Mechanically alloyed Sn-Fe(-C) powders as anode materials for Li-ion batteries - I. The Sn2Fe-C system

We have prepared intermetallic phases and mixtures of such phases in the Sn -Fe-C Gibbs' triangle by mechanical alloying methods or by direct melting o f elemental powders. This first paper in a three-part series focuses on the materials which fall on the two-phase line collecting Sn2Fe and C. Using i n situ X-ray diffraction, Mossbauer spectroscopy, and electrochemical metho ds, we show that Sn2Fe reacts with Li in Li/Sn2Fe cells to form lithium-tin alloys and very small metallic iron grains. The experimental capacity for this reaction is about 800 mAh/g, as expected. During the first charge of s uch cells about 650 mAh/g of Li can be extracted up to 1.5 V vs. Li. The de nsity of these materials is near 7 g/cm(3), so first-cycle volumetric capac ities near 4500 Ah/L have been attained. It was our hope that the formed ir on would act as an electrically conductive, inactive matrix to support the Li-Sn alloy grains and that good cycling behavior would result. However, th e extended cycling life of these materials between 1.5 and 0.0 V is poor. O n the other hand, reasonable cycle life is obtained if the cycling range is restricted to between 0.0 and 0.55 V, but in this case, the irreversible c apacity is about 600 mAh/g and the reversible capacity only about 200 mAh/g . We show strategies to overcome these difficulties in the next papers in t his series. (C) 1999 The Electrochemical Society. S0013-4651(98)04-052-X. A ll rights reserved.