This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 12741–12745 12741 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 12741–12745 Silicon core–hollow carbon shell nanocomposites with tunable buffer voids for high capacity anodes of lithium-ion batteriesw Shuru Chen, Mikhail L. Gordin, Ran Yi, Giles Howlett, Hiesang Sohn and Donghai Wang* Received 2nd July 2012, Accepted 27th July 2012 DOI: 10.1039/c2cp42231j Silicon core–hollow carbon shell nanocomposites with control- lable voids between silicon nanoparticles and hollow carbon shell were easily synthesized by a two-step coating method and exhibited different charge–discharge cyclability as anodes for lithium-ion batteries. The best capacity retention can be achieved with a void/Si volume ratio of approx. 3 due to its appropriate volume change tolerance and maintenance of good electrical contacts. Introduction Silicon has been considered as a promising alternative to graphite-based anodes for next generation lithium-ion batteries because of its natural abundance, environmental friendliness, and most importantly, low discharge potential and the high theoretical capacity (4200 mA h g À1 in Li 4.4 Si). 1,2 However, the practical application of Si anodes has thus far been mainly hindered by low electrical conductivity and low lithium diffusion rate of Si, 3 and by the enormous volume change (300–400%) experienced during the lithiation/delithia- tion process. 4 The volume change can cause bulk Si to be pulverized and lose electrical contact with the conductive additive or current collector, and can also lead to instability of the solid electrolyte interphase (SEI) resulting in continuous consumption of the Li-ion electrolyte for reformation of SEI layers, both of which therefore lead to fast capacity fading. In order to improve the cycling stability of silicon anodes, great efforts have been made to mitigate the pulverization of Si and improve the stability of the SEI layer. These efforts include the development of Si materials composed of nanostructures, 5–9 porous structures, 10–15 or nanocomposites, 16–22 the addition of coating layers, 23–25 and the application of electrolyte additives 26,27 and novel binders. 28–30 Among these efforts, a simple and widely applied strategy is to use Si–C nanocomposites. 20–25 However, the success of this approach is still limited because large Si volume change can only be tolerated to a limited degree, especially during deep charge–discharge processes. Suitable porosity in the Si–C nanocomposite is thus needed in order to further buffer the volume change of the Si. 31 Hollow micro-/nano-structured materials have been recognized as one type of promising material for applications in energy-related systems. 32 For example, Si–C nanocomposites in which Si nanoparticles are encapsulated in hollow carbon materials, such as Si-hollow carbon nanotubes 33 and Si-hollow carbon spheres, 34,35 can not only increase electrical conductivity and provide intimate electrical contact with Si nanoparticles, but also provide built-in buffer voids for Si nanoparticles to expand freely without damaging the carbon layer. However, Si-hollow car- bon nanotubes require a sophisticated binder-free fabrication process. 33 In comparison, direct synthesis of Si–C core–hollow shell nanocomposites in powder, reported by Iwamura S. et al. and Li X. et al., is promising as the materials can be fabricated using a conventional industrial coating procedure. 34,35 Both of these studies indicate the importance of carbon coating and void spaces in order to obtain good cycling performance from Si anodes. In their approaches, however, part of the Si is sacrificed to obtain buffer voids through its thermal conver- sion to SiO 2 and subsequent removal by etching, resulting in the loss of active materials. Moreover, the diameters of the Si cores vary based on different degree of oxidation during the formation of SiO 2 . As diameters of Si nanoparticles have a prominent influence on the cycling performance, 36 it is difficult to illustrate the direct relationship between cycling perfor- mance and various void/Si volume ratios in the nanocompo- site. Thus, it is desirable to develop a bottom-up synthesis approach to produce Si–hollow carbon nanocomposite materials with conformal carbon shells and tunable built-in buffer voids, in order to demonstrate the effectiveness of the Si–hollow carbon structure using a conventional electrode fabrication process and elucidate the relationship between void/Si volume ratio and electrochemical performance of the nanocomposites. Here we report a versatile solution growth method to synthesize silicon core–hollow carbon shell nanocomposites with controllable, built-in buffer voids between the silicon cores and the hollow carbon shells. A similar approach was reported to synthesize Si core–hollow carbon shell nanocomposites during preparation of the manuscript. 37 However, there have been no systematic studies on optimization of buffer voids for Si core–hollow carbon shell structures. As illustrated in Scheme 1, the synthesis process starts with solution growth Department of Mechanical & Nuclear Engineering, the Pennsylvania State University, University Park, Pennsylvania 16802, USA. E-mail: dwang@psu.edu w Electronic supplementary information (ESI) available: Additional XRD, TGA, TEM and electrochemical characterization. See DOI: 10.1039/c2cp42231j PCCP Dynamic Article Links www.rsc.org/pccp COMMUNICATION Published on 27 July 2012. Downloaded by Pennsylvania State University on 18/07/2013 21:07:43. View Article Online / Journal Homepage / Table of Contents for this issue