One-Pot Synthesis of Co-Based Coordination Polymer Nanowire for Li-Ion Batteries with Great Capacity and Stable Cycling Stability
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KeywordsNanowire Coordination polymer Lithium-ion battery Anode Ultra-high capacity
Amide-group-coordinated cobalt–terephthalonitrile (Co-BDCN) coordination polymers, with a diameter distribution of 45–55 nm, were synthesized by a one-pot solvothermal method.
Reversible capacity of 1132 mAh g−1 was achieved at a current density of 100 mA g−1.
Rechargeable lithium-ion batteries (LIBs) have been finding increasing number of applications in a variety of fields, including portable electronic devices, electrical energy storage (EES), electric vehicles (EVs), and hybrid electric vehicles (HEVs) . Given the commercial anode graphite possesses a low theoretical capacity of 372 mAh g−1, developing high-capacity anode materials is vital for aforementioned applications, particularly in areas where application of miniaturization is increasing. To develop futuristic high-performance anode materials, stable structure with abundant lithiation sites is necessary. To this end, metallic oxide-based [2, 3, 4, 5, 6, 7, 8, 9, 10], Sn-based [11, 12], Si-based [13, 14], and P-based [15, 16, 17, 18] anode materials have been widely studied. Although their theoretical capacity is high, their cycling stability is poor due to large volume changes during the charge–discharge process [19, 20, 21, 22].
Metal organic frameworks (MOFs) or coordination polymers (CPs), which are assembled by inorganic metal ions as vertices and organic ligands as linkers, have attracted tremendous attention in recent years [23, 24, 25]. By varying the metal centers and functional linkers, MOFs with various pore sizes and structures can be designed to cater for the increasing demands in the fields of catalysis, sensing, gas storage, drug delivery, and proton conductivity [26, 27]. Recently, the electrochemical applications, especially for LIBs, have attracted significant attention due to their tremendous potential as both cathode [28, 29, 30] and anode materials [31, 32, 33, 34, 35]. MOF-177 , Zn3(HCOO)6 , Mn-LCP , Mn-BTC , Co2(OH)2BDC , BiCPs , and CoBTC  have been applied as anode (Table S1). For example, Co2(OH)2BDC exhibited reversible capacity of 650 mAh g−1 after 100 cycles at current density of 50 mA g−1 . In these CPs or MOFs, carboxylate groups (e.g., 1,3,5-benzenetricarboxylate and 1,4-benzenedicarboxylate) are usually used to coordinate with different metal centers (e.g., Mn, Co, and Zn). During the charging process, Li+ ions are inserted mainly to the organic moiety (including the carboxylate group and the benzene ring) in these MOFs [39, 42]. The electron-donating effect of the carboxylate group and the benzene ring is considered the main impetus in storing lithium ions. Conjugated dicarboxylates can eventually serve as anode materials without any metal center . However, CP- or MOF-based electrodes with other kinds of organic linkers are seldom used.
Herein, we selected terephthalonitrile as the organic linker and Co(NO3)2·6H2O as the metal source, and synthesized an amide-group-coordinated CP with nanowire-like structure using a simple solvothermal method. The coordination participation of the amide group showed a higher Li+ storage performance as compared to Co2(OH)2BDC, which uses the carboxylate group as a linker. A reversible capacity of 1132 mA g−1 was retained after 100 cycles at a rate of 100 mA g−1. The synergistic effect between the organic linker and the Co2+ center, as well as the excellent stability of the nanowire-like structure, may account for the superior electrochemical performance.
3.1 Materials Synthesis
Co-BDCN was solvothermally synthesized with Co(NO3)2·6H2O (5 mmol, Aladdin, 99.99%) and terephthalonitrile (5 mmol, Aladdin, 99%) in N,N-dimethylformamide (DMF, 50 mL, Sinopharm, AR) solution. The reactants were stirred for 10 min at room temperature to achieve complete dissolution and then transferred to a 100 mL Teflon-lined stainless steel autoclave before heating at 150 °C for 3 or 24 h. The samples obtained after 3 and 24 h will be referred to as Co-BDCN-3h and Co-BDCN-24h, respectively. After cooling to room temperature, the product was filtered and successively washed by DMF and ethanol for three times to remove surplus reactants. The product was finally obtained by drying at 70 °C for 12 h. It is noteworthy that the direct synthesis of Co-BDCN-24h from terephthalamide and Co(NO3)2·6H2O failed due to the very low solubility of terephthalamide in the available solvents (DMF, methanol, alcohol, and water).
