We have measured the phase kinetics and stress evolution during potentiostatic lithiation of Sn thin film layers to form the first lithiated phase (Li2Sn5). Real-time measurements of the wafer curvature are used to determine the stress evolution during lithiation; the corresponding layer structure is determined from the measured charge and confirmed with cross-sectional focused-ion beam (FIB) images and X-ray diffraction (XRD). Prior to new phase formation, a 20-hour conditioning step is used to form a solid-electrolyte interphase (SEI) layer. The Stress in the SEI and Sn layers are determined by stress measurements carried on different thicknesses of Sn film. When the potential is lowered to initiate Li2Sn5 formation, both the electrochemical current and stress exhibit a complex transient behavior as Li2Sn5 phase nucleates and grows. The stress evolution in this regime is attributed to rate-dependent plasticity in the Sn layer. At longer times, as the new phase propagates into Sn, a steady-state is reached that enables us to determine the stress in the Li2Sn5 layer. The phase evolution in the steady-state regime is analyzed with a kinetic model that enables us to estimate the Li diffusivity in Li2Sn5 and the reaction rate coefficient controlling the phase transformation. (C) 2017 The Electrochemical Society.All rights reserved.