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 (I (4) over bar 3d, a(x=0) = 10.6322(9) angstrom, a(x=0.63(1)) = 10.6172(4) angstrom, 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 degrees C. However, the decomposition process of Li15Si4, is sluggish and appreciable rates are not observed before temperatures reach 400 degrees C. In contrast Li15-xAlxSi4 is thermodynamically stable. The decomposition temperature is at about 700 degrees 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.