New Route toward POM[6]Catenane Members for Lithium-Ion

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

New Route towards POM[6]Catenane Members for Lithium-Ion Batteries Jing-Quan Sha, Jianzhuang Jiang, Mengting Li, Xiya Yang, Ning Sheng, Ji-Sen Li, Mengliang zhu, and Guo-Dong Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00373 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

New Route towards POM[6]Catenane Members for Lithium-Ion Batteries Jing-Quan Sha,*,†,‡ Meng-Ting Li,§ Xi-Ya Yang,‡ Ning Sheng,‡ Ji-Sen Li,‡ Meng-Liang Zhu,† Guo-Dong Liu,‡ and Jianzhuang Jiang*,† †

Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, University of Science and Technology Beijing, Beijing

100083, China Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong, 273155,China § Institute of Functional Material Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun, 130024 China ‡

ABSTRACT: A new general route was developed for the synthesis of POM-based [6]catenane frameworks. Hydrothermal reaction of 1,2,4-triazole and AgNO3 with the saturated Keggin polyoxoanions [SiW12O40]4-/[PMo12O40]3-/[AsW12O40]3- as tempalte led

to

the

successful

isoaltion

of

POM[n]catenane

members

[Ag(trz)][Ag12(trz)9][HSiW12O40]·2H2O (1), [Ag(trz)][Ag12(trz)9][PMo12O40]·2H2O (2), and [Ag(trz)][Ag12(trz)9][AsW12O40]·2H2O (3). Single crystal X-ray diffraction analysis not only clearly reveals their [6]catenane nature but also discloses their regular three-dimensional infinite polycatenated structure with a self-dual NaCl-type topology. And for the first time, POM[6]catenane (1 and 2) are used as anode materials in lithium-ion batteries (LIBs), which exhibit promising electrochemical performance with the first discharge capacities of 1182 mAh g-1 for 1 and 1355 mAh g-1 for 2, and stabilized discharge capacity of 330 mAh g-1 for 1 and 520 mAh g-1 for 2 after 100 cycles at the current density of 0.5 A g-1. Keywords: Polyoxometalate / Catenane / POM[6]Catenane / Template / Lithium-ion battery

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION Polyoxometalates (POMs), a unique class of important metal oxide clusters with nanoscale size and abundant topologies,1,2 have attracted significant research interests due to their potential applications in catalysis,3-9 magnetism,10-13 electrical conductivity,14,15 and material science.16-19 For the purpose of improving their intrinsic low specific surface area and low stability nature,20,21 POMs have been incorporated with special structural coordination polymers (CPs), resulting in the novel hybrid POMs-CPs showing high catalytic activity in alkene epoxidation, oxidative desulfurization, aerobic decontamination, and asymmetric dihydroxylation of olefins.22-30 On the other hand, catenanes,31,32 as the special supramolecular assemblies that are formed by mechanical interlocked linkages rather than chemical covalent bonds between/among components, have also aroused extensive research interests in the past years due to their potential applications in nanoscale electronic devices, molecular machines, and molecular motors.33-37 Incorporation of POMs and catenanes into the POM-based catenane (POM[n]catenane) system is expected to lead to novel hybrid materials exhibiting intriguing properties due to the combination of respective functionalities. This indeed results in the fabrication of a few POM[n]catenane systems with the degree of catenation (DOC) number n = 2/5 and a planar motif fabricated from the two-dimensional molecular components,38-43 similar to the naturally occurring proteins44 and synthetic DNA assemblies.45 However, other POM[n]catenane systems with n ≠ 2 or 5 still remains extremely rare, limited to the sole example of [PW12O40]3--based POM[6]catenane constructed from the three-dimensional octa-nanocage [Ag24(trz)18]6+,46 to the best of our knowledge. As a consequence, new POM-based polycatenane compounds in particular those with other DOC number than 2 or 5 are highly desired in this field towards enhancing their application-related studies. It is noteworthy that rechargeable Li-ion batteries (LIBs) have been dominating the commercial market of portable electronic devices due to their superior comprehensive battery performance. However, recent efforts towards further continuously improving the LIBs performance (such as increasing their energy


