Kinetic Study of Lithiation-Induced Phase Transitions in Amorphous Germanium Thin Films

Abstract

Mechanisms of the reversible lithiation of amorphous germanium thin films were studied using electrochemical and structural characterization techniques. Cyclic voltammograms (CV) exhibited major lithiation peaks and delithiation peaks, the latter of which turned out to be highly dependent on the lithiation cutoff voltage and closely associated with the formation of the crystalline phase c-Li15Ge4. Through detailed analyses of CV and potentiostatic tests we demonstrate that the features of electrochemically induced phase transitions vary with lithiation voltage. A surprising finding is that two phase transitions occur simultaneously in the 0.29–0.14 V range. In this regime, three phases, two amorphous phases with different stoichiometries and the crystalline phase, can co-exist. The cycle-number dependence of formation of the crystalline phase as well as structural analyses suggest that crack formation may play an important role in reducing the kinetic barrier associated with crystallization. This study sheds new light on the kinetic constraints on lithiation of a-Ge and yields insight for electrode design and development of cycling schemes for optimized performance and cyclability.