Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Metal−Organic Framework Derived Core−Shell Co/Co3O4@N‑C Nanocomposites as High Performance Anode Materials for Lithium Ion Batteries Ming Zhong,†,‡,∥ Wei-Wei He,‡,§,∥ Wei Shuang,†,‡ Ying-Ying Liu,†,‡ Tong-Liang Hu,*,†,‡ and Xian-He Bu*,†,‡,§ †
School of Materials Science and Engineering, Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, National Institute for Advanced Materials, Nankai University, Tianjin 300350, People’s Republic of China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China § College of Chemistry, State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China S Supporting Information *
ABSTRACT: Metal−organic frameworks (MOFs) with diverse structures, adjustable pore sizes, and high surface areas have exhibited awesome potential in many fields. Here we report a simple carbonization strategy to obtain a series of core−shell structured Co/Co3O4 nanoparticles encapsulated into nitrogendoped carbon shells from cobalt-based metal−organic framework precursors at different carbonization temperatures. When it is applied as an anodes for lithium ion batteries, the Co/Co3O4@N-C-700 electrode delivers a maximum initial discharge capacity of 1535 mAh g−1, the highest reversible capacity (903 mAh g−1 at a current density of 100 mA g−1 after 100 cycles), and the best rate performance (i.e., 774 mAh g−1 at a current density of 1.0 A g−1 after 100 cycles) in comparison with those of Co/Co3O4@N-C-600 and Co/Co3O4@N-C-800 electrodes. The excellent electrochemical performance could be mainly attributed to the unique core−shell structure, abundant graphited carbon, and the well-dispersed Co/Co3O4 nanoparticles which can promote the specific capacity through conversion reactions.
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anode materials.3,13−16 Specifically, mixed-valence Co3O4 exhibits a high theoretical capacity of 890 mAh g−1 through an eight-electron conversion reaction, thus showing themselves to be promising anode materials for LIBs.17,18 However, its intrinsic poor electrical conductivity and huge volume expansion/extraction during the discharge and charge processes still hamper the widespread application of this material. To solve the aforementioned issues, many kinds of strategies have been proposed.19 One method is preparing the materials with controlled size and morphology, which can shorten the electronic and ionic diffusion distance and increase the contact area between electrode and electrolyte.20,21 Another strategy is assembling bicomponent-active Co3O4-based materials, such as Co3O4−Fe3O4, Co3O4−CoFe2O4, and Co3O4−ZnO.22−24 In addition, the design of electrode materials with hollow and polyhedral structures is also an effective approach.25−27 In addition to the methods mentioned above, coating carbon to form a core−shell structure has been proven to be a terrific strategy to solve the aforementioned issues because carbon can
INTRODUCTION Rechargeable lithium ion batteries (LIBs), as some of the most promising, highly efficient, and cost-effective energy storage techniques, have received sustained attention and have been applied in portable electronic devices and hybrid electrical vehicles because of their relatively high theoretical energy density, long cycling stability, low self-discharge, and no memory effect.1−5 Though LIBs have been commercialized since the 1990s,6 they still cannot meet the requirements of further markets. For this expectation, more and more efforts have been devoted to developing electrode materials, especially anode materials, to improve LIB performance.7−10 Presently, there have been mainly three kinds of widely studied anode materials, including commercial graphite, transition-metal oxides, and silicon, but all have various dissatisfactory aspects. For example, the commercial graphite anodes have a low theoretical capacity (LiC6, 372 mAh g−1) and low Li ion transport rate,11 while silicon anodes are limited by huge volume expansion (300−400%) during the lithium insertion/ extraction process.12 In contrast, transition-metal oxides, with the advantages of low cost, high theoretical specific capacity (>600 mAh g−1), and high reversible capacity achieved by conversion reactions, have stood out from these common © XXXX American Chemical Society
Received: February 8, 2018
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DOI: 10.1021/acs.inorgchem.8b00365 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. (a) Schematic illustration for the synthetic process of core−shell Co/Co3O4@N-C samples and (b) XRD patterns, (c) Raman spectra, and (d) nitrogen adsorption−desorption isotherms of the as-synthesized Co/Co3O4@N-C at 600, 700, and 800 °C.
has a great influence on the electrochemical performance when these nanocomposites were evaluated as anodes for LIBs. In particular, the optimized sample Co/Co3O4@N-C-700 displays a higher reversible capacity and rate performance in comparison to those of Co/Co3O4@N-C-600 and Co/Co3O4@N-C-800 samples.
