One-Dimensional Zinc-Based Coordination Polymer as a Higher

Sep 21, 2017 - ... 3-methyl-1H-pyrazole-4-carboxylic acid; H2tfbdc = 2,3,5,6-tetrafluoroterephthalic acid). Zn-ODCP shows blue luminescence in the sol...
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One-Dimensional Zinc-Based Coordination Polymer as a Higher Capacity Anode Material for Lithium Ion Batteries Yidan Song,†,‡ Lili Yu,†,‡ Yuanrui Gao,§ Changdong Shi,† Meiling Cheng,† Xianmei Wang,† Hong-Jiang Liu,*,§ and Qi Liu*,†,∥

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†

School of Petrochemical Engineering, Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, and Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, 1 Gehu Road, Changzhou, Jiangsu 213164, P. R. China § Department of Chemistry, College of Science, Shanghai University, No. 99 Shangda Road, Shanghai, 200444, P. R. China ∥ State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, China S Supporting Information *

ABSTRACT: A zinc-based one-dimensional (1D) coordination polymer ([Zn(H2mpca)2(tfbdc)(H2O)], Zn-ODCP) has been synthesized and characterized by spectroscopic and physicochemical methods, single-crystal X-ray diffraction, and thermogravimetric analysis (H2mpca = 3-methyl-1H-pyrazole-4-carboxylic acid; H2tfbdc = 2,3,5,6-tetrafluoroterephthalic acid). Zn-ODCP shows blue luminescence in the solid state. When Zn-ODCP acts as an anode material for lithium ion batteries, it exhibits a good cyclic stability and a higher reversible capacity of 300 mAh g−1 at 50 mA g−1 after 50 cycles. The higher capacity may be mainly ascribed to the metal ion and ligand all taking part in lithium storage. Searching for electrode materials of lithium ion batteries from 1D metal coordination polymers is a new route.

1. INTRODUCTION For sustainable development of society and economy, considerable attention is focused on clean energy utilization, such as wind energy and solar energy. However, these energies are intermittent; the electrical energy produced by them needs to be stored. To meet this demand, lithium ion battery is considered to be a promising energy storage and conversion system.1−3 But, the electrode materials, such as anode material graphite and cathode material lithium transition metal oxide, applied in lithium ion batteries (LIBs) at present have not satisfied the need of developing LIBs with high energy density in the future. A large amount of research work has been centered on obtaining high-safety, high-energy-density, sustainable, renewable, and low-cost electrode materials for LIBs. Various organic and inorganic electrode materials have been reported,4−8 such as conjugated dicarboxylate lithium, coordination polymer, metal oxide, Si, Sn, etc., but metal oxide, Si, and Sn based electrodes often have large volumetric changes in the cycling, which can result in the pulverization and degradation of the material. Coordination polymers (CPs), as a kind of compound constructed by organic ligands and metal ions or metal clusters, contain three-dimensional (3D), twodimensional (2D), and one-dimensional (1D) structures bonded through extended covalent or coordinate interactions.9 Among them, 2D and 3D porous CPs, usually called metal− organic frameworks (MOFs), have exhibited widely potential applications in magnetic materials, energy storage, sensor, gas separation and storage, catalysis, and drug delivery.10−17 Due to their ability to provide metal ions and organic ligands with © 2017 American Chemical Society

