On the basis of density functional theory calculations, we present the energetics, structure, and electronic and mechanical properties of crystalline and amorphous Li-Si alloys. We also discuss the dynamic behavior of the alloys at finite temperatures based on ab initio molecular dynamics. When the Li content is sufficiently high, alloying between Li and Si is energetically favorable as evidenced by the negative mixing enthalpy; the alloy is most stable around 70 atom % Li in the crystalline phase and 70 ± 5 atom % Li in the amorphous phase. Our calculations unequivocally show that the incorporation of Li leads to disintegration of the tetrahedrally bonded Si network into small clusters of various shapes. Bader charge analysis shows that the charge state of Li remains nearly unchanged around +0.8, while that of Si varies approximately from -0.5 to -3.3 depending on the number of Si neighbors as can be understood as Zintl-like phases. Electronic structure analysis highlights that the charge transfer leads to weakening or breaking of Si-Si bonds with the growing splitting between 3s and 3p states, and consequently, the Li-Si alloys soften with increasing Li content.