Carbon-coated silicon as anode material for lithium ion batteries: advantages and limitations Nikolay Dimov, Satoshi Kugino, Masaki Yoshio * ,1 Department of Applied Chemistry, Saga University, 1 Honjo, Saga 840-8502, Japan Received 18 November 2002; received in revised form 6 January 2003 Abstract Carbon coating of silicon powder was studied as a means of preparation of silicon-based anode material for lithium ion batteries. Carbon-coated silicon has been investigated at various cycling modes vs. lithium metal. Ex situ X-ray data suggest that there is irreversible reduction of crystallinity of the silicon content. Since carbon layer preserving the integrity of the particle, the reversibility of the structural changes in the amorphous state Li  /Si alloy provides the reversible capacity. The progressively decreased Coulomb efficiency with cycling indicates that more and more lithium ions are trapped in some form of Li /Si alloy and become unavailable for extraction. This is the main factor for the capacity fading during cycling. Qualitative studies of the impedance spectra of the electrode material at the first cycle for the fresh anode and at the last cycle after the anode capacity faded considerably and provide further support for this model of fading mechanism. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Silicon; Lithium ion batteries; Anode material; Carbon coating 1. Introduction In recent years, alternatives to metallic lithium and carbonaceous anodes in secondary lithium/lithium ion cells are being sought. Metallic lithium has several disadvantages for use in secondary battery systems including high reactivity, particularly with moisture, and dendrite formation. Although carbonaceous mate- rials have solved the safety problem, their capacity is much lower than that of metallic lithium. A number of investigations appeared, dealing with binary lithium alloys as anodes for secondary batteries with much work on the Li /Al, Li /Sn and Li  /Si systems. The common problem in all these cases is the big volume change (of the order of 100  /300% in respect to the initial volume) which occurs during Li  -insertion in the respective solid state, leading to internal cracks in the structure of the material. The latter effect causes dramatic morphological changes. Due to the changes in the morphology and the loose contact between the active material and current collector, these metals have very poor cyclic characteristics. It is believed that decreasing the particle size could improve the perfor- mance of these metals. However, especially in the case of silicon, even the use of mixtures of silicon nanoparticles and some other conducting phases (usually carbon black) was not successful [1]. The reason is the high surface energy of the nanoparticles. During insertion/ extraction and corresponding expanding/contraction, the particles merge together and after some cycles form dense blocks unable to take part in the electro- chemical reaction. The latter process, known as electro- chemical sintering, limits the applicability of reducing particle size itself. Therefore, the particle  /particle inter- action should be avoided. One way to do this is to cover their surface with a layer, which is electronic and Li  conductor. In this work, we employed carbon coating to prepare silicon-based anode material. Carbon-coated silicon was prepared in cooperation with Mitsui Kozan Co. (Japan) and tested under various conditions [2,6]. It has shown superior cyclic performance compared with the conventional silicon anodes. However, after ex- tended cycling it also shows capacity fade. * Corresponding author. Tel.:  /81-952-28-8673; fax:  /81-952-28- 8591. E-mail address: yoshio@ccs.ce.saga-u.ac.jp (M. Yoshio). 1 ISE member. Electrochimica Acta 48 (2003) 1579  /1587 www.elsevier.com/locate/electacta 0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00030-6