A ^(119)Sn Mössbauer Spectrometry Study of Li-SnO AnodeMaterials for Li-Ion Cells

Anodes of SnO were charged reversibly with Li to capacities greater than 600 mAh/g, The anode materials were characterized by 119Sn Mossbauer spectrometry at 11 and 300 K, and by X-ray diffractometry at 300 K. Trends in the valence of Sn were as expected when the Sn oxides are reduced in the presence of Li. At low Li capacities the SnO is reduced to small particles of β-Sn, and with increasing Li capacity an alloy of Li_(22)Sn_5 is formed. Although the Li_(22)Sn_5 develops over a range of Li concentrations in the anode material, the Li_(22)Sn_5 that forms at low Li insertions is not typical of bulk Li_(22)Sn_5 in either its structural or electrochemical properties. The recoil-free fraction of the Sn oxide (and perhaps the metallic Sn) in the anode materials showed an anomalously large temperature dependence. This is indicative of nanoparticles or a severely defective structure. We monitored the changes in the Li-SnO and Li-Sn materials during atmospheric exposure over times up to 2 months. This oxidation process of Sn was very much the reverse of the Sn reduction during the Li insertion, although it occurred over a much longer time scale. We also report the temperature dependencies of recoil-free fractions for standard samples of β-Sn, SnO_2, and the alloy Li_(22)Sn_5.