The mechanisms and kinetics of lithiation and delithiation of amorphous silicon were investigated using potentiostatic techniques and thin films of different thickness, with a focus on the initial lithiation process that occurs in the first cycle. In potentiostatic tests, distinct kinks were observed in the current vs. time curves, and the time at which the kink occurred increased for thicker films. This behavior can be explained using a model in which a sharp interface between an amorphous LixSi phase and Li-saturated amorphous Si propagates through the film. Using this model, the rate-limiting process was determined to be diffusion of Li in the LixSi phase rather than reaction at the lithiation front. The Li diffusivity in the lithiated phase was determined to be in the 10-13 cm(2)/s range, independent of film thickness above 135 nm. The thin-film potentiostatic technique used in this study should prove useful in investigation of the mechanisms and rate parameters for other phase transitions that occur during lithiation of silicon and for kinetic studies of other electrode materials. (C) The Author(s) 2018. Published by ECS.