act as a layer to increase the conductivity and accommodate the volume changes.28,29 Metal−organic frameworks (MOFs) are a class of inorganic− organic hybrid materials assembled by metal ions/clusters and organic linkers through coordination interactions to form an ordered structure. Recently, MOFs have been promising precursors or self-sacrificial templates to obtain various transition metal oxide based nanomaterials with special structures and morphologies,30 due to their advantages of diverse structure, adjustable pore size, and high surface area.31−33 In particular, MOF-derived Co3O4/carbon hybrids have shown high specific capacity and excellent cycling stability and rate performance when they are used as anode materials for LIBs. Han et al.34 reported the nitrogen-rich cobalt-based MOF [Co(diimpym)(npta)]n (diimpym = 4,6-bis(1H-imidazol-1yl)pyrimidine, H2npta = 5-nitroisophthalic acid) and used it as precursor to obtain porous nitrogen-doped carbon-coated Co3O4 fish-scale composites. The results demonstrated that this kind of material delivered excellent electrochemical performance with a high capacity of 612 mAh g−1 at 1000 mA g−1 after 500 cycles. Sun and co-workers35 have also prepared porous hollow Co3O4 polyhedrons with nitrogen-doped carbon-coated composite (Co3O4/N-C) via pyrolysis of a cobalt-based MOF ([Co6O(TATB)4](H3O+)2·Py), which exhibited high lithium storage capacity and outstanding cycling performance when they were applied as anode materials for LIBs. Therefore, using MOFs as precursors to prepare carbon-coated cobalt oxide nanocomposites has great potential. Despite the progress mentioned above, there is still room to design new MOFderived core−shell Co-based electrode materials for superior performance. In this work, a new layered cobalt-MOF (NUM-6) based on mixed ligands of 4,4′,4″-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid (H 3 TATAB) and 2,4,6-tris(pyridin-4-yl)-1,3,5-triazine (TPT) was designed, constructed, and used as a precursor for the preparation of core−shell Co/ Co3O4@N-C nanocomposites. The carbonization temperature
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EXPERIMENTAL SECTION
Chemicals. All of the reagents and solvents, including cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR, Aladdin), N,N-dimethylformamide (DMF, AR, Tianjin Concord), and ethanol (EtOH, 99.5%, Tianjin Concord), were commercially obtained and used without further purification. Distilled water was used in experiments. The organic ligands H3TATAB (C24H18N6O6, 4,4′,4″-((1,3,5-triazine-2,4,6triyl)tris(azanediyl))tribenzoic acid) and TPT (C18H12N6, 2,4,6tris(pyridin-4-yl)-1,3,5-triazine) were synthesized according to the literature procedures.36,37 Preparation of [Co3(TATAB)2(TPT)2(H2O)10]·DMF·4H2O (NUM6). Co(NO3)2·6H2O (29 mg, 0.1 mmol), H3TATAB (24 mg, 0.05 mmol), and TPT (10 mg, 0.03 mmol) in 6 mL of a DMF/EtOH/H2O mixture (2/2/2, v/v/v) was sealed in a 10 mL vial and heated at 100 °C for 24 h. The orange crystals were collected by filtration, washed with DMF, and dried in air. Yield: about 80%. Preparation of Co/Co3O4@N-C Nanocomposites. Typically, the as-prepared Co-MOF (NUM-6) precursors were transferred into a tubular furnace and heated at different temperatures (T = 600, 700, 800 °C, respectively) with a heating rate of 5 °C min−1 under a nitrogen atmosphere for 4 h. After they were cooled to room temperature, the samples were then maintained in air at 250 °C for 30 min. Finally, the targeted materials were obtained and designated as Co/Co3O4@N-C-T (where T denotes the heating temperature, including 600, 700, and 800 °C: i.e., Co/Co3O4@N-C-700). Characterization. XRD patterns were tested by a Rigaku MiniFlex600 X-ray diffraction meter in a 2θ range of 3−80° at 40 kV and 15 mA with a Cu-target tube. The thermal stability of the NUM-6 precursor was conducted on a TGA instrument (Thermo plus EVO 2, TG8121). TEM (Tecnai G2F20S-TWIN) and energy dispersive X-ray spectrum (EDX) elemental mapping were used to identify the morphology of Co/Co3O4@N-C-T. A CHONS elemental analyzer (Vario EL cube) was used to confirm the contents of C and N B
DOI: 10.1021/acs.inorgchem.8b00365 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. (a, b) TEM images, (c, d) HRTEM images, and (e) TEM image with corresponding elemental mapping of Co, O, C, and N of the assynthesized Co/Co3O4@N-C-700 sample. fluoride) (PVDF) at a mass ratio of 7:2:1 with N-methyl-2pyrrolidinone (NMP) as the solvent to form a slurry. Then, the obtained slurry was uniformly coated on copper foil (∼0.9−1.1 mg per electrode) and dried overnight at 110 °C in a vacuum oven. The electrolyte was a solution of 1 mol L−1 LiPF6 dissolved in a mixture of dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and ethylene carbonate (EC) (1/1/1, v/v/v). Celgard 2400 membrane was used as a separator. Cyclic voltammetry (CV) was tested on a VersaSTAT 4 electrochemical workstation (Princeton Applied Research) with a scanning rate of 0.1 mV s−1 between 0.01 and 3.00 V (vs Li/Li+). Discharge/charge tests were performed galvanostatically. Electrochemical impedance spectroscopy (EIS) was carried out on a VersaSTAT 4 electrochemical workstation (Princeton Applied Research) in a frequency range of 105−10−2 Hz at room temperature.
elements. Raman spectra were collected on an Invia Reflex Raman spectrometer with a 632.8 nm laser. XPS was operated on a PHI5000 Versaprobe system. Nitrogen adsorption/desorption measurements were performed on an ASAP 2020 instrument (Micromeritics) at 77 K, and the BET method was utilized to calculate the specific surface areas. X-ray Crystallography. The diffraction data of NUM-6 were collected on a Rigaku 007HF XtaLAB P200 diffractometer at 293 K with Cu Kα radiation (λ = 1.54178 Å) by scan mode. The structure was solved by direct methods using the SHELXTL38 program. Metal atoms in NUM-6 were located from the E maps, and other nonhydrogen atoms were subsequently located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The H atoms of the coordinated water molecules in NUM-6 cannot be added in the calculated positions, and they were directly included in the final molecular formula. Part of the lattice solvent molecules were removed by the SQUEEZE program in PLATON39 and appended in the CIF file. Further details of crystal data and structure refinement for NUM-6 are summarized in Table S1. Selected bond lengths and angles of NUM-6 are given in Table S2. CCDC for NUM-6: 1817893. Electrochemical Measurements. The electrochemical performance was tested on CR2032-type coin cells which were assembled in an argon-filled glovebox (both O2 and H2O have a content
