Polyoxometalates Templated Metal Ag–Carbene Frameworks Anodic

Article pubs.acs.org/IC

Polyoxometalates Templated Metal Ag−Carbene Frameworks Anodic Material for Lithium-Ion Batteries Jingquan Sha,*,† Peipei Zhu,† Xiya Yang,† Xueni Li,‡ Xiao Li,† Mingbo Yue,*,‡ and Kunfeng Zhou† †

Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong, 273155, P. R. China ‡ The Key Laboratory of Life-Organic Analysis, School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong, 273165, P. R. China S Supporting Information *

ABSTRACT: A POMs templated 3D Ag−carbene framework with lvt-a topology was hydrothermally synthesized. The POMs templated MCF combining the advantages of POMs, MOFs, and carbene not only shows excellent thermal and chemical stabilities but also possesses a good discharge capacity of 481 mAh·g−1 after 100 cycles applied as anode material in LIBs.

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MCFs) have been successfully fabricated through employing the Keggin POMs ([PW12O40]3− and [SiW12O40]4−) as templates reported by our group.11 Inspired by the better performance of POM-MOFs as anode materials,16−19 the electrochemical performances of these two POM-based CuMCFs in lithium-ion batteries (LIBs) were evaluated, which reveals that their good charge−discharge stabilities as anode materials for LIBs depend on the synergistic effect of the POMs, carbene, and high-dimensional POM-based MCFs structures. Therefore, the design, synthesis, and further functional exploring of new POM-based MCFs are still investigated. As a continuation of the work, the Ag+ ion and 1,2,4-triazole (trz) with the Keggin polyoxoanions [PW12O40]3− were selected to fabricate the new high-dimensional MCFs for the following reasons: (1) Of the transition metals, the metal silver is the best conductor of electrons at present, which can remedy slightly the weak conductivity shortcoming of POMs complexes. (2) The study of the POM-based compounds as electrode materials in LIBs is still an emerging field, and many factors could influence the discharge performance. Therefore, it is difficult to point out what role each component plays in lithium-ion batteries. The simliar compounds with the different components are desired to the study of their structure−activity relationship. (3) Ag (+I) ions possess the similar chemical environment to the Cu (+I) ions, which can be expected to form the same structure as the reported POM-based Cu-

xploring the highly activated and multifunctional Nheterocyclic carbene (NHC) complexes has been attracting much attention not only stemming from their excellent catalytic activities to various organic reactions but also owing to their high stabilities.1−3 Furthermore, a series of transition metal ions (M = Cu, Fe, Zn, Ag, Ru, etc.)4−9 have also been tried to introduce into the NHCs to form transition metal− carbene complexes (MCCs) or metal−carbene frameworks (MCFs) with specific functionalities due to their strong σelectron-donating properties and enviable coordination abilities with metal ions.9,10 To the best of our knowledge, a large number of reported MCCs are only limited to the lowdimensional structures,1−10 and the only sole example with three-dimensional MCFs based on [XW12O40]n− (X = P or Si, n = 3 or 4) were discovered.11 Additionally, the utilization of templates is regarded as an effective pathway to construct the high-dimensional MCFs, which can be found early through patiently analyzing the MCCs structures.1−11 As a consequence, more high-dimensional MCFs are highly desired in the field toward enhancing their application-related studies. On the other hand, polyoxometalates (POMs),12,13 with abundant surface negative electrons, have been widely used as templates to directly participate in the formation of the high-dimensional metal−organic frameworks (MOFs).14,15 Therefore, it can be expected to be the most effective pathway by introducing POMs as templates to fabricate the high-dimensional MCFs (POM-based MCFs), thereby showing the intriguing properties due to the combination of respective functionalities. Excitingly, the strategy has been well implemented and two threedimensional MCFs based on metal Cu ions (POM-based Cu© 2017 American Chemical Society

Received: August 3, 2017 Published: September 19, 2017 11998

DOI: 10.1021/acs.inorgchem.7b01962 Inorg. Chem. 2017, 56, 11998−12002

Article

Inorganic Chemistry MCFs.11 (4) The well-defined structures of complexes are very much helpful to deeply investigate the charge−discharge mechanism of the LIBs, which can provide experimental guidance to further explore the excellent LIBs materials. Fortunately, a new three-dimensional POM-based Ag−carbene compound (POM-based Ag-MCFs) with the same structure as the POM-based Cu-MCFs was obtained, [Ag10(H3trz)4(Htrz)4](HPW12O40), and its electrochemical performance, and the thermal and chemical stabilities were also explored thoroughly in the work.

