Nanostructured silicon/silicide/carbon composite anodes with controllable voids for Li-ion batteries
Graphical abstract
Introduction
Advances in electric vehicles (EVs), hybrid electric vehicles (HEVs), portable electronic devices, and renewable energy storage have resulted in increasing demands for high-energy and high-power, long-life, and low-cost Li-ion batteries (LIBs) [1], [2], [3]. To date, natural and synthetic graphite powders are the most extensively used anode materials for commercial LIBs [4], [5]. However, graphite has a theoretical maximum capacity of 372 mAh g− 1, which limits its extended applications in high-capacity systems such as EVs and HEVs. Si, Sn, Ge, and their alloys have been studied as anodes for LIBs because these materials possess higher theoretical specific capacities compared with graphite [5], [6], [7], [8], [9]. Among these materials, Si has been most intensively studied because of its high theoretical capacity (3580 mAh g− 1 for Li15Si4), low cost, relatively low reaction potential (< 0.5 V vs. Li/Li+), and natural abundance [5], [6], [10], [11]. Despite these advantages, Si undergoes an extremely large volume change (> 300%) during Li insertion and extraction processes, which leads to pulverization of the material, reduction in electrical contact with the conductive additive or current collector, and continuous consumption of the electrolyte to reform the solid electrolyte interphase (SEI) layers on the newly exposed Si surfaces, thus, eventually resulting in rapid reduction of the capacity [5], [12], [13].
Thus far, several strategies have been proposed to reduce the detrimental effects of the large volume change and suppress the side reactions with the electrolyte to improve the long-term cycling stability of Si. Nanoparticles, nanowires, and nanotubes have been tested [10], [14], [15], [16], [17] as it has been reported that nanosized Si materials tend to exhibit greatly reduce fracturing because of volume change during cycling [18], [19]. There are some reports on the preparation of composites of Si with electrochemically inactive materials to reduce the extent of volume changes [20], [21], [22], [23]. It has also been reported that mixtures of Si and carbon or carbon-coated Si, which exhibit better electrical contact and promote the formation of more stable SEI layers, result in a significant improvement of the cycling performance [21], [24], [25]. In addition, Si with porous structures that can accommodate volume change without fracturing have been used as anodes, thus, resulting in superior cycling properties [26], [27], [28].
Recently, several groups have successfully combined strategies such as size reduction, carbon coating, and incorporation of voids to prepare elaborate structures, such as yolk–shell and pomegranate-like structures as well as non-filling carbon-coated structures [29], [30], [31]. All of these anode materials contain voids between Si nanoparticles and carbon coating layers [32], [33], [34], [35], [36]. As the Si particles are coated with an electrically conducting carbon layer and there is sufficient space to accommodate the volume expansion during the lithiation process, the initial structures can be effectively maintained even after a few hundred cycles. Additionally, the carbon coating layer prevents the direct contact between the liquid electrolyte and Si surface, therefore suppressing the formation of a new SEI layer, resulting in significant improvements in the cycling performance. Even though these structures may be ideal to solve the intrinsic problem of large volume expansion of Si, most of the previous attempts have adopted complicated and multi-step processes that are difficult to apply in large-scale production. For instance, expensive nanosized Si powders are partially oxidized and coated with carbon using various methods and the oxide layer is etched out using hydrofluoric acid solution to produce nano-Si/voids with an encapsulating coating layer [28]. Most recently, porous Si/C composites were prepared through spray drying and carbonization method by using NaCl as pore template. They used 80–160 nm silicon and sucrose as a carbon precursor. As they used water as a solvent and sucrose is highly soluble in water, it would be difficult to control the size of voids [37].
In the present study, we develop an economical and easy-to-scale-up method to prepare three-dimensional (3D) carbon-network-supported Si/silicide nanocomposite powders with controllable voids using inexpensive commercially available ferrosilicon alloy, NaCl, and polyfurfuryl alcohol (PFA) as starting materials. These nanostructured Si/silicide/carbon composites with voids are designed by combining all of the four strategies: size reduction, mixing with inactive materials, carbon coating, and introduction of controllable voids. We believe that our approach is versatile in controlling both the size and volume fraction of voids without using expensive raw materials or environmentally toxic processes.
Section snippets
Synthetic procedure
We prepared composite active anode materials with controllable voids using ball-milled ferrosilicon alloy and fine water-soluble compound mixtures as starting materials. Fig. 1 presents an overview of the preparation process of the Si/silicide composites with and without NaCl. The mixture of ferrosilicon (81 wt% Si, Elkem), Al (99.5%, Wako chemical/3 wt%), Fe powders (99.5%, Sigma Aldrich/4 wt%), and paraffin (melting point 53–57 °C, Sigma Aldrich/1.5 wt%) together with 10-mm-diameter Cr-steel balls
Results and discussion
Fig. 2a presents XRD patterns of the raw ferrosilicon alloy, as-milled alloy, and coated alloy without voids. Si and α-FeSi2 peaks are observed in the XRD pattern of the raw alloy. After the ball-milling process, the Si peaks are much weaker and broader, indicating a substantial decrease in the crystallinity and crystallite size as well as a decrease of the amount of Si due to the reaction between Si and Fe during ball milling. The decrease in Si content, in turn, leads to an increase in the
Conclusion
We have developed an economical and easy-to-scale-up method to prepare 3D carbon-network-supported Si/silicide nanocomposite powders with voids using ferrosilicon alloy, NaCl, and PFA as starting materials. This nanostructured Si/silicide/carbon composite anode with controllable voids has been designed and prepared by combining the strategies: nanostructuring (size reduction and compositing with inactive materials), carbon coating, and the introduction of controllable voids. Coin-half cell
Acknowledgements
This research was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea (10037919).
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