Metal–Organic Framework-Assisted Synthesis of Compact Fe2O3 Nanotubes in Co3O4 Host with Enhanced Lithium Storage Properties
Transition metal oxides are promising candidates for the high-capacity anode material in lithium-ion batteries. The electrochemical performance of transition metal oxides can be improved by constructing suitable composite architectures. Herein, we demonstrate a metal–organic framework (MOF)-assisted strategy for the synthesis of a hierarchical hybrid nanostructure composed of Fe2O3 nanotubes assembled in Co3O4 host. Starting from MOF composite precursors (Fe-based MOF encapsulated in a Co-based host matrix), a complex structure of Co3O4 host and engulfed Fe2O3 nanotubes was prepared by a simple annealing treatment in air. By virtue of their structural and compositional features, these hierarchical composite particles reveal enhanced lithium storage properties when employed as anodes for lithium-ion batteries.
KeywordsMetal–organic framework (MOF) Hierarchical structures Fe2O3 nanotubes Co3O4 Lithium-ion batteries (LIBs)
A metal–organic framework (MOF)-assisted approach is developed for the synthesis of hierarchical composite particles composed of Fe2O3 nanotubes encapsulated in a Co3O4 host matrix.
The hierarchical Fe2O3 nanotubes@Co3O4 composite particles exhibit excellent electrochemical performance when evaluated as an anode material for lithium-ion batteries (LIBs).
Lithium-ion batteries (LIBs) have drawn considerable research attention as a rechargeable power source for portable electronic devices and electric vehicles [1, 2]. Until now, graphite has been the most commonly used anode material in commercial LIBs . However, the relatively low theoretical capacity (372 mAh g−1) of graphite is inadequate to meet the growing demands of energy density and life span in next-generation batteries [4, 5, 6, 7]. Transition metal oxides (TMOs) have been considered as promising electrode materials for LIBs owing to their high specific capacity, low cost, and synthetic versatility to diverse nanostructures [8, 9, 10, 11]. As two representative TMOs, iron oxide and cobalt oxide have been actively investigated [12, 13, 14, 15, 16, 17, 18]. However, the practical application of these anode materials still faces serious challenges, such as fast capacity fading, poor rate performance caused by large volume changes occurring during the lithiation/delithiation processes, and low intrinsic electric conductivity.
To overcome these drawbacks, diverse approaches have been proposed to improve the lithium storage properties. One effective way is to integrate two or more TMO materials into hybrid nanostructures [3, 19, 20]. The hybrid configuration is expected to retain the advantages of each component and, at the same time, provide synergetic effects that enhance the physicochemical properties such as electrochemical reactivity and mechanical stability . Recently, several iron oxide@cobalt oxide hybrid materials have been reported with enhanced lithium storage capability, such as Fe2O3@Co3O4@C composite nanoparticles , Fe2O3@Co3O4 nanowire arrays , and Co3O4@Fe2O3 core–shell nanoneedle arrays . In addition, the construction of hierarchical hollow nanostructures was found to be an effective way to accommodate the large volume changes associated with electrochemical reactions [25, 26]. The permeable shells can reduce Li+ ion diffusion length and guarantee sufficient electrode–electrolyte contact area. Therefore, a rational design and synthesis approach for iron oxide@cobalt oxide hybrid electrodes with hierarchical hollow nanostructures is expected to yield enhanced lithium storage properties.
In recent years, there have been growing research interest for designing advanced electrode materials with controlled architectures and chemical compositions using metal–organic framework (MOF)-based precursors [27, 28, 29, 30, 31, 32, 33, 34, 35]. Most MOF-derived blends are based on simple MOF crystals, and the resulting nanomaterials exhibit relatively simple porous or hollow structures. A rational design of MOF hybrid precursors with novel structures and tailored compositions is highly desirable for the synthesis of high-performance electrode materials [36, 37].