redox activity, and porosity for the diffusion of electrolyte ions, even flexible structures for overcoming the pulverization and volume effect of some electrode materials, MOFs/CPs, which are directly used as electrode materials of LIBs, have also received tremendous attention recently.18−34 In 2006, Chen et al. first investigated the electrochemical performances of 3D Zn-based MOF (Zn4O[1,3,5-benzenetribenzoate]2, MOF-177) as an anode material of LIBs; after the second cycle, the anode only displays a discharge capacity of about 100 mAh g−1, owing to the structure damage of MOF-177 during the cycling process.18 In 2007, a 3D Fe-based MOF (FeIII(OH)0.8F0.2[O2C−C6H4−CO2], MIL-53(Fe)) used as a cathode material of LIBs, was first examined by Tarascon et al.; the cathode displays a lower capacity of 70 mAh g−1, but a good cycling performance. 19 In 2010, a zinc-based MOF [Zn3(HCOO)6] was reported by Vittal et al. as an anode material, having a discharge capacity of 560 mAh g−1 at 60 mA g−1 after 60 cycles.20 [Mn(tfbdc)(4,4′-bpy)(H2O)2] (MnLCP), a layered 2D Mn-based CP, as an anode material for LIBs was first investigated by us; after 50 cycles, the Mn-LCP electrode still exhibits a reversible capacity of 390 mAh g−1.24 Since then, some 3D/2D CPs and MOFs as cathode and anode materials of LIBs have been verified.25−34 Considering that 1D organic polymers, such as polyanthraquinone (PAQ),35 can be used as electrode materials for LIBs, 1D CPs should be potential electrode materials for LIBs based on their structure Received: June 6, 2017 Published: September 21, 2017 11603

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Inorganic Chemistry

isotropic parameter. Crystallographic data of Zn-ODCP are listed in Table 1.

character of redox active metal ions, ligands, and the space between chains for the diffusion of Li+ ions. However, to our knowledge, until now, only one 1D cobalt-based CP ([Co1.5L(H2O)4]n, H3L = 4-hydroxypyridine-2,6-dicarboxylic acid) used as an electrode material for LIBs was investigated.36 It is worthwhile to mention that 1D CPs with magnetic, optoelectronic, and catalytic properties have also received attention.37−39 On the other hand, not only can 3-methyl-1H-pyrazole-4carboxylic acid (H2mpca) coordinate with metal ions acting as a ligand40,41 but also it may provide conjugated dicarboxylates for combining Li+ ions like Li2C8H4O4 (Li terephthalate) when its CPs served as electrode materials for LIBs.4 So, in the present work, we utilized environmentally friendly materials, 3-methyl1H-pyrazole-4-carboxylic acid (H2mpca), tetrafluoroterephthalic acid (H2tfbdc), and Zn(OH)2, to synthesize a 1D coordination polymer [Zn(H2mpca)2(tfbdc)(H2O)] (ZnODCP) and investigate its electrochemical performance as an anode material of LIBs. This is the first report confirming 1D Zn-based CP as electrode material used in LIBs. The ZnODCP electrode shows a higher capacity, a better rate performance, and an excellent cycling stability, and it still maintains the specific capacity of 300 mAh g−1 after 50 cycles.

Table 1. Crystallographic Data for Zn-ODCP empirical formula formula weight T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å 3) Z Dcalcd (g·cm−3) μ (mm−1) F (000) θ (deg) index ranges no. of total reflns unique reflns data/restraints/params GOF (F2) R1, wR2 [I > 2σ(I)] R1, wR2 (all data) largest diff peak and hole/e·Å

