First-order amorphous-to-amorphous phase transitions during lithiation of silicon thin films

First-order amorphous-to-amorphous phase transitions (polyamorphic transitions) are rarely observed and have been associated with pressure or temperature changes. The authors report first-order polyamorphic transitions caused by changes in composition. Observations were made during electrochemical insertion of lithium into amorphous silicon films under potentiostatic conditions. Kinetic analyses using the Johnson-Mehl-Avrami-Kolmogorov model applied to measurements of current as a function of time at different overpotentials indicate nucleation and growth of Li-rich phases throughout the volume of the films. This conclusion is supported by electron microscope images of twophase amorphous films with image contrast provided through preferential high energy electron sputtering of Li from Li-rich phases.