Silicides and Novel Alloys with Open Tetrahedral Framework Structures Hosting Lithium Atoms

Alloys and intermetallic compounds are emerging as new generation anode materials with high lithium storage capacity [1].Due to the higher density of alloys Li insertion results in a higher energy density . Further is the operating voltage of alloy anodes well above the potential of metallic Li and there are less solvent intercalations for alloys.

It is well known that (semi)metals and alloys undergo several phase changes during the intercalation process. For example, the lithitation/delithiation process of Si is highly complex, involving crystalline-to-amorphous phase transitions and various amorphous phases. For a long time Li_3.75Si (78.9 atom % Li) had been considered to be the fully lithiated state in a typical electrochemical reaction. Depending on the applied discharge voltage and the design of the Si anode amorphous Li_3.75Si can undergo a transition to a crystalline but metastable phase Li₁₅Si₄. During our re-investigation of the Li-Si phase diagram we established Li_4.25Si as the Li-richest phase, and found the novel high-temperature phase Li_4.11Si. Further we report that the metastable Li_3.75Si transforms to a stable phase through addition of Al [2].

In the course of our studies we searches for alternatives to pure silicon and have now synthesized novel framework structures consisting of boron and silicon, Zn and Sn, Zn and Ge as well as Ag and Ge which offer all tetrahedral framework structures that host Lithium atoms [3,4].

[1] W. J. Zhang, J. Power Sources 196 (2011) 13. M. N. Obrovac, L. Christensen, Electrochem. Solid-State Lett. 7 (2004) A93.
[2] M.Zeilinger, D. Benson, U. Häussermann, T. F. Fässler, Chem. Mater. 25 (2013) 1960−1967
[3] M. Zeilinger, L. van Wüllen, D. Benson, V. F. Kranak, S. Konar, T. F. Fässler, U. Häussermann, Angew. Chem. Int. Ed. 52 (2013) 5978 –5982
[4]  S. Stegmaier, T. F. Fässler, Inorg. Chem. 52 (2013) 2809-2816.