2. EXPERIMENTAL SECTION 2.1. Materials. All solvents and reagents were purchased from Shanghai Chemical Reagent Corporation and used as received. The ligand 3-methyl-1H-pyrazole-4-carboxylic acid (H2mpca) was synthesized according to the reported literature.40,41 2.2. Synthesis of [Zn(H2mpca)2(tfbdc)(H2O)]. A suspension water solution (4 mL) containing Zn(OH)2 (0.0098g, 0.1 mmol) was added to a methanol−ethanol solution (6 mL, v:v = 1:1) containing H2mpca (0.0126g, 0.10 mmol) and H2tfbdc (0.0238g, 0.1 mmol) and then mixed to obtain a colorless solution. After evaporation of the solvent at room temperature for 3 weeks, colorless needle crystals of [Zn(H2mpca)2(tfbdc)(H2O)] (Zn-ODCP) were collected. Yield: 85% (based on Zn). Anal. Calcd for C18H14F4N4O9Zn (Mr = 571.7): C, 37.81; H, 2.45; N, 9.80. Found: C, 37.83; H, 2.40; N, 9.87. IR (KBr, cm−1): 3467 (s), 1640 (vs), 1552 (s), 1463 (s), 1418 (s), 1380 (s), 1317 (vs), 1184 (m), 1146 (m), 962 (m), 892 (w), 842 (w), 772(m), 576 (m), 512 (w). 2.3. Physical Measurements. An element analyzer (PerkinElmer 2400 Series II) was used to perform elemental analysis (C, H, and N). Under N2 atmosphere, using a heating rate of 10 °C·min−1 from room temperature to 800 °C, thermogravimetric analysis (TGA) was performed on a thermal analyzer (TG 209 F3). A Nicolet 460 spectrometer was used to record IR spectra in the range of 4000−400 cm−1 using KBr pellets. Luminescence spectra of solid samples were recorded on a spectrometer (Varian Cary Eclipse system). Field emission scanning electron microscopy (FESEM) images were collected by using a FESEM system (Zeiss, Supra 55). X-ray photoelectron spectrum (XPS) was recorded on an ESCALABMK II X-ray photoelectron spectrometer. An X-ray diffractometer with Cu Kα radiation (D/max 2500 PC, Rigaku, λ = 1.5406 Å) was used to record powder X-ray diffraction (XRD) patterns. 2.4. X-ray Analysis. A diffractometer equipped with a Bruker Smart Apex CCD area detector was used to perform single crystal Xray diffraction measurement for Zn-ODCP. Intensity reflections were collected adopting Mo Kα radiation (λ = 0.71073 Å) in the range of 1.75° ≤ θ ≤ 33.5°. The structure was solved by direct methods employing the SHELXTL-97 program.42 All the non-hydrogen atoms were refined by using anisotropic parameters. The hydrogen atoms linked to nitrogen atoms and oxygen atoms from H2mpca and the water molecules, respectively, were first found in difference Fourier maps and then refined isotropically. The hydrogen atoms linked to carbon atoms were placed at geometric positions and refined with an

−3

C18H14F4N4O9Zn 571.72 296(2) monoclinic C2/c 8.0304(6) 23.2742(18) 11.8741(9) 90.00 102.0100(10) 90.00 2170.7(3) 4 1.749 1.226 1152 1.75 to 33.50 −12/12, −34/33, −18/17 20825 3910 (Rint = 0.0312) 3910/3/166 1.137 0.0296, 0.0882 0.0331, 0.0904 0.362 and −0.511

2.5. Electrochemical Measurement. The Zn-ODCP electrodes were prepared by the following method: 60 wt % crystalline ZnODCP, acetylene black (30 wt %), and polyvinylidene fluoride binder (10 wt %) were mixed in N-methyl-2-pyrrolidinone (NMP) to produce a slurry; then, the slurry was coated on a copper foil. The coated electrodes were put in a vacuum oven and dried at 80 °C for 12 h and then pressed. Electrochemical measurements were done in CR 2016 coin-type batteries. The battery was assembled in a glovebox filled with argon, in which a porous polypropylene separator was located between the Zn-ODCP electrode and a lithium-foil counter electrode, and 1 M LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (VEC:VDMC:VDEC = 1:1:1) served as the liquid electrolyte. Cyclic voltammetry (CV) was measured at 0.2 mV s−1 between 0.1 and 3.0 V. In the same potential window, the batteries were galvanostatically discharged and charged at 50 mA g−1. Electrochemical impedance spectroscopy (EIS) was measured at the open circuit voltage with an amplitude of 5 mV in the frequency range of 0.01 to105 Hz.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure Description of Zn-ODCP. The asymmetric unit of Zn-ODCP consists of one Zn(II) ion, one H2mpca molecule, a half tfbdc2− anion, and one coordinated water molecule. As displayed in Figure 1a, the Zn(II) ion adopts a slightly distorted triangular bipyramidal coordination geometry, which contains two nitrogen atoms (N1, N1A) from two H2mpca, two oxygen atoms (O1, O1A) from two carboxylate groups of two tfbdc2− anion ligands, and one oxygen atom (O5) from a coordinated water molecule. O5, N1, and N1A atoms constitute the base plane of the triangular bipyramidal structure, and O1 and O1A atoms occupy the axial positions. As shown in Table S1, the bond lengths of Zn1−O (1.9636−2.1540 Å) and Zn1−N (2.0215 Å) are close to the values observed in the Zn(II) compounds reported.41 O1− 11604

