A Novel Coordination Polymer as Positive Electrode Material for Lithium Ion Battery Jiangfeng Xiang, Caixian Chang, Ming Li, Simin Wu, Liangjie Yuan, and Jutang Sun* College of Chemistry and Molecular Science, Wuhan UniVersity, Wuhan 430072, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 1 280–282
ReceiVed April 23, 2007; ReVised Manuscript ReceiVed September 23, 2007
ABSTRACT: A new coordination polymer based on an aromatic carbonyl ligand is prepared and investigated as a positive active material for lithium ion batteries, namely, [Li2(C6H2O4)] (1). It is synthesized by the dehydration of [Li2(C6H2O4) · 2H2O] (2). These compounds are characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), elemental analysis (EA), single crystal X-ray diffraction methods and powder X-ray diffraction (XRD). As positive material, compound 1 has an initial discharge capacity of 176 mAh · g-1 and a columbic efficiency of 93.18% in the first cycle. It might provide a new method for finding new positive-electrode materials in lithium ion batteries. Introduction In recent years, considerable attention has been paid to lithium ion batteries for their applications in cameras, computers, and portable electronics, etc.,1–5 while energy storage in this technology is mainly limited by the positive electrode, which usually shows a much lower capacity than the negative one.6,7 There are intensive interests in finding new positive-electrode materials with high capacity and high stability. Efforts mainly focus on the modification of inorganic transition-metal-oxidebased materials, due to their excellent oxidizing and reducing capabilities.8–14 So far as we know, little attention has been paid on the organic positive-electrode materials.15,16 Recently, we reported an aromatic carbonyl derivative polymer, namely, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) sulfide polymer, which showed both high capacity and high cycling stability.17 On the basis of our previous work, we selected another aromatic carbonyl compound, 2,5-dihydroxy1,4-benzoquinone (2,5-DBQ), with higher theoretical capacity (384 mAh · g-1) than PTCDA (273 mAh · g-1). In this work, we report a 2,5-DBQ based coordination polymer as a novel positive-electrode material for lithium ion batteries. More recently, coordination polymers have undergone tremendous development owing to their interesting crystal structures and potential application in magnetism, luminescence, biology and catalysis, etc.18 Here, it is applied in lithium ion batteries as a new kind of electrode material. In this coordination polymer, namely, [Li2(C6H2O4)] (1), the existence of the carbonyl group brings the possibility for Li ions to be inserted or deinserted reversibly at positions of oxygen atoms when the carbonyl groups are reduced or oxidized, implying a novel organic energy storage system in lithium ion batteries. In the reduction process, as shown in Scheme 1, each carbonyl group can accept one electron and insert one Li ion to form lithium enolate, and the Li ions deinsert in the reverse oxidation process. At the same time, the coordination polymer framework can provide a stable structure for insertion or deinsertion of the Li ions. Furthermore, the charge–discharge tests show the initial discharge capacity of compound 1 is 176 mAh · g-1, which is * Corresponding author. Tel: +86-27-8721-8264. Fax: +86-27-6875-4067. E-mail:
[email protected].
Scheme 1. Diagram for the Proposed Reversible Li Ion Insertion-Deinsertion in Compound 1
much higher than that of former reported polymer based on 2,5DBQ monomer units (90 mAh · g-1).19 Experimental Section Materials and Measurements. All the starting materials were purchased from commercial sources and used without further purification. The powder X-ray diffraction (XRD) patterns were obtained by a Shimadzu XRD-6000 diffractometer with a Ni filter and Cu KR1 radiation (λ ) 1.54056 Å). The IR spectra were recorded from KBr pellets at a range of 400–4000 cm-1 on a Nicolet 5700 FTIR spectrometer with a spectral resolution of 4.00 cm-1. Thermogravimetric analysis (TGA) studies were carried out with a NETZSCH STA 449C at a heating rate 20 K · min-1 under air. The elemental analysis (EA) data were obtained from a Perkin-Elmer 240B elemental analyzer. For the cell measurement, the cathode performance was examined by a Neware battery program-controlled test system. The charge–discharge tests were carried out using the coin-type cell (size: 2016), which consisted of a working electrode and a lithium foil counter electrode separated by a Celgard 2300 microporous membrane. Compound 1 powders, 35 wt% acetylene black and 5 wt% PTEE binder were mixed. Electrode was prepared by compressing the mixture onto an aluminum mesh current collector. The mass loading of the electrode was about 4.63 mg · cm-2. A 1 mol · L-1 solution of LiPF6 dissolved in EC/DMC (1:1 volume ratio) was used as the electrolyte. The cells were assembled in an argon-filled glovebox (Mikrouna, Super 1220/750). The cells were charged and discharged between 1.5 and 3.5 V vs metallic lithium at a constant current density of 100 mA · g-1. Cyclic voltammetry (CV) was determined on CHI760B electrochemical analyzer (CH Instruments Corp. USA) in three-electrode system. Preparations of Compound 1. This compound was prepared from the dehydration of 2 at 250 °C for 6 h. Compound 2 was synthesized as follows: Stirring of 0.50 g of 2,5-DBQ with 2 equiv of lithium hydroxide in 5 mL of water was performed to get a red solution. Then the solution was heated at 80 °C in an open container for 10 h; the single crystal of compound 2 was obtained. X-Ray Crystallography. Crystallographic measurements were obtained on a Bruker SMART CCD area-detector diffractometer and performed at room temperature (293 K) using graphite monochromated
10.1021/cg070386q CCC: $40.75 2008 American Chemical Society Published on Web 11/21/2007
Coordination Polymer for Lithium Ion Battery
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Figure 2. TGA curves for compound 2.
