A Sn-119 Mossbauer spectrometry study of Li-SnO anode materials for Li-ioncells

Anodes of SnO were charged reversibly with Li to capacities greater than 60 0 mAh/g. The anode materials were characterized by Sn-119 Mossbauer spectro metry at 11 and 300 K, and by X-ray diffractometry at 300 K. Trends in the valence of Sn were as expect ed when the Sn oxides are reduced in the prese nce of Li. At low Li capacities the SnO is reduced to small particles of be ta-Sn, and with increasing Li capacity an alloy of Li22Sn5 is formed. Altho ugh the Li22Sn5 develops over a range of Li concentrations in the anode mat erial, the Li22Sn5 that forms at low Li insertions is not typical of bulk L i22Sn5 in either its structural or electrochemical properties. The recoil-f ree fraction of the Sn oxide (and perhaps the metallic Sn) in the anode mat erials showed an anomalously large temperature dependence. This is indicati ve of nanoparticles or a severely defective structure. We monitored the cha nges in the Li-SnO and Li-Sn materials during atmospheric exposure over tim es up to 2 months. This oxidation process of Sn was very much the reverse o f the Sn reduction during the Li insertion, although it occurred over a muc h longer time scale. We also report the temperature dependencies of recoil- free fractions for standard samples of beta-Sn, SnO2, and the alloy Li22Sn5 . (C) 2000 The Electrochemical Society. S0013-4651 (99)05-009-0. All rights reserved.