3.1 Synthesis of MIL-88B@ZIF-67 Composites
The MIL-88B nanorods were synthesized by following a hydrothermal method reported earlier . In this method, 0.16 g of F127 was first dissolved in 15 mL of deionized water to which 0.179 g of FeCl3·6H2O was added. The solution mixture was stirred for 1 h, and 0.6 mL of acetic acid was added to it. After stirring for 1 h, 0.06 g of 2-aminoterephthalic acid was injected. It was stirred for another 2 h, after which the reaction mixture was transferred into an autoclave and crystallized for 24 h at 110 °C. The resulting product was washed with ethanol several times. It was then dispersed with 10 mL of methanol solution containing 0.5 g of polyvinylpyrrolidone (PVP, Mw = 40,000), and the mixture was stirred at room temperature for 12 h. The PVP-functionalized MIL-88B nanorods were collected by centrifugation, washed several times with methanol, and dispersed in 15 mL of methanol for further use. To synthesize the MIL-88B@ZIF-67 composite, 0.8 mL of the MIL-88B nanorod suspension, 5 mL of 80 mM 2-methylimidazole (2-MIM) solution, and 3 mL of 20 mM Co(NO3)2·6H2O solution were mixed and allowed to react at room temperature for 4 h without stirring. The reaction product was extracted by centrifugation, washed with methanol several times, and vacuum-dried overnight.
3.2 Thermal Synthesis of Fe2O3 Nanotubes@Co3O4 Composites
The as-formed MIL-88B@ZIF-67 composite was placed in a ceramic boat and heated to 500 °C at a ramp rate of 5 °C min−1 in a tube furnace under ambient atmosphere. The temperature was maintained for 2 h after which the furnace was naturally cooled to room temperature.
3.3 Materials Characterization
Field-emission scanning electron microscope (FESEM; JEOL-6700F) and transmission electron microscope (TEM; JEOL-2010) were used to examine the morphology and structure of the prepared samples. The composition was analyzed by an energy-dispersive X-ray analysis (EDX) equipment attached to the FESEM instrument. The crystal phase was examined using a Bruker D2 Phaser X-ray diffractometer. Elemental mapping and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed in a JEOL-2100F electron microscope. Nitrogen sorption isotherms were measured using Autosorb 6B.
3.4 Electrochemical Measurements
Electrochemical measurements were carried out using CR2032 coin-type half cells. The working electrode consists of an active material (here, Fe2O3 nanotubes@Co3O4 composite particles), carbon black (Super-P–Li), and a polymer binder (polyvinylidene fluoride) in the weight ratio of 70:20:10. The loading mass of the active material is approximately 0.5–0.8 mg cm−2 for each electrode. Lithium foil was used for both the counter and reference electrodes. LiPF6 (1.0 M) in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate was used as the electrolyte. The cell assembly was placed in an Ar-filled glove box with moisture and oxygen concentrations below 1.0 ppm. The galvanostatic charge–discharge tests were performed with a Neware battery test system.
4 Results and Discussion
Overall, we regard that the outstanding lithium storage properties are attributable to a combination of the following factors. First, the assembly of compact Fe2O3 nanotubes in each Co3O4 host provides synergistic effects between two metal oxides with slightly different redox potentials [22, 23, 24]. This facilitates the electrochemical reactions and guarantees high energy density. Second, the hierarchical multilevel cavities and robust architecture lead to an increase in the electrode/electrolyte contact area and help to accommodate the strain of Li+ insertion/extraction, hence contributing to good cycling stability. Finally, the nanosized subunits facilitate electronic/Li+ transport in the electrode material, ensuring enhanced electrochemical activity. All of the above make the Fe2O3 nanotubes@Co3O4 composite particles a highly promising anode material for LIBs.
A novel MOF-assisted strategy has been developed to construct a complex hierarchical nanostructure consisting of compact Fe2O3 nanotubes encapsulated in Co3O4 host. The synthesis involves incorporation of MIL-88B nanorods in the ZIF-67 polyhedron host followed by a thermal treatment process in air to convert the MIL-88B nanorods and ZIF-67 polyhedron to Fe2O3 nanotubes and Co3O4 host, respectively. Benefiting from the unique structural and compositional advantages, the as-prepared hierarchical Fe2O3 nanotubes@Co3O4 composite exhibits outstanding electrochemical properties with good rate capability and excellent cycling stability as an anode material for LIBs. Our study sheds new light on the controlled synthesis of complex hollow structures for various energy-related applications.
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