In situ X-ray diffraction studies of displacement reactions in intermetallic systems

Kristina Edström, Hanna Bryngelsson and Linda Fransson

Department of Materials Chemistry, Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden

Several studies have been made on intermetallic structures, which can accommodate lithium with minimal structural changes: Cu₆Sn₅ (NiAs-type structure) [1], InSb (Zinc-blende type structure) [2] and Cu₂Sb (P₄/nmm) [3]. Several of these, i. e. Cu₆Sn₅, Cu₂Sb and MnSb have proven to form isostructural intermediate ternary phases on lithiation, Li₂CuSn, Li₂CuSb and Li2MnSb (Zinc-blende type structure). These reactions involve metal extrusion and structural rearrangements of the parent structures. For Cu₆Sn₅, it involves a displacement of 50% of the Sn atoms within the structure accompanied by extrusion of some Cu. This reaction corresponds to a volume expansion of 59%. For Cu₂Sb, the phase transformation to Li₂CuSb is facilitated by the very strong structural relationship between the two phases. The Sb atoms of Cu₂Sb create a slightly distorted face-centered framework that, on lithiation, undergoes small displacements to create the "CuSb" face-centered array of Li₂CuSb. This reaction corresponds to a volume expansion of 25%. Both the Li₂CuSb and Li₂CuSn phases show a solid-solution behavior of lithium within the structures. Further lithiation of Li₂CuSb results in extrusion of the remaining Cu atoms and the formation of Li₃Sb, a process during which the Sb face-centered array is kept virtually invariant. The cubic-cubic phase transformation from Li₂CuSb to Li₃Sb involves a volume expansion of only 13%. The reactions for the Cu₂Sb system can be summarized as follows:

Cu₂Sb + Li ® Li₂CuSb + Cu (1)

Li₂CuSb + 2 Li ® Li₃Sb + Cu (2)

In this presentation, emphasis is placed on the structural transformations of two intermetallic systems: Mn₂Sb (Cu₂Sb-type structure) and MnSb (NiAs-type structure), shown in Fig. 1. The similarities/discrepancies between these systems and the earlier studied structures are explored in terms of structural transformations and electrochemical performance.

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2. J. T. Vaughey, J. O´Hara and M. M. Thackeray, Electrochem. Solid State Lett., 3 (2000) 13.

3. L. M. L. Fransson, J. T. Vaughey, R. Benedek, K. Edström, J. O. Thomas and M. M. Thackeray, Electrochem. Comm., 3 (2001) 317.