Page 2 of 21

Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


density) have met significant difficulties due to the restricted theoretical specific capacity and limited structure stability of the conventional inorganic anode materials such as graphite.47 Despite the great potential revealed for the silicon and electroactive organics/polymers as the anode material of LIBs due to their high theoretical capacity and environmental friendliness,48-50 their practical commercial application has been retarded by their low cycling stability due to the severe volume change of bulk silicon and the dissolution of the active organic materials in the electrolyte. As a consequence, developing new electrode materials with high capacity, good stability, and meanwhile environmental friendliness for the next-generation Li-ion batteries still remains a great challenge. Quite recently, a number of POMs have been incorporated into the LIBs as the electrode materials due to their reversible multielectron redox behavior, structural integrity during the reactions, and environmental friendliness. In 2011, Sonoyama and co-workers tried to employ the crystals of K3PMo12O40 as the cathode material for the first time, affording the capacity over 200 mA h g−1, opening the way of applying POM clusters in LIBs.51 This was followed by the efforts of using a series of POM-containing salts including TBA3[PMo12O40],52 K5.72H3.28[PV14O42],53 and Li7[V15O36(CO3)]54 as the cathode material of LIBs, all of which however gave the discharge capacity in the range of lower than 300 mAh g-1. At the end of this paragraph, it is noteworthy that quite lately, the ε-Keggin POM-based open framework materials (Mo-Zn-Vx-O and Mo-Mn-Vx-O) were utilized as the cathode material of LIBs, also exhibiting relatively lower discharge capacity of ca. 270 mAh g-1.55 Nevertheless, as early as in 2011, various POM frameworks have been incorporated with various kinds of carbon materials including SWNTs,

CNTs,

and

graphene

to

improve

the

battery

capacity

and

charging/discharging capacity of traditional carbon anode materials in LIBs,56-60 with the highest discharge capacity of 932 and 1000 mAh g-1 achieved by the [N(C4H9)4]3[MnMo6O18{(OCH2)3CNH2}2]-decorated

SWNTs61

and

Na3[AlMo6O24H6]-EDA-functionalized reduced graphene oxide,62 respectively. Very



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

lately, the POM-CPs were also applied in LIBs as the anode material, Table S1 (Supporting Information), displaying quite good reversible capacity of 540 mAh g-1 at a current rate of 0.25 C due to the regular structure in the single crystal materials.63 However, POM[n]catenane systems as electrode materials in LIBs have not yet been explored thus far, to the best of our knowledge. In the present paper, a new pathway was developed for the purpose of affording new POM[6]catenane examples in addition to the sole [PW12O40]3--based POM[6]catenane compound reported thus far. Hydrothermal reaction of 1,2,4-triazole and AgNO3 with the Keggin polyoxoanions ([SiW12O40]4-/[PMo12O40]3-/[AsW12O40]3-) as template led to the successful isolation of beautiful octahedron single crystals of [Ag(trz)][Ag12(trz)9][HSiW12O40].2H2O (1),

[Ag(trz)][Ag12(trz)9][PMo12O40].2H2O

(2), and [Ag(trz)][Ag12(trz)9][AsW12O40].2H2O (3), Figure 1. X-ray diffraction analysis clearly reveals their [6]catenane nature. In particular, the POM[6]catenane (1 and 2) exhibit promising electrochemical performance when being employed as the anode material in lithium-ion battery (LIB) with the first discharge capacity of 1182 mAh g-1 for 1 and 1355 mAh g-1 for 2, the second discharge capacity of 793 mAh g-1 for 1 and 939 mAh g-1 for 2, and stabilized discharge capacity of 330 mAh g-1 for 1 and 520 mAh g-1 for 2 after 100 cycles, representing the first application-directed study for POM[6]catenanes and actually also the first trial of POM[n]catenanes in LIBs.

