyl 5-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)isophthalate by the Suzuki coupling reaction to generate ligand H4L.15 Powder X-ray diffraction (PXRD), FT-IR spectra, thermogravimetric analysis (TGA), IR spectra, elemental analyses, UV-Vis absorption spectra and Single-crystal X-ray diffraction characterized the features of compound 1 and composites. Details of compound 1 and instrument types see the Electronic Supplementary Information (ESI). 2.4. Electrochemistry By mixing S/1 composite, conductive carbon black (CB) and poly(vinylidene fluoride) (PVDF) according to the mass ratio 7:2:1 we get the cathodes. The average active sulfur loading mass on each electrode disk is about 1.0 mg cm-2 (based on the geometrical area of electrodes). Then added N-methyl-2-pyrrolidinone (NMP) to the mixture to form a paste with appropriate viscosity. Then the paste was coated on aluminum flakes mildly and vacuum dried at 60 °C at least one day. Button cell was assembled with Li anode, S/1 cathode and approximately 45 μL typical electrolyte for Li–S batteries.16 We used the LAND CT2001A instrument (Wuhan, China) and electrochemical workstation (Princeton Applied Research, Germany) to record the circulation measurements, Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of batteries at constant ambient temperature, respectively.

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2.5. Photoluminescence Experiments We use common solvents including N,N-dimethylformamide (DMF), dichloromethane (CH2Cl2), methanol (MeOH), chloroform (CHCl3), ethanol (EtOH), acetonitrile, ethyl acetate (EA) and 1,4-dioxane (DOX), benzene homologs including toluene (Tol), benzene (Bz), ethylbenzene (EtBz), oxylene, m-xylene, p-xylene, Trimethylbenzene (1,2,3-TMB, 1,2,4-TMB, 1,3,5-TMB), aliphatic alcohols solvents including n-Propanol, i-Propanol, butanol and hexanol to investigated fluorescence of compound 1. 3 mg of the preground sample of compound 1 was soaked in 3 mL solution and treated with ultrasonication for a couple of minutes. After then, the suspension was carried by quartz colorimetric utensil and placed in F-4600 FL Spectrophotometer with a xenon lamp to recorded Fluorescence spectra at room temperature.

3. RESULT AND DISCUSSION Single-crystal data of compound 1 revealed that each cadmium atom coordinated with eight oxygen atoms from four carboxylates in a distorted dodecahedral coordination geometry (Figure 1a). Each L4- ligand coordinated with four cadmium cations (Cd-O, 2.238 (13)-2.744 (16) Å, Table S2, ESI†) to extended a 3D porous framework (Figure 1d). Bond-valence calculation indicated the oxidation state of cadmium atom is +2 calculated by total bond valence (+2.087).17,18 The framework contained two different kinds of channels which one is the open channel, and another is the semi-open channel. The 1D open channels with a transverse section of 0.9×1.4 nm (Figure 1b) along a and b axis.

Figure 1. Crystal structure and underlying network topology of compound 1. a) Four 4-connected Cd2+ cations and L4- ligand. b) 1D open channel along a and b axis. c) The 3D network of compound 1. d) sqc515 topology of host framework. Cd black spheres, C dark blue, O red. H atoms were omitted for clarity. It is interesting that aromatic rings of ligand look like tentacles inside the semi-open channels along c axis with a short distance (face to face, 3.66 Å, as shown in Figure 2). As is well-known, π-π* interaction usually occurs in short distance approximately 3.5 Å. In other words, the

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aromatic rings tentacles were extremely likely to provide a π-π* conjugated matrix for the charge transfer and guest molecule trapping. We speculated that this unique architectural feature of compound 1 might be beneficial to as interaction of host-guest. From the topological angle of vision, the Cd2+ and L4- ligand could be simplified into four-connected and bi-three connected node, respectively. Thus, the whole framework could resembles as a bimodal 3,4-net (Figure 1c) with the unique sqc515 topology. The point symbol for the net was {62·84}{62·8}2. There was 15816.7 Å3 potential solvent area volume (70.5% per unit cell) in compound 1 calculated by PLATON routine.

compound 1 possesses a much higher loading quantitative of sulfur (S content-active material: 72 wt%, electrode: 50.4 wt%) than most of the MOF/sulfur composites, which probably by taking advantage of open channels to trap sulfur. PXRD further ascertained the successful loading of sulfur. As shown in Figure 3b and 3c, the experimental diffraction pattern of compound 1 was compliance with simulated ones, and the structure is unabridged during the sulfur loading procedure. Moreover, characteristic peaks of sulfur can be observed from the ground sample but not from the heated sample, which confirmed the conclusion above.