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CRYSTAL SYNTHESIS

According to the synthetic strategy of the Cu-MCFs template by the Keggin polyoxoanions ([PW12O40]3−/[SiW12O40]4−), the similar initial materials (POMs and 1,2,4-trz) were tried to use except that the Cu(CH3COO)2 was replaced by the CH3COOAg under the similar range of pH values and reaction temperature. Unfortunately, the targeted POM-based Ag-MCF was not isolated instead of the formation of the compound Na[Ag4(trz)4(H2PW12O40)]. Nevertheless, no crystals could be obtained from the reaction of [SiW12O40]4− under the same hydrothermal reaction. In addition, employing AgNO3 instead of CH3COOAg together with [PW12O40]3−/[SiW12O40]4− and trz as starting materials, the abundant POMbased catenanes compounds, {[Agtrz][Ag12trz9][PW12O40]·2H2O} and {[Agtrz][Ag12trz9][HSiW12O40]·2H2O},19 were obtained under the same conditions, in which a small amount of POM-based Ag− carbene frameworks {[Ag10(H3trz)4(Htrz)4](HPW12O40)} were obtained. Therefore, the mass ratio of initial materials, pH value of mixed solution (ca. 1.0−3.0), and the reaction temperature (160−180 °C) were explored, respectively, in order to accurately explore the effective synthetic pathway of POM-based Ag-MCFs. Disappointingly, although a great deal of work had been attempted, the strategy of obtaining single/pure targeted PW12/SiW12 Ag−carbene complexes has not yet been completely implemented at present, shown in Table S1 and Figure S1 (Supporting Information).

Figure 1. (a) Representation of the dodecanuclear [Ag12(trz)8]4+ building units containing 8 carbene bonds (Ag−C bonds, 2.098 Å) constructed by the 12 Ag+ ions and 8 trz ligands. (b) Representation of the [Ag12(trz)8]4+ building units in POM-based Ag-MCFs linking with four adjoined building units via the Ag1−Ag1 interaction. (c) Representation of the three-dimensional Ag−carbene frameworks. (d) Topological representation of three-dimensional Ag−carbene frameworks with lvt-a topology. (e) Representation of the PW12 polyoxoanions as templates encapsulated into the three-dimensional metal−carbene frameworks. (f) Representation of POM polyoxoanions inserted into the three-dimensional metal−carbene frameworks to construct the POM-based MCFs.

stabilize the structure of [Ag12(trz)8]4+ building units, leading to the good thermal and chemical stabilities for Ag-MCFs. The 13 C NMR has been tried to confirm the existence of the Ag−C bonds, but no result was observed due to the enviable stability of the POMs Ag−carbene frameworks in the conventional solution. Then, each [Ag12(trz)8]4+ building unit connects with the adjacent four building units (Figure 1b) through the Ag−Ag interaction (3.0314 Å), to construct the three-dimensional Ag− carbene framework shown in Figure 1c, which is the first example of a 3D carbene framework based on Ag+ ions, to the best of our knowledge. In order to more clearly understand the three-dimensional Ag−carbene frameworks, the topology was analyzed in which each Ag1 ion is regarded to be the threeconnected nodes with the trz ligand and Ag1−Ag1 interactions as connectors, and the three-dimensional Ag−carbene frameworks could be described as a {4.8.10}-connected net with the lvt-a topology (Figure 1d). It is noteworthy that the threedimensional Ag−carbene frameworks contain three carbene channels along a, b, and c axes, respectively (Figure S3). Finally, the PW12 polyoxoanions as templates were encapsulated into the three-dimensional Ag−carbene frameworks to generate the POM-based Ag−carbene frameworks shown in Figure 1e,f, in which the Keggin PW12 polyoxoanions connect with 16 Ag+ ions from 6 [Ag12(trz)8]4+ building units through the powerful Ag−O interactions between PW12 clusters and Ag+ ions (Figures S4 and S5).