Figure 1. Photos of single crystals of compounds 1-3 isolated by the newly developed pathway


Page 4 of 21

Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL SECTION Materials and Methods. All reagents purchased were used without further purification. Elemental analyses (C, H, N) were performed on a Perkin-Elmer 2400 CHN. The IR spectra were obtained on an Alpha Centaurt FT/IR spectrometer with KBr pellet in the 400-4000 cm-1 region. The XRPD patterns were obtained with a Rigaku D/max 2500V PC diffractometer with Cu-Kα radiation, the scanning rate is 4°/s, 2θ ranging from 5-40°. The TG analyses were performed on a SDTQ600 instrument in flowing air with a heating rate of 10°C min-1. The working electrode was prepared by casting a slurry of POM-based [6]catenane (2 mg), super-P carbon (0.57 mg), and poly(vinylidene fluoride) (PVDF) (0.28 mg) at a weight ratio of 7:2:1 on pure Cu foil, which was dried in vacuum at 50°C for 24 h. The loading mass of electroactive materials in electrode slurry is ∼2 mg cm−2. The testing coin cells were assembled in an argon-filled glove-box with the working electrode as-fabricated, metallic lithium foil as the counter electrode, and 1.0 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1v/v) as the electrolyte. Galvanostatic charge/discharge cycles were performed on a LAND 2001A Battery Tester between 0.01 and 3.00 V at various current densities. Cyclic voltammetry measurements were carried out on an electrochemical workstation (CHI750D) in the potential range of 0.01–3.00 V vs. Li+/Li at a scan rate of 0.1 mV s-1. EIS measurements were carried out in the frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV at open circuit voltage. Synthesis of [Agtrz][Ag12trz9][HSiW12O40].2H2O (1). The mixture of H4SiW12O40 (450 mg, 0.15 mmol), AgNO3 (150 mg, 0.88 mmol), Htrz (50 mg, 0.72 mmol), and NH4VO3 (36 mg, 0.31 mmol) was added into 15mL H2O, which was stirred for 1 h in air. The pH value was adjusted to 2-3 by 1 M HNO3. The resulting mixture was sealed in a 20 mL Teflon-lined reactor and kept at 160°C for 4 days. After cooling down to room temperature, the red octahedral crystals of 1 were isolated, washed with water, and dried at room temperature (yield: 41% based on Ag). C20H25N30Ag13SiW12O42 (4994.05): Calcd. C 4.81, H 0.50, N 8.40%; Found C 4.81, H 0.68, N 8.35 %.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

Synthesis of [Agtrz][Ag12trz9][PMo12O40].2H2O (2). The mixture of H3PMo12O40 (300 mg, 0.16 mmol), AgNO3 (150 mg, 0.88 mmol), Htrz (50 mg, 0.72 mmol), and NH4VO3 (36 mg, 0.31 mmol) was added into 15 mL H2O, which was stirred for 1 h in air. The pH value was adjusted to 2-3 by 1 M HNO3. The resulting mixture was sealed in a 20 mL Teflon-lined reactor kept at 160°C for 4 days. After cooling down to room temperature, the red octahedral crystals of 2 were isolated, washed with water, and dried at room temperature (yield: 38% based on Ag). C20H24N30Ag13PMo12O42 (3941.13): Calcd. C 6.09, H 0.61, N 10.66%; Found C 6.11, H 0.57, N 10.62 %. Synthesis

of

[Agtrz][Ag12trz9][AsW12O40].2H2O

(3).

The

mixture

of

Na3[AsW12O40] (450 mg, 0.15 mmol), AgNO3 (150 mg, 0.88 mmol), Htrz (50 mg, 0.72 mmol), and NH4VO3 (36 mg, 0.31 mmol) was added into 15mL H2O, which was stirred for 1 h in air. The pH value was adjusted to 2-3 by 1 M HNO3. The resulting mixture was sealed in a 20 mL Teflon-lined reactor kept at 160°C for 4 days. After cooling down to room temperature, the red octahedral crystals of 3 were isolated, washed with water, and dried at room temperature (yield: 48% based on Ag). C20H24N30Ag13AsW12O42 (5039.87): Calcd. C 4.76, H 0.48, N 8.33%; Found C 4.75, H 0.56, N 8.21 %. X-ray Crystallographic Study. Diffraction data for 1-3 were collected on a Bruker SMART-CCD diffractometer equipped with Mo-Kα monochromatic radiation (λ = 0.71073 Å) at room temperature. The structure was solved by direct methods and refined on F2 by full-matrix, least-squares methods using the SHELXL-97 program package.64,65 The position of the metal atoms and their first coordination spheres were determined from direct-methods. Other non-hydrogen atoms were found using alternating difference Fourier syntheses and least squares refinement cycles. During the refinement, the command 'isor' was used to restrain then on-H atoms with ADP and NPD problems. Additionally, restraint command "simu" was used to average the thermal parameters of atoms with same environments. A summary of the crystallographic data and structural determination for them is provided in Table 1. Selected bond lengths and angles of the three compounds are listed in Table S2-4.


Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CCDC reference no. 966143 for 1, 1502545 for 2, and 1513953 for 3. Table 1. Crystallographic data and structural refinements for 1 - 3. Formula Fw T (K)

1

2

3

C20H25N30Ag13SiW12O42 4994.05 293(2)

C20H24N30Ag13PMo12O42 3941.1 293(2)

C20H24N30Ag13AsW12O42 5039.8 293(2)

19.284(5) 19.284(5) 19.284(5) 90 90 90 7171(3) 4 4.590 22.740 8723.8 0.0753 0.1340

19.1563(12) 19.1563(12) 19.1563(12) 90 90 90 7029(13) 4 3.626 5.709 7060.0 0.1418 0.3412

19.289(5) 19.289(5) 19.289(5) 90 90 90 7177(6) 4 4.640 23.159 8788 0.0727 0.1878

0.0863 0.1858 1.214

0.2187 0.4216 1.038

0.0916 0.2052 1.193

space group a (Å) b (Å) c (Å) α (o ) β (o) γ(o) V (Å3) Z Dc (g⋅cm-3) µ (mm-1) F(000) finalR1a, wR2b [I > 2σ(I)] finalR1a, wR2b(all data) GOF on F2 a

R1=∑║Fo│─│Fc║/∑│Fo│. b wR2 = {∑[w(Fo2─Fc2)2]/∑[w(Fo2)2]}1/2

RESULTS AND DISCUSSION Synthesis. In 2010, the first and thus far sole POM[6]catenane framework, [Ag2(trz)2][Ag24(trz)18][PW12O40]2,

consisting

of

discrete

three-dimensional

[Ag24(trz)18]6+octa-nanocages and [PW12O40]3- anions was fabricated by Lu and co-workers with trilacunary Keggin polyoxoanion [PW9O34]9-, CH3COOAg, and trz as starting materials under hydrothermal reaction.46 Interestingly, replacement of the [PW9O34]9- ion by the saturated Keggin polyoxoanion [SiW12O40]4- led to the formation of a pseudorotaxane framework Ag14(trz)10[SiW12O40] rather than the [SiW12O40]-based POM[6]catenane framework according to the same group.66 However, employment [PW12O40]3- instead of [PW9O34]9- together with CH3COOAg



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and

trz as

starting

materials induced

the

Page 8 of 21

formation

of

the

compound

Na[Ag4(trz)4(H2PW12O40)] rather than the targeted [PW12O40]-based POM[6]catenane framework as detail in Supporting Information, Figure S1 (Supporting Information). Nevertheless, no crystals could be obtained from the reaction of [SiW9O34]10- with CH3COOAg and trz under the same hydrothermal reaction. Fortunately, hydrothermal reaction of the saturated Keggin polyoxoanion [SiW12O40]4- with trz and AgNO3 led to the successful isolation of [SiW12O40]-based new POM[6]catenane framework [Ag(trz)][Ag12(trz)9][HSiW12O40].2H2O (1). Employing the same route with [PMo12O40]3-/[AsW12O40]3-

