Rational design of Co3O4/Co/carbon nanocages composites from metal organic frameworks as an advanced lithium-ion battery anode
Introduction
As one of the most promising energy storage devices, lithium ion batteries (LIBs) have been widely applied in different portable electronics [1]. Since LIBs were introduced into commercial electronic products by Sony [2], graphite has been used as the typical anode material. However, the theoretical capacity (372 mA hg−1) and poor rate performance of graphite is far short of the demands for large scale applications like electrical and hybrid electrical vehicles (EVs and HEVs) [3], [4], [5], [6], where higher energy density and power density are particularly important. A class of emerging anode materials, transition metal oxides (MOx, M = Mn, Fe, Ni, Co, Sn, etc.) have attracted much attention due to their higher specific capacities [7], [8], [9], [10]. However, there are several obstacles of those metal oxides to be used in LIBs [11], [12], [13]: (1) large volume change upon lithiation and delithiation; (2) poor electrical conductivity; (3) low coulombic efficiency. Therefore, various strategies have been carried out for solving those obstacles and enhancing the electrochemical performance. For instance, nanostructured materials, with high specific surface area and robust structure, can accommodate the volume expansion and shrinkage during insertion and extraction process of lithium ion [13], [14], [15], [16], [17], [18]. Hybridization with conductive carbonaceous substrates (amorphous carbon, carbon nanotube and graphene) is a common approach to enhance the conductivity and electrochemical properties of the electrodes materials [19], [20], [21], [22], [23], [24].
Recently, metal organic frameworks (MOFs) was used to prepare transition metal oxide nanostructures due to its high surface areas, large pore volumes and well defined pore size distributions [25]. Through thermal annealing MOFs materials, carbon-coated transition metal oxide with desired structure can be obtained in a facile process [26], [27], [28], [29], [30]. For distance, Zou et al. reported hierarchical yolk-shell NiO/Ni/Graphene nanostructures derived from Ni-based MOF [26], which exhibited nearly no capacity loss for Li-ion storage after 1000 cycles. It was demonstrated that mixed valence metal oxides hollow structures (CuO/Cu2O) were successfully prepared from MOFs, which could provide superior performance as anode materials in LIBs (740 mA hg−1 at 100 mA g−1) [27]. A porous carbon electrode with embedded ZnO quantum dots was prepared by annealing Zn-based MOFs, reaching a reversible capacity of 1200 mA hg−1 [28]. Other metal oxides or sulfides such as Fe2O3, TiO2, CoS and NiS derived from MOFs were also reported and show promising applications as anode materials in LIBs [31], [32], [33], [34]. These studies inspire us to design anode materials derived from MOFs with enhanced Li-ion.
In this work, Co3O4/Co/carbon nanocages (COCCNCs) with hollow polyhedral shape were synthesized through two steps of thermal annealing of Co-based MOFs (ZIF-67). Such composites with rational design have a high surface area and porous structure, robust carbon framework and hollow structure, which can provide a well-contact between electrode and electrolyte and more active sites, possess a good conductive network and structural stability of the electrode. Therefore, COCCNCs was used as anode electrode for LIBs and exhibited a high reversible capacity, improved Coulombic efficiency, superior rate capability and excellent cycling stability.
Section snippets
Materials synthesis
Co3O4/Co/carbon nanocages (COCCNCs) were synthesized by using Co-based metal organic frameworks as a precursor. The Co-based MOF (ZIF-67) was synthesized according to previous report with minor modifications [34]. Typically, 0.45 g Co(NO3)2·6H2O were dissolved in 5 mL deionized water to form solution A. 5.5 g 2-Methylimidazole was dissolved in 20 mL deionized water to form solution B. Then, the solution A was poured into solution B slowly and stirred for 6 h at room temperature. The resulting purple
Results and discussion
Co3O4/Co/carbon nanocages (COCCNCs) were fabricated based on the scheme illustrated in Fig. 1. Firstly, Co-based metal organic frameworks (ZIF-67) were prepared from Co(NO3)2·6H2O and 2-Methylimidazole in deion water with a vividly stir under a room-temperature condition. Secondly, Co/carbon nanocages (CCNCs) were obtained after heat treatment of ZIF-67 precursors under Ar condition. Finally, COCCNCs were prepared after oxidation treatment of CCNCs at Air atmosphere.
Fig. 2a shows the SEM image
Conclusion
In summary, the Co3O4/Co/carbon nanocages (COCCNCs) were successfully synthesized from cobalt based metal organic frameworks precursors, which were prepared through a facile method. Such composite material is composed of the N-doped carbon nanocages with graphitic structure and well-dispersed Co3O4 and Co tiny nanoparticles embedded in the nanocage matrix. COCCNCs also own a large specific surface area and uniform pore size distribution. When COCCNCs was employed as anode in LIBs, it exhibited
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (NSFC 51502038, 21406035 and U1505241), Research Fund for the Doctoral Program of Higher Education of China (RFDP 20133514110002), Natural Science Foundation of Fujian Province (2015J01042), and Education Department of Fujian Province (JA14081 and JA14076).
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