Synthesis of silicon/carbon, multi-core/shell microspheres using solution polymerization for a high performance Li ion battery
Introduction
The development of rechargeable batteries has been the focus of extensive research due to the fast growing market for battery-powered devices, such as power tools, mobile phones, game devices, hybrid electric vehicles and locomotives [1], [2], [3], [4]. Although batteries are commercially available, challenges still exist for producing high power, specific capacity batteries that are light weight and have high energy densities, long time durations and low production costs [5], [6], [7]. Some research groups have reported the replacement of carbon-based anode materials with oxides and alloys using transitional metals because commercial graphite has a low capacity (372 mAh/g) for anode electrodes and a performance that is currently insufficient for use in cars and portable electronic devices [8].
Silicon has been proposed for use as a high-capacity anode material because Si has a theoretical capacity of approximately 4200 mAh/g, which can result in high capacity battery cyclability with increased capacity retention [9], [10]. Despite these advantages, Si is not a satisfactory material because of poor cycle stability, high manufacturing costs and insufficient capacity. When Si forms alloys with Li (4.4 Li atoms per Si atom, Li4.4Si), a large change in volume occurs during lithiation/delithiation. The large change in volume (>300%) upon insertion has caused electrode failure due to the loss of electrical contact between the active materials and the current collector [11], [12], [13]. Previous studies using Si bulk film, which underwent ball-milling and thermal reduction to improve capacity and cyclability of the Si-based materials, resulted in cycle fading and short battery life due to pulverization and loss of electrical contact [14], [15], [16], [17], [18], [19]. Significant improvements have been reported for a Si–graphite composite [20], Si nanowires [21], Si nanotubes [22], and Si 3D porous structures [23] that allow for high capacity and cyclability as anode materials. However, these methods suffer from high synthesis costs as well as difficult and complex reactions that limit the industrial application.
We have designed a simple method to synthesize Si–C composite anode materials using an O/W system. After the polymerization of the silicon nanoparticle/carbon precursor (acrylonitrile) in an organic solvent, the mixture was granulated in an aqueous PVA solution. Si–PAN composite particles were prepared after evaporation of the organic solvent. The Si–C composite particles were obtained by thermal annealing at 800 °C and an etching process. The etching process provided expansion space for the silicon and increased the capacity and cyclability. In addition, the incorporated porous structure of the Si–C composite particles induced the fast transport of Li ions during Li insertion and extraction. As a result, the porous structure and etching process contributed to a high capacity and excellent cyclability for Li ion batteries.
Section snippets
Materials
Acrylonitrile (AN, 99.9%, JUNSEI, Chuo-ku, Tokyo, Japan), 3-(trimethoxysilyl)propyl methacrylate (TMSPM, 98%, Aldrich Chemical Co., Milwaukee, WI, United States), sodium dioctyl sulfosuccinate (aerosol-OT, Wako Chemical, Dalton, GA, United States), divinylbenzene (DVB, isomer mixture; 55%, Aldrich Chemical Co., Milwaukee, WI, United States) 2,2′-azobis-(2,4-dimethylvaleronitrile) (ADVN, Waco Chemical Co., Dalton, GA, United States), polyvinylalcohol (PVA, MW = 8.8–9.2 × 104 g mol−1, 87–89%
Results and discussion
Fig. 1 presents the SEM images of the Si–C multi-core particles after carbonization. The carbonized particle has a rough surface and a polydispersed size distribution (5–80 μm), as shown in Fig. 1a. These features resulted from shrinkage of the suspended droplet during the evaporation of MC. After treatment with a HF solution for 10 min, the surface of the etched Si–C composite particles was rougher than that of the non-etched Si–C composite particles (Fig. 1b) because the Si nanoparticles were
Conclusions
Si/poly(AN–TMSPM) composite particles were synthesized through a simple method using solution polymerization and an O/W (oil in water) system. The overall capacities of the prepared electrode materials increased with increasing silicon content in the composite particles. Moreover, the etching process for the silica oxide layer of the composite particles successfully provided volume expansion for the Si nanoparticles and thus induced a change in cyclability. By incorporating an organic polymer
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2010-0012206).
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These authors contributed equally to this work.