instead

of

[SiW12O40]4-

[PMo12O40]3-/[AsW12O40]3--based

as

starting

material,

POM[6]catenanes,

[Ag(trz)][Ag12(trz)9][PMo12O40].2H2O (2) and [Ag(trz)][Ag12(trz)9][AsW12O40].2H2O (3), were also isolated. It is also noteworthy that utilization of this newly developed general procedure with the saturated Keggin polyoxoanion [PW12O40]3- as template rather than the trilacunary Keggin polyoxoanion [PW9O34]9- also led to the successful isolation of previously reported [Ag(trz)][Ag12(trz)9][PW12O40],46 in detail see Supporting Information, demonstrating the generality of the present pathway. Structural Description. Single crystal X-ray diffraction analysis reveals that 1-3 crystallize in the space group Pn-3m and consist of one-twelfth of a polyoxoanions {[SiW12O40]4- for 1, [PMo12O40]3- for 2, and [AsW12O40]3- for 3}, half each of two Ag ions, half of an independent 1,2,4-triazole (trz) ligand, one-fourth of the second independent trz ligand, and a third disordered Ag ions that is connected to a third trz ligand in the asymmetric unit, Figure S2 (Supporting Information). In line with the previous report,46 the third triazole anion could not be exactly determined in 1-3 due possibly to a highly disordered and statistical arrangement of the third Ag ion, which however was established by elemental analysis and TG curve result to be one-twelfth of the independent composition of the asymmetric unit. Compound 1 consists of [Ag24(trz)18]6+ cationic nanocage, [Ag2(trz)2] fragment, and [SiW12O40]4- anion template. Each nanocage can be broken up into four tridentate (A) and six ditopic (B) subunits, Figure 2a. More specifically, three trz molecules


Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adopt pyrozole-like coordination mode connecting with three Ag+ ions, giving rise to a trigonal building block [Ag3(trz)3] (A), Figure S3 (Supporting Information), in which the remaining three N atoms link three Ag+ ions to facilitate expansion in space. Furthermore, another trz molecule adopts a pyrozole-like coordination mode to bridge two Ag+ ions, forming [Ag2trz] ditopic bent linker (B), Figure S3 (Supporting Information). Eventually four tridentate subunits (A) as the faces and six ditopic subunits (B) as the corners are assembled into the discrete truncated tetrahedron [Ag24(trz)18]6+ nanocage, Figure 2a. The cages possess the nanosize (27.57 Å × 27.57 Å × 27.57 Å) calculated from two N atoms on both ends of the vertices. The most fascinating structural feature of 1 is that each nanocage is chemically independent but physically interlocked with adjacent six equivalent independent nanocages through all its six vertices along a, b, and c axes, Figure 2b, forming the three-dimensional infinite polycatenated structure based on [6]catenanes, Figure S4 (Supporting Information), with an extent of intercalation about 8.292 Å. It is worth noting that the interlocking corners are disposed orthogonally to one another, resulting in the weak aromatic (3.625 Å) and argentophilic (3.0 Å) interactions from the offset stack between [Ag3(trz)3] units, Figure 2b, which stabilize the structure of [6]catenanes to some extent. In order to facilitate the comprehension of three-dimensional infinite polycatenated structure, each [Ag24(trz)18]6+ nanocage is regarded as the six-connected node and the catenation between nodes as linkage, the overall structure then shows a NaCl-type α-Po topological framework, Figure 2c. As can be seen, the framework contains large voids with a diameter of ca. 20.6 Å {the face-to-face distance between [Ag3(trz)3] unit}. As a result, the identical independent frameworks interpenetrate each other, forming a self-dual 3D whole structure, Figure 2c and S4 (Supporting Information).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Ball/stick and schematic representation of [Ag24(trz)18]6+nanocageof 1 (a); [6]catenanes fabricated by each [Ag24(trz)18]6+ nanocage physically interlocking six equivalent nanocages through its vertices and stabilized by π… π stacking and Ag…Ag interactions between the adjacent interlocking nanocages. The interpenetrating corners are disposed orthogonally to one another and overlap by 4.146 Å. The six interlocking nanocages are shown in different color, although they are crystallographically equivalent (b); NaCl-type topological framework of infinite polycatenane and the two-fold interpenetrating 3D infinite polycatenane, in which the cage is seen as cubes, and the catenation is seen as cylinders (c).