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correspond to νas (OCO) and νs (OCO) stretching vibrations of tfbdc2− anions and H2mpca molecules, respectively. The strong peak originated from stretching vibrations of the CN bond appears at 1552 cm−1, and the δ (OCO) bending vibration peak appears at 772 cm−1.41 3.3. Thermal Stability. The TGA curve for Zn-ODCP is shown in Figure S2. The TGA result reveals that there is a weight loss between 60 and 198 °C, corresponding to the loss of one water molecule (calcd 3.15%, found 3.60%); subsequently, the remaining substance is decomposed slowly to give final product ZnO (calcd 14.23%, found 12.12%). 3.4. Luminescent Property. The solid state emission spectrum of Zn-ODCP at room temperature is shown in Figure S3. Upon excitation at 337 nm, intense fluorescence emission with maximum peak appears in the blue region 473 nm for H2mpca ligand, while under excitation at 373 nm, the strongest emission peak of H2tfbdc ligand appears at 424 nm, which is attributed to the intraligand (π → π* and/or n → π*) fluorescence emission.44 In comparison with the free acid ligands, Zn-ODCP exhibits a similar character with the H2mpca ligand, but a red-shift appears, indicating that the luminescence of Zn-ODCP mainly comes from H2mpca. The red-shift of 12 nm should be ascribed to the ligand-to-metal charge transfer.40,45 Because the free H2mpca ligand has a similar peak, the weaker shoulder peak around 520 nm should be ascribed to intraligand transitions. 3.5. Electrochemical Performance. To investigate the electrochemical performances of the Zn-ODCP electrode, a galvanostatic cycling test was carried out. Figure 2a displays the charge/discharge curves of the Zn-ODCP electrode at 50 mA g−1. The first discharging and charging capacities of the ZnODCP electrode are 992 mAh g−1 and 392 mAh g−1, respectively, and an initial Coulombic efficiency of 39.5% can be obtained. The irreversible capacity of 600 mAh g−1 should be ascribed to the formation of solid electrolyte interface (SEI) and the electrolyte decomposition; such phenomena of lower initial Coulombic efficiency have been observed in metal oxide nanomaterials and MOF/CP based materials.20,24,46 For increasing initial Coulombic efficiency and meeting the needs of practical applications, it may be a good practice to prelithiate the Zn-ODCP electrode before full LIB assembly47 and improve the inherent transport property of Zn-ODCP. The cycling performance of the Zn-ODCP electrode is presented in Figure 2b. After initial irreversible capacity decay, the Coulombic efficiency of the Zn-ODCP electrode dramatically reaches 86.4% in the second cycle, and reaches 93.0% in the 50th cycle, demonstrating that it has excellent cycle stability. The second discharge capacity remains at 342 mAh g−1, and the 50th discharge capacity remains at 300 mAh g−1. The capacity of 300 mAh g−1 is also higher, compared with some 2D/3D CPs/ MOFs reported (Table S3). The Zn-ODCP electrode shows also better rate performance (Figure 2c). The average reversible capacities of 295, 247, 203, 172, and 146 mAh g−1 are obtained, respectively, as the discharged/charged test of the battery was carried out at the current density from 50 to 100, 300, 500, and 1000 mA g−1. When the current density turns back to 50 mA g−1, the average capacity of 287 mAh g−1 is still retained. For studying the electrochemical reactions in the Zn-ODCP electrode, cyclic voltammetry (CV), FT-IR spectra, and XPS spectra were investigated. Figure 2d displays the CV profiles of the Zn-ODCP electrode. In the process of the first cathodic scan, a peak at 1.36 V may be assigned to the depletion of the

Figure 1. View of complex Zn-ODCP: (a) the coordination environment of Zn(II). Only hydrogen atoms on oxygen atoms are drawn, and others are omitted for clarity. Symmetry codes: (A) −x + 2, y, −z − 1/2. (b) 1D chain. (c) 3D framework. (Dashed lines represent hydrogen bonds.)

