Revision of the Li-Si Phase Diagram: Discovery and Single-Crystal X-ray Structure Determination of the High-Temperature Phase Li4.11Si

Silicon has been regarded as a promising anode material for future lithium-ion batteries, and Li-Si phases play an important role. A detailed reinvestigation of the Li-rich part of the binary Li-Si phase diagram revealed the existence of a new phase, Li4.106(2)Si (Li16.42Si4). Li16.42Si4 forms through the peritectic decomposition of the Li-richest phase Li17Si4 at 481-486 degrees C and was characterized by single-crystal X-ray diffraction (a = 4.5246(2) angstrom, b = 21.944(1) angstrom, c = 13.2001(6) angstrom, space group Cmcm, Z = 16), differential scanning calorimetry, and theoretical calculations. Li16.42Si4 represents a high-temperature phase that is thermodynamically stable above similar to 480 degrees C and decomposes peritectically at 618 +/- 2 degrees C to Li13Si4 and a melt. Li16.42Si4 can be retained at room temperature. The structure consists of 3 and 10 different kinds of Si and Li atoms, respectively. Two Li positions show occupational disorder. Si atoms are well-separated from each other and have only Li atoms as nearest neighbors. This is similar to Li17Si4 and Li15Si4 compositionally embracing Li16.42Si4. The SiLin coordination polyhedra in the series Li15Si4, Li16.42Si4, and Li17Si4 are compared. Li15Si4 exclusively features coordination numbers of 12, Li16.42Si4 of 12 and 13, and Li17Si4 reveals 13- and 14-coordinated Si atoms. The band structure and density of states of Li16.42Si4 were calculated on the basis of two ordered model structures with nominal compositions Li16Si4 (a hypothetical Zintl phase) and Li16.5Si4. Both reveal a metallic character that is analogous to Li17Si4. In contrast, the electronic structure of Li15Si4 is characteristic of a p-doped semiconductor.