Fe3C@carbon nanocapsules/expanded graphite as anode materials for lithium ion batteries
Introduction
With the fast development of mobile electric application, lithium ion batteries with high energy/power density and better cycling performance are in ever-increasing need. As commercial anode currently, graphite owes high electrical conductivity, better recyclability. However, the low theoretical specific capacity (372 mAh/g) and poor rate capability restrict its application on high performance batteries. Therefore, the development of new carbon-based anode materials with enhanced reversible capacity and fast charge/discharge ability is of utmost importance [1].
Expanded graphite (EG), the most important graphite derivative, has aroused considerable interest [2], [3], [4], [5], [6]. Compared with graphite, EG exhibits much higher capacity owing to its larger pores, larger surface area, high conductivity, structural defects, chemical stability and extra space for lithium-ion storage. Bai et al. applied modified Hummers’ method to prepare EG which maintained 412 mAh/g after 30 cycles at the current density of 0.2 mA/cm2[7]. Mildly EG was synthesized by using perchloric acid as both intercalating agent and oxidizing agent, displaying a rate capacity as high as 397 mAh/g at 0.2C and 250 mAh/g at 1.6C [8]. From the previous work, EG shows a higher reversible capacity than traditional carbon material, but it is still very low. The loss of capacity partly attributes to the irreversible formation of solid electrolyte interface (SEI) film.
Recently, Zhou’s group proposed that the newly-generated transition metal nanoparticles could activate the reversible transformation of some SEI components and further benefited reversible capacity [9], [10]. Especially, some authors pointed out that iron carbide (Fe3C) could improve the electrochemical performance of the carbonaceous anode under similar mechanism [1], [11], [12], [13]. In 2012, an Fe@Fe3C/C composite was first reported as an anode material for lithium ion batteries, which gave a capacity of 500 mAh/g after 30 cycles due to the modification of SEI film by Fe3C [11]. The obvious extra capacity can be attributed to the reversible formation and dissolution of polymeric gel-like SEI film along with the discharge and charge process. Indeed, this reversible gel-like film is part of SEI components. Fe3C shells can reduce some SEI components. Considering the catalysis function of Fe3C, many researches have adopted various methods to prepare Fe3C/C composite with interesting morphologies and excellent electrochemical performances (shown in Table 1, Table 2), including in-situ electrospinning [13], polymerization-pyrolysis [1], [12], sol-gel process [14]. However, these traditional synthesis of Fe3C/C composite are often difficult because of high reaction temperature, using hazardous and expensive chemical precursors, advanced equipment and tedious steps, and the as-obtained composite demonstrates the highest capacity reported so far 1098 mAh/g [12].
In this work, we attempt to design Fe3C@carbon nanocapsules (CNCs)/EG composite with cheap ferroence and EG through a facile process including reflux and heat treatment under low temperature (below 600 °C). The as-obtained Fe3C@CNCs/EG exhibits excellent electrochemical performances, including cycle ability and rate performance.
Section snippets
Materials Synthesis
Fe3C@CNCs/EG was fabricated by an in-situ synthesis method as following: The starting materials were ferrocene (Xi Long Chemical Share Co. Ltd), hydrogen peroxide (Guang Dong Guang Hua Sci-Tech Co. Ltd), EG (Qing Dao Graphite Material Co. Ltd), acetone (Xi Long Chemical Share Co. Ltd). In a typical synthesis, ferrocene (12.00 g) was dissolved in acetone to form a clear solution. EG (1.00 g) was added into above solution with vigorous stirring for 8 h. When the temperature cooled down, H2O2 (11.00
Results and discussion
As shown in Fig. 1a, the crystal structure of pristine EG was studied by X-ray diffraction. In the pattern of EG diffraction peaks are observed at 2θ = 26.5°, 42.2°, 44.4°, 54.5° and 77.2°: these peaks can be indexed to the Graphite-2H (0 0 2), (1 0 0), (1 0 1), (0 0 4) and (1 1 0) faces, respectively (PDF No. 41-1487), which is representative of EG in the crystalline phase. The XRD analyses illustrated in Fig. 1b indicates a similar curve with EG except some peaks located at the range from 35° to 50°,
Conclusion
In summary, a novel Fe3C@CNCs/EG composite with in-situ generated Fe3C nanoparticles and interesting 3-D structure was synthesized by a new and facile method, which includes mix of starting materials and heat treatment under low temperature. It is featured by unique 3-D structure, where EG acts as scaffold to ensure a continuous entity, and Fe3C coated by CNCs are embedded intimately on the EG matrix. The Fe3C nanoparticles encased in CNCs act as catalyst in the modification of SEI film during
Acknowledgements
This work is supported by the National Natural Science Foundation of China (U1401246 and 51364004), and Guangxi Natural Science Foundation(2013GXNSFDA019027).
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