Zn1−O1A, N1A−Zn1−N1, and N1−Zn1−O5 bond angles are 174.27(6), 117.13(7), and 121.44 (3)° respectively. As displayed in Figure 1b, all Zn(II) ions are linked to each other by tfbdc2− anions to produce a 1D chain structure, in which the interval between two Zn(II) ions is 11.365 Å. As presented in Figure 1c, these 1D chains are connected to give a 3D supramolecular network via the interaction of O−H···O and N−H···O hydrogen bonds (Table S2). One structural character of Zn-ODCP is that it includes uncoordinated carboxyl groups from H2mpca molecules, owing to H2mpca adopting a unidentate coordination fashion (Figure 1b). The groups might be used as Lewis acid sites in acid−base and catalytic reactions.41,43 3.2. Synthesis and Infrared Spectrum. Zn-ODCP can be obtained by using evaporation of EtOH−MeOH−H2O solution containing H2mpca, H2tfbdc, and Zn(OH)2 in a molar ratio of 1:1:1. Zn(OH)2 not only provides the center ion but also plays a role of base. Zn-ODCP is insoluble in ethanol, DMF, acetone, and acetonitrile. The IR spectrum of Zn-ODCP is displayed in Figure S1. The broad and strong peak around 3000−3600 cm−1 is ascribed to νs (OH) from H2O molecules. The strong peaks at 1640 cm−1 as well as 1380 cm−1 11605

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Figure 2. Electrochemical performances of the Zn-ODCP electrode: (a) Charge/discharge plots at 50 mA g−1 in the range of 0.01−3.0 V. (b) Cycling performance. (c) Rate performance at varying rates. (d) CV curves.

coordinated water from Zn-ODCP ([Zn(H2mpca)2(tfbdc)(H2O)]), while the peaks at 0.62, 0.45, and 0.1 V are ascribed to the prodution of a solid electrolyte interface (SEI) film, the transfer of the remaining Zn-ODCP ([Zn(II)(H2mpca)2(tfbdc)]) to [Zn(0)Lin+2(mpca)2(tfbdc)], and the formation of Li−Zn alloys,20 meaning Zn2+ ions were reduced to Zn and Li+ ions were inserted in the Zn-ODCP (eqs 1 and 2 below). A wide peak at 1.28 V in the first anodic scan should be attributed to the dealloying process of Li−Zn alloys and the diversion from [Zn(0)Li n (mpca) 2 (tfbdc)] to [Zn(II)(mpca)2(tfbdc)], corresponding to the oxidation of Zn(0) to Zn(II) and the deintercalation of Li+ ions.20,46,48 However, in the second cycle, only the catholic peak at 0.1 V and the anodic peak at 1.28 V can be observed. The third and forth CV curves almost coincide, further indicating that the Zn-ODCP electrode has good stability. Figure S4 displays the FT-IR spectra of the bare Zn-ODCP electrode and the discharged and charged Zn-ODCP electrodes. The peaks at about 1639 cm−1, belonging to the tfbdc2− anions, all appear in these FT-IR spectra. This fact shows that the tfbdc2− and mpca2− anions can cycle reversibly. Figure 3 depicts the Zn 2p XPS spectra of the Zn-ODCP samples from the assembled electrodes taken before and after the discharge/charge processes. As shown in Figure 3a, the characteristic peaks belonged to Zn 2p1/2 and Zn 2p3/2 of Zn(II) ions appearing at 1045.84 and 1022.74 eV, respectively. After the first discharge, the two peaks shift to 1045.99 and 1022.98 eV, respectively (Figure 3b). Besides, the difference value of two binding energies decreases from 23.1 to 23.01 eV. The two kinds of changes should be attributed to the formation of metallic zinc.18,49 After one cycle, there are two peaks appearing at 1046.24 and 1045.53 eV, respectively (Figure 3c), which may imply that Zn(II) ion and metallic zinc all exist in the sample due to the metallic zinc having not been fully