Figure 2. The semi-open channel of compound 1 along c axis within aromatic rings tentacles (face to face distance, 3.66 Å).

Figure 4. a) CV curves of S/1 in a range of 1.5-3 V with a scanning rate of 0.1 mV s-1. b) EIS of S/1 electrode in Li–S battery before and after 100 discharge/charge cycling performance. c), d) and e) EIS measurements at different discharge/charge platforms after activated process at 0.1 C.

Figure 3. Characterization of the sulfur infiltration of compound 1. a) The TG curve of compound 1 and. b) PXRD patterns and c) photos of compound 1, sulfur, compound 1 after ground (S+1) and after heated (S/1). Dye uptake measurements certified the channels of compound 1 has excellent guest molecule adsorption capacity (Figure S2, ESI†). We monitored the encapsulation procedure of methylene blue (MB) and methyl orange (MO) molecules in 180 min at concentration of 2×10-5 mol L-1. FT-IR spectra of compound 1 were also confirmed and see Figure S4, ESI†. TGA of compound 1 indicated that the weight loss before 250 °C was solvent molecules. The whole framework completely collapsed after 380 °C (Figure S1, ESI†). According to TGA (Figure 3a), degassed

CV curves and EIS evaluated the discharge/charge cycling and dynamics performance of batteries with S/1 cathode (Figure 4). As shown in Figure 4a, the characteristic peak at about 2.3 V attributed to the reduction process of S8 to Li2Sn (n≥4), and characteristic peak at 2.0 V are further reduction process to Li2S2 and Li2S, respectively.19,20 The characteristic peak at 2.4 V corresponded to oxidation reaction during the charging process. Notably, the peak intensity of S/1 increased mildly to a higher potential at 2.0 V, which signified an activated process and favorable electrochemical reversibility of S/1 cathode.21-23 EIS reflected the Nyquist profiles of AC impedance of the S/1 cathode. As presented in Figure 4b, the chargetransfer resistance (Rct) of the first cycle (68 Ω cm-2) was considerably larger than 100 cycles (26 Ω cm-2). In compound 1, a conductive matrix, which constructed by open channels and semi-open channels within aromatic rings tentacles, can reduce the resistance of electron transfer. Further more, the channels can facilitate the electrolyte

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infiltration, which means interface ion transfer can easily happen and finally show lower Rct. Besides, we also investigated the Rct of S/1 cathode at different discharge/charge platforms after the activated process at 0.1 C (Figure 4c, 4d, and 4e). The results indicated that the open channels and semi-open channels of comound 1 did not block up by the insoluble polysulphide species.

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vation, especially the MOF/sulfur composites with the higher sulfur loadings.13,24 Owing to the good loading quantitative of sulfur (S content-active material: 72 wt%, electrode: 50.4 wt%) and porosity effect of framework,25 sulfur would gradually escape from inside of the open and semi-open channels within aromatic rings tentacles to become electrochemically active. After 50 cycles the capacity of S/1 cathode as high as 799 mA h g-1. Put another way, electronically insulating compound 1 host effectively slowed down the reaction process of sulfur and electrolyte due to weak interaction between the polysulfides and the aromatic rings tentacles. During the discharge/charge cycling performance, S/1 cathode undergone two typical plateau, namely a upper (approximately 2.3 V) and a lower plateau (2.1 V) (Figure 5c).26,27 Twenty-five percent of the theoretical capacity (419 mAh g-1) mainly output at the upper plateau, and the rest of seventy-five percent capacity output at the lower plateau. To further estimated the electrochemical performance of the S/1 cathode, two parameters of U1 and Q2/Q1 were introduced, which represented onset potential and discharge capacity ratio, respectively.28 U1 and Q2/Q1 were the good reference points to evaluate the interfacial kinetics and Li+/e transfer between the /semiopen channels and sulfur. The values of U1 and Q2/Q1 of the S/1 cathode were 2.54 V and 2.61, which still remain as high as 2.37 V and 1.8 at 1 C, respectively (Figure 5d). We presumed that open channels offered an ideal matrix for diffusion of electrolyte, and semi-open channels within aromatic rings tentacles kept sulfur from leaching.

