Electronic origin for the phase transition from amorphous Li_xSi to crystalline Li₁₅Si₄

Silicon has been widely explored as an anode material for lithium ion battery. Upon lithiation, silicon transforms to amorphous Li_xSi (a-Li_xSi) via electrochemical-driven solid-state amorphization. With increasing lithium concentration, a-Li_xSi transforms to crystalline Li₁₅Si₄ (c-Li₁₅Si₄). The mechanism of this crystallization process is not known. In this paper, we report the fundamental characteristics of the phase transition of a-Li_xSi to c-Li₁₅Si₄ using in situ scanning transmission electron microscopy, electron energy loss spectroscopy, and density function theory (DFT) calculation. We find that when the lithium concentration in a-Li_xSi reaches a critical value of x = 3.75, the a-Li_3.75Si spontaneously and congruently transforms to c-Li₁₅Si₄ by a process that is solely controlled by the lithium concentration in the a-Li_xSi, involving neither large-scale atomic migration nor phase separation. DFT calculations indicate that c-Li₁₅Si₄ formation is favored over other possible crystalline phases due to the similarity in electronic structure with a-Li_3.75Si.