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CRYSTAL STRUCTURES All W atoms have a +VI oxidation state and the Ag atoms have a +I oxidation state in the POM-based Ag-MCFs, which can be confirmed by valence sum calculations,20 with the help of crystal color and coordination chemistry information. Additionally, the protons have been added to the molecular formula to balance the molecular charge, and then the molecular formula of the POM-based Ag-MCFs is [Ag10(H3trz)4(Htrz)4](HPW12O40). The targeted complex crystallized in the tetragonal space group I41/amd consisting of 1 Keggin polyoxoanion [PW12O40]3− (abbr. PW12), 10 Ag+ ions, and 8 trz molecules, in which there are one crystallographic independent PW12 cluster, two crystallographically independent Ag+ ions (Ag1 and Ag2), and two crystallographically independent trz ligands (trz-I and trz-II), and their coordinated modes are shown in Table S2. All bond lengths around the Ag ions center are in normal distances of 2.650−2.754 Å for Ag−O bonds, 2.124−2.170 Å for Ag−N bonds, and 2.098 Å for Ag−C bonds, respectively. Single crystal X-ray diffraction clearly exhibits that the Keggin POMs (PW12) as templates are easily encapsulated into the three-dimensional Ag−carbene frameworks through the powerful interaction between PW12 clusters and 16 Ag+ ions. As shown in Figure 1a, there are 12 Ag+ ions centers (8 Ag1 and 4 Ag2 ions) with 8 trz ligands to generate the regular quadrilateral dodecanuclear [Ag12(trz)8]4+ building units with the dimensions of 19.853 Å × 19.853 Å. Interestingly, eight carbene structures (Ag−C bonds, 2.098 Å) can be clearly found in each building units, shown in Figure S2, which significantly

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BATTERY PERFORMANCE At the present stage, the study of POM-based compounds as electrode material in LIBs is still an emerging field; many factors (including structure and composition of compounds) that might affect the discharge performance still remain unknown. However, the well-defined structure of targeted compounds is very much helpful toward deeply investigating the charge−discharge mechanism in LIBs. In this work, metal 11999

DOI: 10.1021/acs.inorgchem.7b01962 Inorg. Chem. 2017, 56, 11998−12002

Article

Inorganic Chemistry ions of the POM-based MCFs were changed from Cu+ ions to Ag+ ions in order to further clarify the charge−discharge behavior of POM-based MCFs as anode materials. As shown in Figure 2, the first discharge and charge capacities of the PW12-

In addition, the cycling stability and the rate performance of the PW12-based Ag-MCFs as anodic material have also been evaluated at different current densities from 100 to 1000 mA· g−1 to assess the practicability. As shown in Figure 3a, the

Figure 3. (a) Rate performances and Coulombic efficiency of PW12based Ag-MCFs and commercial graphite as anode materials at current densities of 100 to 1000 mA·g−1. (b) Cyclic voltammograms for PW12based Ag-MCFs recorded at a scan rate of 0.1 mV·s−1 in the potential range of 0.01−3 V.

reversible discharge capacity of PW12-based Ag-MCFs was ca. 470 mAh·g−1 at the current density of 100 mA·g−1. However, when the current density was changed to ca. 1000 mA·g−1, the Ag-MCFs retained the good discharge capacity of ca. 234 mA· g−1. Excitingly, when the current density was set back again to 100 mA·g−1 after 50 cycles, the discharge capacity of PW12based Ag-MCFs gets almost fully restored to the stable value. In good contrast, the reversible discharge capacities of POM-based Cu-MCFs,11 and commercial graphite were ca. 553, 425 and 334 mAh·g−1, respectively, but the (NBu4)3[PW12O40] and (NBu4)4[SiW12O40] as anode materials showed the lower reversible discharge capacity of ca. 28 and ca. 35 mAh·g−1, respectively, at the same current density shown in Table 1. Figure 3b shows the cyclic voltammogram (CV) of the POM-based Ag−carbene frameworks with the scan rate of 0.1 mV·s−1 in the range of 0.01−3 V. An obvious and irreversible reduction peak could be observed at ca. 0.7 V, which can hardly be found again in the second cycle in LIBs, indicating the formation of SEI films.21−25 In addition, the peak observed at 0.01−0.2 V in the reduction process is caused by the insertion of Li+ ions. Otherwise, the electrochemical impedance spectroscopy (EIS) of the PW12-based Ag-MCFs was also used to discuss the anodes. As shown in Figure S7, the Nyquist plots display a depressed semicircle and a straight line in the high-frequency region and low-frequency area, respectively. R1 corresponds to the electronic resistance of the electrodes and electrolyte, the high-frequency semicircle is attributed to the SEI film resistance (R2) and the charge-transfer resistance (R3), and the Warburg impedance (W1) represents the diffusion process of the Li+ ions into the solid phase. The resistance value of ca. 86.6 Ω for the POM-based Ag-MCFs was deduced from the EIS plots, indicating that the POMs carbene frameworks can be favorable to the Li+ insertion/extraction and electron transmission. Generally speaking, by comparing the charge and discharge behavior of the POM-based metal−carbene frameworks with the same structure but the different components, the following fact can be clearly found (shown in Table 1): Among these electrode materials, the charge and discharge performance of PW12-based Cu-MCFs is the best, followed closely by PW12based Ag-MCFs and SiW12-based Cu-MCFs, respectively. The electrochemical performance of commercial graphite is better

