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

Abstract

The kinetics and mechanisms of phase transitions that occur during lithiation of amorphous silicon thin films were extensively studied using electrochemical techniques and transmission electron microscopy. Measurements of current made while changing applied potentials at a constant rate (cyclic voltammograms) showed peaks that correspond to the formation of two amorphous Li-Si alloys with different stoichiometries. Peaks associated with these phase transitions were also observed in current-time measurements made in potentiostatic experiments. The times at which currents reached maxima were found to be independent of film thickness, suggesting that the phase transitions occur throughout the volume of the films. Analysis of current-time measurements using the Johnson-Mehl-Avrami-Kolmogorov method indicate that the transitions occur through a first-order nucleation and growth process. This conclusion is supported by cross-section transmission electron microscopy in which high-energy electron-beam-induced sputtering of lithium led to contrast between the parent and product phases. In contrast with other studies, the polyamorphic amorphous-to-amorphous first-order phase transitions observed in this study are driven by changes in composition rather than changes in pressure or temperature. The potentiostatic techniques employed in this study can be used to characterize the nature and kinetics of phase transitions that occur in other electrode materials for lithium-ion batteries.