Cite this: RSC Advances, 2013, 3, 9408 Effect of oxide layer thickness to nano–Si anode for Li- ion batteries Received 27th February 2013, Accepted 8th April 2013 DOI: 10.1039/c3ra41006d www.rsc.org/advances Byeong-Chul Yu, a Yoon Hwa, a Cheol-Min Park, b Jae-Hun Kim c and Hun-Joon Sohn* a Oxide thickness controlled amorphous SiO 2 /nano-Si core–shell structure anode for Li ion batteries was easily prepared by etching with NaOH solution using commercial nano-Si with a native oxide. Based on X-ray diffraction patterns and analysis of high resolution transmission microscope images, the electrode which consists of amorphous SiO 2 /amorphous Si/core crystalline Si after the first cycle showed a self- limiting reaction behavior. Due to this unique dual core–shell structure for a short diffusion path, a high reversible capacity of about 1800 mAh g 21 over 50 cycles and excellent rate capability could be achieved. Introduction Recently, consideration has been made to expand the applications of Li-ion batteries (LIBs) to large scale units including various types of electrical vehicles (EVs) and energy storage for utility grids. LIBs are indispensable for portable electronics, but they cannot meet the requirements for more demanding applications due to limitations in energy density. Among the candidates for anode materials, silicon has been considered as an alternative to graphite (372 mAh g 21 ) for the next generation of Li-ion batteries due to its natural abundance, high capacity (Li 15 Si 4 : y3579 mAh g 21 ), and low operating voltage (y0.4 V). 1–3 However, silicon anode suffers from a large volume expansion (300%–400%) during lithiation/delithiation, 4 which causes pulverization of the bulk Si and a loss of electrical contact with the conducting additive or current collector. A solid electrolyte interphase (SEI) layer continuously formed due to the pulverizing of Si which led to fast capacity fading. Several strategies have been proposed to ease the volume expansion. For instance, nanostructured Si could overcome the pulverization because small-sized Si relaxes the stress during the significant volume expansion. 5–8 However, in the case of using nano-sized Si exclusively, cycle performance was still poor. Among other efforts to enhance electrochemical performance, materials with a core–shell structure have been widely applied. 9–13 The core–shell structure could buffer the severe volume change of active Si upon cycling, preventing it from being pulverized under mechanical stress. However, while the native amorphous surface SiO 2 layer which forms on the Si surface has not attracted a great deal of attention, 14 it is important to design for a nano-structured Si active material. McDowell et al. reported that the use of native SiO 2 on nanowire of a diameter of less than 50 nm could suppress the volume expansion during lithiation, inducing compressive stress that could act to limit the extent of lithiation. 15 Also, using a controlled oxide layer, nano- structured silicon thin film and nanotube showed an excellent cycle performance without carbon or conducting material coating. 16,17 In the case of a spherical Si nano-particle, Xun et al. investigated the effect of surface oxide reduction on its initial performance using HF. 18 However, HF is harmful and difficult to handle due to the fast etching rate of SiO 2 . Also, etched Si still shows poor cycle performance, although its initial reversible capacity increased dramatically compared with bare Si. Since the NaOH solution showed a slow etching rate of SiO 2 while Si was etched anisotropically at a fast rate, 19 the thickness of the amorphous SiO 2 layer formed on nano-Si powders was controlled by simple etching with NaOH solution. The effect of the amorphous SiO 2 layer thickness on the electrochemical performances during cycling was investigated in this study. Also, the self-limiting reaction behavior of nano- Si with oxide layer was proposed. Experimental Material preparation Nano-Si (Aldrich, ,100 nm, n-Si) powders as received were placed under air for 48 h to form a uniform native oxide layer followed by etching with 0.1 M NaOH solution for various times (15, 20, and 25 min) at room temperature. Samples were referred to as n-Si(x), where (x) indicates etching time. The obtained Si powders were washed with distilled-deionized (DI) a Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea. E-mail: hjsohn@snu.ac.kr; Fax: +82-2-885-9671; Tel: +82-2-880-7226 b School of Advanced Materials & System Engineering, Kumoh National Institute of Technology, Gumi, 730-701, Korea c School of Advanced Materials Engineering, Kookmin University, Seoul, 136-702, Korea RSC Advances PAPER 9408 | RSC Adv., 2013, 3, 9408–9413 This journal is ß The Royal Society of Chemistry 2013 Published on 08 April 2013. Downloaded by Seoul National University on 25/06/2013 06:26:01. View Article Online View Journal | View Issue