Journal of Alloys and Compounds 509 (2011) 3074–3079 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Nanocrystalline Fe 1-x Co x Sn 2 solid solutions prepared by reduction of salts in tetraethylene glycol Uche G. Nwokeke a , Alan V. Chadwick b , Ricardo Alcántara a , Maria Alfredsson b , José L. Tirado a,∗ a Laboratorio de Química Inorgánica, Universidad de Córdoba, Edificio C3, Campus de Rabanales, 14071 Córdoba, Spain b Functional Materials Group, School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK article info Article history: Received 23 September 2010 Received in revised form 27 November 2010 Accepted 30 November 2010 Available online 7 December 2010 Keywords: Cobalt tin intermetallic compound Iron tin intermetallic compound Lithium batteries anode Mössbauer spectroscopy abstract In an effort to improve the electrochemical performance of tin intermetallic phases as electrode active material for lithium-ion batteries, Fe 1-x Co x Sn 2 solid solutions with x = 0.0, 0.25, 0.3, 0.5, 0.6 and 0.8 were prepared by chemical reduction in tetraethylene glycol. Precise control of the synthesis conditions allowed single-phase nanocrystalline materials to be prepared, with particle diameters of about 20 nm and cubic, nanorods, and U-shaped morphologies. The substitution of iron by cobalt induced a contraction of the unit cell volume. The hyperfine parameters of the 57 Fe Mössbauer spectra were sensitive to the Co/Fe substitution and revealed a superparamagnetic behaviour. In lithium cells nanocrystalline Fe 1-x Co x Sn 2 active materials delivered reversible capacities above 500 mAh g -1 that depended on the composition and cycling conditions. The intermediate compositions exhibit better electrochemical performance than the end compositions CoSn 2 and FeSn 2 . © 2010 Elsevier B.V. All rights reserved. 1. Introduction Intermetallic compounds containing tin atoms are excellent electrode active materials for lithium ion batteries. The alloys that contain cobalt are particularly promising because they show great ability to form nanostructured particles and exhibit excellent sta- bility and capacity retention [1]. Thus, several studies on CoSn [2], CoSn 2 [3,4], and Co 3 Sn 2 [4] have been reported. Also carbon [5,6] and polymer [7,8] additives were recently found useful to improve the performance. Pure CoSn 3 is difficult to prepare at high temperature [9]. Microcrystalline CoSn [2] and Co 3 Sn 2 [4,10,11] show very little ability to react with lithium, but in the amorphous state or with very small particle size, these solids can exhibit high capacities. Microcrystalline CoSn 2 was studied by Ionica-Bousquet et al. [3] and a high reactivity with lithium was observed. Recently, nano-sized Co–Sn alloy anodes were studied in lithium cells, and a two-step electrochemical reaction mechanism of CoSn 2 alloy with Li + was confirmed [12]. Due to cost and toxicity factors, the substitution of cobalt by iron in successful cobalt–tin intermetallics would be advan- tageous. Similarly, by using only a partial substitution, the best properties of CoSn x phases might be preserved. The intermetal- lic compounds FeSn 2 and CoSn 2 are isostructural (space group I4/mcm). The former was first studied by Mao et al. [13], and it ∗ Corresponding author. Tel.: +34 957218637; fax: +34 957218621. E-mail address: iq1ticoj@uco.es (J.L. Tirado). was found that the reaction of lithium with Sn–Fe alloys leads to Li–Sn alloys and nanometer-sized Fe grains. Recently, thermody- namically metastable structures have been generated during the electro-crystallisation process and this has given rise to a new and novel range of amorphous based Sn–Fe electrodeposited alloys [14]. Nanocrystalline FeSn 2 prepared by chemical reduction in tetraethylene glycol [15] and by reduction in aqueous and ethanol solutions [16] was recently reported. More recently, micro- and nano-FeSn 2 were compared by different researchers [17,18]. On the other hand, Sn 30 (Co 1-x Fe x ) 30 C 40 materials were prepared by Fergu- son et al. [19] using mechanical attrition of FeSn 2 , CoSn 2 and C, and amorphous grains of Fe x Co 1-x Sn in a carbon matrix were found. These authors found that the cobalt-free sample (x = 1) exhibits poor capacity retention on cycling. In this work the formation of nanocrystalline particles com- posed of Fe 1-x Co x Sn 2 solid solutions by using a one-pot method is reported. For this purpose, the reduction of salts in an organic solvent below 200 ◦ C is used. The tin nanocrystals are used as a template for the formation of intermetallic particles. Previous stud- ies showed that the transition metals atoms can diffuse into -Sn nanoparticles to form intermetallic phases [20]. 2. Experimental Powders of Fe1-xCoxSn2 were obtained by following a tetraethylene glycol (TEG) method based on previous works [15,20]. For this purpose, SnCl2, FeCl3·6H2O and CoCl2·6H2O were dissolved in TEG. All the samples were prepared under the same experimental conditions, except the variable ratio between the reagents cobalt chlo- ride and iron chloride in order to modify the x-value in the nominal composition. 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.11.202