In the absence of the POMs, Ag ions and trz ligands form a 2D [Ag(trz)]n network,67 indicating such a fact that [SiW12O40]4- clusters as template initiate and drive the formation and propagation of 3D infinite [6]catenanes, Figure 3, because many coordination oxygen atoms in the POM surface (24 for Keggin POMs) are able to coordinate with considerable Ag ions to induce the interlocking and interpenetrating assemblies of POM[6]catenanes. More specifically, four [SiW12O40]4polyoxoanions as exo-templates occupy the four open windows of one nanocage by twelve Ag2···O1 bonds (2.683Å), which contributes to the formation of nanocages,


Page 10 of 21

Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3a. Then each two adjacent nanocages interlock together to form [6]catenanes via one Ag1···O5 bond (2.891 Å), Figure 3b. As a consequence, each [SiW12O40]4links circumambient four nanocages by three terminal oxygen and three bridging oxygen atoms along a, b, and c axes to form the 3D infinite POM[6]catenanes, Figures 3c and S5 (Supporting Information). It is noteworthy that despite Keggin POMs possess the “Td” symmetry, only six oxygen atoms locating in the half Keggin sphere take part in the construction of the above-mentioned 3D infinite [6]catenanes. This is surely not favor for the stabilization of the whole framework. Fortunately, the remaining six oxygen atoms (3 terminal and 3 bridging oxygen atoms) locating in the remaining half Keggin sphere link with the other nanocages, forming another 3D infinite [6]catenane. As a total result, an intriguing 3D self-dual POM[6]catenane has been perfectly fabricated, in which the Keggin POMs as 12-dentate inorganic ligands link with 8 nanocages, Figures 3d and S6 (Supporting Information), and determine the initiating, propagating, and final formation of the twofold interpenetration of 3D infinite POM[6]catenanes. This is also true for 2 and 3.

Figure 3. Schematic representation of the inducing process of [SiW12O40]4- polyoxoanions as templates to the initiating, propagating and formation of [Ag24(trz)18]6+nanocage (a), and interlocking among the nanocages (b), and POM[6]catenanes (c), and the two-fold interpenetrating 3D infinite polycatenane (d). Where four [SiW12O40]4- polyoxoanions as exo-templates occupy the four open windows of each nanocage (a); Ag1-O5 bonds between each [SiW12O40]4- polyoxoanion and Ag ions from different adjacent nanocage induce the interlocking



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of nanocages (b) and the POM[6]catenanes (c), and the spherical coordination mode of [SiW12O40]4-polyoxoanions result in the formation of 3D self-dual POM[6]catenanes (d).

XRPD, FT-IR and TG-DTA. The XRPD patterns for 1-3 are shown in Figure S7 (Supporting Information). As can be seen, the experimental diffraction pattern matches well with the simulated one obtained according to the single crystal diffraction data except for some variation in the diffraction peak intensities and widths, confirming the purity of the compounds. In addition, in the IR spectra of 1-3, Figure S8 (Supporting Information), characteristic bands of the ν(W=O), ν(Si-O), and ν(W-O-W) vibrations were observed at 961, 909, and 792 cm-1 s for [SiW12O40]4-, while the ν(P-O), ν(Mo=O), and ν(Mo-O-Mo) vibrations show absorptions at 1064, 947, 865, and 789 cm-1 for [PMo12O40]3- and 1072, 968, 870, 790 cm-1 for [AsW12O40]3-, respectively. The trz ligand gives characteristic bands in the region of 1609-1161 cm-1. Nevertheless, in order to fully characterize compounds 1-3, their thermal behavior was studied by TG analysis. As shown in Figure S9 (Supporting Information), the whole weight loss before ca. 550oC for 1-3 was attributed to the decomposition of trz ligands and the loss of two crystallized water molecules due to the good agreement between the experimental result (14.49% for 1, 18.43% for 2 and 14.22% for 3) and the calculated one (14.54 % for 1, 18.42% for 2 and 14.41% for 3), giving additional evidence for the composition of 1-3. Battery Performance. Towards exploring the potential application of these newly prepared POM[6]catenanes, initial studies over the electrochemical properties of 1 and 2 as anode material for LIBs were carried out. Figure 4 shows the cyclic voltammograms (CV) of 1 and 2 at a scan rate of 0.1 mV s-1 over the potential window of 0.01-3.0 V vs. Li/Li+. In the first cycle, an irreversible reduction peak around 0.7 V indicates the formation of solid electrolyte interphase (SEI) films, but the peak disappears in the following cycles, which frequently happens in LIBs. The reduction peak observed at 0.01–0.2 V is due to the insertion of Li+ into the anode materials. This phenomenon is in line with those observed for other POM systems as anode in LIBs,56,61,68,69 Figure S10 (Supporting Information). In addition, there is no obvious peak after 2.5 V. As a consequence, the cutoff voltage during the discharge


Page 12 of 21

Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


process is set from 0 to 2.5 V.