Figure 2. Charge−discharge curves during the initial 2 cycles and the 50th cycles and 100th cycles, discharge capacity, and Coulombic efficiency of PW12-based Ag-MCFs and commercial graphite anodes for LIBs at a current density of 100 mA·g−1.

based Ag-MCFs are ca. 1259 and 768 mAh·g−1 at the current density of 100 mA·g−1, respectively, which give the lower Coulombic efficiency (CE) of ca. 61%. By comparison, the CuMCFs based on PW12 clusters exhibited better initial discharge capacity (ca. 1620 mAh·g−1) than the Ag-MCFs,11 but the CuMCFs based on SiW12 clusters showed poorer discharge capacity (1245 mAh·g−1) than the Ag-MCFs at the same current density. At the second discharge and charge process, the discharge capacity was rapidly reduced to ca. 758 mAh·g−1, and the capacities loss may be mainly attributed to the formation of SEI films and the decomposition of electrolyte, which is extremely common in LIBs.21−25 Conversely, the charge capacity (ca. 690 mAh·g−1) was almost unchanged, and the CE was quickly improved to ca. 91%, suggesting that stable SEI films could be formed.21−25 In addition, the initial discharge capacities of (NBu4)3[PW12O40], (NBu4)4[SiW12O40], and commercial graphite (theoretical capacity of 372 mAh·g−1) as anode materials were disappointing in that their discharge capacities were ca. 434, 436, and 624 mAh·g−1, respectively.11 Otherwise, after 100 times cycles, the reversible capacity of the compound PW12-based Ag-MCFs was stabilized at ca. 481 mAh·g−1, showing the good performance stability than the performance of SiW12-based Cu-MCFs (ca. 426 mAh· g−1), (NBu4)3[PW12O40] (ca. 89 mAh·g−1), (NBu4)4[SiW12O40] (ca. 113 mAh·g−1), and commercial graphite (ca. 323 mAh·g−1), but the discharge capacity of PW12-based Cu-MCFs (ca. 570 mAh·g−1) is better than the discharge capacity of PW12-based Ag-MCFs. Even so, the PW12-based Ag-MCFs, like PW12-based Cu-MCFs and SiW12-based Cu-MCFs,11 exhibit also an obvious electrochemical performance advantage over the (NBu4)3[PW12O40] and (NBu4)4[SiW12O40] as anode materials and even commercial graphite anode, which is largely due to the stable Li+ ions transmission pathways provided by the three-dimensional POM-based MCFs during repeated Li+ ions insertion and extraction processes. 12000

DOI: 10.1021/acs.inorgchem.7b01962 Inorg. Chem. 2017, 56, 11998−12002

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Inorganic Chemistry Table 1. Electrochemical Behavior in LIBs of Different Anode Materials under the Same Conditionsa compounds first-DC first-C first-CE second-DC second-C second-CE 50st-DC 50st-C 100st-DC 100st-C RP

