This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 9149–9151 9149 Direct scattered growth of MWNT on Si for high performance anode material in Li-ion batteriesw Pengfei Gao, a Yanna Nuli, a Yu-Shi He, a Jiazhao Wang, c Andrew I. Minett, b Jun Yang* a and Jun Chen* b Received 28th July 2010, Accepted 8th October 2010 DOI: 10.1039/c0cc02870c A novel Si-MWNT nanocomposite synthesized via a CVD process shows a high reversible capacity of over 1500 mAh g À1 and stable cycling performance, which can be ascribed to the maintenance of a good conductive network by means of the direct scattered growth and pinning of MWNTs on Si particles. The increase of energy density is a key point for extending the application range of lithium-ion batteries, especially for electric/hybrid vehicles. Silicon is one of the most attractive anode materials for lithium batteries on account of its low discharge potential and the well-known highest theoretical capacity of about 4200 mAh g À1 corresponding to the fully lithiated composition of Li 4.4 Si, 1 which is more than ten times higher than that of commercial graphite anodes (372 mAh g À1 ). 2 However, silicon-based electrodes suffer from very poor cyclability due to loss of electronic contact between the active particles resulting from silicon’s volume changes by 400% upon insertion and extraction of lithium. 3 Research results have confirmed that the decrease of particle size of the Li-storage hosts can significantly improve the electrochemical cycling behavior due to the reduced absolute volume change and weaker pulverization tendency. 4–6 In order to improve the electrode stability, nano-size silicon is homogeneously dispersed within a suitable matrix resulting in a nanocomposite, in which the second phase component acts as a buffering matrix to accommodate the large volume changes upon cycling. Among the different matrix materials considered, carbon is a good candidate due to its relatively low mass, good electronic and ionic conductivity, reasonable Li-insertion capability, small volume expansion, softness and compliance. 7 Various methods have been employed for preparing Si/C composite anodes. 8,9 Dahn et al. appear to have been the first to prepare Si/C composites from thermal pyrolysis of various polymers containing silicon and carbon, such as polymethylphenylsiloxane (PMPS) or polyphenyl- sesquisiloxane (PPSSO) in the temperature range of 900–1300 1C. 10 Lately, the Si@C composites with core/shell structure have aroused great interest because the complete carbon shell could buffer the volumetric changes of Si particles and maintain the electric contact. 11,12 Ng et al. reported on the synthesis of carbon-coated Si nanocomposite prepared using a spray pyrolysis method, exhibiting a capacity of 1489 mAh g À1 after 20 cycles. 13 Hu et al. prepared core/shell Si@C nano- composites by hydrothermal treatment of a dispersion of Si nanoparticles and glucose in water, followed by heat treatment in nitrogen. 14,15 This material presented good cycling performance with the help of vinylene carbonate (VC) electrolyte additive, but it was not stable in the conventional electrolyte without VC. Up to now, the cycling performance of core/shell Si@C composites is still unsatisfactory for industrial applications, perhaps due to the destruction of the carbon shell upon silicon volume variations during cycling. 16 More durable composite architectures with strong adherence between silicon and carbon need to be developed to guarantee the close electric contact. More recently, great efforts have been made toward the syntheses of nanocomposites of inorganic materials and carbon nanotubes (CNTs), 17–19 with the aim of exploiting the unique properties of CNTs, such as the high aspect ratio, low mass, flexibility, high mechanical strength, and high electrical and thermal conductivities for applications in lithium ion batteries. 20,21 Shu et al. demonstrated a cage-like CNTs/Si composite with a reversible capacity of 940 mAh g À1 and improved cycling performance for 20 cycles, where micro- size Si particles were enwrapped by the coiled CNTs via a catalytic chemical vapor deposition (CVD) process. 22 Related work was published by Kim et al., wherein the improved cyclability has been attributed to the conductive buffering role of the CNTs layer and the void space within it, which alleviated breakdown of the conductive network in Si negative electrodes. 23 However, the inhomogeneous Ni catalyst deposits prepared in the electroless plating process gave rise to two different types of CNTs and even caused unsuccessful growth of those in some samples. Meanwhile, the number of charge–discharge cycles was limited to only 12. Another kind of one dimensional carbon material, carbon nanofibers (CNFs), were also investigated as candidates for Si/C composites through a CVD approach, 24,25 but showed less stable cycling performance, perhaps owing to their intrinsical a Institute of Electrochemical and Energy Technology, Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. E-mail: yangj723@sjtu.edu.cn; Fax: +86-21-5474 7667; Tel: +86-21-5474 7667 b Intelligent Polymer Research Institute, ARC Centre of Excellence for Electromaterials Science, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW 2522, Australia. E-mail: junc@uow.edu.au; Fax: +61-2-4221 3114; Tel: +61-2-4221 3781 c ARC Centre of Excellence for Electromaterials Science, Institute of Semiconducting and Electronic Materials, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW 2522, Australia w Electronic supplementary information (ESI) available: Experimental details, SEM images of the Si/MWNT mixture, Raman spectra and thermogravimetry profiles of the Si-MWNT composite and pure Si powder, reversible capacities of the Si-MWNT composite and Si/MWNT mixture and schematic illustration of structural changes for different Si-CNT materials. See DOI: 10.1039/c0cc02870c COMMUNICATION www.rsc.org/chemcomm | ChemComm Downloaded by University of Wollongong on 06 December 2010 Published on 28 December 2010 on http://pubs.rsc.org | doi:10.1039/C0CC02870C View Online