Short-range Sn ordering and crystal structure of Li4.4Sn prepared by ambient temperature electrochemical methods

The unique powder diffraction pattern of Li4.4Sn (or Li22Sn5), prepared by the electrochemical reaction of Li with Sn at room temperature, displays both sharp diffraction peaks and intense broad oscillations. This pattern is explained here by an underlying BCC lattice, of cube edge a, on which all the Li and Sn atoms are placed, with the tin atoms positioned in groups of randomly oriented tetrahedra of Sn, having edges of length root 2a. Calculations of the diffraction patterns of model structures show that the sharp peaks arise from the underlying lattice and the broad intense oscillations from the tin tetrahedra. The crystal structure of Li,,Sn, prepared at elevated temperatures has the same underlying BCC lattice but has the tin tetrahedra arranged in a regular fashion, leading to a diffraction pattern showing only sharp peaks. Recent total energy calculations of the binary phases in the Li-Sn system, used to predict the voltage versus composition of Li/LixSn electrochemical cells, assumed that the ordered bulk crystalline phases formed sequentially as the cell discharged (Courtney et al., unpublished results). Although experiment (Courtney and Dahn, J. Electrochem. Sec. 144 (1997) 2045) showed this to be true for Li2Sn5 and LiSn, the other bulk phases were not directly observed experimentally. Instead the pattern which we now recognize as characteristic of an underlying BCC lattice and randomly positioned groups of tin tetrahedra was observed over a wide range of 2.5 <x<4.4 in LixSn. Thus, our work also serves as a warning to theorists, that ordered equilibrium phases are not always observed during room temperature electrochemical reactions. (C) 1998 Elsevier Science B.V. All rights reserved.