3.2 Materials Characterizations
A Rigaku Ultima IV X-ray diffractometer (XRD) with Cu-Kα radiation (V = 35 kV, I = 25 mA, λ = 1.5418 Å) was used to analyze the crystal phase of the as-prepared materials. N2-sorption isotherms and BET surface area were measured at 77 K with a 02108-KR-1 system (Quantachrome). The morphologies of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-2400, Japan). Before initiating the test, the samples were mounted on aluminum stubs and sputtered with gold. Thermogravimetric analysis (TGA) was performed using a STA 449 F3 Jupiter®, which simultaneously acted as a thermo-analyzer. Temperature was varied from room temperature to 800 °C at a heating rate of 10 °C min−1. A Nicolet-Nexus 670 infrared spectrometer was used to perform Fourier transform infrared spectroscopy (FTIR) analysis. The cells for ex situ SEM test were cycled 50 times and discharged to 0.01 V to reduce the reactivity of the electrode. After that, we disassembled the battery in a glove box filled of pure argon and washed the electrode several times with DMC to remove the residual electrolyte. The electrode was tailored and pasted in conductive carbon adhesive tape directly before the test. The inductively coupled plasma (ICP) test was performed on Thermo IRIS Intrepid II XSP spectrometer. Varian 700 M was used to collect 1H nuclear magnetic resonance (1H-NMR) spectra in liquid state. About 1-mg samples were dispersed in 0.5 mL DMSO-6d. Then the liquids were heated at 80 °C for 5 min and ultrasonically vibrated for 5 more minutes before the 1H-NMR test. Bruker 600 M was used to collect 1H-NMR spectra in the solid state.
3.3 Battery Performance Measurements
All electrochemical measurements were taken at room temperature. The active material (weight ratio: 80%), conducting additive (Super-P carbon black, weight ratio: 10%), and the binder (carboxymethyl cellulose sodium or CMC, weight ratio: 10%) were homogenously mixed in deionized water (solvent) for at least 3 h to produce a slurry. The thus-obtained slurry was coated onto Cu foil and dried at 70 °C in vacuum oven for 12 h. The electrodes were punched into round plates (diameter of 14.0 mm). The loading of the as-prepared electrodes is about 1.0 mg cm−2. 1 M LiPF6 in EC–DMC–EMC (1:1:1 in volume) was used as the electrolyte. Finally, a coin cell (CR2032) was assembled by the as-prepared anode, a Celgard 2325 separator (diameter of 19.0 mm), a pure lithium wafer (counter electrode), and electrolyte in an argon-filled glove box, with oxygen and water contents less than 0.1 ppm. The galvanostatic charge and discharge and rate tests were performed on a LAND 2001A battery test system in the voltage range of 0.01–3.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (CHI660e) at a scan rate of 0.2 mV s−1 in the voltage range of 0.01–3.0 V.
4 Results and Discussion
The 1H NMR spectra in liquid state are shown in Fig. 1b. The chemical shift of H in -D2H of DMSO-d6 was set at 2.5 ppm (Fig. S1). A well-defined peak is detected at 8.10 ppm for the H (–Ar) of terephthalonitrile, while three resonances of intensity ratio of 1:2:1 at 7.52 ppm (Ha, –NH2), 7.92 ppm (H, –Ar), and 8.09 ppm (Hb, –NH2) are observed in the terephthalamide (the two protons in –NH2 of the amide group show different chemical shifts due to magnetic anisotropy, electric field, and steric effects) . For the formation of amide groups in Co-BDCN-3h, these three broad peaks appear at the same positions. Due to the successful coordination of Co2+ and terephthalamide, Co-BDCN-24h could not dissolve in DMSO-d6. As a result, no resonance could be detected in the positions. Solid-state 13C NMR spectra of terephthalonitrile, terephthalamide, and Co-BDCN-24h are plotted. In contrast with the well-defined peaks of terephthalonitrile and terephthalamide (Fig. S2), the peaks of Co-BDCN-24h (Fig. 1c) are very broad (FWHM ≈ 200 ppm) due to the effect of paramagnetic Co2+ center.
Rate performance was also studied to further explore the electrochemical capability of Co-BDCN-24h. Figure 5d shows the change of cycling performance with increasing rates: 100, 200, 500, 1000, and 2000 mA g−1. The charge capacities corresponding to these rates are 1000 ± 35, 1020 ± 30, 866 ± 13, 713 ± 7, and 538 ± 10 mAh g−1, respectively. After repeating the rate test at 100 mA g−1 for 50 cycles, the capacity is recovered with a value of about 1100 mA g−1 and is sustained at a steady value in the subsequent cycles, which indicates that the Co-BDCN-24h anode remains stable during the rate cycling process.
In the past, CPs or MOFs based on carboxylate ligands, such as 1,3,5-benzenetricarboxylate and 1,4-benzenedicarboxylate, have shown potential for Li+ storage. In this work, an amide-group-based CP, Co-BDCN-24h, was synthesized and characterized for the first time. The Co-BDCN-24h electrode, with uniform nanowire morphology, demonstrated ultra-high capacity for Li+ storage, i.e., 1132 mAh g−1 at 100 mA g−1 (after 100 cycles). The great reversible capacity and superior cycling stability were attributed to the synergistic effect between metal centers and organic ligands, as well as the preservation of the nanowire morphology during cycling. This work provided an alternative to conjugated dicarboxylate-based MOF anode materials.
This work is supported by Basic Research Project of Shanghai Science and Technology Committee (14JC1491000), the Large Instruments Open Foundation of East China Normal University, National Natural Science Foundation of China for Excellent Young Scholars (21522303), National Natural Science Foundation of China (21373086), National Key Basic Research Program of China (2013CB921800) and National High Technology Research and Development Program of China (2014AA123401).
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