Figure 3. XPS spectra of Zn 2p from the Zn-ODCP samples taken before (a) as well as after the first discharge process (b), one cycle (c), and 50 cycles (d) from the assembled electrodes.

transferred to Zn(II) ion. After 50 cycles, as presented in Figure 3d, two peaks appear at 1045.91 and 1022.74 eV, respectively, and are close to the original locations (1045.84 and 1022.74 eV), indicating that metallic zinc was transferred to Zn(II) ion. From C 1s XPS spectra (Figure S5), it can be observed that the C 1s XPS spectrum of the Zn-ODCP sample from the assembled electrodes has considerable difference in comparison to that of the sample after the first discharge, while the C 1s XPS spectrum of the sample after one cycle is similar to that of the sample after 50 cycles. These results reveal that the carboxylate groups from the organic ligands have taken part in the electrochemical reaction, as observed in the polydopaminederived electrode50 and the [Co1.5L(H2O)4]n electrode.36 Based on aforementioned experimental facts, and the reports that the conjugated carboxylate from terephthalate lithium (Li2C6H4O4) can serve as redox centers and aromatic ring can 11606

DOI: 10.1021/acs.inorgchem.7b01441 Inorg. Chem. 2017, 56, 11603−11609

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Inorganic Chemistry accommodate Li+ ion,4,32,51−53 the probable reaction mechanism in the Zn-ODCP electrode may be expressed as follows: [Zn(II)(H 2mpca)2 (tfbdc)(H 2O)] + 3Li+ + 3e → [Zn(II)Li 2(mpca)2 (tfbdc)] + LiOH

(1)

[Zn(II)Li 2(mpca)2 (tfbdc)] + nLi+ + ne ↔ [Zn(0)Li n + 2(mpca)2 (tfbdc)] +

Zn + Li + e ↔ LiZn

(2)

Figure 4. Higher magnification (a) and lower magnification (b) FESEM images of the Zn-ODCP sample after grinding.

(3)

As shown in eqs 1 and 2, Zn-ODCP reacted first with Li to form [Zn(II)Li 2 (mpca) 2 (tfbdc)], and then [Zn(II)Li2(mpca)2(tfbdc)] further reacted with Li to produce [Zn(0)Lin+2(mpca)2(tfbdc)] via conversion reaction during the first discharge process. Specifically, when 1 mol of H2O reacted with Li, 1 mol of Li was needed, and 2 mol of Li was needed as 1 mol of Zn(II) ion was reduced to Zn; besides, considering that 1 mol of H2mpca and 1 mol of tfbdc2− anion can accommodate 7.5 mol of Li and 8 mol of Li on the average, respectively,51,53 as displayed in Figure S6, so, 1 mol of ZnODCP can accommodate 26 mol of Li in all. The theoretical capacity of [Zn(H2mpca)2(tfbdc)(H2O)] would be 1218 mAh g−1, higher than the first discharge capacity of 992 mAh g−1. Considering that the first discharge capacity of acetylene black is about 178 mAh g−1 at 50 mA g−1 (shown in Figure S7), the actual first discharge capacity of 903 mAh g−1 is obtained for Zn-ODCP by subtracting the capacity delivered from acetylene black (178 mAh g−1 × 30 wt % acetylene black/60 wt % ZnODCP = 88 mAh g−1). Equation 3 stands for the formation and dealloying process of Li−Zn alloys. XRD patterns of assynthesized Zn-ODCP and the Zn-ODCP bare electrode containing Zn-ODCP, polyvinylidene fluoride, and acetylene black after 1 cycle are displayed in Figure S8; obviously, the diffraction peak positions of the Zn-ODCP bare electrode have both similarities and differences, compared to those of assynthesized Zn-ODCP. This fact further demonstrates that ZnODCP has reacted with Li and the product is still crystalline. To get deeper insight into the exact electrochemical reaction mechanism of the Zn-ODCP, more research work is needed. Several factors result in the better electrochemical performances of the Zn-ODCP electrode: one is that Zn(II) ions and organic ligands all participate in redox reaction in the process of charge/discharge; another is that the space between the 1D chains may be flexible due to the interaction force of the interchain being a weak interaction force (hydrogen bonds and intermolecular interaction force), which is favorable to the diffusion of Li ion; the third is that Zn-ODCP nanorods provide more active sites for electrochemical reactions as well as a shorter route for Li ion diffusion. Figure 4 depicts FESEM images of the Zn-ODCP after grinding, which shows that the sample consists of nanorods. The width of nanorods is ca. 70− 150 nm. The electrochemical impedance spectra (EIS) of the ZnODCP electrode are presented in Figure 5. As can be seen, each profile consists of an inclined line and a semicircle. The two equivalent circuits are presented in the inset of Figure 5, and they are used to fit the EIS data after 1 cycle and after 50 cycles, respectively. Using Zview software simulation, the values obtained are displayed in Table S4.The internal resistances (R1) after 1 and 50 cycles are 5.3 Ω and 14.1 Ω, respectively. The difference value of both is small, meaning that the Zn-ODCP electrode has good stability.