Figure 5. Rate capabilities of S/1: a) Discharge/charge curves of the S/1 cathode at different cycles at 0.1 C. b) S/1 cathode at various C-rates. c) Schematic of voltage profile during discharge. d) The relevance of Q2/Q1 and U1. The discharge/charge cycling performance of S/1 cathode studied at 0.1 C and various C-rate (1 C = 1672 mAh g-1) entre 1.5 V and 3 V (Figure 5b). As shown in Figure 5a, after activation process, the discharge capacities of battery with the S/1 cathode exhibited a remarkable increase from 728 mAh g-1 to 1092 mAh g-1 at 0.1 C. The activation phenomenon can be explained that the sulfur is hardly access to the electrolyte at the beginning due to the smaller pore sizes of MOFs materials. It will take much time for sulfur to achieve completely electrochemic acti-

To help understand the relative importance of aromatic rings tentacles to guest molecule, luminescence experiments were carried out in different states. The emission peak of compound 1 shifts to 387 nm at λex=338 nm compared with free H4L ligand, which attributed to the deprotonation and coordination of H4L to Cd2+ ions.29,30 The observed emission colors of compound 1 still located in the blue area and match well with the CIE. Additionally, the quantum efficiency of 24.5% for compound 1 was considerably higher than H4L (2.4 %), which mainly due to the tightly coordination of fluorophore-containing ligand and Cd2+.31,32 The luminescence type of compound 1 belonged to fluorescence due to its typical lifetimes (1.18 ns) (Figure S5, ESI†).33 We use common solvents, benzene homologs reagents and aliphatic alcohols reagents to investigated fluorescence of compound 1. The results indicated that the fluorescence intensity of compound 1 was considerable depended on the guest molecules, particularly for benzene and its homologs. The fluorescence of compound 1 in common solvents and alcohols reagents had no quenching phenomenon happened (Figure 6a and 6b). To the contrary, compound 1 behaved a strong quenching effect in solvents with the same concentration (2 mMol mL-1) of benzene homologs reagents (Figure 6c).

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of our knowledge, the detection of benzene homologs was limited by multiple and complicated techniques.37-39 As we all know, the electron density of 1,2,4-TMB is higher than benzene due to electron donating methyl group, which may give rise to more strong π-π* interaction at quenching procedure. The quenching percentage of compound 1 displayed as high as 96 % with 0.027 % (v/v) of 1,2,4-TMB and the color of the sample under irradiation with UV light (365 nm) almost disappeared. These results indicated that compound 1 could be used as a chemical sensor for distinguishing benzene homologs by naked-eye observations. Compared with the PXRD of fresh sample of compound 1, the pattern after five cycles luminescence quenching process had no variation, which confirmed the superior stability of compound 1 (Figure S5, ESI†).

4. CONCLUSIONS In summary, we designed and fabricated a MOF with aromatic rings tentacles, which as the host material for sulfur trapping in Li-S Batteries and differentiating benzene homologs. By using the fluorophore-containing ligand, we anchored the aromatic rings tentacles into open and semi-open channels of framework, which efficiently provides a π-π* conjugated matrix for the charge transfer and guest molecule trapping. These channels The open channels assured a much higher loading quantitative of sulfur (S content-active material: 72 wt%, electrode: 50.4 wt%) than most of the MOF/sulfur composites, and the semi-open channels within aromatic rings tentacles enlarged an outstanding specific discharge capacity (1092 mA h g-1 at 0.1 C) and mentionable cycling stability. Luminescent studies reveal that compound 1 is the first observation of MOF material as a chemical sensor for differentiating benzene homologs, especially for 1,2,4-TMB, which is a good candidate in areas of food-safety and environment monitor, and so on. Our works may be worthy of use for reference in other porous materials systems to manufacture more long-acting Li–S batteries and sensitive chemical sensor.

Figure 6. a) PL spectra and b) intensities of the samples that were introduced into various common solvents and aliphatic alcohols reagents. λex=338 nm. c) Relative intensities of 1 that were immersed into solvents with the same concentration (2 mMol mL-1) of benzene homologs reagents. d) The PL spectra and relative intensity and quenching percentage of 1 in the sovlents with 1,2,4-TMB at different concentration (none-containing sample as blank).

ASSOCIATED CONTENT

We estimated that the open and semi-open channels with aromatic rings tentacles provided an ideal π-π* conjugated matrix for the guest aromatic molecule, and induced the luminescent quenching phonomenon. Particularly, compound 1 exhibited high quenching sensitivity in the sovlent with 1,2,4-Trimethylbenzene (abbreviate to 1,2,4-TMB) (Figure 6d), which can be used as a chemical sensor to distinguish benzene homologs.34-36 To the best

Notes

Supporting Information. Materials and measurements, Synthesis of compound 1 and S/1 composite, figures and tables see the Supporting Information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the NSFC of China (No. 21471027, 21671034), National Key Basic Research Program of China (No. 2013CB834802), the Fundamental Re-

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search Funds for the Central Universities (2412016KJ041), Changbai Mountain Scholars of Jilin Province.

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