Figure 1. (a) A 2-D layer constructed by Li-O bonds bridged carbonyl rings along the ac plane in compound 2. All H atoms are omitted for clarity. (b) Packing structure of compound 2 with a 3-D architecture along the a-axis. Mo KR radiation (λ ) 0.71073 Å). The structures were solved by direct methods using the program SHELXS-97.20
Results and Discussion X-ray single-crystal determination indicates that compound 2 crystallize in the C2/m space group. Figure 1a exhibits the two-dimensional (2-D) framework of compound 2 along the ac plane, in which aromatic carbonyl rings are bridged via Li-O bonds. In this 2-D layer, 2,5-DBQ has two different coordinated models: (i) All of its carbonyl and hydroxyl groups provide oxygen atoms to coordination. (ii) Only two hydroxyl oxygen atoms connect the Li ion and the other two uncoordinated oxygen atoms can be assigned to CdO, which is also confirmed by the stretching band at 1700 cm-1 in the FT-IR spectrum (Figure S1, Supporting Information). Furthermore, the coordinated water molecules extend the 2-D layers into a threedimensional (3-D) architecture along the a-axis (Figure 1b). According to Scheme 1 and structure analysis above, the existence of the carbonyl group brings the possibility for insertion or deinsertion of Li ions, and Li-O bridged 2-D layer aromatic rings can provide a stable structure for insertion or deinsertion of the Li ions. However, the coordinated water is a problem for this compound as a positive-electrode material because of the reaction of water and electrolyte. To determine the dehydration temperature, TGA was carried out at a heating rate of 20 K · min-1 under air. As shown in Figure 2a, a total weight loss of 18.09% occurs in the range of 120-200 °C, corresponding to the removal of two coordinated water molecules per formula unit (calcd. 19.17%). No obvious mass loss is observed from 200 °C, which indicates a dehydrated phase formulated as [Li2(C6H2O4)] is stable at this stage. Hence, calcination of compound 2 is carried out in air at 250 °C for 6 h, resulting in compound 1. EA for compound 1: (Mr ) 151.96). Anal. calc. (%): C, 47.43; H, 1.33. Found (%): C, 47.48; H, 1.42.
Figure 3. XRD patterns of compounds 1 and 2.
When the coordinated water in compound 2 is removed, the obtained compound 1 is not sufficient for single-crystal diffraction. However, it can be determined by XRD experiments. The XRD patterns of compounds 1 and 2 are shown in Figure 3. Different from compound 2, XRD analysis of compound 1 shows that it crystallize in the P2/m space group. More details of the lattice parameters of compounds 1 and 2 are given in Table S2, Supporting Information. The removal of coordinated water molecules may cause the collapse of the framework along the b-axis. As confirmed by the XRD analysis results, the value of b decreases dramatically [from 6.4354(13) to 3.8713(61) Å]. Electrochemical Performance. CV is a useful technique to evaluate the cycling performance of the [Li2(C6H2O4)] electrode, which is carried in a potential range of 1.0–4.0 V versus Li/ Li+ with lithium metal as the reference and counter electrode. The cyclic voltammogram of electrode at a scanning rate of 50 mV · s-1 is presented in Figure 4. No obvious oxidation is observed on sweeping positive from the open circuit potential of 2.1 V. On the first negative sweep, a cathodic peak corresponding to the insertion of Li+ is observed at about 1.7 V. This process is assumed to be the reduction of the carbonyl groups to form lithium enolate, which is illustrated in Scheme 1. Oxidation process appears in the subsequent positive sweeps as shown in Figure 4. The broad peak around 2.7 V can be ascribed to oxidation of the lithium enolate. Charge–discharge characteristics of the Li/[Li2(C6H2O4)] cells are studied in a voltage range from 1.5 to 3.5 V at a constant current density of 100 mA · g-1, and the charge–discharge curves for the first 10 cycles are shown in Figure 5. The columbic
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fully synthesized a coordination polymer with carbonyl groups and 2-D layered aromatic rings. All of these specialties indicate that can be used in lithium ion batteries. Further investigation based on the aromatic carbonyl derivative is still in progress. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20771087). Supporting Information Available: The IR spectra and X-ray crystallographic information files (CIF) of compound 2 (CCDC No. 625094). Table of lattice parameters of compounds 1 and 2. Table of XRD data for compound 1 calculated from the XRD pattern. This information is available free of charge via the Internet at http:// pubs.acs.org.
References Figure 4. Cyclic voltammogram of compound 1 recorded at 50 mV · s-1.
Figure 5. Charge–discharge curves for the first 10 cycles of [Li2(C6H2O4)] electrodes at a constant current density of 100 mA · g-1 between 2.0 and 3.6 V.
efficiency of the first cycle is 93.18%. All the charge–discharge curves are similar and show clear plateaus. The distinct plateaus can be assumed to structural changes of carbonyl groups to lithium enolate in the charge–discharge process. After 10 cycles, the discharge capacity still remains at 137 mAh · g-1. Conclusions Our work presents a novel Li-O bonds bridged coordination polymer, namely, [Li2(C6H2O4)], prepared from the dehydration of compound 2. X-ray single-crystal determination indicates that compound 2 illustrates a 3-D architecture. When the coordinated water molecules are removed, the 3-D framework may collapse along the b-axis in compound 1. However, the collapsed structure is even stable enough for insertion or deinsertion of the Li ion, which is proved by cyclic voltammogram and charge–discharge experiments. In conclusion, we have success-
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