Figure 5. Nyquist and the equivalent circuit diagrams of the ZnODCP electrode.

R1 includes the solution resistance and battery component resistance. The passivating film resistance and the charge transfer resistance are represented by Rs and Rct respectively, while W, CPE, and C1 are used to stand for the constant Warburg impedance, phase element, and intercalation capacitance, respectively. The charge transfer resistance (Rct) after 50 cycles is 131 Ω, larger than the Rct (73.8 Ω) after 1 cycle, which may be ascribed to the dropping of some ZnODCP active material. Besides, parts of the two plots in the low frequency region are almost overlapped, further indicating good cycle stability of the Zn-ODCP electrode. The Li+ ion diffusion coefficient can be obtained via calculation based on the following formula: DLi =

2 2 1 ⎛ Vm ⎞ ⎡⎛⎜ dE ⎞⎟⎤ ⎜ ⎟ ⎢ − ⎥ 2 ⎝ FAσ ⎠ ⎣⎝ dx ⎠⎦

where F presents the Faraday constant, Vm the molar volume of the active material, A the surface area of the electrode, σ the Warburg coefficient, and dE/dx the slope of the electrode potential (E) in function of the composition (x).54,55 σ stands for the slope of the line Z′ ∼ ω−1/2, and the line of Z′ ∼ ω−1/2 is displayed in Figure S9.56 The DLi value is calculated to be 3.56 × 10−12 cm2 s−1 during the first cycle for the Zn-ODCP, which is comparable to that of M-Nb2O5 nanofibers.54

4. CONCLUSIONS In summary, a zinc-based 1D coordination polymer ([Zn(H2mpca)2(tfbdc)(H2O)], Zn-ODCP) was successfully synthesized, which exhibited blue luminescence in the solid state. As an electrode material for LIBs, the Zn-ODCP electrode exhibited both an excellent cycling stability and a higher capacity, which still retained a specific capacity of 300 mAh g−1 after 50 cycles, and a better rate performance. This is the first example that 1D zinc-based coordination polymer can serve as 11607

DOI: 10.1021/acs.inorgchem.7b01441 Inorg. Chem. 2017, 56, 11603−11609

Article

Inorganic Chemistry

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an electrode material for LIBs. More importantly, our work confirmed that the capacity originated from metal ion and ligand in coordination polymer and 1D coordination polymers could act as electrode materials of LIBs instead of the usual 3D porous materials. The fact will motivate us to find out excellent electrode materials from 1D metal coordination polymers. Related research work is in progress in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01441. IR spectra, TGA curve, solid-state emission spectra, and XPS spectra (PDF) Accession Codes

CCDC 1510364 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.

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AUTHOR INFORMATION

Corresponding Authors

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

Qi Liu: 0000-0003-3996-6489 Author Contributions ‡

Y.S. and L.Y. have equally contribution to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (No. 20971060 and 21101018), the Natural Science Research Key Project of Jiangsu Colleges and Universities (No. 16KJA430005), and the Natural Science Foundation of State Key Laboratory of Coordination Chemistry.

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