Electrochimica Acta 98 (2013) 239–243 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al hom epa ge: www.elsevier.com/locate/electacta Lithiation behavior of single-phase Cu–Sn intermetallics and effects on their negative-electrode properties Atsushi Kitada ∗ , Naoki Fukuda, Takashi Ichii, Hiroyuki Sugimura, Kuniaki Murase 1 Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan a r t i c l e i n f o Article history: Received 27 September 2012 Received in revised form 13 February 2013 Accepted 6 March 2013 Available online xxx Keywords: Cu–Sn intermetallics Reduction-diffusion method Alloy anodes Lithium ion batteries a b s t r a c t Single-phase Cu–Sn intermetallics were used to investigate their phase transformations during lithia- tion/delithiation, together with their negative-electrode properties. The stoichiometric Cu–Sn samples were prepared as thermodynamically warranted phases using a reduction-diffusion method with con- trolled potentials. Potentiostatic lithiation tests suggested that Cu 3 Sn directly changes into Li x Sn, while Cu 6 Sn 5 becomes Li 2 CuSn before Li x Sn forms. We also revealed that separation from Li 2 CuSn into Cu and Li x Sn occurs at +0.11 to +0.10 V vs. Li. Additionally, charging/discharging tests with cutoff potentials of +1.5 V to +0.11 V showed better cycling performance than that with +1.5 V to +0.00 V, probably due to the suppression of Li x Sn formation. Such a tendency can be expected in other Cu 6 Sn 5 electrode materials. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Lithium ion batteries have gained increasing importance associ- ated with the fast progress of mobile devices and electric vehicles. Graphite, the most common negative electrode material reaches about 360 mAh g -1 , near the theoretical capacity of 372 mAh g -1 (or 837 mAh cm -3 ); thus, alternative materials have received consider- able attention. Elemental Sn has been studied by many researchers because of its very large theoretical capacity of 994 mAh g -1 or 7246 mAh cm -3 . However, it is well known that a fairly large vol- ume expansion ratio of 4.6 during the lithiation into Li 4.4 Sn gives rise to poor cycle stability [1]. Instead, Sn-based alloy electrodes (M x Sn y ) take advantages of elemental Sn electrodes, where a sec- ond metal element (=M) buffers the volume change to improve the cell performance [1]: examples include M = Fe [2–4], Ni [4,5], Cu [4,6–16], and Ag [17,18], to name but a few. Among them, Cu–Sn and Ag–Sn intermetallics have intermediate phases i.e. Li–M–Sn ternary alloy [4,6–16], which suppress the effect of the volume change, and the cheaper Cu–Sn materials have attracted many researchers. Lithiation of a Cu–Sn intermetallic Cu 6 Sn 5 results in an interme- diate phase Li 2 CuSn with a much lower volume expansion ratio of 1.6, comparable to that of graphite (1.1); moreover, its theoretical capacity is as large as 275 mAh g -1 or 2285 mAh cm -3 , providing potential applications as compact electrode materials. Since the ∗ Corresponding author. Tel.: +81 75 753 5475; fax: +81 75 753 5463. E-mail address: kitada.atsushi.3r@kyoto-u.ac.jp (A. Kitada). 1 ISE member. Thackeray group first reported on electrochemical performance of Cu 6 Sn 5 [6,7], numerous studies have been done [4,6–16]. Some reports suggest that cycling performance can be improved by mor- phological control (i.e. porous material) to absorb the mechanical strain during cycling [13]. However, most samples reported pre- viously were mixed phase Cu–Sn intermetallics and/or elemental Sn. There are only a few studies on electrochemical performances of single-phase Cu–Sn compounds such as Cu 6 Sn 5 [4,11–13] and Cu 3 Sn [11,14,15]. Moreover, as far as we know, nothing has been discussed in terms of phase transformations of stoichiometric Cu–Sn intermetallics using potentiostatic lithiation tests, where thermodynamically stable phase would appear. Recently, single-phase Cu–Sn intermetallics have been prepared in a Sn-containing ionic liquid bath using a reduction diffusion (RD) method. Here, depending on the potentials (i.e. +5 mV vs. Sn for Cu 6 Sn 5 and +20 mV vs. Sn for Cu 3 Sn) stoichiometric Cu–Sn inter- metallics are selectively obtained as thermodynamically warranted phases [19]. The RD method has advantages over other methods for obtaining stoichiometric Cu 6 Sn 5 electrodes. It requires much lower temperature (150 ◦ C) than the conventional high temper- ature solid-state process (400 ◦ C) [12]. Additionally, the use of a single-metal-containing bath enables simpler and easier treatment of the waste bath than the use of Cu 2+ –Sn 2+ mixture baths [4,13]. In this paper, we compared the lithiation/delithiation behaviors of single phase Cu 3 Sn and Cu 6 Sn 5 and their negative-electrode prop- erties. Cyclic voltammetry suggested that both Cu 3 Sn and Cu 6 Sn 5 can be lithiated, but that Cu 3 Sn is not so active against lithiation compared to Cu 6 Sn 5 . Potentiostatic lithiation also revealed that for Cu 6 Sn 5 , the change from Li 2 CuSn to Li x Sn occurs at +0.11 V to +0.10 V vs. Li. Additionally, lithium charge/discharge tests with 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.035