Synthesis and characterization of SiO2/C composite nanofibers as free-standing anode materials for Li-ion batteries
Introduction
The rapid development of different technologies to meet the needs of today’s information-rich society requires a consecutive improvement of Li-ion batteries (LIBs) as key components of portable, entertainment, and telecommunication equipment [1]. LIBs have been the energy storage system of choice for various applications owing to their high energy and power density, light weight, and compactness [2]. On the other hand, the current demand for energy lies far beyond the capacity of commercially available LIBs, which is limited by their electrode materials [[1], [2], [3], [4]].
Research over the past several decades has been devoted to the development of new electrode materials with special attention to cathodes [[5], [6], [7]]. As a result of extensive studies, novel high-energy technologies with the potential to be implemented in the near future have been proposed [8]. Nevertheless, when coupled with high-capacity cathode materials, the commercially available anode materials still have a limited capacity (372 mA h g−1) [4].
Since the discovery of the electrochemical activity of SiO2 towards Li+ ions by Gao et al. [9], SiO2 has been attracting more attention as a promising alternative anode material owing to its high theoretical capacity (1965 mA h g−1) [10]. Furthermore, the natural abundance, low cost, and environmental friendliness of SiO2 make it a commercially viable electrode material for LIBs [11]. However, there are some limitations hindering the practical application of this material. First, the high capacity of SiO2 cannot be fully utilized owing to its poor electronic conductivity. The actual reversible capacity of pure commercial SiO2 nanoparticles (7 nm diameter) has been reported to be ca. 400 mA h g−1 [9]. Another problem is capacity fading caused by volume expansion during cycling. To overcome these issues, carbon coating and nanostructuring are believed to be effective approaches [[10], [11], [12], [13], [14], [15], [16], [17], [18]].
Among the different techniques for the synthesis of carbon composite nanostructures, electrospinning is recognized as a simple and cost-effective synthesis technique. It can produce one dimensional (1D) carbon composite nanofibers of a large number of materials, including organics and inorganics [[19], [20], [21], [22]]. The 1D fibers are considered to be an excellent conductive substrate for host nanomaterials owing to the very short paths for Li+ on the cross-section of the fibers and the large interior surface area [23]. Polymer composite nanofibers of SiO2 can be easily synthesized via electrospinning as an intermediate product of porous carbon (C), silicon (Si) or Si/C nanofiber synthesis [[24], [25], [26], [27], [28]]. Recently, an electron-conductive SiO2@C nanofiber has been developed and applied as a microporous layer in a proton-exchange membrane fuel cell [29]. However, there are only a few studies on the electrochemical performance of SiO2/C composite nanofibers as an anode material for LIBs [[30], [31], [32], [33]].
Wu et al. [30] developed nanostructured SiO2/C composites by mixing SiO2 nanoparticles with polyacrylonitrile (PAN) as a carbon precursor, which was subjected to electrospinning and subsequent heat treatments. Prepared composite fibers containing 15 wt.% SiO2 showed stable cycling performance with a high reversible capacity of 658 mA h g−1 after 100 cycles at a current density of 50 mA g−1. Another research group [31] further enhanced the cyclability of SiO2/C nanofibers up to 1000 cycles by fabricating flexible and robust N-doped free-standing nanofiber films from a mixture of PAN and tetraethyl orthosilicate (TEOS) as the SiO2 source. On the other hand, the low utilization of SiO2 (23 wt.%), which is caused by the precipitation of PAN upon mixing with the SiO2 precursor solution [32], still limits the overall capacity of the composite. Furthermore, PAN has relatively low solubility in many solvents, and the commonly used solvent dimethylformamide (DMF) is hazardous. The introduction of another polymer source, along with an increase in the overall SiO2 content in the electrode, should be considered.
Wang et al. [34] developed mesoporous carbon nanofibers from thermoplastic polyvinylpyrrolidone (PVP), which is more soluble than PAN in many solvents, including green water and ethanol, making it much easier to develop a wide range of carbon-based composite nanofibers. Ren’s group [33] then mixed SiO2 nanoparticles with PVP and obtained SiO2/C composite fibers by electrospinning with heat treatments. The composite containing 44 wt.% SiO2 showed a reversible capacity of 465 mA h g−1 at a current density of 50 mA g−1 up to 50 cycles. On the other hand, the uniform dispersion of nanoparticles in the polymer solution cannot be ensured, and inhomogeneous mixing of the components may negatively affect the cyclability of the composite [31].
In this work, we have attempted to develop free-standing SiO2/C composite nanofibers with a high SiO2 content from the homogeneous precursor solutions containing PVP and TEOS via electrospinning with two-step heat treatment, comprising preoxidation at 280 °C in the air followed by annealing at 700 °C for 1 h in a reduced atmosphere. The excellent mechanical flexibility and strength of the free-standing nanofibers ensure the stability of the electrode during cycling. Furthermore, elimination of the additive of conductive carbon black and electrochemically inactive binder, and metal current collector, to some extent, decrease the weight of cell and improve the energy density [31,35]. We have investigated the effects of the process parameters on the structure and morphology of the prepared nanofibers and the correlation between the physical and electrochemical properties of SiO2/C nanofibers as the anode material for LIBs. To the best of our knowledge, this is the first report on the effects of the preoxidation process and fiber size on the electrochemical performance of SiO2/C nanofibers.
Section snippets
Precursor solutions
The precursor solution used for the electrospinning was prepared by dissolving 0.24 g of TEOS (99%) in a mixture of 0.48 g of PVP (Mw = 1,300,000) with 8 mL of ethanol, which was followed by stirring for 2 h. The concentration of PVP in the ethanol was varied from 4 to 7 wt.%, while the weight ratio of PVP to TEOS was fixed at 2:1. Nitric acid (HNO3) was added to the solutions as a catalyst with a TEOS: HNO3 molar ratio of 20:1.
Experimental setup and procedure
Fig. 1 shows a schematic diagram of the electrospinning apparatus,
Synthesis of SiO2/C composite fibers
Fig. 3 shows the SEM images of the samples prepared by electrospinning from the precursor solution with 7 wt.% PVP before and after the heat treatments under different conditions. The electrospun sample clearly shows a fibrous morphology with continuous long smooth fibers (Fig. 3(a)). On the other hand, the morphology was drastically changed after the direct heat treatment at 700 °C by the interconnection of the fibers with each other (Fig. 3(b)). To prevent the drastic change in morphology
Conclusions
SiO2/C composite nanofibers were successfully synthesized by electrospinning with two-step heat treatment, comprising the preoxidation at 280 °C in the air followed by annealing at 700 °C for 1 h in a reduced atmosphere. The effects of the preoxidation treatment on the morphology, chemical structure, and electrochemical properties of the synthesized materials were studied by SEM-EDS, TEM, XRD analysis, FTIR spectroscopy, NMR spectroscopy, and electrochemical testing. FS-SiO2/C-CNFMs with
Acknowledgment
This research was supported by Tokyo Institute of Technology. The authors are also grateful to Mr. J. Koki and Mr. Y. Sei, staff members of the Center for Advanced Materials Analysis (Tokyo Institute of Technology, Japan) for the analysis of samples. Moreover, the authors gratefully acknowledge Mr. A. Michishita (Shimadzu Co.) for help in obtaining the pore size distributions of the samples.
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