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 Li₁₅Si₄ and Li_15–xAl_xSi₄ (x = 0.63(1)) were obtained from equilibrated melts with compositions Li_100–xSi_x (x = 10, 15) and Li₈₃Al₁₃Si₄, respectively, and isolated by isothermal centrifugation. Li₁₅Si₄ and Li_14.37(1)Al_0.63(1)Si₄ crystallize with the Cu₁₅Si₄ structure type (I4̅3d, 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 Li₁₅Si₄ 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–xAl_xSi₄ 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 Li₁₅Si₄, decomposing exothermally at temperatures around 200 °C. However, the decomposition process of Li₁₅Si₄ is sluggish and appreciable rates are not observed before temperatures reach 400 °C. In contrast Li_15–xAl_xSi₄ is thermodynamically stable. The decomposition temperature is at about 700 °C. It is speculated that the thermodynamic stability of Li_15–xAl_xSi₄ is a consequence of the increased electron concentration, shifting the Fermi level to a pseudo-gap in the electronic density of states. Since metastable Li₁₅Si₄ plays an important role during electrochemical lithiation of a silicon anode, thermodynamically stable Li_15–xAl_xSi₄ may have interesting properties as anode material in lithium ion batteries.