Figure 4. Cyclic voltammograms for 1 and 2 as anodes with a scan rate of 0.1 mV s-1 at the potential range of 0.01-3 V.

To understand the influence of POM[6]catenanes on the battery performance, the battery performance with insoluble (NBu4)4[SiW12O40] and (NBu4)3[PMo12O40] and commercial graphite (with theoretical capacity of 372 mAh g-1) as reference anodes were studied. As shown in Figures 5 and S11 (Supporting Information), the first discharge capacities of 1 and 2 are 1182 and 1355 mAh g-1, respectively, while the first discharge capacities of (NBu4)4[SiW12O40], (NBu4)3[PMo12O40], and graphite anodes amount to only 438, 587, and 624 mAh g-1 at the current density of 0.1 A g-1. At the second cycle, the discharge capacities reduce to 793 mAh g-1 for 1 and 939 mAh g-1 for 2 due probably to the formation of SEI films,31 while those for (NBu4)4[SiW12O40], (NBu4)3[PMo12O40], and graphite anodes to 176, 256, and 409 mAh g-1, respectively. In order to further explore their battery performances, the cycling stability of 1 and 2 as anode materials was studied. As shown in Figure 6, after a dramatic initial drop, the cycling capacity of 1 anode maintains stable at ca. 330 mAh g-1, while that for the 2 anode at ca. 520 mAh g-1 for 100 cycles at the current density 0.5 A g-1. In comparison, the capacity of (NBu4)4[SiW12O40] and (NBu4)3[PMo12O40] anodes changes to ca. 100 and 110 mAh g-1, respectively, after



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the same 100 cycles. Obviously, the capacities of 1 and 2 anodes are significantly higher than their corresponding matrix due probably to the synergistic effect between POMs and the catenane array. It is worth noting that the coulombic efficiencies of 1 and 2 can reach 99% after 40 cycles and remain stable thereafter, suggesting their commendable cycling stability. As a result, the superior capacity and stability of 1 and 2 anodes over matrix are considered to closely associate with the regular catenane structures, which possess stabilized crystal structure and provide a stable charge transmission pathway.

Figure 5. The charging-discharging curves of (a) 1; (b) 2; (c) (NBu4)4[SiW12O40] and (d) (NBu4)3[PMo12O40] anodes during the initial two cycles at a current density 0.1 A g-1.


Page 14 of 21

Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. The discharge capacity and the coulombic efficiency of 1, 2, (NBu4)4[SiW12O40], and (NBu4)3[PMo12O40] anodes at a current density 0.5 A g-1.

Usually, cycling and rate performance of LIBs are the decisive factor for practical applications. As shown in Figure 7, 1 and 2 anodes also show good cycling stability and rate performance at the current density range from 0.1 to 1 A g-1. The reversible capacities of 1 and 2 are ca. 793 mAh g-1 for 1 and 939 mAh g-1 for 2 at 0.1 A g-1 after an initial drop, and they retain in ca. 300 mAh g-1 for 1 and 500 mAh g-1 for 2 at 1 A g-1. When the current density is set back to 0.1 A g-1 after 40 cycles, the capacities are almost restored to the original specific capacities. In contrast, the discharge capacities for (NBu4)4[SiW12O40] and (NBu4)3[PMo12O40] anodes are only ca. 30-90 mAh g-1 at 1 A g-1. Moreover, the electrochemical impedance spectroscopy (EIS) is used for evaluating the dynamics performance of 1 and 2 anode and the spectra were simulated using the inset equivalent circuit, Figure 7b. As can be seen, 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 EIS plots suggest that the value of total resistance is 167.3 and 57.8



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ω for 1 and 2, respectively，which is much smaller than POMCPs.70 Moreover, the reduction of each POM cluster depends on the energy of its lowest unoccupied molecular orbitals (LUMOs),71,72 which correlates quite well with the electron affinity of different isolated metal ion in the order W6+

New Route toward POM[6]Catenane Members for Lithium-Ion

Recommend Documents