Stabilizing the Phase Li₁₅Si₄ through Lithium–Aluminum Substitution in Li_15–xAl_xSi₄ (0.4 < x < 0.8)—Single Crystal X-ray Structure Determination of Li₁₅Si₄ and Li_14.37Al_0.63Si₄

Single crystals of Li 15 Si 4 and Li 15– x Al x Si 4 ( x = 0.63(1)) were obtained from equilibrated melts with compositions Li 100– x Si x ( x = 10, 15) and Li 83 Al 13 Si 4, respectively, and isolated by isothermal centrifugation. Li 15 Si 4 and Li 14.37(1) Al 0.63(1) Si 4 crystallize with the Cu 15 Si 4 structure type ( I 4̅3 d, a x =0 = 10.6322(9) Å, a x =0.63(1) = 10.6172(4) Å, Z = 4, T = 123 K). The incorporation of Al equally affects both crystallographically distinguished Li positions in the Li 15 Si 4 structure. The replacement of about 4% of Li is firmly established by the refinement of single crystal diffraction data and NMR spectroscopy. The homogeneity range of Li 15– x Al x Si 4 was assessed as 0.4 < x < 0.8 from synthesis experiments using stoichiometric proportions of the elements. Differential scanning calorimetry studies confirm the metastable character of Li 15 Si 4, decomposing exothermally at temperatures around 200 °C. However, the decomposition process of Li 15 Si 4 is sluggish and appreciable rates are not observed before temperatures reach 400 °C. In contrast Li 15– x Al x Si 4 is thermodynamically stable. The decomposition temperature is at about 700 °C. It is speculated that the thermodynamic stability of Li 15– x Al x Si 4 is a consequence of the increased electron concentration, shifting the Fermi level to a pseudo-gap in the electronic density of states. Since metastable Li 15 Si 4 plays an important role during electrochemical lithiation of a silicon anode, thermodynamically stable Li 15– x Al x Si 4 may have interesting properties as anode material in lithium ion batteries.