RDC PV

100 200 500 1000

PW12 Cu-MCFs

SiW12 Cu-MCFs

PW12 Ag-MCFs

PW12-NB

SiW12-NB

CG

1620 858 53 833 804 96 571 559 570 565 553 500 416 298 553 60.9

1245 672 54 662 607 97 446 435 426 421 425 390 295 211 425 80.1

1259 768 61 758 690 91 514 503 481 473 470 493 376 234 470 86.6

434 254 58 164 152 93 90.75 90 89 86 125 96 58 28 113

436 244 56 168 161 95 102.4 102 113 112 115 81 57 35 100

624 372 59 409 378 92 328 322 323 320 350 323 294 197 334

a

PW12 = [PW12O40]3−; SiW12 = [SiW12O40]4−; PW12-NB = (NBu4)3[PW12O40]; SiW12-NB = (NBu4)4[SiW12O40]; CG = commercial graphite; DC = discharge (mAh·g−1); C = charge (mAh·g−1); CE = Coulombic efficiency (%); RDC = reversible discharge capacity (mAh·g−1); RP = rate performance (mA·g−1); PV = resistance value (Ω).

addition, the chemical stability of PW12-based Ag-MCFs at different solvents (DMSO, DMF, and electrolyte) was also explored. As shown in Figure S9, the PXRD of PW12-based AgMCFs dipped by different solvents after 24 h, respectively, matched well with the PXRD pattern of the simulation. The results indicate that the new PW12-based Ag-MCF possesses both good cycle stabilities as anode materials and good thermal and chemical stabilities.

than that of the (NBu4)3[PW12O40] and (NBu4)4[SiW12O40], which could be attributed to the better stability. Moreover, the variable valent Cu ion (+2 ↔ +1 ↔ 0) has a significantly important role on the LIBs performance over the POMs through judging the discharge behavior of POM-based MCFs electrode materials. Therefore, the conclusion can be found that the redox property and stability of the electrode materials in LIBs has an important effect on the performance of the LIBs. To fully evaluate the structural stability of the PW12-based Ag-MCFs, the scanning electron microscopy (SEM) and powder XRD techniques before and after the cycling in LIBs have been tested. As shown in Figure 4, the structure and

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CONCLUSION

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ASSOCIATED CONTENT

In summary, a novel three-dimensional POMs templated Ag− carbene framework with high thermal and chemical stabilities was successfully isolated for the first time under hydrothermal reaction. When applied as an anode material for LIBs, the POM-based Ag−carbene framework exhibits a good discharge capacity of 481 mAh·g−1 after 100 cycles at a current density of 100 mA·g−1, which will open new possibilities for utilizing POM-based metal−carbene frameworks as electrode materials in LIBs.

Figure 4. SEM image of targeted complex (a) as anode materials before cycling. SEM image of targeted complex (b) as anode materials after cycling for 100 times. PXRD patterns of simulated (red), before (black), and after cycling (blue) of complex (c) as anode material for LIBs.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01962. Table of crystallographic data of targeted complex C16H17Ag10N24O40PW12, tables of selected bond lengths and bond angles, and IR and PXRD for targeted compound (PDF)

morphology of PW12-based Ag-MCFs as anode materials remains almost unchanged even after 100 times cycling during the repeated Li+ insertion and extraction reactions, which could be further proved by the PXRD patterns. The PXRD patterns before and after cycles matched well with the simulated one (Figure 4c). In addition, the thermal stability of PW12-based Ag-MCFs was studied by TG analysis. As shown in Figure S8, the carbene frameworks remain still stable before ca. 300 °C, which can also be confirmed by the corresponding PXRD patterns and IR recorded at ca. 300 °C. After 300−500 °C, the weight loss for the POM-based Ag−carbene frameworks was ca. 13.2%, attributed to the decomposition of the trz ligands, which is in good agreement with the calculated result of ca. 12.1%. In

Accession Codes

CCDC 1504085 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 12001

DOI: 10.1021/acs.inorgchem.7b01962 Inorg. Chem. 2017, 56, 11998−12002

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Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Q.S.). *E-mail: [email protected] (M.B.Y.). ORCID

Jingquan Sha: 0000-0002-5925-9565 Mingbo Yue: 0000-0002-8534-7606 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (No. 21271089), the China Postdoctoral Science Foundation (2016M600914), the Talent Culturing Plan for Leading Disciplines of University and University Scientific Research Project (J17KA118), and the Natural Science Foundation (ZR2017LB001) of Shandong Province are acknowledged.

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DOI: 10.1021/acs.inorgchem.7b01962 Inorg. Chem. 2017, 56, 11998−12002