Stabilizing the Phase Li15Si4 through Lithium–Aluminum Substitution in Li15–xAlxSi4 (0.4 < x < 0.8)—Single Crystal X-ray Structure Determination of Li15Si4 and Li14.37Al0.63Si4

Single crystals of Li15Si4 and Li15–xAlxSi4 (x = 0.63(1)) were obtained from equilibrated melts with compositions Li100–xSix (x = 10, 15) and Li83Al13Si4, respectively, and isolated by isothermal centrifugation. Li15Si4 and Li14.37(1)Al0.63(1)Si4 crystallize with the Cu15Si4 structure type (I4̅3d, ax=0 = 10.6322(9) Å, ax=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 Li15–xAlxSi4 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 Li15–xAlxSi4 is thermodynamically stable. The decomposition temperature is at about 700 °C. It is speculated that the thermodynamic stability of Li15–xAlxSi4 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 Li15–xAlxSi4 may have interesting properties as anode material in lithium ion batteries.